40AR/39AR AGES OF MANGANESE OXIDE ORES IN SURFICIAL AND BEDROCK DEPOSITS OF THE CENTRAL APPALACHIAN MOUNTAINS

Nodular manganese (Mn) and iron (Fe) oxide ore deposits along the foot of the Blue Ridge in the central Appalachians are known to be widespread near the contact between the Cambrian Antietam Formation and Shady Dolomite, but an explanation for their origin has remained elusive. Commonly associated with these ore deposits are Fe- and Mn-oxide matrix cements in quartzite breccias of the Antietam Formation. The quartzite breccias have been interpreted either as faults, or collapse deposits associated with karst. The Fe- and Mn-oxide ores have been interpreted as resulting from supergene weathering and/or hydrothermal activity.

New ⁴⁰Ar/³⁹Ar ages of cryptomelane in the matrix cement of a number of quartzite breccias show a distribution within the Eocene, from 47.9±0.3 Ma to 35.8±1.7 Ma, that overlaps with and post-dates the 47.9—47.0 Ma magmatic event in the western Valley and Ridge. Petrographic relations within several samples of Mn- and Fe-cemented quartzite breccias indicate that they are composed of angular clasts of a “crackle” texture and are almost universally matrix supported, inconsistent with the interpretation that they formed through gravitational collapse. However, the cements are concentrically layered around clasts, partially fill void spaces and are themselves undeformed. Therefore, we suggest that Fe- and Mn-oxide cements replaced unknown matrix cement during or following Eocene magmatism by injection of hydrothermal fluids into breccia along fault zones that are possibly of Mesozoic age.

Nodular Mn-oxide ores found within residuum of carbonate bedrock and weathered surficial deposits exhibit evidence for a supergene origin. These Mn-oxides encase host sediments, are typically comprised of more varied mineralogy and are difficult to date; however, careful sampling of a 2 mm-thick vein of cryptomelane in one nodular sample yielded an age of ~28 Ma, younger than the breccia cements. Thus, we hypothesize that Mn was remobilized from the Eocene cement in the Mesozoic fault breccias during later periods of intense weathering in the Neogene, and was redistributed within rocks surrounding the brecciated zones and within overlying residuum and weathered surficial sediments. The ⁴⁰Ar/³⁹Ar dates provide rare absolute age constraints for interpreting these geomorphic and tectonic deposits.