Northam started below Amandebult and was regarded by many as being an exception to the rule, particularly after the struggle the mine went through to ultimately become profitable, so most mining companies “drew” a rough depth limit at ~1200m to 1300m below surface as a conceptual depth limit to mining platinum in the Bushveld Igneous Complex (BIC). This depth limit was also a function of economic return, weighing up refrigeration requirements and grade, especially when the PGM prices were relatively low and costs were steadily rising due to inflationary pressures. For a number of the mining companies, having boundaries at such limits appeared to cover the foreseeable future.

When fresh rock is first exposed it has a temperature called the virgin rock temperature (VRT), and that temperature increases with depth depending on the surrounding strata that it is contained in and to some degree what the surface temperatures are. The increase in temperature with depth is called the geothermal gradient. As mines extend deeper, the VRT increases to a point where refrigeration is required to lower the temperature to provide a better and safer working environment.

Refrigeration or cooling the air circulating in the workings takes a number of different forms, however, refrigeration increases the moisture content or humidity in the air resulting in what is called a wet bulb temperature (the VRT on exposure being effectively a dry-bulb temperature).

There is a limit, to how much moisture air can absorb before it becomes super-saturated and that has been estimated at about 66ºC to 68ºC dry bulb such that super-saturation of the air reduces the temperature to 33ºC to 34ºC wet bulb. At such temperatures, mineworkers need to wear ice-jackets (such as were worn in parts of Western Deep Levels, and which were changed every hour or so) in order for the body to sweat (otherwise the body cannot sweat because the air is super saturated).

If a body cannot sweat, then theoretically the fluids surrounding the brain boil and the person (so exposed) dies. That limit appeared to be around 4km below surface on a South African gold mine (the depth does vary between the various goldfields).

Table 1. Temperatures at Various depths in SA Gold Mines and the BIC. Source : Chamber of Mines Research Organisation Annual Report 1983, p 6.

However, the depth limit for a South African platinum mine is much shallower, because the geothermal gradient is significantly higher in the Bushveld Igneous Complex (BIC) than on the gold mines such that virgin rock temperatures (VRTs) at 1.5km in the BIC can be equivalent to those at 3.5km deep in a South African gold mine as shown in Table 1.

In their presentation at Indaba February 2006 in Cape Town, Roy Pitchford’s Afplats showed a slide of the depths of the existing and proposed shafts of some of the South African platinum companies as shown in Figure 1.

While the mines are going deeper, the costs are rising and the time to achieve full production is taking longer. Impala Platinum names its shafts according to generations, following its shallowly dipping Merensky and UG2 platinum reefs deeper. So after the initial declines came the first generation of vertical shafts to relatively shallow depths and so on, the last shaft complex in the third generation is apparently No 20 (at the northern end of the lease) and is expected to cost ~R3bn (A$680m), while the first shaft complex in the fourth generation No 16 (at the southern end of the lease) is expected to cost ~R3.6bn (A$820m).

The No 16 shaft complex consists of a 10m diameter main shaft to ~1650m which is to service 7 levels from 21 Level (~1215m below surface) to 27 Level (~1535m below surface), and a 6.8m diameter ventilation shaft to 25 Level (~1425m below surface). Sinking the No 16 main shaft started In January 2006, and first production is expected more than 5 years’ later in 2011, building up to full 2.7mtpa production a further 3 years’ later in 2014. VRTs at ~1600m are expected to be ~57ºC and a 30MW refrigeration plant is being installed to meet the expected requirements.

Afplats showed in Figure 1 the position and expected depth of their Leeuwkop Phase 1 shaft. Afplats’ 3mtpa and 300,000ozpa 4E Leeuwkop Phase 1 mine starts at the northern boundary of Lonmin’s old Westplat mine at a depth of about 1000m below surface (downdip from [or below] Lonmin’s Saffy Shaft) and is expected to cost R2.3bn (A$520m) with operating costs of R195/t (excluding royalties and capex). Afplats is currently waiting to receive its new prospecting right before converting it to a new mining right and start construction.

Once Afplats’ construction starts, mining is expected to now start in month 21 (less than 2 years, was previously month 34 or almost 3 years), milling from month 36 (3 years) and then build up to full production by month 51 (just over 4 years, was previously month 68 or >5.5 years).

Afplats’ Leeuwkop shaft system includes a 9.8m diameter main shaft to 1350m below surface, a 7m diameter ventilation shaft to 1270m below surface, a 3mtpa plant to treat the UG2 ore (with recoveries increasing from 79% to 82% over 5 years), and a refrigeration plant that can reduce the ambient working temperatures to ~27ºC and be capable of delivering 1250l/s of ~4ºC chilled water.

We asked a number of different people after Indaba 2006, and all thought that the eastern limb of the Bushveld had a very similar geothermal gradient to the western limb, however, there is some uncertainty over the temperatures on the northern limb (because it has a different style of platinum mineralisation).

Consequently it was interesting to see AfriOre’s presentation at Indaba showing that they expected their Akanani mine to initially start at a depth of 900m to 1km below surface and mine to 1500m and then 2km below surface on Platreef (east and downdip from [or below] Amplats’ [PPRust’s] Zwaartfontein South open-cut) on the northern limb of the BIC.

It can be seen that the perceived depth limit of 1200m to 1300m for South African platinum mines could be revised back up to at least 1700m to 1800m, or possibly even deeper, extending the life of the South African Platinum industry still well into the future at high PGM prices.

Disclosure and Disclaimer : This article has been written by Keith Goode, the Managing Director of Eagle Research Advisory Pty Ltd, (an independent research company) who is an Authorised Representative with Taylor Collison Ltd, and with his associates, may hold interests in some of the stocks mentioned in this article. The opinions expressed in this article should not be taken as investment advice, but are based on observations by the author. The author does not warrant the accuracy or completeness of any information and is not liable for any loss or damage suffered through any reliance on its contents.

Figure 1. Depths of South African Platinum Shafts according to Afplats in February 2006