Mineralogy and Petrology of the Iron Hill Carbonatite Complex, Colorado

Abstract

Bulk rock and mineral chemistry of the rock types occurring in the Iron Hill carbonatite complex have been examined in order to determine the genetic link between carbonatites and the alkaline igneous rocks with which they are ubiquitously associated. A variety of silicate rock types, including pyroxenite, uncom-pahgrite, ijolite, and nepheline syenite is developed at Iron Hill. The carbonatites are composed dominantly of calcite, ankeritic dolomite, or siderite. Only a rare variety of calcite carbonatite contains significant amounts of silicate minerals. Silicate minerals in the calcite carbonatite are generally intermediate in composition to those occurring in ijolite and nepheline syenite. These data, together with the scarcity of rock types gradational between silicate and carbonate varieties, suggest the possible separation of a discrete carbonate fluid phase during the latter part of the crystallization of the silicate series postdating formation of the ijolite but prior to the crystallization of the nepheline syenites. Calcite-dolomite and pyrite-pyrrhotite pairs indicate temperatures of crystallization of late-stage carbonatites of the order of 435° C to 290° C at sulfur fugacities of 10 −7.8 b and oxygen fugacities of the order of 10 −25 to 10 −26 b.