METALLOGENESIS IN BACK-ARC ENVIRONMENTS - THE LAU BASIN EXAMPLE

Geologic investigations with the submersible Nautile in the Lau basin represent one of the first detailed studies of hydrothermal activity o n a modern back-arc volcanic ridge, the Valu Fa Ridge. Three major hyd rothermal fields (Hine Hina, Vai Lili, and White Church) were discover ed in the areas of the greatest differention of volcanic rocks. The ty pe of hydrothermal deposit is controlled by the type of volcanism and by tectonic activity which increases from south to north. Three stages of sulfide formation are proposed. During the first stage (Hina Hina field), diffuse discharge through volcaniclastic material produces ext ensive Fe-Mn oxide crusts covering sulfide deposits within the volcani c material. During the second stage (Vai Lili field), tectonic activit y increases and fault-controlled discharge forms chimneys on the sea f loor. Diffuse discharge is still present. During the third stage (Whit e Church field), hydrothermal activity is completely controlled by maj or faults and results in the development of sulfide mounds. Vent fluid s at Valu Fa have much higher metal contents than those at midocean ri dges. Cl enrichment is best explained by mixing with deep brine rather than by subcritical phase separation and there is no evidence for a m agmatic fluid contribution. Some characteristics of the fluids, such a s low pH (2) and low concentration of H2S, can be explained by subsea- floor sulfide formation. The water-rock reaction zone is estimated to be about 1 km below the sea floor and 2 km above the magma chamber. Ve rtical mineralogical zonation within the mound differs from midocean r idge deposits but is similar to that of a kuroko deposit. At the surfa ce of the deposit, the virtual absence of pyrite and the high amount o f sphalerite, barite, tennantite, galena, and locally, native gold are remarkable. The distinctive chemical characteristics of deposits in e ach volcanic segment are explained by interaction with variable amount s of differentiated rocks. Compared to midocean ridges, Lau basin depo sits are enriched in Ba, Zn, As, Pb, Ag, Au, and Hg and depleted in Mo , Se, and Co. Relative to young intracontinental back-are basins (e.g. , Okinawa trough), the mineralizations are Pb and As poor. Their miner alogy, chemical composition, and geologic setting show that the southe rn Lau basin deposits are intermediate between oceanic and continental back-are deposits.