Genesis of Chaoshan skarn Au deposit, Tongling, eastern China: Insights from mineral geochemistry

Skarn Au deposits are particularly developed along Middle and Lower Yangtze River region, eastern China, but gold behavior and genesis of the skarn Au deposits remain unclear. Whole-rock geochemical, sulfur isotopic and in-situ mineralogical data from the mineralized alteration stages of Chaoshan skarn Au deposit in Tongling re-gion are systemically investigated to reveal Au distribution and precipitation, and useful information for ore -forming process. The altered rocks and ores in the Chaoshan deposit have total REE contents of 7.81-226 ppm and show LREE enrichment patterns with weak negative to positive Eu (delta Eu = 0.71-2.24) anomalies. Electron probe microanalysis results indicate that garnet and pyroxene of the Chaoshan deposit are grossular-andradite and diopside-hedenbergite series, respectively. In situ elemental compositions of the euhedral pyrite analyzed show that invisible Au is mainly of lattice-bound Au+ in As-bearing pyrite. The studied pyrite samples also have high Co/Ni (mainly >1) and Se/Tl (>10) ratios, highly variable Pb and As contents, low Se content (<5.00 ppm), low Tl/Pb and Sb/Pb ratios, and relatively consistent sulfur isotopic compositions (5.0-8.3 parts per thousand). The distinctly geochemical characteristics indicate that the Chaoshan skarn Au mineralization was likely derived from a magmatic-hydrothermal system. A comprehensive discriminant analysis shows that the Au precipitation and enrichment of the Chaoshan deposit are a result of boiling-induced trace element fractionation resulted from fluid boiling during the skarn formation process.