QUARTZ SULFIDE MINERALIZATION OF THE BALD EAGLE FORMATION OF SKYTOP MOUNTAIN, NEAR STATE COLLEGE, PENNSYLVANIA

The construction of I-99 in west central Pennsylvania recently opened a gash in Skytop Mountain that PennDOT wishes would have remained closed. Excavation there intersected a two hundred meter zone in which the Ordovician Bald Eagle Formation is laced with fracture-filled veins heavily mineralized with hydrothermal sulfide. Veins range in width from millimeter scale to several centimeters and are dominated by highly reactive pyrite, accompanied by quartz and minor sphalerite and galena. Surface runoff readily penetrates the fractures, producing substantial acid run-off and concern over acid and heavy metal contamination of surface and ground water.

SEM images reveal pyrite in cubic and dodecahedral habit as well as distinctive thin tablets and needles. XRD analyses confirm that all of these forms have the pyrite crystal structure.

Fluid inclusion homogenization and freezing studies were performed on the hydrothermal quartz. Most inclusions are too small (<2µm) for investigation. Sparse, larger inclusions, 3-10µm, often occur as singlets, and are hard to treat as members of fluid inclusion assemblages. Most are 2-phase aqueous inclusions, and none bear daughter crystals. Microthermometry records a complex mineralization history, and indicates maximum temperatures higher than anticipated. Homogenization temperatures, T_h, which record minimum entrapment temperature, range from 140^oC to greater than 375^oC, with most between 180-350^oC. No petrographic evidence exists to suggest heterogeneous entrapment. Pressure corrections are forthcoming, as are CL studies to clarify entrapment chronologies.

Fluid salinities are also highly variable. The temperature of final ice melting T_m ranges from -6.0^oC to -26^oC. Visible onset of melting in the latter samples occurred near -40^oC. These data indicate total salinities ranging from as little as 9.2% to as least as high as 25%. Salinities cluster at a number of values across this range. The data demonstrate that the solutions are not all simple NaCl brines but include a substantial divalent component, probably Ca.

These studies point to the need for more thorough investigation of the thermal and hydrothermal history of rocks at the western margin of the Valley and Ridge in central PA.