Low-sulfidation System Research Articles

Recently-discovered Bahçecik Au±Ag mineralization in the Eastern Pontides, Gümüşhane-NE Türkiye: geological and geochemical implications on the intermediate sulfidation epithermal deposit

ABSTRACT Intermediate sulfidation epithermal deposits represent significant reservoirs of precious metals, playing a crucial role in global mining operations. The Anatolian Peninsula, predominantly characterized by relatively young (Tertiary) lithologies, hosts numerous occurrences of epithermal base and precious metal mineralization. Within the realm of research in Northeastern Anatolia, several hydrothermal alteration zones have emerged as indicators of concealed epithermal mineralization, with the Bahçecik mineralization standing out as a recent focal point. The Bahçecik mineralization represents an intermediate sulfidation epithermal Au-Ag system, hosted by Late Cretaceous andesitic rocks, as well as Eocene dacitic lava and tuffs. Employing U-Pb LA-ICP-MS zircon dating of the dacitic host rocks, the mineralization is estimated to have occurred during the Lutetian epoch (43.61 ± 0.35 Ma) or later. The genesis of the Bahçecik deposit is believed to be linked to postcollisional regional extensional tectonics, primarily controlling its structural arrangement. Comprising pyrite, chalcopyrite, low-Fe sphalerite, galena, tetrahedrite/tennantite, enargite-luzonite, and gold-silver minerals, the deposit is accompanied by prominent gangue minerals such as Mn- and Ca-carbonates, quartz, and barite. Microprobe measurements have identified a majority of Au-Ag minerals (krennerite, buckhornite, and nagyagite) closely associated with sulphide minerals. Fluid inclusion data reveal that mineralization and fluid transportation occurred at temperatures ranging from 153 to 327°C, with solutions exhibiting salinity levels consistent with those of intermediate sulfidation deposits (1.7–7.3 wt.% NaCl). This study unequivocally classifies the Bahçecik mineralization as an intermediate sulfidation deposit, contrary to the conclusions drawn in previous research. Moreover, our investigation has unveiled the presence of a substantial ore resource, totalling 7.84 million tons, featuring 1.76 g/t Au and 2.24 g/t Ag. As an intermediate-type deposit, it shares characteristics with other subtypes within the epithermal class, bridging the realms of both high and low sulfidation systems.

Characteristics and genesis of phyllosilicate hydrothermal assemblages from Neoproterozoic epithermal Au-Ag mineralization of the Avalon Zone of Newfoundland, Canada

The Avalon Zone hosts well-preserved examples of high- and low-sulfidation epithermal Au-Ag mineralization and associated phyllosilicate alteration. Using examples from the Hope Brook and Hickey's Pond high-sulfidation systems (HSS) and the Heritage low-sulfidation system (LSS), this study presents new data on the mineralogical and chemical characterization of phyllosilicates associated with epithermal Au-Ag deposits. Electron probe microanalysis and X-ray diffraction studies revealed that the HSS examples contain Fe muscovite ((K0.62–0.79Na0.10–0.37Ca0.00–0.01)Al1.31–1.72(Mg0.01–0.08Fe0.05–0.19)(Al1.00–1.24Si2.76–3.00O10)(OH)2), paragonite ((K0.05–0.30Na0.78–1.02Ca0.00–0.01)Al1.66–1.84(Mg0.00–0.01Fe0.02–0.04)(Al1.01–1.11Si2.89–2.99O10)(OH)2), Al clinochlore ((Mg2.42–2.65Fe1.62–1.78)(Si2.57–2.69Al1.31–1.43O10)(OH)8), pyrophyllite, kaolin minerals, illite-smectite (I-Sme) and chlorite-smectite (C-Sme). Conversely, the LSS examples contain Fe-Mg muscovite ((K0.44–0.76Na0.00–0.02Ca0.00–0.01)Al1.55–1.82(Mg0.17–0.31Fe0.05–0.12)(Al0.55–0.71Si3.29–3.45O10)(OH)2), Al clinochlore ((Mg2.17–2.56Fe1.59–2.01)(Si2.71–2.92Al1.08–1.29O10)(OH)8), and I-Sme and C-Sme intermediates. Chlorite geothermometry indicated that HSS and LSS assemblages crystallized at average temperatures of 261 °C and 145 °C, respectively. Mica and chlorite from HSS and LSS are defined by Tschermak substitution and high-temperature polytypism (2M1 and IIb, respectively). In the HSS examples, one or more pulses of hot, acidic fluid reacted with host rocks generating an envelope of advanced argillic alteration (Fe-Ms, Kln, Pg, Prl) around a vuggy silica core. This is bounded by a zone of argillic alteration (Fe-Ms, Kln), with sericitic/chloritic alterations (Fe-Ms, Al-Clc) developed in distal parts of the systems. In the LSS, near-neutral hydrothermal fluids led to the formation of broad phyllic/chloritic zones (Fe-Mg Ms and Al-Clc). A low temperature overprint with 1M mica, Ib clinochlore ((Mg2.48–2.74Fe1.58–1.73)(Si3.05–3.15Al0.85–0.95O10)(OH)8), kaolinite and Sme-poor I-Sme and C-Sme is attributed to late waning stage hydrothermal activity and/or weathering. A close paragenetic link was established between Fe-Mg muscovite and Ag mineralization at Heritage, as well as between Fe-Mg muscovite and bladed calcite, suggesting boiling as the precipitation mechanism. The present study provides guidance on how phyllosilicate assemblages have the potential to discern HSS versus LSS mineralization during early prospection in terranes with limited exposure, and to contribute to the reconstruction of their hydrothermal record.

Mineralogy and microthermometry of the Escondida, Gabriela and Margarita mineralized structures from the Cerro Moro epithermal deposit, Deseado Massif, Argentina

Cerro Moro deposit is a low sulfidation Au–Ag epithermal deposit located in Santa Cruz province, southern Patagonia Argentina. It is hosted by numerous structurally controlled quartz veins developed in close spatial and temporal relationship with the Jurassic extension and magmatism. The main area in Cerro Moro comprises the Escondida horst block, limited by NW-SE listric faults which provided the channels for the hydrothermal system. The Escondida (including its high grade ore shoot Zoe), Gabriela and Margarita prospects are representative areas documenting the dynamics of the major fault systems. Four stages of mineralization were defined in Cerro Moro. Gabriela and Margarita prospects, at the center and NE of the deposit, respectively, show good development of bladed quartz replacement textures with adularia crystals and scarce translucent/opaque minerals from the stage 1 (pyrite-sphalerite > chalcopyrite ≫ galena-gold). The Escondida prospect, representing the southwest boundary of the Escondida horst block, is the most significantly mineralized structure hosting the base metal-rich and high grade Au–Ag mineralization. In this vein and in the Zoe high grade ore shoot, the translucent/opaque minerals are abundant (sphalerite, galena, chalcopyrite, lesser pyrite, marcasite, gold, silver and different quantities of Ag-bearing minerals) and mainly related to the dark grey, finely-grained quartz-muscovite veins from the stage 3 and to a lesser degree, from the stage 2. The presence of porous massive, crustiform, moss and colloform textures are interpreted to be formed by a sudden pressure release, where hydrofracturing and repetitive boiling occurred and lead to mineral deposition. Whitish quartz-adularia veinlets from the stage 4 may crosscut the previous sequences. The transparent, translucent and opaque minerals in Cerro Moro express a typical low sulfidation system, where the vein system is the result of the infilling and deposition of low temperature (174–265 °C) and salinity (0.4–0.7 wt% NaCl(eq)) hydrothermal fluids.

Multiple mineralization events in the Zacatecas Ag-Pb-Zn-Cu-Au district, and their relationship to the tectonomagmatic evolution of the Mesa Central, Mexico

Mineralization in the Zacatecas district is polymetallic (Ag, Zn, Pb, Cu, and Au) and occurs as skarn-type and epithermal veins formed in different metallogenetic stages. The oldest mineralization in the district is skarn-type, Cu-rich with lesser Zn-Pb-Ag, and is considered to be close in age to felsic dikes and plugs dated at ∼51 Ma. Epithermal mineralization occurs in both low- and intermediate-sulfidation styles. Intermediate-sulfidation veins (the Veta Grande, Mala Noche, El Bote, and La Cantera veins) are polymetallic, Ag-rich, hosted in ESE- to SE-striking structures, and were formed at ∼42 Ma (adularia 40Ar/39Ar isochron age from Veta Grande of 42.36 ± 0.18 Ma; 2σ, MSWD = 0.76). Low-sulfidation Au-(Ag) mineralization occurs in the N–S-trending El Orito vein system, which yielded an adularia 40Ar/39Ar isochron age of 29.19 ± 0.20 Ma (2σ, MSWD = 1.8). These ages and the differences in structural orientation indicate that the two styles of epithermal mineralization are temporally and tectonically unrelated.The mineral paragenesis of the Mala Noche deposit consists of early skarn-type Cu mineralization overprinted by later epithermal Pb-Zn-Ag veins. Skarn-type minerals include relicts of prograde silicate minerals (hedenbergite and garnets), retrograde silicate minerals (ilvaite, grunerite, stilpnomelane, epidote, clinochlore), and ore minerals (chalcopyrite, pyrite, sphalerite, galena, magnetite, wolframite, and minor bismuthinite). Epithermal mineralization is characterized by layered to vuggy quartz veins and breccias, with phyllic wallrock alteration (quartz, sericite-illite). The veins consist of quartz, calcite, dolomite, and ankerite with variable amounts of base metal sulfides (sphalerite, galena, pyrite, minor chalcopyrite, and rare acanthite and stromeyerite).The Veta Grande epithermal mineralization was emplaced in two main stages of Ag-rich quartz veining, with narrow selvedges of phyllic (quartz-sericite) wallrock alteration. Stage I consist of quartz, calcite, and minor adularia intergrown with pyrite, followed by sphalerite, galena, and lesser chalcopyrite, acanthite, pyrargyrite, and jamesonite. Stage II mineral paragenesis is similar to stage I but is characterized by amethystine quartz and contains less abundant sulfide minerals.The ore mineral paragenesis of the El Compas vein, within the El Orito System, consists of quartz, adularia, calcite, and chalcedony with minor pyrite, followed by rare aguilarite, naumannite, electrum, and native gold.The skarn-type and intermediate-sulfidation mineralization is coeval with Eocene subduction-related magmatism in the Zacatecas area, which is constrained by zircon U-Pb ages for igneous rocks between 51 and 42 Ma. The emplacement of these magmas was controlled by the same regional-scale, ESE- to SE-trending, transtensional structures that controlled the skarn-type and intermediate-sulfidation deposits. This mineralization is thus interpreted to be related to the last stages of subduction volcanism in central Mexico, under transtensional stress conditions. In contrast, no nearby magmatism is clearly related to the Oligocene low-sulfidation system. However, its age and structural orientation (N–S), combined with a regional change in magma composition from Eocene calc-alkaline to Oligocene bimodal volcanism in central Mexico, suggest that the low-sulfidation mineralization is related to post-subduction continental extension processes, reflecting the beginning of Basin and Range tectonics.

Native gold from the Inagli Pt–Au placer deposit (the Aldan Shield, Russia): geochemical characteristics and implications for possible bedrock sources

The Inagli alluvial Pt–Au placer deposit in the Republic of Sakha (Yakutia), Russia, is linked to the Inagli massif, one of the several Uralian–Alaskan-type alkaline-ultrabasic complexes in the Aldan Shield. Gold from the placer is heterogeneous in composition and is represented by three types. Type 1 gold is the most abundant and is characterized by simple Au–Ag alloys with 4–34 wt% Ag, low Cu (up to 0.08 wt%) and negligible Hg, Pt, and Pd contents, and silver–tellurium sulfosalts (Ag–Cu–Te–S–As compounds) in the inclusion suite. Silicate inclusions are biotite, K-feldspar, Fe–Mg amphibole, chlorite, plagioclase, Fe–Mg pyroxene, zircon, and titanite. Distinctive features of this gold type are most similar to those derived from low-sulfidation systems linked to iron oxide copper–gold or iron skarn types of mineralization. The bedrock source of type 1 gold could be related with monzonite to syenite intrusions surrounding the Inagli massif. Distinctive features of type 2 gold include a wide discontinuous range of Ag content (1–18 wt%), elevated Cu (up to 0.5 wt%), and occasional Pd (up to 0.3 wt%) levels, non-detectable Pt and Hg contents, and rare inclusions of simple sulfides (digenite, pyrrhotite) and Na amphibole. Type 3 gold is distinguished by a narrow range in Ag content (5–8 wt%), elevated Hg (0.5–1 wt%) contents, negligible Cu, Pt and Pd levels, and Au–Pb compounds + K-feldspar inclusions. Microchemical characteristics of type 2 and type 3 gold are interpreted as suggestive of an alkaline–magmatic-related fluid. Based on the grain morphology and microchemical signatures, potential bedrock sources for both gold types could be related to the numerous alkaline veins and potassic alteration zones within the dunite core. A comparison of the Inagli and the Kondyor placer gold allows to generate distinctive generic signatures for gold from Uralian–Alaskan-type alkaline–ultrabasic complexes in the Aldan Shield.

Native sulfur, sulfates and sulfides from the active Campi Flegrei volcano (southern Italy): Genetic environments and degassing dynamics revealed by mineralogy and isotope geochemistry

We investigated sulfur-bearing minerals from the Campi Flegrei caldera, southern Italy, in relation to the increase of hydrothermal activity phenomena since 2006, aimed at providing insights into the volcanic system dynamics. Mineral encrustations and muds were sampled between 2013 and 2015 at the long-standing degassing crater of the Solfatara tuff cone and its recently restless north-eastern Pisciarelli slope. Deep-seated sulfides were further separated from two drill cores (AGIP's Mofete boreholes: 1500m and 2695m depth).The mineral assemblage and texture of sampled encrustations were determined by X-ray diffraction, optical and scanning electron microscopy and X-ray microanalysis by energy dispersive spectrometry. Native sulfur and alunite dominate among the newly formed mineral phases. Other minerals are mostly alunogen, and locally pickeringite, potassium alum, hematite and pyrite. Mereiterite and amarillite sporadically occur. The mud pools are rich in gypsum, potassium alum and pyrite. Quartz and argillic phases, locally with analcime, are dispersed in the outcropping rocks.δ34S values were determined for shallow subsurface native sulfur (−5.5 to 0.0‰) and alunite (−1.7 to −0.2‰), as well as for the deep-seated pyrite (3.3 to 7.4‰ in the depth range:1500–2695m). δ18O values were measured for shallow native alunite (4.2 to 7.0‰). Pisciarelli alunite was finally analyzed for its 87Sr/86Sr ratio and 143Nd/144Nd ratios (0.707517±6 and 0.512459±6, respectively).Textural and isotopic data constrain the genesis of alunite at the expense of K-feldspars through rock alteration by hydrothermal fluids. We suggest that the caldera is a low-sulfidation system hosting acid-sulfate deposits in its active degassing area. The acid-sulfate environment developed on an argillitic facies that thins outwards and is characteristic for steam-heated and magmatic-steam environments. These environments developed in relation to the fractured settings that facilitates the gas emissions from deeper levels and the meteoric infiltration from surface to depth. Newly determined δ34S data are lower than previously published values, suggesting a progressive decrease in the sulfur isotopic composition through time at least since 1956. The isotopic variation is associated with a lowering of the SO2/H2S ratio and the widening of the surface of CO2 degassing, in the absence of significant changes in the average CO2 flux values and in the carbon and helium isotope compositions. Native sulfur and sulfates move from sulfur isotope equilibrium to disequilibrium. We ascribe this transition to enhanced SO2 scrubbing due to a high rock permeability that facilitates a faster gas ascent rate in the shallow crust.

A review of the stable-isotope geochemistry of sulfate minerals in selected igneous environments and related hydrothermal systems

The stable-isotope geochemistry of sulfate minerals that form principally in I-type igneous rocks and in the various related hydrothermal systems that develop from their magmas and evolved fluids is reviewed with respect to the degree of approach to isotope equilibrium between minerals and their parental aqueous species. Examples illustrate classical stable-isotope systematics and principles of interpretation in terms of fundamental processes that occur in these systems to produce (1) sulfate in igneous apatite, (2) igneous anhydrite, (3) anhydrite in porphyry-type deposits, (4) magmatic-hydrothermal alunite and closely related barites in high-sulfidation mineral deposits, (5) coarse-banded alunite in magmatic-steam systems, (6) alunite and jarosite in steam-heated systems, (7) barite in low-sulfidation systems, (8) all of the above minerals, as well as soluble Al and Fe hydroxysulfates, in the shallow levels and surface of active stratovolcanoes. Although exceptions are easily recognized, frequently, the sulfur in these systems is derived from magmas that evolve fluids with high H 2S/SO 2. In such cases, the δ 34S values of the igneous and hydrothermal sulfides vary much less than those of sulfate minerals that precipitate from magmas and from their evolved fluids as they interact with igneous host rocks, meteoric water, oxygen in the atmosphere, and bacteria in surface waters. Hydrogen isotopic equilibrium between alunite and water and jarosite and water is always initially attained, thus permitting reconstruction of fluid history and paleoclimates. However, complications may arise in interpretation of δD values of magmatic-hydrothermal alunite in high-sulfidation gold deposits because later fluids may effect a postdepositional retrograde hydrogen–isotope exchange in the OH site of the alunite. This retrograde exchange also affects the reliability of the SO 4–OH oxygen–isotope fractionations in alunite for use as a geothermometer in this environment. In contrast, retrograde exchange with later fluids is not significant in the lower temperature steam-heated environment, for which SO 4–OH oxygen–isotope fractionations in alunite and jarosite can be an excellent geothermometer. Sulfur isotopic disequilibrium between coexisting (but noncontemporaneous) igneous anhydrite and sulfide may occur because of loss of fluid, assimilation of country-rock sulfur during crystallization of these minerals from a magma, disequilibrium effects related to reactions between sulfur species during fluid exsolution from magma, or because of retrograde isotope exchange in the sulfides. Anhydrite and coexisting sulfide from porphyry deposits commonly closely approach sulfur–isotope equilibrium, as is indicated by the general agreement of sulfur–isotope and filling temperatures (315 to 730 °C) in quartz. The data from anhydrite and coexisting sulfides also record a significant range in H 2S/SO 4 2− and δ 34S ΣS among deposits and even during the course of mineralization at a single deposit. Sulfur isotopic disequilibrium among aqueous sulfur species may occur in any hydrothermal environment except the relatively high-temperature (200–400 °C) low-pH (<3) magmatic-hydrothermal environment, in which SO 4 2− forms along with H 2S from the disproportionation of SO 2 during the condensation of a magmatic vapor plume. Magmatic–steam alunite forms from expanding SO 2-rich magmatic vapors that rise so rapidly that sulfur isotopic exchange between SO 4 2− and H 2S does not occur during disproportionation of SO 2. Such alunite has δ 34S values similar to that of the bulk sulfur in the magma. Residence times of SO 4 2− in steam-heated systems, although seldom long enough to permit more than partial sulfur–isotope exchange with streaming H 2S, often are long enough to permit oxygen–isotope equilibrium with water. In active stratovolcanoes, aqueous sulfate derived from the disproportionation of SO 2 and the oxidation of H 2S can also mix with that derived from the oxidation of pyrite near the surface. In the near-neutral low-sulfidation system at Creede, CO, isotopic exchange among hydrothermal aqueous species was so slow that sulfur and even oxygen isotopic signatures derived from bacteriogenic and thermochemical reduction of sulfate in moat sediments are observed in hydrothermal barite.

Epithermal Alteration and Mineralization in the Comstock District, Nevada

The precious and base metal deposits of the world-renowned Comstock district of western Nevada constitute one of several superimposed Miocene hydrothermal systems. In the district, Mesozoic metasedimentary and igneous rocks are overlain by Oligocene to Miocene ash-flow tuffs. A thick sequence of middle Miocene andesitic volcanic rocks and intrusions host the bulk of the hydrothermal alteration and ore deposits. Some of the magmatic events were directly associated with hydrothermal activity. Secondary biotite alteration locally affects preore dioritic intrusions. Preore barren quartz, alunite, pyrophyllite, and kandite alteration (high-sulfidation style) is zoned outward from mostly discontinuous, crudely radial, fractures associated with andesitic intrusions. Blanket-like cristobalite, alunite, and kaolinite alteration exposed at the periphery of the district may be linked to preore alteration near the paleowater table. Later, large volumes of subore-grade massive quartz veins and quartz-adularia stockworks were deposited in steeply to moderately east-dipping sets of north- and northeast-striking faults that also localized the major precious metal-bearing lodes. Quartz, chlorite, illite, and very localized muscovite alteration (deep low-sulfidation) developed coeval with vein deposition. Epidote-bearing propylitic alteration forms halos around the higher-temperature parts of the lodes. Adularia and rare bladed calcite indicate boiling during deep low-sulfidation hydrothermal activity, but little is apparently associated with ore deposition. The main stage Au-Ag-Cu-Zn-Pb ore was deposited late in the deep low-sulfidation system in irregularly spaced lenticular dilatant zones or structural intersections. Massive, nonbladed(?) calcite was deposited just below and in the lower parts of orebodies in the Comstock lode, but it was deposited in the upper parts of orebodies or was barren elsewhere in the district. Pliocene to Holocene reactivation of faults has disrupted the district, redistributing the relative positions of many of the orebodies and alteration assemblages.

Re–Os isotopes applied to the epithermal gold deposits near Bucaramanga, northeastern Colombia

The epithermal gold mineralization near Bucaramanga, Colombia, is spatially associated with a dacitic porphyry of Upper Cretaceous to Early Tertiary age. Two mining districts in the vicinity of Bucaramanga, the California and the Vetas, host different styles of mineralization that could be interpreted as high- and low-sulfidation style mineralization, respectively. Re–Os isotope systematics were used on sulfide-gold-rich gravity concentrates in an attempt to determine both the ages of the epithermal deposits and the possible genetic relationships between the porphyry and the epithermal mineralization. The concentration of Os for sulfide samples taken from both systems is relatively uniform, ranging from 19 to 35 ppt (parts per trillion), whereas the Re concentration varies significantly, with the California system averaging 10 ppb (parts per billion) and the Vetas system averaging 140 ppb. The samples from the high-sulfidation California deposit form an isochron with an age of 57±10 Ma (MSWD=0.8), which overlaps with the age of the dacitic volcanism. The initial 187Os/ 188Os of the isochron is 1.20±0.13 and indicates a predominately crustal source for the Os and, by inference, gold. The samples from the Vetas low-sulfidation system have very high Re/Os ratios and do not lie on the isochron. These data reveal a difference between two epithermal systems that border one intrusion. Sulfide samples from the high-sulfidation system are isotopically homogenous with respect to Re–Os (form an isochron) and probably represent mineralization linked closely with the high temperature volatiles/fluids from the magma. In contrast, the sulfides from the Vetas system are in disequilibrium with respect to Re–Os and may illustrate mineralization that is not directly related to similar magmatic fluids.

Boiling and vertical mineralization zoning: a case study from the Apacheta low-sulfidation epithermal gold-silver deposit, southern Peru

The Au-Ag (±Pb-Zn) Apacheta deposit is located in the Shila district, 600 km southeast of Lima in the Cordillera Occidental of Arequipa Province, southern Peru. The vein mineralization is found in Early to Middle Miocene calc-alkaline lava flows and volcanic breccias. Both gangue and sulfide mineralization express a typical low-sulfidation system; assay data show element zoning with base metals enriched at depth and higher concentrations of precious metals in the upper part of the veins. Three main deposition stages are observed: (1) early pyrite and base-metal sulfides with minor electrum 1 and acanthite; (2) brecciation of this mineral assemblage and cross-cutting veinlets with subhedral quartz crystals, Mn-bearing calcite and rhombic adularia crystals; and finally (3) veinlets and geodal filling of an assemblage of tennantite/tetrahedrite + colorless sphalerite 2 + galena + chalcopyrite + electrum 2. Fluid inclusions in the mineralized veins display two distinct types: aqueous-carbonic liquid-rich Lw-c inclusions, and aqueous-carbonic vapor-rich Vw-c inclusions. Microthermometric data indicate that the ore minerals were deposited between 300 and 225 °C from relatively dilute hydrothermal fluids (0.6–3.4 wt% NaCl). The physical and chemical characteristics of the hydrothermal fluids show a vertical evolution, with in particular a drop in temperature and a loss of H2S. The presence of adularia and platy calcite and of co-existing liquid-rich and vapor-rich inclusions in the ore-stage indicates a boiling event. Strong H2S enrichment in the Vw-c inclusions observed at –200 m, the abundance of platy calcite, and the occurrence of hydrothermal breccia at this level may indicate a zone of intense boiling. The vertical element zoning observed in the Apacheta deposit thus seems to be directly related to the vertical evolution of hydrothermal-fluid characteristics. Precious-metal deposition mainly occurred above the 200-m level below the present-day surface, in response to a liquid/vapor phase separation due to an upward boiling front.

COMPOSITIONAL VARIATION OF ARSENOPYRITE AND FLUID EVOLUTION AT THE ULSAN DEPOSIT, SOUTHEASTERN KOREA: A LOW-SULFIDATION PORPHYRY SYSTEM

The Ulsan Fe–W mine is located within the Cretaceous Gyeongsang volcano-sedimentary basin at the southeastern edge of the Korean Peninsula. Distinct hydrothermal events resulted in calcic skarn and vein deposits in recrystallized limestone near a Tertiary epizonal granite stock. The deposits of the Ulsan mine present a unique opportunity to document geochemically the complex evolution of a skarn–vein system that is related genetically to a low-sulfidation system. Isochemical contact metamorphism of an early skarn stage (stage I) is displayed by the presence of anhydrous Ca–Al–Mg skarn minerals at the contact between granite and recrystallized limestone. Following magnetite deposition in the main prograde skarn (stage II), the first deposition of arsenopyrite occurs intergrown with rammelsbergite – niccolite – gersdorffite – lollingite – native bismuth – bismuthinite –hexagonal pyrrhotite. These common sulfide assemblages are characterized by an overall low-sulfidation state during the main skarn stage. Retrograde skarn (stage III) is characterized by minor impregnations of scheelite in calcite and quartz, with actinolite and chlorite. During the latest part of stage III, Cu–Zn and polymetallic sulfide mineralization was introduced. The latest episode in the hydrothermal system (stage IV) is characterized by Zn–Pb–Ag mineralization in siderite–quartz veins. Decreasing As contents in arsenopyrite from stages II to IV indicate a decrease in temperature or sulfur fugacity (or both) with time. The various skarn-forming events and ore minerals from various stages are interpreted to have resulted from an evolutionary trend from hypersaline magmatic fluids during prograde skarn formation associated with Fe–As(–Ni) mineralization to low-salinity and low-temperature fluids during the retrograde skarn formation, associated with W–Cu–Zn mineralization. As the influence of magma-derived fluids waned, surficial fluids descended to deeper levels along fractures, resulting in siderite–quartz deposition associated with Zn–Pb–Ag mineralization. These results demonstrate that the Ulsan deposit is likely a skarn deposit that is genetically related to a low-sulfidation porphyry system.