Short-chain carboxylates in high-temperature ore fluids of W-Sn deposits in south China

Abstract

The Xihuashan, Dangping and Piaotang deposits in the Jiangxi Province, south China can be considered as typical high-temperature hydrothermal quartz vein-type W-Sn mineralization, that is temporally and spatially related to Mesozoic S-type granite. Microthermometric studies of fluid inclusions in various minerals from these deposits were conducted to obtain homogenization temperature and salinity data. Major organic and inorganic aqueous components of the fluid inclusions were extracted by a crush-leaching, and concentrations of short-chain carboxylate (formate, acetate, propionate, oxalate), fluoride, chloride and sulfate anions in the leachates were determined using ion chromatography. The results indicate the presence of formate and acetate as well as minor amounts of oxalate and propionate in leachates, and the dominance of formate over other carboxylates. The molar ratio of acetate to formate appears to be a potential indicator of the redox state for the ore fluid system. Oxygen and hydrogen isotopic data show that inclusion water in quartz is mainly magmatic in origin. It is postulated that the dissolved carboxylate anions in the ore fluids for the deposits probably were generated by the breakdown of kerogen in "primitive sedimentary rocks" during the formation of S-type granites. The possibility of inorganic synthesis of the carboxylate species in a C-H-O system can not be excluded.