Structural-stratigraphic setting and U-Pb geochronology of Ni-Cu-Co-PGE ore environments in the central Cape Smith Belt, Circum-Superior Belt

Abstract

The economically important Cape Smith Belt of northern Quebec represents a key segment of the "Circum- Superior Belt", preserving a ~12-15 km thick record of Paleoproterozoic stratigraphy in a south-vergent, mostly north-dipping fold-thrust belt. The central part of this belt is only moderately deformed and experienced merely sub-biotite grade metamorphism. It hosts world-class Ni-Cu-Co-PGE mineralization in both extrusive and intrusive settings that, at present, form the basis for two integrated mining operations. Collectively, these attributes make this belt unique in Canada and the world. A detailed, holistic understanding of this belt has been hampered, however, by conflicting interpretations on the degree of thrust stacking, and a lack of accurate and precise ages for critical elements of the stratigraphy. Here we report on new fieldwork and drill core observations-collected over several summers-that resolve many of the outstanding questions and provide a more detailed stratigraphic framework for the mineralization and the belt as a whole. We integrate these data and observations with new high-precision U-Pb ages for ~10 critical rock units, from the Cape Smith Belt and other parts of Circum-Superior Belt, as well as relevant parts of the overall craton-scale ore system. We briefly describe the different ore settings of the Cape Smith Belt and place these in the structuralstratigraphic framework. We show that the central part of the Cape Smith Belt is more coherent and less imbricated by thrusts than previously interpreted. The important Nuvilik Formation of thinly bedded distal turbidites and sulphidic mudstones forms a stratigraphic unit at the top of the Povungnituk Group, reflecting a phase of basin formation and deepening following volcanism of the ca. 1959 Ma Cecilia Formation. Neither the lower contact nor the upper contact of the Nuvilik Formation is a regional thrust. At 1883-1882 Ma, the Nuvilik sulphidic mudstones formed the ambient seafloor across which high-volume, hot, Mg-rich lavas of the Chukotat Group were emplaced, which included high-flow rate, turbulent, channelized komatiite flows, thus bringing into direct contact the most dynamic magma system of the belt with a ubiquitous, prolific sulphur source. Although Chukotat magmas may have been at sulphur saturation on final ascent, thermo-mechanical erosion of the lava channels into underlying Nuvilik mudstones, and mixing and melting of sulphidic sediments into the channels, led to massive sulphur oversaturation and accumulation of net-textured and massive sulphides. Our ages show that all of this happened during the onset and climax phase of the Chukotat magmatic event, which lasted ~2 Myr and occurred at the same time as similar events more than 1500 km away, in Thompson, Manitoba. At Raglan, we describe one well preserved lava and ore channel where, in present coordinates, flow was demonstrably down-dip and to the north-northeast. This is the first observation of flow polarity. We prefer an overall model of several anastomosing, subparallel lava channels presently plunging down to the north-northeast, not a single, giant, meandering lava channel subparallel to the trace of the basal Chukotat lavas on the present erosion surface. The observed flow direction makes it feasible, if not likely, that the channelized komatiite flows were fed from an eruptive fissure system ~25 km to the south, which would suggest a "processing length" of ~25 km to achieve optimal mixing, sulphide saturation, and segregation at high R-factors. At the craton scale, the overall model that best explains the rich spectrum of phenomena is that of a hot mantle upwelling ascending from a deep thermal boundary layer to impinge on the base of the lithosphere of the Superior craton, or rather its ancestral supercraton Superia, prior to final breakup. Rapid lateral flow of hot buoyant mantle to lithospheric "thin spots", such as active or pre-existing rifts or delamination scars, caused nearly synchronous, high-volume, ultramafic-mafic magmatism around what ultimately became the margins of a fully separated Superior craton. Although the overall magmatic event has a younger tail, to as young as 1878 Ma, and also a distinct younger pulse at ca. 1870 Ma, the early onset and climax phase of high-volume ultramafic-mafic magmatism is most prospective for economic Ni-Cu-Co-PGE mineralization.