MORPHOLOGY AND PARAGENESIS OF SULFIDE MINERALIZATION AT SKYTOP ON BALD EAGLE RIDGE, CENTRE CO., PENNSYLVANIA

Veins bearing pyrite and other sulfide minerals were exposed during the excavation of a more than 200-foot deep road cut through the Bald Eagle ridge at Skytop near Matternville in Centre County, Pennsylvania. The I-99 Skytop road cut uncovered an extensive network of pyrite-laden veins cutting across the strike of overturned sandstone beds of the Ordovician Bald Eagle formation. Pyrite and other iron sulfides (pyrrhotite and possible marcasite) occur in the veins along with minor amounts of sphalerite and galena, and traces of chalcopyrite and a cadmium sulfide (greenockite?).

Pyrite occurs in a wide range of morphologies, possibly indicating a depositional history during which there was a wide variation in temperature and other conditions. Crosscutting pyrite veins ranging from one to 20 mm in thickness are filled with brassy, highly reflective massive pyrite displaying striated faces on roughly equant grains up to 10 mm across. Cubic pyrite up to 8 mm on edge occurs in open spaces in these veins. Other veins contain fine-grained grey-greenish pyrite. At least one fault-zone has a zone about 1 meter thick of sandstone breccia cemented by a fine-grained grey-green masses of very fine-grained iron sulfides composing more than 50% of the rock.

Other veins in the sandstones are lined with well-formed clear quartz crystals roughly 0.5 to 2 mm across. Very fine grains of iron sulfides (less than 0.1 mm) occur as overgrowths on the quartz. Veins may also display one or more of a wide-variety of morphologies of iron sulfides on top of the fine-grained material. These iron sulfide crystals are found with cuboctahedral morphologies, as well as displaying graphic habits, and occurring in the shape of blocks, laths, matchsticks, and fine needles. This site has some of the world’s finest “whisker” pyrites.

Early striated pyrite followed by smooth-faced cubic crystals indicates change from low-supersaturation, higher-temperature (above 250ºC) to moderate-supersaturation, lower-temperature conditions. Late needle-like forms may indicate a subsequent shift to conditions of low-temperature (below 250ºC), low-supersaturation as the main stage of mineralization ceased, or a later remobilization of iron sulfides.