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

Résumé

Most of the world’s platinum-group element (PGE) and Ni-Cu sulfide deposits are found within or in contact with mafic and ultramafic rocks. In most cases these deposits can be related to a large igneous province (LIP), e.g. the Noril’skTalnak Ni deposits are associated with the Siberian flood basalts and the Merensky and UG-2 Pt deposits are found within the Rustenburg Layered series of the Bushveld Complex, which is the size of a LIP.

The reasons for this association are as follows. I) In the mantle source of the Ni and PGE in the ore deposits, Ni is hosted by olivine and PGE are hosted in sulfides and possibly Fe-PGE alloys. High degrees of partial melting are required in order to release all the PGE from the sulfides and to melt as much olivine and possibly alloys as possible. Melts generated by high degrees of partial melting are formed in the stems of mantle plumes and LIPs are thought to form from mantle plumes. II) The magma must be transferred from the mantle to the crust before a sulfide liquid can segregate from it because sulfide segregation would remove the metals and thus improverish the magma. LIPs represent areas of the crust where primary magmas have reached the surface. III) When the magma approaches the surface it should segregate a sulfide liquid that collects the metals. The source of the sulfur is thought to be country rock that is assimilated into the magma. This requires that the sytem have a large amount of heat, heat that can be derived from the LIP magmas. IV) Once the sulfide liquid forms, it must interact with a large volume of silicate magma in order for the metal concentrations (grade) of sulfide liquid to be high enough to mine. In most ore deposits, the ratio of silicate to sulfide magma is thought to have been at least 1000. This requires a very high-energy system, which the heat from the LIP could supply. V) Finally, in order for a deposit to be large enough to mine, a large quanty of magma must undergo all these processes. LIPs represent large quanties of magma.