Experimental Study of Interaction of Carbonic Fluid with Cumulus Minerals of Ultrabasic Intrusions at 950°C and 200 MPa

At the stage of magmatic crystallization, magmas of basic–ultrabasic intrusions of subduction origin and alkaline-ultrabasic intrusions have high oxygen fugacity, which prevents large-scale sulfide formation. Correspondingly, PGE in such intrusions are dispersed among cumulus minerals rather than accumulated in sulfides. It has been shown experimentally that the subsolidus interaction (P = 200 MPa, T = 950°C) of CO2 with olivine, a typical cumulus mineral of basic–ultrabasic intrusions, leads to the oxidation of the fayalitic component and reduction of a fluid. At a low silica activity in the fluid, the content of CO in CO2 reaches a maximum value of 14 mol %, which corresponds to fO2 = QFM–2. With such a CO content, platinum from the capsule walls was dissolved in the fluid in the form of carbonyl and reprecipitated with spinel in olivine cracks. It has been experimentally established that the interaction of CO2–H2O fluid with pyrrhotite under the same P-T conditions is accompanied by the reduction of the fluid with a decrease in oxygen fugacity to QFM buffer. Analysis of the composition of fluid captured in an albite glass trap by micro-Raman scattering showed the formation of saturated (С2Н6 and СН4) and unsaturated (with functional groups СН=СН and =СН2) hydrocarbons, CO, H2 and H2S. The platinum of the capsule walls has buffered the sulfur fugacity at a low level of Pt–PtS buffer, resulting in the low content of sulfur species in the fluid and dissolution of Pt in carbonyl form. Crystallization of the isoferroplatinum from such a fluid was observed experimentally. Preliminary data indicate that the CO-bearing carbonic fluid extracts Cr from the Cr-spinel, which increases the range of the Cr/(Al + Cr) ratio with constant Fe3+/(Al + Cr) at the spinel surface. All established experimental effects of fluid interaction with cumulus minerals of the basic–ultrabasic intrusions have been found in nature. This supports the inferred important role of such interaction in the formation of the low-sulfide PGE deposits.