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

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Abstract

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.

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About the authors

A. G. Simakin

Korzhinskii Institute of Experimental Mineralogy, Russian Academy of Sciences

Author for correspondence.
Email: simakin@iem.ac.ru
Russian Federation, Chernogolovka

O. Yu. Shaposhnikova

Korzhinskii Institute of Experimental Mineralogy, Russian Academy of Sciences

Email: simakin@iem.ac.ru
Russian Federation, Chernogolovka

V. N. Devyatova

Korzhinskii Institute of Experimental Mineralogy, Russian Academy of Sciences

Email: simakin@iem.ac.ru
Russian Federation, Chernogolovka

S. I. Isaenko

Institute of Geology of the Komi Scientific Center, Ural Branch of the Russian Academy of Sciences

Email: simakin@iem.ac.ru
Russian Federation, Syktyvkar

D. D. Eremin

Lomonosov Moscow State University

Email: simakin@iem.ac.ru
Russian Federation, Moscow

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Supplementary files

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2. Fig. 1. Experimental data on the solubility of platinum in a fluid at high temperature, depending on the volatility of oxygen. The dotted circle marks the solubility in brine at low oxygen volatility, obtained by extrapolation (according to [6]).

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3. Fig. 2. Chlorine volatility equilibrium in a metal–chloride pair, calculated using a thermodynamic database [9]. The data for PtCl2 and PdCl2 are extrapolated to high T (dotted line). The volatility of chlorine is shown separately, providing a chlorine content of about 1 wt. % in the dry melt of basalt at 1400 °C [10]. Chlorine volatiles corresponding to FeCl3–FeCl2 (FFC) equilibria with oxygen volatiles equal to NNO and CCO are also shown. The volatility of chlorine in 1M HCl aqueous fluid (P = 200 MPa) calculated by the reaction of 2HCl+1/2O2 = H2O+Cl2 at two oxygen volatiles (CCO and NNO buffers) is shown separately.

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4. Fig. 3. Microprobe analyses (atomic fractions) of aggregates of finely dispersed phases, products of chlorine-containing fluid generation reactions; a) experiment O75 system with iron: on the walls of the inner ampoule, outer ampoule, quartz trap and magnetite, b) two experiments with chromite: the first experiment with 10% NaCl, the second with 1% NaCl. The composition points inside the Fe(Cr)–NaCl–Na triangles reflect the presence of sodium chlorine-free phases (Na2CO3, NaHCO3, etc.). 3b shows that a number of compounds extend beyond the triangle (Fe+Cr)–CrCl3–NaCl, which reflects the presence of chlorides other than sodium and chromium (+ iron) chlorides, presumably magnesium chlorides.

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5. Fig. 4. Micro-Raman spectrum of magnetite and NaCl reaction products in the O89 experiment. There is also a line of undecomposed siderite, the source of the fluid. The lines of wustite and magnetite merge into a solid strip.

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6. Fig. 5. Calculated according to reactions (9) in a low–water fluid C–O-H–Cl (ratio H/(O+H)≈H2O are indicated on the charts): a) oxygen volatility, a dotted line corresponding to the FeCl2–FeCl3 buffer in the presence of magnetite at various fO2; b) molar fractions of H2O and HCl depending on lg(XCl2).

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