A review of the origins of metal-rich Pennsylvanian black shales, central U.S.A., with an inferred role for basinal brines

Abstract

Compared to most shales, even most black shales, numerous thin Pennsylvanian marine black shales of the U.S. Midwest are very enriched in organic C (5–40wt%) and heavy elements. For example, instrumental neutron activation analysis of 74 samples of mostly black and dark gray Pennsylvanian shales of the U.S. Midwest average 1300 ppm Zn, 85 ppm U, 655 ppm Mo, 130 ppm Se and 55 ppm Cd—amounts sufficient to raise concerns about heavy element pollution. Direct precipitation of sulfides and fixation by abundant organic matter during sedimentation and early diagenesis may account for heavy metals contained by shales in most cases. Metal supply calculations, however, indicate that a special source would be needed to supply metals for a strictly syngenetic origin. Possible sources for such metals include submarine hot springs and Pennsylvanian seas that may have been more enriched in metals than modern ocean water. At least at some localities, additional metal values may have been added later from basinal brines (e.g. Zn) or from modern groundwaters (e.g. Mo, Se, U). The actions of modern supergene processes may fix metals, mitigating detrimental effects from black shales.