Metals hosts in the Pennsylvanian Hushpuckney and Stark Black Shales from Selected Areas in Kansas City

Abstract

The Hushpuckney Shale of the Swope Formation and the Stark Shale of the Dennis Formation, two black shales in the Pennsylvanian Kansas City group, contain elevated concentrations of transition and based metal elements, notably zinc, lead, vanadium, and others. Both shales consist of an organic-rich black shale lower part and gray shale, organic poor upper part. The main rock-forming minerals in the shales include silicates (quartz and clay minerals), carbonates (calcite and dolomite), and phosphate (francolite). Various sulfide/selenide minerals (pyrite, sphalerite, chalcopyrite, clausthalite) account for most of the transition and base metal enrichment, though some appears to be hosted in organic material and on clays. Traces of barite, rutile, and mica were also detected. While the rock forming minerals are detrital or diagenetic, most of the sulfides/selenide appear to be hydrothermal in origin. Ni/Co and V/ (V+Ni) ratios indicate the sedimentary environment of Hushpuckney deposition was marginally oxic (in the upper part) to anoxic and euxinic (in the lower part). The same ratios applied to the black shale portion of the Stark indicate deposition in anoxic to euxinic conditions. The presence of hydrothermal sulfides suggest later introduction of hydrothermal fluids and mineralization, probably during the Ouachita uplift to the south during the early Permian. Elemental variation with depth, and interelemental correlations were determined using scanning XRF for a variety of major, minor, and trace elements, including Si, Al, Fe, Mg, Ca, K, and P (major elements), minor Ba, S, Co, Cu, Ni, and Sr, and trace V, Zn, Pb, Sc, La, Ti, Se, Rb and Sb elements. Interelement correlations generally agree with observations made using scanning electron microscopy/energy dispersive spectroscopy and X ray diffraction analyses, and can be explained by the mineralogy. The organic material and clays appear to host vanadium whereas the other metals are present as sulfides (sphalerite, chalcopyrite, and pyrite) or selenide (clausthalite). The hydrothermal minerals appear to be associated with the francolite, suggesting that chemical reactions with the francolite played a role in sulfide deposition. Vanadium is also concentrated in some of the hydrothermal sulfides, suggesting remobilization and deposition of V during sulfide formation. Metal enrichment in these two black shales thus appears to be due to a combination of adsorption or inclusion of elements such as V from seawater during sedimentation or diagenesis, and a later hydrothermal addition of zinc, lead, copper, and selenium.