The Basin quadrangle, in the northern part of the Boulder Mountains between Butte and Helena, Montana, is underlain principally by igneous rocks that include Late Cretaceous quartz latitic and andesitic Elkhorn Mountains volcanics, quartz monzonite and related rocks of the Boulder batholith, Oligocene(?) quartz latitic volcanic rocks, and late Miocene(?)-early Pliocene(?) rhyolitic volcanic rocks. Sedimentary rocks of the Late Jurassic Morrison formation, the Early Cretaceous Kootenai formation, and the Early Cretaceous lower part of the Colorado formation, crop out in the northwest part of the quadrangle. The batholithic rocks include early stage quartz monzonite, intermediate or main stage quartz monzonite and granodiorite, and late stage aplite and alaskite. The rocks of the main stage are in discontinuous layers approximately conformable to the folded Elkhorn Mountains volcanics that form the roof of the batholith, and may be part of a sill-like body rather than of a batholith in the classic sense. Metamorphic changes in the roof rocks are not conspicuous except in one stratigraphic unit that probably was especially susceptible to thermal reorganization.
The batholithic rocks and Elkhorn Mountains volcanics are jointed, and are cut by faults that trend about east, north, N. 20° E., northeast, and northwest. The east-trending faults are most abundant, especially in the eastern part of the quadrangle, and cut only the batholithic and pre-batholithic rocks, whereas many of the faults of other trends cut Tertiary volcanic rocks and a few cut Pleistocene glacial deposits.
A surface of moderate relief was cut before eruption of the Oligocene(?) volcanic rocks, and the late Miocene(?)-early Pliocene(?) volcanic rocks covered a deeply weathered surface of low relief. By the Pleistocene a landscape essentially like that of today had been formed, and during the one period of Pleistocene glaciation, valley glaciers and a mountain ice sheet modified the earlier landforms and left extensive deposits of till and outwash. These deposits have been modified in many places by mass-wasting processes that have dominated post-glacial erosion.
Mineral deposits in the quadrangle include deposits of disseminated auriferous pyrite, base- and precious-metal bearing quartz veins that occupy the east-trending fault zones, placer deposits of gold and tin, and a few non-metallic deposits, chiefly stone, gravel, and dumortierite. Nearly all of the metallic minerals mined in the quadrangle have come from the east-trending quartz veins and from the placer deposits.
