Analysis On Bauxite Sedimentary Environments Of The Benxi Formation In Yuzhou Area

The detailed study were conducted for the bauxite forming enviroments of the Benxi Formation?Late Carboniferous?of Yuzhou areas with the help of polarizing microscope,elemental analysis,EDS and XRD using the principles of mineralogy,geochemistry and sedimentology.The Yuzhou bauxite mine belongs to typical karstic bauxite.The ore-bearing settings underlies the shales of the Shanxi Formation and overlies limestones of ther Majiagou Formation or dolomites of Gushan Formation.The bauxite ores in Yuzhou area can be divided into clastic bauxite,earthy bauxite,bean oolitiic bauxite,densely bauxite.The ores mainly display different grain-size structures,occasionally visible of oolitic structure.The analysis of polarizing microscope,XRD and electron probe revealed that the bauxite ore types in Yuzhou area mainly consist of diaspore,kaolinite,illite,hematite,anatase,and also contain minerals such as chlorite.Geochemical analyses indicate that Al₂O₃,SiO₂,Fe₂O₃,K₂O,TiO₂ and LOI are the main components in bauxite ores.The analysis of trace elements shows that enrichment of B,Li and Zr,Hf,Nb,Ta,W are related to the continental environments during the bauxitization process.It is suggested that bauxites in Yuzhou area are mainly formed in partly closed bay environment based on the evidences of the mineral assemblages,geochemistry of trace elements and REE of bauxite.Their forming environments can transformed between partly closed bay and lacustrine.It appears to be salt-brackish partly closed bay environment when Yuzhou area was connected with Huabei sea with the sea level rising.It appears to be salt-brackish lacustrine environment when Yuzhou area was not connected with the sea with sea level declining.ITt appears to be freshwater environment in part area with the input of atmospheric precipitation and land fresh water.The transformation between partly closed bay and lacustrine was induced the transformation of palaeosalinity of aqueous medium between freshwater and alt-brackish water,which also induced the bauxites have both continental and transitional characteristics.