The Greenstone Mountain area, which includes the Utopia mining
district, is located in the foothills of the Pioneer Range, Beaverhead
County, Montana. Approximately 12, 600 feet of Paleozoic and
Mesozoic sedimentary rocks and an undetermined thickness of Precambrian
sediments and Late Cretaceous igneous rocks are exposed
within the area of investigation.
Most of the sedimentary rocks were deposited along the margin
of the Cordilleran miogeosyncline. Precambrian Beltian sediments
are the oldest exposed rocks in the map area. Pre-Mississippian
Paleozoic rocks have not been differentiated in this study but probably
represent the Cambrian and Devonian Systems. Other Paleozoic rock
units include the Lodgepole and Mission Canyon Formations of
Mississippian age, the Amsden Formation of Mississippian to
Pennsylvanian age, the Quadrant Formation of Pennsylvanian age,
and the Phosphoria Formation of Permian age. Mesozoic sediments
include the Dinwoody and Woodside Formations of Triassic age and
the Kootenai Formation and Colorado Group of Cretaceous age. The
Jurassic System is not represented.
During the Late Cretaceous Laramide orogeny the sedimentary
rocks of the area were folded and faulted. The dominant structures
produced during this period of deformation are northeast- to north-trending
anticlines and synclines that are locally overturned to the
east. Steep faults of small to moderate displacement disrupt the folds.
Quartz monzonite intrusive rocks were emplaced following the
Laramide orogeny and are probably genetically related to the Boulder
Batholith. The intrusion gave rise to mineralization and thermal
metamorphism of the surrounding country rocks.
Quaternary glacial and fluvial deposits mantle parts of the area.
At least three valley glaciers advanced into the northwestern corner
of the area in Late Pleistocene time and left lateral and terminal
moraines. Pleistocene terrace gravels are situated along Birch Creek
and on lowlands in the west, and Recent alluvium is being deposited
along existing streams.