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The Yangshan gold deposit, Gansu Province, is the largest gold deposit in China and the best representative of the western Qinling gold province. Granite dykes can be observed in the orefield, of which some are spatially associated with the gold orebodies. Gold mineralization was debated between magmatic hydrothermal and metamorphic hydrothermal models in previous studies due to a shortage of precise isotopic ages of the granite dykes in the orefield. In this contribution, we report in-situ zircon LA-ICP-MS U-Pb ages combined with Hf isotope and REE data. The age of granite dykes is constrained to be 220.2 ± 3.1 Ma, coeval with the regional development of Triassic granitoids, which resulted from the northward subduction of the Mian-Lue oceanic plate. The εHf(t) values of inherited and new zircon grains from the granite dykes show that the granite dykes have a complex source dominated by sediments which were sourced from the North China Craton, Yangtze Craton and Qinling Orogen, and accumulated in a fore-arc setting. The granite dykes were derived from partial melting of the fore-arc sediment prism induced by the fluids sourced from metamorphic devolatilizaiton of the underthrust Mian-Lue oceanic plate. The Yangshan gold deposit was formed at about 190 Ma, some 30 Myr later than the intrusion of the granite dykes, ruling out the possibility that gold mineralization was caused by Triassic granitic magmatism. The Yangshan gold deposit is a transitional system between Carlin- and orogenic-types that resulted from continental collision orogeny, metallogeny and fluid flow. Copyright © 2014 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The Xunyang Hg-Sb orefield, Shaanxi Province, containing two large (Gongguan and Qingtonggou) and tens of small to medium Hg-Sb deposits, is located in the southern Qinling Orogen. Ore bodies of the deposits are hosted in Devonian dolomite and controlled by a fault system. Ores are mainly present as massive veins, disseminations, breccias and fine veinlets, with cinnabar (Hg) and stibnite (Sb) being the dominant ore minerals. Gangue minerals include quartz, calcite and dolomite. Hydrothermal minerals (quartz and calcite) only contain aqueous fluid inclusions with low homogenization temperature (135-274 °C) and salinity (1.23-12.3 wt.% NaCl equiv.), supporting an epizonogenic hydrothermal origin. The carbon, oxygen and hydrogen isotope data indicate that the ore-forming fluids were mainly sourced from epizonogenic to metamorphic devolatilization of the Sinian−Triassic strata that possibly underthrusted beneath the orefield, with inflow of the circulating meteoric water. Sulphur isotope ratios of stibnite and cinnabar range from 2‰ to 12‰, suggesting a contribution from sedimentary sulphate or sulphate-bearing fluids potted in strata, which is coincident with the δ34S values (3.4-8.6‰) of diagenetic pyrite in the strata. The conclusion drawn from sulphur isotopes is fully supported by the Pb isotope signatures. In addition, the 87Sr/86Sr ratios gradationally increase from ore-hosting dolostones, through ore-barren calcite or low-grade ores, to high-grade ores, indicating that a portion of the fluids originated from a source with higher 87Sr/86Sr values than the host-rocks, which is possibly composed of the Neoproterozoic−Lower Palaeozoic basement of the Xunyang Basin. Integrating the data from ore geology, fluid inclusion microthermometry and stable, radioactive isotope geochemistry, it can be concluded that the deposits in the Xunyang Hg-Sb orefield were formed by epizonogenic hydrothermal fluids mainly sourced from the strata via structural deformation during the North China−Yangtze continental collision. Copyright © 2014 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The Dashui porphyry granitoids in the western Qinling Orogen are spatially associated with the Dashui gold deposit, but their ages and tectonic setting have not been well defined yet in previous studies. This paper reports the results and understandings obtained from our original study in geochronology and isotope and element geochemistry. SHRIMP zircon U-Pb dating yielded ages of 213.7 ± 2.7 Ma for granitic stock and 215.1 ± 2.5 and 210.2 ± 1.6 Ma for granitic dikes. The granitoids are characterized by high LREE but low HREE and HFSE. They have (87Sr/86Sr) i ratios of 0.706 to 0.709 and εNd(t) values of −5.3 to −7.6. The majority of zircons in the Dashui granitoids have εHf(t) values ranging from −6.2 to 6.7 and TDM2(Hf) ages from 743 to 1464 Ma. The isotope signatures, together with La/Y and Mg# values, show that the magmas are mainly sourced from partial melting of a subducted slab, and possibly assimilated immature sediments during upward migration. Thus, the Dashui granitoids most likely formed in a volcanic arc where supra-subduction zone melts assimilated lower crustal material no later than 215 Ma. Copyright © 2014 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The East Qinling region in central China, hosting several tens of Mesozoic magmatic-hydrothermal Mo deposits, is one of the largest molybdenum belts in the world. The Zhifang Mo deposit is hosted in volcanic rocks of the Xiong'er Group in the Waifangshan area, Qinling Orogen. Previous studies variously correlated the mineralization in this deposit with Yanshanian magmatism or Palaeo-Mesoproterozoic volcanic-hydrothermal events. The orebodies are associated with quartz veins and controlled by subsidiary faults of the Machaoying Fault. The ore-forming process can be divided into the early, middle and late stages and is characterized by quartz-pyrite, quartz-polymetallic sulphide and quartz-carbonate veins, respectively. The early-stage quartz is structurally deformed, suggesting a compressional tectonic regime; the middle-stage sulphides fill the fractures of the early-stage assemblages and show no deformation, suggesting a tensional setting; the late-stage veins mostly infill the open-space fissures. Three types of fluid inclusions (FIs) are identified at the Zhifang deposit: H2O-NaCl (W-type), CO2-rich (C-type) and daughter mineral-bearing inclusions (S-type). Fluid inclusions of early-stage quartz homogenize between 380 and 470 °C, with salinities ranging from 0.4 to 9.6 wt.% NaCl equiv., whereas the late-stage calcite contains only the W-type FIs with homogenization temperatures of lower than 240 °C, and salinities of 0.4-8.7 wt.% NaCl equiv. This indicates that the ore fluid system evolved from CO2-rich, probably metamorphic hydrothermal to CO2-poor, meteoric fluid. All three types of FIs can be observed in the middle-stage quartz, and even in the microscopic domain of a crystal, suggesting that this heterogeneous association was trapped from a boiling fluid system. These FIs homogenized at temperatures ranging from 250 to 360 °C and display two salinity clusters of <18.5 and 29.1-29.9 wt.% NaCl equiv. These results suggest that metal precipitation resulted from fluid boiling. The estimated trapping pressures of FIs range from 101 to 285 MPa, suggesting an alternating lithostatic-hydrostatic fluid system, which was controlled by a fault-valve at the depth of 10 km. The δ34S values of ore minerals from the Zhifang Mo deposit show a range between −11.8‰ and 6.0‰, with a bimodal distribution. The early-stage pyrite has a positive δ34S value of 6.0‰ that is similar to the host rocks of the Xiong'er Group and the Taihua Supergroup, suggesting that the wall rocks contributed much of the sulphur to the early-stage pyrite during fluid-rock interaction. However, the δ34S values of the middle-stage sulphides have negative mean and restricted range from −11.8‰ to −4.5‰. The widespread rutile grains coexisting with molybdenite in the middle-stage correlate the negative δ34S values with relatively oxidized fluids. We consider phase separation as an efficient mechanism for ore-fluid oxidation and molybdenum deposition based on fluid inclusions and sulphur isotope data. Geological, fluid inclusion and sulphur isotope data of the Zhifang Mo deposit suggest that the Mo mineralization is unrelated to the Yanshanian magmatism or the Palaeo-Mesoproterozoic volcanic-hydrothermal event. Here we propose that the Zhifang Mo deposit may be considered as an orogenic mineral system, with its formation in an active continental margin related to the northward subduction of the Mian-Lue oceanic plate during the Triassic. Copyright © 2014 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]