ABSTRACT

Kupferschiefer type syngenetic metalliferous organic-rich black shales as well as certain other copper deposits in the Paleozoic are shown to be strongly correlated with anoxic conditions in the water column due to low atmospheric oxygen concentrations. Global anoxicity in the water column must be considered in addition to previously investigated cyclic tectonic relationships. In the mid Mesozoic and younger, local anoxia combined with favorable tectonic settings override atmospheric oxygen concentrations in providing reducing ore-fixing solutions. Local rapid transgression and the availability of organic material in marine sediments appear to be important factors to produce subsesequent mineralization.

INTRODUCTION

Studies of the origin of ore deposits in black shales recently have concentrated on the link to global tectonic events (Titley, 1993, Kibek, 1991, Huyck, 1989 and Gustafson and Williams, 1981) generally relating to the Wilson tectonic cycle. In this paper, we investigate the correlation of syngenetic ore bodies of the Kupferschiefer type (KST: Eugster, 1989) with global atmospheric conditions favoring black shale formation which are seemingly independent of tectonics. By focusing on syngenetic deposits, we avoid the complication of interpreting the time and manner of the introduction of the ores or ore-bearing fluids after diagenesis. Accordingly, dating of both the sediment and the postulated influencing global event is not a problem. Eugster (1989) has summarized the various theories of origin of Kupferschiefer deposits. He describes the geologic setting as an organic-rich marine shale overlying continental red beds.

SOURCES

Russian geologists have provided a wealth of global inventories throughout the Phanerozoic. We use Budyko, Ronov and Yanshin (1987) for the major global atmospheric-sediment interactions. Basically their concept is that the amount of organic matter preserved in the geologic record can be used to predict atmospheric compositions with time. Carbon is stored and preserved during intervals of low amounts of atmospheric oxygen. Budyko et al. (1987, p. 101) calculate three major intervals of low oxygen concentrations [below 75% of Present Atmospheric Level: PAL] in the atmosphere (1)Cambrian-Early Ordovician; (2) Silurian-Devonian; (3) Permian- Early Jurasssic; and one minor one in the Early Cenozoic. As noted by Wilde (1987), Henry's Law predicts that the level of saturation of oxygen in the ocean fluctuates directly with the amount of oxygen in the atmosphere. Thus during deposition of marine sediments, the redox conditions in the overlying water is strongly influenced by atmospheric oxygen concentrations. In the modern global ocean, with connections at depth at high latitudes through temperate to tropical, the redox profile consists of (1) a surface well-mixed layer in equilibrium with the atmosphere; (2) a region of declining oxygen concentration with depth due to its consumption by organic matter raining down below the photosynthetic zone to a minimum value; (3) a region of increasing oxygen caused by advection from (4) a deep well-oxygenated zone to the sea floor as a result of the sinking of cold oxygen-rich water from high latitudes (Sverdrup, Johnson and Fleming, 1942). True anoxia in the water column is only seen in barred basins like the Black Sea and the Cariacao Trench or the Arabian Sea where there is no or only limited connection to deep ventilated waters from high latitudes. However, with reduced oxygen in the atmosphere such as in the pre-Cretaceous, true anoxia would exist in the oceanic water column globally (Wilde and Berry, 1982).

Titley (1993) using Kirkham's (1989) compilation has identified various stratiform ore deposits over time. In Figure 1, we have plotted the Phanerozoic occurrences of Cu-KST depositsand Cu-Pb-Zn deposits from Titley (1993 p. 296) with the atmospheric oxygen curve of Budyko et al. (1989, p. 101). In the Paleozoic-Early Mesozoic, the Cu-KST deposits and the Cu-Pb-Zn clastics (CH-LZS) occur during the three major intervals of low atmospheric oxygen. However, later deposits in the Jurassic and younger do not seem to correlate with low atmospheric oxygen, and in fact are coincident with atmospheric values greater than modern. We focus on the KST deposits, but note in passing the apparent correlation of Titley's (1993) CH-LZS deposits with atmospheric oxygen minima as according to Huyek (1989), they generally are associated with black shales.

DISCUSSION

Jowett (1989) has demonstrated the major KST deposits are synchronous with the rifts producing the continental source beds. All the KST deposits have underlying red-beds associated with rifting albeit not Wilson-cycle rifting. The common thread is the rapid transgression of marine organic-rich shales over the newly-formed continental deposits. Apparently, KST deposits can occur independent of eustatic variations in sea level using curves discusses by Hallam's (1992, p. 49-79). Local sea level fluctuations certainly occur within the Wilson cycle as a result of rifting and accretion of various terranes.

The relatively low atmospheric oxygen concentration would cause a relatively thin aerated surface layer in the ocean with concomitantly lower oxygen concentrations even within the oxygenated zone. Beneath that zone, waters would be anoxic. Thus a relatively minor transgression would bring anoxic waters onto pre-existing continental sediments. At present atmospheric conditions, the oxygen minimum zone is from 600 to 1000 meters. To bring the present day oxygen-mininum zone over present continental sediments, would necessitate a sea level rise both major and eustatic. However for the times of Phanerozoic minimum values, the atmospheric oxygen concentration was at 40% PAL (Fig. 1). Using calculation techniques in Wilde (1987) for tropical waters with atmospheric concentration at 40% PAL, the maximum saturation would be about 2 mL/L for the surface wind-mixed layer as compared with concentrations greater than 5 mL/L in the tropics today. Anoxic water would occur as shoal as 60 meters (Fig. 2). The organic-rich nature of the KST deposits suggests deposition in a high productivity zone, particularly in the pre-Devonian (before land plants). In areas of high productivity, organic matter settling through the only moderately aerated waters rapidly consumes what little oxygen is present. This would decrease the oxygen content even below the low saturation values with the atmosphere. Therefore in the regions where KST deposits are found, masses of organic matter could have been deposited even at shoaler depths augmented by land plants in the post Devonian and seen in the classic Kuperschiefer as kerogen (Eugster, 1989). Thus the change of sea level associated with KST deposits need not have been dramatic or large to bring anoxic or near anoxic waters over continental clastics in the rift. KST deposits occur close to shore suggesting that the transgression occurred rapidly, otherwise the new sediments would be oxidized even with the low but positive saturation values..

After deposition, waters within the relatively porous underlying continental sediments mobilized metals (for example, copper) in the continental sediments. These probably were of continental origin driven by the hydraulic head of the land. Compaction, would express these metal bearing fluids up into the organic-rich muds. The pore waters associated with the organic-rich material would be anoxic containing high concentrations of ionic sulfides. Metals in the ore-bearing fluids expelled from the continental sediments during compaction would precipitate as sulfides in the interface zone between the two types of pore water in the overlying black muds and the top of the continental clastics.. Tidal pumping in shoal waters also could facilitate the exchange of fluids between the layers and produce fluctuations in the location of the mineralized zone. This model is chemically similar to that of Eugster (1989) and others mentioned in his historic summary of theories of origin of the Kupferschiefer. It does explain the timing of the ore deposits as a function of atmospheric and related oceanic redox conditions.

KST deposits apparently occur in a variety of geographic settings within the classic Wilson cycle framework. Figure 3 shows the paleogeography situation for three KST events; two corresponding with atmospheric low concentrations in the Cambrian (ca. 495) and the Permian (ca. 255 MA) and one in the Cretaceous (ca. 100 Ma). In the Cambrian, the KST deposits occur on blocks facing the open ocean but in the equatorial high-productivity zone. For the classic Permian KST locales, all are in shallow seas in tropical to semi-tropical climates near the evaporation maxima near 30^o. The Cretaceous examples from West Africa and Eastern South America occur when the Atlantic was still very small [Figure 3] so that localized anoxic conditions could exist due to a combination of poor circulation, warm water temperatures and high productivity (Herbin et al., 1986). The KST deposits occur in a highly-constricted sea in which the onset of anoxia would be rapid. Such conditions are similar to those found in modern basins such as the Cariocao Trench and Chesapeake Bay estuary.

SUMMARY AND CONCLUSIONS

The combination of anoxic sediments overlying continental source beds produce KST deposits. Anoxic conditions and black shales occur under almost any atmospheric oxygen level. In the Phanerozoic, KST deposits are associated with low levels of oxygen in the atmosphere until the middle Mesozoic when atmospheric values reached or exceeded present levels. Accordingly, both favorable tectonic conditions and oceanic redox conditions are required for ore formation. Such oceanic conditions are more easily attained at low atmospheric levels, but can be locally augmented by high organic productivity (marine or terrestrial in the post-Devonian) or warm water temperatures proxying for initial low saturation values in the ocean. This apparently occurred in the later Mesozoic deposits. Estimates of atmospheric oxygen content are limited to the Phanerozoic using the Budyko et al. (1987) inventory method. May one suggest that the Proterozoic KST deposits listed by Titley (1993) also reflect atmospheric oxygen minima as the atmosphere progressively ventilated?

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