Geology of the Eureka pegmatite, Pennington County, South Dakota

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.

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.

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.