Page 109 - Handbook of Gold Exploration and Evaluation

Geology of gold ore deposits 89

Convergent plate margins

Since continental lithosphere is always less dense than the oceanic crust and
upper mantle from which it derives, oceanic lithosphere is always subducted
beneath continental lithosphere. Subduction may be either of the intraoceanic
island arc type or take place along the edge of a continent (Cordilleran type). At
the site of subduction, a descending slab composed of cold opiolite suite
materials carries with it large quantities of seawater and is gradually heated due
to the heat of friction and because of the geothermal gradient as it descends into
the mantle. Fractional melting of the mantle material takes place just above the
slab at a depth of about 120 km. With increasing heat, the lower temperature
phases (Bowen Reaction Series) in the rock are melted to produce magmas of
intermediate composition, which rise up through the crust to emplace and
solidify as diorites, granodiorites, etc. The unmelted residue has a more mafic/
ultramafic composition and is higher in the Bowen Reaction Series than the
original rock.
Volcanic arcs form time-transgressive wider belts of volcanic rocks and
associated granitoids. Small island arcs merge slowly together as the result of
continual seafloor spreading; over time the continental crust becomes an
amalgamation of small island arcs accreted together to form ever-enlarging
continental assemblies. A string of relatively narrow (tens of kilometres wide)
volcanic islands create a volcanic arc along the subduction zone that may be
hundreds of kilometres long. Batholiths, emplaced early in the volcanic arc
cycle, are fractionated by the heat released from later rising magmas to form
alkali-granites and related rocks. The ultramafic residue, being very dense,
remains in the mantle while the lighter melt rises to form intermediate and then
felsic magma chambers from which magma reaches the surface as lava.
Hydrothermal metamorphism occurs with the formation of pillow lavas at the
interface of hot lava with seawater at the ocean floor.
Two kinds of metamorphism may occur:
· Low- to high-temperature±low-pressure metamorphism, which occurs within
the volcano where rocks are subjected to heat from the batholiths, accom-
panied by intense folding and shearing; since the batholiths invade mafic
oceanic crust, these rocks are converted into greenschist, amphibolite, and
granularite facies rocks according to their proximity to the batholith and with
depth.
· High-pressure±low-temperature (blue schist) metamorphism, which occurs at
a convergent plate boundary where trench sediments are squeezed between
two subducting plates; it is low temperature because the cool rocks are
subducted too rapidly for them to have time to heat.
Japan, islands of the Malaysian Archipelago, and the Aleutian Islands of Alaska
are ancient and modern examples of volcanic island arcs.