Modern orogenic systems
The magmatic arcs of SE Asia contain some of the world’s major gold deposits. These are mainly related to magmatic–hydrothermal activity and include epithermal, porphyry Cu–Au and skarn deposits. Most gold deposits in SE Asian arcs formed during three intervals of tectonic reorganization rather than during periods of normal or steady-state subduction. These plate reorganizations and periods of gold mineralization were caused initially by the collision of the Australian craton with the Philippine Sea plate arc at 25 Ma. A second Mid-Miocene period of mineralization accompanied plate reorganization following maximum rotation or extrusion of Indochina and the cessation of spreading of the South China Sea at 17 Ma. However, the vast majority and largest deposits formed since 5 Ma in a broad belt from Taiwan to the Solomon Islands during an important period of tectonic reorganization. This tectonic reorganization accompanied a postulated change in the relative motion between the Indian–Australian and Pacific plates between 5 and 3.5 Ma following collision of the Philippine arc and the Eurasian plate in Taiwan. Arc-related magmatism in unusual tectonic settings produced the most abundant and largest deposits with many deposits associated with relatively rare high-K calc-alkaline, shoshonite, adakite and alkaline magmatism. In particular peak mineralization appears related to melting of mantle that had been previously modified by subduction. During large-scale plate reorganization this can occur at the end of a period of normal subduction, following the cessation of subduction, following arc collision or accompanying subduction reversal at approximately the same time in different parts of a complex system of magmatic arcs such as those in SE Asia. Such tectonic controls may be impossible to recognize in older orogenic belts where recognition of a major change in tectonic style and unusual magma types may be the best guides to mineralization.