Reduced CO2 Fluid as an Agent of Ore-Forming Processes: A Case Study of Dolomite-Replacement Skarns at the Yoko-Dovyren Massif

Аннотация

The paper presents newly obtained geochemical data on outer-contact rocks and carbonate-replacement 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 = 950ºC 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.