A compiled geological, geochemical, and metallogenic study of a magmatic nickel-copper sulphide occurrence at the Cirque Property, Nain Plutonic Suite, Northern Labrador

Dwyer, Berni Lori (2001) A compiled geological, geochemical, and metallogenic study of a magmatic nickel-copper sulphide occurrence at the Cirque Property, Nain Plutonic Suite, Northern Labrador. Masters thesis, Memorial University of Newfoundland.

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Abstract

Located approximately 40 kilometers south of Okak Bay, the Cirque property lies within an area spanning 40 km long by 5-10 km wide that contains several localized, magmatic Ni-Cu sulphide occurrences. With the exception of the OKG prospect located approximately 35 km north of the Cirque prospect, most of the magmatic Ni-Cu sulphide occurrences are hosted within Mesoproterozoic varieties of leucotroctolite, leuconorite, leucogabbro, and anorthosite of the Nain Plutonic Suite. -- The Cirque property (LBN, Licence #1764M), so named because the gossan lies within a 1 km wide cirque, consists dominantly of massive and layered anorthosite and lesser amounts of leuconorite/leucogabbro and leucotroctolite. Outcrop exposure has defined the mineralized zone to strike approximatelyl km northwest and dip to the west Ground and borehole geophysical surveys have defined a corresponding narrow zone (250 to 300 m wide) plunging steeply to the southeast. Six drill holes at the top and two of six drill holes at the base of the cirque have intersected sulphide zones of variable widths and textures. Mineralization consists of massive, net, and disseminated pyrrhotite with minor chalcopyrite. Trace pentlandite occur as fine flame-lamellae within massive pyrrhotite. -- Overall, the metal contents in all of the holes are low (average Ni/Cu = ≤ 1, Pd and Pt values <50 ppb). The best intersection is from borehole LBN-4 (390.35m): 0.28% Ni, 0.44% Cu, and 0.12% Co over 5.1 m. Localized, elevated Cu values were intersected in several boreholes but were not extensive; the greatest is 3.9% Cu over 20 cm in a 1.5 cm wide sulphide vein @ 923.92 m in borehole LBN-8. Values from neighbouring properties to the north (Canadian States Resources; Licence #1514M) and west (Noranda Mining and Exploration Ltd.; Licence #915M) are anomalous; highest values from grab samples are 0.36-0.85% Ni and 1.8% Ni, respectively. -- Sulphide mineralization at the Cirque gossan consists of thin, irregular lenses and stringers, steeply dipping to the west and east, wrapping around and crosscutting iron stained, unmineralized leucoanorthosite. Fractured and embayed edges of plagioclase crystals by sulphide minerals at the Cirque gossan suggest thermal erosion of semi-crystallized plagioclase-rich fractionate by a Fe-rich sulphide melt which had mobilized from depth. Although not common, thermal erosion textures have also been reported on the neighbouring Canadian States Resources property (Licence #1514M). This epigenetic-style of magmatic sulphide mineralization is important because, although the host magma for the sulphide mineralization is absent, analyses can still be made based on the geology and geochemistry of the gossan. -- Major and trace element geochemistry of the rocks at the Cirque indicate that the there are at least two magma sources: one formed anorthosites, leucotroactolite, and leuconorite/leucogabbro with similar REE signatures and positive Eu anomaly and another, richer in incompatible elements with a slight negative Eu anomaly, formed the ferrodiorite dykes which intrude the anorthosites. The parental (or least fractionated) material for the anorthosites, leucotroctolite, leuconorite/leucogabbro could possibly have a geochemical composition similar to a clinopyroxenite sample located in talus along the cirque wall. Isotopic data (ISr = 0.704 to 0.706 and εNd = -8.76 to -2.72, calculated at 1.39 Ga, and δ³⁴SCDT= 2.5 to 3.0‰) indicate that the crustal contaminants of the magma in the Cirque area are the Paleoproterozoic Churchill Province gneiss and the Nain Province gneiss. The Tasiuyak gneiss may also be a possible source, but to a lesser extent. Estimated total crustal contamination is between 20 to 60%. -- Sulphide geochemistry of the Cirque area indicate that the Canadian States Resources property, Licence #1514M may have a different source magma than that of the Cirque sulphides, however, the epigenetic-style mineralization on the Noranda "Hilltop" property suggests that the source of the sulphides may be to the west where pyrrhotite-bearing pyroxenite sill and dykes have been observed. Low R factors (25 to 100) and PGE ratios indicate that the initial sulphide melt at the Cirque may have segregated from a metal-poor magma possibly due to a low degree of partial melting and extensive crystal fractionation history. Nonetheless, given that extensive zone of localized anomalous Ni and Cu values, it is still possible that a significant Ni-Cu sulphide occurrence may exist in the surrounding area.

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