STRUCTURAL AND STRATIGRAPHIC CONTROLS OF REACTIVE, IRON-BEARING FLUID, CENTRAL APPALACHIANS

Maps of deposits of iron ore and clay, as shown by furnaces, mines, pits and quarries, reveal the distribution of the distinctive association across the central Appalachians, a region almost 400 miles wide. Iron ore was extensively mined from Philadelphia, PA to Cambridge, OH, prior to 1900 whereas the clay deposits commonly were exploited for paper manufacture into the twentieth century. Iron ore was mined as nodules of oxides embedded within clay above and within limestone strata. In the Piedmont province, iron deposits crop out along gently dipping thrust faults, such as the Blue Ridge, and Martic. Footwall rocks, composed of Cambrian limestone that generally composes the sole, e.g. Keedysville mylonite, are strongly altered to clay, which contains cobble- to boulder-sized masses of iron oxides. Rarely, quartzose clastic units near a fault may be extensively stained and remarkably altered. Iron ore from pits in colluvium at the western base of South Mountain and in limestone in Cumberland Valley to the west that previously have been interpreted as sedimentary bog iron deposits are likely residuum that marks the former extent of a mineralized fault surface.

Within the Valley and Ridge province, iron ore has been mined along thrust faults that cut limestone e.g. the Path Valley fault in Franklin County and Birmingham fault in Centre County.

In the Plateau province, the commonly mined, siliceous, buhrstone horizon formed at the upper contact of the Vanport limestone. Coal-bearing Carboniferous strata also commonly contain deposits of pottery clay and iron nodules among nodular calcareous units transitional into “underclay”, which may be siliceous. These deposits are mainly stratigraphically controlled except where cross-strike discontinuities provided accessible pathways.

The iron ore and associated silica enrichment adjacent to or within limestone units suggest passage of a reactive fluid from which Fe and Si precipitated in response to interaction with carbonate beds.