Replacement and primary magmatic carbonatites from the Wet Mountains area, Fremont and Custer counties, Colorado

Abstract

Carbonatites associated with alkalic rocks of the McClure Mountain and Gem Park Complexes, and the complex at Democrat Creek, can be classified into two distinct groups, replacement and primary magmatic, based on distinctive petrographic differences. These two groups also show distinctive differences in mineralogy and geochemistry. Replacement carbonatites show textures indicating nearly complete pseudomorphous replacement of originally porphyritic or hypidiomorphic-granular igneous dike rocks by carbonate minerals. They contain an element suite characteristic of carbonatites as well as a trace element signature indicative of a mafic silicate precursor. Minerals containing thorium, niobium, and rare earth elements as essential constituents are generally rare. Primary magmatic carbonatites, which do not show the distinctive replacement textures, are also enriched in elements characteristic of carbonatites but contain greater average amounts of these elements than replacement carbonatites. Thorite, bastnaesite, synchisite, ancylite, and monazite are chiefly responsible for the high thorium and rare earth element content of these rocks compared to other igneous rocks. Primary magmatic carbonatites are spacially more closely associated with the alkalic intrusions than are replacement carbonatites, which have a somewhat wider distribution. Field data and delta 18 O and delta 13 C values support an igneous origin for primary magmatic carbonatites; isotopic values also indicate that the carbonatite replacing the earlier alkalic dikes also was derived from igneous sources. Exchange of isotopes between both types of carbonatite and nonigneous reservoirs appears to be minimal.Abundances of rare earth elements and niobium in carbonatites of the Wet Mountains are somewhat lower than those at other carbonatite localities containing economic or near-economic concentrations. Thorium is more abundant in closely related vein-type deposits than in the carbonatites.