Roy Eldon Roadifer 1954 <p>The Eureka Pegmatite, two miles southeast of Keystone, Pennington County, South Dakota, is a small but possibly valuable zoned body containing more than 85,000 tons of coarse-grained granite-pegmatite rock.</p><p>The mica in the pegmatite is probably not of any great importance, but the potash feldspar and beryl may comprise a considerable tonnage of valuable reserves. If the pegmatite can be exploited at a profit in spite of the graphite-damaged feldspar, it may yield substantial amounts of strategically important beryl by open-cut mining of the entire body.</p><p>This report contains complete, megascopic mineralogical and structural details of the pegmatite, which are supplemented with drill core logs and a geologic map and cross-sections.</p> application/pdf 10.3133/ofr54256 en U.S. Geological Survey Geology of the Eureka pegmatite, Pennington County, South Dakota reports