Abstract
The Voisey’s Bay nickel-copper-cobalt (Ni-Cu-Co) sulphide deposits constitute a significant resource of orthomagmatic mineralization. The deposits are not exposed at the surface except for in a small ferruginous gossan (Discovery Hill). The subsequent geophysical surveys and diamond drilling led to the discovery of the Ovoid ore body, buried beneath 20 m of till, and other deeper deposits in the bedrock. This study was initiated to characterize the sulphide mineralogy of these deposits through various stages of weathering, erosion, and transport. Because the samples ranged from bedrock through to a variety of surficial sediment types, the automated SEM-based identification provided by the MLA-SEM system was the ideal technique to quantitatively evaluate mineral distributions in the different media. The derived MLA-SEM data indicate that, aside from the Discovery Hill gossan, the surface sulphide mineralization at Voisey’s Bay was weathered in a pre-glaciation regolith at the Mini-Ovoid deposit and, on the surface of the Ovoid deposit, the massive sulphide was unoxidized due to a thin calcite-cemented clay cover. Pentlandite is very preferentially oxidized compared to other sulphides in the Voisey’s Bay ore, to depths of up to 10 m in bedrock. Conversely, within the coarse reject samples of crushed drill cores stored in sealed plastic bags, pyrrhotite was altered, whereas pentlandite and chalcopyrite are stable, presumably due to anaerobic reactions. The MLA-SEM detected trace amounts of minute sulphide grains in surficial sediments, but their contents abruptly decreased with distance from the sulphide mineralization. Microtextures such as troilite and pentlandite exsolution or twinning in pyrrhotite, however, could be observed in the fine sulphide grains from till, suggesting a derivation from orthomagmatic sulphide material, such as the Voisey’s Bay mineralization.
Highlights
The Voisey’s Bay Ni-Cu-Co orthomagmatic sulphide deposits (Figure 1) were discovered in 1993 when a small gossan (Discovery Hill) was sampled by prospectors [1,2,3,4,5,6,7,8].The Ovoid deposit was discovered in a valley below Discovery Hill through geophysical surveys, buried beneath 10–20 m of glacial till [9]
The hole was re-logged in a simple fashion (Figure 2) and thirteen samples were removed from the core and prepared as polished thin section and another portion was crushed for a whole-rock geochemical analysis
The exposed sulphide at Discovery Hill was oxidized into a gossan which led to the discovery of the buried massive sulphide deposits at the Mini-Ovoid and Ovoid deposits, and to the massive sulphide bodies in deeper crustal rocks
Summary
The Voisey’s Bay Ni-Cu-Co orthomagmatic sulphide deposits (Figure 1) were discovered in 1993 when a small gossan (Discovery Hill) was sampled by prospectors [1,2,3,4,5,6,7,8].The Ovoid deposit was discovered in a valley below Discovery Hill through geophysical surveys, buried beneath 10–20 m of glacial till [9]. The Voisey’s Bay Ni-Cu-Co orthomagmatic sulphide deposits (Figure 1) were discovered in 1993 when a small gossan (Discovery Hill) was sampled by prospectors [1,2,3,4,5,6,7,8]. Geological models for the deposit suggest that it was much larger when it formed [3], but the upper portion was essentially sliced off and removed by the last period of glaciation, ca. When the surface of the Ovoid was exposed at the start of the mining operation it was found to be remarkably intact, with no alteration/oxidation whatsoever, as opposed to the Discovery Hill gossan. The mine has evident and probable geological resources (2018) of 32.4 million tonnes of ore grading
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