USGS Home
Contact USGS
Search USGS

U.S. GEOLOGICAL SURVEY
Data Series 280

Back to Table of Contents

Summary and Conclusions

To contribute to a more accurate understanding of regional ground-water flow beneath the Nevada Test Site vicinity in southern Nevada using ⁸⁷Sr/⁸⁶Sr values, chemical and strontium isotope data are reported for rock samples representing water-producing horizons in the lower carbonate aquifer, as well as in the upper carbonate aquifer and lower clastic confining unit. Samples obtained in 2006 include core and cutting fragments from 20 boreholes located around Yucca Flat, Frenchman Flat, Rainier Mesa, and Mercury Valley. Partial digestions using dilute nitric acid were used to quantify MgO, CaO, SiO₂, Al₂O₃, MnO, Rb, Sr, Th, and U concentrations in carbonate fractions, which constituted more than 90 percent of most rocks. Small amounts of SiO₂ and Al₂O₃ in leachates are evidence that clay minerals may be present in many carbonate samples. Three samples containing visible shale or volcanic components have SiO₂ concentrations as high as 5–10 weight percent. Dolomites have median Sr concentrations of 46.5 micrograms per gram (µg/g), substantially lower than the median value of 384.5 µg/g for limestones. Because both rock types have similar Rb concentrations, dolomites tend to have higher Rb/Sr ratios. However, these values are low enough in most samples so that increases in ⁸⁷Sr/⁸⁶Sr caused by decay of ⁸⁷Rb are not substantial. Values of ⁸⁷Sr/⁸⁶Sr analyzed from the same leachates of dolomites and limestones range from 0.70809 to 0.72089 and show no dependency on Sr concentrations.

In addition to subsurface samples from NTS boreholes, 114 samples from outcrop, road cut, and underground exposures of Paleozoic carbonates were collected and analyzed in the early-to-mid 1990s. These samples are from locations to the west, south, and east of the NTS including sites in the Funeral Mountains, Bare Mountain, Striped Hills, Specter Range, Spring Mountains, and ranges east of the NTS. Although results for these samples were discussed earlier (Peterman and others, 1994), the data have not been published. Like the borehole samples, dolomite tends to have lower bulk-rock Sr concentrations (typically 22–96 µg/g), whereas limestone tends to have higher Sr concentrations that commonly range up to about 500 µg/g.

The distribution of Sr isotopic compositions from carbonate fractions of borehole and outcrop samples are skewed toward higher ⁸⁷Sr/⁸⁶Sr values. Many samples have ⁸⁷Sr/⁸⁶Sr values consistent with Paleozoic seawater ⁸⁷Sr/⁸⁶Sr values ranging from about 0.7070 to 0.7092. Samples with ⁸⁷Sr/⁸⁶Sr values greater than 0.7093 have experienced post-depositional modification, which was caused by interaction with fluids containing radiogenic Sr. Carbonate rocks with modified ⁸⁷Sr/⁸⁶Sr values are present throughout the stratigraphic section. However, rocks with elevated ⁸⁷Sr/⁸⁶Sr are most common around areas of known mineralization at Bare Mountain, and in western Yucca Flat where the upper clastic confining unit may contribute radiogenic Sr from clay minerals with elevated Rb/Sr. Paleozoic carbonate rocks with non-marine ⁸⁷Sr/⁸⁶Sr signatures may show mineralogical or textural evidence of alteration; however, some samples with elevated ⁸⁷Sr/⁸⁶Sr show no obvious signs of post-depositional modification. The ranges of Sr concentrations and ⁸⁷Sr/⁸⁶Sr values presented in this report can be used in hydrochemical modeling efforts to represent the composition of carbonate rock dissolved during regional ground-water flow.

Back to Table of Contents