Summary:
Deposit Type
The mineralisation of the Project is consistent with the description of a crystalline flake graphite deposit. These types of deposits are described (Simandl and Kenan, 1997) as being commonly hosted by porphyroblastic and granoblastic marbles, paragneiss and quartzites. The alumina-rich paragneiss and marbles in upper amphibolite or granulite grade metamorphic terrains are the most favourable host rocks. Highest grades are commonly associated with rocks located at the contacts between marbles and paragneisses and deposits are thickest within fold hinges. Minor feldspathic intrusions, pegmatites, and iron formations may also contain disseminated natural flake graphite.
The Lac Knife deposit corresponds to the metamorphic equivalent at the upper amphibolite to granulite facies of the Menihek black shales, plus tectonic remobilisation into higher grade zones in fold hinges. Simandl and Kenan stated that the grade and tonnage of producing mines and development projects can vary substantially.
Mineralisation
Graphite occurs within the Nault Formation as a paragneiss which is a metamorphic equivalent of the graphitic black shales in the Labrador Trough located further north. There is no indication of secondary hydrothermal or other transported, post-metamorphic deposition. The present distribution and crystallinity of the graphite units are due to the Grenville high grade metamorphic events. However, deformation favoured the thickening of graphitic horizons by transposition towards the fold noses.
Birkett et al. examined 28 core samples for petrographic, electron-microscope and chemical studies (Birkett et al., 1989). They noted that the host rocks of the graphite deposit are of the silicate or calcsilicate categories. Tremblay (2014) examined three samples from the deposit and confirmed that the silicate type host rock correlated more with the massive and low-grade mineralization whereas the calcsilicate type is more associated with semi-massive mineralisation.
Mazarin geologists logged the diopside and minor calcite, but did not record the other pale coloured, low-Fe calcsilicates, which can be difficult to identify visually without previous experience or microscopic determination. Thus, the distinction of host-rock lithologies observed in the Birkett study was not reliably reflected in the core logs. Birkett et al. (1989) also noted that within a given host rock, the presence/quantity of graphite and pyrrhotite was the only variable; no other mineral proportions changed with respect to graphite presence/content. These observations have been confirmed since then by Focus and IOS geologists (Block and Gagné, 2014).
Birkett et al. (1989) also noted that the amount of total iron in whole rock analyses was similar to the silicate rocks; the calcsilicate mineralogy suggests that, likely during metamorphism, the iron migrated to the original sulphides, changing pyrrhotite to pyrite, and deriving low-Fe calcsilicate minerals. Another point was that vanadium (V) was enriched in the phlogopite mica near the graphite, which is consistent with a sedimentary origin for the carbon since V is commonly scavenged by carbon in other sedimentary carbon-rich deposits.
The margins of the graphite lenses and bands are sharp to rapid grade changes with background graphite on the order of <1% graphite increasing to ~5% graphite near the lenses contact. Grades within the lenses range from 5-60% graphite with thin waste bands included. The lenses form continuous elongated horizons from 90 to over 300 m in length based on the limited geometry of the target horizons tested to date. The depth of the graphite rich lenses ranges from 40 to over 120 m on the down dip plane, while thicknesses of individual graphite rich horizons range from < 1.5 m to up to 70 m in the fold noses (typically 20-30 m thick).
The mineralisation has been categorized by Focus into three (3) types: massive, semi-massive and low-grade mineralisation categories. All three (3) types are intercalated within the mineralized envelope (repetition of several massive horizons with semi-massive and low-grade type horizons) with both edges of the deposit characterised by low grade type mineralisation. The massive type forms metric scale bands (up to 25 m thick) that contain more than 60% graphite with up to 15-20% sulphides. The semi-massive type contains 20 to 60 percent graphite and is characterised by metric to decametric horizons intercalated with the massive and low-grade types. The low-grade type (5-20% graphite) forms horizons a few meters thick that are intercalated with the two previous mineralisation types and is present on both eastern and western edges of the deposit forming a zone of 5-10 m of transition between deposit and barren host rocks. Transition from the low-grade type to the barren quartzo-feldspathic paragneiss is often less than 1 m.
Graphite occurs as flakes ranging from 2 mm to very fine grain size in hand sample. Commonly the coarser flakes appear to be associated with Cg grades below ~25% and finer flakes above that. The industrial term for coarse flake is 0.2 mm (200 microns), so that even “fine-grained” to the eye can still provide high quality natural flake graphite.
Birkett, et al. (1989) and Tremblay (2014) observed that the graphite occurs in four (4) modes:
1. Independent grains with coarse to medium flakes > 0.7 mm. These are disseminated flakes up to 2 mm in size and rosette clusters up to 9 mm in size.
2. Independent grains in the fine-grained category (<0.7 mm ) includes the higher-grade graphite with ribbons of coarsely crystalline graphite.
3. Graphite inclusions in gangue minerals as scattered fine grains may be relicts of the original, unmetamorphosed graphite protected from metamorphic recrystallisation.
4. Graphite inclusion interlayered with mica, mainly muscovite.
The independent coarse grains (Type 1) are observed within the massive, semi-massive and lowgrade types of mineralisation. Low-grade mineralisation contains only large, isolated flakes. Fine flakes (Type 2) can be found in semi-massive mineralisation but are largely associated with massive mineralisation. Fine flakes of Type 3 represent only a weak proportion of the overall flake categories and Type 4 can be observed within all 3 types of mineralisation associated with schistose rocks.