Magmatic Ore Deposits - Metals and Society: An Introduction to Economic Geology - page 48

3.3 Magnetite and Platinum Group Element Deposits
of the Bushveld Complex
The Bushveld Complex contains two other important types of deposit. Layers of
magnetite, mined for their vanadium contents, occur in the upper part of the
intrusion. These magmatic deposits probably formed in a manner similar to the
chromite deposits. The other type, economically far more important, are deposits of
the platinum group elements (PGE). The Bushveld Complex contains about 60% of
global reserves of these increasing valuable metals, mainly at two specific horizons
in the lower part of the intrusion. The upper layer is the famous Merensky Reef,
a thin (1-10 m) layer of pegmatoid pyroxenite located at the top of the Critical
Zone (Fig. 3.2 ). Seemingly low, but economically viable, concentrations of PGE
(5-500 ppm) are associated with minor sulfides mainly towards the base of the
Reef. Because of the high cost of the platinum group elements, even these low
concentrations of metals can be mined at a profit. The second major mineralized
layer, called UG2, is a series of thick chromite reefs that, in addition to high PGE
content, are also mined for their Cr contents.
Box 3.2 Competing Theories for the Formation of the PGE Deposits
There no consensus about the origin of the PGE deposits. One school,
championed by Tony Naldrett and Ian Campbell (Campbell et al . 1983 ;
Naldrett 1989 ) and Grant Cawthorn and colleagues (Cawthorn et al . 2005 ),
argues that these deposits formed through magmatic processes; the opposing
school, led by Alan Boudreau, Chris Ballhouse, Ed Mathez and others
(Ballhaus and Stumpfl 1986 ; Boudreau 1995 ; Mathez 1995 ) ascribes an
important role to the migration of volatile-rich fluids.
Campbell and Naldrett proposed that a plume of primitive magmatic
liquid was injected into the base of the chamber and then mixed with evolved
liquid to produce a hybrid magma that became saturated in sulfide. Small
droplets of magmatic sulfide segregated from the silicate liquid and these
attracted the chalcophile (sulfur-loving) PGE. The droplets of PGE-enriched
sulfide then settled to the floor of the intrusion to slowly build up the ore-
bearing horizon. Geologists of the opposing school propose that volatile-rich
fluids migrated up through the cumulus pile, leaching out the PGE from the
cumulus minerals then redepositing them at favourable horizons. The various
models are illustrated in Fig. 3.5 .
What do you make of these competing hypotheses? Is it not surprising that
the origin of some of the world's greatest ore bodies, which are well exposed
because of good outcrop and extensive mining operations, and which have
been intensively studied for almost a century, is still so poorly understood?
Why is it that two groups of highly respected, highly experienced geologists
have developed such different models? Which model do you think is the more
plausible (to answer this question will require that you read some of the
abundant literature available on the subject)? What type of research could be
done to (help) resolve the issue?