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, nickel, copper, chrome and vanadium.
The Kruidfontein Project, which is located in the eastern extremity of the Pilanesberg enclave, belongs to the Swartklip facies in the north-western Bushveld. The project area is underlain throughout most of its southern and western portions by the gabbronoritic rocks of the Main Zone, and by the ferrogabbros of the Upper Zone in the north and east. The project area is intruded by a series of NW-trending dykes of various compositions which are visible on available aeromagnetic image. Although traces of Iron-rich Ultramafic Pegmatoid (IRUP) bodies are visible on the aeromagnetic image, drillholes have not intersected any IRUPs and this contrasts with what is observed in Magazynskraal to the north of Kruidfontein.
The Merensky Reef Layer
The MR has been traced over 150 km along strike in the Eastern Bushveld and over 110 km strike in the Southern Sector of the Western Limb. There is also extensive mining of the Western MR from Pilanesberg to Thabazimbi giving a total strike length of approximately 250 km. Generally, two types of MR exist: the normal reef and potholed reef. Where fully developed in the Swartklip Facies, the MR consists of an orthopyroxenitic or harzburgitic pegmatoid, between 1 cm and 5 m thick, bounded top and bottom by thin (1 cm or less) chromitite stringers.
At Kruidfontein, the MR has been intersected at depths ranging from 1 135 to 2 395 m. The MR belongs uniformly to the PUP facies (i.e. the MR potholed on to Upper Pseudo Reef) throughout the Kruidfontein project area, apart from a small area in the extreme south-west where pegmatoidal Merensky Main Reef has been intersected in two drill holes namely KFT012DO and KFT014DO. In this respect, it differs considerably from the properties to the west (e.g. PPM, Sedibelo, Ruighoek), where substantial variations in facies across short distances make the MR difficult to assess as a resource. Generally in the Swartklip pothole facies, the MR erosion surface transects through the footwall anorthosite until it reaches the Upper Pseudo Reef (UPR), but does not penetrate into this unit. Thus, the MR completely eliminates its anorthositic to noritic footwall lithologies, and the Merensky Hangingwall Pyroxenite rests directly on the UPR. The chromitite of the MR coalesces with the upper chromitite of the UPR, the resultant combined chromitite is up to 2 or 3 cm thick. Under these circumstances, here termed the PUP (potholed onto Upper Pseudo) reef facies, elevated PGM grades are common in the UPR and in at least the top few tens of centimetres of the underlying Tarentaal harzburgite. The PUP reef is the dominant Merenskytype reef in the Kruidfontein project area.
Within the MR, the PGMs occur as small (<20 µm) grains, most commonly at the contact between the base metal sulfides (BMS) and silicate minerals. Their composition varies considerably, from sulfides through tellurides to PtFe alloys. Grade varies considerably over short distances in the MR. Where the pegmatoidal pyroxenite of the MR is greater than 50 cm, grade is concentrated at or near the upper chromitite with a smaller peak on the lower chromitite. PGM mineralisation is generally low grade in the body of the pegmatoid itself.
The UG2 Chromitite Layer
On a regional scale, the UG2 is hosted within pyroxenites and typically consists of a main chromitite band, typically 50 to 120 cm wide, often accompanied by a series of smaller chromitite stringers in the immediate hanging wall. These stringers range from 0.5 cm to several tens of cm in width. Additionally, pyroxenite stringers may be developed within the main chromitite layer. The footwall to the UG2 consists of a coarse-grained feldspathic, pegmatoidal pyroxenite or harzburgite unit of variable thickness. Discontinuous chromitite stringers and blebs are present within the pegmatoid footwall. Pyroxenite is developed beneath the pegmatoidal pyroxenite zone. The common signature of the UG2 reef within this ultramafic layer is the massive 1 m thick chromitite found within the package of alternating thin chromite seams. Overlying the UG2 is mostly norite or pyroxenite; the only exception is in the northern part of the Western Limb where the UG2 is overlain by harzburgite.
At Kruidfontein, the UG2 was intersected at depths ranging from 1 150 to 2 409 m. The UG2 interval consists of the UG2 chromitite, separated from a 10 cm thick UG2 leader chromitite layer (UG2L) by a pyroxenite parting (UG2P) approximately 10 cm thick under normal circumstances. The main UG2 layer is generally between 80 cm and 110 cm thick. A pegmatoidal orthopyroxenite is typically developed in the footwall of the UG2. Where there is disseminated chromite in the footwall pegmatoidal orthopyroxenite, it may carry significant grade. In the UG2 pothole condition, which is relatively rare, the UG2L merges with the UG2 and the overall thickness can be significantly reduced.
The PGM mineralogy of the UG2 is simpler than that of the MR, being dominated by PGM sulfides, although the grain size is smaller (<10 µm) than in the Merensky reef. The 4E grade tends to peak (in excess of 10 g/t) at the bottom and top contact of the main UG2 Chromitite seam.