DISTRIBUTION AND PRESUMED ORIGIN OF SIDERITE IN WESTERN PENNSYLVANIA ROCKS

Western Pennsylvania played a significant early role in the history of iron smelting and fabrication with more than 180 charcoal blast furnaces manufacturing pig iron from siderite and hematite ores from the late 1700s to the mid-1800s. These ores commonly are abundant in Pennsylvanian and Permian formations, but production and use eventually shifted to higher-quality ores from the Lake Superior region in the 1870s.

In Late Paleozoic formations, siderite mineralization typically occurs as nodules and thin layers in calcareous clay shales of marine origin, although some ores of terrestrial origin are locally important (e.g., associated with fire clays). Hypotheses for the origin of these ores include: 1) siderite formed diagenetically in aqueous environments exhibiting good biogeochemical depositional conditions (e.g., temperature and pressure, Eh and pH, iron concentration, bacterial activity); 2) iron precipitated where it encountered carbonate rocks as reactive fluids seeped or flushed through permeable sedimentary layers, bedding planes, and fractures in response to tectonic events.

Significant differences exist between terrestrial and marine siderites. Terrestrial siderite often has a greater percentage of Fe than marine siderite, which is always impure due both to replacement of Fe by other elements in the mineral lattice and to “junk” minerals such as silica and phosphorus scattered throughout the siderite deposits. Local limonite ores commonly occur where weathering affected siderite deposits exposed in outcrop. The famous “Buhrstone ore” of the Vanport Limestone (Allegheny Formation) typically is a 15 to 30.5 cm thick siderite, but in some places weathering resulted in pockets of limonite as much as 6 m thick.