IDENTIFICATION OF CUMULATES AND TRAPPED RESIDUAL LIQUIDS IN THE VOISEY'S BAY OVOID NI-CU-CO MAGMATIC SULFIDE DEPOSIT

The 1.3 Ga Voisey's Bay Ni-Cu-Co magmatic sulfide deposit, located in northern Labrador, Canada, consists of several mineralized zones hosted by troctolites and gabbros of the Nain Plutonic Suite. The near-surface Ovoid is a bowl-shaped mass of massive sulfide approximately 800 metres long by 350 metres wide and up to 125 metres thick. It contains 32 million tonnes of proven and probable reserves at 2.8% nickel, 1.6% copper and 0.14% cobalt, and is presently being open-pit mined. Disseminated sulfides are dispersed in the silicate host rocks surrounding the massive ore.

We have used an algorithm to calculate normative mineral abundances in the Ovoid based on 3175 assays from spatially controlled drill core samples. The calculations were verified by comparison with mineral modes determined on selected samples using a Mineral Liberation Analyzer (MLA), and integrated with textural data collected by optical microscopy and the MLA. The results indicate that the Ovoid is zoned with a central core rich in magnetite, but poor in pyrrhotite and with moderate amounts of chalcopyrite and pentlandite; surrounded by a pentlandite- and chalcopyrite-rich mantle containing moderate pyrrhotite and magnetite; and underlain by a basal and marginal zone rich in massive pyrrhotite with little chalcopyrite, moderate pentlandite, and low to moderate magnetite. Most chalcopyrite and pentlandite is present on both macroscopic and microscopic scales as “loops” surrounding massive pyrrhotite grains. The loops are most common above the pyrrhotite-rich, basal/marginal zone.

Quantitative modelling of Ni and Cu tenors in the ores suggests that the mineralogical zonation in the Ovoid is the result of fractional crystallization of monosulfide solid solution (mss), which formed preferentially along the cooler walls and base of the body. Some of the residual sulfide liquid enriched in Cu and Ni was trapped along grain boundaries between the mss cumulates, but most was concentrated into the hotter, more slowly cooling centre of the body. The mss cumulates and residual liquids are now represented by pyrrhotite grains and chalcopyrite-pentlandite loops, respectively. Disseminated ores have Ni and Cu tenors consistent with crystallization from small amounts of residual liquid that escaped from the main sulfide mass into the silicate host rocks.