2005 Salt Lake City Annual Meeting (October 16–19, 2005)

Paper No. 5
Presentation Time: 2:30 PM


ZHOU, Xiaoping, Resources Department, Southern Nevada Water Authority, 1900 E. Flamingo Rd., Suite 255, Las Vegas, NV 89119, YU, Zhongbo, Department of Geoscience, University of Nevada, Las Vegas, 4505 S. Maryland Parkway, Las Vegas, NV 89154-4010, STETZENBACH, Klaus J., Harry Reid Center for Environmental Studies, University of Nevada, Las Vegas, 4505 S. Maryland Parkway, Las Vegas, NV 89154-4009 and JOHANNESSON, Karen H., Department of Earth and Environmental Sciences, The Univ of Texas at Arlington, 500 Yates Street, Box 19049, Arlington, TX 76019-0049, xiaoping.zhou@lvvwd.com

The origin of fracture-coating minerals, mainly secondary calcite and silica, within aquifers at the Nevada Test Site (NTS) and Yucca Mountain in southern Nevada has been debated for the past several years. Whether the secondary minerals in fractures formed from percolating meteoric water or from upwelling groundwater or hydrothermal fluid, they record geochemical changes and the history that occurred in the paleo-groundwater that flowed through the fractures. Therefore, further study of the origins of these secondary minerals will provide crucial information for evaluating hydrogeologic changes and the fate of the future nuclear waste repository.

A total of 26 secondary mineral samples (mainly calcite) were collected from the western Pahute Mesa-Oasis Valley and from underground tunnels at Yucca Mountain, southern Nevada. These samples were analyzed for their rare earth element (REE) concentrations using both ICP-MS and LA-ICP-MS. Different REE patterns in secondary minerals, along with other geological and geochemical data (such as petrographic, mineralogical, stable isotope, and fluid inclusion), suggest that the fracture-coating minerals were precipitated from groundwaters that originated from different sources over a long interval of geologic time. Such secondary minerals in fractures can precipitate either from groundwater percolating through the unsaturated zone, or from upwelling groundwater (both shallow perched and deep aquifer groundwater) with hydrothermal fluid influences, or a combination of these two origins. Mineral samples from different depths in the same borehole may have different origins. A comparative study was performed using ICP-MS and LA-ICP-MS to analyze the same mineral sample for its REE concentrations. The good correlation of the REE results obtained from the two different analytical methods shows a great potential for LA-ICP-MS in high-precision spot analysis of rocks and minerals.