Is the East Ridge above Butte geologically different from the Butte Hill? Why wasn mining done on the East Ridge?
The East Ridge and Butte Hill are both underlain by basically the same granitic rock (called the Butte Quartz Monzonite by geologists). However, under the Butte Hill hot fluids (mostly water) moved through fractures and altered the minerals in the granite. Immense quantities of metals, including copper, molybdenum, zinc, lead, manganese, and others were brought in and deposited in fractures, or in some cases actually replaced previous mineral grains in the granite, creating an orebody. However, all good things must come to an end. The orebody exposed at the surface on the Butte Hill has limits, and the eastern limit mostly stops short of extending under the East Ridge.
