Geology and geochemistry of Jingxi-Yelmand gold deposit in Tulasu basin,North Tianshan,Xinjiang

Jingxi-Yelmand gold deposit is located in the northwest of Tulasu basin,North Tianshan(Xinjiang,NW China),which is hosted in tuff and tuff sandstone in Devonian to Early Carboniferous volcanic-sedimentary strata.Silicification,sericitization,pyritization,brecciation and carbonation are found in the host rocks,in which silicification is closely related to gold mineralization.Orebody in Jingxi-Yelmand gold deposit,mainly composed of hydrothermal breccias and strongly silicified host rocks,is lenticular,stratoid and stratified,parallel to the volcanic-sedimentary strata.The ore-forming process consists of four stages:Silicification and sericitization(stage I),forming abundant disseminated quartz,sericite and pyrite in host rocks;Brecciation and silicification(stage II),made the formation of hydrothermal breccia-a,composed of altered host-rock fragments,which were cemented by chalcedonic quartz,pyrite,arsenopyrite and some auriferous minerals;Brecciation and silicification(stage III),formed hydrothermal breccia-b composed of altered rock and breccia-a fragments cemented by fine-grained quartz,pyrite,arsenopyrite and auriferous minerals.The hydrothermal process is terminated by forming coarse-grained calcite-barite veins(stage IV).Geochemistry study suggests that the original ore-forming fluid enrich in V,Cr,Ni,Cu and Sb,and leach Mn,Co,Zn,Bi and LILE from wall-rocks.Ore-forming fluid is magmatic water reequilibrated with host volcanic-sedimentary rocks,and becomes more oxidized from early to late stage.Model calculation suggests that gold concentration in ore-forming fluid is about 1 × 10-6 ～ 2 × 10-6,and about 1 × 108 to 0.5 × 108 ton magmatic hydrothermal fluid is needed for forming Jingxi-Yelmand gold deposit.Brecciation and the related increase of fO2 value might be the major factors causing gold precipitation.