METALLOGENY OF OCEANIC ARCS

Island arc metallogeny is characterised by an evolution that accompanies the evolution of arc crust. This evolution is related to: 1) chemical changes in the underlying asthenosphere as successive partial melts are removed, and 2) thickening of the crust, allowing magmatic differentiation, crustal contamination of primitive melts, and generation of new magmas in the crust. Oceanic arcs only become highly productive of ores when they have attained substantial crustal thickness: all productive arcs have older basement below the volcanic rocks. It is unlikely that enough crustal thickness can be achieved solely by arc volcanism. Much of the observed thickness of productive arc environments is tectonic, due to docking with crustal fragments of both oceanic and continental origin.

Primitive island arcs are dominantly mafic and submarine. Ore-forming processes are very restricted, with the dominant style being small submarine massive sulfide Cu-Zn-(Au) deposits. VMS deposits continue to form throughout the evolution of the arc where deep marine settings are available, however economically important arc-related VMS deposits only appear with maturing of the arc and onset of substantial felsic volcanism.

At a later stage of arc evolution intermediate composition rocks usually become dominant, then felsic volcanic rocks appear and ultimately may become a significant component. At this stage substantial parts of the arc are subaerial, and the styles of ore deposits become more diverse. Dominantly intermediate composition arcs may host porphyry Cu-Au and associated epithermal gold and base metal deposits of both high- and low-sulfidation style. Low-sulfidation Ag-Au-(Zn-Pb) deposits are exclusively in the subaerial parts of the arc. High-sulfidation Au-Cu deposits are dominantly, but not exclusively, found in the emergent parts of the arc. Low-sulfidation Au-Ag deposits only appear in the mature stage of arc evolution, probably reflecting the presence of large scale felsic plutonism at depth. Locally, centres of alkalic igneous activity may produce porphyry Cu-Au or low-sulfidation epithermal gold deposits.