3D modeling of the Esker intrusive complex, Ring of Fire intrusive suite, McFaulds Lake greenstone belt, Superior Province: Implications for mineral exploration

Abstract

A 3D geological model of the Archean Esker ultramafic–mafic intrusive complex of the Ring of Fire intrusive suite of northwestern Ontario (Canada) is presented, providing insight into the nature of its magmatic plumbing system and allowing for speculation on the formation of its valuable Ni-Cu-(PGE) and chromitite deposits. The 3D model was constrained by exploration drillholes and high-resolution aeromagnetic data, which allowed the interpretation of the broad internal structure of the intrusive complex and two shear zones, which define for a major part of their strike extent the contacts between ultramafic and overlying mafic intrusive rocks. Restoring the post-emplacement component of strike-slip displacement and subsequently rotating the intrusive complex back from its tilted to sub-horizontal orientation of emplacement, provides a three-dimensional rendition of a 14 km-long dominantly tabular-shaped intrusive complex. The complex is defined by intact basal intrusive contacts with localized keel-shaped ultramafic promontories parallel to a nearby blade-shaped magmatic conduit. The keels extend in a subparallel orientation to depths of up to 1 km into the underlying tonalitic rocks and coincide with significant offsets of the basal intrusive contact, leading to speculation that their intrusion may have been guided by pre-existing normal faults. Two of these magmatic conduits host Ni-Cu-(PGE) deposits/prospects that formed from gravitational segregation of sulfides. The chromitite deposits, hosted in ultramafic rocks at higher stratigraphic levels, were modeled based on drill hole assay data. Zones of dominantly massive to semi-massive chromitite were defined by the ≥ 35% Cr2O3 threshold, while intercalated sequences of chromitite in dunite/peridotite were defined by the ≥ 15% to ≤ 35% Cr2O3 percentage range. The 3D modeled surfaces fitted to these constraints exhibit ore zones with lateral extents from several hundreds of meters for massive bodies up to a maximum of 2 km for intercalated chromitite ore that are dominantly conformable to the layering of the ultramafic host rocks. The lack of lateral continuity of chromitite ore zones beyond this scale, suggests that the chromitite layers are not continuous along the entire strike extent of the Esker intrusive complex, which is consistent with bifurcations of the 3D modeled ore shells signifying pinch-outs of both the massive chromitite layers and the dunitic interlayers. This discontinuous distribution of chromitite ore resembles that in the stratiform deposits of the Stillwater Complex and may support recent models of mechanical segregation of chromitite from cotectic chromite-olivine slurries. Based on our 3D model, we propose that the Esker intrusive complex evolved as a series of individual intrusions that were emplaced during multiple magmatic pulses that eventually coalesced to form a composite ultramafic–mafic complex prior to being dissected and partly dismembered by post-ore shear zone deformation.