HIGH-RESOLUTION X-RAY COMPUTED TOMOGRAPHY APPLIED TO MINERALIZED SHEAR ZONES IN THE CRIPPLE CREEK DIATREME, COLORADO

Understanding the mechanisms that result in the localization of high-grade veins is of great importance in targeting new and potential resources in ore districts. At the University of Texas at Austin, new techniques utilizing high-resolution X-ray computed tomography (HRXCT) are being developed to, among other things, evaluate preferred mineralization sites within kinematic zones to explore the relationship between structural fabric and high-grade ore distribution. HRXCT technology allows for non-destructive, three-dimensional examination of geologic specimens. HRXCT scans highlight density contrasts in samples, providing an excellent medium by which to examine the distribution of mineral grains in rocks. Scanned images reflect differences in X-ray attenuation though an object, which are caused by density and atomic number variations among phases. Scanning produces a series of two-dimensional slices through an object, which are stacked to produce a three-dimensional data set that can be rotated, sliced and viewed from any angle. Using custom-developed software for measurement of volumetric data sets (“Blob3D”), information about mineral grains, such as volume and major/minor axes orientations, can be extracted for in-situ analysis. This technology was applied to material collected from a high-grade mineralized sheared vein found within the Cripple Creek diatreme, located in southern Colorado. Preliminary results from three-dimensional analyses of individual grains indicates a planar distribution of maximum length axes and a statistically significant correlation between the long-axis maximum orientation and the orientation of shear zone dilation in the diatreme, as well as related fault striae orientations. Given that the telluride grains show no indication of deformation, we interpret these results to demonstrate a first-order relationship between post-shearing growth of telluride grain orientations and structural fabric. Evaluation of the application of this new technology continues as we explore relationships between shear zone kinematics and the formation of high-grade ores.