Platinum in Layered igneous complexes/deposits
One of the best examples is the Bushveld igneous complex, in South Africa, which supplies 63 % of the world's supply of newly mined platinum group metals (pgm's). This group of rocks is the source of 85 % of the world's production of platinum, and hosts 82 % of the world's economic resources of pgm's.
The Bushveld complex is a lode deposit, and pgm's are found in association with nickel and copper sulphide. It was formed when a large body of mafic magma(molten rock) was emplaced in the earth's crust. Thereafter the mafic magma slowly cooled. During the cooling silicate, oxide and sulphide minerals crystallised and sank to the bottom of the magma chamber to form texturally distinctive layers. The removal of the more refractory minerals in this way depletes or enriches the residual melt in various elements. The magma thus changes composition continuously until solidification is complete.
As a result, the platinum group metals, nickel and copper when in presence of a sulphide phase, can become sufficiently enriched to form mineralised horizons at predictable levels within the intrusion.
The Bushveld complex consists of four lobes, the northern Potgietersrus lobe, the Western and Eastern lobes and a hidden sequence in the south-east and west. All dip towards the centre of the Transvaal. Originally it was thought that the lobes were interconnected to form a large saucer-shaped intrusion with a 300 km diameter. Recently it has been suggested that each lobe has its own feeder system through which magma was channelled.
In the Eastern lobe of the Bushveld complex the outcrop is good, but surface weathering has obscured much of the geology in the western lobe. A sequence of more than 8 km in thickness is formed by the rocks in these lobes. A much more extensive surface expression of the Bushveld is, however, flat dips. A characteristic of the Bushveld is the continuity shown by many of the layers,including economically important horizons, over tens of km's.
Exploitation of pgm mineralisation in the Western Bushveld complex is currently associated with two main horizons, the Merensky Reef and the 60 cm wide UG2 chromite layer which is located 100m to 350 m below the Merensky Reef.
The Merensky Reef in the eastern limb of the Bushveld complex is very much thicker than in the western limb. The Merensky Reef is the main source of ore at present for the South African producers. Overall grades for the Merensky and UG2 Reef are similar, but platinum and palladium in the UG2 are present in equal proportions compared to the platinum rich Merensky Reef. The UG2 reef does, however, contain more than double the amount of rhodium. The uniformly layered appearance of both the Merensky and the UG2 horizons are at points disrupted by circular depressions known as potholes. A third pgm horizon is the Platreef, which is a nickel-rich ore body which has a platinum-palladium ratio close to 1:1. It has mineralogical similarities to the Merensky Reef. As the Reef is up to 15 m wide in places, there is potential for low cost open-cast or bulk underground mining.
magmatic segregation.
"As magmas cool crystallization occurs over a range of temperatures. In basic and ultrabasic magmas at about 1000oC crystals of chromite (Fe2CrO4), Magnetite (Fe2O3) and Ilmenite (FeTiO3) may form first. These sink to the bottom of the magma chamber and form workable concentrations.
Crystals can be swept through the magma chamber by slow moving currents and behave as a sediment, giving rise to layering, grading, and current bedding. Accumulations of this kind are seen in magnetite, chromite and platinum-rich ultrabasic bodies such as the Bushveldt intrusion in South Africa."
geology.gla.ac.uk
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