Precious metals in magnesian low-Ti lavas: Implications for metallogenesis and sulfur saturation in primary magmas

Boninites and related magnesian low-Ti magmas are generally regarded as partial melts of a moderately to severely depleted peridotite source. Incompatible lithophile element abundances indicate that this source was variably enriched in LREE, Zr, Sr, Ba and alkalis by some mantle metasomatic process. Low-Ti lavas from the Bonin-Mariana arc system, Cape Vogel, New Caledonia, Cyprus, Newfoundland and SE Australia have been analysed for Pd, Ir, Au, Cu, S and Se. Comparison of fresh glassy material with variably altered samples suggests sporadic loss of Au and Cu and essentially inert behaviour for Pd, Ir and Se during seawater and subsequent alteration. They are uniformly enriched in Pd (mean 15 ppb) and depleted in Cu (mean 20 ppm), S (mean < 54 ppm) and Se (mean 53 ppb) compared to average MORB (<0.8 ppb Pd, 72 ppm Cu, 800 ppm S and 196 ppb Se) and exhibit incompatible-like behaviour for these elements and Au.The data are compatible with fractionation of the chalcophile elements during multi-stage mantle melting. Primary MORB liquids are S-saturated in their mantle source and an immiscible sulfide component is retained in the mantle residue. This results in the preferential removal of metals having low D^S/L- values (base metals) and concentration of those metals with high D^S/L values (precious metals) in the residual mantle sulfide fraction. Subsequent remelting of this refractory source produces S-deficient precious metal-enriched magmas, as exemplified by boninites. The absence of correlation between incompatible lithophile element enrichment and chalcophile element abundances suggests that the latter were not added to the source during mantle metasomatism.The constraints imposed by the nature of the source region result in two fundamentally contrasting patterns of behaviour for exclusively chalcophile elements. Magmas generated in mildly depleted to undepleted source regions by low to moderate degrees of partial melting (e.g. MORB) are S-saturated and become rapidly impoverished in precious metals during the early stages of silicate fractionation, owing to the co-precipitation of an immiscible sulfide component. Magmas generated from a strongly depleted source are initially S-undersaturated and concentrate chalcophile metals in their liquid residua.The contrasting behaviour of chalcophile metals during the early crystallisation stage of MORB and low-Ti magmas lead to divergent predictions concerning the primary distribution of these metals in oceanic crust generated by these magmas. The similarity in composition of early Bushveld magmas and boninites suggests that these S-deficient, PGE-enriched magmas may be essential to the formation of platiniferous horizons in layered intrusions.