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 x 10(-6) similar to 2 x 10(-6), and about 1 x 10(8) to 0.5 x 10(8) ton magmatic hydrothermal fluid is needed for forming Jingxi-Yelmand gold deposit. Brecciation and the related increase of f(o2) value might he the major factors causing gold precipitation.