Introduction

In the minerals sector of world economics, the Bushveld Complex in South Africa (Figure 1) is renowned for its overwhelming deposits of platinum group elements (PGE) and chromium (over 80% of the world's deposits of each according to Crowson (1)). Inevitably, from time to time, the question is raised as to how reliable those estimates are. The occurrences of all other mineral deposits are scattered around the world in an erratic way. Each deposit type has well-understood geological processes that operated to form them, but those processes have operated usually all over the world and in many cases throughout long periods of Earth's 4.6 billion-year history, so that most commodities are mined in many different countries. Therefore, the PGEs, and especially platinum itself, are unusual in that they are largely concentrated in a single location.

Mining companies may only publish ‘reserves’ and ‘resources’ of platinum. However, as discussed below (see box) (2, 3), this figure represents only what has been rigorously quantified in the short- to medium-term mining plans of these companies, and excludes all the geologically known extensions of these deposits for which an in-depth (and expensive) evaluation is not justified. Current legally enforced definitions operative in several countries, including the USA, Canada, Australia, the UK and South Africa, define ‘reserves’ and ‘resources’ quite precisely, and exclude what geologists know to be identified deposits. In the case of those deposits in the Bushveld Complex there is simply no need to rigorously quantify them at this point in time, because there are adequate proven reserves already identified.

A number of estimates of the potential deposits of the PGE in the Bushveld Complex have been published in scientific journals over the last thirty years, and the quoted numbers have not changed significantly. A summary of these compilations is presented in Table I (1, 4–7). These reports have come mainly from South African-based geologists, and their figures have been reproduced or validated by independent organisations such as the United States Geological Survey (USGS), that have staff qualified to make critical assessments of these estimates.

The Geology of the Bushveld Complex

Before discussing how these estimates are calculated, it is worth considering the origins and geology of the Bushveld Complex. The economic potential of many ore deposits is difficult to evaluate, but this is not so in the case of the Bushveld ores. The Bushveld Complex (see Figure 2) is an enormous irruption of magma (molten rock) sourced deep within the Earth. The extent of the magma flow was at least 300 km in diameter. In the order of 1 million km³ of magma was emplaced in a (geologically) very short period of time. As this enormous volume of Bushveld magma slowly cooled, different minerals began to solidify and accumulated in thin, parallel layers at the bottom of this huge magma ocean. The maximum thickness ultimately was about 8 km.

Most minerals in the Bushveld Complex have no economic importance, but two types are important here: chromite and the sulfide group of minerals. Both of these mineral types concentrate the PGE. The first economically important discovery of platinum in the Bushveld Complex was found in a single layer, associated with the sulfides, that we now call the Merensky Reef after its discoverer, Dr Hans Merensky (8). Figure 3 shows a specimen of a Merensky Reef section. The distribution of the PGE vertically through the Merensky Reef layer is somewhat variable, but mining companies would aim to extract a layer in the order of 1 m in thickness that contains the majority of the total PGE. Lower-grade ore below and above this zone has to be left behind as it is not economical to process. For that reason it is usually excluded from any resource calculations.

Even before the Merensky Reef was discovered, the presence of platinum in chromite-rich layers in the Bushveld Complex was quite well known, but never found to be economic. There are a number of these chromitite layers in the Bushveld Complex, some reaching up to 1 m in thickness. They contain up to 3 grams per tonne (g t⁻¹) of PGE, often with quite high proportions of rhodium and lesser amounts of iridium and ruthenium. One layer, called the Upper Group 2 (UG2) chromitite layer, is a possible hybrid of the sulfide- and chromite-hosted reefs. The distribution of the PGE associated with the chromitite layers is sharply controlled by the chromite. There is essentially no PGE above or below the chromitite reef, and so the mining operations and resource calculations are much easier to define.

Assaying of a borehole intersection of the Merensky and/or UG2 Reefs usually involves taking several consecutive lengths, each about 20 cm long, which are individually analysed, so that the best mineralised, 1 m-thick interval, especially for the Merensky Reef, can be identified. Often two or three deflections are drilled for each borehole. This involves putting a wedge into the hole some 20–30 m above a reef intersection, and redrilling. This produces another section of reef within a metre of the first (mother) hole, providing more analytical and statistical data without the cost of drilling long, deep holes. Since many boreholes are drilled to intersect the reef at up to 2 km depth such deflections provide more information extremely cheaply. Over the many years of mining, confidence in the statistical variability of reef intersections permits reliable information on predicted grade. Until ten years ago most such exploration was based around the major existing mines in the western Bushveld. Since then, because of the rise in the platinum price (9), much exploration has been undertaken on areas in the eastern Bushveld, at great depths in established mining areas, and also where there are geological complications, for both the Merensky and UG2 Reefs. Two such examples of the latter would be around the Pilanesberg intrusion in the west, and near the Steelpoort fault in the east.

Estimating Reserves and Resources

Paradoxically, although there are now more data available than previously, the ability to publish estimates of the platinum content of the Bushveld Complex is much more restricted. When the first estimates were calculated (Table I) they included predictions, projections and interpretations of geological continuity which are geologically plausible. Now, based on statutory resource codes, it is necessary to report all information only in terms of ‘reserves’ and ‘resources’, and whether proven or inferred. All the early calculations would have contained ore that would fall far outside what is now considered ‘inferred’. No exploration or mining company is going to expend unnecessary effort on proving deposits that might become mineable only a long way into the future. As a result, it appears as if the currently quoted platinum deposits of the Bushveld Complex are less than they actually are (compare Table II with Table I), purely because only ‘reserves’ and ‘resources’ may be reported and geologically known deposits are formally excluded by statutory codes. Even with this limitation, the ‘reserves’ and ‘resources’ reported by the four major mining companies in South Africa amount to at least 1200 million oz of PGE of which more than 50% is platinum (Table II).

From a purely geological perspective, we can make the following calculation from the following simple assumptions. We can assume the thickness of mineable Merensky and UG2 Reefs to be 0.8 m each. Geological maps show the two reefs to occur at outcrops with a strike length of about 100 km in both eastern and western limbs, giving a measure of the horizontal extent of the reef. We know that the typical dips of the reefs are less than 15°, so the reef can be mined for 4 km down the dip before a vertical depth of 1 km is reached. The densities of Merensky and UG2 Reefs are 3.2 t m⁻³ and 4.0 t m⁻³ respectively (7). We can take a conservative grade of 2 g t⁻¹ (0.06 oz t⁻¹) of extractable platinum. Simple multiplication of these figures (strike length along outcrop × distance into the Earth × thickness × density × grade, using appropriate units) yields 350 million oz of platinum per km depth (see box below). I use this method of presenting the information (in oz per km depth) because it is not known what mining depths may be ultimately viable. This simple calculation ignores what are called geological losses (assorted features such as faults and potholes, as described by Vermaak and van der Merwe (7)). Taking a mining depth to 2 km, and adding what might be present in the Platreef which occurs in the northern limb of the Bushveld Complex (see Figure 2), gives the rounded figure of 800 million oz of platinum, similar to that reported by Crowson (1) in Table I. For comparison, the annual refined production of the major platinum producers in South Africa is shown in Table II, and amounts to only around 9 million oz of PGE, of which less than 5 million oz are platinum.

Probably nearly half that figure of 350 million oz has already been mined from the Bushveld Complex and most of it from depths of less than 1 km. So we can assume that there are around 200 million oz remaining in the upper one km, and 350 million ounces in the second km. Since mining in some cases is already at more than 2 km vertical depth (as at the Northam Platinum Mine), assuming material to that depth can all be mined is technically straightforward. Mining to greater depth will encounter high temperatures and serious rock stresses. However, mining of the Witwatersrand gold reefs has progressed to a depth of 4 km. Hence, mining of Bushveld reefs to comparable depths should not be considered implausible during this century.

The Platreef

In calculating a figure of 350 million oz platinum per km depth I have excluded the Platreef. Evaluating its total PGE content is more difficult. Evaluating the mineable PGE is even harder. The PGE mineralisation is distributed over vertical intervals that can reach 100 m in some borehole intersections, but more usually is intermittent over 50 m. Tracing the best mineralised horizons from one borehole to another is difficult – grade varies, thickness varies, and their location is not at a constant geological elevation. Best mining methods then also become an issue. Currently, all mining on the Platreef is by open pit methods, but it is limited to probably 500–800 m depth. Drilling has shown that the Platreef continues to at least 2 km depth. Methods of underground mining of this wide ore body are still being developed. As a result the estimates in Table I for the Platreef show considerable variation.

Other Chromitite Layers

Every chromite-rich layer of the Bushveld Complex contains some PGE. Very little systematic work has been done to evaluate these layers since the grade is in the range 1–3 g t⁻¹ of PGE (10). The chromite ore itself is also currently sub-economic for chromium. The layer thickness and chromium-to-iron ratio are the two most important parameters in evaluating the economic potential of chromite deposits, and the chromitite layers of the Bushveld Complex generally decrease in this ratio upward, and are in the range 2.0 to 1.2 (10). The layers with the highest values are too thin to mine. The thickest layers, which are mined, have ratios close to 1.6. Layers with a ratio of less than 1.4 are not economic at present. There is an inverse correlation between the PGE grade and the chromium-to-iron ratio in these layers. As with the PGE, there are extremely large proven reserves of chromium in the Bushveld Complex (1), even down to vertical depths of less than several hundred metres. Thus, if the chromium and PGE reefs that are currently being mined were ever exhausted one could mine the lower-grade chromitite layers for both chromium and PGE. Conservatively, we could suggest that that these chromitite layers host nearly as much PGE and chromium as is presently considered potentially economic.

The tailings dumps from these chromite mines can be reprocessed to provide additional PGE. However they contain very minor PGE and, while some recovery is taking place, it will never represent a major additional supply.

Other Deposits of the Platinum Group Elements

The PGE contained in other deposits and exploration areas around the world tend to be much harder to define. They do not occur in such well-constrained layers of uniform thickness and great lateral extent. The mineralised layer in the Great Dyke of Zimbabwe is the most similar to the Merensky Reef, but the area of the intrusion is very much smaller. The Stillwater Complex in North America is also similar to the Bushveld Complex, but the ore zone is patchily concentrated along and vertically within a thicker layer than the Merensky Reef. The length of that entire intrusion is about 40 km, which is similar to that of one single mining operation in the Rustenburg area (Anglo Platinum, Impala Platinum or Lonmin). The dip of the rocks in Stillwater is much greater, again reducing the ore tonnage to maximum mining depth.

The total potential of all the other PGE mining areas is much harder to quantify because they are not layered. Extensive exploration during the last ten years has failed to produce any new major targets or mines. Also, all occurrences in the Bushveld Complex and Great Dyke have high platinum-to-palladium proportions (platinum greater than palladium), whereas all other occurrences in the world are dominated by palladium.

Other Issues

Maintaining and increasing future production rates represent significant challenges. Resolving social, political and environmental issues, together with ensuring water and electrical supply capacities, needs ongoing monitoring and careful planning (11). These challenges are the unknowns and unpredictables in the future of platinum mining in South Africa, not the availability of the ores.

Conclusions

The estimates of the PGE presented here are not intended to be rigorous or quantitative. They are designed to show that the broad estimation of the PGE in the Bushveld Complex is extremely easy to make and to understand, and that the remarkably disproportionate concentration of the PGE in one geographic location, South Africa, is genuine. Even with current mining methods, technology and prices, there are many decades to a century of extractable PGE ore already known in the Bushveld Complex. With around 350 million oz of platinum per vertical km depth, the enormous deposits of PGE in the Bushveld Complex can be confidently relied upon to provide a major proportion of the demand needs for a long time into the future.