Abstract

Manganese and Fe have similar geochemical properties in the supergene environment. Separation of Mn and Fe is an important process for the formation of high-grade sedimentary manganese deposits. Large-scale manganese carbonate deposits (total reserves of approximately 700 Mt) were formed during the interglacial of the Sturtian and Marinoan in South China. The orebodies are hosted in the black rock series at the basal Datangpo Formation of the Cryogenian period. The Fe contents in ores range from 1.15 to 7.18 wt.%, with an average of 2.80 wt.%, and the average Mn/Fe ratio is 8.9, indicating a complete separation of Mn and Fe during the formation of manganese ores. Here, we present element data of manganese carbonates and sulfur isotopes of pyrite from the Dawu deposit, Guizhou, China, aiming to investigate the separation mechanism of Mn and Fe and the ore genesis. The Fe in ores mainly occurs as carbonate (FeCO3) and pyrite (FeS2). The Mn, Ca, Mg and Fe exist in the form of isomorphic substitutions in manganese carbonate. The contents of FeCO3 in manganese carbonates are similar in different deposits, with averages of 2.6–2.8 wt.%. The whole-rock Fe and S contents have an obvious positive correlation (R = 0.69), indicating that the difference of whole-rock Fe content mainly comes from the pyrite content. The δ34SV-CDT of pyrite varies from 40.0 to 48.3‰, indicating that the pyrite formed in a restricted basin where sulfate supply was insufficient and the sulfate concentrations were extremely low. Additionally, the whole-rock Fe content is negatively correlated with the δ34S values of the whole-rock and pyrite, with correlation coefficients of −0.78 and −0.83, respectively. Two stages of separations of Mn and Fe might have occurred during the mineralization processes. The reduced seawater became oxidized gradually after the Sturtian glaciation, and Fe2+ was oxidized and precipitated before Mn2+, which resulted in the first-stage separation of Mn and Fe. The residual Mn-rich and Fe-poor seawater flowed into the restricted rift basin. Mn and Fe were then precipitated in sediments as oxyhydroxide as the seawater was oxidized. At the early stage of diagenesis, organic matter was oxidized, and manganese oxyhydroxide was reduced, forming the manganese carbonate. H2S was insufficient in the restricted basin due to the extremely low sulfate concentration. The Fe2+ was re-released due to the lack of H2S, resulting in the second-stage separation of Mn and Fe. Finally, the manganese carbonate deposit with low Fe and very high δ34S was formed in the restricted basin after the Sturtian glaciation.

Highlights

  • The abundance of Fe in the continental crust is 7.08 wt.%, which is much higher than that of Mn at 0.14 wt.% [1]

  • In the gradual oxidation of the reduced water, Fe is oxidized and precipitated before Mn, which results in the separation of Mn and Fe

  • The Cryogenian manganese deposits in South China are characterized by low Fe contents and very high δ34 S

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Summary

Introduction

The abundance of Fe in the continental crust is 7.08 wt.%, which is much higher than that of Mn at 0.14 wt.% [1]. Separation of Mn and Fe is very common in sedimentary manganese deposits [2,3,4], such as the Archean Noamundi Fe-Mn deposit in India, the. Mn and Fe are dissolved in water in the form of Mn2+ and. In the anoxic and sulfur-rich environment, Mn occurs as soluble manganese bicarbonate and manganese sulfide with poor stability, while Fe mainly occurs as stable sulfide [10,11]. Some microorganisms selectively promote manganese oxidation and may result in the separation of Mn and Fe [10,11,12]. The study of the separation of Mn and Fe can trace the Mn mineralization process and deepen the understanding of ore genesis

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