Hydrothermal mobility of In, Sn, Cu, Cd, Fe, Pb, Zn during interactions between HF- and HCl-bearing fluids and granitic melts: experimental constraints on In mineralization

Abundant indium (In) is frequently observed as an impurity element within sphalerite and closely associated with granite-related Sn-polymetallic deposits. Magmatic-hydrothermal processes have been proposed to govern In mineralization; however, the underlying mechanisms remain poorly understood. This study experimentally investigates geochemical behaviors of In, Sn, Cu, Cd, Fe, Pb, and Zn during interactions between granitic melts and 0.5–2.0 mol/L HF-bearing or 0.5–11.0 mol/L HCl-bearing solutions at 850 ℃ and 200 MPa. The results reveal a significant covariation relationship between In and these elements. Specifically, In, Sn, Zn, Cu, and Cd were enriched in all granitic melts, with their concentrations progressively increasing with higher HF concentrations but decreasing with higher HCl concentrations in both melts and solutions, while Fe and Pb were roughly depleted. This distinction is attributed to the low hydrothermal mobility of In, Sn, Zn, Cu, and Cd, which promotes their synchronous enrichment within acidic magmatic-hydrothermal systems. The results also highlight a strong pH dependence on hydrothermal mobility of In. Acidic F- or Cl-bearing fluids can enrich In within the granitic melts by a factor of 5 to 20 or 1 to 10 times, respectively. In contrast, neutral-alkaline Cl-bearing fluids may cause substantial depletion of In by factors of up to 1000 times. Consequently, the exsolution of acidic F- or Cl-bearing fluids could be an essential prerequisite for generating an In-rich magma reservoir. Subsequent activities involving neutral-alkaline fluids, such as exotic brine extraction or saline fluid exsolution, can efficiently transport In from the In-rich magmas into hydrothermal fluids, thereby facilitating In mineralization.