Abstract

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.

中文翻译：

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减少的CO 2流体作为成矿过程的代理人：以Yoko-Dovyren地块的白云石替代矽卡岩为例

摘要

本文介绍了北贝加尔湖地区Yoko-Dovyren层状超镁铁质-镁铁质岩浆侵入的外部接触岩石和碳酸盐置换矽卡岩的最新地球化学数据。岩石最初包含具有高氧气逸度（高达NNO + 3-4）的富含CO _2的流体，这是由于白云石的部分分解以及SiO ₂之间的反应而产生的和碳酸盐。矽卡岩蓝透辉石富含Pt（最高0.2 ppm）和V（300 ppm），而矽卡岩的硅灰石带中的Re浓度较高（最高0.4 ppm）。接触区石英岩的REE模式与Yoko-Dovyren地块下部接触部分的含金云母的锂铁矿的REE模式相同。侵入岩的这些地球化学特征可以通过减少的H ₂ O–CO ₂流体对物质的转移和再沉积来解释。根据热力学计算，在固相线温度T =950ºC和压力P下，H ₂ O–CO ₂流体与高镁橄榄石之间的反应= 2 kbar应该导致氧逸度降低至QFM – 2，从而产生大量的CO。根据计算，还可以通过H _2的黄铁矿氧化来维持低氧逸度（接近QFM + 0.7）。积P – T参数下的O和CO ₂流体成分。这些反应的结果是，流体中应富含从岩浆硫化物中提取的Pt，并且该Pt可以重新沉积在岩石中，包括构成矽卡岩带的岩石中。