Geochronology of the Koloula porphyry copper prospect, Guadalcanal, Solomon Islands

Abstract

K-Ar ages are reported for the major intrusive and mineralization episodes from the Koloula Igneous Complex. Two major cycles of igneous activity occurred. Cycle 1 rocks (gabbro including olivine pyroxenite, diorite, Vasuata Quartz Diorite, dolerite dikes, granodiorite, and aplite dikes) are at least 4.5 m.y. old. The major cycle 2 intrusions (quartz diorites and tonalites) were emplaced 2.4 m.y. ago. In one of these bodies (Inamumu Zoned Pluton) hydrothermal alteration and mineralization followed within a short interval not exceeding 0.3 m.y. Within this mineralized area (A system) the average age from primary biotite of 1.55 m.y. indicates a protracted cooling history. A satellite pluton (Kolokemau Tonalite), that on field evidence is younger than the main cycle 2 intrusive episode, has an age of 2.0 m.y. A second episode of mineralization (B system) is spatially and temporally associated with the Chikora Tonalite Porphyry and a series of trondhjemite dikes. The preferred age for strongly altered whole rocks from this system is 1.7 to 1.6 m.y. An andesitic dike that locally postdates this alteration has an age in the range 1.7 to 1.5 m.y.A date of 24.4 m.y. is reported for tonalite from the Poha Diorite Complex, another igneous body on the island of Guadalcanal.