Linkage of distal vein-type Au mineralization in carbonate rocks with Cu-Au skarn mineralization in the Fengshan area, Eastern China: Mineralogy and stable isotope geochemistry

Numerous limestone-hosted vein-type Au deposits (12 million tonnes at 5.0 g/t Au) are located peripheral to Cu-Au skarn deposits around the Late Jurassic granodiorite intrusions in the Fengshan area, Middle-Lower Yangtze River metallogenic belt, Eastern China. Both the Cu-Au skarn and distal vein-type Au deposits have a complex mineral assemblage, consisting of base metal sulfides, sulfosalts and Au-Ag-(Tl) telluride minerals. In the Cu-Au skarn deposits, grains of electrum, Au-Ag tellurides (hessite and petzite) coexist with Bi-Te minerals (native bismuth, tetradymite, Phase C, and aikinite), sulfides (chalcopyrite, sphalerite, and galena), occurring as grain aggregates or veinlets. In the distal vein-type Au deposits, fine-grained Au-Ag-Tl tellurides (calaverite, petzite, hessite, and Tl-bearing Au telluride) occur as ore minerals in assemblages of sulfides, and carbonate minerals. The mineralogical Bi-Te signature characterizes the skarn assemblages, while the Te-Tl signature is typical of the distal vein-type Au mineralization. Textural relations and mineral assemblages allow the recognition of five generations of hydrothermal carbonate: (1) Cu sulfides and tellurides-bearing calcite assemblages; (2) sphalerite, galena and tellurides-bearing dolomite assemblages; (3) sericite, quartz and tellurides-bearing carbonate veinlets; (4) realgar and fluorite-bearing carbonate veinlets; (5) pure calcite veinlets. Electron microprobe analysis indicates that the tellurides-bearing, ore-related carbonate contains variable Mn content (up to 80 mol.% MnCO3). The C-O isotope compositions of carbonate from the Cu-Au skarn mineralization, to the carbonate-replacement Pb-Zn and distal vein-type Au mineralization, and to the post-ore, realgar-bearing veinlets in the country rocks display a tendency towards heavier isotopic compositions with time, which could be best interpreted as the effect of fluid-rock interaction between magmatic fluid and the Triassic carbonate host rocks. New S isotope data on sulfate minerals (anhydrite, gypsum, and celestite), and C-O isotopic data suggest a common magmatic fluid source for both skarn and distal vein-type mineralization.