Peculiar Knob

South Australia, SA, Australia

Main commodities: Fe
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The Peculiar Knob iron deposit is located 23 km northwest of the Prominent Hill copper-gold deposit and 87 km SE of Coober Pedy in northern South Australia, ~700 km north of Adelaide.
(#Location: 29° 35' 18"S, 135° 23' 2"E)

Peculiar Knob lies within the Skylark Domain of the Mount Woods Inlier, elongated parallel, and adjacent, to the contact with the Kennedy Dam sub-domain. It is ~5 km NW of the White Hill Complex which comprises pyroxenite, norite and gabbro, with pronounced plagioclase and pyroxene-rich layers, and interleaved disseminated to massive magnetite-ilmenite bands. See the regional setting, magnetic and geologic images in the Prominent Hill record (Peculiar Knob is located in the top NW corner of part b of the first figure in that record).

The deposit and surrounding area is completely covered by Quaternary red-brown clay, sand and silt, overlying flat-lying pale brown shale with minor gypsum of the Cretaceous Bulldog Shale and locally some thin, up to 1 m thick, wedges of the basal sandstone and conglomerate of the Cadna-owie Formation. The thickness of these sedimentary cover rocks varies from 12 m at the western end of the deposit, to 32 m at the eastern end. The underlying host sequence comprises a northeasterly-trending Palaeoproterozoic metasedimentary sequence of upper amphibolite facies BIF, quartzite and quartz-microcline-sillimanite gneiss, correlated with the 1760 to 1740 Ma Wallaroo Group. Mesoproterozoic porphyritic granodiorite of the Balta Granite, a member of the 1600 to 1580 Ma Hiltaba Suite intrudes the metasediments with associated intense chloritic alteration. These granites are coeval with the mafic intrusions of the White Hill Complex detailed above.

The Peculiar Knob deposit is located on the southern margin of the Mount Woods Inlier in a region of complex linear, and elongate to equidimensional magnetic anomalies, some of which correspond to banded iron formations. Other of these magnetic anomalies reflect large hydrothermal iron oxide replacement deposits, e.g., Manxman. The Peculiar Knob deposit occurs as a zone of high grade iron ore mineralisation contained in two sub-parallel elongate lensoidal bodies of massive specular (micaceous) hematite, thought to have formed as a result of hydrothermal enrichment of a metamorphosed BIF (Joubert and Eadie, 2013). This hydrothermal enrichment is postulated to be related to the Hiltaba iron oxide ±copper-gold event found throughout the Mt Woods Inlier.

In addition to the steeply dipping massive hematite mineralisation, the countryrock comprises magnetite-bearing quartzite, granite, aplite and minor basic dykes with a metamorphic grade in the middle to upper greenschist facies. The ore is predominantly massive specular hematite. Leaching of silica and concentration of iron produced a coarse mosaic of 0.3 to 4 mm in diameter specular hematite with minor remnant banding and some residual magnetite. Hematite has largely replaced magnetite with the remainder subsequently replaced by martite. Minor interstitial, subrounded, 0.1 to 0.4 mm diameter quartz grains are also present. Breccia textures with magnetite quartzite clasts are found mainly in the outer contact zones of the massive hematite bodies, supporting a hydrothermal replacement interpretation for the formation of the hematite ores (Morris et al., 1998).

The high grade iron mineralisation at Peculiar Knob occurs as two sub-parallel elongate lensoidal bodies of massive specular hematite with grades that are consistently >60% Fe. Both hematite lenses strike NE-SW and pinch and swell along a strike length of ~1100 m. They overlap over a length of ~200 m in the centre of the deposit to form two parallel lenses, with one persisting to the NE and the other to the SW away from the centre, and taper at the extremities. The mineralisation has been drilled to a depth of >300 m, and generally dips steeply to the NW, although dip reversals are evident locally. The lenses are generally ~35 m thick, to a maximum of 63 m. A series of NW-SE faults have resulted in a sinistral offset of the lenses (Joubert and Eadie, 2013).

JORC compliant Mineral Resources in 2013 to a depth ~120 m (Joubert and Eadie, Parsons Brinckerhoff report to Arrium Mining, Sept. 2013) at a cut-off grade of 55% Fe were:
  Measured resource - 13.60 Mt @ 64.0% Fe, 0.01% P, 7.11% SiO2, 0.27% Al2O3, 0.42% LOI;
  Indicated resource - 4.10 Mt @ 63.8% Fe, 0.02% P, 7.69% SiO
2, 0.20% Al2O3, 0.38% LOI;
  Inferred resource - 1.50 Mt @ 64.6% Fe, 0.03% P, 6.15% SiO
2, 0.20% Al2O3, 0.24% LOI;
  TOTAL resource - 19.2 Mt @ 64.0% Fe, 0.01% P, 7.16% SiO
2, 0.25% Al2O3, 0.40% LOI.

Mining commenced in 2012, but was put on care and maintenance due to low iron prices in 2015. Operations resumed in 2020 under a new owner.

The most recent source geological information used to prepare this summary was dated: 2013.    
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:
Morris, B.J., Davies, M.B. and Newton, A.W.,  1998 - Iron ore deposits of the northern Gawler Craton: in Berkman D A, Mackenzie D H (Ed.s), 1998 Geology of Australian & Papua New Guinean Mineral Deposits The AusIMM, Melbourne   Mono 22 pp. 401-406.

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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