PALEOGEOTHERMAL AND PALEOHYDROLOGIC CONDITIONS IN SILICIC TUFF FROM YUCCA MOUNTAIN, NEVADA

The clay mineralogy of tuffs from Yucca Mountain, Nevada, the potentia l 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 a nd clay-mineral separates from three drill holes at Yucca Mountain (US W G-1, USW G-2, and USW GU-3/G-3) were studied using X-ray powder diff raction, and supporting temperature information was obtained using flu id inclusion data from calcite. Twelve K/Ar dates were obtained on ill ite/smectite (I/S) separated from the tuffs from the two northernmost drill holes, USW G-1 and G-2. The predominant clay minerals in the Yuc ca Mountain tuffs are interstratified I/S, with minor amounts of chlor ite and interstratified 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 g reater-than-or-equal-to 3 interstratifications to illite in USW G-2 an d to R greater-than-or-equal-to 3 I/S in USW G-1. 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 dep th in the northern portion of Yucca Mountain were altered 10.0-11 my a go, soon after creation of the Timber Mountain caldera to the north. B oth I/S geothermometry and fluid inclusion data suggest that the rocks at depth in USW G-2 were subjected to postdepositional temperatures o f at least 275-degrees-C, those in USW G-1 reached 200-degrees-C, and rocks from USW GU-3/G-3 probably did not exceed 100-degrees-C. These d ata suggest that no significant hydrothermal alteration has occurred s ince Timber Mountain time, approximately 10.7 my ago. Estimates of the temperature of formation of illite/smectites yield probable stability limits for several minerals at Yucca Mountain. Clinoptilolite apparen tly became unstable at about 100-degrees-C, mordenite was not a major phase above 130-degrees-C, and analcime transformed to albite above 17 5-degrees-200-degrees-C. It appears that cristobalite transformed to q uartz at 90-degrees-100-degrees-C 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 clinoptiloli te and cristobalite disappear approximately simultaneously, supporting aqueous silica activity as a controlling variable in the clinoptiloli te-to-analcime reaction. The reaction of clinoptilolite to analcime al so coincides with the appearance of calcite, chlorite, and interstrati fied 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 c hlorite, K for the I/S found deeper in the section, and Na for analcim e and albite. Using the rocks in USW G-1, G-2, and GU/G-3 as natural a nalogs to repository-induced thermal alteration suggests that the bulk of the clinoptilolite- and mordenite-bearing rocks in Yucca Mountain will not react to less sorptive phases such as analcime over the requi red lifetime of the potential repository. The zeolites in zeolite inte rval I, directly underlying the proposed repository horizon, may trans form at the predicted repository temperatures. However, the reaction o f clinoptilolite to analcime in interval I may require the transformat ion of all of the abundant opal-CT and glass to quartz, an unlikely sc enario considering the unsaturated nature of these rocks and the predi cted temperatures of < 100-degrees-C.