Mineralogy and geochemistry of the Proterozoic Wafangzi ferromanganese deposit, China

Abstract

In the Proterozoic Wafangzi deposit, China, sedimentary manganite ores are interstratified with red shale and silty limestone, whereas sedimentary rhodochrosite ores are interbedded with organic-rich black shale. Lateral change in mineralogy of the ores accompanying lateral changes in the host rocks demonstrates a classic example of facies variation in a "zoned" deposit. The manganite ores show a distinct Ce depletion reflecting a hydrothermal contribution in the seawater from which they precipitated. Absence of volcanics in the sequence suggests that the source of the hydrothermal input should be remote. It is likely that manganite precipitated directly from seawater in these deposits above the redox interface, whereas rhodochrosite was possibly early diagenetic formed in an anoxic environment. Manganite has been converted to braunite through a deoxidation-dehydration reaction during late diagenesis. Both types of ores have been thermally metamorphosed at the contact with igneous intrusions. Four distinct mineral assemblages have been identified in the thermally metamorphosed ores which were produced as a result of variations in bulk compositions (including volatiles) of the premetamorphic ores. In the final stage, the ores have been affected by supergene processes to give rise to pyrolusite-vernadite assemblages in the weathering zone.