Distribution of cogenetic iron and clay deposits in the\ud central Appalachian region\ud \ud

Maps of more than 500 abandoned iron mines, 350 early iron furnaces, and numerous clay mines in the central part of the Appalachian region reveal the distribution and close association of siderite-limonite bearing ores and clay deposits. The deposits crop out from Lancaster County, Pennsylvania to Scioto County, Ohio, a distance of more than 300 miles. The geologic settings of the deposits are diverse. In the Valley and Ridge Province and Piedmont Province, mineralization follows structures such as major sub-horizontal thrust faults (e.g. Martic) and steep thrust faults, (e. g. Path Valley), that juxtapose carbonate units against other rocks. Carbonate units within the Plateau Province also contain economic deposits of iron ores and clay. The ores are commonly siderite and limonite principally in the form of nodules and other irregular masses in clayey, calcareous beds. Illite and kaolinite are the main clay minerals. Silica is a common constituent of the clayey rocks and in some of the iron-rich, ore horizons (e.g. Buhrstone). The working hypothesis for the formation of the iron ore and clay is that reactive fluids probably moved westward along structural and stratigraphic horizons in response to tectonic events and where they encounter carbonate rocks, clay formed and iron precipitated under favorable geochemical conditions. This hypothesis differs from other ideas such as weathering and consequent development of leached paleosols, and bog iron formation. Modern models of iron deposition are based on shallow groundwater transporting iron to the precipitation site. The possible outflow of basinal fluids along deep thrust faults and cross strike discontinuities was considered as an alternative process for iron deposition and clay formation