The Nebo-Babel Ni-Cu-PGE Sulfide Deposit (West Musgrave Block, Australia): Pt. 2. Constraints on Parental Magma and Processes, with Implications for Mineral Exploration

Abstract

The Nebo-Babel Ni-Cu-platinum-group element (PGE) sulfide deposit (West Musgrave, Australia) is hosted in a gabbronorite chonolith emplaced into sulfur-free country-rock orthogneiss at ca. 1068 Ma. Five different types of mafic dikes are found in the area or are spatially associated with the Nebo-Babel intrusion. The mafic dikes are divided into three groups based on petrography, whole-rock major, trace, and PGE chemistry, Sm-Nd isotope ratios, and silicate mineral chemistry: (1) low Ti basalts (NB-1, NB-2, and NB-3); (2) high Ti basalts (NB-4); and (3) alkali basalts (NB-5). Recent age constraints on similar magma suites intruding in the west Musgrave indicate the low and high Ti basalts are coeval with the Nebo-Babel intrusion and hence can potentially be equivalent to its parental magma composition(s). Based on this, we use whole-rock major, trace, and PGE chemistry, Sm-Nd isotope data, silicate mineral chemistry, and geochemical modeling to constrain the petrogenesis of each mafic dike type and its potential relationships with the Nebo-Babel intrusion. Our results indicate that crustal contamination did not play a major role in the generation or evolution of the different magma series. The geochemical variations observed are rather interpreted to reflect different mantle source compositions and different degrees of partial melting. The low Ti basalts exhibit subduction-related geochemical signatures and are interpreted to have been generated by 5 to 10 percent partial melting of a hydrous spinel-bearing mantle. The melting is inferred to have been triggered as a result of mantle plume impingement in an area of the mantle that has previously been metasomatized. The high Ti basalts are interpreted to have formed by mixing of the sublithospheric mantle with deeper asthenospheric mantle melts from the mantle plume. The decompressional melting of the plume head led to the formation of alkali basalts (characterized by typical ocean island basalt (OIB)-like compositions) generated by 4 to 5 percent of partial melting of a garnet-bearing lherzolite. Our results show that the Nebo-Babel intrusion and its associated Ni-Cu-PGE sulfide mineralization formed between the intrusion of the low- and high Ti basalts and may have originated from either mixing between these two magma types, or as a result of continuous change in the melting conditions between these two magmas types. The spatial analysis of the repartition of the different magma suites allowed highlighting zones where the two magma types are spatially associated and where Ni-Cu-PGE sulfide anomalies were discovered, suggesting that magma mixing may be a key factor in the ore genesis at Nebo-Babel. This observation is strengthened by the modeling of the sulfur concentrations at sulfide saturation, which indicates that the assimilation of country-rock orthogneiss by the mixed magma caused a drop in sulfur solubility which in turn led to sulfide saturation in the early stages of the mixing and/or assimilation and potentially to the formation of the Nebo-Babel Ni-Cu-PGE sulfide deposit. Calculated metal concentrations of the parental magma from which the Ni-Cu-PGE mineralization formed are: 165 ppm Ni, 0.15 ppb Ir, 0.30 ppb Ru, 0.22 ppb Rh, 3.40 ppb Pt, 3.30 ppb Pd, and 150 ppm Cu. These metal concentrations are similar to those obtained from the low Ti basalt compositions after they experienced a small amount (<0.001%) of sulfide segregation prior to their final emplacement.