CALCITE TWIN GEOMETRY AND TEMPERATURE REGIMES IN THE UWHARRIE GOLD FIELDS: McAllister MINE, ASHEBORO, NORTH CAROLINA

The Uwharrie Formation (586 ± 20 Ma) is a dominantly felsic pyroclastic suite of rocks found in the North Carolina Piedmont. Rhyolitic to dacitic in composition, these rocks have undergone mild greenschist metamorphism and hydrothermal alteration dated to ~480 Ma. They have a characteristic mineral assemblage of quartz ± plagioclase ± alkali feldspar ± clinozoisite ± calcite, as well as disseminated trace pyrite, arsenopyrite, chalcopyrite, and gold. The Phelps Dodge Corporation prospected the McAllister Mine, located 3.5 km southwest of Asheboro, in the mid 1980’s, resulting in five NX cores. This study re-examines 398 feet of core PDM-3 using petrographic analysis, selective staining techniques for mineral identification, and XRD analysis. Calcite staining, using alizarin red-s, potassium ferrocyanide blue, and a solution of 10% HCl, on slabs and thin sections revealed variable amounts of carbonate (calcite) ranging up to 30 modal percent. The calcite occurs as fracture fill and disseminated crystals (possibly replacement) and is suggested to be of hydrothermal origin. Thin section and XRD analysis samples were chosen based upon the carbonate staining results. The majority of the calcite is twinned. The types of twinning suggest temperatures of deformation from 250 to 350 °C using the methods described by Ferrill et al. (1991). Variability of twin type with depth may suggest deformational control by structural entities within the boundaries of the study area.