Structural Geologic Evolution of the Butte District, Montana

Abstract

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. Stratigraphic relationships, isotopic ages, hornblende barometry, and fluid inclusion data indicate that exposures of the Butte Granite, which hosts mineralization, crystallized at ~6- to 9-km depth and that early porphyry Cu-Mo deposits formed at similar depths of 5 to 9 km. A period of rapid uplift and erosion removed about 7 km of rock from above the ore deposit between 66 to 64 Ma and 53 Ma, but was mostly completed by 59 Ma. Estimated exhumation rates range from 0.5 to 2 mm/yr, and uplift was likely most rapid between ~63.5 and 62 Ma during Main stage mineralization, which occurred at depths of 2 to 6 km. The Butte Granite and Boulder batholith were emplaced at 79 to 76 Ma into thrust fault ramps, which later became inactive near Butte as Laramide crustal shortening moved eastward in the Helena salient. The accelerated period of uplift and unroofing is attributed to renewed deformation along the Montana disturbed belt south of Butte that uplifted the south end of the Boulder batholith and produced ~20° of gentle NE tilting (episode I) in the district. The exhumation history of the deposit is therefore broadly contemporaneous with ore formation, and is consistent with the general transition from lithostatic to hydrostatic conditions that is evident from the telescoping of early porphyry Cu-Mo ores cut by the Main stage lode veins. Within a 5-m.y. period, the Laramide compressional regime near Butte evolved into an extensional environment characterized by normal faulting. The normal faults were synchronous with eruption of the 53 to 48 Ma Lowland Creek Volcanics, which directly overlie the Main stage veins in the northwest part of the district. Structural reconstruction of the district at the time of mineralization produces orientations of quartz porphyry dikes, porphyry Cu-Mo veinlets, Main stage veins, and alteration zonal patterns that are steeply dipping and symmetrical, similar to observations from other upright or restored porphyry copper deposits. Early porphyry Cu-Mo veins and dikes are oriented E-W and likely originated as tensile hydrofractures above a ductile magma carapace during Laramide shortening. Additionally, during the Laramide compressional phase, two conjugate fault systems occupied by Main stage veins are normal oblique-slip faults that record minor E-W and vertical shortening and N-S extension. These normal faults with minor offset likely resulted from brittle failure caused by doming above a volatile-rich and buoyant granitic magma chamber and cupola at depth. The structural evolution of the Butte district therefore reflects the interaction of a far-field compressive stress regime with local buoyant magmatic forces.