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Selebi-Phikwe - Pikwe, Selebi, Selebi North, Dikoloti, Lentswe, Phokoje |
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Botswana |
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Ni Cu
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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All papers now Open Access.
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The Pikwe - Selebi (or Selebi - Phikwe) copper-nickel deposits are located in eastern Botswana, 400 km NNE of Gabarone (#Location: 21° 56' 45"S, 27° 51' 25"E).
The Selebi-Phikwe district lies within the 250 x 550 km block of granulite facies metamorphic rocks that forms part of the Archaean to Palaeoproterozoic Limpopo Belt separating the Kaapvaal and Zimbabwe cratons (Barton et al., 2006 and references therein). The Limpopo Belt is made up of several terranes with different tectono-metamorphic histories and ages of formation that range from 3.2 to 2.6 Ga. These terranes were accreted to the Zimbabwe and Kaapvaal cratons along wide, steeply dipping shear zones over a period of ~700 m.y., from 2.7 to 2.04 Ga (Barton et al., 2006). They comprise the Central Zone, Southern Marginal Zone and Northern Marginal Zone. The Phikwe-Selebi Complex is located within the Central Zone and largely consists of Archaean hornblende-bearing tonalitic and trondhjemitic gneisses, and the Selebi-Phikwe belt of mafic–ultramafic intrusions. The latter are intruded into a suite of 2.65 to 2.60 Ga (U-Pb SHRIMP, McCourt et al., 2004) medium- to coarse-grained, massive to weakly foliated, granoblastic to porphyroblastic granite gneiss and a variety of banded supracrustal gneisses comprising hornblende-gneiss, quartzo-feldspathic gneiss and anorthositic gneiss. The protoliths of the hornblende-gneisses are interpreted to have been volcanic rocks and shallow intrusions of tholeiitic basaltic and Ti-rich ferro-basaltic composition, whereas the protoliths to the quartzo-feldspathic gneisses may have been calc-alkaline volcano-sedimentary rocks (Brown 1988). Subordinate pelitic schists, marbles, impure quartzites and ironstones are also mapped. Most of the above rocks are very sulphide poor, containing <200 ppm S (Brown 1988).
The Selebi-Phikwe belt includes intrusions hosting the Selebi, Selebi North, Dikoloti, Lentswe and Phokoje deposits which are spread over a 15 km interval. The ore bearing bands over this interval is from 0 to 45 m thick and comprises an amphibolite hosting mineralisation ranging from disseminated (0 to 40%) to semi-massive (40 to 70%) and massive (70 to 95%) pyrrhotite with associated pentlandite and chalcopyrite. In all of these deposits, the sulphide ores are predominantly associated with boudinaged, generally relatively thin lenses and layers of fine- to medium-grained amphibolite interlayered with various types of gneisses (Gordon 1973; Wakefield 1976; Key 1976; Gallon 1986; Brown 1988, Maier et al., 2008).
At Phikwe, the ore-bearing intrusions average ~11 m in thickness (Lear 1979), although it is considered this may largely be the result of intense folding and shearing. The amphibolites are mainly composed of hornblende, feldspar, gedrite and mica. Minor metamorphic orthopyroxene and olivine are also found. Brown (1988) estimated from the geochemistry of the intrusion that their parental magmas were tholeiitic basalts (with ~8 wt.% MgO) that crystallised variable proportions of olivine (Fo85), pyroxene (En85) and plagioclase (An83). Disseminated and massive sulphides are found throughout the amphibolites, locally forming up to 38 m thick concentrations of massive and semi-massive sulphides, particularly adjacent to the contacts between the amphibolites and the host gneisses. The sulphides are intensely tectonised, as evidenced by their concentration in crosscutting veins and fold closures, common durchbewegung fabrics, and the concentration of silicate inclusions along traces of isoclinal folds within the massive sulphides. This has been interpreted to suggest that the massive sulphide zones focussed at least three phases of shearing resulting in intense tectonisation (Brown 1988). The principal sulphide minerals within the massive and disseminated mineralisation are pyrrhotite, pentlandite and chalcopyrite which occur in variable relative proportions. Chalcopyrite may be relatively concentrated around and within silicate inclusions. Minor sulphides formed during hydrothermal and supergene alteration include bravoite, marcasite, pyrite and violarite (Lear 1979). Euhedral, subhedral or anhedral magnetite grains constitute up to 20 vol.% of the opaque fraction (Maier et al., 2008).
Image below: Polished slab of Selebi-Phikwe net textured semi-massive sulphide mineralisation (Image by Mike Porter 2021; sample taken 1994).
The Dikoloti-Lentswe-Phokoje deposits are located a few kilometres to the SW and W of the Selebi-Phikwe deposits and occur within tectonised lenses and layers of partially serpentinised / amphibolised peridotite, pyroxenite and melagabbronorite that may be as much as 45 m in thickness. These intrusions are surrounded by hornblende gneiss, quartzofeldspathic gneiss and magnetite quartzite. Based mainly on geochemical data, Brown (1988) interpreted the Dikoloti, Lentswe and Phokoje intrusions to have crystallised from magma of broadly similar composition as the Phikwe intrusions, but with a larger cumulate component. Lithostratigraphic correlations, led Brown (1988) to propose that the Dikoloti, Lentswe and Phokoje bodies were intruded at a higher stratigraphic level than those at Selebi-Phikwe, although as the contacts between the mafic–ultramafic intrusions and the gneissic host rocks tend to be intensely sheared, lithostratigraphic reconstructions were regarded to be contentious, and other authors have favoured a model whereby all intrusions in the Selebi-Phikwe belt belong to the same tectonically dismembered sill (Gordon 1973). Sulphides are disseminated and occasionally form thin, <1 m thick, irregular massive lenses. The latter are particularly located along the basal and upper contacts of the intrusions. Massive sulphides concentrations also tend to be localised in the nose and adjacent limbs of isoclinal antiforms, e.g. at Dikoloti. Locally, some massive sulphide lenses can be as much as 10 m in thickness. As at Phikwe, the sulphides contain abundant silicate inclusions of hornblende, biotite, carbonate, epidote and chlorite. Pyrrhotite is the principal sulphide mineral, with pentlandite, occurring mainly as flame-like exsolutions in pyrrhotite, and chalcopyrite significantly, i.e., <5 vol.% less abundant than at Phikwe. Magnetite constitutes ~5 vol.% of the massive ore but >10 vol.% of the disseminated ore. Both the magnetite and the silicate inclusions may be distinctly rounded (Maier et al., 2008).
Reserves remaining in 1985 were 74 Mt @ 1% Cu, 0.75% Ni, after a startup in 1973 and production rate of 3 Mtpa.
The Selebi-Phikwe deposits were discovered during a regional geochemical soil sampling exploration program which detected a 1500 m long pronounced Ni/Cu anomalous zone at Selebi in 1963. Further sampling to the north outlined another impressive anomaly 14.5 km NE of Selebi at Phikwe by 1966. Diamond drilling continued from 1966 to 1971. Underground exploration commenced in 1967 (Gallon, 1986). Mining operations at Pikwe-Selebi began in 1973 and ceased in September 2016.
The most recent source geological information used to prepare this decription was dated: 2008.
Record last updated: 2/6/2021
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
Phikwe
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Fiorentini M L, Bekker A, Rouxel O, Wing B A, Maier W and Rumble D, 2012 - Multiple Sulfur and Iron Isotope Composition of Magmatic Ni-Cu-(PGE) Sulfide Mineralization from Eastern Botswana: in Econ. Geol. v.107 pp. 105-116
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Gallon M L 1986 - Structural re-interpretation of the Selebi-Phikwe nickel-copper sulphide deposits, Eastern Botswana: in Anhaeusser C R, Maske S, (Eds.), 1986 Mineral Deposits of South Africa Geol. Soc. South Africa, Johannesburg v2 pp1663-1669
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Maier W D , Barnes S-J, Chinyepi G, Barton J M, Eglington B and Setshedi I, 2008 - The composition of magmatic Ni-Cu-(PGE) sulfide deposits in the Tati and Selebi-Phikwe belts of eastern Botswana: in Mineralium Deposita v43 pp 37-60
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