Geology and Mineralization of the Southeastern Part of the Black Pine Mountains, Cassia County, Idaho
Date of Award:
Master of Science (MS)
Donald R. Olsen
Donald R. Olsen
Clyde T. Hardy
Richard R. Alexander
The southeastern part of the Black Pine Mountains is located in the southeastern part of Cassia County, southern Idaho. The Utah-Idaho state line is three miles south of the studied area and the Cassia-Oneida county line bounds it on the east. The area is nearly square and encompasses about 30 square miles.
Devonian, Mississippian, Pennsylvanian, Permian, Tertiary and Quaternary sedimentary rocks are exposed within the area. Limestone, dolomitic limestime, quartzite, and bedded chert represent the Jefferson (?) Formation of Devonian age. It is 400 feet thick, however, the base is not exposed. The Milligen Formation is Early Mississippian in age and is black argillite with interbedded orthoquartzite. The Milligen is about 1,850 feet thick. The Late Mississippian White Knob Formation is 2,400 feet thick and has two members. The lower member is limestone interbedded with calcareous siltstone. Massive blue-gray limestone with some chert nodules characterizes the upper member. The undifferentiated Pennsylvanian-Permian unit is 1,800 feet of mostly sandy limestone. Quartzite and calcareous sandstone are also present. Tertiary rocks are present in the form of an orangish-white tuff which is considered part of the Salt Lake Formation. Lake Bonneville Group, alluvial, and landslide deposits represent the Quaternary System. Most of these are unconsolidated silt, sand, and gravel deposits. However, the Lake Bonneville Group displays a tightly cemented shore-line deposit in places.
The effects of metamorphism are common in the area. The Milligen shows signs of contact and tectonic metamorphism. In places it has been bleached or altered to slate and phyllite. The White Knob Formation has been marblized at several locations.
Igneous activity has emplaced two small dikes on the eastern flank of the Black Pine Mountains. Although they are highly altered, the original rock was apparently a diabase.
The structure of the area is complex. Three low-angle thrust faults are present which are generally situated along bedding planes. The lower thrust fault separates the Jefferson (?) and Milligen formations. The middle thrust fault intervenes at the Milligen-White Knob contact. Locally, this thrust fault has cut out the lower member of the White Knob. The upper thrust fault is present at the base of the undifferentiated Pennsylvanian-Permian strata. The upper thrust fault overlies the White Knob and, locally, the Milligen. Several high-angle faults are present which displace the low-angle thrust faults. A major range-front fault is present on the southeastern side of the range. Displacement on it may be as much as 6,500 feet.
Mineralization in the area occurred during two episodes. The first was guided by fractures related to Laramide structure. This episode was characterized by mesothermal deposits of sphalerite, tetrahedrite, and jamesonite. Following the first mineral deposition Basin-and-Range faulting began. New fractures provided a locus for mesothermal and epithermal deposits of the second episode. Calcite, barite, and gold were deposited at this time. Emplantation of the dikes probably accompanied this episode.
French, Don E., "Geology and Mineralization of the Southeastern Part of the Black Pine Mountains, Cassia County, Idaho" (1975). All Graduate Theses and Dissertations. 1903.
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