Study of the Diagenetic Characteristics of Uraniferous Sandstone and Its Relationship with Uranium Mineralization of the Bayin Gobi Formation, Lower Cretaceous Tamusu Deposit

Abstract

The Tamusu uranium deposit in the southern part of the Bayin Gobi Basin is a typical hard-sandstone uranium deposit， but with very different diagenetic characteristics from other sandstone-type uranium deposits in northern China. The characteristics of uraniferous sandstone （K1b2-2） and its relationship to uranium mineralization in the Tamusu deposit have been studied In recent years by petrology， petrography， geochemical， scanning electron microscope （SEM） and so on. The detritus is composed of quartz （average content 2.38%） and feldspars （average content 50.33%）. The cements comprise gypsum （average content 12.57%）， ankerite （average content 3.20%）， ferruginous dolomite （average content 9.33%）， dolomite （average content 9.76%） and a small amount of complex matrix material （average content 2.62%）， causing both basal cementation and porous cementation. The diagenetic process is characterized by early stage B diagenesis and later stage A diagenesis， with alkaline water （pH 7.52）. Early stage B diagenesis is characterized by chemical cementation， water-rock interaction， dissolution and metasomatism. The formation sequence of the cements is ankerite-ferruginous dolomite-dolomite-gypsum； the carbonate cements， from center to edge， are automorphic ankerite， automorphic-hypidiomorphic ferruginous dolomite， and hypidiomorphic-xenomorphic dolomite. Gypsum forms last， with metasomatism of quartz， feldspar and dolomite. In this process， plagioclase is altered to albite due to the presence of Na+ in the highly saline water replacing Ca2+， with dissolution microcavities formed on the surface of the plagioclase and microcracks along cleavage planes. During early stage B diagenesis， no auto-cementation takes place， and clay minerals are affected by alkaline groundwater. During late stage A diagenesis， acidic surface water containing S O 4 2 - dissolves the carbonate cement， forming cavities. The uraniferous sandstone does not favor interlayer oxidation due to its low porosity and permeability. It is inferred that the genesis of uranium mineralization in the Tamusu deposit occurs by sedimentary diagenesis and interlayer oxidation.