Structural Geologic Evolution of the Butte District, Montana

摘要

The Butte district contains two porphyry Cu-Mo deposits and the younger zoned Main stage base metal lode deposits that formed in the period from 66 to 62 Ma, synchronous with Laramide compressional thrust faulting and folding in southwest Montana. New geologic mapping of the Butte district reveals three distinct episodes of deformation: 1. Gentle (20° ± 10°) NE tilting produced during Laramide compressional deformation, uplift, and exhumation during and after ore formation in the Late Cretaceous to Paleocene (-66-53 Ma); 2. Moderate (10°-50°) NW tilting of Eocene age produced by NE-striking normal faults that are limited to the northwest part of the district and synchronous with the eruption of the Lowland Creek Volcanics; and 3. Miocene to present gentle (15°) ESE tilting in the hanging walls of the Continental, East Ridge, and Rocker normal faults.