Fluid inclusion study of the Gaofeng tin-polymetallic deposit in the Dachang ore field, Guangxi, China

The Dachang tin-polymetallic district, Guangxi, China, is one of the largest tin ore district in the world and contains the Lamo Zn-Cu proximal skarn deposit, the Tongkeng-Changpo and Gaofeng tin-base metal deposits, and the Huile and Dafulou black shale-hosted cassiterite-sulfide deposits. They are hosted by Devonian carbonate-rich sediments near the underlying Cretaceous (91 similar to 96Ma) Longxianggai granite. The Gaofeng deposit in the district occurs in the Middle Devonian reef limestone with higher grade ore and giant resource of Sn. The mineralization includes the early ore stage of cassiterite-arsenopyrite-pyrrhotite and the late ore stage of carbonate-sulfide-sulfosalt. Petrography, microthermometry, and Laser Raman spectroscopy, combined with scanning electron microscope-cathodoluminescence (SEM-CL) analyses of fluid inclusions are used to characterize the chemical evolution of ore fluids at the Gaofeng deposit. Two types of fluid inclusions are recognized in quartz and cassiterite : CO2-CH4 vapor-rich fluid inclusions are mainly observed in quartz and cassiterite associated with cassiterite-arsenopyrite-pyrrhotite stage and have high homogenization temperatures of 360 similar to 410 degrees C and salinities of 3% similar to 6% NaCleqv, whereas two-phase, liquid-rich aqueous fluid inclusions only occur in quartz related to sulfide-sulfosalt-carbonate stage and have relatively low homogenization temperatures ranging from 270 to 310 degrees C and salinities of 3% similar to 6% NaCleqv. The range of salinity for all fluid inclusions is similar to those of CO2 vapor-rich fluid inclusions (7% similar to 9% NaCleqv) in diopside and garnet in the Lamo skarn Zn-Cu deposit nearby the Gaofeng deposit. Oxygen and hydrogen isotopes of ore fluids from quartz and cassiterite indicate magmatic water in origin. Base on the physical and chemical evolution of ore fluids, it is concluded that the mineralizing fluids for the Gaofeng deposit were probably derived from intermediate density, supercritical single-phase fluid that exsolved from the deep crystallizing granite. This magmatic fluid reacted with carbonate-rich host rocks in the depth that led to phase segregation to form brine and vapor-like fluid. Vapor contraction and cooling of vapor-like fluids may deposit cassiterite and arsenopyrite, and followed by sulfides and sulfosalts at the Gaofeng deposit.