Ni-Cu-PGE Mineralizations within a Komatiitic Subvolcanic and Volcanic Succession in the Mesoarchean Venus Greenstone Belt, Superior Province, Northern Québec, Canada

English Abstract:

The Mesoarchean Venus greenstone belt (VGB), located in the northern part of the La Grande Subprovince within the Superior Province, consists of a lower amphibolite facies volcano-sedimentary succession which extends over a strike length of 15 km. It includes two main lithological packages: 1) a lower one, dated at ca. 2880 Ma and structurally overlying the younger Favard tonalitic Suite, composed of calc-alkaline felsic to intermediate volcanic and sedimentary rocks intruded by differentiated komatiitic dikes and sills (Lower Komatiite unit: LKU) and 2) an upper one that consists of a thick sequence of tholeiitic basalts intercalated with thin flows of spinifex-textured komatiitic basalts (Upper Komatiite unit: UKU). The limit between the two lithological packages is marked by sulfide-rich exhalative horizons.

The LKU consists of peridotites, with local harrisites, pyroxenites and gabbros which are interpreted to be intrusive in origin based on the lack of volcanic features and local discordant relationships with the country rocks exhibiting contact metamorphism and metasomatism. Poorly-developed pillowed flows and volcanic breccias support an extrusive origin for the very fine-grained ultramafic rocks of the UKU. The VGB is an excellent example where komatiitic magmas were emplaced as lower intrusive and upper extrusive komatiitic units into a felsic-dominated volcanic succession similar to other greenstone belts (e.g. Abitibi, Agnew-Wiluna).

Twelve significant Ni-Cu-PGE occurrences are recognized in the Nancy magmatic feeder and subconcordant sills of the LKU. Mineralization occurs as disseminated to net-textured to massive sulfides hosted in ultramafic rocks and within the adjacent country rocks. The high metal tenors (>20% Ni and 20 g/t PGE) combined with the presence of MgO-rich komatiites that exhibit evidences of interaction and contamination with surrounding S-rich country rocks testify the potential of the VGB to host additional Ni-Cu-PGE mineralizations.