On thorite in Nubian granodiorite (Southwestern Egypt)

Abstract

Thorite, as a principally thorium (Th)-bearing mineral, is an important indicator for Th mineralization. However, its occurrence and enrichment processes are still discussed and debated. Here, a unique occurrence of thorite, discovered in Nubian granodiorite rather than in highly evolved granites from southwestern Egypt, is reported. This report presents data derived from optical and backscattered electron (BSE) microscopy and energy-dispersive X- ray spectrometry (EDS) analyses conducted on the thorite and its host rock. The Nubian granodiorite thorites are viewed as secondary, not primary products. Two distinct types of secondary thorites are identified that are referred to as type A thorite and type B thorite herein. Type A thorite occurs as small grains that are enclaved in a fine-grained matrix of altered oligoclase and ferrohornblende, and clinochlore. Thorite grains, up to 100 μm in size are characterized by corona-type structures comprising of clinochlore and hematite with some barite. Their sources are most likely hydrothermal solutions occurring during an alteration stage and having relatively high conditions of sulfate activity. Type B thorite, on the other hand, forms crystallites in altered domains of magmatic allanite-(Ce), ranging in size from ~ 0.1 to ~ 10 μm. Formation of Type B thorite is a direct result of fluid-driven alteration processes, since it requires the in situ-redistribution of elements, particularly thorium, silicon, and uranium. Thorite types A and B are composed mainly of thorium uranium silicate, with variable minor amounts of Y, Al, Ce, Nd, Fe, Ca, Na, Mg, P, and Cl. Thorite compositions are within the range reported for uranothorites from other occurrences.