OLDER HYDROTHERMAL ACTIVITY ALONG THE NORTHERN YELLOWSTONE CALDERA MARGIN AT SULPHUR CREEK, YELLOWSTONE PARK, WYOMING

The Tuff of Sulphur Creek (480 ka) is well exposed in the Sevenmile Hole area of the Grand Canyon of the Yellowstone River, Yellowstone National Park, Wyoming. The rhyolitic tuff erupted after the collapse of the Yellowstone Caldera (640 ka) and hosts more than 350 vertical meters of hydrothermal alteration. Two epithermal alteration assemblages with varying mineral associations have been identified in the area: an illite-silica-pyrite phase and a kaolinite-alunite-silica-pyrite phase. Kaolinite and opal occur along the canyon rim, montmorillonite and other smectites are found at intermediate depths, and illite and sulfides (pyrite/marcasite) are found deepest in the section. Our work on the north side of Sevenmile Hole has found a complex system of veining. Brecciated cross-cutting veins ranging from 2 to 30cm wide are found at the base of the canyon. Vertically up the canyons walls, the veining style becomes less complex. These veins are about 1 to 1.5cm wide and lack brecciation. The canyon walls and rim mainly contain millimeter-scale cross-cutting silica veinlets. These “stockwork” veinlets are the most abundant fracture fill found in the canyon walls. Veins at the base of the system (in the stream bed) contain abundant sulfides. Sulfides are present in three forms: disseminated in a silica matrix, massively in healed fractures, and encrusting clays and silica. Disseminated and massive sulfides are typically associated with the matrix in brecciated veins. Breccias include angular clasts of altered tuff with argillized feldspar phenocrysts and fragments of earlier vein-filling opal. Hydrothermal silica is pervasive in the veins throughout the entire system. At the base of the system, it is dark grey and found in the matrix of the veins. On the canyon walls, the veins are white to cream in color and rarely cross-cutting other veins. The stockwork veinlets found throughout the system are usually bluish in color (rarely clear) and contain minor sulfides. Though these veins are narrow, the wall rock surrounding them is pervasively silicified. Wall rock alteration associated with all of the vein types does not vary significantly across the field area. The tuff deeper in the system is completely silicified. The canyon walls are mostly silicified, with occasional argillized pockets, and the top of the system is not silicified.