Magmatic architecture of the Esker intrusive complex in the Ring of Fire Intrusive Suite, McFaulds Lake greenstone belt, Superior Province, Ontario: Implications for the genesis of Cr and Ni-Cu-(PGE) mineralization in an inflationary dyke-chonolith-sill complex

Abstract

One of the dominant geological features in the arcuate, >175 km long, Mesoarchean to Neoarchean McFaulds Lake greenstone belt in northern Ontario is the semi-continuous trend of mafic to ultramafic intrusions belonging to the Ring of Fire intrusive suite, which hosts world-class Cr mineralization, major Ni-Cu-(PGE) mineralization, and potentially significant Fe-Ti-V-(P) mineralization. It appears to have been emplaced over a relatively short time interval of approximately 4 to 4.5 million years. The intrusive suite contains two subsuites: the less widely distributed Koper Lake subsuite, which consists of komatiitic ultramafic- dominated intrusions and typically hosts Cr and Ni-Cu-(PGE) mineralization (e.g. Esker intrusive complex), and the more widely distributed Ekwan River subsuite, which consists of tholeiitic high-Fe-Ti mafic-dominated intrusions and typically hosts Fe-Ti-V-(P) mineralization (e.g. Thunderbird intrusion). The Esker intrusive complex contains the majority of the known Cr and Ni-Cu-(PGE) mineralization in the Ring of Fire intrusive suite. It is a semi-continuous, structurally rotated, subvertical ultramafic-mafic sill-like body that is composed of multiple intrusions with morphologies that vary from bladed dyke morphologies (e.g. Eagle's Nest), transitional dyke/chonolith morphologies (e.g. Double Eagle, AT-3, and AT- 8), to some with transitional chonolith/sill morphologies (e.g. Black Thor). It extends over more than 16 km, youngs to the south-southeast, and is bordered to the north-northwest by several keel-like ultramafic intrusive bodies (e.g. AT-12, C-6, AT-5, AT-1). Clear connections between AT-12 and AT-1 and the overlying Black Thor and Double Eagle intrusions, respectively, and the continuous spectrum of intrusion morphologies suggest that the keels were originally subhorizontal blade-shaped dykes (e.g. Eagle's Nest), the upper parts of which expanded laterally to form transitional dykes/chonoliths (e.g. Double Eagle intrusion) and chonoliths/sills (e.g. Black Thor intrusion), which inflated laterally and coalesced over time to form the silllike Esker intrusive complex. Most of the Ni-Cu-(PGE) mineralization in the Esker intrusive complex appears to have formed by incorporation of sulphur from footwall oxide-silicate-sulphide iron formations, a process that is similar to most other komatiite-associated Ni-Cu-(PGE) deposits worldwide. A fundamental issue in the genesis of all stratiform chromite deposits is how to form thick layers of massive to semi-massive chromite, an issue exacerbated by the vast amounts of chromite in the Esker intrusive complex. A genetic model that resolves the mass balance problem involves partial melting of Fe+/-Ti oxide-rich rocks (oxide-facies iron formation or ferrogabbro) and conversion of fine-grained oxide xenocrysts to chromite by reaction with Cr-rich komatiitic magma in a dynamic magma conduit. This model has been recently challenged based on the capacity of komatiitic magma to dissolve large amounts of magnetite, which would prevent upgrading. However, alternative models cannot explain the presence of composite chromite-silicate-sulphide grains with textures like those in footwall magnetite-silicate-sulphide facies iron formations. More research is required to reconcile the discrepancies. Regardless of their origin, the wide diversity of mineral deposit types in the McFaulds Lake greenstone belt, including world-class Cr, significant Ni-Cu-(PGE), and potential Fe-Ti-V-(P) mineralization related to mafic and ultramafic rocks, make the Ring of Fire region an excellent exploration target to increase the world's supply of critical minerals.