Abstract

The Qianjiadian (QJD) uranium ore field, located in the southwest of the Songliao Basin of east China, was discovered within the red-variegated depositional layer of the Upper Cretaceous Yaojia Formation (Fm). It preformed some particular and complex characteristics in respect of the occurrence location, ore-hosting strata, alteration, and morphology of the ore body, compared with the sedimentary uranium deposit generated in the traditional dark coal-bearing clastic strata. In this study, the categories of organic matter (OM) within the Lower Member of Yaojia Fm (LYF), and their effects on the rock alteration and uranium mineralization process, were researched macro-microscopically. The aim of the study was to confirm the co-associated relationships between the gray layer, rock-faded zone, and tabular-shaped ore body, in addition to their genesis. Results showed that the OM mainly consists of synsedimentary carbonaceous detritus (kind-I), amorphous movable humic substances (kind-II), and deep-originated hydrocarbon fluid (kind-III), which widely led to rock-bleaching, green alteration, and different forms of pyritization and carbonation throughout the geological history. OMs of kind-I and kind-II were inherited from terrigenous higher plants, and did not directly adsorb and bio-reduce hexavalent uranyl ions during the metabolic processes of sulfate reducing bacteria (SRB), but indirectly promoted uranium precipitation related to a H₂S-rich secondary reducing barrier, produced by bacterial sulfate reduction (BSR) and the pyrolysis of the diabase dikes. III-OM was derived from deep reservoirs and exhibited multiple positive effects of increasing the reduction capacity of the primary weak reducing strata, and protecting and enriching the earlier ores. Uranium in the ores is mainly composed of pitchblende, followed by a small amount of coffinites and uranium-bearing titanium minerals. Combined with the geological setting of the QJD ore field, a six-stage ore-forming model for the LYF was constructed from the perspective of the reducing medium, i.e., (1) development of ore-bearing sand-bodies in the Yaojia Fm period; (2) formation of a tabular grey layer before the Nenjing Fm period; (3) initial bleaching and uranium pre-concentration during the diagenetic period; (4) initial mineralization and hydrocarbon reduction from the end of the Nenjing period to the end of the Mingshui period; (5) main mineralization and thermal transformation in the Paleogene period; and (6) superimposed mineralization in the Neogene period. The proposed new model does not enrich the uranium metallogenic mechanism for the red-variegated bed, but plays a key role in guiding the uranium prospecting for similar red beds, both in the Songliao Basin and in other sedimentary basins globally.