MICROSTRUCTURAL STUDY OF CONTROLS ON URANIUM MINERALIZATION AT COLES HILL, VIRGINIA

Coles Hill in southern Virginia is a large uranium deposit hosted by granitic gneiss and amphibolite of the Silurian Martinsville Igneous Complex. Very fine-grained uranium (U) mineralization of undetermined age inhabits cataclastic zones and related veins in the complexly deformed footwall of the Chatham fault, a normal fault that forms the western boundary of the Late Triassic Dan River Basin.

Understanding the detailed orientation relations and the relative temporal evolution of mineralized veins and fractures will be key to developing a model for ore deposition. For example, is the structural evolution consistent with ore formation before, during, or after the Late Triassic rifting? Although the deposit is not exposed at the surface, a large inventory of rock core is available. In the early 1980s, the original owner of the deposit used clay-impression and Craelius tool techniques to obtain several oriented rock cores. Using oriented cores, they performed autoradiography on selected intervals to identify and orient numerous U-bearing structures. Though incomplete, the records of this early work formed the basis for a sampling strategy for selected brittle deformation zones in the core. From each sample, we obtained three mutually perpendicular oriented thin sections, which allow detailed characterization and orientation of structures. For some of the samples we have autoradiography records that clearly indicate the presence of radioactive minerals in specific structures. In oriented thin sections, we investigate detailed orientations of brittle microstructures, relative timing of brittle structures, and micro-scale kinematics. We are integrating these petrographic observations with SEM and electron microprobe analyses and with imaging using alpha particle-sensitive film to verify the presence or absence of very fine-grained U mineralization in association with these structures. Types of structures evident in the thin sections include opening-mode veins, complex shear zones with extensive comminution, and stylolite-like structures. The techniques and approaches outlined here will contribute critical data and interpretations to an understanding of this U deposit.