Northwest Territories Geological Survey

Originally discovered in the mid 1960s, regional geological mapping has identified direct shipping iron ore (DSO) within high-grade gneisses correlated to the Mary River Group (MRG) on Northern Baffin Island. The MRG forms the northern extension of the central Rae Committee Bay Belt. On north Baffin the MRG is characterized by lower metagreywacke overlain by a BIF-komatiite-quartzite cover sequence. Regionally, the BIF member hosts high-grade magnetite ores, with grades averaging 65 wt % Fe.

Baffinland Iron Mines’ direct shipping ore deposits and prospects cover a vast extent of Northern Baffin Island, covering variable metamorphic and structural terranes. This study uses Deposit No. 1, Baffinland’s flagship deposit, as a type-section for MRG stratigraphy and a comparative benchmark for fluid-rock interactions in the DSO forming processes. Detailed petrography, bulk rock geochemistry and microprobe mineral analyses have been employed to determine the protoliths of the host-rock units to try to correlate them to traditional MRG stratigraphy.

LA-ICP-MS analysis has been used to determine that least-altered BIFs across the study areas in the MRG have very similar major oxide, trace element and rare earth element (REE) patterns, typical of Algoma-type BIFs found in the Prince Albert and Woodburn Lake groups on mainland Nunavut as well as other districts globally. As there is no global standard for Algoma-type BIFs, Deposit No. 1 serves as the baseline for regional comparisons. At the mineral scale, enrichment and depletion trends of major oxides are typical of DSOs globally, however significant regional variation is observed in trace elements and REEs. These variations are attributed to fluid source and country rock differences as well as disparate metamorphic grade. Fluid chemistry, metamorphic grade and structural preparation/control are seen to have an effect on the physical characteristics of the ore.

Timing of the metamorphic events that led to the natural beneficiation of the DSO is congruent across the MRG. Metamorphic zircon, monazite and titanite dating has tied the ore forming event to the Transhudson Orogen at ~1.83 Ga. LA-ICP-MS techniques were used for in-situ dating of monazites in DSO, as well as a novel new approach of directly dating hematite itself. Dates from rocks hangingwall and footwall to the orebodies have been used to correlate MRG stratigraphy across North Baffin, decipher regional metamorphic events and to help shape deposit formation models.