Geology, petrology, geochemistry and platinum-group element-gold-copper-nickel ore asselmblage [i.e. assemblage] of the Roby Zone, Lac Des Iles Mafic-Ultramafic Complex, Northwestern Ontario

Abstract

The Archean Lac des lies Complex is a mafic to ultramafic intrusion emplaced into gneissic tonalite. The Lac des lies Complex Is the largest of several mafic to ultramafic intrusions that form a circular outcrop pattern approximately 30 kilometers In diameter. The Lac des lies Complex is composed of two ultramafic intrusions exposed at Lac des lies and a gabbroic intrusion located south of the lake. The gabbroic rocks contain the economically significant Roby Zone PGE-Au-Cu-Ni deposit. The Roby Zone deposit is composed of two texturally- and compositionally-distinct portions. The northern portion of the deposit is composed of a relatively unaltered layered gabbroic sequence consisting of leucogabbro, gabbronorite, gabbro and clinopyroxenite. Field data, including the orientation and type of geolgic contacts, indicate that the layers represent an intrusion of magma into a largely crystallized mush. In-situ fractionation was identified within individual layers. The southern portion of the Roby Zone consists of a lithologically and texturally complex unit containing numerous rounded and angular fragments varying in composition from leucogabbroic to pyroxenitic and grain size ranging from medium-grained to pegmatitic. These rocks have experienced pervasive deuteric alteration that modified the original magmatic textures and compositions. Numerous pegmatitic dikes and patches occur throughout the heterolithic gabbro. PGE-Au-Cu-Ni mineralization within the northern layered sequence often forms net-textured sulphides and represents primary magmatic mineralization. Within the heterolithic gabbro, PGEs occur as primarily sulphides and tellurides. These PGE minerals occur as blebs within pegmatitic pods and as fine-grained inclusions and streaks within secondary silicates suggesting that deuteric fluids have concentrated and deposited the metals within the heterolithic gabbro. Within the southern portion of the Roby Zone, higher PGE concentrations are associated with altered areas. The model for the development of the Roby Zone and its attendant PGE-Au-Cu-Ni mineralization consists of 1) fractionation of tholeiitic magma in lower magma chamber and exsolution of immiscible sulphide liquid with associated PGE-Au-Cu-Ni, 2) intrusion of fractionated magma into Roby Zone and subsequent in-situ fractionation, 3) prior to complete solidification of the layers, a volatile-rich gabbroic magma injected the Roby Zone resulting in brecciation of the layered sequence and formation of the heterolithic gabbro composed of rounded and angular fragments within a gabbroic matrix, 4) partially solidified rounded fragments and partial melting of some of the remaining fragments by the gabbroic magma triggered liquid immiscibility, 5) deteuric fluids percolated through the fragmented gabbroic rocks modifying the original magmatic textures and compositions and concentrated and redeposited metals within the heterolithic gabbro. Subsequent regional deformation tilted the Roby Zone to the east and shearing occurred within a portion of the clinopyroxenite. Late-stage local Suiting and hydrothermal fluids further modified the original magmatic textures, compositions and PGE-Au-Cu-Ni mineralization.