Other
Scientific paper
Mar 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993gecoa..57.1277b&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 57, Issue 6, pp.1277-1290
Other
7
Scientific paper
The Munni Munni Complex in the Archaean Pilbara Block contains PGE mineralization in the form of disseminated sulphides within pyroxenite. The uppermost sulphide-bearing layer shows a characteristic vertical distribution of PGE and Au. Peak values for Pd, Pt, and the other PGEs are systematically offset by up to 5 metres below the horizon of maximum S, Ni, and Cu abundance. The Main Sulphide Zone of the Great Dyke of Zimbabwe, which occurs in an identical stratigraphic setting, shows very similar features. This distribution of PGEs with respect to the sulphide peak cannot be explained in terms of postcumulus mobilisation from an initially homogenous sulphide layer, for a number of reasons: the offset profiles are remarkably consistent along several km of strike; remobilization would be expected to affect Pd and Au more than the other PGEs; and underlying sulphide concentrations show coincident trends for all the PGE, Au, Cu, Ni, and S. The offset between PGE and Ni-Cu-S peaks has been interpreted by workers on the Great Dyke as the result of fractional sulphide segregation, combined with different values for sulphide liquid-silicate melt partition coefficient (SSD) for the various PGEs, Au, Ni, and Cu. A series of model calculations has been performed in an attempt to apply this interpretation to the Munni Munni case, with the object of making quantitative estimates of SSD values for the PGEs and Au. Results in general support the conclusions of Great Dyke studies that apparent SSDs decrease in the order Ir = Ru = Rh = Os Pd > Pt > Au > Cu = Ni. However, it proves impossible to duplicate some highly regular and persistent features of the Munni Munni offset sulphide layer with any model that assumes constant values for these SSDs. The inadequacy of the fractional sulphide segregation model implies that sulphide liquid is not the sole collector of the PGEs. The observed behaviour of the PGEs requires a sudden increase in the apparent affinity of the PGEs for the sulphide fraction at the base of the main sulphide peak, unattributable to any significant change in parent magma composition or crystallization conditions. Natural profiles can be modelled successfully by describing this apparent affinity of PGE for the sulphide fraction in terms of widely variable "virtual" partition coefficients. This behaviour can be explained if the silicate melt, and consequently the co-existing sulphide melt, is saturated with respect to solid PGE phases. The association of PGE with sulphides is necessary because of the extremely low concentrations of PGE in the silicate melt, which makes it kinetically impossible to nucleate solid phases even at substantial degrees of supersaturation. Pre-concentration by several orders of magnitude due to partitioning into sulphide liquid allows solid PGE phases to nucleate within sulphide droplets, giving rise to an apparently very strongly PGE-enriched sulphide fraction.
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