Magmatism and associated hydrothermal activity during the evolution of the Tavua Caldera; 40 Ar- 39 Ar dating of the volcanic, intrusive, and hydrothermal events

The Tavua caldera in Viti Levu, Fiji, occurs in the upper part of the shoshonitic Tavua volcano. Precaldera rocks forming the edifice of the volcano are predominantly mafic (absarokite) in composition, whereas intracaldera rocks are intermediate (shoshonite and banakite) in composition. The caldera hosts subeconomic, porphyry-style copper-gold mineralization and minor weakly auriferous acid sulfate epithermal alteration, both associated with monzonite intrusions in the center of the caldera, and adularia-sericite epithermal mineralization (the Emperor gold deposit) on the caldera margin.Ten Tavua samples, ranging from hydrothermally altered or mineralized to relatively fresh material, were dated by the 40 Ar- 39 Ar incremental heating technique. These dates establish that caldera collapse occurred between 4.6 and 4.5 Ma. Precaldera absarokites have an age greater than 4.6 Ma and probably as much as 4.7 Ma, while intracaldera shoshonites and banakites have a maximum age of 4.51 Ma, so there was a gap of at least 0.1 m.y. between mafic and major intermediate volcanism in this area. Monzonite intrusion and concurrent porphyry-style alteration in the center of the caldera occurred between 4.51 and 4.44 Ma, essentially synchronous with intracaldera volcanism. The last magmatic event within the caldera, a banakite vent-phase pyroclastic unit, occurred at 4.14 + or - 0.08 Ma. Adularia deposited during epithermal mineralization at Emperor was formed at 3.71 + or - 0.13 Ma. This history, which is clearly indicated by 40 Ar- 39 Ar data, would not have been possible to deduce by conventional K-Ar dating. The results are similar to those obtained in other porphyry and epithermal systems, where porphyry-style magmatic hydrothermal activity is deemed to be synchronous with magmatism, but a measurable interval occurs between magmatism and epithermal mineralization. Tavua is unusual in that samples of both types of mineralization are available, are amenable to dating techniques, and can be tied to the history of the caldera.