THE NATURE AND SIGNIFICANCE OF SULFIDE MINERALIZATION IN BALD EAGLE RIDGE AT SKYTOP, NEAR STATE COLLEGE PENNSYLVANIA

Sulfide-bearing veins are known from a number of localities in Central Pennsylvania, and Pb-Zn was mined . at Fort Roberdeau (War of Independence), at the Keystone Mine, near Birmingham, and in the Milesburg Gap. Serious exploration has targeted localities along the Tyrone – Mt. Union lineament, and near Bakers Summit. A new road-cut through a gossan zone, near Skytop, has revealed an extensive cross-strike vein network in the well-jointed sandstones of the Ordovician Bald Eagle Formation, amongst the overturned beds of the Ordovician Reedsville and Juniata formations, and Silurian Tuscarora Formation on the western limb of the Nittany anticlinorium. The dominant joints are an early set, J1 parallel to strike and antithetic to bedding, and a later cross-strike J2 set formed from a gas hydrofracing event during late Alleghanian deformation. Pyrite veins are associated spatially with the J2 joint set and a later cross-strike lineament. A late, NNE-trending, essentially strike-slip fault has displaced pyrite veins. Pyrite occurs in generally NNW-trending veins up to 1 cm thick, and locally in zones several feet thick of interlocking networks of veins cutting across the bedding. Pyrite coats a myriad of fracture surfaces throughout the entire 900 foot section of Bald Eagle Sandstone. Traces of sphalerite, galena, chalcopyrite, marcasite and pyrrhotite have been identified. Fluid inclusion temperatures on co-genetic vein quartz range from 150-250 C, suggesting a depth of 5-8 km at the time of sulfide fluid migration. Whole-rock elevated transition element concentrations suggest deep connate fluids from a black shale provenance; the underlying Reedsville Formation or younger Plateau shales to the west are likely sources.