Uranium deposits in the Espanola Basin, Santa Fe County, New Mexico

present time. However, many private water wells in the region produce water with concentrations of uranium (up to 1,820 g/L (ppb)) that exceed the safe drinking water standard of 30 g/L. Therefore, it is important to understand the source of the uranium in the groundwater and the processes involved. Potential sources for uranium in the groundwater include 1) uranium occurrences in the Tesuque Formation (San Jose mining district), 2) rhyolitic volcanic ash beds and sandstones with volcanic detritus found interbedded within the Tesuque Formation, 3) veins, replacements, and pegmatites in Proterozoic rocks (San Jose and Nambe mining districts), and 4) Proterozoic granitic rocks in the Sangre de Cristo Mountains. The sandstone uranium occurrences in the Tesuque Formation represent natural precipitation and concentration from uraniferous groundwaters, likely derived from 1) rhyolitic volcanic ash beds within the Tesuque Formation, 2) the alteration of granitic and/or volcanic detritus within the sedimentary host rocks, and 3) Proterozoic rocks in the Sangre de Cristo Mountains to the east. One property, the San Jose No. 13 (NMSF0033), produced 12 lbs (5 kg) of U3O8 at a grade of 0.05% U3O8 in 1957. Uranium in modern groundwaters likely was derived from the same sources, as well as from leaching and oxidation of older uranium occurrences in the Tesuque Formation. Uranium then precipitated from the waters to form the geochemical anomalies found in the prospects.