The Mt Weld carbonatite intrudes greenstones 30 km SSE of Laverton, in the central part of the fault-bounded, north-south trending Laverton tectonic zone, which lies in the eastern part of the Laverton greenstone belt, part of the greater Norseman-Wiluna greenstone belt within the Archaean Yilgarn Craton of Western Australia.
The Laverton greenstone belt comprises three major cycles of ultramafic to mafic volcanic rocks, separated by thin heterogeneous units of one or more of banded iron formation, chert and carbonaceous shale. The third cycle, which surrounds the Mt Weld carbonatite, is overlain by conglomerate along the western margin of the Laverton tectonic lineament. These greenstones are bounded by a variety of granitoids with intrusive contacts. The greenstone belt has been divided into a series of sectors of open folding and minimal metamorphism, separated by linear to sinuous tectonic zones, characterised by periodic shearing, faulting and peralkaline igneous activity, and subsequent sediment filled grabens. The host sequence and carbonatite have been overprinted by greenschist facies metamorphism (Duncan and Willett,1990).
According to Hoatson et al. (2011) the mid-Palaeoproterozoic Mount Weld Carbonatite belongs to a regional alkaline province represented by coeval low- to high-MgO alkaline ultramafic-mafic igneous rocks, kimberlites, and carbonatites. Isotopic data from oxide minerals, and carbonatite, kimberlite and melonite rock types, yield a precise Re-Os isochron of 2025±10 Ma (Graham et al., 2003; 2004). Incompatible element geochemistry, stable isotope and Sm-Nd isotope data suggest all these rocks are strongly enriched in incompatible elements and are comagmatic. The genetic and spatial relationships between the alkaline ultramafic and carbonatitic melts appear to be controlled by the depth and composition of the sub-continental lithospheric mantle, without significant crustal contamination (Graham et al., 2004).
The Mt Weld carbonatite has been dated at 2021±13 Ma (Rb-Sr from fresh rock; K.D. Collerson, 1982, unpublished data, reported by Hoatson et al., 2011); 2064±40 Ma (K:Ar from a biotite-rich calcite and apatite bearing fragment; A.W. Webb, 1973, unpublished data, reported by Hoatson et al., 2011); 2090±10 Ma (Pb-Pb; Nelson et al., 1988). It comprises a steeply plunging cylindrical body of primary carbonatite, ~3 to 4 km in diameter, with an outer 500 m wide annulus of fenitic glimmerite alteration which has a gradational outer margin with the intruded greenstones. The glimmerite is composed of strongly brecciated ferriphlogopite. Carbonatite dykes persist for up to 5 km from the main body. The carbonatite is dominantly calcitic (sövite with beforsite) and dolomitic (rauhaugite) igneous rock types that contain more than 50% carbonate, while biotite and apatite may locally predominate. A thick weathering/regolith layer (10 to >70 m) of laterite overlying the unweathered carbonatite contains high-grade REO deposits and concentrations of niobium, tantalum, zirconium, and other 'rare' metals.
The mineral assemblage within the fresh carbonate locally preserves relict cumulus textures, accompanied by minor intercumulus apatite, biotite, carbonate and magnetite. The surrounding altered annulus exhibits a gradational outward transition, from predominantly potassium-rich micaceous rock (glimmerite) to mafic volcanic country rocks. In addition to phlogopite-rich rocks, this zone of alteration also incorporates brecciated wall-rocks, and is characterised by alkali metasomatism under strongly oxidising conditions, typical of fenitisation. However, minor, but widely disseminated sulphides within the carbonatite, suggest the periods of carbonatite invasion, occurred under mildly reducing conditions. Duncan (1990) has estimated that since its emplacement, ~4 vertical kilometres of the carbonatite intrusion have been removed by erosion, with a thick residual regolith profile preserved, in contrast to the mineralised Ponton Creek carbonatite ~190 km to the SE, where scouring by Permian glaciation completely removed the palaeo-regolith, without a significant subsequent lateritic profile developed.
All the currently known economic REE resources at the Mount Weld deposit are hosted within the lateritic regolith above the intrusive carbonatite. Carbonates have been leached from the primary carbonatite and removed by groundwater activity, resulting in the progressive accumulation of primary igneous apatite and minor oxides, sulphides and silicates, accompanied by the replacement, decomposition and oxidation of primary igneous minerals, crystallisation of secondary minerals, and the formation of ferruginous cap rocks. These processes resulted in the formation of a mineralogically and chemically zoned laterite profile, the base of which is defined by a relatively sharp, karst-like interface with the underlying carbonatite. The unweathered carbonatite is overlain by a residual zone containing relict igneous minerals (apatite, magnetite, ilmenite, pyrochlore, monazite, silicates) concentrated by the removal of carbonate. The residual layer is overlain by a zone of supergene-enrichment containing abundant insoluble phosphates, aluminophosphates, clays, crandallite group minerals, iron and manganese-bearing oxides that contain elevated concentrations of REE, Y, U, Th, Nb, Ta, Zr, Ti, V, Cr, Ba and Sr, including economic accumulations of REE, niobium-tantalum and phosphatic minerals. Extreme lateritic weathering prevailed in the supergene zone over a protracted period of time and resulted in the degradation of the residual magmatic REE-bearing minerals.
Very high-grade lanthanide concentrations (up to 45% combined lanthanide oxides) in the regolith are due to secondary monazite which is particularly rich in LREE and low in thorium, and is found as polycrystalline aggregates that often pseudomorph apatite (Lottermoser, 1990). Other REE-bearing minerals recognised include crandallite, rhabdophane, cerianite and churchite. Churchite contains large amounts of high-grade yttrium (up to 2.5% Y2O3) and is an important host to the HREE. The LREE-bearing minerals monazite and rhabdophane are found in the upper part of the residuum, whilst the HREE and Y are preferentially concentrated at depth as xenotime and churchite (Richardson and Birkett, 1995). Yttrium and lanthanides are distributed throughout the carbonatite regolith and are derived from low grade occurrence in the primary carbonatite of the major REE-bearing minerals monazite, apatite and trace synchysite, and to a lesser degree, from other silicate and carbonate minerals (feldspar, calcite, dolomite) that contain trace amounts of REE (Lottermoser and England, 1988; Lottermoser, 1990). The primary apatite within the carbonatite has an average content of ~0.5% lanthanide oxide. REE-bearing phosphatic Lacustrine sedimentary rocks are apparently derived from weathering of fine-grained detrital pyrochlore and apatite that were deposited in the deeper levels of broad palaeodrainage channels (Fetherston, 2004).
The highest niobium-tantalum grades are found in 6 to 15 m thick horizontal units of unconsolidated, highly phosphatic lacustrine sedimentary rocks that grade upwards into smectite clays. Apatite occurs as a residual layer near the base of the regolith, varying from 6 to 30 m thick, at depths of 50 to 90 m below surface. The best beneficiable type is a lightly cemented fine sand of apatite, magnetite and vermiculite. The primary carbonatite contains around 3.5% apatite. Niobium and tantalum bearing pyrochlore, ilmenite and niobium rutile from the primary carbonatite are concentrated in the apatite and magnetite rich residual zone. Grades are generally variable and locally as high as 1.5% Nb2O5 and 0.05% Ta2O5(Duncan and Willett, 1990). Higher grades of niobium (up to 6% Nb2O5) characteristically form within the supergene zone as crandallite and goethite that have been derived in part from the lacustrine-fluviatile sedimentary rocks.
The most important rare earth resource, the 'Central Lanthanide Deposit' is in the regolith above the central part of the carbonatite, while the higher grade Nb and Ta zones are closer to the outer margins of the pipe. The majority of the REOs are contained within secondary, low Th phosphate minerals with low levels of deleterious elements (e.g., F and Ca). The Central lanthanide deposit contains an indicative mix of predominantly LREE with CeO2 (46.7%), La2O3 (25.5%), Nd2O3 (18.5%), Pr6O11 (5.32%), Sm2O3 (2.27%) and Eu2O3 (0.443%), together with minor components of HREE: Dy2O3 (0.124%) and Tb4O7 (0.068%: Lawrence, 2006).
The lower-grade Duncan REE deposit, SE of Central Lanthanide contains about 25% of the total REO resource, but it has a relatively higher component of the more valuable HREE.
The 'Crown Deposit' in the northern part of the complex has economic resources of niobium, tantalum indicated and inferred resource, while phosphorous mineralisation in the 'Swan Deposit', also in the northern half of the complex has an indicated and inferred resource of phosphorous oxide (MiningNewsPremium.net, 16th March 2011). The 'Anchor Deposit' in the southwest of the Mount Weld complex contains niobium-tantalum mineralisation.
The age of the mineralised laterite is uncertain, although the up to 70 m thick overlying sequence of bioturbated lacustrine clay and sand, is equated to similar sequences in the Eastern Goldfields Province, that are considered to be Late Cretaceous to Early Cenozoic in age (Bunting et al., 1974; Duncan and Willett, 1990). It is suggested the development of the carbonatite weathering profile postdates the Permian glaciation event, which would have removed any pre-existing weathering products. Consequently, the exposure of the carbonatite intrusion and subsequent weathering are interpreted by Hoatson et al. (2011) to have taken place during the Late Mesozoic to Early Cenozoic.
REE enrichment is interpretd to have been the product of long-term leaching and redeposition by groundwater movement (Cassidy et al., 1997). The REE are consider to have undergone pronounced lateral and vertical mobility during weathering, favoured by high fluid:rock ratios, long fluid residence times, abundant REE complexing agents, and the decomposed nature of the primary igneous carbonatite minerals (Lottermoser, 1990). The wide range of pH and alkaline conditions, and the carbonate anion concentrations in the groundwater variably influenced the different stabilities of LREE and HREE complexes which caused fractionation, separation, and deposition of LREE and HREE (Lottermoser, 1990). The development of the strongly REE-enriched central laterite zone was facilitated by lateral groundwater flow towards a central topographic low, whilst the associated decreasing pH of the solutions favoured the mobilisation of large amounts of REE (Hoatson et al., 2011).
The regolith over the carbonatite was estimated (1989) to contain an indicated resource of (Duncan and Willett, 1990):
250 Mt @ 18% P2O5,
270 Mt @ 0.9% Nb2O5,
145 Mt @ 0.034% Ta2O5,
15.2 Mt @ 11.2% lanthanide + yttrium oxides.
Resource estimates in 2010 at a cut-off of 2.5% ReO were quoted as (Lynas Corp. web site, 2013):
Central Lanthanide deposit
Measured - 3.55 Mt @ 14.4% REO, 14.3% TLnO, 820 g/t Y2O3
Indicated - 1.44 Mt @ 8.2% REO, 8.1% TLnO, 960 g/t Y2O3
Inferred - 4.884 Mt @ 8.6% REO, 8.5% TLnO, 1120 g/t Y2O3
Total - 9.88 Mt @ 10.7% REO, 10.6% TLnO, 990 g/t Y2O3
Duncan deposit
Measured - 3.65 Mt @ 14.4% REO, 5.2% TLnO, 2700 g/t Y2O3
Indicated - 3.56 Mt @ 8.2% REO, 3.9% TLnO, 2460 g/t Y2O3
Inferred - 0.41 Mt @ 8.6% REO, 4.1% TLnO, 2360 g/t Y2O3
Total - 7.62 Mt @ 10.7% REO, 4.5% TLnO, 2570 g/t Y2O3
Combined total measured + indicted + inferred resource
Total - 17.49 Mt @ 8.1% REO, 7.9% TLnO, 1680 g/t Y2O3
In 2010 the total REE resource amounted to 1.416 Mt ReO.
Crown Deposit niobium, tantalum resources at January 2010 (Lynas Corp.)
indicated + inferred resource - 37.7 Mt @ 1.07% Nb2O5, 1.16% TLnO, 0.09% Y2O3, 0.3% ZrO, 0.024% Ta2O5, 7.99% P2O5;
Swan Deposit phosphate, as at March, 2011 (Lynas Corp.)
indicated + inferred resource - 77 Mt @ 13.5% P2O5.
(Source: Porter GeoConsultancy, www.portergeo.com.au, 2013)
