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

Citation
Vs. Kamenetsky et al., Parental basaltic melts and fluids in eastern Manus backarc Basin: implications for hydrothermal mineralisation, EARTH PLAN, 184(3-4), 2001, pp. 685-702
Citations number
47
Categorie Soggetti
Earth Sciences
Journal title
EARTH AND PLANETARY SCIENCE LETTERS
ISSN journal
0012821X → ACNP
Volume
184
Issue
3-4
Year of publication
2001
Pages
685 - 702
Database
ISI
SICI code
0012-821X(20010130)184:3-4<685:PBMAFI>2.0.ZU;2-H
Abstract
The eastern Manus Basin is an actively forming backarc extensional zone beh ind the New Britain Island are, which hosts a number of submarine volcanic edifices and hydrothermal fields. Isotopic and trace element geochemical ch aracteristics of the edifices are comparable with those of the adjacent sub aerial New Britain are, and differ significantly from those of MORE-like la vas on and near the Manus Spreading Ridge in the central part of the basin. Fractional crystallisation dominates magma evolution from primitive basalt s to andesites, dacites and rhyodacites in the eastern Manus Basin, but sev eral lineages with differing trace element enrichment have been delineated. Melt inclusions within olivine phenocrysts (Fo(82-92)) Of two representati ve 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 proportio nal to the bubble size reaching a maximum in fluid bubbles with little or n o melt. Among the precipitates, detected by laser Raman spectroscopy and ED S-scanning electron microscopy, carbonates are common and include magnesite , calcite, ankerite, rhodochrosite and nahcolite (NaHCO3). Gypsum, anhydrit e, barite, anglesite, pyrite, and chalcopyrite have also been found. Some a morphous precipitates recrystallise after bubbles are opened to Na-Ca carbo nates, halite and Na-K-Ca alumine-silicates. Copper abundances decrease fro m 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 de creases during further fractionation. Loss of Cu especially from the fracti onating magmas, in the absence of immiscible sulphide liquid, strongly impl ies 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 f rom the crystallising are-like magmas at this immature backarc basin, and a re able to transport and concentrate major amounts of ore metals, particula rly Cu. (C) 2001 Elsevier Science B.V. All rights reserved.