FIBROUS AND NONFIBROUS AMPHIBOLES AT THE IRON HILL CARBONATITE COMPLEX, POWDERHORN, COLORADO

The United States Geological Survey has begun a reconnaissance study of asbestiform minerals in vermiculite deposits throughout the United States. Preliminary findings have revealed a consistent relationship between alkalic complexes, including carbonatites, and fibrous sodic to sodic-calcic amphiboles. The Iron Hill Carbonatite Complex, Powderhorn, Gunnison County, Colorado, was chosen for its location and ideal ring-like intrusive sequence typical of many carbonatites. The occurrence of amphibole minerals throughout the various stages of formation of the carbonatite complex from the early-intruded ultramafic pyroxenites to the later intruded carbonate sequence is being investigated. Chemically, the early-formed amphiboles found in the ultramafic and mafic rocks of the carbonatite complex are calcium and aluminum rich and alkali and ferric iron poor. Physically, these amphiboles are nonfibrous. Later formed amphiboles occurring with the intrusion of the carbonatite body are alkali and ferric iron rich, calcium and aluminum poor, and occur more commonly in the fibrous habit. Physical and chemical factors, including temperature, pressure, and the ratio of water to carbon dioxide activity may be responsible for the differing crystallization habits of the early and late-formed amphiboles. Results of this study will be used to determine the likelihood of fibrous amphiboles in various geologic settings to allow for appropriate exploration, exploitation, and remediation of economic deposits and possible impacts upon nearby public developments.