Tilting of Urad-Henderson and Climax porphyry molybdenum systems, central Colorado, as related to northern Rio Grande rift tectonics

Abstract

Asymmetric orientation of stocks, radial dikes and veins, and attendant molybdenite mineralization and hydrothermal alteration indicates that Oligocene Urad-Henderson (Red Mountain) and Climax intrusive-hydro-thermal systems were tilted approximately 25° southeast and 30° west-southwest, respectively. Field and age relationships suggest that tilting resulted from displacement on normal faults associated with the northern Rio Grande rift. Minimal amounts of extension in central Colorado argue for listric configuration for these normal faults below the level of observation. Both intrusive systems formed during incipient rift development and are associated with early and scarce lamprophyre (kersantite) dikes and with later and numerous high-silica rhyolite stocks and dikes. Magmatic activity was initiated slightly earlier at Climax (33 Ma) than at Red Mountain (29 Ma). Post-ore tilting of Red Mountain was controlled by a set of subparallel, north-northeast- to northeast-trending normal faults with west side down. At Climax, deformation was more complex, with a reversal in tilt direction. The resultant tilt of Climax deposit cannot be associated simply with one set of faults. Reconstruction of Oligocene paleotopography near Red Mountain suggests a minimum depth of emplacement of 1.6 km (41 MPa confining pressure) for the apices of principal mineralizing stocks in the Urad deposit and of 2.8-3.1 km (72-80 MPa) in the Henderson deposit. Depths of emplacement for Climax system are similar to those of Urad-Henderson, 2.3-3.2 km. Exploration for buried Climax-type molybdenite deposits should allow for the probability of tilted ore systems.