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The paper presents newly obtained geochemical data on outer-contact rocks and carbonatereplacement skarns of the Yoko-Dovyren layered ultramafic-mafic intrusion in the northern Baikal area. The rocks initially contained CO2-rich fluid with a high oxygen fugacity (up to NNO + 3–4), which was generated by the partial decomposition of dolomite and by reactions between SiO2 and carbonates. The skarn blue diopside is enriched in Pt (up to 0.2 ppm) and V (300 ppm), and the wollastonite zone of the skarns contains elevated Re concentrations (up to 0.4 ppm). The REE pattern of the contact-zone quartzite is identical to the REE patterns of phlogopite-bearing lherzolites from the lower contact part of the Yoko-Dovyren massif. These geochemical features of the rocks of the intrusion may be explained by the transfer and redeposition of material by reduced H2O-CO2 fluid. According to thermodynamic calculations, a reaction between H2O-CO2 fluid and high-Mg olivine at a subsolidus temperature of T = 950оC and pressure P = 2 kbar should result in a decrease in the oxygen fugacity to QFM – 2 and, hence, generate much CO. According to the calculations, a low oxygen fugacity (close to QFM + 0.7) can also be maintained by pyrrhotite oxidation with H2O and CO2 fluid components under cumulus P-T parameters. As a result of these reactions, the fluid should enrich in Pt extracted from magmatic sulfides, and this Pt can be redeposited in rocks, including those composing the skarn zones.

Reduced CO2 fluid as an agent of ore-forming processes: a case study of dolomite-replacement skarns at the Yoko-Dovyren massif)