Mineral resources of the Beartooth Primitive Area and vicinity were studied in 1969-72 as part of a program to evaluate the resource potential of primitive and wilderness areas in the United States. Geologic study was made by the U.S. Geological Survey, and appraisal of known mines, prospects, and mining claims was made by the U.S. Bureau of Mines. The area studied comprises the Beartooth Primitive Area of 950 square kilometers (365 square miles) in south-central Montana and contiguous areas in Montana and Wyoming aggregating 610 square kilometers (235 square miles), or a total area of 1,560 square kilometers (600 square miles), herein referred to as the Beartooth study area. The entire area is within the drainage basin of the Yellowstone River. Most of it is in Custer National Forest, and parts are in Gallatin and Shoshone National Forests. The Beartooth Mountains are a block mainly of amphibolite-facies Precambrian granitic gneiss, amphibolite, and subordinate metasedimentary rocks intruded by many Precambrian mafic dikes. The block is about 120 kilometers (75 miles) long and 40 to 65 kilometers (25 to 40 miles) wide. Paleozoic and Mesozoic sedimentary rocks, about 3,000 meters (10,000 feet) in combined thickness, are exposed around the periphery of the mountains but have been almost completely eroded from the interior; within the Beartooth study area, the total outcrop area of these rocks is less than 26 square kilometers (10 square miles). The Precambrian rocks are intruded by numerous silicic dikes and plugs of Tertiary and possibly Cretaceous age and, along the west edge of the study area, are overlain by small patches of Tertiary volcanic rocks, part of the extensive Absaroka volcanic field to the southwest. The total extent of Tertiary rocks is less than 8 square kilometers (3 square miles). The Beartooth block was uplifted as much as 4,500 to 6,000 meters (15,000 to 20,000 feet) above the adjoining basins along a system of reverse and normal faults in Late Cretaceous and early Tertiary time. The present rugged topography of extensive high plateaus, jagged peaks and divides, and deep steep-walled canyons resulted from fluvial and glacial erosion that followed regional uplift in the late Tertiary. Much of the bedrock is now covered by glacial deposits, rock glaciers, talus, and felsenmeer. An aeromagnetic survey of the Beartooth study area made in 1969 revealed several positive anomalies. Some are attributable to Tertiary intrusive rocks, and others to the ultramafic zone of the Stillwater Complex on the northeast edge of the area. But the sources of other anomalies within the Precambrian gneiss terrane are uncertain; they may be caused by fades changes Fl F2 STUDIES RELATED TO WILDERNESS PRIMITIVE AREAS within the Precambrian rocks or by concealed intrusive bodies. The anomalies within the Beartooth study area do not seem either to be associated with or to suggest the occurrence of mineral deposits. The areas in, adjoining, or near the Beartooth study area known to contain mineral resources are the Cooke ...
