Mineralogy and geochemistry of carbonatites from the Gem Park complex, Fremont and Custer counties, Colorado

Abstract

The Gem Park carbonatite complex is located 11 miles northwest of Westcliffe, Colorado and is hosted within Precambrian X metasedimentary gneisses and amphibolites. The complex of Cambrian age gabbros and pyroxenites contains a series of carbonatite dikes and masses. There are two other major carbonatite complexes in the Wet Mountain Valley. Parker and Sharp (1970) and Armbrustmacher and Brownfield (1978) have surveyed the surface mineralization of the mafic rocks and carbonatites. The present study has dealt with the evaluation of three diamond drill holes in the south central portion of the complex. A total of 29 samples were analyzed for mineralogic content, and geochemical analyses were performed. Results have been used to determine a rough paragenetic sequence for the development of the carbonatites. Dolomitic carbonatites, with a wide range of iron contents, form the earliest stage of carbonatite development. Apatite, magnetite, and phlogopite are common constituents with associated pyrochlore and zircon. Later dikes contain strontianite, barite, celestite, dolomite, and calcite with associated monazite, ancylite, bastnasite, and low albite. Lamprophyllite has been identified as an alteration of phlogopite. The total rare earth content in the samples studied ranges from 121 ppm to 757,000 ppm with an average of 13,700 ppm. Niobium ranges from 10 ppm to 7,000 ppm with an average of 740 ppm. Average thorium and uranium contents are 104 ppm and 18 ppm, respectively, with ranges from 2.0 ppm to 590 ppm and 0.2 ppm to 131 ppm. Geochemical analyses indicate that the intercepts studied are enriched in the elements Cu, Sr, Ga, P, Co, Sc, and rare earth elements (REE) and depleted in Mo, Ti, Si, Al, Nb, Zr, and K as compared to the average values for carbonatites (Gold, 1963). These results do not necessarily represent the entire complex; the cores studied were chosen for their varied mineralogy and carbonatite occurrences.