Magmatic Ore Deposits - Metals and Society: An Introduction to Economic Geology - page 49

a

b

Pyroxenites

Pyroxenites

Merensky Reef

Merensky Reef

PGE-rich
late
magmatic
liquid

PGE- and Cl-
rich brines

Underlying
cumulates

Underlying
cumulates

Enrichment of existing sulfudes by
ascending hydrothermal fluids

Enrichment of existing sulfides by
ascending late magmatic liquid

c

d

less dense magma

magma crystallizing and
forming sulfides

turbulent plume

more dense magma

PGE- and Cl-
rich brines Underlying

cumulates

Underlying
cumulates

Enrichment of zone of crystallization
and sulfide liquid segregation from
ascending hydrothermal fluids

Segregation of PGE-ric sulfides
from a plume of new magma
injected into the chamber

Fig. 3.5 Models for the formation of the Merensky Reef (From Naldrett ( 2004 ))

Platinum-group elements are recovered from other large layered mafic-ultra-
mafic intrusions like the Stillwater in the USA and the Great Dyke in Zimbabwe,
and these constitute the major source of these metals. The same metals are also
recovered, though in differing proportions, as a valuable by-product of mining
magmatic sulfide deposits, our third example of a magmatic ore deposit, and in
placer deposits, which are discussed briefly in Chap. 5.

3.4 Magmatic Sulfide Deposits

When a mafic magma cools it crystallizes. A series of solid phases appear, typically
olivine, pyroxene, feldspar and oxides, which become the constituents of mafic and
ultramafic magmatic rocks. Under some circumstances second liquid separates, in
rare cases another silicate liquid, which is immiscible with the first silicate liquid,
or, if the sulfur content of the liquid is high, an immiscible sulfide liquid. In the
remarkable example shown in Fig. 3.6 , droplets of sulfide liquid have been frozen in