MINERALOGICAL AND ISOTOPIC ANALYSIS OF FRACTURE-COATING AND ALTERATION MINERALS IN NONWELDED TUFFS OF YUCCA FLAT, NEVADA

Yucca Flat was the location of 661 underground nuclear tests conducted by the United States between 1957 and 1992. Most of these tests were conducted within low-permeability, Miocene-age, non-welded tuffs (the Tuff Confining Unit, or TCU) or within overlying alluvial and tuff aquifers. Extensive secondary alteration of the TCU provides a potential barrier to contaminant transport into the underlying regional carbonate aquifer. SEM-EDS, XRF, XRD, and isotope data are used here to characterize the abundance and distribution of fracture-coating and alteration minerals in the TCU. Textures indicate that alteration minerals resulted from the higher temperature processes of devitrification and vapor-phase crystallization (alkali feldspar, quartz, and cristobalite), and lower temperature diagenesis (clinoptilolite, smectite, mordenite, calcite, analcime, and Mn-, Fe-oxides, in decreasing order of abundance), but not from hydrothermal alteration. Oxygen isotope analyses of fracture-lining calcite suggest depositional temperatures < 90oC. Whole rock oxygen and strontium isotope analyses of the TCU are consistent with pervasive, low-temperature alteration and remobilization of high solubility elements. Calcite and analcime are confined to fractures, whereas clinoptilolite, mordenite, smectite, and oxides are found within the tuff matrix as well as coating fractures. Mineral textures suggest a dominant paragenesis of cristobalite + feldspar, clinoptilolite, mordenite, smectite, oxides, although numerous exceptions exist. Secondary minerals in Yucca Flat are zoned with respect to depth, with an upper clinoptilolite-dominant zone, a lower mixed mineral zone, and a basal argillic zone. Correlation between secondary mineral assemblages and volcanic stratigraphy suggest that welding, reworking, and chemical composition of the tuff all exert some control on the distribution of certain alteration minerals. Partly to moderately welded tuffs typically are low in smectite, whereas bedded tuffs typically are dominated by clinoptilolite. The basal argillic zone is composed predominantly of smectite.