Magmatic Ore Deposits - Metals and Society: An Introduction to Economic Geology - page 45

Fig. 3.3 Formation of chromite veins in the Muskox Intrusion, Canada
the continental crust is only a few tens of ppm, but this figure is not relevant because
chrome deposits are found in ultramafic rocks in which concentrations are much
higher. But even taking a typical level for ultramafic intrusions of 1,000 ppm means
that the enrichment factor; i.e. the ratio of the ore grade over the background level,
is 250-350. In other words, to form the ore deposit, the concentration of Cr had to
be increased by several hundred percent. What geological process could have
produced this degree of enrichment?
Box 3.1 Deposits of the Bushveld Complex
The Bushveld complex in South Africa (Fig. 3.2 ) is the world's largest mafic-
ultramafic intrusion. As shown in the figure it has a complex form and
consists of a collection of intrusions that are roughly circular in plan in plan
view; in section it probably comprises a series of amalgamated funnel-shaped
(lopolithic) magmatic bodies. The dip around the periphery is persistently
towards the interior and thus the base of the intrusion is exposed along the
margin. The stratigraphic section shows that the rock-types change from
ultramafic at the base to intermediate at the top. The lowermost rocks are
olivine and pyroxene cumulates; these pass upwards through pyroxene pla-
gioclase cumulate to the diorites that occupy the upper third of the complex.
Three types of ore deposit are found in the complex. (1) Chromite deposits,
such as the layers illustrated in the photo, are located in the ultramafic
(continued)