STRUCTURAL ELEMENTS WITHIN CRETACEOUS AND TERTIARY PLUTONS IN THE FRONT RANGE PORTION OF THE COLORADO MINERAL BELT

Internal brittle structures in Cretaceous and Tertiary plutons such as joints, veins, dikes, and foliations record tectonic stresses that either accompanied crystallization of the bodies or post-date them. The principal stress-producing event(s)affecting stocks is thought to be regional, the result of plate-tectonic processes. Analysis of internal plutonic structures can help estimate the orientation and relative strength of tectonic compression, as well as interpret controls on syn-and post-crystallization mineralization. A large data base of joint, fault, vein orientations and characteristics (spacing, length, mineral fillings, kinematic indicators) were collected in plutons from 2004-2007. Data were analyzed using stereographic and rose diagrams for systematic structural trends, some of which are coincident with vein hosted mineralization, to characterize the plutons. In plutons probably emplaced between about 70 and 50 Ma, vein and joint sets are remarkably similar in orientation to structural trends in Arizona stocks emplaced during the Laramide Orogeny. The Colorado data indicate a significant east-west to northeast-southwest regional compression during that time as interpreted from regional intra-pluton consistency in extensional joints, hydrofractures, and veins oriented northeast to east-west. Preliminary data from younger Eocene-Oligocene plutons show different structural patterns in terms of joint-vein orientations, intensities, and physical characteristics, suggesting a change in stress regimes and tectonism. At least one pluton, formerly thought to be of Laramide age, may be younger, based on the similarity of its interval brittle structure pattern to that of the younger plutons. If a re-orientated and distinct stress field is substantiated by further structural and geochronology studies, it will contribute significant understanding to the tectonics of a poorly known time period (30-40 Ma) in the geologic history of the western United States. Because this relatively young age of plutonism is temporally related to one of the periods of substantial gold mineralization in Colorado, and in Nevada, understanding the relative orientations and timing of tectonic stress and delineating mineral controls during this period may aid in tectonic reconstructions and metals exploration.