DIVERSE ORIGINS FOR ECONOMIC MINERALS IN CENTRAL PENNSYLVANIA

Recent mapping in central Pennsylvania has revealed a variety of mineral species associated with residual iron ore and clay deposits in the carbonate-floored valleys, as well as vein and replacement base metal deposits in the clastic ridges.

The Scotia Mine in Centre Co. was the major source of iron in Pennsylvania from 1851-1899 (with brief revivals as late as WWII) with more than one million tons of iron processed. Ore occurs primarily as masses of goethite/chert breccia (wash ore) in clayey sands above Upper Cambrian and Lower Ordovician carbonates and sandstones. The ores are residual in nature and transgress formational contacts. Angular cherts with sharp contacts within the breccia ore suggest open space collapse. Local pipe ore (stalactitic goethite) indicates reprecipitation of iron oxides due to Eh/pH changes during weathering. Ores appear to be associated more with linear topographic depressions sub-parallel to regional strike than with specific bedrock strata.

Nearby associated “bauxite” deposits in the Gatesburg Formation were exploited as late as the 1950s. Pisolites within the bauxite are predominately gibbsite with minor nacrite and kaolinite. The matrix consists mainly of dickite, nacrite and kaolinite, with minor gibbsite and traces of quartz, anatase, and rutile.

Recent excavation of the I-99 Skytop road cut exposed a pyrite vein complex approximately 250 m wide. Pyrite occurs in steeply-dipping dominantly 140° trending veins up to 2 cm thick in sandstone of the Bald Eagle Formation. At least 20 minerals have been identified at the Skytop site, including “whisker pyrite” and a host of secondary efflorescent minerals. Minor sphalerite and galena occur and correlate with other base metal sulfide deposits along the western section of the Valley and Ridge. In Huntingdon County, lead was mined at the Revolutionary War-era Fort Roberdeau, and zinc was mined at the Keystone Mine near Birmingham.

Currently aggregates and chemical-grade lime and mined from the Middle Ordovician carbonates. Calcite and dolomite-filled veins, within carbonate rocks occur as dominantly 140° trending veins. Some may carry minor quartz, fluorite, and traces of barite and pyrite. Vug-filling strontianite occurs in strata-bound layers within the Middle Ordovician Loysburg Formation.