The
magmatic arcs of SE
Asia and SW Pacific contain some of the world's major
magmatic-hydrothermal gold deposits. Most gold deposits in SE Asian and SW
Pacific arcs formed during periods of plate reorganisation rather than during
periods of normal or steady state subduction. These plate reorganisations were
caused initially by the collisions of the Australian Craton with the
Philippines-Halmahera Arc and the Ontong Java Plateau with the Melanesian Arc at
~25 Ma. A second Mid-Miocene period of mineralisation accompanied plate
reorganisation following maximum rotation or extrusion of Indochina and
cessation of spreading in the South China Sea
at ~17 Ma. However, the vast majority of deposits from New Zealand to Taiwan
formed since 7 Ma during an important period of tectonic reorganisation that
accompanied and followed changes in the relative motion between the
Indian-Australian and Pacific plates between ~8 and 3.5 Ma. Magmatism in unusual
tectonic settings produced the most abundant and largest deposits with many
deposits associated with high-K calc-alkaline, shoshonite, adakite and alkaline
magmatism. In particular peak mineralisation appears related to melting of
sub-arc lithosphere that has been previously modified by subduction. During
large-scale plate reorganisation this can occur towards or at the end of a
period of normal subduction, following arc collision or accompanying subduction
reversal at approximately the same time in different parts of a complex system
of arcs such as in SE Asia and the SW
Pacific.