An Overview of Platinumin the Bushveld Complex

Tony NaldrettUniversity of the Witwatersrand

The largest nugget of platinum; mined at Konder, Siberia, weight 3.521 kg

So where do our Platinum and other Platinum Group Elements (PGE)

come from?

The largest and richest resource of PGE is the earth’s core,2900 km away, vertically below us!

1. The part of the Earth in whichthe PGE are reasonablyabundant and yet with whichwe have some communicationis the mantle.

2. Geological processes provide this communicationand serve to transfer the PGE from the mantle andconcentrate them to minable concentrations in theCrust

3. The most important process is that of partialmelting in the mantle, and the intrusion of thismelt into the crust. This accounts for the originof the PGE in the Bushveld complex and Great Dyke

What kind of geological environmentsdo we find them in?

The vast majority of the PGE are found in layered intrusions, with the Bushveld complex being the prime example

4321

Roof Rocks

Floor Rocks

Magma spreads out at acertain level and crystallizes

How do layered intrusions form?

NorthernChamber

In the Bushveld complex several hundreds of magma pulses have entered the chamber.Groups of consecutive pulses have similar features that differ from other groups so that thelayers that these pulses give rise to can be grouped together as “Zones”.

A

C

B

Different parts of the Bushveld have evolved in separate chambers that may be entirelyseparate, may join up with one another as more magma pulses are injected into them.

These chambers have evolved at different levels, so that erosion has removed different amounts of the upper rocks of each chamber.

What does igneous layering look like?

It is the result of variable proportions different minerals in a body of rock

Differing proportions of the minerals plagioclase and pyroxene

UG-1, Dwars RivierA layer composed of >80% chromite within rockcomposed of 90% plagioclase

The Merensky Reef

Leuconorite/ Anorthosite

Top Chrome Cnct

Bottom Chrome Cnct

Pyroxenite

Top of Merensky Pyroxenite

Merensky Reef in Eastern Bushveld

9 g/tOver 85cm(mining width)

Footwall anorthosite

Bottom contact CrTop contact Cr

Hanging wall pyroxenite

Merensky Reef

First Pulse

Second pulse

Merensky Reef, Rustenburg area

Pentlandite

Pyrrhotite

SperrylitePtAs2

Photo courtesy of Martin Slabbert, Lonmin

Unconformity at base of Merensky Reef

A brief note on how the Merensky Reef formed

To demonstrate what I mean, go to your kitchen, and take some clear, rather salty meat stock, and shake it up with a cooking oil. Taste the water and the oil separately. The water will be salty, but have only a slight taste of the meat. The oil will not be salty at all but will have a strong meaty aroma.

This is because the chemical properties of salt and water are compatible, both are polar, while the molecules that give rise to smell and taste are non-polar, and are more at home in the oil, which is also non-polar. The tastiest gravies always contain droplets of fat, which acts in the same way as oil, attracting the smell and taste components of a joint of meat.

First, a word about “Partitioning”

(shown in next slide)

A body of magma becomes saturated in sulfur anddroplets of liquid iron sulfide develop in the magma. Platinum group elements, along with nickel, copper and some other metals present in the silicate magma, are chemically much more at home in a sulfide liquid (we call them chalcophile, or sulfur-loving metals) than in the silicate magma and therefore will partition into it. The sulfide droplets will therefore become quite enriched in the chalcophile metals, becoming richer and richer with the more magma they come into contact with.

The UG-2 Chromitite

UG2 Grobelaarsdal Area

Photograph Courtesy of Ron Hieber, Angloplatinum

The Platreef

The Northern Limbof the Bushveld

The Platreef is a composite unit, 50-300 m thick, that cuts down through units of the Transvaal Supergroup to come to rest on Archean basement. The upper unit, a cumulus-textured pyroxenite shown in dark green, contains sulfides with PGE tenors resembling those of the Merensky Reef. Overlying gabbro-norites of the Main Zone have intruded, and eliminated the upper Platreef pyroxenite in places.

Schematic cross-section through the Platreef

From: Sello Kekana M.Sc. Thesis, Univ. of Witwatersrand, 2013

Grade and sulfide tenor inHole UMT277, Turfspruit

And where does the future lie?The data in the next 2 slides are of holes chosen at random and do not constitute grades.They have been chosen to illustrate a point, not as the basis of a quantitative evaluation.

AngloAmericanPlatinum reckon that mining constitutes 68% of cost of producing PGE, Concentrating 20%, Smelting 8% and Base Metal and PGE refining 5%. (July Ndlovu, Executive Head of PGM processing, AngloAmerican Platinum, November 2014, “An overview of PGM processing”)

So how does it all come together geologically?

45

The Pudding Basin model

Calculations based on the mass of chromite in the Critical Zone indicatethat this zone must have fored from at least 12 times more magma thanis now represented by the rocks forming the Zone. I.e. that magma has beenlost from the complex.

The “Pudding basin”Model