Distribution of radioactive elements (Th, U) and formation mechanism of the bottom of the Lopingian (Late Permian) coal-bearing series in western Guizhou, SW China

Abstract

A positive anomaly natural gamma-ray deposition has been identified at the bottom of the Lopingian (Late Permian) coal-bearing series (Longtan Formation) in Panxian, western Guizhou, China. It is divided into two subunits, the lower Ka1 and the upper Ka2 layers. Mineralogical and geochemical analyses and petrological investigations were conducted on the drilling core samples of such sedimentary series in order to understand the distribution characteristics of radioactive elements (Th and U) and determine the formation mechanism. The results showed that the samples were predominantly composed of clay minerals (illite, berthierine, and kaolinite; contributing on average 80.53 wt%) with minor and rare contents of anatase and calcite, marcasite, rutile, pyrite, diaspore, and boehmite, respectively. The Al2O3/TiO2 and SiO2/Al2O3 ratios in the Ka2 layer were lower than those in the Ka1 layer, and exhibited good negative and weak positive correlations with the natural gamma-ray magnitude in the Ka2 and Ka1 layers, respectively. The high matching degree of radioactive elements Th and U with the natural gamma-ray magnitude indicates that the formation of highly radioactive features is closely related to the enrichment of radioactive elements Th and U. In particular, Th is mainly concentrated in clay minerals represented by berthierine, whereas U is primarily hosted by organic matter. The provenance of the anomalous natural gamma-ray deposition mainly includes weathering basalts and syndepositional falling acidic volcanic ash, wherein the abnormally high radioactive elements contents (Th, U) are attributed to acidic volcanic ash deposition during the Late Permian Emeishan mantle plume activity. Further, the abnormal vertical redistribution of some trace elements (REE-Y-Nb) may be caused by leaching.