CARBONIFEROUS MN DEPOSITS FROM THE SOUTH CHINA BLOCK: A NEW TYPE OF UPWELLING-RELATED ORE

The Lower Carboniferous Luzhai and Baping formations (ca. 354 Ma) of the South China Block, Guangxi Province, comprise a ca. 170-m-thick siltstone-carbonate succession capped by Mn ore. The excellent exposure and nearly complete stratigraphy provide an unparalleled opportunity to investigate the origin of a large carbonate-hosted Mn deposit with reserves in excess of 4.8 million tonnes. Manganese is an important micronutrient and required for steel production.

Lithofacies associations suggest deposition occurred on a mesotrophic shelf capable of sustaining a thriving community of filter feeders. Proximal facies are siltstones with spiculitic chert that change distally into carbonate packstones and grainstones hosting diverse assemblages of bryozoans, crinoids, and brachiopods. Vertical facies trends indicate accumulation during marine transgression, culminating with the precipitation of high-grade Mn deposits that define the maximum flooding surface. Ore horizons are composed of amalgamated alabandite-bearing rhodochrosite, Mn-calcite, and braunite laminae. Paragenetic relationships indicate these Mn minerals are authigenic and record the influence of an anoxic, manganiferous water mass on the distal to middle shelf. Such anoxia is interpreted to have shut down the carbonate factory and diminished sedimentation, which is a prerequisite for the concentration and precipitation of Mn in pore water.

Sedimentologic data support recent research that suggests areas of the Paleozoic ocean were persistently anoxic and periodically tapped by coastal upwelling to produce Fe and Mn-rich deposits. Application of this emerging model to other economically important Mn ores may improve resource exploration and extraction strategies. This insight will provide a more complete understanding of the Earth system feedbacks that governed ocean-atmosphere oxygenation and the development of giant, sediment-hosted mineral deposits.