Recognition, genesis and evolution of manganese ore deposits in southeastern China

• Numerous oolitic and pisolitic structures are clearly observed in manganese ores.
• Mn enrichment is probably related to microbial activity under the oxic conditions.
• Mn solution and precipitation are redox–controlled corresponding to sea level change.

Manganese ore deposits are common in the Falang Formation of Ladinian age (Middle Triassic), southeastern Yunan, China. The deposits containing manganese oxide and manganese carbonate ores are not metamorphic in origin and they commonly contain bioclasts. Concentric ring structures are clearly observed in oolitic and pisolitic manganese ores. An integrated approach using sedimentology and paleoenvironment analysis provides new insight into the genesis of these ores. Systematic research has been conducted on typical deposits (Dounan, Yanzijiao, Laowu, Tujichong) using cathodoluminescence and scanning electron microscope techniques, which allow elucidation of changes in laminated microtextures at the mm–μm scale. Preliminary results show that manganese enrichment in Triassic deposits of this region may be related to microbial activity and mineralization was a consequence of bacterially mediated diagenetic reactions. In addition, findings of biodetritus in the manganese ore indicate that manganese ore is close to the redox level. The concentration of manganese in solution and in precipitates in Triassic is controlled by bacterial oxidation and redox conditions corresponding to sea level changes during the Ladinian.