SEM ANALYSIS OF SILURIAN CLINTON IRONSTONE FROM BELL FURNACE, PENNSYLVANIA: PRELIMINARY ASSESSMENT OF MINERALIZATION AT A LEGACY JUNIATA IRON MINE SITE

Recent quarrying in the Tuscarora State Forest at the historic Bell Furnace (BF) east of Mt. Union, PA, uncovered an underground mine excavated in Silurian shales. Hundreds of mine sites from the Juniata Iron industry are scattered across the area. The extent of these mines is poorly documented, although workings can be identified using LiDAR. Iron mining produces multiple disturbances from surface/subsurface workings, and mine wastes have potential to impact air, soil, and water quality. Of concern is the mobilization of metals from waste rock into soils or streams. Depending on source ores and gangue minerals, metals such as Pb, Zn, Cu, Cd, etc. may be mobilized with acid rock drainage, especially when host rocks contain sulfide minerals such as pyrite. After oxidation, mobilized metals become bioavailable in the environment and acid production can be harmful.

Unfortunately, little is known about the legacy of Juniata Iron mine sites nor the impacts to soil/water quality in the region. This study is an initial investigation of iron ores from the BF site to better understand the mineralogy and potential for mobilization of metals from Silurian Clinton Group ironstones. Research by Matheson and Pufahl (2021) on Clinton ironstones from the NY type area suggests two ironstone types exist: granular ironstones composed of quartz-cored, iron-coated grains, and fossiliferous forms composed of carbonates coated by iron cements. Mineralogically, these are hematite (Fe2O3) and chamosite (Fe2+, Mg, Fe3+) 5 Al (Si3Al) O10 (OH, O)8. In our study, rock sections and SEM-EDS analysis reveal complex mineral associations with similarities and differences to the work of Matheson and Pufahl. Initial SEM-EDS analysis shows BF ironstones contain Fe-coated and inter-layered grains cored by both quartz and calcite as well as siderite (FeCO3), indicating mineralization during deposition of the grains. These mineral assemblages are stable in the weathering environment. However, our analyses reveal more diverse mineralogies exist as well. Dissolution of crinoid skeletal elements during diagenesis allowed for subsequent infilling and replacement of original material with a silvery mineral. Although SEM analyses are ongoing, the mineral is likely marcasite (FeS2), and as such, has the potential to generate acid rock drainage.