Impala Platinum - Bafokeng North and South, Wildebeestfontein South and North - Bushveld Complex
North West Province, South Africa
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The Impala Platinum operation is distributed over an interval of ~30 km from Rustenburg in the south, to the margin of the younger Pilanesberg Intrusive Complex to the north in the Northwest Province of South Africa. It comprises a complex of at least 15 shafts (including the early Bafokeng North, Bafokeng South, Wildebeestfontein South and Wildebeestfontein North mines).
The Impala operations exploit both the UG2 and Merensky reefs on the south-western limb of the Western Lobe of the Bushveld Complex.
For background on the geology and mineralisation of the Bushveld Complex, see the separate Bushveld Complex record. See also the Rustenburg Section for a description of the two reefs in the adjacent section of the complex immediately to the east.
Both the Merensky Reef and UG2 Reefs, which are present throughout the lease area, sub-outcrop on the mining area and dip approximately 10 to 12°NE towards the centre of the Complex, although locally dips may increase to 15°. The strike of the reefs is generally NNW, but changes to NW as the Pilanesberg Complex is approached. The vertical separation between the Merensky and UG2 reefs varies from about 125 metres in the south to some 45 metres in the north (Implats, 2012).
The UG2 Reef is a massive chromitite layer varying from 56 to 68 cm in thickness at Impala, with grade in general increasing as the chromitite layer width thins. It is commonly underlain by a coarse pegmatoidal feldspathic pyroxenite, while a characterisitc feature of the hangingwall is the presence of 3 or 4 thinner chromitite stringers which form potential partings during mining. These are known as the Leader Chromitite Layers. The relative abundance of precious metals in the UG2 is Pt - 47.4%, Pd - 25.7%, Rh - 9.0%, Ru - 13.6% Ir - 3.7% and Au - 0.7% (Blakrishna, 2006, and sources quoted therein).
There is a vertical zonation of grade within the reef, often with two peaks. The precious metals are concentrated towards the lowermost 20 cm of the reef, with lower values in the middle and higher grades towards the top. This PGE distribution tends to correlate with a textural change in the chromitite-silicate gangue from a granular to a poikilitic texture. The PGMs are interstitial to the chromitite grains, with laurite the only PGM mineral enclosed by chromite (Viljoen and Schurmann, 1998). The grain size of the PGM averages 9.3 µm (Lee, 1996). Despite the low sulphide content of the reef, most PGMs are attached to small base metal sulphides. PGMs are aso known to occur within the pyroxenite gangue. The major silicate gangue minerals are orthopyroxene and feldspar (Blakrishna, 2006).
Most of the Impala mining area is underlain by the Merensky Reef, which outcrops over a strike length of about 16 km and dips at 9° toward the northeast. It occurs toward
the top of the sequence of alternating layers of peridotite, pyroxenite and norite that constitute the Critical Zone of the Rustenburg Layered Suite within the Bushveld Complex. The Reef averages 46 cm in thickness and consists from bottom to top of leuconorite, anorthosite, chromitite and a very coarse-grained melanorite. The footwall overlies a thin basal chromitite stringer and leuconoritea to northosite, while the hanging wall is melanorite. The only hydrous mineral present is biotite, which amounts to 1%, or less, of the rock. All of the rocks contain 0.1 to 5% interstitial sulphide blebs (41% pyrrhotite, 37% pentlandite, 18% chalcopyrite and 4% pyrite), with the Reef rocks containing the most sulphides (1 to 5%) (Barnes and Maier, 2002). The core of a typical base metal sulphide bleb consists of pyrrhotite surrounded by a mosaic of pentlandite grains, which in turn may be enclosed by a rim of chalcopyrite. The relative volumetric abundance of the PGMs have been determined to be as follows: 44% coopetite, 21% laurite, 17% moncheite and 1% braggite. Thirteen additional PGMs comprise the remaining 6%. Cooperite and laurite occur preferentially in the chromitite layers, whereas moncheite is most commonly found in the silicate part of the Merensky Reef. Observed textures include the marked tendency of PGMs to occur with chalcopyrite, the exsolution of laurite and moncheite from pyrrhotite, and stringers of moncheite and sperrylite cutting across silicate and base metal sulphide grain boundaries. Pt-Fe alloy commonly forms the core with cooperite as a rim of compound zoned PGM grains (Mostert et al., 1982).
The ore reserves and mineral resources at the Impala Mines at 31 December, 2011 (Implats, 2012) were:
Proved Reserve - 10.9 Mt @ 4.10 g/t 4E, 4.57 g/t 6E, over 129 cm thickness,
Probable Reserve - 32.9 Mt @ 4.25 g/t 4E, 4.25 g/t 6E, over 131 cm thickness,
Measured Resource - 153.7 Mt @ 5.90 g/t 4E, 6.59 g/t 6E, over 125 cm thickness,
Indicated Resource - 87.0 Mt @ 6.15 g/t 4E, 6.86 g/t 6E, over 107 cm thickness,
Inferred Resource - 65.2 Mt @ 5.46 g/t 4E, 6.09 g/t 6E, over 130 cm thickness,
Proved Reserve - 15.9 Mt @ 4.07 g/t 4E, 4.89 g/t 6E, over 98 cm thickness,
Probable Reserve - 123.2 Mt @ 3.87 g/t 4E, 4.65 g/t 6E, over 99 cm thickness,
Measured Resource - 135.9 Mt @ 7.32 g/t 4E, 8.80 g/t 6E, over 64 cm thickness,
Indicated Resource - 68.3 Mt @ 7.22 g/t 4E, 8.67 g/t 6E, over 63 cm thickness,
Inferred Resource - 33.3 Mt @ 7.40 g/t 4E, 8.89 g/t 6E, over 63 cm thickness,
TOTAL Combined Reserve - 263.3 Mt @ 4.05 g/t 4E, 4.70 g/t 6E,
TOTAL Combined Resource - 543.4 Mt @ 6.50 g/t 4E, 7.53 g/t 6E.
The ore reserves and mineral resources at the Impala Mines/Royal Bafokeng Resources JV at 31 December, 2011 (Implats, 2012) were:
Measured Resource - 5.3 Mt @ 6.52 g/t 4E, 7.28 g/t 6E, over 152 cm thickness,
Indicated Resource - 7.0 Mt @ 6.84 g/t 4E, 7.64 g/t 6E, over 150 cm thickness,
Inferred Resource - 23.4 Mt @ 6.51 g/t 4E, 7.27 g/t 6E, over 122 cm thickness,
Measured Resource - 2.3 Mt @ 7.49 g/t 4E, 9.00 g/t 6E, over 53 cm thickness,
Indicated Resource - 1.8 Mt @ 7.86 g/t 4E, 9.44 g/t 6E, over 54 cm thickness,
Inferred Resource - 8.6 Mt @ 7.43 g/t 4E, 8.93 g/t 6E, over 57 cm thickness,
TOTAL Combined Resource - 48.5 Mt @ 7.78 g/t 4E, 10.6 g/t 6E.
Note: 4E = Pt+Pd+Rh+Au, 6E = Pt+Pd+Rh+Ru+Ir+Au.
For detail see the reference(s) listed below.
The most recent source geological information used to prepare this summary was dated: 2006.
Record last updated: 22/11/2012
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.
References to this deposit in the PGC Literature Collection:
Barnes S-J and Maier W D 2002 - Platinum-group Elements and Microstructures of Normal Merensky Reef from Impala Platinum Mines, Bushveld Complex : in J. of Petrology v43 pp 103-128|
Leeb-Du Toit A 1986 - The Impala Platinum mines: in Anhaeusser C R, Maske S, (Eds.), 1986 Mineral Deposits of South Africa Geol. Soc. South Africa, Johannesburg v2 pp 1091-1106|
Mostert A B, Hofmeyr P K, Potgieter G A 1982 - The Platinum-Group mineralogy of the Merensky Reef at the Impala Platinum mines, Bophuthatswana: in Econ. Geol. v77 pp 1385-1394|
Naldrett A J, Gasparrini E C, Barnes S J, Von Gruenewaldt G, Sharpe M R 1986 - The upper critical zone of the Bushveld complex and the origin of Merensky-type ores: in Econ. Geol. v81 pp 1105-1117|
Prichard H M, Barnes S-J, Maier W D, Fisher P C, 2004 - Variations in the nature of the platinum-group minerals in a cross-section through the Merensky Reef at Impala Platinum: implications for the mode of formation of the reef - **CURRENTLY UNAVAILABLE**: in The Canadian Mineralogist v42 pp 423-437|
Von Gruenewaldt G, Hatton C J, Merkle R K W 1986 - Platinum-group element-Chromitite associations in the Bushveld complex: in Econ. Geol. v81 pp 1067-1079|
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