THE ROLE OF SEAFLOOR HYDROTHERMAL VENTING IN THE GENERATION OF ELEVATED METAL CONCENTRATIONS IN PENNSYLVANIAN BLACK SHALES

It has long been recognized that black shales are commonly enriched in transition metal concentrations, although the causal mechanisms are highly debated. Pennsylvanian black shales of midcontinent North America are no exception, with many beds enriched in transition metals by as much as two to three orders of magnitude relative to the Post-Archean Shale Standard (PAAS). High-resolution sampling of the Missourian Hushpuckney Shale Member of the Swope Limestone in several cores—focusing on the transitions between bioturbated, organic-lean, gray shales and laminated, phosphatic, organic-rich, black shales—was undertaken to evaluate the relative roles of different enrichment mechanisms. Concentrations of a broad suite of transition metals (Fe, Zn, Pb, U, V, Cr, Ni, Mo) are highly enriched within the black shale facies compared to the over- and underlying gray shales, but the level of enrichment varies in cores from different locations (Iowa, Missouri, Oklahoma, Kansas). Fe/Al and Pb/Al ratios are highest in Oklahoma, where they are as much as six and thirty times higher, respectively, than those calculated for PAAS. In Iowa, Fe/Al ratios exhibit only minor enrichment relative to PAAS, but the concentrations of other metals, such as Zn, U, V and Mo, are several hundred times higher than PAAS. Petrographic observations and d³⁴S values of sulfide imply that the enrichments are likely not caused by post-depositional (epigenetic) mineralization.

Previously, others have suggested that scavenging from normal ocean water was not the sole source of metals. We hypothesize that white-smoker hydrothermal vents, with fluids rich in metals but not sulfide, were present in the Pennsylvanian sea and raised bottom-water metal concentrations contemporaneous with shale deposition. The composition of the potential Pennsylvanian vent fluids is explored using isotope mass balance calculations and a comparison between modern vent fluid compositions and observed metal enrichments. While mechanisms of metal sequestration and concentrations in black versus gray shales were controlled by low-temperature paleoceanographic conditions, the presence of metal-rich hydrothermal fluids venting into Pennsylvanian seas might have been a critical factor in determining the absolute level and spatial variation of metal enrichment.