Traces of epigenetic hydrothermal activity at Yucca Mountain, Nevada: preliminary data on the fluid inclusion and stable isotope evidence

Abstract

Samples of opal–quartz–calcite crusts from the 7.8-km-long tunnel excavated under Yucca Mountain were studied to determine their origin. Calcite in these crusts commonly forms relatively large (up to 1.5 cm), euhedral crystal terminations. In some crusts, layers of chalcedony and patches of opal alternate with calcite layers. Chalcedony commonly grades into euhedral quartz crystals. Quartz also forms microdruses of individual crystals up to 8 mm in size. Fluorite was identified in four samples. Fluid inclusion studies on calcite revealed formation temperatures of 35–85°C. Gas-filled inclusions are apparently of low-density ( P<1 bar at room temperature) and contain aromatic hydrocarbons. In four studied calcite samples, values of δ 18O remain virtually constant across crusts (−10.6‰ to −12.1‰ V-PDB). In one sample, the earliest part of calcite had δ 18O of −19‰; in this part of the sample, the highest fluid inclusion temperatures, 70–85°C, were measured. The secondary calcite found at Yucca Mountain is commonly interpreted as being formed through a “pedogenic” process from rain waters percolating in the vadose zone and carrying dissolved carbonate from overlying soils [e.g., Stuckless, J.S., Peterman, Z.E., Muchs, D.R., 1991. U and Sr isotopes in ground water and calcite, Yucca Mountain, Nevada: evidence against upwelling water. Science 254, 551–554; NAS/NRC, 1992. Ground Water at Yucca Mountain: How High Can It Rise? National Academy Press, Washington, DC; Roedder, E., Whelan, J.F., Vaniman, D.T., 1994. Fluid inclusion studies of calcite veins from Yucca Mountain, Nevada, tuffs: environment of formation. High-Level Radioactive Waste Management. Proc. Int. Conf., Am. Nucl. Soc. La Grande Park, IL, pp. 1854–1860]. Our data indicate that the calcite–opal–quartz (with minor fluorite) crusts from the presently unsaturated (vadose) zone of Yucca Mountain were formed from low-temperature hydrothermal aqueous fluids in the saturated (phreatic) environment. The issue has direct bearing on the suitability of Yucca Mountain as a site for permanent geological disposal of the high-level nuclear waste.