REGIONAL MINERALOGY OF SULFIDE DEPOSITS NORTH AND SOUTH OF THE I-99 TOADCUT AT SKYTOP, CENTRE COUNTY, PENNSYLVANIA

Construction of the new Interstate Highway I-99 through Centre County, Pennsylvania, uncovered sulfide-laden rocks at the Skytop road cuts resulting in a significant acid drainage problem. Pyrite at the Skytop site occurs as a cross-strike vein network in the Late Ordovician Reedsville-Bald Eagle-Juniata sequence and the Early Silurian Tuscarora Formation. Most of the mineralization occurs in the Bald Eagle Sandstone. These strata are part of the northwest limb of the Nittany anticlinorium, and underlie a SW-NE trending ridge in central Pennsylvania, along Bald Eagle and Brush Mountains. At Skytop, pyrite typically occurs on joint faces in the Bald Eagle Sandstone as vein-filling masses, small (1 mm) crystals, and unusual small (< 1 mm) filiform crystals. Pyrite is associated with minor dark-red to black sphalerite crystals and masses, and traces of galena. Other minerals at the site include chalcopyrite in barite and dolomite-filled veins in the Juniata sandstones, and wavellite and variscite accompanying traces of pyrite and other sulfide minerals in the Tuscarora sandstones. Other sulfide localities include the Shad prospect (sphalerite and galena) in the Milesburg water gap; pyrite in Interstate I-80 road cuts northeast of Milesburg, as well as along joint surfaces in new I-99 road cuts south of Port Matilda. In the Birmingham area, galena occurs as spider-web vein networks in carbonate rocks along Rt. 453, and sphalerite was extracted from the nearby Keystone Zinc Mine. The Revolutionary-era lead mine known as Fort Roberdeau is located farther southwest in Sinking Valley, Blair County. Here, lead-zinc mineralization occurs in veins in the transition zone between the Ordovician limestones and dolomites. Pyrite veins were exposed recently in excavations for the Logan Centre Mall near Altoona. Although these sulfide mineral occurrences have not yet been dated, similarities between these sites and the Skytop site imply that they are all likely to be the result of a single episode of mineralization.