Sulfur and oxygen isotopic evidence of country rock contamination in the Voisey's Bay Ni–Cu–Co deposit, Labrador, Canada

Abstract

The emplacement of basaltic magma into sulfide-bearing country rocks provides a favorable geologic environment for magmatic sulfide ore formation related either directly to assimilation of country rock sulfur or indirectly to a depression of sulfide solubility caused by assimilation-induced changes in magma composition. Pelitic country rocks of the Proterozoic Tasiuyak Gneiss in the area of the Voisey's Bay Ni–Cu–Co deposit contain sulfidic layers that may have provided sulfur to basaltic magmas during emplacement of the Voisey's Bay intrusion. Sulfur isotopic compositions of the Tasiuyak Gneiss range from −0.9 to −17.0‰, values typical for sulfides produced via bacterial sulfate reduction in an open marine environment. Archean gneisses in the area contain low amounts of sulfide and are less likely to have served as a source of externally-derived sulfur. Sulfur isotopic compositions of the sulfide minerals from the Voisey's Bay deposit show consistent variations, both spatially and with rock types. Disseminated and massive sulfides show a decrease in δ 34 S to the west, with values typically between 0 and −2‰ in the Eastern Deeps, Ovoid, and Discovery Hill zone, and between −2 and −4‰ in the Reid Brook zone. δ 34 S values of the Mushua intrusion to the north and the Normal Troctolite in the Eastern Deeps are more positive, ranging between −0.5 and 1.8‰. This range is taken to represent the isotopic composition of primary mantle-derived sulfur in the area because the Mushua intrusion and Normal Troctolite show the least geochemical evidence for contamination by country rocks. Sulfur isotopic data from the Reid Brook zone are consistent with up to a 50% sulfur contribution from the Tasiuyak Gneiss. Correspondingly lower proportions are indicated for the eastern portion of the deposit where country rocks are predominantly low-sulfide enderbitic and quartzofeldspathic gneisses. Oxygen isotopic values of gneiss fragments in the Basal Breccia Sequence and Feeder Breccia suggest that the assimilation process involved a greater proportion of high− 18 O contaminant to the west. δ 18 O values of the Tasiuyak Gneiss (5.9 to 14.0‰), enderbitic gneiss (6.4 to 8.7‰), and Archean quartzofeldspathic gneiss (9.5 to 9.7‰) are consistent with an increased proportion of Tasiuyak Gneiss contaminant to the west. Isotopic data strongly indicate that sulfur from the Tasiuyak Gneiss has been involved in ore deposition at the Voisey's Bay deposit. However, sulfur and oxygen isotopic data also strongly suggest that the addition of externally derived sulfur was not the sole process responsible for mineralization, and that assimilation of both Proterozoic and Archean country rocks played a key role in depressing sulfide solubility prior to sulfide localization via dynamic, physical mechanisms.