湘西民乐地区大塘坡组锰矿沉积氧化还原条件OA

[Objective]The western Hunan region hosts extensive Neoproterozoic marine sedimentary manganese de-posits.Manganese(Mn)is a typical redox-sensitive metallic element whose enrichment in sediments is strongly corre-lated with the redox condition of the bottom water in the basin.A detailed study of the sedimentary redox conditions of such deposits helps to explain their metallogenic mechanisms and the constraints on their marine cycling during the mineralization period,and also provides theoretical support for the assessment of regional mineralization potential.[Methods]This study involved mineralogical,geochemical and inorganic carbon isotope analyses of 16 manganese ore samples of various grades from the Nanhua System Datangpo Formation in the Minle area,western Hunan.[Results]Mineralogical and carbon isotope analyses revealed that high-grade(MnO>20%)and medium-grade(10%<MnO<20%)manganese ores consist of predominantly subhedral granular microcrystalline rhodochrosite(grain size<5 μm)and are relatively enriched in light carbon isotope(δ13C=-10.4‰～-8.3‰;n=10).Conversely,low-grade(MnO<10%)manganese ores contain larger-grained(>20 μm)subhedral platy rhodochrosite enriched 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 the high-grade manganese ores were deposited in oxic condi-tions,the medium-grade ore in suboxic conditions,and the low-grade ore in anoxic conditions.Additionally,high-grade ores exhibit geochemical characteristics indicating hydrothermal deposition(e.g.,high Eu* values and high Fe/Ti ratios).[Conclusions]These findings suggest that,in the Minle area of western Hunan,the rhodochrosite in the medium-to high-grade manganese ores was formed as a result of the reaction between Mn oxides with organic matter during diagenesis,whereas in the low-grade manganese ores the rhodochrosite was probably precipitated from Mn2+and CO2-3 in highly alkaline and anoxic conditions.Submarine gas-hydrothermal activity in the deep-water basin of northeastern Guizhou most likely supplied the Mn2+ions for these manganese deposits;oxidation of bottom water in the basin was the key factor determining manganese enrichment and mineralization in the region.