Patrick Moran MSc thesis abstract

The Musselwhite gold deposit, 100% owned and operated by Goldcorp Inc., has cumulatively produced in excess of 2 million ounces since opening in 1997 and has a projected mine life through 2013. It is situated in the North Caribou Lake metavolcanic/metasedimentary belt in the central northwestern portion of Superior Province, approximately 430 km northwest of Thunder Bay, Ontario, Canada. The belt occurs along the contact between the North Caribou Terrane and the Island Lake Domain, with a large, crustal-scale deformation zone forming its eastern margin. The Musselwhite gold deposit is hosted by amphibolite facies metamorphic rocks dominated by banded iron formation (BIF). This study primarily focuses on the Northern Iron Formation (NIF) metasedimentary assemblage, host to the majority of gold mineralization at Musselwhite.

Stratigraphic and geochemical analyses suggest that the lithologies of the NIF assemblage were deposited on Mesoarcheaen mafic to ultramafic volcanic rocks forming the ocean floor. The NIF assemblage and another iron formation lower in the stratigraphy, the Southern Iron Formation (SIF), record hydrothermal regimes associated with, and interrupted by, eruptive volcanic activity. The stratigraphically lowest lithologies in the NIF assemblage, meta-argillite, quartz-grunerite BIF, and magnetite-dominant BIF, were deposited in deep, calm water, in association with venting hydrothermal fluids. These ancient chemical sediments are analogous in geochemistry to modern day deposits in places such as the Red Sea and the East Pacific Rise. Differing Eu contents between chert and magnetite layers indicate that rhythmically changing temperature variations drove the hydrothermal system, imparting the banded nature. The chemically pure chert and magnetite layers of the lower portion of the NIF assemblage contrast with silicate-dominant banded iron formation; the silicate-dominant BIF increases in frequency up stratigraphically. It represents a decreasing hydrothermal system and/or an increase in the rate of clastic sedimentation. Hornblende-garnet and biotite-garnet schists were formed by metamorphism of mudstones composed of eroded material. The sediment that formed the hornblende-garnet schist is the same sediment that comprises the siliciclastic component of the silicate-dominant NIF. Similarly the biotite-garnet schist represents a mudstone, but unlike the hornblende-garnet schist, it is primarily derived from intermediate to felsic volcanic rocks. Lastly the garnet-bearing quartzite represents metamorphosed sandstone eroded from the same intermediate to felsic igneous source rocks as the biotite-garnet schist. Just as there is an overall increase in clastic content upwards through the approximately 30 m thick sedimentary succession, there is also a change from more mafic-sourced debris to a more intermediate/felsic source.

The majority of samples collected from Musselwhite did not experience significant remobilization of typically immobile elements. This is indicated by the relatively linear geochemical ratios between the immobile elements (Al₂O₃, TiO₂, Zr, U, Th, etc.). Even elements that are commonly more mobile (K₂O, Na₂O, etc.) appear to have remained relatively immobile at Musselwhite. The only samples that show significant geochemical change are from shear zones. The gold mineralization is primarily associated with shear zones within the siliciclastic-rich, upper NIF assemblage, where pyrrhotite (possibly originally pyrite) replaced iron oxides and iron silicates. This indicates that the control on areas of gold mineralization was a combination of: 1) the presence of structural zones allowing gold-bearing fluids to move through the NIF, which could act as a geochemical trap for gold; and 2) structural conditions in the siliciclastic-rich NIF that favoured hydrothermal fluid involvement with this unit.