RATTLESNAKE HILLS AND BEAR LODGE MOUNTAINS CARBONATITE-ALKALIC IGNEOUS COMPLEXES, WYOMING: REE'S AND OTHER TRACE-ELEMENT GEOCHEMISTRY

The Rattlesnake Hills (RH) alkalic igneous complex (44-45 Ma) consists of a central alkalic group with carbonatite, phonolite, trachyte, trachyandesite, and trachydacite, flanked on the east and west by more silica-rich intrusive rocks including rhyolite. The Black Hills (BH) alkalic belt (including Bear Lodge Mountains (BLM)) (<46-58 Ma) includes these and additional tephrite, basaltic trachyandesite, shoshonite, various lamprophyres, as well as alkali and subalkalic rhyolite. Both complexes contain gold, but only the BLM contains significant REE deposits.

REE, Th, and Nb concentrations in rocks of the Rattlesnake Hills (RH) are lower than those of carbonatites and other rocks in the BLM. In a BLM drill-core sample of carbonatite, Ce is >10,000 ppm and La is 8000 ppm, whereas in two RH drill-core samples of carbonatite, concentrations of Ce are 845 and 952 ppm, and those of La are 696 and 960 ppm. The RH REE patterns are less LREE-enriched: Ce/Yb of phonolites, trachytes, and trachyandesites in the RH is <50, whereas in the BH, this ratio is 100-200. Black Hills thorium concentrations are high—775 ppm Th in BLM carbonatite, 104 ppm Th in phonolite in the central BH, and 83 ppm Th in the Missouri Buttes phonolite (westernmost BH intrusions). In the RH, 20 ppm Th is the highest value in carbonatite from drill core, whereas 49 ppm Th is found in phonolite from a surface sample. Nb content is lower in RH samples (<40 ppm) than in BH samples (mostly <100 ppm, with some phonolites and trachytes between 120-220 ppm).

Barium concentrations are greater in the RH (65-1079 ppm in surface samples of trachyte-phonolite). In central RH drill-core, phonolite contains 23,000 ppm Ba, and carbonatite contains 51,000 ppm Ba. In the BH, Ba concentrations are 1750 ppm in phonolite outcrop, and one BLM carbonatite outcrop contains 30,000 ppm Ba. Higher Ba concentrations in RH rocks appear to be related to assimilation of high-Ba mantle of the Wyoming Archean craton (similar to the Crazy, Highwood, and Bearpaw Mtns., Montana).

Initial ε_Nd and ⁸⁷Sr/⁸⁶Sr isotopic values from the RH are lower than those in most BH samples. Isotope and trace-element contrasts between the RH and BH/BLM reflect differences between the two complexes in (1) age and composition of crust and mantle lithosphere, (2) evolution of magmas, and (3) depth of sampling (i.e. emplacement depth).