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R Sharpe - One of the best experts on this subject based on the ideXlab platform.
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alteration characteristics of the archean golden grove formation at the Gossan hill deposit western australia induration as a focusing mechanism for mineralizing hydrothermal fluids
Economic Geology, 2001Co-Authors: R Sharpe, Bruce J GemmellAbstract:The Archean Golden Grove Formation is a 550-m-thick rhyodacitic tuffaceous volcaniclastic succession that hosts the Cu-Zn-rich Gossan Hill volcanic-hosted massive sulfide (VHMS) deposit in the Yilgarn craton, Western Australia. The Golden Grove Formation consists of volcanic quartz and altered pumice and shards, which were deposited during successive episodes of subaqueous mass flow. Coherent volcanics are absent from the Golden Grove Formation at Gossan Hill but form the main rock type in the hanging-wall Scuddles Formation. A massive dacite dome that overlies its volcanic feeder dike is inferred to occupy a synvolcanic structure that focused mineralizing fluids during the formation of the Gossan Hill deposit. The Gossan Hill deposit consists of two stratigraphically separate ore zones interconnected by stockwork. The Cu-rich lower ore zone and the Zn-rich upper ore zone occur in the middle and upper parts of the Golden Grove Formation, respectively. Podiform zones of massive magnetite occur in the Cu-rich ore zone, where the formation of magnetite predates massive sulfide. The asymmetry of massive sulfide, massive magnetite, and alteration zones at the Gossan Hill deposit attest to synvolcanic structural control during mineralization. The principal lithofacies of the Golden Grove Formation are sandstone and pebble breccia, which have a regionally extensive quartz, Fe-rich chlorite, and lesser muscovite alteration. At Gossan Hill, this alteration has resulted in near-complete replacement of tuffaceous components, causing substantial chemical modification of the primary lithologies. Quartz-chlorite (±muscovite) alteration is characterized by severe K2O, Na2O, and CaO depletion, with rocks consisting principally of SiO2, FeO, Al2O3, and MgO, along a quartz-chlorite mixing trend. Widespread preservation of pumice and shard volcanic textures within the Golden Grove Formation indicates that quartz-chlorite (±muscovite) alteration occurred soon after, or possibly during, sedimentation. The absence of diagenetic compaction textures further suggests induration of the succession during this early alteration stage, with the tuffaceous succession largely sealed from the texturally destructive effects of subsequent hydrothermal alteration, except where mineralizing fluids were locally channeled along synvolcanic feeder conduits. Local intense hydrothermal alteration zones surround the Gossan Hill deposit and overprint earlier quartz-chlorite (±muscovite) alteration. These local alteration zones have the same extent as the sulfide vein envelope and represent hydrothermal alteration formed during sulfide-magnetite mineralization. Intense Fe-rich chlorite (ankerite-siderite) alteration occurs as a strata-bound envelope around massive magnetite, Cu-rich veins, and massive sulfide in the lower ore zone. This chlorite-rich alteration has strong FeO and MgO enrichment with minor chloritoid and andalusite that reflect intense acid leaching during hydrothermal alteration. Iron chlorite (ankerite-siderite) alteration grades upward into discordant to strata-bound intense quartz alteration. Intense quartz alteration forms an envelope around Zn-rich veins and massive sulfide in the stockwork and upper ore zone. The trend from Fe chlorite-ankerite-siderite to quartz alteration toward the top of the deposit is consistent with the cooling of hydrothermal mineralizing fluids nearing the sea floor. Rhyodacite and dacite volcanics of the hanging-wall Scuddles Formation have a pervasive muscovite-calcite alteration. Muscovite-calcite alteration led to Na2O depletion and CaO and K2O enrichment associated with burial of the Gossan Hill mineralizing system. We propose that the Gossan Hill sulfide-magnetite VHMS deposit formed during an evolving Archean hydrothermal system that began as part of a regional-scale, low-temperature seawater convection-alteration system. Initially, this system caused extensive replacement of the Golden Grove Formation by quartz and Fe chlorite (±muscovite); a process that sealed and indurated the volcaniclastic rocks by infilling of primary porosity and permeability structures. Due to subsequent impermeability of the host-rock succession, later and hotter mineralizing fluids that generated alteration and massive magnetite and sulfide at the Gossan Hill deposit were constrained to, and focused upward along, a synvolcanic feeder structure.
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sulfur isotope characteristics of the archean cu zn Gossan hill vhms deposit western australia
Mineralium Deposita, 2000Co-Authors: R Sharpe, J B GemmellAbstract:Gossan Hill is an Archean (∼3.0 Ga) Cu–Zn–magnetite-rich volcanic-hosted massive sulfide (VHMS) deposit in the Yilgarn Craton of Western Australia. Massive sulfide and magnetite occur within a layered succession of tuffaceous, felsic volcaniclastic rocks of the Golden Grove Formation. The Gossan Hill deposit consists of two stratigraphically separate ore zones that are stratabound and interconnected by sulfide veins. Thickly developed massive sulfide and stockwork zones in the north of the deposit are interpreted to represent a feeder zone. The deposit is broadly zoned from a Cu–Fe-rich lower ore zone, upwards through Cu–Zn to Zn–Ag–Au–Pb enrichment in the upper ore zone. New sulfur isotope studies at the Gossan Hill deposit indicate that the variation is wider than previously reported, with sulfide δ34S values varying between −1.6 and 7.8‰ with an average of 2.1 ± 1.4‰ (1σ error). Sulfur isotope values have a broad systematic stratigraphic increase of approximately 1.2‰ from the base to the top of the deposit. This variation in sulfur isotope values is significant in view of typical narrow ranges for Archean VHMS deposits. Copper-rich sulfides in the lower ore zone have a narrower range (δ34S values of −1.6 to 3.4‰, average ∼1.6 ± 0.9‰) than sulfides in the upper ore zone. The lower ore zone is interpreted to have formed from a relatively uniform reduced sulfur source dominated by leached igneous rock sulfur and minor magmatic sulfur. Towards the upper Zn-rich ore zone, an overall increase in δ34S values is accompanied by a wider range of δ34S values, with the greatest variation occurring in massive pyrite at the southern margin of the upper ore zone (−1.0 to 7.8‰). The higher average δ34S values (2.8 ± 2.1‰) and their wider range are explained by mixing of hydrothermal fluids containing leached igneous rock sulfur with Archean seawater (δ34S values of 2 to 3‰) near the paleoseafloor. The widest range of δ34S values at the southern margin of the deposit occurs away from the feeder zone and is attributed to greater seawater mixing away from the central upflow zone.
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the archean cu zn magnetite rich Gossan hill vhms deposit western australia evidence of a structurally focussed exhalative and sub seafloor replacement mineralising system
1999Co-Authors: R SharpeAbstract:The Archean Cu-Zn Gossan Hill volcanic-hosted massive sulphide deposit is situated on the northeast flank of the Warriedar Fold Belt in the Yilgarn Craton, Western Australia. The deposit is hosted within re-deposited rhyodacitic tuffaceous volcaniclastics of the Golden Grove Formation and is overlain by rhyodacite-dacite lavas and intrusive domes of the Scuddles Formation. The Gossan Hill deposit consists of two discrete subvertical ore zones situated stratigraphically 150 m apart in the middle and upper Golden Grove Formation. The stratigraphically lower Cu-rich ore zone (7.0 Mt @ 3.4% Cu) consists of stratabound, podiform to discordant massive pyrite-chalcopyrite-pyrrhotite-magnetite. In addition to massive sulphides, the lower ore zone also contains discordant to sheet-W,e zones of massive magnetite-carbonate-chlorite-talc (-12 }\·ft). The upper Zn-Cu ore zone (2.2 Mt @ 11.3% Zn, 0.3% Cu, 15 glt Au and 102 glt Ag) is mound-shaped with sheetW, e, stratabound, massive sphalerite-pyrite-chalcopyrite overlying discordant massive pyrite-pyrrhotite-chalcopyrite-magnetite. A sulphide-rich vein stockwork connects the upper and lower ore zones. Metal zonation grades from Cu-Fe (±Au) in the lower ore zone to Zn-Cu-nch sulphides at the base of the upper ore zone. The upper ore zone grades upwardS and laterally from Zn-Cu to Zn-Ag-Au (tCu, tPb)-rich sulphides. Regional preservation of primary tuffaceous volcanic textures within the Golden Grove Formation is attributed to an early syndepositional, quartz-chlorite alteration. Induration and differential permeabilityI porosity reduction of the succession during the early alteration W,ely promoted more-focussed pad1\vays for successive hydrothermal fluids. Subsequent hydrothermal alteration related to mineralisation at Gossan Hill has a limited lateral extent, and forms a narrow Fe-chlorite-ankerite-siderite envelope to the massive magnetite and sulphide of the lower ore zone, and an intense siliceous envelope surrounding d,e stockwork and upper ore position. Pervasive calcite-muscovite alteration is recognised Ul d,e hangingwall volcanics of the ScuddIes Formation. The nature of deformation and metamorphism (greenschist facies: 454 t 4°C at I kbar based on andalusite-chloritoid-quartz equilibrium) is uniform throughout d,e massive magnetite, massive sulphide and host succession. Sedllnent-sulphide-magnetite relationships at Gossan Hill suggest d,e formation of magnetite and sulphide during deposition of the upper Golden Grove Formation. Massive magnetite formed entirely by sub-seafloor replacement processes as inferred from gradational upper and lower contacts and interdigitating volcaniclastics. Replacement occurred along permeable tuffaceous strata outward from a discordant feeder. Massive magnetite was later veuled, replacedand cut by massive sulphide. The synchronous formation of both upper and lower sulphide ore zones is indicated by the connecting sulphide stockwork. Both sulphide ore zones formed by sub-seafloor replacement, although stratiform hydrothermal chertsulphide- sediment layers in, and adjacent to, the upper sulphide zone attest to some exhalation of fluids onto the seafloor. The thickest occurrence of massive magnetite, massive sulphide and stringer stocb.-work spatially coincide and support a common feeder conduit during massive magnetite and sulphide mineralisation. The asymmetry of hydrothermal alteration envelopes, massive magnetite and massive and veins sulphide zones are consistent with synvolcanic structural controls, with a growth structure occupied and obscured by a younger dacite dome from the Scuddles Formation. A systematic increase in sulphide 8;4S values (range of -4.0 to 7.8%0, average 2.1 ± 1.7%0) stratigraphically upwards through massive and vein sulphide is suggestive of progressive mixing of upwelling ore fluids with entrained seawater. Homogeneous 8"s values of -1.5%0 in the lower ore zone have a consistent homogeneous rock sulphur source with possible magmatic contributions. The 8180 H20 values of ore fluids responsible for deposition of magnetite in massive magnetite and disseminated magnetite in the sulphide zones range from 6%0 to 13%0. This data is inconsistent with the direct input of Archean seawater, and favours derivation of hydrothermal fluids by rock buffering of circulating fluids, or by direct magmatic contribution. Thermodynamic considerations suggest massive magnetite and sulphide formed from high temperature (300° to 350°C), reduced (low f 0,), slightly acidic hydrothermal fluids. HzSdeficient fluids formed massive magnetite, whilst HzS-rich fluids formed massive sulphides. Fluid chemistry differences are attributed to magmatic sulphur contributions during sulphide mineralisation. Precipitation of sub-seafloor sulphide in the lower ore zone resulted from chemical entrapment by the interaction of upwelling HzS-rich fluids with pre-existing massive magnetite. It is suggested that shallow parental magma chambers to the Scuddles Formation drove hydrothermal convection of seawater and may have supplied volatiles and HzS to the ascending hydrothermal fluids. The Gossan Hill sulphide-magnetite deposit represents an evolving hydrothermal system in an environment characterised by rapid volcaniclastic sedimentation and changing structural and magmatic processes. An important influence on this hydrothermal system was the creation and destruction of porosity and permeability in the host succession. The hydrothermal system initiated as part of a regional seawater convection-alteration system that led to VHMS mineralisation at Gossan Hill by (1) synsedimentary metasomatism and progressive heating of convecting fluids, (2) formation of massive magnetite by host rock replacement above a buried synvolcanic conduit, and (3) structural re-activation and tapping of deeper HzS-rich and metal-bearing fluids, leading to the sub-seafloor sulphide replacement and local exhalation of hydrothermal fluids forming sulphide and chert. Burial by proximal felsic volcanism led to preservation of the deposit.
Carmelo Gomez - One of the best experts on this subject based on the ideXlab platform.
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The high-grade Las Cruces copper deposit, Spain: a product of secondary enrichment in an evolving basin
Mineralium Deposita, 2017Co-Authors: Fernando Tornos, Nieves Gomez-miguelez, Juan Manuel Escobar, Francisco Velasco, John F. Slack, Antonio Delgado, Carmelo GomezAbstract:The Las Cruces deposit (Iberian Pyrite Belt) includes a large, high-grade cementation zone capped by unusual rocks that contain carbonates, galena, iron sulphides, and quartz. Between the Late Cretaceous(?) and Tortonian, the volcanogenic massive sulphides were exhumed and affected by subaerial oxidation that formed paired cementation and Gossan zones. Onset of Alpine extension produced accelerated growth of the cementation zone along extensional faults, leading to formation of the high-grade copper ore at ca. 11 Ma. Later, replacement of the overlying Gossan by sulphide- and carbonate-rich rocks beneath sealing marl sediments is thought to have involved microbial processes, occurring between the Messinian (ca. 7.2 Ma) and today. Isotope data show that the cementation zone formed by the mixing of descending acidic waters derived from oxidation of the massive sulphides, with upwelling geothermal waters flowing at temperatures above 100 °C. The C, O, and Sr isotope values of the mineralization (^87Sr/^86Sr 0.7101–0.7104) and of the local groundwater (0.7102–0.7104) reflect equilibration with basement rocks, and indicate that influence on the ore-forming process by marl-equilibrated water (0.7091–0.7093) or Miocene seawater (0.7086–0.7092) was negligible. The high sulphur isotope values of the sulphides in the biogenic zone (most +19 to +24 ‰) are well above those of the primary sulphides (δ^34S ca. −6.8 to +10.3 ‰) and likely reflect formation of the biogenic sulphides by reduction of aqueous sulphate in the groundwaters. Sulphur isotope values of the cementation zone (δ^34S ca. −2.4 to +21.7 ‰) are also consistent with some contribution of sulphur from the biogenic reduction of aqueous sulphate.
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mineralogical evolution of the las cruces Gossan cap iberian pyrite belt from subaerial to underground conditions
Ore Geology Reviews, 2017Co-Authors: Lola Yesares, Carmelo Gomez, R Saez, G R Almodovar, Jose Miguel Nieto, Gobain OvejeroAbstract:Abstract The Las Cruces VMS deposit is located at the eastern corner of the Iberian Pyrite Belt (SW Spain) and is overlain by the Neogene–Quaternary sediments of the Guadalquivir foreland Basin. The deposit is currently exploited from an open pit by Cobre Las Cruces S.A., being the supergene Cu-enriched zone the present mined resource. The Las Cruces orebody is composed of a polymetallic massive sulfide orebody, a Cu-rich stockwork and an overlying supergene profile that includes a Cu-rich secondary ore (initial reserves of 17.6 Mt @ 6.2% Cu) and a Gossan cap (initial reserves of 3.6 Mt @ 3.3% Pb, 2.5 g/t Au, and 56.3 g/t Ag). The mineralogy of the Las Cruces weathering profile has been studied in this work. Textural relationships, mineral chemistry, deposition order of the minerals and genesis of the Las Cruces Gossan are described and discussed in detail. A complex mineral assemblage composed by the following minerals has been determined: carbonates such as siderite, calcite and cerussite; Fe-sulfides including pyrite, marcasite, greigite and pyrrhotite; Pb–Sb sulfides and sulfosalts like galena, stibnite, fuloppite, plagionite, boulangerite, plumosite, and the jordanite–geocronite series, Ag–Hg–Sb sulfides and sulfosalts including miargyrite, pyrargyrite, sternbergite, acanthite, freibergite, cinnabar, Ag–Au–Hg amalgams; and Bi–Pb–Bi sulfides and sulfosalts such as bismuthinite, galenobismutite, others unidentified Bi–Pb-sulfosalts, native Bi and unidentified Fe–Pb–Sb-sulfosalts. Remains of the former oxidized assemblage appear as relicts comprised of hematite and goethite. Combining paragenetic information, textures and mineral chemistry it has been possible to derive a sequence of events for the Las Cruces Gossan generation and subsequent evolution. In that sense, the small amount of Fe-oxyhydroxides and their relict textures replaced by carbonates and sulfides suggest that the Gossan was generated under changing physico-chemical conditions. It is proposed that the Las Cruces current Gossan represents the modified residue of a former Gossan mineralization where prolonged weathering led to dissolution and leaching out of highly mobile elements and oxidation of the primary sulfides. Later, the Gossan was subject to seawater-Gossan interaction and then buried beneath a carbonated-rich cover. The basinal fluids-Gossan interaction and the equilibration of fluids with the carbonated sediments brought to the carbonatization and sulfidation of the Gossan, and thus to the generation of Fe-carbonates and Pb–Sb-sulfides. The Las Cruces mineral system likely represents a new category within the weathering class of ore deposits.
J B Gemmell - One of the best experts on this subject based on the ideXlab platform.
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sulfur isotope characteristics of the archean cu zn Gossan hill vhms deposit western australia
Mineralium Deposita, 2000Co-Authors: R Sharpe, J B GemmellAbstract:Gossan Hill is an Archean (∼3.0 Ga) Cu–Zn–magnetite-rich volcanic-hosted massive sulfide (VHMS) deposit in the Yilgarn Craton of Western Australia. Massive sulfide and magnetite occur within a layered succession of tuffaceous, felsic volcaniclastic rocks of the Golden Grove Formation. The Gossan Hill deposit consists of two stratigraphically separate ore zones that are stratabound and interconnected by sulfide veins. Thickly developed massive sulfide and stockwork zones in the north of the deposit are interpreted to represent a feeder zone. The deposit is broadly zoned from a Cu–Fe-rich lower ore zone, upwards through Cu–Zn to Zn–Ag–Au–Pb enrichment in the upper ore zone. New sulfur isotope studies at the Gossan Hill deposit indicate that the variation is wider than previously reported, with sulfide δ34S values varying between −1.6 and 7.8‰ with an average of 2.1 ± 1.4‰ (1σ error). Sulfur isotope values have a broad systematic stratigraphic increase of approximately 1.2‰ from the base to the top of the deposit. This variation in sulfur isotope values is significant in view of typical narrow ranges for Archean VHMS deposits. Copper-rich sulfides in the lower ore zone have a narrower range (δ34S values of −1.6 to 3.4‰, average ∼1.6 ± 0.9‰) than sulfides in the upper ore zone. The lower ore zone is interpreted to have formed from a relatively uniform reduced sulfur source dominated by leached igneous rock sulfur and minor magmatic sulfur. Towards the upper Zn-rich ore zone, an overall increase in δ34S values is accompanied by a wider range of δ34S values, with the greatest variation occurring in massive pyrite at the southern margin of the upper ore zone (−1.0 to 7.8‰). The higher average δ34S values (2.8 ± 2.1‰) and their wider range are explained by mixing of hydrothermal fluids containing leached igneous rock sulfur with Archean seawater (δ34S values of 2 to 3‰) near the paleoseafloor. The widest range of δ34S values at the southern margin of the deposit occurs away from the feeder zone and is attributed to greater seawater mixing away from the central upflow zone.
Gobain Ovejero - One of the best experts on this subject based on the ideXlab platform.
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mineralogical evolution of the las cruces Gossan cap iberian pyrite belt from subaerial to underground conditions
Ore Geology Reviews, 2017Co-Authors: Lola Yesares, Carmelo Gomez, R Saez, G R Almodovar, Jose Miguel Nieto, Gobain OvejeroAbstract:Abstract The Las Cruces VMS deposit is located at the eastern corner of the Iberian Pyrite Belt (SW Spain) and is overlain by the Neogene–Quaternary sediments of the Guadalquivir foreland Basin. The deposit is currently exploited from an open pit by Cobre Las Cruces S.A., being the supergene Cu-enriched zone the present mined resource. The Las Cruces orebody is composed of a polymetallic massive sulfide orebody, a Cu-rich stockwork and an overlying supergene profile that includes a Cu-rich secondary ore (initial reserves of 17.6 Mt @ 6.2% Cu) and a Gossan cap (initial reserves of 3.6 Mt @ 3.3% Pb, 2.5 g/t Au, and 56.3 g/t Ag). The mineralogy of the Las Cruces weathering profile has been studied in this work. Textural relationships, mineral chemistry, deposition order of the minerals and genesis of the Las Cruces Gossan are described and discussed in detail. A complex mineral assemblage composed by the following minerals has been determined: carbonates such as siderite, calcite and cerussite; Fe-sulfides including pyrite, marcasite, greigite and pyrrhotite; Pb–Sb sulfides and sulfosalts like galena, stibnite, fuloppite, plagionite, boulangerite, plumosite, and the jordanite–geocronite series, Ag–Hg–Sb sulfides and sulfosalts including miargyrite, pyrargyrite, sternbergite, acanthite, freibergite, cinnabar, Ag–Au–Hg amalgams; and Bi–Pb–Bi sulfides and sulfosalts such as bismuthinite, galenobismutite, others unidentified Bi–Pb-sulfosalts, native Bi and unidentified Fe–Pb–Sb-sulfosalts. Remains of the former oxidized assemblage appear as relicts comprised of hematite and goethite. Combining paragenetic information, textures and mineral chemistry it has been possible to derive a sequence of events for the Las Cruces Gossan generation and subsequent evolution. In that sense, the small amount of Fe-oxyhydroxides and their relict textures replaced by carbonates and sulfides suggest that the Gossan was generated under changing physico-chemical conditions. It is proposed that the Las Cruces current Gossan represents the modified residue of a former Gossan mineralization where prolonged weathering led to dissolution and leaching out of highly mobile elements and oxidation of the primary sulfides. Later, the Gossan was subject to seawater-Gossan interaction and then buried beneath a carbonated-rich cover. The basinal fluids-Gossan interaction and the equilibration of fluids with the carbonated sediments brought to the carbonatization and sulfidation of the Gossan, and thus to the generation of Fe-carbonates and Pb–Sb-sulfides. The Las Cruces mineral system likely represents a new category within the weathering class of ore deposits.
Keith M Harris - One of the best experts on this subject based on the ideXlab platform.
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genesis of pyrite au as zn bi te zones associated with cu au skarns evidence from the big Gossan and wanagon gold deposits ertsberg district papua indonesia
Economic Geology, 2005Co-Authors: Kylie Prendergast, Gavin W Clarke, Norman J Pearson, Keith M HarrisAbstract:The Ertsberg district hosts multiple skarn and porphyry-related deposits, which together comprise one of the largest Cu-Au resources in the world. Earlier skarn Cu-Au deposits at Big Gossan and 2 km along strike to the northwest at Wanagon Gold are overprinted by distinctive late-stage pyrite, sphalerite, arsenopyrite, and native gold with local Bi and Te minerals. The Wanagon Gold deposit contains an estimated 2 million ounces (Moz) of gold; reserves at Big Gossan are 33 million tonnes (Mt) at 2.63 percent Cu, 0.92 g/t Au, and 15.72 g/t Ag. Phlogopite from the Big Gossan occurrence is younger than 2.82 ± 0.04 Ma, based on a new 40Ar/39Ar age from the Big Gossan skarn, and K-feldspar from the Wanagon Gold deposit has a 40Ar/39Ar age of 3.62 ± 0.05 Ma. A K-Ar date (3.81 ± 0.06 Ma) from the Wanagon sill constrains formation of the overprinting skarn Cu-Au and late-stage Wanagon Gold deposit to a period of ca. 0.2 m.y. At Big Gossan, earlier skarn Cu-Au mineralization displays three-dimensional mineralogical, chemical, and temperature zonation. The high-temperature core (defined by low Zn/Cu) plunges to the northwest and is open at depth. Highest Cu grades and greatest development of the overprinting pyrite-Au-As-Zn-Bi-Te association occur to the northwest coincident with northeast-striking faults. Pyrite-Au-As-Zn-Bi-Te occurrences are also distributed in faults and fractures to the north and south of the Big Gossan skarn Cu-Au deposit. At Wanagon Gold, leaching of skarn and sandstone preceded introduction of the pyrite-Au-As-Zn-Bi-Te occurrences. In the sandstone, the pyrite-Au-As-Zn-Bi-Te mineralization was accompanied by K-feldspar (adularia) and minor quartz gangue. In carbonate rocks, no leaching or secondary K-feldspar is apparent; instead, sulfides are accompanied by quartz and dolomite gangue. The {delta}34S of sulfide from skarn Cu-Au and overprinting pyrite-Au-As-Zn-Bi-Te occurrences at both deposits range from –0.7 to +5.1 per mil. Laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) analyses show that later pyrite overprinting both occurrences is distinct from that in the earlier skarn Cu-Au deposits and contains up to 60 ppm Au, 2 percent As, 680 ppm Bi, and 40 ppm Te. The mineralogy of the overprinting occurrences includes native gold, argentian tetrahedrite and tennantite, a silver-antimony sulfide, and Bi and Te-(Ag-Au) minerals including cosalite, bismuthinite, petzite, hessite, altaite, and tetradymite. The fineness of native gold varies with sulfide association. The lowest fineness gold (737–863) occurs with Pb minerals (galena and sulfosalts), and the higher fineness gold (904–974) occurs trapped within pyrite or in association with bismuthinite. Fluid inclusions in sphalerite and quartz in the Big Gossan pyrite-Au-As-Zn-Bi-Te occurrence have an average salinity of 8 wt percent NaCl equiv and an average homogenization temperature of 245°C. Stable isotopes indicate that the inclusion fluids were magmatic. However, a direct genetic relationship to earlier skarn Cu-Au mineralization is not obvious. The pyrite-Au-As-Zn-Bi-Te occurrences are considered to have formed from a fluid with a different composition, possibly the magmatic precursor to fluids commonly recognized in low- and high-sulfidation epithermal deposits that develops at shallow levels and contains significant nonmagmatic (i.e., meteoric) water.
