U-Pb dating of a football-sized, dogtooth spar, calcite crystal collected from a cave in Big Canyon, Guadalupe Mountains. New Mexico. USA, gave an age estimate of 87 - 98 Ma for calcite deposition. This Upper Cretaceous (Laramide) date is important because: (1) it implies that there may have been a major karsting episode in the Guadalupe Mountains in the Laramide; (2) it implies that the Laramide was a time of heating and deeply circulating hydrothermal water; (3) it relates to the possible time of regional uplift above sea level of the Guadalupe Mountains along with the rest of the western United States: and (4) it relates to a time of possible hydrocarbon maturation and migration in the Delaware Basin.