Magmatic Ore Deposits - Metals and Society: An Introduction to Economic Geology - page 53

and sulfide ores in ultramafic rocks have high Ni/Cu. More important, however, is
the relative proportion of sulfide to silicate liquid, which influences the proportion
of the chalcophile elements that are present in the sulfide. The situation is best
illustrated by the extremely chalcophile PGE, which, at equilibrium, will be almost
entirely contained in the sulfide phase. However, in a static system, the PGE
migrate into the sulfide by diffusion, which is inefficient. In the absence of
mechanical mixing of the two phases, each sulfide droplet will be surrounded by
a zone of silicate liquid that is effectively stripped of PGE. Only if the sulfide is
mixed with a large volume of silicate liquid can it realize the high PGE content
promised by the high partition coefficients. To describe this process, Campbell and
Naldrett ( 1979 ) introduced the R-factor, which measures the relative proportion of
silicate liquid that interacted with sulfide liquid. If the R-factor is low; i.e. a small
volume of silicate liquid is mixed with a large volume of sulfide, the content of PGE
and other chalcophile elements of the sulfide is low. This is the case for ore deposits
in small intrusions or lava flows, like the Kambalda example discussed below. If,
however, a small volume of sulfide can mix with a large volume of silicate liquid,
the chalcophile element content of the sulfide is high. This is the case for the
magmatic sulfide of the Merensky Reef, which we discussed in the previous section
(Campbell et al . 1983 ).
3.4.3 Kambalda Nickel Sulfide Deposits
We chose not to start with the very largest and richest Ni-Cu sulfide deposits, which
are found in intrusive rocks of various types and origins, but will first investigate the
ores at Kambalda, one of very few deposits that occur a volcanic setting. Kambalda is
located in hot, dry savannah of the Western Australian outback, in the Archean
(2.7 Ga) Yilgarn Craton. The geological make-up of this region, summarized from
Marston et al. ( 1981 ), Lesher ( 1989 ) and Lesher and Keays ( 2002 ), is shown in
Fig. 3.8 : a series of ultramafic lava flows (komatiites), is underlain by tholeiitic basalts
and overlain by magnesian basalts. The complete sequence is exposed in a small
structural dome. Figure 3.9 is a schematic cross section through the lava pile. The ore
deposits are mainly restricted to the lowermost komatiite flow and they are localized at
the base of this flow, within troughs in the underlying basaltic sequence. Away from
the ore deposit, thin bands of sulfide-rich cherty sediment intervene between basalt
and komatiite but these sediments are missing within the troughs that contain the ore.
The ores themselves have features that need to be catalogued because they
provide important clues as to the ore-forming process. Strictly speaking, the ore
minerals should be described as Fe-Ni-Cu-PGE sulfides because they contain all of
these metals. The main ore minerals are pentlandite (Fe,Ni) 9 S 8 and chalcopyrite
CuFeS 4 which coexist with the barren Fe sulfide pyrrhotite (Fe (1 x) S). In many ore
sections, a layer of massive 100% sulfide lines the base of the komatiite flow and is
overlain by “net-textured” ore, in which serpentinized olivine grains are enclosed in
a sulfide matrix, and in turn by (serpentinized) olivine cumulate containing
disseminated sulfides. Veins and lenses of Cu-rich sulfides penetrate into the floor