GEOCHEMISTRY OF SUBLACUSTRINE HYDROTHERMAL FLUIDS, VENT CRATERS, AND SILICEOUS DEPOSITS IN YELLOWSTONE LAKE

The geochemical composition of Yellowstone Lake water is strongly influenced by sub-lacustrine hydrothermal vent activity. The evidence for this conclusion is twofold. First, mass balance calculations indicate that Yellowstone Lake is enriched in dissolved As, B, Cl, Cs, Ge, Li, Mo, Sb, and W relative to inflowing stream waters. Stable isotopic (δD and δ¹⁸O) and mass balance calculations indicate about 13 percent evapo-concentration in the lake, which is inadequate to account for the enrichment of these elements in the water column. Second, linear relationships between the concentration of Cl and many other elements in the lake and in hydrothermal vent fluids suggest that Yellowstone Lake water is a mixture of inflowing surface water and hydrothermal source fluid. Flux estimates indicate that ~10% of the total hydrothermal flux in YNP occurs in Yellowstone Lake.

Geochemical and mineralogical studies of hydrothermal deposits and altered vent muds from the floor of Yellowstone Lake indicate that these features form due to hydrothermal fluid quenching in shallow flow conduits or upon egress into bottom waters. Siliceous precipitates occur as conduits within the uppermost sediments, as tabular deposits that form along sedimentary layers, and as spires up to 8-m-tall that grow upward from the lake bottom. These deposits are enriched in As, Cs, Hg, Mo, Sb, Tl and W. Spires and many vent deposits and conduits contain filamentous micro-structures that probably represent silicified bacteria. Diatoms are important components in many siliceous hydrothermal deposits.

Variations in major element geochemistry indicate that spire interiors and subaerial sinters from West Thumb are nearly pure SiO₂, whereas sublacustrine conduits are less SiO₂-rich and are similar in some cases to normal Yellowstone Lake sediments due to incorporation of sediments into conduit walls. Vent muds, which are hydrothermally altered lake sediments, and some outer conduit walls, show pervasive leaching of silica, with ~63 wt% SiO₂ removal on average. This hydrothermal leaching process explains the occurrence of most sublacustrine vents in deeps or craters.