Skarn mineralogical characteristics and ore sulfur isotope geochemistry of Tianmashan sulfur-gold deposit, Tongling area

The Tianmashan sulfur-gold deposit is a typical stratabound hydrothermal superimposed reformed deposit in the Tongling ore concentration area and hosts abundant skarn minerals in major bedded ore body. In order to ascertain the skarn minerals types and ore-forming setting of Tianmashan deposit and explore the relationship between skarnization and mineralization, the mineralogy and mineral chemistry of main skarn minerals, and the geochemistry of its sulfur isotope were studied. The research shows that the skarn minerals in Tianmashan deposit are mainly comprised of garnet and pyroxene. Most of garnet are andradites, belonging to andradite-grossular series (Gro_0~18.73And_80.54~99.00Spe+Pyr+Alm_0.54~1.47). Pyroxene is dominated by diopside, followed by hedenbergite, thus can be listed into diopside-hedenbergite series (Di_62.35~97.65Hd_1.89~36.27Jo_0.31~1.55). The mineral assemblage(andradite and diopside)in Tianmashan sulfur-gold deposit is attributed as oxidized skarn, indicating the skarn diagenetic conditions of relatively high temperature and high oxygen fugacity. The characteristics of the end-member components in garnet and pyroxene, and the lower pyroxene Mn/Fe ratio (0.02~0.07) were similar to those in the typical skarn copper and gold deposits. The sulfur isotopes of ore with magmatic hearth features were consistent with Yanshanian magmatic-metallogenic ore, but obviously different from sedimentary exhalative deposit, indicating that the mineralization was closely related to Yanshanian magmatic activity. Due to the metasomatism of Yanshanian acidic magma, a large amount of skarn was formed in the carbonate surrounding rocks. The formation of skarn minerals increased the porosity and permeability of rock, which were conducive to the later formation of sulfur-gold deposits.