English: Rhyolite plug in the Tertiary of Montana, USA.

The town of Butte, Montana is known as the “Richest Hill on Earth” and "The Mining City". The Butte Mining District has produced gold, silver, copper, molybdenum, manganese, and other metals.

The area's bedrock consists of the Butte Quartz Monzonite (a.k.a. Butte Pluton), which is part of the Boulder Batholith. The Butte Quartz Monzonite ("BQM") formed 76.3 million years ago, during the mid-Campanian Stage in the Late Cretaceous. BQM rocks have been intruded and altered by hydrothermal veins containing valuable metallic minerals - principally copper sulfides. The copper mineralization has been dated to 62-66 million years ago, during the latest Maastrichtian Stage (latest Cretaceous) and Danian Stage (Early Paleocene). In the supergene enrichment zone of the area, the original sulfide mineralogy has been altered.

In addition to quartz monzonite, the Butte area's bedrock also includes rhyolite, in the form of lava flows, shallow intrusions, and volcanic plugs. Seen here is Big Butte, the namesake of the town itself. The hill is composed of Tertiary-aged porphyritic rhyolite that dates to 52 to 59 million years.

Locality: Big Butte (seen from near the Syndicate Pit), hill on the western side of the town of Butte, Silver Bow County, western Montana, USA