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Savage River Mines, Corinna-Savage River district, Circular Head municipality, Tasmania, Australia

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Latitude & Longitude (WGS84): 41° 28' South , 145° 13' East
Latitude & Longitude (decimal): -41.46667,145.21667
Köppen climate type:Cfb : Temperate oceanic climate


The Savage River iron mines (SRM) consist of a number of small to large open-cut workings operated by Grange Resources Limited, in north-western Tasmania. The mines are exploiting a group of magnetite-rich lenses within a series of highly metamorphosed ferromagnesian rocks, including carbonates and mafic rocks, in the Arthur Metamorphic Complex, of probable late Proterozoic age . These magnetite rich magnesian rocks occur as a series of discontinuous zones in a shear zone ab few hundred metres wide, that extends for some 25 kilometres, from north of the Savage River to north of the Pieman River. The main workings are the North, central and South pits, and presently the deepest pit floor is at about 320m below surface level at North Pit (Grange Resources pers. comm.).

History
The first discovery of magnetite outcrops in the Savage River area were made in early 1877 by the State Government Surveyor C.P. Sprent, who also found gold in the same area. The magnetite deposits at Savage River and Rocky River were originally prospected and mined for gold, and were first described in detail by Twelvetrees (1900, 1903), who considered them to be a auriferous and a primary source for much of the widespread alluvial gold in the area. Grades up to 55g/t gold have been reported in the magnetite ores (Twelvetrees, 1900, 1903; Shannon et al., 1985; Turner, 1997), but no production has been recorded.
Regional exploration in the 1950's and 60's, particularly by the Tasmanian Mines Department, found the magnetite deposit to be very large and Savage River Mines Limited, a consortium largely driven by Pickands Mather & Co. International, took up the lease in 1965. After developing a then novel ore pipeline (about 80 km long, to the north coast), the mine was brought into operation in 1967-8 (Duncan and Weatherstone, 1990) and it has operated almost continually since. Due to falling iron prices and lack of accessible reserves, ownership of the Savage River Project was transferred to the Tasmanian Government on 26 March 1997, but was subsequently leased to Australian Bulk Minerals (ABM). Their original mine plan contemplated completion of mining operations in 2009, but during 2006 a feasibility study was completed on a possible extension of the mine well beyond this. In 2009 Grange Resources Limited acquired a controlling interest in the mine, and they are extending the mine workings, with a long future planned with current high iron prices.

The Savage River Project currently produces about 2.3 Mt pa of iron ore pellets and concentrates, with a plan to expand production to 2.6 Mt pa. The current mine plan extends to 2026, with potential for a further 10 years (Grange Resources Limited, Report to the Australian Stock Exchange for the Quarter ending 31 December 2010). The Savage River magnetite deposits have been mined since 1967. The total resource as at July 2010 was 306 Mt @ 52.3% magnetite ((Davis Tube Recovery, DTR) and total reserves were 119 Mt @ 51.2% DTR (Grange Resources Limited Annual Report 2010). The main mining operations to date are in three open pits known as the North, South and Central Pits. Smaller, unexploited deposits are known to occur at Long Plains, Rocky River and several other areas along the Lineament.

Location
The Savage River mine, mill, concentrator plant, and privately owned town site are located approximately 100km south west of Burnie. The magnetite concentrate is piped as a slurry to a pelletising plant and dedicated port facility at Port Latta, located 70 kilometres north-west, linked by a sealed road to Burnie (Figure 1). Local topography surrounding the mine is characteristically rugged with steep valleys highly dissecting an old (Permian) peneplain, and is heavily forested. The Savage River bisects the deposit. The economic magnetite deposits have been classified on the basis of minable areas, namely North Pit, South Lens, Centre Pit North, Centre Pit South, Sprent and South Pit.

Magnetite – rich ore bodies of the Savage River mine occur within highly metamorphosed ferromagnesian rocks, in the Bowry Formation of the Arthur Metamorphic Complex (AMC). The Bowry Formation contains metamafics, siliceous and carbonate rich sediments of upper greenschist to lower amphibolite and blueschist facies. The Main Host Assemblage (MHA) is the most intensely deformed part of the Bowry Formation and hosts the NNE-striking magnetite deposits. It is discontinuous over the length of ore bodies, highly variable in width, up to about 300m wide and is about 5 km long. The Main Host Assemblage is bounded by two steeply dipping faults, and surrounding rocks are metamafics and carbonates and siliciclastic sediments, probably Mesoproterozoic to Neoproterozoic in age. The MHA is effectively a fault mélange and has variable contents of magnetite, amphiboles, dolomite, chlorite, serpentine, talc, pyrite and apatite. Lithologies include metamafics, dolostones and magnesite-stones, ultramafic-like magnesian skarns and magnetite ore.

The magnetite deposits consist of a series of discontinuous lenses and bodies of irregular shapes of variable length (<40m to ~ 1.5km) and width (<20 to 140m) and extend to a depth of around 1200m ( from geophysical modelling). The orebodies in the North and Centre Pits are connected at depth whereas the south deposits are separated by non-magnetic rocks. The Ti/V ratios of the North Pit differ slightly from those of Centre Pit and also support a 3 D magnetic inversion model of ore which indicates that the South Deposit West and South Deposit East are fault displaced repetitions of the North Pit and Centre Pit magnetite respectively.

Much of the MHA is highly deformed and there are at least two phases of Cambrian deformation preserved in talc and amphibole-rich ore bodies, with magnetite and pyrite generally predating the main deformations. The main gangue minerals are antigorite, talc, chlorite and tremolite; apatite is locally abundant; chalcopyrite is very minor. The quartz content of the Main Host Assemblage is characteristically low, suggesting dissolution and reaction to form magnesian silicates.
The main hydrothermal alteration products in the wall rocks include albite, dolomite, tourmaline, hematite, chlorite and biotite. Albitisation is the most pervasive alteration in the Savage River mine area, in particular the mafic and siliciclastic-rich units of the Western Wall contain up to 95% albite. Some albitite bodies are coarse grained, dyke-like bodies but detailed study shows most are simply intensely recrystallised metasomatised zones and veins, which locally grade into the finer grained, more mafic albitites that appear to have mostly replaced pelitic metasediments, or possibly some intermediate volcanics. Some contain minor magnetite veining and replacement. These albitite bodies are thus of complex, highly altered, polygenetic origin, predating mineralisation, and including siliceous, mafic and pelitic rocks, and possibly intermediate igneous rocks, but there is no definite evidence for a granitic or other intrusive igneous origin for any of these bodies.

The proposed genetic model for the Savage River magnetite deposits suggest they were formed as magnetite-rich magnesian skarn by the reaction of high temperature magmatic - dominated fluids, assumed to be intrusion-related, with variably iron- rich magnesite and dolomite - rich sedimentary sequences. The original skarns were probably variably dominated by diopside +/- forsterite +/- spinel assemblages (now mostly retrogressed, perhaps largely by late magmatic fluids).
During the Early Cambrian, the Bowry Formation was subjected to subduction to about 20km, with relatively high pressures locally forming blueschist facies, but this is not readily evidenced in the ores. During the rapid, extrusive emplacement of the Bowry Formation, later in the Early Cambrian, these high grade rocks experienced lower grades of metamorphism (greenschist facies), replacing most of the early formed skarn silicates and blueschist facies assemblages with assemblages of serpentine ± talc ± chlorite ± tremolite assemblages). Strong folding, faulting, shearing, boudinaging and brecciation of magnetite layers and surrounding lithologies (as part of Bowry Formation) during and particularly after the emplacement, caused the current configuration of the near vertical magnetite ore lenses.

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Regional Geology

This geological map and associated information on rock units at or nearby to the coordinates given for this locality is based on relatively small scale geological maps provided by various national Geological Surveys. This does not necessarily represent the complete geology at this locality but it gives a background for the region in which it is found.

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Neoproterozoic
541 - 1000 Ma



ID: 781456
chloritic schist 37515

Age: Neoproterozoic (541 - 1000 Ma)

Description: Chlorite schist with minor phyllite, dolomite and magnetite

Comments: meta-igneous mafic; synthesis of multiple published descriptions

Lithology: Meta-igneous mafic

Reference: Raymond, O.L., Liu, S., Gallagher, R., Zhang, W., Highet, L.M. Surface Geology of Australia 1:1 million scale dataset 2012 edition. Commonwealth of Australia (Geoscience Australia). [5]

Data and map coding provided by Macrostrat.org, used under Creative Commons Attribution 4.0 License



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References

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McAndrew, JA, 1965, Geology of Australian ore deposits. AIMM.
Rocks & Minerals: 22:326.
Trevor H. Green and Anthony R. Spiller(1977):Blue amphibole from Precambrian metabasalts, Savage River, Tasmania. American Mineralogist, Volume 62, pages 164_166.
BOTTRILL, R. S.; Taheri, J., 2006. The Savage River iron deposits and other mineral deposits of the Arthur Metamorphic Complex – A brief summary. Mineral Resources Tasmania report, UR2006_05R
Mining Annual Review (1985): 367-368.
Duncan, D.McP. , Weatherstone, N. 1990. Exploration history of the Savage River iron ore deposits. IN Glasson K.R., Rattigan J.H. (eds.)- Geological aspects of the discovery of some important mineral deposits in Australia. AusIMM. Monograph Series. 17 p287-292.

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