Parental basaltic melts and fluids in eastern Manus backarc Basin: implications for hydrothermal mineralisation

The eastern Manus Basin is an actively forming backarc extensional zone behind the New Britain Island arc, which hosts a number of submarine volcanic edifices and hydrothermal fields. Isotopic and trace element geochemical characteristics of the edifices are comparable with those of the adjacent subaerial New Britain arc, and differ significantly from those of MORB-like lavas on and near the Manus Spreading Ridge in the central part of the basin. Fractional crystallisation dominates magma evolution from primitive basalts to andesites, dacites and rhyodacites in the eastern Manus Basin, but several lineages with differing trace element enrichment have been delineated. Melt inclusions within olivine phenocrysts (Fo82–92) of two representative east Manus basalts, respectively, with modest (0.2 wt%) and high (0.8 wt%) potassium contents, host ubiquitous CO2-bearing vapour bubbles, denoting presence of an immiscible fluid phase at early stages of crystallisation. Bubbles often carry precipitate phases whose abundance is broadly proportional to the bubble size reaching a maximum in fluid bubbles with little or no melt. Among the precipitates, detected by laser Raman spectroscopy and EDS-scanning electron microscopy, carbonates are common and include magnesite, calcite, ankerite, rhodochrosite and nahcolite (NaHCO3). Gypsum, anhydrite, barite, anglesite, pyrite, and chalcopyrite have also been found. Some amorphous precipitates recrystallise after bubbles are opened to Na–Ca carbonates, halite and Na–K–Ca alumino-silicates. Copper abundances decrease from basalt to dacite across the eastern Manus fractionation spectrum, whereas Pb behaves as an incompatible element, increasing to highest values in the dacites. Zinc abundance reaches maximum concentrations in andesite, and decreases during further fractionation. Loss of Cu especially from the fractionating magmas, in the absence of immiscible sulphide liquid, strongly implies metal partitioning into CO2–H2O fluid, which is degassed significantly during magma fractionation. Hydrothermal fluids in the PACMANUS system may carry a direct contribution of the magmatic metal-bearing fluid, exsolved from the crystallising arc-like magmas at this immature backarc basin, and are able to transport and concentrate major amounts of ore metals, particularly Cu.