Queensland University of Technology School of Natural Resource Sciences
Volcanic Stratigraphy, Alteration Zoning, and
Vein Paragenesis of the Sascha-Pelligrini Low-
Sulphidation Epithermal System, Santa Cruz,
Argentina
By
Quinn Eric Smith B. App. Sc. (QUT)
2009
Supervisor: Assoc. Prof David Gust
A Thesis submitted for the degree of Master of Applied Science (Queensland University of Technology)
I
KEYWORDS Epithermal, Low-sulphidation, Gold-Silver, Vein Paragenesis, Deseado Massif, Chon Aike, Santa Cruz, Argentina
II
Abstract The Sascha-Pelligrini low-sulphidation epithermal system is located on the
western edge of the Deseado Massif, Santa Cruz Province, Argentina.
Outcrop sampling has returned values of up to 160g/t gold and 796g/t silver,
with Mirasol Resources and Coeur D‟Alene Mines currently exploring the
property.
Detailed mapping of the volcanic stratigraphy has defined three units that
comprise the middle Jurassic Chon Aike Formation and two units that
comprise the upper Jurassic La Matilde Formation. The Chon Aike Formation
consists of rhyodacite ignimbrites and tuffs, with the La Matilde Formation
including rhyolite ash and lithic tuffs. The volcanic sequence is intruded by a
large flow-banded rhyolite dome, with small, spatially restricted granodiorite
dykes and sills cropping out across the study area.
ASTER multispectral mineral mapping, combined with PIMA (Portable Infra-
red Mineral Analyser) and XRD (X-ray diffraction) analysis defines an
alteration pattern that zones from laumontite-montmorillonite, to illite-pyrite-
chlorite, followed by a quartz-illite-smectite-pyrite-adularia vein selvage.
Supergene kaolinite and steam-heated acid-sulphate kaolinite-alunite-opal
alteration horizons crop out along the Sascha Vein trend and Pelligrini
respectively.
Paragenetically, epithermal veining varies from chalcedonic to saccharoidal
with minor bladed textures, colloform/crustiform-banded with visible electrum
and acanthite, crustiform-banded grey chalcedonic to jasperoidal with fine
pyrite, and crystalline comb quartz. Geothermometry of mineralised veins
constrains formation temperatures from 174.8 to 205.1°C and correlates with
the stability field for the interstratified illite-smectite vein selvage.
Vein morphology, mineralogy and associated alteration are controlled by host
rock rheology, permeability, and depth of the palaeo-water table.
Mineralisation within ginguro banded veins resulted from fluctuating fluid pH
III
associated with selenide-rich magmatic pulses, pressure release boiling and
wall-rock silicate buffering.
The study of the Sascha-Pelligrini epithermal system will form the basis for a
deposit-specific model helping to clarify the current understanding of
epithermal deposits, and may serve as a template for exploration of similar
epithermal deposits throughout Santa Cruz.
IV
CONTENTS
INTRODUCTION ........................................................................................... 1
REGIONAL GEOLOGICAL SETTING ............................................................................................ 3 SASCHA-PELLIGRINI EPITHERMAL SYSTEM ............................................................................... 7
METHODS ..................................................................................................... 9
FIELD INVESTIGATIONS ........................................................................................................... 9 SAMPLE PREPARATION AND ANALYTICAL TECHNIQUES ............................................................ 10
Short Wave Infrared (SWIR) spectrometry ................................................................... 10 X-ray Diffraction ............................................................................................................. 11 Microscopy .................................................................................................................... 12 Geochemistry ................................................................................................................ 13 Remote sensing ............................................................................................................. 14
RESULTS .................................................................................................... 16
STRATIGRAPHY .................................................................................................................... 16 PETROGRAPHY .................................................................................................................... 21
Chon Aike Formation ..................................................................................................... 21 La Matilde Formation ..................................................................................................... 22 Other units ..................................................................................................................... 23
GEOCHEMISTRY ................................................................................................................... 26 STRUCTURAL SETTING .......................................................................................................... 33 EPITHERMAL VEINING ........................................................................................................... 36
Quartz textures .............................................................................................................. 36 VEIN GEOCHEMISTRY AND MINERALOGY ................................................................................ 39 GEOTHERMOMETRY ............................................................................................................. 42 ALTERATION ........................................................................................................................ 43
Regional Alteration – Multispectral Mineral Mapping .................................................... 43 Prospect Alteration – PIMA and XRD............................................................................ 46
ALTERATION GEOCHEMISTRY ................................................................................................ 55
DISCUSSION ............................................................................................... 61
VOLCANOLOGY .................................................................................................................... 61 Depositional setting ....................................................................................................... 61 Eruption styles ............................................................................................................... 63 Magma Petrogenesis..................................................................................................... 68
HOST ROCK CONTROL AND STRUCTURAL MODEL .................................................................. 70 ALTERATION ZONING ............................................................................................................ 74 VEIN PARAGENESIS .............................................................................................................. 82 SUPERGENE OVERPRINT ...................................................................................................... 88 SUMMARY ............................................................................................................................ 89
CONCLUSION ............................................................................................. 93
REFERENCES ............................................................................................ 96
APPENDIX 1.............................................................................................. 109
APPENDIX 2.............................................................................................. 115
APPENDIX 3.............................................................................................. 119
APPENDIX 4 (MAP) .................................................................................. 121
APPENDIX 5 (MAP) .................................................................................. 122
V
List of Figures Figure 1. Conceptual models and genetic classifications of epithermal deposits .................... 3
Figure 2. Location of the Sascha-Pelligrini study area. ............................................................ 6
Figure 3. Sample and prospect locations ................................................................................. 7
Figure 4. Comparison of accepted results for rock standard SEQG566 with returned
analysis. ...................................................................................................................14
Figure 4. Stratigraphy for the Sascha-Pelligrini study area. ...................................................19
Figure 5. Interpretive geology, structure and mapped veining for the Sascha-Pelligrini
study area. ...............................................................................................................20
Figure 7. XRF major element results for Sascha-Pelligrini volcanic samples ........................29
Figure 8. ICP-MS trace element results for Sascha-Pelligrini volcanic samples. ..................30
Figure 9. REE spider diagrams for Sascha-Pelligrini volcanic samples. ...............................31
Figure 10. REE geochemistry normalised to upper crust, Bajo Pobre and lower crust
xenoliths ................................................................................................................32
Figure 11. Vein trace orientation in Sascha Main indicating dextral oblique-slip
movement..............................................................................................................33
Figure 12. Sascha Main saccharoidal and chalcedonic vein phases ....................................35
Figure 13. Sascha Main ginguro vein phases ........................................................................35
Figure 14. Sascha Main pyritic-chalcedonic vein phases ......................................................35
Figure 15. Backscattered SEM images of characteristic vein mineral assemblages.............41
Figure 16. Backscattered SEM images of coexisting electrum-sphalerite grains ..................42
Figure 17. Selected end-member spectra compared to known library spectra .....................44
Figure 18. Aster mineral mapping results, simplified geology and gold geochemistry ..........45
Figure 19. Selected end-member spectra used for mineral mapping and spectral
unmixing ................................................................................................................47
Figure 20. PIMA and XRD profiles of individual vein phases from the Sascha Main
vein zone ...............................................................................................................50
Figure 21. PIMA and XRD profiles of individual vein phases from the Sascha Ginguro
vein zone ...............................................................................................................51
Figure 22. PIMA and XRD profiles of individual vein phases from the pyrite-chalcedony
vein zone ...............................................................................................................52
Figure 23. PIMA and XRD profiles of individual vein phases from the Sascha Sur
vein zone ...............................................................................................................53
Figure 24. ESEM images of alteration mineral morphologies ................................................54
Figure 25. Selected immobile elements and geochemical mass-changes for rhyolite
crystal ash tuff alteration within Sascha Main. ......................................................57
Figure 26. Selected immobile elements and geochemical mass-changes for rhyodacite
ignimbrite alteration within Sascha Sur. ................................................................58
Figure 27. Selected immobile elements and geochemical mass-changes for rhyolite
ash tuff alteration within Pelligrini. .........................................................................59
VI
Figure 28. Bar graph comparing net mass-changes for immobile elements Dy and Zr ........60
Figure 29. Styles of explosive eruptions ................................................................................67
Figure 30. Structural model for the Sascha-Pelligrini study area ...........................................72
Figure 31. Riedel shear model for the Sascha – Pelligrini and Huevos Verdes systems ......72
Figure 32. Alteration zoning and mineral assemblage model ................................................75
Figure 33. Alteration zoning and mineral assemblages of the Hishikari epithermal
system ...................................................................................................................78
Figure 34. Vein paragenetic relationships for the Sascha-Pelligrini epithermal system ........82
Figure 35. Vein mineral paragenetic relationships for the Sascha-Pelligrini epithermal
system ...................................................................................................................83
Figure 36. Conceptual epithermal model for the Sascha-Pelligrini epithermal system..........92
List of Tables Table 1. End-member alteration mineral spectra locations ....................................................14
Table 2. Representative whole-rock geochemical analysis. ...................................................25
Table 3. Summary geochemical signatures of Sascha Main vein phases. ............................39
Table 4. Calculated electrum-sphalerite formation temperatures...........................................42
Table 5. PIMA and XRD results of characteristic alteration assemblages .............................47
Table 6. Tabulated correlation coefficient values for immobile elements Dy, Sm and Y. ......60
List of Appendices Appendix 1. Petrography .....................................................................................................110
Appendix 2. Quartz textures. ...............................................................................................116
Appendix 3. Digital Dataset. .................................................................................... CD Pocket
Appendix 4. Sascha-Pelligrini Fact Geology Map. ................................................ Map Pocket
Appendix 5. Sascha-Pelligrini Interpretive Geology Map. ..................................... Map Pocket
VII
Statement of Original Authorship The work contained in this Thesis has not been previously submitted for a degree of diploma at any other higher education institution. To the best of my knowledge, this contains no material previously published or written by another person except where due reference is made. Signed: ___________________________ Date: ___________________________
VIII
Acknowledgements There have been numerous people who have provided invaluable assistance
throughout the duration of my project who I would like to thank. Firstly, I
would like to sincerely thank my supervisor Associate Professor David Gust
for his time, work and commitment over the duration of the project.
I would also like to acknowledge the support and technical advice provided
by Stephen Nano, Daryl Nunn and Mirasol Resources during field visits to
Argentina. The project would not have been able to succeed without them.
Thank you to the QUT technical staff, in particular Luke Nothdurft, Loc Duong
and Bill Kwiecien for their assistance and time on all the machines at QUT.
Thanks to Peter Cole from the UQ rock prep lab for helping to process all the
thin sections and samples.
Finally, special thanks must go to my family and friends for their support and
encouragement along the way.
Quinn Smith Master of Applied Science Thesis
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Introduction
Epithermal deposits provide significant gold for world reserves. Individual
deposits can exceed 40 million ounces of contained gold (Lihir, Papua New
Guinea), produce over 1 million ounces of gold per annum (Porgera, Papua
New Guinea), and contain spectacular gold grades in excess of 100 ounces
per ton (Midas, Nevada; Hishikari, Japan). Epithermal deposits vary
significantly in size and form. They are often characterised by relatively
small, banded quartz-chalcedony veins with spectacular visible gold, or large
scale disseminated mineralisation associated with residual „vuggy‟ silica.
Their importance provides impetus for understanding how they form.
Epithermal deposits are defined as being „formed by ascending hot waters
near the surface in or near effusive rocks at relatively low temperature and
pressure‟ (Lindgren, 1922) and are analogous to modern geothermal
systems, with formation conditions of less than 200°c and less than 100 bars
(Lindgren, 1933). Recent fluid inclusion studies suggest temperatures of ore
deposition are less than 300°C, stable isotope analysis are consistent with a
meteoric source of water and a magmatic volatile source for sulphur and
carbon (Cooke and Simmons, 2000).
Initial classifications of epithermal deposits were based on geologic studies
that summarise common features and formulate schematic or conceptual
models. Conceptual models portray the anatomy of an epithermal deposit,
showing the vertical and horizontal mineral and alteration zoning typically
observed in epithermal districts (Buchanan, 1981). End-member models
were formulated to incorporate deposit variability, and lead to classification
based on depositional settings (Berger and Eimon, 1982). Detailed
paragenetic studies outline variations in mineralogy, and indicate that
epithermal deposits can be classified by observed mineral assemblages
irrespective of depositional setting (Bonham, 1986; Heald et al, 1987; Berger
and Henly, 1988).
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Studies on phase relationships of observed mineral assemblages suggest
that the variability of epithermal deposits is due principally to variations in
fluid chemistry. Genetic classifications incorporate this observation and relate
the deposit type to the oxidation state of the mineralising fluid (White and
Hedenquist, 1990; Sillitoe, 1993; White and Hedenquist, 1995). Low-
sulphidation (LS) epithermal mineralisation forms from reduced, near-neutral
pH conditions, with H2S(aq) the predominant sulphur species. Temperatures of
ore deposition are less than 300°C and salinities are usually less than 3.5
weight percent NaCl equivalent (Cooke and Simmons, 2000). High-
sulphidation (HS) epithermal mineralisation forms from oxidised, acidic
conditions, with SO2(g) formed from the disproportionation of magmatic gases.
Temperatures of ore deposition vary from greater than 400°C to 100°C, with
salinities generally less than 5 weight percent NaCl equivalent (Cooke and
Simmons, 2000).
Current research in epithermal mineralisation indicates that HS and LS
deposits are end-members of a transitional environment, with the existence
of deposits characterised by mineral assemblages intermediate between HS
and LS deposits (Hedenquist and Arribas, 2000). Genetic classifications of
epithermal deposits are subject to continual debate. Corbett (2002, 2004, and
2005) suggests epithermal mineralisation forms largely from the same fluid
source, with end-member deposits due to differing tectonic regime, host
rocks, depth of formation, relation to intrusive bodies and dominance of
circulating meteoric fluids (Figure 1).
Specific deposits often differ from the general model, with conceptual and
genetic classifications of epithermal deposits continually evolving.
Examination of individual deposits both test and refine the general model.
This thesis aims to develop a deposit specific model for the geology, zoning
of vein mineral textures, mineral assemblages and associated geochemistry,
and alteration assemblages for the Sascha – Pelligrini LS epithermal system
in Santa Cruz, Argentina. The deposit specific model will help to clarify the
current understanding of epithermal deposits by providing a test of the
genetic and conceptual classifications. The deposit specific model may also
Quinn Smith Master of Applied Science Thesis
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serve as a template for exploration of similar epithermal deposits throughout
Santa Cruz.
Regional geological setting
The Sascha-Pelligrini epithermal system falls within the Deseado Massif - a
large region of subdued upland physiography that is flanked by the Austral
Basin to the south, the San Jorge Basin to the north, the Andean cordillera to
the west and the Atlantic Ocean to the east. The Deseado Massif is host to
thick sequences of Permo-Triassic rift sediments emplaced in north- to
northwest-trending basins in Precambrian and Lower Palaeozoic rocks
(Echavarria et al, 2005). Precambrian and Lower Palaeozoic upper
greenschist to amphibolite facies metamorphic basement crops out within or
near the valley of the Rio Deseado. Sedimentation occurred at the onset of
widespread extensional tectonics that eventually resulted in the
Gondwanaland break-up. The Middle Triassic El Tranquilo Formation
represents the last traces of this phase of sedimentation as uplift commenced
Figure 1. Conceptual models and genetic classifications of epithermal deposits showing end-member high- and low-sulphidation styles to be part of a broader range
of hydrothermal systems. (Corbett, 2005).
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in the Lower Jurassic. Calc-alkalic granitic stocks and dikes of the La Leona
Formation, which are found in rare localities to the east, (Sanders, 2000)
were emplaced during this uplift.
After the lower Jurassic uplift, widespread subsidence and deposition of a
range of continental-fluvial sedimentary rocks occurred in the Deseado
Massif.
Volcanic activity commenced in the Lower-Middle Jurassic. This activity was
characterised by the extrusion of flood basalts and the intrusion of mafic
dikes and sills that, together with minor clastic sedimentation, compose the
Bajo Pobre Formation. The thickness of this sequence ranges from 200 to
1,000 meters and is controlled by northeast-trending half-graben structures of
which the southern sides show the thickest accumulations (Sanders, 2000).
These structures were created by a new kinematic regime that persisted
through to the Neocomian that produced a structural fabric that is
approximately normal to the Permo-Triassic graben trends.
Unconformably overlying the Bajo Pobre Formation is the Bahia Laura
Group, which is comprised of the Middle Jurassic Chon Aike Formation and
the Middle to Upper Jurassic La Matilde & Bajo Grande Formations. These
rocks form a large ignimbritic plateau composed of lava flows, pyroclastic
rocks, ash-flow tuffs and re-worked volcanic and non-volcanic epiclastic
sequences. The volcanic rocks range in composition from basaltic-andesite
lavas and rhyodacite to rhyolitic ash-flow tuffs. The sequence represents a
marked increase in the volume and areal extent of volcanic deposition as
magmatic activity migrated westward towards the Andean continental margin
(Gust et al, 1985).
The Deseado Massif epithermal veins are contemporaneous with the waning
stages of the volcanism represented by the Bahia Laura Group rocks. Dating
of various mineral deposits (Echavarria et al, 2005) indicates that the
volcanic host rocks are only several millions of years older than the
hydrothermal systems responsible for the economic mineralisation.
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Circular caldera structures, irregular partial-collapse features and linear
fissures are identified within the Chon Aike Formation and are locally
associated with mineralised hydrothermal alteration systems. A late-stage
resurgent dome activity has emplaced the rhyolitic and dacitic intrusive rocks
and ash-flow tuffs of the La Matilde and Bajo Grande Formations (Sanders,
2000). This period of hypabyssal volcanic activity is closely related to
economic mineralisation throughout the Massif.
A series of rock formations have been deposited within the Deseado Massif
subsequent to the Late Jurassic. These form cover sequences to the
mineralisation and represent the end of subsidence and the establishment of
„cratonic‟ stability across the Massif. These cover sequences include the
continental sediments and pyroclastic rocks of the Middle Cretaceous
Baqueró Formation and a series of back-arc, olivine basalt flows that inter-
finger with continental and shallow marine volcano-clastic sediments of
Upper Cretaceous to Upper Tertiary transgression-regression cycles (Gorring
et al, 1997). The Andean stage of Late Cainozoic uplift resulted in further
widespread eruption of olivine basalt flows and tuffs that form the dissected
plateaus of the modern landscape (Panza and Franchi, 2002). The Deseado
Massif is covered in the north and south by Late Pliocene to Recent coarse
gravels known as the “Rodados Patagónicos”.
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Figure 2. Location of the Sascha-Pelligrini study area, distribution of Chon Aike
volcanics, operating mines and advanced exploration projects of Santa Cruz, Argentina.
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Figure 3. Locations of samples used for whole-rock geochemistry, and alteration analysis. Also shown are the prospect locations across the study area. Sascha Main, Sascha Central and Sascha Sur comprise the Sascha Vein Zone (SVZ). Map projection WGS84 SUTM19.
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Sascha-Pelligrini epithermal system
The Sascha-Pelligrini epithermal system comprises an area of approximately
70 square kilometers on the western edge of the Deseado Massif, north-
central Santa Cruz Province, Argentina (Figure 2). The Sascha-Pelligrini
epithermal system is expressed as intermittent outcropping epithermal veins
and pervasive silicification, and consists of the Sascha Vein Zone (SVZ),
Marcellina and Pelligrini prospects (Figure 3).
The SVZ was initially discovered during reconnaissance exploration for
Orvana in 1997. Mirasol subsequently visited the area and staked the
property in October 2003 during the inception of its Santa Cruz exploration
program (Smith et al, 2006).
The SVZ is centered on a 4.4 kilometer long vein trend and encompasses the
Sascha Main, Sascha Central and Sascha Sur zones. Sascha Main is a 1.7
kilometer-long, northwest-trending zone of intermittently outcropping, sub-
parallel veins and structural splays that collectively define a corridor reaching
300m in width. Exposed veins are up to 2 meters wide, and display classic
low-sulphidation crustiform-colloform quartz textures. Assay results from 50
vein samples average 14.23g/t gold and 89.8g/t silver, with values of up to
160g/t gold and 796 g/t silver.
Sascha Central is a continuation of the SVZ, and is expressed as small
discontinuous veinlets and un-mineralised goethite-rich shears. Sascha
Central encompasses a 1.2 kilometer long un-mineralised corridor between
the Sascha Main to the north and Sascha Sur to the south.
Sascha Sur is a 1.2 kilometer long zone of semi-continuous multi-directional
veinlets. Individual veinlet zones are up to 40 meters wide. Individual
veinlets are typically 1 to 30 centimeters wide with rare veining up to 1.5
meters wide. Assay results from 120 composite veinlet samples average 0.2
g/t gold and 3.4 g/t silver with values of up to 1.6 g/t gold and 158 g/t silver.
Quinn Smith Master of Applied Science Thesis
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The Marcellina prospect occurs on a parallel structure to the SVZ and crops
out as a small veinlet and vein breccia zone. The veinlet zone reaches 20
meters in width, with individual veinlets typically being 1 to 20 centimetres
wide. Vein sampling has returned assays of up to 0.25 g/t gold and 1.16 g/t
silver.
The Pelligrini prospect forms a predominant topographic high within the study
area and is manifest as a large zone of intense silica replacement,
brecciation and minor veining. Assayed samples contain up to 1.47 g/t gold
and 11 g/t silver from minor quartz veins located stratigraphically below the
silica replacement horizon.
Methods
Field investigations
Geologic mapping has been completed across the study area including
detailed outcrop and vein facies mapping at 1:1000 and 1:2500 scales over
the vein zones (Appendix 3). Prospect scale geologic mapping was
completed at 1:5000 with regional mapping completed at 1:25,000 (Appendix
4 & 5). Detailed mapping was constrained by GPS-surveyed tape and
compass grids, with prospect and regional mapping constrained with rectified
air photo images and GPS. Spatial positioning of the air photo was achieved
through the use of ER Mapper, using a cubic polynomial rectification
constrained to ASTER satellite imagery with approximately 120 control
points. Original detailed outcrop mapping was subsequently repositioned with
differential GPS (DGPS) control.
Trench sections were mapped at 1:200 scale, with vein windows mapped at
1:50 scale. Trench start and end points were DGPS-positioned, with trench
length, orientation and topography measured with tape, compass and
clinometer. Survey data for individual sections were plotted on graph paper
for field mapping. Vein windows were mapped on a measured tape grid, with
Quinn Smith Master of Applied Science Thesis
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control at 50cm intervals. Trench floor and wall geology, veining, structure
and alteration were mapped with tape and compass control along the trench
floor. 39 trenches were mapped over 1485 meters (35 at Sascha Main, 4 at
Sascha Sur)(Appendix 3). All trenches were photographed on 1 to 2 meter
intervals, with photos combined into composite images for individual
trenches.
Sample preparation and analytical techniques
Short Wave Infrared (SWIR) spectrometry
Alteration sampling was conducted across the study area for analysis by a
Portable Infrared Mineral Analyser (PIMA). Regional traverses perpendicular
to the vein trend at 250 meter sample spacing were undertaken to determine
the extent of the alteration halo and define background geological response.
Detailed traverse lines perpendicular to the vein at 5 and 1 meter sample
intervals were conducted to identify zonation within the alteration system.
Vein wall rock and vein clay samples were collected to define individual vein
phase assemblages. A total of 199 hand samples were collected across the
study area for PIMA analysis (Appendix 3). PIMA hand samples were air
dried for 48 hours, with PIMA sample surfaces cleaned of loose material prior
to analysis.
Alteration samples were analysed using a PIMA II, operated by Integrated
Spectronics control software version 3.4.0, at IAMGOLD in Mendoza,
Argentina. The PIMA II cycle count was left at the standard 0.2409, with
controller version of 1.58. An internal calibration was conducted on start-up,
and then after every 10-20 samples. Calibration was performed on samples
SP223, SP210, SP189, SP179, SP159, SP149, SP139, SP125, SP109,
SP079, SP059, and SP039. Samples were held to the sight window for an
analysis of approximately 30 seconds, with the internal reference sample
following for another 30 seconds. The PIMA II operating temperature was
maintained below 38ºC.
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PIMA spectra were analysed with „The Spectral Geologist‟ (TSG) computer
program. Raw PIMA II spectra were interpreted through „The Spectral
Assistant‟ (TSA) within TSG. Output information included mineral 1, weight 1,
mineral 2, weight 2, TSA Error, AlOH 2200nm Absorption Wavelength, and
ALOH 2200nm Absorption Depth (Appendix 3).
X-ray Diffraction
Samples were prepared for chemical analysis and x-ray diffraction at the
University of Queensland (UQ) sample preparation laboratory. Samples were
dried for three days at 60ºC, and then crushed using a hardened steel jaw
crusher and disc mill. Rock chips were pulverised using a hardened steel
swing mill, with approximately 100 grams of material pulverised for 45
seconds to obtain an ideal particle size of 100 microns.
Samples of vein and wall rock alteration were prepared for clay and mineral
assemblage identification. Orientated clay samples were prepared by
ultrasonic dispersion of approximately 2g of pulverized material in ten times
its volume of distilled water. Material left in suspension after 5 minutes was
separated by pipette, and spread over a glass slide. The samples were left to
dry on top of a warm surface until the water had evaporated, leaving a
gravimetrically separated clay fraction.
Randomly orientated powder samples for quantitative XRD analysis were
prepared by micronisation. Approximately 3g of pulverized material and 12ml
of alcohol were placed into the micronisation mill using agate cylinders and
milled for 5 minutes. The slurry obtained is homogenous and the particle size
is ideally 1-5 microns. The slurry was placed into pre-labeled glass beakers
and left to dry in an oven at 60°c. Once dried, about 1.5-2g of sample was re-
mixed and lightly packed into circular aluminum sample holders.
58 PIMA and hand samples of vein and wall rock alteration were analysed for
clay and mineral assemblages by X-Ray Diffraction (XRD) at the X-Ray
Analysis Facility (XAF) Queensland University of Technology (QUT). The
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XRD analyses were carried out on a Philips wide-angle PW 1050/25 vertical
goniometer using Co Kα radiation. The samples were measured with steps of
0.02° 2θ and a scan speed of 1.00° per minute from 3 to 75° 2θ. Spectra
were analysed with the software packages TRACES and SIROQUANT for
mineral identification (Appendix 3).
Microscopy
Twenty-four cover slipped sections were prepared for petrographic analysis.
Nineteen polished sections were made of vein samples for petrographic and
microprobe analysis. Twelve alteration samples were prepared by breaking
rock fragments to expose fresh surfaces, and mounting on aluminium stubs
with carbon tape. Polished sections and mounted rock fragments were
carbon coated prior to microprobe analysis.
Microprobe analyses were undertaken at the QUT Analytical Electron
Microscopy Facility using a JEOL-JXA-840A Scanning Electron Microprobe
with an Energy-Dispersive Spectrometry (EDS) detector. Operating
conditions for the quantitative determination of mineral chemistry were: 20kV
accelerating voltage, beam current of approximately 1.7nA, count time of 100
seconds, 38mm working distance, 40° take off angle for the EDS detector
and a focused beam of <10μm in diameter. EDS spectra were collected and
interpreted through Moran Scientific quantitative EDS software. Vein minerals
were probed between 2 and 8 times from core to rim, with a total of 381
spectra collected from 8 sections (Appendix 3).
Clay morphology analyses were undertaken at the QUT Analytical Electron
Microscopy Facility using a FEI Quanta Environmental Scanning Electron
Microscope with an Energy-Dispersive Spectrometry (EDS) detector
Samples were analysed in high vacuum with operating voltage between 15
and 20kV. Working distance was set to 10mm with a spot size of 3 to 4
angstroms (Appendix 3).
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Geothermometry
Formation temperatures are calculated from compositional relationships
between coexisting electrum and sphalerite mineral grains. The equation
derived by Shikazono (1985) uses the mole fraction of silver in electrum and
the mole fraction of FeS in sphalerite to calculate a pressure independent
temperature and is expressed as:
T = (28,765 + 22,600 x (1- NAg)2 – 6,400 x (1- NAg)
3) /(49.008 – 9.152 log XFeS + 18.2961 log
NAg + 5.5 x (1- NAg)2),
Where NAg, XFeS, and T denote mole fraction of silver in electrum, mole
fraction of FeS in sphalerite and absolute temperature in degree Kelvin (+/-
20º) respectively.
Geochemistry
Twenty-four whole rock samples, including 2 quartz blanks, 1 standard and 1
duplicate were analysed for major, trace and rare-earth geochemistry. Rock
sample SEQG566 (Moultrie, 1995), with known major and trace element
geochemistry, was used as an internal standard for comparison (Figure 4)
(Appendix 3). Major elements (Si, Al, Ca, Fe, K, Mg, Mn, Na, P, Ti, S, and
Cr) were determined by X-ray fluorescence (XRF) Silicate Fusion at Ultra
Trace Analytical Laboratories in Perth, Western Australia. Glass beads were
prepared using a sample/flux ratio of 12:22.
Trace and rare earth elements (Ag, As, Ba, Ce, Co, Cs, Cu, Dy, Er, Eu, Ga,
Gd, Hf, La, Li, Nb, Nd, Rb, Sb, Sm, Sr, Th, Y,Yb, and Zr) were determined by
Inductively Coupled Mass Spectrometry (ICP-MS) at Ultra Trace Analytical
Laboratories.
Samples were digested in hydrofluoric, nitric, hydrochloric and perchloric
acids allowing a total digestion in most samples. Loss on ignition (LOI) was
determined with samples heated between 105 and 1000 degrees Celsius.
LOI results were determined gravimetrically and reported on a dry sample
basis.
Quinn Smith Master of Applied Science Thesis
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Remote sensing
Image acquisition
ASTER Level 1B data was acquired through NASA‟s data acquisition request
(DAR) process. A formal research proposal was submitted to NASA for
ASTER data acquisition. The proposal was submitted through;
http://asterweb.jpl.nasa.gov/gettingdata/authorization/proposal.asp
The proposal was accepted by NASA, with Michael Abrams, ASTER Science
Team Leader Jet propulsion laboratory, uploading two level 1B scenes to the
asterweb.jpl.nasa.gov FTP site for download. The scenes were downloaded
and contained the following image identification numbers;
AST_L1B_003_03122003143108_03272003165841.hdf
And
AST_L1B_00301192005143600_01312005113946.hdf
y = 1.0618x
R2 = 0.9987
0
50
100
150
200
250
300
350
0 50 100 150 200 250 300 350
Trace Element Analysis (ppm)
Tra
ce
Ele
me
nt
Sta
nd
ard
(p
pm
)
Figure 4. Comparison of accepted results for rock standard SEQG566 with returned analysis performed by Ultra Trace Laboratories, Perth.
Quinn Smith Master of Applied Science Thesis
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Image processing
ASTER scene „AST_L1B_00301192005143600_01312005113946.hdf‟ was
cloud-free over the entire study area and was subsequently chosen for
multispectral image processing.
The level 1B ASTER scene was atmospherically corrected through the use of
the computer software „FLASH‟, utilising a modified transform 5 process. The
atmospherically corrected ASTER scene was processed in ENVI4.2 and
registered from the ephemeral satellite information. Mineral spectra were
identified through band rationing outlining the presence of kaolinite, illite and
alunite. The most spectrally pure pixels were determined through the use of
the „Pixel Purity Index‟ (Broadman et al, 1995), highlighting end-members of
kaolinite, illite and alunite. End-member spectra were chosen from the image
for comparison with the USGS spectral library and subsequently used as a
standard for further processing.
The locations for each of the end-member mineral spectra are as follows;
Mineral Spectra Easting Northing Datum/Projection
Alunite 415106E 4713855N WGS84/SUTM19
Illite 410096E 4705185N WGS84/SUTM19
Kaolinite 412646E 4705425N WGS84/SUTM19
Table 1. End-member mineral spectra locations used for ASTER image processing.
The ASTER scene was subset to the study area and transformed to
„minimum noise fraction‟ (MNF) space to reduce background noise and
spectral scatter (Broadman et al, 1995). Natural surfaces are rarely
composed of a single uniform material and spectral mixture modelling is
necessary to identify areas of mixed spectral signatures (Kruse and
Hintington, 1996). The subset MNF image was processed with Global Ore
Discovery‟s proprietary „mixture tuned matched filter‟ (MTMF) analysis to
identify pixels containing variable amounts and mixtures of kaolinite, illite and
alunite. The MTMF analysis produced information related to abundance, and
infeasibility for the mineral within a given pixel. Pixels containing a low
Quinn Smith Master of Applied Science Thesis
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infeasibility and high abundance for a given mineral were subjectively chosen
by evaluating populations from a scatter plot of infeasibility versus
abundance. The MTMF data was smoothed with a convolution median filter
on a 3x3 pixel matrix.
Abundance grids for kaolinite and illite were exported as high resolution
geotiff's and subsequently opened in MapInfo. The alunite shape files were
opened in MapInfo and converted to vector data. The ASTER VNIR 231
image was compressed and exported to ecw format (Appendix 3).
Results
Stratigraphy
The volcanic stratigraphy of the Sascha-Pelligrini area is divided into five
units; of these, three units comprise the Chon Aike Formation and two
comprise the La Matilde Formation (Figure 5). The Chon Aike Formation
consists of a basal massive, biotite rhyodacite welded ignimbrite, a middle
pumiceous, biotite rhyodacite welded tuff, and an upper clast-rich, welded
rhyodacite crystal ash tuff. The La Matilde Formation comprises a basal lithic
ash rhyolite tuff which grades into a crystal ash rhyolite tuff, and is overlain by
a unit of finely laminated rhyolite ash tuffs with spherulitic and accretionary
lapilli horizons. Unit thickness is highly variable and the thicknesses reported
are representative of maximum exposed thickness. The Chon Aike Formation
is 678 meters thick and the La Matilde Formation is 130 meters thick in the
Sascha-Pelligrini area. Reported Chon Aike Formation and La Matilde
thicknesses range from 300 to >900 meters and 15 to >175 meters
respectively (Echavarria et al, 2005; Sanders, 2000).
The lower unit of the Chon Aike Formation crops out throughout the study
area and is relatively homogenous in appearance (Figure 6; Appendix 4 & 5).
Internal variations include variably welded horizons and the inclusion of small
clasts of rare mica schist. Welding within the rhyodacite ignimbrite varies
both vertically and horizontally, and forms composite welding horizons
Quinn Smith Master of Applied Science Thesis
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preserved as topographic highs across the study area. The estimated
thickness of the unit is 570 meters. The overlying unit is similar in
composition, but contains pumice and occasional hematite-chlorite altered
juvenile lava clasts. It is also welded with compaction ratios of fiamme of up
to 10:1. The estimated maximum thickness of this unit is 100 meters. The
top of the Chon Aike Formation is composed of a welded rhyodacite crystal
ash tuff that contains juvenile and accidental clasts comprised of fine grained
granite, hematite-chlorite, and mica schist. The unit also contains abundant
large angular spherulitic devitrified volcanic glass and pumice fragments. The
estimated thickness of the horizon is 8 meters.
The La Matilde Formation exhibits paraconformable contacts with the
underlying sequence. The basal unit of the La Matilde Formation is a rhyolitic
tuff that varies from a sparsely distributed lower lithic –rich horizon (~10 to 20
meters thickness) to a more geographically widespread upper ash-flow tuff
(~75 meters thickness). The lithic-rich horizon contains accidental clasts of
angular and rounded metamorphic rocks. Clast size and angularity increase
towards the northwest with the unit becoming pumiceous towards the
southeast. The upper ash-flow is a rhyolite tuff with occasional devitrification
textures. Finely laminated rhyolite ash tuffs with locally developed spherulites
and accretionary lapilli horizons crop out within the Pelligrini prospect (Figure
6; Appendix 4 & 5) and overlie the ash-flow tuff. The laminated ash tuff
mantles the topography of the underlying unit, and attains a maximum
exposed thickness of 55m. The La Matilde Formation exhibits extreme
vertical and lateral variation.
A large flow-banded to spherulitic rhyolite dome with auto-brecciated margins
crops out within the Pelligrini prospect and intrudes into the upper-most unit
of the La Matilde Formation (Figure 5 & 6; Appendix 4 & 5). The volcanic tuff
sequences are intruded by small, spatially restricted, biotite-albite porphyritic
granodiorite dykes and sills.
The Jurassic volcanic sequence is unconformably overlain by a 15 meter
thick Oligocene feldsarenite to sparry grainstone that is preserved in
Quinn Smith Master of Applied Science Thesis
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topographic depressions. The unit has a conglomeritic base comprised of
rounded tuff fragments to 10 centimeters in diameter, grading upward to a
sandy feldsarenite intercalated with bivalve, gastropod and bryzoan
fragments. Epiclastics and fine laminated rhyolitic ash surges with prominent
cross stratification and carbonized plant fragments are locally developed at
the top of the unit (Appendix 1, plate 12). The unit is best exposed to the
west of the SVZ and intermittently crops out under overlying cover
sequences (Figure 6; Appendix 4 & 5).
Pliocene olivine tholeiite basalt forms a large plateau that runs through the
middle of the study area and also intermittently crops out as remnant plugs
and dykes. Pleistocene gravels and recent sediments cover most of the low-
lying areas, with gravels being best preserved as plateau caps to the
Oligocene feldsarenite (Figure 6; Appendix 4 & 5).
Quinn Smith Master of Applied Science Thesis
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Figure 5. Stratigraphy of the Sascha-Pelligrini study area showing correlated unit
age, diagrammatic relationship of rock units, unit thicknesses and grain size.
Quinn Smith Master of Applied Science Thesis
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Figure 6. Interpretive geology, structure and mapped veining of the Sascha-Pelligrini study area. Unit colours are the same as figure 5.
Quinn Smith Master of Applied Science Thesis
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Petrography
Least altered samples from each lithology within the study area were
selected for petrographic examination.
Chon Aike Formation
The lower ignimbrite of Chon Aike Formation is crystal rich with subordinate
lithic clasts. The phenocryst assemblage comprises sodic plagioclase (30%),
sanidine (20%), quartz (30%), biotite (10%), muscovite (5%), ilmenite and
magnetite (5%). Plagioclase and sanidine phenocrysts occur as moderately
altered, subhedral to euhedral, broken fragments ranging up to ~4mm in
length. Quartz phenocrysts are large, ranging up to ~5mm in size, and are
significantly embayed. Biotite and rare muscovite occur as small plates and
books ranging up to ~2mm in size, and are usually altered to chlorite and
sericite respectively. Ilmenite and magnetite constitute minor phenocryst
phases but are generally abundant in the less altered groundmass. Very
small accessory phases including zircon and apatite rarely occur in the
samples. Devitrified glass shards (<1mm) comprise the majority of the
groundmass. The glass shards are intensely welded with no original texture
preserved (Appendix 1, Plate 1).
The middle welded tuff of the Chon Aike Formation is pumice and ash rich
with rare juvenile lava clasts. Strongly welded and deformed devitrified glass
shards (<1mm) comprise the majority of the unit with no original x, y or
cuspate shapes preserved. Pumice glass is totally altered, with rare axiolitic
devitrification developed on pumice margins and spherulites developed within
pumice interiors. Pumice clasts are strongly flattened, range in size of up to
12cm in length and define a strong eutaxitic texture. The phenocryst
assemblage of the middle unit comprises albite (20%), sanidine (15%),
quartz (30%), biotite (10%), pumice (20%), ilmenite and magnetite (5%).
Albite and sanidine phenocrysts are subhedral, strongly sericitised, and
range up to ~2mm in length. Biotite occurs as subhedral to euhedral plates
and minor books ranging up to ~2mm in length, and is also strongly
Quinn Smith Master of Applied Science Thesis
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sericitised. Quartz occurs as angular subhedral fragments ranging up to
~4mm in size (Appendix 1, Plate 2).
The upper crystal ash tuff of the Chon Aike Formation is crystal and ash rich
with abundant accidental and juvenile lava clasts. The unit is comprised of
abundant moderately welded and strongly deformed devitrified glass shards
(<1mm). The horizon is unique with rare cuspate shapes preserved within the
glass shards. Strong mantling and welded textures occur around larger lithic
and juvenile clasts. Phenocryst are comprised of albite (20%), sanidine
(25%), quartz (35%), biotite (15%), ilmenite and magnetite (5%). Albite and
sanidine phenocrysts occur as moderately altered subhedral to euhedral
broken fragments ranging up to ~3mm in length. Quartz phenocrysts range
up to ~4mm in size, and are moderately embayed. Biotite and rare muscovite
occur as small plates and books ranging up to ~2mm in size, and are usually
altered to chlorite and sericite respectively. Ilmenite and magnetite constitute
minor phenocryst phases and are generally abundant only in the lesser
altered groundmass. Lithic clasts distinct to the horizon are comprised of rare
mica schist and strongly altered fine-grained granitic fragments. Abundant
juvenile lava clasts range up to 15mm in diameter and are strongly altered to
chlorite and hematite. Angular volcanic glass fragments contain abundant
spherulitic devitrification textures and range up to 10mm in size. Silicification
replaces pumice glass with fine grained quartz, preserving devitrification
textures (Appendix 1, Plate 3).
La Matilde Formation
The basal unit of the La Matilde Formation is crystal rich with abundant lithic
clasts. The phenocrysts comprises feldspar (35%), quartz (45%), muscovite
(15%), ilmenite and magnetite (5%). Feldspars are subhedral broken
fragments, completely sericitised, and range up to 5mm in size. Quartz
phenocrysts are subhedral broken fragments and range up to 2mm in size.
Muscovite occurs as small plates and is strongly sericitised. Weakly-welded
devitrified glass shards (<1mm) comprise the groundmass, with no original
shard textures preserved. Lithic clasts are comprised of distinct rounded
muscovite schist and range up to 10cm in size (Appendix 1, Plate 4).
Quinn Smith Master of Applied Science Thesis
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The lithic basal unit of the La Matilde Formation grades into an ash-rich
crystal rhyolite tuff. The phenocryst assemblage of the crystal rhyolite tuff
comprises sanidine (30%), quartz (50%), and muscovite (20%). Sanidine
phenocrysts occur as large subhedral to euhedral broken fragments and
range up to 3mm in size. Quartz phenocrysts occur as strongly embayed
subhedral broken fragments and range up to 3mm in size. Muscovite
phenocrysts range up to 1mm in length and occur as small plates. Poorly
welded devitrified glass shards comprise the majority of the groundmass,
with rare cuspate textures preserved. Rare axiolitic to bow-tie devitrification
textures form around phenocrysts (Appendix 1, Plate 5).
The upper unit of the La Matilde Formation is comprised of abundant ash
with minor phenocrysts. Broken, subhedral to euhedral sanidine crystals are
the only phenocrysts phase in the unit. Intensely altered glass shards
comprise the majority of the tuff, with no original shard textures preserved.
Silicification is strong within the unit and replaces volcanic glass, preserving
spherulitic devitrification and accretionary lapilli textures (Appendix 1, Plate
6).
Other units
The flow-banded rhyolite contains euhedral phenocrysts within a finely
crystalline groundmass. Euhedral sanidine crystals are the only phenocryst
phase present. The strongly sericitised groundmass is comprised of very fine
feldspar and quartz crystals. Flow-banded textures are distinguished by
alternating quartz-rich and feldspar-rich layers. Large spherulitic
devitrification textures (<5mm) develop within the unit in areas that are glass-
rich and phenocryst-poor. Silicification is strong within the unit preserving
flow-banded and spherulitic textures (Appendix 1, Plates 8 & 9). Auto-breccia
is locally developed around the margins of the flow-banded rhyolite. Large
angular clasts of flow-banded and spherulitic rhyolite up to 2 meters in
diameter are hosted within a finely crystalline groundmass composed of
euhedral sanidine and quartz crystals (Appendix 1, Plate 7).
Quinn Smith Master of Applied Science Thesis
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Granodiorite dykes and sills intrude the tuff sequence and contain large
euhedral phenocrysts within a crystalline groundmass. The phenocryst
assemblage is composed of sodic plagioclase (50%), hornblende (20%),
biotite (15%), quartz (10%), ilmenite and magnetite (5%). Plagioclase
phenocrysts are moderately altered to sericite, occur as euhedral crystals,
rarely show concentric zoning, and range up to ~4mm in size. Hornblende
and biotite phenocrysts are strongly altered to chlorite, sericite and minor
calcite and range up to ~3mm in size. Quartz occurs as small euhedral
phenocrysts and range up to ~1mm in size. Ilmenite and magnetite constitute
minor phenocryst phases and range up to ~0.5mm in size. Phenocrysts
occur in a feldspar lath groundmass (Appendix 1, Plate 10).
The feldsarenite to sparry grainstone is well sorted with variably rounded
clasts and high porosity. The clasts comprise feldspar (20%), quartz (35%),
lithics (5%), echinoderm, gastropod and mollusc fragments (40%). Clasts are
variably altered, mostly matrix supported, and cemented with sparry calcite.
Quartz fragments range from euhedral crystals to well rounded and embayed
clasts up to ~2mm in size. Lithic fragments are comprised of altered tuffs and
angular chalcedonic quartz fragments (Appendix 1, Plates 11 & 12).
Basalts occur as remnant dykes, plugs and flows, and are aphanitic to
slightly porphyritic. Basalt flows are vesicular, and comprised of fine grained-
olivine and feldspar. Feldspars range in composition from oligoclase to
labradorite, with minor zeolites infilling vesicles. Basalt dykes and plugs are
slightly porphyritic and weakly chloritised. Euhedral feldspars are weakly
albitised, range from oligoclase to labradorite, and are in a feldspar lath
groundmass (Appendix 1, Plates 13 & 14).
Quinn Smith Master of Applied Science Thesis
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Rhyodacite ignimbrite
Pumice Rhyodacite
Tuff
Pumice Ash Rhyodacite
tuff
Crystal Lithic
Rhyolite Tuff
Crystal Ash
Rhyolite Tuff
Rhyolite Ash Tuff
Flow-banded Rhyolite
Sample QR25 QR05-27 QR1 QR23 QR05-26 QR05-21 QR05-37
SiO2 63.2 74.5 70.3 84.9 76.1 78.3 77.1
TiO2 0.6 0.28 0.23 0.17 0.07 0.18 0.15
Al2O3 15.4 14.8 13.6 8.28 13.1 10.6 12.6
Fe2O3 5.14 1.16 2.39 1.46 1.55 1.46 0.39
MnO 0.13 BDL 0.06 BDL 0.02 0.03 0.02
MgO 1.52 0.27 0.46 0.23 0.14 0.26 0.2
CaO 4.41 0.14 1.79 0.43 0.21 0.14 0.09
Na2O 2.63 1.38 2.9 0.1 0.22 1.5 0.99
K2O 3.23 4.36 4.45 0.94 5.41 3.24 4.9
P2O5 0.172 0.019 0.068 0.032 0.03 0.049 0.035
SO3 BDL 0.02 0.29 0.54 0.03 1.06 0.12
LOI 3.54 2.18 2.38 3.43 2.67 3.13 1.55
Total 99.97 99.11 98.92 100.51 99.55 99.95 98.15
Ag BDL BDL BDL BDL BDL BDL 2
As 1 2 14 127 248 3 4
Co 32 8 22 BDL 6 2 12
Cu 9 2 2 13 3 4 4
Mo 8.5 3 7 6.5 3.5 1.5 3.5
Ga 16.4 15.2 14.4 9.4 11.8 13.8 18.6
Sb 0.6 1 1 14.6 2.4 0.4 2
Rb 115 182 159 74.6 220 117 191
Sr 315 82.5 215 23 24.5 137 137
Y 20.9 15.6 19.3 11.3 18.6 6.7 11.7
Zr 60 67 70 43 87 100 86
Nb 6.5 4.5 7 5 10 9 10
Cs 4.9 8.6 8.2 4.1 5.6 2.4 4.3
Ba 902 943 996 81 889 830 1350
La 29.9 29.3 39.2 20.3 29.8 25.2 31.1
Ce 61.1 57.1 69.7 39.7 60.2 41.2 64.4
Nd 25.1 20.9 25.4 14.8 26.1 14.4 23.8
Sm 5.05 4 4.65 3 5.5 2.6 4.5
Eu 1.2 0.8 0.85 0.55 0.5 0.4 0.7
Gd 4.2 3 3.6 2.4 3.8 2 3
Dy 3.75 2.7 3.25 2.05 3.8 1.55 2.45
Er 2.2 1.65 1.95 1.25 2.35 0.8 1.35
Yb 2.15 1.9 2.1 1.3 2.8 0.85 1.5
Hf 1.8 2.4 2.6 1.4 3.6 4.8 3
Th 12.5 17.3 18.8 9.7 21.5 6.6 21.1
Table 2. Representative whole-rock geochemical analyses for Jurassic volcanic units of the Sascha-Pelligrini study area.
Quinn Smith Master of Applied Science Thesis
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Geochemistry
Least altered representative samples of each of the volcanic tuffs were
analysed for both major and trace elements (Table 2). REE values for Bajo
Pobre andesite (Pankhurst and Rapela, 1995) were used to test for fractional
crystalisation trends within the volcanic suite, with REE values for the Sierra
Los Chacays xenoliths (Pankhurst and Rapela, 1995) used to test for partial
melting of the upper crust.
The volcanic rocks of the Sascha-Pelligrini area are high-K rhyodacites and
rhyolites with K2O contents ranging from 3.2 to 5.4 weight percent. SiO2
contents vary from 63 to 78 weight percent, with most of the samples being
greater than 70 weight percent SiO2. Na2O concentrations are low, and show
and inverse correlation with K2O concentration (Figure 7). The Sascha-
Pelligrini volcanic tuffs have a broad range in Al2O3 content (8.28 to 15.4
weight percent), Fe2O3 content (1.16 to 5.14 weight percent) and CaO
content (0.14 to 4.41 weight percent).
Groups identified on the basis of stratigraphy and petrography are easily
discernable on most major and trace element graphs (Figures 7 and 8). Chon
Aike Formation rhyodacites are distinguished primarily by their relatively low
SiO2 contents (63 to 74 weight percent). Abundances of all major elements
except for K2O have correlations with SiO2, however Al2O3, TiO2, NaO and
K2O show decrease in concentrations above 70 weight percent SiO2. Trace
elements Sr and Yb decrease with increasing SiO2, with Rb, Cs and Zr
showing correlation with SiO2. Trace elements Ba, Ce, Hf, Nb and Th
increase in concentration to 70 weight percent SiO2, with a marked decrease
in concentration above 70 weight percent SiO2.
La Matilde Formation rhyolites are distinguished by their very high SiO2
contents. The rhyolites have a limited SiO2 content ranging from 76.1 to 78.3
weight percent. Abundances of Al2O3, Fe2O3, CaO and K2O decrease with
increasing SiO2 content, while TiO2, MgO, NaO and P2O5 correlate with SiO2.
Trace elements Sr, Zr, and Hf correlate with SiO2, with Ba, Cs, Rb, Ce, Yb,
Nb and Th contents decreasing with increasing SiO2.
Quinn Smith Master of Applied Science Thesis
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Chondrite-normalised REE patterns of the Sascha-Pelligrini samples are
light-REE enriched (Figure 9). LREE concentrations range between 85 and
165 times chondritic levels, with Yb concentrations approximately 5.2 to 17.4
times chondritic levels. La/Yb ratios vary from 7.2 to 20.2. REE patterns
shallow towards the heavy-REE, with La/Gd values from 5.9 to 10.6 and
Gd/Yb values from 1.09 and 1.90. Chondrite-normalised REE patterns for the
Chon Aike Formation are smooth and near parallel (Figure 9), with minor
negative Eu anomalies. Chondrite-normalised REE patterns for the La
Matilde Formation have a distinct negative Eu anomaly. La Matilde rhyolites
diverge towards the heavy-REE, with Dy/Yb values between 0.88 and 1.19.
Chondrite-normalised REE values for the flow-banded rhyolite are similar to
the La Matilde rhyolite tuffs, and show a negative Eu anomaly.
The Rhyodacite suite REE pattern is relatively flat normalised against crustal
abundances, with slight negative Nd and Y, and positive Yb anomalies
(Figure 10). REE patterns show a decrease in total REE concentrations with
increasing SiO2. The Rhyodacite suite of samples is slightly enriched relative
to upper crust and shows a small positive Eu anomaly. La concentrations
range from 0.9 to 1.3 times upper crust. Ce/Yb values range from 0.85 to
1.14. Increasing SiO2 and K2O in the rhyolite suite leads to a slight depletion
in REE relative to upper crust, with Eu inverting to a small negative anomaly.
La concentrations in high SiO2 rhyolites range from 0.6 to 1.0 times upper
crust, with Ce/Yb values from 0.73 to 1.66.
REEs for the Rhyodacite suite are slightly enriched, with small negative Eu
and Y anomalies (Figure 10). La concentrations range between 2.0 and 2.7
times Bajo Pobre. Ce/Yb values range from 1.27 to 1.49. REE patterns show
a progressive depletion in heavy REE‟s with increasing SiO2, with negative
Eu, Y and positive Yb anomalies becoming more pronounced. Highest SiO2
rhyolites are enriched in light, and depleted in heavy REE‟s relative to Bajo
Pobre andesite. High SiO2 rhyolites have La concentrations which range from
1.7 to 2.1 times Bajo Pobre. Ce/Yb values range from 0.96 to 2.18.
Quinn Smith Master of Applied Science Thesis
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Sierra Los Chacays xenolith normalised REE patterns are light-REE enriched
(Figure 10). The Rhyodacite suite show slight negative Eu, Y and positive Yb
anomalies. La concentrations range between 13.7 and 18.3 times xenolith
levels. Ce/Yb values range from 2.78 to 3.25. REE patterns shallow towards
the heavy-REE, with Ce/Sm values from 2.53 to 3.14 and Sm/Yb values from
0.98 to 1.10. REE patterns show a progressive depletion in heavy REE‟s with
increasing SiO2, with negative Eu, Y and positive Yb anomalies becoming
more pronounced. High SiO2 rhyolites have La concentrations which range
from 11.81 to 14.57 times Sierra Los Chacays xenoliths. Ce/Yb values range
from 2.10 to 4.75. REE patterns of high SiO2 rhyolites shallow towards the
heavy-REE, with Ce/Sm values from 2.29 to 3.32 and Sm/Yb values from
0.91 to 1.43.
Quinn Smith Master of Applied Science Thesis
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Figure 7. XRF major element results for Sascha-Pelligrini volcanic samples in weight percent plotted against SiO2. Least altered samples for each of the stratigraphic units
are shown in red.
TiO2
MgO
NaO
P2O5
SiO2
0
0.2
0.4
0.6
0.8
60 65 70 75 80 85 90
0
0.5
1
1.5
2
60 65 70 75 80 85 90
0
1
2
3
4
60 65 70 75 80 85 90
0
0.05
0.1
0.15
0.2
60 65 70 75 80 85 90
Chon Aike
La Matilde
Flow Banded Rhyolite
Chon Aike Least Altered
La Matilde Least Altered
Al2O3
Fe2O3
(total)
CaO
K2O
SiO2
6
8
10
12
14
16
18
60 65 70 75 80 85 90
0
1
2
3
4
5
6
60 65 70 75 80 85 90
0
1
2
3
4
5
60 65 70 75 80 85 90
0
1
2
3
4
5
6
60 65 70 75 80 85 90
Quinn Smith Master of Applied Science Thesis
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Figure 8. ICP-MS trace element results (Sr, Ba, Cs, Rb, Ce, Yb, Zr, Hf, Nb & Th) for
Sascha-Pelligrini volcanic samples in parts per million (ppm) plotted against SiO2.
Cs
Ce
Zr
Nb
2
4
6
8
10
12
60 65 70 75 80 85 90
0
100
200
300
60 65 70 75 80 85 90
0
2
4
6
8
10
12
60 65 70 75 80 85 90
0
10
20
30
40
50
60
70
80
90
100
60 65 70 75 80 85 90
40
60
80
100
120
60 65 70 75 80 85 90
0
200
400
600
800
1000
1200
1400
1600
1800
60 65 70 75 80 85 90
0
50
100
150
200
250
300
60 65 70 75 80 85 90
0
1
2
3
60 65 70 75 80 85 90
1
2
3
4
5
6
60 65 70 75 80 85 90
5
10
15
20
25
60 65 70 75 80 85 90
Chon Aike
La Matilde
Flow Banded Rhyolite
Chon Aike Least Altered
La Matilde Least Altered
SiO2 SiO2
Sr Ba
Rb
Yb
Hf
Th
Cs
Ce
Zr
Nb
Quinn Smith Master of Applied Science Thesis
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Rhyolite Tuff (La Matilde)
1
10
100
1000
La Ce Nd Sm Eu Gd Dy Y Yb
Rhyolite Ash Tuff (QR05-21)
Crystal Ash Rhyolite Tuff (QR05-26)
Lithic Rhyolite Tuff (QR23)
Rhyodacite Ignimbrite (Chon Aike)
1
10
100
1000
La Ce Nd Sm Eu Gd Dy Y Yb
Rhyodacite Ash Tuff (QR1)
Pumaceous Rhyodacite Tuff (QR05-27)
Crystal Ash Rhyodacite Ignimbrite (QR25)
Rhyolite Intrusive (La Matilde)
1
10
100
1000
La Ce Nd Sm Eu Gd Dy Y Yb
Flow Banded Rhyolite (QR05-37)
Figure 9. Chondrite-normalised REE diagrams for least altered Sascha-Pelligrini volcanic stratigraphic units. Normalised to C1 chondrite of McDonough and Sun (1995).
Quinn Smith Master of Applied Science Thesis
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Figure 10. REE geochemistry for least altered stratigraphic units from the Sascha-Pelligrini study area normalised to upper crust, Bajo Pobre andesite, and Sierra Los
Chacays xenoliths with increasing primitive lower crustal signatures respectively.
Upper Crust Normalised REE (Rudnick & Gao, 2003)
Bajo Pobre Normalised REE (Pankhurst & Rapela, 1995)
Sierra Los Chacays Xenolith Normalised REE
(Pankhurst & Rapela, 1995)
Rh
yo
da
cit
e Ig
nim
bri
te (
Ch
on
Aik
e)
0.11
10
La
Ce
Nd
Sm
Eu
Gd
Dy
YY
b
Pu
mic
e A
sh
Rh
yo
dacit
e T
uff
(S
022)
Pu
maceo
us R
hyo
dacit
e T
uff
(S
023)
Cry
sta
l A
sh
Rh
yo
dacit
e I
gn
imb
rite
(S
024)
0.11
10
La
Ce
Nd
Sm
Eu
Gd
Dy
YY
b
0.11
10
10
0
La
Ce
Nd
Sm
Eu
Gd
Dy
YY
b
Rh
yo
lite
Flo
w D
om
e (
La M
ati
lde)
La
Ce
Nd
Sm
Eu
Gd
Dy
YY
b
Flo
w B
an
ded
Rh
yo
lite
(S
007)
La
Ce
Nd
Sm
Eu
Gd
Dy
YY
b
La
Ce
Nd
Sm
Eu
Gd
Dy
YY
b
Rh
yo
lite
Tu
ff (
La M
ati
lde)
La
Ce
Nd
Sm
Eu
Gd
Dy
YY
b
Lam
inate
d R
hyo
lite
Ash
Tu
ff
Cry
sta
l A
sh
Rh
yo
lite
Tu
ff (
S008)
Cry
sta
l L
ith
ic R
hyo
lite
Tu
ff (
S018)
La
Ce
Nd
Sm
Eu
Gd
Dy
YY
b
La
Ce
Nd
Sm
Eu
Gd
Dy
YY
b
Quinn Smith Master of Applied Science Thesis
33
Structural setting
The SVZ is hosted on a right-lateral, oblique-slip fault system termed the
Sascha Fault (Figure 11). Sascha Main is hosted within a normal fault-
bounded graben trending approximately 315°. Mineralised veins are
developed at right-stepping structural splays along the southwesterly dipping
315° trend. North-trending left-lateral and east-trending right-lateral structures
within the normal faulted block are often unmineralised and offset the vein
trend. The graben is comprised of La Matilde rhyolite tuffs which are inferred
to thicken to the southeast and thin towards the northwest. Structural studies,
based on airphoto and satellite image interpretation, indicate the block is
bound by regional northeast-trending left-lateral basement transfer
structures. The 315° trending Sascha fault rotates through to a north-trending
360° orientation in Sascha Central. The orientation change of the Sascha
fault is controlled by, and bounded by, northeast-trending left-lateral transfer
structures. Epithermal veining is limited to small, unmineralised quartz
veinlets, within the north-trending Sascha fault.
Figure 11. Mapped surface vein trace orientation in Sascha Main indicating dextral oblique-slip movement with mineralised veins forming on right-stepping structural
splays to the main 315° trending Sascha fault.
Quinn Smith Master of Applied Science Thesis
34
The Sascha fault changes orientation from north-trending within the transfer
structure corridor, back to 315°on the northern margin of Sascha Sur.
Epithermal veining is hosted within the hanging-wall of a half-graben. The
half-graben is bounded to the west by the northeast dipping, oblique-slip,
normal Sascha fault. The strike extent of the veinlet zone is controlled by
northeast-trending left-lateral transfer structures that offset the Sascha fault.
North-trending left-lateral, and east-trending right-lateral structures offset the
vein trend with the half-graben block.
The Marcellina veinlet zone parallels Sascha Sur, and is hosted within a 315º
trending normal fault. Epithermal veining is hosted within the hanging wall of
a half graben structure. The normal fault dips towards the northeast,
extending under basalt cover to the north, and recent sediment to the south.
Pelligrini is hosted within a similar structural setting to Sascha Sur and
Marcellina. Epithermal veining is hosted within the hanging wall blocks of
315º trending half grabens. Veining is controlled by northeast dipping
oblique-slip normal structures, bounded to the north and south by northeast-
trending right-lateral transfer structures.
Quinn Smith Master of Applied Science Thesis
35
1 cm
A B
1 cm
A B D C E F G H
0.1 mm
I
A. Wall rock silicification B. Initial crustiform gold-silver ginguro band
(Ginguro Stage I) C. Pseudoacicular fine bladed quartz D. Colloform/crustiform chalcedony E. Colloform/crustiform chalcedony-ginguro bands
with minor adularia (Ginguro Stage II) F. Secondary kaolinite G. Coarse lattice bladed quartz H. Saccharoidal and vuggy quartz I. Photomicrograph of gold and acanthite within
chalcedony-ginguro band (Reflected PPL x40)
0.25 mm
1 cm
A
B
C
A. Saccharoidal and vuggy quartz B. Crustiform banded chalcedonic quartz
with disseminated pyrite infilling cavity within saccharoidal vein phase
C. Photomicrograph of fine disseminated pyrite within chalcedonic quartz (Reflected PPL x10)
Figure 12. Sascha Main saccharoidal vein phase (B) overprinting the earlier
chalcedonic (A) vein phase.
Figure 13. Sascha Main ginguro vein phase.
Figure 14. Pyritic chalcedonic vein phase overprinting saccharoidal vein phase.
Quinn Smith Master of Applied Science Thesis
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Epithermal veining
Quartz textures
Veins in the Sascha Main prospect are subdivided into a series of vein-types
with different textural and geochemical signatures. These veins vary from
chalcedonic to crystalline comb quartz.
Chalcedonic veins are white-grey to brown, massive to finely-banded chaotic
veins and veinlets; they generally occur on the western side of the vein trend
(Figure 12). Angular leached clasts of argilllised wall rock that include
strongly colloform-banded, fine saccharoidal silica define zones of single
pulse tectonic breccia. These zones generally occur as large deflation
surfaces with no outcrop expression and are considered equivalent to the
chalcedonic phase (Appendix 2, Plate 1).
Saccharoidal veins with minor bladed textures are white and consist of
medium- to coarse-grained saccharoidal and crystalline silica with prominent
bladed carbonate pseudomorphs (Appendix 2, Plates 3 & 4). The
saccharoidal veins consist of massive to weakly crustiform-banded quartz
exhibiting local ghosted breccia textures. These veins overprint the
chalcedonic veins (Figure 12), occur through the center of the vein trend, and
are considered to be equivalent to the ginguro mineralization event (see
below).
Fine-grained, strong colloform / crustiform-banded vein and vein breccia with
bands of adularia, clay and bladed quartz textures supersede the
saccharoidal veins. Dark grey to black, sulphide-rich bands with native gold
and silver sulphides occur with colloform-banded chalcedonic silica. The
sulphide bands are several millimeters in width and are similar in character to
the „ginguro‟ bands described by Izawa et al (1990). This phase typically
forms as an initial pulse of several repeated bands on the margin of vuggy
saccharoidal and bladed quartz. Colloform / crustiform-banded chalcedonic
quartz and ginguro deposition occurs prior to the formation of bladed quartz
textures (Figure 13). Wall rock breccias proximal to the colloform / crustiform
Quinn Smith Master of Applied Science Thesis
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veins contain ginguro-like bands encrusting wall rock fragments, followed by
saccharoidal to crystalline silica fill (Appendix 2, Plate 2).
Crustiform-banded grey chalcedonic veins with fine pyrite and hematite
typically occur as chaotic veins and breccia zones on vein margins (Appendix
2, Plate 5). Rare saccharoidal and cockade-textured quartz is present.
Outcropping veins have strong iron oxide gossanous zones (Appendix 2,
Plate 6). This phase is associated with pervasive kaolinite alteration and
silica-pyrite flooding of the lithic tuff on the eastern side of the vein trend. The
chalcedonic veins with fine pyrite and hematite are observed overprinting the
saccharoidal with minor bladed texture quartz vein phase (Figure 14).
Jasperoidal veins are massive to moderately banded, contain disseminated
pyrite and are distinguished by their unique cryptocrystalline matrix amongst
vuggy quartz cavities (Appendix 2, Plate 7). The jasperoidal veins crop out
along strike from the chalcedonic veins with fine pyrite.
Late stage veins of euhedral, axiolitic, clear to milky quartz crystals (comb
quartz) are typically less than 5 cm wide and grow perpendicular to vein
margin. The comb quartz veins overprint all other vein phases (Appendix 2,
Plates 8 & 9). Silica-poor, limonite-rich, tectonic breccias and veins
commonly occur within structural intersections associated with major faulting.
Limonite-stained structures are observed cross-cutting veining.
Sascha Sur exhibits multiple quartz textures expressed as multiphase
veinlets, veins and vein breccias. Grey-white chalcedonic quartz with patchy
disseminated pyrite is associated with anomalous gold-silver values and
occurs as a late infill to paragenetically earlier saccharoidal to crystalline
quartz veins (Appendix 2, Plate 10). Amethystine quartz and euhedral
axiolitic comb quartz veinlets cross-cut both chalcedonic with disseminated
pyrite and saccharoidal vein phases (Appendix 2, Plates 8 & 9).
The Marcellina veinlet zone is expressed as multiphase veinlets which
collectively define a stockwork. The veinlets form with an initial
Quinn Smith Master of Applied Science Thesis
38
crustiform/colloform phase, followed by axiolitic comb quartz growth, and are
infilled with ladder-banded chalcedonic quartz with banding perpendicular to
vein margin (Appendix 2, Plate 11).
The Pelligrini prospect forms a predominant topographic high within the study
area, and is a large zone of intense silica replacement, brecciation and minor
veining. A stratigraphically controlled zone of pervasive silica replacement
forms within the rhyolite ash tuff. Silicification is texturally destructive, with
minor rock textures locally preserved (Appendix 2, Plate 17). Massive
chalcedonic to weakly banded chalcedonic veins and associated wall-rock
breccias occur within the same stratigraphic level as the pervasive
silicification. The chalcedonic veins have a distinctive red-black colour due to
inclusions of hydrothermal hematite and pyrolusite (Appendix 2, Plate 13).
Weakly banded chalcedonic to saccharoidal quartz veins with prominent
bladed textures occur stratigraphically below the pervasive silicification and
contain anomalous gold and silver values (Appendix 2, Plate 12). Multiple-
pulse, milled-clast hydrothermal breccia veins occur directly below the
pervasive silicification. Breccia veins contain rock, vein and earlier breccia
clasts within a chalcedonic quartz matrix (Appendix 2, Plates 13 to 16).
Zones of clast-supported, silica-flooded jigsaw breccias occur within all
stratigraphic levels, and contain angular rock fragments with a hydrothermal
quartz matrix (Appendix 2, Plate 16).
Vein morphology along the SVZ is found to be primarily controlled by host
rock rheology. The least competent lithic rhyolite tuffs consistently host
discontinuous, chaotic veining. Crystal-ash rhyolite tuffs host upward
terminating veins with high grade ginguro bands that encompass the vein
margins. Pumiceous rhyodacite tuffs at Sascha Sur and Marcellina host
broad zones of discontinuous multi-directional veinlets, with ash tuffs at
Pelligrini being pervasively silicified and brecciated.
Quinn Smith Master of Applied Science Thesis
39
Vein geochemistry and mineralogy
The vein phases across the Sascha-Pelligrini area have distinct geochemical
signatures (Table 3). Early phase comb and chalcedonic quartz veins are
anomalous in gold and silver, and have elevated concentrations of arsenic,
barium and manganese. The early phase comb and chalcedonic quartz veins
average 0.52g/t gold and 3.2g/t silver. Colloform/crustiform and ginguro-
banded veins exhibits high gold and silver values associated with low
concentrations of arsenic, barium and antimony. Colloform/crustiform and
ginguro-banded veins average 14.23g/t gold and 89.8g/t silver. The
colloform/crustiform-banded vein phase is hosted on the margins of the more
voluminous saccharoidal and bladed vein phase. The saccharoidal and
bladed vein phase is anomalous in gold and silver, averaging 0.19g/t gold
and 0.5g/t silver. The chalcedonic with disseminated pyrite phase is
anomalous in gold and silver, averaging 0.76g/t gold and 9.5g/t silver, and
also has high concentrations of antimony and arsenic. The jasperoidal phase
is anomalous in silver with high concentrations of barium and moderate
concentrations of arsenic. The flow-banded rhyolite and auto-breccia at
Pelligrini is anomalous in silver, with vein samples characterised by local
mercury values of up to 633ppb. On the basis of 639 rock chip samples
(Appendix 3) collected from the SVZ, the generalized multi-element signature
is strongly elevated in gold, silver and arsenic, moderately elevated in
antimony and barium, weakly elevated in copper, lead and zinc, and locally
anomalous in mercury.
Vein Phase Au Ag As Ba Cu Hg Mn Pb Sb Zn Au/Ag Ratio
As/Au Ratio
No. of Samples
Comb & chalcedonic
0.52 3.2 409 201 11 1 236 56 8 28 6 786 408
Ginguro 14.23 89.8 186 107 12 1 133 20 7 17 6 2 50
saccharoidal & bladed
0.19 0.5 511 94 12 1 132 38 7 25 3 1022 123
chalcedonic & pyrite
0.76 9.5 1124 97 17 1 135 30 41 21 13 118 44
Jasperoidal & pyrite
0.06 1.7 465 486 13 1 170 24 4 29 27 273 14
Table 3. Summary geochemical signatures of Sascha Main vein phases with key
epithermal elements. Elements are reported as ppm.
Quinn Smith Master of Applied Science Thesis
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Element ratios of exploration rockchip geochemistry indicate that the textural
variants of the different veins are geochemically distinct. While their
differences are evident in a range of elements, it is most pronounced in the
arsenic:gold ratio (Table 3). The arsenic:gold ratio clearly differentiates the
early phase chalcedonic, the colloform/crustiform ginguro-banded phase, and
the jasperoidal and chalcedonic with disseminated pyrite phases. The
arsenic:gold ratio shows a separation of three orders of magnitude between
the ginguro and jasperoidal and chalcedonic with disseminated pyrite
phases.
Individual vein phases are also characterised by unique mineral
assemblages. The colloform/crustiform ginguro-banded veins are comprised
of the ore minerals, acanthite and selenium-rich acanthite, electrum, silver
halides, uytenbogaardtite, petrovskaite and jalpaite, with gangue minerals
jamesonite, hematite, calcite, sphalerite, galena, chalcopyrite, dufrenoysite,
barite, adularia and muscovite (Figure 15). The chalcedonic veins with
disseminated pyrite contain ore minerals acanthite, selenium-rich acanthite
and jalpaite, and gangue minerals, pyrite, arsenopyrite, hematite, barite,
gypsum, muscovite and jarosite.
Electron microprobe analysis of individual mineral grains from samples from
across the Sascha-Pelligrini area show geochemical zoning. Acanthite
incorporates selenium contents ranging from 1.65 to 6.21 weight percent and
has compositional ranges of Ag2S0.96Se0.04 to Ag2S0.83Se0.17. Iron content of
sphalerite ranges from 0.71 to 2.75 weight percent with calculated
compositional ranges from Zn0.95Fe0.05S to Zn0.99Fe0.01S . Sphalerite also
incorporates manganese (<1.86 weight percent Mn) and copper (<3.08
weight percent Cu); calculated compositions range from Zn0.91Fe0.05Cu0.04S to
Zn0.93Fe0.04Mn0.03S. Silver content of electrum varies over the entire ginguro
event, and ranges from 6.49 to 87.17 weight percent; electrum varies from
Ag0.05Au0.95 to Ag0.88Au0.12.
Quinn Smith Master of Applied Science Thesis
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E
1
2
3
A
1
2
3 4
5
B
1
2
3
4
G
1
2
3
4
5
F
1
2
3
4
H
1
2
3
4
C D 1
2
2
1
3
A. Ginguro Stage I (QR05-4A) 1-Se rich Acanthite, 2-Jamesonite, 3-Acanthite, 4-Barite, 5-Quartz B. Ginguro Stage I (QR05-4A) 1-Se Acanthite, 2-Iodoembolite, 3-Barite, 4-Quartz C. Ginguro Stage I (QR05-4A) 1-Ag rich electrum, 2-acanthite and uytenbogaardtite intergrowth D. Ginguro Stage II (QR05-4B) 1-Acanthite, 2-Iodoargyrite, 3-Barite E. Ginguro Stage II (QR05-4B) 1-Iodoargyrite, 2-Gold, 3-Quartz F. Ginguro Stage II (QR05-4B) 1-Iodoembolite, 2-Hematite, 3-Calcite, 4-Quartz G. Pyrite Chalcedony (QR05-6) 1-Pyrite, 2-Hematite, 3-Hematite & Gypsum, 4-Acanthite, Se Acanthite &
Jalpaite, 5-Quartz H. Pyrite Chalcedony (QR05-6) 1-Pyrite (As zoning), 2-Hematite & Gypsum, 3-Acanthite, 4-Quartz
Figure 15. Backscattered SEM images of characteristic vein mineral assemblages.
Quinn Smith Master of Applied Science Thesis
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Geothermometry
Shikazono (1985) proposed that formation temperatures can be calculated
from coexisting electrum and sphalerite by combining the thermodynamic
equations for electrum in equilibrium with argentite (Barton and Toulmin
1964), and equilibrium between FeS in sphalerite and pyrite (Barton and
Skinner 1979).
In order to obtain reliable formation temperatures from coexisting electrum
and sphalerite grains, Shikazono (1985) suggests that the grains must be in
direct contact with each other without mutual replacement textures; that the
FeS content of sphalerite and the Ag content of electrum have not changed
during the post depositional period; and that trace element impurities in
electrum and sphalerite are of low concentration.
Table 4. Silver content of electrum, iron content of sphalerite, and calculated
electrum-sphalerite formation temperatures from the Sascha ginguro vein phase.
Sample Grain Site Ag content of
electrum Fe content of
sphalerite NAg XFes
Electrum-sphalerite geothermometer
Electrum-Sphalerite
(atm fraction) (atm fraction) (mol fraction) (mol fraction) (Shikazono eq. 6,
1985) (°C)
4B-18 Core-Core 81.12 2.25 0.887 0.041 204.8
4B-18 Rim-Rim 79.45 2.54 0.876 0.040 205.1
4B-19 Core-Core 72.64 0.79 0.829 0.012 177.6
4B-19 Core-Rim 72.64 0.71 0.829 0.011 174.8
4B-20 Core-Core 87.15 3.05 0.925 0.047 204.2
4B-20 Rim-Rim 86.67 1.91 0.922 0.030 190.3
E
E E
E
S
S S
J
Figure 16. Backscattered SEM images of coexisting electrum-sphalerite grains used for
geothermometry calculations. E-electrum, S-sphalerite, J-jalpaite.
Quinn Smith Master of Applied Science Thesis
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Formation temperatures are estimated from compositional relationships
between coexisting electrum and sphalerite mineral grains. Mineral grains
considered to be in equilibrium are presented in figure 16, with calculated
temperatures of formation presented in table 4. Coexisting electrum and
sphalerite were found within the Sascha ginguro stage II mineralising phase.
Electrum in equilibrium with sphalerite has a silver content which ranges from
72.64 to 87.15 weight percent, with calculated composition varying from
Ag0.829Au0.171 to Ag0.925Au0.075. The iron content of sphalerite in equilibrium
with electrum ranges from 0.71 to 3.05 weight percent, with calculated
composition varying from Zn0.953Fe0.047S to Zn0.998Fe0.018S. Calculated
electrum-sphalerite formation temperatures range from 174.8°C to 205.1°C
+/- 20°C, and vary less than 13.2°C from core to rim.
Alteration
Regional Alteration – Multispectral Mineral Mapping
Using the spectral information recorded by the ASTER sensor, mineral maps
for the remotely sensed alteration minerals kaolinite, illite and alunite are
created for the entire study area (Figure 18). Surface cover across the study
area includes localised woody shrubs and grasses, rock outcrop, gravels and
recent alluvium, allowing good exposure of the alteration system. End-
member spectra of minerals used for mineral mapping and spectral un-
mixing correlate well with known library spectra (Figure 17). Areas of
mineralogically homogenous and mixed pixels are abundant over the
prospect areas. ASTER-derived mineral maps readily define the regional
extent of alteration over the known prospects.
Sascha Main is characterised by abundant, moderate intensity kaolinite, with
small areas of mixed, moderate intensity illite and kaolinite (Figure 18). Pixels
containing pure illite and alunite are located in small areas to the north and
west of the area respectively. Vein distribution and associated anomalous
gold values correlate with areas of mixed kaolinite and illite.
Quinn Smith Master of Applied Science Thesis
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ASTER Selected Endmember Spectra - Library Spectra Comparison
1.7 2.2 2.2 2.3 2.3 2.4
Wavelength μm
Refl
ecta
nce
Illite (This Study) Kaolinite (This Study) Alunite (This Study)
Illite IL101 Kaolinite CM92 Alunite GDS84
Sascha Sur is characterised by a broad zone of mixed illite and kaolinite,
which is zoned outwardly to pure illite (Figure 18). Illite and kaolinite intensity
is strong over the veinlet zone and is dominated by illite. Similar to Sascha
Main, vein distribution and associated anomalous gold values correlate with
areas of mixed kaolinite and illite.
The Marcellina veinlet zone is characterised by abundant kaolinite, with
zones of mixed kaolinite and illite associated with veining and weakly
anomalous gold values (Figure 18). The alteration at Pelligrini is a large area
of mixed kaolinite and illite, in combination with zones of pure kaolinite or illite
(Figure 18). The pervasive silicification correlates to areas of high abundance
kaolinite, mixed kaolinite and illite, and scattered alunite. Mineral mapping
has highlighted a zone of intense kaolinite and illite that corresponds to an
outcropping chalcedonic vein and vein-breccia. In the western part of the
Pelligrini prospect, and stratigraphically below the zone of pervasive
silicification, the alteration assemblage is dominated by illite.
Figure 17. Selected end-member spectra compared to known library spectra.
Quinn Smith Master of Applied Science Thesis
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Figure 18. ASTER mineral mapping results showing kaolinite, illite and alunite intensities and illite-kaolinite ratios. Simplified geology and gold assays are also presented from the
Sascha-Pelligrini study area.
Quinn Smith Master of Applied Science Thesis 46
Prospect Alteration – PIMA and XRD
In combination with the regional scale ASTER multi-spectral mineral
mapping, detailed analysis of individual prospects reveal a variety of
hydrothermal alteration minerals occur within the Sascha-Pelligrini epithermal
system. PIMA spectral analysis across the study area identifies spectrally
pure end-members of kaolinite, illite, muscovite and alunite (Figure 19).
Mixtures of end-member spectra are common, and characterise distinct
alteration assemblages at each prospect location. XRD analysis confirms
PIMA results and identifies mixed-layered illite-smectite vein selvage
assemblages (Table 5). Hydrothermal alteration across the Sascha-Pelligrini
area is strong to intense. Hydrothermal minerals completely replace primary
phenocrysts and glass in intensely altered wall-rocks, except for primary
quartz and zircon. Strongly altered rocks contain relict plagioclase and mica,
with primary rock textures preserved.
Montmorillonite and iron-chlorite alteration of the host ignimbrite unit can be
detected several kilometers away from the outcropping veins at Sascha
Main. Veins are hosted within a strong pervasive kaolinite +/- iron oxide
alteration halo of several hundred meters. Individual vein phases have
distinct alteration selvages at the vein/wall-rock margin, extending
centimeters to meters into the host sequence. Silicification of the wall-rock
occurs within the alteration selvage and becomes more pervasive and
texturally destructive within 30 centimeters of the main vein.
Quinn Smith Master of Applied Science Thesis 47
1.3
1.4
1.5
1.6
1.7
1.8
1.9
2.0
2.1
2.2
2.3
2.4
2.5
Wavelength (μm)
Re
fle
cta
nc
e
Illite
K-Alunite
Kaolinite
Muscovite
Sample PIMA XRD SP196 Kaolinite Quartz (37.3%), Kaolinite (44.8%), Illite (17.9%) SP197 Illite, Muscovite Quartz (59.9%), Illite (+illite-smectite mixed layer)(40.1%) SP198 Illite, Phengite Quartz (71.4%), Illite (+illite-smectite mixed layer)(25.6%),
Kaolinite (2.9%) SP199 Muscovite Quartz (71.5%), Illite (28.5%) SP200 Kaolinite Quartz (40.8%), Kaolinite (44.5%), Illite
(+illite-smectite mixed layer)(14.8%)
SP054 Kaolinite Quartz (51.5%), Kaolinite (48.5%) SP216 Kaolinite, Halloysite Quartz (74.8%), Kaolinite (25.2%) SP053 Kaolinite Quartz (68.9%), Kaolinite (31.1%)
SP103 Kaolinite Quartz (48.8%), Kaolinite (31.3%), Illite (19.9%) SP201 Kaolinite, Halloysite Quartz (40.7%), Kaolinite (44.1%), Illite (15.2%) SP104 Kaolinite Quartz (56.1%), Kaolinite (43.9%) SP202 Kaolinite Quartz (46.7%), Kaolinite (40.7%), Illite (12.6%) SP105 Kaolinite Quartz (44%), Kaolinite (52.7%), Illite (2.4%), Calcite (0.9%)
SP160 K Alunite, Opal Quartz (74.3%), Alunite (16.5%), Jarosite (9.3%) SP159 Illite, NH Alunite Quartz (57.3%), Mica (24.9%), Orthoclase (12.6%), Albite
(1.1%), Illite (2.6%), Jarosite (1.4%) SP158 Illite, NH Alunite Quartz (46.6%), Mica (24.5%), Orthoclase (12.3%), Albite
(13.7%), Illite (1.7%), Jarosite (1.3%) SP157 Dickite, Nacrite Quartz (49.1%), Kaolinite (44.5%), Hematite (6.3%), Anatase
(0.2%)
QR05-23 NA Quartz (28.5%), Albite (26.8%), Orthoclase (17.2%), Mica (9.5%), Chlorite (8.8%), Calcite (4.6%), Sanidine (3%), Illite (1.7%)
QR11 NA Quartz (27.9%), Albite (29.5%), Mica (14.9%), Orthoclase (13.6%), Chlorite (10.6%), Calcite (1.8%), Illite (1.7%)
SP021 Illite, Kaolinite Quartz (56.7%), Mica (23.5%), Kaolinite (7.4%), Orthoclase (3.9%), Albite (3.9%), Illite (2.3%), Jarosite (2.3%)
SP023 NA Quartz (56.8%), Mica (23.6%), Kaolinite (6.4%), Orthoclase (6.3%), Albite (3.3%), Illite (2%), Jarosite (1.2%), Calcite (0.3%)
Table 5. PIMA results compared to quantitative XRD results for samples containing characteristic alteration assemblages from individual prospect areas. Sample locations are shown on figures 3, 20, 21, 22 and 23.
Figure 19. End-member PIMA mineral spectra foriIllite, k-alunite, kaolinite and muscovite from the Sascha-Pelligrini area. Spectra presented across the short wave infrared (SWIR) band width of 1.3μm to 2.5μm.
Quinn Smith Master of Applied Science Thesis 48
At Sascha Main, PIMA and XRD analysis of alteration associated with the
chalcedonic vein phase indicates the vein phase is hosted within the
background kaolinite-dominated alteration halo. Alteration outside the zone of
intense silicification on the margin of the saccharoidal to bladed and
colloform / crustiform ginguro-banded vein phase is characterised by illite
overprinted by kaolinite (Table 5, Figures 20, 21 & 24). Wall-rock alteration
within the zone of intense silicification is characterised by illite-smectite
mixed-layered clays, with the vein assemblage comprised of muscovite and
Illite, with secondary kaolinite infilling vugs. The chalcedonic and jasperoidal
vein phases are hosted within pervasive kaolinite, illite and halloysite
alteration. Overprinting crystalline to comb quartz veins are associated with
pervasive, texturally preserving halloysite (Table 5, Figures 22 & 24).
Sascha Sur is hosted within a background laumontite alteration halo of
several kilometers. The alteration halo over the veinlet zone is characterised
by illite (+/- mixed-layer illite-smectite) > kaolinite +/- laumontite, alunite and
lesser gypsum. PIMA and XRD analysis of alteration assemblages of better
developed veins indicates alteration outside the zone of intense silicification
is characterised by illite, patchy laumontite and minor chlorite overprinted by
kaolinite. Wall-rock alteration within the zone of intense silicification is
characterised by illite>kaolinite/halloysite>gypsum. Vein assemblages are
dominated by illite, with kaolinite overprinting illite and infilling vugs (Table 5,
Figures 23 & 24).
A background alteration halo of kaolinite that overprints laumontite around
the periphery of the pervasive silicification is characteristic of Pelligrini. The
alteration assemblage proximal to the pervasive silicification is characterised
by kaolinite, minor jarosite and alunite overprinting illite. Pervasive hematite
alteration forms a small halo on the northern periphery of the kaolinite-
jarosite-alunite alteration zone. Minor veins and veinlets cross-cutting the
pervasive silicification are associated with potassium alunite, natroalunite,
opal and dickite. (Table 5, Figure 24).
Quinn Smith Master of Applied Science Thesis 49
Detailed PIMA analysis of individual vein selvages along the SVZ indicate
each alteration assemblage is associated with distinct Al-OH absorption
wavelengths and absorption depths within the short wave infrared (SWIR)
spectrum. PIMA results from samples taken along a traverse perpendicular to
the saccharoidal and bladed vein phase shows the Al-OH spectra absorption
wavelength increases towards the vein, with decreases in the Al-OH
absorption depth in samples of silicified wall-rock (Figure 20). PIMA results
from adjacent to the ginguro-banded vein shows the Al-OH spectra
absorption wavelength and absorption depth increases on the vein selvage,
and decreases in samples of silicified wall-rock (Figure 21). The Al-OH
spectra absorption wavelength and absorption depth decrease in samples of
silicified wall-rock for the pyrite-chalcedony PIMA traverse (Figure 22). PIMA
sampling across gossanous chalcedonic veins at Sascha Sur shows the Al-
OH spectra absorption depth increases towards the vein selvage, and the Al-
OH spectra absorption wavelength decreases in samples of silicified wall-
rock adjacent to the vein (Figure 23).
Quinn Smith Master of Applied Science Thesis 50
00.0
5
0.1
0.1
5
0.2
0.2
5
0.3
0.3
5
22
06
22
07
22
08
22
09
22
10
22
11
22
12
22
13
SP
19
6S
P1
97
SP
19
8S
P1
99
SP
20
0S
P11
3
Absorption Depth
Absorption Wavelength
Sa
mp
le
Sa
sc
ha M
ain
PIM
A-X
RD
Pro
file
AL
(OH
) A
bso
rptio
n
Wa
ve
len
gth
AL
(OH
) A
bso
rptio
n D
ep
th
Saccharo
idal quart
z v
ein
w
ith b
laded t
extu
res
PIM
A s
am
ple
location
Com
b q
uart
z v
ein
Wall-
rock s
ilicifi
cation
Rhyolit
e T
uff
LE
GE
ND
LE
GE
ND
FeO
x v
ein
Vein
Analy
sis
0.8
m @
22
.5g
/t A
u
0.8
m @
22
.5g
/t A
u
0.8
m @
22
.5g
/t A
u
0.8
m @
22
.5g
/t A
u
0.8
m @
22
.5g
/t A
u
0.8
m @
22
.5g
/t A
u
0.8
m @
22
.5g
/t A
u
0.8
m @
22
.5g
/t A
u
0.8
m @
22
.5g
/t A
u
50
cm
50
cm
50
cm
50
cm
50
cm
50
cm
50
cm
50
cm
50
cm
SP
113
SP
113
SP
113
SP
113
SP
113
SP
113
SP
113
SP
113
SP
113
SP
200
SP
200
SP
200
SP
200
SP
200
SP
200
SP
200
SP
200
SP
200
SP
199
SP
199
SP
199
SP
199
SP
199
SP
199
SP
199
SP
199
SP
199
SP
198
SP
198
SP
198
SP
198
SP
198
SP
198
SP
198
SP
198
SP
198
SP
196
SP
196
SP
196
SP
196
SP
196
SP
196
SP
196
SP
196
SP
196
SP
197
SP
197
SP
197
SP
197
SP
197
SP
197
SP
197
SP
197
SP
197
Figure 20. PIMA and XRD profiles of individual vein phases from the Sascha Main vein zone showing sample location, geology and veining, gold/silver assays, and
SWIR absorption wavelength and absorption depth.
Quinn Smith Master of Applied Science Thesis 51
00.0
5
0.1
0.1
5
0.2
0.2
5
0.3
22
05
22
06
22
07
22
08
22
09
22
10
22
11
22
12
22
13
SP
10
5S
P2
02
SP
10
4S
P2
01
SP
10
3
Absorption Depth
Absorption Wavelength
Sa
mp
le
Sa
sc
ha G
ing
uro
PIM
A-X
RD
Pro
file
AL
(OH
) A
bso
rptio
n
Wa
ve
len
gth
AL
(OH
) A
bso
rptio
n
De
pth
50cm
50cm
50cm
50cm
50cm
50cm
50cm
50cm
50cm
0.5
m @
0
.5m
@
0.5
m @
0
.5m
@
0.5
m @
0
.5m
@
0.5
m @
0
.5m
@
0.5
m @
4
8.5
1g
/t A
u /
79
6g
/t A
g4
8.5
1g
/t A
u /
79
6g
/t A
g4
8.5
1g
/t A
u /
79
6g
/t A
g4
8.5
1g
/t A
u /
79
6g
/t A
g4
8.5
1g
/t A
u /
79
6g
/t A
g4
8.5
1g
/t A
u /
79
6g
/t A
g4
8.5
1g
/t A
u /
79
6g
/t A
g4
8.5
1g
/t A
u /
79
6g
/t A
g4
8.5
1g
/t A
u /
79
6g
/t A
g
0.3
m @
0
.3m
@
0.3
m @
0
.3m
@
0.3
m @
0
.3m
@
0.3
m @
0
.3m
@
0.3
m @
1
7.7
9g
/t A
u /
10
4.5
g/t
Ag
17
.79
g/t
Au
/ 1
04
.5g
/t A
g1
7.7
9g
/t A
u /
10
4.5
g/t
Ag
17
.79
g/t
Au
/ 1
04
.5g
/t A
g1
7.7
9g
/t A
u /
10
4.5
g/t
Ag
17
.79
g/t
Au
/ 1
04
.5g
/t A
g1
7.7
9g
/t A
u /
10
4.5
g/t
Ag
17
.79
g/t
Au
/ 1
04
.5g
/t A
g1
7.7
9g
/t A
u /
10
4.5
g/t
Ag
1m
@
1m
@
1m
@
1m
@
1m
@
1m
@
1m
@
1m
@
1m
@
3.8
8g
/t A
u /
29
.5g
/t A
g3
.88
g/t
Au
/ 2
9.5
g/t
Ag
3.8
8g
/t A
u /
29
.5g
/t A
g3
.88
g/t
Au
/ 2
9.5
g/t
Ag
3.8
8g
/t A
u /
29
.5g
/t A
g3
.88
g/t
Au
/ 2
9.5
g/t
Ag
3.8
8g
/t A
u /
29
.5g
/t A
g3
.88
g/t
Au
/ 2
9.5
g/t
Ag
3.8
8g
/t A
u /
29
.5g
/t A
g
SP
104
SP
104
SP
104
SP
104
SP
104
SP
104
SP
104
SP
104
SP
104
SP
201
SP
201
SP
201
SP
201
SP
201
SP
201
SP
201
SP
201
SP
201
SP
103
SP
103
SP
103
SP
103
SP
103
SP
103
SP
103
SP
103
SP
103
SP
202
SP
202
SP
202
SP
202
SP
202
SP
202
SP
202
SP
202
SP
202
SP
105
SP
105
SP
105
SP
105
SP
105
SP
105
SP
105
SP
105
SP
105
Au-A
g B
anded Q
uart
z-G
inguro
Rhyolit
e C
rysta
l A
sh T
uff
Saccharo
idal &
Bla
ded Q
uart
z
Wall-
rock s
ilicifi
cation
PIM
A s
am
ple
location
Vein
Analy
sis
LE
GE
ND
LE
GE
ND C
om
b Q
uart
z
0.1
6
0.1
65
0.1
7
0.1
75
0.1
8
0.1
85
0.1
9
0.1
95
0.2
0.2
05
22
06
.5
22
07
22
07
.5
22
08
22
08
.5
22
09
22
09
.5
SP
02
2S
P0
21
SP
02
0
Absorption Depth
Absorption Wavelength
Sa
mp
le
Sa
sc
ha S
ur P
IMA
-XR
D P
rofi
le
AL
(OH
) A
bso
rptio
n
Wa
ve
len
gth
AL
(OH
) A
bso
rptio
n D
epth
70
70
7070
7070
70
70
70
1m
@ 0
.23g/t
Au /
1.8
g/t
Ag
1m
@ 0
.23g/t
Au /
1.8
g/t
Ag
1m
@ 0
.23g/t
Au /
1.8
g/t
Ag
1m
@ 0
.23g/t
Au /
1.8
g/t
Ag
1m
@ 0
.23g/t
Au /
1.8
g/t
Ag
1m
@ 0
.23g/t
Au /
1.8
g/t
Ag
1m
@ 0
.23g/t
Au /
1.8
g/t
Ag
1m
@ 0
.23g/t
Au /
1.8
g/t
Ag
1m
@ 0
.23g/t
Au /
1.8
g/t
Ag
2m
2m
2m2m
2m2m
2m
2m
2m
SP
021
SP
021
SP
021
SP
021
SP
021
SP
021
SP
021
SP
021
SP
021
SP
022
SP
022
SP
022
SP
022
SP
022
SP
022
SP
022
SP
022
SP
022
SP
020
SP
020
SP
020
SP
020
SP
020
SP
020
SP
020
SP
020
SP
020
Rhy
od
acite
Pum
ice
Tuff
Ve
in A
na
lysis
PIM
A s
am
ple
loca
tio
n
Co
mb
qua
rtz
vein
LE
GE
ND
LE
GE
ND F
eO
x st
ain
ed
cha
lce
do
nic
vein
s w
ith
dis
se
min
ate
d p
yrite
Wa
ll-ro
ck s
ilicific
atio
n
Rhy
od
acite
Cry
sta
l Tuff
Figure 21. PIMA and XRD profiles of individual vein phases from the Sascha ginguro vein zone showing sample location, geology and veining, gold/silver assays, and
SWIR absorption wavelength and absorption depth.
Quinn Smith Master of Applied Science Thesis 52
0.2
0.2
2
0.2
4
0.2
6
0.2
8
0.3
0.3
2
0.3
4
0.3
6
0.3
8
22
05
22
06
22
07
22
08
22
09
22
10
22
11
22
12
SP
05
4S
P2
16
SP
05
3
Absorption Depth
Absorption Wavelength
Sa
mp
le
Sa
sc
ha P
yri
te-C
halc
ed
on
y P
IMA
-XR
D P
rofi
le
AL
(OH
) A
bso
rptio
n
Wa
ve
len
gth
AL
(OH
) A
bso
rptio
n
De
pth
1m
@ 1
.37g/t
Au /
12.4
g/t
Ag
1m
@ 1
.37g/t
Au /
12.4
g/t
Ag
1m
@ 1
.37g/t
Au /
12.4
g/t
Ag
1m
@ 1
.37g/t
Au /
12.4
g/t
Ag
1m
@ 1
.37g/t
Au /
12.4
g/t
Ag
1m
@ 1
.37g/t
Au /
12.4
g/t
Ag
1m
@ 1
.37g/t
Au /
12.4
g/t
Ag
1m
@ 1
.37g/t
Au /
12.4
g/t
Ag
1m
@ 1
.37g/t
Au /
12.4
g/t
Ag
50cm
50cm
50cm
50cm
50cm
50cm
50cm
50cm
50cm
SP
054
SP
054
SP
054
SP
054
SP
054
SP
054
SP
054
SP
054
SP
054
SP
216
SP
216
SP
216
SP
216
SP
216
SP
216
SP
216
SP
216
SP
216
SP
053
SP
053
SP
053
SP
053
SP
053
SP
053
SP
053
SP
053
SP
053
FeO
x s
tain
ed c
halc
edonic
to
opalin
e s
tockw
ork
vein
s
with d
issem
inate
d p
yrite
Lithic
Rhyolit
e T
uff
Wall-
rock s
ilicifi
cation
Cry
sta
l A
sh R
hyolit
e T
uff
Vein
Analy
sis
PIM
A s
am
ple
location
Com
b q
uart
z v
ein
LE
GE
ND
LE
GE
ND C
om
b q
uart
z v
ein
0.1
6
0.1
65
0.1
7
0.1
75
0.1
8
0.1
85
0.1
9
0.1
95
0.2
0.2
05
22
06
.5
22
07
22
07
.5
22
08
22
08
.5
22
09
22
09
.5
SP
02
2S
P0
21
SP
02
0
Absorption Depth
Absorption Wavelength
Sa
mp
le
Sa
sc
ha S
ur P
IMA
-XR
D P
rofi
le
AL
(OH
) A
bso
rptio
n
Wa
ve
len
gth
AL
(OH
) A
bso
rptio
n D
epth
70
70
7070
7070
70
70
70
1m
@ 0
.23g/t
Au /
1.8
g/t
Ag
1m
@ 0
.23g/t
Au /
1.8
g/t
Ag
1m
@ 0
.23g/t
Au /
1.8
g/t
Ag
1m
@ 0
.23g/t
Au /
1.8
g/t
Ag
1m
@ 0
.23g/t
Au /
1.8
g/t
Ag
1m
@ 0
.23g/t
Au /
1.8
g/t
Ag
1m
@ 0
.23g/t
Au /
1.8
g/t
Ag
1m
@ 0
.23g/t
Au /
1.8
g/t
Ag
1m
@ 0
.23g/t
Au /
1.8
g/t
Ag
2m
2m
2m2m
2m2m
2m
2m
2m
SP
021
SP
021
SP
021
SP
021
SP
021
SP
021
SP
021
SP
021
SP
021
SP
022
SP
022
SP
022
SP
022
SP
022
SP
022
SP
022
SP
022
SP
022
SP
020
SP
020
SP
020
SP
020
SP
020
SP
020
SP
020
SP
020
SP
020
Rhy
od
acite
Pum
ice
Tuff
Ve
in A
na
lysis
PIM
A s
am
ple
loca
tio
n
Co
mb
qua
rtz
vein
LE
GE
ND
LE
GE
ND F
eO
x st
ain
ed
cha
lce
do
nic
vein
s w
ith
dis
se
min
ate
d p
yrite
Wa
ll-ro
ck s
ilicific
atio
n
Rhy
od
acite
Cry
sta
l Tuff
Figure 22. PIMA and XRD profiles of individual vein phases from the Sascha pyrite-chalcedony vein zone showing sample location, geology and veining, gold/silver
assays, and SWIR absorption wavelength and absorption depth.
Quinn Smith Master of Applied Science Thesis 53
Figure 23. PIMA and XRD profiles of individual vein phases from the Sascha Sur vein zone showing sample location, geology and veining, gold/silver assays, and SWIR
absorption wavelength and absorption depth.
0.1
6
0.1
65
0.1
7
0.1
75
0.1
8
0.1
85
0.1
9
0.1
95
0.2
0.2
05
22
06
.5
22
07
22
07
.5
22
08
22
08
.5
22
09
22
09
.5
SP
02
2S
P0
21
SP
02
0
Absorption Depth
Absorption Wavelength
Sa
mp
le
Sa
sc
ha S
ur P
IMA
-XR
D P
rofi
le
AL
(OH
) A
bso
rptio
n
Wa
ve
len
gth
AL
(OH
) A
bso
rptio
n D
epth
70
70
7070
7070
70
70
70
1m
@ 0
.23g/t
Au /
1.8
g/t
Ag
1m
@ 0
.23g/t
Au /
1.8
g/t
Ag
1m
@ 0
.23g/t
Au /
1.8
g/t
Ag
1m
@ 0
.23g/t
Au /
1.8
g/t
Ag
1m
@ 0
.23g/t
Au /
1.8
g/t
Ag
1m
@ 0
.23g/t
Au /
1.8
g/t
Ag
1m
@ 0
.23g/t
Au /
1.8
g/t
Ag
1m
@ 0
.23g/t
Au /
1.8
g/t
Ag
1m
@ 0
.23g/t
Au /
1.8
g/t
Ag
2m
2m
2m2m
2m2m
2m
2m
2m
SP
021
SP
021
SP
021
SP
021
SP
021
SP
021
SP
021
SP
021
SP
021
SP
022
SP
022
SP
022
SP
022
SP
022
SP
022
SP
022
SP
022
SP
022
SP
020
SP
020
SP
020
SP
020
SP
020
SP
020
SP
020
SP
020
SP
020
Rhy
od
acite
Pum
ice
Tuff
Ve
in A
na
lysis
PIM
A s
am
ple
loca
tio
n
Co
mb
qua
rtz
vein
LE
GE
ND
LE
GE
ND F
eO
x st
ain
ed
cha
lce
do
nic
vein
s w
ith
dis
se
min
ate
d p
yrite
Wa
ll-ro
ck s
ilicific
atio
n
Rhy
od
acite
Cry
sta
l Tuff
0.1
6
0.1
65
0.1
7
0.1
75
0.1
8
0.1
85
0.1
9
0.1
95
0.2
0.2
05
22
06
.5
22
07
22
07
.5
22
08
22
08
.5
22
09
22
09
.5
SP
02
2S
P0
21
SP
02
0
Absorption Depth
Absorption Wavelength
Sa
mp
le
Sa
sc
ha S
ur P
IMA
-XR
D P
rofi
le
AL
(OH
) A
bso
rptio
n
Wa
ve
len
gth
AL
(OH
) A
bso
rptio
n D
epth
70
70
7070
7070
70
70
70
1m
@ 0
.23g/t
Au /
1.8
g/t
Ag
1m
@ 0
.23g/t
Au /
1.8
g/t
Ag
1m
@ 0
.23g/t
Au /
1.8
g/t
Ag
1m
@ 0
.23g/t
Au /
1.8
g/t
Ag
1m
@ 0
.23g/t
Au /
1.8
g/t
Ag
1m
@ 0
.23g/t
Au /
1.8
g/t
Ag
1m
@ 0
.23g/t
Au /
1.8
g/t
Ag
1m
@ 0
.23g/t
Au /
1.8
g/t
Ag
1m
@ 0
.23g/t
Au /
1.8
g/t
Ag
2m
2m
2m2m
2m2m
2m
2m
2m
SP
021
SP
021
SP
021
SP
021
SP
021
SP
021
SP
021
SP
021
SP
021
SP
022
SP
022
SP
022
SP
022
SP
022
SP
022
SP
022
SP
022
SP
022
SP
020
SP
020
SP
020
SP
020
SP
020
SP
020
SP
020
SP
020
SP
020
Rhy
od
acite
Pum
ice
Tuff
Ve
in A
na
lysis
PIM
A s
am
ple
loca
tio
n
Co
mb
qua
rtz
vein
LE
GE
ND
LE
GE
ND F
eO
x st
ain
ed
cha
lce
do
nic
vein
s w
ith
dis
se
min
ate
d p
yrite
Wa
ll-ro
ck s
ilicific
atio
n
Rhy
od
acite
Cry
sta
l Tuff
Quinn Smith Master of Applied Science Thesis 54
A. Pelligrini (SP157) 1-Laumontite, 2-Kaolinite B. Pelligrini (SP158) 1-Illite, 2-Quartz C. Pelligrini (SP160) 1-Alunite, 2-Opal D. Sascha Main (SP196) Kaolinite E. Sascha Main (SP197) Illite F. Sascha Main (SP199) 1-Muscovite, 2-Illite, 3-Quartz G. Sascha Sur (SP021) Laumontite H. Sascha Sur (SP023) 1-Illite, 2-Gypsum
H
1
2
10 μm
G
5 μm
F
1
2
3
20 μm
E
E
5 μm
D
5 μm
C 1
2
10 μm
B
1
2
10 μm
A
1
2
5 μm
Figure 24. ESEM images of alteration mineral morphologies from individual
alteration systems within the study area.
Quinn Smith Master of Applied Science Thesis 55
Alteration geochemistry
Mass balance alteration geochemistry is calculated for individual alteration
zones, within stratigraphically homogeneous units using the methods of
MacLean and Barret (1993). Least altered samples for each of the
stratigraphic host units were chosen through petrographic and XRD
examination. Least altered precursor samples for the rhyolite crystal ash tuff,
rhyodacite ignimbrite, and rhyolite ash tuff are QR05-26, QR25 and QR05-21
respectively. Sample locations are shown in figure 3.
The Sascha Main alteration profile shows a general mass gain with
increasing alteration intensity proximal to veining. Immobile element
concentrations decrease with increasing alteration intensity and mass gain
(Figure 25 A). Sample SP196 is located 160cm from the vein margin (Figure
19), and is characterised by moderate mass gains in SiO2 (16.5%), Al2O3
(14.5%), small mass gains in TiO2 (1%) and Fe2O3 (4.6%), and a small mass
loss in K2O (-2.2%) (Figure 25 B). SP197 and SP198 are located 90cm and
25cm from the vein respectively, and show the largest mass gains in the
profile. SP197 shows large mass gains in SiO2 (235.9%), Al2O3 (42.6%),
moderate mass gains in Fe2O3 (14.4%), K2O (10.2%), and small mass gains
in MgO (3.9%), TiO2 (2.1%), Na2O (0.9%) (Figure 25 B).
Similar to Sascha Main, the Sascha Sur alteration profile shows mass gains
adjacent to epithermal veins. Increasing alteration intensity decreases
immobile element concentrations associated with mass gains (Figure 26 A).
Sample QR05-23 is located approximately 2.3km from the Sascha Sur
veinlet zone (Figure 3), and is characterised by small mass gains in Al2O3
(2.5%), Fe2O3 (2.6%), CaO (3.8%), Na2O (2.9%), and MgO (0.6%), with
small mass losses in SiO2 (-5.2%) and K2O (-1.5%) (Figure 26 B). The
largest mass gains in the profile are observed in samples located directly on
the vein margin. SP023 shows large mass gains in SiO2 (163.3%), Al2O3
(22%), a moderate mass gain in K2O (6.2%), and small mass gains in Fe2O3
(1.6%), MgO (1.1%), and TiO2 (0.6%) (Figure 26 B).
Table XX. Selected PIMA and XRD results
Quinn Smith Master of Applied Science Thesis 56
The Pelligrini alteration profile is unique, with a mass loss in the kaolinite-illite
peripheral alteration zone, and a mass gain in the zone of pervasive
silicification with alunite and opal. Increasing alteration within the host
lithology at Pelligrini causes immobile element concentrations to increase
with mass losses, and decrease with mass gains (Figure 27 A). The
alteration of the rhyolite ash tuff around the periphery of the silicified-
alunite/opal zone is characterised by a large mass loss in SiO2 (-45.9%),
small mass losses in Al2O3 (-3.7%), K2O (-3.2%), and Na2O (-1.4%), with a
small mass gain in Fe2O3 (4.5%) (Figure 27 B). SP160 is located directly
within the silicified-alunite/opal zone and shows the largest mass gain in the
profile. SP160 shows large mass gains in SiO2 (63.3%), moderate mass
gains in SO3 (16.2%), small mass gains in Al2O3 (5.4%), Fe2O3 (3.4%), and
K2O (1.3%), with a small mass loss in Na2O (0.6%) (Figure 27 B).
Quinn Smith Master of Applied Science Thesis 57
Rhyolite Crystal Ash Tuff Immobile Elements
y = 1.4372x
R2 = 0.9792
0
1
2
3
4
5
6
0 0.5 1 1.5 2 2.5 3 3.5 4
Dy ppm
Sm
pp
mSm vs Dy
Linear (Sm vs Dy)
A
QR05-26
SP196
SP197
SP199
SP198
-50
0
50
100
SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 SO3 NET
Ma
ss
Ch
an
ge
(g
/10
0g
)
Sascha Main Alteration
SP196 SP199 SP198 SP197235.92
311.05101B
Figure 25. Selected immobile elements and geochemical mass-changes for
rhyolite crystal ash tuff alteration within Sascha Main.
Selected immobile elements showing single precursor, highlighted by red circle, and altered samples along linear alteration trends for homogenous stratigraphic horizons. Bar graphs showing mass-changes of major elements in samples representing increasing alteration intensity for individual alteration systems.
A. Immobile elements Sm and Dy within the rhyolite crystal ash tuff which hosts Sascha Main.
B. Mass changes for major elements within the Sascha Main alteration system based on the single precursor QR05-26
Quinn Smith Master of Applied Science Thesis 58
Figure 26. Selected immobile elements and geochemical mass-changes for
rhyodacite ignimbrite alteration within Sascha Sur.
Selected immobile elements showing single precursor, highlighted by red circle, and altered samples along linear alteration trends for homogenous stratigraphic horizons. Bar graphs showing mass-changes of major elements in samples representing increasing alteration intensity for individual alteration systems.
A. Immobile elements Dy and Y within the rhyodacite ignimbrite which hosts Sascha Sur
B. Mass changes for major elements within the Sascha Sur alteration system based on the single precursor QR25
Sascha Sur Alteration
-50
0
50
100
SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 SO3 NET
Ma
ss
Ch
an
ge
(g
/10
0g
)
QR05-23 QR11 SP021 SP023163.3 196.16
D
Rhyodacite Ignimbrite Immobile Elements
y = 0.1788x
R2 = 0.9951
0
0.5
1
1.5
2
2.5
3
3.5
4
0 5 10 15 20 25
Y ppm
Dy
pp
m
Dy vs Y
Linear (Dy vs Y)
CQR25
QR05-23
QR11
SP021
SP023
A
B
Quinn Smith Master of Applied Science Thesis 59
Selected immobile elements showing single precursor, highlighted by red circle, and altered samples along linear alteration trends for homogenous stratigraphic horizons. Bar graphs showing mass-changes of major elements in samples representing increasing alteration intensity for individual alteration systems.
A. Immobile elements Dy and Y within the rhyolite ash tuff which hosts Pelligrini
B. Mass changes for major elements within the Pelligrini alteration system based on the single precursor QR05-21
Figure 27. Selected immobile elements and geochemical mass-changes for rhyolite ash tuff alteration within Pelligrini.
Rhyolite Ash Tuff Immobile Elements
y = 0.193x
R2 = 0.9703
0
0.5
1
1.5
2
2.5
3
3.5
0 2 4 6 8 10 12 14 16 18
Y ppm
Dy
pp
m
Dy vs Y
Linear (Dy vs Y)
E
QR05-21
SP160
SP157
SP158
SP159
A
B
-50
0
50
100
SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 SO3 NET
Ma
ss
Ch
an
ge
(g
/10
0g
)
Pelligrini Alteration
SP160 SP158 SP159 SP160
-50.87
F
Quinn Smith Master of Applied Science Thesis 60
Dy vs Zr net mass change comparison
-60
-40
-20
0
20
40
60
80
100
120
SP198 SP199 QR05-23 QR11 SP158 SP160
Ma
ss
Ch
an
ge
(g
/10
0g
)
Zr net mass-change
Dy net mass-change
The trace element Zr is generally considered as immobile in many alteration
systems (Maclean and Kranidiots, 1987; Cail and Cline, 2001, Grant, 1986;
Grant 2005). Comparisons of possible immobile elements Dy, Sm and Y with
Zr show good correlations (Table 6). Mass change calculations using Zr
show similar trends to mass-changes calculated from Dy, however absolute
values differ by up to 58% (Figure 28).
Samples Outliers Dy vs Zr R
2
Sm vs Zr R
2
Y vs Zr R
2
Rhyolite Crystal Ash Tuff
QR05-26, SP198, SP199
SP196, SP197 0.94 0.96 0.84
Rhyodacite ignimbrite
QR25, QR05-23, QR11
SP021, SP023 0.92 0.91 0.92
Rhyolite Ash Tuff
QR05-21, SP158, SP160
SP157, SP159 0.83 0.34 0.98
Table 6. Tabulated correlation coefficient values for immobile elements Dy, Sm, and Y plotted against Zr.
Figure 28. Bar graph comparing net mass changes for immobile elements Dy and Zr.
Quinn Smith Master of Applied Science Thesis 61
Discussion
Epithermal deposits are characteristically hosted within complex geological
environments subject to multiple episodes of deformation (Begbie et al, 2007)
and overprinting alteration (Mauk and Simpson 2007; Gemmel 2007; Warren
et al,2007; Ducart et al, 2006). Host sequences exhibit strong controls on the
type and nature of alteration and the morphology of veining. Understanding
the local geology within the study area is vital in generating a realistic and
accurate generic model of the epithermal system.
Volcanology
Depositional setting
Large silicic volcanic environments have complex and diverse facies
associations and stratigraphic relationships. Facies associations within the
Sascha-Pelligrini area comprise two compositionally distinct volcanic
sequences of rhyodacite and rhyolite ash-flows and minor air-fall deposits
(Chon Aike and La Matilde Formations). The rhyolite sequence is deposited
on a minor unconformity on the rhyodacite sequence and represents
deposition of significantly less material. The rhyodacite sequence is very
thick (~670m) strongly welded, and laterally continuous across the study
area. The upper rhyolite sequence is relatively thin (~130m), poorly welded
and spatially restricted within the study area.
Classification into broad facies models is achieved by the unique
characteristics within the volcanic stratigraphy. Stratigraphic associations, or
facies associations, can be used to reconstruct palaeogeological
environments and depositional settings aiding in the interpretation of eruption
styles. Continental silicic provinces are composed of rhyolite domes and
flows of subdued topography rising above large ignimbrite shields (Cas and
Wright, 1987). The focal element of a silicic province is the caldera, which
may contain multiple eruption points and a basin for accumulation of volcanic
material. The critical facies association within silicic volcanic terrains is the
Quinn Smith Master of Applied Science Thesis 62
recognition of volcanics formed in intracaldera or extracaldera environments
(Cas and Wright, 1987). Intracaldera deposits are comprised of lavas and
domes, thick crystal-rich ignimbrites and associated near vent co-ignimbrite
breccias and intercalated epiclastics. Extracaldera successions are
dominated by thin sheet-like outflow ignimbrites interspersed with pyroclastic
fall and abundant epiclastic deposits.
The deposition of the homogenous and strongly welded rhyodacite ignimbrite
of the Chon Aike formation within the study area requires the pre-existence of
a large topographic depression proximal to an eruption source (Cas and
Wright, 1987). The lithic concentration zone, or lag deposit, within the La
Matilde rhyolite ash tuff would suggest eruption proximal to source vents and
characteristically shows a decrease in maximum clast size away from the
source (Sheriden, 1979). Resurgent rhyolitic flow domes, such as at
Pelligrini, form within or at the margins of the caldera structure, however they
may also be erupted outside the caldera margin (Cas and Wright, 1987). On
the basis of facies models (Cas and Wright, 1987), the thick, homogenous
and strongly welded nature of the rhyodacite ignimbrite, lithic concentration
zones within the rhyolite sequence, and the resurgent flow dome at Pelligrini
is consistent with an intracaldera depositional setting.
Although facies associations within the study area suggest an intra-caldera
setting, Guido et al (2004) proposes that the distribution of the Jurassic
volcanics of the Chon Aike and La Matilde formations are associated with
intersections of regional east-northeast and west-northwest basement
fractures. Regional east-northeast and west-northwest basement transfer
structures traverse the study area, with spatially restricted rhyolite tuffs
confined within grabens and half-grabens bound by the structures. Maximum
size lithic clast vectors from the base of the rhyolite sequence suggest a
source close to the intersection of the basement structures in the north-west
of the study area. Echeveste et al (1999) and Guido et al (2004) propose the
large volume of volcanic material that comprises the Chon Aike and La
Matilde formations were produced through a complex system of Jurassic
extensional graben-forming structures with only minor caldera formation.
Quinn Smith Master of Applied Science Thesis 63
Therefore the facies associations of the volcanics within the study area may
closely represent an intra-caldera setting, with volcanics erupted from a
complex series of graben-forming extensional fractures instead of a classic
caldera margin.
Eruption styles
Eruption styles depend strongly on the physical properties of magma. Magma
properties are related to temperature, melt composition, proportion of
crystals, amount of dissolved volatiles and the abundance of gas bubbles
(Sparks et al, 1997). Explosive eruptions of dacitic-rhyolitic material usually
occur through degassing of dissolved volatiles with decreasing lithostatic load
(Sparks et al, 1997). Highly explosive dacitic-rhyolitic eruptions can produce
tremendous amounts of pyroclastic material. Collapse of the giant La Garita
caldera in the San Juan Volcanic Field erupted as much as 5000km3 of
material in individual eruptions, producing the well studied Fish Canyon Tuff
(Lipman, 2000).
Pyroclastic deposits can be classified into genetic groups of fall, flow and
surge according to their mode of transport and deposition (Sparks and
Walker, 1973) (Figure 29). Pyroclastic fall deposits are formed from material
that has been explosively ejected from a vent (Sheriden, 1979) and usually
mantle topography, with the geometry and size of the deposit controlled by
eruption column height and wind conditions (Cas and Wright, 1987).
Pyroclastic flows are formed by the collapse of a convective ash cloud, with a
lateral deposition of pyroclastic material away from the eruptive centre
(Sheriden, 1979). Pyroclastic flows are controlled by topography with material
filling valleys and depressions; flows emplaced at extremely high velocities
may mantle topography (Cas and Wright, 1987). Pyroclastic surges are
deposited from directed blasts caused by plug explosions, phreatomagmatic
eruptions or changing vent geometry (Sheriden, 1979).
The volcanic units of the Sascha-Pelligrini area record several distinct
eruption styles. The rhyodacite crystal ignimbrite forms a thick, homogeneous
Quinn Smith Master of Applied Science Thesis 64
composite welded package with no clear lithological separation between
welded horizons. Lithic clasts are rare and occur as small discrete fragments
within a much finer-grained crystal-rich matrix. The rhyodacite unit may
represent a composite flow formed by eruption column collapse from multiple
vents or fissures. Alternatively, this unit may have been produced by multiple
rapid eruptive events from a single vent or fissure. Both models imply a
relatively synchronous and rapid deposition.
Welding within the rhyodacite ignimbrite varies both vertically and
horizontally, and forms composite welded horizons preserved as topographic
highs across the study area. Complex welding patterns develop in pyroclastic
flows due to varying temperature and load stress and vary irregularly from
proximal to distal vent facies (Christiansen, 1979; Wilson and Hildreth, 1997).
The composite welding within the lower rhyodacite ignimbrite of the Chon
Aike Formation suggests deposition originated from a succession of small
eruptions that quickly deposited material at relatively hot temperatures. The
lack of air-fall horizons within the ignimbrite package suggests that the
pyroclastic flows were associated with low eruption columns (Guido et al,
2004), or alternatively, the volcanic ash was elutriated from the eruption
column.
An increase in lithic and juvenile lava clasts towards the top of the rhyodacite
ignimbrite coupled with the transition into a massive strongly welded pumice
tuff represents a change in the vent geometry producing a more energetic
eruption sequence. Thin air-fall ash horizons at the top of the pumiceous unit
may indicate that buoyant lift-off occurred within the pyroclastic flow,
producing a co-ignimbrite plume. Buoyant lift-off is thought to be the main
mechanism for producing co-ignimbrite plumes, elutriating fine ash and
preferentially enriching pyroclastic flows in crystal material (Sparks et al,
1997). Dilute gravity flows can form on the sides of an eruption column, and
deposit thin ash layers proximal to eruption centres in a similar manner to co-
ignimbrite plumes (Sparks et al, 1997).
Quinn Smith Master of Applied Science Thesis 65
The transition at the top of the Chon Aike from a massive welded pumice tuff
to welded crystal ash rhyodacite tuff with abundant juvenile chlorite-hematite,
granitic and lithic clasts represents renewed vent activity. Lithic clasts signal
a possible change in vent geometry or vent clearing. Abundant deep-origin
chlorite-hematite and granitic clasts represent a tapping of deeper parts of an
exhausted magma chamber. The welded crystal ash rhyodacite tuff is the last
eruptive unit of the Chon Aike sequence, and subsequently has the least
amount of lithostatic load. Strong welding with abundant undeformed clasts
and minimal lithostatic load suggests the crystal ash rhyodacite package was
deposited at high temperatures proximal to source. Current exposure of
highly welded ignimbrites and disconformable relationships with the overlying
un-welded rhyolite ash package, represents a period of erosion and volcanic
quiescence.
The spatially restricted La Matilde rhyolite ash tuffs show disconformable
relationships with the underlying Chon Aike Formation, suggesting they were
deposited after a brief period of volcanic quiescence. Stratigraphic
relationships between the La Matilde and Chon Aike formations are
debatable, with significant lateral and vertical facies changes within the units
obscuring depositional contacts. The laminated tuffs of the La Matilde
Formation often interdigit with the ignimbrites of the Chon Aike Formation,
leading Sanders (2000) to suggest that the two units were deposited
concurrently. Epiclastic deposits of the La Matilde show disconformable
contacts with the underlying Chon Aike, however they do not represent a
significant hiatus in volcanic activity, but rather reworking of pyroclastic
material between eruptions (Pakhurst et al, 1998; Guido et al, 2004).
The massive rhyolite package of the La Matilde Formation formed from a
single, violent eruption filling a pre-existing valley. The lithic rhyolite tuff at the
base of the La Matilde represents a proximal lag breccia, sourced from a
single vent following a transition from rhyodacite to rhyolite volcanism.
Metamorphic muscovite schist clasts compose >95% of the total lithics and
represent a deep vent source of pre-Jurassic basement. Lithic fragments in
pyroclastic deposits result from conduit and vent erosion during explosive
Quinn Smith Master of Applied Science Thesis 66
eruptions, recording changes in mechanics and timing of caldera collapse
(Suzuki-Kamata et al, 1993: Browne and Gardner, 2003). The lithic unit has
extreme vertical and lateral geometry variations, with clast size vectors
identifying a single vent in the north-west of the study area. The lithic lag
breccia is contemporaneous with the massive valley-filling un-welded rhyolite
crystal ash tuff.
Eruption column collapse after the initial explosion that deposited the lithic
lag breccia produced a relatively cool pyroclastic flow that travelled along a
pre-existing valley towards the southeast. The flow deposited the un-welded
rhyolite crystal ash tuff, the most extensive unit of the La Matilde Formation.
The overlying rhyolite ash unit at the top of the La Matilde sequence mantles
topography and contains zones of accretionary lapilli. Mantled topography
and accretionary lapilli indicate the rhyolite ash is an air-fall sequence
deposited during a rain event. The planar-bedded rhyolite ash is
contemporaneous with the air-fall ash, and represents the settling of the
convecting cloud from the initial transitional eruption that produced both the
lag breccia and the un-welded crystal ash rhyolite tuff.
Several distinct eruption styles are recorded within the volcanic sequence.
The rhyodacite crystal ignimbrite exposed at the base of the sequence was
rapidly deposited by eruption column collapse from multiple vents or fissures
creating a composite welded unit. An increase in lithic and juvenile lava
clasts towards the top of the unit, and the transition into a massive strongly
welded pumice tuff represents a change to a more energetic eruption
sequence. The transition at the top of the Chon Aike to welded crystal ash
rhyodacite tuff with abundant undeformed juvenile and lithic clasts represents
renewed vent activity, with deposition under minimal lithostatic load proximal
to source. The spatially restricted La Matilde rhyolite sequence was
deposited from a single violent eruption depositing the lag breccia proximal to
the vent. The pyroclastic flow produced by the eruption column collapse
deposited the un-welded rhyolite crystal ash tuff, with settling of the
convective cloud depositing the planar-bedded rhyolite ash tuff.
Quinn Smith Master of Applied Science Thesis 67
A. Convective – Development of Plinian eruption column, fine grained pyroclastic fallout covers wide area.
B. Transitional – Development of convective plume with some pyroclastic fallout, oscillating fountain feeds some pyroclastic flows with associated co-ignimbrite plumes.
C. Collapsing – Sustained fountain of pyroclastic material feeds continual massive pyroclastic flows that develop large co-ignimbrite plumes. Small convecting cloud above collapsing fountain.
(Modified from Purdy 2003, after Neri et al 2002)
Figure 29. Styles of explosive eruptions with vent development restricted to half-graben
and graben forming regional basement structures.
Quinn Smith Master of Applied Science Thesis 68
Magma Petrogenesis
Two petrographically and geochemically distinct groups (rhyodacite and
rhyolite) compose the Sascha-Pelligrini suite of the study area. Hydrothermal
alteration affects rocks throughout the majority of the study area and
precludes quantitative petrochemical modeling the volcanic rock suite.
Sorting, welding, post emplacement crystallisation and devitrification, as well
as erosion prior to hydrothermal alteration contribute to the complexity of the
suite‟s genesis.
The pyroclastic rocks of the study area belong to the Jurassic Chon Aike and
La Matilde Formations which form a silicic large igneous province (LIP) with
an estimated volume of 235,000km3 (Pankhurst et al, 1998). The generation
of large volumes of silicic magma reflects large-scale crustal melting
controlled by the water content and composition of the crust, and a large
thermal (+/- mass) input from the mantle (Bryan, 2007). Jurassic rhyolites of
the Chon Aike Formation were produced through anatexis of sedimentary
source material, with silicic melts generated from heat input of mantle-derived
basaltic melts ponding at the base of the crust (Gust et al, 1985). The Chon
Aike silicic LIP is characterised by the absence of mantle-derived rocks
(Pankhurst el at, 1998), with basaltic andesite of the Bajo Pobre Formation
being the least evolved and restricted to only a few localities (Pankhurst and
Rapela, 1995; Pankhurst et al, 1998; Sanders, 2000; Sharpe et al, 2002).
Trace element similarities between the basaltic andesite of the Bajo Pobre
and the rhyolites of the Chon Aike suggest the rhyolite formed from partial
melting of the basaltic andesite (Storey and Alabaster, 1991; Pankhurst and
Rapela, 1995; Pankhurst et al, 1998).
Pankhurst and Rapela (1995) demonstrate that the sequence from basaltic
andesite (Bajo Pobre) to dacite/rhyodacite to rhyolite (Chon Aike) can be
modeled by a combination of partial melting and fractional crystallisation.
Batch partial melting of a depleted Bajo Pobre andesite produces Chon Aike
equivalent dacite through 20% melt extraction, with a residual composition
composed of orthopyroxene and plagioclase. Fractional crystallisation of
dacite to rhyodacite is accomplished with the removal of 50% crystal
Quinn Smith Master of Applied Science Thesis 69
assemblage composed of 10% amphibole and 90% plagioclase. High-silica
rhyolite is produced from fractional crystallisation of the rhyodacite, with the
removal of 40% crystal assemblage composed of approximately 58%
plagioclase, 40% hornblende, 1% apatite and trace allanite.
The exposed rhyodacite sequence of the Chon Aike Formation exhibits some
variation in REE abundances. The last ash-flow unit of the sequence, the
pumice ash welded rhyodacite tuff, is enriched in the LREE, depleted in the
middle REE with a pronounced negative Eu anomaly relative to the first ash-
flow rhyodacite. These REE differences are consistent with the proposed
fractionation trends of Pankhurst and Rapela (1995).
The rhyolitic tuff sequence of the La Matilde Formation shows a general
decrease in REEs from the massive crystal ash tuff to the planar bedded air-
fall ash tuff. The REE pattern for the lag breccia lithic tuff deviates from the
general rhyolite sequence patterns and may represent contamination from a
variety of components. The lag breccia is considered to be deposited through
the initial explosive eruption of the rhyolitic sequence, contains vent derived
material, and outcrops within the intense alteration zone of Sascha Main. The
discrepancies observed for the lag breccia REE trend may be a complex
product of obscuring alteration and vent contamination. Similar to the
rhyodacite package, the rhyolite sequence REE patterns show a negative Eu
anomaly, decrease in REE concentrations with continued eruptive units, and
depressed HREEs. The negative Eu anomaly may be controlled by the
fractionation of sanidine, with the depletion of REEs controlled by the
fractionation of hornblende. Compositional zoning observed in sequence
suggests eruption of a zoned magma chamber.
Eruptive sequences commonly show an orderly progression from most
evolved to least evolved magmatic ejecta, representing eruption from
compositionally zoned magma chambers (Hildreth, 1979). Individual ash-flow
sheets of the rhyodacite sequence within the study area have a limited
compositional range. Large volume, phenocryst-rich ignimbrites such as the
rhyodacite sequence rarely develop from zoned magma chambers, with the
Quinn Smith Master of Applied Science Thesis 70
fractionation process prematurely aborted by venting of the dominant magma
volume (Hildreth, 1979). This may be the case for the rhyodacite sequence,
however the volcanic stratigraphy below the ignimbrite is not observed, and a
more evolved silicic unit may exist at the base of section below the
rhyodacite.
The rhyodacite and rhyolite units form two distinct groups within the study
area. Strong hydrothermal alteration, sorting, welding, post emplacement
crystallisation and devitrification, as well as erosion throughout the majority of
the study area precludes quantitative petrochemical modeling the volcanic
rock suite. Variation in REE abundances across the rhyodacite and rhyolite
sequences is consistent with the proposed fractionation trends of Pankhurst
and Rapela (1995). Pankhurst and Rapela (1995) demonstrate that the
sequence from basaltic andesite (Bajo Pobre) to dacite/rhyodacite to rhyolite
(Chon Aike) can be modeled by a combination of partial melting and
fractional crystallisation.
Host Rock Control and Structural Model
Host rock rheology controls vein distribution and morphology within the
Sascha-Pelligrini system and is a common control in many epithermal
deposits (Brathwaite et al, 2001; Christie et al, 2007; Izawa et al, 1990).
Outcrop distribution of ginguro-banded veins at Sascha Main is limited, with
quartz veins and lenses hosted within poorly welded crystal ash rhyolite tuff.
The heavily altered and relatively friable tuff restricts coherent vein
development within the current stratigraphic exposure. Outcrop morphology
of discontinuous quartz veins within the rhyolite tuff suggests that veins are
upward terminating within a friable tuff that does not host continuous brittle
fractures. The lithic rhyolite tuff hosts discontinuous and chaotic veining, with
friability and high porosity favouring multiple discontinuous fractures and
veinlets. Pumiceous rhyodacite tuffs at Sascha Sur and Marcellina host
broad zones of discontinuous multi-directional veinlets, while the ash tuffs at
Pelligrini are pervasively silicified and brecciated.
Quinn Smith Master of Applied Science Thesis 71
Lithological controls are well documented at numerous epithermal deposits,
with coherent 2 to 3 meter wide veins in andesite at the Hauraki goldfield
changing to stockwork veining in rhyolite with individual veins less than 10
centimeters in width (Christie et al, 2007). Well developed coherent
epithermal veins are developed within crystalline and brittle Bajo Pobre
andesite that underlies the Chon Aike Formation (Dietrich et al, 2008) as well
as felsic intrusives within the Chon Aike Formation (Wallier and Tosdal,
2008). The rheologically brittle massive crystalline rhyodacite ignimbrite that
stratigraphically underlies the current outcrop exposure at Sascha Main may
host larger epithermal veins in more continuous fractures. The rhyodacite
ignimbrite at Sascha-Pelligrini is analogous to the massive quartz-feldspar
porphyritic, densely welded, pumiceous ignimbrite (Granosa) within the Chon
Aike Formation that hosts the majority of veining within the Cerro Vanguardia
epithermal deposit (Sharpe et al, 2002).
Epithermal veining at Sascha-Pelligrini was emplaced during a period of
extensional tectonics that formed northwest-trending grabens. The
extensional event was contemporaneous with the last stages of the Chon
Aike (Echavarria et al, 2005) with the stratigraphically high La Matilde rhyolite
filling the grabens and half-grabens. Maximum dilation and associated fluid
flow formed at right-stepping structural spays along the right-lateral, oblique-
slip, 315° trending normal Sascha Fault (Figure 30). The tension axis
trended toward the northeast quadrant, producing left-lateral movement in
north- and northeast-trending fault and shear zones, and right-lateral
movement in structures oriented between 330° and 250°.
Quinn Smith Master of Applied Science Thesis 72
Figure 30. Structural model for the Sascha – Pelligrini study area showing vein
styles and kinematic indicators for observed structural orientations.
Figure 31. Riedel shear model for the San Jose district applied to the Sascha –
Pelligrini study area. Mineralised veins striking at 315° parallel to 1 represent extension fracture (T) in the model. Veins striking >315° are observed with a sinistral strike-slip component (San Jose), whereas veins striking <315° are observed to have
a dextral strike-slip component (SVZ). (After Dietrich et al 2008).
Sascha
Vein Zone
San Jose
District
Quinn Smith Master of Applied Science Thesis 73
Contrary to recent structural interpretations of the Desseado Massif that
suggest right-lateral structures develop only narrow discontinuous and en
echelon veins of no economic significance (Echavarria et al, 2005), detailed
mapping of the SVZ indicates mineralised veins are hosted by right-lateral
fault structures. A recent structural study of the Huevos Verdes veins within
the San Jose district also supports the concept that economic significant
epithermal veins are hosted on right-lateral fault structures. The structural
model of the SVZ correlates well with the structural model of the San Jose
district for the northwestern edge of the Desseado Massif. This structural
model suggests right-lateral west-northwest-trending faults (280°) and left-
lateral north-northwest-trending faults (350°) act as a conjugate shear pair.
The orientation of 1 is modelled as the acute bisector of the conjugate shear
pair and is orientated at 315°. The majority of the veins at Huevos Verdes
within the San Jose district are modelled as purely extensional, and similar to
the SVZ, are orientated parallel to 1 at 315°. The trend of the Huevos
Verdes and Sascha systems varies from 300° to 340° with an overall left-
lateral strike-slip component observed for the Huevos Verdes veins striking at
325°(Dietrich et al, 2008), and an overall right-lateral strike-slip component
observed for the Sascha veins striking at 315°. The Huevos Verdes model
correlates with the observed structural pattern of the SVZ and fits well with
the Riedel shear model (Figure 31). Left-lateral north-northwest-trending
lineaments represent the main shear plane and north-northwest and west-
northwest structures represent R and R‟ shears respectively (Dietrich et al,
2008).
Host rock rheology controls vein morphology and alteration patterns across
the study area. The current exposure along the SVZ indicates the friable
rhyolite tuffs of the La Matilde Formation do not host continuous brittle
fractures or veins, and instead host upward terminating veins, stockwork vein
zones, and pervasive silicification. Brittle fractures and more massive veining
may be developed within the crystalline rhyodacite ignimbrite that
stratigraphically lies below the current vein exposure. Structural analysis of
the SVZ indicates the tension axis trended toward the northeast quadrant,
producing right-lateral movement along the 315° trend. The structural pattern
Quinn Smith Master of Applied Science Thesis 74
of the SVZ conforms to the Riedel shear model, with 1 orientated at 315°
and modelled as the acute bisector of the conjugate shear pair.
Alteration zoning
Hydrothermal alteration within active geothermal fields is well documented
(eg. Browne, 1978; White, 1981; Hedenquist and Henley, 1985; Spycher and
Reed, 1989; Reyes, 1990; Fulignati et al, 1997; Cox and Browne, 1998;
Ruggieri et al, 1999; Simmons and Browne, 2000; Patrier et al, 2003; and
Bignall et al, 2004), with observed alteration assemblages and mineral
zoning forming the basis of epithermal alteration models (Berger and Eimon,
1982; Heald et al, 1987; Berger and Henley, 1988 etc).
Hydrothermal alteration of the SVZ is characterised by quartz, adularia, illite,
pyrite and minor calcite, and is overprinted by a late stage kaolinite-
dominated assemblage. Throughout the Sascha-Pelligrini area, the common
hydrothermal minerals define distinct alteration zones around epithermal
veins (Figure 32). Each zone is characterised by a unique mineral
assemblage and can be grouped into two major alteration types according to
fluid chemistry. Clay abundance progressively increases above and towards
the vein margins, with formation of alteration assemblages driven by reduced
neutral or oxidized acid fluids.
Quinn Smith Master of Applied Science Thesis 75
Figure 32. Schematic cross section of alteration zoning and mineral assemblage
observed for the Sascha-Pelligrini epithermal system.
Quinn Smith Master of Applied Science Thesis 76
The primary alteration assemblage along the SVZ formed from a near neutral
to weakly alkaline chloride water, and is characteristic of the alteration found
in many active geothermal systems (Simmons and Browne, 2000). Primary
vein calcite, quartz pseudomorphs after bladed calcite and adularia occur
within the saccharoidal vein phase at Sascha Main and form from gas loss
and cooling associated with boiling (Browne and Ellis, 1970). Platy calcite
scales, analogous to bladed calcite within epithermal veins, form in
geothermal wells 100 to 300 meters above the point where the geothermal
water first flashes to steam (Simmons and Browne, 2000). The weakly
alkaline chloride-rich geothermal waters at Broadlands-Ohaaki contain
approximately 1,300 mg/kg chloride and 1,900 mg/kg CO2 (Hedenquist and
Henley, 1985), and produce a characteristic alteration assemblage
dominated by quartz, adularia, illite, calcite, chlorite and pyrite (Simmons and
Browne, 2000).
Abundant kaolinite, alunite and native sulphur are observed at many
epithermal deposits, and occur overlying or towards the periphery of inferred
fluid up-flow zones (Schoen et al, 1974; Love et al, 1998; Simpson et al,
2001). Steam-heated, acid-sulphate waters commonly occur in the vadose
zone above boiling up-flow points, with alteration formed by leaching of rocks
from fluids concentrated in H2SO4. Fluids concentrated in H2SO4 can be
produced by atmospheric oxidation of sulphides, oxidation at the water table
by H2S release via boiling, and the condensation of magmatic vapor (Rye et
al, 1992). The acid-sulphate water reacts with the host-rock to produce
kaolinite, cristobalite, alunite, pyrite and native sulfur (Scheon et al, 1974).
Stable isotope studies of alunite from acid-sulphate alteration zones suggest
precipitation from dominantly meteoric waters with magmatic SO2 (Rye et al,
1992; Love et al, 1998; Mykietiuk et al, 2004).
Calcium-rich zeolites that characterise the peripheral alteration halo of the
Sascha-Pelligrini system form within the shallow and peripheral zones of
geothermal systems (Simmons and Browne, 2000; Steiner, 1977).
Condensation of CO2 gas and absorption into cool ground waters produces
Quinn Smith Master of Applied Science Thesis 77
these zeolites as well as low-temperature clays and carbonates (Simpson et
al, 2001). Country rock exposed to residual water after steam separation has
a high H2S/H2 ratio and also favors pyrite formation (Browne and Ellis, 1970).
The distal laumontite-montmorillonite alteration assemblage of the Sascha-
Pelligrini system passes to illite-dominant alteration proximal to epithermal
veins. The H2O content of calcium zeolites progressively decreases with
increasing temperature, zoning from laumontite to wairakite (Steiner, 1977).
Laumontite forms at temperatures above 110°C, passing to wairakite at
150°C, with illite forming above 200°C (Reed, 1994). The replacement of Ca-
zeolites (laumontite) with calcite and illite proximal to epithermal veins within
the Sascha-Pelligrini system indicates an increase in temperature and
dissolved CO2 content towards fluid up flow points (Browne and Ellis, 1970;
Cox and Browne, 1998).
The alteration assemblage of quartz, adularia, illite, calcite, chlorite and pyrite
along the SVZ is in equilibrium with deep chloride-rich waters (Simmons and
Browne, 2000). This alteration assemblage results from the recrystallisation
of the original rock with uptake of variable amounts of H2O, CO2 and H2S.
Mass balance geochemistry across the study areas shows hydrothermal
fluids proximal to veins introduced variable amounts of SiO2, Al2O3, Fe2O3,
K2O, MgO, TiO2 and Na2O, and correlates well to the chemistry of the
observed alteration assemblages.
PIMA alteration profiles adjacent to the SVZ show a decrease in the SWIR
AlOH clay absorption feature towards the silicified vein selvages representing
a decrease in total clay abundance (Pontual et al, 1997; Herrmann et al,
2001). Although total clay abundance decreases towards the vein, the
increase in the absorption wavelength position corresponds to an increase in
the amount of Fe and Mg in illite attributable to increasing temperature (Post
and Noble, 1993). Compositional variations of white micas studied in various
geothermal systems generally show an increase in K, Si, Fe and Mg and a
decrease in Al with increasing temperature (Bishop and Bird, 1987;
Cathelineau and Izquierdo, 1988; McDowell and Elders, 1983). Interstratified
illite-smectite vein selvages at Sascha Main confine fluid temperature to
Quinn Smith Master of Applied Science Thesis 78
below 220°C (KRTA, 1990) with illite-smectite alteration located above
mineralised veins at Golden Cross (Simpson et al, 1998) and Hishikari
(Izawa et al, 1990; Ibaraki and Suzuki, 1993) (Figure 33). Interstratified illite-
smectite usually occurs at the top of the gold mineralised interval
(Hedenquist and White, 2005) and suggests the current exposure at Sascha
Main is at a high level within the epithermal system.
The disassociation of aqueous CO2 during pressure release boiling within the
SVZ provides H+ into solution. The subsequently acidic hydrothermal
environment hydrolyzes feldspars within the host rock adjacent to the
epithermal system, forming the dominant clay-illite alteration halo. Pressure
release boiling, disassociation of aqueous CO2, and acidity buffering from
wall-rock feldspars releases Ca+ into solution to form bladed calcite within the
vein system. Calcite in equilibrium with adularia, illite and muscovite at
Sascha Main results from the reduced acidity buffering of host-rock K-
feldspar (sanidine) and K-mica (muscovite) with deep chloride waters
(Browne and Ellis, 1970; Browne, 1978; Simmons and Browne, 2000).
Figure 33. Alteration zones and mineral assemblages of the Hishikari epithermal system showing alteration zoning from sericite and or kaolinite through to illite-smectite mixed layer clays above the gold-silver veins. The presence of chlorite-sericite-adularia and minor mixed layer clays marks the alteration within the gold mineralised interval.
Quinn Smith Master of Applied Science Thesis 79
The Sascha Main and Sur systems are characterised by abundant kaolinite
associated with halloysite, hematite, goethite and very minor alunite and
gypsum. This acid-sulphate alteration overprints illite assemblages and is
spatially related to weathered pyritic veins and wall-rocks. Altered rocks with
acid-sulphate assemblages contain abundant cubic voids after pyrite with
mass-balance geochemistry indicating no introduction of sulphate. SWIR
analysis of alteration adjacent to pyritic veins show a systematic increase in
both absorption wavelength and absorption depth, attributed to an increase in
overprinting kaolinite (Pontual et al, 1997). The Sascha Main acid-sulphate
blanket represents a supergene alteration assemblage formed by oxidation of
wall-rock pyrite above the palaeo-water table. During weathering of pyrite,
iron released by carbonic acid-bearing rainwater precipitates almost
immediately as ferric hydroxide due to the low pH and Eh (Schoen et al,
1974). Bright yellow-orange amorphous iron hydroxides, hematite and
goethite characteristic of weathered pyrite are abundant throughout the
kaolinite blanket over Sascha Main.
The Pelligrini acid-sulphate alteration characterised by abundant kaolinite,
with lesser alunite, natroalunite and opal, forms a broad circular alteration
halo around pervasive silicification. The acid-sulphate alteration passes
vertically to illite-pyrite alteration, and represents overprinted alteration
assemblages. The Pelligrini alteration system is similar to that formed above
blind orebodies within the Fresnillo district (Simmons, 1991) and is most
likely associated with H2S oxidation in a steam-heated environment. Base
cation leaching and mass loss within the pervasively altered ash tuff at
Pelligrini is indicative of steam-heated acid-sulphate leaching (Reyes, 1990;
Rye et al, 1992), with large mass gains in sulphate corresponding to the
introduction of alunite. The intense pervasive silicification is associated with
potassium enrichment and is characteristic of ore-related hydrothermal
silicification within the Desseado Massif (Echavarria et al, 2005). Intense
silicification occurs within stratigraphically high La Matilde and Chon Aike
units above epithermal veining at Manantial Espejo in Argentina (Wallier and
Tosdal, 2008). Silicification within these units is interpreted to form at the
Quinn Smith Master of Applied Science Thesis 80
palaeo-water table that channeled lateral flow of the hydrothermal system
and led to silica precipitation (Wallier and Tosdal, 2008). The stratigraphically
high silicification associated with acid-sulphate alteration forming by oxidation
of H2S within a steam-heated environment places the current exposure at
Pelligrini above the water table with formation temperatures less than 100°C
(Rye et al, 1992).
Hematite-rich rocks developed along the northern periphery of the kaolinite-
alunite-jarosite alteration zone at Pelligrini suggest horizontal permeability
with products of hydrolysis migrating laterally from the source. Steamboat
Springs provides a modern analogue to the alteration at Pelligrini, with
hematite-rich zones developed at the transition from acid-sulphate to
montmorillonite alteration (Schoen et al, 1974). High temperature dickite in
outcropping polymict vein breccias associated with pervasive illite-dominant
alteration adjacent to low-temperature alunite-opal at Pelligrini suggest the
acid-sulphate assemblage has encroached downward into hotter parts of the
epithermal system. The complex alteration at Pelligrini represents a well
preserved high-level epithermal alteration system at the palaeo-water table.
Hematite-rich rocks along the northern periphery of the alteration system
represent the location of the palaeo-water table, and the boundary between
the downward migrating low-temperature alunite-opal assemblage with the
higher temperature illite-dominant assemblage. The overprinting alteration
assemblages may represent a descending water table in a waning
geothermal system with low-temperature alunite-opal alteration forming in
illite altered rocks. Oxidation of H2S vapors produce dilute, acid SO4 waters
and acid-sulphate alteration above fluid up flow points (Pelligrini), with
outflow of neutral Cl waters (Sascha) discharged at a considerable distance
from up flow points (Giggenbach, 1992).
Hydrothermal minerals define distinct alteration zones across the study area.
Formation of alteration assemblages is driven by fluid chemistry, and can be
characterised on the basis of an assemblage in equilibrium with reduced
neutral or oxidized acid fluids. The alteration system around the SVZ is
characterised by a broad distal laumontite-montmorillonite alteration halo that
Quinn Smith Master of Applied Science Thesis 81
passes to illite-dominant alteration proximal to epithermal veins. Vein
selvages are characterised by smectite and interstratified illite-smectite, with
the alteration assemblage indicating a reduced neutral environment. The
initially acidic hydrothermal fluid is buffered by wall-rock feldspars, forming
the dominant clay-illite alteration halo. The Sascha Main acid-sulphate
blanket represents a supergene alteration assemblage formed by oxidation of
wall-rock pyrite above the palaeo-water table. The acid-sulphate alteration
passes vertically to illite-pyrite alteration, and represents overprinted
alteration assemblages. Base cation leaching and mass loss within the
pervasively altered ash tuff at Pelligrini is indicative of steam-heated acid-
sulphate leaching, with large mass gains in sulphate corresponding to the
introduction of alunite. The overprinting alteration assemblages across the
study area indicate a lowering of the water table during the waning stages of
the epithermal system.
The study of alteration minerals through the use of field portable PIMA
equipment has provided a qualitative estimate of alteration mineralogies.
Based on the methodology employed through the use of PIMA equipment,
potential errors can occur in mineral identification. PIMA alteration sampling
is a useful field based tool for rapid identification of alteration mineralogies
sufficient for a mineral exploration program. Where possible, ground truthing
of PIMA data should be validated by quantitative XRD analysis of selected
individual alteration assemblages. Depending on the use and need of
alteration data, PIMA can be effectively used a quick field base tool for
exploration programs dealing with strong clay alteration of host rocks. The
comparison between PIMA and XRD data shows detailed clay mineralogies
and mixed layer clay assemblages can only be identified through the use of
XRD analysis. Detailed alteration studies must employ lab based XRD
sampling to quantitatively outline the alteration mineralogies.
Quinn Smith Master of Applied Science Thesis 82
Vein Paragenesis
Detailed trench and outcrop mapping identifies six temporally and texturally
distinct vein phases along the SVZ. The paragenesis of the vein phases
along the SVZ form six main stages: stage one, chalcedonic; stage two,
colloform-crustiform +/- ginguro; stage three, saccharoidal; stage four,
chalcedonic with disseminated pyrite; stage five, jasperoidal; and, stage six,
crystalline and comb veins (Figure 34). The colloform- crustiform-banding of
stage two forms as an initial mineralising phase on the margins of stage three
saccharoidal veins. The main mineralising events are stage two, associated
with two colloform-crustiform ginguro bands, and stages four and five,
associated with chalcedonic and jasperoidal veins with disseminated pyrite.
Individual mineralising events are comprised of complex mineral paragenetic
sequences with mineral relationships giving insight into system evolution
(Figure 35).
Vein Phase
Chalcedonic
Saccharoidal with minor bladed textures
Colloform/Crustiform +/- ginguro
Chalcedonic with patchy disseminated pyrite
Jasperoidal
Crystalline and Comb
> TIME >
Figure 34. Vein phases plotted against time outlining the vein paragenetic relationships for the Sascha-Pelligrini epithermal system.
Vein
Phase
Quinn Smith Master of Applied Science Thesis 83
Min
era
lF
orm
ula
Gin
gu
ro
Gin
gu
ro
Pyri
te C
ha
lce
do
ny
Sta
ge
IS
up
erg
en
eS
tag
e I
IS
up
erg
en
eS
up
erg
en
e
Se
Aca
nth
ite
Ag
4S
eS
Ele
ctr
um
AgA
u
Ja
me
so
nite
Pb
(Ag) 4
Fe
Sb
6A
sS
14
He
ma
tite
Fe
2O
3
Uyte
nb
oga
ard
tite
Ag
3A
uS
2
Go
ldA
u
Aca
nth
ite
Ag
2S
Ca
lcite
Ca
CO
3
Ch
alc
op
yrite
Cu
Fe
S2
Ga
len
aP
bS
Sp
ha
lerite
Zn
S
Pe
tro
vska
ite
AgA
u(S
eS
)
Du
fre
no
ysite
Pb
2A
s2
S5
Ba
rite
Ba
SO
4
Ad
ula
ria
KA
lSi3
O8
Mu
sco
vite
KA
l 2(S
i 3A
l)O
10
(OH
) 2
Iod
oe
mb
olit
eA
g(C
l,B
r,I)
Em
bo
lite
Ag(C
l,B
r)
Iod
oa
rgyrite
AgI
Va
rla
mo
ffite
(Sn
,Fe
)(O
,OH
) 2
Pyrite
Fe
S2
Ars
en
op
yrite
AsF
eS
2
Gyp
su
mC
aS
O4
Ja
lpa
ite
Ag3
Cu
S2
Ja
rosite
KF
e3+(S
O4)2
(OH
)6
Figure 35. Vein mineral paragenetic relationships for the Sascha-Pelligrini epithermal system.
Quinn Smith Master of Applied Science Thesis 84
Ginguro stage I mineralisation is characterised by the initial precipitation of
selenium-rich acanthite. Mineral assemblages observed in selenide-bearing
epithermal deposits suggests oxygen fugacities were below or very close to
the hematite-magnetite buffer, ƒSe2(g)/ƒS2(g)
ratios were lower than unity, and
temperatures of formation were between 150° to 210°C (Simon et al, 1997).
Under these conditions, selenium cannot be separated from sulphur; the
early substitution of selenium in sulphide minerals prevents its concentration
in hydrothermal fluids, and limits precipitation to silver selenides (Simon et al,
1997). Selenide-bearing minerals are often associated with gold and silver
mineralisation (Simon et al, 1997) and commonly occurr in many epithermal
deposits throughout Indonesia (Kieft and Oen, 1973), Japan (Shikazono,
1978), Kunashir Island (So et al, 1995), Nevada (Saunder et al, 1988),
Mexico (Petruk and Owens, 1974), and New Zealand (Main et al, 1972).
Selenium-rich acanthite is associated with gold and silver mineralisation
across the SVZ, with silver selenide restricted to a single stage that predates,
or is contemporaneous with gold and silver deposition. The presence of silver
selenide in the ginguro stage I ore marks an acidic-oxidised magmatic-
sourced fluid that rapidly hydrolyses feldspars.
Following the introduction of an acidic-oxidised magmatic-sourced fluid, the
fluid pH increases during equilibration with wall-rocks associated with silicate-
buffered dilution (Reed and Palandri, 2006). The solution pH increases as H+
ions are consumed in breaking down the primary rock silicates, resulting in
an exchange of aqueous H+ for cation from the rock. The overall reaction
changes the initially acidic aqueous phase (magmatic composition) to a
composition in equilibrium with a propylitic assemblage. The H+ is exchanged
for base cations, and sulphate is reduced to sulphide. The molar ratios of
sulphate to sulphide change from 24.5 to 0.00018 and the concentration of
aqueous H2 increases 3 orders of magnitude. Sulphate concentration
decreases due to the combined effects of fractionation of early-formed
sulphate minerals (barite, jarosite, gypsum, and alunite) and the reduction of
sulphate to sulphide with reaction of ferrous iron to form hematite, magnetite
or epidote. The high concentration of SO2 in magmatic gases renders them
much more oxidising than equilibrium with ferrous iron in wall rocks allows
Quinn Smith Master of Applied Science Thesis 85
(Reed, 1994). The outward traverse of such fluids inevitably yields reduced
fluids and a relatively more oxidised wall rock (Giggenbach, 1992; Reed,
1994).
The concentration of Ba2+ in the hydrothermal fluid increases as sulphate is
reduced to sulphide through acid neutralisation from wall-rock buffering
(Reed, 1994). The precipitation of barite within ginguro stage I indicates a
reduced fluid that has mixed with shallow acid-sulphate waters above the
water table. Following the increase in pH, hematite replaces pyrite (Spycher
and Reed, 1989), with gold and silver precipitated as electrum (Reed and
Palandri, 2006). Silver-rich electrum coexisting with intergrown
uytenbogaardtite and acanthite represents disequilibrium cooling of a high-
termperature gold-bearing argentite and electrum assemblage (Figure 14 A,
B) (Barton, 1980). This disequilibrium assemblage may be associated with
conduit sealing and system quiescence.
Vein filling associated with ginguro stage I mineralisation sealed the fluid
conduit allowing fluid pressure to increase. Incremental structural dilation and
associated fracturing resulted in rapid pressure release with isoenthalpic
boiling of the hydrothermal fluid depositing fine pseudo-accicular bladed
calcite prior to deposition of ginguro stage II (Figure 11). Calcite is a common
soluble phase in epithermal veins (Dong et al, 1995), with the precipitation of
vein calcite driven by the loss of CO2 due to boiling and the subsequent
generation of CO32- ions from the dissociation of HCO3 (Henley, 1985).
Primary pseudo-acicular bladed calcite is replaced by quartz, preserving the
original crystal morphology.
Mineralisation during Sascha ginguro stage II is characterised by the initial
precipitation of hematite in association with barite. Electrum and acanthite
are the first ore minerals to be depositied, followed by precipitation of gangue
calcite (Figure 14 C, D). Gold and uytenbogaardtite are precipitated after
calcite, followed by selenium rich acanthite. Base metal sulphides,
chalcopyrite, sphalerite and galena, are deposited after uytenbogaardtite,
with the lead sulphosalt dufrenoysite marking the end of ginguro stage II
Quinn Smith Master of Applied Science Thesis 86
mineralisation. Gangue minerals adularia and muscovite characterise the
final stages of mineralisation. Quartz and barite are deposited throughout
most of the ginguro mineralising event.
Ginguro stage II mineralisation followed isoenthalpic boiling from 280°C,
decreasing pH and favoured the precipitation of acanthite instead of arsenic
and antimony sulphosalts (Drummond and Ohmoto, 1985; Spycher and
Reed, 1989). Continued wall-rock silicate buffering of the low pH fluid
increases Ba2+ in solution as sulphate is reduced to sulphide through acid
neutralisation (Reed, 1994). The occurrence of gangue barite throughout
ginguro stage II indicates the continuation of fluid mixing between the
reduced hydrothermal fluid and shallow acid-sulphate waters above the water
table. Following an increase in pH, hematite replaces pyrite (Spycher and
Reed, 1989), with gold and silver precipitating as electrum (Reed and
Palandri, 2006). Calculated electrum-sphalerite formation temperatures
range from 174°C to 205°C and indicate a decrease in fluid temperature from
isoenthalpic boiling at 280°C associated with increasing pH. Chalcopyrite
precipitates at pH 3.5-3.8, consuming bornite and pyrite, with sphalerite
precipitating at pH 5.4, and galena precipitating at pH 5.7 (Reed and
Palandri, 2006; Reed, 1994). The rise in pH associated with wall-rock
buffering and acid neutralisation decreases metal concentrations by many
orders of magnitudes as the fluid approaches neutral pH (Reed and Palandri,
2006). The sequential precipitation of chalcopyrite-sphalerite-galena in
ginguro stage II is characteristic of an increase in fluid pH as wall-rock
feldspars neutralise acidic metal-bearing fluids (Reed and Palandri, 2006).
Neutral fluid is marked by the presence of adularia and muscovite
precipitated as the last phases within the ginguro stage II event. Following
fluid neutralisation and precipitation of ginguro bands, the fluid conduit may
have sealed allowing fluid pressure to increase once again.
Ginguro stage II mineralisation is followed by coarse lattice bladed calcite
pseudomorphs and saccharoidal quartz. Rapid pressure release boiling
associated with structural dilation and/or hydrothermal eruptions deposited
coarse bladed and crystalline calcite after the ginguro stage II event. The
Quinn Smith Master of Applied Science Thesis 87
current outcrop exposure suggests the epithermal system was sealed after
deposition of coarse bladed and crystalline calcite.
Overprinting pyritic chalcedonic quartz veins represents renewed
hydrothermal activity focused along parallel fractures to the preceding
ginguro event. The presence of pyrite associated with chalcedonic and
jasperoidal silica indicates meteoric incursion within the upper level of the
epithermal system. Chaotic pyritic chalcedonic and jasperoidal quartz veins
are hosted within silicified lithic rhyolite tuff in Sascha Main. Spatially
restricted and laterally discontinuous lithic tuff concentration zones may have
acted as a confined aquifer between underlying strongly welded tuff and
overlying rhyolite crystal ash tuffs, introducing cool, oxygenated meteoric
water at depth below the palaeosurface. Simultaneous dilution and cooling by
cold water mixing yields substantial pyrite in association with acanthite below
temperatures of 177°C (Reed and Palandri, 2006). Copper-rich acanthite
inverts to jalpaite at temperatures below 117°C, and exists as a 2 phase
region with acanthite limiting temperatures of formation to below 106°C
(Skinner, 1966). Strong arsenic zoning in pyrite represents local
disequilibrium during growth, representing local fluctuations in the S2/As2
ratio (Kretchmar and Scott, 1976).
Individual mineralising events of ginguro-banded and pyritic chalcedonic and
jasperoidal veins are comprised of complex mineral paragenetic sequences
that give an insight into systems geochemical evolution. Ginguro stage I ore
is characterized by an initially acidic-oxidised magmatic-sourced fluid. The
solution pH increases with wall-rock buffering, driving the fluid to equilibrium
with a propylitic assemblage. The precipitation of barite within ginguro stage I
marks the point at which the now reduced fluid has mixed with shallow acid-
sulphate waters above the water table. The deposition of fine pseudo-
accicular bladed calcite after ginguro stage I, and prior to ginguro stage II,
suggests isoenthalpic boiling lowering the fluid pH once again. The
occurrence of gangue barite throughout ginguro stage II indicates the
continuation of fluid mixing between the reduced hydrothermal fluid and
shallow acid-sulphate waters above the water table. The presence of lead
Quinn Smith Master of Applied Science Thesis 88
sulfosalts Jamesonite and Dufrenoysite indicate a transgression from the
typically low sulphur activity during the systems evolution, indicating brief
intervals of intermediate sulphidation. Electrum-sphalerite formation
temperatures indicate a decrease in fluid temperature associated with
increasing pH. The sequential precipitation of chalcopyrite-sphalerite-galena
in ginguro stage II indicates an increase in fluid pH, with adularia and
muscovite marking the return to neutral conditions in equilibrium with a
propylitic assemblage. The presence of pyrite associated with chalcedonic
and jasperoidal silica indicates simultaneous dilution and cooling by cold
water mixing above the water-table.The progression from deeper level
veining exposed adjacent to shallow level veining correlates with the
overprinting alteration at both Sascha and Pelligrini and represents the
downward migration of the water table across the study area over the
systems evolution.
Supergene Overprint
Destabilisation of wall-rock pyrite by oxygenated ground water produces acid
sulphate water associated with kaolinite. Acid sulphate groundwater
destabilises pyrite within chalcedonic quartz mineralisation, with pyrite being
replaced by hematite. Selenium-rich acanthite, acanthite and jalpaite persist
as inclusions within supergene hematite and gypsum. The silver halide
assemblage within outcropping veins at Sascha is uncharacteristic of silver
chloride-rich supergene zones within many silver-rich deposits. Embolite,
iodoembolite and iodoargyrite variably overprint acanthite, and in conjunction
with free gold comprise the supergene assemblage of the Sascha Ginguro
veins. Silver halides are common secondary minerals in supergene zones of
silver rich mineral deposits, zoning from silver chloride to silver iodide with
depth (Gammons and Yu, 1997). Silver iodides are uncommon in
outcropping supergene-enriched veins with iodide rapidly oxidizing to iodate
(Gammons and Yu, 1997), however silver iodide persists within the
supergene zone at Sascha Main. Preservation of silver iodide within
outcropping veins across the Sascha-Pelligrini area can be attributed to the
dry, cold and arid climate of Patagonia.
Quinn Smith Master of Applied Science Thesis 89
Summary
Facies associations of the volcanics within the study area closely represent
an intra-caldera setting, with volcancis erupted from a complex series of
graben-forming extensional fractures instead of within a classic caldera
margin. Several distinct eruption styles are recorded within the volcanic
sequence. The rhyodacite sequence was rapidly deposited by eruption
column collapse from multiple vents or fissures creating a composite welded
unit. The spatially restricted La Matilde rhyolite sequence was deposited from
a single violent eruption, and associated column collapse, with settling of the
convective cloud.
Strong hydrothermal alteration, sorting, welding, post emplacement
crystallisation and devitrification, as well as erosion throughout the majority of
the study area precludes quantitative petrochemical modeling the volcanic
rock suite. Variation in REE abundances across the rhyodacite and rhyolite
sequences is consistent with the proposed fractionation trends of Pankhurst
and Rapela (1995). Pankhurst and Rapela (1995) demonstrate that the
sequence from basaltic andesite (Bajo Pobre) to dacite/rhyodacite to rhyolite
(Chon Aike) can be modeled by a combination of partial melting and
fractional crystalisation.
Host rock rheology controls vein morphology and alteration patterns across
the study area. The current exposure along the SVZ indicates the friable
rhyolite tuffs of the La Matilde Formation host upward terminating veins,
stockwork vein zones, and pervasive silicification. Brittle fractures and more
massive veining may be developed within the crystalline rhyodacite
ignimbrite that stratigraphically lies below the current vein exposure.
Structural analysis of the SVZ indicates the tension axis trended toward the
northeast quadrant, producing right-lateral movement along the 315° trend.
The structural pattern of the SVZ conforms to the Riedel shear model, with 1
orientated at 315° and modelled as the acute bisector of the conjugate shear
pair.
Quinn Smith Master of Applied Science Thesis 90
Hydrothermal minerals define distinct alteration zones across the study area.
The alteration system around the SVZ is characterised by a broad distal
laumontite-montmorillonite alteration halo that passes to illite-dominant
alteration proximal to epithermal veins. Vein selvages are characterised by
smectite and interstratified illite-smectite, with the alteration assemblage
indicating a reduced neutral environment. The initially acidic hydrothermal
fluid is buffered by wall-rock feldspars, forming the dominant clay-illite
alteration halo. The Sascha Main acid-sulphate blanket represents a
supergene alteration assemblage formed by oxidation of wall-rock pyrite
above the palaeo-water table. Alteration assemblages at Pelligrini are
indicative of steam-heated acid-sulphate leaching.
Individual mineralising events of ginguro-banded and pyritic chalcedonic and
jasperoidal veins are comprised of complex mineral paragenetic sequences
that give an insight into systems geochemical evolution. Ginguro stage I and
II is characterised by an initially acidic fluid, with wall-rock buffering driving
the fluid to equilibrium with a propylitic assemblage. The precipitation of
barite indicates fluid mixing with shallow acid-sulphate waters above the
water table. Electrum-sphalerite formation temperatures indicate a decrease
in fluid temperature associated with increasing pH. The sequential
precipitation of chalcopyrite-sphalerite-galena in ginguro stage II is
characteristic of an increase in fluid pH, with adularia and muscovite marking
the return to neutral conditions in equilibrium with a propylitic assemblage.
The presence of pyrite associated with chalcedonic and jasperoidal silica
indicates simultaneous dilution and cooling by cold water mixing above the
water-table.The progression from deeper level veining exposed adjacent to
shallow level veining correlates with the overprinting alteration at both
Sascha and Pelligrini and represents the downward migration of the water
table across the study area over the systems evolution. The unique
supergene, silver iodide assemblage, preserved within outcropping veins
across the Sascha-Pelligrini area can be attributed to the dry, cold and arid
climate of Patagonia.
Quinn Smith Master of Applied Science Thesis 91
The Sascha-Pelligrini study area consists of a well preserved, high-level,
Jurassic low-sulphidation epithermal system hosted within the Chon Aike
Formation. The presence of lead sulfosalts Jamesonite and Dufrenoysite
indicate intermittent intervals of higher sulfur fugacities, with the epithermal
system progressing from typical low-sulphidation to intermediate-sulphidation
assemblages. The transgression between sulphidation states supports the
idea that end-member deposits form as part of a continuum between the two.
The Sascha-Pelligrini epithermal system was produced by a complex
interaction of pyroclastic volcanics, host-rock rheology and geochemistry,
structural setting, paleo-water table and hydrothermal fluid evolution. The
conceptual model for the Sascha-Pelligrini study area is presented in figure
36, with epithermal veining forming on graben and half-graben structures
below the palaeo-water table. The model depicts the association between
host rock, alteration zoning including vein selvages and depth of palaeo-
water table. Each individual aspect plays an important role in the nature and
occurrence of the systems evolution, of which each is essential in combining
to form an epithermal deposit.
Quinn Smith Master of Applied Science Thesis 92
Fig
ure
36. C
on
cep
tual ep
ith
erm
al m
od
el fo
r th
e S
as
ch
a –
Pell
igri
ni
stu
dy a
rea.
Mode
l d
ep
icts
altera
tion a
nd v
ein
selv
ag
e z
on
ing, p
ala
eo w
ate
r ta
ble
, str
uctu
ral a
nd s
tratigra
phic
rela
tionsh
ips, rh
eo
log
ical contr
ol, v
ein
dis
trib
utio
n a
nd c
urr
ent ero
sio
na
l surf
ace.
Quinn Smith Master of Applied Science Thesis 93
Conclusions
The Sascha-Pelligrini low-sulphidation epithermal system is located on the
western edge of the Deseado Massif, Santa Cruz Province, Argentina.
Outcrop sampling has returned values of up to 160g/t gold and 790g/t silver.
Detailed mapping of the volcanic stratigraphy has defined three units that
comprise the middle Jurassic Chon Aike Formation and two units that
comprise the upper Jurassic La Matilde Formation. The Chon Aike Formation
consists of rhyodacite ignimbrites and tuffs, with the La Matilde Formation
including rhyolite ash and lithic tuffs. The volcanic sequence is intruded by a
large flow-banded rhyolite dome, with small, spatially restricted granodiorite
dykes and sills cropping out across the study area.
The Chon Aike rhyodacite sequence shows an enrichment of LREE through
the fractionation of hornblende, allanite and apatite, with the Eu anomaly and
drepssion of the middle REE controlled by fractionation of plagioclase and
hornblende respectively. Decreasing REE trends in the La Matilde rhyolite
might be due to the eruption of a zoned magma chamber. Similar to the
rhyodacite package, the rhyolite sequence REE patterns show a negative Eu
anomaly, decrease in REE concentrations with continued eruptive units, and
depressed HREE‟s. The negative Eu anomaly may be controlled by the
fractionation of sanidine, with the depletion of REE‟s controlled by the
fractionation of hornblende. High-silica rhyolite can be produced from
fractional crystalisation of rhyodacite, with the removal of 40% crystal
assemblage composed of 58.05% plagioclase, 40% hornblende, 1% apatite
and 0.05 allanite (Pankhurst and Rapela, 1995).
XRD analysis in combination with PIMA and ASTER spectral analysis defines
an alteration pattern that zones from laumontite-montmorillonite, to illite-
pyrite-chlorite, followed by a quartz-illite-smectite-pyrite-adularia vein
selvage. The alteration assemblage of quartz, K-feldspar, illite, calcite,
chlorite and pyrite across the Sascha-Pelligrini area is commonly observed in
geothermal systems and is in equilibrium with near neutral, deep chloride
Quinn Smith Master of Applied Science Thesis 94
waters (Simmons and Browne, 2000). Condensation of CO2 gas and
absorption into cool ground waters within the periphery of the epithermal
system has produces low-temperature clays, carbonates and calcium
zeolites (Simpson et al, 2001).
The supergene kaolinite blanket over Sascha Main is charcterised by
hematite, goethite and bright yellow-orange amorphous iron hydroxides, with
the acid-sulphate assemblage formed through the weathering of pyrite. Base
cation leaching and mass loss within the pervasively altered ash tuff at
Pelligrini is indicative of steam-heated acid-sulphate leaching (Reyes, 1990;
Rye et al, 1992), with large mass gains in sulphate corresponding to the
introduction of alunite. The oxidation of H2S within a stream heated
environment takes place above the water table at temperatures not
exceeding 100°C (Rye et al, 1992), with opal at Pelligrini confining
temperature of formation to less than 120°C (KRTA, 1990).
ASTER mineral mapping is an invaluable tool in rapidly identifying alteration
systems across large mineral districts. PIMA spectral analysis enables rapid
field identification of alteration minerals, however cannot identify
interstratified clays. XRD analysis readily identifies and quantifies alteration
minerals, however does not highlight end-member mineral compositions.
Characterisation of epithermal alteration systems should incorporate ASTER,
PIMA and subsequent XRD analysis. Individual methods can readily identify
alteration minerals, however the realisation of the complete alteration
assemblage and spatial distribution can effectively be compiled with all three
tools.
Paragenetically, epithermal veining varies from chalcedonic to saccharoidal
with minor bladed textures, colloform- crustiform-banded with visible electrum
and acanthite, crustiform-banded grey chalcedonic to jasperoidal with fine
pyrite, and crystalline comb quartz. Mineralisation during the ginguro events
is controlled by fluctuating pH, driven by a combination of magmatic gases,
pressure release boiling and silicate wall-rock buffering. Pyrite-rich
chalcedonic veins formed through simultaneous dilution and cooling by cold
Quinn Smith Master of Applied Science Thesis 95
water mixing, yielding substantial pyrite in association with acanthite below
temperatures of 177°C (Reed and Palandri, 2006). Electrum-sphalerite
geothermometry of ginguro mineralised veins constrains formation
temperatures from 174.8 to 205.1°C and correlates with the stability field for
the interstratified illite-smectite vein selvage.
Vein morphology, mineralogy and associated alteration are controlled by host
rock rheology, permeability, and depth of the palaeo-water table.
Mineralisation within ginguro banded veins resulted from fluctuating fluid pH
associated with selenide-rich magmatic pulses, pressure release boiling and
wall-rock silicate buffering. The Sascha-Pelligrini epithermal system provides
a deposit specific model helping to clarify the current understanding of
epithermal deposits, and may serve as a template for exploration of similar
epithermal deposits throughout Santa Cruz.
Quinn Smith Master of Applied Science Thesis 96
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Quinn Smith Master of Applied Science Thesis 109
Appendix 1
Petrography
Quinn Smith Master of Applied Science Thesis 110
Plate 1. Rhyodacite crystal ignimbrite with large albite, sanidine, biotite (chlorite after biotite) and embayed quartz phenocrysts in a quartz-feldspar-glass ash matrix. Photo A XPL x50. Photo B PPL x50. Field of view approximately 2mm.
A B
A B
Plate 2. Rhyodacite pumice ash tuff with sericitised albite, sanidine and biotite phenocrysts. Large, devitrified flattened pumice clasts and embayed quartz phenocrysts in a quartz-feldspar-glass ash matrix. Strong compaction textures. Photo A XPL x50. Photo B PPL x50. Field of view approximately 2mm.
A B
Plate 3. Rhyodacite crystal ash tuff with albite, sanidine, biotite (chlorite after biotite) and embayed quartz phenocrysts in a quartz-feldspar-glass ash matrix. Large fine-grained quartz-feldspar-biotite juvenile clasts prominent. Photo A XPL x50. Photo B PPL x50. Field of view approximately 2mm.
Quinn Smith Master of Applied Science Thesis 111
A B
Plate 4. Lithic rhyolite tuff with sericitised sanidine, minor muscovite and embayed quartz phenocrysts in a quartz-feldspar-glass ash matrix. Large mica schist lithic clasts prominent. Photo A XPL x50. Photo B PPL x50. Field of view approximately 2mm.
A B
Plate 5. Rhyolite crystal ash tuff with sericitised sanidine, minor muscovite and embayed quartz phenocrysts in a quartz-feldspar-glass ash matrix. Rare devitrification textures in glassy matrix. Photo A XPL x50. Photo B PPL x50. Field of view approximately 2mm.
Plate 6. Rhyolite ash tuff with fine sanidine laths, quartz and glass shards. Rare spherulitic and accretionary lapilli layers. Photo A XPL x50. Photo B PPL x50. Field of view approximately 2mm.
A B A B
Quinn Smith Master of Applied Science Thesis 112
A B
Plate 7. Flow-dome auto-breccia fine sanidine-quartz crystalline matrix, with large clasts of flow-banded and spherulitic rhyolite to 2m in diameter. Photo A XPL x100. Photo B PPL x100. Field of view approximately 1mm.
A B
Plate 8. Flow-banded rhyolite with euhedral sanidine phenocrysts in a crystalline quartz-feldspar matrix. Photo A XPL x100. Photo B PPL x100. Field of view approximately 1mm.
A B
Plate 9. Spherulitic rhyolite with spherulites to 5mm in a quatz-feldspar-glass matrix. Photo A XPL x50. Photo B PPL x50. Field of view approximately 2mm.
Quinn Smith Master of Applied Science Thesis 113
A B
Plate 10. Porphyritic granodiorite dyke with sanidine and sodic plagioclase phenocrysts. Photo A XPL x50. Photo B PPL x50. Field of view approximately 2mm.
A B
A B
Plate 11. Marine fossiliferous feldsarenite with bivalve, gastropod and bryzoan fragments with a micritic cement. Photo A XPL x50. Photo B PPL x50. Field of view approximately 2mm.
Plate 12. Epiclastic with mica, quartz, feldspar and calcite clasts. Photo A XPL x50. Photo B PPL x50. Field of view approximately 2mm.
Quinn Smith Master of Applied Science Thesis 114
Plate 13. Vesicular olivine tholeiite plateau basalt. Photo A XPL x50. Photo B PPL x50. Field of view approximately 2mm.
Plate 14. Basaltic dyke with microcline and oscilitory zoned anorthite phenocrysts in a feldspar lath ground-mass. Photo A XPL x50. Photo B PPL x50. Field of view approximately 2mm.
A B
A B
Quinn Smith Master of Applied Science Thesis 115
Appendix 2
Quartz Textures
Quinn Smith Master of Applied Science Thesis 116
Plate 1 Chalcedonic single pulse tectonic breccia. Angular leached clasts of argilllized wall rock, including strongly colliform banded fine saccharoidal to chalcedonic silica. Scale bar equals 10mm.
Plate 3 White, fine-grained saccharoidal silica with fine, prismatic-bladed carbonate pseudomorphs. Scale bar equals 10mm. A – Quartz-clay wall rock alteration B – Fine bladed pseudomorphs
Plate 2 Single phase wall rock breccia proximal to main veins contain sulphide bands in matrix to wall rock fragments. Sulphide bands occur on clast edges, grading to late crystalline quartz lining inter clast voids. Scale bar equals 10mm. A – Wall rock clast B – Sulphide band
A
Plate 5 Chalcedonic with disseminated pyrite vein/vein breccia with supergene iron oxide gossanous zones. Scale bar equals 10mm.
Plate 4 White, fine-grained saccharoidal silica with vuggy cavities. Scale bar equals 10mm.
Plate 6 Banded quartz-iron oxide gossanous crust on chalcedonic with disseminated pyrite vein. Scale bar equals 10mm.
A B
A
B
Quinn Smith Master of Applied Science Thesis 117
Plate 7 Massive to banded jasperoidal silica with disseminated sulphides and a saccharoidal, moderately banded pyrite matrix. Scale bar equals 10mm. A – Jasperoidal silica B – Pyritic crystalline to saccharoidal quartz
Plate 9 Euhedral axiolitic clear to milky comb quartz crystals growing perpendicular to vein margins. Scale bar equals 10mm.
Plate 8 Amythestine to milky, euhedral axiolitic comb quartz crystals growing perpendicular to vein margins. Scale bar equals 10mm. A – Silicified wall rock B – Amythestine quartz C – Large euhedral zoned quartz crustals
Plate 10 Grey-white fluidised vein breccia with crustiform banded chalcedonic to crystalline quartz. Scale bar equals 10mm.
Plate 11 Chalcedonic vein breccia cross-cut by crustiform chalcedony, followed by milky comb quartz with iron oxide staining, and infilled with „ladder‟ banded cream chalcedonic quartz. Cream chalcedonic quartz preserves meniscus like texture. Horizontal vein slice in formation orientation with top of photo towards palaeosurface. Scale bar equals 10mm.
A B
A
B
C
Quinn Smith Master of Applied Science Thesis 118
Plate 17 Intensely silicified tuff with relict rock textures preserved. Scale bar equals 10mm.
Plate 12 Crustiform chalcedonic, bladed and colloform banded quartz vein overprinted by axiolitic comb quartz. Scale bar equals 10mm.
Plate 16 Monomict chalcedonic quartz jigsaw breccia containing wall rock clasts Scale bar equals 10mm.
Plate 13 Chalcedonic fluid streaming breccia with crystalline quartz overprint. Scale bar equals 10mm.
Plate 14 Matrix supported vein breccia with massive to banded chalcedonic vein fragments and silicified tuff fragments within chalcedonic silica. Scale bar equals 10mm.
Plate 15 Monomict vein breccia containing clasts of pervasively silicified wall-rock in chalcedonic to jasperoidal silica matrix. Scale bar equals 10mm.
Quinn Smith Master of Applied Science Thesis 119
Appendix 3
Digital Dataset
Quinn Smith Master of Applied Science Thesis 120
DVD-ROM includes:
ASTER raw scenes and processed images and alteration maps
Geothermometry calculations and excel spreadsheet
Mapinfo dataset including
o Exploration geochemistry
o Geological mapping
o PIMA and XRD dataset
o Remotesensing including rectified airphoto and ASTER
products
o Workspace folders of maps presented in the thesis
Microprobe data including Jeol840 mineral probe data and Quanta
ESEM alteration probe images
PIMA data files and processed excel spreadsheet
Scanned trench maps
Whole rock data including tables and spreadsheets included within the
thesis
XRD raw data and processed excel spreadsheet
Quinn Smith Master of Applied Science Thesis 121
Appendix 4
Sascha-Pelligrini Fact Geology Map 1:10,000 WGS82 SUTM19
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Epiclastics and juvenile clast
ash rhyodacite tuff
Ch
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Aik
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r Ju
rassic
)
Biotite rich rhyodacite ignimbrite
Biotite, pumice/juvenile
clast rhyodacite tuff
Dacite-Andesite dykes and sills
Lower Miocene feldsarenite
La M
atild
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Upper
Jura
ssic
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Flow-banded/spherulitic
lavas (Flow-Dome Complex)
Sperulitic ash tuff with
accretionary lapilli
Crystal ash rhyolite tuff
Flow-banded/spherulitic auto-
breccia (Flow-Dome Complex)
Pliocene olivine tholeite
Pliestocene gravel
Recent alluvium
STRATIGRAPHYSTRATIGRAPHY
Playa
Minor Airphoto Linearment
Major Airphoto Linearment
Flat lying stratigraphy
Statigraphic/Structural Dip
STRUCTURESTRUCTURE
Normal Fault
Mapped Structure
%
!15
Colloform/crustiform +/- ginguro
Chalcedonic quartz and single pulse
banded cockade breccia
HYDROTHERMAL SILICA CLASSIFICATIONHYDROTHERMAL SILICA CLASSIFICATION
Silica matrix phreatic
jigsaw breccia
Pervasive silicification
Silica fill, banded
hydrothermal breccia
Structurally controlled
silicification
Silica matrix phreatic
breccia with metamorphic clasts
Chalcedonic to opaline
+ diss. pyrite
Crystalline and Comb quartz
Jasperoidal + diss. pyrite
VEIN CLASSIFICATIONVEIN CLASSIFICATION
Weakly banded and bladded
saccharoidal quartz
TIM
E
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Sascha-Pelligrini Fact Geology1:10,000
WGS84 SUTM19Quinn Smith 2006
SASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGY
Quinn Smith Master of Applied Science Thesis 122
Appendix 5
Sascha-Pelligrini Interpretive Geology Map 1:10,000 WGS82 SUTM19
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00 m
N4,7
00,0
00 m
N
Epiclastics and juvenile clast
ash rhyodacite tuff
Ch
on
Aik
e (
Lo
we
r Ju
rassic
)
Biotite rich rhyodacite ignimbrite
Biotite, pumice/juvenile
clast rhyodacite tuff
Dacite-Andesite dykes and sills
Lower Miocene feldsarenite
La M
atild
e (
Upper
Jura
ssic
)
Flow-banded/spherulitic
lavas (Flow-Dome Complex)
Sperulitic ash tuff with
accretionary lapilli
Crystal ash rhyolite tuff
Flow-banded/spherulitic auto-
breccia (Flow-Dome Complex)
Pliocene olivine tholeite
Pliestocene gravel
Recent alluvium
STRATIGRAPHYSTRATIGRAPHY
Playa
Minor Airphoto Linearment
Major Airphoto Linearment
Flat lying stratigraphy
Statigraphic/Structural Dip
STRUCTURESTRUCTURE
Normal Fault
Mapped Structure
%
!15
Colloform/crustiform +/- ginguro
Chalcedonic quartz and single pulse
banded cockade breccia
HYDROTHERMAL SILICA CLASSIFICATIONHYDROTHERMAL SILICA CLASSIFICATION
Silica matrix phreatic
jigsaw breccia
Pervasive silicification
Silica fill, banded
hydrothermal breccia
Structurally controlled
silicification
Silica matrix phreatic
breccia with metamorphic clasts
Chalcedonic to opaline
+ diss. pyrite
Crystalline and Comb quartz
Jasperoidal + diss. pyrite
VEIN CLASSIFICATIONVEIN CLASSIFICATION
Weakly banded and bladded
saccharoidal quartz
TIM
E
0000000000000000000000000000000000000000000000000 0.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.5 1111111111111111111111111111111111111111111111111
kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers * Sascha-Pelligrini Interpretive Geology
1:10,000WGS84 SUTM19Quinn Smith 2006
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