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

Résumé

The Platreef is a platinum group elements and base metal enriched mafic/ultramafic layer situated along the base of the northern limb of the Bushveld Complex. The present study contains a detailed petrographic and geochemical investigation of a borehole core intersection through the Platreef on the farm Townlands. At this locality, the Platreef rests on metasedimentary rocks of the Silverton Formation of the Transvaal Supergroup, and is comprised of three medium-grained units of gabbronorite/feldspathic pyroxenite that are separated by hornfels interlayers. We refer to the three platiniferous layers as the Lower, Middle and Upper Platreef. The Middle Platreef is the main mineralised layer, with total PGE contents up to 4 ppm. The Lower and Upper Platreefs are less well mineralised (up to 1.5 ppm). Trace element and S-isotope data show compositional breaks between the different platiniferous layers suggesting that they represent distinct sill-like intrusions of pyroxene and sulphide enriched crystal mushes. The study also reveals a reversed differentiation trend of more primitive rocks towards the top of the succession. For example, orthopyroxene shows an increase in Cr (sub 2) O (sub 3) from 0.07 to 0.37 weight % with height and the whole rock concentration of incompatible trace elements such as Y and Zr decreases. This pattern is interpreted to reflect enhanced crustal contamination of the lower Platreef layers. All three Platreef layers are enriched in heavy S (delta (super 34) S of 2.6 to 9.1 per mil) indicating addition of crustal sulphur, and they have elevated K, Ca, Zr and Y contents and high Zr/Y ratio relative to Critical Zone rocks from elsewhere in the Bushveld Complex, suggesting a model of crustal contamination in ore formation. Well defined correlations between the concentrations of the individual PGE, and between the PGE and S suggest that the concentration of the PGE was controlled by segregating sulphide melt. Alteration of the rocks, possibly due to infiltration by fluids derived from the floor rocks, caused localized redistribution of Cu, S and, to a lesser degree, the PGE. However, alteration, sulphur and metal mobility was apparently much less pronounced at Townlands than at other Platreef localities further to the north, notably at Sandsloot mine where the PGE are largely hosted by PGM (Armitage et al., 2002). We suggest that this is due to more pronounced devolatisation of the dolomites relative to the shales, implying that the nature of the floor rocks plays an important role in ore formation.