Pyritic ash-flow tuff, Yucca Mountain, Nevada -- A discussion

Technical Report · Sat Dec 31 04:00:00 EST 1994

DOI:https://doi.org/10.2172/240931· OSTI ID:240931

Weiss, S. I.; Larson, L. T.; Noble, D. C. ^[1]

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1. Univ. of Nevada, Reno, NV (United States)

Textural and mineralogic evidence exists for at least one episode of widespread hydrothermal alteration of volcanic rocks deep in Yucca Mountain, Nevada. Despite this evidence, Castor et al. infer that most of the pyrite found in tuffs at Yucca Mountain was introduced as ejecta (lithic fragments) incorporated during the eruptions of the tuffs, rather than by in-situ hydrothermal activity. Their conclusions appear to be based on their observation that most of the pyrite resides in unaltered to variably altered and veined lithic fragments, whereas pyrite-bearing veins are absent in the tuff matrix, titanomagnetite and mafic phenocrysts in the matrix are generally not replaced by pyrite, and feldspar phenocrysts in the pyritic tuff matrix are generally unaltered. Castor et al. dismiss the much smaller quantities of pyrite disseminated in the tuff matrix, including relatively rare pyritized hornblende and biotite grains, as xenolithic as well. The pyritic tuffs belong to large-volume, subalkaline rhyolite ash-flow units (ca. > 150 to 250 km{sup 3} each). The interpretation of Castor et al. has broad implications for the temperature, fO{sub 2} and fS{sub 2} of major ash flow eruptions. Pyrite origin also bears on the nature of past fluid flow and water-rock reactions at Yucca Mountain, which in turn are important factors in assessing the potential for currently undiscovered mineral resources in the area of the proposed nuclear waste repository. We have studied core and cuttings from the same drill holes studied by Castor et al., as well as other drill holes. It is our contention that the inconsistent lateral and stratigraphic distribution of the pyrite, textural features of the pyrite, and phase stability considerations are incompatible with the {open_quotes}lithic{close_quotes} origin of Castor et al., and are more reasonably explained by in-situ formation from hydrothermal fluids containing low, but geochemically significant, concentrations of reduced sulfur.

• Research Organization: Nevada Univ., Reno, NV (United States). Center for Neotectonic Studies
• OSTI ID: 240931
• Report Number(s): DOE/NV/10461--T46; ON: DE96006638
• Country of Publication: United States
• Language: English

Related Subjects

05 NUCLEAR FUELS
FLUID FLOW
GEOCHEMISTRY
HIGH-LEVEL RADIOACTIVE WASTES
HORNBLENDE
HYDROTHERMAL ALTERATION
NEVADA
ORIGIN
PROGRESS REPORT
PYRITE
SITE CHARACTERIZATION
TUFF
UNDERGROUND DISPOSAL
VOLCANIC ROCKS
YUCCA MOUNTAIN