Uranium in Granites

Radioactive decay generates thermal energy, the driving force for most of the earth's internal processes. The radioelements, principally potassium, uranium and thorium have been progressively transferred {rom the mantle to the continental crust through time. Granitic rocks provide the largest repository for these elements. A small proportion of granitic (and syenitic) rocks contain above normal radioelement concentrations. Under a favourable combination of structural and hydrological conditions the heat generating capacity of these 'hot' granites is significant with respect to low temperature mineralizing processes. Uraniferous granites constitute a special class and Hercynian examples suggest that certain gross compositional and alteration features are indicative of genetically associated uranium mineralization (e.g. a high U/Th ratio combined with uranium levels 2 to 5 times the clarke; muscovite-biotite; accessory uraninite; strong negative Bouguer gravity anomalies). The South Mountain batholith of SW Nova Scotia meets several of these criteria. In western Canada a linear negative gravity anomaly extending 1600 km {rom Edmonton to Baker Lake, is coincident with a zone of high uranium, mostly in granitoid rocks, which is in the exposed Shield NE of the Athabasca basin, and present in the prairie Precambrian basement to the SW. The gravity anomalies can be traced beneath the Athabasca basin with the inference that there is associated high uranium. This combined gravity-radioelement linear anomaly is termed the Athabasca axis. There is some evidence to suggest it developed through crustal tension. The rich mineralization of the Athabasca basin can be attributed to the favourable conjunction of heat sources, source material, hydrology, and structure, in turn attributable to the Athabasca axis which has deep crustal or mantle connections. General questions are posed as to the distribution of 'hot' granites in time, their origins and tectonic associations, and their possible significance with respect to the stabilization of crotons.