Petrogenesis and Ni-Cu-PGE sulphide potential of the Bell Rock Range, Giles Complex, central Australia Roland E. B. Seubert, Reid R. Keays, Simon M. Jowitt School of Geosciences, Monash University, Victoria, 3800, Australia Abstract. The layered Bell Rock Range in the Giles Complex, west Musgrave Province, central Australia, is a ~5500 m thick troctolitic intrusive body. It comprises at least 6 subunits with distinct rare earth chemistry, but no cyclicity was observed in the geochemical trends. Each of the different subunits was formed by multiple magma injections which caused the magmatic system to alternate between a S-saturated and undersaturated state. Abundant olivine controls Ni; Cu and PGE are depleted in many layers and their geochemical trends are controlled by sulphide segregation. Saturation with S was most likely achieved by assimilation of crustal rocks. Keywords. PGE, troctolite, sulphide, Giles Complex 1 Introduction The Giles Complex is one of the world’s largest mafic to ultramafic layered igneous complexes, comparable in size to the Bushveld Igneous Complex of South Africa and as such the Giles Complex is considered by many Ni-Cu-Platinum Group Element (PGE) explorers to be highly prospective for such magmatic sulphide mineralisation. However, it is located in one of the least explored and least studied areas in Australia and little economic geology-type research has been undertaken on these intrusive bodies apart from Seat et al. (2007, 2009) on the Nebo-Babel Ni-Cu-PGE sulphide deposit. This study focuses on the geochemistry of the Bell Rock Range, undertaken to understand the igneous evolution and Ni-Cu-PGE prospectivity of the troctolitic intrusions of the Giles Complex. 2 Geological setting The Mesoproterozoic Giles Complex comprises at least 20 mafic to ultramafic layered intrusions of either peridotitic, gabbroic or troctolitic composition. The complex is > 550 km from east to west and > 100 km from north to south, with individual intrusions up to 10 km thick (Glikson et al. 1996; Evins et al. 2010). The tectonic setting in which the intrusions formed is ambiguous. Wingate et al. (2004) suggested that the complex had formed in proximity to the plume head of the Warakurna Large Igneous Province (LIP), a newly defined LIP which is exposed over an area of 1,5 × 10 6 km² throughout central and western Australia. However, Evins et al. (2010) proposed that a mantle plume setting might be an oversimplification and the Giles Complex, as part of the west Musgrave Pronvince, had formed in the failed Ngaanyatjarra Rift, due to the complex magmatic and tectonic history of the area with bimodal volcanism and plutonism over an extended period of min. 50 Ma (Evins et al. 2010). The Bell Rock Range, the focus of this study, is a ~30 km long and ~5500 m thick troctolitic-type intrusion. 3 Analytical methods Detailed sampling along a traverse across the igneous layering has been done. Whole-rock geochemistry was undertaken at Acme Analytical Laboratories in Vancouver, Canada. Major elements were determined by ICP-ES, trace elements by ICP-MS, base metals Ni and Cu by ICP-ES and ICP-MS, respectively, Au, Pt and Pd by fire assay followed by ICP-MS and S has been determined by LECO. 4 Petrology The majority of the samples of Bell Rock Range (Fig. 1) are coarse-grained troctolites with interstital clinopyroxene and opaque minerals. Olivine often shows reaction rims with orthopyroxene, opaque minerals (magnetite according to Glikson et al. 1996) and a symplectitic texture; rutile can occur. The troctolites are interlayered with few anorthosites and fine- to medium-grained sub-ophitic gabbros with minor olivine and interstitial clinopyroxene and opaque minerals. Gabbros occur more frequently towards the base and the top of the intrusion. 5 Geochemistry A consistent HREE depleted and LREE enriched signature is present in all samples (Fig. 1); samples with the lowest REE generally display the largest positive Eu-anomaly (Eu/Eu*). However, the REE patterns indicate several coherent sample populations. A basal unit with moderate Eu/Eu* but relatively high REE overall is followed by a large lower troctolitic unit with high Eu/Eu* and moderate HREE fractionation. A sample on top of the lower troctolitic unit represents a clear discontinuance with moderate Eu/Eu* and distinct flat LREE. The upper troctolitic unit is geochemically similar to the lower troctolitic unit. Again, one sample at the top represents a dicontinuance with moderate Eu/Eu* and flat LREE. At the top of the magmatic succession a low Eu/Eu* unit occurs with flat LREE but high REE overall. It must be noted that there is a large gap in outcrop between the upper discontinuance and the top unit. Neither a clear single fractionation trend nor a distinct cyclicity can be seen in the magmatic logs of Bell Rock Range (Fig. 2). The magmatic logs show a good correlation between Ni and MgO and also between Cu and PGEs, although Pd tends to exhibit peak values towards the bottom of the intrusion. The Eu-anomaly and (La/Sm) PM generally correlate with each other; the