Rare-earth element geochemistry of fluorite-carbonate deposits in western Montana, U.S.A

The fluorite-carbonate-quartz deposits of Mineral, Missoula and Ravalli Counties in western Montana, U.S.A., at one time described as carbonatitic, have recently been shown to be hydrothermal in origin based on chemical, O- and Sr-isotope, and field studies. We report the results for rare-earth elements (REE) in the fluorite and carbonate minerals and discuss the implications for their origin. Fluorite from Lime Gulch, White Cloud, Swallow, Spar, Wilson Gulch and Snowbird deposits exhibits REE/chondrite patterns with a pronounced enrichment in the middle REE (MREE), highly depleted light REE (LREE) and slightly depleted Lu, often accompanied by a similar small depletion in Yb. La is depleted relative to Sm by an order of magnitude. Calcite is also MREE-enriched, although La is not as depleted relative to Sm as in the associated fluorite; dolomite does not show La depletion relative to Sm. Calcite and dolomite have negative Eu anomalies whereas some of the fluorite does not. REE patterns for the fluorite and carbonates are typical of those from other hydrothermal deposits reported in literature. Chemical zonation caused by complexation of MREE and HREE with carbonate and fluoride ions and a high degree of REE fractionation apparently produced the REE distribution patterns observed for these deposits.