Paleogeothermal and Paleohydrologic Conditions in Silicic Tuff from Yucca Mountain, Nevada

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Abstract
--The clay mineralogy of tufts from Yucca Mountain, Nevada, the potential site of the nation's first high-level radioactive waste repository, has been studied in order to understand the alteration history of the rocks and,to predict potential future alterations. Bulk-rock samples,and,clay-mineral separates from three drill holes at Yucca Mountain (USW G-l, USW G-2, and USW GU-3/G-3) were studied using X-ray powder diffraction, and supporting temperature information was obtained using fluid inclusion data from,calcite. Twelve K/Ar dates were obtained,on illite/smectite (US) separated,from,the tufts from the two northernmost drill holes, USW G-I and G-2. The predominant clay minerals in the Yucca Mountain tufts are interstratified I/S, with minor amounts of chlorite and interstratitied chlorite/smectite. The I/S reactions observed,as a function,of depth,are similar to those observed,for pelitic rocks; I/S transforms,from,R = 0 interstratifications through,R = 1 and R -> 3 interstratifications to illite in USW G-2 and,to R > 3 I/S in USW G-I. The R = 0 I/S clays in USW GU-3/G-3 have,not significantly transformed. K/Ar dates for the I/S samples,average,10.4 my. These data suggest that the rocks at depth in the northern portion of Yucca Mountain were altered 10.0-11 my ago, soon after creation of the Timber,Mountain,caldera to the north. Both I/S geothermometry,and,fluid inclusion,data suggest that the rocks at depth in USW G-2 were,subjected to postdepositional temperatures,of at least 275~ those in USW G- 1 reached,200~ and rocks from,USW GU-3/G-3 probably,did not exceed,100~ These data suggest that no significant hydrotbermal alteration has occurred since Timber Mountain time, ~ 10.7 my ago. Estimates of the temperature,of formation,of illite/smectites yield probable,stability limits for several minerals,at Yucca Mountain. Clinoptilolite apparently,became,unstable,at about,100~ mordenite,was not a major,phase above,130~ and analcime,transformed,to albite above,175"-2000C. It appears,that cristobalite transformed,to quartz at 90*--100~ in USW G-2 but must,have reacted at considerably,lower temperatures,(and for longer times) in USW GU-3/G-3. The reactions,with increasing depth,appear coupled, and clinoptilolite and cristobalite disappear approximately simultaneously, supporting aqueous silica activity as a controlling variable in the clinoptilolite-to-analcime reaction. The reaction of clinop- tilolite to analcime also coincides with the appearance of calcite, chlorite, and interstratified chlorite/ smectite. Although the hydrothermal fluids may have been a source for some cations, breakdown of clinoptilolite (and mordenite) probably provided the source of some of the Ca for calcite, Mg for chlorite,