Most of the world’s platinum group elements (PGE) are produced from PGE-reef layer within an ultramafic to mafic intrusion. A textbook example of a PGE reef is the Merensky Reef, within the Bushveld Complex, South Africa. It contains about 75% of the known platinum deposits in the world.
A comparison of the copper-palladium values across the Bushveld Complex reveals that, going from the rocks below the UG-1 into the area where PGE reefs are common (between the UG-1 and the Merensky Reef), the copper-palladium ratio falls. Above the Merensky Reef, the ratio rises again. (See Figure A.)
The low copper-palladium ratios in the area where PGE reefs are common is partly due to the fact that sulphides concentrate PGE more efficiently than they concentrate nickel and copper. By applying this concept, Sarah-Jane Barnes of the Dept. of Earth Sciences, Universite du Quebec at Chicoutimi, has developed a geochemical method of prospecting for PGE reefs within ultramafic and mafic intrusions based on ratios between nickel and PGE and copper and PGE. The method uses the ratios of nickel and copper to PGE to decide whether the sequence of geological processes that occurred in a mafic or ultramafic intrusion favored development of a PGE reef. First of all, to determine the PGE reef potential of any intrusion, the nickel-palladium and copper-iridium ratios must be similar to values of the extrusive rocks. To locate the PGE reef within the intrusion, examine the change in nickel and copper to PGE ratios. PGE reefs and rocks in the vicinity of a reef have very low nickel-PGE and copper-PGE ratios.
Current samplings of mafic and ultramafic intrusions often reveal a few specimens of 100 parts per billion, or more, of platinum and palladium. Although such levels are anomalous in mafic and ultramafic rocks, they could simply correspond to the original sulphide content of the rocks and need not indicate a favorable PGE reef environment. This potential misinterpretation may be prevented by applyiig Barnes’s new geochemical method of prospecting for platinum.
Most mafic and ultramafic intrusions are large, with stratigraphic thicknesses of over three kilometres. Above and below the reefs, the PGE content of the rocks is not generally anomalous thus a recurring problem for the exploration geologist is, first, to decide whether the intrusion has the potential to contain a PGE reef and, second, to locate the PGE-rich strata of one metre or less in a section of four kilometres or more.
The method relies on the fact that PGE have very much greater partition coefficients into sulphides than nickel or copper (20,000 as opposed to 200); that is to say, sulphides concentrate PGE more efficiently than they do nickel and copper. The sulphides are thought to act as a collector of PGE during the formation of PGE deposits. Therefore, from a prospecting point- of-view, determining if and when sulphide segregation of a magma occurred is important.
If a magma is to contain sufficient PGE to form a reef deposst within an intrusion, it should not have experienced any sulphide segregation before emplacement at the present location. If prior sulphide segregation occurred, the PGE would have been scavenged from the magma by the sulphides and deposited elsewhere, presumably at depth. If sulphides have not been removed from a magma, then the nickel-palladium and copper-iridium ratios of the chilled rocks from the margins of the intrusion will be similar to those of extrusive rocks (defined by komatiites, high-mgo basalts, flood basalts) and, therefore, the intrusion has potential for a PGE reef deposit.
In order to decide whether an intrusion has the potential to contain a PGE-reef deposit, the nickel and copper to PGE ratios of grab samples should be considered. If the samples have low nickel-palladium and copper-iridium ratios, the intrusion has the potential to contain a PGE reef deposit. These samples would plot in one of the three zones of extrusive rocks (komatiites, high-MGO basalts, flood basalts), or below them. If the magma that formed the intrusion had lost sulphides, the nickel-palladium and copper-iridium ratios would be high and the intrusion would not be a good target. These samples would plot above the extrusives as illustrated in Figure B.
Once good PGE reef potential has been established, the exploration geologist must locate the reef within the intrusion. In order for a PGE-reef deposit to have developed, a process such as sulphide segregation must have concentrated the PGE within the intrusion at the site where the sulphides first formed. The segregation of sulphides from a magma causes nickel-palladium and copper-iridium to decrease in the corresponding cumulate containing the sulphides, relative to initial magma.
In order to locate the PGE reef within the intrusion, the ratios of nickel and copper to PGE in the rock samples across a stratigraphic section can be used to find the level where sulphide saturation first occurred. A cumulate containing the first formed sulphides should be enriched in PGE relative to nickel and copper and so will tend to plot below the field defined by extrusive rocks in Figure B. If cumulate rocks are found to plot in this area, then it is probably safe to assume that processes favored PGE reef formation.
The fractionated magma may later form a cumulate containing a second generation of sulphides, but this rock is expected to be depleted in PGE because the earlier-formed sulphides will have stripped the magma of PGE. Rocks formed from a magma depleted in PGE by an earlier sulphide segregation event will plot above the field of extrusive rocks (komatiites, high-MGO basalts, flood basalts) in Figure B. Such rocks do not make good exploration targets themselves, but they do suggest that PGE-enriched rocks might lie stratigraphiially below them.
This geochemical method of PGE exploration is complicated by the fact the nickel-copper-PGE ratios are also influenced by olivine and chromite crystallization. To eliminate the effects of olivine and chromite and their complicating effect on the ratios in Figure B, one must take into account such ratios as palladium-iridium or palladium-rhodium or palladium-platinum vs. nickel/copper.
Considering all these potential stratigraphic variations, the use of nickel and copel and cop ratios could be a worthwhile tool for exploration geologists interested in PGE deposits.
REFERENCE:
Barnes, S.J. (1990). The use of metal ratios in prospecting for platinum-group element deposits in mafic and ultramafic intrusions. Journal of Geochemical Exploration, Vol. 37, pp. 1-9.
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