Western Australia, WA, Australia
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The Greenbushes tantalum-tin-lithium deposit is located ~250 km south of Perth In Western Australia (#Location: 33° 51' 23"S, 116° 3' 48"E).
Mining in the Greenbushes area started with tin production in 1888 by the Bunbury Tin Mining Company, which continued until 1930. Over the period from 1935 to 1943 the Greenbushes tin mine transitioned to Vultan Mines which began sluicing operations of the weathered tin oxides. Later from 1945 to 1956, modern earth moving equipment was introduced, and tin dredging initiated. In 1964 a new corporation, Greenbushes Tin NL was formed and open cut mining of the softer oxidised rock commenced in 1969.
Tantalum mining commenced in 1992 with an ore processing capacity of up to 0.8 Mt per annum. By the late 1990s demand for tantalum reached all-time highs and the existing high grade Cornwall Pit was nearing completion. In order to meet increasing demand a decision was made to expand the mill capacity to 4 million tonnes per annum and develop an underground mine, to provide higher grade ore for blending with the lower grade ore from the Central Lode pits. An underground operation began at the base of the Cornwall Pit in April 2001 to access high-grade ore prior to the completion of the available open pit high-grade resource. In 2002, the tantalum market collapsed due to a slow-down in the electronics industry and subsequently the underground operation was placed on care and maintenance. The underground operation was restarted in 2004 due to increased demand but again placed on care and maintenance the following year before briefly resuming mining to be put on care and maintenance again in 2008. The lithium open pit operation has continued throughout recent times and mining is now focused on the Central Lode zone. The tantalum mining operation and processing plants have been on care and maintenance since 2005.
Lithium mining by Greenbushes Limited commenced in 1983 with the commissioning of a 30 000 tpa lithium mineral concentrator in 1984-1985. The lithium assets were acquired by Lithium Australia Ltd in 1987 and Sons of Gwalia in 1989. Production capacity was increased to 100 000 tpa of lithium concentrate in the early 1990s and to 150 000 tpa by 1997, which included the capacity to produce a lithium concentrate for the lithium chemical converter market. The Talison Minerals Group was incorporated in 2007 for the purpose of acquiring the assets of the Advanced Minerals Division of Sons of Gwalia by a consortium of US private equity companies led by Resource Capital Funds. Upon completion of the reorganisation of the Talison Minerals Group in 2010, the company was split into Talison Lithium and Talison Tantalum, which became Global Advanced Metals. The capacity of the lithium operation has progressively increased to1.5 Mtpa of ore feed yielding ~740 000 tpa of lithium concentrates. The tantalum operation was re-opened in 2011.
The Greenbushes district lies within the NNW-SSE trending, 15 to 20 km wide Donnybrook-Bridgetown shear zone which has a strike length of ~150 km within the Balingup Metamorphic Belt, located near the southwestern margin of the 3100 Ma Western Gneiss Terrane. This terrane forms the southwestern corner of the Yilgarn Craton, and is one of four main divisions that make up the craton. Greenbushes is one of the largest rare metal pegmatite deposits in the world.
The Balingup Metamorphic Belt has limited exposure, being largely obscured by Tertiary sediments and laterite. The Donnybrook-Bridgetown shear zone and the Balingup Metamorphic Belt are both truncated to the west by the Darling Fault and Phanerozoic rocks of the Perth Basin. To the south, the metamorphic belt is bounded by the Proterozoic Albany-Fraser Mobile Belt, and to the east by the Hester Lineament. It extends as far north as the Loguebrook Granite, where it is truncated by intrusions of the Darling Range Batholith.
The Donnybrook-Bridgetown shear zone is an ancient structure, characterised by steeply dipping mylonitic textures, horizontal stretching lineations, assymetric folds and evidence of sinistral strike-slip movement. It corresponds to a sequence of sheared gneiss, orthogneiss, amphibolite and migmatite outcrops along the trace of the lineament. A series of syn-tectonic granitoid intrusives also occur within the Balingup Metamorphic Belt, elongated along the Donnybrook-Bridgetown Shear Zone.
The Greenbushes pegmatites have been dated at ~2525 and 2610 to 2589 Ma, and appear to were intruded during shearing, thereby accounting for the fine grain size and internal deformation (Jacobson et al., 2007). They contain the same fabric as the shear zone and exhibit evidence of syntectonic crystallisation of minerals such as tourmaline, tantalite, garnet and cassiterite. The pegmatites have also been affected by subsequent deformation and/or hydrothermal recrystallisation, the last episode of which has been dated at ~1100 Ma (Partington et al., 1995).
The principal country rock enclosing the pegmatites include dioritic gneiss, which appears to be basement to Archaean greenstone-like sequences of fine-grained amphibolite and associated banded iron-formation, ultramafic schist, coarse-grained amphibolite and felsic massive to banded paragneisses, which are termed 'granofels' by the Geological Survey of Western Australia (Wilde and Walker, 1979).
The supracrustal lithologies have been intruded by quartz-biotite-feldspar porphyry dykes, dolerite sills, dolerite dykes and granitoids, which predate the intrusion of the mineralised pegmatites at Greenbushes, and barren pegmatites and dolerite dykes, which post-date intrusion. The later dolerite dykes are oriented east-west, and where they cut the Greenbushes pegmatite, are reintruded by pegmatite veins, interpreted to have formed by remelting of the pegmatite at the time of dolerite intrusion (Bettenay et al., 1985).
The granitoid rocks of the Greenbushes district have been subdivided into an older suite, predating the Greenbushes pegmatites, and a younger suite which is apparently synchronous with emplacement of the mineralised pegmatites (Partington, 1988, 1990). The younger granitoids are aligned parallel to the Donnybrook-Bridgetown shear zone, associated with linear belts of migmatite, and are interpreted to form part of the Wheatbelt batholith (Wilde and Walker, 1979). Mesoscopic and macroscopic relationships suggest the younger granitoids and the Greenbushes pegmatites were intruded synchronously with movements on the Donnybrook-Bridgetown shear zone (Partington, 1988, 1990). The Logue Brook granitoid, one of the suite of younger granitoids, is locally deformed by both the proto-Darling fault zone and the Donnybrook-Bridgetown shear zone, and has been dated at 2612±5 Ma (SHRIMP U-Pb zircon; Compston et al., 1986).
A second generation of pegmatites in the Greenbushes district includes the Late Proterozoic Ferndale and Mullalyup pegmatites, that are only weakly mineralised. These pegmatites were introduced during Proterozoic reactivation along the Donnybrook-Bridgetown shear zone, accompanied by amphibolite facies metamorphism (Kepert, 1985; Seet, 1986).
Deposit Geology and Mineralisation
The principal country rock lithologies in the deposit area may be summarised as follows:
Granofels - composed of metasediments, quartz-feldspar-biotite gneiss ±hornblende, generated from a mixed granitoid-mafic origin, containing interbedded fine-grained amphibolites. It is generally competent despite highly foliated to laminated nature. These are dated at ~3100 Ma.
Amphibolite - a hornblende-tremolite-Ca plagioclase rock with minor biotite. Metabasaltic flows with pillow structures and co-genetic subvolcanic intrusive metadolerite are evident.
Dolerite - Proterozoic dykes and sills which trend east west and were intruded into the pegmatite and host rocks at about 1100 Ma. They vary in
width from a few centimetres to tens of metres, and are composed of plagioclase-augite-hornblende with minor quartz-apatite-titanite-holmquistite. They are heterogeneously deformed and metamorphosed, although primary igneous textures are preserved in low strain zones. Structural, lithological and geochemical evidence suggests that dykes and sills were generated from different pulses of the same magma (Ingham et al., 2011; Partington et al., 1995).
The Greenbushes deposits comprise a main, rare-metal zoned pegmatite with numerous smaller pegmatite dykes and pods in the footwall of the main body. The main pegmatite and its subsidiary dykes and pods are concentrated within shear zones formed along the contact between sequences of granofels and amphibolite (Partington et al., 1995).
The main pegmatite body strikes NNW and dips variably from 30 to 70° towards the WSW. It has a strike length of ~3 km and 300 m width. Its syn-tectonic development has produced variable poorly to well-developed mylonitic fabrics, particularly along host rock contacts, with low strain zones preserving primary igneous textures, whilst highly strained domains exhibit re-crystallised and mylonitic fabrics.
Other pegmatites are also 2 to 3 km in length and 10 to 300 m in thickness, and persist to at least 500 m depth, with a 40 to 50°W dip, all within a 7 x 1 km, north-south elongated, enveloping zone.
In general the hanging wall of the main pegmatite is an amphibolite, whereas the footwall is granofels. The amphibolites and dolerites ('greenstones') contain occasional stringers and pods of sulphides such as pyrite, pyrrhotite and chalcopyrite. Arsenopyrite and arsenolamprite (native arsenic) are ubiquitous in some areas, particularly within granofelsic and amphibolitic inliers in the main pegmatite. Some of the granofels is distinctly garnetiferous (Ingham et al., 2011).
The pegmatite bodies of the Greenbushes deposit are mineralogically zoned. They are generally lenticular in nature and interfinger along strike and down dip. Major minerals and approximate average abundances in the main Greenbushes pegmatite are quartz 28%, spodumene 26%, albite 23%, K feldspar 20%, tourmaline 1%, mica 1% and apatite 0.5%.
The main pegmatite in the south is characterised by an outer 'Lithium or Spodumene Zone' that occurs both in the footwall and hanging wall, sandwiching the asymmetric development of an upper 'K feldspar Zone' that decreases in thickness northward, and a lower 'Sodium or Albite Zone' towards the footwall of the intrusion. The upper lithium zone also decreases in thickness northward, and eventually disappears completely from the hanging wall (Partington et al., 1995). The 'Sodium or Albite' and the lower 'Lithium or Spodumene' (contact) zones are separated by a 'Mixed Zone'.
The primary ore minerals are found in specific mineral assemblages displaying complex mineralogical zoning. The 'Lithium or Spodumene Zone' is enriched in the lithium-bearing silicate mineral spodumene. High-grade tantalum (>420 ppm) and cassiterite mineralisation is generally concentrated in the 'Sodium or Albite Zone' which is characterized by albite, tourmaline and muscovite. The 'K feldspar Zone' is of lesser commercial importance, containing concentrations of the potassium feldspar microcline. Other accessory minerals present in the pegmatite are phosphates such as apatite, minor beryl and garnet (Ingham et al., 2011; Partington et al., 1995).
The outer contact zones are usually aplitic, with a granular texture, and all but the most resistant minerals annealed, whilst in the undeformed parts of the pegmatite these two zones are similar, containing albite, quartz, biotite, tourmaline, holmquistite, tin with tantalite inclusions, garnet, zircon, calcite, and scapolite. The minerals from these zones appear to be the earliest to have crystallised. Textures suggest that deformation, early crystallization of the pegmatite in the contact zone, including crystallisation of tin and tantalum minerals and zircon, were all essentially synchronous with the metamorphism (Partington et al., 1995).
The hanging-wall lithium zone in the main pegmatite is generally richer (up to 5% Li2O, equivalent to 60 to 80% spodumene) compared to the footwall lithium zone, which is laterally more continuous. The lithium zones in the main pegmatite generally contain coarse-grained euhedral spodumene intergrown with quartz, which forms white and pink lustrous units at the top of both the hanging-wall and footwall zones. Spodumene crystals are finer grained at the centre of the lithium zones, occurring as granular intergrowths with quartz and K feldspar. Intercalations of quartz-albite or monomineralic lenses of blocky perthitic microcline also occur in this part of the lithium zone. Accessory minerals include apatite, tourmaline, muscovite, beryl and tantalite.
The principal lithium mineral is spodumene (LiAlSi2O6 containing ~8% Li2O) and its related varieties, e.g., kunzite and hiddenite. Minor to trace minerals include lepidolite [K(Li,Al,Rb)3(Al,Si)4O10(F,OH)2], amblygonite and lithiophilite (phosphates). The iron-bearing species hiddenite appears to increase towards the south of the deposit. Lithium is strongly leached in the weathering environment and is virtually absent in weathered pegmatite. Lithium grade decreases towards the footwall of the high-grade Lithium Zone. The higher lithium grade ore generally consists of 50% quartz and 50% spodumene, having a Li2O grade of around 4%.
The K feldspar zone is found at, or near, the upper contact of the main pegmatite, reaching a maximum thickness of >75 m in the centre. This assemblage is rare in the other pegmatites, and where present, occur as discrete pods and smaller dykes, commonly concentrated in the hanging-wall of subsidiary pegmatites. No significant mineralisation has been found in any of the K feldspar zones, with only minor tantalite and spodumene. In the main
pegmatite, this zone is largely composed of pods of perthitic microcline and coarse-grained quartz, which are locally intergrown with muscovite. Fluid inclusions suggest perthitic microcline crystallised at 700°C and 5 Kbars (Han, 1991; Partington et al., 1995).
The albite zone has two main assemblages: albite-quartz, and quartz-albite±microcline±muscovite (Bettenay et al., 1985). It occupies the lower footwall region of the main pegmatite, although further south it is found in a more central position within the pegmatite. Albite pegmatites also commonly occur as dykes and pods to the north of the main pegmatite. The larger albite crystals behaved in a brittle fashion, and consequently many crystals with well-developed albite twins are heavily deformed. These crystals commonly contain inclusions of mica, tantalite and cassiterite, and are also embayed by K feldspar and spodumene. They contain fractures infilled by K feldspar, and form pulled-apart fragments cemented by K feldspar. Tourmaline is the first mineral to crystallise, formed at 860°C and 5.5 Kbars (Han, 1991), but continued with the coeval crystallisation of albite, tantalite and cassiterite±zircon, whereas spodumene, apatite, beryl and K feldspar formed late in the crystallisation sequence (Partington et al., 1995).
Tantalum and niobium in the albite zone, mainly occur as columbo-tantalite and in numerous trace tantalum minerals. Columbo-tantalite grains commonly have Ta-rich cores and more Nb-rich rims. Grains are generally less than 350 µm across. Columbo-tantalite occurs as elongate blebs or crystals, chiefly at the grain boundaries of quartz/albite, albite/mica and K-feldspar/spodumene. The Ta:Nb ratio is typically 2:1. Mineral species include microlite [Na,Ca)2Ta2O6(O,OH,F)], stibiotantalite, ilmenotantalite, manganotantalite and tapiolite.
The 'Mixed Zone' is composed of quartz, albite, K feldspar and spodumene with minor tourmaline, mica and apatite, and represents a transition from the 'Sodium or Albite' to the lower 'Lithium or Spodumene' (lower contact) zones and sometimes replaces the latter.
More than ten Ta-bearing phases have been identified in the main pegmatite (Bettenay et al., 1985), including Ta ilmenite and Ta rutile (striiverite). Cassiterite is the main Sn-bearing phase, occurring as euhedral swallow-tailed crystals. Early formed tantalum minerals, mainly wodginite and ixiolite, occur as inclusions within cassiterite and tourmaline crystals. In contrast to the early tantalum minerals, the later coexisting tantalum phases (tantalites and tapiolites) in silicates are Sn free and generally occur in fractures and pull-aparts in the early silicate phases. Characteristic ore zone accessories associated with early crystallised mineralisation include zircon, monazite, uraninite and probable euxenite (Bettenay et al., 1985; Partington et al., 1995).
Micron-scale inclusions of Ta-rich cassiterite is characteristic of some of the spodumene-quartz zones, containing low U, Ce and Th, with variable Ti (Bettenay et al., 1985). This Ta mineralisation is generally late and is associated with zircon, monazite, pollucite and lepidolite.
Three phases of Sn and/or Ta mineralisation are suggested by paragenetic and fluid inclusion studies (Han, 1991; Partington, 1988, 1990). The first occurred with the initial crystallisation of tourmaline at 750°C at 5 Kbars. The preserved low-strain textures in this phase are regarded as typical of magmatic crystallisation, where ore minerals crystallised at an early stage, with other enriched accessories (notably zircon and uraninite). The second phase appears to have occurred during deformation and metamorphism, but still during crystallisation of the pegmatite at a temperature of 680°C at 5 Kbars. The third is associated with the greisen and metasomatic zones in the pegmatite and appears to be related to a hydrothermal event at 620°C at 5 Kbars. The second and third phases were the result of hydrothermal processes at the end of, or after, magmatic crystallization of the pegmatite, and these upgraded the preexisting early mineralization, which is more typically associated with microlite rather than tantalite.
Weathering and erosion of the pegmatites has produced adjacent alluvial deposits in ancient drainage systems. These are generally enriched in cassiterite. All of the rocks have been extensively lateritised during Tertiary peneplain formation. The laterite profile locally reaches depths in excess of 40 m below the original surface (Partington et al., 1995).
Structure - Shear zones within the pegmatites are most strongly developed at pegmatite margins and within the albite rich zones, producing foliation fabrics of variable intensity. The enclosing greenstones also exhibit foliation fabrics, which are generally conformable to layering observed within the pegmatites. The orientations of shear fabrics are sub-parallel to the regional Donnybrook-Bridgetown Shear Zone. North of the main lithium mineralisation, asymmetric macrofolds are evident, post-dating mylonisation of the albite zone, and pre-dating or are synchronous with later stages of crystallisation. Dilatant zones formed in the footwall albite zone during folding were infiltrated by late-stage fluids and provide the focus for a second stage of mineralisation. The evidence appears to confirm that the pegmatite intrusion was synchronous with the formation of the Donnybrook- Bridgetown Shear Zone.
Distribution of mineralisation - The Greenbushes pegmatite deposits, which extend over a strike length of ~3 km north-south, are sub-divided, for mining purposes, into four sectors, from north to south, the Cornwall (tantalum only), C3, C2 and C1 areas.
C3 contains the main lithium deposit and is about 400 m south of the tantalum/tin enriched pegmatites in the Cornwall Pit. The C3 deposit occurs in the upper section of a 250 m wide lithium-enriched pegmatite. Spodumene in the ore zone makes up about 50% of the rock with the remainder being largely quartz. The spodumene ore is hard and massive. The spodumene ore body is ~600 m long and up to 100 m wide, dipping at 30 to 80°W to SW. At the northern end of the orebody a highly felspathic (K feldspar) zone separates the high grade lithium zone from the hanging wall amphibolites and a dolerite sill. Tantalum/tin and lithium mineralisation are conformable with the trends of the pegmatites both along strike and down dip.
C1 contains a second lithium deposit, ~500 m long x 150 m wide, dipping moderately to the west.
C2 lies between C1 and C3. Limited mining has been undertaken in this part of the lithium bearing pegmatite which extends for about 600 m, varies in width up to about 30 m and dips moderately to the west.
Continuity of lithium mineralisation at a 1.5% Li2O cut-off is >2 km from
the south of C1 to beyond the north of C3. The lithium mineralisation narrows in the southern end of C2. At a 2.5% Li2O cut-off, the lithium body breaks into a 490 m long zone in C1 and a 1280 m long zone covering in C3.
Reserves, Resources and Production
Between 1888 and 1987 the field had produced 26 000 tonnes of 72% Sn concentrate, 2300 tonnes of Ta2O5 concentrate and 39 000 tonnes of 7.2% Li2O concentrate.
In 1990 soft rock reserves (in the weathered, decomposed upper 50 m of the pegmatite (Hatcher and Clynick, 1990) were:
5.4 Mt @ 0.24 kg/t cassiterite, 0.07 kg/t tantalite.
In 1990 hard rock proven+probable reserves totalled (Hatcher and Clynick, 1990):
13.46 Mt @ 0.15% Sn, 0.059% Ta2O5, 0.44% Nb2O5.
Mining reserves in 1991 (Sons of Gwalia, 1991) were:
7.1 Mt @ 4.06% Li20; 4.7 M t @ 0.06% Ta, 10.8 Mt @ 0.42% Nb, 4.7 Mt at 0.24% Sn, and 2.3 Mt @ 30% Kaolin.
Lithium mineral resources and ore reserves at March 31, 2011 (Ingham et al., 2011) were:
Proved reserve - 0.2 Mt @ 3.9% Li2O,
Probable reserve - 31.3 Mt @ 3.1% Li2O,
TOTAL reserve - 31.4 Mt @ 3.1%Li2O,
Measured resource - 0.6 Mt @ 3.2% Li2O,
Indicated resource - 117.9 Mt @ 2.4% Li2O,
TOTAL Measured + indicated resource - 118.4 Mt @ 2.4%Li2O,
Indicated resource - 2.1 Mt @ 2.0% Li2O.
The most recent source geological information used to prepare this summary was dated: 2012.
Record last updated: 9/6/2016
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
Hatcher M I, Clynick G 1990 - Greenbushes Tin-Tantalum-Lithium deposit: in Hughes F E (Ed.), 1990 Geology of the Mineral Deposits of Australia & Papua New Guinea The AusIMM, Melbourne Mono 14, v1 pp 599-603|
Partington G A 1990 - Environment and structural controls on the intrusion of the giant rare metal Greenbushes Pegmatite, Western Australia: in Econ. Geol. v85 pp 437-456|
Partington G A, McNaughton N J, Williams I S 1995 - A review of the geology, mineralization, and geochronology of the Greenbushes Pegmatite, Western Australia: in Econ. Geol. v90 pp 616-635|
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