SKYTOP: A CASE HISTORY OF HIGHWAY CONSTRUCTION THROUGH A SULFIDE VEIN DEPOSIT IN CENTRE COUNTRY, PENNSYLVANIA

A new millennium saga involving highway construction across Bald Eagle Ridge is evolving in Central Pennsylvania. During the excavation of Interstate I-99 through the Skytop wind gap more than 1 million cubic yards Bald Eagle sandstone containing approximately 5% sulfide-bearing veins. Fluid inclusion temperatures indicate a hydrothermal system of up to 400ºC of short duration. Massive pyrite adjacent to a fault yielded a 35 Ma emplacement date. Most of the pyritic rock was dumped into nearby waste sites (Abrogast, Siebert, and Skytop), a substantial amount was used as fill in at least 6 and many minors site along an 8-mile section of the highway, before its toxic nature was realized. The toxic nature of the regolith at some of these sites is evident by the lack of or stunted vegetation, and the appearance of efflorescent minerals such as gypsum, epsomite, pickeringite, alunogen, melanterite, hexahydrite, wattevilleite. thenardite after prolonged dry spells. Pyritic rock is exposed in two large cut faces; (800 x 500 feet, and 300 x 150 feet, probably will be covered with synthetic and natural materials.

An environmental constraint that mandates “movable” toxic material to be moved out of the Buffalo Run drainage basin (Class A stream) require scientific compromises and engineering challenges. In addition to removal of “acid” rock to proposed new land-fill, remediation plans must accommodate in situ treatment of transported material in already completed sections of the highway, where it was used as embankment fill, road-bed sub-base to elevate the north-bound lane up to 17 feet over a distance of 5000 feet, and as buttress material to load the toe of major cut face in a land slide prone slope.

In situ treatment experiments were compromised by a lack of penetration of the slurry into the “fill”, and by a permeability anisotropy (layering) engendered by blading and covering with a “bag house lime” (BHL) a waste product from a local lime plant.