Log fS2 Research Articles

Characteristics of the Kematu epithermal gold deposit at Tboli, Mindanao, Philippines

AbstractThe Kematu Au‐Ag epithermal deposit at Tboli area, Province of South Cotabato, Mindanao, Philippines, contains an inferred mineral resource of 3.8 million tonnes, at 8.7 g/t Au and 27 g/t Ag in 2012. The deposit is spatially and genetically linked to the volcanism related to the Parker Volcanic Complex of Pliocene to Holocene age. Local extension of the northwest‐trending splays of the faults in the Tboli area are associated with epithermal mineralization, including the Kematu Au‐Ag deposit. The study aimed to provide a detailed description of the characteristics and physicochemical processes involved in the formation of the Kematu Au‐Ag deposit on the basis of field occurrence, mineralogy, ore composition, mineral chemistry, and sulfur isotope data. The deposit is characterized by Au‐Ag bearing quartz‐adularia‐clay‐sulfide veins hosted in argillic‐altered andesite porphyry and coarse tuff of the Parker Volcanic Complex. The mineralization in the Kematu deposit is in the form of veins as well as open space fill and dissemination. Based on the observation of the ore bodies and petrographic analysis, three paragenetic stages with different vein characteristics have been identified. Stage 1 is dominantly quartz, adularia, and clay mineral veins, with electrum, sphalerite, pyrite, chalcopyrite, tetrahedrite‐tennantite, and galena. Stage 2 is coarse‐grained sulfide‐quartz‐adularia veins and in‐fill of vugs, consisting of sphalerite, pyrite, chalcopyrite, galena, tetrahedrite‐tennantite, and electrum. Stage 3 is massive calcite veins, with minor amounts of electrum, sphalerite, pyrite, chalcopyrite, and galena. The alteration mineral assemblage of the host rocks is chlorite‐illite‐adularia, indicative of neutral pH conditions. The FeS content of the Fe‐poor sphalerite in the stage 2 ranges from 0.03 to 1.07 mol%. Sulfur isotope compositions (δ34SCDT) of sulfides in all the stages range from 0.1‰ to 1.9‰ for pyrite, and −5.3‰ to 0.5‰ for galena, sphalerite, and chalcopyrite, where those in the stage 2 tend to have relatively lower values compared to those in the stages 1 and 3. The relatively narrow and close to zero δ34SCDT values suggest that the sulfur was mainly derived from a magma. The mineral assemblages of the stages 1, 2, and 3, and the sulfur fugacity (log fS2 = −12.5 to −8.8) and temperature (200–280°C) of ore formation based on the FeS mol%, electrum composition (at.%), and alteration mineral assemblage suggest that the Kematu deposit is of intermediate sulfidation type.

Mineralogy of the Imalia Au-Sn-bearing polymetallic sulfide deposit, Mahakoshal belt, Central India

The Au-Sn-bearing, polymetallic sulfide deposit at Imalia in the western part of Mahakoshal belt, Central India is hosted by recrystallized dolostone and subsidiary phyllitic dolostone. These host rocks are transected by shallow level intrusion of quartz porphyry dykes. There are two major NS trending ore zones (∼1.5 km cumulative length) and a subsidiary-one, confined to fractures and shears in the dolostones or along its contact with the intrusive dykes. Polyphase mineralization include: irregularly disseminated pyrite crystals of diagenetic/metamorphic origin, patchy to stringery Pb-Zn sulfide ores showing pervasive metamorphic fabric, dominant massive vein type pyrite-arsenopyrite ores with significant amounts of invisible gold and tin. Other ore minerals include cassiterite, molybdenite, wolframite and roquesite in a magnetite-rich oxidic halo, and electrum, enargite/luzonite, wittichenite, ourayite, eclarite, aikinite and idaite in the sulfidic veins. Some tentatively identified ore minerals in these veins include calaverite, proustite, famatinite, sakuraiite, and tenorite. Pyrite contains structurally located invisible gold as high as 17.6 ppm whereas arsenopyrite contains a maximum of 20–24 ppm Au. Pyrite also contains unusually high Sn (upto 2673 ppm). The mineralogical assemblage of rare phases and their paragenesis in the Imalia vein type ores reflect the high sulfur (log fS2 = −9 to −13 bar) and oxygen (logfO2 = −32 bar) fugacities at a temperature range of 350 °C to 250 °C (according to thermometric calculations) during their emplacement.

Chromite Composition and Platinum-Group Element Distribution in the Proterozoic Chimalpahad Anorthosite Complex, South India: Implications for Magmatic Processes and Discrimination of Tectonic Setting

Abstract The Proterozoic Chimalpahad Anorthosite Complex (CAC) is a deformed and dismembered complex in South India comprising of anorthosite-leucogabbro-gabbro-ultramafic rocks. It is located in the Nellore-Khammam Schist Belt (NKSB) within the contact zone between the Eastern Dharwar Craton (EDC) and the Eastern Ghats Belt (EGB). The chromitites occur as small dismembered lensoidal bodies within the ultramafic rocks. Presence of micrometric inclusions of iridium-like platinum-group minerals (IPGM), such as laurite (RuS2), erlichmanite (OsS2), irarsite (IrAsS), and Ir-Os-Ru alloys, is reported for the first time in the CAC chromites. The chromite cumulates preserve magmatic character (Cr#: 0.61 to 0.77; Mg#: 0.29 to 0.55), while disseminated chromites and rims of some grains (Cr#: 0.59 to 0.89; Mg#: 0.10 to 0.25) show postmagmatic modifications. The chromites are characterised by higher Cr and Al contents and lower Fe and Ti contents. The mineral chemistry of chromites such as Cr# and Mg# contents of Al2O3 (10.05–20.07 wt.%) and TiO2 (0.1 to 0.25 wt.%) typically suggests that the CAC chromitites and their ultramafic hosts were generated from the S-undersaturated boninite melt in a magmatic arc in suprasubduction tectonic setting. A general negative slope of bulk-rock PGE distribution with upward convex pattern is observed for the CAC chromitites in the chondrite-normalized plot. These geochemical and mineral chemical characters in addition to location of the chromitites and the CAC in the tectonic zone between Eastern Dharwar Craton and Eastern Ghats Mobile Belt point towards ophiolitic affinity of the CAC chromitites. Laurite (RuS2) grains are trapped close to the cores of chromites, whereas erlichmanites (OsS2) are trapped away from the cores. Textural relations indicate primary orthomagmatic nature of laurite and Or-Ir-Ru alloys and their precipitation prior to or concomitant with chromite from an S-undersaturated melt at ~1200–1300°C and log fS2 varying between −2 and −1.3. Ru-Os disulfide and erlichmanite crystallized subsequently with a gradual decrease in temperature in the differentiating ultramafic magma. Precipitation of irarsite and base-metal sulfides can be attributed to local and rapid fluctuations of temperature, fS2, and fAs during the formation of the chromitites due to influx of fresh batch of melts or crystallization in an open magmatic system. The compositional variations of laurite-erlichmanite record heterogeneous physicochemical environment during their crystallization. In addition to zoning, preservation of IPGMs in the CAC chromites points towards fast cooling of the magma possibly associated with rapid exhumation of the chromitites and their ultramafic hosts from the roots of the subcontinental lithospheric mantle (SCLM) towards their final emplacement into the crust. Occurrence of the CAC along a linear extension with the Kondapalli Ultramafic Complex and deformed remnants of ophiolites (Kandra and Kanigiri complexes) along the eastern margin of the Eastern Dharwar Craton (EDC) bordering the Eastern Ghats Belt (EGB) indicates this zone to be a Proterozoic convergent margin.

Comparison of petrological and geochemical characteristics of three different types of Eocene copper-gold mineralization in eastern Iran

The Maherabad, Qaleh Zari, and Hired deposits in eastern Iran are porphyry (Cu-Au)-, Iron oxide copper–gold (IOCG)-, and reduced intrusion-related gold system (RIRGS)-type mineralization, respectively. They are located within the Lut block magmatic assemblage, which is the main metallogenic province in eastern Iran. The mineral assemblages and paragenesis of the deposits indicate that highest log fO2 and log fS2 contents belong to the Qaleh Zari deposit. In contrast, the lowest values of oxygen and sulphur fugacity are related to the RIRGS system in the Hired deposit. The Maherabad mineralization has moderate log fO2 and log fS2 contents relative to the Qaleh Zari and Hired deposits.Geochemically, these three deposits are related to different types of granitoids, which occurred within the short time interval from 37 to 41 Ma. The Maherabad and Qaleh Zari intrusions are related to oxidized magnetite-series granitoids, whereas the mineralization at Hired deposit is related to the reduced ilmenite-series granitoids. Magnetite-series intrusive rocks at Hired are barren. Intrusive bodies in the three studied sites are metaluminous, high-K calc-alkaline to shoshonitic, and formed in a volcanic arc setting. However, there are distinct differences in the radiogenic isotopic composition, content of some elements and elemental ratios reflecting the effect of different sources in the formation of intrusions. The reduced granite of the Hired deposit has the lowest Eu/Eu* of <0.7, P + Sr < 1000 ppm, and Sr/Y < 20 associated with (La /Yb)N < 12. Crustal contamination played an important role in magma evolution (87Sr/86Sri = 0.70627 and −εNd(t) = -2.60). P + Sr and Sr/Y contents in the productive oxidant magnetite-series intrusive rocks, such as the Maherabad and Qaleh Zari deposits, are more than 1500 ppm and 35, respectively. On this basis, petrological characteristic, geochemical indicator elements and mineral assemblage and paragenesis can be used for discrimination of productive and barren intrusions in the three different deposit types studied in this research. This can be used as an exploration criteria for the hydrothermal deposits associated with different magmatic intrusions in some areas.

Two types of sediment-hosted Pb-Zn deposits in the northern margin of Lanping basin, SW China: Evidence from sphalerite trace elements, carbonate C-O isotopes and molybdenite Re-Os age

Lanping basin is a large Pb-Zn-Cu-Ag polymetallic mineral province in SW China, with major deposit types including vein-type Cu deposits, basinal brine-related Pb-Zn deposits (e.g., Liziping, Fulongchang) and minor quartz-vein-type Pb-Zn deposits (e.g., Shangnuluo, Baiji). In this study, we have conducted EPMA and LA-ICP-MS sphalerite trace element analyses, together with gangue carbonate C-O stable isotope analysis and molybdenite Re-Os dating. Using these data, we discuss the mineral assemblage, sphalerite minor/trace element composition, ore-fluid source, and mineralization age of these two types of Pb-Zn deposits. Except sphalerite and galena, major ore minerals from Liziping and Fulongchang are As-rich (e.g., gratonite, jordanite, bournonite, As-rich pyrite), whereas those from Shangnuluo and Baiji are Sb-rich (e.g., boulangerite, jamesonite, stibnite, zinkenite). The trace element compositional characteristics in sphalerite of these deposits (low Mn, Fe, Co, In and Sn, and high Cd) mimic those of medium-low-temperature hydrothermal deposits (MVT and Jinding deposits). For the Liziping zoned sphalerite, the dark-brown zones have relatively high Co, Cu, Ga, As, and Sb; whilst the light-colored zones have relatively high Fe, Mn and Cd. The calculated sphalerite formation temperature based on sphalerite geothermometer – GGIMFis (Frenzel et al., 2016) are basically in agreement with those homogenization temperature of inclusions, indicating that the contents of these elements (Ga, Ge, Fe, Mn and In) in sphalerite are mainly controlled by temperature. Moreover, the estimated sulfur fugacity (log fS2) for Liziping (−14.7) and Fulongchang (−13.6) are distinctly lower than those for Shangnuluo (−7.1) and Baiji (−8.5). Carbonate C–O isotope compositions indicate that the dissolved carbon and oxygen of ore forming-fluids of these Pb-Zn deposits were mainly derived from sedimentary carbonate dissolution. Early-ore-stage molybdenite from Baiji deposit yielded two Re-Os weighted average model ages of 21.4 Ma and 12.93 Ma, and a Re-Os concordia age of 11.88 ± 0.77 Ma was obtained from four younger molybdenite samples. These ages are distinctly younger than those reported mineralization age (ca. 30 Ma) for basinal brine-related Pb-Zn deposits. All these results indicate that there are two different types of Pb-Zn metallogenic events in the northern margin of Lanping Basin.

Early Fimiston and late Oroya Au–Te ore, Paringa South mine, Golden Mile, Kalgoorlie: 4. Mineralogical and thermodynamic constraints on gold deposition by magmatic fluids at 420–300 °C and 300 MPa

The Golden Mile deposit (1767 t Au) in the Archean Yilgarn Craton, Western Australia, is controlled by D2 strike-slip and D3 reverse faults displacing folded tholeiitic greenstones. Granodiorite and monzodiorite dykes emplaced into the faults predate and are synchronous with propylitic and sericite–ankerite alteration overprinted by the ore. Arsenopyrite and chlorite thermometry and fluid-inclusion data indicate ore formation at 420 ± 30 °C and 300 MPa and cooling to the ambient temperature at 10 km depth (250–300 °C). Fimiston refractory ore (7 g/t Au) in propylitic Golden Mile Dolerite is zoned from inner albite–ankerite to outer ankerite–phengite–quartz replacement. Pyrite–magnetite is overprinted by pyrite–hematite ± anhydrite. Arsenical pyrite encloses chalcopyrite, tennantite, gold and tellurides. Gold thio-sulfide complexes, buffered by magmatic SO2 and H2S in the fluid, were destabilised during the ankerite–pyrite replacement of propylitic chlorite, a reaction releasing hydrogen ions for the hydrolitic alteration of adjacent albite. Gold deposition was assisted by declining gold, silver, tellurium and sulfur solubility as the fluid cooled. Dissolved sulfate is recorded in pyrite by negative δ34S (− 10 to − 5‰) at magmatic Δ33S (0–0.2‰). Oroya ore bodies are subdivided into oxidised pyrite lodes (5 g/t Au) in propylitic GMD and reduced lodes (30–120 g/t Au) characterised by Stage 1 silica–pyrite and siderite–chlorite replacement in ankerite-rich wall rocks. Rapid cooling, an acidic fluid and the retrograde solubility of ankerite facilitated replacement and the deposition of minor arsenopyrite, pyrrhotite, chalcopyrite, sphalerite and gold. Locally, the H2S-rich fluid (log fS2 = − 5.8 ± 0.5 bar at 420 °C) was reduced by organic methane. Oroya Stage 2 ore fills crosscutting veins and cements breccia. Quartz, chalcedony, dolomite–ankerite, calcite, V-muscovite, V-chlorite and V-tourmaline form the gangue of pyrite–telluride ± magnetite ore. Altaite–tellurium myrmekites were deposited at ≥ 400 °C as melt droplets together with tellurantimony, altaite, calaverite, montbrayite and petzite. As fluid temperature and tellurium fugacity declined, free gold, krennerite, coloradoite and melonite also crystallised joined by sylvanite and hessite below 350 °C. Finally, trace covellite and digenite precipitated at high fluid sulfidation states.

Age and source of the Küre/Ödemiş arsenopyrite-gold mineralization (Menderes Massif, Turkey) determined by Re-Os-He isotopes

The Menderes Massif (MM) is considered to be one of the principal regions hosting vein-type quartz-arsenopyrite orogenic gold deposits and occurrences in Turkey. Gold mineralizations in the MM can be grouped into: (i) schistosity-conformable E-trending veins and (ii) shear-controlled NW-trending veins. Arsenopyrite is one of the major gold-bearing minerals in these deposits or occurrences. Rhenium–Osmium isotopic dating of two types of arsenopyrite from the schistosity-conformable and vein-type Küre (Ödemiş) gold deposit has yielded two groups of model ages (maximum ages) at 557–574 Ma and 246–250 Ma, respectively. The data suggested that the quartz-arsenopyrite gold deposit formed in the Neoproterozoic and Early Triassic, with the former and the latter corresponding to Pan-African compressional orogenesis and Palotethys rifting, respectively. Fluid inclusions in Küre arsenopyrite have 3He/4He ratios of ~0.08–0.09 Ra (Ra = 1.39x10-6, the 3He/4He ratio of air) being within the range of middle to upper crustal values. A minor helium contribution of mantle fluids to the Küre gold deposit is possible because the 4He concentrations in arsenopyrite are enriched 170 to 9520 times relative to argon and typical atmospheric values, indicating that contribution of atmospheric He to the mineralizing fluids is negligible, and 0.08–0.09 Ra is slightly above typical crustal 3He/4He ratios. Arsenopyrite yields relatively non-radiogenic initial Os isotopic compositions, also indicating possiblecontribution from mantle fluids. Carbonic fluid inclusion type-2 (H2O-CO2-NaCl ± CH4) in quartz of NW-trending veins at Küre are gas–liquid-hydrate-crystal-rich and their homogenization temperatures (Th) range from 244 to 387 °C. Salinity values range from 0.2 to 5.7 wt% NaCl equiv. The homogenization temperatures (Th) of aqueous fluid inclusions type 1 (H2O-NaCl) in quartz from NW-trending veins at Küre vary between 237 and 358 °C whereas salinity values range from 1.7 to 7.7 wt% NaCl equiv. Quantitative EPMA spot analyses on arsenopyrite minerals have shown that As concentrations in EW-trending and NW-trending veins range from 32.5 to 33.8% and 32.3–33.8%, respectively. Based on a phase diagram for the Fe–As–S system along with the arsenopyrite geothermometer, the ranges of temperature and corresponding logf S2, at which arsenopyrite is in equilibrium with pyrite are between 380 and 480 °C, and −7.4 and −4.3 for Apy-1, respectively. These values are between 390 and 475 °C, and −7.0 and −4.5 for Apy-2, respectively.

The gold occurrence in pyrite and Te–Bi mineralogy of the Fancha gold deposit, Xiaoqinling gold field, southern margin of the North China Craton: Implication for ore genesis

The Fancha lode gold deposit is hosted in the Neoarchean Taihua Group metamorphic rocks in the Xiaoqinling gold field, on the southern margin of the North China Craton. Pyrite, as the predominant sulphide mineral and Au‐hosting mineral, was investigated using a combination of ore microscopy and in‐situ laser ablation inductively coupled plasma mass spectrometry (LA‐ICP‐MS). Three types of pyrite were identified during the ore‐forming process, that is, the coarse euhedral grains in milky quartz veins (Py1), the fine to medium, euhedral to subhedral grains in light grey quartz (Py2), and the fine grains intergrown with abundant sulphide minerals in light grey quartz (Py3). The trace elements exhibit systematic different concentrations among three types of pyrite, implying different crystallization processes. The Py1 grains have the lowest concentrations of Au and other accompanying elements such as Ag, Te, Bi, Cu, Pb, and Zn, and the trace elements of pyrite in wall rock are similar to those of Py1. The Py2 and Py3 grains show consistently high contents of Au, Ag, Te, Bi, Cu, Pb, and Zn. The uneven distribution of gold and positive correlation between Au, Ag, Te, and Bi within Py2 and Py3 grains, indicating that gold mainly occurs as submicroscopic inclusions of Au–Ag–Te–Bi assemblages in the pyrite. This interpretation is supported by the microscopic observation that large quantities of telluride and Bi‐sulfosalt minerals contact directly with gold. Observed Te–Bi minerals include petzite, sylvanite, hessite, calaverite, rucklidgeite, altaite, volynskite, tellurobismuthite, buckhornite, tetradymite, krupkaite, and bismuthinite, and native bismuth is also found. Bi (Te) melts, as an excellent gold scavenger, have played a certain role in governing gold distribution patterns and leading to high concentrations during the mineralization process in the Fancha gold deposit. The Te–Bi mineral assemblages also reveal that the gold was precipitated under variable redox conditions with logfTe2ranged from &lt;−11.0 to −6.4, and logfS2ranged from &lt;−11.8 to −8.6 at 300°C, vertically upward from the deep fluid source. Combined with available geochronological and geochemical data, enrichment of Te and Bi indicates that the ore‐forming fluids and metals might be stemed from a mantle‐derived magmatic system.

The geochemistry of Au–Ag minerals and base‐metal sulphides as indicators for gold precipitation: Case study of the Shihu gold deposit, central North China Craton

The Shihu quartz vein‐type gold deposit, the largest gold deposit in the central North China Craton, occurs within wall rocks of Neoarchean TTG (tonalite–trondhjemite–granodiorite) gneiss and shows a close relationship with the early Cretaceous Mapeng granitoid pluton and various intermediate–mafic dykes. The occurrence, morphology, composition, and precipitation mechanism of the Au–Ag minerals are short of research, which is the purpose of this study. The primary mineral assemblage of orebody contains quartz, calcite, and base‐metal sulphides including pyrite, galena, sphalerite, chalcopyrite, and minor pyrrhotite and enargite. The Au–Ag minerals can be classified into three distinct types, namely, (a) quartz‐hosted, (b) pyrite‐hosted, and (c) base‐metal sulphidehosted, corresponding to formation sequence. Electron microprobe analysis indicates that these Au–Ag minerals show fineness (=Au/[Au + Ag] × 1,000) ranging from 976 to 366 with an average of 667 and are rich in Bi, Te, and Hg while deficient of Cu, As, and Se. Bismuth and tellurium in the Au–Ag minerals show a linear relationship with Au, whereas in the coexisted sulphides are almost below detection limits. We suggest that Bi and Te fundamentally influenced the transportation, enrichment, and precipitation of gold as scavengers to concentrate and deposit gold at the location with an “appropriate distance” from magma. Phase analyses of native bismuth and Bi–Te minerals included in pyrite, reported for the first time in Shihu Gold deposit, imply that the gold precipitation witnessed the ore‐forming fluid evolution with log fTe2 decreasing from approximately −11 to approximately −15 and log fS2 increasing from approximately −13 to more than −11. Mineralogical mapping with the fineness of Au–Ag minerals (fineness contours) suggest that the ore‐forming fluids came from the deep earth beneath the north of the mining area with excellent gold potential, which has been verified by later drilling.

Platinum-group minerals and Fe–Ni minerals in the Sartohay podiform chromitite (west Junggar, China): Implications for T–pH–fO2–fS2 conditions during hydrothermal alteration

Complex assemblages consisting of native iron + pentlandite, pentlandite + heazlewoodite + magnetite, heazlewoodite + magnetite + platinum-group minerals (PGM), and millerite ± heazlewoodite are associated with three types of altered chromites (partly altered chromite, double-rimmed chromite, and porous chromite) discovered in the Sartohay podiform chromitite hosted by the Sartohay ophiolitic mélange in west Junggar of China. We propose a two-stage genetic model to explain the chromite alteration processes based on detailed petrographic observations and thermodynamic calculations. Reactions between primary chromite and olivine formed porous Fe2+-rich chromite and chlorite patches under low-temperature, reducing conditions. Afterwards, oxidizing Fe3+- and CO2-rich fluids derived from host serpentinite were incorporated into the Fe2+-rich chromite, shifting its composition to heterogeneous Fe3+-rich chromite (ferrian chromite). Thermodynamic calculations suggest that the co-evolution of fO2 and fS2 played a crucial role in controlling the formation and/or transformation of the Fe–Ni minerals coevally with the sulfides, antimonides, and alloys of platinum-group elements (PGE). Pd and Pt precipitated as sulfides, while Ir precipitated as an alloy when log fS2 ranged from –16.0 to –14.2 within the heazlewoodite stability field. Compared with the host serpentinite, the fO2 in the Sb-bearing fluid equilibrated with chromitite was low enough to decrease the Pd solubility, leading to co-precipitation of Sb and Pd.

Early pyrite and late telluride mineralization in vanadium-rich gold ore from the Oroya Shoot, Paringa South mine, Golden Mile, Kalgoorlie: 3. Ore mineralogy, Pb-Te (Au-Ag) melt inclusions, and stable isotope constraints on fluid sources

The Oroya Shoot (> 62 t Au) in the giant Golden Mile deposit, Yilgarn Craton, Western Australia is controlled by reverse faults, which offset altered quartz diorite porphyry but are crosscut by a late-mineralization kersantite dyke (> 2642 ± 6 Ma). Oroya stage 1 ore (12–14 g/t Au) is characterized by chert-like quartz, Fe-chlorite, siderite and pyrite (10–20 vol%), minor arsenopyrite, chalcopyrite and sphalerite, and accessory pyrrhotite, gersdorffite, and melonite. Dendritic pyrite and arsenopyrite-chlorite thermometry indicate rapid precipitation from an H2S-dominant fluid of intermediate sulfidation and low oxidation state cooling from 450–400 to 340 °C. Oroya stage 2 ore (120–600 g/t Au) forms veins and breccia filled or cemented by quartz, muscovite (≤ 13.2 wt% V2O3), ankerite, chlorite, and tourmaline (≤ 14.7 wt% V2O3). The assemblage pyrite-arsenopyrite-chalcopyrite brackets telluride deposition. Bornite-chalcopyrite aggregates, sphalerite, and tetrahedrite are associated with native gold, montbrayite, altaite, calaverite, petzite, tellurantimony, coloradoite, and melonite. Myrmekitic altaite-tellurium ± sylvanite and altaite-krennerite symplectites represent melt droplets deposited at > 400 °C and log fTe2 = − 4 bar (400 °C, 200 MPa). Tellurium fugacity declined as the fluid cooled during the deposition of native gold and free telluride grains terminating in melonite replacement at 340–300 °C. Pyrite-nolanite- and pyrite-magnetite-telluride assemblages suggest oxidation states up to 2 log units higher than during stage 1. After telluride deposition, the fluid evolved to a high sulfidation state (log fS2 = − 5 bar at 300 °C) and increased salinity indicated by digenite-covellite and Cl-bearing altaite. The average ore is enriched in lithophile (K, Rb, Cs), siderophile (Fe, V, Ni, W), and chalcophile elements (e.g., Te, Se, Cu, Zn, Pb) implicating local monzodiorite plutons as the fluid source. Stable isotope data from the Oroya kersantite and from Cu-Au skarn and monzodiorite-granodiorite stocks southeast of Kalgoorlie constrain the composition of the magmatic fluid to δ13CPDB = − 3.0 to − 2.2‰ and δ18OSMOW = 8.3 to 9.7‰. In the Golden Mile, the fluid oxygen isotope ratios of the gold ore bodies (8.2 to 9.8‰) are consistent with I-type magmatic water. The fluid carbon isotope ratios of all altered rocks (δ13CPDB = − 1.7 to − 0.5‰) are 13C-enriched, perhaps due to the reduction of fluid CO2 to CH4 by interaction with ferrous greenstones close to the intrusion.

Composition of Pyrrhotite as an Indicator of Gold Ore Formation Conditions at the Sovetskoe Deposit (Yenisei Ridge, Russia)

Abstract —We present results of an investigation into the composition and parageneses of pyrrhotite at the Sovetskoe gold–quartz deposit (Yenisei Ridge, Russia). The variability of parameters (temperature T and sulfur fugacity fS2) during the stage crystallization of pyrrhotite-containing assemblages has been assessed from the composition of this mineral (Fe0.873±0.02S–Fe0.885±0.02S) and its parageneses. The compositions Fe0.873–0.875S close to Fe7S8 (Apy + Po + Rut + Qz), for which the estimated formation parameters are T = 486–465 °C and log fS2 = –4.71 to –5.28, are typical of early pyrrhotite in the form of microinclusions in arsenopyrite, associated with rutile and quartz. According to the composition of inclusions of pyrrhotite microcrystals (Fe0.873–0.881S) associated with pyrite in native gold (950‰) (Au + Po + Py), the formation parameters are T = 489–410 °C and log fS2 = –4.63 to –6.98. Coarse pyrrhotite grains containing microinclusions of relict arsenopyrite and galena, sometimes, in aggregate with siderite (Po + Apy + Ga + Sid), and pyrrhotite in aggregate with pyrite and siderite (Py + Po + Sid) have composition Fe0.874–0.878S and form at 479–443 °C and log fS2 = –4.9 to –5.9. The xenomorphic pyrrhotite microinclusions present together with galena and native gold (950‰) in pyrite crystals (Py + Po + Ga + Au) are characterized by higher contents of iron (Fe0.878–0.885S) and, correspondingly, lower temperatures of formation, 432–382 °C, and log fS2 = –6.27 to –7.95. The log fS2–T diagrams have been calculated for the systems Fe–S and Ag–Au–S in the temperature range 25–700 °C with regard for the stability fields of iron sulfides (pyrite FeS2, troilite FeS, and pyrrhotite Fe7S8), phases Fe11S12, Fe10S11, and Fe9S10, metallic iron, native sulfur, uytenbogaardtite, petrovskaite, and solid-solution phases Fe1–xS (0 &amp;lt; x &amp;lt; 0.125), Ag1–zAuz (z = 0, 0.25, 0.5, and 1), and Ag2–yAuyS (y = 0, 0.5, 1, and 2). The calculation results have demonstrated that there is a field of petrovskaite and uytenbogaardtite solid solutions and Au–Ag alloys (&amp;gt;670‰, Ag0.5Au0.5–Au) in the stability field of the pyrrhotite–pyrite parageneses of the Sovetskoe deposit. The gold and silver contents in iron sulfides of the Sovetskoe deposit show that the Au/Ag ratios in pyrrhotites (0.002–2.4) and pyrites (0.004–13) are lower than those in high fineness (950–980‰) gold (19–50). The difference in the Au/Ag ratios in these minerals and the results of thermodynamic calculations indicate the possible presence of Au–Ag sulfides and Au–Ag alloys of lower fineness in the pyrrhotite–pyrite ores of the studied deposit. The absence of visible mineral forms of gold sulfides from the ores suggests that these sulfides are present in finely dispersed or invisible microscopic forms. The pyrrhotite compositions in pyrite-containing parageneses as well as Au/Ag in pyrites, pyrrhotites, and visible native gold in sulfide ores of other gold and gold–silver deposits can be used to assess the possible presence of nanosized solid microinclusions of sulfide and other gold and silver forms.

Physicochemical model for the genesis of Cu-Ag-Au-Hg solid solutions and intermetallics in the rodingites of the Zolotaya Gora gold deposit (Urals, Russia)

In this contribution we examine the compositions of solid solutions and intermetallics of the system Cu-Ag-Au-Hg and the physicochemical conditions of their formation in rodingites from the Zolotaya Gora gold deposit (Southern Urals, Russia). Thermodynamic calculations, modeling the formation of mineral assemblages of rodingite and Cu-Ag-Au-Hg mineralization, were carried out using a “Selektor-C” software package. Two probable models for the genesis of Au-Ag-Cu-Hg solid solutions and Au-Cu intermetallics in rodingites are: 1) hydrothermal; the result of single-stage discharge in open space of deep-sourced gold-bearing fluid with the composition corresponding to rodingite, taking into account its interaction with host serpentinites. 2) metasomatic; deep-seated gold-bearing fluid (W) rising to the surface interacts with early formed rodingite (R) at different ratios (W/R). T and P-conditions of modeling: 450 °C, 3 kbar; 350 °C, 2 kbar; 250 °C, 1 kbar. Results of the calculations on the “hydrothermal” and “metasomatic” models showed different degrees of similarity of natural and theoretical model associations of rodingites. The metasomatic model is better for corresponding to real mineral compositions and mineral paragenesis in the natural Cu-Ag-Au-Hg system observed at the deposit. In this model the chlorite-garnet-pyroxene rodingite is replaced by a chlorite-rich rock with increasing W/R. In this case all gold minerals of Zolotaya Gora deposit (Au-Cu intermetallics and Au-Ag solid solutions) are formed at 250–450 °C. Gold-copper solid solution formed at a temperature of 450 °C (W/R >10). Au-Ag-Hg solid solutions and native copper are formed only at 250 °C. According to the hydrothermal model native copper and AuCu3 were absent phases, but other Au-Cu intermetallics (AuCu, Au3Cu) precipitate if gold concentration in the solution is higher than 0.5 ppm. Thermodynamic calculations proved the possibility of formation of equilibrium assemblages of rodingite minerals and gold-bearing minerals with the participation of water-chloride complexing and low CO2 fluids. At a temperature <350 °C the main status of gold in solution are Au(HS)2− and AuHS0, while at higher temperatures it occurs as AuOH0. Formation of Au-Cu intermetallics occurred under the effect of weak-acid hydrothermal solutions (pH = 3.5 ÷ 5) with low fugacity of O2 and S2: log fO2 = −26 ÷ −47, log fS2 = −8 ÷ −20. Both models (hydrothermal and metasomatic) explain the formation of Au-bearing rodingites and can be used for predicting potential gold-bearing rodingite targets.

Geology and geochemistry of the Jianchaling hydrothermal nickel deposit: T–pH–fO2–fS2 conditions and nickel precipitation mechanism

The Jianchaling nickel deposit in the Bikou Terrane (Shaanxi Province, China) occurs along the boundaries between granite porphyry and carbonated ultramafic rocks (carbonated serpentinite, talc–carbonate rocks, and listwaenite). Serpentine– magnetite, serpentine– magnesite– magnetite, and magnesite– talc– quartz– pyrite– violarite– millerite– chalcopyrite assemblage formed in carbonated ultramafic rocks during hydrothermal activities. Ni-bearing sulphides, coexisting with magnesite, postdated magnetite in carbonated ultramafic rocks. Compared with serpentinite, Ni, Co, Cu, Mn, and Pb concentrate in talc–carbonate rocks. The fact that the NiO contents of magnetite decrease with progressive carbonation of serpentinite suggests that Ni from magnetite concentrated in fluid and contributed to the formation of the Jianchaling nickel deposit. Sulphides precipitated from fluid with log fO2 value varying from −34.5 to −31.8 and log fS2 value varying from −10.3 to −9.2. High pH and HS− activities triggered by transformation of serpentine into magnesite–talc–quartz assemblage promoted precipitation of Ni-bearing sulphides, and finally formed the Jianchaling hydrothermal nickel deposit.

Mineralogy and mineral chemistry of Bi-minerals: Constraints on ore genesis of the Beiya giant porphyry-skarn gold deposit, southwestern China

The Beiya deposit, located in the Sanjiang Tethyan tectonic domain (SW China), is the third largest Au deposit in China (323t Au @ 2.47g/t). As a porphyry-skarn deposit, Beiya is related to Cenozoic (Himalayan) alkaline porphyries. Abundant Bi-minerals have been recognized from both the porphyry- and skarn- ores, comprising bismuthinite, Bi–Cu sulfosalts (emplectite, wittichenite), Bi–Pb sulfosalts (galenobismutite, cosalite), Bi–Ag sulfosalt (matildite), Bi–Cu–Pb sulfosalts (bismuthinite derivatives), Bi–Pb–Ag sulfosalts (lillianite homologs, galena-matildite series), and Bi chalcogenides (tsumoite, the unnamed Bi2Te, the unnamed Ag4Bi3Te3, tetradymite, and the unnamed (Bi, Pb)3(Te, S)4). Native bismuth and maldonite are also found in the skarn ores. The arsenopyrite geothermometer reveals that the porphyry Au mineralization took place at temperatures in the range of 350–450°C and at log fS2 in the range of −8.0 to −5.5, respectively. In contrast, the Beiya Bi-mineral assemblages indicate that the skarn ore-forming fluids had minimum temperatures of 230–175°C (prevailing temperatures exceeding 271°C) and fluctuating fS2–fTe2 conditions. We also model a prolonged skarn Au mineralization history at Beiya, including at least two episodes of Bi melts scavenging Au. We thus suggest that this process was among the most effective Au-enrichment mechanisms at Beiya.

Solubility of molybdenite in hydrous granitic melts at 800 °C, 100–200 MPa

The solubility of molybdenite, MoS2, in fluid-saturated, subaluminous to peraluminous granitic melts was determined experimentally using rapid-quench cold-seal pressure vessels at 800°C and 100–200MPa, and analysis by laser-ablation ICP-MS. Molybdenite solubility seems to be independent of pressure, but it shows strong variations with oxygen and sulfur fugacity. At constant log fS2=−1.3 it increases from 0.1–0.7ppm by weight Mo at the Co–CoO buffer to 29–38ppm by weight Mo near the MnO–Mn3O4 buffer. The solubility isopleths are nearly parallel to the pyrrhotite–magnetite equilibrium, along which the solubility varies only slightly, from 10 ppmw Mo at the quartz–fayalite–magnetite buffer to 29–38ppmw Mo at the MnO–Mn3O4 buffer. The observed solubility variations are consistent with the equilibrium MoS2(s)+3/2O2=MoO3(l)+S2 and thus confirm that molybdenum(VI) oxide is the predominant species in subaluminous silicate melts at log fO2=−16 to −11. In addition, the experimental results are well reproduced by a simple thermodynamic model employing the Burnham eight-oxygen formulation for silicate melt species and assuming ideal mixing of dissolved MoO3. The thermodynamic calibration can be used to estimate the molybdenum solubility in subaluminous silicic melts or, for pyrrhotite- and molybdenite-saturated assemblages, the oxygen and sulfur fugacities during magma crystallization.

Au–Ag–Te Mineralization of the Low‐Sulfidation Epithermal Aginskoe Deposit, Central Kamchatka, Russia

AbstractMineralogic studies of major ore minerals and fluid inclusion analysis in gangue quartz were carried out for the for the two largest veins, the Aginskoe and Surprise, in the Late Miocene Aginskoe Au–Ag–Te deposit in central Kamchatka, Russia. The veins consist of quartz–adularia–calcite gangue, which are hosted by Late Miocene andesitic and basaltic rocks of the Alnei Formation. The major ore minerals in these veins are native gold, altaite, petzite, hessite, calaverite, sphalerite, and chalcopyrite. Minor and trace minerals are pyrite, galena, and acanthine. Primary gold occurs as free grains, inclusions in sulfides, and constituent in tellurides. Secondary gold is present in form of native mustard gold that usually occur in Fe‐hydroxides and accumulates on the decomposed primary Au‐bearing tellurides such as calaverite, krennerite, and sylvanite. K–Ar dating on vein adularia yielded age of mineralization 7.1–6.9 Ma. Mineralization of the deposit is divided into barren massive quartz (stage I), Au–Ag–Te mineralization occurring in quartz‐adularia‐clays banded ore (Stage II), intensive brecciation (Stage III), post‐ore coarse amethyst (Stage IV), carbonate (Stage V), and supergene stages (Stage VI). In the supergene stage various secondary minerals, including rare bilibinskite, bogdanovite, bessmertnovite metallic alloys, secondary gold, and various oxides, formed under intensely oxidized conditions. Despite heavy oxidation of the ores in the deposit, Te and S fugacities are estimated as Stage II tellurides precipitated at the log f Te2 values −9 and at log fS2 −13 based on the chemical compositions of hypogene tellurides and sphalerite. Homogenization temperature of fluid inclusions in quartz broadly ranges from 200 to 300°C. Ore texture, fluid inclusions, gangue, and vein mineral assemblages indicate that the Aginskoe deposit is a low‐sulfidation (quartz–adularia–sericite) vein system.

Greenstone Metamorphism, Hydrothermal Alteration, and Gold Mineralization in the Genetic Context of the Granodiorite-Hosted Gold Deposit at Jonnagiri, Eastern Dharwar Craton, India

Abstract Gold mineralization at Jonnagiri is hosted in laminated quartz veins within sheared granodiorite in a typical Archean greenstone-granitoid ensemble. The greenstones and the intrusive granodiorite have been affected by two folding and shearing events, respectively. THERMOCALC-aided P-T calculations suggested that lower amphibolite-grade metamorphism affected the greenstones, involving peak conditions at ~5 kbar and 500°C. Gold mineralization is characterized by a chlorite + biotite-rich proximal alteration zone and a muscovite-dominant inner zone among laminated auriferous quartz veins. Whereas the proximal zone had significant gain in S, C, K, Si, and Rb and depletion of Ca, Mg, and Fe, the inner alteration zone is typified by major addition of Si, K, S, C, and Au. General depletion of rare earth elements in the altered granodiorites is ascribed to high ambient fluid/rock during hydrothermal alteration. Results of the computed (aMg2+/aH+2) vs. (aK+/aH+) and (aNa+/aH+) vs. (aK+/aH+) diagrams, along with an increase in carbonation and alkali enrichment indices in the altered rocks, explain the observed hydrothermal alteration-induced mineralogical changes and constrain the possible fluid composition. Gold occurs as fracture fillings in the quartz matrix and pyrite grains, together with other sulfides, and associated with the fS2-buffering assemblage pyrite + pyrrhotite. Arsenopyrite compositions in the pyrite-arsenopyrite assemblage yielded temperature and log fS2 ranges of 305° to 335°C and −10.9 to −8.6, respectively. These temperature estimates compare well with the published results of chlorite thermometry (263°–323°C). Mineral-fluid equilibrium computations place narrow constraints on the ore fluid fO2 (and aH2S). Gold precipitation was a consequence of postpeak metamorphic fluid phase separation involving immiscibility of gaseous spies, fluid-rock interaction, and decrease in fO2 of the fluid. Published δ34S values in pyrite fall in a narrow range of 1.4 to 7.1‰, and the calculated δ34SH2S varies from 0.2 to 5.8‰, at log fO2 = −32.6 and pH = 5.15 to 5.95; these imply that Au(HS)2− was the dominant gold complex. The narrow range of δ34SPy values is indicative of magmatic (± mantle) source, or involvement of fluid with an average crustal sulfur composition. The geologic events responsible for Jonnagiri started with accretion of the greenstone package at the subduction boundary between the eastern and western blocks of the Dharwar craton, followed by their deformation, shearing, and metamorphism, with subsequent emplacement of the Pagadarayi granodiorite and continued shearing, culminating with fluid flow focused along the shear fractures, which resulted in postpeak metamorphic hydrothermal alteration and precipitation of gold-bearing quartz lodes. From the observed craton-scale uniformity in the chemical and sulfur isotope compositions of the ore fluids, we propose a model involving contamination of an auriferous metamorphic fluid with crustal sulfur during its upward transport.

Au and Cr mobilization through metasomatism: Microchemical evidence from ore-bearing listvenite, South Eastern Desert of Egypt

Listvenite is an uncommon rusty-red-weathering quartz-fuchsite-carbonate rock formed by the metasomatic transformation of mafic and ultramafic rocks, typically at the margins of fault-bound ophiolitic blocks. In the highly deformed orogenic belts, listvenite is commonly associated with metal enrichments and auriferous quartz veins. Electron microprobe measurements are used to study the mobility and dispersion of Cr, Au and other elements in listvenite associate with the Haimur gold deposit, South Eastern Desert of Egypt. The results suggest that the Cr-Fe3+-enriched spinel rims and Cr-chlorite aureoles around relict Cr-rich cores are the result of destabilization of chromite during metasomatism. The association of disseminated Cr-chlorite, fuchsite and Fe-Mg carbonate in silicified wallrock and slivers within the auriferous quartz lodes implies Cr mobilization by CO2-rich hydrothermal fluids. Intergrowth textures suggest that listvenitization was concomitant with quartz veining.Early paragenetic idiomorphic Co-gersdorffite crystals disseminated in the quartz lodes show similar compositions, generally containing several hundreds ppm Au, while dispersed, irregular gersdorffite grains, Ni-bearing arsenopyrite and As-bearing pyrite are considered relatively late. These late paragenetic sulfarsenide minerals commonly contain up to 2000ppm Au, and less common Sb. The distribution of Ni, Co, As, and Au in these minerals suggests mobilization and re-distribution by infiltrating hydrothermal fluids with sufficient CO2 to form the abundant carbonate in lodes and wallrocks. The positive correlation between Ni and As in the disseminated pyrite and arsenopyrite argues for the dissolution of Ni-bearing phases in serpentinite during carbonatization and co-precipitation.Temperature estimates based on the composition of gersdorffite, arsenopyrite and hydrothermal chlorite reflect the development of the observed mineral associations as the system cooled down from ~450 to 250°C. The oxygen and sulfur fugacity (log fO2=−30 and log fS2=−8bars at 350–400°C) corresponds to the low sulfidation and oxidizing environment typical for orogenic, silica-rich ore-bearing listvenite.

Alteration of Platinum-Group and Base-Metal Mineral Assemblages in Ophiolite Chromitites from the Dobromirtsi Massif, Rhodope Mountains (Bulgaria)

The Dobromirtsi Ultramafic Massif, located in the Rhodope Mountains (SE Bulgaria), is a portion of a Paleozoic sub-oceanic mantle affected by polyphase regional metamorphism. This massif contains several small, podiform chromitite bodies which underwent the same metamorphic evolution as their host peridotites. Like other ophiolite chromitites, those found in Dobromirtsi carry abundant platinum-group minerals (PGM) and base-metal minerals (BMM). The PGM consist mainly of Ru-, Os-, and Ir-based PGM (laurite RuS2-erlichmanite OsS2, Os-Ru-Ir alloys, irarsite [IrAsS], Ru-rich pentlandite, and an unknown Ir-sulfide) but minor Rh- and Pd-based PGM (hollingworthite [RhAsS] and a series of unidentified stannides and sulfantimonides) are also present. In contrast, the BMM are dominated by pentlandite (Ni,Fe)9S8, followed by heazlewoodite (Ni3S2), breithauptite (NiSb), maucherite (Ni11As8), godlevskite (Ni7S6), gersdorffite (NiAsS), millerite (NiS), undetermined minerals containing Ni, As and Sb, orcelite (Ni5-XAs2), awaruite (Ni3Fe) and chalcopyrite (CuFeS2). The detailed study of the textural relationships, morphology and composition of the PGM and BMM inclusions indicate the existence of two different PGM-BMM assemblages: (i) a primary or magmatic; and (ii) a secondary related with postmagmatic alteration. The PGM and BMM inclusions in unaltered zones of chromite crystals (mainly laurite-erlichmanite and pentlandite) are considered to be primary magmatic minerals formed under variable temperature (1200–1100°C) and sulfur fugacity (between −2 and −0.5 log fS2). In contrast, PGM and BMM located along altered edges of chromite and serpentinised silicate matrix are considered to be secondary, formed from or re-equilibrated with altering fluids. Secondary PGM and BMM assemblages are considered as result of the combination of reducing and oxidising events related with regional metamorphism. Under low fO2 states, fS2 also drops giving place to the formation of S-poor Ni-rich sulfides and secondary Ru-alloys by desulfurisation of primary S-containing minerals. In contrast, predominance of platinum-group elements and/or base-metal arsenides and sulfarsenides associated with the altered edges of chromite (chromite strongly enriched in Fe2O3) is related with the fixing of remobilised PGE (mainly Ir, Rh and Pd) and base-metals (mainly Ni and Fe) when late oxidising fluids supplied As as well as Sb and Sn.