Radiometric surveying for Th and U mineralization in southwestern, Sri Lanka: radiological, mineralogical and geochemical characteristics of the radioactive anomalies

Abstract

Radioactive emanations (gamma-ray) from minerals due to the natural decay of radioactive elements (U, Th, K, etc.), is used as a tool to identify radioactive mineral occurrences. Sri Lanka is famed for occurrences of thorium-bearing minerals. A car-borne gamma-ray spectrometric survey has been initiated by Geological Survey and Mines Bureau (GSMB) to trace promising Th, U mineralization of the country. Herein we report and interpret the radiological, mineralogical, geochemical characteristics, and the possible origin of the radioactive anomalies identified during the survey in southwestern part of Sri Lanka covered by the Aluthgama-Galle geological sheet area. The typical gamma dose rate (GDR) recorded for the study area is 50-80 nSv/h and the GDR values vary with the local geology. The identified anomalies were categorized into three groups based on the GDR and associated geology as; Group I: 100-300 nSv/h, Group II: 300-600 nSv/h, and Group III: > 600 nSv/h (Inland and beach/raised beach locations). The highest GDR value of 2300 nSv/h (Th-432 nSv/h, U-141 nSv/h, and K-49 nSv/h) is recorded at Uragasmanhandiya. Inland radioactive anomalies of Group III are associated with pegmatite occurrences (e.g. Uragasmanhandiya, Pituwala, Makubura, etc.) with characteristic mineralogy of quartz + feldspar + monazite + zircon ± biotite ± garnet ± ilmenite while higher monazite and zircon contents have been reported for beach/raised beach anomalies. The average Th/U ratio for the Group III anomalies is 32.99, which is higher than the average ratio for the upper crust, indicating the high abundance of Th compared to U in the terrain. Higher enrichment factors compared to the average upper crust have been reported for high field strength elements especially for Th, La, Ce and Zr (e.g. 28-1400 for Th, 5-407 for La, 5-445 for Ce and 1-291 for Zr). The combined analysis of radiology, mineralogy, and geochemistry shows that the radioactive anomalies mainly depend on the monazite content at each locality, and monazite served as the main source for the high concentration of Th, La and Ce. The frequent distribution of radioactive anomalies and occurrences of pegmatites along the shear zones, faults and major lineaments of the terrain suggests a structurally influenced radioactive mineralization. The results of the analysis show that the main source for the monazite placer deposits in the southwestern coastal region is the monazite-rich granitic pegmatites and associated gneisses of the region. A widespread study is suggested to evaluate the U, Th, La and Ce mineralization in the country in terms of the extent and economic potential.