AGE AND ORIGIN OF FLUORITE-BEARING, SNOWBIRD-TYPE VEINS, WESTERN MONTANA

The Snowbird-type deposits are a series of hydrothermal, quartz-carbonate veins that intrude Belt metasediments, on a strike line extending NW-SE from the Coeur d’Alene Mining District along the Idaho-Montana border toward the Idaho Batholith.

In the NW they are Proterozoic (Ramos and Rosenberg 2012), open space fillings with pegmatitic textures and similar parageneses: outer ankerite, peripheral euhedral quartz and massive calcite cores (Stage I). All deposits to the SE are characterized by later fluorite, carbonate, quartz assemblages (Stage II). Stage I and II mineralization are both present only at the Spar(SP) and Snowbird (SB) deposits.

Laser ablation studies of xenotime in calcite associated with fluorite have yielded several concordia ages for the fluorite-bearing assemblages. At the SB deposit the massive calcite core is transected by veins of quartz, ankerite and purple fluorite (or parisite). An age of 72±10 Ma for this assemblage is in agreement with the U-Th-Pb parisite age, 71.1±1.0 Ma, reported by Metz et al. (1985). At the SB the green fluorite was found to have an age of 65.58±0.61 Ma while to the SE an age of 61.5±5.1 Ma was obtained. Thus the massive white to green fluorite, the ore mineral at the SB deposit, is Cretaceous to Eocene in age. These ages suggest that the F-bearing fluids were derived from the Idaho Batholith approximately 22 miles south of the SB trend.

Carbonates are generally LREE depleted, HREE enriched, and retain negative Eu anomalies (i.e., early carbonates Eu/Eu*>0.7, late carbonates and all fluorites Eu/Eu*<0.5). Normalization of SB deposit calcite REE patterns to those of adjacent Belt wall rocks results in a similar REE distribution and virtually no Eu anomaly implying that REEs from Belt metasediments were scavenged by fluids responsible for early carbonates (Ramos and Rosenberg, 2012). Ar/He/N₂ ratios of fluid inclusions in late quartz reflect a magmatic signature (Norman and Sawkins, 1987) supporting the conclusion that fluids, derived from the Idaho Batholith, intruded and metasomatized pre-existing Proterozoic veins and contributed additional REEs to both calcites and fluorites.