Damaged Goods - Deformation fabrics in a gold-bearing ...€¦ · Damaged Goods - Deformation...
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Damaged Goods - Deformation fabrics in a gold-bearing damage zone Nils R. Backeberg*, Naomi A. Barshi and Christie D. Rowe McGill University, Montreal, Canada *contact: [email protected] Acknowledgements: Our thanks go to Osisko Mining Corporation, specifically Anne Charland, Robert Wares and Zoran Madon for hosting us during our field seasons and ongoing support for this study. Special thanks to Denver Stone for sharing his insights and expertise with us in the field once again. A big thanks to the Bjorkman family for providing local knowledge of outcrops and gold showings. Thanks to Eric Bellefroid and Ben “Silky Chicken” Melosh for field assistance and discussions during field mapping. References: Backeberg et al. (2014). Structural and metamorphic evidence for Mesoarchaean subduction of the Finlayson Lake greenstone belt, Superior Province, Ontario. Precambrian Research, 249, 100 - 114 Caine et al. (1996). Fault zone architecture and permeability structure. Geology, 24 (11), 1025 - 1028 Faulkner et al. (2003). On the internal structure and mechanics of largestrike-slip fault zobes: field observations of the Carboneras fault in southeastern Spain. Tectonophysics, 367, 235 - 251 Percival et al. (2006). Strike-slip juxtaposition of ca. 2.72 Ga juvenile arc and > 2.98 Ga continent margin sequences and its implications for Archean terrane accre- tion, western Superior Province, Canada. Canadian Journal of Earth Sciences, 43, 895 - 927 Stone (2010). Precambrian geology of the central Wabigoon subprovince area, northwest Ontario. Technical Report, Ontario Geological Survey Vearncombe (1998). Shear zones, fault networks, and Archean gold. Geology, 26 (9), 855 - 858 CON FI DO Kamb Contours C.I. = 2.0 Sigma Equal Area Lower Hemisphere N = 27 1 % Area Contours C.I. = 2.0 % / 1 % area Equal Area Lower Hemisphere N = 311 Kamb Contours C.I. = 2.0 Sigma Equal Area Lower Hemisphere N = 34 Marmion gneiss intrusive Finlayson Lynx Head fault damage zone 1 km Late shallow thrust faults schematic block model quartz vein quartz vein with sericite altered xenoliths breccia zone qtz cemented brecciated hanging wall unaltered tonalite foliated tonalite 8 m 1 m 30 3 m 2 m 9 m 50 55 decreasing foliation intesity Lynx Head Fault shallow southeast dipping thrust fault 2 zones of quartz veins are emplaced along Lynx Head Fault with brecciated hangingwalls Lynx Head Fault consistent with regional late shortening event across the Marmion Shear Zone (Backeberg et al. 2014) Is it tectonically related to the dextral transpression across the Quetico fault during 2.7 Ga Superior Province amalgamation? cross cuts anastomosing fracture network eastern limit of regional chlorite retrogression after amphibole shallow thrust fault with slicks N A gold-bearing damage zone, Hammond Reef Marmion Shear Zone terrane boundary between 2.93 Ga Finlayson Lake GSB and 3.00 Ga Marmion gneiss late stage reactivation (brittle) probably did not accommodate significant offset wider damage zone developed in the tonalites Possible reactivation during 2.7 Ga Quetico fault activity during docking of Wawa subprovince from the south with northward subduction multiple reactivation events intruded by Diversion Stock granitoids (? 2.8 Ga) early ductile deformation by at least 2.92 Ga Hammond Reef disseminated within pervasive altered tonalite disseminated mineralization not found along the entire length of the fracture network sericite with pyrite, ankerite, calcite, quartz, hematite, fuchsite and chlorite alteration hosted within an anastomosing fracture network located across intrusive contact of Marmion gneiss with younger Diversion Stock granitoid Hammond Reef localized cleavage foliation zones (0.1-5 m) unfoliated lenses (5-30 m) Marmion Shear Zone / Fault Zone and terrane boundary overall decreasing alteration intensity (2km) inspired from Faulkner 2003 dates from Stone (2010) and Percival et al. (2006) N Hammond Reef SW Superior Province (approx.) 200 km Lake Superior Percival 2007 Brittle fracture pattern in the tonalites “long” fractures “short” fractures undifferentiated fractures Fractures in the tonalites fracture density varies significantly from 10s to 100s of fractures per meter pattern of high density “short” fractures perpendicular to lower density ”long” fractures all the fracture data plots two dominant NNE and ESE orientations very variable fracture orientations locally and regionally mutual cross cutting relations short fractures terminate at long fracture intersect overall pattern matches the “long-short” pattern many fractures are vein-filled (epidote, chlorite, sericite, quartz, carbonate, quartz-carbonate) long short long 1 3 4 2 1mm 1mm increasing deformation static alteration fracturing and alteration Alteration and spaced cleavage foliation Anastomosing Fracture Network brittle deformation of unaltered tonalites (1) spaced cleavages promote further localisation of alteration and deformation (5) unfoliated lenses are protected from flatening/shearing, but show pervasive sericitisation of feldspars (pseudomorphs) (6) intensely foliated units show aggregates of quartz ribbons with long axes parallel to foliation (5) progressive alteration of feldspars (3,6) sericite dominant zones promote flattening and cleavage (4) sericitisation of feldspars (Fsp) along fractures and grain boundaries (2) flattening and minor shearing of sericite spaced cleavage defines the foliation fabric (5) foliation trends NE, subparallel to Marmion Shear Zone 1cm Qtz ribbon in sericite altered tonalite and localizing cleavage foliation 1 cm altered undeformed tonalite altered and sheared tonalite 1 2 3 6 4 5 Fsp Fsp Fsp Fsp quartz ribbons sheared sericite fractures (0.5-10 cm) increasing deformation schematic of localisation 1 2 3 4 5 5 5 6 Why are we here? Permeability of fault zones varies based on localized, distributed and sealed conduits defined by the fault core and related damage zones (Caine et al. 1996). Archean gold deposits are common in fault zones (Vearncombe 1998), but often associated with late sub-static damage not related to the major slip events of the nearby fault. Here we explore the structural and micro-structural controls on permeability related to the Archean Hammond Reef gold deposit in the southwestern Superior Province in order to under- stand the timing and location of gold mineralization in a previously under-explored tonalite setting. Lynx Head Fault Large quartz veins and gold 5 quartz vein altered, foliated tonalite Quartz veins multiple events visible gold associated with large quartz veins dextral and sinistral offset associated with quartz veins, consistent with NW-SE shortening latest structural activity in the Marmion tonalite cross cuts anastomosing fracture network locally quartz-cemented brecciated wall-rock emplaced along pre-existing structures N qtz vein (1 m) older qtz vein weak foliation horse tail veins historic gold mine 10 m 75 dilational jog brecciated hanging wall chlorite retrogression front dextral vein and gold mine opening quartz vein parallel to cleavage foliation. - opening-closing strain? Implications and open questions brittle damage zone related to shallow reactivation of Marmion Shear Zone (see Backeberg et al. 2014) damage zone and thrusts consistent with late shallow NW-SE shortening around Marmion Shear Zone Quartz veins carry gold and are last event. Is the disseminated alteration and Au-mineralization due to the Lynx Head Fault fluid barrier at Hammond Reef? Why is disseminated gold not along entire damage zone? does static alteration and localized flattening of Feldspars promote pervasive permeability in the tonalites? Hammond Reef gold is not a porphyry style deopsit - possibly related to 2.7 Ga Superior Province tectonism. late shallow faults cut altered tonalites; Does Lynx Head Fault act as fluid barrier to upward fluid flow? Is feldspar alteration promoted by the early perpendicular fracture pattern? Unloading/exhumation anastamosing fracture network accommodates flattening along sericite altered feldspars Diversion Stock intrusive acts as rigid block along shear zone - fluid barrier? 1 % Area Contours C.I. = 2.0 % / 1 % area Equal Area Lower Hemisphere N = 104
Transcript of Damaged Goods - Deformation fabrics in a gold-bearing ...€¦ · Damaged Goods - Deformation...
Damaged Goods - Deformation fabrics in a gold-bearing damage zone Nils R. Backeberg*, Naomi A. Barshi and Christie D. RoweMcGill University, Montreal, Canada
*contact: [email protected]
Acknowledgements:Our thanks go to Osisko Mining Corporation, speci�cally Anne Charland, Robert Wares and Zoran Madon for hosting us during our �eld seasons and ongoing support for this study. Special thanks to Denver Stone for sharing his insights and expertise with us in the �eld once again. A big thanks to the Bjorkman family for providing local knowledge of outcrops and gold showings. Thanks to Eric Bellefroid and Ben “Silky Chicken” Melosh for �eld assistance and discussions during �eld mapping.
References:
Backeberg et al. (2014). Structural and metamorphic evidence for Mesoarchaean subduction of the Finlayson Lake greenstone belt, Superior Province, Ontario. Precambrian Research, 249, 100 - 114
Caine et al. (1996). Fault zone architecture and permeability structure. Geology, 24 (11), 1025 - 1028
Faulkner et al. (2003). On the internal structure and mechanics of largestrike-slip fault zobes: �eld observations of the Carboneras fault in southeastern Spain. Tectonophysics, 367, 235 - 251
Percival et al. (2006). Strike-slip juxtaposition of ca. 2.72 Ga juvenile arc and > 2.98 Ga continent margin sequences and its implications for Archean terrane accre-tion, western Superior Province, Canada. Canadian Journal of Earth Sciences, 43, 895 - 927
Stone (2010). Precambrian geology of the central Wabigoon subprovince area, northwest Ontario. Technical Report, Ontario Geological Survey
Vearncombe (1998). Shear zones, fault networks, and Archean gold. Geology, 26 (9), 855 - 858
CONFIDO
Kamb ContoursC.I. = 2.0 Sigma
Equal AreaLower Hemisphere
N = 27
1 % Area ContoursC.I. = 2.0 % / 1 % area
Equal AreaLower Hemisphere
N = 311
Kamb ContoursC.I. = 2.0 Sigma
Equal AreaLower Hemisphere
N = 34
Marmion gneissintrusiveFi
nlay
son
Lynx Head fault
damagezone
1 km
Late shallow thrust faults
schematic block model
quartz vein
quartz vein with
sericite altered xenoliths
breccia zone
qtz cemented brecciated
hanging wall
unaltered tonalite
foliated tonalite
8 m1 m
30
3 m2 m9 m
5055
decreasingfoliation intesity
Lynx Head Faultshallow southeast dipping thrust fault
2 zones of quartz veins are emplaced alongLynx Head Fault with brecciated hangingwalls
Lynx Head Fault consistent with regional lateshortening event across the Marmion Shear Zone(Backeberg et al. 2014)
Is it tectonically related to the dextral transpressionacross the Quetico fault during 2.7 Ga SuperiorProvince amalgamation?
cross cuts anastomosing fracture network
eastern limit of regional chlorite retrogressionafter amphibole
shallow thrust fault with slicks
N
A gold-bearing damage zone, Hammond Reef
Marmion Shear Zoneterrane boundary between 2.93 Ga Finlayson Lake GSB and 3.00 Ga Marmion gneiss
late stage reactivation (brittle) probably did notaccommodate signi�cant o�set
wider damage zone developed in the tonalites
Possible reactivation during 2.7 Ga Quetico faultactivity during docking of Wawa subprovincefrom the south with northward subduction
multiple reactivation events
intruded by Diversion Stock granitoids (? 2.8 Ga)
early ductile deformation by at least 2.92 Ga
Hammond Reefdisseminated within pervasive altered tonalite
disseminated mineralization not found along the entire length of the fracture networksericite with pyrite, ankerite, calcite, quartz, hematite, fuchsite and chlorite alteration
hosted within an anastomosing fracture networklocated across intrusive contact of Marmion gneiss with younger Diversion Stock granitoid
HammondReef
localized cleavage foliation zones (0.1-5 m)
unfoliated lenses (5-30 m)
Mar
mio
n Sh
ear Z
one
/ Fau
lt Zo
ne a
nd te
rran
e bo
unda
ry
overall decreasing alteration intensity (2km)
inspired from Faulkner 2003
dates from Stone (2010) and Percival et al. (2006)
N
HammondReef
SW Superior Province
(approx.)
200 km
Lake Superior
Percival 2007
Brittle fracture pattern in the tonalites“long” fractures “short” fractures undi�erentiated fractures
Fractures in the tonalitesfracture density varies signi�cantly from 10s to 100s of fractures per meter
pattern of high density “short” fractures perpendicular to lower density ”long” fractures
all the fracture data plots two dominant NNE andESE orientations
very variable fracture orientations locally and regionally
mutual cross cutting relations
short fractures terminate at long fracture intersect
overall pattern matches the “long-short” pattern
many fractures are vein-�lled (epidote, chlorite,sericite, quartz, carbonate, quartz-carbonate)
long
short
long
1
3
4
2
1mm
1mm
increasing deformation
static alteration
fracturing and alteration
Alteration and spaced cleavage foliation
Anastomosing Fracture Networkbrittle deformation of unaltered tonalites (1)
spaced cleavages promote further localisation of alterationand deformation (5)
unfoliated lenses are protected from �atening/shearing, butshow pervasive sericitisation of feldspars (pseudomorphs) (6)
intensely foliated units show aggregates of quartz ribbonswith long axes parallel to foliation (5)
progressive alteration of feldspars (3,6)
sericite dominant zones promote �attening and cleavage (4)
sericitisation of feldspars (Fsp) along fractures and grain boundaries (2)
�attening and minor shearing of sericite spaced cleavage de�nes the foliation fabric (5)
foliation trends NE, subparallel to Marmion Shear Zone
1cm
Qtz ribbon in sericite
altered tonalite and localizing cleavage foliation
1 cm
altered undeformed tonalite
altered and sheared tonalite
123
6
45
Fsp
Fsp
FspFsp
quartz ribbons
sheared sericite fractures
(0.5-10 cm)increasing deformation
schematic of localisation1
2
34
5
5
5
6
Why are we here?Permeability of fault zones varies based on localized, distributed and sealed conduits de�ned by the fault core and related damage zones (Caine et al. 1996). Archean gold deposits are common in fault zones (Vearncombe 1998), but often associated with late sub-static damage not related to the major slip events of the nearby fault. Here we explore the structural and micro-structural controls on permeability related to the Archean Hammond Reef gold deposit in the southwestern Superior Province in order to under-stand the timing and location of gold mineralization in a previously under-explored tonalite setting.
Lynx H
ead Fa
ult
Large quartz veins and gold5
quartz vein
altered, foliatedtonalite Quartz veins
multiple events
visible gold associated with large quartz veinsdextral and sinistral o�set associated with quartzveins, consistent with NW-SE shortening
latest structural activity in the Marmion tonalite
cross cuts anastomosing fracture network
locally quartz-cemented brecciated wall-rock
emplaced along pre-existing structures
N
qtz v
ein
(1 m
)
olderqtz vein
weak foliation
horse tailveins
historicgold mine
10 m
75
dilational jog
brecciated hanging wall
chlo
rite r
etro
gress
ion fr
ont
dextral vein and gold mine
opening quartz vein parallel to cleavage foliation.- opening-closing strain?
Implications and open questionsbrittle damage zone related to shallow reactivation of Marmion Shear Zone (see Backeberg et al. 2014)
damage zone and thrusts consistent with late shallow NW-SE shortening around Marmion Shear Zone
Quartz veins carry gold and are last event. Is the disseminated alteration and Au-mineralization due to theLynx Head Fault �uid barrier at Hammond Reef? Why is disseminated gold not along entire damage zone?
does static alteration and localized �attening of Feldspars promote pervasive permeability in the tonalites?
Hammond Reef gold is not a porphyry style deopsit - possibly related to 2.7 Ga Superior Province tectonism.
late shallow faults cut altered tonalites; Does Lynx Head Fault act as �uid barrier to upward �uid �ow?
Is feldspar alteration promoted by the early perpendicular fracture pattern? Unloading/exhumation
anastamosing fracture network accommodates �attening along sericite altered feldspars
Diversion Stock intrusive acts as rigid block along shear zone - �uid barrier?
1 % Area ContoursC.I. = 2.0 % / 1 % area
Equal AreaLower Hemisphere
N = 104
