The giant Vergenoeg fluorite deposit in a magnetite-fluorite-fayalite REE pipe: a hydrothermally-altered carbonatite-related pegmatoid?

The world-class Paleoproterozoic Vergenoeg fluorite deposit in South Africa is hosted in a breccia pipe comprising units with varying proportions of pegmatoidal fayalite, magnetite, fluorite and siderite. The adjacent A-type Bushveld granites also have associated fluorite deposits containing fluorite with similar REE patterns, fluid inclusion and Sr isotope compositions to those at Vergenoeg, leading to the proposal that there is a genetic relationship. This is despite the silica-undersaturated nature (SiO2<30%) of the Pipe and its extreme enrichment in Ca, F, Fe, Nb, P and REE compared to granites. Both liquid immiscibility from a granitic melt and granitic magmato-hydrothermal activity have been proposed as genetic mechanisms to explain this exotic composition. However, the Vergenoeg Pipe shows greater similarities to alkaline rocks, in particular the Phalaborwa carbonatite of similar age, including: i) size and shape, ii) associated maars, iii) mineralogical zoning, iv) geochemical, radiogenic and stable isotope composition, and v) presence of both high-T and low-T fluid inclusions. This suggests an alternative genetic relationship with alkaline magmas, in which some geochemical and radiogenic isotopic similarities to Bushveld granites are the consequence of broadly contemporaneous development in the same tectonic setting within the same lithosphere, whereas others may be due to hydrothermal overprinting. Similarities with Phalaborwa and also with Bayan Obo, Mongolia, indicate that the Vergenoeg pegmatoid pipe could be an extreme carbonatite-associated member of the Fe-oxide Cu-Au (&PLUSMN;REE &PLUSMN;P) group of deposits.