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

Résumé

The Bravo intrusion is a komatiitic sill that forms part of a feeder system to the Chukotat komatiitic basalts of the Cape Smith Fold Belt in northern Quebec. The sill is of interestb ecauseo f the large changesin the distribution of chalcophilea nd siderophilee lementsw ithin the intrusion. The differences in their distribution are interpreted to be due to the differences in the history of crystallization. contamination and alteration of each of the zones of the intrusion. The concentrations of the lithophile elements, platinum-group elements (PGE), Ni and Cu in tle upper zone of the sill may be modeled as the product of 2590 batch partial melting of a fertile mantle, followed by emplacement of this magma into the crust and accumulation of 40 to 6090 olivine and 0.5 to 190 sulfide during crystallization. The lithophile elementso f the lower-zoner ocks may be modeled in a similar manner, but the rocks are much richer in Cu, Ag, Rh, Pt and Pd than most komatiites. This enrichment could have been caused by the conttmination of the komatiite magma with a Cu-rich sulfide, which was formed either by paflial melting of a pre-existing igneous sulfide or by fractionation of an igneous sulfide liquid, At the base of the intrusion, there is a chlorite-sulfide zone that is depletedin Si, Mg, Ca, Na, K and Cr, and enriched in Ti, Al, REE, Sc, Y and Zr relative to the enclosing komatiite. The Ni, Os, Ir and Ru contents of the sulfide are similar to those found in sulfide associated with komatiites,b ut the rock is depletedi n Pd, Pt and Au, and richer in Cu and Ag than typical komatiite-associated ores. The chlorite-sulfide zone may represent the product of hydrothermal leaching of a komatiite rich in sulfide.