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Magnetite-apatite deposits associated with the Atacama Fault Zone of northern Chile are interpreted here, on field criteria, as being the products either of hydrothermal fluids with a strong magmatic signature, or of latestage Fe-rich magmas mixed with an aqueous fluid. Even in the Chilean iron belt, apatite-rich magnetite deposits are a rarity. Variations in F-and C1-contents in apatites, strongly zoned with respect to halogens, are indicative of primary variations in fHF and fHCI in the hydrothermal fluids. Small variations in halogen fugacities in the aqueous fluid are capable of buffering large variations in halogen content within apatite crystals in equilibrium with that fluid. The recorded halogen zonation profiles are inconsistent with crystallization of the apatites simply from a volatile-rich, late-stage fractionation Fe-rich magma, or its derived magmatic vapour. It is more likely that they are the result of mineral-fluid buffering with a fluid that represents the mixing of a magmatically-derived aqueous fluid with a meteoric fluid that has variably scavenged Ca and C1 from within the country rocks. The source magma of the former is probably an Fe-P enriched acidic magma, derived by fractionation of primary calc-alkaline basic magmas.

The Wainimala Group rocks of southwestern Viti Levu, Fiji, represent part of an Oligocene-Miocene island arc. Some of the early volcanism in this arc took place on a substrate of Eocene-Oligocene frontal-arc crust. Mainly andesitic, transitional calcalkaline lavas were erupted from major subaerial volcanic edifices, while elsewhere tholeiitic magmas formed low-lying basaltic lava fields and small felsic volcanic centres on the seafloor. The intrusion of a dense bimodal basalt-dacite dyke swarm into the underlying frontal-arc crust indicates that significant crustal extension accompanied the eruption of the inter-edifice lavas.Geochemical variations within the Wainimala volcanic suite are thought to have been generated through fractional crystallisation processes. While relatively slow ascent and mixing of magmas took place beneath the major edifices, efficient shallow-level fractionation and rapid ascent of magmas occurred in the areas between them. The contrasting styles of volcanism may reflect the interaction of ascending magmas with pre-existing heterogeneities in the frontal-arc substrate.