Petrology and petrogeochemistry of the A'gyi gold deposit, Zoige, Sichuan

The A'gyi gold deposit in Zoige, Sichuan is located in the "golden triangle" as metallogenic clusters at the juncture of Sichuan, Gansu and Shaanxi Provinces, where the Middle Triassic strata are considered to be the most important source rocks and host rocks. The gold deposit resulted from polyphase geological processes and has polyphyletic origins, and thus recorded the complex evolution from deposition through diagenesis to enrichment and finally to mineralization. The study of the host rocks indicates that the gold deposit was formed in the bathyal-slope environment. The rocks cropped out in the region include the sedimentary and metamorphic rocks composed of fine-grainded sandstones, graywacke, sedimentary tuff, carbonate rocks, siliceous rocks, leptynite, quartzite, hornfels, marble, skarns and breccias. The magmatic rocks consist dominantly of dacite porphyry (porphyrite) and diorites. The quartzites as the most important host rocks exhibit massive and irregular banded structures, brownish yellow/light gray blastobedded structures, and consist of quartz (75%-80%), kaolinite (10%-15%), sericite (2%-3%), ilmenite (3%-4%), chalcedony (2%-3%), and a small amount of carbonaceous components. Petrochemically, the quartzites can be distinguished from the representative siliceous rocks by lower SiO₂, higher Al₂O₃ and highly kaolinization. The quartzites are interpreted to be a kind of altered products and their primitive rocks should be normal clastic sedimentary rocks.