geochemistry
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Australia's Large Igneous Provinces (LIPs) span most of Earth's geological history, ranging from Early Archean to Recent. LIPs in continental Australia are represented by continental flood basalts, fragments of oceanic plateaux, layered mafic-ultramafic intrusions, sill complexes and dyke swarms. It is only in the last decade that geologists have started to focus on LIPs in Australia, mainly from the perspective of their mineral potential, particularly after the discovery of the Nebo-Babel Ni-Cu-PGE deposit in the West Musgrave Province, central Australia. The list of LIPs increased by including other well-known igneous provinces, such as the Fortescue, Warakurna, Hart-Carson, Kalkarindji (formerly known as Antrim Plateau Volcanics) and various dyke swarms (e.g., Widgiemooltha, Marnda Moorn, Gairdner). The Bunbury Basalt, although only covering a small area in the Cape Naturaliste-Cape Leeuwin peninsula, joined the list of LIPs, due to its age links with the huge Kerguelen oceanic plateau magmatism. As indicated by the world-class Nebo-Babel deposit and further discoveries in the West Musgrave and in the Kimberley region, the mineral potential of LIPs is very high. In the case of orthomagmatic mineral systems, the selection of areas or specific intrusions requires focusing on isotope systematics and trace- and major-element geochemical trends to filter out mafic-ultramafic intrusions that may not have undergone sulphur saturation from those that have experienced sulphur saturation from processes, such as crustal contamination. In eastern Australia, there are two major volcanic provinces: the Early Cretaceous Whitsunday volcanic province, which is a good example of a silicic LIP, and a 4400 km long belt characterised by recent (youngest volcano is 4600 years ago) intraplate alkaline volcanism. The mineral potential associated with these provinces is as yet not fully assessed.
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Catchment outlet sediments (0-10 cm depth, sieved to <2 mm) collected at a very low density over most of the Australian continent have been analysed using the Mobile Metal Ion (MMI®) partial extraction technique. Of the 54 elements analysed, eight are generally regarded as essential nutrients for plant growth: Ca, Cu, Fe, K, Mg, Mn, P and Zn. For these, 'bioavailability', defined here as the ratio of the partial digest concentration to the total concentration, has been investigated. This estimation of 'bioavailability' gives results comparable with standard agricultural measurements. Average 'bioavailability' ranges from 15.0% for Ca to 0.1% for Fe. Smoothed (kriged) colour contour maps for continental Australia have been produced for these eight nutrients and interpreted in terms of lithology (e.g., presence of carbonates in the MMI® Ca map), mineralization (e.g., well known and possibly less known mineral districts in the Cu, P and Zn maps), environmental processes (e.g., salinity in K map, weathering and acid generation in Fe map) and agricultural practices (e.g., application of fertilizers in P and Zn maps). This first application of a partial extraction technique at the scale of a continent has yielded meaningful, coherent and interpretable results.