Electrochemical reactions driving Mn-enrichment in Fe-Mn supergene ores: A mineralogical perspective

Iron and manganese oxides embody a geochemical system of great environmental, biological, and economical relevance. Chemical equilibria and the stability of Fe-Mn phases under surface or near-surface conditions can influence the fate of contaminants in the environment, impact biological metabolic processes, or Fe and Mn phase distribution in weathered ores. In the present work, we focus on the textural, mineralogical, and chemical study of Fe-Mn ores from the weathered zone of a vein-hosted deposit outcropping in the Iberian Pyrite Belt, SW Portugal, by means of micro-Raman spectroscopy coupled with electron microprobe microanalysis. The aims of our investigation are i) identifying the several Fe and Mn phases occurring in differently enriched ores samples, ii) relating their chemical composition with possible mineralization mechanisms, and iii) defining the mineral paragenetic pathway related to the observed textural features. Our approach enabled both the identification of the coexisting Fe and Mn phases, and unravelling paragenetic pathways leading to supergene enrichment mineralizations. Collected evidence demonstrates that changes in Eh/pH can lead to goethite dissolution under reductive conditions, promoting the release of Fe2+ into solution, whose electrochemical interaction with Mn4+ results in the formation of several types of Mn oxides, and secondary goethite. Our data shows a clear rela-tionship between the type of Mn oxide crystallized and the ratio of aqueous Mn3+/Mn4+, alongside other prevalent cations, incorporated into tunnel or interlayer structural sites, which may be also desorbed/solubilized from primary goethite.

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This study focuses on the textural, mineralogical, and chemical analysis of iron-manganese ores from a vein-hosted deposit in the Iberian Pyrite Belt, SW Portugal. The research identifies the different Fe and Mn phases present in the enriched ores, explores their chemical composition, and defines the mineral paragenetic pathway. The study demonstrates that changes in Eh/pH conditions can lead to goethite dissolution and the formation of various Mn oxides and secondary goethite through electrochemical reactions between Fe2+ and Mn4+.