Geochemistry of Yellowstone Lake Hydrothermal Vent Deposits

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 sublacustrine hydrothermal system. Here we focus on the chemical and mineralogical composition of deposits/alteration and coexisting vent fluid chemistry associated with two distinct hydrothermal upflow zones on the lake floor. Located at a depth of 121 meters in the northern part of Yellowstone Lake, the Stevenson Island Deep Hole is a vapor driven system where vent fluid temperatures measure at 174 °C. The sublacustrine fumarole is highly enriched in dissolved H₂S and CO₂, while depleted in dissolved chloride. Accordingly, the acidic fluids enhance mass transfer reactions (H⁺ metasomatism), effectively transforming the sedimentary substrate to an assemblage predominantly characterized by kaolinite, boehmite, and pyrite. In contrast, in the Western area of the Lake, the West Thumb hydrothermal field (53 m depth) is a liquid dominated system, with 141 °C vent fluids. The vent fluids have near neutral pH and are enriched in dissolved chloride and silica. Predictably, the associated vent deposits are silica rich with a bulk SiO₂ content of approximately 80-wt%, largely due to the presence of amorphous-silica bearing phases. The deposits also contain illite, pyrite, quartz, and plagioclase. Hydrothermal vent deposits from both locations provide insight into the temporal evolution of the two hydrothermal systems. In particular, isocon plots, which quantitatively define chemical gains and losses within a system, reveal opposite trends of total iron and silica for individual vents. Within Stevenson Island Deep Hole vents, this can be attributed to chemical and physical processes (excavation and/or degassing effects) at the site of venting. Thus, data indicate the overwhelming importance of alteration of the precursor lake sediment and the replacement by minerals stable at chemical and physical conditions intrinsic to the composition of the corresponding vent fluids at each site.