Magmatic Ore Deposits - Metals and Society: An Introduction to Economic Geology - page 55

Fig. 3.9 Schematic cross-section through a typical Kambalda ore shoot, showing distributions of
interflow metasediments, interspinifex ores, and felsic ocellites (After Groves et al. ( 1986 ), Frost
and Groves ( 1989 ))

rock and in places invade the upper part of the komatiite flow. The composition of
the disseminated sulfide corresponds to “monosulfide solid solution”, a term that
refers to the immiscible sulfide liquid that separates from the silicate liquid. The
massive sulfide liquid undergoes fractional crystallization (just like the silicate
liquid fractionally crystallizes). The first solid sulfide that crystallizes is relatively
rich in Ni and Fe and this material remains as a “cumulate” layer at the base of the
flow while the late-solidifying Cu- and PGE-rich sulfide liquid may leak out to form
veins in surrounding rocks.
Komatiite lava flows occur throughout the 1,500 km long Yilgarn Craton, but ore
deposits are known in only some of them. And komatiites are common in the much
larger Abitibi belt in Canada, but there the ore deposits are small and rare. With this
background we are in the position to ask a number of questions:

• Why do the sulfide deposits occur preferentially in the lower part of the lower-
most komatiite flow?
• Why are sedimentary rocks present between each komatiite flow, except in the
troughs that contain the ore deposits?
• Why do Ni deposits form in komatiites and not in basaltic lava flows?
• Why are Ni sulfide deposits more common in the Yilgarn than in the Abitibi belt?
• What distinguishes

the ore-bearing Kambalda komatiites from barren

komatiites?

Table 3.2 compares the chemical and physical properties of komatiitic and
basaltic magmas. The ultramafic magma has much a higher MgO content and a
lower SiO 2 content than the basaltic magma, which means that it erupts at much