Evolution of porphyry copper mineralization in an oceanic island arc; Panama

Abstract

Petrologic, isotopic, and radiometric studies of intrusive rocks in the Panamanian oceanic island arc indicate that magmatism and associated porphyry copper mineralization were episodic and that they exhibit systematic compositional changes. The earliest stage of igneous activity took place about 60 to 70 m.y. ago and involved emplacement of quartz diorites with tholeiitic chemical characteristics, which did not form porphyry copper mineralization. Later magmatism was calc-alkaline in character and commenced with 50 m.y. quartz diorite and K-poor granodiorite. Later events at 35 m.y. and 5 m.y. emplaced granodioritic intrusions that are more potassic than their local predecessors. Porphyry copper mineralization was most intense and most widespread during the last intrusive event.Comparison of the Panamanian results with information on other island arcs indicates that porphyry copper mineralization is essentially absent from earliest stage, tholeiitic(?) intrusions and, with few but prominent exceptions, becomes more important in successively younger stages of arc evolution. Not all arcs exhibit potassium-enrichment trends like Panama, and late-stage quartz diorite-type porphyry copper deposits can develop. Shoshonitic or alkaline porphyry copper deposits, though absent from the tholeiitic phase of arc evolution, can form at any time during the calc-alkaline stage. Gold values are high in shoshonitic and possibly in quartz diorite-related calc-alkaline porphyry copper deposits, and molybdenum appears to be most abundant in granodiorite-related calc-alkaline deposits.