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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.

Abstract: The Na2O–B2O3–SiO2–H2O system has been studied at 520 °C and 1.5 kbar in order to understand boron solubility and speciation under hydrothermal conditions at different sodium content. Within the studied compositional range (excess of SiO2; Na2O – 0–5.8 wt.%; B2O3 – 3.5–6.7 wt.%, and H2O – 50.5–68.2 wt.%) the system contains three phases: aqueous fluid, quartz and borosilicate liquid. At Na2O<3 wt.% (low-Na subsystem) it consists of quartz and aqueous fluid, while at higher Na2O (high-Na subsystem) all three phases coexist. The colloidal nature of the borosilicate liquid is suggested on the basis of properties of its vitreous quench product. In the high-Na subsystem significant amount of sodium and almost all boron partition into the borosilicate liquid. Raman spectroscopy has not detected known sodium borate and borate polyanionic (meta-, ortho-, tri-, tetra-, and pentaborate) species in inclusions from all experiments within 20–400 °C. Dissolved orthoboric acid line 876 cm−1 was the only Raman spectral feature determined in fluid inclusions within the region typical for aqueous borate species (500–1200 cm−1). High-temperature Raman spectroscopy along with microthermometrical data suggests that a significant part of boron in the fluid and borosilicate liquid at the experimental temperature and pressure is bonded to sodium borosilicate species. It was shown that boron has no significant effect on the phase relations within the studied part of the system Na2O–B2O3–SiO2–H2O and under the studied conditions the system can be regarded as Na2O–SiO2–H2O to a first approximation. The obtained experimental data show that under natural hydrothermal conditions slightly alkaline aqueous fluids can transport significant amounts of boron and silica far away from the hydrothermal fluid source. On the other hand in natural systems with relatively high alkalinity one can expect that boron does not migrate too far from the fluid source and, being accumulated in the borosilicate liquid, will form borosilicate mineralization at appropriate conditions. [Copyright &y& Elsevier]