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Title: Mineralogy and petrology of tuff units from a UE25a-1 drill site, Yucca Mountain, Nevada

Abstract

Drill hole UE25a-1 has penetrated tuffs of Tertiary age which contain two major zeolitized horizons at depths below 380 m. These horizons are restricted to low-density, high-porosity nonwelded tuffs below the basal vitrophyre of the Topopah Springs Member of the Paintbrush Tuff (approximately 70 m above the current water table), and interfinger with more-densely-welded devitrified tuffs of granophyric mineralogy. Zeolites occur as glass pyroclast replacement, vug linings, and fracture fillings. Nonwelded units above the welded portion of the Topopah Springs Member are essentially unaltered, indicating that they have never been ground water-saturated for any significant length of time. Zeolite mineral assemblages appear to be characteristic of low temperature (<100{sup 0}C) ground water alteration of glass in an open hydrologic system. The principal zeolite phase is high-Si clinoptilolite with Si/Al ratios of 4.7 to 6.0. Ca tends to be the dominant large-radius cation, but grains with dominant K or Na are not uncommon, particularly with increasing depth. Compositional variations in clinoptilolite may be due to ground water composition or original pyroclast composition. Minor amounts of mordenite, characterized by lower silica content (<55 wt %) and high alkali content (>10 wt % Na{sub 2}O + K{sub 2}O), occur as vug fillings atmore » depths below 500 m. Presence of mordenite may indicate slightly elevated alteration temperatures, but more likely reflects enrichment of ground water in alkalis with depth. Mineralogical, compositional, and textural similarities of the zeolitized tuffs from UE25a-1 and J-13 are compatible with a single episode of crystallization. 16 figures.« less

Authors:
; ;
Publication Date:
Research Org.:
Los Alamos Scientific Lab., NM (United States)
OSTI Identifier:
59108
Report Number(s):
LA-8139-MS
DOE Contract Number:
W-7405-ENG-36
Resource Type:
Technical Report
Resource Relation:
Other Information: PBD: Nov 1979
Country of Publication:
United States
Language:
English
Subject:
58 GEOSCIENCES; 05 NUCLEAR FUELS; TUFF; MINERALOGY; NEVADA TEST SITE; ZEOLITES; PETROLOGY; BOREHOLES; CLINOPTILOLITE; GROUND WATER; Yucca Mountain Project

Citation Formats

Sykes, M.L., Heiken, G.H., and Smyth, J.R. Mineralogy and petrology of tuff units from a UE25a-1 drill site, Yucca Mountain, Nevada. United States: N. p., 1979. Web. doi:10.2172/59108.
Sykes, M.L., Heiken, G.H., & Smyth, J.R. Mineralogy and petrology of tuff units from a UE25a-1 drill site, Yucca Mountain, Nevada. United States. doi:10.2172/59108.
Sykes, M.L., Heiken, G.H., and Smyth, J.R. 1979. "Mineralogy and petrology of tuff units from a UE25a-1 drill site, Yucca Mountain, Nevada". United States. doi:10.2172/59108. https://www.osti.gov/servlets/purl/59108.
@article{osti_59108,
title = {Mineralogy and petrology of tuff units from a UE25a-1 drill site, Yucca Mountain, Nevada},
author = {Sykes, M.L. and Heiken, G.H. and Smyth, J.R.},
abstractNote = {Drill hole UE25a-1 has penetrated tuffs of Tertiary age which contain two major zeolitized horizons at depths below 380 m. These horizons are restricted to low-density, high-porosity nonwelded tuffs below the basal vitrophyre of the Topopah Springs Member of the Paintbrush Tuff (approximately 70 m above the current water table), and interfinger with more-densely-welded devitrified tuffs of granophyric mineralogy. Zeolites occur as glass pyroclast replacement, vug linings, and fracture fillings. Nonwelded units above the welded portion of the Topopah Springs Member are essentially unaltered, indicating that they have never been ground water-saturated for any significant length of time. Zeolite mineral assemblages appear to be characteristic of low temperature (<100{sup 0}C) ground water alteration of glass in an open hydrologic system. The principal zeolite phase is high-Si clinoptilolite with Si/Al ratios of 4.7 to 6.0. Ca tends to be the dominant large-radius cation, but grains with dominant K or Na are not uncommon, particularly with increasing depth. Compositional variations in clinoptilolite may be due to ground water composition or original pyroclast composition. Minor amounts of mordenite, characterized by lower silica content (<55 wt %) and high alkali content (>10 wt % Na{sub 2}O + K{sub 2}O), occur as vug fillings at depths below 500 m. Presence of mordenite may indicate slightly elevated alteration temperatures, but more likely reflects enrichment of ground water in alkalis with depth. Mineralogical, compositional, and textural similarities of the zeolitized tuffs from UE25a-1 and J-13 are compatible with a single episode of crystallization. 16 figures.},
doi = {10.2172/59108},
journal = {},
number = ,
volume = ,
place = {United States},
year = 1979,
month =
}

Technical Report:

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  • The Topopah Spring Member of the Paintbrush Tuff and the Lithic-rich tuff and two Tertiary volcanic units that occur in cores from drill holes UE25a-1 and USW-G1 at Yucca Mountain, Nevada. Recently they have been suggested as possibly suitable for the permanent storage of high-level radioactive waste. Earlier petrologic characterization of these units is augmented here. The Topopah Spring Member (approximately 350 m thick) has two compound cooling units. The upper, thinner unit is densely welded to vitrophyric. The lower unit ranges from nonwelded to vitrophyric, and its nonwelded base is extensively zeolitized to clinoptilolite and mordenite. Heulandite occurs asmore » fracture fill in the overlying vitrophyric part, but zeolites are absent above that vitrophyre. Here primary devitrification plus vapor-phase crystallization dominate the mineralogy. Vapor-phase effects are especially prominent between the two vitrophyres in both cores and include numerous large lithophysal cavities throughout most of this moderately to densely welded tuff. The Lithic-rich tuff extends from 1203 to 1506 m in the USW-G1 drill core. It is nonwelded to partly welded but is well indurated due to pervasive intergrowths of authigenic minerals. These phases are analcime, albite, alkali feldspar, sericite, chlorite and quartz. The transition from analcime to secondary albite corresponds to Iijima`s zeolite Zone IV boundary, and this boundary appears in USW-G1 at 1326 m. However, analcime remains as a prominent phase through most of the Lithic-rich tuff. Further work is necessary to assess the suitability of either of these horizons for a waste repository. In the Topopah Spring Member, both mechanical and hydrologic properties of thick lithophysal zone must be studied, as well as the complete sequence of fracture fill. For both units, zeolite and clay mineral stabilities need to be investigated.« less
  • Thermal data from two sites about 20 km apart in the Nevada Test Site indicate that heat flow both within and below the upper 800 meters is affected significantly by hydrothermal convection. For hole UE25a-1, Yucca Mountain, the apparent heat flow above the water table ({similar_to}470 m) is 54 mWm{sup -2} ({similar_to}1.3 HFU). Below the water table, the temperature profile indicates both upward and downward water movement within the hole and possibly within the formation. Hole UE25a-3, Calico Mountain, is characterized by conductive heat flux averaging 135 mWm{sup -2} ({similar_to}3.2 HFU) to a depth of about 700 meters below whichmore » water appears to be moving downward at the rate of nearly 1 ft y{sup -1} (255 mm y{sup -1}). Between 735 and 750 meters, the hole intersected a nearly vertical fault along which water seems to be moving vertically downward. The nearly threefold variation in conductive heat flow over a lateral distance of only 20 km suggests the presence of a more deeply seated hydrothremal convective system with a net upward flow beneath Calico Hills and a net downward flow beneath Yucca Mountain.« less
  • The J-13 drill hole, located in Jackass Flats, Nevada Test Site, has penetrated 125 m of alluvium and 932 m of tuff. Most of the tuff deposits consist of welded tuffs; glass phases in the tuffs have been replaced by authigenic minerals, mainly K-feldspar, silica, and zeolites. The zonation of authigenic minerals, with depth, indictes that alteration of glass phases and filling of vugs occurred during welding and compaction of tuff units soon after deposition and by interaction with groundwater. Zonation of authigenic minerals in tuff deposits at Jackass Flats is similar to mineral zonation in tuffs elsewhere at themore » Nevada Test Site and in tuff deposits of west Texas. All appear to have been developed by leaching of glass phases and deposition of authigenic minerals in open hydrologic systems. 10 figures, 38 tables.« less
  • A subsurface geologic study in connection with the Nevada Nuclear Waste Storage Investigations has furnished detailed stratigraphic and structural information about tuffs underlying northeastern Yucca Mountain on the Nevada Test Site. Drill hole UE25a-1 penetrated thick sequences of nonwelded to densely welded ash-flow and bedded tuffs of Tertiary age. Stratigraphic units that were identified from the drill-hole data include the Tiva Canyon and Topopah Spring Members of the Paintbrush Tuff, tuffaceous beds of Calico Hills, and the Prow Pass and Bullfrog Members of the Crater Flat Tuff. Structural analysis of the core indicated densely welded zones to be highly fractured.more » Many fractures show near-vertical inclinations and are commonly coated with secondary silica, manganese and iron oxides, and calcite. Five falt zones were recognized, most of which occurred in the Topopah Spring Member. Shear fractures commonly show oblique-slip movement and some suggest a sizable component of lateral compression. Graphic logs are included that show the correlation of lithology, structural properties, and geophysical logs. Many rock units have characteristic log responses but highly fractured zones, occurring principally in the Tiva Canyon and Topopah Spring Members restricted log coverage to the lower half of the drill hole.« less
  • Magnetic susceptibility measurements have been shown to be dependent upon the magnetite content of rocks with variations in rock susceptibility arising from changes in the shape, size, composition, and quantity of the contained magnetite grains. The present study was undertaken to determine the factor(s) responsible for the variation in magnetic susceptibility measurements from borehole UE25a-5 on the Nevada Test Site (NTS). The well logs and sample analyses presented in this paper form part of a larger geophysical well-logging project studying the physical properties of welded tuffs at NTS. The ash-flow sheets at NTS appear to be the products of singlemore » compositionally zoned magmas that tend, within a cooling unit, to erupt hotter, more mafic, and more crystal-rich with time. These factors, however, have little effect on the degree to which the tuffs become welded. Furthermore, zones of crystallization and alteration are superimposed upon the welded units. X-ray data show poor correspondence between the relative abundance of magnetite in a sample and the borehole magnetic susceptibility measurement associated with it. Curie balance experiments demonstrate no change in the magnetic mineralogy that could account for the susceptibility variation. Thin-section observations corroborate the x-ray data, but indicate a proportional relationship between the borehole susceptibility measurements and the grain-size distribution of magnetite. The association of magnetic susceptibility anomalies with the crystal-rich zones of the welded tuffs will aid in the identification and correlation of the eruptive sequences at NTS.« less