Organic matter the major sink of redox-sensitive trace elements in Upper Devonian black shale

The Bakken Formation is a hydrocarbon-rich, metalliferous shale with subordinate sandstone and limestone that was deposited in the Williston Basin in Canada and the USA. Deposition of the Bakken Formation spanned the Devonian-Carboniferous boundary - a key interval in Earth's history representing profound biotic and geochemical changes in the terrestrial and marine realms, leading to deposition of metal-rich shales in the lower part of the sequence. Most studies of metal enrichment in the Bakken Formation have focused solely on the bulk rock litho-geochemistry, with little attention given to the organic matter, mineralogical, and diagenetic controls on metal sequestration and distribution. The current study combined bulk sulfur isotope and litho-geochemistry, along with reflected light and scanning electron microscopy (SEM) petrography to elucidate organic matter and mineralogical controls on trace element geochemistry. We have also used laser ablation-inductively couple plasma-mass spectrometry (LA-ICP-MS) to quantitatively assess the distribution of trace elements among different mineral phases, shale matrix, and organic matter. We show that sphalerite is the main zinc host, and that sphalerite sequesters Cu, Ga, Ge, and Cd within its crystal structure. Pyrite is the main Fe mineral and has typical abundances for sedimentary pyrite, however, the total bulk rock abundances of Ni, Mo, and Cu cannot be accounted for in pyrite alone, and indeed, are present within early mature organic matter in the shale matrix. Vanadium (V) is associated with organic carbon but is also high in aluminosilicates within the matrix. Collectively, this study demonstrates the need for detailed petrographic analyses even in seemingly monotonous black shales.