Geochemistry of komatiites from Kambalda, Western Australia; I, Chalcophile element depletion, a consequence of sulfide liquid separation from komatiitic magmas

Abstract

The type examples of Archean volcanic peridotite-associated Ni sulfide deposits occur at or toward the base of the ultramafic lava pile at Kambalda, Western Australia. Spinilex-textured komatiites, considered to represent silicate liquids derived from the original magma, are significantly depleted in chalcophile elements throughout the entire ultramafic sequence relative to unmineralized and poorly mineralized komatiites elsewhere and to a theoretically modeled sulfide-undersaturated komatiite system.This depletion is not attributed to postformational modification of the komatiites by sea-floor alteration or metamorphism, although very low sulfur contents in the same rocks probably result from such processes. The depletion is best interpreted as the result of scavenging of metals by sulfide liquid in equilibrium with the silicate magma, implying that all the silicate magma now represented by the ultramafic sequence equilibrated with sulfide liquids at the magmatic stage. These results provide convincing evidence against formation of the sulfide ores by in situ sulfide separation from the thick komatiitic peridotitc host. The magnitude of the depletion in the spinilex-textured komatiite divisions of the peridotitc hosts is much less than that expected for such an origin, but for the entire sequence it is greater than that expected from metal partitioning relations and mass balance calculations for the volume and composition of the ultramafic sequence and the volume of associated sulfides. This suggests that a proportion of the sulfide liquid that formed from the ultramafic magma(s) was lost from the magmatic system prior to extrusion. Such loss is a natural consequence of models for ascent and eruption of sulfur-rich komatiitic magmas that involve prior concentration of sulfides (plus olivine) in high-level magma chambers or separation and segregation of sulfide liquid during ascent from the upper mantle.