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Geology of gold ore deposits 83

of some relatively high-grade pay streaks to presently uneconomical mining
depths

2.2.2 Lithospheric plates in motion
Lithosphere, which is composed of sial (p ˆ 2:65±2.7) and sima (p ˆ 2:9±3.3) is
the general term given for the entire solid earth realm. Sial is the lighter granitic
layer of the lithosphere that forms the continents and is rich in silica and
alumina. Sima, which underlies the sial forms the floor of the oceans and is the
denser mafic part of the lithosphere. The average thickness of the oceanic crust
overlying basaltic lavas and intrusive gabbro is some 10±16 km. Continental
crust is about 33 km thick with thicknesses of up to 50 km in high mountain
regions. Figure 2.9 is a schematic representation of crust and mantle.
The first global system of lithospheric plates appeared in published form in
1968 but there have since been many minor changes and revisions. A recent
diagram (Fig. 2.10) identifies seven major plates of continental dimensions and
numerous minor plates ranging in size from intermediate to very small. The
requirement for identifying a particular plate is good evidence of activity along
all of its boundaries and of its interaction with adjourning plates. Continental
plates are named for the continents embedded in them (North America, South
America, Eurasia, Africa, Indo-Australia, and Antarctica). Oceanic plates are
named for their oceans (e.g. Pacific, Nazca).

2.9 Schematic representation of crust and mantle.