Shales, black

The deposits occurring in the sedimentary rocks (mainly black shale) are distributed mainly in three districts Kitakami, Yamizo and Koma (Fig. 1.185). The few deposits in the metamorphic region are the Suwa, Kinkei, Amo and Hashidate (Fig. 1.185). 

Black shale Uraniferous alum shale in Sweden, the Chatanooga shale in the USA, the deposit of Gera-Ronneburg in the eastern part of Germany. 

U3Og). The concentration during the period 400-300 M years occurs in marine black shales, while that in the 300 M years to recent period is accounted for by sedimentary-type deposits, an important example being the Colorado Plateau deposits of USA. 

Fig. 7. Relationship between polarity index, (N+0)/C, and the Koc of a-Naphthol sorption to peat, black shale, and humic acids (HA) from different horizons of a soil. Modified from Chen et al. (1996).

High concentrations of lead were detected in the wine-coloured and black shales of the Volcanic Sedimentary Complex with 96 mg kg 1 and 52 mg kg"1, respectively. The highest concentrations were reported for the Cu-Pb-Zn massive sulphide ore samples from the abandoned mines of Cabezas del Pasto, Romanera and Serrencillas where concentrations of Pb exceeded 5000 mg kg"1. 

Clastic sediments from anoxic environments (black shales) Il L. 

At this point, nothing is known about Cu and Zn isotopic variability in seawater. Zn is very depleted in surface waters because it behaves as a nutrient. A substantial amormt of Zn isotope data is available for sediments. Marechal et al. (2000) found that the 5 Zn values of clay minerals from different environments (Paleozoic shales, including a black shale, Mediterranean sapropels. Pacific and Atlantic sediments, a eolian dust) fall within a narrow range (0.17-0.35%o) centered around the magmatic values and therefore reflect the Zn isotope composition... 

Marechal (1998) reports a relatively large seasonal fluctuation of 0.2%o in sediment trap material at the EUMELI site (see above) at depths of 1000 and 2500 m with maximum depletion of Cu during Spring and Summer. In contrast to Zn, Archer and Vance (2002) found that Cu (and Fe) in Belingwe black shales is signiflcantly lighter with 8 Cu values down to -1.0%o. 

Devonian black shale Mid-Proterozoic black shales ... 

Initial efforts to apply the Mo isotope system have targeted sediments deposited rmder oxic conditions (marine ferromanganese crusts) as well as sediments deposited under redueing conditions (black shales). Results of these studies are summarized below. 

As seen in Figure 6, mean 8 Mo in these ancient sediments is significantly offset from that of modem seawater and from mean 8 Mo of recent euxinic sediments. The offset is in the direction of less fractionation of Mo isotopes in the oceans (closer to the likely 5 MOjj ), as would be expected if removal of Mo to euxinic sediments expanded at the expense of removal to oxic (and suboxic) sediments. The result is consistent with the hypothesis of expanded ocean anoxia, and consequent increase in the areal extent of euxinic depositional settings, during this time. A similar, albeit smaller, offset is seen in repeated measurements of a Devonian black shale sample (USGS SDO-1 Fig. 6). This shift could he interpreted as consistent with somewhat expanded euxinic deposition during this time—an interpretation consistent with other evidence of expanded Paleozoic ocean anoxia in inland hasins. While further work is needed, these initial results are promising. 

Helz GR, Miller CV, Chamock JM, Mosselmans JEW, Pattrick RAD, Gamer CD, Vaughn DJ (1996) Mechanism of molybdenum removal from the sea and its concentration in black shales EXAFS evidence. Geochim Cosmochim Acta 60 3631-3642 Hille R (1996) The mononuclear molybdenum enzymes. Chem Rev 96 2757-2816... 

Figure 11.28 shows the rhenium and osmium isotopic compositions of black shales and sulfide ores from the Yukon Territory (Horan et al., 1994). The black shale and sulfide layers are approximately isochronous. The superimposed reference isochrons bracket the depositional age of the enclosing shales. One reference line represents the minimum age (367 Ma) with an initial ( Os/ Os)q ratio of one, consistent with the mantle isotopic composition at that age (see later). The other reference isochron is drawn for a maximum age of 380 Ma, with ( 870s/ 860s)o = 12 (the maximum value measured in terrigenous sediments). Further examples of application of Re-Os dating of sediments can be found in Ravizza and Turekian (1989). 

Figure 11.28 Re-Os isochron for black shales and sulfide deposits of Yukon Territory (Canada). Reprinted from M. F. Horan et ah, Geochimica et Cosmochimica Acta, 58, 257-265, copyright 1994, with kind permission from Elsevier Science Ltd., The Boulevard, Langford Lane, Kidlington 0X5 1GB, UK.

Horan M. E, Morgan J. W, Grauch R. I., Coveney R. M. Jr., Murowchick J. B., and Hulbert L. J. (1994). Rhenium and osmium isotopes in black shales and Ni-Mo-PGE-rich sulfide layers, Yukon Territory, Canada, and Hunan and Guizhou provinces, China. Geochim. Cosmochim. Acta, 58 257-265. 

Ravizza G. and Tnrekian K. K. (1989). Application of the Re- Os system to black shale geochronometry. Geochim. Cosmochim. Acta, 53 3257-3262. 

Geochemistry of Archean sulfidic black shale horizons combining data at multiple scales for improved targeting in VMS exploration... 

Keywords black shale, VMS, exploration vectoring, LA-iCP-MS, portableXRF... 

Metalliferous sediments are a common component of modern ocean-floor sedimentary sequences, recording halos of metal dispersion from seafloor hydro-thermal vent systems (Gurvich, 2006). Sulfidic black shales are also commonly present as intercalations in ancient subaqueous volcanic sequences, where each likely represents a significant hiatus in volcanic activity and deposition. These shale horizons form geophysical anomalies (conductors) that are routinely drilled during exploration for volcanogenic massive sulfide (VMS) base metal deposits. 

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