BA MOBILITY IN A GEOTHERMAL ENVIRONMENT, YELLOWSTONE NATIONAL PARK

Argillic and localized advanced argillic alteration redistributed Ba concentrations in the Ba-rich tuff of Sulphur Creek (Tsc). 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 area has been altered by circumneutral fluids creating pervasive illite + quartz with hydrothermal feldspars in upwelling zones as well as acid-sulfate fluids generating pervasive kaolinite ± sulfate minerals. Acid-sulfate alteration is most pronounced at higher elevations and along the South Fork of Sulphur Creek, to the Grand Canyon where it overprints early circumneutral alteration. Circumneutral alteration is best seen roughly 100 m below the current canyon rim on a topographic high, interpreted as a major upwelling zone, between the rim and Yellowstone River. Replacing Ba-rich material with clays in a reducing environment, such as those in deep circumneutral conditions, liberates Ba to the aqueous phase. As a result, it migrates into more permeable areas where conditions are feldspar-stable, notably hyalophane ((Ba_0.5,K_0.5)Al(Si ,Al)₃O₈). Given the proper conditions, hyalophane becomes the first product for Ba and the only sink in this regime. Higher in the system, typical oxidizing and acidic conditions of acid-sulfate fluids prevail. These conditions are likely amplified in the South Fork of Sulphur Creek due to the proximity to the highly permeable caldera margin. As the water table lowered, the pervasive alteration from acidic fluids extended to greater depths where they overprinted previous alteration, liberating much of the Ba remaining in sanidine. However, the abundant sulfate quickly sequesters Ba as walthierite ((Ba_0.5□_0.5Al₃(SO₄)₂(OH)₆) and barite. Walthierite is usually found with other alunite group minerals where, similarly to the hydrothermal feldspars, speciation is likely controlled by cation ratios. Pseudocubic habits indicate these alunite group minerals precipitated in steam-heated conditions potentially generated from a descending water table. Barite is primarily found in veins and veinlets near clusters of marcasite and pyrite, but is paragenetically younger. Barite and walthierite are typically found in the South Fork of Sulphur Creek in vuggy silica and occasionally from the canyon rim to about 100 m below the rim. However, only walthierite was found on the topographic high with hydrothermal feldspars, potentially indicating waning fluid flow or maybe deep oxidation to produce the necessary sulfate.