Uranium, thorium isotopic analyses and uranium-series ages of calcite and opal, and stable isotopic compositions of calcite from drill cores UE25a 1, USW G-2 and USW G-3/GU-3, Yucca Mountain, Nevada

Abstract

Fracture and cavity filling calcite and opal in the unsaturated zone of three drill cores at Yucca Mountain were analyzed for uranium and stable isotope contents, and were dated by the uranium-series method. Stable isotope data indicate that the water from which the calcite precipitated was meteoric in origin. The decrease in {sup 18}O and increase in {sup 13}C with depth are interpreted as being due to the increase in temperature in drill holes corresponding to an estimated maximum geothermal gradient of 43{sup 0} per km. Of the eighteen calcite and opal deposits dated, four of the calcite and all four of the opal deposits yield dates older than 400,000 years and ten of the remaining calcite deposits yield dates between 26,000 and 310,000 years. The stable isotope and uranium data together with the finite uranium-series dates of precipitation suggest a complex history of fluid movements, rock and water interactions, and episodes of fracture filling during the last 310,000 years. 10 refs., 7 figs., 4 tabs.