EPITHERMAL GOLD DEPOSITS OF THE SOUTHWEST PACIFIC

Since the freeing of the market price of gold in 1971, exploration for epithermal gold deposits in the southwest Pacific region has been int ense, and has been encouraged by some major successes. Sufficient disc overies have been made to summarise what has been found to date, and t o assess what implications may be drawn for future exploration. Data a re tabulated for 137 epithermal precious metal deposits and prospects in Australia (30), Fiji (2), Indonesia (43), New Zealand (22), Palau a nd Yap (2), Papua New Guinea (18), the Philippines (19), and Solomon I slands (1). Comparison of the character of epithermal deposits in the southwest Pacific with those in other regions (e.g., the northeastern Pacific margin) suggests that they are in most respects similar with, however, three differences: (i) through much of the region (especially the Philippines and Indonesia) low-sulfidation style deposits commonl y show characteristics suggestive of formation at deeper levels than i s typical elsewhere; (ii) high-sulfidation deposits appear to be more common than along the northeast Pacific margin; (iii) many of the larg est deposits show atypical features, particularly those suggestive of a transition to porphyry or mesothermal conditions, or indicating over printing of an epithermal system on a porphyry environment. These diff erences can be partly understood in terms of the tectonic setting and evolution of the volcanic arcs of the southwest Pacific, and the conse quent hydrological conditions that existed during mineralization. The results of exploration in the southwest Pacific indicate that, while t he principles of formation of epithermal deposits still apply, deposit models developed in other tectonic/volcanic settings should not be ri gidly applied. Gold explorers should first consider the character of t he geologic environment at the time of mineralization to try to predic t the hydrological conditions during mineralization. Once the likely v olcanic environment and paleorelief have been established, then the ty pes, distribution, and zoning of hydrothermal alteration, coupled with observations of deposit form, vein textures and mineralogy, will allo w a judgement to be made on the level of system that is now exposed, a nd on the probable controls that localized mineralization. The very dy namic character of the tectonic and volcanic setting throughout much o f the southwest Pacific has resulted in deposits that reflect changes in the environment of the deposit, in some case even while deposition was occurring. This has produced some deposits with unusual characteri stics which could not have predicted before exploration.