Grassroots exploration under cover
Spinifex geochemistry leads to discovery of a new Australian metallogenic province
Introduction
Mineral exploration in arid environments is challenging due to wind-transported deposits and resulting equivocal surface geochemistry (Dunn, 2007). However, research by CRC-LEME (Reid et al, 2008; 2009; Reid and Hill 2010; Reid and Hill 2013) demonstrated the potential of Spinifex (Triodia Sp.) as a biogeochemical medium showing excellent promise for the identification of gold and base metal anomalism. To confirm the technique, the Queensland Department of Natural Resources and Mines supported this study in the Simpson Desert, where little was known about the underlying geology, nor its mineral potential.
Spinifex in the study area exhibited distinctive trace element chemistries indicative of three styles of mineralisation: (1) Au, Cu and Ni in mafic and ultramafic rocks on a terrane boundary; (2) Au, Ag and Cu epithermal mineralisation in calc alkaline granitoids; and (3) Sc, Cu, PGE and REE in alkaline intrusions characterised by phoscorite and carbonatite pipes. This report focuses on the latter association, the Devonian age phoscorite and carbonatite pipe intrusions and their geodynamic significance.
The phoscorite and carbonatite intrusions are interpreted to be part of a Silurian-Devonian plume track that extends from central NSW through south-west Queensland into the Northern Territory. They were emplaced when proto-Australian lithosphere traversed the Pacific Superplume (Maruyama et al, 2007; Torsvik et al, 2010). As phoscorites commonly host economic mineralisation (eg, Wall and Zaitsev, 2004; Fontana, 2006), this new Australian metallogenic province (Diamantina Alkaline Province) could emerge as an exciting new postcode for greenfield exploration.
Spinifex as a sampling medium in arid environments
Spinifex (Figure 1) is widely distributed in Australia, occurring in approximately 30 per cent of the continent (Reid and Hill, 2013). In regions of central and western Australia that are highly prospective for mineralisation it is almost ubiquitous. Roots penetrate > 70 m, providing a point of anchorage and a means for acquisition of water and nutrients (Raven and Edwards 2001). Roots support mycorrhizal fungi that contribute to element transport (Marschne and Dell, 1994) by facililitating dissolution and transport of cations from the basement rock/root tip interface. This involves downward movement of carbohydrates to support bacterial and fungal activity, and the upward movement of water and chemical nutrients into the growing fronds. As roots seek out deep sources of water, spinifex is potentially an ideal sampling medium for biogeochemical exploration in arid environments.
Study design and analytical details
The study was carried out on a tenement that straddles the terrane boundary between the eastern Arunta Block and the north Australian Craton/southern Mount Isa Block (Figure 2). More than 3000 spinifex samples were collected along NE-SW oriented traverses normal to the basement magnetic grain.
Ashed spinifex samples (to increase detection limits by removing volatiles) were analysed by Inductively Coupled Plasma Mass Spectrometry (ICPMS) at ACME Labs in Victoria BC, Canada. Concentration ranges are given in Table 1. Elements concentrated in spinifex fronds include Au, Ag, PGEs, Cr, Ni, Sc, Cu, Fe, Mg, Ca, Pb, Zn, the lanthanides, Y and the actinides. Except for the low atomic number elements (eg Li, Na, K, Ca) no significant concentration differences were observed between fresh (green) and dry (orange to brown) spinifex (Collerson, 2014).
Chemical vectoring indicates an alkaline igneous source
Figure 3 shows scandium, lanthanides plus yttrium, copper, platinum-palladium-rhenium, actinides (U and Th), and yttrium data. Spinifex exhibits significant variation in composition, reflecting variation in basement geology.
The distinctive association of Sc, Cu, Pt, Pd, Re, P, Th, U and REE plus Y occurs in four areas. Elevated Sc, Ni and Cr values suggest pyroxene-bearing mafic and ultramafic units. Based on the distinctive multi-element association (PGEs, REEY, Cu and actinides) these are most likely differentiated alkaline intrusions, possibly containing phoscorite and carbonatite (cf., Wall and Zaitsev, 2004).
What are phoscorite-carbonatite pipe complexes?
Phoscorite-carbonatite intrusions are steeply dipping, zoned, multi-phase pipe-like alkaline intrusions. They are circular to elliptical in shape, between 3 and 6 km in width and exhibit metasomatic alteration haloes of fenite up to 2 km in width. These aureoles are caused by interaction with magmatic fluorine- and carbon dioxide-rich fluids. Phoscorites are spectacular phosphate-rich (apatite-bearing) medium to coarse plutonic rocks containing carbonate (calcite, dolomite or ankarite), olivine, diopside, tetraferiphlogopite, sodic amphibole (magnesio-arfvedsonite and richterite), magnetite and apatite. They are associated with calcite carbonatite (sövite) or dolomite carbonatite (beforsite). Related alkaline silicate lithologies include dunite (olivinite), pyroxenite, feldspathoid-bearing gabbro (ijolite), foid-diorite, foid-monzonites and foid-syenite (Krasnova et al, 2003; Wall and Zaitsev, 2004). The possible presence of such alkaline intrusions in the study area is important. They are rare with only approximately 30 occurrences reported compared to more than 527 alkaline intrusions containing carbonatite and related mafic and ultramafic alkaline lithologies (Woolley and Kjargaard 2008 a, b). Furthermore, they invariably contain economic mineralisation (eg, Fontana 2006).
Confirmation of alkaline pipes in south-west Queensland
Two large coincident magnetic and gravity anomalies, identified as Mulligan and Lake Machattie intrusions in Figure 2, were drilled by AusQuest Limited as IOCG targets. Although cores showed variable alteration, IOCG lithologies were not encountered, and the exploration program was abandoned (Sherrington & others, 2008a, b). However, zircons recovered from ‘pyroxenites’ and ‘gabbros’ yielded SHRIMP U-Pb ages of 386±2 Ma (Carson et al, 2011) showing that the intrusions were Devonian and very young for this part of Australia.
In view of the Sc, PGEs, REEs, Y, Cu, U and Th anomalism shown in Figure 3 and because one of these areas coincided with a magnetic feature, the potential importance of the AusQuest cores and chemistry became apparent. It was immediately reconised that Mulligan and Lake Machattie intrusions contained extreme and rare lithologies that plot outside the field of most igneous rocks with significantly lower silica values (Figure 4). Compositions range from ultramafic cumulates, to pyroxenites, foid-gabbros (ijolite), phoscorite, carbonatite, foid-diorite to foid-syenite.
Chemically, they are identical to phoscorite and carbonatite suites from the Kola Peninsula (Downes et al, 2005, Dunworth and Bell 2001, Zaistsev et al, 2015), and Brasil (Barbosa et al, 2012; Brod et al, 2013).
Geodynamic interpretation and significance
The phoscorite and carbonatite intrusions in southwestern Queensland are very large intrusions up to 12 km in diameter defined by pronounced magnetic and gravity anomalies. They define a trend (Figure 5) with decreasing age of magmatism from Fifield and Owendale (444±4 Ma; Glen et al, 2007) and Gilgai (442±4 Ma; Fraser et al, 2014) in New South Wales, to Mulligan and Lake Machattie intrusions (386±2 Ma; Carson et al, 2011) in Queensland, to the Merlin kimberlite field in the Northern Territory (368±4 Ma; McInness et al, 2009).
This belt of the alkaline intrusions defines an approximately 2000 km long and approximately 200 km wide plume track (Figure 5). The Diamantina Plume track (Figure 5) formed during passage Gondwana (proto-Australian lithosphere) over the Pacific Superplume during the Silurian to Devonian (Torsvik et al, 2010). Based on the length of the track (approximately 2010 km) and the duration of magmatism (76 Ma), the plate velocity over the plume is approximately 2.6 cm/year. As kimberlite magmas form during mantle plume events (Collerson et al, 2010), the presence of micro-diamonds and diamond indicator minerals in eastern Northern Territory (Hutchison, 2013) and western Queensland (Tompkins 2002) likely also reflects the impact of the Pacific Superplume. The plume track defines a new metallogenic province in Australia, termed the Diamantina Alkaline Province.
Metallogenic Potential
Diamantina Alkaline Province has considerable economic potential, as phoscorite-carbonatite complexes contain both magmatic and metasomatic mineralisation (Wall and Zaitev, 2004; Fontana, 2006), They are enriched in platinum group elements (PGE; including platinum, palladium, rhodium), rare earth elements (REE), yttrium (Y), high field strength elements (HFSE; zirconium, niobium and tantalum), scandium (Sc), iron (Fe), phosphorus (P) and the actinides (U and Th). Important mines in phoscorite-carbonatite complexes are Phalaborwa (RSA) for Cu and P, Catalão, Araxá (Brazil) for P and Nb, and Kovdor (Russia) for Fe, Sc, Zr and Hf.
Although Mulligan and Diamantina cores were not systematically assayed, the mineral potential Diamantina Alkaline Province intrusion is suggested by elevated levels of zirconium (500 ppm), P2O5 (4.5 wt. per cent), Nb (250 ppm), and total REE + Y (3000 ppm). The mean Sc concentration of 27.4 ± 10 ppm is similar to values in Brazilian intrusions (Brod et al., 2013). Furthermore, the cores have elevated precious metals with Au ranging up to 11 ppb and one lithology returned an assay of 33 ppb Pt plus Pd. Furthermore, kimberlite pipes, are also associated with phoscorite-carbonatite alkaline intrusions e.g., on the Kola Peninsula in Russia and northern Finland (Downes et al., 2005). Importantly, the Merlin diamondiferous kimberlite pipes as well as diamond indicator minerals and microdiamonds suites in the eastern Northern Territory (Hutchison 2013) and 382 Ma kimberlite-derived rutile from the Toko Range (Tompkins 2002) also occur along the plume track.
Conclusions
Discovery of the Diamantina alkaline province using spinifex biogeochemistry and its relationship to a plume track provides another example of the direct link between mantle plumes and the location of alkaline magmatic provinces previously demonstrated in Africa (Hartnady and le Roex, 1985) and North America (Heaman et al., 2004). Given the rarity of phoscorites and the fact that they are commonly associated with economic mineralisation, this new metallogenic province represents an excellent new postcode for Greenfield exploration.
As spinifex covers large tracts of the Australia, this study has demonstrated that biogeochemical exploration using spinifex as a sampling medium could emerge as a low-cost and low-impact ‘grassroots’ greenfield exploration technique to identify elemental anomalism and lithologies under cover.
Acknowledgements
We acknowledge Krucible Metals for providing access to their tenement and for logistical support. Colin Dunn and Helen Waldron provided invaluable advice regarding QA/QC protocols. Dr Harvey Merchant, RSBS, ANU provided valuable details of element transport processes in spinifex.
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