Sedimentary environments and uranium enrichment in the Yaojia Formation, Qianjiadian depression,Kailu Basin,Nei Mongol

The sedimentary environments and palaeoclimates in the uranium-bearing Yaojia Formation in the Qianjiadian depression,Kailu Basin are discussed on the basis of well logs,electric logs and laboratory data.The Yaojia Formation is heaped up by the medium-to fine-grained sandy braided stream deposits.The host rocks of ore beds consist of lithic arkose and arkosic arenite.The grain size probability cumulative curves for the sandstones display the features of saltation and suspension populations,i.e.,those of typical fluvial deposits. Sedimentary structures include scour filling structure,scour-faceted structure, large-scale trough cross-bedding,wedged cross-bedding,parallel bedding,massive bedding,graded bedding,and wavy cross-bedding.The vertical sequences exhibit a fining-upward normal rhythmic sequence from gravelly sandstones or medium-to fine-grained sandstone passed upwards into siltstone and mudstone.The analysis of clastics and heavy minerals shows that the sediments are derived from the acidic magmatic rocks and metamorphic rocks on the palaeouplift in northwestern basin.The chemical analysis and colour index of mudstone and sporopollen analysis indicate that the palaeoclimates are dominated by the arid climates of the subtropical zone in which there occurred weak and ephemeral moistening events here and there.The arid-semiarid climates facilitate the formation of interstratal oxidized zones.The acidic volcanic rocks,old crystalline rocks and metamorphic rocks from the above-mentioned palaeouplift or distal mountain areas may serve as the uranium sources,and the braided stream sandstones as good reservoir rocks for uranium mineralization.