Boulder Batholith

The Boulder Batholith is a small batholith in southwest Montana, exposed at the surface as granite and serving as the host rock for rich mineralized deposits at Butte, Montana and other locations.

The batholith is composed of at least seven and possibly as many as 14 discrete rock masses called plutons, which, according to scientific theories, had formed beneath the Earth’s surface during a period of magma intrusion about 73 to 78 million years ago (Late Cretaceous time). [ [http://www.unc.edu/~breckj/gsa_2004_denver_abstract.htm Johnson, B.R., and others, 2004, Reexaming the geochemistry and geochronology of the Late Cretaceous Boulder Batholith, MT] ] The rising buoyant plutons resulted from subduction along what was then the west coast of North America. Regional uplift brought the deep-seated granite to the surface, where erosion exposed the rocks and the mineral veins they contained.

The Boulder Batholith was named for the prominent rounded boulders that typify its landscape, the result of spheroidal weathering of fractured granite. It measures approximately 75 miles (121 km) north-south by about 25 miles (40 km) east-west, rather small in comparison to most batholiths.

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