ORIGINS OF HYPER-ENRICHED DEPOSITS OF HEAVY METALS IN BLACK SHALES

Certain black shale-hosted deposits contain exceptional metal values that are several orders of magnitude greater than average shales of Turekian and Wedepohl (1961) and Vine and Tourtelot (1970) (e.g. >300 ppm Zn; >10 ppm Mo). For example in Illinois and Indiana, bed B of the Carboniferous Mecca Quarry Shale Member of the central USA contains an average of 1100 ppm Mo as well as erratic values for Zn in Illinois and Indiana that average 3000 ppm but in some cases exceed 10,000 ppm (Coveney and Martin, Econ. Geol., 1983; Schultz and Coveney, Chem. Geol., 1992). Elsewhere, the Ni-Mo sulfide layer(s) in Cambrian shales of southern China contain more than four weight percent Mo and nearly as much Ni (Lott et al., Econ. Geol., 1999). We suggest that beds which contain such highly concentrated deposits of metals be called “hyper-enriched”. Known examples are all located in exceptionally metal-rich geological environments (i.e. in the Mississippi Valley of North America, and the rich ancient mining areas of southern China in Guizhou and Hunan).

Although hyper-enriched deposits of metals in black shales have been studied since the days of Goldschmidt’s work on the Kupferschiefer, a consensus on the mode or modes of origin has yet to emerge. Workers have variously attributed the hyper-enriched deposits of heavy metals to a wide range of agents including syngenetic hydrothermal activity, epigenetic mineralization, bolide impact, or accumulation from sea water, despite the lack of clear modern analogs (Emsbo and Breit, Goldschmidt conference 2011). We note, however, that the hyper-enriched black shale deposits that we cite are associated regionally with coeval hydrothermal ore systems that have left evidence in the form of hot saline fluid inclusions that are found associated with the shales and nearby strata. For these and other reasons, we infer that the metalliferous black shale deposits, such the Late Paleozoic American black shales, and the unusual Ni-Mo deposits of the Chinese Cambrian, primarily owe their origins to hydrothermal processes.