FLUID GENERATION AND GOLD MINERALIZATION DURING PROGRESSIVE DUCTILE DEFORMATION AT JINSHAN GOLD DEPOSIT, SOUTH CHINA

Jinshan gold deposit, the largest gold deposit south of Yangzi River in Jiangxi Province of China, is strictly hosted by a regional ductile shear zone. The shear zone is superimposed on Shuangqiaoshan Group, a Mesoproterozoic subgreenschist facies turbiditic and subaqueous volcanic and volcanoclastic sequence, and present as a mylonite zone which resembles a fine-grained muscovite-quartz schist. Gold mineralization occurs as “auriferous altered mylonite” and laminated quartz veins in high strain centers of the shear zone.

Deformation structures observed in the shear zone and ores at various scales indicate an in-situ source of auriferous fluids and a syn-deformational mineralization associated with a progressive deformation process. Firstly, pressure solution associated with early stage ductile shearing is common and pervasive within the shear zone, and provided significant volume of in-situ mineralization fluids. Quartz grains were dissolved at stressed surfaces where muscovite-defined foliation domain touches quartz and where the shear strain was high. The volume of dissolved silica subsequently precipitated as quartz veins of various scales at places with lower stress after diffusive transport as indicated by voluminous “differentiation” quartz veins within the shear zone especially within high strain subzones. Dissolved silica also precipitated in pressure shadows and produced pyrite pressure shadows as observed throughout the shear zone including the “auriferous altered mylonite” orebodies. Secondly, auriferous quartz veins at various scales experienced intensive progressive ductile shearing. Once precipitated, the “differentiation” quartz veins were sheared and boudinaged due to subsequent progressive deformation. For example, microscopic-scale ductile shear zones are common and pervasive within the quartz veins mined as high-grade ores. Frequently sheared and deformed quartz veins are always rich in gold.