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Abstract:

Magmatic Fe-Ni-Cu-(PGE) sulfide deposits have been produced during all major phases of mafic-ultramafic magmatism on Earth. Structural deformation hampers precise interpretations of their tectonic settings, but most appear to occur in continental rift zones or along rifted continental margins. They are hosted by mafic-ultramafic cumulate rocks (e.g., gabbros, pyroxenites, peridotites, dunites) in lava channels, feeder sills, and magma conduits, are associated with chalcophile metal-rich magmas derived by high degree partial melting of the mantle (e.g., Al-depleted and Al-undepleted komatiites, komatiitic basalts, ferropicrites, picrites, high-Mg basalts), and appear to have incorporated (via melting and/or volatilization) significant amounts of crustal sulfur (sedimentary and/or volcanic). There is a general progression from predominantly extrusive settings, more magnesian parental magmas and host rocks, and higher Ni/Cu and lower Pd/Ir ores in the Archean to predominantly intrusive settings, less magnesian parental magmas and host rocks, and lower Ni/Cu and higher Pd/Ir ores in the Phanerozoic. These changes probably reflect fundamental changes in the thermal and compositional structure of the Earth's mantle and crust over time, but it appears that deposits of this type formed wherever and whenever metal and heat sources encountered suitable crustal sulfur sources.

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