The Occurrence of Asphaltite and Thucholite in the Precambrian Bedrock of Sweden

Abstract

Carbonaceous matter has frequently been formed in most parts of the Precambrian bedrock of Sweden. Preferentially it is deposited in fracture zones or breccias. Occasionally the organic matter is associated with uranium or thorium and is then called thucholite. In chemical and physical respects the thucholite is different from the rest of the organic material which is called asphaltite. For both types infrared studies, carbon and hydrogen analyses, carbon-13 and carbon-14 measurements have been performed, also two age measurements by the U/Pb method. The result of the work shows that the carbonaceous matter has been transported by groundwater streaming in fractures in the Precambrian bedrock and deposited down to a depth of several hundred metres. This organic material had a composition which varied within certain limits but is essentially represented by asphaltite. The carbonaceous matter is of plant or animal origin and consists of many different organic compounds. The deposition took place during several stages of the geologic evolution in Phanerozoic time. It is also shown that the organic compounds which constitute the thucholite probably have been subordinate constituents of the organic material. This carbonaceous matter was transported by the ground water to the location of the radioactive minerals and by some unknown chemical reaction U, Th, and Pb were dissolved and a fraction of the dissolved uranium again deposited together with the reactive organic compounds of the carbonaceous matter. Evidently also the compounds with an affinity to uranium are able to extract this element from e.g. radioactive groundwater.