CLAY AND ZEOLITE RECORDS OF NATURAL GEOCHEMICAL PROCESSES AT YUCCA MOUNTAIN, NEVADA

Strata at Yucca Mountain consist of rhyolitic to latitic ash flows and bedded tuffs. In the thick unsaturated zone (~500-600 m), rhyolitic compositions predominate, with latitic compositions restricted to a zone ~50 m thick and ~150 m above the proposed repository horizon. Underlying strata down through the repository horizon and extending below the water table are relatively uniform in rhyolitic composition, with formation of thick zeolitized horizons from initially vitric rhyolites near the water table. The latitic zone has a distinct geochemical character (lower Si/Al and Rb/Sr ratios; higher Fe, Mg, Ti, Zr, Ba; higher light lanthanide content and little or no Eu anomaly) compared with the rhyolites. Clays (dominantly smectites) within the latitic horizon inherit most of this latitic geochemical character, whereas smectites in rhyolitic units inherit rhyolitic characteristics. These differences point to in situ clay formation, predominantly by dissolution of local latitic or rhyolitic glass. The clays can have either positive or negative Ce anomalies, in contrast to other alteration minerals that have no Ce anomalies (zeolites) or only negative Ce anomalies (calcites). The development of Ce anomalies provides information on whether alteration took place with or without influence from Ce-accumulating Mn-oxides. Although modeling, 36Cl data, and field experiments indicate that nonwelded portions of the latitic horizon play a significant role in accumulating and moderating recharge, the alteration in this zone is only partial, dominated by clays rather than zeolites, and bears evidence of dissolution leaching (e.g., Na depletion in clays relative to source glass). In contrast, the thick zeolitized horizons below the repository horizon represent complete and early alteration of glass within ~100 m of the present water table, with little geochemical exchange and minor clay formation. After essentially isochemical formation, this zeolitized zone has been variably overprinted by cation-exchange profiles. The record of zeolite and clay alteration indicates that the chemical composition of recharge waters is affected by glass dissolution in the vitric latitic horizons; these groundwaters are then modified by cation exchange in zeolitized horizons below the repository horizon.