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Technical Report

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In the near future a decision will be made as to whether or not Yucca Mountain, 90 miles northwest of Las Vegas, Nevada is a suitable site for a permanent, underground, high level nuclear waste repository. A major factor in determining the suitability of Yucca Mountain as a repository is the potential for the site to be flooded by water during the regulatory lifetime. The current study was undertaken to examine the past fluid history at the site, to gain a better understanding of the possibility of flooding in the near geologic future. To estimate the past fluid flux into the repository horizon, research has focused on secondary minerals that precipitated in open space in lithophysal cavities, fractures, and breccias in the host Miocene tuffs. U.S. Geological Survey researchers concluded that secondary minerals formed from descending surficial meteoric fluids in a vadose environment. State of Nevada scientists observed 2-phase fluid inclusions with homogenization temperatures of 35 to 85 °C in secondary minerals and concluded that these minerals formed in the phreatic environment from upwelling hydrothermal fluids. They further concluded that upwelling hydrothermal fluids repeatedly invaded the site, have invaded the site in the recent geologic past, and could do so again making Yucca Mountain an unsafe site for high level nuclear waste storage. These studies did not constrain the timing of incursion of the fluids with elevated temperatures or the extent of this fluid flux across the site. This report provides the geologic context for subsequent fluid inclusion and geochronological studies (Wilson et al., 2002) that identified the temperature and extent of the fluid incursion and placed absolute temporal constraints on the fluid history at Yucca Mountain. Here we describe a detailed paragenetic study that determined the depositional history of secondary minerals at Yucca Mountain. One hundred and fifty-five samples of secondary minerals were collected from lithophysal cavities, fractures, and breccias at Yucca Mountain. Extensive petrography, paragenetic studies, and microprobe mapping indicate that early secondary minerals were heterogeneously distributed across the site and consist of variable amounts of calcite, opal, chalcedony, fluorite, and quartz. Early calcite contained variable trace amounts of Mg (up to 1.3 wt. %). Intermediate minerals consist of mainly calcite, often in bladed habits, with minor opal, and chalcedony and quartz. These minerals contain no diagnostic trace element variations. The latest secondary minerals deposited across the site consist of sparry calcite and minor intergrown opal. This sparry calcite exhibits fine (~ 50 u.m) Mg-enriched and depleted growth zones and is chemically distinct from all other calcite. Mg-enriched growth-zoned sparry calcite (MGSC) contains up to ~ 1.0 wt. % Mg and has been identified in > 65 % of the samples collected from across the site. MGSC and associated opal are always the paragenetically youngest minerals; where MGSC is not present, young secondary minerals did not precipitate. Calcite exhibits ranges for 513C from -8.5 %o to 9.5 %o, and for 818O from 5.2 %o to 22.1 %o. Samples exhibit generally consistent trends of decreasing C and increasing O isotopic compositions from paragenetically older to younger calcite. C and O isotope signatures for MGSC are between 16 %o and 20 %o for 618O and -3 %o and -8.5 %o for 513C. However, signatures for the various stages are not unique and are not diagnostic in correlating secondary mineral stages across the site. Early calcite is generally more luminescent than later calcite, but luminescence was not sufficiently consistent to aid in constraining the paragenetic sequence. LA-ICP-MS analyses indicate that higher levels of U, Th, and Sr are locally present in MGSC compared to paragenetically early calcite, however, this variation is not present in all samples. An important observation is that 90% of primary and secondary open space in the tuffs at Yucca Mountain contains no secondary mineral record. Where secondary minerals are present, the older secondary mineral record is heterogeneous across the site. However, MGSC, which forms the youngest part of the secondary mineral record, is present in a majority of samples and exhibits a more homogeneous distribution across the site. Secondary mineral abundances and textures indicate that secondary minerals precipitated in a vadose environment. The observed features are not consistent with secondary mineral precipitation in a phreatic environment saturated with aqueous fluids. Growth zoning in the outermost MGSC is consistent with formation from discontinuous influx of small fluid volumes with variable Mg content from surficial fluids that percolated downwards. Fluctuations in the Mg content in MGSC may be related to climate changes that occurred in the last few million years.


Calcite; Fluid inclusions; Geochemistry; Groundwater; Hydrothermal deposits; Nevada – Yucca Mountain; Paragenesis


Geochemistry | Hydrology | Mineral Physics




Document No.: TR-02-005,l
Document Revision: 0
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