Proterozoic mafic–ultramafic intrusions in the Arunta Region, central Australia: Part 1: Geological setting and mineral potential

Abstract

Proterozoic mafic–ultramafic intrusions of the Arunta Region record a protracted period of magmatism during the evolution of this geologically complex and tectonically long-lived terrane in central Australia. New U–Pb zircon geochronology data highlight the episodic emplacement of the mafic–ultramafic systems. Five major events of dominantly tholeiitic mafic magmatism have been recognised at ∼1810 Ma, ∼1780 Ma, ∼1690 Ma, and ∼1635 Ma, and a much younger event of probably early Palaeozoic age. A sixth event at ∼1135 Ma has alkaline-ultramafic affinities. Chondrite- and mantle-normalised multi-element patterns of rocks with melt-like compositions have refined the correlations of the magmatic systems indicated by the geochronological framework. Continental margin environments (e.g. subduction or back-arc related) are generally indicated for the magmatic events at ∼1810 Ma, ∼1780 Ma, and ∼1635 Ma, whereas the mafic magmatism at ∼1690 Ma and in the early Palaeozoic is more characteristic of intracontinental settings (e.g. extensional rifts). The intrusions occur in proximity to major province-wide faults. Differential vertical movements along these faults provide the opportunity to examine geological processes at crustal depths ranging from ∼5 km to 25 km (2 kb to 8 kb). The intrusions form large homogeneous mafic granulite bodies, granulite bodies interfolded with felsic units, contaminated gabbroic sheets, stacked sequences of high-level doleritic sills, small pods and laterally extensive sheets of amphibolite, and rare plug-like ultramafic bodies. Metamorphic overprints range from granulite to sub-amphibolite facies, with a concentration of high-grade mafic granulite bodies in the central Arunta Region attributed to deeper levels of crust uplifted north of the Redbank Thrust. Mafic rocks in the intrusions of high-metamorphic grade are dominantly two-pyroxene mafic granulites with high clinopyroxene to orthopyroxene ratios and variable amounts of minor hornblende, biotite, quartz, and garnet. In contrast, gabbroic rocks in the lower-grade intrusions of the western Arunta Region contain more orthopyroxene, alkali feldspar, quartz, and Fe–Ti oxides. LREE-enrichment trends with decreasing 147Sm/ 144Nd ratios and initial ɛ Nd values (+1.5 to −4.7) indicate that felsic crustal contamination processes were particularly important during the evolution of the latter intrusions. Incompatible trace-element trends show that the Arunta intrusions fall into two major geochemical groups that for the first time highlight geographical differences in mineral prospectivity. 1. A S-rich group (∼1200 to 300 ppm S: Andrew Young Hills intrusion, Mount Hay Granulite, Mount Chapple Metamorphics) from the western and central Arunta regions that has potential for basal Ni–Cu–Co-sulphide associations. 2. A relatively S-poor (<300 ppm S), slightly more primitive group (Attutra Metagabbro, Mordor Complex) from the eastern Arunta Region that has greater potential for stratabound PGE-sulphide associations. Some potential also exists for structurally controlled hydrothermal deposits of Cu–Au ± PGEs ± Ag ± Pb spatially associated with mafic–ultramafic rocks (Riddock Amphibolite). The major challenges for finding massive Ni–Cu–Co sulphides in the lower parts of prospective S-saturated intrusions (e.g. Andrew Young Hills, Mount Hay Granulite) are to determine the pre-deformational geometries of the bodies and to locate favourable mineralised environments, such as embayments in basal contacts and feeder conduits, concealed by thin Cainozoic alluvial deposits.