Hydrothermal Alteration and Self-Sealing in Y-7 and Y-8 Drill Holes in Northern Part of Upper Geyser Basin, Yellowstone National Park, Wyoming

Keith, Terry; White, Donald; Beeson, Melvin

Of the 13 research holes drilled in Yellowstone's thermal areas in 1967 and 1968, 2 holes, Y-7 and Y-8, are the most closely spaced pair, thus providing opportunities for detailed correlations and interpretations of mineralogy and chemistry as affected by differences in original rocks (generally minor), temperatures, pressures, and compositions of the fluid phases. All of these factors, except the detailed fluid chemistry, are considered here. Y-7 drill hole, with moderately high temperatures and no excess fluid pressures, is outside the present local upflow system. Y-8 drill hole, 130 m away, has temperatures that exceed the simple hydrostatic boiling curve and water overpressures of more than 2.1 kg/cm2 , indicating direct involvement of upflowing fluids. Permeability in obsidian-bearing sands and gravels between the two holes has decreased greatly in response to devitrification, solution, and deposition of hydrothermal minerals, largely in initial pore spaces. The principal hydrothermal minerals of Y-7 are clinoptilolite, opal, cristobalite, mordenite, and montmorillonitic clays. The more intense alteration of Y-8 includes zones of the same minerals, but in other prominent zones, quartz and analcime have replaced the less stable (more soluble) clinoptilolite and cristobalite; celadonite is the dominant clay mineral. Below -24.8 m in Y-8, hydrothermal potassium feldspar is associated with the analcime-quartz zones, probably having formed from excess potassium as potassium-rich clinoptilolite altered to analcime, quartz, and potassium feldspar. Local concentrations of calcite in Y-8 are associated with analcime-quartz-potassium feldspar zones in the altered sediments. Rapid upward decrease in fluid overpressure caused boiling, which resulted in partitioning of CO2 into the vapor phase, and an increase in pH and conversion of bicarbonate to carbonate in the water. Thus, calcite formed by reaction of calcium with carbonate from the water. The sediments from both drill holes are more altered than the underlying flow breccia of the Biscuit Basin flow, because the initial permeability of the sediments was much greater than that of the flow breccia. The pumiceous tuff of the Biscuit Basin flow is also completely altered to potassium feldspar, quartz, mordenite, and celadonite. The tuff probably had high initial permeability owing to its pumiceous texture, but its permeability is now generally very low. A detailed study of veinlet mineralogy and paragenesis indicates that first-formed minerals are generally the most soluble and least stable and, in time, are converted into more stable minerals. This succession is most strongly indicated for the five common species of silica minerals. Major factors influencing formation of the hydrothermal minerals are abundance and reactivity of obsidian as the dominant unstable starting material, temperature, permeability, silica activity, and fluid composition. The initial lithologies of Y-7 and Y-8 are very similar and indicate high horizontal permeabilities. Thus, present contrasts in wellhead pressures between the two drill holes at equivalent depths provide strong evidence for horizontal self-sealing.

Hydrothermal Alteration and Self-Sealing in Y-7 and Y-8 Drill Holes in Northern Part of Upper Geyser Basin, Yellowstone National Park, Wyoming

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