Mulga Rock - Princess, Ambassador, Emperor, Shogun
Western Australia, WA, Australia
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The Mulga Rock uranium deposits are hosted by organic rich muddy sediments within an Eocene buried palaeochannel cut into sediments of the Officer Basin, 230 km ENE of Kalgoorlie in Western Australia. The individual deposits include Princess, Ambassador, Emperor and Shogun.
(#Location: 29° 59' 0"S, 123° 48' 8"E).
The NNE trending, glacially scoured palaeo channel at Mulga Rock, which is up to 600 m deep, is incised into Permo-Carboniferous sediments of the Paterson Formation in the SW corner of the Officer Basin, overlying granitoids and metamorphics belonging to the Archaean Yilgarn Craton to the west and the Proterozoic Albany-Fraser orogen to the east. The channel has been traced below Quaternary cover for more than 100 km. In the Mulga Rock area it is horseshoe shaped and varies from 5 to 15 km in width. It contains up to 100 m of flat lying mudstones and fine sandstones disconformably overlying the Paterson Formation, although locally at Ambassador it directly overlies Albany-Fraser rocks where the Paterson Formation has been eroded.
The base of the channel is generally occupied by 2 to 3 m of pebble gravel, grading upward into fining quartz sand up to 40 m thick, with lenses of carbonaceous and pyritic silts and clays. The sand is conformably followed by up to 30 m of interbedded clay, carbonaceous clay, peaty clay and peat, with the organic content increasing upwards. The base of this unit has sand lenses, while the upper 2 to 3 m has been oxidised to white kaolinitic clay. The bulk of the uranium is hosted by the peats and peaty clay near the top of this unit, with radium in the kaolinitic oxidised layer marking the previous mineralisation at the redox front.
The kaolinitic clay is followed by up to 30 m of interbedded quartz sand, silt and clay which is wholly oxidised and silicified to varying degrees and has been lateritised nearer surface. All of these are masked by a blanket of up to 20 m of aeolian sand and alluvial clay.
The uranium mineralisation in each of the deposits is found immediately below the redox front at the base of weathering hosted by richly carbonaceous muddy sediments, commonly peat, clayey peat and peaty clay and at a depth to top varying from 20 to 50 m below surface. Dry ore has up to 60% organic carbon, with kaolinitic clay and minor quartz. The ore contains clastic resistates (quartz, zircon, rutile, etc), authigenic sulphides (pyrite, chalcocite, chalcopyrite, sphalerite, polydymite and violarite), barite, gypsum and anglesite, selenides (of Pb, Hg, Ag, Cu, Ni), diffuse concentrations of Se, Ti, V, U, and Cu, and rare authigenic uranium oxides, U-Ti oxides, U arsenates, and U silicates.
The bulk of the ore is as fine disseminations, too fine to discern, disseminated through the organic sediments, held by adsorption on the organic matter as urano-organic complexes.
The three deposits contain a total resource (Fulwood and Barwick, 1990) of:
13 000 t of U at a grade averaging 0.12% U (at a cut-off of 0.03% U and minimum 0.1 m% U thickness
Average grades are:
Shogun - 0.09% U, Emperor - 0.11% U, Ambassador - 0.14% U,
The areal extent of mineralisation at these grades and cut-offs totals 7 km2 and averages 2 m in thickness.
Published JORC compliant ore reserves and mineral resources at a cut-off of 0.15 kg/t U3O8 (Vimy Resources website, viewed November, 2016), were:
Probable ore reserves at Mulga Rock East, as at March 2016, included within mineral resources,
Princess - 1.3 Mt @ 0.64 kg/t U3O8 for 830 tonnes of U3O8;
Ambassador - 13.9 Mt @ 0.66 kg/t U3O8 for 9175 tonnes of U3O8;
TOTAL reserves - 15.2 Mt @ 0.66 kg/t U3O8 for 10 005 tonnes of U3O8.
Resources at Mulga Rock East, as at June 2016,
Princess - Indicated resource - 1.3 Mt @ 0.69 kg/t U3O8 for 860 tonnes of U3O8;
Inferred resource - 2.5 Mt @ 0.38 kg/t U3O8 for 950 tonnes of U3O8;
Ambassador - Indicated resource - 19.8 Mt @ 0.72 kg/t U3O8 for 14 290 tonnes of U3O8;
Inferred resource - 10.4 Mt @ 0.33 kg/t U3O8 for 3490 tonnes of U3O8;
TOTAL resources - 34.1 Mt @ 0.58 kg/t U3O8 for 19 590 tonnes of U3O8.
Resources at Mulga Rock West, as at June 2016,
Emperor - Inferred resource - 28.4 Mt @ 0.45 kg/t U3O8 for 12 780 tonnes of U3O8;
Shogun - Inferred resource - 4.1 Mt @ 0.55 kg/t U3O8 for 2225 tonnes of U3O8;
TOTAL resources - 32.6 Mt @ 0.46 kg/t U3O8 for 15 005 tonnes of U3O8.
TOTAL Resources at Mulga Rock - 66.6 Mt @ 0.52 kg/t U3O8 for 34 595 tonnes of U3O8
For detail consult the reference(s) listed below.
The most recent source geological information used to prepare this summary was dated: 1990.
Record last updated: 22/11/2016
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.
Mulga Rock area
References to this deposit in the PGC Literature Collection:
Fulwood K E, Barwick R E 1990 - Mulga rock Uranium deposits, Officer Basin: in Hughes F E (Ed.), 1990 Geology of the Mineral Deposits of Australia & Papua New Guinea The AusIMM, Melbourne Mono 14, v2 pp 1621-1623|
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