This complex of ancient magmas is known to have formed some two billion years ago, but the source of its metallic riches has been a matter of scientific dispute. Now researchers from the Carnegie Institution and the University of Cape Town have traced the origin of the unique ore deposits by using another of South Africa's treasures—diamonds.
The study, published in the June 12 issue of Nature, suggests that the source of these valuable ores may be ancient parts of the mantle beneath the African continent.
Platinum group elements (PGEs), which include platinum, palladium, rhodium, ruthenium, osmium and iridium, are extremely rare in the Earth's crust. Platinum, the most abundant, is 30 times rarer than gold. Mined only in a few places in the world, these elements are becoming increasingly important in applications ranging from pollution control (they are key components of catalytic converters in automobiles) to microelectronics.
Previous isotopic studies of rocks from the Bushveld Complex had suggested that a significant fraction of the magma that formed the complex and deposited the ores came from shallow parts of the crust, despite the rarity of PGEs there compared to the Earth's mantle. "But the ore layers are extremely homogeneous over hundreds of kilometers," says Steven Shirey of the Carnegie Institution's Department of Terrestrial Magnetism. "The crust is very heterogeneous. That suggests a deeper source for the platinum."
To test this idea, Shirey and Stephen H. Richardson of the University of Cape Town studied minute mineral inclusions in about 20 diamonds mined from areas surrounding the Bushveld Complex. The diamonds formed at depths of 150-200 kilometers within the Earth's mantle. By measuring the ratios of certain isotopes of strontium, osmium, and neodymium in the mineral inclusions, the researchers were able to determine the isotopic "signatures" of the different regions of the mantle where the diamonds grew. They then compared these signatures with those of ore rocks in the Bushveld Complex.
Richardson and Shirey found that the isotopic signatures of the ores could be matched by varying mixtures of source rocks in the mantle beneath the continental crust. That these parts of the mantle were involved in producing the magmas is also suggested by seismic studies, which reveal anomalies beneath the complex. The anomalies were likely the result of magmas rising through these parts of the mantle.
"This helps explain the richness of these deposits," says Richardson. "The old subcontinental mantle has a higher PGE content than the crust and there is more of it for the Bushveld magmas to traverse and pick up the PGEs found in the ores."
The results of this study may be applicable to similar ore deposits elsewhere, such as the Stillwater Complex in Montana. "Knowing how these processes work can lead to better exploration models and strategies," says Shirey.
Steven Shirey | EurekAlert!
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