Marc Davies and Marc Twining
Geological Survey of South Australia, Government of South Australia
Download this article as a PDF (7 MB); cite as MESA Journal 86, pages 30–44
Published 2018
Introduction
South Australia has a significant and continuous history of iron ore mining, commencing in the late 1800s to provide hematite flux for the Broken Hill smelting operations. The first smelting of iron ore to produce pig iron occurred in 1873 from the Mount Jagged deposit, located 52 km south of Adelaide. Deposits of iron ore in the Middleback Range, northern Eyre Peninsula, first provided ore for the Australian steel manufacturing industry in January 1915 with an interstate shipment of ore to steelworks in Newcastle, New South Wales. Mining of Middleback Range deposits continues through to the present day as the major supply of iron ore to Australia’s largest integrated steelworks at Whyalla. Whilst hematite has historically been the mainstay of South Australia’s iron ore production, the mining of magnetite commenced in the mid 2000s as the principal source of iron ore for the Whyalla steelworks (Talbot, Rowett and Abbot 2004).
The potential for significant magnetite resources has long been recognised in South Australia, but it was not until the mid 2000s that these were seriously considered as a new and viable source of iron ore for the global market. In the mid 2000s significant changes occurred in the iron ore market when product pricing changed from fixed contract to spot-market pricing and the demand for iron ore rapidly expanded, principally due to activity in China. Since then a resource base totalling 15.9 Bt of magnetite ore has been defined across three regions in the state with potential to significantly extend this (Table 1; Fig. 1). In 2015 Geoscience Australia reported that magnetite represented 44% of our nations ‘economically demonstrated resource’ base of iron ore resources (Geoscience Australia, pers. comm. 2017). South Australia’s economically demonstrated resource of magnetite ore currently totals over 6 Bt. Complementing the expanding magnetite resource base and related commercial feasibility work on these deposits, the South Australian Government’s magnetite plan aims to maximise the opportunities for the state in line with increasing global demand for magnetite. Production has commenced at two deposits (Iron Magnet and Cairn Hill), with several others at the advanced feasibility and financing stage.
Table 1 JORC-compliant magnetite resources and conceptual targets, South Australia.
Figure 1 Total magnetic intensity (TMI) image of South Australia showing geological domains and magnetite ore occurrences.
The iron oxide mineral magnetite as Fe3O4 has a mass percent of 72.36% Fe and 27.64% O and typically occurs as a natural ore containing 15–40% Fe. Historically hematite direct shipping ore (DSO) has been the preferred source of iron ore globally, with significant resources located on several continents, including Australia in the Hamersley Range, Western Australia. DSO as its name implies is mined, crushed and blended, requiring minimal or no beneficiation. High-grade hematite DSO ore reserves (current benchmark grade set at >62% Fe) are being steadily depleted. Australia’s hematite iron ore resources as of 2009 are forecast for exhaustion in 130 years (Yellishetty et al. 2012).
Use and demand
Magnetite ore has a prolonged history of use as a source of iron ore, in particular, in historically important iron and steel producing countries such as the USA, Canada, Russia, Sweden and China. Magnetite ores require varying degrees of processing and beneficiation to produce an iron ore suitable for steelmaking purposes, ultimately contributing to higher production costs. Magnetite concentrates typically range between 65 to 70% Fe and are increasingly being sought as a preferred feedstock for steelmaking, particularly those concentrates with low impurities. Steelmaking using magnetite has lower environmental impacts and this too is increasing demand for magnetite concentrate. Magnetite concentrate is also being sought to blend with and upgrade lower grade iron ore. The Chinese market represents a significant component (~40%) of global iron ore demand for steelmaking and is showing preference for both iron ore and steelmaking operations that meet higher environmental standards.
Much of South Australia’s magnetite ore is characterised by a unique combination of metallurgical characteristics including relatively soft ore and/or large grain size which results in concentrates with comparatively lower input costs, higher iron grade and lower levels of deleterious impurities.
Global iron ore production in 2015 was ~2 Bt of which 28% was magnetite (CRU cited in Department of the Premier and Cabinet 2017, p. 7). Global production of crude steel was ~1.7 Bt (World Steel 2018) of which 50% was produced by China, having risen from ~15% of world steel production in 2000 (Holloway, Roberts and Rush 2010). Australia produced ~27 Mt of magnetite concentrate production in 2017, with South Australia contributing over 2 Mt of this.
The premium paid for high-grade concentrates (>65% Fe) has steadily increased over the last decade, being up to 150% of the benchmark 62% Fe CFR (see Footnote 1) index price. Furthermore, three distinctly different markets for iron ores are developing (Löf and Magnus 2017) – below 58% Fe; around 62% Fe; and high-grade and low impurity ore (>65% Fe).
South Australia has a unique opportunity, by virtue of the changing global market and the geological and unique compositional characteristics of its magnetite resources, to become a significant global supplier of quality magnetite ore and concentrates in the foreseeable future.
Magnetite ore and processing
A general processing circuit for magnetite ore is initial crushing and screening of run of mine (ROM) ore to <6 mm. The ore is then milled to a specified grind size to liberate the iron minerals from the gangue, then processed further to produce a magnetite concentrate, commonly by a wet magnetic separation process. For deposits with a significant component of hematite, the hematite ± magnetite may be recovered by use of gravimetric separation technology. The concentrate may be exported as is, or processed further to hematite sinter or pellets. Concentrates with a grain size <75 µm are used as pellet feed and concentrates ranging from 150 µm to 6.3 mm are considered suitable for processing to an agglomerate (sinter). Intermediate grain sizes have various uses, often being blended with other feedstock.
Magnetite ore beneficiation circuits are optimised to a grind size that achieves optimal concentrate specifications (i.e. minimal impurities) for the least amount of power input (i.e. cost). Select South Australian magnetite ores are typically relatively soft when compared with ores from other Australian sources and (will) require less power inputs to achieve market specifications.
Defined resources and production profile
Magnetite producing mines, advanced projects and JORC (Joint Ore Reserves Committee) defined resources in South Australia are shown in Figure 1 and summarised in Table 1. Further details on individual deposits can be found from company websites, ASX releases and MINDEP, the South Australian mineral deposits database.
Production of magnetite ore commenced in South Australia in 2007 and has averaged around 2 Mtpa since then. Production has been derived predominantly from the Iron Magnet mine (South Middleback Range), a massive magnetite-bearing metasomatite deposit underlying the Iron Duke and Iron Duchess hematite deposits. ROM ore is processed on site to produce a magnetite concentrate that is transported by slurry pipeline to the Whyalla steelworks where it is dewatered, then used as feedstock in the manufacture of hematite pellets.
Production has also occurred from the Cairn Hill deposit (North Gawler iron region), another magnetite metasomatite deposit, with a head grade of ≳50% Fe. Cairn Hill ROM magnetite ore is crushed, screened and transported by rail to Port Adelaide for overseas export. The operation is currently on care and maintenance, although anticipated to restart in 2018.
Metallurgical parameters such as the concentration of deleterious elements in magnetite concentrates and comminution parameters are typically intrinsically related to grain size. Discussion around the specific details for individual deposits is beyond the scope of this article and in some cases is the focus of ongoing project optimisation studies and not publicly available. However, there are many characteristics associated with South Australian magnetite ores that are broadly similar. From an ore hardness and comminution perspective, the majority of South Australian magnetite ores have a rock strength classified as soft to medium with Bond crushing work index (CWi) values typically 10–20 kWh/t, abrasion index (Ai) values 0.2–0.6 (majority <0.5) and Bond ball mill work index (BWi) values 11–19 kWh/t. Magnetite ores from the Braemar iron region are notable for being comparably softer than magnetite ores from the other iron regions, enabling grinding to finer grain size without incurring significant additional power inputs.
All magnetite ores from South Australia produce concentrates with low levels of deleterious elements inclusive of those deposits with relatively coarse grinds in the range 75–125 µm. Magnetite concentrate grades range from 65 to 70% Fe with the variation in iron grade largely sympathetic according to the silica content. SiO2 is <5% from nearly all concentrates, with the majority in the range 2.3–3.5%. Al2O3 is <1.9%, with the majority in the range 0.15–0.51%. Sulfur concentrations vary across the range of deposits with most deposits at or below the benchmark specification of 0.05%; Braemar iron region concentrates are <0.01% S. Phosphorus ranges from 0.002 to 0.02%, well below the benchmark of 0.07%.
The more significant South Australian deposits are characterised by relatively simple ore body geometries which are readily amenable to efficient mining. Cover and overburden thicknesses vary, but are typically 30–50 m over the Gawler iron regions and low to negligible in the Braemar iron region.
Geological setting
Figure 2 Time–space diagram of magnetite occurrences in South Australia.
In South Australia most iron ore resources occur in the Mesoarchean–Mesoproterozoic Gawler Craton, the Paleo-Mesoproterozoic Curnamona Province and the Neoproterozoic–Cambrian Adelaide Rift Complex (Figs 1, 2). The Gawler Craton comprises a Meso- to Neoarchean basement overlain predominantly by Paleoproterozoic volcano-sedimentary cover sequences that underwent major tectonic reworking and magmatic events during the Sleafordian Orogeny (2465–2410 Ma), Kimban Orogeny (1730–1690 Ma) and finally the Kararan Orogeny (1610–1575 Ma), which was synchronous with magmatism of the Hiltaba Suite (1595–1575 Ma) and Gawler Range Volcanics (1595–1585 Ma). Within the Gawler Craton the basement rocks have been subdivided into a series of domains and subdomains, defined by their contrasting magnetic character and lithostratigraphy (Ferris, Schwarz and Heithersay 2002). To the east the Adelaide Rift Complex contains a thick succession of Neoproterozoic–Cambrian continental to marine siliciclastic and evaporative sediments (Preiss 2000) which separate the Gawler Craton from the Curnamona Province. The Curnamona Province Paleoproterozoic sedimentary succession shares a similar geodynamic history to the Gawler Craton during the earliest Mesoproterozoic, deformed during the Olarian Orogeny (1620–1570 Ma) (Forbes et al. 2011; Rutherford, Hand and Barovich 2007), synchronous with magmatism of the Ninnerie Supersuite (1600–1570 Ma) (Wade 2011; Wade et al. 2012).
Magnetite-bearing rocks are primarily found exposed or beneath thin cover sequences on the Gawler Craton and within the Nackara Arc region of the Adelaide Rift Complex. Three designated geographic iron regions are recognised – the North Gawler iron region, the South Gawler iron region, and the Braemar iron region (Fig. 1).
Deposit types
Magnetite ores in South Australia are characterised by two principal varieties of magnetite-bearing lithologies:
- magnetite banded iron formation (BIF) and/or magnetite metasediment
- magnetite metasomatite to skarn.
A key feature of many iron formations in South Australia is that they have typically been emplaced early in the geological record and subsequently modified by later tectonic and magmatic processes into their present day configuration. Figure 2 presents an interpretation of the time and space distribution of the magnetite ore deposit types in relation to the geological architecture of South Australia. The figure illustrates the interpreted depositional age of primary BIF, the distribution of meta-BIF, indicating the initial phase of formation of the metamorphic mineral magnetite, and the distribution of magnetite metasomatite shown with its associated igneous and/or tectonic event.
Resources of magnetite BIF are more prevalent than magnetite metasomatite, with both deposit styles exhibiting weathering profiles with attendant variable degrees of oxidation of magnetite to hematite. The overall form of both magnetite BIF and magnetite metasomatite deposits is tabular. Deposits on the Gawler Craton have dips variable from moderate to steep, in contrast to deposits in the Adelaide Rift Complex with generally shallow dips. Controls on the internal distribution of magnetite are both stratigraphic and structural.
Magnetite BIF and magnetite metasediment
Research publications on the origin of BIF are prolific in number. The consensus from recent publications proposes the deposition of iron minerals in a marine environment, either from solution or as particulate matter (Becker et al. 2010; Krapež, Barley and Pickard 2003; Lascelles 2007; Pickard, Barley and Krapež 2006). The iron is proposed as being of volcanic origin, leached out by hydrothermal fluids then distributed into marine waters in a depositional setting of mid to lower shelf to abyssal plain. Associated metasediments reflecting this offshore environment include graphitic to quartzo-feldspathic metapelite (-schist), metapsammite (-gneiss), calcareous metasediments (-calc-silicate) and carbonates (-marble or dolomite) (Chalmers 2007; England and Thomas 2017; BHP Steel Pty Ltd 2008; Centrex Metals Ltd 2017; Miles 1954).
The protolith BIFs and iron-bearing sediments were subsequently subjected to metamorphism resulting in the formation of magnetite meta-BIF and magnetite metasediment, with the chemical composition and original sedimentary textures of the protolith remaining largely unchanged.
Several intrinsic characteristics of the protolith BIF and subsequent geological processes are likely to have contributed favourably to the economic considerations of these deposits. The protolith BIF characteristically demonstrates very fine grain size and extensive lateral facies continuity. The significance of the facies continuity is that the original distribution of iron is inferred to be preserved during metamorphism resulting in a consistent distribution of magnetite throughout a deposit. Additionally, during metamorphism the replacement and recrystallisation of a significant proportion of primary iron minerals to crystalline iron oxides including magnetite and hematite was accompanied by an increase in particle size.
Magnetite BIF prospects are often recognised by their prominent linear aeromagnetic signatures sourced from strike ridges commonly extending for many kilometres, or forming clusters of discrete, shorter anomalies that also can extend for many kilometres (e.g. Braemar ironstone facies; see Footnote 2). Variations and examples of magnetite BIF deposits in South Australia are discussed below according to their metamorphic facies (granulite, amphibolite, greenschist) and depositional age.
Figure 3 Meta-BIF of granulite facies, early Paleoproterozoic outcrop at Mount Christie. (Photo 047688)
Magnetite BIF of granulite facies demonstrates compositional banding with alternating magnetite and quartzo-feldspathic dominant layers, possibly reflecting original sedimentary layering. Examples include the Neoarchean Bungalow–Minbrie deposits on eastern Eyre Peninsula, Sequoia on the northern Gawler Craton and the Mount Christie region west-northwest of Tarcoola (Fig. 3). Preliminary petrological investigations indicate an increased magnetite grain size associated with granulite facies magnetite BIF when compared to amphibolite facies magnetite BIF of Proterozoic age (Whitehead 1978).
The giant 4.5 Bt Warramboo deposit on central Eyre Peninsula is an example of granulite facies magnetite BIF of late Paleoproterozoic age (Lane et al. 2015). The magnetite BIF formed during intense deformation and metamorphism of sediments to granulite facies, characterised by a dominantly banded to gneissic texture. Morrissey et al. (2016) suggest the ore formation is related to melt loss during granulite facies metamorphism, with subsequent upgrading of subeconomic units of iron-rich phyllite.
Figure 4 Well-laminated BIF (32% Fe), late Paleoproterozoic Skylark Metasediments, drillhole DD88EN29, 123 m. (Photo 047690)
Magnetite BIF of amphibolite facies has a characteristic well-laminated texture reflecting alternating magnetite and quartz dominant layers. Other minerals include hematite, iron silicates, iron carbonates, iron sulfides, carbonates and apatite (BHP Steel 2008; Davies, Morris and Crettenden 1997; Morris, Davies and Crettenden 1997; Whitehead 1978). Examples from the South Gawler iron region include the informally named Middleback iron formation within the early Paleoproterozoic Hutchison Group. North Gawler iron region examples include the early Paleoproterozoic Wilgena Hill Jaspilite and the late Paleoproterozoic Skylark Metasediments (Fig. 4) from the Mount Woods Domain. Unpublished recent research at the University of Adelaide has identified very small zircons in Proterozoic magnetite BIF (K Lane, PhD student, pers. comm. 2018).
Interesting magnetite BIF variants include the early Paleoproterozoic Kite prospect within the Hawks Nest district in the North Gawler iron region. Petrology descriptions (Morris, Davies and Crettenden 1997) confirm a magnetite BIF, with no evidence of metasomatic alteration and with iron grades from 55 to 65% Fe. Lascelles (2012) proposes such an iron-enriched BIF as protore to hematite deposits in the Hamersley Basin, Western Australia. In the Mount Woods Domain and eastern Mabel Creek Ridge the late Paleoproterozoic Skylark Metasediments contain a significant proportion of iron-rich clastic rocks which include minor extrusive mafic igneous rocks (Chalmers 2007).
Figure 5 Fissile ironstone of greenschist facies, Neoproterozoic Braemar ironstone. (Courtesy of Magnetite Mines; photo 414859)
Magnetite BIF of greenschist facies is predominantly located within the Neoproterozoic Pualco Tillite and Benda Siltstone (Braemar ironstone facies), and Holowilena Ironstone of the Adelaidean and Sturtian glacial sequence of the basal Umberatana Group. Past workers studying the Braemar ironstone facies of the Nackara Arc have identified a stratigraphic succession containing multiple horizons of magnetite BIF within a sequence of metasiltstone and dolomitic metasiltstone. The magnetite BIF varies from a persistent, basal, massive, tillitic facies to an overlying more fissile facies (Fig. 5). Iron oxide mineralogy is dominated by euhedral magnetite over minor hematite which occur as ultrafine flakes. Gangue mineralogy is dominated by quartz, with trace muscovite, sericite, chlorite and feldspar (Whitten 1970). Lottermoser and Ashley (2000) describe the origin of the Braemar ironstone facies as the result of precipitation of iron oxide from marine waters during episodes of renewed marine currents in restricted marine basins marginal to land masses, following melting of a snowball earth. Lechte and Wallace (2016) propose a similar process for formation of the iron formations although deposition is proposed to have taken place under an ice shelf.
Figure 6 Well-laminated jaspilite of greenschist facies, early Paleoproterozoic Wilgena Hill Jaspilite. (Photo T022046)
A variant of the greenschist facies magnetite BIF is the early Paleoproterozoic Wilgena Hill Jaspilite which displays a distinctive finely laminated sequence of alternating dark grey iron oxide dominant and red jasper layers (Fig. 6). Petrological descriptions have identified scattered minute spherules of quartz aggregates, commonly with an iron oxide centre (Daly 1979).
Magnetite metasomatite
Magnetite metasomatite deposits are formed by the alteration of a host rock by hydrothermal fluids from metamorphic and/or igneous sources. Metasomatism involves degrees of replacement of a protolith by another rock via processes of mineral dissolution and deposition. In South Australia iron metasomatism is associated with hydrothermal mineral systems related to orogenic episodes. Factors that control the degree and distribution of iron metasomatism include the presence of iron-bearing rocks in the host sequence, the mineralogy and texture of the host rocks, as well as the presence of structures or fluid pathways and proximity to igneous intrusions. The majority of the iron associated with these deposits is considered likely to be sourced principally from the iron-rich host rocks, with relatively minor contributions from magmatic sources (Keyser et al. 2018; Leevers, Gaughan and Bubner 2005; McPhie et al. 2011). Examples of metasomatic iron deposits are discussed below, categorised in the first instance by the initial orogenic event that contributed to metasomatism (some deposits are likely to have experienced repeated episodes of iron metasomatism).
The Iron Magnet deposit is an example of iron metasomatism with clear stratigraphic and structural controls, possibly associated with mafic intrusives. Leevers, Gaughan and Bubner (2005) propose the Iron Magnet deposit (South Gawler iron region) is a hypogene-enriched metasomatite developed in host magnetite BIF of the Middleback iron formation. Magnetite mineralisation occurs as pervasive and selective replacement of layers at all scales from fine laminations to bands of iron carbonates, carbonates, iron silicates and quartz. The metasomatic events are interpreted to have been episodic, having been initially related to deformation and the introduction of large volumes of mafic intrusive during the Kimban Orogeny (1730–1690 Ma) (Reid, McAvaney and Fraser 2008; Wade and McAvaney 2017), with evidence of minor metasomatism associated with the Hiltaba magmatic event (1610–1575 Ma) (Keyser et al. 2018).
Iron metasomatism is associated with the iron oxide – copper–gold (IOCG) hydrothermal mineralising systems associated with the Hiltaba event. In the Mount Woods Domain metasomatism of iron-bearing metasediments of host Skylark Metasediments is a prominent feature. Reid and Fabris (2015) suggest the host rocks were at a low metamorphic grade and contained formation waters and porosity at the time of the Hiltaba event. Magnetite metasomatite subsequently formed as the result of the intrusion of a high temperature igneous intrusive into a fertile geochemical environment with associated hydrothermal cells. The dominant metasomatic rock type is a hydrothermal breccia, with a matrix of hydrothermal iron oxides cementing metasedimentary clasts. Magnetite that occurs as a relatively minor byproduct of Cu–Au-dominated IOCG systems has not been included in the inventory of South Australia’s magnetite resources. There are minor occurrences of magnetite metasomatite within the Adelaide Rift Complex, often associated with mafic intrusive within diapiric structures or marginal to granitic plutons, with both possibly related to deformation and magmatism associated with the Delamerian Orogeny (514–490 Ma) (Desertstone NL 2000; Foden et al. 2006).
Exploration and iron regions
Three iron regions have been designated based on the geographical location of iron ore deposits and prospects. Exploration for magnetite iron ore has largely been driven by the targeting of aeromagnetic anomalies with follow-up ground geophysical surveys, geological mapping, drilling, geochemical sampling and metallurgical test work.
Prior to the upswing in iron ore price in the mid 2000s, exploration specific to magnetite ore was confined to investigation of the magnetite resource beneath the Iron Duke deposit of the South Middleback Range and statewide reconnaissance drilling and beneficiation test work by the Geological Survey of South Australia in the 1950s to early 1960s. The Geological Survey also undertook exploration work in the Hawks Nest district during 1994–96 as part of the South Australian Steel and Energy project in the North Gawler iron region.
Following the discovery of Olympic Dam in 1972 there was considerable drilling of bullseye magnetic anomalies for IOCG-type mineralisation resulting in the identification of various occurrences of magnetite metasomatite in the Olympic and Mount Woods domains, eastern Gawler Craton.
Since the early 2000s company exploration activity for magnetite ore has increased dramatically, resulting in a significant increase in the state’s magnetite resource inventory, with potential for further discovery of (significant) additional resources also identified.
North Gawler iron region
Figure 7 North Gawler iron region magnetite occurrences shown over TMI image.
Within the North Gawler iron region there are currently six deposits with JORC-defined resources and a further 12 magnetite prospects (Fig. 7). The early Paleoproterozoic Wilgena Hill Jaspilite is host to magnetite BIF style occurrences. Exploration and mapping indicates this formation occurs as a series of discontinuous strike ridges from Hawks Nest district in the north through to the Giffen Well and Bulgunnia Iron deposits further south, and continuing further south again to the Coolybring, Wilgena Hill and Hicks Hill prospects for a total distance of ~150 km.
Aeromagnetic imagery, geological mapping and drilling have confirmed significant strike ridges of late Paleoproterozoic magnetite BIF in the Mount Woods Domain and the eastern Mabel Creek Ridge. Magnetite BIF within a sequence of calc-silicate, metasediment, carbonate and minor basalt has been penetrated at the Skylark South occurrence and Tomahawk prospect.
Significant resources of magnetite metasomatite occur in the Mount Woods Domain. The Cairn Hill deposit is developed on a magnetite-bearing tectonite developed on a splay off the eastern extension of the regional Karari Shear Zone (Morris and Davies 2011; Fig. 8). Whilst the resource is relatively small, the head (ROM) grade is exceptional at >50% Fe, together with copper and gold credits.
The largest defined magnetite metasomatite resource is the Snaefell deposit. It has a broad, linear magnetic response, with resource drilling to date confirming a tabular-shaped body, with an average thickness of 90 m, a moderate to steep southerly dip and consistent iron grades throughout (IMX Resources Ltd and OZ Minerals Ltd 2012). There are several other magnetite metasomatite occurrences within the Mount Woods Domain that remain poorly defined (e.g. Manxman A1, Joes Dam, Penrhyn; Fig. 9). All the magnetite metasomatite deposits in the Mount Woods Domain display a prominent breccia texture, including fine to coarse-grained magnetite in the matrix, and at metamorphic facies ranging from lower amphibolite to granulite (Chalmers 2007). The source of the hydrothermal iron is problematic, with a component possibly of magmatic origin complementing what is inferred to be a significant component remobilised from magnetite BIF of the host late Paleoproterozoic Skylark Metasediments (Williams, Kendrick and Xavier 2010; Gale et al. 2012).
Neoarchean magnetite BIF prospects include the Mount Christie district west of Tarcoola (Whitten et al. 1965) together with isolated occurrences at Ooldea and Sequoia.
Figure 8 Massive, coarse crystalline magnetite ore with sulfides, Mesoproterozoic iron metasomatite, Cairn Hill. (Photo 409485)
Figure 9 Mesoproterozoic iron metasomatite (~35% Fe, 0.23% Cu), drillhole DD86EN43, 302 m, Manxman A1 prospect. (Photo 048716)
South Gawler iron region
Figure 10 South Gawler iron region magnetite occurrences shown over TMI image.
Defined resources within the South Gawler iron region exceed 7 Bt in 19 deposits with the majority of these being magnetite BIF style deposits (Fig. 10). Historical iron ore production in South Australia has been dominated by deposits within this region with the mining of several hematite deposits in host hematite BIF or magnetite BIF of the Middleback iron formation.
Early Paleoproterozoic magnetite BIF forms a discontinuous series of strike ridges that constitute the Middleback Range on the northeastern Eyre Peninsula, continuing south along the east coast of the Eyre Peninsula to near Port Lincoln, and recognised by its prominent linear aeromagnetic signature (Fig. 10). The forms and textures of the deposits are comparable to that of the early Paleoproterozoic BIF described for the North Gawler iron region, being tabular overall and moderate to steeply dipping. The magnetite BIF has the typical well-laminated texture with mineralogy dominated by magnetite and quartz and with variable iron silicate, carbonate, iron carbonate and iron sulfide. Much of the magnetite BIF resource has a weathering profile of oxidised magnetite BIF in the order of tens of metres to >100 m, within which magnetite is oxidised to hematite and goethite.
The largest magnetite BIF resource is the giant >4.5 Bt Warramboo deposit, a late Paleoproterozoic magnetite metasediment with a head grade of ~17% Fe and characterised by coarse magnetite grain size (106 µm), a consequence of upper granulite facies metamorphism and intense deformation related to the Kimban Orogeny. Aeromagnetic imagery suggests further potential for similar magnetite BIF may occur south of Warramboo.
The most significant magnetite metasomatite deposit is the Iron Magnet deposit located at the southern extremity of the South Middleback Range and to date is the sole magnetite metasomatite deposit identified within the Middleback Range. Related magnetite metasomatite deposits and prospects include those in the Wilcherry Hill district (northern Eyre Peninsula) associated with Hiltaba-age metasomatism of host metasediment of the Hutchison Group. Minor IOCG-type magnetite metasomatite prospects also occur on central Yorke Peninsula. Age dates suggest a Neoarchean age for magnetite BIF or magnetite metasediment deposits at Bungalow–Minbrie (Reid and Jagodzinski 2011).
Braemar iron region
Figure 11 Braemar iron region magnetite occurrences shown over TMI image.
Magnetite resources within the Braemar iron region (Fig. 11) are dominated by magnetite BIF (Braemar ironstone facies) within the Pualco Tillite and Benda Siltstone. Defined mineral resources of magnetite BIF total 6.9 Bt, excluding the Hawsons deposit (2.5 Bt; located south of Broken Hill in New South Wales, near the South Australia border) which represents the eastern extremity of known Braemar ironstone facies.
Exploration since the mid 2000s has identified many magnetite BIF prospects within the Nackara Arc, with several deposits being defined together with accompanying advanced feasibility work to assess their economic merits. Compared with the magnetite BIF resources from the other two iron regions, the geological form of the defined resources within the Braemar iron region is relatively simple, characterised by gently dipping slabs with some deposits displaying open folding. Whilst total iron percentages are typically 15–20+% Fe, favourable factors for mining include very large resource inventories, relatively simple geology and lower values for hardness and grind indices that should enable economic recovery of a high-grade magnetite concentrate with low levels of impurities.
Exploration potential
Within each region there is potential for significant increases in the resource inventory. Table 2 summarises exploration target estimates publicly released in accordance with JORC guidelines. Additional exploration potential associated with other known deposits and prospects is recognised but has not been estimated in accordance with JORC guidelines.
Table 2 Company magnetite exploration targets, South Australia
| Company | Iron region | Conceptual exploration target range (Bt) |
| Magnetite Mines Ltd | Braemar | 16–32 |
| Lodestone | Braemar | 1–20 |
| Minotaur Exploration | Braemar | 2.4–4.0 |
| Carpentaria Resources | Braemar | 0.58–0.96 |
| Simec Mining | South Gawler | 0.3–0.6 |
| Iron Road | South Gawler | 10–21 |
| Total | 30–79 |
Conclusion
In the geological landscape of South Australia there are globally significant resources of magnetite ore preserved at relatively shallow and mineable depths below surface. The geological record identifies two dominant styles of magnetite ore, namely magnetite BIF and magnetite metasomatite to skarn, which by virtue of their geological evolution are characterised by magnetite which is relatively soft and coarser grained when compared with magnetite ores from other identified global magnetite regions. The relatively coarse grain size of magnetite ores and concentrates are complemented by favourable comminution parameters and relatively low concentrations of deleterious elements, further reinforcing the globally competitive properties of magnetite ores from South Australia. From a mining perspective the South Australian magnetite deposits have significant scale and relatively simple and favourable mining geometries with minimal overburden.
Exploration work to date has outlined three major iron ore regions within the state. Total statewide JORC compliant resources are in the order of 15.9 Bt (excluding the Hawsons deposit with 2.5 Bt), with economically demonstrated resources comprising over 6 Bt of this.
Footnotes
1 Benchmark price set at the Tianjin exchange market, China, inclusive of the cost of freight and rail. (Back)
2 The term ‘Braemar ironstone facies’ refers to similar ironstone facies found within two separate formations, the Pualco Tillite and the Benda Siltstone. It is, however, commonplace for the term ‘Braemar Iron Formation’ to incorrectly be used as a collective reference to any of these Braemar ironstone facies units. (Back)
Acknowledgements
The proponents of the various magnetite projects in South Australia are acknowledged for their cooperation and assistance in providing technical data to assist in the preparation of this article. Teena Dunn (Geological Survey of South Australia, GSSA) is acknowledged for research assistance in the preparation of this article. Stacey Curtis (GSSA) is thanked for constructive input into the discussion around the geological framework presented herein.
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