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Jan 2004 - Platreef

Platreef is different

In late November / early December 2003 I visited the northern limb of South Africa’s Eastern Bushveld, and concluded that the northern limb appears to consist of different types of Platreef that formed when a type of Merensky reef ingested different footwall rocks over which it passed.

I was unaware that for some such a conclusion has been known knowledge whereas others debate any possible proof. I remember once when I saw angular blocks of Merensky “A” – type reef (an Implats classification for the conventional Merensky reef that consists of a pyroxenite above a 1 to 2cm thick chromite band, then a 15cm to 50cm thick pegmatoid pyroxenite, then a 1mm thick chromite band over a mottled anorthosite/norite footwall) within UG2 on a mining visit to Wesplat many years’ ago. However, I was later told that without proof of what I saw “it cannot exist, and you must have been mistaken” (despite mining that ore for almost a year).

The reason being that what I saw (or thought I saw) demolishes a number of erudite theories on the formation of the mega basin-shaped Bushveld layered intrusive Igneous Complex of South Africa that contained the world’s largest source of platinum (and other PGMs).

The formation of South Africa’s Bushveld complex has been widely studied and theorised and remains open to heatedly debated controversy as to how it could have actually formed. However, what is clear is that something very different occurred in the almost north-south trending northern limb of the Eastern Bushveld, resulting in Platreef. As far as I knew (prior to my visit) Platreef consisted of layered A, B and C reef bands within a large mass (20m to 30m thick) of low grade 3 to 4g/t PGE ore, that was mined in total since the total package was above the cut-off grade.

After visiting Pan Palladium’s properties of Aurora and Volspruit at the respective north and south extremities of the limb and the similarities that they showed (both with a fine grained pyroxenite host containing some olivine bands, yet totally different mineralisation being discrete high grade bands at Aurora in the north, compared to general lower grade at Volspruit in the south). I was intrigued by what happened in the middle and managed to speak to two of Amplat’s most senior geologists in the Johannesburg head office.

Amplat’s Platreef mineralisation lies north-south across the farms Overysel, Zwaartfontein, Sandsloot and Tweefontein in the Eastern Bushveld’s northern limb north of Potgeitersrust (now called Mokopane).

Amplats showed me a specimen of their conventional Merensky reef (a) from Rustenburg, and their Platreef – type of Merensky reef (b) as shown in Figure 1. The Platreef Merensky – type has an upper 4cm thick chromite band above 3 to 4 times larger crystals (in the pegmatoid) than the more usual Merensky reef (a). This pegmatoid crystalline sequence is apparently up to 4m to 5m thick and exists on the farm Tweefontein (north) with a bif (banded-iron formation) footwall The Merensky pegmatoid has a variable thickness in the Western Bushveld too, being usually up to 4m thick on the western side of Wesplat, it can increase to 9m thick as on the eastern side of Lonplats’ Wesplat.

Amplats’ Platreef varies across its farms depending on what footwall has been ingested. The more commonly known and described Platreef is based is that in which dolomite has been ingested to result effectively in a “smelter flux” such as on the farm Sandsloot with the commonly described A, B and C reefs (in a hybrid sequence) which are regarded by some as imaginative, especially when attempting commonality over distances. Where interaction with the dolomite has occurred, horizontal slabs of undigested dolomite can also be found included within the ore (usually with cu-ni mineralisation around them).

It may be noted that our text varies in that sometimes PGE is used (especially 4EPGE) and at other times PGM. PGM refers to platinum group metals and should only include platinum, palladium, rhodium, ruthenium, (and iridium and osmium which are usually overlooked). PGE refers to platinum group elements and includes other minerals that can be found in platinum reefs such as gold. 4EPGE or 4 element platinum group elements hence usually consists of platinum, palladium, rhodium and gold. Osmiridium is not a PGM or a PGE, it is a name formed from osmium and iridium to represent the precious metal sludge of PGEs that exist when producing gold bars by a gold mine.

PGE and base metal grades (nickel and copper) vary across the different farms. Sandsloot can contain 6g/t to 8g/t 4EPGE, as in 6g/t over 15m to 20m, although it usually reduces to 4g/t to 4.5g/t over 40m thick in the farm’s south. While Zwaartfontein, just north of Sandsloot has patches of grade up to 25g/t 4EPGE. Further north into the sedimentary sequence (away from the granites in the south), Overysel has two platinum reefs, an upper with ni/cu in the 0.4% vicinity and a lower with 1% to 2% cu (but with lower PGM grades). Tweefontein’s ni/cu grades are typically about 0.4%. The platinum reefs dip at 45 degrees, with their strike varying according to line of the reef especially where it encounters the dolomite “tongue” which can be seen on geological plans.

Paint splashes of pentlandite can be seen on core (it looks literally as if a paint brush has been dipped in pentlandite paint and slapped across the drill-core), as can also be seen on core from Pan Palladium’s Aurora prospect.

Aside from grade and rock-type differences, Platreef in the northern limb also has two other restrictions (compared to the platinum mines of the Western Bushveld), that greatly influence its capability to be mined, namely water and townships. The Potgeitersrust area is dry, as illustrated by the number of bridges over dry riverbeds, and consequently approval to mine is required from the State who control the water rights. The Platreef line of reef is also dotted with many townships of varying sizes.

It has been estimated that one of the other companies could have to resettle 40,000 to 140,000 people before it can consider mining its property, since the accepted rule of thumb is no open-cut workings within 1km of a township.

It can be seen that Platreef is very different in type, location and content with higher base metal credits too, such that it should not be lumped in with the UG2 rich palladium reefs of the remainder of the Eastern Bushveld that are facing an uncertain future against the backdrop of a stronger rand exchange rate against the US$.

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, holds interests in Pan Palladium (PPD). 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.

  • Written by: Keith Goode
  • Thursday, 01 January 2004