Reevaluation of the Role of External Sulfur Addition in the Genesis of Ni-Cu-PGE Deposits: Evidence from the Nebo-Babel Ni-Cu-PGE Deposit, West Musgrave, Western Australia

Abstract

The Nebo-Babel Ni-Cu-PGE sulfide deposit, located in the West Musgrave Block, Western Australia, is hosted within a tube-like, concentrically zoned, gabbronorite magma conduit emplaced at 1068 Ma. The Nebo-Babel deposit is associated with the ca. 1078 Ma Giles Complex which is part of the Warakurna large igneous province. Previous geologic and geochemical studies have shown that the Nebo-Babel consists of chemically related but temporally distinct magma pulses. The initial magma pulses, which were the most primitive and the most contaminated, formed the marginal units of the conduit. The later pulses were emplaced into the core of the conduit. The chilled margin rocks contain sulfide globules. Mineralization is confined to the early magma pulses and consists of massive sulfide breccias and stringers and disseminated gabbronorite-hosted sulfides. The Ni tenor of the massive sulfides, which have undergone sulfide liquid fractionation, and the disseminated sulfides are almost the same, indicating that both formed at similar R -factors. The whole-rock massive and disseminated sulfides contain ~650 ppb and 60 to 500 ppb total PGE, respectively. All of the Nebo-Babel rock units are slightly PGE depleted with Cu/Pd ratios higher than primitive mantle, indicating early saturation and segregation of a sulfide and/or PGM. Rocks with trace cumulus sulfides have the lowest Cu/Zr and highest Cu/Pd ratios and the most fractionated metal patterns, indicating that these units have undergone the highest degrees of Ni, Cu, and PGE depletion. The average δ 34S value of all Nebo-Babel disseminated and massive sulfides is 0.4 ± 0.2 per mil, which is consistent with the S originating in the mantle. Sulfur saturation did not occur in situ and is inferred to have been triggered by assimilation of felsic orthogneiss, perhaps in the parental magma chamber. This initial magma pulse with entrained sulfides formed marginal units of the Nebo-Babel intrusion. Some of the entrained sulfides accumulated within physical traps on the way into the conduit and/or at the exit from the chamber. These sulfides were later emplaced along the footwall contact as massive and/or marginal fragment-laden sulfide liquid. Distribution of the disseminated sulfides was influenced by the conduit geometry and magma flow dynamics. The final and volumetrically dominant barren magma pulse underwent crystal fractionation and oxide precipitation in the core of the Nebo-Babel intrusion, which in turn caused sulfide saturation and formed trace but PGE-depleted sulfides in these units. The Nebo-Babel deposit has a number of features in common with other Ni-Cu-PGE deposits hosted in dynamic magma conduits (e.g., Voisey’s Bay), such as multiple magma pulses and sulfide entrainment from depth, rather than in situ sulfide segregation. However, Si addition was the primary cause of sulfide saturation and external S addition did not play a role in the genesis of the Nebo-Babel deposit.