Chromite is mined almost exclusively from massive to semi massive accumulations in ultramafic
or mafic igneous rocks. Hard rock chromite deposits are normally assigned either to stratiform or
podiform on the basis of deposit geometry, petrological character, and tectonic setting.
Stratiform chromite deposits are sheet-like accumulations of chromite that occur in layered
ultramafic to mafic igneous intrusions. The best examples of stratiform chromite deposits in
Canada occur in the Bird River Sill in south-eastern Manitoba and in the Big Trout Lake
intrusion in north-western Ontario. Other intrusions in Canada with chromitite layers include the
Muskox complex in the Northwest Territories, the Lac des Montagnes body in Quebec, and the
Puddy Lake and Crystal Lake intrusions in Ontario.
There is no past or current production from stratiform chromite deposits in Canada apart from
approximately 6000 tons of material which was mined at Puddy Lake, Ontario in the 1930’s.
Stratiform chromite deposits occur in large, layered intrusions which are commonly
differentiated into a lower ultramafic zone and an upper mafic zone. The intrusions fall into two
broad categories with respect to morphology. The first includes essentially tabular bodies which
were emplaced as sill-like intrusions in which igneous layering is conformable to the floor.
Examples include Kemi, Campo Formoso, Stillwater Complex, Bird River Sill, and Big Trout
Lake.
The intrusions which host stratiform chromite deposits occur in a variety of tectonic settings. The
Bushveld Complex, Great Dyke, and Muskox Intrusion are unmetamorphosed and were
emplaced into stable cratonic settings. The Kemi and Campo Formoso intrusions are
prekinematic and occur at the unconformable contact between Archean granitic basement and
overlying, mainly sedimentary Proterozoic supracrustal rocks. The Bird River Sill and Big Trout
Lake body are synvolcanic intrusions in Archean greenstone belt settings.
Most stratiform chromite deposits comprise laterally extensive chromite-rich layers which,
despite local irregularities are generally conformable to and form an integral part of the igneous
layering that characterizes such intrusions. The individual chromite rich layers range from less
than 1 cm to more than 1 m in thickness, but their lateral extent is measured in kilometres or tens
of kilometres. Chromite bearing horizons may be interlayered with a variety of rock types
including dunite, peridodite, orthopyroxenite, anorthosite and norite and may occur at various
stratigraphic levels within the host layered intrusion. However because chromite in the most
primitive rocks tends to be the most Cr –rich mineral species the immediate host rocks of
economically significant chromitites are peridotites, or less commonly, pyroxenites. Orebodies
may comprise discrete layers of massive chromitite rocks as in the Bushveld complex and Kemi
deposit or a number of closely spaced chromite-rich layers separated by UM rocks.
The association of chromite with magmatic platinum group element deposits is well known. The
association is two-fold. Firstly, the intrusions hosting the most prominent PGE “reefs” (Bushveld
Complex, Stillwater Complex, and Great Dyke) also contain significant stratiform chromite
deposits, normally at lower levels in the igneous stratigraphy.
7.2.1 BLACKBIRD ONE CHROMITE DEPOSIT
The following description is taken directly from a report by Dr. James E. Mungall, Noront’s
Chief Geologist. In the winter of 2008, Noront encountered massive chromitite mineralization in
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boreholes drilled to test airborne anomaly AT2 on the Double Eagle Property. The AT2 anomaly
is a coincident magnetic and conductive feature that was recognized as paired linear AeroTEM
conductors striking parallel to the margins of a highly magnetic body several kilometres long.
The magnetic body is connected directly to the Eagle One magmatic massive sulfide deposit and
as such was considered a prime target for further discoveries of peridotite-hosted magmatic
sulfide mineralization. Diamond drilling into the conductive anomalies confirmed that the
magnetic anomaly corresponds to a large body of metadunite and metaharzburgite that has been
altered to magnetite-rich tremolite serpentinite and talc-carbonate rock. Drill holes encountered
extensive Ni-rich sulphide mineralization hosted by shear zones parallel to the contact between
the ultramafic rocks and their felsic plutonic (granodiorite, sensu lato) host rocks. The sulphide
deposit at the AT2 anomaly area was named the Eagle Two deposit.
Below the Eagle Two shear-hosted sulphide deposit the drilling unexpectedly intersected
chromite mineralization. The chromite mineralization has been named the Blackbird One
Deposit. Blackbird One mineralization consists of massive chromitite layers interbedded with
chromite-rich metadunite, now entirely replaced by talc carbonate minerals, chromite, and minor
ferrochrome overgrowths.
Several drill holes intersected massive chromitite mineralization. The layers vary widely in
thickness, from centimetres on the margins of the Blackbird One deposit to continuous massive
chromitite intersections approaching true thicknesses of 30 metres at its central axis. The
mineralization is thickest along an axis plunging steeply to the northwest from a near-surface
expression near the southeast extremity of the drill pattern. To the southwest of this axis the
mineralization thins rapidly to nothing; along the northeast side there are insufficient data to tell
the shape of the body at the time of writing.
The chromite mineralization at Blackbird One shows several different textural and structural
styles. In the host ultramafic silicate rocks there is abundant chromite which stands out in drill
core as isolated or disseminated submillimetric black euhedra in the white talc-carbonate host
rock. The modal abundance of chromite varies from less than 1% to as much as 25% and locally
shows evidence of primary layering. When chromite abundance reaches 25% the rock typically
shows antinodular texture, with submillimetre chromite grains forming a closely packed matrix
around larger pseudomorphs of olivine. Within domains showing disseminated or antinodular
texture there are common cognate xenoliths of chromitite or dunite up to several centimetres in
size. Where xenoliths are larger than the diameter of the core they may become difficult to
distinguish from primary layers, however, most primary layering preserves some combination of
centimetric layering and fine laminations, which make identification unequivocal.
The chromitite mineralization does not have a notably strong magnetic susceptibility, compared
with serpentinized dunite and peridotite which are both common in the area around Eagle One,
Eagle Two, and the Blackbird One Deposits. Chromite is an electrical insulator hence there is no
EM expression from the chromite deposit despite the presence of traces of interstitial sulfide
minerals in the massive chromitite.
A useful characteristic of chromite is its high density, around 4.5, which is similar to that of
magnetite and pyrrhotite. Massive chromite therefore has an anomalously high density
compared even with ultramafic rocks and is detectable by gravity survey when it exists in
sufficient tonnages.
The demonstration of a detectable gravitational response to the massive chromitite at the
Blackbird One Deposit has been used to infer the presence of another body of chromitite some
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900 metres long striking 60° away from the AT2 area. A single diamond drill hole through the
highest part of the density anomaly at line 4000E cut two bodies of massive chromitite, each
with apparent thicknesses of approximately 20 metres. The new mineralized zone has been
named the Blackbird Two Deposit.
Massive layers of chromitite vary in thickness from centimetres to apparent thicknesses
exceeding 70 metres and true thicknesses probably exceeding 30 metres. Within massive layers
there are occasional xenoliths and thin beds of talc after olivine. The chemical composition of
the chromitite mineralization has been assessed by three methods; whole-rock assay by
instrumental neutron activation analysis (INAA), electron microprobe analysis of individual
chromite grains (EMP), and hand-held semi-quantitative X-ray fluorescence spectrometry (HXRF).
Assay results through the chromitite zone in drillhole 1G17 are shown in Figure 7.3. Cr was
determined by INAA and is not subject to matrix effects; the results are therefore considered to
be quantitative. The main mineralized zone remains well above 40% Cr2O3 for more than 30
metres, and includes a zone greater than 5 metres wide that averages above 45% Cr2O3. Similar
results have been reported in other holes, and further assay results are pending. The Cr/Fe
elemental weight percent ratio in the whole rock is consistently above 2 and averages about 2.6
over the massive chromitite interval.
Electron microprobe analyses were conducted on polished thin sections of samples taken at
intervals through several chromitite intersections. Ten grains were analyzed at random from
each thin section. Notable results include the observations that the Cr/Fe ratio of the chromite
ranges from 1.6 to 2.2, and that the Cr2O3 concentration in a chromite mineral concentrate
would exceed 52%.
DDH NOT 08 1G17
175
185
195
205
215
225
235
2450.0 10.0 20.0 30.0 40.0 50.0
Cr2O3 (wt%)
Depth in hole
Series1
Figure 7.3: Concentration of Cr2O3 (wt%) in chromite
mineralization in hole NOT- 08-1G17. Note sharp increase from
background levels near 4% Cr2O3 to a plateau above 40%,
including several metres above 45% Cr2O3.
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Figure 7.4: Cross Section 1987 through Blackbird One Chromite Deposit
Figure 7.5: Cross Section 2025 through Blackbird One Chromite Deposit
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7.2.2 ECONOMIC CONSIDERATIONS FOR CHROMITE MINING
Annual production of chromite ores is approximately 19 Mt, 90% of which is used to make
ferrochrome alloy for use in steel. Chromium ores and products are sold in three principal forms
worldwide. Direct shipping ore is referred to as lumpy ore and is sold by contract only. The
lump ore is defined as the > 6 mm size fraction; < 6 mm fractions are sold as fines and carry a
lower price. High-grade lump ore can be fed directly into smelters and there is a global trade in
this material, particularly from developing countries to a growing metallurgical market in China.
Fine-grained chromite concentrates are not amenable to smelting and must be pelletized first;
most pelletizing operations are integrated with smelters and there is little trade in concentrates.
However, the bulk of trade in chromium products is in the ferrochrome product of smelting.
Most ferrochrome is produced by vertically integrated large-scale mining, pelletizing and
smelting operations that produce the ferrochrome on site or very near to the mine site.
The dominant player in the world ferrochrome market is South Africa, which accounted for 43%
of global production in 2005. Major producers include Assmang Ltd., Kermas Group Ltd.,
(formerly Samancor Chrome Mines, South Africa), Xstrata South Africa Proprietary Ltd., and
International Ferro Metals Ltd. Most ferrochrome produced in South Africa is made from ores
mined from the stratiform chromitite deposits of the Bushveld Complex of South Africa.
Annual production in 2005 was 7.645 Mt. These ores have moderate grades compared with
some smaller deposits worldwide but are economic due to economies of scale and simple deposit
geometry. Other important producers are India (3.640 Mt), Kazakhstan (3.566 Mt), Finland
(0.572 Mt), Turkey and India.
Lump ore prices are reported in units called dry metric ton units (dmtu). The price of an ore is
found by multiplying the concentration of Cr2O3 in wt% by the price in dmtu; for example, a
Turkish lump ore containing 39% Cr2O3 was sold in May 2008 at a price of USD14.97/dmtu for
a price of $583/Mt ex ports (https://www.asianmetals.com). The value of lump ore increases
dramatically as the grade increases above 30%; no significant international trade takes place at
grades below this value. The value of a concentrate containing 50% Cr2O3 is approximately
$750/ton (https://www.asianmetals.com).
The smelting process adds considerable value to chromite ores if a deposit is large enough to
justify the capital cost of a smelter. The product of smelting is ferrochrome alloy, whose price
has rapidly increased in recent years due to interruptions of the electricity supply in South Africa
and mounting demand from China. The Q3 contract price for high-carbon 65% ferrochrome is
$US $2.05, up from about $US 0.60 in mid-2005
(https://www.metalprices.com/FreeSite/... The contained metal value in a ton of
ore at 40% Cr2O3 therefore is $1,890 a considerable increase over the May 2008 lump ore value
of about $600 ex ports.
The closest geological analogs to the Blackbird One Deposit are the Kemi Mine in Finland, the
Ipueira-Medrados Deposit in Brazil, and the Sukinda Valley Deposit in India. All consist of
massive chromitite bodies up to several tens of metres thick and hundreds of metres long, hosted
by ultramafic rocks predominantly consisting of dunite and harzburgite, and all are currently
being mined.
The Kemi Mine is the best described of these similar deposits. It is a massive body 40 metres
thick dipping 70°. Ore reserves in January 2006 were 41.1 Mt grading 24.5% Cr2O3 and
Inferred Resources of 86.1 Mt at 29% Cr2O3. The Cr/Fe ratio is 1.53. From 1968 to 2005
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mining was from an open pit, but with stripping ratios becoming intolerable in the late 1990's, an
underground operation was begun, and since 2006 all production has been from underground,
(Duke, 1998).
Production from Kemi in 2005 was 1.1 Mt, recovering 572,000 t of lumpy ore (36% Cr2O3) and
metallurgical grade concentrate with a grade of 44% Cr2O3, which were used to produce
235,000 t of ferrochrome (Duke, 1998).
Grades at Noront’s Blackbird One compare favourably with those at the world's most important
chromite producers. The deposit is the latest example of the Kemi Deposit type, represented by
important producing mines on three continents that collectively account for approximately one
third of global chromite production. A geophysical anomaly known to correspond to the
presence of massive chromitite persists for more than one kilometre along strike from the
Blackbird One Deposit, indicating that the potential exists for a multimillion ton resource
comparable in size to the Kemi, Sukinda Valley, or Ipueira-Medrados Deposits.