Porphyry copper mineralization at the Koloula igneous complex, Guadalcanal, Solomon Islands

The Koloula Igneous Complex on the island of Guadalcanal consists of a low-K calc-alkaline sequence of 26 different intrusive phases. The oldest phase is a leucogabbro within which occurs a tabular cumulate body of olivine pyroxenite. Younger phases include diorites, quartz diorites, tonalites and trondhjemite, and aplite dikes. Two major cycles of igneous activity are recognized, as well as at least four periods of andesitic dike intrusion. Four distinct episodes of hydrothermal alteration occurred within the sequence of igneous events. Two of these episodes were associated with porphyritic intrusions of tonalitic composition that show textural and fluid inclusion evidence consistent with boiling. These intrusions are spatially and temporally associated with minor copper mineralization.The earlier, or A, alteration system has five distinguishable concentric zones. The innermost is sulfide free and contains alkali feldspar-quartz-magnetite-actinolite + or - pyroxene (salite). It is succeeded by zones consisting of potassic and propylitic alteration assemblages, containing chalcopyrite and bornite in the potassic zones. The later, or B, system is characterized by four zones in which chlorite, seriate, and lesser amounts of clay minerals have overprinted an earlier potassic alteration. Here, copper mineralization (average grade 0.17ppm Cu) is best developed in chloritic fractures and quartz veins.Within the B system, the fluids responsible for hydrothermal alteration became progressively less saline and to some extent cooler during the system's evolution. It is likely that ocean water played a significant role in the later stages of the B system. The gradual decrease in the K (super +) /H (super +) ratio of the B system fluids is responsible for the observed alteration veinlet sequence: phlogopite --> chlorite --> sericite. Peripheral vein assemblages of the B system were formed at lower temperatures than the final sericite and sericite-clay alteration within the center of the system. These peripheral assemblages are now composed mainly of poorly crystalline illite and interstratified illite-smectite. The final-stage veins consist of laumontite + or - smectite that postdate at least part of the B system and are widespread throughout the Koloula Igneous Complex. The solutions responsible for laumontite deposition were low temperature ( nearly equal 250 degrees C) and water rich (X (sub CO 2 ) < or =0.03). Electron probe studies of alteration minerals indicate that hydrothermal pyroxene, amphibole, and usually biotite are more magnesian and contain less TiO 2 than primary (magmatic) mafic silicates from the same, or similar, rock types. Secondary biotites from the B system are progressively less magnesian and richer in octahedral aluminium with increasing distance from the center of the B alteration area. Secondary alkali feldspar from the A system is somewhat more sodic in the outer alteration zones than in the center of the system.Within the B system, illite crystallinity and molybdenite polytypism of samples are related to distance from the center of alteration. The compositions of chlorite (usually pycnochlorite or ripidolite), sericite, and epidote (22 to 30 mole % pistacite) are not related to spatial position within alteration zones but reflect the initial chemical composition of the rock or mineral which they have replaced.