Summary:
The Iron Baron mining area (IBMA) lies at the northern end of the Middleback Ranges (MBR). The local geology in the vicinity of the IMBA, as described in SIMEC Mining (2015a) is as follows:
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, although 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; a process which 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).
Central Iron Baron mining area (IBMA) geology
The Central Iron Baron deposits include the Little Baron, Big Baron, East Baron, Iron Prince and Iron Prince North ore bodies.
The Iron Prince deposit is located at the northern end of the IBMA. Prior to mining the resource was approximately 1500 m long and 180 – 240 m wide. Most of the ore was located in a north/north-east striking syncline which plunges 8º south. The syncline is fault bounded to the east and west in a graben-like structure. The western fault is more prominent and dips 60 – 80º east. The footwalls to the bounding faults consist of granite gneisses that, from previous petrological studies, are described as granodiorite-adamellite granitoids which represent both late syntectonic granitoids possibly of the Lincoln Complex and pre-early syntectonic granitoids possibly equivalent to the Minbrie Gneiss. A number of later stage cross faults and folds further complicate the structure, creating an undulating base to the ore. These structures terminate the ore along strike. The main ore body is made up of hematite lenses intercalated with amphibolite dykes.
The deeper and more extensive southern end of the ore body is underlain predominantly by pink-red, ferroan, (and to a lesser extent by grey-white) dolomite. Minor argillaceous schists and banded iron formation also occur. The shallower northern end (Prince South Stage 2 deposit) is more variable and is intercalated with more amphibolite and schists and less dolomite. Red ore is more common in the northern part of Iron Prince. Red ore is generally more schistose and has a ‘greasier’ texture than the harder more massive blue ore common in the South Prince. Blue ore often retains relict banding and varies from fine grained hard to soft, friable hematite.
The Little Baron and Big Baron deposits occupy a similar structure to the Iron Prince and lie adjacent to the Western (NNE striking) Fault. These deposits are the southern strike extension of the Iron Prince deposits. The geology of the ore bodies is similar to Iron Prince although red ore is less common and the Baron deposits are predominantly underlain by schists and banded iron formations (BIF) with minor dolomite.
Several amphibolite dykes intrude the deposit, including a possible ultrabasic rock identified by petrological work. The eastern ore body occupies a second synclinal structure to the east of the Big Baron. The syncline is faulted off to the north and does not appear adjacent to the Little Baron or the Iron Prince deposits. The two synclines are separated by an anticline which has been extensively intruded by amphibolite. The eastern ore body is extremely irregular. Hematite pods are intercalated with clay schists and amphibolite. Remnants of BIF and dolomite are less common.
Substantial scree ore deposits have been identified around the flanks of the current mining operations, particularly on the north and western slopes of the Central IBMA. Two major resources have been identified; the Empress to the north and the Baroness to the west. A smaller resource has also been identified to the east of the East Baron deposits. The Baroness and Empress scree deposits are the detrital concentrations of iron ore scree from the Iron Baron and Iron Prince deposits respectively. The scree deposits are formed by the erosion of well exposed iron ore deposits such as the Iron Prince and Big Baron ore bodies and have accumulated over time in alluvial fans at the foothills of the in-situ iron deposits.
Both the Empress and Baroness deposits are wide in nature and have lateral extents greater than a kilometre. Typical iron grades have been shown to vary from ~30-60% Fe with sand, clays, calcrete and other gangue rock types as the main components considered to be deleterious to the iron grade. The smaller scree deposit on the eastern flanks of the East Baron pits is similar nature to the Empress, with a clay cemented product. A substantial paleochannel of scree ore in the vicinity also contains a good quantity of high grade material flowing east from the faces.
Iron Queen Geology
The Iron Queen deposit is on the far eastern flanks of the northern MBR.
The geology consists predominantly of the Lower Middleback Iron Formation (LMIF), the preferred host to mineralisation. The Katunga Dolomite outcrops in several places to the north and west and at depth beneath the deposit, and significant volumes of amphibolite occur to the north and east of the mined area. The Iron Queen deposit was mined during the 1980’s and consists of two main pods of hematite ore. The southern pod has been almost completely mined out, while the northern pod remains unexploited. Significant amounts of amphibolite intrude the deposit including high magnesium olivine/pyroxene porphyritic mafic to ultramafic intrusives. Hematite ore is associated with schists and BIF. The overall structure is broadly synclinal and plunges approximately 20º south. The two main ore pods appear to have been dislocated along a north trending strike slip fault. The western block appears to have moved approximately 100 m north. Granitic rocks appear to confine the ore to the north and east and a prominent north-south fault/breccia zone exists on the western side of the deposit.
There are scree ore deposits on the eastern flank of the Iron Queen deposit. The upper sections of the scree appear to be cemented calcrete agglomerations materials, whilst the lower flanks are more loosely consolidated hematite cobbles.