Geology, fluid inclusion, and isotope constraints on ore genesis of the Neoproterozoic Jinshan orogenic gold deposit, South China

Abstract

AbstractThe Jinshan gold deposit is located in the Neoproterozoic Jiangnan orogen between the Yangtze and Cathaysia blocks. Gold‐bearing disseminated ores are associated with the earlier stage of NWW‐trending ductile zone, and auriferous quartz vein‐type ores show an intimate relationship with the later stage of NE‐trending brittle‐ductile zones. Fluid‐inclusion studies were conducted on the quartz veins. Three types of fluid inclusions can be identified: H2O–CO2 inclusions (type I), CO2‐rich inclusions (type II), and aqueous inclusions (type III). The pre‐ore stage, quartz‐pyrite veins primarily contain type I inclusions with constant CO2 bubble volumetric proportions. The main gold mineralization‐stage veins have all three types of inclusions with variable gas‐phase ratios and CO2 contents. The post‐ore stage carbonate±chlorite veinlets only contain type III inclusions. Type I inclusions in the pre‐ore stage display homogenization temperatures (Th) of 285–340°C, with salinities of 1.4–6.1 wt.% NaCl equivalent. In the main gold mineralization stage, type II and III inclusions show similar Th at 208–277°C, but contrasting salinity values with 0.6–3.6 and 3.5–8.9 wt.% NaCl equivalent, and type I inclusions show variable CO2‐phase proportions and have Th of 241–292°C and salinities of 1.0–7.0 wt.% NaCl equivalent. In the post‐ore stage, type III inclusions yield Th of 109–201°C and salinities of 1.1–6.4 wt.% NaCl equivalent. Petrological observations and microthermometric results show that fluid immiscibility primarily occurred during the gold mineralization stages. The oxygen and hydrogen isotope compositions (δ18 O = +6.9‰ to +11.2‰, δD = −71‰ to −46‰) of inclusion water in quartz grains imply that ore fluids were principally metamorphic in origin. The sulfur and lead values of sulfide from the ores are analogous to those from the basement strata, suggesting a predominantly crustal source of the ore sulfides. The Jinshan deposit is a typical orogenic gold deposit.