Fluid inclusion studies in relation to hydrothermal alteration and mineralization at the Koloula porphyry copper prospect, Guadalcanal

Abstract

The Koloula Igneous Complex on the island of Guadalcanal, part of the British Solomon Islands Protectorate, consists of a calc-alkaline sequence of 26 intrusive phases ranging from leucogabbro through diorites and granodiorites to andesite dikes. The intermediate and felsic rocks of the younger central part of the complex are host to porphyry copper mineralization, characterized by two well-defined separate alteration systems.The earlier or system has five distinguishable concentric zones. The innermost is sulfide free and contains K feldspar-quartz-magnetite-actinolite-pyroxene (salite). It is succeeded by zones containing potassic and propylitic assemblages, with chalcopyrite and bornite associated with the potassic zones. The later, or system, is characterized by four zones in which sericite and lesser amounts of clay minerals predominate.Fluid inclusion assemblages are closely similar to those from porphyry copper deposits of the Bingham type. Three types of inclusions are common: gas-rich, moderately saline, highly saline inclusions which contain NaCl + or - KCl + or - hematite + or - birefringent phases. Homogenization temperatures obtained from moderately and highly saline inclusions range up to approximately 700 degrees C in the A system and up to 480 degrees in the B system. There is a continuous variation in salinity from 2 percent equivalent NaCl in some two-phase inclusions up to approximately 60 percent equivalent NaCl in some highly saline inclusions.The evolution of fluids in the A hydrothermal system has not proved easy to elucidate, but the best development of high-salinity inclusions containing KCl daughter crystals is found in areas of strong potassic alteration. Information derived from the sequence of secondary fluid inclusion planes and from the coincidence of alteration and late-intrusive activity suggests that in the B system high-salinity fluids dominated the early phase of alteration and moderate-salinity fluids dominated the later phases. It is considered that both hydrothermal systems developed predominantly under lithostatic pressure conditions with a cover of about 3 km.Counts of inclusions in primary quartz in standard petrographic thin sections reveal a close relationship between fluid inclusion abundance and the location of Cu soil geochemistry anomalies. Such counts may well prove to be a simple preliminary exploration technique for porphyry copper prospect evaluation.