Studies On The Granites And Gold Mineralization Of The Wangfeng Gold District In Xinjiang

The Wangfeng gold deposit is located in the Bingdaban ductile shear zone along the northern margin of Middle Tianshan in Xinjiang. This paper focuses on three parts related to gold mineralization, and then preliminarily discusses tectonic evolution and mineralization process based on setting study area to macroscopical tectonic background. On the part of the granites occurring as ore-hosted countryrocks in Wangfeng area, detailed petrology、 geochemistry and zircon U-Pb-Hf isotopes will be operated so as not only to denote rock-forming age, genesis and tectonic setting, but also to probe into influences on gold mineralization. As for studies on characteristics of gold mineralization, classic orebody will be analysed in different scales accompanying with He, Ar isotopic compositions of fluid inclusions in pyrite in order to make mineralization more detailed and trace ore-forming fluid as well as discussions on macroscopical tectonic setting and mineralization potential. Some advances have been obtained below:(1) Granites yielded as ore-hosted countryrocks in the Wangfeng gold deposit consist of two-mica monzonitic granite and biotite monzonitic granite, which systematically show geochemical characteristics of high-K calc-alkali peraluminous S-type granites. Zircon LA-ICP-MS U-Pb dating result exhibits granites crystallized in439.9±2.2Ma equal to Early Silurian. Hf isotopic compositions of zircon and experimental petrological results indicate granites in the Wangfeng gold district origined from partial melting of Middle Mesoproterozoic crystallized base parablastesis with less-mature graywack which suffered from weathering as primary rock. Synthetical research results denote granites intruded at the stage of syn-collision during orogeny, which stand for Caledonian accreation of continental crust in the north margin of Middle Tianshan. Rock-forming process has no relation with gold mineralization, while geochemical compositions of granites and orogeny in Late Caledonian play an important role in tracing ore-forming fluid.(2) Gold mineralization in Wangfeng betides in middle and upper layer of brittle-ductile strain domain which shows as a part of Bingdaban ductile shear zone. There existes condese relationships between gold mineralization and typical alterations such as silification and pyritization. Ore types are mainly composed of silification alterated mylonite, silification alterated rock and quartz-vein-like rock, which show diffusive alteration and filling. Meanwhile, a new mineralization type namely cataclastic mylonite was observed under microscope with unclear prospecting potential. There is a synthetically marked trend that ore type reveals stronger silification and concomitant lower gold grade in vertical from below with weaker silification in transverse from orebody to countryrock. Ore types changing from silification alterated mylonite to quartz-vein-like rock mean different mineralization stages in time or order.(3) Helium and Argon isotopic compositions of fluid inclusions in pyrite were analyzed to trace ore-forming fluid by using an inert gas isotopic mass spectrometer after analyzing variables that may affect He-Ar original isotopic compositions of the ore-forming fluid. The analytical results indicate that3He/4He ratio and40Ar/36Ar ratio vary respectively from0.00473Ra to0.01079Ra and from301to413, helium and argon isotopic compositions synthetically exhibit the crust-derivation of the ore-forming fluid, which was the product of the deep circulation of atmospheric water. Meanwhile, in contrast to typical crust-derived fluid, Wangfeng ore-forming fluid was richer in both4He and4Ar, resulting in lower He/4He and higher40Ar/36Ar than typical He-Ar isotopic compositions of crust-derived fluid. An analysis of main country rocks and ore-forming fluid inclusions reveals that ore-hosting countryrocks with high U, Th concentrations produced much more radiogenic4He which reduced3He/4He ratio in comparison with normal radiogenic He isotopic composition, that atmospheric water with atmosphere-derived He-Ar isotopic compositions subsided through brittle crannies and captured radiogenic He existing in country rocks, and that some radiogenic40Ar was obtained synchronously, which reduced3He/4He ratio preliminarily and raised40Ar/36Ar ratio at the pre-mineralization stage. Then, ore-forming fluid boiled due to hydrothermal decompression at the ore-forming stage, which brought3He/4He ratios down. In summary, the ore-forming fluid of the Wangfeng gold deposit was crust-derived and reformed during the process of fluid evolution.(4) This paper preliminarily resumes processes of tectonic evolution in late Early Paleozoic and ductile shearing with gold mineralization in middle-late Late Paleozoic i n Wangfeng area. In late Early Paleozoic, Junggar terrane and Yili-Middle Tianshan te rrane collided due to the closure of ancient ocean along the northern margin of Middl e Tianshan, which resulted in remelting of middle Mesoproterozoic graywack metamor phosed into one part of the active parametamorphic base and then magma characterize d by S-type granites intruded as ore-hosted rocks. Meanwhile, regional geochemical an omaly including Au generated associated with subduction and collisional orogeny, whic h behaved as probable material source. Ater the entire closure of ancient ocean in Tia nshan in late Devinian-early Carboniferous, northern Xinjiang getted into the post-colli sonal stage with extensive tectonics and mineralization. The Bingdaban ductile shear z one was superimposed on the former orogenic belt in Wangfeng area with gold miner alization. Primary ore-forming fluid namely atmospheric water infiltrated along shallow deformation structures of shear zone, and captured ore material like Au, orogenic Pb as well as4He,40Ar due to the radioactive decay of U, Th riched in Early Silurian S-type granites, then becomed ore-forming fluid. Under the influence of some uncertai n geological factors so far, ore-forming fluid cycled and ascended in brittle-ductile do main till comfortable mineralization environment, and then gold mineralization happene d in weaker strain zone and transitional zone between tectonic zone with different stra in intensities on account of tectoinic decompression, fluid boiling and so on. Because of pulsant tectonics and hydrothemalism, various contact relations generated among diff erent ore types shown as distinct ore-forming stages. Gold mineralization may still acc ur in deeper layer in brittle-ductile transitional domain of the Wangfeng gold district. There are also some clues that porphyry-type copper mineralization may be found alon g Gulungou-Wangfeng in the northern margin of Middle Tianshan.