In situ Raman spectroscopic investigation of copper speciation in hydrothermal fluids at temperatures up to 300 °C

The formation of copper deposits is closely related to hydrothermal processes. Understanding the migration of copper in hydrothermal fluids aids in reconstructing mineralization processes and deciphering deposit genesis. Copper primarily exists as Cu + and Cu 2+ in hydrothermal solutions, with redox conditions governing their interconversion. In chloride-rich geological fluids, Cu–Cl complexes are considered critical for copper transport. However, the specific types and valence transitions of Cu–Cl complexes under varying hydrothermal conditions remain poorly understood. This study employed in situ Raman spectroscopy to systematically analyze Cu + HCl and CuCl 2  + K 2 S 2 O 3 /H 2 systems under saturated vapor pressure at 25–300 °C, elucidating the effects of temperature, Cl − concentration, and redox conditions on copper speciation. In the Cu + HCl system, copper dissolved as monovalent Cu–Cl complexes. At high temperatures (> 200 °C), [CuCl 2 ] − is the dominated species, whereas [CuCl 3 ] 2− becomes prevalent at lower temperatures and higher HCl concentrations. For the Cu 2+ –Cl system, the dominant species transitioned from [Cu(H 2 O) n ] 2+ (< 50 °C) to [CuCl 4 ] 2− (100 °C) and further to [CuCl] + and [CuCl 2 ] 0 at 300 °C. The introduction of reducing agents (K 2 S 2 O 3 /H 2 ) facilitated Cu 2+  → Cu + reduction, thereby stabilizing Cu + –Cl complexes and inducing partial copper precipitation. The behavior of copper in chloride-rich hydrothermal fluids observed in this study indicates that high-temperature oxidizing fluids facilitate Cu mobilization, while cooling and redox changes promote deposition and ore minerals formation.