A major negative gravity anomaly with a minimum of —325 mgal and an amplitude of —70 mgal occurs in west-central Colorado between Aspen and Gunnison. The gravity minimum is closely associated spatially with Late Cretaceous to Oligocene granitic rocks and continues along the Colorado mineral belt to the northeast. Gradients indicate that the source of the negative anomaly is in the upper crust, but part of the negative anomaly is attributed to crustal thickening that is a result of isostatic compensation. The most plausible explanation for the negative gravity anomaly is that most of it is caused by a granite batholith 8 to 25 km thick and that the numerous granitic stocks in the area are protrusions from this batholith, so that the mineral belt occurs along the roof zone of the batholith. Although the stocks appear to have been emplaced primarily by stoping, the gravity effect of the sloped material must be at the base of the crust or dispersed, because the gravity effect is minimal. Temperatures in the lower crust may be high enough for the granite to have been formed by partial melting. The postulated batholith is a major crustal feature that cuts obliquely across many Laramide structural trends.