PB ISOTOPIC EVIDENCE FROM GALENA AND THE TACONIC FORELAND SEDIMENTS FOR A BASEMENT SOURCE OF METAL IN FORT ROBERDEAU REVOLUTIONARY MINES, CENTRAL PA

Fort Roberdeau, located outside of Altoona, PA, was built to support local area Pb mines. The genesis of the Pb deposits was thought to be related to MVT processes in which Pb was derived from the nearby and underlying Ordovician Gatesburg Formation. To determine the source of lead for the ore in these mines, we compared the lead isotope ratios from the galena and the Cambrian through Silurian stratigraphy in the area. ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁶Pb/²⁰⁴Pb plots of the data reveal that the lead from the galena is not sourced from the Gatesburg limestone formation. Unexpectedly, the galena lies on the same trend as the Taconic Clastic wedge sedimentary rocks (Reedsville, Juniata, Bald Eagle) and oldest Cambrian limestone formation, the Waynesboro. Assuming that the fluids associated with mineralization originated from depth and permeated up gradient, the Pb isotope data support a model whereby the brines acquired Pb from deeper in the section then previously suggested. In other words, the Pb found in the Taconic Clastic wedge sedimentary rocks and the galena from fort Roberdeau is derived from the Waynesboro formation and surrounding basement rocks.