BIOLOGICAL FORMATION OF SUPERGENE MANGANESE OXIDE DEPOSITS IN EAST TENNESSEE

Mn oxide ore deposits are commonly found as discrete and disseminated deposits below the middle Ordovician unconformity throughout the Valley and Ridge from GA to VA. Deposits in are associated with brecciated jasperoid and residual clays of the Shady dolomite and are commonly exposed in the hanging wall of northeast trending thrust faults that juxtapose the Shady dolomite atop Erwin Formation quartzites. The ore occurs primarily as matrix material in brecciated jasperoid and as vug infillings, with minor occurrences along bedding planes.

Primary ore mineralogy includes the Ba-rich Mn oxides hollandite and romanèchite, with minor cryptomelane and accessory goethite, limonite, and clays. Electron microscopy shows a variety of µm- to mm-scale Mn oxide morphologies commonly associated with biological origin: micropores with imprints of gas bubbles, acicular needles radiating from irregular chains and interconnected clumps of clay spheres, ropes of interconnected and gravity-draped filaments, and stromatolitic layering. Nucleation sites consist of irregular sheets, encrusted ropes, and networked filament structures. Electron and X-ray diffraction analysis indicates that individual Mn and Fe oxide crystals are highly ordered, while FT-IR analysis shows the presence of aliphatic methylene compounds. The scale and shape of these structures indicates that microcrystalline Mn oxide crystals likely nucleated on cell surfaces or extracellular polymeric substances, catalyzing subsequent abiotic, highly ordered crystal growth via oriented attachment. The lack of fluid inclusions in associated secondary quartz mineralization indicates fluid temperatures < 120°C. The ore mineralogy, µm scale morphology, and presence of residual biopolymers indicate that the deposit is supergene in origin.

⁴⁰Ar/³⁹Ar ages of the cryptomelane fraction of the ore indicate formation at 15.1 to 8.8 Ma. These ages are consistent with published dates on other supergene cryptomelane in PA (1), and Europe (2-4), and indicate a period of increased weathering in the northern Atlantic during the middle-late Miocene.

1. Bikerman et al., Jour. of the PA Acad. of Sci.72, 109 (1999)

2. Hautmann & Lippolt, Chem. Geol. 170, 37 (2000).

3. Dill & Wemmer, Sed. Geol. (in press).

4. Jordan & Burgess, Irish Jour. of Earth Sci., 55 (2007).