cavity-based secondary mineralization in volcanic tuffs of yucca mountain, nevada: a new type of the polymineral vadose speleothem, or a hydrothermal deposit?

Abstract

Secondary minerals (calcite, chalcedony, quartz, opal, fluorite, heulandite, strontianite) residing in open cavities in the Miocenerhyolite tuffs of Yucca Mountain, Nevada have been interpreted by some researchers as "speleothemic" formations, deposited as aresult of downward infiltration of meteoric waters (DOE, 2001, Whelan et al., 2002). The major mineral of the paragenesis, calcite,shows spectacular trend of the textural and crystal morphology change: from anhedral granular occurrences, through (optional)platelet, bladed and scepter varieties, to euhedral blocky morphologies. The trend is consistent with the overall decrease in thesupesaturation of the mineral forming solution. Stable isotope properties of calcite evolve from 13C-enriched (δ13C = +4 to +9 ‰ PDB)at early stages of growth to 13C-depleted (-5 to -10 ‰) at late stages. The non-cyclic character of the isotope record and extremevariations of isotopic values argue against the meteoric origin of mineral forming fluids. The δ13C >4 ‰ PDB require isotope partitioningbetween dissolved CO2 and CH4, which is only possible in reducing anoxic environment, but not in aerated vadose zone.Fluid inclusions studied in calcite, quartz and fluorite revealed that the minerals were deposited from thermal solutions. Thetemperatures were higher at early stages of mineral growth (60 to 85oC) and declined with time. Most late-stage calcites containonly all-liquid inclusions, suggesting temperatures less than ca. 35-50oC. Minerals collected close to the major fault show the highesttemperatures. Gases trapped in fluid inclusions are dominated by CO2 and CH4; Raman spectrometry results suggest the presenceof aromatic/cyclic hydrocarbon gases. The gas chemistry, thus, also indicates reduced (anoxic) character of the mineral formingfluids.Secondary minerals at Yucca Mountain have likely formed during the short-term invasion(s) of the deep-seated aqueous fluidsinto the vadose zone. Following the invasion, fluids, initially equilibrated with the deep (reduced, anoxic) environment, evolvedtoward equilibrium with the new environment (cooling, degassing, mixing with shallow oxidizing waters, etc.). While some featuresof mineralization are compatible with the "speleothemic" or "meteoric infi ltration" model, most of the evidence does not lend itself torational explanation within this model.