Sedimentological setting of sandstone-type uranium deposits in coal measures on the southwest margin of the Turpan-Hami Basin, China

Abstract

Sandstone-type uranium deposits usually exist in coal measures. This stratigraphy study found that the Jurassic coal measures (Shuixigou Group) at the southwest margin of the Turpan-Hami Basin consist of the Badaowan, Sangonghe and Xishanyao formations, which correspond to sequences A, B and C, respectively. Sequence C can be divided into the lowstand systems tract (LST), expanding lacustrine systems tract (EST) and highstand systems tract (HST). Uranium mineralization mainly develops in sequence C, in which LST and HST are the main mineralization strata while EST is the secondary mineralization stratum. High-resolution study of depositional systems tracts indicates that the fine sediments around the boundaries of parasequences can form the confining beds of uranium metallogenetic fluid systems. Within parasequences, the braided channel sandstones and part large-scale braided distributary channel sandstones have a close relationship with strong interlayer oxidation and uranium mineralization. Periodical development of parasequences conduces to the frequent growth of large skeletal sandstones and confining beds. That is the essential reason for the complexity of uranium mineralization in depositional systems tracts. The study also found that different depositional systems lead to obvious differences in the spatial distribution structures of skeletal sandstones and confining beds. LST and HST mainly comprise braided stream depositional systems, where uranium skeletal sandstones develop and are always located in the middle and lower parts of the parasequences, while confining beds are always located in the upper part of parasequences. While EST mainly comprises braided delta depositional systems, where uranium skeletal sandstones have a relatively small scale and are always located in the middle part of parasequences, confining beds are always located in the lower and upper parts of parasequences. Thus, the study of depositional systems tracts in sequence is only suitable for the primary exploration stage of sandstone-type uranium deposits, and parasequences are the best evaluation units in the advanced exploration stage.