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- Geological background to the copper-bearing strata of southern Shaba (Zaïre)
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Geological background to the copper-bearing strata of southern Shaba (Zaïre)
Abstract
The Katangan (Katanga System or Sequence) is defined and a threefold division is adopted and justified. It is shown that each of the component supergroups is linked to a distinct uplift of the Kibaride Belt. The Upper Katangan or Kundelungu Supergroup is at least in part the result of the erosion of the Kibarides during uplift of the earlier belt under the influence of the Lufilian orogeny; the Middle Katangan or Grand Conglomérat and Mwashya Supergroup is mainly the result of uplift and erosion of the Kibarides caused by the Lomamian orogeny which affects the Lower Katangan in western and northern Shaba; the Lower Katangan or Roan Supergroup was deposited in part as a consequence of uplift and erosion of the Kibarides during the late stages of the Kibaran orogenic cycle and in part as a platform deposit during the waning phases and cessation of this uplift.
Whereas in southern Shaba deformation is practically entirely due to the successive phases of the Lufilian orogeny connected with the Kundelungu Supergroup, the situation near the Kibaride Belt and on both sides of it is different: the Lufilian orogeny is weaker and in some areas non-existent; earlier tectonic movements are detectable and sometimes important.
Stratiform copper is known in minor quantities in beds of the Upper and Middle Katangan. The important stratiform deposits are in strata belonging to the Lower Katangan.
The Lower Katangan is known from several distinct areas in Shaba. Only the Bushimay Supergroup deposited in a basin immediately to the north-west of the Kibaride Belt and the Roan Supergroup of the Shaba basin to the south-east of the same belt are examined and compared to each other and to what is known of the uplift history of the Kibaride Belt.
It is concluded that in both basins of deposition each uplift of the Kibaride Belt is accompanied by deposition of arenaceous sediments, the terrigenous character of which decreases as the uplift slows down. Slight sinking of the belt brings in all basins decrease of arenaceous sedimentation, increase of pelites and introduction of a carbonate component: this is followed by a more or less stable period characterized by predominantly carbonate deposition; finally the early stages of the Lomamian orogeny are marked by a volcanic activity and an increase of pelitic and arenaceous sedimentation which heralds the advent of the Middle Katangan, consequence of the past-Lomamian uplift of the Kibarides.
The stratified mineralization in the Bushimay Supergroup and in the Roan Supergroup appears to occur at the same general level. For the areas immediately to the north-west and to the south-east of the Kibaride Belt, this is shown by the study of columnar stromatolites which yields results in keeping with the model age of the syngenetic lead mineralization in the Bushimay Supergroup.
The same may be said of the southeastern Shaba (and Zambian) Copperbelt in view of
the striking parallelism between the depositional histories in all basins examined and the movements of the Kibaride Belt as deduced from the dating of various intrusive bodies.
The ore-bearing beds in all three areas are coeval with the top of the Middle Riphean of the U.S.S.R. (circa 1 100 - 1 050 m.yr.).
The mineralized groups appear at the close of the first uplift of the Kibaride Belt. The mineralization belongs to type 4 of Pelissonnier (1972) and thus conforms to this author's conclusion linking this type of mineral deposit to the post-orogenic stage of orogenic evolution. The reader will find three appendices at the end of the paper: 1. Notes on radiometric age data; 2. Isotopic composition of common leads; 3. On the limit between Lower and Middle Katangan.
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A propos de : Lucien Cahen
Musée royal d'Afrique Centrale, B-1980 Tervuren.