Structural and economic aspect of the Vatukoula caldera, Fiji

Abstract

The Vatukoula caldera is semi-elliptical in shape with the long axis trending north-easterly and occupies about 14 square miles of an undulating topographical basin located near the central north coast of Viti Levu, the largest island of the Fiji Group. The caldera formed when Tertiary basalts collapsed after prolonged explosion from a central vent area. The ensuing subsidence, which appears to have been cyclic, was accompanied by the deposition of andesitic volcanic material to form 5,000 to 7,000 feet of rhythmic tuffs, breccias and agglomerates partly under lacustrine conditions. The peripheral basalts were shattered during the stages of collapse forming a ring fault zone around the caldera. The depositional and subsidence stages were followed by an intrusive augite andesitic one from which extensive cone sheets formed in the caldera rocks. Radial and tangential dykes formed around the caldera in the peripheral basalts. After a time interval, the comparatively shallow central depression of the caldera received biotite andesitic pyroclastics and flows. Biotite andesite dykes followed a similar structural pattern to the augite andesitic ones. Finally, plug like bodies of porphyrite and monzonite intruded into the highly fractured zones, particularly the ring fault zone in the peripheral basalts. An important younger structural development with economic significance was the formation of a north-westerly shear system across the caldera. Flatly dipping structures formed in the peripheral basalts from the resettling of major blocks around the caldera. After the monzonite intrusions, epithermal mineralisers were liberated with economic amounts of gold in the form of telluride and auriferous pyrite. The mineralisers favoured the north-westerly shear system and, in the peripheral basalts, the accompanying flatly dipping structures. Thermal spring activity appears to mark the last phase of volcanicity.