Geology and genesis of Kafang Cu–Sn deposit,Gejiu district,SW China

Kafang is one of the main ore deposits in the world-class Gejiu polymetallic tin district, SW China. There are three main mineralization types in the Kafang deposit, i.e., skarn Cu–Sn ores, stratiform Cu ores hosted by basalt and stratiform Cu–Sn ores hosted by carbonate. The skarn mainly consists of garnet and pyroxene, and retrograde altered rocks. These retrograde altered rocks are superimposed on the skarn and are composed of actinolite, chlorite, epidote and phlogopite. Major ore minerals are chalcopyrite, pyrrhotite, cassiterite, pyrite and scheelite. Sulfur and Pb isotopic components hint that the sources of different types of mineralization are distinctive, and indicate that the skarn ore mainly originated from granitic magma, whereas the basalt-hosted Cu ores mainly derived from basalt. Microthermometry results of fluid inclusions display a gradual change during the ore-forming process. The homogenization temperature of different types of inclusions continuously decreases from early to late mineralization stages. The salinities and freezing temperatures exhibit similar evolutionary tendencies with the T _{homogenization}, while the densities of the different types keep constant, the majority being less than 1. Oxygen and hydrogen isotopic values (δ¹⁸O and δD) of the hydrothermal fluids fall within ranges of 3.1 to 7.7‰ with an average of 6.15‰, calculated at the corresponding homogenization temperature, and − 73 and − 98‰ with an average of − 86.5‰, respectively. Microthermometry data and H–O isotopes indicate that the ore-forming fluid of the Kafang deposit is mainly derived from magma in the early stage and a mixture of meteoric and magmatic water in late stage. Molybdenite Re–Os age of the skarn type mineralization is 83.4 ± 2.1 Ma, and the stratiform ores hosted by basalt is 84.2 ± 7.3 Ma, which are consistent with the LA-ICP-MS zircon age of the Xinshan granite intrusion (83.1 ± 0.4 Ma). The evidence listed above reflects the fact that different ore styles in the Kafang deposit belong to the same mineralization system.     

The origin and evolution of skarn-forming fluids from the Phu Lon deposit,northern Loei Fold Belt,Thailand: Evidence from fluid inclusion and sulfur isotope studies

The Phu Lon skarn Cu–Au deposit is located in the northern Loei Fold Belt (LFB), Thailand. It is hosted by Devonian volcano-sedimentary sequences intercalated with limestone and marble units, intruded by diorite and quartz monzonite porphyries. Phu Lon is a calcic skarn with both endoskarn and exoskarn facies. In both skarn facies, andradite and diopside comprise the main prograde skarn minerals, whereas epidote, chlorite, tremolite, actinolite and calcite are the principal retrograde skarn minerals.Four types of fluid inclusions in garnet were distinguished: (1) liquid-rich inclusions; (2) daughter mineral-bearing inclusions; (3) salt-saturated inclusions; and (4) vapor-rich inclusions. Epidote contains only one type of fluid inclusion: liquid-rich inclusions. Fluid inclusions associated with garnet (prograde skarn stage) display high homogenization temperatures and moderate salinities (421.6–468.5 °C; 17.4–23.1 wt% NaCl equiv.). By contrast, fluid inclusions associated with epidote (retrograde skarn stage) record lower homogenization temperatures and salinities (350.9–399.8 °C; 0.5–8 wt% NaCl equiv.). These data suggest a possible mixing of saline magmatic fluids with external, dilute fluid sources (e.g., meteoric fluids), as the system cooled. Some fluid inclusions in garnet contain hematite daughters, suggesting an oxidizing magmatic environment. Sulfur isotope determinations on sulfide minerals from both the prograde and retrograde stages show a uniform and narrow range of δ³⁴S values (?2.6 to ?1.1 ‰ δ³⁴S), suggesting that the ore-forming fluid contained sulfur of orthomagmatic origin. Overall, the Phu Lon deposit is interpreted as an oxidized Cu–Au skarn based on the mineralogy and fluid inclusion characteristics.     

Alteration and mineralization at the Zhibula Cu skarn deposit,Gangdese belt,Tibet

The Zhibula Cu skarn deposit contains 0.32 Mt. Cu metal with an average grade of 1.64% and is located in the Gangdese porphyry copper belt in southern Tibet. The deposit is a typical metasomatic skarn that is related to the interaction of magmatic–hydrothermal fluids and calcareous host rock. Stratiform skarn orebodies occur at the contact between tuff and marble in the Lower Jurassic Yeba Formation. Alteration zones generally grade from a fresh tuff to a garnet-bearing tuff, a garnet pyroxene skarn, and finally to a wollastonite marble. Minor endoskarn alteration zonations are also observed in the causative intrusion, which grade from a fresh granodiorite to a weakly chlorite-altered granodiorite, a green diopside-bearing granodiorite, and to a dark red-brown garnet-bearing granodiorite. Prograde minerals, which were identified by electron probe microanalysis include andradite–grossularite of various colors (e.g., red, green, and yellow) and green diopside. Retrograde metamorphic minerals overprint the prograde skarn, and are mainly composed of epidote, quartz, and chlorite. The ore minerals consist of chalcopyrite and bornite, followed by magnetite, molybdenite, pyrite, pyrrhotite, galena, and sphalerite. Three types of fluid inclusions are recognized in the Zhibula deposit, including liquid-rich two-phase inclusions (type L), vapor-rich two-phase inclusions (type V), and daughter mineral-bearing three-phase inclusions (type S). As the skarn formation evolved from prograde (stage I) to early retrograde (stage II) and later retrograde (stage III), the ore-forming fluids correspondingly evolved from high temperature (405–667 °C), high salinity (up to 44.0 wt.% NaCl equiv.), and high pressure (500–600 bar) to low-moderate temperature (194–420 °C), moderate-high salinity (10.1–18.3 and 30.0–44.2 wt.% NaCl equiv.), and low-moderate pressure (250–350 bar). Isotopic data of δ³⁴S (− 0.1‰ to − 6.8‰, estimated δ³⁴S_fluids = − 0.7‰), δD_H2O (− 91‰ to − 159‰), and δ¹⁸O_H2O (1.5‰ to 9.2‰) suggest that the ore-forming fluid and material came from magmatic–hydrothermal fluids that were associated with Miocene Zhibula intrusions. Fluid immiscibility likely occurred at the stage I and stage II during the formation of the skarn and mineralization. Fluid boiling occurred during the stage III, which is the most important Cu deposition mechanism for the Zhibula deposit.     

Ore genesis and hydrothermal evolution of the Huanggang skarn iron–tin polymetallic deposit,southern Great Xing'an Range: Evidence from fluid inclusions and isotope analyses

The southern Great Xing'an Range is one of the most important metallogenic belts in northern China, and contains numerous Pb–Zn–Ag–Cu–Sn–Fe–Mo deposits. The Huanggang iron–tin polymetallic skarn deposit is located in the Sn-polymetallic metallogenic sub-belt. Skarns and iron orebodies occur as lenses along the contact between granite plutons and the Lower Permian Huanggangliang Formation marble or Dashizhai Formation andesite. Field evidence and petrographic observations indicate that the three stages of hydrothermal activity, i.e., skarn, oxide and sulfide stages, all contributed to the formation of the Huanggang deposit.The skarn stage is characterized by the formation of garnet and pyroxene, and high-temperature, hypersaline hydrothermal fluids with isotopic compositions that are similar to those of typical magmatic fluids. These fluids most likely were generated by the separation of brine from a silicate melt instead of being a product of aqueous fluid immiscibility. The iron oxide stage coincides with the replacement of garnet and pyroxene by amphibole, chlorite, quartz and magnetite. The hydrothermal fluids of this stage are represented by L-type fluid inclusions that coexist with V-type inclusions with anomalously low δD values (approximately − 100 to − 116‰). The decrease in ore fluid δ¹⁸O_H2O values with time coincides with marked decreases in the fluid salinity and temperature. Based on the fluid inclusion and stable isotopic data, the ore fluid evolved by boiling of the magmatic brine. The sulfide stage is characterized by the development of sphalerite, chalcopyrite, fluorite, and calcite veins, and these veins cut across the skarns and orebodies. The fluids during this stage are represented by inclusions with a variable but continuous sequence of salinities, mainly low-salinity inclusions. These fluids yield the lowest δ¹⁸O_H2O values and moderate δD values ( − 1.6 to − 2.8‰ and − 101 to − 104‰, respectively). The data indicate that the sulfide stage fluids originated from the mixing of residual oxide-stage fluids with various amounts of meteoric water. Boiling occurred during this stage at low temperatures.The sulfur isotope (δ³⁴S) values of the sulfides are in a narrow range of − 6.70 to 4.50‰ (mean = − 1.01‰), and the oxygen isotope (δ¹⁸O) values of the magnetite are in a narrow range of 0.1 to 3.4‰. Both of these sets of values suggest that the ore-forming fluid is of magmatic origin. The lead isotope compositions of the ore (²⁰⁶Pb/²⁰⁴Pb = 18.252–18.345, ²⁰⁷Pb/²⁰⁴Pb = 15.511–15.607, and ²⁰⁸Pb/²⁰⁴Pb = 38.071–38.388) are consistent with those of K-feldspar granites (²⁰⁶Pb/²⁰⁴Pb = 18.183–18.495, ²⁰⁷Pb/²⁰⁴Pb = 15.448–15.602, ²⁰⁸Pb/²⁰⁴Pb = 37.877–38.325), but significantly differ from those of Permian marble (²⁰⁶Pb/²⁰⁴Pb = 18.367–18.449, ²⁰⁷Pb/²⁰⁴Pb = 15.676–15.695, ²⁰⁸Pb/²⁰⁴Pb = 38.469–38.465), which also suggests that the ore-forming fluid is of magmatic origin.     

Origin and evolution of hydrothermal fluids in the Taochong iron deposit,Middle–Lower Yangtze Valley,Eastern China: Evidence from microthermometric and stable isotope analyses of fluid inclusions

The Middle–Lower Yangtze River Valley is one of the most important metallogenic belts in China, hosting numerous Cu–Fe–Au–Mo deposits. The Taochong deposit is located in the northern part of the Fanchang iron ore district of the Middle–Lower Yangtze River metallogenic belt. The Fe-orebody is hosted by Middle Carboniferous to Lower Permian limestones. Skarns and Fe-orebodies occur as tabular bodies along interlayer-gliding faults, at some distance from the inferred granitic intrusions. Field evidence and petrographic observations indicate that the three stages of hydrothermal activity—the skarn, iron oxide (main mineralization stage), and carbonate stages—all contributed to the formation of the Taochong iron deposit. The skarn stage is characterized by the formation of garnet and pyroxene, with high-temperature, hypersaline hydrothermal fluids with isotopic compositions similar to those of typical magmatic fluids. These fluids were probably generated by the separation of brine from a silicate melt instead of the product of aqueous fluid immiscibility. The iron oxide stage coincides with the replacement of garnet and pyroxene by actinolite, chlorite, quartz, calcite and hematite. The hydrothermal fluids at this stage are represented by saline fluid inclusions that coexist with vapor-rich inclusions with anomalously low δD values (− 66‰ to − 94‰). The decrease in ore fluid δ¹⁸O_water with time and decreasing depth is consistent with the decreases in fluid salinity and temperature. The fluid δD values also show a decreasing trend with decreasing depth. Both fluid inclusion and stable isotopic data suggest that the ore fluid during the main period of mineralization was evolved by the boiling of various mixtures of magmatic brine and meteoric water. This process was probably induced by a drop in pressure from lithostatic to hydrostatic. The carbonate stage is represented by calcite veins that cut across the skarn and orebody, locally producing a dense stockwork. This observation indicates the veins formed during the waning stages of hydrothermal activity. The fluids from this stage are mainly represented by a variety of low-salinity fluid inclusions, as well as fewer high-salinity inclusions. These particular fluids have the lowest δ¹⁸O_water values (− 2.2‰ to 0.4‰) and a wide of range of δD values (− 40‰ to − 81‰), which indicate that they were originated from a mixture of residual fluids from the oxide stage, various amounts of meteoric water, and possibly condensed vapor. Low-temperature boiling probably occurred during this stage.We also discuss the reasons behind the anomalously low δD values in fluid inclusion water extracted by thermal decrepitation from quartz at high temperatures, and suggest that calcite data provide a possible benchmark for adjusting low δD values found in quartz intergrown with calcite.     

Isotope and fluid inclusion geochemistry of the Cangyuan Pb-Zn-Ag polymetallic deposit in Yunnan,SW China

The Cangyuan Pb-Zn-Ag polymetallic deposit is located in the Baoshan Block, southern Sanjiang Orogen. The orebodies are hosted in low-grade metamorphic rocks and skarn in contact with Cenozoic granitic rocks. Studies on fluid inclusions (FIs) of the deposit indicate that the ore-forming fluids are CO₂-bearing, NaCl-H₂O. The initial fluids evolved from high temperatures (462–498 °C) and high salinities (54.5–58.4 wt% NaCl equiv) during the skarn stage into mesothermal (260–397 °C) and low salinities (1.2–9.5 wt% NaCl equiv) during the sulfide stage. The oxygen and hydrogen isotopic compositions (δ¹⁸O_H2O: 2.7–8.8‰; δD: −82 to −120‰) suggest that the ore-forming fluids are mixture of magmatic fluids and meteoric water. Sulfur isotopic compositions of the sulfides yield δ³⁴S values of −2.3 to 3.2‰; lead isotopic compositions of ore sulfides are similar to those of granitic rocks, indicating that the sulfur and ore-metals are derived from the granitic magma. We propose that the Cangyuan Pb-Zn-Ag deposit formed from magmatic hydrothermal fluids. These Cenozoic deposits situated in the west of Lanping-Changdu Basin share many similarities with the Cangyuan in isotopic compositions, including the Laochang, Lanuoma and Jinman deposits. This reveals that the Cenozoic granites could have contributed to Pb-Zn-Cu mineralization in the Sanjiang region despite the abundance of Cenozoic Pb-Zn deposits in the region, such as the Jingding Pb-Zn deposit, that is thought to be of basin brine origin.     

Mineral chemistry and H–O–S–Pb isotopic compositions of skarn type copper deposits in the Jiurui district of the Middle-Lower Yangtze River metallogenic belt,Eastern China

The skarn type copper deposits are widespread in the Jiurui district in the Middle-Lower Yangtze River metallogenic belt. This paper reports a detailed study on mineral chemistry, and H, O, S and Pb isotopic compositions on skarn silicate and sulfide minerals in the three major skarn dominant deposits (Wushan, Dongleiwan and Dengjiashan). The Wushan skarn deposit is characterized with prograde garnet-dominated and clinopyroxene limited skarns with average andradite content of 83% and hedenbergite content of 10%, whereas the Dongleiwan and Dengjiashan deposits are featured with retrograde skarn alteration with abundant hydrous minerals such as epidote and chlorite. The garnet and clinopyroxene compositions show 59% andradite and 15% hedenbergite for the Dongleiwan skarns, and 43% and 22% for the Dengjiashan skarns respectively. The pistacite components (Ps value) defined as Fe^3 +/(Fe^3 ++ Al) and Fe^3 +/Fe^2 + value of epidote are 0.12 and 1.63 for the Wushan skarns, 0.30 and 32.73 for the Dongleiwan skarns, and 0.17 and 42.85 for the Dengjiashan skarns. It is suggested that the prograde skarn mineralization in the three deposits was all formed in a relatively oxidizing environment, with the Wushan showing the highest oxidation potential and the Dengjiashan having the least oxidation potential. However, in the retrograde skarns, the Dongleiwan and Dengjiashan deposits show higher oxidation potential than that of Wushan. The three deposits show similar sulfur isotopic compositions of − 2.9 to + 1.4‰ and similar lead isotopic compositions with ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb ratios of 17.900 to 18.205, 15.538 to 15.649 and 38.170 to 39.025, respectively. All the three deposits should have similar magmatic origin for the ore-forming materials based on their S and Pb isotopes. The oxygen isotopic compositions of the prograde and retrograde fluids in the three deposits show some differences, with δ¹⁸O_Fluid values of + 8.13‰ and + 7.81‰ for the Wushan, + 6.47‰ and + 2.33‰ for the Dongleiwan, and + 8.27‰ and + 4.43‰ for the Dengjiashan. But the hydrogen isotopic compositions are similar for the prograde (− 65 to − 31‰) and retrograde (− 64 to − 33‰) fluids. Therefore, the fluid origins and evolution may be different in each deposit. The hydrothermal fluids for the prograde skarns in all three deposits were likely derived from magmatic–hydrothermal sources, but the Dongleiwan and Dengjiashan show a higher proportion of meteoric water input in the retrograde stage. Considering the similar average prograde temperatures (574 to 560 °C) as calculated from coexisting garnet–clinopyroxene pairs, and similar retrograde temperatures (281 to 246 °C) as calculated from chlorite chemistry for the three deposits, we suggest that the trigger for deposition of sulfide ores in the Wushan garnet-dominated skarn deposit was mainly caused by system cooling with temperature drop along with magmatic intrusion and crystallization process. The Dongleiwan and Dengjiashan skarn deposits constitute a well developed retrograde skarn system with abundant epidote, chlorite, quartz and calcite, which probably caused by fluid mixing of high-temperature saline magmatic–hydrothermal fluids with cooler, oxidizing and dilute meteoric water.     

Origin of fluids in the Hetaoping Pb–Zn deposit,Baoshan–Narong–Dongzhi block metallogenic belt,Yunnan Province,SW China

The Hetaoping skarn type Pb–Zn deposit is located in the Baoshan–Narong–Dongzhi block metallogenic belt (BND belt), a belt between the Tengchong terrane and the Lanping basin. The deposit is hosted by marble of the upper Cambrian Hetaoping Formation and there are no outcrops of plutonic rocks present. This deposit is one of two large Pb–Zn deposits recently discovered in the BND belt. The Hetaoping deposit is a high Mn skarn. Four types of fluid inclusions were recognized in quartz from the deposit: vapor-rich inclusions (Type I), liquid-rich inclusions (Type II), pure vapor inclusions (Type III), and pure fluid inclusions (Type IV). The coexistence of Type I and Type III inclusions in Stage I (pre-ore stage) and Stage II (main ore stage) shows evidence of fluid boiling. Quartz-hosted fluid inclusions (Stage I and Stage II) display high homogenization temperatures and salinities (134–315 °C; 3.7–18.6 wt% NaCl equivalent) but calcite-hosted fluid inclusions in Stage III (post-ore stage) record lower homogenization temperatures and salinities (85–214 °C; 0.5–5.4 wt% NaCl equivalent). These data suggest a possible mixing between primary magmatic water and meteoric water. Based on chromatography data, the fluid inclusions in quartz contain abundant CO₂ and O₂ and subordinate CO, CH₄ and C₂H₂ + C₂H₄, suggesting an oxidizing environment. Based on their Na/K and Cl/SO₄ ratios, fluids contained in fluid inclusions are similar to volcanic spring waters. The low Na/K ratios (0.40–1.34) of the ore-forming fluids may have resulted from interaction with a deep alkaline intermediate-acid intrusion. Hydrogen and oxygen isotope determinations on quartz from different ore stages show low δ¹⁸O and δD values relative to VSMOW (−4.3‰ to 2.3‰; −109‰ to −91‰), indicating that the ore-forming fluids were diluted by external fluid sources as the skarn system cooled. Overall, geological and geochemical interpretations suggest that the Hetaoping deposit is a distal manganese skarn Pb–Zn deposit related to concealed intrusions.     

Geology,mineralization, stable isotope,and fluid inclusion characteristics of the Vostok-2 reduced W-Cu skarn and Au-W-Bi-As stockwork deposit,Sikhote-Alin,Russia

The large (>180 Kt WO₃ and at least 10–15 t Au) Vostok-2 deposit is situated in a metallogenic belt of W, Sn-W, Au, and Au-W deposits formed in late to post-collisional tectonic environment after cessation of active subduction. The deposit is related to an ilmenite-series high-K calc-alkaline plutonic suite that, by its petrologic signatures, is transitional between those at W-dominant and Au-dominant reduced intrusion-related deposits. Consistently, besides large W-Cu skarns of the reduced type, the deposit incorporates quartz stockworks with significant Au-W-Bi mineralization also formed in a reduced environment. The hydrothermal stages include prograde and retrograde, essentially pyroxene skarns, hydrosilicate (amphibole, chlorite, quartz) alteration, and phyllic (quartz, sericite, albite, apatite, and carbonate) alteration assemblages. These assemblages contain abundant scheelite associated with pyrrhotite, chalcopyrite and, at the phyllic stage, also with Bi minerals, As-Bi-Sb-Te-Pb-Zn sulfides and sulfosalts, as well as Au mineralization. The fluid evolution included hot, high-pressure (420–460 °C, 1.1–1.2 kbar), low-salinity (5.4–6.0 wt% NaCl-equiv.) aqueous fluids at the retrograde skarn stage, followed by lower temperature cyclic releases of high-carbonic, low salinity to non-carbonic moderate-salinity aqueous fluids. At the hydrosilicate stage, a high-carbonic, CH₄-dominated, hot (350–380 °C) low salinity fluid was followed by cooler (300–350 °C) non-carbonic moderate-salinity (5.7–14.9 wt% NaCl-equiv.) fluid. At the phyllic stage, a high-carbonic, CO₂-dominated, moderately-hot (330–355 °C, 0.9 kbar) low salinity fluid was followed by cooler (230–265 °C) non-carbonic moderate-salinity (6.6–12.0 wt% NaCl-equiv.) fluid. A homogenized magmatic source of water (δ¹⁸O_H2O = +8.3 to +8.7‰), and a sedimentary source of sulfur (δ³⁴S = −6.9 to −6.2‰) and carbon (δ¹³C_fluid = −20.1 to −14.9‰) at the hydrosilicate stage are suggested. A magmatic source of water (δ¹⁸O = +8.6 to +9.2‰) and a sedimentary source of sulfur (δ³⁴S = −9.3 to −4.1‰) but a magmatic (mantle- to crustal-derived) source of carbon (δ¹³C_fluid = −6.9 to −5.2‰) are envisaged for fluids that formed the early mineral assemblage of the phyllic stage. Then, the role of sedimentary carbon again increased toward the intermediate (δ¹³C_fluid = −16.4 to −14.5‰) and late (δ¹³C_fluid = −16.3 to −14.7‰) phyllic mineral assemblages. The magmatic differentiation was responsible for the fluid enrichment in W, whereas Au and Bi could also have been sourced from mafic magma. The decreasing temperatures, together with elevated Ca content in non-boiling fluids, promoted scheelite deposition at the early hydrothermal stages. The most intense scheelite deposition at the phyllic stage was caused by CO₂ removal due to boiling of CO₂-rich fluids; further cooling of non-boiling fluids favoured joint deposition of scheelite, Bi and Au.     

The Vilalba gold district,a new discovery in the Variscan terranes of the NW of Spain: A geologic,fluid inclusion and stable isotope study

This work defines the Vilalba gold district, a new gold region in the NW of Spain comprising three mineralized areas (Castro de Rei, Valiña–Azúmara and Arcos) and two different types of mineralization: 1) W–Au skarn in Castro de Rei, and 2) Au–As (Ag–Pb–Zn–Cu–Sb) vein/disseminated mineralization in Valiña–Azúmara and Arcos. Mineralization is mainly hosted by impure limestone and black slates (lower Cambrian) and spatially related to Variscan and Late-Variscan structures. Rhyolite dikes occur in Arcos (299 ± 6 Ma) and Castro de Rei, both consisting of peraluminous, high-potassium and calc-alkaline rocks. These dikes represent the fractionated expression of unexposed post-tectonic granitoids located at depth. The Castro de Rei area exhibits a contact aureole with biotite ± cordierite and calc-silicate hornfels. Prograde skarn consists of garnet, pyroxene, quartz, wollastonite and scheelite. The retrograde alteration took place in two stages and mainly consists of amphibole, epidote, zoisite, quartz, calcite, chlorite and sulfides. Gold mineralization in the skarn occurs as invisible gold in sulfoarsenides and as electrum related to Bi–Te–S minerals. The mineralization of Valiña–Azúmara and Arcos comprises two stages. The first consists of As-rich pyrite and arsenopyrite with invisible gold. The second stage mainly consists of base metal sulfides and Ag–Pb–Zn–Cu–Sb sulfosalts.Hydrothermal fluids involved in the skarn formation are low salinity (up to 6.6 wt.% NaCl eq.) aqueous carbonic fluids (H₂O + CO₂ ± CH₄ ± N₂) evolving into aqueous fluids during the last stage of gold mineralization. Stable isotope geochemistry indicates the importance of a magmatic fluid in the early stages (δ¹⁸O_SMOW from 7.5 to 11‰; δ³⁴S_CDT from 2.7 to 5.9‰) that evolved through interaction with host-rocks and mixing with metamorphic and/or meteoric fluids. The δ³⁴S_CDT values (7.8–10.9‰) of second retrograde stage sulfides suggest the incorporation of ³⁴S-enriched from host-rocks. In Valiña–Azúmara and Arcos, the fluid inclusion and isotope studies indicate similar hydrothermal fluids to those associated with the second retrograde skarn stage. Taking into account the fluid inclusion study and mineral geothermometry, an attempt was made to determine the P–T conditions of ore formation. The prograde skarn of Castro de Rei formed at temperatures and pressures ranging from 520 to 560 °C and from 1.7 to 2.5 kb. The mineralization of Valiña–Azúmara formed at minimum P–T conditions of 1.7 kb and 355 °C. The estimated temperatures for the Arcos mineralization are similar to those of Valiña–Azúmara (up to 383 °C) but at lower pressures.We propose the model of intrusion related gold systems (IRGSs) to explain the mineralizations of the Vilalba gold district. In this model, the Castro de Rei skarn and the Valiña–Azúmara mineralization represent a proximal and a distal deposit, respectively, relative to an unexposed granitoid situated at depth. We also consider Arcos as a distal deposit in relation to another unexposed granitoid located further south in the district.     

Fluid flow and mass transport at the Valentine wollastonite deposit, Adirondack Mountains, New York State

The Valentine wollastonite skarn in the north-west Adirondack Mountains, New York, is a seven million ton deposit which resulted from channellized infiltration of H₂O-rich, silica-bearing fluids. The wollastonite formed by reaction of these fluids with non-siliceous calcite marble. The skarn formed at the contact of the syenitic Diana Complex and was subsequently overprinted by Grenville-age granulite facies metamorphism and retrograde hydrothermal alteration during uplift. Calcite marbles adjacent to the deposit have generally high δ¹⁸O values (c. 21‰), typical of Grenville marbles which have not exchanged extensively with externally derived fluids. Carbon isotopic fractiona-tions between coexisting calcite and graphite in the marbles indicate equilibration at 675d? C, consistent with the conditions of regional metamorphism. Oxygen isotopic ratios from wollastonite skarn are lower than in the marbles and show a 14‰ variation (-1‰ to 13‰). Some isotopic heterogeneity is preserved from skarn formation, and some represents localized exchange with low-δ¹⁸O retrograde fluids. Detailed millimetre- to centimetre-scale isotopic profiles taken across skarn/marble contacts reveal steep δ¹⁸O gradients in the skarn, with values increasing towards the marble. The gradients reflect isotopic evolution of the fluid as it reacted with high δ¹⁸O calcite to form wollastonite. Calcite in the marble preserves high δ¹⁸O values to within <5 mm of the skarn contact. The preservation of high δ¹⁸O values in marbles at skarn contacts and the disequilibrium fractionation between wollastonite skarn and calcite marble across these contacts indicate that the marbles were not infiltrated with significant quantities of the fluid. Thus, the marbles were relatively impermeable during both the skarn formation and retrograde alteration. Skarn formation may have been episodic and fluid flow was either chaotic or dominantly parallel to lithological contacts. Although these steep isotope gradients resemble fluid infiltration fronts, they actually represent the sides of the major flow system. Because chromatographic infiltration models of mass transport require the assumption of pervasive fluid flow through a permeable rock, such models are not applicable to this hydrothermal system and, by extension, to many other metamorphic systems where low-permeability rocks restrict fluid migration pathways. Minimum time-integrated fluid fluxes have been calculated at the Valentine deposit using oxygen isotopic mass balance, reaction progress of fluid buffering reactions, and silica mass balance. All three approaches show that large volumes of fluid were necessary to produce the skarn, but silica mass balance calculations yield the largest minimum flux and are hence the most realistic.     

Geology,fluid inclusion and stable isotopes (O,S) of the Hetaoping distal skarn Zn-Pb deposit,northern Baoshan block,SW China

The Hetaoping zinc–lead deposit is located in the northern Baoshan block, Sanjiang region, SW China. The ore deposit comprises massive orebodies in the lower part and lenticular and vein-like orebodies in the upper part, both of which are hosted in the marbleized Upper Cambrian limestone and slate of the Hetaoping Formation. Three mineralization stages of Hetaoping skarn system have been recognized based on petrographic observation, which are pre-ore stage (pyroxene–garnet–actinolite–epidote–magnetite), syn-ore stage (sulfides–quartz–calcite–fluorite), and post-ore stage (calcite–quartz–chlorite). Andradite and hedenbergite are dominant in pre-ore garnet and pyroxene, respectively. Ore minerals consist of mainly pyrite, sphalerite, chalcopyrite, bornite and galena. Three types of fluid inclusions have been identified in Hetaoping, including primary two-phase (A type), primary three-phase (B type) and secondary two-phase (C type) inclusions. Based on fluid inclusion microthermometric study, the fluids forming the Hetaoping skarn minerals and sulfides evolved from high-moderate temperature (255–498 °C) and low-moderate salinity (5.0–18.0 wt.% NaCl equiv) in pre-ore stage, through moderate-low temperature (152–325 °C) and low salinity (0.4–14.2 wt.% NaCl equiv) in syn-ore stage, to low temperature (109–205 °C) and low salinity (0.9–10.0 wt.% NaCl equiv) in post-ore stage. The sulfide δ³⁴S values range from 3.7 to 7.1‰ (mean = 5.2‰, n = 29), indicative of a dominantly magmatic sulfur origin. Silicate and carbonate oxygen isotopes give calculated δ¹⁸O_H2O ranges of 3.9–11.1‰ in prograde stage, − 0.9 to 4.6‰ in early retrograde stage, and − 1.3 to 2.9‰ in late retrograde stage (syn-ore stage), The oxygen isotope data reveal that the prograde fluid in Hetaoping could be primarily magmatic, which has been mixed significantly with meteoric water in the late retrograde stage. Such a fluid mixing process is considered to be a key factor controlling ore precipitation.     

The Cipoeiro gold deposit,Gurupi Belt,Brazil: Geology,chlorite geochemistry,and stable isotope study

The Cipoeiro gold deposit, located in the Gurupi Belt, northern Brazil, is hosted by tonalites of 2148 Ma. The deposit is controlled by splays related to the major strike-slip Tentugal shear zone, and at the deposit scale, the mineralization is confined to ductile–brittle shear zones. Mineralization style comprises thick quartz veins and narrow and discontinuous quartz-carbonate veinlets associated with disseminations in altered host rocks. The postmetamorphic hydrothermal paragenesis is composed of quartz, calcite, chlorite, white mica (phengite), pyrite, and minor albite. Electron microprobe analysis of chlorites reveals a relatively uniform chemical composition at depths of more than 100 m. The chlorites are characterized by (Fe + Mg) ratios between 0.37 and 0.47 and Al^IV ranging between 2.22 and 2.59 a.p.f.u. and are classified as Fe-chlinochlore. Temperatures calculated by applying the Al^IV contents of chlorites yield a relatively narrow interval of 305 ± 15°C. Stable isotope (O, H, C, S) compositions have been determined in silicate, carbonate, and sulfide minerals. The δ¹⁸O and δD values of the mineralizing fluid range from +2.4 to +5.7 and from −43‰ to −20‰, respectively, and are interpreted as having a metamorphic origin. The δ¹³C values of fluid CO₂ are in the range −10.7‰ to −3.9‰, whereas the fluid δ³⁴S is around 0‰. Carbon and sulfur compositions are not diagnostic of their sources, compatible as they are with mantle, magmatic, or average crustal reservoirs. The hydrothermal paragenesis, chlorite–pyrite coexistence, temperature of ore formation, and sulfur isotope evidence indicate relatively reduced fO₂ conditions for the mineralizing fluid. Geologic, chemical, and isotopic characteristics of the Cipoeiro deposit are compatible with the class of orogenic gold deposits.     

Oxygen Isotopic Study on the Date‐Nagai Skarn‐Type Tungsten Deposit,Northeastern Japan

The skarn‐type tungsten deposit of the Date‐Nagai mine is genetically related to the granodiorite batholith of the Iidateyama body. Skarn is developed along the contact between pelitic hornfels and marble that remains as a small roof pendant body directly above the granodiorite batholith. Zonal arrangement of minerals is observed in skarn. The zonation consists of wollastonite, garnet, garnet‐epidote, and vesuvianite‐garnet zones, from marble to hornfels. Sheelite is included in garnet, garnet‐epidote, and vesuvianite‐garnet zones. The oxygen isotope values of skarn minerals were obtained as δ¹⁸O = 4.2–7.7‰ for garnet, 5.9–6.9‰ for vesuvianite, ?0.3–3.4‰ for scheelite, 6.0–10.9‰ for quartz, and 8.2‰ for muscovite. The temperature of skarn‐formation was calculated from oxygen isotopic values of scheelite‐quartz pairs to be 288°C. Calculated oxygen isotope values of fluid responsible for skarn minerals were 6.1–9.5‰ for garnet, 1.2–4.8‰ for scheelite, ?1.3‐3.6‰ for quartz, and 4.5‰ for muscovite. Garnet precipitated from the fluids of different δ¹⁸O values from scheelite, quartz, and muscovite. These δ¹⁸O values suggest that the origin of fluid responsible for garnet was magmatic water, while evidence for the presence of a meteoric component in the fluids responsible for middle to later stages minerals was confirmed.     

Fingerprinting a multistage metamorphic fluid–rock history: Evidence from grain scale Sr,O and C isotopic and trace element variations in high-grade marbles from East Antarctica

Granulite grade marble layers interlayered with metapelitic granulites from Lützow Holm Bay, East Antarctica, provide insight into fluid–rock interactions during burial to and exhumation from lower crustal levels. Sub-millimeter scale strontium, oxygen and carbon isotope variations along with LA-ICPMS trace element geochemistry and mineral chemistry of texturally characterized carbonates and associated minerals helped to reconstruct the multistage metamorphic fluid history.Fluid–rock interaction dating back to prograde metamorphism are still preserved in consistently low oxygen and high strontium isotope compositions (δ¹⁸O = 12‰; ⁸⁷Sr/⁸⁶Sr_(550Ma) = 0.7248) within a massif dolomitic marble layer that escaped significant later metasomatism. In most marbles, total re-crystallization and isotopic resetting occurred in the presence of “externally derived” hyper-saline fluids that circulated along the carbonate layers during the early stages of prograde metamorphism. This leads to a trend of increased radiogenic Sr in marbles towards the value of associated metapelitic rocks that have ⁸⁷Sr/⁸⁶Sr_(550Ma) of 0.764.LA-ICPMS studies on trace elements in carbonate and associated silicate minerals at different textural settings, distinguished using cathodoluminescence microscopy, revealed multiple metasomatic events during retrograde metamorphism. Trace element contents of Ba, Sr, Pb and U gave compelling evidence for metasomatic alteration that postdate the exsolution of carbonate at ~ 600 ºC, which can be correlated with the fluids released from the crystallization of anatectic melts and pegmatites. Subsequently, meteoric fluid infiltration occurred at a shallower level of the crust and caused extreme oxygen isotopic heterogeneity (δ¹⁸O = 14.7 ~ ? 4.9‰) and imprinted high concentration of fluid mobile elements. Taken together our results emphasize the importance of integrating textural and chemical heterogeneities to reveal the multiple episodes of fluid–rock interaction processes in a dynamic continental crust, which has major implications on migration of fluids and material and help in formulating models on the geodynamic evolution of crust.     

Intermediate sulfidation epithermal gold-base metal deposits in Tertiary subaerial volcanic rocks,Sahinli/Tespih Dere (Lapseki/Western Turkey)

Gold in the Sahinli and Tespih Dere intermediate sulfidation gold-base metal deposits in Western Turkey occurs in relatively deep epithermal quartz veins along with base metal minerals which have epithermal textures, including plumose quartz, vug infills, comb and cockade textures and matrix-supported milled breccias. The total sulfide content of the veins in the area is variable ranging from < 1% to 60% and is dominated by pyrite, galena, sphalerite and chalcopyrite. Sphalerite is Fe-poor (0.6 to 1.4 mol% FeS). Minor amounts of Ag-rich tetrahedrite are present. Primary hydrothermal alteration minerals include illite/muscovite, mixed-layer illite/smectite (11.6 Å) and clinochlore towards the east and, alunite, dickite/nacrite and pyrophyllite towards the west at Sahinli; major illite/muscovite and dickite occur at Tespih Dere and Sarioluk, respectively.Fluid inclusions in main-stage quartz at Sahinli are only liquid-rich, with homogenization temperatures ranging from 220 to 322 °C and the majority of T_h values between 250 and 300 °C. Salinity ranges from 4.3 to 6.9 wt.% NaCl equiv. First ice-melting temperatures (T_mf) between ?24.5 and ?19.0 °C indicate that the fluids were dominated by NaCl – H₂O during mineralization. The relatively higher average T_h at the Tespih Dere deposit (295 °C) is attributed to a relatively deeper level of exposure.Calculated δ¹⁸O values indicate that ore-forming hydrothermal fluids in the study area had δ¹⁸O_H2O ranging from + 1.1 to + 9.7‰ (average = 3.8‰), strongly ¹⁸O-enriched compared with present-day hydrothermal meteoric water in the area (δ¹⁸O = ?8.5‰). δD values of fluid inclusions in quartz range from ?58 to ?93‰ and δD values of clay minerals and alunite from ?40 to ?119‰. δD values from intermediate argillic alteration (average = ?68‰) in the study area are very similar to δD values of the present-day local geothermal system (average δD = ?54‰) whereas δD values from advanced-argillic alteration (average δD = ?33‰) are very different from the present-day local geothermal system.The δ³⁴S values in samples from the Sahinli and Tespih Dere deposits average ?2.9‰ for pyrite; ?3.3‰ for chalcopyrite; ?5.4‰ for sphalerite and ?7.6‰ for galena. These data are consistent with derivation of the sulfur from either igneous rocks or possibly from local wallrock.     

Hydro-geochemical and isotopic fluid evolution of the Los Azufres geothermal field,Central Mexico

Hydrothermal alteration at Los Azufres geothermal field is mostly propylitic with a progressive dehydration with depth and temperature increase. Argillic and advanced argillic zones overlie the propylitic zone owing to the activity of gases in the system. The deepest fluid inclusions (proto-fluid) are liquid-rich with low salinity, with NaCl dominant fluid type and ice melting temperatures (T_mi) near zero (0 °C), and salinities of 0.8 wt% NaCl equivalent. The homogenization temperature (T_h)  = 325 ± 5 °C. The boiling zone shows T_h = ±300 °C and apparent salinities between 1 and 4.9 wt% NaCl equivalent, implying a vaporization process and a very important participation of non-condensable gases (NCGs), mostly CO₂. Positive clathrate melting temperatures (fusion) with T_h = 150 °C are observed in the upper part of the geothermal reservoir (from 0 to 700 m depth). These could well be the evidence of a high gas concentration. The current water produced at the geothermal wells is NaCl rich (geothermal brine) and is fully equilibrated with the host rock at temperatures between T = 300 and 340 °C. The hot spring waters are acid-sulfate, indicating that they are derived from meteoric water heated by geothermal steam. The NCGs related to the steam dominant zone are composed mostly of CO₂ (80–98% of all the gases). The gases represent between 2 and 9 wt% of the total mass of the fluid of the reservoir.The authors interpret the evolution of this system as deep liquid water boiling when ascending through fractures connected to the surface. Boiling is caused by a drop of pressure, which favors an increase in the steam phase within the brine ascending towards the surface. During this ascent, the fluid becomes steam-dominant in the shallowest zone, and mixes with meteoric water in perched aquifers. Stable isotope compositions (δ¹⁸O–δD) of the geothermal brine indicate mixing between meteoric water and a minor magmatic component. The enrichment in δ¹⁸O is due to the rock–water interaction at relatively high temperatures. δ¹³C stable isotope data show a magmatic source with a minor meteoric contribution for CO₂. The initial isotopic value δ³⁴S_RES = −2.3‰, which implies a magmatic source. More negative values are observed for shallow pyrite and range from δ³⁴S (FeS₂) = −4‰ to −4.9‰, indicating boiling. The same fractionation tendencies are observed for fluids in the reservoir from results for δ¹⁸O.     

Listvenite-related gold deposits of the South Urals (Russia): A review

The Urals is a complex fold belt, which underwent long geological evolution. The formation of most gold deposits in the Urals is related to the collision stage. In this paper, we review some relatively small listvenite-related gold deposits, which are confined to the large Main Uralian fault zone and some smaller faults within the Magnitogorsk zone. The Mechnikovskoe, Altyn-Tash, and Ganeevskoe deposits are studied in detail in this contribution. They comprise the ore clusters along with other numerous small gold deposits, and constituted the sources for the gold placers exploited in historical time. The gold is hosted by metasomatites (listvenites, beresites) and quartz veins with economic gold grades (up to 20 g/t Au). Listvenites are developed after serpentinites and composed of quartz, fuchsite, and carbonates (magnesite, dolomite) ± albite. Volcanic and volcanoclastic rocks are altered to beresites, consisting of sericite, carbonates (dolomite, ankerite), quartz and albite. Pyrite and chalcopyrite are major ore minerals associated with gold; pyrrhotite, Ni sulfides, galena, sphalerite, arsenopyrite and Au-Ag tellurides are subordinate and rare. Gold in these deposits is mostly high-fineness (>900‰). The lower fineness (∼800‰) is typical of gold in assemblage with polymetallic sulfides and tellurides. The ores have been formed from the NaCl–CO₂–H₂O ± CH₄ fluids of low (∼2 wt% NaCl-equiv.) to moderate (8–16 wt% NaCl-equiv.) salinity at temperatures of 210–330 °C. The oxygen isotopic composition of quartz (δ¹⁸O) varies from 14.7 to 15.4‰ (Mechnikovskoe deposit), 13.2 to 13.6‰ (Altyn-Tash deposit) and 12.0 to 12.7‰ (Ganeevskoe deposit). The oxygen isotopic composition of albite from altered rocks of the Ganeevskoe deposit is 10.1‰. The calculated δ¹⁸O_H2O values of the fluid in equilibrium with quartz are in a range of 5.7–6.3, 4.2–4.6 and 6.3–6.7‰ respectively, and most likely indicate a magmatic fluid source.     

The geology and genesis of the iron skarns of the Turgai belt,northwestern Kazakhstan

The magnetite deposits of the Turgai belt (Kachar, Sarbai and Sokolov), in the Valerianovskoe zone of the southern Urals, Kazakhstan, contain a combined resource of over 3 Gt of iron oxide ore. The deposits are hosted by carbonate sediments and volcaniclastic rocks of the Carboniferous Valerianovka Supergroup, and are spatially related to the gabbroic to granitoid composition intrusive rocks of the Sarbai–Sokolov intrusive series. The magnetite deposits are developed dominantly as metasomatic replacement of limestone, but also, to a lesser extent, of volcanic rocks. Pre-mineralisation metamorphism and alteration resulted in the formation of wollastonite and the silicification of limestone. Magnetite mineralisation is associated with the development of a high temperature skarn assemblage of diopside, grossular–andradite garnet, actinolite, epidote and apatite. Sub-economic copper-bearing sulphide mineralisation overprints the magnetite mineralisation and is associated with deposition of hydrothermal calcite and the formation of an extensive sodium alteration halo dominated by albite and scapolite. Chlorite formation accompanies this stage and further later stage hydrothermal overprints. The replacement has in places resulted in preservation of primary features of the limestone, including fossils and sedimentary structures in magnetite, skarn calc-silicates and sulphides.Analysis of Re–Os isotopes in molybdenite indicates formation of the sulphide mineral assemblage at 336.2 ± 1.3 Ma, whilst U–Pb analyses of titanite from the skarn alteration assemblage suggests skarn alteration at 326.6 ± 4.5 Ma with re-equilibration of isotope systematics down to ~ 270 Ma. Analyses of mineral assemblages, fluid inclusion microthermometry, O and S isotopes suggest initial mineralisation temperatures in excess of 600 °C from hypersaline brines (45–50 wt.% NaCl eq.), with subsequent cooling and dilution of fluids to around 150 °C and 20 wt.% NaCl eq. by the time of calcite deposition in late stage sulphide-bearing veins. δ¹⁸O in magnetite (− 1.5 to + 3.5‰) and skarn forming silicates (+ 5 to + 9‰), δ¹⁸O and δ¹³C in limestone and skarn calcite (δ¹⁸O + 5.4 to + 26.2‰; δ¹³C − 12.1 to + 0.9‰) and δ³⁴S in sulphides (− 3.3 to + 6.6‰) and sulphates (+ 4.9 to + 12.9‰) are all consistent with the interaction of a magmatic-equilibrated fluid with limestone, and a dominantly magmatic source for S. All these data imply skarn formation and mineralisation in a magmatic–hydrothermal system that maintained high salinity to relatively late stages resulting in the formation of the large Na-alteration halo. Despite the reported presence of evaporites in the area there is no evidence for evaporitic sulphur in the mineralising system.These skarns show similarities to some members of the iron oxide–apatite and iron oxide–copper gold deposit classes and the model presented here may have implications for their genesis. The similarity in age between the Turgai deposits and the deposits of the Magnitogorsk zone in the western Urals suggests that they may be linked to similar magmatism, developed during post-orogenic collapse and extension following the continent–continent collision, which has resulted in the assembly of Laurussian terranes with the Uralide orogen and the Kazakh collage of the Altaids or Central Asian Orogenic Belt. This model is preferred to the model of simultaneous formation of very similar deposits in arc settings at either side of an open tract of oceanic crust forming part of the Uralian ocean.     

Geology,mineralogy, and geochemistry of fault-controlled hydrothermal Cu–Au mineralization in the Shanmen Volcanic Basin,SE China

The Yukeng–Banling deposit is a typical fault-controlled hydrothermal Cu–Au deposit in the Shanmen Volcanic Basin (SVB), SE China. Ore bodies commonly occur as lodes, lenses and disconnected pods dipping SW with vertical zonation of ore minerals. Ore-related hydrothermal alteration is well developed on both sides of the veins, dominated by silicification, sericitization, chloritization and argillation with a banded alteration zonation. The mineralization can be divided into three stages (stages I, II and III). Native gold is present as veinlets in fractures of fine-grained pyrite from stage II.Zircon U–Pb and Rb–Sr isochron ages indicate that the Cu–Au mineralization is coeval with the Caomen alkaline granite and Xiaokeng quartz-diorite, both emplaced at ca. 102 Ma. Microthermometric measurements of fluid inclusions in quartz and sphalerite from stage II veins indicate that the Yukeng–Banling deposit is an epithermal deposit. Six ore-related quartz grains have δD_H2O values of − 69 to − 43‰, and δ¹⁸O_H2O values calculated using total homogenization temperatures that range from − 2.0 to 0.7‰. All samples plot in an area between the magmatic field and the meteoric line, suggesting that the ore-forming fluids are derived from a mixed source of magmatic and meteoric waters. δ³⁴S values for eight pyrite separates range from − 2.1 to + 4.1‰ with an average of + 1.7‰, and δ³⁴S values for galena and sphalerite are 2.3‰ and 2.2‰, similar to magmatic sulfur. Four alkaline granite samples have Pb isotopic ratios (²⁰⁶Pb/²⁰⁴Pb)_t = 18.175–18.411, (²⁰⁷Pb/²⁰⁴Pb)_t = 15.652–15.672 and (²⁰⁸Pb/²⁰⁴Pb)_t = 38.343–38.800. Three quartz-diorite samples have ratios (²⁰⁶Pb/²⁰⁴Pb)_t, (²⁰⁷Pb/²⁰⁴Pb)_t and (²⁰⁸Pb/²⁰⁴Pb)_t of 18.277–18.451, 15.654–15.693 and 38.673–38.846, respectively. These age-calculated lead isotopic data for alkaline granite are similar to those for the analyzed sulfides. Co/Ni ratios for stage II pyrites range from 1.42 to 5.10, indicating that the Yukeng–Banling deposit records the past involvement of magmatic hydrothermal fluids. The isotope data, together with geological, mineralogical and geochronological evidence, favor a primary magmatic source for sulfur and metals in the ore fluids. Mixing of the Cu- and Au-rich fluids with meteoric water led to precipitation of the Cu–Au veins along NW-trending faults.The Yukeng–Banling deposit, the contemporaneous Caomen alkaline granite and Xiaokeng quartz-diorite in the SVB formed under an extensional setting, due to high-angle subduction of the paleo-Pacific plate. The extensional setting facilitated the formation of Cu- and Au-rich magmas which was derived from enriched mantle and lower crust.