Geochemistry of Inkpot Spring, Sulphur Creek-Sevenmile Hole area, Yellowstone caldera, Wyoming

Allen K. Andersen

The Yellowstone hydrothermal system consists primarily of meteoric water circulating to deep levels within and just outside of the Yellowstone caldera. Inkpot Spring is a small group of bubbling pools located just outside the hypothesized northern margin of the Yellowstone Caldera. Here the Yellowstone hydrothermal system is vapor-dominated. Inkpot Spring fluids have previously been classified as acid-sulfate waters. This study presents evidence for multiple water types contributing to surface fluids at Inkpot Spring. The complex chemistry of fluids at Inkpot Spring can be attributed to mixing of multiple water types, boiling, and water-rock interaction. The geologic setting of Inkpot Spring is situated so that fluids may react with several lithologies during their ascension to the surface. High concentrations of mercury, boron, ammonia, and volatile light hydrocarbons at Inkpot Spring suggest that petroleum is flushed from Paleozoic or Mesozoic sediments by hot water and then distilled at high temperatures. The fluids probably also react with basaltic-andesites of the Eocene Absaroka volcanics, contributing high levels of iron, calcium, and magnesium, and producing a fluid supersaturated with pyrite. Pebbles recovered from Inkpot Spring pools exhibit coatings of layered pyrite bands, indicating multiple episodes of pyrite precipitation from the fluids. Although considered to be acid-sulfate waters, many of the pools at Inkpot Spring are near neutral. Excess ammonia combines with sulfuric acid, produced from oxidation of hydrogen sulfide, to produce ammonium sulfate and neutralize the fluids. Other possible factors controlling the amount of sulfuric acid and pH are oxidation of sulfur or sulfide, disproportionation of SO2 in vapor, and sulfuric acid production from sulfur-consuming bacteria (sulfolobus) in native sulfur deposits of buried solfataras from previous hydrothermal activity. An examination of fluid-mineral equilibria in Inkpot Spring fluids and suspended sediment has revealed several minerals at or near equilibrium with the fluids including kaolinite, alunite, opal, montmorillonite (beidellite), and pyrophyllite. This is consistent with an advanced argillic alteration mineral assemblage observed in the Grand Canyon of the Yellowstone River consisting of an association with quartz (opal) + kaolinite ± alunite ± dickite.

Geochemistry of Inkpot Spring, Sulphur Creek-Sevenmile Hole area, Yellowstone caldera, Wyoming

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