Anatomy of an Epithermal Ore

Economic Geology Vol. 90, 1995, pp. 1776-1798

The Anatomy of a Carboniferous Epithermal Ore Shoot at Pajingo, Queensland: Setting, Zoning, Alteration, and Fluid Conditions

RENATO E. Bores% SUBHASH JAIRETH **, AND GREGG W. MORroSON*** Gold Research Group, Geology Department, James Cook University of North Queensland, Townsville, Queensland 4811, Australia

Abstract

The Paleozoic Scott lode gold-silver epithermal deposit, situated at Pajingo, northeast Queensland, is hosted by feldspathic-volcanolithic sandstones, quartzose sandstones, ignimbrites, andesitic voltanits with interbedded block and ash deposits, lapilli tuffs, and andesitic to dioritic intrusive rocks. The Scott lode deposit contains 1.23 million metric tons of 9.9 g/t gold and 38.9 g/t silver.

There are six principal types of alteration assemblages which are zoned with respect to the main vein structure: propylitic, potassic (adularia), intermediate argillic, silicic, kaolinitic, and ferroan carbonate. Over- printing textural relationships indicate that the propylitic type is earliest, progressively followed by potassic (adularia), then intermediate argillic, and finally silicic alteration. Auriferous veins are largely coeval with the silicic and intermediate argillic alteration types. Kaolinitc and ferroan carbonate occur as late-stage alteration products, postdating the precious metal mineralization.

Principal ore and sulfide minerals include pyrite, sphalerite, galena, chalcopyrite, hessitc, tennantite-tetrahedrite, argentitc, petzite, electrum, and native gold. Gangue minerals in the veins are quartz, illitc, adularia, calcite, and locally, kaolinitc. The main-stage quartz is zoned with respect to mineralogy, mineralization styles, metal distribution patterns, and vein textres. Three major zones can be recognized: zone I--an upper precious metal, moss chalcedonic crustiform-colloform quartz zone, zone II--intermediate precious + base metal, crystalline quartz-coarse-grained sulfide-dominated, crustiform-colloform quartz zone, and zone III-- deep sparse base metal, weakly banded crystalline comb quartz zone. The ore interval occurs in zones I and II with the transition to zone III effectively defining the limit of mineralization.

Economic mineralization at the Scott lode deposit was laid down from dilute fluids (0.7-2.5 wt % NaC1 equiv) with a very low concentration of CO2. Homogenization temperatures of 170 to 315C exhibit vertical variation with higher temperatures at greater depth. The presence of coexisting liquid- and gas-rich inclusions in mineralized quartz, together with adularia, vein brecciation, and pronounced crustiform-colloform banding, indicates hydrothermal boiling as the principal precipitation mechanism for gold and silver deposition. Thermo- dynamic modeling of adiabatic boiling of a fluid with an initial concentration of 2 to 5 ppb gold at 300C indicates that such a fluid can deposit ores containing 10 ppm gold and 260 ppm silver at the temperature intervals recorded by the fluid inclusion work.

The 5SO values of the fluids in equilibrium with the vein quartz, calculated from the quartz SlSO values and median homogenization temperatures lie within the range -7.0 to -1.0 per mil. The SD values of fluid extracted from primary inclusions in quartz range between -59.5 and -64.9 per mil. The compositions of altered rocks show significant depletion in oxygen and deuterium which can be attributed to isotopic reactions with meteoric water. Isotopic calculations indicate that meteoric water of a composition 51SO = - 12 per mil and SD = -86 per mil alone cannot explain the observed values. It is suggested that the epithermal fluid in Pajingo was the result of mixing between meteoric water and isotopically magmatie water.

Introduction

THE Carboniferous gold-silver deposit at Pajingo, located in the northern Drummond basin of eastern Australia (Fig. 1), is one of the oldest documented epithermal veins. The ore shoot, called the Scott lode, contains 1.23 million metric tons of 9.9 g/t gold and 38.9 g/t silver (i.e., about 12 t of gold and 47 t of silver). Developed by Battle Mountain (Australia), Inc., in 1987, it is the first epithermal deposit to be mined in northeast Queensland and its discovery in 1983 has been considered the most significant geologic find in eastern Queensland in recent years (Ingram, 1989).

There is no previous history of prospecting recorded at Pajingo (Porter, 1991). The Pajingo area was explored by

* Present address: Kidston Gold Mines, Cairns, Queensland 4870, Aus- tralia.

** Present address: Bureau of Resource Sciences, A.C.T. 2600, Australia. *** Present address: 7 Mary Street, West End, Townsville, Queensland,

Australia.

Battle Mountain (Australia), Inc., in 1983 on the basis of the presence of a northeast-trending belt of Pertoo-Carbonifer- ous intrusive rocks. Many porphyry and breccia-hosted gold deposits and prospects in north Queensland are associated with these intrusive complexes. An example of such a gold deposit is the diatreme-hosted Mount Leyshon gold deposit (Morrison et al., 1988) which lies in a parallel belt of Pertoo- Carboniferous intrusive rocks. Prior to the discovery of this deposit, the possibility of epithermal mineralization received little attention because of the belief that the deep erosional levels in Paleozoic terranes would not have allowed preserva- tion of shallow-level mineralization. It was also noted that the majority of reported epithermal deposits occur in Cenozoic to Recent andesitic volcano-plutonic arcs.

Auriferous veins with distinct epithermal characteristics carrying up to 15 g/t gold and with an associated high-level elemental suite of As, Sb, and Hg were recognized at the Doongara homestead, in the eastern part of Pajingo (Fig. 1). On this encouragement, Battle Mountain (Australia), Inc.,

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1778 BOBIS ET AL.

and alteration-mineralization characteristics of the Scott lode ore shoot. The information presented here on the mineralogical and textural zoning of quartz veins may be useful in the exploration of epithermal deposits in eastern Australia.

Geologic Setting Host rocks for the auriferous veins in the Scott lode area

are Late Devonian to early Carboniferous feldspathic-volcanolithic sandstones, quartzose sandstones, iguimbrites, andesitic lavas with interbedded block and ash deposits, lapilli tuffs and andesitic to dioritic intrusive rocks (Figs. 2, 3, and 4). In general, the Mount Molly Darling sediments and the dacitic ignimbrite unit occur in the footwall, whereas the andesitic volcanics dominate in the hanging wall (Fig. 2).

The feldspathic sandstone does not crop out and was encountered only in deep (750 m) drilling (Fig. 3). The oldest outcropping unit is a clastic sedimentary sequence consisting of generally shallow-dipping interbeds of quartzose sandstones, micaceous fine to coarse sandstones, siltstones, con- glomerates, and carbonaceous shales. Coherent clastic rocks are exposed immediately north of the pit area, extending to Mount Molly Darling, and are faulted and altered toward the Scott lode deposit. Deep holes drilled from the footwall of the Scott lode show that these sediments are present at depth where they are downthrown in the hanging wall along major east-west and northeast-striking normal faults. At surface, the sedimentary sequence has a general shallow northwesterly dip but with locally steep dips due to folding and faulting, particularly in the vicinity of the Scott lode deposit.

The Mount Molly Darling sediments are overlain by an ignimbrite-dominated unit (Figs. 3 and 4). A typical ignimbrite is dacitic, displays eutaxitic texture, and contains flattened pumiceous lapilli, lithic fragments (including Mount Molly Darling sediments), glass lenses formed by flattening of pumice lapilli (compacted around larger lithic fragments), and crystals of plagioclase and quartz. The general attitude of the pumiceous clasts is subparallel to the bedding of the sediments. This ignimbrite flow unit is 70 to 100 m thick and contains beds of epiclastic rocks and sediments. The main body of the unit is moderately to poorly sorted with 20-cm-diameter clasts. The abundance of flattened pumice clasts increases upward in the sequence. Larger clasts are concentrated at the base of the sequence. The fiamme-rich unit usually grades downward to well-sorted, imbricated con- glomerate beds.

The sediments and iguimbrite are overlain by andesitic volcanics which are mainly porphyritic andesitc flows with intercalated fragmental pyroclastic flows and finer grained lapilli tuffs. In places, the andesitc carries blocks as big as 10 m in diameter of sedimentary rocks incorporated during flow. Although bedding is seldom observed, the correlation between transverse sections suggests that the volcanics dip south at low to moderate angles. A distinctive fragmental pyroclastic rock unit, resembling a block and ash-flow deposit, occurs within the andesitc pile (Figs. 3 and 4). This fragmental unit has a homogeneous clast composition of nonvesiculated cog- nate andesitc blocks which can exceed 2 m in diameter. The unit has a predominant ash matrix.

The sediments and overlying volcanic rocks were intruded by shallow-level andesitc to diorite porphyry. Several dikelike

bodies occur in the main vein structure and along the faulted sediment-volcanic contact.

Hydrothermal breeeiation at the Scott lode deposit occurred before, during, and after gold mineralization. Most of the hydrothermal breeeia bodies predate the vein emplaee- ment but are poorly mineralized. Premineralization breedas occur mainly as linear bodies transgressing the andesitie voleanies and sedimentary rocks (Figs. 2, 3, and 4). They are mainly concentrated on the walls of the subvolcanic intrusions and faulted contacts between the andesitie voleanies and sedimentary elastic rocks. Most of the breedas are heterolitho- logic, fragment-supported, and poorly sorted. The elast composition reflects local lithologie units. The largest body occurs immediately north and northeast of the Scott lode's hinge zone (Fig. 2) and is spatially associated with subvolcanic andesitc porphyry intrusions. Disposed mainly in the footwall, it is irregularly shaped but defines a continuous breeeia body around the andesite porphyry intrusion. This breeeia becomes highly linear extending several kilometers farther east, defining the sediment-volcanic contact (Stephens, 1988).

Synmineralization breedas occur as irregular bodies confined within, and as narrow envelopes to, the main lode structure (Figs. 3 and 4). They are generally heterolithologie, com- posed of angular to subrounded elasts of intensely hydrother- mally altered wall rocks (mainly andesitic volcanics) and premineralization breccias in a matrix of quartz-illite-pyrite _ chalcopyrite _ sphalerite _ galena altered wall rock or vein quartz. Within the main lode, fragments of rocks are surrounded by concentric bands of quartz and sulfide (cockade texture).

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of the individual faults merge toward the hinge area. The rocks between the faults are tectonically breeeiated. Subsid- iary and minor, steeply dipping antithetie faults are also developed along the footwall. Displacements on the individual faults form steps and the displaced rock units indicate normal movements (Figs. 3 and 4). The dip separation in the voleanies cannot be unambiguously established because of the gra- dational contacts and interealation between the lava flows and fragmental units. Within the Scott lode area, the amount of displacement in the fault zone increases to the south, toward the main vein structure. For example, the biggest stratigraphic displacement documented from cross section recon- struction is 290 m down-to-the-south stratigraphic offset in the southernmost east-west fault (occupied by the western segment of the Scott lode deposit; Fig. 3). The displacement successively decreases in a stepwise fashion to less than 50 m as the northeast-striking faults are encountered northward. Parallel faults covered by Tertiary laterite are inferred from drilling and interpretation of aeromagnetic data.

Southeast-dipping and northeast-striking high-angle faults are generally present as short segments between east-striking faults. In the pit area, there are at least three individual faults approximately 80 m apart constituting a northeast-striking fault zone. The faults are approximately 350 m long with

stratigraphic offsets clearly showing normal, down to the south displacements (Figs. 3 and 4). The northeast-striking faults link the east-west faults by accommodating coordinated changes in dip separation along their strikes. For example, the separations in the east-west faults successively decrease as the northeast-striking faults are encountered northward. Displacement in the northeast faults accordingly increases eastward with the largest dip displacement being around 130 m. Hydrothermal intrusion breeeias associated with the subvolcanic intrusions are localized in a rhomb defined and bounded by the northeast-striking and east-west faults suggesting a dilational jog generated along the northeast-striking accommodation faults. The premineralization intrusion breeeias along this dilational jog contain erustiform fissure veins indicating incremental extensional opening.

Steep-dipping veins and siliceous bodies southwest of the Scott lode area are associated with northwest-striking frae- tures and faults. The major east-west faults curve at the western end (Janet "A" west area; Fig. 1) passing westward to intersect the northwest-striking faults and fractures (now mostly filled by quartz veins and siliceous bodies). The precise relationships between the east-west and northwest faults and fractures cannot be easily established. The similarity in quartz textures and reported normal dip separation (Porter, 1991)


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strongly suggest that the northwest-striking fractures form part of, and are coeval with, the normal fault system. The curvature cannot be related to mechanical inhomogeneity because the faults are within the same lithological unit (i.e., andesitic volcanics). Rigby (1987) reported dextral strike-slip displacement of around 5 m in one of the northwest-trending silicified zones at Mount Janet. Porter (1991) interpreted the northwest-trending faults transecting the Pajingo field as normal faults bounding faulted blocks developed in the basin margin.

Interpretation: Kinematics of faults and regional relations The absence of a southwest continuation of the northeast-

trending faults across the east-west fault zone indicates that it is not a simple case of intersecting conjugate fractures. The steep-dipping fault sets at Pajingo are probably coeval because the patterns of displacement show that the faults were connected, and in some places, merge with each other. The northeast-striking faults accommodate movement on the east-striking faults. It is therefore considered that the observed geometry and displacements predating vein emplacement are the result of a single structural event.

The east-west, northeast, and probably northwest-striking faults mainly record down-to-the-south dip displacement of

strata, and the few striae indicate mainly dip-slip with minor sinistral strike-slip along the east-west faults. Despite the variation in strike and curvature of several faults, all but minor faults show down-to-the-south displacement, suggesting that much of the offset along these faults represents a single faulting event. The kinemarie connections between the principal east-striking faults via northeast-striking faults indicate that all the high-angle normal faults were contemporaneous. Because the east-west faults appear to be primarily dip-slip and are predominantly normal with respect to bedding, we interpret them as normal faults formed by extension in an approximate north-northeast direction parallel to the bedding. The orien- tation of the main faults can be related to the orientations of the major structural trends in the basement rocks. The dominant structural grain in the Lolworth-Ravenswood block is east-west (Henderson, 1986). Ore shoot at the Scott lode: Morphology and eraplacement mechanisms

The Scott lode is predominantly a vein infill ore shoot developed in a northeast-trending, 9..5-km-long structure (Ja- net "A" ) characterized by lesser vein infill and by silieified faults and shear zones (Fig. 1). The orebody occurs where the vein structure changes in strike from northeast to almost

1782 Bo3s ET AL.

east-west. Such changes are known to be a locus for dilatancy (e.g., Hulin, 1929; Newhouse, 1942).

The productive ore shoot at the Scott lode is up to 330 m in length and plunges 70 to 80 south-southeast (Fig. 2). It is enveloped by lower grade veins and is thicker and richer in the center. The shoot has a restricted maximum vertical interval of approximately 150 m. Economic mineralization is present along a vein from premining levels of around 1,260 m RL dowmvard to a well-defined base at 1,100 m RL in the central segment (Fig. 3). Below the 1,100 m RL level, the principal vein structure tapers to a set of parallel, less than 2.0-m-wide steeply south-dipping subeconomic veins.

The vein generally coincides with the 1.0 g/t Au contour and its thickness varies from a maximum of 23 m in the central portion (i.e., the hinge area) to about 4 m at each end. The thickest parts have the highest gold and silver grades. In the hinge area, the 23-m-wide principal vein is flanked in the hangingwall and footwall by discontinuous 1- to 2-m-wide auriferous quartz veins and moderate to strong multidirec- tional quartz veinlets (5.0 g/t) occurs in the immediate west-central portion of the hinge area and tapers to the north- eastern and western ends.

At Scott lode, there is a strong spatial coincidence between faulted zones, dikes, and auriferous veins (Figs. 2, 3, and 4). The same dilational openings permitted the eraplacement of poorly mineralized andesitic dikes. Commonly shear faults and dikes coincide along part of their course. Over such intervals, it is common for the fault structure to be partly confined to a dike that is weaker than adjacent quartzose sediments. Dikes disposed along major vein structures have been reported in many vein deposits (e.g., Gunnar mine, Manitoba, Newhouse, 1942; Calera epithermal deposit, Peru, Gibson et al., 1990; E1 Bronce epithermal vein system, Chile, Camus et al., 1991). Poulsen and Robert (1989) suggested that the jogs created by this geometry, coupled with the physical im- possibility of maintaining a simple slip history at every point in the structure, probably account for the increased tendency for dilational sites to occur where faults and dikes interact.

Zoning in the Ore Shoot Systematic evaluation of vein intersections from drilling

has led to the recognition of a distinct vertical zonation of the main-stage vein quartz with respect to mineralogy, mineralization styles, and vein textures. These are designated zones I, II, and III (Fig. 5). Textural characteristics of quartz in the principal zones

Morrison et al. (1990) and Dong et al. (1995) have established a formal definition and classification of epithermal quartz textures based on quartz in different Queensland epithermal deposits. This classification has been adopted in the present study. The classification is primarily descriptive, but textures are grouped into genetic classes of primary growth, replacement, and recrystallization textures. Primary growth textures indicate precipitation in open space, replacement textures result from silica products partially or completely replacing earlier mineral precipitates (e.g., carbonates, sul- fates, adularia), and recrystallization textures encompass mor- phological attributes resulting from the transformation of es-

sentially metastable phases (e.g., chalcedony, silica gel, opal, cristobalite, opal-CT) to quartz.

Quartz textures were observed in drill core froin the Scott lode deposit at hand specimen scale with supplementary microscope work. Proportions of textures were estimated and the predominant textures were used to define textural assemblages. Primary growth textures at the Scott lode include crustiform-colloform banding, chalcedonic, crystalline granu- lar, and crystalline comb quartz. Replacement textures in- elude mould, parallel, and lattice bladed quartz. Recrystalliza- tion textures include all quartz attributes formed by recrystallization froin an amorphous silica or chalcedony precursor. These include radial, fibrous, anomalous extinction (plumose) textures and rounded concentric zones of fluid-solid inclusions and/or impurities constituting moss bands.

Boundaries between textural zones are generally grada- riohal but may be telescoped or internally complex. Thus, textures belonging to both zones occur in the transition. How- ever, in any given vein interval, a dominant theme defining a particular zone is almost always evident.

The zone I textural assemblage is characterized by pronounced colloform-crustiform banding (Fig. 6a-d). Chalce- donic quartz and moss bands are dominant over crystalline bands. Moss and fine needle adularia pseudomorphs and replacement lattice blades (after calcite) occur within the chalcedonic bands. Subsidiary textures inchide crystalline granu- lar quartz and some crystalline comb quartz in discrete bands with chalcedony. Individual bands can include all types of bladed pseudomorphs, moulds, and recrystallization textures (i.e., moss plumose). Relic adularia occurs as fine needles forming discrete bands with chalcedony. Aggregates of radiating acicular (needlelike) adularia are also sometimes observed to transgress crustiform-colloform quartz. Rare moss adularia is associated with chalcedony and moss bands. Vein breccias in zone I contain clasts of microcrystalline to crystalline quartz-pyrite-veined wall rocks and chalcedonic quartz veins cemented by banded chalcedonic quartz.

The zone II textural assemblage is characterized by pronounced development of alteruating (crustiform _ colloform) 0.01- to 1.5-era bands of milky and clear crystalline quartz, chalcedony, adularia and sulfides (Fig. 7a-d). Under the microscope, bands are defined by alternating fine- and coarse- grained quartz. Crystalline quartz typically displays recrystallization features (e.g., plumose). Pyrite, sphalerite, galena, chalcopyrite, hessRe, tetrahedrite-tennantite, and electrum occur as bands with quartz, blebs or matrix in breccias, and intergrowths with gangue quartz. Needlelike and fine-grained adularia (mainly replaced by late-stage kaolinitc) occurs mainly along vein margins. Crystalline quartz also includes bladed pseudomorphs, moulds, and recrystallization texttires. Vein breccias in the zone II assemblage contain clasts of silicified wall rock and chalcedonic quartz veins cemented by bands of crystalline quartz (with crystalline comb quartz phases), and coarse sulfide grains and blebs. High-grade ore (>30.0 g/t) occurs with strong vein brecciation and a high sulfide content.

The zone III assemblage is defined by the association of crystalline comb quartz with coarse euhedral adularia, sparse fine- to coarse-grained disseminated sulfides, and minor calcite, intergrown with vein quartz (Fig. 8a-d). Subsidiary


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FIG. 5. Longitudinal section of the Scott lode showing the distribution of the principal zones with respect to quartz vein textural characteristics. Dashed line with arrows = 6 g/t Au contour, dashed line = 30 g/t Au contour, full line = 100 g/t Au contour, filled triangles = zone of intense breeeiation in the vein, filled squares = sample location.

textures include bladed calcite and lattice blade pseudomorphs of quartz, presumably after bladed calcite. The lattice textures are characterized by numerous intersecting thin blades with polyhedral cavities lined with quartz crystals. The progression from zone II to zone III is marked a dramatic decrease in the intensity of erustiform banding. zone III is characterized by poorly developed erustiform banding in dominantly crystalline quartz. Coarse-grained adulada is dominantly euhedral and occurs mainly as trains of rhombie crystals. Individual quartz crystals may exhibit growth zones defined by trails of fluid inclusions or alternating clear and milky quartz. In the hinge area, development of ernstiform banding is generally a function of depth. Near the transition to zone III, there is weak to moderate banding. In the deepest portions, the banding is weak to absent with crystalline comb quartz predominating. Mineralization in zone III consists of fine-grained sulfides (sphalerite, galena, ehaleopyrite, and pyrite) occurring as sparse disseminations within gangue quartz. Mineralization characteristics of the principal zones

The principal vertical zones also show characteristic mineralization features which can be summarized as: zone I--upper pyrite-precious metal zone (maximum thickness of 60 m), zone II--an intermediate base metal-rich and precious metal zone (maximum thickness of 110 m), and zone III--deep sparse base metal zone (at least 300 m thick) (Table 1, Fig. 5).

Zone I is characterized by electrnm, gold, pyrite, and argentitc. Gold, electrum, and argentitc have been observed within and interstitial to quartz grains, in fractures, as anhe- dral blebs in quartz, and as inclusions in pyrite. Bonanza grade intervals (>100 g/t Au) in zone I are commonly associated with intense vein brecciation, finely banded colloform

chalcedonic quartz, and fine delicate adularia needles. In these intervals, aggregates of electrum and gold grains (up to 1.2 mm across) form bands and clots within vein quartz. In the vein breccias, gold and electrum are found both in the brecciated veins and in the vein matrix.

The base metal-rich zone II contains base metal sulfides, electrnm, hessitc, tetrahedrite-tennantite, petzite, and subor- dinate adularia, kaolinitc, and illRe. The veins in zone II are well developed in the hinge area and consist of alternating 0.1- to 1.5-cm-wide bands of milky and clear crystalline quartz, chalcedony and generally coarse sulfides (Fig. 6c). Sulfides attain 2 to 5 vol percent; however, the total base metal content does not reach economic grades (30 g/t Au; Fig. 5) in zone II coincide with intense vein brecciation, crustiform banding, and high sulfide contents (up to around 5 vol %). The sulfides occur as coarse crystalline bands in veins, and as black blebs and disseminations in the matrix.

The veins in zone III are generally weakly mineralized (i.e.

1784 BOBIS ET AL.

a ib

FiG. 6. Typical quartz vein textures in zone I, Scott lode, Pajingo. (a). Crustiform and colloform banded quartz with bands dominated by chalcedony (gray) and crystalline quartz (white). (b). Crustiform and colloform banded quartz with bands dominated by moss texture. Bands of chalcedony (gray) and crystalline quartz, typical of high-grade ore. (c). Radiating aggregate of acicular (needlelike) cavities transgressing crustiform-colloform banded chalcedonic quartz. Cavities contain quartz and kaolinitc pseudomorphous after adularia needles which grew originally with the banded quartz. (d). Cryptocrystal- line quartz (chalcedony) with crudely banded aggregate of spheroidal grains and an overall appearance simfiar to moss vegetation.

6.0 g/t Au contour) essentially corresponds to the gbom- etry of zones I and II with sharp bottoming of the ore interval at around 150 m below the premining surface (1,100 m RL; Fig. 9). This bottoming of the ore interval coincides with the transition from zone II to zone III.

Hydrothermal Alteration The focused ascent of hydrothermal fluids along the east-

west to northeast-trending extensional fractures generated zoned alteration around the principal veins (Fig. 10). The alteration assemblages associated with the epithermal regime are of six principal types: propylitic, potassic (adularia), intermediate argillic, silicic, kaolinitic, and ferroan carbonate alteration. They are best exhibited in the andesitic volcanics and comagmatic intrusive rocks.

Altered lithologies contain several overprinting hydrothermal mineral assemblages as well as supergene minerals. The sequence of alteration assemblages can be established from the geometry, overprinting relationships of minerals, and crosscutting relationships of coeval veins-veinlets. Propylitic alteration is earliest, progressively followed by the low-temperature potassic (adularia), then intermediate argillic and silicic alteration (Fig. 11). Main stage ore-bearing vein eraplacement immediately followed adularia alteration and was probably largely coeval with silicic and intermediate argillic

ORE SHOOT, PAIINGO, QUEENSLAND 1785

FIG. 7. Typical quartz vein textures in zone II, Scott lode, Pajingo. (a). Partial to complete replacement of carbonate lattice blades by aggregates of quartz grains. (b). Vein breccia dasts of crustiform-colloform banded vein quartz sulfide (center and upper right) and veined and silicified wall rock (left) overgrown by crustiform bands of fibrous chalcedony (gray) and saccharoidal quartz (white). Ore zone at Scott lode. (c). Crustiform-colloform texture with bands of crystalline quartz (white) with disseminated sulfides and massive to moss chalcedonic quartz (buff) and telluride sulfide (black). High- grade zone. (d). Crustiform banded vein quartz with coarse-grained adularia (now largely kaolinRe) along selvage of milky crystalline quartz.

alteration. This is indicated by the presence of argillically altered clasts in the vein structures and similarity in mineralogical components. Kaolinitc and ferroan carbonate occur as late-stage alteration products postdating the main precious metals mineralization. Supergene minerals consist of alunite, gypsum, kaolinitc, and smectite.

The propylitic assemblage in the andesitic rocks and feldspathic sandstone is characterized by partial to complete albite- chlorite-quartz-epidote-calcite replacement of the plagioclases, chlorite-calcite-ankerite-epidote _ actinolite alteration of au- gitc, pyrite-pyrrhotite-chalcopyrite _+ rutfie replacement of primary magnetite-fimenite, and an elevated content of calcite as a replacement mineral and as crosscutting veinlets. Selective replacement of alteration minerals within this assemblage pre- serves primary phenocrysts and groundmass textures. Propyli- tic alteration is well developed in the hanging wall below the laterite cover and grades outward to weakly propylitized andesitic volcanics. Although pervasive in detail, the well-developed propylitic assemblage must have been guided by the major

structures that confined the Scott lode. This is indicated by the increase in intensity of propylitic alteration toward the principal vein structures. Hydrothermal albite and quartz be- come prominent alteration products after plagioclase and mafic minerals with proximity to the major vein. With increasing depths below 1,100 m RL, this propylitic assemblage is accom- panied by epidote and actinolite.

Well-developed potassic (adularia) alteration of the andesitic rocks occurs below the ore interval (i.e., 1,100 m RL downward; Fig. 10). The potassic (adularia) alteration assemblage includes adularia, quartz, chlorite, calcite, muscovite (2M mica), pyrite, chalcopyrite, and rutile-anatase. This alteration type is conspicuous at hand specimen scale as coalescing reddish pink envelopes around the quartz-adularia-calcite veinlets. The key textures to this alteration transition include the adularia replacement of albite and the dramatic increase in silicification with incipient to well developed destruction of plagioclase laths by medium-grained equigranular quartz.

At depth, the degree of potassic (adularia) alteration, to-

1786 BOBlS ET AL.

FIG. 8. Typical quartz vein textures in zone III, Scott lode, Pajingo. (a). Cockade rims of weakly banded quartz around fragments of wall rocks. Fine bands of mfiky and clear crystalline quartz show progression to crystalline comb quartz with dogtooth texture. (b). Crystalline comb quartz with weak crustiform texture due to a milky to clear variation in quartz. Carbonate fills in vugs. Barren or low-grade zone. (c). Weakly banded, crystalline comb quartz with wall rock and clasts consisting of quartz, chlorite, adularia, magnetite, rutile-anatase, and chalcopyrite. (d). Weakly banded crystalline comb quartz hosting sparse sphalerite-galena-chalcopyrite-pyrite (dark) disseminations.

T^]L 1. Description of Principal Zones in the Scott Lode

Zone Mineralogy Quartz textures Mineralization

I Upper

II Intermediate

III Lower

Quartz, illitc, adularia, kaolinitc, mixed layer filite-smectite, pyrite, argentitc, electrum, gold, chalcopyrite

Quartz, filite, adularia, kaolinitc, mixed layer filite-smectite, pyrite, sphalerite, galena, chalcopyrite, hessitc, tetrahedrite-tennantite, petzite, electrum

Quartz, illRe, calcite, pyrite, sphalerite, galena, chalcopyrite, ___ hessitc ___ electrum

Dominant textures: colloform and crustiform banding; chalcedonic and moss bands dominant over crystalline bands; moss and needle adularia and lattice blades after bladed carbonate

Minor textures: zoned crystalline quartz and plumose quartz (mainly recrystallization)

Dominant textures: colloform and crustiform banding; microcrystalline to crystalline banding dominant over chalcedonic and moss bands; needle and crystalline adnlaria, replacement lattice blades (after bladed carbonate); coarse-grained sulfide bands and sulfide disseminations

Minor textures: amethystinc quartz, zoned crystalline quartz, plumose quartz (mainly recrystallization)

Dominant textures: weakly banded; crystalline comb quartz with coarse-grained adularia, sparse fine- to coarse-grained sulfides, and calcite

Minor textures: amethystinc quartz, plumose quartz (mainly primary)

Gold and electrum as inclusions in pyrite and interstitial to quartz grains

Electrum inclusions in galena, hessitc, sphalerite, chalcopyrite, tetrahedrite-tennantite; electrum in contact with base metal sniffdes and hessitc; electrum grains interstitial and within quartz

Electrum associated with base metal sulfides; electrum interstitial to quartz


MIDlaOIN T OF HINGE AIqEA

1ooo (m,RL)

o eo m I l:r""':' > I00

SAMPLE LOCATION

FIG. 9.Longitudinal section along the Scott lode showing the distribution of Au (ppm).

EXPLANATION

$1LICIC

*.e INT IrRMEDIA'rE ARGILLIC

--800 1 LOW-TEMPERATURE POTAeeI

pRopYLrrlc

1, ,'"j' KAOLINrl'E OVERPRINT --'r00 I OUTLIN OI e INTENIE INTERMEOIATE m, RL J A/OILLIC

I

, o

/ IIRINClPAL IN-FILL QUARTZ VEIN o o o

o

o o / AURIFEFtOUS QUARTZ VEIN . CALCITE QUARTZ-ADULARIA-PYRrrE V1NLETS

/ FAULT

UNCON II'ORMITY

$ THIN SECTION SAMPLE LOCATION

XRO JIPL le LOCTION

DIAMOND 0RILL HOLE

FIG. 10. North-south traverse section of the Scott lode vein showing the distribution of the alteration types and zones. The same section as A-A' in Figure 3.

1788 BOBIS ET AL.

qnaJ'lz chloritc albic calcite

cpidotc aclinolitc adularia muscovitc(2M mica) illitc

mixed-layer US kaolinitc ankcrilc sideritc

mtilc/anatasc pyritc pyrrhotitc chalcopyritc

Propylitic J Low-Temperature Potassic Intermediate $ilicic Late-stage Late-stage Argillic Kaolinitic Carbonate

Early ................................................................ -Lale

FIG. 11. Alteration paragenesis at the Scott lode, Pajingo. No scale is intended.

gether with the number of v, eins, increases toward the principal vein. The widest zone of preore adularia coincides with the andesitc porphyry intrusion in the footwall. In the footwall, the potassic zone measures 150 to 180 m around the principal vein and assumes a steep, narrow geometry away from the footwall. This geometry, however, is inherited from the more easily altered porphyry intrusions. The assymetrical distribution or the absence of corresponding low-temperature potassic adularia in the hanging wall could be attributed to the paucity of reactive minerals in the hanging-wall sandstones and the pervasive overprinting argillic alteration which affects both the dacitic ignimbrites and quartzose sandstones. Alteration symmetry, however, is suggested by the propylitized andesitic volcanics in the hanging wall (Fig. 10) which show increasing adularia replacement of the plagioclases toward the main vein. Genetic relationship with porphyry emplacement is highly unlikely because of the following features: (1) intensity of adularia replacement increases toward the main vein, (2) the other intrusions in the footwall and in the Mount Molly Darling area do not host significant adularia veins although they do exhibit patchy K silicate alteration facies, and (3) potassic adularia alteration replaces epithermal-related propylitic alteration.

The main intermediate argillic alteration envelope in the upper 200 m is 165 m across at the widest point and significantly tapers at a depth of around 1,000 m RL (Fig. 11). Away from the principal vein structure, veins, faults, lithologic boundaries, and permeable rock units have narrow envelopes of intermediate argillic alteration. The extent of the argillic envelope varies in proportion to the width of the quartz vein. The intermediate argillic alteration is a texture-destructive aggregate of illitc (showing broad 10 reflection in X-ray

diffractograms), and/or mixed layer illite-smectite, quartz, and pyrite. The relic phenocryst sites (i.e., of plagioclase, alteration albite, and adularia) in andesitic volcanics and co- genetic intrusive rocks consist of

ORE SHOOT, PAJINGO, QUEENSLAND

T^BLE 2. Heating-Freezing Data for Quartz

1789

Th (C) Drill hole/ Elevation s Tm,,e (C)

m (m RL) Vein type '3 Type 4 n Range Median Range 291/379.3 1,001 3 (deep, crystalline) P 21 243-315 275 -0.5 to - 1.0 291/424 967 3 (deep, crystalline) P 4 283-305 291 -0.6 to -0.8 234/63.8 1,191 3? P 12 195-260 222 -0.4 to -1.5 234/116.7 1,159 3 P 8 240-265 249 -0.3 to -0.8 242/45.6 1,214 27, 17 P 5 187-227 203 - 1.0 to - 1.4 1/62.5 1,232 1 P 6 212-265 232 -0.7 to - 1.2 171/32.4 1,252 1 P 20 186-257 218 -0.2 to - 1.4 204/102 1,190 2 P 4 245-265 256 -0.3 to -0.6 293/114 1,200 3 P 2 235-240 237 -0.6 to -0.7 292/675.3 783 3 (deep, crystalline) P 2 296-315 305 -0.6 to -0.7 175/121 1,166 27, 17 P, S 14 171-285 245 -0.8 to -1.3 171/7.0 1,277 Late stage P, PS 18 236-262 235 -0.4 to -0.7 173/46.3 1,243 Late stage P 9 238-167 246 -0.7 to - 1.1 153/125 1,163 2 P 3 270-283 274 -0.5 to -0.8 254/159 1,136 2?, 3? P 5 255-287 270 -0.6 to -0.8 243/153 1,169 3 P '2 262-280 267 -0.2 to -0.3

S 4 183-191 187 -0.6 to -0.7 172/130 1,159 2 P 5 246-275 254 -0.7 to - 1.2 234/129 1,149 3 P 8 236-287 255 -0.7 to - 1.3 202/20.3 1,254 1 P 3 153-187 173 -0.7 to - 1.1 169/50 1,214 2 P 4 230-270 260 -0.6 to -0.9 146/178.2 1,115 2 P 250-264 257 -0.8 86, surface 1,245 2 P 239-256 -0.6 to -0.9 QP7-60 1,200 3 P 235-266 -0.6 to -0.8

146/178.2; 86, surface sample; and QP7-60 data from an unpublished report of the Bureau of Mineral Resources 2 The present surface elevation is around 1,270 m RL 3 Vein types: 1 = pronounced colloform and crustiform banding (chalcedonic and moss bands dominant over crystalline bands), Au interstitial to quartz;

2 = pronounced crustiform and colloform banding (microcrystalline to crystalline bands dominant over chalcedonic and moss bands), well-developed sulfide bands, gold in galena, hessitc, sphalerite, tetrahedrite; 3 = weakly banded crystalline comb quartz carrying sparse base metal sulfide

4p = primary, PS = pseudosecondary, S = secondary

throughout the deposit and strongly developed along faulted host rocks, lithologie boundaries, and within fragmental voleanies (e.g., block and ash flow and daeitie ignimbrite).

Fluid Inclusion Study Fluid inclusions were observed in more than 40 drill core

samples from different intersections of the Scott lode vein. Of these, 23 doubly polished plates contained inclusions suitable for mierothermometrie work. Samples were from 10 to 680 m below the present surface and from the whole strike length of the Scott lode vein (Table 2). Suitable materials were obtained from representative quartz veins hosting base metal sulfides and precious metals within zones I, II, and III.

Thermometrie measurements were made on a U.S.G.S.- designed heating-freezing stage, calibrated with synthetic fluid inclusion standards manufactured by SYNFLINC. The data presented here are reproducible to +0.1C for freezing runs and __3.0C for heating runs. Calculations of fluid salinity and density from the mierothermometrie data were performed using a modified version of the program HALWAT (Nieholls and Crawford, 1985). Nature of fluid inclusions

Studied inclusions mostly varied from 8 to 20/am in length with a few as big as 25/am across. Primary, pseudosecondary, and secondary inclusion recognition was based on the criteria of Roedder (1984) and Shepherd et al. (1985). Most of the

fluid inclusions studied were either of primary or pseudosecondary origin, although in some eases this distinction was difficult to make.

The dominant fluid inclusion type is two phase and liquid rich with gas generally less than 15 percent by volume of the cavity. These inclusions show variable morphology from irregular to regular, equidimensional to elongated. Within a trail of inclusions, one form generally predominates. Gas-rich inclusions contain a large gas bubble which is more than 50 percent by volume of the cavity. A few observed inclusions are three-phase types with liquid, gas, and a captive solid. The solids do not resemble common daughter minerals (e.g., NaC1, KC1, CaSO4). They occur either as an irregular shape or as triangular and rhombohedral crystals but were too small to be optically identified. Laser Raman techniques indicated that they were not carbonate nor sulfate (Etminan et al., 1987).

During freezing studies, melting of solid CO.2 was not observed. Similarly, repeated runs did not detect the presence of COs clathrates. Hedenquist and Henley (1985) have stressed that most of the apparent salinity in fluid inclusion measurements in the epithermal environment is probably due to dissolved CO2 which does not appear as a discrete gas phase. They have shown that the minimum COs concentration necessary for liquid COs to form in a fluid inclusion at 10C is around 2.2 molar in pure water and lower in saline fluids. Absence of clathrate would indicate that the COs con-

1790 BOBIS ET AL.

tent was less than 0.85 molar during entrapment of the inclusions. This could translate to around - 1.0C in freezing point depression measurements (Hedenquist and Henley, 1985). Thus if fluids at Scott lode contain similar amounts of CO2, dissolved CO2 may account for the majority of the last ice- melting temperatures (T .... ) measurements (i.e., -0.5 to -1.0C). However, analysis by mass spectrometer of gases released by thermal deerepitation shows that the inclusions contain dominantly water with very minor CO2 (Etminan et al., 1987). Methane or hydrogen sulfide was below the detec- tion limits. Thus, the extremely low levels of CO2 observed in the Scott lode fluid inclusions will not significantly affect results of T .... measurements. Temperatures of homogenization

Fluid inclusion homogenization temperatures show a con- sistent relationship with depth (i.e., homogenization temperatures generally decrease with decreasing depth; Fig. 12). Ho- mogenization temperatures decreased from a median of 305C at 783 m RL (around 500 m below the present surface) to 218C at 1,252 m RL.

The crystalline comb quartz veins hosting sparse base metal sulfides, which generally occur in zone III, contain a dominant population of liquid-rich inclusions with generally uni- form liquid/vapor ratios. This population of homogenization temperatures varies from 315 to 230C depending on depth (Fig. 13).

Upward from 1,100 to around 1,200 m RL, the crystalline quartz-dominated crustiform-colloform quartz veins hosting precious and base metal mineralization occurring in zone II give homogenization temperatures of 187 to 285C (Fig. 12). The fluid inclusions show a population of coexisting gas- and liquid-rich types. These generally occur within a plane of inclusions, confined within a single quartz crystal and show no evidence of necking down. The ubiquitous presence of these coexisting inclusion types throughout the precious and base metal-bearing veins can be taken as evidence for boiling of the hydrothermal fluid. The homogenization temperatures of gas-rich inclusions could not be observed because of their small size. The biggest population of data for this boiling stage is from 220 to 275C. Boiling is also suggested by vein textures consisting of breccias, adularia, bladed quartz replacing carbonates, and crustiform-colloform banded quartz.

The medium-grained crystalline quartz associated with chalcedonic quartz bands at shallow levels (zone I; 1,200 m RL upward) and hosting precious metal mineralization, also contains a population of coexisting liquid-rich and gas-rich inclusions (Fig. 12). However, the temperatures ofhomogeni- zation are widely scattered. The Th varies from a narrow 153 to 187C subset generally related to fine- or medium-grained crystalline quartz to a large range of filling temperatures of 187 to 265C, related to the more crystalline quartz veins within the crustiform-colloform banded quartz veins. Salinity measurements

Ice-melting temperatures range from -0.6 to -1.6C, indicating that inclusion fluids in quartz are very dilute and contain less than 3 wt percent NaC1 equiv (Table 2). The apparent salinity values average 1.2 wt percent NaC1 equiv

at 783 m RL and 1.75 wt percent NaC1 equiv at 1,252 m RL; this is a 31 percent increase over a 470-m vertical interval. Pressure regime during mineralization

Because the hydrothermal fluids were boiling and the hy- drostatic pressure regime was prevalent in shallow open con- duit systems, the Th of liquid-rich inclusions is used to suggest fluid pressures of approximately 22 bars (220C) at 1,252 m RL, to a maximum of 89 bars (300C) at 967 m RL.

The histograms of homogenization temperatures are plot- ted for all the vein types and adjusted to the hydrodynamic boiling for depth curves characteristic of low-salinity fluid composition (Henley, 1985; Fig. 12). These curves represent temperature-depth relations in the upfiow region in which ascending hydrothermal solutions are governed by hydrodynamic gradients (Hedenquist and Henley, 1985). Erosion has probably removed the markers (e.g., sinters, chalcedonic cap) that can be used to constrain the paleosurface. The position of the histograms with respect to the boiling curves is con- strained by the temperature and vertical intervals between the sample points. The boiling curves were adjusted to best fit samples in which boiling demonstrably occurred. Thus for the 1,100-m RL where boiling is indicated by coexisting gas- and liquid-rich inclusions, the corresponding paleoforma- tional depth is 500 m. These relations place the paleoforma- tional depths of the whole documented vertical interval (i.e., 1,270-780 m RL) to approximately 300 to 800 m below the paleosuface (Fig. 12). Thus the palcosurface elevation at the time of mineralization was a minimum of 300 m above the present-day surface.

Stable Isotope Composition Oxygen isotope ratios were determined from 31 mineral

samples (predominantly quartz, siderite, and kaolinitc) and four weakly propylitized to argillic-altered andesitic rocks. Hydrogen isotope ratios were determined for all the analyzed wall-rock samples. Two SD values were obtained for fluid inclusions from vein quartz. Banded vein quartz was sampled for 51sO determinatidns by cutting a strip across bands from a slabbed drill core. Chalcedonic and crystalline quartz bands were then carefully cut out using a fine diamond-impregnated saw. The quartz samples were crushed and handpicked under a binocular microscope.

Minerals and whole rocks were analyzed using conven- tional preparation techniques and a Micromass 602D mass spectrometer for oxygen and hydrogen. Routine analytical precision for oxygen is +0.1 per mil and hydrogen is +2.0 per mil. Isotopic data on quartz, altered wall rocks, and siderite

The 51sO values of the 27 quartz vein samples show isotopic homogeneity with a narrow range of 4.7 to 7.88 per mil (Table 3). The 51sO values of the mineralizing fluids, based on quartz-water fractionation factors of Matsuhisa et al. (1979), and median fluid inclusion homogenization temperatures range from -0.26 to -6.90 per mil.

The liquid-rich fluid inclusions analyzed were from the economic and subeconomic veins in the Scott lode and have homogenization temperatures of 170 to 315C (Table 2). Fluid inclusion waters were extracted by differential heating

ORE SHOOT, PAJLYGO, QUEENSLAND 1791

present IOO- surfGce

J

E ilOO-

900-

700-

m,RL

18Burned

poleosurfoce

-200

8

--800

m below poleosur face

4.0wt C02

water

eq wt e/e NaCl

I I I I00 200 :00

Temperature, C

!! i histogram - qtz- py- c py- e lec veins histogram- qtz- py - base metal sulf- hess-tenn -elec vein s histogram- qtz- sparse 1:,3se metal veins

FIG. 12. Histograms of fluid inclusion homogenization temperatures from the various quartz veins superimposed on hydrodynamic depth curves for different fluid compositions. Curves from Hedenquist and Henley (1985).

and fluids extracted at 250 to 350C were analyzed for the H and D ratio. It is assumed that the fluids extracted at temperatures between 250 and 350C came dominantly from primary fluid inclusions since the secondary inclusions in quartz homogenize below 150C. The D values of the extracted fluids are -59.5 and -64.9 per rail (mean = -62.0%0; Table 3).

The weakly and strongly propylitized andesitc samples have sO values of 3.94 and 3.56 per mil and D values of -95.20 and -100.90 per rail, respectively (Table 4). The isotopic composition of the pervasively altered rocks has been used to estimate the isotope composition of the fluid in isotopic equilibrium with these rocks. For propylitized andesires, an-

orthite-water (O'Neil and Taylor, 1967) and chlorite-water (Taylor, 1979) fraetionation factors were used to estimate the 5sO and SD composition of the fluid, respectively. For pervasively kaolinized andesites and for kaolinite from the vug infills, kaolinite-water fraetionation factors were used for estimating 5sO and SD values of the fluid. Interpretation of the isotopic results

The calculated 5sO compositions of the fluids show a dear homogeneity among the fluids that are responsible for the various vein types, with values spanning a narrow range of -7 to -1 per mil with only one sample near the 1 per mil contour. These values do not reveal any significant difference

1792 BOBIS ET AL.

o

Z * AW -20 1 '3

-40 'O MWL . o

/ 2,00 3doo I I ,oo/ I I

-100

-120

-140 .... ' .... ' .... ' .... -20 -10 0 10 20

$180 (%0) FIG. 13. Isotopic exchange curves for the analyzed and calculated mete-

oric water (6D = -86%c, 6s0 = -12%c) with andesitic rocks (6D = -70%c, 6s0 = 7%c) at 200 and 300C and various water/rock ratios (0.01.0.1, 1, 10, 100). Stippled rectangle represents isotopic composition of the fluid responsible for the precipitation of vein quartz. Points marked "k" represent a fluid in equilibrimn with kaolinitc (Table 4) and points marked "o" represent fluids in equilibrimn with propylitized andesitc (Table 4). The points 3, 4, and 5 represent meteoric fluid equilibrated with unaltered andesires at very low water/rock ratios at 300 , 400 , and 500C. The point AW corresponds with Giggenbach's (1992) andesitic water. A line joining AW with the initial meteoric water will indicate the isotope mixing line.

between the economic and subeconomic veins and fail to delineate any distinct cooling trend.

In most epithermal deposits, a close similarity between 6isO and 6D compositions of the fluids and moderu-day meteoric water is used to indicate meteoric derivation of the epithetreal fluids. The ]sO and D depletion in the 'altered rocks and the corresponding shift in the 6]SO composition of the fluid is used as supporting evidence. These shifts in the isotopic compositions are explained by an interaction between meteoric water and the surrounding volcanic or sedimentary rocks. Although meteoric water has been proposed as the dominant source of epithetreal fluids in a number of epithermal deposits (e.g., Comstock lode district), a significant com- ponent of hypothetical magmatic water has not been ruled out (O'Neil and Silberman, 1974).

In the case of younger epithetreal deposits (e.g., the Ter- tiary deposits of the Great basin, Nevada), the isotopic composition of present-day meteoric water is reliably known but for geologically older epithetreal systems, such as Pajingo, this information is not available. This, together with uncertainties about the temperature of formation, the equilibrium isotope fractionation factors, and the models of water-rock interaction, complicates any reliable interpretation of the isotopic colnpositions. In a recent paper, Giggenbach (1992) has ques- tioned the basis for arguing that meteoric waters are the source of many geothermal and epithetreal fluids. Based on a study of isotopic shifts in waters from a number of geother-

TABLE 3. Oxygen and Hydrogen Isotope Data (%c) for Hydrothermal Quartz and Carbonate Samples from the Scott Lode Deposit Elevation 6Dwater

Drill holehn (m BL) Sa,nple description 61SOo,.k 61SOw.acr (extracted fluid) JMD 171/14.2 1,271 JMD 220/39.7A 1,257 JMD 220/39.7B 1,257 JMD 171/32.4A 1,254 JMD 171/32.4B 1,254 JMD 202/26 1,254 JMD 173/37.6 1,251 JMD 221/72.5 1,232 JMD 171/58 1,230 JMD 163/94 1,198 SRD 242/45.6 1,214 JMPD 84/94.45 1,203 MJD 204/102 1,190 JMPD 103/106.3 1,190 JMD 188/81 1,187 JMPD 181/110 1,162 JMPD 101/133.5 1,156 JMPD 146/153.2 1,155 JMPD 146/158.9 1,122 JMPD 101/139.8 1,151 SRD 234/129 1,149 JMPD 101/148 1,145 JMPD 146/178 1,135 JMPD 176/256 1,088 SRD 291/381 1,000 SRD 291/424 967 SRD 29'2/674.3 783 JMPD 176/256.8 1,257

Chalecdony quartz in banded quartz veins 4.7 -6.9 (200 ) Chalcedony quartz in banded quartz veins 7.8 -3.8 (200 ) Crystalline quartz in banded quartz veins 6.9 -4.7 (200 ) Chalcedony quartz in banded quartz veins 6.4 -4.1 (218 ) Crystalline quartz in banded quartz veins 5.6 -4.9 (200 ) Chalcedony quartz in banded quartz veins 6.5 -5.2 (200 ) Chalcedony quartz in banded quartz veins 6.5 -2.6 (246 ) Chalcedony quartz in banded quartz veins 7.7 -2.1 (232 ) Crystalline comb quartz, late vein infill 5.3 -5.2 (218 ) Chalcedony quartz in banded quartz vein 6.6 -3.7 (222 ) Crystalline quartz + chalcedony + electrum 4.8 -6.6 (203 ) Crystalline quartz + sulfide in banded vein 5.9 -3.6 (237 ) Crystalline quartz + sulfide 5.3 -3.3 (256 ) Crystalline quartz + sulfide 6.1 -3.6 (237 ) Crystalline quartz in brecciated chalcedony vein 6.4 -3.2 (235 ) Crystalline quartz in brecciated quartz vein 7.0 -1.9 (254 ) Crystalline quartz + sulfide 5.0 -3.8 (254 ) Milky crystalline quartz 4.7 -3.9 (257 ) Crystalline quartz + sulfide in banded vein 7.1 -1.5 (257 ) Crystalline quartz + sulfide in banded vein 5.7 -3.2 (250 ) Crystalline quartz + carbonate infill 7.0 -2.0 (249 ) Crystalline quartz + sulfide in banded vein 5.8 -2.9 (255 ) Crystalline quartz + sulfide in banded vein 5.2 -3.7 (250 ) Crystalline quartz in banded quartz vein 6.5 -1.5 (270 ) Crystalline comb quartz tips 6.2 -1.6 (275 ) Crystalline comb quartz 7.4 -0.3 (291 ) Crystalline quartz comb quartz 5.3 -1.4 (305 ) Siderite-calcite ankerite 10.2 -2.0 (249 )

-65

-60

Oxygen isotope composition of water was calculated using the equilibrium fractionation factor by Matsuhisa et al. (1979); numbers in parentheses indicate temperatures in C used in the calculations; oxygen isotope composition of water in equilibrium with carbonates was calculated using calcite-water fraetionation f;aetor by O'Neil et al. (1969); hydrogen isotope composition of water was analyzed for fluid inclusions extracts obtained at temperatures between 250 and 350C


T^BL; 4. Oxygen and Hydrogen Isotope Composition (%0) of Altered Rocks Elevation 6SO,,t tSD,vat

Drill hole/m (m BL) Sample description 61SOro& 6Dok (calculated) SRD 291/235.4 1,112 SRD 236/48.8 1,209 SRD 231/119 1,159 JMPD 101/194 1,112 JMPD 204/97 1,193 JMPD 60/140.05 1,164 JMPD 172/187 1,116 SRD 293/415.9 968

Weakly propylitized andesitc 3.9 -95 -0.9 -51 Strongly propylitized rock 3.6 - 101 - 1.3 -57 Intermediate argillic-altered andesitc 6.7 -94 Pervasive kaolinRe _+ illitc 'altered andesRe 7.6 -98 Argillized andesitc -77 Argillized andesitc -73 Kaolinitc concentrate; replaces adularia + vug infill 10.1 -91 -1.0 -86 Kaolinite concentrate; replaces adularia + vug infill 8.0 -94 -3.1 -89

Bulk of the samples were analyzed in the Division of Exploration Geoscience, CSIRO, North Ryde, Australia; JMPD 204/97.0 and JMPD 60/140.05 were analyzed in the Bureau of Mineral Resources, Canberra (Etminan et al., 1987); isotope composition of water in equilibrium with propylitized andesitc has been calculated using anorthite-water (for oxygen) and chlorite-water fractionation factor by O'Neil and Taylor (1967) and Taylor (1979); isotope composition of water in equilibrium with kaolinitc concentrate has been calculated at 150C using kaolinitc-water fraction factors for oxygen and hydrogen, tabulated in Rey et al. (1992)

Present surface elevation is around 1,270 rn RL

mal and volcanic systems along convergent plate boundaries, Giggenbach (199) has highlighted the presence of significant deuterium shifts and has proposed the generation of andesitic waters with 6sO and 6D values of 10 and -0 per mil, respectively. The oxygen and deuterium shifts observed in the geothermal waters are explained as a result of mixing between meteoric and andesitic waters.

In the Pajingo epithetreal system, the altered rocks show a depletion in both SO and D compared to the unaltered andesires (Table 4). The calculated 6SO and 6D values of the fluid responsible for the formation of quartz in the Scott lode lie in a region between the meteoric water line and hypothetical magmatic water (Fig. 14). Hence, it can be ar- gued that meteoric water was a dominant soume of water in these fluids, but in order to establish whether this meteoric water underwent only oxygen shift or both oxygen and deuterium shifts, it is necessary to know the isotopic compositions of the paleometeorie waters.

Sun and Eadington (1987) reported that Permian meteoric water, occurring as liquid-rich fluid inclusions in late quartz- sphalerite-galena veins in the Mole Granite, New South Wales, has a 6sO value of -15 per mil and 6D of -110 per mil. This meteoric water is isotopically light because of the near-polar (70 ) paleolatitude of eastern Australia in the Per- mian (Golding and Wilson, 1984). The south polar paleoconti- nental maps showing the position of Australia in the late Paleozoic indicate that Pajingo would be located around 50 to 55 paleolatitude during the mid-Carboniferous epoch. This means that the meteoric waters at Pajingo are expected to be heavier because the position of the Pajingo area was more equatorial than the Permian meteoric waters. In this paper meteoric water with 5sO and 6D values of -1 and -86 per rail has been assumed.

The changes in the isotopic composition of meteoric water have been calculated by using Taylor's model (Taylor, 1979) modified by Field and Fifarek (1985). In the water-rock interaction calculations, the anorthite-water (O'Neil and Taylor, 1967) and chlorite-water (Taylor, 1979) equilibrium fraetionation factors have been used to model the partitioning of 5sO and 6D between fluid and rocks. The 6sO and 6D compositions of the unaltered andesitic rocks have been as-

sumed to be of 7.0 and -70.0 per mil, respectively (Field and Fifarek, 1985).

The isotope evolution curves for meteoric water were calculated at 100 , 200 , and 300C (Fig. 13; only 200 and 300C are shown for illustrative purposes). Although, the isotopic composition of the fluid responsible for the Scott lode mineralization (narrow rectangle in Fig. 13), lies close to the calculated curves, the water/rock ratios of

1794 oI$ ET AL.

-5

300

a) % Fluid boiled 10 20 30

H2 (g) :::t - ! - . - [ - i - ,

280 260 240 220 200 180

30000

20000

10000

c)

% Fluid boiled 10 20 30

I!1 Adularia

300 280 ;20 240 220 200 180

-6

-7

-8

-9

-lO

3o0

b)

I

H+

20 30

280 260 240 220 ZOO 180

d) 30 10 20 30

Temperature (C) Temperature (C) FIG. 14. Results of chemical equilibrium computations modeling transport, and precipitation of the significant species

and mineralogical components associated with adiabatic boiling of a fluid saturated with respect to gold, galena, and sphalerite and undersaturated with respect to silver at 300C. (a). Changes in the activity of the dissolved gases. Fugacity of H2 reflects the changes in the redox state of the fluid on boiling. (b). Changes in pH and the molalities of total dissolved Au, Ag, Pb, and Zn. (c). Amount of gangne minerals (g/t) precipitating during boiling. (d). Amount of gold, silver, zinc (as ZnS) and lead (as PbS) precipitating during boiling.

ported by the presence of magmatic bodies in the northeast- trending corridor of Permo-Carboniferous felsic to intermediate subvolcanic intrusions.

Transport and Metal Precipitation Mechanisms Fluid inclusion studies indicate that precious and base

metal mineralization developed at the Scott lode as the hydrothermal fluids cooled from 275 to 315C to 170 to 260C. The homogenization temperatures decrease with decreasing depth. Cooling by boiling is indicated by the presence of coexisting liquid- and gas-rich fluid inclusions in mineralized quartz, extensive vein brecciation, vein adularia, bladed calcite, and pronounced crustiform-colloform banding in quartz veins.

Chemical equilibrium and mass balance calculations were carried out to model adiabatic boiling of a fluid closely resembling the ore-froming fluid in Pajingo (Table 5). Thermody- namic data on the stability of various aqueous species used

in these calculations have been obtained from the datafile SOLTHERM (Reed, 1982) which was modified by Heinrich (1987) to be compatible with the datafile CPDMRL of the CSIRO-SGTE-THERMOCHEMISTRY system (Turnbull and Wadsley, 1988) and with the data on aqueous ions calculated by Cobble et al. (1982). Data for chloride complexes of gold and silver were taken from Helgeson (1969) and Seward (1973), respectively. For sulfide complexes of gold and silver, data were obtained from Shenberger and Barnes (1989) and Gammons and Barnes (1989), respectively. Thermodynamic data on solids were taken from Bowers et al. (1984) and Helgeson (1969). Activity coefficients for the charged aqueous species were estimated using the extended Debye-Huckel equation of Helgeson (1969). The distance of closest approach (i.e, parameter ft in angstroms) for AgC1-2, AuCI-, Au(HS)-, Ag(HS)-, and Au(HS)-'2 was taken to equal 4.0, 4.0, 5.0, 4.0, and 4.0, respectively (Seward, 1976). The fi for other charged aqueous species were obtained from Truesdell

ORE SHOOT, PAJINGO, QUEENSLAND

TABLE 5. Fluid Composition Used in the Calculations

1795

Adularia-sericite deposits

Scott lode

Determined Rotokawa Broadlands (adopted value) Estimated

T (C) 200-310 (250)* Salinity (vet % NaC1 equiv) 0-8.0 (1.6) CO, (moles/kg)

1796 BOBIS ET AL.

TABLE 6. Adiabatic Boiling of the Scott Lode Hydrothermal Fluid

Temperature (C) 300 292 285 274 259 252 242 230 216 206 181 % Fluid boiled 0 2.36 4.30 7.30 11.52 13.06 15.40 18.06 21.00 23.00 27.62 pH 5.81 6.07 6.20 6.31 6.38 6.39 6.41 6.43 6.46 6.49 6.57

Aqueous species (log molality) C1- - 1.04 -0.66 -0.64 -0.62 -0.59 -0.58 -0.56 -0.54 -0.52 -0.50 -0.47 SOl- -8.51 -6.03 -5.11 -4.30 -4.20 -4.12 -4.04 -3.97 -3.91 -3.88 -3.83 HS- -4.51 -4.40 -4.35 -4.33 -4.36 -4.38 -4.40 -4.42 -4.44 -4.45 -4.47 H2S -2.50 -2.70 -2.90 -3.10 -3.40 -3.40 -3.60 -3.80 -3.90 -4.10 -4.40 CO.- -8.47 -8.20 -8.11 -7.81 -7.65 -7.59 -7.51 -7.40 -7.27 -7.17 -6.89 CO2 -1.00 -1.42 -1.65 -1.9 -2.2 -2.35 -2.52 -2.70 -2.9 -3.05 -3.40 CH4 -4.47 -2.73 -2.89 -3.11 -3.38 -3.48 -3.62 -3.78 -3.97 -4.09 -4.40 HC1 -5.07 -5.47 -5.72 -6.02 -6.36 -6.48 -6.66 -6.87 -7.10 -7.27 -7.66 SiO -2.03 -2.05 -2.07 -2.11 -2.16 -2.19 -2.22 -2.28 -2.34 -2.39 -2.51 ZK -1.22 -1.21 -1.20 -1.19 -1.17 -1.16 -1.15 -1.13 -1.12 -1.10 -1.08 ZNa -0.71 -0.70 -0.69 -0.68 -0.66 -0.65 -0.64 -0.63 -0.61 -0.60 -0.57 ZCa -3.74 -3.76 -3.74 -3.71 -3.69 -3.70 -3.67 -3.65 -3.64 -3.62 -3.60 ZFe -5.39 -6.25 -6.65 -6.99 -7.30 -7.40 -7.52 -7.67 -7.84 -7.96 -8.23 ZA1 -4.91 -5.02 -5.10 -5.23 -5.40 -5.46 -5.56 -5.67 -5.78 -5.86 -6.01 AuCL; -12.34 -12.63 -12.80 -13.00 -13.44 -13.61 -13.89 -14.24 -14.68 -15.01 -15.91 Au(HS) -7.72 -7.71 -7.70 -7.74 -7.99 -8.09 -8.25 -8.42 -8.63 -8.78 -9.16 AgCI -6.19 -6.19 -6.19 -6.39 -6.81 -6.96 -7.21 -7.49 -7.83 -8.08 -8.69 Ag(HS) -8.00 -7.67 -7.48 -7.50 -7.75 -7.85 -7.99 -8.16 -8.34 -8.47 -8.76 Zn + -10.32 -10.71 -10.92 -11.09 -11.20 -11.23 -11.28 -11.33 -11.41 -11.48 -11.68 ZnC] + -6.59 -7.14 -7.49 -7.89 -7.33 -8.49 -8.74 -9.03 -9.39 -9.65 -10.30 ZnC] - -6.63 -7.18 -7.54 -7.96 -8.40 -8.56 -8.81 -9.09 -9.43 -9.68 -10.33 ZnCl- -7.08 -7.53 -7.80 -8.12 -8.51 -8.66 -8.94 -9.27 -9.70 -10.03 -10.85 Zn(HS)2 -6.27 -6.60 -6.85 -7.20 -7.66 -7.82 -8.08 -8.36 -8.67 -8.88 -9.35 Pb + -10.41 -10.80 -11.00 -11.18 -11.29 -11.33 -11.39 -11.48 -11.61 -11.72 -12.06 PbCI+ -8.44 -8.91 -9.18 -9.45 -9.71 -9.80 -9.95 -10.12 -10.35 -10.54 -11.03 PbCI- -7.65 -8.24 -8.61 -9.04 -9.50 -9.66 -9.91 -10.20 -10.53 -10.78 -11.41 PbC14 - -9.89 -10.38 -10.66 -10.94 -11.19 -11.26 -11.38 -11.52 -11.69 -11.83 -12.22 Pb(HS) -9.64 -9.79 -9.86 -9.90 -9.88 -9.85 -9.80 -9.70 -9.59 -9.52 -9.40

Gasses (log fugacity) H2 -1.284 -1.86 -2.08 -2.27 -2.47 -2.53 -2.64 -2.77 -2.92 -3.03 -3.31

Minerals (g/t) Quartz ( x 103) 939 967 980 984 985 987 988 989 990 991 K felsdpar 20,479 17,730 14,906 12,345 11,623 10,650 9,762 8,930 8,442 7,561 Pyrite 9,845 6,063 3,719 2,435 2,168 1,857 1,615 1,417 1,312 1,140 Calcite 28,870 8,194.9 0 0 0 0 0 0 0 0 Zns 1,589 1,145 773 529 474 408 356 312 289 251 PbS 107 73 47 31 28 24 21 18 17 15 Au 0 0 3.18 9.9 10.49 10.66 10.35 9.77 9.35 8.46 Ag 0 0 201.5 267.8 260.0 241.2 219.4 197.4 184.5 161.7

an extensional brittle regime. The ore shoot occurs in a 2.5- kin-long generally northeast-striking structure and was developed in a dilational zone created by the intersection of east- west normal faults and northeast-striking accommodation normal faults. Extension direction is north-northeast.

3. The three-dimensional characterization of the veins at Scott lode shows that distinct quartz textures which are verti- cally zoned accompany ore shoot emplacement. The textural zoning, in conjunction with the metal ratios and styles of mineralization, can be used as a tool to estimate the relative vertical position of any vein sample in the Pajingo field and potentially to estimate those of other epithermal systems as well. The significant quartz textural indicators of good gold grades include: increased vein brecciation, presence of moss- needle adularia, moss quartz, and pronounced crustiform- colloform banding in quartz. The following textural features effectively indicate limits of gold mineralization: decrease in the intensity or absence of crustiform-colloform banding in quartz, paucity of vein brecciation, crystalline comb quartz

becoming more dominant, paucity of adularia, and sparse sulfides.

4. It is inferred that the main mechanism responsible for the precipitation of precious and base metals at the Scott lode is boiling. The boiling of hydrothermal fluids has been established from coexisting liquid- and gas-rich inclusions (without evidence of necking), widespread vein breeeiation, and presence of adularia in the veins. Thermodynamic modeling of adiabatic boiling of the ore fluid at the Scott lode indicates that this fluid could have deposited ores containing an average of 10 ppm gold and 260 ppm silver. The gold/ silver ratio of the ores would have increased with continuous boiling which possibly represents the trend commonly observed in man 5. The 51sOy epithermal deposits. values of the fluid in equilibrium with vein quartz as calculated from quartz values and median homogenization temperatures lie within the range -7.0 to -1.0 per mil. The SD values of fluid extracted from primary inclusions in quartz range between -59.5 and -64.9 per mil. The eom-

ORE SHOOT, PA]IZVGO, 0 UENSLAND 1797

positions of altered rocks show significant depletion in oxygen and deuterium which can be attributed to isotopic reactions with meteoric water. It is suggested that the epithermal fluid at Pajingo was the result of mixing between meteoric water and isotopic magmatic water.

Acknowledgments This study forms part of a project, Epithermal Gold Depos-

its in Queensland, funded by the Australian Mineral Industry Research Association (AMIRA). The member companies, particularly Battle Mountain (Australia), Inc., are thanked for funding this postgraduate study. R.E.B. benefited from discussions with R. Porter, K. Washburn, I. Campbell, and B. Jones of Battle Mountain. We also benefited from discussions with the academic, research, and support staff of the Geology Department of James Cook University, particularly M. Rubenach, C. Cuff, R. Henderson, S. Ness, M. Worsley, D. Guoyi, A. Aliibone, M. Hinman, and W. Huang. R.E.B. is grateful to P. Mckibben of RGC Exploration Pty. Ltd. for providing additional funding. The bulk of the oxygen isotope analyses was performed by A. Andrew in the stable isotope laboratory of the Division of Exploration Geoscience of CSIRO, in North Ryde, Australia. Some of the oxygen and hydrogen isotope analyses were performed in the Division of Petrology and Geochemistry of the Australian Bureau of Mineral Resources in Canberra by H. Etminan. B. Harrold assisted with the drafting of Figures 13 and 14. The manu- script has benefited considerably from the critical reviews of Economic Geology referees.

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Anatomy of an Epithermal Ore

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