14
~ INTRODUCTION
black organic radioactive shale that is remarkable for its lateral uniformity across the
North Sea basin. It overlies the eolian sands of the Rotliegende, and is itself overlain by
the Zechstein evaporites. Locally the Kupferschiefer is sufficiently rich in copper minerals such as chalcopyrite and malachite to become an ore. A syngenetic origin for the
ore is widely accepted (e.g., Gregory, 1930; Brongersma Sanders, 1967).
In Zambia, relatively unmetamorphosed Pre-Cambrian sediments of the Roan Group
overlie an igneous and metamorphic basement. Extensive copper mineralization in the
Roan Group sediments includes pyrite, chalcopyrite, bornite, and chalcocite. These occur within shallow marine shales and adjacent fluvial sandstones, but not in eolian sandstones or basement rocks. The mineralization is closely related to a paleocoastline. The
ore bodies are mineralogically zoned with optimum mineralization in sheltered embayments (Fig. 1.2B). Early workers favored a hydrothermal epigenetic origin for the ore
due to intrusion of the underlying granite. It was demonstrated, however, that the irregular granite/sediment surface is due to erosion, not intrusion, Criteria now quoted
in favor of a syngenetic origin for the ore include its close relationship with paleotopography and facies, and the occurrence of reworked ore grains in younger but penecontemporaneous sandstones (Garlick, 1969; Garlick and Fleischer, 1972). Analogous
deposits at Kamoto in Katanga have been interpreted as shallow marine in origin, but
extensive diagenesis is invoked as the mineralizing agent (Bartholome et al., 1973).
Though the source of the Copper belt ores is unknown, a syngenetic origin is now
widely accepted, with the proviso that subsequent diagenesis has modified ore fabrics
and mineralogy (Guilbert and Park, 1986).
1.3.2.3 Epigenetic Sedimentary Ores
Epigenetic or exogenous ores in sedimentary rocks are those that formed later than the
host sediment. Epigenetic ores can be generated by a variety of processes. These include the concentration of disseminated minerals into discrete ore bodies by weathering, diagenetic, thermal, or metamorphic effects. Epigenesis also includes the introduction of metals into the host sediment by meteoric and hydrothermal solutions resulting
in the replacement of the country rock by ore minerals.
Criteria for differentiating syngenetic and epigenetic ores have been mentioned in
the preceding section. Epigenetic ores are characteristically restricted to modern topography, to unconformities or, if hydrothermal in origin, to centers of igneous activity. The ore may not contain a dwarfed and stunted fauna as in syngenetic deposits. Isotopic dating will show the ore to postdate the host sediment (Bain, 1968).
Once upon a time, epigenetic ores were very largely attributed to hydrothermal
emanations from igneous sources, except for obvious examples of shallow supergene
enriched ores. Within recent years it has been shown that many epigenetic ores lie far
distant from any known igneous center and show no evidence of abnormally high geothermal gradients (Sangster, 1995). It has been argued that these ores formed from concentrated chloride-rich solutions derived from evaporites and from residual solutions
derived from the final stages of compaction of clays (Davidson, 1965; Amstutz and Bubinicek, 1967). The two main groups of ore attributed to these processes are the Mississippi Valley type lead-zinc sulfides (Fig. 1.2C), and the uranium "roll-front" ores.
These are described in more detail in Section 6.3.2.8 and Section 6.3.2.2.4, respectively.
~ INTRODUCTION
black organic radioactive shale that is remarkable for its lateral uniformity across the
North Sea basin. It overlies the eolian sands of the Rotliegende, and is itself overlain by
the Zechstein evaporites. Locally the Kupferschiefer is sufficiently rich in copper minerals such as chalcopyrite and malachite to become an ore. A syngenetic origin for the
ore is widely accepted (e.g., Gregory, 1930; Brongersma Sanders, 1967).
In Zambia, relatively unmetamorphosed Pre-Cambrian sediments of the Roan Group
overlie an igneous and metamorphic basement. Extensive copper mineralization in the
Roan Group sediments includes pyrite, chalcopyrite, bornite, and chalcocite. These occur within shallow marine shales and adjacent fluvial sandstones, but not in eolian sandstones or basement rocks. The mineralization is closely related to a paleocoastline. The
ore bodies are mineralogically zoned with optimum mineralization in sheltered embayments (Fig. 1.2B). Early workers favored a hydrothermal epigenetic origin for the ore
due to intrusion of the underlying granite. It was demonstrated, however, that the irregular granite/sediment surface is due to erosion, not intrusion, Criteria now quoted
in favor of a syngenetic origin for the ore include its close relationship with paleotopography and facies, and the occurrence of reworked ore grains in younger but penecontemporaneous sandstones (Garlick, 1969; Garlick and Fleischer, 1972). Analogous
deposits at Kamoto in Katanga have been interpreted as shallow marine in origin, but
extensive diagenesis is invoked as the mineralizing agent (Bartholome et al., 1973).
Though the source of the Copper belt ores is unknown, a syngenetic origin is now
widely accepted, with the proviso that subsequent diagenesis has modified ore fabrics
and mineralogy (Guilbert and Park, 1986).
1.3.2.3 Epigenetic Sedimentary Ores
Epigenetic or exogenous ores in sedimentary rocks are those that formed later than the
host sediment. Epigenetic ores can be generated by a variety of processes. These include the concentration of disseminated minerals into discrete ore bodies by weathering, diagenetic, thermal, or metamorphic effects. Epigenesis also includes the introduction of metals into the host sediment by meteoric and hydrothermal solutions resulting
in the replacement of the country rock by ore minerals.
Criteria for differentiating syngenetic and epigenetic ores have been mentioned in
the preceding section. Epigenetic ores are characteristically restricted to modern topography, to unconformities or, if hydrothermal in origin, to centers of igneous activity. The ore may not contain a dwarfed and stunted fauna as in syngenetic deposits. Isotopic dating will show the ore to postdate the host sediment (Bain, 1968).
Once upon a time, epigenetic ores were very largely attributed to hydrothermal
emanations from igneous sources, except for obvious examples of shallow supergene
enriched ores. Within recent years it has been shown that many epigenetic ores lie far
distant from any known igneous center and show no evidence of abnormally high geothermal gradients (Sangster, 1995). It has been argued that these ores formed from concentrated chloride-rich solutions derived from evaporites and from residual solutions
derived from the final stages of compaction of clays (Davidson, 1965; Amstutz and Bubinicek, 1967). The two main groups of ore attributed to these processes are the Mississippi Valley type lead-zinc sulfides (Fig. 1.2C), and the uranium "roll-front" ores.
These are described in more detail in Section 6.3.2.8 and Section 6.3.2.2.4, respectively.