Constellation, le dépôt institutionnel de l'Université du Québec à Chicoutimi

Résumé

Two main models have been proposed to explain the collection of platinum-group elements (PGE) from silicate liquid by an immiscible sulfide liquid. In the first model, the distribution of PGE is chemically controlled by their very high partition coefficients into an immiscible sulfide liquid. Alternatively, it has been proposed that PGE self-organize as nanometer size clusters in the sulfide liquid, which could be captured by an immiscible sulfide liquid. The main difference between both models is that in the cluster model the distribution of PGE would be physically controlled by the surface properties of nanometer-sized particles. Consequently, the partition coefficients of PGE, and other cluster-forming elements, would not affect their distribution in the cluster model. A way of testing which mechanism is more appropriate is to model the expected distribution of chalcophile elements with different partition coefficients in a sulfide deposit, and then compare with measured values. If the cluster model is valid, the distribution of the elements should not be controlled by their partition coefficients. We have measured the concentrations of Te, As, Bi, Sb (TABS), Se and PGE in the parental magmas from which the Bushveld Complex have formed (Marginal Zone), and used the results to model the composition of the Merensky Reef. The components of the reef have been modelled as cumulate sulfide liquid, silicate liquid and cumulate (silicate minerals and oxides). The cumulate sulfide liquid was calculated using the zone refining equation and taking into account the different partition coefficients of PGE, Se and TABS between silicate and sulfide liquids, and an N factor of 20,000. The modeled concentrations closely resemble the measured values obtained for a section across the Merensky Reef at the Impala mine. The modeling also reveals that the distributions of PGE, Se, Te and Bi in the reef are essentially controlled by the presence of sulfide minerals, whereas As and Sb distributions are controlled by both sulfide minerals and melt component. This is because PGE, Se, Te and Bi are moderately to highly chalcophile elements, whereas As and Sb are only slightly chalcophile elements. Therefore, our results support the model of distribution of PGE, Se and TABS controlled by their different chalcophile nature, and do not favor their distribution to be physically controlled as suggested by the cluster model.