New South Wales, NSW, Australia

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The Hawsons Iron deposit is located approximately 60 km southwest of Broken Hill in New South Wales, Australia, close to the border with South Australia.

The magnetite-rich host rocks of the Hawsons deposit belong to the Braemar Iron Formation, section of the basal sedimentary sequence of the Neoproterozoic Yudnamutana Sub-Group (Umberatana Group), located within the Nackarra Arc of the Adelaide Fold Belt. The north-south oriented Adelaide Fold Belt represents a complex system of successive intracratonic rifting and subsequent basin formation associated with the prelude to the break-up of Rodinia. It persists from Kangaroo Island in the south, to the northernmost Flinders Ranges to the north, while the Nackarra Arc forms a curvilinear structural arc that extends from east of Adelaide to Broken Hill (NSW) in the east, forming the southeastern limit of the Adelaide Fold Belt, immediately inboard of the Tasman Line that marks the margin of the Australian segment of Rodinia following breakup of the supercontinent.

The Yudnamutana Sub-Group (Umberatana Group) contains diamictitic siltstones (tillites), quartz sandstones, calcareous siltstones, dolomite and magnetic ironstone units of the Braemar Ironstone unit. The Braemar ironstones are examples of the world-wide glaciomarine Rapitan-Sturtian sedimentary iron-formation facies.

The Hawsons deposit is reflected in regional aeromagnetic data by a large, pronounced, curvilinear, high amplitude, magnetic anomaly, apparently representing a regional scale fold of magnetite-rich Braemar Iron Formation.   Exposure within the deposit area is limited to a window of folded, greenschist-metamorphosed, Neoproterozoic strata located on the southeast limb of the aeromagnetic anomaly, occurring as an irregularly exposed succession of steep, west-northwest to south-dipping strata.   From the structural base of this succession, the lowermost unit, an orthoquartzite is overlain by interbedded magnetite-hematite ironstone, siltstone-shale and rare carbonate units.   Siltstones are recessive, but where exposed, occur as cleaved grey to green mica- or chlorite-bearing, quartz-rich rocks. Regionally, this type of siltstone is often diamictite, containing granitic erratics, and interbedded carbonate units. Although erratics were not noted in any of the siltstone exposures mapped at the Hawsons Prospect, core from drilling, located close to the exposure window, contains siltstone matrix diamictite with angular granitic erratics.   Ironstone units are numerous and form protruding, track-like exposures up to 10 m wide and many hundreds of metres long, which are interbedded with recessive siltstone.

The ironstone units are composed of generally dark grey to black, fine-grained, diffusely laminated magnetite and, less commonly, specular hematite-rich rocks. Differentiated quartz and silicate rich bands are rare within mapped ironstones. The number and thickness of the ironstone units increases up section and northward along strike, where they are obscured by younger ferricrete duricrust/regolith (Cenozoic?), overlain by recent unconsolidated sheet-wash and aeolian sand.

The exposed Neoproterozoic rocks are cut by two penetrative fabrics and deformed by infrequent mesoscopic parasitic folds, with a later, cross-cutting, non-penetrative axial plane cleavage. The early crenulated foliation is best developed in hematitic ironstone, and is occasionally observable in magnetite ironstone. The later mesoscopic fold-related, spaced cleavage, which is most obvious in recessive siltstone. The mesoscopic folds vary from tight to open, and plunge moderatley to steeply to the southwest and suggest the Hawsons magnetic anomaly is a southwestplunging synform.

The ferricrete duricrust occurs as rounded mesas with a well-cemented cap of pisolitic goethite-limonite, with an apron of less well-cemented, coarse, angular ironstone saprock fragments. Ferruginous regolith and recent sandy cover entirely obscure the high amplitude peaks of the Hawsons aeromagnetic anomaly, suggesting the ferruginous regolith has been developed preferentially and preserved over thicker and potentially higher magnetite content, bedrock ironstone units responsible for the peak amplitude aeromagnetic anomalies.

The magnetite-dominant ironstone contains abundant blocky 25 to 150 µm subhedral to euhedral magnetite grains. The magnetite is generally inclusion free, and dispersed to weakly-banded within a simple quartz-biotite+chlorite silicate matrix of similar grain size. The texture is granoblastic to weakly lepidoblastic, with biotite and/or chlorite forming an irregular schistosity. Biotite is dominant, with regressive chlorite mostly replacing biotite. Few other minerals are present. In all samples studied to 2009, the magnetite is extensively, but irregularly, pseudomorphed by martite (hematite), which is interpreted to be the result of surface oxidation. Magnetite-primary hematite ironstone samples contain both blocky magnetite (martitised) and plate-like micaceous hematite. Hematite is always significantly finer-grained, and more irregular in shape, than associated magnetite. Silicates are the same as those in the magnetite dominant ironstones, defining diffusely differentiated micaceous hematite, and silicate mineral bands. In all the magnetite-primary hematite ironstone samples, hematite defines a distinct crenulated schistosity, which both wraps around, and is truncated by, the coarser (>100 µm), porphyroblastic magnetite grains.

In late 2010, a JORC compliant inferred resource had been established within one part of the project area, of:

    1.4 Gt @ 15.5% Fe as magnetite (DTR) with a concentrate grade of 69.9% Fe and 2.5% SiO2.

The mineralisation is soft and readily breaks along grain boundaries, easily crushed and separated by magnetic beneficiation. The deposit is close to the major Broken Hill-Port Pirie railway line and trunk power transmission lines linking the nearby mining town of Broken Hill to the national grid.

This description is largely paraphrased from Brewster, Hill and Donohue (2009).

The most recent source geological information used to prepare this summary was dated: 2011.    
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:
Brewster DC, Hill Q S and Donohue J G,  2009 - The Forgotten Hawsons Iron Prospect: Is there significant magnetite iron-ore mineralisation in the Neoproterozoic at Broken Hill?: in Korsch R J, 2009 Broken Hill Exploration Initiative: Abstracts for the 2009 Conference, Geoscience Australia,   Record, 2009/28, pp. 12-21
Koenig, R.L. and Broekman, K.T.,  2011 - Development of the Hawsons Low-Grade Magnetite: in   Proceedings, Iron Ore 2011 Conference, 11-13 July 2011, Perth, Western Australia, The Australasian Institute of Mining and Metallurgy, Melbourne,    pp. 633-638

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 takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.

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