Graphite: properties, uses and South Australian resources
Geological Survey of South Australia, Department of the Premier and Cabinet
Background and properties
Graphite is a versatile industrial mineral with unique properties that have facilitated technological innovation, beginning in the 16th century with discovery of high-grade lump graphite at Borrowdale, England. Borrowdale graphite was carved into pencil sticks for convenient durable markers and had strategic importance as a refractory lining in moulds that produced superior, smooth and round cannonballs with greater projectile range. Today, natural graphite is a key component in high-performance refractory linings for steel manufacture, high-charge capacity anodes for lithium-ion batteries, and a source of graphene to inspire a new generation of smart materials.
Graphite, like diamond, is a crystalline form of carbon. Graphite and diamond are natural allotropes of carbon (i.e. different molecular forms of the same element) that arise from the way the carbon atoms are joined together and arranged to form regular structures. In diamond, each carbon atom is bound to four other carbon atoms by strong covalent bonds in a regular isometric structure that gives rise to the hardest known mineral. In graphite, carbon atoms are bonded to only three other carbon atoms to form strong, two dimensional layers, that are extremely stable, but where each layer is only weakly linked to adjacent layers by van der Waal’s forces. The resulting hexagonal layered structure forms one of the softest minerals. The presence of an unpaired valence electron makes graphite an excellent electrical conductor within the plane of the layers. Graphite is inert towards most chemicals and has a high melting point of ~3,550 °C, but in the presence of oxygen will begin to oxidise at temperatures >300 °C and can be induced to sustain burning above 650 °C, given suitable conditions; the rate of thermal oxidation is slow but increases with increasing temperature. Thermal conductivity in graphite is anisotropic but is very high in the direction parallel to the plane of the layers. The calculated crystal density of graphite is 2.266 g/cm3 but the measured specific gravity is typically between 2.20–2.30 depending on purity; high values are due mostly to impurities, low values are associated with trapped porosity. Graphite is soft, Mohs hardness of 1–2, greasy to the feel, and is flexible and sectile, but not elastic. It commonly forms as foliated or scaly masses but may be radiated or granular, and is opaque black with a metallic lustre but can also be dull and earthy.
The graphite content in commercial graphite samples is determined routinely by LECO furnace method with pre-treatments, as necessary, to eliminate carbon associated with carbonate minerals and non-graphitic organic matter (e.g. Sader et al. 2015). Analyses are reported as percentage carbon (C) or as graphitic carbon (Cg); the trend in reporting resources data for natural graphite deposits is to use the term ‘total graphitic carbon’ (TGC).
Commercial sources of natural graphite are commonly classified as flake graphite (Fig. 1), vein or ‘lump’ graphite, and ‘amorphous’ (microcrystalline) graphite (Table 1). Broadly, these reflect the geological setting and conditions under which the graphite formed.
Flake graphite is associated mostly with high-grade metamorphic rocks where organic carbon deposited within sediments was transformed into graphite by pressures typically exceeding 5 kbar and temperatures above 700 °C. The process of metamorphic graphitisation is controlled largely by temperature and is essentially complete above 500 °C (Wang 1989), but pressure, duration of the metamorphic event, increasing temperature and shear strain influence the degree of crystallisation and the size of graphite flakes (Buseck and Huang 1985; Beyssac et al. 2002). Graphite-bearing rocks include orthoquartzite and marble but more commonly are quartz–biotite schist and gneiss, sometimes with sulfide minerals, and usually with a variable content of associated metamorphic minerals including feldspar, muscovite, chlorite, garnet, orthopyroxene or sillimanite. The graphite is often present as flakes or elongate patches aligned with the schistosity or banding to form lenses or layers where the graphite content can exceed 10% of the rock.
Vein graphite occurs as vein, fracture fill or pipe-like bodies where graphitic carbon and/or carbon-rich fluids have migrated and precipitated as graphite masses. High-grade, vein-style graphite is known from several countries, but at present is produced only from Sri Lanka, a long-term supplier. Former sources include the historic Borrowdale mine where highly crystalline graphite forms pipe-like bodies, fracture fill and disseminated replacements. Recent studies concluded that these formed as the result of dehydration and rehydration reactions of CO2 and CH4 fluids, generated during dioritic magma assimilation of organic-rich metapelites at comparatively low temperature and pressure of 400–500 °C and 1–2 kbar, respectively (Luque et al. 2009; Ortega et al. 2010). Vein graphite from Sri Lanka shows needle-like macro morphology and flake-like micro morphology and is produced as centimetre-sized lumps of mostly high purity graphite (94–99% Cg).
Amorphous graphite is produced mostly from anthracitic coal seams that have undergone variable graphitisation during contact or regional metamorphism. This form of graphite is typically massive and has comparatively high levels of fine-grained impurities that are not easily separable from the graphite. Commercial grades typically range from 75 to 85% Cg. Some ‘amorphous graphite’ is also produced as a fine-grained graphite byproduct from flake graphite mining and processing.
Total production of natural graphite in 2015 was estimated to be 1.19 Mt (United States Geological Survey 2017) with China the largest producer (Fig. 2). Natural graphite product was comprised of 50% amorphous graphite – predominantly from China and Mexico; 49% flake graphite – dominated by China with significant production also from India and Brazil; and <1% vein graphite from Sri Lanka.
Synthetic graphite is made by heating amorphous carbon materials, such as calcined petroleum coke of suitable crystalline quality (i.e. high proportion of naturally occurring benzene rings), in a reducing environment at temperatures between 2,300 to 3,000 °C to convert it to graphite. The process of preparation, heating and cooling takes several weeks to several months. Graphite fibres are manufactured by high-temperature conversion of polyacrylonitrile, pitch or rayon precursors.
Table 1 Summary of characteristics of different forms of commercial natural graphite
|Description||Crystalline graphite flakes; coarse >150 µm; fine <150 µm||Interlocking aggregates of coarse graphite crystals; typically >4 cm||Microcrystalline, soft earthy graphite; mostly <40 µm|
|Origin||Syngenetic; regional metamorphism of organic matter in metasedimentary rocks||Epigenetic; regional metamorphism; metasomatism involving CO2–CH4–H2O fluids||Syngenetic; contact and/or regional thermal metamorphism of coal seams|
|Ore||2–30% graphite; stratabound, tabular or lenses||>90% graphite; veins and fracture infill||>70% graphite; in anthracitic coal layers, typically folded and faulted|
|Product grade||85–97% Cg||90–99% Cg||75–90% Cg|
|Main uses||Refractories, batteries, brake linings, flame retardants||Carbon brushes, brake linings, batteries, lubricants||Steel recarburiser, foundry mould facing, lubricants, pencils|
|Major producers||China, Brazil, India, Canada||Sri Lanka||China, Mexico, North Korea, Turkey|
Source: Updated from Mitchell (1993).
Synthetic graphite makes up over 90% of the value of the global graphitic carbon market, which was estimated during peak demand in 2012 at around $US14 billion (Fig. 3). Natural graphite accounted for less than 10% by value but the quantities produced were similar with ~1.2 Mt natural graphite and ~1.5 Mt synthetic graphite (Fig. 4). Independent estimates of natural graphite demand for 2014 and 2015 were <700,000 tpa (Fig. 5), with significant oversupply driving lower prices (Schodde 2016; Spencer and Hill 2016). Demand for flake graphite is presently under 400,000 tpa, but this is anticipated to rise substantially in line with the growth in manufacture of lithium-ion batteries for portable devices, electric vehicles and energy storage (Fig. 5).
Synthetic vs natural graphite
Synthetic and natural graphite supply mostly separate markets but there is increasing overlap in uses (Fig. 3), influenced mostly by factors of price and purity. Synthetic graphite is engineered to specific specifications with high purity and predictable properties, but is less conductive and more expensive than natural graphite. Synthetic graphite is used to make electrodes for metal refining, principally in electric arc furnaces used for steel production and large cathodes that line electrolytic cells used in aluminium smelting. Other uses include graphite blocks and powders, carbon brushes and bearings, and graphite fibre reinforcement in polymer composites. Natural amorphous graphite is used as a carbon raiser in iron casting and steelmaking, as a facing on foundry moulds, and for pencil manufacture. Flake graphite is consumed mainly in the production of high-performance refractories. These are used in steelmaking to line ladles and the zones most vulnerable to slag corrosion in basic oxygen and electric arc furnaces, usually in the form of magnesia-carbon bricks or alumina-graphite casting ware and monolithic refractories (Ewais 2004; Stein and Aneziris 2014). Coarser grades of flake graphite (Table 2) have a slower rate of oxidation and are preferred by refractory manufacturers, but surface treatments and antioxidants are widely used that also improve the performance of more readily available medium-flake graphite (Ewais 2004; Lee and Zhang 2004).
Table 2 Flake graphite particle size ranges widely applied for industrial uses
|Particle size range|
|Very fine (amorphous)||<75||<0.075||–200|
Both synthetic and natural graphite make effective, stable anodes for batteries. Increased demand for graphite in this market is being driven by the growth in manufacture of lithium-ion batteries. High electrical storage and good charge cycling characteristics can be achieved using natural graphite after treatment by impact milling to crumple the flakes into spherical shapes (Fig. 6), followed by acid or thermal purification (>99.5% Cg), then coating with a thin film of amorphous carbon (Yoshio et al. 2004; Mundszinger et al. 2017). The shaping and coating reduce the surface area of graphite flakes and increase the anode packing density. This improves the rate and extent of intercalation of lithium ions into the graphite, reduces the tendency for exfoliation, and controls the formation of solid electrolyte interphase, which consumes lithium ions and can lower battery capacity (Joho et al. 2001; An et al. 2016). Anode technologies continue to evolve and further improvement in reversible charge capacity and cycling performance is achieved by coating processed natural graphite with silicon nanoparticles (e.g. Zhang et al. 2007).
Processing of natural graphite into spherical shapes currently has low efficiency, resulting in high quantities (50–70%) of very fine grained, relatively high-grade graphite ‘waste’ that will likely displace amorphous graphite in some traditional markets. Despite the high wastage, natural spherical graphite anodes offer cost savings over synthetic graphite. An estimated >35,000 t of natural graphite anode was used in 2015 out of a total lithium battery anode demand of >75,000 t (Pillot 2016). Anode graphite demand is anticipated to rise to between 250,000 and 400,000 t by 2020, mostly to service projected increased sales of portable electronic devices and electric vehicles (Benchmark Mineral Intelligence 2016). Assuming use of natural graphite in anodes is 50%, a conversion efficiency of 50% would require ~400,000 t of quality flake graphite to meet the upper projection. This is in line with the Industrial Minerals Research forecast of ~360,000 t of flake graphite for anodes by 2020 (Patel 2017). Improvement in production efficiency of spherical graphite is expected from new resource developments that will produce higher grades of flake graphite product (95–98% Cg) and offer greater choice of flake size in the range 50–300 µm. Being more selective in the flake size range used to produce spherical graphite is one approach to achieving higher yields of anode product with specified median particle size (varies from 5 to 25 µm depending on the battery device; Moores 2016).
Expandable flake graphite
Demand for expandable natural flake graphite is also expected to increase, principally for use as a fire retardant additive in foam, plastic and various construction materials (Ghilotti 2016). For this use, suitable flake graphite is intercalated with a chemical species (e.g. halogens, sulfate, nitrate, organic acids, ferric chloride) which will rapidly gasify at high temperatures causing the graphite to exfoliate/expand to between 80 and 300 times. Coarse flakes (>300 µm) show the highest expansion but finer sizes (<150 µm) are preferred for some applications. On expansion, graphite retards the spread of fire and minimises the creation of toxic gases and fumes. Expanded graphite can also be interlocked and compressed to give an essentially flat, flexible, integrated graphite foil, which is widely used as gaskets and seals (Chung 1987), with potential use also as a separation layer in fuel cells and redox flow batteries.
Graphene is a single layer of carbon atoms with tensile strength >100 times that of steel, and with much higher electrical and heat conductivity than copper or silver (Novoselov et al. 2012). Potential applications include batteries, supercapacitors, electronic, energy storage, water desalination, adsorbents, surface coatings and biomedical, although new products are mostly still in research and development. Graphene, or graphene-like products comprising several layers of carbon atoms, can be derived from natural graphite by chemical oxidation and exfoliation to give a graphene oxide product (Tung et al. 2016), which can then be reduced using amino acids to give nanosheets with fewer oxygen groups and good stability in aqueous dispersions (Tran, Kabiri and Losic 2014). Highest quality graphene coatings are produced synthetically (Novoselov et al. 2012). Demand for graphene from natural graphite or synthetic sources is still exceedingly small and the impact on existing markets or the requirements for new markets await further commercial developments.
Other developing markets
Technologies already developed that could impact future demand for graphite include fuel cells, where hydrogen and oxygen are reacted via a proton-exchange graphite membrane to generate electrical energy and water, redox flow batteries, and pebble-bed nuclear reactors, where uranium fuel pellets are encased in nuclear-grade graphite pebbles, ~60 mm in diameter. Further commercialisation of these technologies would substantially increase the demand for graphite, although the specification for very low levels of impurities presently favours synthetic graphite over purified natural graphite.
The current slowdown in steel manufacture worldwide has resulted in an oversupply of graphite and a reduction from the high prices achieved during 2011–12 (Fig. 5). Growth in the near term will be driven by developments in the battery market and the steel industry, but future opportunities for growth in graphite markets clearly exist across various emerging technologies.
South Australian natural graphite
Numerous graphite occurrences have been recorded in Neoarchean to Paleoproterozoic high-grade metamorphic rocks on Eyre Peninsula in the southern Gawler Craton (Fig. 7). Occurrences of graphite from Sleaford Bay, south of Port Lincoln, to Kimba in the north were reported shortly after European settlement of the region in the late 1800s to early 1900s (Brown 1908). Many of these were prospected extensively during the 1910s and 1920s when imports of graphite from Sri Lanka and India were restricted during World War I. Minor graphite production from the Uley and Koppio mines took place between 1928 and 1951 to supplement Australian requirements for graphite in steelmaking, when imports from Sri Lanka and Madagascar were halted during World War II (Valentine 1994). Historic production of a few thousand tonnes has come largely from the Uley graphite mine, which has been revived on two occasions. Market demand and prices for natural graphite, however, have, to date, not been sustained at sufficiently high levels to justify the cost of extraction and beneficiation, despite the confirmation of substantial resources.
A dramatic increase in the price for graphite during 2010–12 sparked renewed interest in graphite exploration. The latter price rise was due to strong demand for graphite in steelmaking, restructure of Chinese graphite mines that restricted output from the world’s leading suppliers, and anticipated additional increase in demand for natural flake graphite, principally for lithium-ion batteries. Exploration resulted in further discoveries on Eyre Peninsula and subsequent resource drilling at Uley, Kookaburra Gully, Koppio, Campoona, Wilclo South, Oakdale, Oakdale East and Siviour deposits (Fig. 7).
Total graphite resources across all Eyre Peninsula deposits, as at June 2017, exceeds 100 Mt with grades ranging from 3 to 17% TGC, for ~8.6 Mt of in situ graphite (Table 3). The resources and grades are significant in terms of current world demand for natural crystalline flake graphite of ~0.5 Mtpa, but are an order of magnitude smaller than resources with comparable, or higher, grades discovered recently in northern Mozambique and southern Tanzania. Graphite deposits are not rare globally. The viability of individual deposits is a function of competitive supply and demand, influenced by economic factors of development, processing and transport costs, and also physical characteristics of the graphite product in terms of particle size and shape and the composition and characteristics of impurities (Fig. 8).
Table 3 Resource estimates (JORC) for graphite deposits on Eyre Peninsula at June 2017
|Deposit||Mineral Resources*||Million tonnes (Mt)||Grade (% TGC)||Contained graphite (t)||Cutoff (% TGC)|
|Campoona Shaft (Archer Exploration 2014)|| Measured|
|Kookaburra Gully (Lincoln Minerals 2017a)|| Measured|
|Koppio (Lincoln Minerals 2015)|| Inferred (D1)|
| Oakdale (Oakdale 2015)|
Oakdale East (Oakdale 2015)
|Siviour (Renascor Resources 2017a)|| Indicated|
|Uley – Main Road deposit / Pit 2 (Valence Industries 2015)|| Measured|
|Wilclo South (Monax Mining 2013)|| Inferred|
* Mineral resources data rounded down to nearest 1,000 t, and one decimal place for grade.
Exploration and mining
Graphite exploration and mining activity in South Australia prior to 1993 is described in Valentine (1994), which includes a summary of known graphite occurrences. While early discoveries were the result of surface observations, most deposits are highly conductive and their extent below shallow cover is effectively mapped using airborne and ground electrical geophysical methods (Barrett and Dentith 2003). Graphite content and quality are assessed by drilling, usually a combination of rotary air blast, aircore, reverse circulation (RC) and diamond core. Structural complexity of individual deposits is a factor in the amount of core drilling required for resource estimation. Throughout the region, thick regolith is widespread, predominantly as weathered bedrock but also as thin cover of unconsolidated aeolian and fluvial sediments. Deeply weathered bedrock can provide an advantage in low-cost open pit excavation with little or no requirement for blasting in the upper 30–100 m of orebodies. The variable effects of weathering, however, need to be assessed along with other factors affecting graphite recovery, and the grade and quality of the graphite product.
Graphite mineralisation on Eyre Peninsula occurs as disseminated flake graphite in late Archean Sleaford Complex granulite formed during the Sleaford Orogeny (c. 2450 Ma), and in Paleoproterozoic metasedimentary rocks recrystallised at high metamorphic grade during the 1730–1690 Ma Kimban Orogeny. Peak metamorphic conditions of 8–9 kbar and 820–850 °C (Dutch, Hand and Kelsey 2010) affected Paleoproterozoic rocks forming a north–south belt of tightly folded metasedimentary rocks adjacent to the Kalinjala Mylonite Zone (Fig. 7). Flake graphite deposits within this belt are hosted by Hutchison Group metasedimentary rocks, originally deposited as marine sand and silty clay, with carbonate and banded iron formation. Carbon isotope δ13C values for graphite from the Uley and Koppio mines cluster around –25‰ (Taylor and Berry 1990). This is consistent with carbon derived from an organic sedimentary source, principally algal or bacterial remains that accumulated during times of relatively low detrital or chemical sediment input. The organic matter was converted to graphite during high-grade metamorphism. Graphite distribution is essentially stratigraphically controlled, with minor remobilisation within shear zones.
Deposits and occurrences
Uley graphite mine was worked initially in 1928–29 and again in 1941–45 and 1990–93. Re-opening the Uley mine in the early 1990s was in response to increasing price for refractory-grade flake graphite. The price dropped sharply in 1992 when China entered the global graphite market with access to local, large resources and the support of a rapidly expanding local steel industry. Most recent operations to restart the Uley mine in 2015, by Valence Industries Limited, were suspended in early 2016 pending modifications to the metallurgical circuit; attempts to raise the necessary capital failed and the company was placed in voluntary administration in July 2016.
Disseminated crystalline flake graphite, 0.1–2 mm diameter, is present in biotite–quartz schist and quartz–feldspar–biotite ± garnet gneiss, equated with Hutchison Group, metamorphosed at upper amphibolite to lower granulite facies. Ore zones are graphitic lenses typically grading >6% Cg, which are up to 12 m thick and tightly folded. Open-cut mining during the 1990s was on the hinge of a broad anticlinorium that plunges shallowly to the north-northeast (Fig. 9). Graphitic layers are locally thickened in asymmetric, tight to isoclinal, upright to overturned mesofolds, commonly with shearing along the synclinal axes (Fig. 10). Weathering of the ore zone extends to at least 50 m depth; within the weathered zone, graphite-bearing biotite schist is mostly altered to clay with variable iron oxide content and residual primary quartz. The upper 15 m is dominated by kaolinite with secondary patches of carbonate that become aggregated to form a 0.5–3.0 m thick hardpan directly above the watertable. The patches of dominantly calcite cement, present throughout the vadose zone and forming the extensive hardpan, result from precipitation of calcium remobilised from calcarenite dunes, which form a thin surficial cover across the site. At depths below ~15 m, kaolinite and nontronite clays predominate with remnant patches of weathered biotite (Keeling, Raven and Gates 2000). Graphite recovery by flotation is more effective, in terms of high product grade and coarse particle size, for ore extracted from below the zone of secondary carbonate cement (Keeling 2000). Distribution of coarse flake graphite (>250 µm) is irregular, but occurs particularly at the margins of pegmatitic leucosomes, formed by localised melting, and in the hinge zone of late-stage folds. Fine-grained graphite (<150 µm) is dominant in the upper section of the weathered profile and in graphitic shear zones. Uley graphite ore can be processed by conventional flotation to give products with 91–95% Cg in various size ranges from >500 µm through to 45–75 µm (McNally 1997).
Uley is one deposit in a broad zone of graphite mineralisation on southern Eyre Peninsula termed the Mikkira Graphite Province (Fig. 7). The province was outlined by CRA Exploration in the 1980s using airborne electromagnetic (EM) methods with follow-up drilling to confirm graphite as the cause of several large conductive anomalies. The SIROTEM ground EM technique was effective at Uley to map the continuation of subsurface graphitic units (Fig. 9), leading to discovery of high-grade extensions to the ore zone, ~0.5 km south of the existing workings (Barrett and Dentith 2003). The original open cut was later abandoned and flooded to serve as water storage for the refurbished graphite treatment plant. A new pit, Main Road Pit or Uley Pit 2, was commenced in 2015 on a high-grade extension on the eastern limb of the anticline. At this site the fold limb is interpreted as overturned to recumbent, with tightly folded graphitic layers in an overall ~30° dip west-northwesterly. Total graphite resources to ~100 m depth are 4.54 Mt at 11.6% TGC for 520,000 t of contained graphite at cutoff grade 3.5% TGC (Table 3).
Koppio graphite mine, 32 km north of Port Lincoln, was worked in the 1940s with 100 t mined from underground workings and trucked to Port Lincoln for processing. The ore zone is a series of steeply dipping (60–75° ESE) lenses of graphitic schist of aggregate thickness 10–30 m within Hutchison Group metasediments. Aircore and RC drilling during 2014 by Lincoln Minerals confirmed graphite over a strike length of >570 m and extending from surface to at least 100 m depth. The inferred resource for a nominal cutoff grade of 5% TGC is 1.85 Mt at 9.8% TGC for 180,000 t of contained graphite. Historic flotation and gravity separation tests on representative ore taken from a drive at the base of a 12 m deep shaft reported only 12% of recovered graphite was >150 µm flake size grading 90.0% C, which included 3.7% >500 µm (Gartrell and Blaskett 1943).
Kookaburra Gully deposit
Graphite at Kookaburra Gully, 3 km north of Koppio mine, is hosted by high metamorphic grade schist and gneiss of Hutchison Group. Graphitic schist forms a series of lenses that strike NE–SW and dip southeasterly at 40–50°, but locally steepen to 80°. Preliminary evaluation of the deposit was made by Pancontinental Mining Ltd in the 1980s, but not pursued due to the high proportion of <150 µm flake graphite (Meares 1987). The project was revisited by Lincoln Minerals in 2012 following increase in the price for graphite and the possibility of new markets (Lincoln Minerals 2012).
The deposit is located in gently undulating topography of the Koppio Hills, with maximum relief ~90 m. Bedrock is variably weathered, depending on lithology and landscape position. Depth of weathering exceeds 30 m in the uplands where the watertable is ~40 m below ground surface. The ore zone is typically 15–20 m wide with graphite grading 5–35% Cg. Discontinuous lenses of dolomitic marble underlie portions of the graphitic schist, which is broadly intercalated with quartz–biotite schist and biotite–quartz–feldspar gneiss with minor interlayered amphibolite. Garnet, sillimanite and tourmaline are present in minor to trace amounts.
The graphitic lenses extend along strike for >500 m and are interpreted as tightly folded layers of an antiformal structure that plunges south-southwesterly. An airborne EM survey, flown in 2012 for Lincoln Minerals, shows conductive structures that continue to the southwest, with repetition of highly conductive zones at the Kookaburra Gully Extended prospect (Fig. 11). Aircore and RC drilling during early 2017 confirmed extensive graphite mineralisation of variable grade and thickness below 2–30 m alluvium and weathered bedrock cover (Lincoln Minerals 2017).
Preliminary metallurgy on surface trench and aircore samples from Kookaburra Gully deposit indicate that a graphite concentrate with 93–97% Cg and flake size 20–150 µm diameter can be recovered after grinding the ore to <600 µm followed by four stages of flotation, cleaning and regrinding (Parsons Brinckerhoff 2015). Recovery of flake sizes >150 µm varied between 2 and 24% at grades 93–98% Cg. Drilling and trenching have outlined a resource of 2.94 Mt at 11.4% TGC, at cutoff grade >2% TGC, containing 335,000 t of graphite. A mineral lease (ML 6460) covering the Kookaburra Gully deposit was granted on 3 June 2016.
Siviour deposit, Arno Bay
The Siviour graphite deposit is a well-defined EM conductor anomaly, elongated ~3 km W–E and ~0.6 km wide, that was outlined during an EM survey flown in 2006–07 by Cameco Ltd in the search for uranium mineralisation. Follow-up drilling by Cameco included one cored hole, CRD0090, near the eastern extent of the conductor. This intersected graphite mineralisation (Paxtons prospect) commencing at 67.7 m, but the graphite was not tested at the time. Subsequent analyses in 2014, on the core sample lodged with the South Australia Drill Core Reference Library, returned 12.4 m at 8.1% Cg, with over 40% of the graphite concentrate having flake size >150 µm. In 2015 a drill traverse across the central portion of the EM anomaly intersected high-grade, coarse-flake graphite at depths of 26 to 55 m below surface. Resource definition drilling by Renascor Resources Limited, during 2015–17, delineated a tabular, mostly flat-lying, graphitic biotite–quartz–feldspar schist with abundant coarse flake graphite for a total resource of 80.6 Mt at 7.9% TGC, at cutoff grade 3% TGC, containing 6.4 Mt of graphite (Table 3). The graphite ore zone averages 20 m thickness, at a depth of 10 to 25 m below surficial cover sediments and weathered calc-silicate bedrock capped by calcrete. The extent of graphite mineralisation was reassessed in early 2017 using a helicopter EM survey to confirm the overall flat-lying shape of the orebody and to locate near-surface extensions to mineralisation along strike (Renascor Resources 2017b; Fig. 12).
Metallurgical test work on drill core samples using a conventional graphite flotation circuit gave graphite concentrates averaging 94% Cg, including recovery of a high proportion of coarse flake graphite (33% >180 µm; Table 4; Renascor Resources 2017a). The deposit is the largest coarse-grained, high-grade, flake graphite resource yet discovered on the Eyre Peninsula.
Table 4 Siviour deposit flake graphite size distribution
|Flake category||Particle size range (µm)||Distribution (%)|
|Very fine (amorphous)||<75||13|
Source: Renascor Resources (2017a).
Oakdale graphite prospect is the only graphite resource reliably associated with Neoarchean (c. 2540 Ma) rocks on the Eyre Peninsula. Graphite was noted in shallow drilling by BHP Billiton in 2001 while investigating magnetic anomalies associated with mafic volcanics prospective for volcanic-hosted massive sulfide mineralisation. Subsequent diamond drilling on a strong, ground EM conductor confirmed the high conductance was due to thick intervals of graphite and sulfide, modelled as dipping at 50° to the southwest. Graphitic schist and gneiss are intercalated over a 200 m wide zone of lower granulite facies, feldspar–sillimanite–quartz–pyrrhotite gneiss and marble, with calc-silicates and interbedded mafic volcanic rocks. The graphitic zones typically have a high pyrrhotite content, and are flanked by komatiite.
Projection of the graphitic zone, up dip, outlined areas of near-surface graphite at sites designated as Oakdale and Oakdale East. These were tested by shallow aircore and diamond drilling. Oakdale deposit is up to 500 m wide and extends along strike for 1.5 km. A combined resource of 6.31 Mt at 4.7% TGC, containing 296,000 t of graphite was established within mostly weathered bedrock, below ~20 m of Cenozoic sedimentary cover. The graphitic saprolite, or oxidised zone, is a maximum 30–40 m in thickness, clay-rich and can be mined without blasting or primary crushing. Preliminary metallurgy on ore from the saprolite zone yielded a high proportion (~60%) of coarse to fine flake graphite >75 µm, including ~1% jumbo-sized flakes >425 µm (Oakdale Resources 2015). Clay (kaolinite and nontronite) coating and intercalated with graphite can be reduced by desliming to remove the <15 µm fraction, followed by secondary grinding/attritioning and flotation, to give a product with >90% Cg. Further upgrading to >98% TGC is achievable with acid treatment. The graphite product has a high proportion of thin flakes, mostly <30 µm thick, with many flakes only 3–6 µm in thickness.
Numerous graphite occurrences exist throughout the Cleve–Kimba district (Fig. 13). Drilling by Archer Exploration Limited in 2008 on copper–gold targets in the region returned thick intersections of weathered graphitic schist, which subsequently became a focus of exploration across the company’s tenements.
Sugarloaf prospect. At Sugarloaf, existing airborne EM and drillhole data were reinterpreted, in light of graphitic carbon analyses on recently acquired drill samples. A geological target, with potential for 40–70 Mt grading 10–12% graphite, was developed based on a tight, upright antiformal structural model with steeply dipping limbs in which graphitic layers average 40 m thickness over 2.5–4 km strike length, along a NE–SW-trending fold axis. The graphitic schists are interlayered with banded iron formation and chloritic schist and gneiss. The schists are deeply weathered to ~80 m, coinciding with the present-day watertable. Subsequent investigations, during 2011–12, showed the graphite content is up to 30%, but is present mostly as very fine grained aggregates of variably crystalline carbon arranged in a matted and porous structure with poor electrical conducting properties. The mineralogy is dominantly silica and graphite with subordinate clay, chlorite, biotite and trace levels of sulfur from weathered sulfide. The graphite appears to have low commercial value but the potential large size of the deposit and its unusual properties raised the possibility for use as a soil conditioner on sandy soils. Preliminary investigations show that Sugarloaf graphite, in raw or modified form, improves soil wettability and water retention, and is a source of leachable macronutrients (Ca, Mg, K, S, P) and micronutrients (B, Cu, Fe, Mn, Mo, Zn) required for plant growth (Archer Exploration 2015).
Campoona Shaft deposit. The Campoona Shaft deposit is a zone of steeply dipping graphitic schist layers hosted in upper amphibolite facies paragneiss, equated with Mount Shannan Iron Formation within the Hutchison Group. The near-surface graphitic zone is 10–40 m true width, extends to beyond 100 m depth, and is a minimum 560 m in length. The zone is a composite of one thick tabular graphite schist unit and five thinner graphitic layers that strike 045°and dip 85° northwest. Total resources of 2.23 Mt at 12.3% TGC, at cutoff grade 5% TGC, provide 273,000 t of graphite contained within a zone extending from the surface to 150 m depth. The graphitic host rocks are highly weathered to 50–60 m depth with kaolinite and quartz as the principal gangue minerals. Effects of weathering extend to at least 100 m. Preliminary metallurgy, by Archer Exploration, on graphite from the highly weathered zone favours conventional fine-grinding and flotation to liberate a mostly fine-grained graphite product, <75 µm diameter, that is well-crystalline and can be separated at high grades of 95% to 99% Cg without acid treatment (Archer Exploration 2013). Acid treatment, using dilute hydrofluoric acid, yields a product with >99% Cg.
Central Campoona and Lacroma prospects. Central Campoona is located 2 km southwest of Campoona Shaft and is a 1.4 km linear zone of steeply dipping pods of graphitic schist, offset by faults. One pod, ~200 m long, was tested by drilling to establish an inferred resource of 0.5 Mt at 11.6% TGC. The graphite ore characteristics are similar to ore from the Campoona Shaft deposit. Lacroma prospect is 30 km north-northwest of Campoona Shaft and on a north–south linear EM anomaly that extends for ~12 km. Drill intersections of up to 60 m of graphitic schist grading 6.8% TGC provided graphite samples with similar characteristics also to graphite ore from Campoona Shaft. These and other graphite prospects in the district are regarded as possible satellite sources for a proposed processing facility to produce high-grade, fine-grained crystalline graphite, based initially on development of resources at Campoona Shaft (Archer Exploration 2015).
Wilclo South prospect. Historical observations of graphite occurrence and previous drillhole data were used by Monax Mining Limited, together with regional airborne EM data, to identify anomalies prospective for high-grade graphite. Several targets were identified during 2012, which included the Wilclo South anomaly in probable Hutchison Group. Evidence of graphite was located in surface outcrop and depth continuity was tested by RC drilling, which confirmed graphitic schist with ~15% TGC over 8.5 and 15 m intervals. A ground EM survey (NanoTEM method) with 20 m loop and 10 m stations was used to map the extent and attitude of subsurface conductors. The presence of a continuous zone of shallow, easterly dipping conductors was identified over 2 km strike length. Inferred resources of 6.38 Mt at 8.8% TGC were outlined in a program of 79 RC drill holes, to a maximum depth 120 m, and 2 diamond core holes (Monax Mining 2013). A zone ~70 m thick with up to seven individual layers of graphitic quartz–mica schist, 1–17 m in thickness (av. 3.7 m) and dipping 20–30° easterly was traced over a strike length of ~1.4 km. The graphitic units are interlayered with quartz–feldspar gneiss and minor amphibolite above a footwall of dominantly amphibolite. Tight folding, reverse faulting and variable depth of oxidation are evident in drill samples. Coarse-grained graphite is present in the deposit with ~30% of flake sizes >180 µm, including ~5% with flake size >425 µm. The prospect was included with the sale of the exploration tenement to Archer Exploration in July 2014.
Recent mineral exploration in South Australia has confirmed that various styles of high-grade graphite mineralisation are broadly distributed across the Eyre Peninsula. The discovery, in 2015, of the Siviour graphite deposit highlights the potential for further discovery of globally significant graphite resources in the region and the role of airborne EM as an effective exploration approach in locating and delineating shallow deposits.
Graphitic carbon markets are experiencing major change, driven by slowing global steel production, rapid growth in demand for batteries, and uncertainties around the adoption of emerging technologies. Evolving markets offer new opportunities for natural graphite, but with increased competition from synthetic graphite and alternative technologies. Participation in growth markets will be complex and dynamic. Large high-grade resources of flake graphite with low mining costs and good infrastructure are more likely to be successfully developed, although smaller deposits with special properties could find a market niche. Careful study of the characteristics of individual graphite deposits will assist in determining whether metallurgical options and processes can deliver products that are preferred in the marketplace, offer flexibility across various markets, or might justify further processing for higher value products with improved returns.
Recent geological information on graphite deposits was compiled from public company releases, in particular reports and presentations by J Parker (Lincoln Minerals), G Anderson (Archer Exploration), G Ferris (Monax Mining), J Lynch (Oakdale Resources) and D Christensen (Renascor Resources). The paper was improved with constructive feedback provided by Marc Twining.
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