An estimate of chlorine fugacity in the low water fluid of the system С-О-(Н)-NaCl in the cumulus of ultrabasic-basic intrusions

At high PT parameters of the cumulates of ultramafic-mafic intrusions at low fO2 (below the QFM buffer), platinum dissolves in the fluid with CO as a carbonyl complex of the native metal. The high solubility of platinum as PtCl2 in brines with NaCl, which is associated with the formation of low-sulfide PGE deposits, is achieved at high oxygen fugacity (above the NNO buffer). It is assumed that at low oxygen fugacity in the low water CO–CO2 fluid, native Pt can also be converted into a cation-soluble form by chlorination. Experimental data (Р = 200 MPa, Т = 950oC, fO2 QFM and fluid CO–CO2) on the reaction of NaCl with magnetite and chromite, accessor minerals of mafic-ultramafic intrusions, with the formation of iron and chromium chlorides are presented. As shown by thermodynamic calculations, the equilibrium in the FeCl3–FeCl2 pair provides the high chlorine fugacity (fCl2). This fugacity is only 3–4 orders of magnitude lower than fCl2 in the Pt–PtCl2 equilibrium and 2.5–3 orders of magnitude higher than in the aqueous fluid 1 M HCl at the same P–T–fO2 parameters.