Charoite-(Y) and a uranium hot spot in the City of Aberdeen: oxidative mineralization of an orogenic unconformity

The manganite (manganese ore) deposit at Grandhome, City of Aberdeen, has an anomalously high uranium content and negligible thorium content. The very low Th/U ratio is characteristic of a supergene environment, in this case during Devonian exposure of basement rocks at the Caledonian unconformity. The uranium exceeds 100 ppm in mineralized whole rock, and up to 360 ppm in manganite ore. In manganite uranium resides in the yttrium arsenate mineral chernovite-(Y) as a substituent along with rare earth elements. Arsenic levels are an order of magnitude above background. The occurrence exemplifies low-temperature mineralization on orogenic exposure surfaces.