The South Middleback Ranges (SMR) lies at the southern end of the Middleback Ranges (MBR). Hematite in the MBR occurs as stratabound Palaeoproterozoic deposits of the Lower Middleback Iron Formation (LMIF), which is part of the Hutchison Group. The Hutchison Group forms part of the Cleve Subdomain of the Gawler Craton, and lies on its western edge. The Cleve Subdomain comprises tightly folded, high-grade metamorphic rocks that are mainly derived from marine shelf sediments and mafic and acidic volcanics (Parker 2012).

The Hutchison Group in the MBR is composed of the Warrow Quartzite and the Middleback Subgroup. The Warrow Quartzite is not identified at all locations. The Middleback Subgroup comprises the Katunga Dolomite, the LMIF, the Cook Gap Schist and the Upper Middleback Iron Formation (UMIF). The LMIF hosts the Middleback Ranges’ hematite deposits.

MBR iron ores formed through supergene enrichment; the process selectively dissolved waste minerals and replaced them with iron ore mineralisation. Preferential enrichment occurred in carbonate facies iron formation, dolomitic marble and, to a lesser degree, silicate facies iron formation. The silicates were much less soluble than the carbonates and resulted in patchy mineralisation in the silicate iron facies (Yeates 1990).

Magnetite was recrystallised and remobilised during a period of metamorphism and deformation. Multiple periods of uplift, erosion and weathering resulted in the oxidation of magnetite to hematite and martite through supergene processes.

The formation of iron ore requires fluids to move through the rock. Most deposits lie on the western side of the range, adjacent to a major fault or mylonite zone along the western edge of the range, which may have provided this pathway. The process was most intense where the dolomite and carbonate facies were thickened and then subsequently exposed during the supergene process (Yeates 1990).

Iron Chieftain
The Iron Chieftain deposit is located on the eastern flank of the SMR, 11 km north-north-east of the Iron Duke pit. The deposit forms a 50–300 m outcrop of massive goethitic hematite dipping at 30 degrees to the west. The goethitic hematite is overlain by quartz hematite and quartz magnetite banded iron formation (BIF) and clays. These are interpreted to be basal LMIF. Clay schists beneath the ore body contain quartz hematite BIF lenses. Highly weathered mafic intrusives cross-cut the stratigraphy. Drill holes located north-east of the ore body intersects quartz feldspar granite at a depth of 55 m. The goethitic hematite deposit is bounded to the west by a northerly trending fault. Along the strike the deposit grades into goethitic material, which is graded at approximately 50% iron.

Iron Duchess
The Iron Duchess iron ore deposit is located at the southern end of the MBR within the Gawler Craton geological province. This deposit is hosted within the Palaeoproterozoic Hutchison Group metasediments.

The Hutchison Group comprises a basal quartzite passing up into the Middleback Subgroup, which commences with the Warrow Quartzite overlain by the Katunga Dolomite.

These are overlain by the Lower Middleback Jaspilite which is a carbonate facies iron formation comprised of iron carbonate, silica and iron oxides that weather to porous goethite-limonite rocks at surface. Sulphide facies iron formation and graphitic metasediments are intercalated at depth. This unit becomes more siliceous and iron oxide rich higher in the sequence with prominent iron bearing silicates including iron rich talc and cummingtonite-grunerite series amphiboles. The Lower Middleback Jaspilite is found across the Gawler Craton, but is thickest and best developed in the Middleback Ranges. The Lower Middleback Jaspilite is overlain by the Cook Gap Schist, a poorly outcropping quartz-biotite-muscovite-sillimanite-garnet-tourmaline schist, which is in turn overlain by the thick Corunna Conglomerate.

High-grade hematite ore occurs in a keel structure between the limbs of a major bifurcating amphibolite dyke, which strikes roughly north–south. Hematite mineralisation also occurs beneath, and to the east of, the main amphibolite dykes, however the mineralisation zones are generally thinner and lower grade. The deeper drill-holes indicate that iron ore mineralisation preferentially replaced carbonate facies BIF. Thus, it appears the principle controls on hematite mineralisation at the Iron Duchess are the presence of a favourable host, proximity to the amphibolite intrusions and proximity to supergene weathering processes. The deposit consists of massive hematite with minor goethite and limonite forming a steeply dipping planar body.

The deposit is overlain by massive quartz hematite BIF and chert. The footwall to the deposit is made up of minor BIF, various schist units and ferruginous dolomite, interpreted as the Katunga Dolomite sequence. Highly weathered mafic dykes and sills intrude the stratigraphy. The deposit is interpreted to be a northern extension to the Duchess deposit and occurs in the basal LMIF. A granitic body occurs to the west of the deposit and most of the outcrop of the enriched hematite lens is almost completely masked by BIF scree. The deposit is also masked by the fact that the ore zone often terminates before reaching the surface (Bubner et al. 1998).

The magnetite mineralisation occurs in a mix of relatively thinly-bedded underlying and overlying rock and ore types within the Katunga Dolomite. Magnetite carbonate rock is the most important ore type in the area. The strike length is approximately 2.9 km.

Hematite in the various hematite-bearing rocks is thought to be due primarily to supergene alteration of primary magnetite (although the presence of primary hematite cannot always be discounted). The hematite deposits exist in discontinuous zones along the western side of the orebody, and along faults and fractures, and dyke contacts. The hematite ore varies greatly in dip (from 30° to almost vertical at 90°) and in width (from 5 m to more than 50 m) but generally dips in an easterly direction. This supergene process at depth is especially apparent at the base of the sequence in a strata-bound sense, and along both faults and fractures, and dyke contacts in a cross-cutting sense.

The hematite deposits occur up-dip from the carbonate and silica facies BIF. Hematite mineralisation has formed in a discontinuous zone along the western flanks of the ridges, with a variable easterly dip from 30° to almost vertical (90°). Hematite is the dominant ore mineral with significant goethite and minor limonite present. Ore widths are highly variable from greater than 50 m in the central areas to less than 5 m in some of the eastern areas of the Iron Duke.

Iron Magnet
Iron Magnet is a stratabound Palaeoproterozoic magnetite deposit of the Lower Middleback Iron Formation (LMIF), part of the Hutchison Group. The Hutchison Group forms part of the Cleve Subdomain of the Gawler Craton, and lies on its western edge. The Cleve Subdomain comprises tightly folded high-grade metamorphic rocks mainly derived from marine shelf sediments and mafic and acidic volcanics (Parker, 1993).

Iron Magnet lies adjacent to and below the now depleted Iron Duke hematite deposit. The predominant gangue mineral plus magnetite forms the basis for the Iron Magnet rock and ore classification. Classification boundaries are gradational, laterally and up-sequence, particularly from carbonate to talc dominated and talc to silica dominated gangue.

Major north-south shear zones (Eastern and Western Shear Zones) provide the east and west limits. East-west meso- and macroscopic parasitic folds on the easterly dipping west limb of a much larger syncline are dominant structural features controlling the distribution and morphology of host rock units, as are the north-striking, steeply westerly dipping amphibolite dykes.

South Middleback Range (SMR) includes Iron Chieftain, Iron Knight, Iron Duchess, Iron Magnet and Iron Duke.

During 2020 mining continued in the Iron Magnet and Iron Duchess Pits. Mining occurred at Iron Chieftain for two months only during 2020. Mining at Iron Knight did not occur over 2020.

Iron Duchess region
The Iron Duchess region includes the Duchess North and Duchess South pits.

The Duchess South pit was mined via an open pit cut-back mining process using traditional drill, blast, load and haul methods. The mine sequencing showed a continuous supply of ore from this pit leading to depletion. Ore was stockpiled and then crushed and screened using mobile processing plants. Low-grade ore and waste materials are stockpiled.

The Duchess North area was mined in earlier decades and is known to contain a significant resource. Current mine development commenced in July 2018.

The Duchess North pit has been and can be used to store reclaimed water from the magnetite concentrator and OBP tailings operations. As described in SIMEC Mining’s June 2020 Program notification (WPC-198), reclaimed water from the Duchess North pit has recently (2021) either been used or relocated to the Duchess South pit, as to allow mining of the Duchess North resource to continue, under Stage B Part 2 of the mine plan.

It is proposed that the Duchess North mining be extended to the North, with a new pit, Duchess North D, to be developed within the bounds of ML6431, ML2705, ML2707 and ML2708. New supporting infrastructure, including WRDs and access roads, will be constructed in order to support these new operations. The resource within the new pits is comprised of the same type of ore being mined in the existing Duchess North operations and will be mined using the same methods.

Iron Duke/Magnet
Iron Duke consists of Duke ‘a pit’ and Duke ‘b pit’. The Iron Duke pits are not currently mined due to economic conditions, but the Duke area is active in terms of the connection to the Iron Magnet mining area by pit and by infrastructure (roads, stockpiles and dumps) and are still within the operations area. Ore was last sourced from the Iron Duke pits during December 2013 and January 2014. The pit areas are bunded.

The Iron Magnet deposit is being mined by conventional open pit mining. The ore is recovered by pre-stripping of overburden material, followed by drill, blast, load and haul activities. The sequencing of the material ensures a continuous source of magnetite ore for processing. Low-grade ore and waste materials are stockpiled.

Three stages of the current Iron Magnet pit have been designed and scheduled. These stages provide a continual source of magnetite ore to the end of the pit life.

Further topsoil and subsoil reclaiming and stockpiling is planned for the remaining development of DW03. Various sections of this area are developed simultaneously (as required) and will be rehabilitated upon completion.

Iron Chieftain mining operations
The Iron Chieftain deposit is mined using a traditional staged open pit drill, blast, load and haul method, with pre-strip overburden activities and pits developed in a sequence that allows for a continuous presentation and delivery of ore as required, minimising waste movements. Ore from Iron Chieftain is crushed and screened using mobile processing plants.

Iron Knight mining operations
Iron Knight consists of the Knight North and Knight South pits. The Knight South pit is no longer mined but has the potential for remaining ore reserves to be accessed if economic conditions support this approach.

The Knight North deposit is mined by an open pit mining method. A cut-back (widening of the open pit) included drill, blast, load and haul activities to recover ore. Ore is stockpiled and then crushed and screened using mobile processing plants. Low-grade ore and waste materials are stockpiled.

The current Knight North pit is complete, with no currently economically viable reserves remaining. The completed WRDs (KSW01 and KW01) are undergoing rehabilitation with further rehabilitation works in the planning phase.

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The two primary products are hematite, which can be further classified as either direct shipping ore (DSO) or low grade ore (LGO), and magnetite (concentrate).

Operations (2020) within the SMR included hematite and magnetite mining and the associated activities of crushing, screening, production of magnetite concentrate and low-grade hematite ore beneficiation.

The Ore Beneficiation Plant (OBP) ceased operation in late-2020 and LGO is no longer processed prior to export. Magnetite ore is crushed and processed through the concentrator and the magnetite product continues to be pumped to the Pellet Plant in Whyalla as a slurry.

Ore Beneficiation Plant (OBP)
Beneficiation plant has a capacity of 0.9Mtpa.
The hematite OBP is designed to upgrade low-grade hematite (i.e. 47 to 53% Fe) to a high-grade (i.e. 62% Fe) lump and fines products for export. As of November 2020, the fixed plant at the OBP has been taken offline until a review of the current ........