The Bokoni Mine is an operating mine located on the North-Eastern Limb of the Bushveld Igneous Complex (BIC). The platiniferous horizons of economic significance occur within the Merensky and the UG2 horizons. PGM mineralization is specifically located within the Merensky horizon and the UG2 horizon, which forms part of the Upper Critical zone of the Rustenburg Layered Suite.

The mining area is located within the Zeekoegat, Middelpunt, Umkoanesstad, Brakfontein and Klipfontein farms. The north-eastern portion of the mining area is located below a range of pyroxenite hills while the south-western portion of the mining area is below the valley floor and overlain by black turf.

Both horizons subcrop and in some instances outcrop in the project area along a 16.5 km strike length. The BIC layering dips from northeast to southwest at approximately 25° in the north-western areas (Zeekoegat), and gradually decreases to approximately 18° in the south-eastern area (Brakfontein). The general structural geology is characterized by northeast and east trending dykes and faults with associated conjugated joint sets.

The Merensky is a feldspathic pyroxenite reef horizon and is stratigraphically situated 350 m above the UG2 and is near the top of the Upper Critical Zone. The Merensky is located below the three to six meter thick Merensky Pyroxenite layer and above the Merensky Norite layers. Two thin chromite stringers are discontinuously developed with the upper stringer positioned 20 cm to 25 cm from the Merensky Pyroxenite hanging-wall contact, and the lower stringer located on or just above the Merensky Pyroxenite’s basal contact. In the absence of a consistently developed chromite stringer, the upper contact of the Merensky Pyroxenite layer assists to define the top position of the Merensky horizon and is a guide for sampling purposes and on-reef mining. The Merensky hanging wall stratigraphic sequence is typified by medium to coarse grained feldspathic lithologies, ranging in composition from mela-norites to anorthosites. The Merensky footwall stratigraphic sequence has a sharp footwall contact, usually marked by the lower chromite stringer. While the top contact tends to be planar, the basal contact is undulating as a result of thermo-chemical erosion of the more mafic Merensky lithologies with their underlying felsic lithologies. This contact is often associated with a thin anorthosite layer (approximately 3 cm thick) that probably formed as a secondary reaction product of thermal erosion.

The UG2 is stratigraphically situated approximately 350 m below the Merensky and is separated by a series of well layered sequences. The UG2 is comprised mainly of this well-defined Chromitite layer together with minor hanging wall and or footwall constituents. The average width of the UG2 is 70 cm. It is overlain by a medium-grained poikilitic feldspathic pyroxenite that averages 9.85 m in width, and hosts a variable number (generally up to four) of very thin Chromitite layers. The position of these stringers is important to the mining of the UG2. The UG2 is underlain by a pegmatoidal feldspathic pyroxenite layer of approximately 0.75 m in width which is commonly host to disseminated chromite and some base metal sulphide occurrence within close proximity to the UG2. The UG2 elevation isopachs indicate a relatively undisturbed tabular and gently dipping layer. UG2 widths generally increase to the west from an average of 67 cm on Umkoanesstad to 74 cm on Zeekoegat. There is no evidence of severe undulations to this layer that would adversely affect the planned mining method. Severe undulations of the UG2 are known to hamper mining by increasing dilution and off-reef mining.

Potholes are magmatic disturbances of the reef plane that are generally deep eroded depressions that have serious structural implications in respect of reef continuity. Merensky potholes, including those at the Bokoni Mine, have been well documented. Potholes account for approximately 16% of the estimated total geological loss of 20%.

As with the Merensky, the UG2 is known to be affected by potholes. UG2 potholes typically have a “soup-bowl” profile. The characteristics of normal UG2 are not preserved in the Bokoni Mine potholes and the succession often occurs as a variably thickened feldspathic pyroxenite package, containing disrupted and discontinuous Chromitite layers. As a result, grades within potholes are highly erratic and, invariably, sub-economic. UG2 potholes at the Bokoni Mine are commonly destructive and are not economically mineable. Pothole losses for the UG2 are at 9% of the estimated total geological loss of 15% for the Bokoni Mine UG2.

The weathered overburden (soil and calcrete) depth across the Bokoni Mine is highly variable ranging from no overburden in the rocky outcrops and hill areas, to greater than 50 m in the valley areas. The average overburden depths below surface are Zeekoegat 10 m, Middelpunt 22 m, Umkoanesstad (valley) 30 m, Umkoanesstad (mountain) 2 m and Brakfontein 40 m. The depth of oxidation may be reasonably estimated by adding 25 m to the overburden depth. A mineralogical study by Paetz & Reinecke (December 2002) has confirmed that the depth of oxidation in the vicinity of the Vertical Shaft is approximately 40 m.

The geological structure at the Bokoni Mine is not complicated with faulting. According to existing workings, minor faulting is expected to occur, and would consist of dextral and sinistral strike-slip faults, normal and reverse dip-slip faults and faults with more complex combinations of these components. Displacements are expected to be small, at generally less than or equal to 1 m. Major conjugate joint set orientations were measured from strong macro-lineament features evident from an aeromagnetic survey image and land satellite imagery which provided orientations in the order of 99° and 159°. Joint sets may result in poor ground conditions for mining but are not considered a geological loss.

At the Bokoni Mine the mineralisation within the Merensky occurs at both the upper and lower Chromitite stringers. Most of the PGMs are associated with the upper chromite stringer and often extend over wider intervals to below the chromite stringer. Mineralisation associated with the lower chromite stringer at the base of the Merensky is generally over a very narrow interval and is sometimes absent. High PGM grades are often associated with the lower chromite stringer, but due to its greater separation from the upper stringer, it was not included in the Mineral Resource estimates. The Merensky has visible base metal sulphides (commonly pyrite and pyrrhotite) and, as a result, may have viable concentrations of copper and nickel.

PGMs are commonly associated with base metal sulphides and are associated with the silicate and chromite minerals. The relative proportions of PGM content for the Merensky are colloquially known as the ‘prill split’. Prill splits are determined as part of the Mineral Resource estimation process. At the Bokoni Mine, the Merensky PGM prill split is Pt 61%, Pd 29%, Rh 4% and Au 6%.

The UG2 mineralisation is comprised mainly of PGM accumulations that are hosted within the Chromitite layers and have variable occurrences in the immediate footwall rocks, but very little in the hanging wall rocks. A 95 cm resource cut in most instances allows for the complete extraction of the mineral content. In the case of the presence of internal lenses (bifurcation) of pyroxenite, anorthosite or Norite, the resource cut width may have to be increased to ensure that the UG2 is completely extracted.

The PGM mineralization occurs in solid solution with sulphides, sulpharsenides, arsenides, bismuthides, tellurides, bismuthotellurides and alloys. PGM-sulphides, tellurides, and alloys are the main constituents of mineralization in the UG2. The PGM prill split for the UG2 is broadly Pt 42%, Pd 46%, Rh 9%, and Au 2%.

The mining operation consists of a vertical shaft and three decline shaft systems to access underground mine development on the Merensky a nd UG2 reef horizons. The Bokoni Mine has installed road, water and power infrastructure, as well as two processing concentrators, sufficient to meet its operational requirements up to completion of its first phase growth plans to reach 160 ktpm. The Bokoni Mine has an extensive shallow ore body, capable of supporting a life-of-mine plan that is estimated to exceed 22 years (as Bokoni’s mining license expires in 2038). Current mining operations are being conducted at shallow depths, on average 300m below surface. This benefits the Bokoni Mine’s operations in that there are no major refrigeration (and consequent power) requirements at shallower mining depths.

The older Vertical and UM2 shafts made use of conventional mining methods for narrow tabular ore bodies. Ore broken in stopes is transported laterally by means of track bound equipment and then hoisted through a vertical shaft system at Vertical shaft and an incline shaft system at UM2 shaft.

The Brakfontein shaft is being developed on a semi-mechanized basis, using a hybrid mining method, whereby ore broken in stopes is loaded directly onto a strike conveyor belt and taken out of the mine through a main decline conveyer belt system. This results in less human intervention in the hoisting process and a lower unit operating cost of production. Development of haulages and crosscuts are effected by means of mechanized mining methods, and stoping is conducted using hand held electric drilling machines. The Brakfontein shaft is in a ramp up phase and is planned to increase from its current production levels of 50 ktpm, to a steady state production level of 90 ktpm by Q4 2019.

The Middelpunt Hill shaft is in the process of converting the transport of broken ore from its current mechanized hauling system to a conveyor belt transport system similar to that of Brakfontein shaft. Vamping opportunities in the older adit areas are being investigated to supplement underground mining production. Currently, UG2 ore is produced exclusively from the Middelpunt Hill shaft, which comprises a number of adits and underground development. Production from the adits will be exhausted in the near future and UG2 production will be replaced by production from underground development. Production at Middelpunt Hill is currently 50 ktpm and will be increased to a steady state level of 60 ktpm by Q2 2018.

The Klipfontein Merensky Opencast Mine operation was a short life operation that mined the Merensky subcrop to a depth of about 50 m below surface. The ore from this operation, about 40,000 tpm, was used to fill the spare mill capacity until underground operations at Brakfontein and Middelpunt Hill ramp up to their steady state levels of 90 ktpm and 60 ktpm respectively. The Klipfontein Merensky Opencast operation has been affected by a number of challenges including delays in obtaining a water use licence, stoppages at the eastern pit due to community disruptions and intersection of a large number of potholes on the western portion of the pit. These challenges have impacted the operation’s ability to deliver sufficient volumes to generate a profit. Mining at the Klipfontein Merensky Opencast Mine operation was ceased at the end of Q1 2017 with rehabilitation continuing until the end of the 2017 fiscal year.

The Bokoni Mine currently has two concentrator plants, one of which is used for the processing of concentrate from the Merensky ore and the other for the processing of ore from the UG2. These concentrators are situated adjacent to one another close to the Vertical shaft.

The Merensky concentrator (capacity 100 ktpm) is currently dedicated to processing Merensky ore from the Brakfontein shaft. The UG2 concentrator (capacity 65 ktpm) is dedicated to processing ore from the Middelpunt Hill UG2 adits and decline.

Merensky MF3 Plant.
The current 100 ktpm Merensky concentrator, built in 1990-1991, had a capacity upgrade in 2009 from 85 ktpm to 100 ktpm. The concentrator includes three milling stages with inter-stage flotation circuits.

UG2 MF2 Plant.
The 65 ktpm UG2 concentrator includes two milling stages with inter-stage flotation circuits (MF2). It is a dedicated concentrator, constructed in 2000 to treat UG2 ore that has subsequently been mined ........