Magmatic Ore Deposits - Metals and Society: An Introduction to Economic Geology - page 50

Fig. 3.6 Blobs of magmatic
sulfide from Noril'sk-Talnakh

place, suspended in the surrounding mafic liquid which has solidified to gabbro.
The droplets of sulfide liquid contain elevated concentrations of elements such as
Ni, Cu and the PGE, which are chalcophile or sulfide-loving, and because the
sulfide liquid is denser than the silicate liquid, they tend to settle to the base of
the magma body.

If enough of the sulfide liquid segregates, and if the

concentrations of metals are high enough, it becomes an ore deposit.
This process seems straightforward, yet once again we are faced with a problem:
mafic-ultramafic intrusions are known throughout the world, and from petrological
and geochemical data we can infer that their parental magmas contained high
concentrations of Ni, Cu, and the PGE. Inspection of polished thin sections shows
that many of these rocks do indeed contain sulfides that separated as an immiscible
liquid, but in most cases this phase appears only at a very late stage in the
crystallization sequence and only in very low quantities. What was the particularity
of certain magmas that led them to segregate copious amounts of metal-rich
sulfide? Or to put it another way, how was the normal magmatic differentiation
sequence perturbed so as to form an ore deposit?
To answer these questions we will first discuss the processes that govern whether
or not a sulfide liquid separates from a mafic or ultramafic magma, and, equally
important, the concentrations of ore metals in this liquid. Then we consider how the
sulfide segregates to form an ore body, taking as our first example the deposits of
the Kambalda region in Western Australia.

3.4.1 Controls on the Formation of Magmatic Sulfide Liquid

Mafic and ultramafic magmas form by partial melting at great depth in the mantle,
from about 30 to 300 km. Most of the magmas that yield magmatic ore deposits
result from melting in mantle plumes, which are cylinders or more irregular masses
of solid mantle that are hotter than the surrounding ambient mantle and ascend
because of their low density. Only part of the mantle peridotite melts, between 5%