Geological setting of the world-class Musselwhite gold mine, Superior Province, northwestern Ontario: implications for exploration

Abstract

The Musselwhite world-class Au deposit is hosted in polydeformed amphibolite-facies banded iron formation of the Opapimiskan-Markop metavolcanic assemblage, part of the Mesoarchean North Caribou greenstone belt (northwestern Superior Province). The deposit is located approximately 2 km west of the tectonic boundary with the gneissic Island Lake Domain. Major and trace element geochemical data show that the South Rim and Opapimiskan-Markop metavolcanic assemblages have variable magmatic affinities and diverse normalized rare earth element patterns. The bulk of the Au at Musselwhite is hosted in silicate-rich banded iron formation and occurs in association with stratabound pyrrhotite replacements and associated silica flooding, with local discordant quartz}pyrrhotite veins. The ore zones are associated with D2 high-strain zones that are preferentially developed along hinges and strongly attenuated fold limbs of tight F2 folds. The layered anisotropy induced by the presence of competent banded iron formation layers in mafic and ultramafic volcanic rocks has clearly influenced the rheological response to deformation at all scales, and hence played an important role in Au-bearing fluid flow and ore formation and distribution. A new U-Pb preliminary age of 2666 Ma on late-M2 monazite provides a minimum age constraint for the regional D2 metamorphic/ deformation event to which most of the Au mineralization at Musselwhite is associated. Reappraisal of stratigraphic relationships, supported by U-Pb geochronology, indicates that the mine stratigraphy is inverted and is part of the overturned limb of a kilometre-scale F1 syncline, which is in agreement with multiple occurrences of mesoscopic refolded F1 folds. Previously unrecognized regional F1 folding, which is strongly overprinted by the dominant D2 deformation, has influenced the distribution and geometry of the banded iron formation, which hosts the bulk of the Au at Musselwhite, and provides new vectors for regional exploration. Future work will focus on additional documentation of mineral chemistry and microscopic textural relationships, as well as further analyses of lithogeochemical data in order to ultimately define key exploration vectors for iron formation-hosted Au deposits in other Precambrian terranes.