FEN COMPLEX CARBONATITES, TELEMARK, NORWAY: MINERALOGY OF NIOBIUM AND THORIUM BEARING MINERALS

The thorium fuel cycle has a number of advantages over the traditional U-Pu fuel cycle [1]. A principal advantage is that it does not produce Pu; hence, the possibility of proliferation of nuclear weapons is much reduced. In addition, Th-based fuels, mainly ThO₂, are much more stable than UO₂ in most geologic environments [2]. Norway has substantial reserves of thorium, an estimated 170,000 metric tonnes of ThO₂ [3], and there is active interest in developing these Th-reserves to support the Th-fuel cycle.

We present mineral compositions of the Fen carbonatite determined by election microprobe analysis (EMPA). The Fen Complex, at the site of an extinct volcano, in Telemark, Norway, is ~ 583 ma old [4]. The alkali-rich Fen central complex magma intruded Mesoproterozoic gneisses and formed pegmatite dikes that cover an area of roughly 1500 square kilometers and contain a high proportion of iron, rare-earth, thorium, and uranium-bearing minerals [4]. Calcite, dolomite, and apatite dominate the carbonatite, but do not contain REEs, thorium or uranium. Trace amounts of barite are associated with the dolomite. Minerals of interest, columbite crystals (~1-5 mm) host thorite inclusions (~10-50 micrometers) in a pattern in which the thorite grains are most abundant in the vicinity of calcite grains. Based on EMPA results, the calculated formulas for columbite and thorite are: (Fe_0.99Mn_0.01)(Nb_1.90, Ti_0.09,Ta_0.01)O₆ and (Th_0.8,Zr_0.2)SiO₄, respectively. Systematic core-to-edge analyses of a columbite crystal reveal a decrease in MnO and Ta₂O₅ (~1.5 oxide wt. %) and an increase in FeO and Nb₂O₅ (~1 oxide wt. %). Based on the Th-content and age, the thorite radiation dose was calculated as 6.089 x 10²⁰ alpha decay-events per gram, exceeding the alpha-decay dose required for complete amorphization [5]. The amorphous structure of the thorite was confirmed using transmission electron microscopy (TEM).

[1] Thorium Report Committee (2008). [2] Demkowicz et al. (2003) Nuclear Technology 147 157-70 [3] USGS (2009) [4] Meert et al. (1998) The Journal of Geology 106 553-564 [5] Ewing et al. (2003) Reviews in Mineralogy & Geochemistry 53, 387-425.