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

Résumé

Sulphides act as collectors of platinum-group elements (PGE), therefore, if a magma is to contain sufficient PGE to form a deposit within an intrusion, it should not have experienced sulphide segregation prior to emplacement in its present position. Prior sulphide segregation will have scavenged the PGE from the magma and deposited them elsewhere, presumably at depth. If the PGE have not been scavenged from the magma by a prior sulphide segregation, then they may be concentrated within the intrusion at the site where the first sulphides formed. Platinum-group elements have much higher partition coefficients into sulphides than do Ni and Cu and therefore the Ni and Cu to PGE ratios are strongly influenced by the segregation of sulphides. Thus, in order to decide whether an intrusion has the potential to contain a PGE deposit, the Ni and Cu to PGE ratios of the chilled margins should be considered. If the chilled rocks are not depleted in PGE relative to Ni and Cu, the intrusion may contain a PGE deposit. In order to locate the PGE within the intrusion, the Ni and Cu to PGE ratios of the rocks across a stratigraphic section can be used to find the level where sulphide saturation first occurred. The situation is complicated by the fact the Ni and Cu to PGE ratios are also influenced by olivine and chromite chrystallization. To eliminate the effects of removal of PGE (Os, Ir, Ru) and Ni by olivine and chromite it is necessary to consider the ratios of Pd/Ir or Pd/Rh or Pd/Pt versus Ni/Cu.