Summary:
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.