A KINEMATIC INVESTIGATION IN THE CRIPPLE CREEK DISTRICT, CENTRAL COLORADO: STRUCTURAL CONTROLS INFLUENCING THE LOCATION AND DISTRIBUTION OF HIGH GRADE GOLD ORE ZONES

The Cripple Creek epithermal deposit within central Colorado consists of high-grade Au-telluride bearing veins and disseminated deposits hosted within an Oligocene-aged alakaline volcanic center. We have used regional and mine scale field studies, as well as computed tomography analysis of oriented samples, to test the kinematic influence on the location and distribution of high-grade ore zones. Results reveal that structural development, dominated by strike-slip faulting, played an integral role in the location and distribution of high-grade ore concentrations. Fault kinematic data illustrate that NE-directed shortening influenced early structural development of the deposit, and the main phase of structural development occurred during a transitional tectonic regime, between the waning stages of Laramide contraction and the onset of Rio Grande Rift-related extension. Sub-vertical strike-slip faults moving in response to this extensional tectonic regime were the most efficient pathways for mineralizing fluids. Fault kinematics indicate that faults that contain low-grade mineralization in the NE portion of the deposit formed early (in response to NE-directed shortening) and were not favorable pathways for mineralizing fluids. The NW-striking Cresson fault is the most prominent structural feature in the district. This large-scale right-lateral fault (associated with E-W extension) is believed to have served as a conduit for large volumes of fluid flow through the district and contains very low gold concentrations. Fractures in Riedel shear geometries formed during dextral slip along the fault zone and appear to have allowed fluids to escape through the high permeability zone to form high-grade ore zones. Transtensional tectonic development influenced by Rio Grande Rift-related extension acted to open the plumbing system in the district and resulted in the formation of interconnected networks of faults that allowed for large volumes of fluid to penetrate the shallow crust. Structural plumbing is a key factor influencing the development of many ore deposits, and these results show that the presence of preferentially oriented sub-vertical pathways is a major factor in the development of local high-grade ore zones in the Cripple Creek district.