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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 south trend.

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.