A uranium-bearing layer from the lower ordovician, Sweden

Abstract A uranium-bearing sedimentary phosphorite rock from the Lower Ordovician was discovered in 1957 by the Swedish Atomic Energy Company in Västernorrland County, about 200 km N of the town of Östersund. The probable origin of the uranium-bearing zone is discussed with reference to previously published descriptions of the regional geology. Subzones 5 a, 5 b and 5 c of the Olenid Series (Upper Cambrian) are exposed at Sjougdälven River. Twenty km to the SE only subzone 5 a is present. The uppermost two subzones, especially 5 c which contains Peltura scarabeoides, are richest in uranium. From a maximum of 200 ppm, the uranium content decreases rapidly downward in the series. The partial destruction of subzones 5 c and 5 b was a result of a regression. This was followed by a transgression during the early Ordovician. Most of the uranium in the radioactive zones is presumed to have originated in subzones 5 c and 5 b. A maximum concentration of about 1 000 ppm uranium was produced in the radioactive Ordovician rock through uranium substitution in phosphorite, a carbonate-fluorapatite. The sedimentary rocks richest in uranium should be sought in the most southeasterly parts of the Lower Ordovician, where the coastal areas of the Upper Cambrian were exposed to erosion and where the overlying Lower Ordovician units are phosphorite bearing.