Evaluation of Past and Future Alterations in Tuff at Yucca Mountain , Nevada ,

The tufts at Yucca Mountain in south-central Nevada are being studied by the Yucca Mountain Project (YMP) to determine their suitability for a high-level radioactive waste repository. For predictive purposes, it Is important to understand the alteration history of Yucca Mountain and to know how the minerals in Yucca Mountain tufts respond to changing conditions such as elevated temperatures. The clay mineralogy of these tuffs has been examined using xray powder diffraction, and approximate temperatures of alteration have been determined using available clay mineral data and fluid inclusion analyses. Also, several illites from drill holes USW G-1 and G-2 have bean dated using KlAr techniques, yielding ages of about 11 Myr. The clay minerals In Yucca Mountain tufts are predominantly interstratifled illite/smectites, with minor amounts of chlorite, kaolinite, and nterstratifled chlorite/smectite at depth In USW G-1 and G-2. The reactions observed for these illite/smectites are similar to those observed in pelitle rocks. With depth, the illitesmectites transform from random interstratiflcations (R=0) through ordered intermediates (R=1) to illite in USW G-2 and to Kalkberg (R?3) interstratifications in USW G-1. The illite/smectites in USW G-3 have not significantly transformed. It appears that the illites in deeper rocks result from hydrothermal and diagenetic reactions of earlier-formed smectites. These data demonstrate that the rocks at depth in the northern end of Yucca Mountain were significantly altered about 11 Myr ago. Both clay mineralogy and fluid inclusions suggest that the rocks at depth in USW G-2 have been subjected to postdepositional temperatures of at least 275°C, those in USW G-1 have reached 2000C, and USW G-3 rocks probably have not exceeded 1 OOC. The temperature estimates from clay mineral and fluid inclusion data suggest stability limits for several minerals at Yucca Mountain. Clinoptilolite became unstable at about 1 00C, mordenite was not a majorphase above 1300C, and analcime transformed to albite above 1750 to 200-C. Ippears that cristobalite transformed to quartz at about 90' to 100'0 in USW G-2 but mustsave reacteO alwnshmbw tner~i-Iw eatures in USW G-3. Comparison of the clay mineral data with the bulk-rock mineralogy shows that the reactions seen with Increasing depth are coupled. Clinoptilolite and cristobalite disappear approximately simultaneously in USW G-2 and G-3, supporting aqueous silica activity as the controlling variable in the clinoptilolite-toanalcime reaction. The rtann nf rlinnpiblitA to anaLc ad h the appearance of calcite, chlorite, and interstratifled chlorite/smectite. Breakdown of cliroptin e p provide e Source ofor calcte 3 for chlorite, K or ite/smectites found deeper in the secio--nd Na foranalcime and albite. _ verticl distribution of minerals across Yucca Mountain demonstrates that the alteration at depth is most profound to the north in USW G-2, and there is little evidence for elevated-temperature alteration in USW G-3. Drill cores USW G-1 and UE-25a#1 show the