Wang, Yan-Jun, Zhu, Wei-Guang, Huang, Hui-Qing, Bai, Zhong-Jie, Zhong, Hong, Yao, Jun-Hua, and Fan, Hong-Peng (2020) Geochemistry of magnetite from the giant Paleoproterozoic Dahongshan Fe-Cu deposit, SW China: constraints on nature of ore-forming fluids and depositional setting. Ore Geology Reviews, 118. 103361.

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Abstract

The Dahongshan Fe-Cu ore deposit, hosted in the Paleoproterozoic meta-volcanic and meta-sedimentary sequences, is a giant deposit in the Fe-Cu metallogenic province of southwestern China. Two ore types have been identified: (1) massive and disseminated Fe ores hosted dominantly in meta-volcanic rocks and (2) disseminated and banded Fe-Cu ores in meta-sedimentary rocks. Magnetite presents in all orebodies, and is dominant in the Fe ores. Chemistry of magnetite suggest that the fluids for iron mineralization in the Fe and Fe-Cu orebodies are likely chemically similar and cogenetic. Minor sulfides (e.g., molybdenite) occur at the end of iron mineralization stage, indicating the gradually decrease in oxygen fugacity for the mineralizing fluids with magnetite precipitation. Along with evolution, mineralizing fluids generate magnetite with depletions in Cr, Ni, Ga and V, but enrichments in Mn, Sn and Co, which are primarily reflective of the temperature decrease of fluids.

We have discovered some magnetite grains that are extremely rich in V (similar to 10000 ppm), but low in Ti and Cr. Geochemical patterns suggest that they were formed by fluids similar to those of ordinary ores. Available knowledge about the unique high-V magnetite indicates highly-reduced marine environments for its generation. Ore bulk REE patterns showing remarkable positive Eu anomalies and absence of obvious Ce anomalies suggest a BIF-like marine environment, consistent with the morphology of both Fe and Fe-Cu orebodies. Our results provide independent evidence for submarine environment in the formation of the giant Precambrian stratiform re(-Cu) ore deposit.

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