MAGMATIC SULFIDE MINERALIZATION AT THE VOISEY'S BAY NI-CU-CO DEPOSIT, LABRADOR: SE/S RATIOS AND C-S ISOTOPIC STUDIES OF COUNTRY ROCKS AND ORE

Much of the Ni-Cu-Co sulfide mineralization at the Voisey's Bay deposit occurs in troctolitic to gabbroic dikes that link or are related to two larger troctolitic bodies. Country rocks for the sulfide-silicate intrusive system include Archean-aged orthogneiss, and Paleoproterozoic ortho- and paragneiss. Considerable debate has been generated over the possible role of Paleoproterozoic Tasiuyak paragneiss in providing sulfur to the magma system. ³⁴S values of the sulfide ores are only locally anomalous (values of -2 to -4‰ in the western Reid Brook zone, whereas values to the east are -2 to 1‰). Additional studies of magma contamination at Voisey's Bay are in progress, and we here report new ³⁴S measurements, and both ¹³C values and S/Se ratios of ores and country rocks. An expanded study of sulfur isotopic distribution in the Tasiuyak gneiss has shown that the range of values is much larger than initially proposed. ³⁴S values in the paragneiss range from -17.0 to 18.3‰, with a mode at -6.5‰ and an average of -0.9‰. It is clear that our evaluations must now address if local isotopic heterogeneity should be expected to be preserved in a dynamic magma system, and how fast S isotopic exchange may occur in such systems. Values of Se/S x 10⁶ for the sulfide ores range from 113 to 273, whereas values in the Tasiuyak gneiss show a broad range of values <~400. Se/S ratios are consistent with S introduction from the Tasiuyak gneiss, but are not conclusive. C isotopic values for all of the rocks in the Voisey's Bay Intrusion are similar to those found in the Tasiuyak gneiss. ¹³C values of the paragneiss are in the range expected for organic-C-rich sedimentary rocks (-22 to -32‰). Carbon abundance in the igneous rocks is not high (combined low-T and high-T carbon < 1800 ppm), but ¹³C values similar to those in the paragneiss suggest that all of the magmatic system experienced at least low degrees of C contamination by country rocks, most likely as a result of devolatilization reactions.