Summary:
The Properties’ deposits are classified as Lake Superior-type. Such iron formations are the principal sources of iron throughout the world. Iron formation deposits in the FIOD include ArcelorMittal’s Mont-Wright and Fire Lake Mines, Mont Reed iron deposits and Cliffs Natural Resources Bloom Lake Mine, (formerly owned by Consolidated Thompson Iron Mines Ltd.) and the Lamêlée Lake and Peppler Lake iron deposits.
Iron formations are classified as chemical sedimentary rock containing greater than 15% iron consisting of iron-rich beds, usually interlayered on a centimetre scale with chert, quartz, or carbonate. Ore is mainly composed of magnetite and hematite, and commonly associated with mature sedimentary rocks.
Stratiform iron formations are distributed throughout the world in the major tectonic belts of the Precambrian shields, and in many Paleozoic and Mesozoic fold belts, as well as parts of the present day ocean floor. Gross (2009) noted that the enormous size of some of the Archean and Paleoproterozoic iron formations reflected the unique global tectonic features and depositional environments for iron formation that were distinctive of the time.
Although various models have been used to explain the deposition of iron formations in the past, current thinking (summarized in Cannon, 1992, Gross, 1996, Gross, 2009) supports the idea of iron formation deposition, resulting from the syngenetic precipitation of iron-rich minerals in a marine setting due to hydrothermal exhalative activity on the ocean floor. The iron is thought to have formed in stable tectonic-sedimentary environments where silica, iron, ferrous and non ferrous metals were available in abundance, mainly from hydrothermal sources, and where conditions were favourable for their rapid deposition with minimal clastic sediment input.
Hydrothermal processes related to volcanism and major tectonic features are thought to be the principal source of iron and other metals. Deep fractures and crustal dislocations over hot spots and high thermal gradients penetrating the upper mantle enabled convective circulation, alteration and leaching of metals from the upper crust, including possible contributions by magmatic fluids. Iron formations are important hosts of enriched iron and manganese ore, gold deposits, and are also marker horizons for massive-sulphide deposits. Deposition of the iron was influenced by the pH and Eh of the ambient water, and biogenic anaerobic processes may have also played a role (Gross, 1996, Gross, 2009).
Post depositional events such as weathering, groundwater circulation and hydrothermal circulation can modify the deposits, and the mineralogy is usually recrystallized and coarsened by medium- to high-grade regional metamorphism. Protracted supergene alteration can be an important economic fact in upgrading the primary iron formation (Gross, 1996).
Iron formations can be subdivided into two (2) types, related to two (2) major types of tectonic environments: the Lake Superior-type on the continental shelf and marginal basins adjacent to deep-seated fault and fracture systems and subduction zones along craton borders; and the Algoma-type along volcanic arcs and rift systems and other major disruptions of the earth’s crust. Development of Lake Superior-types was related to global tectonic systems that caused the breakup of cratons, shields or plates in the Paleoproterozoic. Rapitan-type have distinctive lithological features being associated with diamictite, and were deposited in grabens and fault scarp basins along rifted margins of continents or ancient cratons in sequences of Late Proterozoic and Early Paleozoic rocks.