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Location: 33 km NW from Rustenburg, South Africa
Portion 11 Farm Frischgewaagd 96, Division JQ, R565 Road, LedigRustenburgSouth Africa
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The mine is located on the Western Limb of the Bushveld Complex immediately south of the Pilanesberg Alkaline Intrusion and is underlain by the Merensky reef and Upper Group 2 Chromitite layer. The Bushveld Complex is the largest and most economically significant layered igneous complex in the world.The Bushveld Complex consists of a layered sequence of igneous rocks known as the Rustenburg Layered Suite which was derived from differential crystallisation of multiple magma injections.The Rustenburg Layered Suite (mafic rocks) can be divided into five zones known as the Marginal, Lower, Critical, Main and Upper Zones from the base upwards. The mafic-rich Critical Zone hosts the multiple Chromitite and platinum group metal layers. The dominant economic platinum group metal mineralisation, the Merensky reef and Upper Group 2 Chromitite layer occur within the multi-layered norite-pyroxenite-anorthosite-chromitite Upper Critical Zone and are continuous over tens of kilometres. Merensky reefFour facies or types of Merensky reef were identified at the mine with their rock type composition:The Normal Merensky reef facies consist of a thin (2cm) pegmatoidal feldspathic pyroxenite occurs. The pegmatoidal feldspathic pyroxenite becomes more harzburgitic–troctolitic (olivine/ feldspar assemblage) towards the basal Chromitite. The basal Chromitite is underlain by a poikiolitic anorthosite which is typically barren. The upper Chromitite is overlain by approximately 3m of medium-grained (>2cm) pegmatoidal feldspathic pyroxenite occurs. The pegmatoidal feldspathic pyroxenite becomes more harzburgitic–troctolitic (olivine/ feldspar assemblage) towards the basal Chromitite. The basal Chromitite is underlain by a poikiolitic anorthosite which is typically barren. The upper Chromitite is overlain by approximately 3m of medium-grained (<5mm) feldspathic pyroxenites, termed the Merensky Pyroxenite. Macroscopic base metal sulphides (“BMS”) mineralisation is located within the pegmatoidal feldspathic pyroxenite bounded by the upper and lower Chromitites and displays local enrichment within and directly above the upper Chromitite. This facies type is similar to those occurring at the surrounding Implats and Amplats operations, although the average mineralised intersection width intersected at BPM is 1.2m.The Normal Footwall Merensky reef facies is similar to the Normal Merensky reef facies type with a pegmatoidal feldspathic pyroxenite, bounded by upper and lower Chromitite layers. The pegmatoidal feldspathic pyroxenite may be more harzburgitic-troctolitic in character than the Normal Merensky reef facies. The significant difference between the Normal facies and Normal FW facies is that the basal Chromitite is joined to the FW 6 Chromitite and the entire package is directly underlain by alternately layered olivine norites of the stratigraphically consistent FW 7 Chromitite. Significant PGE and BMS mineralisation are associated with this unit and the width over which mineralisation extends can be extensive (approximately 2m in places). The average mineralised intersection width intersected at BPM is 1.6m but the facies include more geological losses than the Normal Merensky reef facies. The Single Chromitite Merensky reef facies consist of one chromite layer with the usual two bounding upper and lower Chromitites joining to form a single Chromitite layer. Occasional intersections were observed where the upperlower Chromitite separation allows for the observation of the internal pegmatoidal feldspathic pyroxenite, highlighting the gradational nature of this facies type close to the facies boundary. The Chromitite(s) are underlain by poikioliticspotted anorthosites and norites which comprise a severely “compressed” FW stratigraphy. Whereas the FW stratigraphy thickness underlying the Normal Merensky reef facies is approximately 12m, (to the regionally significant and consistent FW 6 boulder bed Chromitite), the stratigraphic separation pertaining to the Single Chromitite Merensky reef facies is approximately 3m. This is evidence of a transgression of the Merensky reef FW stratigraphy regionally. Mineralisation occurs in the underlying anorthosites and norites and the overlying feldspathic pyroxenites, giving rise to isolated mineralisation widths in excess of 2m. The average mineralised width intersected at BPM is 1.4m. The Detached Merensky reef facies consist of a lower Chromitite layer overlain by 10cm to 20cm pegmatoidal feldspathic pyroxenite (which occasionally shows evidence of harzburgite) and approximately 8m of fine-grained (<2mm) orthopyroxenite with occasional coarse-grained phenocrysts of clinopyroxene which is overlain by pegmatoidal feldspathic pyroxenite and the upper Chromitite layer. The basal Chromitite is underlain by the same stratigraphy as the Normal Merensky reef facies. Mineralisation is localised to the pegmatoidal feldspathic pyroxenite immediately below the upper Chromitite and upwards (20cm) into the overlying Merensky Pyroxenite. The average mineralised intersection width intersected at BPM is 1.6m.Upper Group 2 Chromitite layerTwo facies of the UG2 Chromitite layer were identified with the distinction between them being the nature and thickness of the immediate FW:The Normal UG2 Chromitite layer facies consist of a main Chromitite layer (approximately 65cm thick) underlain by a pegmatoidal feldspathic pyroxenite (containing laterally discontinuous Chromitite lenses and stringers) and subsequently underlain by a poikiolitic anorthosite and a series of leucocratic norites. This FW sequence can attain a thickness of approximately 7m before encountering a single Chromitite that overlies the UG1 hangingwall (“HW”) pyroxenite. The UG2 main Chromitite layer is consistently overlain by three Chromitite layers (ranging from 10cm to 20cm thick), locally termed “the triplets” which can vary in their parting distance to the UG2 main Chromitite layer between 0.25m to 0.75m. Typically, the Chromitite layers are mineralised and the intervening fine-grained orthopyroxenite contains lower concentrations of the PGMs. The Regional Pothole facies of the UG2 Chromitite layer has similar characteristics to the normal facies but generally has reduced stratigraphy between the UG2 Chromitite layer and the UG1 Chromitite layer. The typical separation of the UG2 and UG1 Chromitite layer is presented in Figure 5-6. The main chromitite layer is slightly thicker than the Normal UG2 reef facies, with an average width of 75cm. The thickness from the HW to the main chromitite layer is generally greater, with various Chromitite layers present.
