Page 105 - Handbook of Gold Exploration and Evaluation

Geology of gold ore deposits 85

The `Pacific plate' occupies much of the Pacific Ocean and is almost
entirely composed of oceanic lithosphere. Its relative motion is northwesterly
so that it has an important converging boundary with the `North American
continental plate' along the coastal portion of California. An important smaller
oceanic plate, the `Nazca plate' subducts beneath the `South American
continental plate' leading to the formation of the Andes Mountains. A belt of
oceanic lithosphere fringes the largely continental Eurasian plate on the north
and east. The `Antarctic plate' is almost entirely enclosed within a spreading
boundary fringed by oceanic lithosphere. The continent of Antarctica
comprises a central core of continental lithosphere completely surrounded by
oceanic lithosphere.
In their simplest forms, there are three types of plate boundaries divergent,
convergent and transform (sliding) at which plates meet and interact.
· Divergent (extentional) boundaries are created where two plates separate and
magma rises from the mantle to fill the gaps between them. New ocean crust
is formed and with continued activity the ocean becomes wider.
· Convergent plate boundaries occur where plates thrust against one another
with one plate bending and sinking down into the mantle to be destroyed in
an oceanic trench subduction zone.
· Transform plate boundaries are where plates slide past one another without
either creating or destroying lithosphere. Movement by sliding results in
repositioning of different lithologies on each side of the plates and the
creation of major earthquake activity along their boundaries.

Figure 2.11 shows three schematic block diagrams (Strahler, 1981) cited by
Strahler and Strahler (1992) that demonstrate how continental rupture leads to
the formation of an ocean basin.

Divergent plate margins
Divergent plate margins are found in both oceanic and continental settings
where they occur most notably in the mid-Atlantic Ocean Ridge, East Pacific
Rise in the Pacific Ocean, the Mid-Indian Ridge in the Indian Ocean and the
Great Rift Valley of Africa. The Mid-Atlantic Ridge rises where American
continents are separating from Europe and Africa at a late stage of rifting.
Involvement of crustal magma is evidenced by the high percentage of silicic and
intermediate volcanics as in the Andes and Cascades. In arc and back-arc
settings, sediments are largely continental, e.g. fluvial, alluvial fan and
lacustrine; and range in the fore-arc area from continental ± to shallow marine
± to deep. However, silicic magmas may be generated in areas where the
basement is oceanic as well as where it is continental, and significant
proportions of mafic magmas in continental settings are also not uncommon.