Distribution and enrichment of radioactive elements (Th, U) in Nb-Zr-REY-Ga polymetallic clay horizons of the Late Permian (Lopingian) coal-bearing strata, eastern Yunnan, SW China: natural gamma-ray anomalies response to critical metals

• High concentrations of Th and U, are the primary causes of natural gamma-ray logging positive anomalies in Nb-Zr-REY-Ga polymetallic clay horizons. • The radioactive elements (Th, U) and critical metals (Nb, Ta, Zr, Hf, REEs, Y) are mainly concentrated in accessory minerals, including zircon, parisite, monazite, and anatase. • The host minerals and distribution of Th and U exhibits a significant spatial overlap with critical metals (REEs, Y, Zr, Nb, Hf, Ta) Natural gamma-ray logging is a key geophysical method used to measure the intensity of natural radioactivity in coal-bearing strata. The Lopingian Nb-Zr-REY-Ga polymetallic clay horizons in eastern Yunnan exhibit significant positive anomalies in gamma-ray logs, with anomaly intensity positively correlated with critical metal concentrations. However, the underlying cause of these geophysical responses has remained unclear. Therefore, this study investigated the distribution and enrichment of radioactive elements in polymetallic clay horizons. The results indicate that Th and U, rather than K 2 O, are the primary contributors to the gamma-ray anomalies. The radioactive elements (Th, U) and critical metals (Nb, Ta, Zr, Hf, REEs, Y) are primarily concentrated in accessory minerals, including zircon, parisite, monazite, and anatase, with minor amounts occurring in clay minerals. Moreover, Th and U exhibit highly consistent vertical variation trends and show significant positive correlations with critical metal elements. Importantly, the strong spatial overlap in both host minerals and distribution indicates that Th and U were more likely incorporated into these minerals through the substitution of critical metal elements. The symbiotic relationship and common host phases provide a theoretical basis for utilizing natural gamma-ray logging to explore critical metal resources within the Lopingian coal-bearing strata of southwestern China.