THE ORIGIN OF MANGANESE OXIDE DEPOSITS IN NORTHEAST TN

The Cambrian Shady, Erwin and Rome Formations in northeast TN have abundant hydrothermal Mn oxide deposits, which were mined commercially during WWII but have not yet been mapped or described in detail. These deposits are associated with jasperoid alteration (replacing dolomite, quartz, and clay minerals in the host rocks), which are located in Alleghenian-age, high-angle thrust faults and in the hinges of outcrop- and map-scale (m to km scale) anticlines. The Mn oxide minerals are found primarily as the matrix material of jasperoid breccias, and as infilling in vugs, although minor mineralization also exists along bedding planes in the host rock that connect with the jasperoid bodies. Mn oxide samples from a single location < 10 cm² within a vuggy 3×10 m² jasperoid breccia pod in the Rome Formation include a) dendrites up to 3 mm thick; b) “coral” structures up to 5 cm in length emanating from the dendrites; c) loose, round pellets enclosed by quartz-lined geodes; d) < 5 µm thick drusy partially covering the quartz in the geodes and vugs; and e) botryoidal aggregates with nodules up to 3 cm in diameter.

XRD, SEM, and TEM analyses indicate that the coral structures and round pellets are composed of columnar crystals of a cryptomelane-hollandite solid solution (~ 1×10 µm, with square cross sections). The botryoidal aggregates are composed of thin (< 1 µm) sheets of romanechite enveloping and infiltrating cracked microcline crystals up to 20 µm in diameter. The dendritic and drusy Mn oxides are composed of tiny (< 1 µm in diameter) acicular romanechite needles. The dendrites and coral structures appear spongy and porous, with imprints of gas bubbles. The presence of both romanechite to hollandite at this scale requires that the ore fluid temperature be < 550°C, and the proximity of the two is possibly due to localized vapor effects of the ore fluid. At this time, the specific chemical and thermal properties of this fluid are not yet resolved, but the presence of Ba and lack of other metals in the Mn oxides in this region suggests a supergene rather than hypogene or marine origin. Age dating of the cryptomelane in the pellets and coral structures will help constrain the regional fluid flow pathways via paleoreconstruction, and allow for estimation of ore fluid temperatures and chemistry.