Fluid inclusion constraint on sulfide precipitation in wolframite-quartz veins: A case study of the giant Dajishan W polymetallic deposit, South China

Abstract

Wolframite-quartz vein-type W deposits commonly produce wolframite accompanied by variable amounts of sulfides with industrial value. In such deposits, the characteristics of sulfide-forming fluids and precipitation mechanisms are poorly understood. Dajishan, located in the east of the Nanling metallogenic belt, is a giant W-polymetallic deposit with the total reserve of 190,000 t WO3, 6253 t Bi, and 1199 t Mo. Individual ore vein at Dajishan has been divided into three hydrothermal stages according to mineral paragenesis: wolframite-quartz stage, scheelite-sulfide stage, and fluorite-calcite stage. Except for molybdenite which occurs in the wolframite-quartz stage, most sulfides occur in the scheelite-sulfide stage and consist of pyrrhotite, bismuthinite, cosalite, chalcopyrite, sphalerite, and galena. Typical quartz samples coexisting with both wolframite and sulfides from the No. 23 vein in 417 m level were studied in detail to rebuild the hydrothermal history of the first two mineralization stages. While the studied quartz is paragenetically later than coexisting wolframite and likely molybdenite, SEM-CL reveals two quartz generations in which precipitation of other sulfides is intimately associated with the later generation. One type of primary fluid inclusions (type LQ1) and two types of pseudosecondary fluid inclusions (types VQ2 and LQ2) forming immiscible assemblage are identified in the first (Qz1) and second (Qz2) quartz generations, respectively. Microthermometry and chemical composition data of quartz-hosted fluid inclusion were measured in combination with previously reported data from wolframite-hosted fluid inclusions to quantitatively determine the fluid evolution during ore formation. Fluid major and trace element signature shows clear inheritance of sulfide precipitating fluid from early wolframite mineralizing fluid. Early molybdenite precipitation (61.2–142.8 ppm drop to 5.6–6.2 ppm) is accompanied by mixing of meteoric water into W unloaded early magmatic fluid whose pH was buffered by fluid-rock interaction. As bracketed by type LQ1 and LQ2 inclusions, deposition of late Bi, Cu, Zn, and Pb sulfides is primarily controlled by fluid immiscibility and the induced drop of fluid temperature and acidity.