PRELIMINARY INVESTIGATION OF SULFIDE TEXTURES IN CARBONATITE FROM THE BEAR LODGE ALKALINE COMPLEX, WYOMING

The Bear Lodge alkaline complex (BLAC) in northeast Wyoming has received considerable attention for its potential as an economic source of rare earth elements (REEs) hosted in carbonatite. Prior to interest in REEs, the BLAC was considered a potential source of Au, Ag, Cu, Mo, Th, U, Mn, and fluorspar. Gold mineralization and alteration styles (potassic, ferric-iron metasomatism or fenitization) have been compared to other epithermal Au-Te or Au-rich porphyry systems related to alkaline magmatism (e.g., Cripple Creek, Colorado; Porgera, Papua New Guinea; Emperor, Fiji). Recent exploration reveals Au enrichment in some shallow, oxidized portions of the carbonatite stockwork. This demonstrates the importance of supergene processes to the concentration of Au, and suggests sulfide-rich carbonatite may be a suitable proto-ore for Au in addition to REEs. Gold concentrations as high as 10 ppm occur in Bear Lodge carbonatite, and the more typical Au concentrations of ~50-100 ppb are some of the highest reported for carbonatites worldwide. Macroscopic observations of drill core and examination by reflected light microscopy and SEM-BSE imaging suggest multiple stages of sulfide mineralization prior to oxidation. Sulfides in carbonatite are typically attributed to late-stage hydrothermal processes. However, in Bear Lodge carbonatites, early magmatic-stage sulfides are distinguished by, 1) an association with early REE-bearing phases (burbankite and carbocernaite), 2) massive pyrrhotite+pyrite+chalcopyrite segregations associated with spinifex-like platy calcite textures, 3) pyrrhotite and euhedral pyrite in close association with magmatic K-feldspar, biotite, and ilmenite phenocrysts and 4) the presence of pyrrhotite in carbonatite with primary igneous or mantle derived C and O isotope values unaffected by hydrothermal processes. Sphalerite contains drop-like, crystallographically-controlled segregations of galena similar to “chalcopyrite disease.” Preliminary electron microprobe analyses of galena reveal 0.4-0.7 wt. % Ag, 2.60-2.78 wt. % Bi, and a positive correlation between Ag and Te. Gold concentrations in sphalerite, galena, and pyrite are below detection (20 ppm) by microprobe, but native gold is found in cubic hematite pseudomorphs suggesting it is initially substitutional in pyrite.