New insight into the origin of manganese oxide ore deposits in the Appalachian Valley and Ridge of northeastern Tennessee and northern Virginia, USA

Abstract

Manganese oxide deposits have long been observed in association with carbonates within the Appalachian Mountains, but their origin has remained enigmatic for well over a century. Ore deposits of Mn oxides from several productive sites located in eastern Tennessee and northern Virginia display morphologies that include botryoidal and branching forms, massive nodules, breccia matrix cements, and fracture fills. The primary ore minerals include hollandite, cryptomelane, and romanèchite. Samples of Mn oxides from multiple localities in these regions were analyzed using electron microscopy, X-ray analysis, Fourier transform infrared spectroscopy, and trace/rare earth element geochemistry. The samples from eastern Tennessee have biological morphologies, contain residual biopolymers, and exhibit REE signatures that suggest the ore formation was due to supergene enrichment (likely coupled with microbial activity). In contrast, several northern Virginia ores hosted within quartz-sandstone breccias exhibit petrographic relations, mineral morphologies, and REE signatures indicating inorganic precipitation, and a likely hydrothermal origin with supergene overprinting. Nodular accumulations of Mn oxides within weathered alluvial deposits that occur near to breccia-hosted Mn deposits in Virginia show geochemical signatures that are distinct from the breccia matrices, and appear to reflect remobilization of earlier-emplaced Mn and concentration within supergene traps. Based on the proximity of all of the productive ore deposits to mapped faults or other zones of deformation, we suggest that the primary source of all of the Mn may have been deep-seated, and that Mn oxides with supergene and/or biological characteristics result from the local remobilization and concentration of this primary Mn. Carmichael ABSTRACTManganese oxide deposits have long been observed in association with carbonates within the Appalachian Mountains, but their origin has remained enigmatic for well over a century. Ore deposits of Mn oxides from several productive sites located in eastern Tennessee and northern Virginia display morphologies that include botryoidal and branching forms, massive nodules, breccia matrix cements, and fracture fills. The primary ore minerals include hollandite, cryptomelane, and romanèchite. Samples of Mn oxides from multiple localities in these regions were analyzed using electron microscopy, X-ray analysis, Fourier transform infrared spectroscopy, and trace/rare earth element geochemistry. The samples from eastern Tennessee have biological morphologies, contain residual biopolymers, and exhibit REE signatures that suggest the ore formation was due to supergene enrichment (likely coupled with microbial activity). In contrast, several northern Virginia ores hosted within quartz-sandstone breccias exhibit petrographic relations, mineral morphologies, and REE signatures indicating inorganic precipitation, and a likely hydrothermal origin with supergene overprinting. Nodular accumulations of Mn oxides within weathered al...

Analysis of soil geochemistry to better understand geogenic manganese contamination in the Shenandoah valley

“…It is thought that this Mn originates from Mn dissolved in water percolating downward, into more reducing conditions, allowing it to remain as soluble Mn 2+ . As water then upwelled and discharged through springs along faults, or directly into streams, the manganese was oxidized, forming the oxide ores that exist today (Carmichael et al, 2017). This model explains the occurrence of manganese oxides in fault breccia and ancient alluvium within the Shenandoah Valley.…”

“…In the Shenandoah Valley, Mn oxide ores are commonly found in fault breccias contained within the Antietam Sandstone (Carmichael et al, 2017). It is thought that this Mn originates from Mn dissolved in water percolating downward, into more reducing conditions, allowing it to remain as soluble Mn 2+ .…”

Co–Mn Mineralisations in the Ni Laterite Deposits of Loma Caribe (Dominican Republic) and Loma de Hierro (Venezuela)

“…According to IR spectra (Figure 6), samples were mostly poorly crystalline, although peaks around 3400, 1600 and 1000 cm −1 could be assigned to lithiophorite, while those around 3600-3500 cm −1 were related to gibbsite [43,44]. The shadowed area (800-600 cm −1 ) in samples LHA-1 and LHA-2 included some peaks related to Mn-oxides and clays [45], and the peak around 1200 cm −1 could be related to kaolinite. It must be noted that mineral contamination can also affect IR spectra.…”