The Relationship between Jurassic Coal Measures and Sandstone‐type Uranium Deposits in the Northeastern Ordos Basin, China

Abstract

Outcrop and drill hole data show that the Jurassic coal measures in the northeastern Ordos Basin are composed mainly of the Yan'an Formation and the lowstand system tract of the Zhiluo Formation, and there is a regional unconformity between them. The Dongsheng uranium deposit is associated with the Jurassic coal measures. Research data indicate that the Jurassic coal measures in the study area have a certain hydrocarbon‐generating capacity, although the metamorphic grade is low (Ro=0.40%–0.58%). In the Dongsheng region alone, the accumulative amount of generated coalbed methane (CBM) is about 2028.29 × 108 −2218.72 × 108 m3; the residual amount is about 50.92 × 108 m3, and the lost amount is about 1977 × 108 m3. Analysis of the burial history of the host rocks and the evolutionary history of the Dongsheng uranium deposit suggests that the Jurassic coal measures generated hydrocarbon mainly from Middle Jurassic to Early Crataceous, which is the main mineralization phase of the Dongsheng uranium deposit. By the Late Cretaceous, a mass of CBM dissipated due to the strong tectonic uplift, and the Dongsheng uranium deposit stepped into the preservation phase. Therefore, the low‐mature hydrocarbon‐containing fluid in the Jurassic coal measures not only served as a reducing agent for the formation of sandstone‐type uranium deposits, but also rendered the second reduction of paleo‐interlayer oxidation zone and become the primary reducing agent for ore conservation. Regional strata correlation reveals that the sandstone‐type uranium reservoir at the bottom of the Zhiluo Formation is in contact with the underlying industrial coal seams in the Yan'an Formation through incision or in the form of an unconformity surface. In the Dongsheng region with poorly developed fault systems, the unconformity surface and scour surface served as the main migration pathways for low‐mature hydrocarbon‐containing fluid migrating to the uranium reservoir.

Petrogenetic Constraints of Early Cenozoic Mafic Rocks in the Southwest Songliao Basin, NE China: Implications for the Genesis of Sandstone-Hosted Qianjiadian Uranium Deposits

“…Sandstone-hosted uranium deposits in the Ordos Basin are mainly distributed in the northern margin of the basin, including the Dongsheng, Nalinggou, and Daying deposits. The U mineralization of these deposits is located in the contact zone between primary reduced grey sandstones and secondary reduced green sandstones within the Zhiluo Formation [31]. Similar to the Qianjiadian deposit, altered green sandstones are spatially closely related to the U mineralization [99].…”

“…The primary reduced host sandstones are mainly composed of lithic fragments and individual detrital minerals such as quartz, K-feldspar, plagioclase, biotite, muscovite, and chlorite. In the Qianjiadian area, the U mineralization occurs at the redox interface between the primary reduced and the secondary oxidized sandstones (Figure 2b) or close to a secondary reduced green-white zone [18,27,31] related to mafic rock intrusions that are spatially distributed along the redox boundary (Figures 1c and 2b). Besides, the content of U from Yaojia Formation sandstones is inversely correlated with REEs' content [17].…”

“…Previous studies proposed that the intrusion of these mafic rocks triggered subsequent circulation of mantle-derived CO 2 -rich hydrothermal fluids, which could have been responsible for U leaching from the surrounding sandstones [34]. In addition, it has also been proposed that the genesis of the U mineralization in the Qianjiadian area was controlled by BSR processes within organic matter (OM)-bearing sandstones [28][29][30][31][32][33]. A recent study on carbonaceous debris from the Qianjiadian U deposit showed the increase of the vitrinite reflectance of carbonaceous debris, hence, revealing the heat effect of diabase dikes and related fluids [20] and suggesting the close spatial relationship between the dikes and the U mineralization.…”

“…In the Qianjiadian area, uranium is mainly adsorbed or reduced on carbonaceous debris or migrated oil, and U minerals such as pitchblende and coffinite are intimately associated with iron disulphides [22][23][24][25][26][27]. Several studies were conducted on the micromorphological observations, in situ sulfur isotope analysis of pyrites, and carbon isotope composition of calcite cement, emphasizing the role of bacterial sulphate reduction (BSR) in the genesis of the U mineralization in the Qianjiadian area [28][29][30][31][32][33]. Nevertheless, the tabular U ore bodies in the Qianjiadian deposit are spatially associated with secondary reduced gray sandstones or bleached white sandstones [34] ( Figure 2) together with the widely distributed mafic rocks [17], which significantly differ from the two predominant models of redox front sandstone-hosted U deposits presented in [33].…”

Adsorption Mechanism and Kinetic Characterization of Bituminous Coal under High Temperatures and Pressures in the Linxing‐Shenfu Area

“…All of the coal samples were collected from the Linxing -Shenfu area, which is located in the north eastern Ordos Basin, and tectonic location of the Linxing-Shenfu area is on the Jinxi flexural fold belt of the eastern Ordos Basin (Fig. 1a, b) (Jiao et al, 2016;Tao et al, 2017;Gao et al, 2018;Li et al, 2019). The No.…”