The complex of alkaline rocks at Iron Hill, Powederhorn district, Gunnison County, Colorado
— Theodore J. Armbrustmacher

The alkaline rocks, and especially the carbonatite stock at Iron Hill, near Powderhorn, Colorado, have been studied since 1912 (Singewald, 1912), not only because of their unusual lithologies but also because of the economically significant mineral concentrations associated with them. Various hypotheses regarding the origin of the Iron Hill rocks have paralleled the development of ideas concerning carbonatite petrogenesis from assimilation of marble followed by crystal differentiation (Larsen, 1942), to meta-somatism of preexisting pyroxenite (Temple and Grogan, 1965) to formation through immiscibility of magmatic liquids (Nash, 1972). Economic minerals associated spatially and genetically with these rocks include vermiculite and minerals that contain thorium, iron, titanium, niobium, and rare-earth elements. Thorium deposits have been discussed by Armbrustmacher (1980), Staatz and others (1979, 1980), Hedlund and Olsen (1961), and Wallace and Olson (1956). Iron and titanium deposits have been discussed by Rose and Shannon (1960) and Singewald (1912). Niobium resources have been mentioned by Armbrustmacher (1980) and Hedlund and Olson (1961). Rare-earth minerals have been discussed by Olson and Wallace (1956) and Hedlund and Olson (1961).