Nanhuan manganese deposits within restricted basins of the southeastern Yangtze Platform, China: Constraints from geological and geochemical evidence

• The manganese carbonates mainly originated hydrothermal processes and were reduced from Mn-oxyhydroxides.
• The carbon was mainly derived from the oxidation of organic carbon.
• Data suggest that the deposits are hydrothermal–sedimentary/diagenetic type deposits.

The Nanhuan manganese deposits in the southeastern Yangtze Platform occur in the black shale series in the lower part of the Datangpo Formation. In order to constrain the genesis of the deposits, a detailed study was undertaken that involved field observations, major and trace element analyses, organic carbon analyses, and isotope analyses (C, O, S). The major findings are as follows. (1) The ore-bearing rock series, morphology of the ore bodies, and characteristics of ores in several deposits are similar. The ore minerals are rhodochrosite and manganocalcite. The gangue minerals are mainly quartz, feldspar, dolomite, and illite. Minor apatite and bastnaesite occur in the manganese ores. (2) The ores are enriched in Ca and Mg, whereas they are depleted in Si, Al, K, and Ti compared to wall rocks. The ores normalized to average Post-Archean Australian shale (PAAS) are enriched in Co, Mo, and Sr. The chondrite-normalized rare earth element (REE) patterns for ores and wall rocks are between those of typical hydrogenous and hydrothermal type manganese deposits. Additionally, the ores have positive Ce anomalies with an average Ce/Ce* of 1.23 and positive Eu anomalies with an average Eu/Eu* of 1.18 (normalized to PAAS). (3) The average content of organic carbon is 2.21% in the samples, and the average organic carbon isotopic value (δ13CV-PDB) is − 33.44‰. The average inorganic carbon isotopic value (δ13CV-PDB) of carbonates in Gucheng is − 3.07‰, while the values are similar in the other deposits with an average of − 8.36‰. The oxygen isotopic compositions (δ18OV-PDB) are similar in different deposits with an average of − 7.72‰. (4) The sulfur isotopic values (δ34SV-CDT) of pyrite are very high and range from + 37.9‰ to + 62.6‰ (average of 52.7‰), which suggests that the pyrite was formed in restricted basins where sulfate replenishment was limited. The sulfate concentrations in the restricted basins were extremely low and enriched in δ34S, which resulted in the very high δ34S values for the pyrite that formed in the manganese deposits. Therefore, a terrigenous weathering origin for manganese can be excluded; otherwise, the sulfate would have been introduced into the basins together with terrigenous manganese, which would have decreased the δ34S values of pyrites. The manganese, which originated from hydrothermal processes, was enriched in the restricted and anoxic basins, and then, it was oxidized to manganese oxyhydroxide in the overlying oxic waters whereby the products precipitated into the sediments. The manganese oxyhydroxide in the sediment was then reduced to Mn2 + and released to the pore waters during the process of diagenesis. Some organic carbon was oxidized to CO32 −, which made the depletion of 13C in manganese carbonates. Therefore, we suggest that the Nanhuan manganese deposits are hydrothermal–sedimentary/diagenetic type deposits.

There is a great deal of uncertainty surrounding the genesis of the Nanhuan manganese deposits in the southeastern Yangtze Platform, China. In our study, detailed mineralogical, organic carbon, and isotopic (C, O, S) analyses were conducted for eight typical deposits in the region. The extremely low concentrations of sulfate in the restricted basins resulted in the high δ34S values of pyrite. The manganese mainly originated from submarine hydrothermal processes. Volcanism and terrigenous weathering probably provided minor amounts of manganese to the deposits. The manganese was enriched in the deep anoxic waters of the basins as dissolved Mn2 +. Then, the Mn2 + was oxidized to Mn-oxyhydroxide in the overlying oxic seawater and it precipitated into the sediments. In the diagenetic reactions, the buried Mn-oxyhydroxide was reduced to Mn2 + and bicarbonate was derived from the oxidation of organic carbon in the pore water. In such a process, the Mn carbonates, which were depleted in 13C, were formed. Therefore, the Nanhuan manganese deposits in the southeastern Yangtze Platform, which are hosted in the black shale series, are hydrothermal–sedimentary/diagenetic type deposits.Figure optionsDownload as PowerPoint slide