Uranium mineralization and granite magmatism in the British Isles

Abstract

Uranium mineralization associated with granites in the Caledonian and Hercynian provinces of the British Isles is shown to be genetically related to the uranium content and distribution, age, and structural setting of these granites. The uranium content of whole rock samples, analysed by the delayed neutron method, is used to demonstrate that mineralization is associated with intrusive complexes with a high mean content of uranium which also exhibit a high concentration of incompatible elements, low K /Rb ratio, low total Sr, low initial 87 Sr/ 86 Sr ratio and high geothermal gradient. The standard deviation for uranium is greater where such intrusives are mineralized but the mean value is relatively unaltered. Therefore mineralization is the result of uranium redistribution, and does not involve further introduction of uranium. Fission track studies indicate that the high ‘background’ uranium content of granites, away from mineralization, is due to the occurrence of uranium in resistate primary phases such as zircon. Uranium is released by the dissolution of these resistate minerals. The processes of greisenization and tourmalinization which have been shown by oxygen isotope studies to involve massive interaction with meteoric water all extract uranium, which is redeposited down the PT gradient as 'primary’ uraniferous ore minerals in vein-type mineralization. It is suggested, therefore, that mineralization involves leaching of granite magma enriched in metals and fluorine by water of meteoric origin containing dissolved carbonate. The breakdown of primary zircon is attributed to a phase of short duration of high temperature interaction of granite magma with meteoric water, and uranium mineralization is thought to have occurred at this time. However, the high concentration of uranium, thorium and potassium of the ‘background' granite which produce hot rock districts may cause redistribution of uranium by hydrothermal mineralization during periods of high average heat flow from the mantle (as in the Tertiary of southwest England) or during dyke emplacement. An extensive system of channels for heating and circulating water is necessary for this system to function, and faults in granite would be particularly favourable. The regional trend of uranium and incompatible elements shown by late Caledonian (Devonian) and Hercynian granites in Britain is related to dehydration reactions during subduction of oceanic crust. The importance of phlogopite breakdown in accounting for the characteristics of uraniferous granites is discussed in relation to magma generation, with the use of closed and open system models with partial fusion of ocean crust or upper mantle. Uranium enrichment by scavenging of subcontinental lithosphere is considered important, but late stage assimilation of uranium from higher levels in the crust is relatively insignificant. The applications of the models for uranium mineralization to exploration at a regional and local scale are discussed.