Origin, timing, and temperature of secondary calcite-silica mineral formation at Yucca Mountain, Nevada

摘要

The origin of secondary calcite-silica minerals in primary and secondary porosity of the host Miocene tuffs at Yucca Mountain has been hotly debated during the last decade. Proponents of a high-level nuclear waste repository beneath Yucca Mountain have interpreted the secondary minerals to have formed from cool, descending meteoric fluids in the vadose zone; critics, citing the presence of two-phase fluid inclusions, argued that the minerals could only have formed in the phreatic zone from ascending hydrothermal fluids. Understanding the origin, temperature, and timing of these minerals is critical in characterizing geologically recent fluid flux at the site, and has significant implications to whether waste should be stored at Yucca Mountain. Petrographic and paragenetic studies of 155 samples collected from the Exploratory Studies Facility (ESF) and repository block cross drift (ECRB) tunnels indicate that heterogeneously distributed calcite with lesser chalcedony, quartz, opal, and fluorite comprise the oldest secondary minerals. These are typically overgrown by intermediate-aged calcite, often exhibiting distinctive bladed habits. The youngest event recorded across the site is the deposition of Mg-enriched (up to similar to1 wt%) and depleted, growth-zoned calcite intergrown with U-enriched opal. The cyclical variation in Mg enrichment and depletion is probably related to climate changes that have occurred during the last few million years. The distribution of secondary minerals is consistent with precipitation in the vadose zone. Fluid inclusion petrography of sections from the 155 samples determined that 96% of the fluid inclusion assemblages (FIAs) contained liquid-only inclusions that formed at ambient temperatures (< 35degreesC). However, 50% of the samples (n = 78) contained relatively rare FIA that contain both liquid-only and liquid plus vapor inclusions (herein termed two-phase FIAs) that formed at temperatures above 35degreesC. Virtually all of these two-phase FIAs occur in paragenetically old calcite; rare two-phase inclusion assemblages were also observed in early fluorite and quartz, and early-intermediate calcite. Homogenization temperatures (drop trapping temperatures) across Yucca Mountain are generally 45 to 60degreesC, but higher temperatures reaching 83degreesC were recorded in calcite from the north portal and ramp of the ESF. Cooler temperatures of similar to35 to 45degreesC were recorded in the intensely fractured zone. Multiple populations of two-phase FIAs from lithophysal cavities in the ESF and ECRB cross drift indicate early fluid cooling with time from temperatures > 45degreesC in early calcite, to < 35 to 45degreesC in paragenetically younger calcite. Freezing point depressions range from -0.2 to - 1.6degreesC, indicating trapping of a low salinity fluid. The majority of intermediate calcite and all outermost Mg-enriched calcite contains rare all-liquid inclusions and formed from ambient temperature (< 35degreesC) fluids. Carbon and oxygen isotope data reveal a consistent trend of decreasing delta(13)C (from 9.5 to -8.5parts per thousand) and increasing delta(18)O (from 5.2 to 22.1parts per thousand) values from paragenetically early calcite to Mg-enriched growth-zoned calcite. Depleted deltaD values (-131 to -90parts per thousand) of inclusion fluids from intermediate and the youngest Mg-enriched calcite indicate derivation from surface meteoric fluids. Recalculation of delta(18)O(H2O) values of -12 to -10parts per thousand is consistent with derivation from paleometeoric fluids. Results of integrated U-Pb dating (opal and chalcedony) and fluid inclusion microthermometry indicate that two-phase FIAs that trapped fluids of > 50degreesC are older than 6.29 +/- 0.30 Ma. Two-phase FIAs in paragenetically later calcite, which formed from fluids of 35 to 45degreesC, are older than 5.32 +/- 0.02 Ma. There is no evidence for trapping of fluids with elevated temperatures during the past 5.32 my. The yo