Distribution and Chemistry of Diagenetic Minerals at Yucca Mountain, Nye County, Nevada

Abstract

Abstract--Yucca Mountain is being studied as a potential site in southern Nevada for an underground, high-level nuclear waste repository. A major consideration for selecting this site is the presence of abundant zeolites in Miocene ash-flow tufts underlying the region. Beneath Yucca Mountain four diagenetic mineral zones have been recognized that become progressively less hydrous with depth.Zone I, the shallowest zone, is 375-584 m thick in the central part of Yucca Mountain, but 170 m thick to the north. Zone I contains vitric tufts that consist of uualtered volcanic glass and minor smectite, opal, heulandite, and Ca-rich clinoptilolite. Zone II thins south to north from 700 to 480 m and is characterized by complete replacement of volcanic glass by clinoptilolite with and without mordenite, and by lesser amounts of opal, K-feldspar, quartz, and smectite. Zone III thins south to north from 400 to 98 m thick and consists of analcime, K-feldspar, quartz, and minor calcite and smectite. Heulandite occurs locally at the top of zone III in the eastern part of Yucca Mountain. Zone IV occurs in the deepest structural levels of the volcanic pile and is characterized by albite, K-feldspar, quartz, and minor calcite and smectite.Clinoptilolite and heulandites in zone I have uniform Ca-rich compositions (60-90 mole % Ca) and Si:AI ratios that are mainly between 4.0 and 4.6. In contrast, clinoptilolites deeper in the volcanic sequence have highly variable compositions that vary vertically and laterally. Deeper clinoptilolites in the eastern part of Yucca Mountain are calcic-potassic and tend to become more calcium-rich with depth. Clinoptilolites at equivalent stratigraphic levels on the western side of Yucca Mountain have sodic-potassic compositions and tend to become more sodium-rich with depth. Despite their differences in exchangeable cation compositions these two deeper compositional suites have similar Si:A1 ratios, generally between 4.4 and 5.0. Analcimes have nearly pure end-member compositions, typical of these minerals formed by diagenetic alteration of siliceous volcanic glass; however, K-feldspars are Si-rich compared to the ideal feldspar formula.Bulk-rock contents of Si, Na, K, Ca, and Mg of zeolitic tufts generally differ significantly from stratigraphically equivalent vitric tufts, suggesting that zeolite diagenesis took place in an open chemical system. Both the whole rock and the clinoptilolite are relatively rich in Ca and Mg in the eastern part of Yucca Mountain and rich in Na in the western part. The Ca-and Mg-rich compositions of the zeolitized tufts in the eastern part of Yucca Mountain may be due to cation exchange by the sorptive minerals with ground water partially derived from underlying Paleozoic carbonate aquifers.Diagenetic zones become thinner and occur at stratigraphically higher levels from south to north across Yucca Mountain, probably due to a higher geothermal gradient in the northern part of the area. The diagenetic zones were established when the geothermal gradient was greater than it ...

Geology in the Vicinity of the TYBO and BENHAM Underground Nuclear Tests, Pahute Mesa, Nevada Test Site

“…This contrasts with the generally systematic, downwards increase in alteration intensity within Yucca Mountain (Broxton et al, 1987), which is a manifestation of regional rather than local control. Cross sections that represent alteration in the TYBO-BENHAM area indicate no trends in alteration that are recognizable as regional, even at the deepest levels penetrated (Figures 3-13 and 3-14).…”

“…These alteration zones appear to be controlled by proximity to local heat sources, such as overlying ash flows and lava flows, 4-2 thus alteration zones tend to follow lithologic zones in the TYBO-BENHAM area. The local correspondence of alteration to lithology at Pahute Mesa stands in striking contrast to a regional systematic downwards increase in alteration intensity seen within Yucca Mountain (Broxton et al, 1987). Nonwelded Ash-Flow Tuff: Grayish-red (10R 4/2); vitric; abundant white (N9) pumice up to 10 mm in size; minor felsic phenocrysts of feldspar and lesser quartz; minor mafic minerals of biotite and much less clinopyroxene; rare dusky-red (5R 3/4) lithic fragments up to 2 mm in diameter; conspicuous dark-yellowish-orange (10YR 6/6) glass shards.…”

“…Within the southwestern Nevada volcanic field, alteration initially produces zeolites in the progressive sequence, clinoptilolite, mordenite, and analcime (Broxton et al, 1987;Moncure et al, 1981). As alteration progresses, zeolites are replaced by a quartzo-feldspathic assemblage of quartz and secondary feldspar.…”

Geochemical and Isotopic Evaluation of Groundwater Movement in Corrective Action Unit 97: Yucca Flat/Climax Mine, Nevada Test Site, Nevada, Rev. No.: 0

“…Mineral dissolution and precipitation constraints were imposed in the models to limit the reactions to those that were considered thermodynamically plausible. Limiting reactions are based on mineral saturation indices and mineralogic studies from the NTS area that have described textural relations between primary and secondary minerals (e.g., Bish, 1989;Broxton et al, 1987).…”

A Natural Analogue for Thermal-Hydrological-Chemical Coupled Processes at the Proposed Nuclear Waste Repository at Yucca Mountain, Nevada

“…The presence of a thick zone of amorphous silica in the vicinity of the intrusion is intriguing; however, the origin of the opal is highly ambiguous, since it formed parallel to bedding within the tuff and subparallel to the basal contact of the sill. The host tuff in this area is part of the base of the Paintbrush Group or the top of the Calico Hills Formation; the latter formation has been found to have extensive diagenetic alteration to zeolites and amorphous silica (including opal) in other localities (Broxton et al, 1987) due to variable exposure to groundwater in the intervening 12.9 Ma since it was deposited. The presence of minor amounts of the zeolite clinoptilolite in both glassy vitrophyre and completely devitrified tuff, located equal distances beneath a sill, suggests that the zeolitization occurred prior to intrusion and was not related to devitrification and alteration associated with the contact metamorphic event .…”