Chalcophile Element Heterogeneity in Ni-Cu-(Platinum Group Element) Orebodies of Raglan Horizon in Cape Smith Belt: Implications for Ore-Forming Processes

Abstract

Abstract The komatiitic Raglan Horizon in northern Quebec hosts one of the largest komatiite-associated magmatic sulfide deposits in the world. Several studies have suggested that the sulfide ores were formed by melting and transporting sedimentary sulfides from upstream after the komatiite melt had assimilated a large amount of S-rich country rocks. In this study, we report new compositional data of the sulfide ores in Zones 5-8 and 13-14 along with the legacy assay database to investigate variability of the ore-forming processes at Raglan at the camp scale. More than 90,000 drill core samples were selected, and the rocks are categorized into disseminated, net-textured, and massive sulfide based on their sulfide abundance. We demonstrate that the majority of the compositional variability of the sulfide-bearing rocks can be attributed to the difference in the mass ratio between silicate and sulfide melts (R factor), by applying thermodynamic modeling of silicate and sulfide magma evolution. The massive sulfides display depletions in Cu, Pd, and Pt that are best explained by sulfide liquid fractionation, which differentiates the residual Cu-, Pd-, and Pt-rich sulfide liquid from the Ni-rich monosulfide solid solution (MSS) cumulate now preserved within the massive sulfide ores. A small population of sulfide-bearing samples that is strongly depleted in all the economically important metals has been interpreted to represent externally derived xenomelts that were poorly equilibrated with the metal-rich host magma due to low residence time. A portion of massive sulfides has been found to be depleted in Pd and Cu but not Pt. This decoupling of these metals can be well explained by a postmagmatic process that persisted to temperatures as low as 300°–350°C. Similar depletions of Pd have also been observed in the Expo Intrusive Suite immediately to the south. The regional variations in the compositions of individual orebodies suggest that they occurred in highly localized environments that record only local magmatic conditions and that the sulfide mineralization did not reach equilibrium with a single regionally continuous silicate magma system. Each zone preserves a unique signature of assimilation, sulfide segregation, and sulfide crystallization; hence, they likely represent separate magmatic pulses within the larger Raglan system. Lastly, based on the spatial locations of the low tenor sulfides, we tentatively suggest that Zone 2 and the Katinniq zone are more proximal to the as-yet-unidentified eruption site of the Raglan komatiite lava flow field.