Estimation of Chlorine Fugacity in Low-Н₂О Fluid of the C–O–(H)–NaCl System in the Cumulus of Ultramafic–Mafic Intrusions

At high PT parameters of cumulates of ultramafic–mafic intrusions and low oxygen fugacity (below QFM buffer), Pt in the form of a carbonyl complex is dissolved in a CO₂-bearing fluid. The high solubility of Pt chloride in brines with NaCl, which is related to the formation of low-sulfide deposits of platinum group elements, is attained only at high oxygen fugacity (above the QFM buffer). It is suggested that native platinum at low oxygen fugacity in low-Н₂О СО–СО₂ (Н₂О) fluid can also transit into a cation-soluble form due to the reaction of chloration. The experimental data are provided for the interaction of NaCl with magnetite and chromite (accessory minerals of ultramafic–mafic intrusions) at Р = 200 MPa, Т = 950°С, and fO₂ <QFM with the formation of Fe and Cr chlorides. As shown by thermodynamic calculations, the FeCl₃ and FeCl₂ equilibrium provides a high chlorine fugacity (fCl₂), which is only 3–4 orders of magnitude lower than fCl₂ in Pt–PtCl₂ equilibrium and 2.5–3.0 orders of magnitude higher than in 1 M HCl aqueous fluid at the same Р‒Т–fO₂ parameters.