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Georgina Basin - Phosphate Hill, Duchess |
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Queensland, Qld, Australia |
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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The Phosphate Hill deposit is located ~50 km south of the town of Duchess and ~140 km SSE of Mount Isa in Northwest Queensland, Australia. It one of a generally north-south, 6 km long string of similar deposits distributed over an area of ~180 sq. km that constitutes the Duchess phosphate field. The other deposits include Rimmer Hill, Rogers' Ridge, Mount Murray and Bluck's Bluff. Phosphate Hill is the only one of these deposits that is currently (2012) economically exploited.
(#Location: 21° 53' 3"S, 139° 58' 6"E).
The deposit lies within an exposed embayment of the Cambrian Georgina Basin into the Mesoproterozoic basement of the Mount Isa Block, bounded by Mesozoic cover sequences to the south. The Georgina Basin covers an area of more than 100 000 sq. km. in the states of Queensland and Northern Territory, and contains sediments that range from Late Neoproterozoic to Middle Palaeozoic in age and are in excess of 4500 m in thickness.
The other Georgina Basin records detail the stratigraphy and temporal and spatial distribution on phosphogenic rocks within the sequence and basin.
The centre of the basin is characterised by well developed, massive, Cambrian dolomites, particularly the Camooweal Dolomite, with thin-bedded limestones occurring around the periphery, passing in turn into mudstones and cherts near the shoreline. Although several Cambrian units are slightly phosphatic, the thick high grade phosphorites are restricted to the marginal mudstone-chert facies and in particular to the Middle Cambrian Beetle Creek Formation and its lateral equivalents.
The Middle Cambrian (Templetonian) Beetle Creek Formation consists of chert, siltstones, shaly-mudstone, phosphatic-siltstone, phosphatic-limestone, limestone and grainstone phosphorite, reaching a maximum thickness of about 120 m. It is usually underlain by the massive dolomitic Thorntonia Limestone, and disconformably overlain in most areas by the lithologically similar, although usually non-phosphatic Inca Formation which largely comprises black shale and chert. The Thorntonia Limestone represents a period of emergence and regression and is slightly phosphatic in the northern and eastern parts of the basin.
Although phosphorites are present throughout the Beetle Creek Formation they are especially common in the upper part, where they have an average grade of ~20% (but locally up to 35%) P2O5.
The Phosphate Hill deposit lies within the Burke River Inlier of the Georgina Basin. It is hosted by the Beetle Creek Formation sequence in a 30 km wide by 100 km long, north-south elongated graben within the southern Mount Isa Block that formed the Duchess Embayment of the Georgina Basin. Within the embayment, the sequence of Cambrian marine sediments is generally less than 150 m thick and is largely obscured by a superficial cover of alluvium. These rocks overlie a local basement of Lower Cambrian Mount Birnie Beds that in turn unconformably rest on Palaeo- to Mesoproterozoic basement, principally the Kalkadoon Granite. The Cambrian, marine sedimentary succession was deposited in a shallow to moderate depth marine shelf environment and contain a significant proportion of carbonate minerals. It has been subjected to periods of structural deformation, including both extension, reflected by faults that bound the embayment to the east and west and subsequent compression which produced open folds within the Cambrian sequence.
The Cambrian sequence at Phosphate Hill comprises, from the base:
Mount Birnie Beds - Composed of a basal, more than 20 m thick tilloid member, comprising unsorted boulder clay of silty mudstone with polymict pebbles and boulders derived from the Proterozoic basement. This is followed by an up to 15 m thickness of dolomite, marly dolomite, siltstone and chert, and up to 35 m of ferruginous sandstone, silty and shaly sandstones, up to 15 m of othoquartzite and conglomerate, and finally by an upper, up to 20 m thick, massive ferruginous silty mudstone member.
Thorntonia Limestone - A lower, up to 12 m thick chert member composed of silicified carbonate rock and coquina, overlain by a carbonate member, made up of up to 20 m of dolomitic limestone and dolomite, and minor chert. These rocks are locally siliceous due to replacement of dolomite, with sub-metre thick chert bands within the mine area.
Beetle Creek Formation - Disconformably overlies the Thorntonia Limestone, with an up to 55 m thick lower fossiliferous siltstone member, comprising both calcareous and siliceous phosphatic facies siltstone, chert, fetid dolomitic phosphatic limestone and minor phosphorite. The siliceous phosphatic facies consists of thinly bedded cherty phosphatic siltstone intercalated with brown and black cherts and phosphatic chert, with some phosphorite and very minor shale, accompanied by minor kaolin, illite and montmorillonite. The laterally equivalent calcareous phosphatic facies consist of fetid, weakly dolomitic phosphatic limestone, with less abundant calcareous phosphorite and calcareous siltstone, and minor chert and shale. The carbonate is micritic. A third facies of the lower siltstone member, comprises non-phosphatic cherty siltstone and chert, which interfingers with the siliceous phosphatic facies along the western margin of the embayment. The lower siltstone unit is overlain by the upper Monastery Creek Phosphorite Member that has a maximum thickness of 38 m and hosts the main ore body. Sedimentation was essentially non-clastic and was dominated by carbonaceous black chert, fetid limestone and phosphorite with phosphatic cherty siltstone, chert and fetid phosphatic limestone. Phosphorite deposition occurred in a shallow marine environment with restricted access to the open sea. As with the lower siltstone member, both siliceous- and calcareous-phosphatic facies are represented. The siliceous-phosphatic facies are white to brown (weathered) or dark grey to black (unweathered) phosphorite beds with intercalated cherty phosphorite, phosphatic cherty siltstone, chert and very minor phosphatic shale, with clayey and slightly ferruginous facies along the western margin of the embayment. The calcareous phosphatic facies comprises calcareous phosphorite with phosphatic and fetid cherty limestones and intercalated phosphatic chert and chert beds. The limestone is also slightly dolomitic. These rocks are generally pale grey in the weathered zone and dark grey to black below the base of oxidation. The carbonates are micritic. The calcareous facies predominates to the east of the siliceous facies and further into the basin, with local development of the siliceous facies over and rimming basement highs that coincide with the known deposits. The mined section of the sequence is 8 to 14 m thick.
Inca Formation - Comprising an upper limestone member of fetid, cherty dolomitic limestone and micritic-dolomite, with minor calcareous shale, and a maximum thickness in excess of 150 m, overlying a lower shale member composed of thinly laminated siltstone, siliceous siltstone, shale, cherty shale and siltstone with minor chert that reaches a maximum thickness of ~100 m and disconformably overlies the Beetle Creek Formation.
Chatsworth Limestone - Calcarenite, calcilutite, sandy and oolitic limestones, coquinite and marl, with a maximum thickness of ~600 m, containing Upper Cambrian (Franconian) fossils, and disconformably overlying the Inca Formation.
Ninmaroo Formation - Sandy calcarenite, calcilutite, oolitic limestone, dolomitic limestone and marl, with a maximum thickness in excess of 150 m, containing Ordovician (Tremadocian) fossils, and disconformably overlying the Chatsworth Limestone.
Swift Beds - Sandstone, siltstone with limestone and chert, with a maximum thickness 9 m, containing Ordovician (Lower Arenigian) fossils, and overlying the Ninmaroo Formation above an angular unconformity.
The Duchess Embayment is a fault-bounded sub-basin developed on the shelf of the basin of the Burke River Outlier. The major fault elements are the: (i) NE-trending Western Fault which forms the western margin of the embayment with the basement Kalkadoon Granite; (ii) North-south trending Rogers' Ridge Fault, and its progressive southern extensions, the; (iii) Eastern and (iv) Mehaffy Creek faults, which together constitute the interpreted hinge line between the shelf and the main basin of the Burke River Outlier. All of the deposits are closely to the west of this composite north-south structure. Other faults, generally with NE-SW-trends, are found in the Mt. Murray and Phosphate Hill areas, and around the southern margin of the embayment. With the exception of the 45°E dipping Mount Murray Fault, these structures are dominantly high-angle, block faulting, with no appreciable lateral movement. The throw on the Western Fault exceeds 100 m in north, and is ~30 m near the westernmost point of the embayment. The throw along the Rogers' Ridge Fault is 100 m, and that along the Eastern and Mehaffy Creek faults is probably >300 m.
The dominant folding in the embayment is manifested as broad anticlines and synclines, generally with well-defined north-south axial trends changing to NW to the south. More complex folding is evident in the Rogers' Ridge, Mt. Murray and Phosphate Hill areas, where cross-folding on general east-west axes produced many dome and basin structures. Dips on the fold limbs of the broad structures are only a few degrees and, in the more complexly deformed areas, generally from 5 to 10°, with abnormal dips of up to 30° at Rogers' Ridge.
Siliceous and calcareous facies were developed within the phosphorite units of the Monastery Creek Phosphorite Member, and the lithological variations were controlled by north-south basement structures active during sedimentation. The member is found over most of the Duchess Embayment, but pinches out to the west. The siliceous phosphatic facies occurs in the central and western area of the shelf, in zones of broad structural highs, and over a string of north-south-trending, structurally controlled basement highs further east. These eastern basement highs broadly coincide with the known deposits and are found just to the west of the north-south hinge-line transition from the western shelf to the eastern basinal facies along the composite Rogers' Ridge-Eastern-Mehaffy Creek faults. The siliceous facies pass into siliceous-clayey phosphatic facies in the extreme western part of the embayment over a very gently shelving positive area interpreted as being in proximity to the strand line. The calcareous phosphatic facies occurs mainly in the eastern part of the embayment coinciding with prominent north-south trending basement lows and near the thickening of the unit along the interpreted major hinge line. High grade phosphorite sections coincide with the maximum development of the siliceous phosphatic facies and, conversely, the lower phosphate contents occur in areas of maximum development of carbonate.
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 predominate at Phosphate Hill and 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 place during deposition.
Collophane (crypto-crystalline carbonate fluorapatite) is the dominant phosphate mineral, with minor crandallite and other aluminium phosphates of secondary origin. Most of the collophane of the sediments of the Beetle Creek Formation occurs in particulate form as pellets and minor replacement of fossil fragments. However, some of the collophane occurs as collophane mudstone beds or as a cement binding breccia fragments or clastic constituents. Fossil fragments replaced by collophane are more common as inclusions in the pellets, some of which consist almost entirely of fossil debris. Banded collophane overgrowths encase some of the fossil debris.
Randomly orientated anhedral and angular clastic quartz and sericite silt grains, generally <10 µm, are found in some pellets, particularly within the more silty lithologies. Rarely, overgrowths are developed on quartz grains. Rhombic carbonate crystals also occur within collophane pellets, although carbonate inclusions are generally restricted to the calcareous phosphatic facies where replacement of collophane by calcite is common.
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 uniform fine-grained structureless collophane groundmass, with siliceous (quartz and sericite silt), 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.
The Phosphate Hill operation produces ~1 Mt of phosphate per annum (2008-09).
Proven + probable reserves in 2004 totalled 86.1 Mt @ 24.4% P2O5, for 21 Mt of phosphate, plus
Measured + indicated + inferred resources of 45.4 Mt @ 23.3% P2O5 for 78.5 Mt of phosphate.
Pre JORC resources in 1990 (Dreissen, 1990) were quoted at: >1 Gt @ 17.3% P2O5.
The most recent source geological information used to prepare this decription 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.
Phosphate Hill
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Selected References:
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Cook P J, 1972 - Petrology and Geochemistry of the Phosphate Deposits of Northwest Queensland, Australia : in Econ. Geol. v.67 pp. 1193-1213
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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
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Russell R T and Trueman N A, 1971 - The Geology of the Duchess Phosphate Deposits, Northwestern Queensland, Australia : in Econ. Geol. v.66 pp. 1186-1214
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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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