Vein Orientation and Structural Geology of the Boulder Batholith, Mount Thompson Quadrangle, southwestern Montana
By N. Mankins
The Mount Thompson 7.5 Minute Quadrangle is located in the central Boulder Batholith, between the cities of Butte and Helena. The Boulder batholith is host to numerous metalliferous vein deposits, including those in the Butte District. Recent USGS (EDMAP)-funded geologic mapping at 1:24,000 scale in the Mount Thompson Quadrangle during the summer of 2016 focused on new measurements of vein and fault orientation and compilation of previous data. The orientations of vein systems observed in the Mount Thompson Quadrangle are similar to the orientations reported elsewhere in the Boulder Batholith.
Most of the veins in the quadrangle shortly post-date the 76 Ma Butte Granite unit of the Boulder Batholith, and cut this unit and older rocks in the quadrangle. Two distinct vein orientations have been identified on stereonet plots: 1) massive milky-grey East-West (azimuth 98˚) quartz veins that have dips that range from 73˚ North to 72˚ South, and 2) NE-SW (azimuth 157˚) striking quartz-chalcedony veins with dip range 65˚NW-76˚SE. Vein mineralogy includes three subsets: 1) massive grey quartz 70-80% of which locally contain quartz vugs with abundant sulfides (Pyrite > Galena), 2) quartz-sulfide-poor veins 65% with chalcedony, 3) cm-scale milky quartz veins 70% with tourmaline. At the Montana Tunnels Mine a younger set of sulfide-rich (Pyrite > Sphalerite > Galena) veins, Eocene in age, cut the ~50 Ma diatreme breccia and a Lowland Creek Volcanics pyroclastic vent. The Boulder batholith veins are systematically zoned on the km scale around heat centers from high to low temperature from central tourmaline- to quartz-sulfide- to quartz-chalcedony in the periphery.