The Mary River mine is operated by Baffinland Iron Mines Corporation, a privately owned Canadian mining company. As of December 31, 2024, the mine is owned by Nunavut Iron Ore Inc. (NIO), which is the sole parent company of Baffinland. ArcelorMittal and The Energy and Minerals Group (EMG) are shareholders of NIO, with ArcelorMittal holding a 25.23% interest. Therefore, Baffinland Iron Mines Corporation operates the mine, while NIO owns it, with ArcelorMittal and EMG as key stakeholders.
Summary:
The Mary River deposits can be described by the depositional model proposed for Algoma-type iron formation. The characteristic stratigraphy for Algoma-type iron formations comprise of a lower succession of typically intermediate volcanics interbedded with acid volcanics, volcaniclastic sediments, and their sedimentary derivatives. This succession is followed by banded iron formation comprised of alternating cherty bands and iron oxides or bedded or banded pure oxides ranging from metres to hundreds of metres in thickness, depending on depths of the original basins. This succession in turn is overlain by a sequence of volcanics and volcaniclastic sediments often intruded by ultramafics and granitic intrusive bodies.
Mary River Group greenstone belts across northern Baffin Island consist of variable amounts of banded iron formation (BIF), which are associated with metasedimentary, metavolcanic and metaigneous rocks (Jackson et. al., 1978a, b, c; Jackson and Morgan, 1978). The banded oxide and silicate facies iron formations comprise the lateral equivalents of the high-grade Mary River deposits, which are almost entirely composed of iron oxides, suggesting a significant enrichment of iron through secondary processes.
Duke (2010) suggests growing evidence for a secondary Fe enrichment mechanism associated with widespread retrograded mylonitic detachments along the base of the high grade and enriched BIF across north central Baffin Island in places such as the Mary, Turner, Cockburn and Rowley River areas. These detachment faults are marked by a chlorite retrograded mylonitic structural contact between underlying ductile rising domes of polydeformed mafic-felsic gneisses and overlying supracrustal sinking keels which host the Mary River iron deposits (Duke and McCleod, 2009). These detachments are thought to be post regional folding of the Hudsonian orogeny around 1,850 Ma (Duke, 2010).
The high grade massive and enriched iron oxide ores occur where Mary River BIF border directly on detachment schist that separates the mylonitic roof of infrastructural domes from the high strained keels of Mary River Group. At detachment schist contacts, magnetite BIF shows evidence of undergoing extreme pressure solution resulting in the highly efficient leaching of quartz from magnetite. In large deposits, the primary banding defining early isoclinal folding is completely destroyed by metamorphic differentiation and recrystallization, resulting in >100m thicknesses of massive magnetite/hematite. On the flanks of the high grade iron deposits, Feenrichment is marked by various degrees of silica loss due to continued silica removal. Several regional examples of exposed giant quartz veins occurring along detachment schist boundaries adjacent to high grade magnetite zones account for the removal of silica from the banded iron formations. Duke (2010) suggests that the hydrothermal removal of silica was likely facilitated by dewatering of the nearby footwall serpentinized komatiites.
The Mary River deposits are comprised of a number of iron formations which have been enriched and altered to varying degrees. Original banded iron formations comprised of alternating layers of magnetite and hematite, are preserved in several locations along strike of existing high grade deposits. The regional metamorphism and folding associated with the Hudsonian Orogeny resulted in significant crustal thickening. This period also resulted in zones of weak to very efficient leaching of silica. Subsequent hypogene and metamorphic events led to the alteration of magnetite to hematite and specular hematite. Surface outcrops differ in iron, silica and sulphur content and in the proportions of their main oxide minerals – hematite, magnetite and specularite. The microscopic examination of drill core specimens by R.A. Blais (1964) suggests that magnetite was the stable mineral of the iron formations during metamorphism (amphibolite facies), and was replaced to various degrees by hematite (martitization), resulting in a range of hematite-magnetite (martite) compositions. This alteration process occurred during a late stage of metamorphism, under conditions of localized stress and high oxygen pressure, and may have been associated with folding and faulting of the tabular high-grade magnetite iron formation.
The strong structural influence on channelling of oxidizing solutions is supported by the alignment of lineations produced by tubular pores in the hard high-grade hematite iron formation (i.e., parallel to the axial plane and plunge of the synform) and the concentration of high-grade hematite ore at the core of the fold. Euhedral specularite crystals in magnetite rock indicate that specularite did not form by a process similar to martitization, but through the possible metamorphic replacement of magnetite. It is also possible that the specularite and other hematite mineralization formed as a primary rather than secondary mineral. However, what caused the crystallization of flaky, course-grained specularite as compared to fine-grained massive hematite is not fully understood. Field evidence suggests that higher fluid or vapour pressures in zones of shearing may have facilitated the growth of specularite.