AGE AND ORIGIN OF SNOWBIRD-TYPE VEINS, WESTERN MONTANA

The Snowbird-type deposits include 9 small, hydrothermal quartz-carbonate veins that intrude Wallace Fm. (Belt) metasediments, along a strike extending NW to SE from the Coeur d’Alene Mining District, along the Idaho-Montana border. All are open space fillings with pegmatitic textures and similar parageneses; outer ankerite, peripheral quartz, calcite core ± later fluorite. At least 4 originated from breccia dikes and some are characterized by a later, fluorite, carbonate, quartz overprint.

Ankerite and calcite from the Cold Creek and Snowbird Pod veins lie on the 1391±280 Ma Sm-Nd isochron and a ²⁰⁷Pb/²⁰⁴Pb-²⁰⁶Pb/²⁰⁴Pb mixing line defined for similar, ore-barren veins associated with the Coeur d’Alene Mining District. Sm-Nd analysis of fluorite from the Spar deposit suggests a much younger age reflecting the late appearance of fluorite. At the Snowbird Pod late quartz + F-phlogopite (⁴⁰Ar/³⁹Ar age, 1014 Ma) intrudes earlier quartz, ankerite, calcite veins. Laser ablation results of xenotime from Cold Creek yield a ²⁰⁶Pb/²⁰⁷Pb age of 1600±400 Ma whereas zoned xenotimes from the Snowbird deposit give approximate ages that range between 906 Ma (core) and 87 Ma (rim). The latter age is consistent with its parisite 71 Ma U/Pb age (Metz et al., 1985). Thus, the Snowbird deposit appears to be Proterozoic and Cretaceous in age.

Initial ⁸⁷Sr/ ⁸⁶Sr ratios of carbonates are <0.80 for veins with only ankerite, quartz and calcite but most are >0.80 where fluorite is present. Thus, early mineralization could be due to Proterozoic leaching of Belt metasediments but later fluorite requires a more recent source of radiogenic Sr, probably pre-Belt basement.

Carbonates are generally LREE depleted and 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 Snowbird deposit calcite REE patterns to those of adjacent Belt wallrocks 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. Ar/He/N₂ ratios of fluid inclusions in late quartz reflect a magmatic signature (Norman et al., 2002) suggesting that fluids, derived from the nearby Idaho batholith, intruded and metasomatized prior-existing Proterozoic veins.