RIGHT PLACE-RIGHT TIME: THE ROLE OF TECTONICS IN ORE DEPOSIT FORMATION AND PRESERVATION

It is now possible to analyse the role of tectonics in ore deposit formation using the best-constrained tectonic reconstructions for the Phanerozoic. When this is done it is clear that the links between regional tectonics and the location of mineral deposits are more complex and dynamic than generally assumed. For example in SE Asian magmatic arcs epithermal Au and Au-rich porphyry deposits are known to be linked to subduction in the last few million years, but when these deposits are plotted on an animated tectonic history for the last 50 Ma it is clear that the distribution of deposits is not simply related to steady state subduction, but rather pulses of mineralisation occur at times that correspond to changes in the regional tectonic regime (i.e. major changes in plate movement velocity and direction). In many cases mineralisation cannot be directly related to ongoing subduction, with arc-related magmatism, involving mantle that had been previously modified by subduction, in unusual tectonic settings producing the most abundant and largest deposits. Similarly these deposits are only likely to be preserved in the geological record if plate geometry allows the mineralised arc segments to escape the future continental collision between Australia and Eurasia/America in the way mainland SE Asia has escaped the India/Eurasia collision. Peripheral orogens such as those facing the Pacific Ocean also appear to become more metal rich with time. For previous tectonic cycles these peripheral orogens and parts of older collision zones where plate geometry has allowed preservation (tectonic escape and stabilisation) of low metamorphic grade terranes provide the best chances of preserving metal deposits formed in oceanic, magmatic arc or collision related settings.