Summary:
The Bushveld Complex (BC) is a magmatic layered mafic intrusion. As one of the largest known differentiated igneous bodies, it hosts world class deposits of PGMs, Ni, Cu, Cr and V.
The Mphahlele deposit is situated along the east-west trending, northern part of the Eastern Limb of the BC. The PGM mineralization occurs within the UG2 and the Merensky (MR), lying within the Upper Critical Zone of the RLS.
The rocks of the Main Zone and the upper parts of the Critical Zone underlie the Mphahlele project. The main structural controls of the northern parts of the Eastern Limb are the Wonderkop and Dwarsrand faults which traverse the Mphahlele project. East of the project, in the vicinity of the Lebowa Platinum Mine (previously known as Atok), the igneous stratigraphy is shallow dipping with a northwesterly strike. West of the faulted region, the Critical Zone trends east-west and on the Mphahlele project dips at an average of 51°, which increases to near vertical, 20 km to the west at Lonmin Platinum’s Limpopo mine.
The main Mineral Resource block of the Mphahlele project is underlain by the Main Zone and the Critical Zone of the RLS. There are no outcrops of either reef because a large alluvial fan emanating from the hills of Transvaal sediments to the north covers the Critical Zone on the Mphahlele project. Aeromagnetic data indicate that the MR and the UG2 continue for an estimated strike length of almost 8 km through the Mphahlele area and terminate to the east against floor lithologies of Magaliesberg Quartzite that have been dragged against the Wonderkop Fault.
The two reefs are separated on average by 120 m of stratigraphy (190 m vertical separation). The lateral extent of both reef horizons within the project area is approximately 8 km along strike, and have been modelled over a vertical extent of approximately 2 km. The depth extent of the reefs has not been limited by drilling and is open at depth.
Both the MR and UG2 exhibit disturbances that include potholing and the intrusion of pegmatoid, Iron-Rich Ultramafic Pegmatoids (IRUPs) and serpentinized harzburgite bodies. The main harzburgite intrusion has not been intersected by drilling but the smaller apophyses emanating from this severely affect the MR.
The major sulfide minerals present are pyrrhotite, pentlandite, chalcopyrite and pyrite with base metal sulfides (bornite, chalcocite, digenite, covellite, violarite and bravoite) in some areas. PGMs in the MR are contained within a complex set of minerals including the arsenide species, sperrylite (PtAs2) and the sulfide species, braggite (Pt,Pd,Ni S), laurite (PdS2) and cooperite (PtS), as well as tellurium and bismuth bearing minerals such as michnerite (Pd,Pt)BiTe, merenskyite (Pd,Pt)(Te,Bi)2 and moncheite (Pt,Pd)(Te,Bi)2.
Merensky Reef Layer
The MR occurs within the 3 m to 6 m thick feldspathic pyroxenite layer (the Merensky Pyroxenite), between a hangingwall of norite-anorthosite and a footwall of norite. Two thin chromitite stringers are present; an upper stringer, 20 cm to 25 cm from the hangingwall contact, and a lower stringer on or just above the basal contact. Both chromitite stringers are typically discontinuous, unlike in other areas of the BC.
Three Merensky Pyroxenite Facies types have been identified: the “A” Facies that occupies the western half of Mphahlele, the “B” Facies to the east while the “C” Facies is central to the two:
• The Merensky Pyroxenite A Facies stratigraphic unit averages 9.3 m in thickness and comprises a medium- to coarse-grained, poikilitic feldspathic pyroxenite with a lensoidal and discontinuous chromitite stringer developed near the upper contact termed the Merensky Upper Chromitite. This stringer varies from 1 to 4 mm in thickness. The upper portion is coarser-grained and contains serpentinized olivine, which is termed the Merensky Olivine Pyroxenite, often highly decomposed with a strongly developed joint fabric, with the result that this contact represents a significant plane of weakness. A thin irregular chromitite stringer (1 to 4 mm thick) may be present on or just above the lower contact, termed the Merensky Lower Chromitite. A pyroxene pegmatoid, some 0.20 m to 0.7 m thick with disseminated sulfides, is often present on the basal contact;
• The Merensky Pyroxenite B Facies averages 12.8 m in thickness and consists of fine- to medium-grained feldspathic pyroxenite with the development of the Merensky Upper and Lower Chromitite stringers varying from 1 to 4 mm in thickness;
• The Merensky Pyroxenite C Facies stratigraphic unit averages 58.9 m in thickness and contains intercalations of serpentinized harzburgite and dunite, feldspathic pyroxenite, pyroxene pegmatoid, norite, iron-rich ultramafic pegmatoid, chromitite stringers, thin chromitite layers and fragmented lenses of chromitite stringers.
The mineralization within the Merensky Pyroxenite A and B Facies is similar. The highest PGM-base metal concentration occurs towards the top of the Merensky Pyroxenite and is referred to as the M1 value zone. Maximum values occur across or immediately below the Merensky Upper Chromitite and correspond to the highest visible concentrations of sulfides. The M2 value zone occurs towards the base of the Merensky Pyroxenite, often associated with a pyroxene pegmatoid and the Merensky Lower Chromitite, and values may extend into the anorthosite footwall. A considerable thickness of barren Merensky Pyroxenite occurs between the M1 and M2 value zones.
PGM-Ni-Cu mineralization within Merensky Pyroxenite C Facies is of a lower tenor and dispersed throughout the thickened stratigraphic sequence.
The lower unit is narrow and too far removed from the economic zone (disseminated mineralization in the top metre of the pyroxenite) to be exploitable. The bulk of the PGM mineralization is associated with the upper chromitite stringer and here often occurs over wider intervals below the chromitite stringer. On the Mphahlele property, the MR is defined as the mineralization at the top of the Merensky Pyroxenite unit and associated with the upper chromitite stringer. In the absence of a well-defined chromitite stringer, the upper contact of the Merensky Pyroxenite defines the top of the reef for sampling purposes and will ultimately probably be used to identify the top of the reef visually during mining. Consequently, the MR Mineral Resources have been defined around the upper chromitite stringer only. Sporadic mineralization is also present within the central parts of the Merensky Pyroxenite, but its erratic distribution precludes its inclusion in the value interval or Mineral Resource.
UG2 Chromitite Layer
The UG2 comprises a coherent chromitite layer with no parting, approximately 1.2 m thick, normally overlying a norite footwall and underlying a feldspathic pyroxenite hangingwall. The hangingwall contact tends to be planar (although often sheared) but the footwall contact undulates, and this can be seen on a small scale in the core. The upper part of the UG2 is fine-grained, granular and devoid of visible sulfides whereas the lower portion is coarse-grained with visible sulfides. The UG2 is subdivided into two facies:
• The UG2 Upper Facies, an upper, fine-grained, poikilitic massive chromite, sometimes accompanied by fine, disseminated sulfide mineralization;
• The UG2 Lower Facies, a lower facies, with a distinctive poikilitic texture, higher silica content, sulfide-rich oikocrysts and significant disseminated sulfide mineralization. The UG2 Chromitite Layer may contain one or more intermittent pyroxenite layers termed “UG2 Middling”, which separate the Upper and Lower UG2 Facies. Typically, the mineralization peaks in the lower part of the layer. There is a general increase in the 4E grade proportionate to the increase in sulfide content but this is accompanied by a decrease in the Pt/Pd ratio.