Growth rims in uraninite: Evidence of episodic changes in hydrothermal fluid composition in Rohil uranium deposit, Rajasthan, India

Growth rims in uraninite from a metasomatic type uranium deposit at Rohil in the North Delhi Fold Belt of Aravalli craton have been observed, studied and documented. The ore mineral assemblage includes sulfides, fluorite, U-phosphosilicate, quartz and magnetite along with uraninite. Compositional characterization of uraninite (n = 31) by electron probe micro analyzer (EPMA) shows a range of 77.12–87.84% UO2, 0.20–4.51% ThO2 and 0.02–1.61% Y2O3. We propose a broad classification of uraninites into three types based on microtextural relationship and mineral chemistry. The high-temperature uraninite-I (U-I) and moderate temperature uraninite-II (U-II) are distinguished based on the U/Th ratio. The uraninites with high alteration factor (CaO + SiO + FeO) are classified as uraninite-III. Empirical thermometric studies based on U/Th ratio and REE content indicate that the minimum crystallization temperature of uraninites to be >350°C. The EPMA U-Th-Pb (n = 28) chemical ages revealed mean value of (784±11 Ma) with a mean square of weighted deviation (MSWD) of 1.3. The backscattered electron (BSE) images and elemental X-ray maps (Y Lα, Ce Lα and Th Mα) on a euhedral uraninite crystal (∼800 µm size) demonstrate concentric growth rims developed due to variation in chemical composition. Such compositional zoning is relatively rare in uraninite and it distinctly documents the episodic changes in fluid composition during the mineralization process