Genetic implications for the D amajianshan W‐Cu‐As polymetallic deposit in L vchun, S outhwest C hina: C onstraints from H–O , He–Ar , S, and Pb isotopes
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The Sanjiang Tethyan domain in SE Asia is one of the most important mineral belts in China. The Damajianshan (DMJS) W‐Cu‐As polymetallic deposit is located in the southern part of Sanjiang Tethyan domain, related to Triassic quartz porphyry. Detailed exploration thereafter shows that the reserves of W, Cu, and As are 0.09, 0.42, and 0.12 Mt, respectively. The W is a typical oxyphile element and always coexisted with Sn–Li–Be–Nb–Ta. The Cu–As are typical sulphophile elements and always coexisted with Au–Ag–Pb–Zn–Sb–Hg. It is rarely reported that a deposit preserves tungsten and copper–arsenic simultaneously in the southern Sanjiang region. In this study, the S–Pb isotopic ratios of sulphides, the H–O isotopic compositions of fluid inclusions in hydrothermal quartz, and the He–Ar isotopic ratios of fluid inclusions in sulphides were analysed to constrain the origin of the DMJS deposit. Studies have shown that the δ 34 S values of sulphides from ores are mainly between −6.17‰ and +0.02‰; the sulphur isotope compositions indicate that the ore‐forming materials might originate from deep sources. The Pb isotope characteristics indicate that the ore metals might originate from deep sources and be closely related to the quartz porphyry. The δ 18 O fluid values of ore‐forming fluids calculated from hydrothermal quartz range from −5.5‰ to +7.5‰, and the δD values of the fluid inclusions in quartz are from −81.7‰ to −50.2‰. The H–O isotope systematics indicate that the ore fluids in the DMJS deposit were probably initially sourced from magmatic water and later gradually mixed with Mesozoic meteoric water. Fluid inclusions in pyrite, chalcopyrite, arsenopyrite, and pyrrhotite possess 3 He/ 4 He ratios of 0.02–0.05 Ra, and their 40 Ar/ 36 Ar ratios range from 562.47 to 4159.15, indicating a complete crustal fluids and with higher radiogenic 40 Ar. The noble gas isotopic data, along with the stable isotopic data, suggest that the ore‐forming fluids have a deep source. The unique geologic structure of the Sanjiang Tethys tectonic metallogenic region and the evolution of ore‐forming fluids both promoted the formation of this rare DMJS deposit.Keywords:
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Sapphirine + quartz bearing garnet granulite from the Central Zone of the Neoarchean Limpopo Belt contains three categories of fluid inclusions: dominant pseudosecondary inclusions in garnet and plagioclase, secondary inclusions in matrix quartz, and least dominant primary inclusions in the same quartz grains. The melting temperatures of the trapped fluids lie in the range of −58.6 to −56.4 °C. Homogenization of the pseudosecondary inclusions into the liquid phase occurs within the temperature range of +9.1 to +28.4, which correspond to CO2 densities of 0.72 to 0.87 g/cm3. In contrast, primary inclusions homogenized at +6.1 ± 3.1 °C, which translates into slightly higher densities of 0.87-0.91 g/cm3. The fluid densities, when computed into isochores, indicate entrapment of CO2 at lower pressure than the peak P-T condition of the rock. The results imply either influx of CO2-rich fluids during post-peak uplift or density reversal of peak metamorphic carbonic fluid. Lack of aqueous fluid inclusions and abundant pseudosecondary carbonic inclusions in the rock confirmed that low-H2O activity condition is necessary to form unique sapphirine + quartz assemblage.
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There are three types of fluid inclusions in the quartz of quartz-vein ores,Baiganhu-Jialesai W-Sn mineralization belts,Qimantage: Liquid-rich two-phase inclusions,CO2 and CH4-rich three-phase inclusions and water inclusions,of which the first two types are primary inclusions and are closely related with W-Sn mineralization.The peak homogenized temperatures and salinity of liquid-rich two-phase inclusions in quartz,with liquid component of H2O and gas component of CO2 and CH4,range from 220℃ to 260℃ and from 0.88% to 20.82% NaCleqv respectively,the total density of fliuds range from 0.72 to 1.06g·cm-3.The peak homogenized temperatures and salinity of CO2 and CH4-rich three-phase inclusions in quartz range from 260℃ to 280℃ and from 6.63% to 15.212% NaCleqv respectively,the total density of fliud range from 0.60 to 0.91g·cm-3.The gas components of CO2 and CH4-rich three-phase inclusions are mainly CO2,CH4 and less H2S,N2,H2,while the liquid components are mainly H2O and less CO2,CH4,H2S.The value of δ18OH2O and δD of quartz-vein ores range from 4.02‰ to 6.32‰ and -75.5‰ to -42.8‰ respectively,both of which indicate that the fluid is significantly magmatic water.In all,the initial fluid of quartz-vein ores is from magmatic hydrothermal fluid,which belongs to NaCl-H2O-CO2 system with middle-high temperature (220~280℃),middle salinity (10%~14% NaCleqv) and lower density.The W-Sn mineralization is closely related with the Caledonian magmatism,and the uneven fluids are captured when the quartz-vein ores formed.The significant factors of forming of quartz-vein ores are immiscibility of ore-forming fluids,and changes of temperature,pressure and value of pH,Eh,fO2 and fS2.
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Abstract Cu–Mo mineralization occurs in southern part of the Chatree Au–Ag deposit, central Thailand. Quartz veins of Cu–Mo mineralization are divided into five types: Types A, B, C, D and E. Quartz veins of Types A, B and C are hosted in altered granodiorite porphyry, and quartz veins of Types D and E occur in altered andesite lava. Mineral assemblages of Types A, B and C quartz veins are composed of qz–chl–ilt–mol–py–ccp, qz–chl–ilt–ccp–py and qz–chl–ilt–ccp–py–sp–po, respectively. Types D and E quartz veins consist of qz–chl–py–ccp–sp–po and qz–ep, respectively. Fluid inclusions of quartz veins are divided into liquid‐rich two‐phases fluid inclusion, vapor‐rich two‐phases fluid inclusion and multiphase solid‐bearing fluid inclusion. Coexistence of a halite‐bearing fluid inclusion having salinity of 37 equiv. wt.% NaCl and a vapor‐rich two‐phases fluid inclusion having salinity of 1 equiv. wt.% NaCl suggests that the Cu–Mo‐bearing quartz veins were formed at temperature of 450°C and pressure of 250 bars (depth of approximately 1.5 km from the paleosurface). Based on the formation temperature of 450°C of quartz veins and the δ 18 O values of quartz of the quartz veins, the δ 18 O value of fluid responsible for the Cu–Mo‐bearing quartz vein is estimated to be +9.9‰. The origin of fluid forming the Cu–Mo‐bearing quartz veins in the N prospect of the Chatree mining area would be magmatic water. Based on the characteristics of geology, age, mineral assemblage and the formation environment, Cu–Mo mineralization would be different from the epithermal Au–Ag mineralization of the Chatree mining area.
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