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 not, vert, similar1810 Ma, not, vert, similar1780 Ma, not, vert, similar1690 Ma, and not, vert, similar1635 Ma, and a much younger event of probably early Palaeozoic age. A sixth event at not, vert, similar1135 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 not, vert, similar1810 Ma, not, vert, similar1780 Ma, and not, vert, similar1635 Ma, whereas the mafic magmatism at not, vert, similar1690 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 not, vert, similar5 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 var epsilonNd 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.
A S-rich group (not, vert, similar1200 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.
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.
