Uranium in the old red sandstone of Orkney (with notes on a radiometric reconnaissance of the O.R.S. of Shetland)

A reconnaissance survey for uranium in Orkney located several hundred low-grade radioactive occurrences within the Devonian succession and a few of higher grade. Primary methods of reconnaissance were the analysis of stream sediments and waters, together with car-borne and on-foot scintillometer traversing. The limitations of these techniques are discussed and it is concluded that the radiometric methods were the more effective. Uranium concentrations were discovered associated with black shale and phosphatic horizons within the fluvio-lacustrine cyclic sequences. These horizons are normally thin and of low average grade. Higher uranium concentrations are associated with basal conglomerates, hydrocarbon-rich sandstones and a breccia zone, all close to the periphery of Precambrian inliers in the Stromness area. Investigation of this area has led to the recognition of an extensive sequence of sediments beneath the local Middle Devonian unconformity. Smaller uranium concentrations are associated with ash bands, mineral veins and a vent agglomerate. Other metals, especially lead, zinc and barium, are associated with some of the mineralisation, particularly in the Stromness area. Uranium occurrences in this area were considered to be the most promising and were investigated in more detail. Assessment of the prospects by radiometric gridding, radon in soil-air measurements and drilling has not so far established the presence of deposits exploitable under present economic conditions, but further investigations are merited. (author)