Black shales are common constituents of sedimentary basins and can often be enriched in metals compared to average crust. However certain black shales can contain a wide variety of metal enrichments, some of which can reach economic concentrations of Co, Ni, Zn, Cu, Mo, U, V and/or REE, Y and PGE (e.g., Mao et al., 2002; Coveney, 2003). Hyuck (1989) proposed a designation of ‘metalliferous black shale’ as being a shale that is enriched in metals by a factor of 2x (except for beryllium, cobalt, molybdenum, and uranium, for which 1x is sufficient) relative to the U.S. Geological Survey Standard SDO-1. Approximately 25% of global black shale examples can fit this description and some are currently being exploited.
Thin sulfide layers in Lower Cambrian shales of South China have been actively mined for decades and contain grades of over 4 wt % Mo and up to 4 wt % Ni, 2 wt % Zn, along with appreciable Pt, Pd and Ir concentrations (Coveney and Nansheng, 1991). Similar stratiform mineralized zones occur in black shales of Middle to Late Devonian age at the Nick horizon in the Yukon as well as other metal-rich horizons in the Selwyn Basin of North Western America (e.g. Magnall et al., 2015). The most economically significant metalliferous black shale (in terms of tonnage) is the Talvivaara Zn-Ni-Cu deposit in Finland that was mined from 2008 to 2013.
High-grade and high-tonnage black shales like those listed above are rare and can be argued to reflect unusual origins compared to ‘typical’ Phanerozoic black shale occurrences. For example, metals in the deposits in South China are proposed as being derived predominantly from seawater (Lehmann et al., 2007; Xu et al., 2013), but syn-depositional hydrothermal processes may have also provided some metal input (Pi et al., 2013, and references therein). In contrast, the Talvivaara deposit contains a thick but disseminated metal-rich precursor black shale, developed in an anoxic, metal-rich basin before being subjected to metamorphic mobilization (this study; Loukola-Ruskeeniemi and Lahtinen, 2013).
Metalliferous black shales are numerous in the Phanerozoic. At present metal prices the majority are sub-economic. However, recent exploration has continued searching for significant deposits in metalliferous black shales (e.g., Cretaceous shale units in the Alberta, Canada (Sabag, 2008); the Cambrian to Ordovician Alum Shale in Sweden (e.g., Leventhal, 1991; Dyni, 2005; Aura Energy Ltd.,
2014) and the Cretaceous Julia Creek deposit of Queensland, Australia (Lewis et al., 2010). Economic potential of metalliferous black shales is often limited by the thickness of the shales, e.g., the Mecca Quarry Shale (United States of America) contains up to 10,000 ppm V (Coveney and Martin, 1983). However, it is only a few tens of centimeters thick, and consequently is not economic.
It is becoming clear that metalliferous black shale deposition can be broadly constrained to certain time periods in Earth history, namely, the Paleproterozoic, the Cambrian, Devonian, Permian and Cretaceous. This begs the question as to whether there is a temporal evolution of metal-rich black shales and whether wider geodynamic processes may influence their evolution?
2.3.1. Metal sources in metalliferous black shales
Debate on whether metals enriched in metalliferous black shales are sourced directly from the seawater or hydrothermally derived, has been ensuing for many years (e.g., Coveney and Glascock, 1989; Lehmann et al., 2007). Metalliferous black shales with high concentrations of Mo, V, Zn, and other trace metals are common in the Phanerozoic. Modern analogues are known in euxinic basins (e.g., the Black Sea, Lyons et al., 2003), strengthen the suggestion that the source of many metals in most black shales are likely derived from seawater (i.e. Lehmann et al., 2007; Fig. 2.3.). Black shales with high metal concentrations derived from the interaction of hydrothermal solutions (such as those near present- day hydrothermal vent systems, i.e., the TAG Hydrothermal Field; Rona et al., 1993; Petersen et al., 2000) can exhibit elemental zonations and enrichments of Cr, Co, Mn Sb, and Ba, as well as base metals such as Ni, Zn Cu and Pb, and silica and/or Fe-oxide phases. Concurrent enrichments of these metals with U, Mo and V would require a very unique fluid history and composition if they were entirely derived form hydrothermal fluids. Therefore contributions of seawater and primary organic matter would still be required to account for enrichments of Mo, V and U. Furthermore, pyrites derived from hydrothermal solutions should hold much higher δ34S values (i.e. Herzig, and Hannington, 1995; Elderfield and Schultz, 1996; Large, 2000; Slack et al., 2007; Saez et al., 2011).
Biogenic sulfide (H2Saq) is produced in most modern carbonaceous, low-oxygen sediments, and pyrite is a common phase in black shales. However, not all pyritic black shales are the same, and the processes
of sulfide formation and the amount of H2Saq generated can exert influence on their formation of (Scott and Lyons, 2012). Where organic carbon deposition rates are low, bottom waters are more oxidizing and bacterial sulfate reduction is restricted to pore waters in the sediment below the sediment-water interface (Scott and Lyons, 2012). In that environment, H2Saq concentrations can be low and trace metal enrichments tend to be small (Scott and Lyons, 2012). In contrast, where organic carbon sedimentation rates are high, bottom waters can become more reducing and sulfate reduction takes place closer to the sediment-water interface. In zones within highly reduced settings, microbial sulfate reduction leads to H2Saq build up and the establishment of euxinic conditions in the water column itself. It is under these conditions that the highest concentrations of trace metals can form (Scott et al., 2008; Sahoo et al., 2012; Scott and Lyons, 2012; Scott et al., 2013).
Euxinic conditions are more common in the Neoarchean (Reinhard et al., 2009; Scott et al., 2010) and the Proterozoic (Poulton et al., 2004; Scott et al., 2008; Planavsky et al., 2012; Scott et al., 2014) than in other periods of Earth history. However, trace metal contents (V, Cr, Zn, Mo, U) are generally lower than those of typical Phanerozoic black shales. This depletion of metals in Precambrian euxinic black shales, relative to the Phanerozoic, can be attributed to many factors including lower weathering flux into the oceans and a higher biological turnover in sulfidic environments (Scott et al., 2008; Sahoo et al., 2012; Partin et al., 2013; Reinhard et al., 2013).
Anbar and Knoll (2002) proposed that redox-sensitive trace metals would be low in late Paleoproterozoic and Mesoproterozoic oceans, and also within the expansion of sulfidic environments in the wake of the Great Oxidation Event (GOE; Canfield, 1998). Anbar and Knoll (2002) further suggested that the bio-limitation of critical trace metals (i.e. Mo and V) might have been responsible for the delay in the diversification of eukaryotes.
The transition from generally metal-poor Precambrian black shales to metal-rich Phanerozoic black shales appears to correlate with widespread oxygenation of the deep oceans at the end of the Neoproterozoic, expansion of the seawater sulfate reservoir, and the first appearance of metazoans (Scott et al., 2008; Sahoo et al., 2012). Therefore, the distribution of metalliferous black shale appears to a function of the evolution of biogeochemical cycles and the dynamics of Earth’s redox state.
The Phanerozoic record includes many examples of black shales that have trace metal hyper- enrichments (>1,000 ppm V, Zn, and Mo; Scott et al., 2014). These hyper-enrichments, relative to typical black shale, are enigmatic as they are not present prior to 635 Ma, with the exception of Talvivaara, and do not occur in modern environments. It has been suggested by some workers that such extreme metal enrichments require a hydrothermal component (e.g., Coveney and Glascock, 1989; Emsbo, 2009). However, hydrothermal fluids in oceanic settings tend not to be enriched in V, Mo, or Cr relative to seawater (Douville et al., 2002). Furthermore, because hyper-enriched black shales are found almost exclusively in the Phanerozoic, it is plausible that these trace metal enrichments may record first order variations in major biogeochemical cycles and therefore, do not require a hydrothermal component.
The ultimate trigger that produced these hyper-enrichments remains debatable. However, the enrichments of Zn in modern sediments (Skei, 1988), and work on the reduction of V4+ to V3+ (Wanty and Goldhaber, 1992), propose that high concentrations of dissolved H2Saq are required to facilitate enrichments of these elements in black shales. Possible processes relating the association of trace metal enrichment and hyper-sulfidic conditions are discussed in later sections of this thesis (within Chapters 3, 5, 6 and 8). This relationship may provide a new and useful tool in understanding paleo-redox conditions as well as in the study and exploration of various sediment-hosted ore deposits.