Flinders Ranges Sedimentary Magnesite - Mount Hutton, Witchelina, Myrtle Springs, Copley, Screechowl

South Australia, SA, Australia

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The sedimentary magnesite deposits of the North Flinders/Willouran Ranges are located 650 to 750 km north of Adelaide in South Australia. The larger deposits are at Witchelina and Mount Hutton, whilst smaller resources have been indicated at Copley, Mount Hutton South, Myrtle Springs, Mount Playfair, Pug Hill, Termination Hill, West Mount Hutt and Screehowl Creek.

  Magnesite mining in the region was first recorded in 1919 at Camel Flat near Copley in the northern Flinders Ranges (McCallum, 1988). Subsequently, BHP Limited produced 4500 t of magnesite from shallow open pits and underground workings in the southern Flinders Ranges at Mundallio and Port Germein Gorge, 300 km to the south, in the 1940s and 50s. Some 2778 t was mined from four prominent 1 to 2 m thick exposed beds at Witchelina prior to 1984 (Crettenden, 1985). Commercial Minerals Ltd mined 30 000 t from Myrtle Springs in 1984 for water filtration in a Queensland aluminium refinery. Annual production since 1990 has averaged 700 t, principally for agricultural purposes and rockwool manufacture (McCallum, 1990). Total production of magnesite from South Australia, mostly from the northern Flinders Ranges from 1915 to 2015 has been 113 740 t (South Australian Department of State Development, 2017). Campaign mining is currently undertaken at the Myrtle Springs quarry as detailed below..

  Sedimentary magnesite deposits of the northern Flinders/Willouran Ranges occur as interbeds within the Cryogenian (<790 to >660 Ma) Skillogalee Dolomite, a widespread sedimentary carbonate sequence within the Mundallio Subgroup towards the middle of the Neoproterozoic Burra Group of the Adelaide Rift Complex. The sag stage carbonates of the Mundallio Subgroup overlie the rift-facies coarse clastic sedimentary rocks of the Emeroo Subgroup, the lowermost unit of the Burra Group, which is composed of conglomerates and sandstones/quartzite grading into siltstones and interbeds of dolostone. Further south in the southern Flinders Ranges, the Skillogalee Dolomite is overlain by the Woolshed Flat Shale and the Undalya Quartzite of the upper Mundallio Subgroup. In the northern Flinders Ranges, the Skillogalee Dolomite is directly overlain by the 1100 m thick sequence of green siltstone with minor quartzite and dolomite interbeds of the shallow marine Myrtle Springs Formation. This unit may, in part, be equivalent to the Woolshed Flat Shale and Undalya Quartzite, but also the Belair Subgroup which is the uppermost member of the Burra Group. The Burra Group is discordantly overlain by the Sturtian glacial sequence of the Umberatana Group, Yudnamutana Subgroup, dated at ~660 Ma.
  The Skillogalee Dolomite carbonate sequence, which has a maximum thickness of 4000 m, and can be traced over a distance of 700 km from the Torrens Gorge in the Mount Lofty Ranges near Adelaide in the south, to the Leigh Creek-Marree area in the Willouran Ranges of the northern Flinders Ranges in the north. However, the carbonate component of the Mundallio Subgroup is best developed and reaches it maximum thickness in the NW trending Willouran Trough in the northern Flinders Ranges. In contrast, to the south in the Clare Valley closer to Adelaide, in its type section, the Skillogalee Dolomite is only 330 m thick.
  The Skillogalee Dolomite is characterised by repetitive lithologies of intraclastic dolomite and magnesite, cryptalgal laminated dolomite (including stromatolite biostomes), siltstone and sandstone. Early diagenetic chert replacement is common. Sedimentary features indicative of a shallow water regime, such as mud cracks, tepee structures and disrupted carbonate crusts are common. Less abundant are oncolites, stromatolite bioherms, halite and shortite moulds, arenaceous conglomerates and sedimentary chert beds.
  The Skillogalee Dolomite is therefore interpreted to have been deposited in a shallow-marine to lagoonal environment and is divided into:
• Lower section which is a pale hued to white dolostone with minor quartzite and slate, interbedded with minor grits, rhyolites and rhyolitic tuffaceous units.
• Upper section composed of dark grey, fine-grained dolostone with ripple marks, slumps and mudcracks.

  Magnesite interbeds are concentrated in the upper section of the Skillogalee Dolomite. Although magnesite is present throughout, beds are more nurnerous, and reach their thickest development in the Willouran Trough, comprising the Copley to Witchelina area and the Willouran Ranges to the north which include West Mount Hut and Screechowl Creek. The magnesite predominantly occurs as 1 to 5 µm cryptocrystalline particles, interpreted to have been deposited as a chemical precipitate in shallow, marginal marine lagoons and mudflats. Keeling et al. (1998) conclude the fine grain size of the magnesite particles and poor packing were due to rapid crystallisation and sedimentation possibly under agitated conditions. This is supported by their observation that magnesite may be interbedded with dolomite of similar grade size. Early diagenetic changes included consolidation through desiccation, and frequently, breakup of the magnesite sediment leading to the development of intraformational conglomerate or, more often, reworking of the disrupted sediment to form pebble conglomerate, and locally, coarse magnesite sand. The conglomerates consist of a framework of clasts of aphanitic magnesite mudstone, typically 2 to 20 mm in diameter, in a poorly sorted sandy matrix of variable composition comprising dolomite, magnesite and rare quartz. Intermixing of magnesite and dolomite clasts is common. Disrupted micritic laminated magnesite crust are, on rare occasions, preserved at the base of some intraclastic beds. Common sedimentary structures include torrents bedding, graded bedding and inverse grading, basal mudcracks, shrinkage cracks, plastic injection features and rip-ups. These reworked sediments are thinly interbedded with dolostone (Preiss et al., 1993).
  Albite crystals up to 1 mm long were formed during the early stages of diagenesis and contain the aluminium and sodium in the magnesite beds. In magnesite bearing conglomerate, microcrystalline dolomite and magnesite form the cementing matrix. At Termination Hill, minor trace amounts of fine-grained quartz occur in the matrix of the conglomerates. Keeling et al. (1998) conclude quartz was unstable in the sedimentary environment as it displays severe replacement by dolomite around its margins, leaving only small angular remnant quartz kernels. This they suggest, is indicative of the alkaline conditions at the time of deposition and later diagenesis.
  Late diagenesis led to the formation of talc and cross cutting veinlets of sparry carbonate or rarely talc. Carbonate veinlets comprise coarsely crystalline dolomite, Mg-calcite or magnesite. The veinlet also contain Sr and Ba, with Sr substituting for Ca in Mg-calcite and Ba as barite. Talc occurs in a wide range of grain sizes from sub-micron particles associated with cryptocrystalline magnesite, to millimetre sized lath-shaped crystals growing outward from the margins of magnesite clasts. Talc flakes occur in both magnesite clasts and matrix, and less frequently in late stage veins with fine grained K feldspar. Talc is interpreted to have formed during very late diagenesis or very low grade metamorphism, either by reaction between colloidal silica and magnesite or through recrystallisation of Mg clays that may have been deposited together with the magnesite. With few exceptions, the majority of the SiO2 in magnesite beds is in the form of talc (Keeling et al., 1998).
  The thickest development of magnesite is along a 120 km strike length to the northwest of Leigh Creek, where, the calcium content is relatively high, ranging from 2% at Mount Hutton to 4.5% at Screechowl Creek, present as dolomite or magnesian calcite.
  The deposition environment is illustrated in the Copley area, where 1 to 20 m thick sequences of dolomitic mudstone, magnesite mudstone, intraclastic magnesite and dolomitic sandstone have been deposited in repetitive depositional cycles. Each shallowing cycle generated a sequence from subtidal cryptalgal dolomite and stromatolites through trough cross bedded dolomitic grainstone to intertidal intraclastic dolomite to deposit widespread, low energy, marginal to shallow marine dolomitic mud units. Further shallowing produced isolated supratidal and ephemeral lagoons which became the sites of magnesite deposition from sulphate-poor Proterozoic marine waters (Preiss et al., 1993; Frank and Fielding, 2003). The semi-lithified magnesite mudflats were disrupted by high-energy flooding events which deposited sand near-shore and led to the formation of the intraclastic magnesite conglomerates. These were, in turn, reworked by tidal currents and subsequently covered by renewed deposition of dolomitic mud of the next cycle (Uppill, 1990; Frank and Fielding, 2003; after Horn et al., 2017).

Mount Hutton (#Location: 30° 27' 0"S, 138° 15', 36'E)

  The bedded magnesite/dolomite sequence at Mt Hutton strikes NW-SE and dips at 60 to 68°NE. Some 86 individual magnesite beds have been delineated within a package sporadically exposed along the south-western limb of a regional syncline. This same limb also contains the following deposits, from SSE to NNW - Camel Flat, Mount Hutton South, Mount Hutton, Myrtle Springs, Mt Playfair and Pug Hill deposits over a strike length of 55 km.
  The economic magnesite package at Mount Hutton comprises magnesite beds towards the middle of the package, which have been the focus of geological and structural mapping and resource definition drilling. At Mount Hutton, 52 magnesite beds, each between 0.1 and 2.4 m thick occur over a 120 m thick stratigraphic interval, averaging 42.9% MgO. Magnesite has been mined over a strike length of 400 m from four magnesite beds, each of ~1 m thickness dipping 65°E.
  Textural characteristics of individual beds may be identifiable and traced along strike in exposure and drill core. The lower contacts of individual magnesite beds tend to be sharp, passing down into dolostone, whilst the upper contacts are generally more gradational with interbedded dolostone over a width of 5 to 30 cm. The deposit is structurally simple, without localised folding, and has consistent dips with only minor variation over a 5 km strike length. A set of sinistral, NE-SW trending strike-slip faults with 1 to 14 m offsets are the dominant local structural features. In addition, minor dextral strike-slip faults are mapped with offsets up to 1 m.
  The magnesite ore zone comprises 80 to 90% magnesite, 5 to 10% dolomite, 3 to 7% talc and 1 to 3% albite, with trace quartz and tourmaline. The average SG is 2.95 for magnesite and 2.86 for dolomite.
Mineral Resources at 31 December, 2016 were:
  Measured - 18.3 Mt; Indicated - 42 Mt; Inferred - 53 Mt; for a total of 113.3 Mt @ 42.9% MgO (Horn et al., 2017).

Witchelina (#Location: 30° 6' 36"S, 137° 58', 48'E)

  The Witchelina deposit is located in the core of a shallow-dipping syncline. Outcrop is generally subdued and the sequence is not fully exposed, largely overlain by alluvium that is generally <2 m thick. However, the geological structure and outcrop/subcrop traces have been mapped with airborne visible short wave infrared imaging. A sequence up 350 m thick has been delineated with 23 magnesite beds, each of 0.3 to 11.0 m in thickness, interbedded with dolostone. This package is composed of four thick magnesite beds, each to 10 m thick, and a further 19 thinner beds that persist over a 4000 m strike length. The thickest of these, bed 4, averages 8 m true width. Minor parasitic folds and occasional small reverse faults with offsets of <5 m occur in the gently folded syncline. The western limb has a consistent strikes at 340° and dip of 35 to 40°E, whilst the eastern limb strikes at 340 to 360°. The latter dips at ~20°W near the centre of the syncline and up to 35°W, 700 m east of the fold axis.
  The magnesite of the deposit typically comprises 1 to 10 mm intraclasts of cryptocrystalline magnesite, set within an interstitial cement of fine-grained crystalline magnesite and dolomite. Minor talc is pervasive throughout the clasts and matrix. Drilling of cored holes was centred on bed 4 on the east and west limbs and the northern hinge of the syncline.
  Representative samples from the magnesite ore zone at Witchelina averaged 46.5% MgO; 3.05% CaO; 1.75% SiO
2; 0.08% Fe2O3; 0.13% Al2O3; 0.023% Na2O; 0.01% K2O; <0.01% MnO; 0.04% P2O5; <0.01% TiO2; 49.9% CO2; 0.07% SO4 (Keeling et al., 1998).
Mineral Resources at 31 December, 2016 were:
  Measured - 23.7 Mt; Indicated - 94 Mt; Inferred - 99 Mt; for a total of 216.7 Mt @ 40.0% MgO (Horn et al., 2017).

Myrtle Springs

  Myrtle Springs is located 30 km by road NW of Leigh Creek, and is immediately along strike to the NNW from the Mount Hutton deposit. Magnesite has been mined over a strike length of 400 m from four magnesite beds each of ~1 m thickness, dipping 65°E.
Mineral Resources at 31 December, 2016 were:
  Measured - 1 Mt; Indicated - 5 Mt; Inferred - 5 Mt; for a total of 11 Mt @ 42.9% MgO (Horn et al., 2017).
JORC compliant Ore Reserves 31 March, 2018 were:
  Proved - 0.167 Mt; Probable - 0.302 Mt; for a total of 0.469 Mt of Ore Reserves (Calix Limited website, viewed March 2019).
  The deposit is campaign mined to feed a 50 000 t per annum capacity Calix Flash Calciner with pre-treatment crushing and grinding circuits, that can produce 25 000 tpa of ISO2001 accredited MgO, and a downstream Magnesium Hydroxide Liquid production and storage facility (Calix Limited website, viewed March 2019). While the mine is in northern South Australia, the treatment and manufacturing plant is at Bacchus Marsh in Victoria, ~950 km to the SE.

Copley or Camel Flat

  This deposit is located 5 km west of the town of Copley. It comprises 60 magnesite beds, varying from 0.05 to 3.0 m thick, developed over a strike length of 1.5 km within the upper 300 m of the Skillogalee Dolomite. The central zone, which includes 21 beds totalling 13.4 m in thickness, has an average chemical composition of is 89.4% MgCO
3 which equals 42.7% MgO.
Mineral Resources at 31 December, 2016 were:
  Indicated - 5 Mt; Inferred - 8 Mt; for a total of 13 Mt @ 42.0% MgO (Horn et al., 2017).

Pug Hill

Diamond drilling undertaken in 1998 defined a resource with a strike length of 2.5 km comprising a 240 m thick sequence with 47 magnesite beds, ranging from 0.1 to 2.1 m thick interbedded with dolostone. The average magnesite grade is 42.7% MgO.
Mineral Resources at 31 December, 2016 were:
  Indicated - 10 Mt; Inferred - 10 Mt; for a total of 20 Mt @ 42.7% MgO (Horn et al., 2017).

Termination Hill

Diamond drilling in 1998 defined a resource with a strike length of 6.9 km comprising a 124 m thick sequence with 45 magnesite beds, ranging from 0.1 to 2.9 m thick interbedded with dolostone.
Mineral Resources at 31 December, 2016 were:
  Measured - 4 Mt; Indicated - 5 Mt; Inferred - 20 Mt; for a total of 29 Mt @ 42.8% MgO (Horn et al., 2017).

Screechowl Creek – West Mount Hut

A 93 m thick sequence, which includes 25 magnesite beds ranging from 0.2 to 3.9 m in thickness, interbedded with dolostone, is exposed in the deeply incised banks of Screechowl Creek, 105 km northwest of Leigh Creek. Detailed mapping has established continuity of the sedimentary sequence for a total strike length of 18 km.
Mineral Resources at 31 December, 2016 were:
  Screechowl Creek - Inferred - 36 Mt @ 44.3% MgO (Horn et al., 2017).
  West Mount Hut - Inferred - 6.7 Mt @ 44.3% MgO (Horn et al., 2017).

The total JORC compliant Mineral Resources in the deposits of North Flinders/Willouran Ranges, including those detailed above, is estimated at:
  Measured - 47 Mt; Indicated - 187 Mt; Inferred - 346 Mt; for a total of 580 Mt @ 42.0% MgO (Horn et al., 2017).

The information in this summary is largely drawn from Horn et al., 2017; the South Australian Department of Energy and Mining website (viewed Feb. 2019); and Keeling et al., 1998.

The most recent source geological information used to prepare this summary was dated: 2017.    
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.

  References & Additional Information
   Selected References:
Horn, C.M., Keeling, J.L. and Olliver, J.G.,  2017 - Sedimentary magnesite deposits, Flinders Ranges: in Phillips, G.N., 2017 Australian Ore Deposits, The Australasian Institute of Mining and Metallurgy,   Mono 32, pp. 671-672.
Keeling, J., Horn, C.M. and Wilson, I.,  2019 - New kiln technology expands market opportunities for cryptocrystalline magnesite: in    Mesa Journal   v.89, pp. 22-38
Keeling, J.L., McClure, S.G. and Raven, M.D.,  1998 - Mineralogy and chemistry of Proterozoic magnesite ores from South Australia: in    CSIRO Land and Water   Technical Report 7/98, 70p.

Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge.   It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published.   While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants:   i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and   ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.

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