Sedimentology of Triassic Dounan-type manganese deposits, western margin, Yangtze Platform, China

Abstract

Manganese (Mn) deposits formed during the Ladinian (Middle Triassic) are widely distributed along the western margin of the Yangtze Platform. All the Mn deposits occur in condensed sections (CS) and are formed in similar depositional and structural settings related to late-stage incipient spreading of the Tethys seaway. Differences in the degree of development and sedimentary facies in different areas determined the size, ore-body strike and dip, and ore composition. Four sedimentary facies occur: (1) dolomite-rich Mn rocks, (2) glauconite-bearing oolitic Mn rocks, (3) phosphatic Fe-rich Mn rocks, and (4) silica-rich Mn rocks. Type 1 formed along the margin of a passive, stable margin; type 2 formed on basin slopes with relatively low topography and deep water; and types 3 and 4 formed mainly in the Ma'erkang Basin, a deep depression on the shelf. Ore constituents partly derived from a western source area formed medium- to low-grade ores in offshore areas during the Ladinian maximum sea-level rise. Ocean currents and lower-cycle sea-level changes produced composite ore beds. Seafloor hydrothermal systems typical of Tethyan rifting may have provided Mn-bearing fluids that ascended along faults and mixed with bottom currents. Those fluids and/or diagenetic oxidation reformed and concentrated the previously formed medium- to low-grade ore bodies.