Summary:
The stratiform, iron-rich deposits in the world are classified as Lake Superior and Algoma types banded iron formations (“BIF”). The BIF are interpreted as finely bedded sedimentary rocks of chemical origin.
The Canadian Lake Superior-type iron formations are the most important source of iron, followed by the deposits of Algoma-type iron formations. Algoma-type iron formations are predominantly found in Archean greenstone belts, while the Lake Superior type is generally of Paleoproterozoic age.
The Lake Superior type of iron formation formed in stable continental shelf and platform settings, whereas the Algoma type is associated with volcanic arcs (tectonically active environment, greenstone belts) or rift zones and related volcanism. However, a complete gradation exists from one type to the other, as the Algoma type iron formation can be deposited at considerable distances from the volcanic centres.
The Algoma deposits are interbedded with felsic to ultramafic volcanic and volcaniclastic rocks, greywacke, black shale, argillite and chert. The Lake Superior iron formation is associated with a variety of shallow marine sedimentary rocks including quartzite, shale and carbonate rocks, and subordinate amounts of felsic to mafic volcanic rocks.
The Lake Superior type, formed on the large areal extents of the platforms, typically have higher lateral continuity and thickness than the Algoma type BIF originating from the more restricted volcanic environment.
The DLIP hosts iron deposits typical of the Algoma type oxide facies, considering the presence of an important volcanic rocks component. However, the large extent of the BIF in the DLIP deposits suggests a setting of deposition in a relatively stable environment. Consequently, the Duncan Lake BIF is interpreted to represent a type half-way between the Algoma and Superior types, sometimes referred to as Carajas iron mineralization. The oxide, silicate, carbonate and sulphide facies are common to both of these groups that contain a minimum of 15% Fe that typically varies between 30 and 35% Fe.
Banded iron formation consists of thin alternating beds of silica (quartz, chert) and iron oxides (magnetite and hematite), with variable amounts of silicate, carbonate and sulphides. The sequences are commonly metamorphosed. Grain size varies according to the degree of metamorphism and iron amphiboles are commonly developed in middle greenschist or higher metamorphic grade rocks.
Iron mineralization within the DLIP property consists of BIF type represented by alternating millimetric to centrimetric bands of quartz and of magnetite with minor amounts of hematite. The DLIP deposits are also associated with silicate and sulphide facies iron formations. The silicates consist mainly of actinolite, tremolite, diopside, grunerite, hornblende, chlorite, epidote, biotite, as well as chlorite-rich mudrocks. The BIF is black to dark blue and becomes dark-green with increased silicate content. It is generally fine-grained, with local variations due to recrystallization in response to contact metamorphism by the felsic intrusions near Deposits 3 and 6. Average head grade varies from 15 to 35% total Fe.
Disseminated sulphides, mostly pyrite, are widespread in the BIF sequence and some of the adjacent waste rocks and represent the main contribution in the sulphur deleterious element to the system.
The chemical analysis established that, on average, the iron formation at DLIP contains very low levels of deleterious elements, in particular phosphorus (0.02% P2O5), manganese (0.03% MnO) and magnesium (0.23% MgO). However, the average sulphur content is elevated. The unweighted average of 3,662 analyses for all deposits yielded 0.78% S in the head grade from core samples. The sequence of the iron-rich units on the DLIP is not clearly defined and is related to a mix of Algoma and Lake Superior types BIF.