Boulder Batholith

The batholith is composed of at least seven, and possibly as many as 14, discrete rock masses called plutons, which had formed beneath the Earth's surface during a period of magma intrusion about 73 to 78 million years ago (Late Cretaceous time).

Regional uplift brought the deep-seated granite to the surface, where erosion exposed the rocks and the extremely rich mineral veins they contained.

The Butte Granite makes up the bulk of the batholith (about 70%), with additional named intrusions along the periphery consisting of granodiorite, monzogranite, and minor syenogranite.

The origin of these igneous rocks is related to subduction and continental magmatic arc processes associated with the Late Cretaceous western margin of North America.

Later hydrothermal vein activity along fault and shear zones within the Butte quartz monzonite and its contact with adjacent rocks, resulted in major ore deposits of lead, zinc, and silver.