Study On The Mineralization Mechanism Of The Baiganhu W-Sn Orefield In East Kuniun Orogenic Belt,Xinjiang,NW China

The newly discovered Baiganhu W-Sn orefield is the only one with a large-scale and potentially superlarge-scale reserve in the East Kunlun Orogenic Belt.Based on a summary of previous research,a systematic study was carried out that included geochronology,petrology,mineralogy,geochemistry,and fluid/melt inclusion microthermometry.It is aim to investigate the mineralization age,tectonic setting,and the origin,evolution and precipitation of ore-forming material of this orefield.A preliminary mineralization model also has been proposed to act as a guide to the targeting of similar ore systems in the adjacent area.The Baiganhu W-Sn orefield consists of Kekekaerde,Baiganhu,Bashierxi,and Awaer deposits and the W-Sn mineralization generally occurs in the inner and outer contact zones between the syenogranite and the wallrock of Xiaomiao Formation.Three types of W-Sn mineralization,seven vertical petrographic zones,and four stages of mineralization are recognized in the orefield.Hydrothermal muscovite associated with W-Sn mineralization yields plateau 40Ar/39Ar ages of 411.7 ± 2.6?413.8 ±2.6 Ma,which is consistent with the zircon U—Pb age of 413,6±2.4 Ma of the parental syenogranite and indicates that the W-Sn mineralization developed during the late stage of East Kunlun Caledonian Orogenic Cycle.The age data obtained in this study and previously published,along with geochemical data,for the granitoids in the Bashierxi magmatic series show a nearly contemporaneous evolution of A-and S-type granites,which were emplaced in a post-orogenic setting at ca.432-413 Ma.Two types of magmatic garnets?almandine-spessartine solid-solutions?in the parental syenogranites and two types of hydrothermal garnets?andradite-grossularite solid-solutions?have been identified.Magmatic garnet composition shows that they crystallized under relatively low temperature and pressure conditions.LA-ICP-MS analyse indicates the substitution of Sn4+ for Fe3+ within these garnets.These data suggest that Sn-rich andradite-grossular garnets in skarns may be used as an indicator mineral for W-Sn exploration when combined with other geological and geochemical signatures.Fluid inclusions study shows that melt inclusions coexist with fluid inclusions in the skarn-type scheelite mineralization whereas greisen-type and quartz-vein-type mineralization were mainly precipitated from hydrothermal fluids.From skarn-type,to greisen-type,to quartz-vein-type mineralization,a gradual decrease trend in homogenized temperature?450?200??and a relatively small change in salinity?20%-10%NaCleq?have been observed.Hydrogen and oxygen isotope study indicates magmatic origin of ore-forming fluids of the three types of mineralization,and probably involving contribution of meteoric and metamorphic water in post-ore stage.Taking all the above into consideration,the ore-forming material of the Baiganhu W--S_n orefield is considered to be mainly derived from the S-type magma.The syenogranites with S-type affinity probably resulted from a lower degree of partial melting of metagreywackes,which was more likely to be enriched in the ore-forming elements W and Sn,as well as volatiles such as B and H₂O.In addition,the syenogranites exhibit low oxidation states and underwent high degrees of factional crystallization,both of which favor post-magmatic W-Sn mineralization.Before the magma completely consolidated,skarn-type scheelite mineralization had already started and the "magma-hydrothermal transition fluid",resulting from the liquid immiscibility between the magma and hydrothermal,plays an important role for this type mineralization.Whereas hydrothermal fluid immiscibility of N_aCI-H₂O-CO₂ probably exerts an significant effect on the quartz-vein-type W-Sn mineralization.