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