A kinematic investigation in the Cripple Creek District, central Colorado : implications regarding the structural controls influencing the location and distribution of gold ore zones

The Cripple Creek epithermal deposit is a historic gold camp located at the southern end of the Colorado Front Range in central Colorado. Mineralization in the Oligocene-aged alkaline volcanic center is characterized by high-grade Au-telluride bearing veins and disseminated deposits. The purpose of this investigation is to evaluate the tectonic conditions during volcanic development and test the kinematic influence on the location and distribution of high-grade ore zones. Results of this investigation 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 interpreted as a high-permeability zone that served as a conduit for large volumes of fluid flow through the district. Fault kinematics show that fractures in Riedel shear geometries were forming during dextral slip along the fault zone. These fractures appear to have allowed fluids to escape along this zone of high permeability, where physiochemical changes in the ore-bearing fluids resulted in the precipitation of gold from solution, forming high-grade ore zones. In-situ measurements of grains within a high-grade sheared vein were produced using high-resolution X-ray computed tomography. Three-dimensional analysis of telluride grains indicates a preferential distribution of major and minor axis orientations, suggesting that telluride mineral fabric orientations are related to shear zone kinematics. Volumetric data indicate that more than half of the grade in the sample is derived from a small percentage of grains, i.e. 60% of ore grade is derived from 17% of the grains, or the largest grains.