Page 83 - Handbook of Gold Exploration and Evaluation

64 Handbook of gold exploration and evaluation

2.1.1 Magma-forming processes

The molten or partly molten rock materials making up magma have varying
compositions, temperatures, crystal contents, volatile contents and thereby
varying rheological properties (McBirney and Murase, 1984). In their simplest
form magmas are produced at mid-ocean ridges when hot mantle material rises
from the asthenosphere to fill the gaps between diverging plates. The process
begins with a mantle convection cell rising to the surface bringing with it
ultramafic parent magma. The parent magma fractionally melts as it approaches
the surface creating a mafic melt, which forms the oceanic crust while leaving
an ultramafic residue behind in the mantle. Magma may reside for some time in
high-level volcanic magma chambers, which are periodically replenished and
tapped, and continuously fractionated to provide the mixing of individual
magmas or development of composition zonation (Cas and Wright, 1995).
Magmas filling the rift between spreading plates either solidify as vertical
sheeted dykes or spill out at the seafloor to form pillow lavas.
Heat energy from the interior of the Earth rises to the surface due to the
action of convection cells within the asthenosphere. The hot plastic rock cools
and is turned over slowly at the base of tectonic plates carrying continents, and
moves parallel to the Earth's surface at about 10 cm/year before descending back
into the mantle at subduction zones to be reheated. In the plate tectonic model,
the Earth's crust is broken into seven major and numerous minor lithospheric
plates, which continuously jostle against one another as they move as indepen-
dent, rigid units across the partly molten asthenosphere. The direction and rate of
movement of any one plate is influenced by its size and shape and by the size,
shape and motion of the surrounding plates. New oceanic crust is in process of
formation by the upwelling of basaltic material at extentional plate margins
(e.g., mid-ocean ridges, back-arc basins) constructive plate margins, while older
crust is being consumed at convergent margins where subducting plates sink
back into the asthenosphere. Figure 2.1 is a conceptual cross-section of the
seafloor hydrothermal system showing the driving force of seafloor spreading
when a plume of hot magma rises under the ocean rift forcing the plates to move
apart, and the involvement of divergent and convergent plate settings.
Composition and mineralogical characteristics of erupted magma are the
end result of a complex history of processes causing chemical and physical
change. Widely different geological histories include the degree of partial
melting of the source rocks and other melting events and the nature and extent
of the sedimentary cover. Additional factors are the amount of contamination
from the wall rock and subducting slabs, periodic replenishment of fresh
magma, and tapping and fractionating of magmas in a succession of magma
chambers as they rise to the surface. Figure 2.2 is a schematic representation of
the principal components of magma genesis, fluid flow and metallogenesis in
convective plate settings where oceanic crust is subducted beneath continental
lithosphere.