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Abstract
The Xialei Mn deposit occurs in the Late Devonian (Famenian) Wuzhishan Formation and consists of three ore horizons composed of three ore types: (1) Mn carbonate, (2) Mn silicate–Mn carbonate, and (3) Mn-oxide. Mn silicate–Mn carbonate ore beds consist of Mn carbonates and associated rhodonite, stilpnomelane, actinolite, chlorite, Mn epidote, biotite, and Mn–Fe antigorite. These minerals are mixed with each other and form thin beds, bands, pelletal and pisolitic grains, and horizontal or wavy laminae. Abundant oolitic and pisolitic grains in Mn ore beds show distinct boundaries and nuclei of Mn carbonate and Mn silicate minerals, rarely clastic grains. The ore deposit is divided into three zones: (1) an inner zone composed of Mn carbonate, silicate, and oxide minerals; (2) a transitional zone consisting of Mn carbonate and stilpnomelane; and (3) an outer zone characterized by Mn carbonate. Argillaceous limestone at the base of the ore beds has a mean δ 13 C PDB of about 0.0‰, indicating that the carbon was derived from seawater bicarbonate. The δ 13 C PDB of Mn carbonate ranges from −2.8 to −14.3‰ (mean −7.1‰), indicating that the carbon was derived from both degradation of organic matter and seawater bicarbonate. Ores formed in a trough on a carbonate platform. Primary sedimentary Mn carbonates precipitated from alkaline and negative Eh waters in the diagenetic zone of sulfate reduction, which occurred either during shallow burial or at the seafloor. Oolites and pisolites formed by sedimentary processes on the seafloor. In places, deposits were reworded and transported by gravity-flow processes. The region where these sedimentary-diagenetic Mn deposits formed was intruded by magma, and hydrothermal fluids with dissolved metals leached from volcanic and sedimentary rocks ascended along fractures. The Mn silicate–Mn carbonate ore may have been produced by contact metamorphism of primary carbonate ore and metal sulfides deposited by hydrothermal fluids, which overprinted the primary sedimentary ore.
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