SOME CRITICAL MINERAL DISTRIBUTIONS IN A LITHOCAP ENVIRONMENT, YELLOWSTONE NATIONAL PARK

In the Seven Mile Hole area of the Grand Canyon of the Yellowstone River, erosion has exposed older epithermal alteration associated with an active thermal area. This system is primarily hosted by the Tuff of Sulphur Creek, one of the oldest post-0.639 Ma caldera collapse ignimbrite flows. Illite and kaolinite are the dominant alteration minerals, separated by a transition zone approximately 70 m below the rim of the canyon. Because alteration in the study is confined to the Tuff of Sulphur Creek, mobility and concentration of critical minerals, such as barite and Rare Earth Elements in a lithocap environment can be assessed. Without significant replacement by Al-oxides, e.g., gibbsite and diaspore, whole-rock aluminum concentrations in altered rock (11.20 ± 7.90 weight percent Al₂O₃) are only approximately 8% lower than unaltered rock (12.14 ± 1.63 weight percent Al₂O₃). Other immobile elements, like titanium, show a similar trend in altered and unaltered rock, 0.27 ± 0.12 and 0.32 ± 0.05 weight percent TiO₂, respectively. In the unaltered Tuff of Sulphur Creek, barium is mostly concentrated in sanidine phenocrysts, 1.60 weight percent BaO, however, acid-sulfate alteration overprinting early alkaline-chloride alteration liberated barium from unaltered sanidine and secondary feldspar before aqueous sulfate sequestered barium in barite and alunite group minerals. Still, there is roughly a 12% loss in Ba between altered, 879.57 ± 605 ppm Ba, and unaltered samples, 993.39 ± 112 ppm Ba. This trend in more mobile elements is visible in whole-rock geochemical data as well. Rare earth elements, such as neodymium and lanthanum, follow similar trends to barium, generally, with lower concentrations in altered rock. These results suggest shallow systems with dilute alkaline and later acidic fluids may weakly redistribute more soluble critical minerals, with the potential to enrich less soluble elements through significant leaching.