RE-OS MOLYBDENITE AGES FOR THE ANTIETAM RESERVOIR, EASTERN PENNSYLVANIA, A STORY OF OPEN-SYSTEM BEHAVIOR RE-OS ISOTOPES IN MOLYBDENITE

The Reading Prong is the southwestern portion of a geologically complex physiographic upland in Pennsylvania. It is composed predominantly of Precambrian rocks and continues into adjacent New Jersey and New York. In order to further understand the geochronology of the area, Re-Os molybdenite ages were obtained from dikes that appear to crosscut potential Byram Intrusive Suite of Rocks. The data can be separated into two groups: 1) Samples with low concentrations of Re and Os (0.25-1 ppm and 0.003-.128 ppm respectively) that yielded non-viable geologic ages. 2) Samples with higher concentrations of Re and Os (12 ppm and 0.426 ppm respectively) that yielded a ‘reasonable' age of 975 ± 2.4 MA. SEM with EDS analyses from the first group revealed predominantly molybdenite, but with intergrown micron sized blebs of supergene and therefore younger powellite and ferrimolybdite. These secondary minerals were not found in the second group of samples that yielded reasonable ages. This study further confirms the notion that molybdenite samples can be isotopically altered and this disequilibrium can be recognized by the presence of secondary molybdenum minerals. The ‘reasonable' Re-Os age is younger than typical for the Reading Prong, but approximately coeval with a zircon U-Pb date for a chevkinite- and molybdenite-bearing dike from elsewhere in the Reading Prong.