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The Waifangshan Mo-quartz vein cluster occurs at the northernmost margin of the Qinling Orogen. The orebodies occur as quartz veins and are controlled by low-angle faults. We identify three hydrothermal stages, characterized by veinlets of quartz–pyrite, quartz–molybdenite–pyrite–chalcopyrite–galena–sphalerite, and quartz–carbonate assemblages, respectively. The early-stage quartz veins were structurally-deformed, brecciated, and filled by non-deformed middle- or late-stage veinlets, suggesting that the mineralization is associated with a tectonic transition from compression to extension. Five molybdenite separates from the Zhifang Mo deposit yield individual Re–Os ages from 241.2 ± 1.6 to 247.4 ± 2.5 Ma, with a weighted mean age of 243.8 ± 2.8 Ma. Individual Re–Os isotope ages of four molybdenite samples from the Badaogou Mo deposit range from 238.9 ± 2.1 to 255.8 ± 2.2 Ma, with a weighted mean age of 246 ± 10 Ma. Four analyses of molybdenite separates from the Xiangchungou Mo deposit yield individual Re–Os isotope ages of 243.8 ± 8.8 to 247.7 ± 4.4 Ma, with a weighted mean age of 246.0 ± 1.1 Ma. All of these data yield a weighted mean age of 241.9 ± 1.9 Ma, coeval with the accretionary orogeny. We thus conclude that the Waifangshan Mo-quartz veins formed during a transition from continental arc to back-arc setting. The Waifangshan Mo-quartz veins share similar geological, geochemical and geochronological features with the orogenic-type systems. We propose that the Waifangshan Mo-quartz veins are an orogenic-type Mo mineralization, and possibly represent the deepest member in the crustal continuum model of the orogenic-type mineral systems. The molybdenite samples from the Mo-quartz veins in the Waifangshan area have low Re contents (1.457 to 39.16 ppm), suggesting a continental crust-dominated source. [ABSTRACT FROM AUTHOR]

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 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]