Insights into the indium enrichment of the Ashele VMS Cu-Zn deposit, Altay, NW China

Indium (In) is a critical metal used in the photovoltaic and semiconductor industries, which have exhibited extraordinary growth in demand. Indium is produced as a by-product of mining from different ore deposits (e.g., epithermal, sediment-hosted, and skarn). Volcanogenic massive sulfide (VMS) deposits are an important source of In; however, the mechanism of In enrichment is not fully understood. Here, we combine mineralogy with in situ trace element and S-Pb isotope geochemistry to reveal the enrichment of indium in the Ashele VMS Cu-Zn deposit (1.08 Mt. Cu, 0.43 Mt. Zn) located in Altay, NW China. The Ashele deposit is hosted in the metamorphosed Devonian felsic-bimodal volcanic rocks. This deposit consists of seafloor hydrothermal, metamorphic hydrothermal, and supergene stages. The seafloor hydrothermal stage comprises macro-scale Cu-rich bands (chalcopyrite, pyrite, and minor sphalerite) and Zn-rich bands (sphalerite, pyrite, and minor chalcopyrite). Indium is mainly hosted by chalcopyrite (mean 178 ppm) and sphalerite (mean 214 ppm) and occurs in the lattice. Mineral assemblages and trace element geochemistry suggest that the Cu-rich bands were deposited under high temperatures (> 300–350 °C) and sulfur fugacity (−7.2 to −4.9), whereas the Zn-rich bands were formed under lower temperatures (180–220 °C) and sulfur fugacity (−15.7 to −11.5). The interlayered Cu-rich and Zn-rich bands may reflect the oscillating temperature and sulfur fugacity variations. In situ S isotopic compositions of sulfides cluster within two ranges: 1–3 ‰ and 3–6 ‰, suggesting two endmembers: volcanic origin and reduced seawater sulfate. Pb isotopic ratios are similar to those of the host volcanic rocks, indicating that the metals may be derived from the felsic volcanic system. During metamorphism, the indium may be retained, but Cu contents of sphalerite become more homogeneous. Most In-rich VMS deposits worldwide are hosted by the felsic-dominant system in island arc and back-arc settings. These tectonic settings are conducive to the production of felsic volcanic systems, which are more likely to contain In mineralization. This study highlights the enrichment mechanism of indium in VMS deposits and suggests that the South Altay could become an important source of In.