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DISCLAIMERThin raprrrt wm prapamd an mr qxmunl O( work yrrwrsord by an n~oncy of the Unhed SIalU (Jovarnmant, Nalthwr Iha (Mcd SImaa(iovammenl nor qny agency tharcof, nor qny of Ibdr amploycan, mak-qry wnrmnly, enpraauor Impllod, or assumcamry IeSnl Ilablllty or ralpomlbllity for Iha mxurmy, m,mplatan~, or UAIIIW of any Infnrmnllon, qppwrstun, prxhrct, or _ dlmdmrl, or raprmantn that itn uw would noi Irfrhrl= prlvntaly owned rights. Rafar. anca harahr 10 nrry mpdllc commarclol IWWIuct, prrrocM,or Mrvlca by trada nmna, tmdammrk, mnnu~nciuror, wr othcrwim rloan n~naccsmrlly urrrntimta or Imply its andomamant, mawrI. This study evaluates the susceptibility of an underlying 50 ft-thick vltrophyre to thermal alteration by examining alteration that occurred in the rock as it cooled after deposition.A 10"C temperature rise should have no mineralogical effects on the vitrophyre, but an increase of 60°or more is likely~to result in alteration.Expected mineralogic changes in the vitrophyre caused by this amoun+ of thelmal loading include crystallization of zeolites and smectite.Alteration will be concentrated in a thin interval near the top of the vltrophyrt and along fractures.Adsorbed water and water in preexisting hydrous minerals and in glass may contribute to hydrothermal alteration of underlying vitrophyre. Bulk porosity change would be slight and local porosity incr~sase would probably be restricted to the upper part of the vitroph,yre. Although some fracture filling could occur, such a minor sealing effect would be balanced by development of secondary porosity.Zeolites and smectite, newly-crystallized along fluid flow paths below the waste repository, could provide an enhanced sorptive barrier to radionuclide migration.4.,

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By mid-1995, the Exploratory Studies Facility (ESF) extended about 1.1 km from Exile H i l l westward toward Yucca Mountain, mostly within densely welded, devitrified Tiva Canyon Tuff. Secondary mineral occurrences in this unit include breccia cements of mordenite, a fibrous zeolite, and vapor-phase deposits of silica, alkali feldspar, apatite, hollandite, amphibole, and zircon. Calcite is also a common secondary mineral in faults and fractures. Studies of water and gas contents in fluid inclusions in calcites from a fault in nonwelded hdf and a fracture in densely welded tuff suggest mineral deposition under transient locally saturated conditions. Calcite in the nonwelded tuff incorporated air from the unsaturated tuff adjacent to the fault.