CHARACTERIZING ANCIENT HYDROTHERMAL SYSTEMS OF SEVENMILE HOLE IN THE GRAND CANYON OF THE YELLOWSTONE RIVER, YELLOWSTONE NATIONAL PARK, WITH MINERALOGY, STABLE ISOTOPES, AND FLUID INCLUSIONS

The Grand Canyon of the Yellowstone River displays regions of pervasive hydrothermal alteration that formed in the shallow parts of an ancient hydrothermal system. The altered protolith, the Tuff of Sulfur Creek, is a 480 ka, high silica, low δ¹⁸O rhyolite tuff that erupted during resurgent doming of the 640 ka Yellowstone caldera. Incision of the canyon exposed 350 vertical m of altered rock in the Sevenmile Hole vicinity. At depths >100m the mineralogy is dominated by illite + qtz with disseminated pyrite and variable alunite, adularia and buddingtonite. Above 100m to the present day canyon rim kaolinite + opal dominate with localized alunite, qtz, and dickite. The 100m transition of kaolinite to illite likely corresponds to a paleo-water table, below which boiling of neutral-pH and/or acid-sulfate fluids created a steam-heated acid-sulfate environment above the water table. Fluid inclusion homogenization temperatures in qtz samples range from 160^° to 350^°C, with most primary inclusions in prismatic vugs ranging from 190^° to 270^°C. Homogenization temps generally increase with increasing depth of samples. Freezing of fluid inclusions yielded salinities of 0.35 to 0.71 wt % NaCl eq. δ¹⁸O values of qtz were measured for 50 samples using laser fluorination techniques. Unaltered magmatic qtz phenocrysts have δ¹⁸O values of 1 to 2‰; hydrothermal alteration slightly decreases δ¹⁸O values to 0.5 ‰. Hydrothermal qtz averages -2.1 ‰. Prismatic vug-filling qtz yielded the lightest average values of -4.2 ‰, while massive qtz at the edge of vugs ranges up to -1.0 ‰. Homogenization temps from massive vug fill are noticeably cooler than prismatic counterparts; therefore temp changes are likely responsible for variable δ¹⁸O values. Calculated fractionation between qtz and water yields δ¹⁸O values ranging from -10.4 to -16.4 ‰ for ancient hydrothermal waters. Low salinities and negative δ¹⁸O values indicate a dominantly meteoric water source for ancient hydrothermal fluids. Calculated δ¹⁸O water values are similar to present day thermal waters in Yellowstone (δ¹⁸O = -8 to -20 ‰), and slightly heavier than present day cold meteoric water (δ¹⁸O = -15 to -20 ‰), which indicates some exchange of O during alteration may have occurred between ancient water and the host tuff.