Abstract

The pear-shaped outcrop of the alkalic rock complex at Powderhorn, Gunnison County, covers approximately 12 square miles. About 70 percent is underlain by pyroxenite which is intruded and/or replaced by concentric suites of nepheline and melilite-bearing rocks, magnetite-perovskite concentrations, biotite-rich zones, and by carbonatite. The central carbonatite occupies an area of about two and one-half square miles; radial and concentric carbonatite dikes are spatially related to it. The granitic country rock is partially fenitized in proximity to the complex. The complex is located within a region characterized by the presence of thorium-bearing veins. The central carbonatite constitutes a major reserve of columbium contained in the mineral pyrochlore. Columbium is also contained in the magnetite-perovskite concentrations in the pyroxenite, and the mineral columbite has been identified in thorium veins close to the complex. In addition, a substantial reserve of titanium is contained in the pyroxenite, principally in the minerals perovskite and sphene. Significant quantities of rare earth elements occur mostly in the apatite of the carbonatite and in the magnetite-perovskite rocks. Drilling indicates that the pyroxenite is conical in shape and that the carbonatite was asymmetrically emplaced along the inclined pyroxenite-country rock contact. Faulting has exposed two structural levels within the complex, the upper characterized by magnetite-perovskite concentrations, and the lower by melilite-bearing rocks and carbonatite.

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