Georgina Basin - Wonarah, Alroy and Alexandria

Northern Territory, NT, Australia

Main commodities: P
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A series of phosphate deposits are hosted around the margins of the Georgina Basin in northwest Queensland and Northern Territory in northern Australia. These include the Phosphate Hill / Duchess (currently being exploited - see separate record); Lady Jane / Thorntonia / Paradise North; Lady Annie / Paradise South; Ardmore; D-Tree; Lily Creek; Mt. O'Connor; Phantom Hills; Sherrin Creek; Babbling Brooke Hill; Mount Jennifer; Riversleigh; Highland Plains and Quita Creek in Queensland, and Wonarah, Alroy and Alexandria in the Northern Territory.

The NW-SE elongated Georgina Basin covers an area of more than 100 000 sq. km. in the state of Queensland and the Northern Territory. It contains sediments that range from Late Neoproterozoic to Middle Palaeozoic in age which reach a maximum thickness of ~9000 m, although in most areas totals much less than 2000 m. In Queensland the bulk of the sequence is Cambrian in age.   It is rimmed by Palaeo- to Mesoproterozoic basement of the Mount Isa-McArthur blocks to the northeast and is separated from the similar aged Wiso Basin to the SW by the Davenport-Tennant Creek Inliers, while being partially bounded to the south by the Arunta Block. It is unconformably overlain by Late Jurassic to Cretaceous sedimentary rocks of the Eromanga Basin.

The Phosphate Hill / Duchess deposit is located ~140 km SSE of Mount Isa, within an exposed, narrow (~30 km wide), long (~150 km) north-south elongated embayment of the Cambrian Georgina Basin into the Mesoproterozoic basement on the southern margin of the Mount Isa Block. The more restricted Ardmore and Quita Creek are within outliers of the Georgina Basin sedimentary rocks on the Mount Isa Block ~120 km SSW of Mt Isa. Sherrin Creek, Lily Creek, Lady Annie / Paradise South, Lady Jane / Thorntonia / Paradise North, D-Tree, Riversleigh, Babbling Brooke Hill, Phantom Hills, Mount Jennifer, Mount O'Connor and Highland Plains are located in that order around a broad open, semi-circular embayment on the northeastern margin of the Georgina Basin from ~70 km WNW of, to ~285 km NW of Mt Isa respectively. The Alexandria, Alroy and Wonarah deposits are to the WSW of Highlands Plains, 40 to 180 km from the basin margin, within the Northern Territory.

The base of the sequence is occupied by late Neoproterozoic continental and possibly some marine sandstones, carbonates and glacial sediments. Unconformably overlying these are Cambrian to Early Ordovician marine carbonate and fine-grained siliciclastic rocks, which host the phosphorites, followed by Ordovician clastic sedimentary rocks. These rocks are in turn unconformaby overlain by a Devonian sequence.

While significant economic phosphorites (e.g., at Phosphate Hill / Duchess) are localised within the Middle Cambrian (Templetonian) Beetle Creek Formation, vertically, phosphatic rocks have been found in all Middle Cambrian (Ordian to Boomerangian) sedimentary sequences, and are characterised by disconformity surfaces related to discrete but widespread tectonic pulses and related sea-level variations, periods of exposure and associated erosional and regolith surfaces.

Laterally, the centre of the basin is characterised by well developed, massive, Cambrian dolomites, particularly the Camooweal Dolomite, grading shoreward into peripheral thin-bedded limestones, which in turn pass into mudstones and cherts near the shoreline. Although several Cambrian units are variably phosphatic, the thick high grade phosphorites are restricted to the marginal mudstone-chert facies and in particular to the Middle Cambrian Beetle Creek Formation.

The first major flooding of the basin at the base of this succession in the early Middle Cambrian (Ordian), saw the development of carbonate banks e.g., the Thorntonia Limestone, which is locally slightly phosphatic, and is found in the northern and eastern parts of the basin. These banks are separated by interbank channels which are accompanied the deposition of anoxic black sediments e.g., the laterally equivalent black shale and carbonates of the Lower Hay River Formation to the south which has weak (~2.6% P2O5) phosphorites. The phosphorites of Border Waterhole Formation that are equivalents of the Thorntonia Limestone on the northeastern and northern margins of the basin hosts the Riversleigh, Babbling Brooke Hill, Phantom Hills, Mount Jennifer, Mount O'Connor and Highland Plains deposits. This same unit, where it continues south from Riversleigh has previously been correlated with the disconformably overlying Beetle Creek Formation and hosts the D-Tree, Lady Jane / Thorntonia / Paradise North, Lady Annie / Paradise South, Sherrin Creek and Lily Creek deposits, although all may belong to the Thorntonia Limestone and equivalents. The top of the Thorntonia Limestone is marked by shallowing of the depository, culminating in evaporitic deposition and subsequent regolith development and subaerial erosion, representing regression and emergence.

After an erosional break following the termination of the Thorntonia Limestone, a second phosphogenic cycle followed a subsequent sea level rise during the Templetonian and deposition of the Beetle Creek Formation. This formation is composed of fossiliferous siltstone, comprising siliceous and calcareous siltstones, and shaly-mudstone, with phosphatic cherts and dolomites in calcareous facies, passing upwards into chert, phosphatic-siltstone, phosphatic-limestone, limestone and grainstone phosphorite, reaching a maximum thickness of about 120 m. Weak phosphatic shales are found in the lower shales of the Beetle Creek Formation that disconformably overlies the Thorntonia Limestone, although the grainstone phosphorites, phosphatic siltstones and chert, containing up to 35% P
2O5 are largely restricted to the uppermost Monastery Creek Member of the Beetle Creek Formation, where it hosts the Phosphate Hill / Duchess deposit in the south. The equivalent Simpson Creek Phosphate Member hosts Ardmore as an outlier on the Mt Isa Block basement. The Wonarah and Burton Beds to the west on the Barkly Tableland to the west into the Northern Territory are also of Templetonian age and as such correlated with the Beetle Creek Formation. These beds host the Wonorah and Alexandria deposits.

The disconformably overlying Inca Formation black shales and chert incorporate reworked phosphate minerals and clasts derived from the underlying Beetle Creek Formation, and comprises mainly black shales and chert of latest Templetonian to early Floran age, with locally associated siltstone, siliceous siltstone, shale and micritic dolomite.

The disconformably overlying Late Floran Gowers Formation in the NE of the basin reflects a further rise in sea level, and has further weak phosphorites, and is the last phosphatic event in the Cambrian. Further phosphate is not recorded until the Middle Ordovician (Llanvirn). No post-Ordovician phosphorites have been recorded.

Two basic types of phosphorite are recognised, a pelletal and a non-pelletal form, with a third brecciated type composed of clasts of either (or both) of these forms. The pelletal forms are most common in the Monastery Creek Member of the Beetle Creek Formation, as at Phosphate Hill, while the non-pelletal variety are of increasing abundance in the phosphorite of the Thorntonia Limestone and equivalent in the north and NE of the basin.

The pelletal phosphorites are primarily composed of ovules, with minor ooliths, and rare compound pellets. The matrix is composed of chert, calcite, detrital clay and silt-size fragments, or collophane mudstone. The ovules generally have little or no internal structure and although well rounded, are seldom spherical. The occurrence of ooliths indicates that at least some accretionary growth took placed during deposition. Collophane (crypto-crystalline carbonate fluorapatite) is the dominant phosphate mineral, with minor crandallite and other aluminium phosphates of secondary origin.

Much of the non-pelletal phosphorite is thinly bedded and exhibits scour and fill structures and intraformational breccias, but also occurs as an irregularly laminated form, rich in iron oxides, and as a fairly massive type, or as a breccia matrix. The non-pelletal phosphorites are predominantly composed of a fine-grained structureless collophane groundmass, with siliceous, calcareous or ferruginous inclusions.

Young phoscretes occur as a thin encrustation on limestone or dolomite or as an extensive deposit ranging in thickness from a few centimetres to a metre, locally blanketing the modern topography, with siliceous or phosphatic clasts derived from the Tertiary weathering profile.

Much of the testing of these deposits took place prior to the development of the JORC code for reporting exploration results, resources and reserve. Most were regarded as sub-economic, and have only recently attracted further interest and additional testing to JORC standards.

Estimated and reported resources and reserves are as follows:

Phosphate Hill (previously known as Duchess)
      Pre JORC 'demonstrated' resources in 1990 (Freeman et al., 1990) - 1.115 Gt @ 17.3% P
2O5 plus
            inferred resource of 304 Mt @ 18.7% P
      Proven + probable JORC compliant reserves in 2004 (Incitec Pivot) - 86.1 Mt @ 24.4% P
2O5, for 21 Mt of phosphate, plus
      Measured + indicated + inferred JORC compliant resources in 2004 (Incitec Pivot) - 45.4 Mt @ 23.3% P
2O5 for 78.5 Mt of phosphate.
      Pre JORC 'demonstrated' resources in 1990 (Freeman et al., 1990) - 47 Mt @ 15.6% P
Quita Creek
      Pre JORC 'demonstrated' resources in 1990 (Freeman et al., 1990) - 30 Mt @ 17.0% P
Lily Creek
      Pre JORC inferred resources in 1990 (Freeman et al., 1990) - 191 Mt @ 14.9% P
Sherrin Creek
      Pre JORC inferred resources in 1990 (Freeman et al., 1990) - 15 Mt @ 16.5% P
Lady Annie / Paradise South
      Pre JORC 'demonstrated' resources in 1990 (Freeman et al., 1990) - 293 Mt @ 16.6% P
      JORC compliant indicated + inferred resources in 2004 - 72 Mt @ 16.9% P
2O5 (12% P2O5 cut-off; Legend International Holdings, Inc.)
Lady Jane / Paradise North
      Pre JORC 'demonstrated' resources in 1990 (Freeman et al., 1990) - 193 Mt @ 17.6% P
      JORC compliant indicated + inferred resources in 2004 - 15 Mt @ 23.9% P
2O5 (12% P2O5 cut-off; Legend International Holdings, Inc.)
      Pre JORC inferred resources in 1990 (Freeman et al., 1990) - 339 Mt @ 16.0% P
      JORC compliant indicated + inferred resources in 2004 - 305 Mt @ 15.0% P
2O5 (10% P2O5 cut-off; Legend International Holdings, Inc.)
      Pre JORC 'demonstrated' resources in 1990 (Freeman et al., 1990) - 191 Mt @ 14.9% P
Babbling Brooke Hill
      Pre JORC 'demonstrated' resources in 1990 (Freeman et al., 1990) - 38 Mt @ 16.8% P
Phantom Hills
      Pre JORC 'demonstrated' resources in 1990 (Freeman et al., 1990) - 46 Mt @ 16.0% P
Mount Jennifer
      Pre JORC 'demonstrated' resources in 1990 (Freeman et al., 1990) - 20 Mt @ 15.3% P
Mount O'Connor
      Pre JORC 'demonstrated' resources in 1990 (Freeman et al., 1990) - 15 Mt @ 17.4% P
      Pre JORC reserve in 1989 (USGS) - Proven 15 Mt @ 17.4% P
2O5 + Probable 27 Mt @ 17.4% P2O5
Highland Plains
      Pre JORC 'demonstrated' resources in 1990 (Freeman et al., 1990) - 84 Mt @ 13.4% P
      Pre JORC inferred resources in 1990 (Freeman et al., 1990) - 15 Mt @ 10% P
      Pre JORC inferred resources in 1990 (Freeman et al., 1990) - 14 Mt @ 10% P
      Pre JORC inferred resources in 1990 (Freeman et al., 1990) - 880 Mt @ 15.7% P

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:
Cook P J,  1972 - Petrology and Geochemistry of the Phosphate Deposits of Northwest Queensland, Australia : in    Econ. Geol.   v.67 pp. 1193-1213
Freeman M J, Shergold J H, Morris D G and Walter M R,  1990 - Late Proterozoic and Palaeozoic basins of central and northern Australia - regional geology and mineralisation: in Hughes F E (Ed.), 1990 Geology of the Mineral Deposits of Australia & Papua New Guinea The AusIMM, Melbourne   Mono 14, v2 pp. 1125-1133

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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