GEOLOGY AND MINERAL DEPOSITS OF THE IRON HILL (POWDERHORN) CARBONATITE-ALKALINE INTRUSIVE COMPLEX, GUNNISON COUNTY, COLORADO, USA

The Iron Hill carbonatite-alkaline intrusive complex crops out across 31 km² near the small town of Powderhorn, about 35 kilometers south-southwest of Gunnison, Colorado. It has a ring-like structure with alkaline intrusive masses surrounding a central stock of carbonatite. Iron Hill has been described as the best example of a carbonatite-alkaline rock association in the United States because it displays classic alkaline igneous lithologies and petrology and is well exposed. The primary rock types of the complex are, from oldest to youngest: pyroxenite, uncompahgrite, ijolite, nepheline syenite, and the central stock of ankeritic dolomite carbonatite. Our Pb/U isotope study indicates that the pyroxenite formed 572±8 million years ago, intruding Early Proterozoic granite, syenite, and older Proterozoic metamorphic rocks. In addition, we found low strontium isotopic initial ratios (Sr_i generally <0.7038) and positive Ɛ_Ndi of mineral phases within the alkaline intrusions and carbonatite, which is consistent with a mantle source for these magmas.

The Iron Hill complex contains a variety of mineral deposits and has been prospected intermittently from the 1880s to the late 1990s. It has been evaluated for its potential iron, titanium, rare earth element, niobium (columbium), vermiculite, and thorium resources by several companies and government geologists, but to date, none have been deemed economic. In particular, the complex is thought to contain the largest hard-rock resource of titanium in the United States, hosted by magnetite-ilmenite-perovskite-rich zones within the pyroxenites. Economic development of these titanium deposits would depend on efficient separation of perovskite (Ca-Ti-oxide) from the pyroxenite followed by cost-effective methods to liberate the titanium. The central carbonatite stock contains a resource of about 2.6 million metric tons of rare-earth oxides, averaging about 0.4 percent total rare earth oxides. The Iron Hill carbonatite is preferentially enriched in light rare earth elements, which is common for carbonatites. The carbonatite stock also represents one of the largest niobium resources in the United States.