STRUCTURAL AND STRATIGRAPHIC CONTROLS ON THE DISTRIBUTION OF SILICEOUS IRON ORE, ALLEGHENY PLATEAU, WEST CENTRAL PENNSYLVANIA

Quartz-rich iron ore; one to fifty cm thick, was mined in the first half of the 1800’s to supply iron furnaces of Western Pennsylvania. The siliceous ore occurs at the top of the Pennsylvanian Vanport limestone. Postulated sources of the ore include: 1) hydrated oxide of iron or limonite formed in swamps or bogs, 2) a product of groundwater leaching of coal seams, 3) paleosols and 4) hydrothermal fluids. The purpose of this study is to: 1) determine the extent of the ore, 2) develop a comprehensive ore emplacement model and 3) distinguish structural and/or stratigraphic controls of deposition as well as pathways for mineralizing fluids. The Vanport, a marine limestone, is limited to the Allegheny Plateau of west central Pennsylvania and east central Ohio. The Vanport may be as thick as 6.2 m; the variability may reflect pre-depositional topography or post-depositional erosion. Strata above the limestone are principally shale with channels of sandstone that locally cut through the Vanport. The top of the Vanport may be modestly deformed by faults and slump structures. The siliceous ore horizon is restricted to the Vanport, but is not developed at all Vanport occurrences as shown by outcrops and well data. The distribution of ore does not correlate well with either regional fold structures or thickness variations as shown and by structural contour and isopach maps covering fifteen 7.5-minute quadrangles. The ore consists of oxides of iron, manganese and silica and occurs as thin irregular layers and as fracture fillings. Ore filled fractures occur both in the Vanport and overlying strata that may be permeated by Fe-staining. Manganese occurs as small pockets or crust intermingling with Fe-rich ore. At the surface the ore weathers to limonite.