Redox conditions of manganese deposits in Datangpo Formation, Minle Area, western Hunan Province: constraints from mineralogy and geochemistry

Key words: 
• Neoproterozoic /
• sedimentary manganese deposits /
• redox proxies /
• carbon isotope /
• hydrothermal activity

Abstract: [Objective] Manganese (Mn), as a typical redox-sensitive metal element, shows a strong correlation between its enrichment in sediments and the redox conditions of the bottom water in basin. The western Hunan region hosts extensive Neoproterozoic marine sedimentary manganese deposits. A detailed study of the sedimentary redox conditions of these manganese deposits aids in elucidating their metallogenic mechanisms, constraining the marine manganese cycling during the mineralization period, and providing theoretical support for regional mineralization potential assessments. [Methods] This study involved mineralogical, geochemical, and inorganic carbon isotope analyses of 16 manganese ore samples of varying grades from the Datangpo Formation of the the Nanhua System from the Minle area in western Hunan. [Results] Mineralogical and carbon isotope analyses reveal that high-grade (MnO>20%) and medium-grade (1020 μm) subhedral platy rhodochrosite, with relative enrichment in heavier carbon isotope (δ13C=-7.4‰~-5.3‰, n=6). Pyrite morphology and element geochemical proxies (Mo, V, Ni concentrations and Ni/Co, V/Cr ratios) indicate that high-grade manganese ore layers were deposited in an oxic condition, medium-grade manganese ore layers in a suboxic condition, and low-grade manganese ore layers in an anoxic condition. Additionally, high-grade manganese ores exhibit geochemical characteristics indicative of hydrothermal deposition, such as high Eu* values and high Fe/Ti ratios. [Conclusions] These findings suggest that rhodochrosite in medium- to high-grade manganese ores formed through the reaction of manganese oxides with organic matter during diagenesis, while the rhodochrosite in low-grade manganese ores likely precipitated from Mn2+ and CO32- under high alkalinity and anoxic conditions in the Minle area of western Hunan. The submarine gas-hydrothermal activity in the deep-water basin of northeastern Guizhou likely supplied Mn2? for manganese deposit in western Hunan, while the oxidation of bottom water in the basin was the key controlling factor for manganese enrichment and mineralization in the region.