Characteristics and Genesis of Supergene Manganese Ores in Xialei,Guangxi

The Xialei Mn-oxides ore deposit,located in Guangxi,is the largest manganese producer in China.Manganese ores consist of sedimentary Mn-bearing carbonates and supergene Mn oxides.The supergene ores are mainly composed of hollandite,cryptomelane,lithiophorite,pyrolusite,braunite,todorokite,and jacobsite.Other secondary minerals intergrown with the Mn-oxides are hematite,goethite,quartz,kaolinite,and montmorillonite.Hollanite typically contain high barium contents,while cryptomelane is high in potassium concentrations resulting in high w(K)/w(Ba) ratio in the mineral.The highest manganese concentrations were seen in pyrolusite.However,this mineral precluded incorporation of other trace elements into its 1×1 tunnel structure.Lithiophorite is characterized by the high aluminum but relatively low manganese contents.Macroscopically,Mn-oxides commonly show botryoidal,massive,or banded structures,with cryptomerous or metasomatic texture.Contrasting with carbonate ores,manganese grade of the oxide ores are significantly enhanced due to supergene enrichments of the ore metal.A few kinds of trance elements such as Co,Ni,Cu and Zn are also enriched to some extent.Compared with Fe-oxides,Mn-oxides typically contain low P and Si contents,reflective of preferential incorporation of these impurity elements into Fe-oxides.The high w(Mn)/w(Fe) ratios suggest intense weathering and sufficient chemical separation between the Mn~-and Fe-oxides,which would have been important in the formation of high-quality Mn-ores.This study,when combined with existing data,suggests that formation and distribution of supergene Mn-oxides were jointly controlled by pre-enrichment of manganese in sedimentary rocks of wide distribution,wet and warm paeloclimate,relatively slow tectonic uplift and development of structure,and favorable geomorphologic units.