Distribution of Ni-Cu-PGE mineralization in
the Cape Smith Belt
(Ungava Orogen): avenues of exploration
Daniel Bandayayera and Abdelali Moukhsil
Direction de Géologie Québec
The Cape Smith Belt offers exceptional mineral
potential. This area of the North of Québec is the target of
an ever-increasing number of exploration activities. The most recent
results obtained by exploration companies confirm the excellent
potential for discovering of new Ni-Cu-PGE showings. Until now,
exploration was mainly concentrated in the southern part of the
belt. Other more northern sectors, however, also deserve to be explored.
With this in mind, the Department recently defined the geological
trends for each part of the Cape Smith Belt (Raglan Trend).
|The distribution of Ni-Cu-PGE mineralization in the Raglan
Trend displays two distinct sub-trends: north and south. The
north trend is clearly richer in nickel than the south trend,
but the latter contains higher levels of platinum and palladium.
The trends also display different Ni/Cu ratios, with values
of about 1 for the south trend and values ranging from 2
to 54 for the north trend. It is reasonable to assume that
mineralized occurrences within the two trends were not fed by
the same feeder dyke system. The magmas associated with the
north trend are more primitive than those associated with the
||The north trend consists of Ni-Cu-PGE showings
along the Raglan horizon located at the interface between the
Povugnituk and Chukotat groups. It includes a string of 19 massive
sulphide lenses of which the most important are the Katinniq,
Donaldson and Lac Cross deposits. These deposits are found at
the base of komatiitic lavas or are associated with cumulates
or ultramafic sills that crosscut the Chukotat Group.
Nickel grades are generally high and in places
attain 8% over a thickness of about 10 m. Copper contents are
fairly low and rarely reach 1%. Several samples of massive or disseminated
sulphides display average grades of 3 g/t Pd and 1.3 g/t Pg.
According to the classification of magmatic sulphide deposits, the
north trend represents magmatic Ni-Cu sulphide deposits.
|The south trend is mainly associated with
differentiated sills of peridotite-pyroxenite-gabbro and occasional
relatively zoned feeder dykes of peridotitic or gabbroic composition.
It includes several mineralized showings of which the most significant
are Mesamax, Expo-Ungava, TK and Méquillon. Mineralization
is mostly in the form of disseminated to semi-massive sulphides,
with occasional massive sulphides, located at the base of intrusions.
||An alternation between massive and disseminated sulphides
has been noted in places, which suggests a cyclic process for
these occurrences, probably due to repeated injections of magma.
Coarse-grained granular pentlandite accounts for more than 20%
of the observed sulphides . It appears, however, that there
is no difference in Ni-Cu-PGE grades between coarse-grained
and fine-grained granular mineralization.
|Mafic to ultramafic sills are locally intercalated by metasedimentary
horizons. It is therefore not rare to drill a section containing
several levels of massive sulphide. The continuation of a drill
hole will often depend on finding stockwork sulphides in these
metasediments (T. Keast, personal communication).
||In contrast to the north trend, nickel grades of the south
trend rarely exceed 3% Ni, although copper is often locally
recorded at more than 4% over 6 m, including intervals
of 8% over 0.5 m. Ni/Cu ratios for individual samples are
fairly constant, hovering around 1. The relatively constant
Pd/Pt ratios of about 4 in massive or disseminated mineralizations
rise abruptly to 15-20 in horizons that are richer in Cu and
Pd. Also observed is a positive correlation between Cu and Pd.
The highest Pd values (> 10 g/t)
are generally associated with Cu-rich mineralization. The presence
of massive chalcopyrite in the form of veins or thin centimetre-scale
horizons suggests hydrothermal remobilization of Cu and Pd from
primary magmatic mineralization.
Some of the magmatic sulphide deposits in the south
trend containing low nickel grades could therefore prove to be economic
due to their high Pt and Pd contents. Systematic analysis for these
elements is thus strongly recommended in the south zone, even in
areas with very low nickel values.