Getty Oil Company well 52-21 in the Roosevelt Hot Springs thermal area was drilled to 7500 feet in predominantly upper amphibolite facies metamorphic rocks. All lithologies in the drill hole are pervasively but weakly altered: the alteration assemblage is chlorite + sericite + clays with occasional traces of calcite, above 2300 feet, and chlorite + sericite + clays + calcite +- epidote below 2500 feet. A zone of increased alteration intensity from approximately 1800 feet to 2300 feet occurs within and adjacent to a dacite dike which cuts the metamorphic rocks. A second zone of stronger alteration extends from 6000 feet to the bottom of the drill hole. The drill hole which is located approximately 5000 feet south of the center of the silica apron known as the Opal Mound was apparently drilled beyond the influence of acid, high-sulfate brines such as have affected the upper portions of drill holes 72-16, 76-1 and University of Utah 1A and 1B.