Geology Reference
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cumulates. These deposits are mined for chrome and in some cases for
platinum group elements as well. (2) Platinum group elements are also
mined in the famous Merensky Reef, which is located at the top of the Critical
Zone, where the rocks change from ultramafic to mafic in composition.
(3) Finally, magnetite veins, which are mined for the vanadium, are hosted
in the mafic rocks in the upper part of the complex.
These deposits supply a very large proportion of the global demand for
these metals. The chrome production is about 50% of the global total and
platinum and palladium production from the complex represents 72% and
34%, respectively, of annual global production.
Neil Irvine, in two papers published in the 1970s (Irvine 1975 , 1977 ), developed
an interesting and important model for the origin of chromite deposits in the
Bushveld Complex and in other layered intrusions. His diagram, modified as
Fig. 3.3 , shows that under normal conditions 1-2% chromite crystallizes together
with olivine. Mafic-ultramafic liquids, such as the parental magma of the Bushveld
Complex, plot in the field of olivine. Such a liquid initially crystallizes olivine,
whose removal drives the residual liquid composition to the cotectic, at which point
chromite starts to crystallize. Chromite and olivine crystallize together, in the
proportion given by the intersection of a tangent to the cotectic with the olivine-
chromite side of the diagram. The proportion of chromite ranges from 1.8% to
1.4%, the amount observed in normal olivine cumulates of the complex. To produce
a layer of almost pure chromite requires suppression of the accumulation of olivine
and other silicate minerals. Irvine suggested two ways this might happen, both
illustrated in the phase diagrams of Fig. 3.4 .
In this simple diagram, a strongly curved cotectic separates the primary phase
fields of olivine and chromite. For chromite to crystallize alone, the composition
of the liquid must be driven off the cotectic and into the chromite field. Irvine
( 1975 ) proposed that one way that this might happen is when the magma
becomes contaminated with granitoid country rock like that which makes up
most of the Archean continental crust into which the Bushveld magma intruded.
Granite plots at the SiO 2 apex in the diagram. A hybrid, contaminated magma
plots on a line between the magma composition and the apex, at the point
labelled D in the chromite field. This magma crystallizes chromite alone until
its composition regains the cotectic. The interval in which chromite crystallizes
seems small, but in a large intrusion like the Bushveld, the chromite that
crystallizes during the interval is enough to form a layer of appreciable thickness
monomineralic chromite cumulate.
The second process depends on the strongly curved shape of the cotectic. Irvine
( 1977 ) argued that because of this shape, when an evolved liquid residing in the
chamber mixes with a more primitive liquid that enters the chamber, the hybrid
liquid plots within the chromite field. Like the contaminated liquid, the hybrid
 
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