HYDROTHERMAL LEACHING OF BARIUM FROM FELDSPAR IN A SHALLOW HYDROTHERMAL ENVIRONMENT, YELLOWSTONE NATIONAL PARK

Argillic and localized advanced argillic alteration redistributed Ba concentrations in the Ba-rich Tuff of Sulphur Creek (Tsc) from sanidine phenocrysts to adularia, alunite supergroup minerals and barite. Nearly 300 vertical m of Tsc is exposed in Seven Mile Hole, Yellowstone National Park, between Washburn Hot Springs and the Grand Canyon of the Yellowstone River. The host rock has been altered by alkali-chloride fluids to pervasive illite + quartz with hydrothermal feldspars in upwelling zones, as well as acid-sulfate fluids generating pervasive kaolinite ± opal ± sulfate/sulfide minerals. Acid-sulfate alteration is most pronounced at higher elevations and along the South Fork of Sulphur Creek. In the Grand Canyon, acid-sulfate alteration overprints earlier alkali-chloride alteration. Alkali-chloride alteration is best seen roughly 100 m below the current canyon rim on a topographic high between the rim and Yellowstone River, interpreted as a prominent upwelling zone. Replacement of Ba-bearing sanidine, unaltered average 1.6 wt% BaO, by adularia and illite liberates Ba to the fluid. Some Ba is retained in crystals that were partially altered, and some crystals completely replaced by adularia can contain up to 1.5 wt% BaO but is typically less than 1 wt% BaO. Shallow acidic conditions found in the South Fork of Sulphur Creek leached significant quantities of cations (Na, K, Ba, etc.) from the wall rock, but the presence of aqueous sulfate acted to sequester a portion of those cations. Here, sanidine are replaced by kaolinite, quartz and alunite supergroup minerals (ASM). Depending on the location, paleo-depth and proximity to upwelling areas, Al₂Si₂O₅(OH)₄ may be kaolinite, dickite, or nacrite and SiO₂ as opal species or quartz. ASM have an average of 1.12 wt% BaO, slightly lower than sanidine phenocrysts and display both tabular and pseudocubic crystal habits, indicating formation under hypogene and steam-heated conditions. The presence of pseudocubic ASM on alkali-chloride assemblages plus sinter fields extending from the canyon rim to the current elevation of the Yellowstone River, indicate the area was affected by a lowering water table allowing the incursion of acidic fluids deeper into the system. Barite, found mainly in the South Fork of Sulphur Creek, has not been seen replacing sanidine.