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Tati District - Phoenix, Selkirk |
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Botswana |
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Ni Cu Pd Pt PGE PGM
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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 Phoenix and Selkirk nickel-copper sulphide deposits are 15 km apart and are located ~30 km east of Francistown in the Tati area of northeastern Botswana, within a few kilometres SW of the Zimbabwe border.
(#Location: Phoenix - 21° 12' 49"S, 27° 46' 12"W; Selkirk - 21° 22' 10"S, 27° 42' 40"W).
The Tati area has a mining history that dates back as far as 600 AD, when the gossan outcrops of the present operations were first exploited for copper. Mining began at the Selkirk underground mine in 1989, and at the Phoenix open pit in 1995. LionOre acquired an initial interest in the operation in 1996, which was increased to 85% in 2002 after it purchased Anglo American's 44.5% share. Norilsk Nickel acquired the 85% stake in Tati Nickel as a result of the acquisition of LionOre on July 1, 2007. The remaining 15% was held by the Botswana Government. In 2015 Norilsk Nickel attempted to dispose of its interest by a sale to Botswana smelter and mining company BCL Investments Proprietary Limited. However, soon after the sale was approved, BCL went into liquidation and defaulted on payment for the Tati Nickel Mines subsidiary in 2016, which reverted to Norilsk Nickel ownership and the mine was closed.
The deposits are hosted by the Archaean Tati greenstone belt of the Zimbabwe Craton. The Tati and adjacent adjacent Vumba greenstone belts are part of what is know as the Francistown Arc Complex (Carney et al., 1994; McCourt et al., 2004) located along the southwestern margin of the Zimbabwe craton. Carney Carney et al. (1994) correlated the volcano-sedimentary rocks of the Francistown Arc Complex with the ~2.7 Ga Upper Bulawayan greenstones of Zimbabwe on the basis of lithostratigraphic similarities. Tonalite-trondhjemite gneisses in the Vumba greenstone belt have been dated at 2647±4 to 2696±4 Ma (U-Pb SHRIMP zircon; Bagai et al. 2002), whilst gabbro at the Phoenix Mine returned dates of 2703±30 Ma (electron microprobe of monazite grains; Van Geffen 2004), consistent with this correlation. The principal lithologies of the Tati greenstone belt are lower greenschist to lower amphibolite facies volcanic and sedimentary rocks intruded by granitoids of unknown age. These granitoids are predominantly tonalitic orthogneiss, tonalitic paragneiss and post-tectonic tonalite-adamellite plutons. The volcano-sedimentary sequence has been split into three formations containing a progressively increasing proportion of felsic volcanic rocks upwards through the succession (Key 1976). The <1600 m thick Lady Mary Formation at the base is predominantly composed of altered komatiite and komatiitic basalt with lesser quartzitic schist, limestone and iron formation. The succeeding >10 km thick Penhalonga Formation comprises basaltic, andesitic and rhyolitic volcanics and volcaniclastic rocks, as well as phyllites, black shales (containing vartiable accessory pyrite, Key 1976), limestones and jaspilites. These two units are capped by the >1 km thick Selkirk Formation that mainly comprises dacitic and rhyolitic volcaniclastic rocks with minor mafic volcanic rocks, quartzites and quartz sericite schists. The Selkirk Formation hosts the Phoenix, Selkirk and Tekwane meta-gabbronoritic and the Sikukwe meta-peridotite intrusions (Maier et al., 2007).
The Phoenix sulphide Ni-Cu-PGM deposit comprises a series of narrow, stacked, structurally controlled massive sulphide lenses enveloped within a broad zone of disseminated sulphides hosted by a meta-gabbro intrusion. Individual massive sulphide lenses may have grades as high as 8% Ni, enhancing the lower grade disseminated mineralisation which typically has a grade of around 0.40% Ni.
The intrusion hosting the deposit occurs on the northern periphery of the Tati greenstone belt, and crops out as an elongated, north-striking body that is ~5 km long and 400 to 1500 m wide. It is composed of medium- to coarse-grained, weakly deformed, meta-gabbronorite that has been pervasively altered to an assemblage of hornblende, albite, oligoclase, chlorite, epidote-sericite and quartz. It is cut by abundant late aplitic-pegmatitic veins that are of granitic composition (1022±16 Ma, U-Pb SHRIMP-II on zircon; Van de Wel et al. 1998) and by dolerites. The surrounding country rocks are tonalitic paragneisses that may contain accessory chalcopyrite (Key 1976).
The magmatic sulphide mineralisation is present as disseminations, and as massive sheets and vein-like accumuolations throughout the gabbronorites and, less commonly, in the granite. The disseminated sulphides occur as submillimetre- to centimetre-sized intergranular masses of pyrrhotite, chalcopyrite and pentlandite. The massive sulphides occur as a series of mostly relatively thin, up to 1.5 m thick, sub-vertical sheets and veins, interpreted to possibly represent fissure and fracture fill that predominantly strike north-south and east-west. The sulphides of the latter have considerable compositional variation on a centimetre to metre scale. Most are dominated by pyrrhotite with flame-like lamellae and granular aggregates of pentlandite, although chalcopyrite-rich (10 to 90% modal) massive/vein-type sulphides are also present. Accessory sulphides include sphalerite, molybdenite, galena and altaite (PbTe; Van Geffen 2004). Van Geffen (2004) also recognised a number of platinum-group minerals in the ores, including merenskyite [(Pd,Pt)(Bi,Te)2], hollingworthite [(Rh,Pt)AsS] and sperrylite (PtAs2), as well as hessite (Ag2Te), argentopentlandite [Ag(Fe,Ni)8S8], rheniite (ReS2) and gold (Maier et al., 2007).
In 2003 the probable mineral reserve at Phoenix was 37.6 Mt @ 0.55% Ni, 0.34% Cu, while the indicated resource was published as 142 Mt @ 0.29% Ni, 0.17% Cu. Production in 2003 involved the removal of 14 Mt of waste and ore, at a waste:ore ratio of 3.9:1. The mill has a capacity to treat 3.6 Mt of ore per annum and in 2003 produced 278,746 tonnes of concentrate at 5.68% Cu, 3.57% Cu containing 11,509 tonnes of Ni, 8,141 tonnes of Cu, 0.102 tonnes of Pd, 0.167 tonnes of Pt, 71 tonnes of Co, 203 kg of Ag 29.5 kg of Au.
Subsequent exploration outlined (Norilsk Nickel Mineral Reserves and Resources Statement as of 31 December, 2010):
Measured + Indicated Mineral Resources - 208.9 Mt @ 0.21% Ni, 0.19% Cu;
Probable Ore Reserves - 111.8 Mt @ 0.22% Ni, 0.18% Cu.
Remaining reserves and resources at 31 December, 2014 were (Norilsk Nickel Mineral Reserves and Resources Statement, 2014):
Measured + Indicated Mineral Resources - 106 Mt @ 0.21% Ni, 0.17% Cu;
Inferred Mineral Resources - 10.756 Mt @ 0.18% Ni, 0.14% Cu.
The Selkirk deposit was mined out by late 2002. It comprised a 30° plunging 200 x 100 x 25 metre zone of massive sulphides grading 2.6% Ni and 1.6% Cu, which extended down to a maximum depth of 150 m below the surface. The massive sulphides were enveloped by a broad zone of low grade disseminated sulphides, hosted by a meta-gabbro intrusion.
The Selkirk host intrusion is located in the centre of the Tati greenstone belt, near the Penhalonga to Selkirk Formation contact. It comprises medium-grained, layered metagabbronorite and anorthosite, and occurs as a ~3 x 2 km wedge-shaped body of undetermined thickness (Key 1976; Johnson 1986). Its contact relationships with the surrounding volcano-sedimentary host rocks are poorly exposed. The principal silicate minerals are sericite, chlorite and amphibole, which are the alteration products of plagioclase and pyroxene. Quartz is an accessory phase. Some parts of the intrusion have ophitic textures where narrow laths of altered plagioclase that are up to 2 mm long, are included in altered pyroxene oikocrysts that are up to 1 cm wide.
The intrusion is host to an ~20 m thick and up to ~250 m long lens of massive sulphide that is surrounded by a halo of disseminated (~20 vol.%) sulphides of undetermined width. Pyrrhotite constitutes up to 90 vol.% of the massive sulphides. Pentlandite is present as flame-like lamellae and granular aggregates within pyrrhotite. Chalcopyrite predominantly occurs as disseminated sulphides. Magnetite locally comprises up to 15% of the opaque fraction, occurring as subhedral grains that may be distinctly rounded. In some cases, pyrite comprises ~5% of the sulphides, occurring as late-stage veins and as euhedral or subhedral crystals. The massive sulphides can also contain distinctly rounded silicate inclusions similar to durchbewegung textures (Maier et al., 2007).
Subsequent exploration outlined (Norilsk Nickel Mineral Reserves and Resources Statement as of 31 December, 2010):
Measured + Indicated Mineral Resources - 124 Mt @ 0.23% Ni, 0.27% Cu, 0.57 g/t 4PGM;
Probable Mineral Resources (?) - 11.3 Mt @ 0.27% Ni, 0.3% Cu, 0.56 g/t 4PGM.
Remaining reserves and resources at 31 December, 2014 were (Norilsk Nickel Mineral Reserves and Resources Statement, 2014):
Measured + Indicated Mineral Resources - 124 Mt @ 0.21% Ni, 0.17% Cu, 0.57 g/t 4PGM;
Inferred Mineral Resources - 11.3 Mt @ 0.23% Ni, 0.27% Cu, 0.56 g/t 4PGM;
The most recent source geological information used to prepare this decription was dated: 2005.
Record last updated: 4/8/2020
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.
Phoenix Selkirk
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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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Johnson R S 1986 - The Phoenix and Selkirk nickel-copper sulphide ore deposits, Tati greenstone belt, eastern Botswana: in Anhaeusser C R, Maske S, (Eds.), 1986 Mineral Deposits of South Africa Geol. Soc. South Africa, Johannesburg v1 pp 243-248
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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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