THE MURRAY GOLD DISTRICT, SHOSHONE COUNTY, IDAHO: A POSSIBLE REDUCED INTRUSION-RELATED GOLD SYSTEM
The Murray district appears to have many of the characteristics of a reduced intrusion related gold system (RIRGS). These characteristics include low total sulfide content, a back-arc tectonic setting, an association with As, W, Te and formation from low salinity mixed CO2–H2O fluids. Geochronology gives conflicting ages but the ages of intrusion and mineralzation appear to be broadly similar.
Some aspects of the Murray district do not match the RIRGS model. The stocks contain disseminated magnetite and the district scale zoning from base metal rich deposits near the intrusions to more distal precious metal mineralization is reversed in comparison to the RIRGS type localities (Hart 2007). However, previous authors (Parkinson, 1984; Leach et al., 1998) have suggested that at least some of the magnetite in and adjacent to the stocks is secondary. Similarly, most of the base metals are Proterozoic and so predate the intrusions (Leach et al. 1998).
Overall, while the Murray district is not an exact match to the RIRGS model, it shares enough characteristics that further investigation may be worthwhile. Additionally, other small syenite-monzonite stocks from the Middle Cretaceous intrude Belt sediments from Gold Hill north of Moscow, Idaho up to Wall Mountain near Bonners Ferry, Idaho. Except for the syenite stock on the Montana-Idaho border northeast of Hope, Idaho, these intrusions are associated with historic gold-tungsten production. Logan et al. (2000) identified a suite of Cretaceous syenite intrusions associated with RIRGS within the Purcell Supergroup. The Idaho intrusions may be a southern extension of this same RIRGS belt though geochemical, isotopic, and geochronologic study is needed to test this idea.