THORIUM AS AN ENERGY SOURCE - Opportunities for Norway ...

Thorium as an Energy Source - Opportunities for Norway
3. THORIUM RESOURCES IN NORWAY
The element thorium (Th) was first identified in 1828 by the Swedish chemist Jöns Jakob
Berzelius (1779 - 1848) when he examined rock samples from Norway. He named the element
thorium and the mineral thorite after Thor, the Norse god of thunder. In 1890, Professor
Waldemar Christopher Brøgger (1851 - 1940) described thorium rich minerals, also containing
niobium (Nb), yttrium (Y), zirconium (Zr) and rare earth elements (REE), from the Permian Oslo
Province (the so called Oslofeltet) [10]. In 1895, thorium mining was performed at Kragerø on the
South East coast of Norway, for production of thorium oxide for gas burners. About 2000 kg of
thorite was produced. The production declined after one year, due to the low market prices of
thorium associated with large monazite (a mineral containing thorium) resources identified in
Brazil.
In Norway, thorium has never been specifically explored for. The knowledge of geological
environments with elevated levels of thorium is mainly based on results from uranium (U)
exploration carried out by the Geological Survey of Norway (NGU) during two periods. The first
period was after the Second World War and up to the mid 1960s, while the second period was
from 1975 to 1985. Based on analyses of uranium in a large number of mineral samples, the
correlation between uranium and thorium in minerals and on helicopter and ground gamma
surveys, the thorium levels were estimated. Recent helicopter gamma surveys covering the
Permian Oslo Province as well as the Fen Complex in Telemark County confirmed in principle the
old data, although the spatial resolution was improved. Additional information on thorium in
Norwegian bedrocks is based on previous exploitation of rare earth elements (REEs) that are
commonly positively correlated with thorium (Megon, 1973 [11]). In the MEGON Report it was
shown that most of the minerals with elevated levels of thorium are situated within 3 main
regions: the Fen Complex in Telemark County, the Permian Oslo Province, and on the Southeast
coast of Norway, the Kragerø and Langesund areas. Investigations performed by the MEGON
group confirmed previous data produced by Norsk Bergverk A/S, investigating the Fen Complex
in 1955 - 1956 [11]. A series of thorium bearing minerals have been identified at the different
sites in Norway, while the Fen Complex was considered as the most promising resource, with
thorium amounting to about 0.1 – 0.4 wt%.
3.1 Resources - Geological Environments with Elevated Thorium Levels
The average concentration of thorium in the Earth's crust is about 7.2 ppm (parts per million),
which is about 2 to 4 times more abundant than uranium, reflecting the difference in half-lives of
thorium-232 (1.4·10 10 years) and uranium-238 (4.5 10 9 years). Thorium (Th) and uranium (U) are
enriched in magmatic processes, and the highest concentrations are found in silica (SiO2)-rich
rocks such as granites. The concentration of thorium and uranium in rocks such as gabbro and
granite can vary by a factor of more than ten. Due to small variations in their chemical
behaviours, thorium and uranium are enriched along different paths during weathering and
near-surface geological processes: uranium is mobile under oxidizing conditions and is deposited
under reducing conditions, while thorium is less mobile and is often concentrated in heavy
mineral sands [12]. The most important geological environments in which thorium are enriched
include:
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