Testing the feasibility of K-Ar dating of Pennsylvania cryptomelanes [potassium manganese oxides]

Michel Bikerman, Todd Myers, Akilah A. Prout, Department of Geology and Planetary Science, University of Pittsburgh, Pittsburgh, PA 15260-3332, and Robert C Smith II (retired), Pennsylvania Geological Survey, 3240 Schoolhouse Road, Middletown, PA 17057

Conventional K-Ar dating on seven samples of cryptomelane [K(Mn++++, Mn++)8O16] from south-central Pennsylvania was done to determine if the method yields meaningful dates and, if so, to place limits on the times of manganese ore formation. The preliminary results are consistent with previous studies in that the method works, but yields relatively large errors because of the high atmospheric 40Ar content. The dates* determined for the six southerly samples are Cenozoic, ranging from late Paleocene/early Eocene through Miocene, tending to confirm a residual origin for the manganese oxides during postAlleghanian periods of surface exposure of the host rocks. There is no correlation of cryptomelane date with the age of the host rock, which is Cambrian for five samples and Mississippian for the sixth. The most northerly sample gave a Triassic date. That deposit may be related in origin to the nearby Pb-Zn ore deposits formed by regional hydrothermal fluids.

*15±1, 24±2, 12.5±1.5, 57±12, 58±5, and 6±0.5 Ma (million years ago)

[from Journal of the Pennsylvania Academy of Science 72 (3):109-114, 1999] Used by permission.