THE AMBIGUOUS TEMPORAL RELATIONSHIP BETWEEN METALLIFEROUS PENNSYLVANIAN BLACK SHALES AND THE CLASSIC LEAD-ZINC ORES OF THE MISSISSIPPI VALLEY

Metalliferous Pennsylvanian black shales of the Midwestern United States contain an abundance of heavy elements such as zinc, vanadium, molybdenum, selenium, and uranium in amounts exceeding ordinary euxinic shales by one hundred to a thousand fold. These metal enrichments are best attributed to hydrothermal processes rather than more prosaic causes associated with euxinic sedimentation. The timing and specific sources of individual metals remain in question. Given their geographic location it is likely that the metals in the shales came from the ore-forming hydrothermal systems that deposited the Mississippi Valley-type (MVT) ores of the American Midwest. However, based on current evidence it is not clear whether metals were introduced syngenetically with shale sedimentation, came later during diagenesis of preexisting organic-rich sediments, or were of mixed origin.