CALCITE TWIN GEOMETRY AND TEMPERATURE REGIMES IN THE ALLRED MINE, RANDOLPH COUNTY, ASHEBORO, NORTH CAROLINA

Located in south-central North Carolina, the Uwharrie Formation (586 ± 20 Ma) was a region of gold exploration in the mid 1980’s by the Phelps Dodge Corporation. Primarily a felsic pyroclastic suite ranging from rhyolitic to dacitic in composition, these rocks experienced mild greenschist metamorphism and hydrothermal alteration circa 480 Ma. The Allred Mine, located in Grays Chapel Quadrangle, Randolph County, North Carolina was drilled with NX core to a depth of 523 feet. The predominantly crystal-lithic tuff contains quartz + calcite ± pyrite ± chalcopyrite ± arsenopyrite ± epidote and possibly gold. Carbonate abundance, distribution, and identity were determined using alizarin-red and potassium ferrocyanide blue staining techniques. Calcite was present only sporadically throughout the core and was mostly found as fracture/vein fill or as disseminated crystals. Calcite twin geometry, using the method of Ferrill et al. (1991), was evaluated to provide an estimate for deformation temperatures in this core. Type I twins form at temperatures under 150°C, while Type II form between 150°C and 200°C. Type III form at temperatures between 200°C and 250°C and Type IV form at temperatures over 250°C. At shallower depths, Type I, II, III, and IV twins were observed (between 204’ and 214’). However, at greater depths, only Type I and II twins were observed (389’). One explanation for the higher-grade deformation temperature higher in the core, as well as the distribution of vein-fill calcite, could be introduction of a hydrothermal fluid. These results, and further study, will help define the metamorphic alteration and deformation in these rocks.