Origin, metal deposition mechanisms and short wavelength infrared (SWIR) spectroscopy study at the Yinzishan epithermal Au deposit, Yunnan Province, SW China

The Yinzishan epithermal Au deposit discovered in the Simao block, is dominated by telluride minerals, and contains a total of ≥ 20 t of Au with ore grading at 8.90 g/t Au. The mineralization is hosted within thick accumulations of steeply dipping andesitic to rhyodacitic volcaniclastic units. The distribution of Au orebodies in this deposit is largely controlled by NNW-oriented faults and the higher porosity and permeability of the volcaniclastic host rocks. A detailed study of the Yinzishan deposit geology, occurrence of gold, and short wavelength infrared (SWIR) data from the northern to southern sections of the deposit was performed to characterize the hydrothermal minerals and alteration zonation, and assess the deposition mechanisms of the metal. The dominant country-rock alteration of the deposit is silicification, advanced argillic alteration, chloritization, sericitization, and pyritization and the main ore mineral combination is pyrite–enargite–covellite–chalcopyrite–calaverite. Au mainly occurs as native gold and calaverite or micronsized inclusions in tellurides (e.g., melonite and tetradymite) and pyrite. The characteristics of the ore minerals and alteration assemblages indicate that the ore-forming fluid was relatively oxidized, with SO2-, H2S-, and F-rich acidic hydrothermal fluids. This feature supports the high-sulfidation origins of the Yinzishan Au deposit. Tellurium was introduced into the site of ore deposition via vapor in F-rich fluid and HS–complexes are thought to have been the main carriers of Au. The mixing of magmatic fluids with meteoric waters was the crucial control on the precipitation of the Au telluride ore. The SWIR spectroscopy revealed that the mineral combination of topaz + alunite has a good coupling relationship with Au mineralization. Moreover, the SWIR scalars of four characteristic single minerals/mineral assemblages, including topaz (Dep2086 > 0.2), alunite (Dep1470 > 0.06), chlorite (Pos2250 > 2250 nm), and kandite and/or pyrophyllite (Dep2170 > 0.07), provide a visualization tool for determining the ore-forming environment and the location of hydrothermal/mineralization centers. Therefore, we proposed that the combined application of detailed SWIR-based analyses of alteration minerals and interpretations of the SWIR scalars of characteristic minerals is an effective approach for the exploration of epithermal hydrothermal deposits.