The origin of uranium and other metal concentrations in the Helmsdale granite and the Devonian sediments of the north-east of Scotland
An examination of the radiometrically anomalous Caledonian Helitssdale granite and certain radiometric localities in the Devonian sediments of Caithness has been undertaken in order to discover the processes of U concentration in the different rock types. The study is based mainly on geochemical examination, with analyses for U, Th, Cu, Zn, Pb, Mo, Zr, Rb, Sr, Bi, Se, P205, and K20 being carried out. The granite was found to have a high mean U content (9ppm), approximately twice that expected from comparison with the world granite average (4ppm), whilst zones of intense alteration gave U enrichments of up to 71ppm. These latter have a mineralogy and geochemistry distinct from the normal granite. A limited Cu-Mo stockwork mineralisation was discovered almost coincident with a highly radioactive hydrocarbon vein. It is suggested that the alteration,and addition of U, in the granite'is due to a hydrothermal event that operated-shortly after the emplacement of the intrusion. Other U concentrations are shown to be due to the redistribution of the metal during weathering. The main U concentrations in the Caithness sediments occur in thin (1-2cm) phosphatic horizons. In these U ranges from 100 to 1000ppm and is paralleled by the phosphate in an approximately linear relationship of 100ppmU/1% P205. Individual horizons appear to have characteristic U/P205 ratios, in the range of 30 to 200, which can be used to correlate across-fault stratigraphy. Black shales contain low level (25-30ppm) U concentrations with anomalously high Zn and/or Mo. The U-phosphate horizons were probably formed in a shallow playa lake in which special pH conditions allowed the deposition of phosphate. Uranium (and other metals) was rapidly absorbedinto the phosphate structure. There is no evidence for any great mobilisation of U in diagenetic or later processes.