Abundance and distribution of thorium in the carbonatite stock at Iron Hill, Powderhorn District, Gunnison County, Colorado

The carbonatite stock at Iron Hill is part of an alkalic complex of Cambrian or latest Precambrian age containing pyroxenite, uncompahgrite, ijolite, nepheline syenite, and related rocks characterized by their subsilicic composition and by their lack of calcic plagioclase. Thorium averages 36.2 parts per million (ppm) Th (0.0041 percent Th0₂) in the carbonatite, which also contains considerably more Ba, Ce, Nd, La, Nb, P, Mn, Mo, Sr, U, and rare-earth elements than average igneous rocks. Thorium content of the carbonatite ranges from 6 ppm Th (0.0007 percent Th0₂) 150 ppm Th (0.017 percent Th0₂), and several areas of anomalous concentration are apparent. The distribution of thorium is not the same as the distribution of niobium and rare-earth elements. Thorium abundances of the level found in the carbonatite at Iron Hill are of little economic interest unless considered as a byproduct of potential niobium, rare-earth, and uranium exploitation.