Summary:
The Greenbushes pegmatite deposit consists of a primary pegmatite intrusion with numerous smaller, generally parallel pegmatite dikes and pods to the east. For the purposes of this report, the term Greenbushes pegmatite deposits relate to the property-scale pegmatites. Central Lode refers to the primary pegmatite area which has been the focus of mining activity while the Kapanga deposit refers to the area of sub-parallel pegmatite located to the east of the Central Lode. The primary Central Lode deposit intrusion and the subsidiary Kapanga deposit dikes and pods are concentrated within shear zones on the boundaries of granofels, ultramafic schists and amphibolites. The pegmatites are crosscut by mafic dolerite dikes that range from 1 to 50 m wide and over 2 km long. The dolerite dikes occur in both swarms and linear trends with two principal orientations, north to south and east to west. The broader pegmatite body is over 3 km long (north by northwest), up to 300 m wide (normal to dip), strikes north to north-west and dips moderately to steeply west to south-west. The syn-tectonic development of the pegmatite has given rise to mylonitic fabrics, particularly along host rock contacts.
The Greenbushes pegmatite is mineralogically segregated into five primary zones. Internally, the Greenbushes pegmatite consists of the Contact Zone, Potassium Feldspar (Potassium) Zone, Albite (Sodium) Zone, Mixed Zone and Spodumene (Lithium) Zone. The zones differ from many other rare-metal pegmatites in that they do not appear concentric, but are lenticular in nature, with inter-fingering along strike and down dip. They do not have a quartz core which is typical of other deposits. The mine sequence was later subjected to the transgressive east-west dike and conformable sill dolerite intrusions.
The highest concentrations of primary Li-bearing minerals are found in specific mineralogical zones or assemblages within the pegmatite. The Lithium Zone within the main pegmatite body exhibit variable dips from 80 to 20° towards the west and south-west. Tantalum (tantalite) and tin (cassiterite) mineralization is concentrated in the Sodium Zone which is characterized by albite (Na-plagioclase), tourmaline and mica (muscovite). The Lithium Zone is enriched in the lithium bearing silicate spodumene. The mixed zone contains lower concentrations of tantalum and lithium. The final major zone is the potassium feldspar microcline which is not considered currently economic.
The predominant rock units on the Greenbushes property are a package of Archean amphibolite and metasediments above the basement Bridgetown Gneiss. Locally, this is present as the hanging wall Amphibolite and Footwall Granofels. Numerous Archean granitoid intrusions are present, all of which are cut by the Donnybrook-Bridgetown Shear zone represented onsite as the roughly N – S trending shear-zone gneiss. Pegmatite intrusions which host Li mineralization have intruded this package of Archean rocks. Post-mineralization dolerite dikes intrude older units, dated at approximately 1.1 Ga. Lastly, recent cover material of lateritic conglomerates, older alluvium, and recent alluvium are present as shallow cover. A simplified stratigraphic column is presented in Figure 6-4. Weathering and erosion of the pegmatites has produced adjacent alluvial deposits in ancient drainage systems. These are generally enriched in cassiterite. All rocks have been extensively lateritized during Tertiary peneplain formation; the laterite profile locally reaches depths in excess of 40 m below the original surface.
The Central Lode lithium deposit occurs within a large (250 m wide) lithium enriched pegmatite. Spodumene in the Lithium ore zone can make up more than 50% of the rock with the remainder being largely quartz. Toward the northern end of C3 pit, a highly felspathic (K-feldspar) zone separates the high-grade lithium zone from the hanging wall amphibolite and the dolerite sill. Tantalum/tin and lithium ore body mineralization are conformable with the trends of the pegmatites both along strike and down dip.
Between C3 and C1 is the mining area referred to as C2. The pegmatite in this area dips approximately 40° west and has an intermediate composition with moderate lithium oxide Li O values and moderate tantalum pentoxide (Ta O ) values. This is in contrast to C1 and C3 which have large distinct zones of separate Li O and Ta O high-grade.
At the southern end of the Central Lode pits is the C1 pit area. It contains the next largest concentration of high-grade spodumene lithium mineralization after C3. The eastern footwall contact in the south of the C1 area dips 35° west steepening toward the north and with depth. The internal grade domains in C1 parallel the eastern footwall contact. The immediate footwall is enriched in tantalum with typical accessory minerals tourmaline and apatite visible. Weathering has locally resulted in argillic alteration of pegmatites near-surface, although this has only limited effects on current operations with the depth of current mining. Moving north, the dip of the pegmatite shallows and the lithium domain at more than 1% Li O is discontinuous.
The Kapanga deposit sits approximately 300 m east as a sub-parallel pegmatite to the Central Lode deposit, and represents a thinner zone of spodumene mineralization, nearsurface, but with reduced volume compared to the Central Lode. It has been interpreted over a northerly strike length of approximately 1.8 km. The pegmatite intrusives within Kapanga typically dip at 40° to 50° with some steepening to 60° toward the southern end of the deposit. The pegmatite has been interpreted as several sub-parallel stacked lodes of varying thickness and length, as well as numerous smaller pods, with an overall thickens of approximately 150 m. Current drilling has identified depth continuity to approximately 450 m below surface.
Mineralogy
As stated above, internally the Greenbushes pegmatite displays up to five mineralogically defined zones; the Contact Zone, K-Feldspar (Potassium) Zone, Albite (Sodium) Zone, Mixed Zone and Spodumene (Lithium) Zone. Zones generally relate to multiple phases of intrusion and crystallization of the pegmatites.
The zones occur as a series of thick layers commonly with a lithium zone on the hanging wall or footwall, K-feldspar towards the hanging wall and a number of central albite zones. High-grade tantalum mineralization (more than 420 grams per tonne [g/t]) is generally confined to the Albite zone within the deposit. The Spodumene and K-Feldspar Zones typically have tantalum-tin grades of less than 100 ppm.
Major minerals hosted in the pegmatites are quartz, spodumene, albite, and K-feldspar. Primary lithium minerals are spodumene, LiAlSi O (approximately 8% Li O) and spodumene varieties kunzite and hiddenite. Minor lithium minerals include lepidolite (mica), amblygonite and lithiophilite (phosphates). Spodumene is hard (6.5 to 7) with an SG of 3.1-3.2. Highest concentrations (50%) of Spodumene occur in the C1 and C3 pits
When spodumene is weathered and oxidized the lithium ions leach into the environment, the result is spodumene pegmatite weathered to clay. This is of little to no economic value to the current operation. Oxidation of the pegmatites has generally occurred in near-surface weathering or along selected structures internal to the pegmatites. Only the near-surface weathering is considered to materially affect the pegmatite from a process mineralogy standpoint.
Tailings Storage Facility (TSF1) ‘geology’ is characterised by a ~7m thick upper layer of higher-grade ‘enriched’ tailings overlying a ~7.5m lower grade layer ‘depleted’ layer, which in turn overlies a layer of clay tailing which in turn overlies the pre-existing natural surface.