Metallogenetic conditions and exploration criteria of Dongsheng sandstone type uranium deposit in Inner Mongolia, China

The study area is located in the north of the Ordos basin. The Ordos basin is a large Meso-Cenozoic depression basin developed in North China Platform, with its size of approximately 250000 km2 and is well known as important 'energy resources basin' because of abundance of coal, oil and gas deposits. The pre-Jurassic basement of the basin can be divided into two parts: one is the crystalline basement consisting of Archean and Proterozoic highly metamorphic rocks and migmatic granites, and the another is Paleozoic lightly metamorphic rocks. The Mesozoic sedimentary covers of the Dongsheng area are Upper-Triassic series, Jurassic system and Lower-Cretaceous series. The Yanchang Formation, Upper-Triassic series, is mainly composed of gravel-bearing sandstone with siltstone and mudstone interbeds, being oil- and coal-productive. The Jurassic system can be divided into three series: (1) Fuxian Formation, the Lower Jurassic, mainly composed of siltstone and arkosic sandstone, with a parallel unconformity to the underlying Yanchang Formation. (2) Yanan Formation, coal-productive beds with arkosic sandstones, siltstone and mudstone interbeds, in which uranium anomalies have been found, being one of uranium target horizons; Zhiluo Formation, the Middle Jurassic, composed of gray, greenish sandstone, siltstone and mudstone, the main uraniumproductive horizon in Dongsheng area, with a parallel unconformity and angular unconformity in local place to the underlying Yanan Formation; Anding Formation, the Middle Jurassic, mainly composed of fine-grained red or brownish red siltstone and mudstone, with a parallel unconformity to the underlying Zhiluo Formation. (3) Fenfanghe Formation, Upper Jurassic, poorly developed in the study area, mainly composed of brownred conglomerate, with an angular unconformity to the underlying Anding Formation. The Lower Cretaceous series is mainly composed of red and gray sandstone and mudstone of alluvial, fluvial and eolian origins, and with an angular unconformity to overlying and underlying horizons. In general, Upper Cretaceous and Lower Tertiary series are poorly developed in the study area. The Quaternary sands and soils are exposed in different thickness from zero to tens of meters. The Dongsheng area underwent multiple tectonic events, leading to changes of contact relations between sediment beds, sedimentary facies, hydrological conditions and erosion etc., having close relationship to uranium mineralization. Metallogenetic conditions of Dongsheng sandstone type of uranium deposit is discussed according to tectonic conditions, sedimentary formation conditions and and subsequent reworking conditions. It is concluded that the crtiteria for exploration are (1) tectonic slope: The slope must have a favorable dipping angle not larger than 10 degrees and undergo suitable subsidence and uplifting movements to keep acceptable depth of target horizons and erosion period for uranium mineralization, (2) connected sandstone bodies and lithologically transitional zone: Braided stream sandstone bodies should be well connected as stable sandstone horzons, with suitable occurrence state. Uranium mineralization often occurs in the transitional zone in lithologic, grain-size, facies or color changes and (3) paleo-oxidation zone identified from presently displayed reduction area: Greenish or green sandstones are special features, which reflect reducing environment due to secondary reduction process. However, they are actually secondary-reduced paleo-oxidization zone. Uranium mineralization is controlled by transitional zone between gray (original rock) and green (paleo-oxidized) sandstones. To identify this kind of transitional zone is very important in exploration