Black chert-hosted manganese deposits from the Bistritei Mountains, Eastern Carpathians (Romania): petrography, genesis and metamorphic evolution

摘要

The East Carpathian manganese deposits (ECMD) occur in the Tulghes Group, a Cambrian-Ordovician island arc complex metamorphosed under greenschist facies conditions during the Silurian. The deposits consist of ferromanganese carbonate-silicate orebodies (Fe-rhodoehrosite, Mn-amphibole, spessartine, stilpnomelane, rhodonite, pyroxmangite and tephroite), occurring within larger and lower-grade mineralized areas. Although hosted by organic matter-rich cherts (C_(org) 1 - 1.5 percent), the ore is devoid of organic matter. The delta ~(13)C_(carbonate) values are from - 3.5 per thousand to - 15.7 per thousand (PDB) in the carbonate ore and - 1.7 per thousand to + 0.9 per thousand in the neighboring carbonate schists, while the hosting organic matter-rich cherts exhibit delta ~(13)C_(org) values from -22.7 per thousand to -27.8 per thousand (PDB). The ore shows positive Ce and negative Eu anomalies on chondrite-normalized plots. The manganese was supplied by both terrigenous and igneous sources stored in oxygen-deficient bottom water of a back-arc basin and precipitated as Mn carbonate and oxide. The metamorphism of ECMD is polyphase with contrasting tendencies. The formation of silicate minerals by decarbonation reactions was followed by the extensive replacement of silicate phases by Mn carbonate and, locally, by iron sulfide, as a result of reducing fluids circulation.