The geological features of the destructive plate margins of
the southwest Pacific favour the formation of porphyry type deposits. The major
controlling factor favouring the formation of gold-rich deposits of this type is
a dense crust. This results in shallow intrusion of large magma bodies beneath
andesite stratovolcanoes. Where the intrusives have near vertical sides, the
comparatively low temperature of transfer of volatiles from the solidifying melt
favours the partitioning of gold into bubbles of volatiles. They reduce the
viscosity and density of the melt, resulting in the upward movement of a
mineralised melt. With the drop of pressure with upward movement the bubbles
separate into a two phase mixture, triggering freezing of the mineralised melt
and the formation of a gold mineralised stockwork.