Summary:
The Greenbushes pegmatite deposit consists of several large pegmatite intrusive bodies which are separated into two main lodes, namely the Central and Kapanga lodes. Both areas consist of several pegmatite bodies, however, the Central lode displays significantly more continuity and thickness as compared to the Kapanga lode.
The Greenbushes pegmatite deposit consists of a primary pegmatite intrusion with numerous smaller, generally parallel pegmatite dikes and pods to the east. 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 Central lode consists predominately of a single main body which is defined by drilling over a strike length of 3 km with thickness ranging from a few 10 m to up 300 m and dips moderately to steeply 40-60° to the southwest. This body contains the majority of the reported Mineral Resource; however, recent drilling (but not included in the Mineral Resource estimate) supports the interpretation of a southern plunge. The westerly extent of the Central lode is limited by a north south structure which potentially offsets mineralization.
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 Li2O values and moderate Ta2O5 values. This is in contrast to C1 and C3 which have large distinct zones of separate Li2O and Ta2O5 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 operations with the depth of mining. Moving north, the dip of the pegmatite shallows and the lithium domain at more than 1% Li2O is discontinuous.
The Kapanga lode is located 300 m to the east of the Central lode and consists of a series of subparallel bodies that strike to the northwest and dip between 40-60° to the southwest. The Kapanga pegmatite lodes consist of a series of relatively continuous semi-parallel bodies interpreted over a northerly strike of approximately 1.8 km with a combined thickness ranging between 120-150 m up to 450 m below surface.
Both the host and pegmatite bodies are intruded by a series of cross-cutting dolerite dykes and sills. The intrusions range from 1 to 50 m wide. Both the host and dolerite intrusives have iron (Fe) content ranging from 9 to 20% with averages of approximately 15%. The inclusion of iron in dilution and feed to the plants has a significant impact on processing recoveries.
Weathering and oxidation are prevalent in the area reaching depths of up to 40 m. The oxidation of lithium-bearing minerals results in the inability to achieve a marketable product, and as such, these oxidized areas are excluded from the Mineral Resources. The majority of rocks in the area are typically covered to shallow depths (a few meters) by lateritic conglomerates and alluvium.
Pegmatite Zonation
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 economic.
Within the thinner stacked Kapanga pegmatites, zonation varies as expected for the style of mineralization. Generally, these pegmatites are less fractionated, with only three distinct zones defined. The elevated spodumene (lithium-rich), zones in individual pegmatite lenses are generally located near the footwall contacts (and to a lesser extent the hanging wall contacts), usually a K-feldspar rich zone occurs close to the hanging wall contact with the core regions generally being albite rich zones. Variation and zonation in mineralogy (and importantly in spodumene), between individual lenses within the Kapanga group of pegmatites is also evident, with the higher lithium concentrations generally in the upper part (hanging wall).
Mineralization
The primary lithium ore mineral within the main mineralized areas is spodumene (LiAlSi2O6) with very little lithium-bearing micas observed. The sodium-rich zone contains the highest concentrations of tantalum (tantalite) and tin (cassiterite). This zone is characterized by dominant mineralogy of albite (sodium-plagioclase), tourmaline and muscovite mica. The mixed zone contains lower concentrations of tantalum and lithium, and intermediate values of sodium and quartz, showing variable mineralogy partly similar to both the lithium-rich zone and the sodium-rich zone.
The potassium-rich zone, which is dominated by the Feldspar microcline, does not have any minerals of economic interest.
The Kapanga pegmatites show less distinction in mineralogy, spodumene content does not necessarily align with specifically low potassium as in the Central lode, which is a common feature in regional pegmatite field fractionation.
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.