MINERALOGICAL CONTROLS ON ACID-ROCK DRAINAGE AT SKYTOP, CENTRE CO., PA

Recent I-99 highway construction exposed Zn-Pb occurrences along a 40-60-m deep roadcut through a 270-m long section of the Ordovician Bald Eagle Sandstone at Skytop, near State College, PA. Other Zn-Pb deposits in Blair and Centre Cos. were prospected in the past. Below a >10-m thick gossan, the cut exposed fresh pyrite and other sulfides to oxidative weathering. Pyritic material was removed from the cut and used at the site as road base and fill. Within months, acidic (pH<3), metal-laden seeps and surface runoff led to a halt in road construction, raised concerns about surface- and ground-water contamination, and prompted costly remediation efforts. Mineralized sandstones from the cut contain as much as 34 wt. % Fe, 28 wt. % S, 3.5 wt. % Zn, 1% wt. Pb, 88 ppm As, and 32 ppm Cd. A composite of <2 mm material sampled from the cut face contains 8.1 wt. % total sulfide S, 0.6 wt. % sulfate S, and is net acidic by acid-base accounting (net neutralization potential –234 kg CaCO3/t). Primary minerals include pyrite, marcasite, sphalerite (2 to 12 wt. % Fe) and traces of chalcopyrite and galena. Pyrite occurs in mm- to cm-scale veinlets and disseminated grains in sandstone, as needles, and in a locally massive pyrite-cemented breccia along a fault. Inclusions (<10 µm) of CdS and Ni-Co-As minerals in pyrite and minor amounts of Cd in sphalerite (0.1 wt. % or less) explain the primary source of trace metals. Wet/dry cycles promote oxidative weathering and dissolution of primary sulfides. Resulting sulfate solutions evaporate to form intermittent “blooms” of soluble, yellow and white efflorescent sulfate salts (copiapite, melanterite, and halotrichite) coating the cut face during dry periods. Salts incorporate Fe, Al, S, and minor Zn and readily dissolved in deionized water in the laboratory to form solutions with pH <2.5, consistent with field observations. In addition to elevated dissolved Fe and sulfate concentrations (>1,000 mg/L), seep waters at the base of the cut contain >100 mg/L dissolved Zn and >1 mg/L As, Co, Cu, and Ni. Pb is relatively immobile (<10 µg/L in seep waters). The salts temporarily sequester metals and acidity in between rainstorms. Episodic salt dissolution contributes pulses of contamination to surface runoff. The Skytop experience highlights the role of mineralogy in anticipating environmental impacts associated with construction in sulfidic rocks.