Montana Tech THE UNIVERSITY OF MONTANA |
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Research 97 In This Issue Montana Decontamination Research Activity ________________ |
Geology
of the Elkhorn Mountains Ron M. Dixon Approximately 77 million years ago, southwest Montana was the home of an active volcanic complex. The Elkhorn Mountains Volcanics (EMV), as they are now known, once contained one of the largest ash-flow volcanic fields on earth. This volcanic material once covered an estimated 10,400 square miles (26,000 km 2) to a depth of approximately 2.5 miles (4.0 km). By comparison, the Yellowstone National Park volcanics are estimated to cover only 4,000 square miles (10,000 km2) to a depth of 500 feet (150.0 m). The EMV are Cretaceous in age and are genetically related to the Boulder Batholith of Butte mining fame. The current EMV study area covers approximately 72 square miles (187 km2) and is located just north of the Golden Sunlight Mine near Whitehall, Montana. The Golden Sunlight Mine currently extracts gold from a breccia pipe associated with Elkhorn Mountains volcanism.The purpose of the EMV project is to provide a detailed geologic map of the area and to investigate previous claims of the possible existence of a volcanic caldera within the study area. In light of nearby mining activities, the economic mineral resources within the area are also being evaluated. Currently, the geology of the area is being mapped at a 1:12,000 scale, and detailed thin section analysis is being performed on collected samples. The mapping portion of the project is funded both by the United States Geological Survey through its EDMAP program and by Montana Tech. The project is coordinated through the Montana Bureau of Mines and Geology, and the final map will be published as an open file report. Field work began in May of 1996 on the lower western slopes of Bull Mountain (Figure 1); and the upper elevations were mapped during the summer months. Previous authors have divided the EMV into the lower, middle, and upper members. However, the detailed stratigraphy of these members has not been fully identified. During the 1996 field season, 31 separate geologic units were identified and mapped. Of those units, 25 are volcanic. The lower member has been divided into 20 separate units ranging in composition from mafic olivine-rich units to silicic units. Deposit types are also quite varied and include coherent flows, volcaniclastic tuffs, volcanic mudflows, and volcanogenic sediments. Of the middle member mapped to date, there are five separate units which also range in composition from mafic to silicic. Deposit types are similar to those found in the lower member; however, a very thick welded ash-flow unit is also present (Figure 2). Ash flow tuffs of the magnitude seen in the EMV are believed to result from fissure eruptions associated with caldera formation. During the fall of 1996, the welded ash flow tuff was studied in more detail. Optical microscopy methods determined the petrology of 13 hand samples which represented a cross-section of the ash-flow unit. On the basis of this analysis, the unit has been subdivided into three different zones which may or may not represent three separate eruption events. These zones are separated by two pyroxene-rich crystal tuffs (Figure 3). Texturally, the lower zone consists of non-welded to partially welded rhyodacitic ash-flow tuffs. Samples decrease in porosity up-section due to an increase in welding. The middle zone is characterized by non-welded to densely welded ash-flow tuffs which are also rhyodacitic in composition. Deformation structures and eutaxitic fabrics (Figure 4), which are characteristic of densely welded tuffs, are increasingly more common up-section. The rhyodacitic tuffs of the upper member are densely welded to partially welded. |
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The 1997 field season will complete the mapping of the middle and upper members on the eastern flanks of Bull Mountain and will perform a detailed stratigraphic measurement of the welded ash-flow tuff unit. To date, no evidence has been found to support the hypothesis that a caldera existed within the immediate area. Although two breccia pipes have been mapped within the study area, no economically significant mineralization has been observed in either pipe. Secondary copper has been noted in a unit that is at least spatially related to one of the breccia pipes. Other indications of possible mineralization include moderate to strong alteration by hydrothermal fluids in limited areas, both along contacts between volcanics and along contacts between volcanics and sedimentary units. |
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