A Fertile Palaeoproterozoic Magmatic Arc Beneath The Central African Copperbelt
A Fertile Palaeoproterozoic Magmatic Arc Beneath The Central African Copperbelt
A Fertile Palaeoproterozoic Magmatic Arc Beneath The Central African Copperbelt
C.Rainaud
Department of Geology, University of the Witwatersrand, Pvt. Bag 3, WITS 2050, South Africa
R.A.Armstrong
Research School of Earth Sciences, Australian National University, Canberra 0200, ACT, Australia
S.Master
Department of Geology, University of the Witwatersrand, Pvt. Bag 3, WITS 2050, South Africa
L.J.Robb
Department of Geology, University of the Witwatersrand, Pvt. Bag 3, WITS 2050, South Africa
ABSTRACT: Chemical characteristics and new U-Pb SHRIMP zircon age data from the basement of the
Katangan Sequence in the Central African Copperbelt confirm the existence of a Palaeoproterozoic calc-
alkaline volcanic arc. The Lufubu schists, comprising intermediate to acid metavolcanics, have been dated at
circa 2000 Ma. Granitic gneiss from the Mkushi copper mine south of the Zambian Copperbelt yields an age of
2049 Ma, whereas the Mufulira granite in the Copperbelt is 1991 Ma. The Samba copper porphyry is dated at
circa 1960 Ma. Quartzite from the Muva Supergroup, which unconformably overlies the crystalline basement,
exhibits a broad range of detrital zircon ages from 3180 Ma down to 1941 Ma, indicating both Archaean and
Palaeoproterozoic provenances. The basement to the world class copper-cobalt deposits of the Katangan
Sequence is regarded as a magmatic arc that is, in itself, copper enriched. This has implications for ore-genesis
models in the region.
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The Muva Supergroup unconformably overlies the
Lufubu schists and includes mainly quartzites with
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Sr age of 1777 ± 89 Ma had been obtained by 44 analyses have been carried out on 42 detrital
Ng’ambi et al., 1986) and detritus from the Muva zircons from a sample of the Muva quartzite close to
quartzites. In addition we have age data for the Mufulira. The results are plotted on the concordia
Samba porphyry and a granite intersected in diagram in Figure 5, where it is seen that a wide
boreholes through the Chambishi Basin, as well as range of ages exist. A significant Archaean
intermediate to felsic metavolcanics from the Lufubu component is observed with ages clustering at 2550
schists. to 2700 Ma and also at around 3000 to 3180 Ma.
Palaeoproterozoic zircons range down to 1941 Ma,
the youngest zircon in the population and an
2.1 The Lufubu Schists
indication, therefore, of the maximum age of
The Lufubu schists, sampled from Kinsenda (in the deposition for the Muva Supergroup. The
DR Congo) and Mufulira (Zambia) yielded similar provenance for the Muva sediments clearly
ages at circa 2000 Ma. The existence of material of comprises both the local Palaeoproterozoic basement
similar age from a large sampling area indicates as well as Archaean crust.
widespread intermediate to acidic volcanism in
Central Africa.
2.2 Granitoids
The Mkushi gneiss yielded a weighted mean
207
Pb/206Pb age of 2049 ± 6 Ma (Fig. 3) which can be
interpreted as the age of emplacement of the gneiss
protolith. Aplites which cut through the gneisses
contain significant economic copper mineralization
(now mined out), but their age has not yet been
obtained.
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African Copperbelt, comprises a Palaeoproterozoic mineralization. Abstract, Québec 1998, GAC
calc-alkaline magmatic arc. Tonalite-granodiorite MAC-APGGQ Annual Meeting, Québec,
gneisses (Mkushi) together with more evolved Canada, May 1998.
granitoids (Mufulira, Samba, and Chambishi)
represent the plutonic phases of a major intermediate Ng’ambi, O., Boelrijk, N. A. I. M., Priem, H. N. A.
to acid volcanic province (Lufubu schists) emplaced & Daly, M. C. 1986 Geochronology of the
in the range 2050 to 1950 million years ago. This Mkushi Gneiss Complex, Central Zambia.
basement complex is unconformably overlain by the Precambrian Res. 32: 279-295.
Muva Supergroup which is younger than 1940 Ma
and contains detrital zircons, the age of which Ngoyi, K., Liégeois, J. P., Demaiffe, D & Dumont,
indicate the presence of both Archaean and P. 1991. Age tardi-ubendien (protérozoique
Palaeoproterozoic provenances. This basement inférieur) des dômes granitiques de l'arc cuprifère
complex is characterized by the presence of Cu zaïro-zambien. C. R. Acad. Sci. Paris: 313:
mineralization in the form of the economically
significant Samba porphyry copper deposit and the Pienaar, P. J. 1961. In Geology of the Northern
enigmatic aplite-related copper mineralization in the Rhodesian Copperbelt: Macdonald, London, 30-
Mkushi gneisses. The basement to the Katangan 41.
Sequence is, therefore, fertile with respect to copper
and this has implications for the origin and formation Sweeney, M., Binda, P. L. & Vaughan, D. J. 1991.
of the enormous metal concentrations in the latter. Genesis of the ores of the Zambian Copperbelt.
Ore genesis models which favour the circulation of Ore Geol. Rev. 6: 51-76.
diagenetic or metamorphic brines through permeable
basal Katangan sediments, enabling fluids to interact Wakefield, J. 1978 Samba: a deformed porphyry-
with detrital metal-rich components derived from the type copper deposit in the basement of the
fertile basement, and then re-precipitate sulphide ores Zambian Copperbelt. Trans. Inst. Min. Metall.
at redox interfaces in the overlying sedimentary 87: B43-B52.
sequences (Master, 1998a) receive added impetus in
the light of the data presented above. Winchester, J. A. & Floyd, P. A. 1977. Geochemical
discrimination of different magma series and
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