ABSTRACT

The Sweet Grass Hills are a northwest-trending group of five intrusive-cored buttes near the Canadian border north of Shelby, Montana. West Butte and Middle Butte are cored predominantly by diorite. East Butte has a core mainly of syenite and syenite porphyry. Grassy Butte was formed by the intrusion of a plug of felsite breccia. Haystack Butte is cored by a plug of hornblende vogesite. The domal structure of the buttes is the result of the vertical force produced during the emplacement of the igneous rocks. A complicated pattern of northwest- and north-east-striking faults has disrupted the general domal structure of the buttes. The faults were probably controlled by Precambrian basement structures and show evidence of repeated movement.

Sedimentary rocks have been intruded and metamorphosed to varying degrees by the intrusive rocks. The oldest rocks exposed in the Sweet Grass Hills are limestones of the Mississippian Madison Group, which are unconformably overlain by the Jurassic Ellis Group. The Upper Jurassic Morrison Formation is absent. The Cretaceous Kootenai, Blackleaf, and Marias River formations occur in a roughly concentric pattern around the buttes. Younger Cretaceous rocks of the Telegraph Creek Formation, Eagle Sandstone, Claggett Shale, and Judith River Formation occur in the lower slopes around the buttes. In general, the best exposures and most complete stratigraphic section of sedimentary rocks occur at East Butte.

The Sweet Grass Hills are important to the economy of the surrounding region. Gold has been produced near the Gold Butte town site on the west side of Middle Butte and lode mineralization containing low-grade gold is known to occur in East, Middle, and West Buttes. Gold produced at Middle Butte is from a placer deposit in pediment gravel and alluvial gravel, both of which are derived from lode deposits about 1/2 mile (0.8 km) to the east. Gold mineralization in lode deposits in all three of the Buttes is associated with contacts between igneous rocks and sedimentary rocks. At East Butte the gold is probably controlled by the contact between syenite and Madison Limestone. Fluorite is associated with this mineralization. Mineralization in Middle and West buttes is associated with the contact of diorite with sedimentary rocks mainly of the Colorado Group.

Diamond exploration is being conducted just across the border in Canada and potential for diamond occurrences does exist in the Sweet Grass Hills area on the Montana side of the border. No favorable host rocks for diamonds are known in the Sweet Grass Hills, but the composition and thickness of the Archean basement, the likely presence of thick lithosphere in the area, and regional relations suggest diamond occurrences are possible beneath the cover of glacial till.

Anticlinal and domal structures cored by intrusive rocks in the plains surrounding the Sweet Grass Hills have produced significant volumes of oil and gas.