Geochemistry of Vapor-Dominated Hydrothermal Vent Deposits in Yellowstone Lake, Wyoming

University of Minnesota M.S. thesis. 2020. Major: Earth Sciences. Advisor: William Seyfried. 1 computer file (PDF); 59 pages.Yellowstone Lake hydrothermal vent systems have been studied recently in connection with the HD-YLAKE program, a multidisciplinary project investigating the feedback between chemical and physical processes characterizing the sub-lacustrine hydrothermal system. Here we focus on the chemical and mineralogical composition of deposits/alteration and coexisting vent fluid chemistry associated with Stevenson Island Deep Hole vents. Remote in its location, at 120 m below lake level, Stevenson Island Deep Hole vents provide an excellent natural laboratory to assess fluid-mineral interaction and phase separation phenomena that contribute to the chemical evolution of a particularly active region of the lake floor hydrothermal system. Hydrothermal fluids issuing from Stevenson Island Deep Hole are enriched in dissolved H2S and CO2, enhancing mass transfer reactions and transforming diatomaceous sediment, which primarily lines the Yellowstone Lake basin floor, to an assemblage characterized by kaolinite, boehmite or smectite, and pyrite. Accordingly, this process results in significant volume loss up to 80%. Theoretically, this may contribute to the conical depressions which define the Stevenson Island vent system. Kinetic consideration are consistent with this interpretation. Hot water discharged (demonstrated by 1D modelling) from the system to lacustrine sediment simulates amorphous silica and quartz dissolution increasing porosity, thus enhancing downward excavation of sediment. This additionally may increase flow, leading to greater amounts of alteration. With the addition of steam derived H2S and CO2, alteration mineralization progresses in the sequence of amorphous silica, quartz, smectite, kaolinite, then boehmite and is intensified with flow volume. Identification of minerals in this alteration sequence in relation to venting proximity is suggestive of a dynamic hydrothermal system with complex fluid networks convectively circulating throughout the subsurface of this area and significant venting that may displace altered sediment. Additionally assessment of deuterium/hydrogen stable isotope data of kaolinite and coexisting vent fluids is most consistent with analogous data from Sheppard and Gilg (1996). The combination of chemical, isotopic, and mineralogical data reported here helps to constrain the spatial and temporal evolution of the sub-lacustrine Stevenson Island vent system in Yellowstone Lake