Caledonian Geology and Uranium-bearing Strata in the Tåsjö Lake Area, Sweden

Abstract

Uranium indications in the Tåsjö Lake area, Västernorrland County, discovered in 1957 by the Swedish Atomic Energy Company, have been the subject of comprehensive geological studies by this company from 1960 through 1965.

Attention has been directed mainly to the basal Ordovician units within the Cambro-Ordovician of the area. Electromagnetic, magnetic and radiometric aerial prospecting has been carried out over the entire concession area, with a line spacing of 125 m at an altitude of 30 m.

By means of field studies, including 68 boreholes comprising in all 2131 m of core, geological, tectonic, radiometric and geochemical ground studies and investigations of the aerial anomalies, it has been possible to map the extension of the uranium-bearing unit within an area 20–30 km long and 10 km wide. Palaeontological studies in the area have shown that the uranium-bearing unit is Early Ordovician in age.

The structural geology of the area is characterised by short overthrusts of the Schuppen type, although gentle folding is also found. Electromagnetic anomalies have been of critical importance in interpreting large-scale tectonic features. The study shows that the “Varegian” quartzite interpreted previously as an Uppermost Precambrian autochthon should be classified as Lower Cambrian. The investigations also indicate that the Vemdal quartzite, which had previously been thought to form nappes, is part of the autochton. These quartzites are best classified as para-autochthonous. Drilling through the “graywacke nappe of the Föllinge-Holmsjö facies” shows clearly that this graywacke is autochthonous.

A generalized geological map of the concession area has been produced.

The larger structural features of the sedimentary rocks in the border zone to the Caledonides are discussed from the viewpoint of the miogeosyncline's character.

Several attempts have been made to enrich and leach the uranium and phosphorous of the Lycophoria shale. However, no method, which would be suitable both technically and economically has been found.