Formation of platiniferous sulfide horizons by crystal fractionation and magma mixing in the Munni Munni layered intrusion, West Pilbara Block, Western Australia

Abstract

The Munni Munni Complex (T Nd CHUR model age 2.85 Ga), located in the west Pilbara block of Western Australia, is one of the best preserved layered intrusions in Australia. Exposed over an area of 4 X 9 km, it is composed of a lower 1,850-m-thick ultramafic zone and an overlying gabbroic zone which has a minimum thickness of 3,630 m. The ultramafic zone contains rhythmically layered dunite, lherzolite, olivine websterite, clinopyroxenite, and websterite, with orthopyroxenite, norite, chromitite, and platiniferous websterite prominent near the top of the zone. The gabbroic zone consists of gabbronorite, anorthositic gabbro, and minor anorthosite which display a pronounced tholeiitic fractionation trend. The order of appearance of cumulus mineral assemblages in the complex is olivine, olivine + clinopyroxene, clinopyroxene + olivine, clinopyroxene, clinopyroxene + orthopyroxene, orthopyroxene + chromite, and plagioclase + clinopyroxene + ?orthopyroxene. This sequence is at variance with major platinum-group element-bearing intrusions in which crystallization of orthopyroxene generally precedes that of clinopyroxene.Trace-element data, obtained on samples collected across the entire intrusion to investigate the effects of crystal fractionation and S evolution on the distribution of the platinum-group elements, show that in the sulfide-undersaturated ultramafic zone, Pt, Pd, Au, Cu, S, Se, Cs, Rb, St, and Zr behaved incompatibly and were concentrated in the melt during fractionation. The S content of the melt began to increase above the 700-m stratigraphic level of the ultramafic zone, but Pt, Pd, and Au contents increased above background levels of approximately 3 ppb to 3 ppm Pt + Pd only with the attainment of sulfide saturation at approximately the 1,830-m stratigraphic level. The concentration trends of Zr, St, Cs, Rb, and Cu paralleled that of S, but Ir and Ni largely partitioned with early crystallizing olivine and decreased in concentration with increasing fractionation. In contrast to the ultramafic zone, Pt, Pd, It, and Au have depletion trends in the sulfide-saturated gabbroic zone. Hence, the evolution of S largely governed the behavior of the platinum-group elements during the fractionation of the Munni Munni magma(s).The platinum-group element mineralization occurs immediately below the ultramafic-gabbroic contact. It resulted from the combined magmatic processes of crystal fractionation (as evidenced by increasing Cu/(Cu + Ni) ratios and incompatible element trends with stratigraphic height), and magma mixing. Two models are presented. In model 1, a hot, buoyant sulfide-saturated tholeiitic magma (containing 1,700-2,600 ppm S) rose through the density stratified platinum-group element-enriched, sulfide-undersaturated resident ultramafic magma (containing 530 ppm S) until reaching its own density level near the top of the chamber, where it spread out laterally for a distance of at least 12 km. Due to crystallization of plagioclase and subsequent Fe-enrichment [of the melt], the density of the gabbroic melt increased until it overturned and mixed with the platinum-group element-enriched fractionated parts of the ultramafic magma. Model 2 is similar to model 1, except that it involves the fractionation and internal mixing of one magma. In both models, magma mixing triggered sulfide saturation in the hybrid magma and established a high R factor (the silicate/sulfide mass ratio). The chalcophile platinum-group elements, due to their high sulfide/silicate partition coefficients, were scavenged by immiscible sulfide droplets which precipitated slowly in a porphyritic plagioclase websterite in a zone up to 20 m below the ultramafic-gabbroic zone contact. The platinum-group minerals comprising arsenides, sulfarsenides, tellurides, native metals, and mercury-bearing phases form small (<10 mu m) grains now enclosed within silicate minerals or along silicate grain boundaries (78% of occurrences), or are associated with chalcopyrite-pyrrhotite-pentlandite blebs (22%).The Munni Munni mineralized websterite layer is defined by discontinuities in (Pt + Pd)/ Se, (Pt + Pd)/S, (Pt + Pd)/Cs, (Pt + Pd)/Zr, and Cu/Zr ratios and Cu, Cs, Zr, Sr, S, and Se concentrations. Hence, these parameters together with information on the intercumulus component to the cumulates are useful for indicating potentially mineralized units in layered intrusions. Sulfide saturation also occurred along the basal contact of the ultramafic zone, but platinum-group element concentrations (< or =92 ppb Pt + Pd) are low since sulfide precipitation was probably induced by rapid cooling and the sulfides equilibrated with a small volume of silicate melt, implying a low R value. The different settings of sulfide mineralization in the Munni Munni Complex illustrate that timing and physical characteristics of the sulfide-saturation event are critical for the development of platinum-group element-enriched sulfide units in layered intrusions.