Iron-Ore Deposits of Southwestern Labrador

J. Conliffe

St. John’s, Newfoundland, August, 2019

Abstract

The iron-ore deposits of southwestern Labrador have been mined continuously since 1962, and in 2016 they accounted for more than half of the annual gross value of mineral shipments for Newfoundland and Labrador. These deposits, classified as metataconite deposits, are a distinct group of iron-ore deposits only described from this region. They are hosted in the Sokoman Formation iron formation, and were metamorphosed and deformed during the Grenville Orogeny at ca. 1.0 Ga. Three broadly defined end-member facies describe the Sokoman Formation: oxide facies (the main ore-bearing unit), silicate facies, and carbonate facies. The iron formation recrystallized during regional metamorphism, and the oxide-facies iron formation consists of medium- to coarse-grained magnetite, specular hematite, and quartz, and is easily beneficiated into iron concentrates (approximately 65% Fe) ideal for pellet production. Deformation and associated structural thickening during the Grenville Orogeny were also important in the development of mineable thicknesses of oxide-facies iron formation.

Forty individual iron-ore occurrences have been described from the study area (NTS 23B/14, 23B/15, 23G/02, 23G/03 and part of 23G/07). These are subdivided into a number of distinct basins, which based on stratigraphic correlations between individual occurrences and geochemical variations, may represent separate depositional centres. The Carol Lake Basin, which hosts all occurrences in the Iron Ore Company of Canada’s Carol Lake Project, is subdivided into a Lower Iron Formation (LIF) dominated by carbonate-facies iron formation, a Middle Iron Formation (MIF) predominantly of oxide-facies iron formation, and an Upper Iron Formation (UIF) with carbonate- and silicate-facies iron formation, and rare oxide-facies bands. The Wabush Basin, hosting the idled Scully Mine and developed prospects at the Julienne Lake and Rose deposits, is composed of a distinctive Basal Silicate Iron Formation (BSIF) that is overlain by a thick sequence of oxide-facies iron formation; carbonate-facies LIF is absent. A smaller third basin, the Mills Lake Basin, is located in the southwest of the study area, whilst a number of occurrences on the western margin of the study area are not included in any basin due to the lack of detailed stratigraphic information.

Geochemical data indicate that the oxide-facies iron formation represents the local ocean chemistry during deposition, and samples from the Wabush and Mills Lake basins generally have elevated Mn and Ba, lower Y/Ho ratios, and are relatively enriched in LREE compared to samples from the Carol Lake Basin. These geochemical trends indicate that deposition in the Wabush and Mills Lake basins occurred in deeper water than that in the Carol Lake Basin, below the redoxcline in a stratified ocean (having an oxic upper layer and reduced lower layer).

Some iron-ore occurrences have been extensively altered, resulting in magnetite in oxide-facies iron formation being partially to completely oxidized to secondary martite with common goethite, and Mn being remobilized into discrete layers and veins. The close spatial association between altered iron formation and brittle faults suggests that these faults may have been reactivated during tectonic activity, and late-stage fluid movement may have focussed along these structures. This late-stage fluid flow is not associated with any iron enrichment, unlike high-grade supergene deposits in the Schefferville area, where late-stage fluid flow, intense alteration and transformation of magnetite to hematite and goethite are responsible for upgrading the iron formation from ~30% Fe to >55% Fe. This may be due to the chemistry of the fluids or the larger quartz grain size in the metamorphosed iron formation, which would impede dissolution.

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• Occasional Paper Report (77 MB)
• Appendix A: Sample Descriptions
• Appendix B: Major and Trace Element Data for Outcrop, Drillcore and Pulp Samples
• Appendix C: Major-element ICP-OES-FUS Standards and Duplicate Data
• Appendix D: Trace-element ICP-OES Standards and Duplicate Data
• Appendix E: Trace-element ICP-MS-FUS Standards and Duplicate Data
• Appendix F: Exploration Drill-log Database (post-2000)

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