Genesis of the Dounan manganese deposit of southeast Yunnan, China: Constraints from the mineralogy and geochemistry of micronodules

The Dounan manganese deposit in the Middle Triassic Falang Formation has prominent features like oolitic Mn ores, which are in micronodule form. Detailed petrological, mineralogical, and geochemical analyses were carried out on bulk samples and typical micronodules to clarify the deposit's material source, metallogenic environment, genesis, and formation mechanism of the micronodules. The ore minerals are mainly braunite, manganite, Ca-rhodochrosite, kutnahorite, and manganocalcite. The micronodules usually exhibit a detrital nucleus and concentric rings consisting of braunite and Mn carbonate. Good correlations were detected among the Si, Al, Ti, and Sigma REE in the ore compositions. The trace elements in the bulk samples and micronodules exhibited similar distribution patterns, and both are enriched in Co, Ni, and Sr and depleted in Cr, Rb, Zr, Nb, Ba, and Th. The micronodules, which are dominated by braunite, are more enriched in V, Co, Ni, Cu, Zn, Mo, and U than the Mn-carbonate-based ones. The Sigma REE of the micronodules was higher than that of the bulk samples, but REE patterns were similar. delta Ce showed weakly positive or negative anomalies, and delta Eu showed weakly positive anomalies. These results suggest that the ore-forming materials were from various sources, which could have included weathered Mn-bearing rocks from the Yuebei palaeoisland and Emeishan basalts from adjacent ancient lands, as well as a hydrothermal source; mineralisation occurred in a marginal shallow-marine environment with weakly alkaline seawater during fluctuating redox conditions. The micronodules formed via a combination of chemical and biological processes.