Three sets of experiments were performed to test the stability and geologic importance of ferric (Fe^III) chloride complexes in acidic, Cl-rich solutions at 150 to 300°C, p = p_sat. Experiment Set A used the change in solubility of AgCl(s) in the presence of FeCl₃ or FeCl₂ to determine the stoichiometry of Fe(III) and Fe(II) chloride complexes at ΣCl = 0.1 to 3.0 molal. Results show that FeCl₄^- and FeCl₂(aq) are the dominant Fe(III) and Fe(II) species, respectively, at T > 200°C and ΣCl ≥ 1m. Set B experiments used the solubility of elemental gold as a redox sensor to determine the aO₂ of the FeCl₄^-/FeCl₂(aq) boundary and log K values of +6.56 and +7.19 for the following reaction at 250 and 300°C, respectively: FeCl₂(aq) + ¼O₂(g) + 2Cl^- + H⁺ = FeCl₄^- + ½H₂O(l). Ferric chloride complexes are stable under conditions of aCl^-, aO₂, and pH where gold is soluble as AuCl₂^-. Set C experiments measured the solubility of hematite in NaCl-HCl solutions at 200 to 300°C by preparing a series of silica tubes with identical matrix chemistry (0.9m NaCl + 0.1m HCl) and increasing concentration of FeCl₃. Hematite saturation was constrained by the tube with the lowest FeCl₃ concentration that precipitated hematite at high temperature. Set C results were used to compute log K values of +3.6, +5.2, and +7.4 for the following reaction at 200, 250 and 300°C, respectively: 0.5Fe₂O₃(s) + 4Cl^- + 3H⁺ = FeCl₄^- + 1.5H₂O.

Hematite solubility as FeCl₄^- is independent of redox state, and increases quickly with an increase in temperature, increase in Cl^- concentration (power of 4), and decrease in pH (power of 3). Concentrations in excess of 100 mg/L ΣFe are attainable under geologically realistic conditions. Once formed, FeCl₄^- is an extremely effective oxidizing agent, capable of destabilizing any sulfide mineral and dissolving Fe, Au, and other metals (e.g., Cu, Pt, Pd) as chloride complexes. In deep hydrothermal systems, FeCl₄^- is a more viable oxidant than dissolved O₂ gas, although its presence requires a source area with abundant hematite and a lack of reductants such as organic carbon, sulfides, or silicate minerals containing ferrous iron. Dissolved ferric chloride could be an important and previously overlooked reactant in the formation of certain types of hematite-rich hydrothermal mineral deposits, including iron oxide-copper-gold (IOCG) deposits.