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

Résumé

The Bushveld Complex of South Africa contains approximately 80 % of the world's Pt and almost half of its Pd resources in the form of three large ore deposits, the Merensky Reef, the UG-2 reef and the Platreef. Two questions arise a) were the magmas that formed the Bushveld Complex particularly Pt- and Pd-rich? and b) what is the origin of these magmas? In order to consider these questions we have estimated the composition of the Bushveld magmas based on 40 new whole rock analyses of quench textured rocks from the margins of the intrusion. Broadly speaking, there are two types of magmas present, a Mg-rich basaltic andesite and a tholeiitic basalt. Both of these magmas are enriched in large ion lithophile elements, light rare earth elements and in Pb. Both magmas have negative Ta, Nb, P and Ti anomalies. The PGE contents of both magmas are similar to primary basalts and they do not appear to be enriched in PGE, except for Pt. The Pt/Pd ratio and Pt/Ti ratios are 1.5 to 2 times that of most basalts. It is possible to model the lithophile element composition of the two magma types by up to approximately 50 % crustal contamination of komatiitic basalt or picrite. This conclusion is supported by Sr, Nd and oxygen isotopic work. However, the Pt contents of the model magmas and the Pt/Pd and Pt/Ti ratios are much lower than those observed. An alternative is to suggest that the Bushveld magmas formed by partial melting of the metasomatised lithosphere. But, modeling using Kaapvaal mantle xenoliths compositions and MELTS shows that this magma would be too Al (sub 2) O (sub 3) rich and SiO (sub 2) poor. Two possible solutions to this are: a) the melts formed by zone refining melting of the lithosphere (to attain usually high Pt concentrations) followed by contamination with crustal melts; OR b) the current estimate of primitive upper mantle with a Pt/Pd of 1 is incorrect and it should be closer to 2, in which case mixing of a plume derived magma with continental crust melts approximates the composition of the Bushveld magmas.