Evidence of early oxidation related to sandstone-type uranium mineralization within the Zhiluo Formation (J₂z), Ordos Basin, China

Abstract

Based upon research on alteration in the Dongsheng and Huangling uranium mineralised areas, Ordos Basin, three stages have been distinguished in the lower member of the Zhiluo Formation: an early-stage red oxidation of hematization and goethitization; a middle-stage yellowish green and greyish blue hydrocarbon related reduction of smectitization, chloritization, pyritization; and a latestage yellow oxidation of geothitization and limonitization. The fact that the mean TOC value of red sandstone and yellowish green and greyish blue sandstone are evidently lower than that of the original grey sandstone may indicates that the yellowish green and greyish blue sandstone have suffered oxidation before hydrocarbon reduction. In addition, residual red alteration sand bodies are observed as inclusions in the yellowish green and greyish blue sandstone in Dongsheng area, while in Huangling area the oil-immersed sands occur a long fractures and unconformity interfaces in red widespread altered sandstone. The brown-black oil-immersed sands are surrounded by yellowish green sandstone, which have locally been bleached due to later reduction. Evidence favouring the existence of early-stage oxidation also include that the yellowish green and greyish blue sandstone with abundant oil stains on the outcrop, and rock slices contain much more acidolysis hydrocarbon content than that of red and yellow sandstone; and that the early formed limonite is replaced by secondary pyrite in yellowish green and greyish blue sandstone. The major Umetallogenic epoch has been dated at 120±11 Ma and 80±5 Ma. During this time, the surface oxidation water supply remained abundant and flowed from the north southwards as the Ordos was not uplifted and the Hetao basin was not down-faulted until Eocene. Exploration has verified that the uranium ore bodies are situated at the front of yellowish green and greyish blue alteration zone in Dongsheng area. The yellowish green and greyish blue sandstone were sourced from the early-stage red sandstone by hydrocarbon reduction, so the front of yellowish green and greyish blue alteration zone is considered to be an indicator of the early-stage red alteration front zone. This might be helpful to outline promising areas for exploration in the Ordos, Basin.