PANDORA'S BOX: THE UNROOFING OF SULFIDE VEINS AT SKYTOP, CENTRE COUNTY, PENNSYLVANIA

Recent excavations along Interstate Route I-99 through the Skytop wind gap, near State College, have exposed an 800-ft wide zone of cross-strike vein sets, dominated by pyrite with lesser amounts of sphalerite and galena. During construction 1x10⁶ cubic yards of Bald Eagle Sandstone, hosting up to 5% sulfides in veins, was excavated and distributed as fill or waste rock to 5 nearby localities. Skytop is a world–class locality for “whisker “ pyrite, and contributes to an acid rock drainage problem.

To the east of the Skytop wind gap the telescoped stratigraphic section of the Ordovician strata (Reedsville Shale, Bald Eagle Sandstone, Juniata shale siltstone and sandstone), occur in a homoclinal domain on the steeply dipping, overturned western limb of the Nittany anticlinorium (Alleghanian orogeny). In contrast, the shallower dipping beds of the Silurian Tuscarora and Rose Hill formations on the western slope underlie the Ordovician formations disharmonically as a series of mesoscopic scale chevron folds. These are inferred to represent lystric splays off a regional thrust plate. Excavation for Route I-99 along a synclinal axis led to unstable slope conditions in both road cuts.

A major east-west trending fault is evident from offsets in the wind gap. A number of low-angle oblique-slip faults, exposed in the road-cuts through the Ordovician rocks, are the locus of local landslides.

A REDOX event that accompanied the sulfide veins coincides spatially with the late Alleghanian J2 joints, and their weathering products also occur in the early Alleghanian J1 joints to depths of 30-50 feet in an oxidized cap rock zone. Evidence from fluid inclusions in quartz and Re-Os isotopes from coexisting pyrite revealed unexpectedly high homogenization temperatures (>375ºC), and a non-radiogenic, Cenozoic mineralization age.