Gold mineralization of the archean Orfée showing, Baie-James, Québec

English Abstract:

The Orfée gold deposit (~0.2 Mt at 14.5 g/t Au) is part of the La Grande Subprovince, in the Baie-James region. Discontinuous gold-rich zones are associated with an oxidized/silicate iron formation and with graphitic sediments containing up to 20% disseminated to massive pyrrhotite and pyrite, and are edged by basaltic amphibolites to the north and by wackes to the south. The graphitic sediments enclose <1% visible gold, as well a maximum gold content of 54.6 g/t in a gold-rich mylonitic corridor. The mineralization and its hostrocks, metamorphosed to the amphibolite facies, is 300 m north of the La Grande and Opinaca Subprovinces contact. The basaltic amphibolites are composed of hornblende, plagioclase and, locally, of biotite and garnet, and have an east-west oriented foliation. Plagioclase glomeroporphyritic horizons have been observed in the basaltic amphibolite unit. Tholeiitic and calc-alkaline geochemical signatures divide the basaltic amphibolites into two populations. Weakly plagioclase-phyric dioritic dykes, containing fine-grained mafic enclaves, cut across the amphibolites. These east-west foliated dykes intersect a pervasive garnet, hornblende, chlorite and pyrrhotite alteration affecting the amphibolites. The oxidized/silicate iron formation displays alternation facies characterized by quartz-plagioclase-rich, magnetite-rich and grunerite-hedenbergite-hornblende-garnet-sulphide-rich bands. The sub-vertical regional fabric was affected by a late dextral strike-slip movement, which explains the local thickening and boudinage of the iron formation. The wackes are composed of quartz, plagioclase and biotite and are affected by a S1 schistosity and a S2 foliation. Most of the wackes have geochemical signatures suggesting sources-derived intermediate component, while a small percentage have relatively high Mg, Cr, Ni content suggesting some sources-derived ultramafic component. The amphibolites, iron formation and wackes have been injected by a wide variety of deformed sterile quartz veins. Sheared and boudinaged pegmatite cuts across the wacke and amphibolite units. The metallogenic study of the Orfée showing will enable us to develop a model for the gold mineralization paragenesis, in relation with the geodynamic history of the La Grande – Opinaca contact.