The distribution, nature and control of metal concentrations in black shales, and relationships with hydrocarbon source rocks

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Black, carbon-rich shales are conspicuous for their unusual metal enrichment, including Ni, Mo, Zn, Pb, Cu, Co, U, V, As, Ag, Au and PGE. They are also the source beds for hydrocarbon liquids and gases. In some cases, notably in the early Cambrian black shales of South China, the concentrations of some of these metals are locally high enough for economic exploitation. For example, in the Zunyi district, there are thin, sulphide-rich layers within that contain about 4 wt.% Ni, 2 wt.% Zn, 0.7 ppm Au, 50 ppm Ag, 0.3 ppm Pt and 0.4 ppm Pd. Nearby, there are other black shale-hosted deposits containing in excess of 4 wt.% V2O5. In Canada, black shales of middle to late Devonian age at the Nick prospect, Yukon are also known to host similarly high concentrations of Ni, Zn, Au and PGE. There are also extensive units of black shale in northeastern Alberta that have been considered recently for economic exploitation, albeit with relatively low metal grades. These shales, which form part of the Cretaceous Colorado Group of the Buckton Creek area, constitute a resource of ~4.5 billion tonnes containing Ni, Mo, U, V, Zn, Cu, Co, Li, REE, Y, Th and Sc with concentrations of metals at the ppm to low 100's of ppm levels. An important question that we wish to address in this research is how black shales are enriched in metals, especially to levels that would allow their economic exploitation. In particular, as such shales have passed through the oil window, we will examine the role of liquid hydrocarbons and their interplay with hydrothermal fluids/brines in mobilising and potentially enriching shales in some and possibly a large proportion of the metals referred to above, to concentrations well above those normally expected in black shales. However, we will exclude the relatively oxidized basins that give rise to Cu-Co-U deposits, as well as the SEDEX-type Pb-Zn deposits. Instead we will focus on deposits containing mainly Ni, Co, Mo, V, Zn, PGE and Au that form in anoxic/euxinic basins. A second question that we will investigate is the relationship between metal concentrations and ratios in black shales and their utility as vectors toward hydrocarbon accumulations, and as tools to evaluate redox and organic preservation models.********