Expanded Abstract of poster session given at IGCP-SEG Congress in Turku, Finland, August 1997: In: In Mineral Deposits: Research and Exploration Where do they meet? H. Papunen (ed.), Balkema, Rotterdam, p. 125-128 1997
The prediction of the location of organic rich marine shales could be a useful exploration tool in the search for metal rich deposits. The affinity between many stratiform metal-rich deposits and organic-rich shales is well documented. The organic content of marine shales generally is controlled by primary productivity in the overlying water column during deposition and redox conditions in the water column and sediments during deposition and early diagenesis. Regions of high productivity and thus potentially organic-rich sediments are found in areas of nutrient upwelling in the water column to the photic zone. The intersection of regions of high primary productivity with appropriate depositional environments can be predicted throughout geologic time. The following outlines a method for the prediction of the sites of organic rich shales using physical oceanographic principles combined with paleogeography.
The first order assumption is that upwelling is controlled by the major wind patterns and wind driven currents (Wilde et al. 1990). The planetary oceanic vertical advection systems are fixed latitudinally by the rotation of the earth and the resultant planetary wind patterns in a band centered about 30o N and S (convergences: downsinking and low productivity) and the Equator and 60o N and S (divergences: upwelling and high productivity). Additionally, upwelling phenomena are observed at 30o N and S on western coasts. Seasonal conditions due to Monsoonal or local weather patterns or unique land-ocean boundary conditions can produce either upwelling or downsinking which may blur or override the planetary signature. For seasonal cases, the geologic record will preserve conditions during major sedimentary events. Further local conditions, particularly water depth, also influence conditions within the sediments.
These local, regional, and global patterns are modified by the positions of the edges of the continental blocks, which shift in time. Thus the intersection of an upwelling oceanographic site with a continental edge with a wide continental shelf produces the maximum possibility for deposition of organic rich shales. Such shales provide the host for either syngenetic ore deposits or those produced at a later time by invasion by ore bearing fluids.
Preservation of organic rich deposits depends strongly on the redox conditions of both the water column and sediments during deposition and early diagenesis. Sediments can be deposited from oxic to anoxic conditions under fresh to estuarine to marine waters. Organic matter is best preserved under anoxic conditions, hence sediments deposited under high productivity conditions under an anoxic water column are most likely to be preserved - such conditions prevailed in marine waters in the pre-Devonian. It is reasonable to assume that sediments deposited under these conditions with appropriate sources can tend to be metalliferous. Well-known examples (Loukola-Ruskeeniemi, 1992) include the Dictyonema shales of Balto-Scania and the Kupferschiefer (Table 1). In Table I, three of the sites, the Kupferschiefer, the Alum Shale, and the Songlin occur on planetary divergences. The Nick Property at 30o S is on western coast and therefore would occur at a secondary upwelling site.
One may test this hypothesis by an examination of black shales from a variety of sites through time. The marine black shales used in this study are finely-divided indicating slow rate of deposition, low permeability after compaction, and lack of not-bioturbated. Lack of bioturbation indicates that there was insufficient oxygen in the sediments to support aerobic organisms. Thus to aerobic organisms, the marine black shales of this study would be considered anoxic, however, chemically their conditions of deposition range from oxic, to nitrate- and sulfate-reducing to possibly methanogenic (Quinby-Hunt and Wilde, 1994, 1996). Accordingly, the chemical signature is preserved.
Table 1. Phanerozoic Sites in which organic-rich deposits might be expected, with their paleo-geographic latitudes. Deposit Ore Time Paleolatitude Kupferschiefer Cu-Ag (a) Permian 0-5o N Nick Property Ni-Zn-Pt gp (b) Devonian 30o N Alum Shale U-V (c) Cambro/Ordovician 60o S Songlin, China Mo-Ni-Pt gp (d) Early Cambrian 0o
(a) Jowett et al., 1991.
(b) Hulbert et al., 1992.
(c) Andersson et al. 1985.
(d) Coveney and Chen, 1991; Grauch et al., 1991
The samples to be discussed include approximately 100 upper Cambrian/Early Ordovician samples from Balto-Scania, Wales, Scotland, Belgium, New Brunswick, Quebec, and Bolivia (Figure 1). The samples with boxes around the designator (N = Norway, S = Sweden, D = Denmark, E = Estonia, NB = New Brunswick) are on the planetary divergence. The sites with circles around the designator (B = Belgium, L = Levis, Canada, Bo = Bolivia, and Norway-Caledonides = N-Ca) occur off the divergence. The samples from Wales (W) occur close to the divergence but are shoal and out of the anoxic zone. We will also examine the composition of black shales from Baltica in the Cambro-Ordovician to examine the effect of its changing latitude on the alum shale and its equivalents (Figure 2).
Figure 1 (left). Tremadoc sample locales and concentrations of anoxic zone indicators (Quinby-Hunt and Wilde, 1994).
Figure 2 (above to the right) Drift of Balto-Scania centered on Oslo district with time from the middle Cambrian through the late Ordovician. Concentrations of anoxia indicators found in the samples from that site are given (Quinby-Hunt and Wilde, 1994).
2.1 Anoxic influence
The chemistry of anoxic waters differs vastly, resulting in wide variations in dissolved metal content and speciation depending on the most oxidative species present. We have demonstrated (Quinby-Hunt and Wilde, 1994) that depth of anoxia and degree of organic preservation can be determined for black shales based upon the concentrations of three metals -iron, manganese, and vanadium. As this paper will demonstrate, when a site is located at a latitude associated with major upwelling and highly anoxic conditions, the vanadium concentration is extremely high. An example of this condition was found in the Alum-Dictyonema shales of Balto-Scania.
2.2 Latitudinal effects
The influence of latitude is demonstrated by an examination of the concentrations of the anoxia indicators in the black shale samples of the Tremadoc about the Iapetus Ocean. The samples from sites located at the planetary upwelling, those from Balto-Scania and New Brunswick have very high concentrations of V and low levels of Mn and Fe. Samples from other sites around Iapetus generally show the more common low concentrations of V. That they are generally of low pE is indicated by the low Mn and Fe concentrations. The samples from Wales have higher concentrations of both Mn and Fe because the samples were shoaler and are out of the anoxic zone (Quinby-Hunt and Wilde, 1994).
The latitudinal effect can also be seen in an examination of the concentrations of the anoxic indicators in black shale samples from the Oslo site with time (Figure 2). In the middle Cambrian, the Oslo site was north of the planetary upwelling zone at 60o S and the V concentration is close to the norm for black shales, 260 ppm. From the late Late Cambrian through the Tremadoc, the site was at this upwelling latitude and the V concentration is many times higher, reaching more than 10x (2740 ppm) at the beginning of the Tremadoc. By the Arenig, the site had drifted north of 60o S and the V concentration had dropped significantly to roughly 300 ppm.
We propose that this knowledge can readily be used to identify sites at which metalliferous organic-rich deposits might be expected due to syngenesis. The presence of highly developed anoxia would have lead to precipitation of many minerals that occur in the deposits. The likelihood of economic deposits would increase if the site is in proximity to ridge rise systems or ore-bearing intrusion which would provide sources for the metals.
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