GEOLOGICAL FEATURES OF SOME EPITHERMAL GOLD SYSTEMS, PHILIPPINES

The Philippine Archipelago, with numerous epithermal gold deposits and prospects, consists largely of Cenozoic and locally Cretaceous arc systems built on basement composed mainly of ophiolite and metamorphic rocks. Descriptions of the Baguio, Paracale, Surigao and eastern Negros-Masbate districts indicate major characteristics of the epithermal systems. Epithermal gold deposits are mostly Pliocene, and concentrated along the axes of late Cenozoic volcanic arcs. Quartz-illite vein ores, and stockworks amenable to bulk mining, predominate, but one deposit is in massive cherty silica, and at least two deposits are carbonate-hosted. These are all interpreted as low-sulphidation systems. High-sulphidation deposits are also present but are not described here. Stratigraphic reconstructions indicate that during mineralization all or most epithermal systems intersected a regional uncomformity; This unconformity lies between immediately pre-mineral Pliocene or latest Miocene subaerially erupted, predominantly clastic hornblende andesitic rocks and underlying basement, with stockworks and extensive quartz-illite alteration largely confined to the subaerial cover. Veins and stockworks formed from near-neutral pH, chloride hydrothermal fluids which ascended through fracture-permeable crystalline and in part ophiolitic rocks at depth. The fluids, commonly focussed in anticlinal crests within overlying relatively impermeable marine successions or serpentinite, rose along reactivated faults into the subaerial volcanics. Acid-sulphate fluids, probably generated surficially, formed advanced argillic blankets and in Negros and Masbate mixed with rising deep fluids to deposit cherty silica bodies. The subaerial volcanism was not accompanied by significant subsidence, implying a syn-epithermal near-neutral or only weakly extensional regional stress regime. In contrast, stratigraphic evidence suggests that porphyry copper deposits in the gold districts formed before the Late Miocene in subsiding arcs undergoing stronger extension. The abundance of epithermal gold in the Philippines is related to Late Miocene-Pliocene subaerial volcanism above folded basement units with contrasting permeability; this favourable geology is best explained by Paleogene arc reversal following emplacement of ophiolites, commonly as thrust sheets. © 1990.