Western Australia, WA, Australia
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The Pilgangoora lithium tantalum pegmatite deposits are hosted within the Palaeoarchaean East Strelley Greenstone Belt in the central Pilbara craton of Western Australia, located ~30 km NE of the Wodgina Tantalum mine and ~85 km SSE of Port Hedland in Western Australia (#Location: 21° 4' 19"S, 118° 54' 5"E).
The North Pilbara Craton consists of large, domal, multiphase granitoid-gneiss complexes bordered by sinuous synformal to monoclinal greenstone belts (Hickman, 1983; Griffin, 1990; Barley, 1997). The greenstone belts range in age from ~3560 to ~2940 Ma, with the granitoids emplaced over a similar but slightly younger time span (e.g., Champion and Smithies, 1998). Although the supracrustal rocks are structurally complex, the primary stratigraphic units may be correlated between greenstone belts (Hickman, 1983, 1990). The granitoid-greenstone terrane of the North Pilbara Craton has been subdivided into tectonostratigraphic domains with boundaries defined by NNE-SSW to NE-SW trending structural lineaments that regionally have a sinistral shear sense. The lithotectonic units identified are the i). East Pilbara granite-greenstone terrane, ii). Central Pilbara tectonic zone, and iii). West Pilbara granite-greenstone terrane. At least seven episodes of granitic magmatism have been identified between 3470 and 2800 Ma. During this period, granitic magmatism became more potassic and large ion lithophile element enriched, with increased compositional variability from tonalite-trondhjemite granodiorite to calc-alkaline and alkaline granite compositions due to cyclic crustal reworking and growth (Champion and Smithies, 1998). Most of the granitoid-gneiss complexes have tectonic margins, with little evidence of contact metamorphism of adjacent supracrustal sequences (Hickman, 1983, 1990). Granitic magmatism culminated with emplacement of a suite of 2890 to 2830 Ma granite plutons at (e.g., Blockley, 1980; Pidgeon, 1984; Bickle et al., 1989; Smithies and Champion, 1998, 2001) and a 2760 Ma suite of small A-type granites and stocks of tourmaline rich S-type peraluminous granites (Smithies and Champion, 1998). There is good spatial, geochemical, and geochronological evidence to link rare metal pegmatites in the North Pilbara Craton with emplacement of the younger granite suite (e.g., Blockley, 1980; Kennedy, 1998; Kinny, 2000; Sweetapple et al., 2000, 2001).
Pilgangoora is one of a series of major tantalum deposits, including Wodgina, Mount Cassiterite, Tabba Tabba and Strelley that are distributed along a NNE trending corridor, the Wodgina-Strelley lineament, within the same tectonostratigraphic domain.
The Pilgangoora pegmatites are hosted within the Palaeoarchaean East Strelley (formerly Pilgangoora) greenstone belt in the central Pilbara craton of Western Australia. The country rock within the greenstone belt comprises a series of steeply dipping, mafic meta volcanic rocks and amphibolites. In this area the greenstone belt is ~10 km wide, and is composed of the,
Table Top Formation - a ~3515 Ma succession of up to 2500 m of amphibolite to metamorphosed, locally schistose mafic volcanic and intrusive rocks and jaspilitic chert.
Double Bar/Euro Basalts - a ~3498 Ma, sequence up to 2000 m thick of massive to pillowed tholeiitic basalts, local mafic tuffaceous rocks and rare chert.
Coucal Formation - a 3515±3 Ma, sequence up to 6000 m thick of metamorphosed mafic volcanic and intrusive rocks and andesitic to felsic volcanic rocks, with interbedded black and white-layered cherty iron formations.
These successions are repeated by folding and faulting and sandwiched by the
Callina Supersuite - a 3490 to 3450 Ma suite of predominantly metadiorite and metasyenogranite, but also including a range of other granitoids, to both the east and NW; and
Cleland Supersuite - a 3275 to 3225 Ma suite of monzogranite and granodiorite, but also including leucogranite, tonalite, granite, pegmatite, mixed granites and minor gneiss found to the SW.
At Pilgangoora, the greenstones have been intruded by a swarm of north-trending, east-dipping, zoned pegmatites, extending over a strike length of >7 km, and width of ~1 km, mostly cross cutting regional foliation.
These pegmatites are considered to have been emplaced into a regional north-south trending fault system (Ellis, 1950). Individual pegmatite dykes range in size up to 600 m long by up to 300 m wide at surface, with many of the larger being as long as ~1250 m. Typical sizes of pegmatite bodies are ~100 to 250 m long x 1 to 8 m wide at surface. They have irregular dyke-like forms, often with splayed, forked or poddy terminations, and are hosted entirely in amphibole (actinolite?) bearing mafic/ultramafic metavolcanic rocks.
A regional zonation pattern of pegmatite types, outward from the nearest granite-greenstone contact was identified by Nisbet (1984). This zonation comprises simple quartz-microcline-muscovite pegmatite → albite-spodumene pegmatite at ~2 km from the contact. The bulk of significant tantalum-tin mineralisation is observed to be hosted in the more distal of these, the albite-spodumene class pegmatites.
The typical composition of the major spodumene bearing pegmatite bodies comorises megacrystic spodumene and microcline (orthoclase also reported, Ellis, 1950) in a fine-coarse grained albite-quartz matrix. cleavlandite [a variety of albite] sometimes evident in these pegmatite, often where spodumene is locally absent, whilst accessory beryl and spessartine garnet are occasionally present. Spodumene [(LiAlSi2O6] containing ~8% Li2O) is frequently altered to lepidolite [K(Li,Al,Rb)3(Al,Si)4O10(F,OH)2] or other mica species, being readily susceptible to alteration by late hydrothermal (pegmatitic) solutions (e.g., Ginzburg and Lugovskoi, 1977). Spodumene crystals are aligned both typically perpendicular to contacts, as well as random orientations. In addition, these crystals do not to have the characteristic pull-apart structures commonly noted of spodumene in pegmatites (e.g., Mt. Cassiterite pegmatite group; Sweetapple, 2000), although fracturing across crystal faces is observed.
These pegmatites are typically mostly unzoned or weakly zoned. The most common internal characteristics are: i). fine-grained aplitic textured border units and banding on the margins of the pegmatites; ii). the development of vague K-feldspar core units; and iii). unusual lenses of K felspar crystal aggregates observed at the #4 pegmatite. K feldspar units are normally much more resistant to hydrothermal alteration than spodumene.
Mineralisation within these weakly zoned pegmatites appears to be restricted to alteration zones, mainly localised along vein margins containing quartz, albite, muscovite and spessartine garnet, with varying amounts of associated spodumene, lepidolite, tantalite, cassiterite and minor microlite, tapiolite and beryl.
Tantalum and tin ore minerals are principally columbite-tantalite group minerals and cassiterite respectively, with minor tapiolite and microlite [Na,Ca)2Ta2O6(O,OH,F)] (Miles et aI., 1945). Columbite-tantalite compositions range from 24 to 53% Ta2O5 and 56 to 30% Nb2O5, representing compositions ranging from manganotantalite to manganocolumbite and ferrocolumbite. Ferrotantalite (tapiolite?) has been reported rarely (Miles et al., 1945; Ellis, 1950).
Cassiterite has a greater tendency to more frequently occur in the hanging wall side of the pegmatite dykes, while columbite-tantalite series minerals tend to be concentrated on the footwall side of the pegmatite dykes, a feature that is probably related to the paragenetic sequence of the mineral species. There is a close association between spodumene and both tantalum and tin minerals (Ellis, 1950). This same association has also been noted in the Mount Cassiterite pegmatite group, 30 km to the SW. Sweetapple (2000) notes that he association of tantalum minerals with lithium has been explained in experiments by Linnen (1998), who showed that columbite/tantalite mineral solubility in water saturated granitic melts increases with lithium abundance. That author suggested that tantalum mineral crystallisation is preceded by the crystallization of a Li±P±F phase which lowers the solubility product of the tantalite in the melt.
In the main Pilgangoora pegmatite field, the pegmatites containing the known resource comprise a series of extremely fractionated dykes and veins from 5, up to 25 m thick. Two groups of pegmatites along the same trend were tested within the Pilgangoora district, by Altura Mining Limited to the south and Pilbara Minerals Limited to the north. The dykes and veins in the north dip at 20 to 60°E, and are parallel to sub-parallel to the main schistose fabric within the greenstones. In a typical section, up to 5 such dykes may be distributed over a width of 150 m.
The main pegmatite dykes to the south are broadly oriented NNE-SSW over a strike of 1.6 km, distributed over zones that are up to 300 m wide. At least 15 dykes, each of from <500 to >1000 m, are developed in an en echelon pattern over these widths and strike length. Dyke thickness ranges from 5 to 50 m over their strike length. In a characteristic section, there are generally 4 dykes from 15 to 50 m thick over a thickness of ~265 m normal to the 30 to 40° E dip of the pegmatites.
JORC compliant mineral resources at the Pilbara Minerals Pilgangoora deposits (Pilbara Minerals Limited website, 2016), in the north, are:
17.9 Mt @ 182 ppm Ta2O5;
35.7 Mt @ 1.31% Li2O;
24.3 Mt @ 205 ppm Ta2O5;
44.5 Mt @ 1.21% Li2O;
42.3 Mt @ 195 ppm Ta2O5;
80.2 Mt @ 1.26% Li2O;
JORC compliant mineral resources and ore reserves at the Altura Mining Pilgangoora deposits (Altura Mining Limited release, May, 2016), in the south, are:
26.7 Mt @ 1.05% Li2O;
9.0 Mt @ 1.02% Li2O;
35.7 Mt @ 1.05% Li2O.
18.4 Mt @ 1.07% Li2O.
This summary is partly drawn from "Sweetapple, M.T., 2000 - Characteristics of Sn-Ta-Be-Li-Industrial Mineral Deposits of the Archaean Pilbara Craton, Western Australia; Australian Geological Survey Organisation, Canberra, Record 2000/4, 59p."
The most recent source geological information used to prepare this summary was dated: 2016.
This description is a summary from published sources, the chief of which are listed below.
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