Northern Territory, NT, Australia

Main commodities: U
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The Nabarlek uranium deposit was located in the Alligator Rivers uranium field of the Northern Territory, Australia, some 270 km due east of Darwin.

The oldest rocks in the vicinity of the deposit are a succession of metamorphosed semi-pelitic sediments and amphibolite (meta-gabbro) of the Paleoproterozoic Myra Falls Metamorphics. The lowest units within this sequence are ~200 m of muscovite-quartz-biotite schists, followed by ~100 m of interlayered hornblende amphibolite and muscovite-biotite-quartz schist exposed in the footwall of the Nabarlek fault. To the south of the deposit there are rare occurrences of graphite-garnet schist. In the hangingwall of the Nabarlek fault, there are at least 75 m of altered biotite-muscovite-quartz-feldspar schist and hornblende amphibolite. These latter two lithologies host the bulk of the ore.

The metamorphic rocks are in fault contact with the Nabarlek Granite which occurs below the deposit at a depth of more than 450 m. Adjacent to the fault, the granite is fine grained and mylonitic, but away from it has an igneous texture, similar to an exposed 1780 Ma granite 6 km to the east. This granite contains ~40 ppm U in the vicinity of the deposit, which may be secondary related to the same mineralised event that formed orebody.

Several hundred metres to the north of the deposit, the host sequence is cut by an east-west trending, 220 to 250 m thick gabbroic facies of the 1688±13 Ma Oenpelli Dolerite which occupies aroud 50% of the mine sequence. The Oenpelli Dolerite is composed of sparse, tabular phenocrysts of plagioclase in a groundmass of randomly oriented plagioclase laths enclosed by ophitic pale green augite and dark green hornblende, with coarse disseminated ilmenite and pyrite. A coarse pink, 20 m thick granophyric layer occurs towards the upper margin of the dyke.

All of these rocks are overlain by the shallow dipping sandstones of the Mesoproterozoic Kombolgie Formation above a sharp angular unconformity. Outliers of this unit are found immediately to the north, west and south of the deposit and comprise weatherd orange-cream sndstone with abundant goethite and kaolinite. Drilling indicates that the unconformity surface had considerable relief. Extrapolation of the unconformity surface indicates that the orebody was originally within 50 m below it.

Within the mine area, the host schists are characterised by tight, isoclinal and recumbent folds of interlayered altered quartz rich psammitic schist and amphibolite, the metamorphism of which is indicated to date at approximately 1790 to 1737 Ma.

The deposit was confined to a shear zone within strongly deformed and metasomatised Palaeoproterozoic semi-pelitic schists and amphibolite of the Myra Falls Metamorphics, near the unconformity with the overlying Mesoproterozoic Kombolgie (sandstone) Formation.

The orebody was deposited within and adjacent to the Nabarlek fault breccia and as an envelope of low grade disseminated mineralisation.   The orebody had a strike length of 250 m, thickness averaging 7 m and down plunge extent of 105 m. It comprises a high grade core of >1.0% U3O8 within the breccia, surrounded by a low grade disseminated envelope of around 0.1% U3O8 which extends for around 7 m into the adjacent country rock. The orebody has an irregular shape, occurring to greater depths to the south, while being wider to the north where the fault bifircates. The Nabarlek fault also cuts the Oenpelli Dolerite which is only weakly mineralised and poorly altered.

Massive breccia-cementing ore is locally present, although in general the ore occurs as millimetric, filamentous veinlets. Coarser veins contain colloform, concentric banding and radial fracturing containing uraninite and rare brannerite. Vein margins vary from sharp, parallel, to serrated, with adjacent, finely disseminated angular uraninite, or as films parallel to chlorite cleavage.

Massive chlorite±sericite±hematite rocks, breccias and intensely altered schists were characteristic of the ore with at least 3 generations of chlorite. The primary ore is dark green or dull black in colour, but may be mottled with pink or white patches where overprinted by later mica and hematite alteration. The gangue is predominantly magnesian chlorite, anatase and rare hematite, apatite and white mica. Veins of primary sulphides comprise <1%, mainly galena, chalcopyrite, but may rarely be up to 5%, and overprint the uraninite. In addition, swarms of 0.5 mm thick fine grained dolomite and calcite cut the primary ore.

Residual primary ore comprises rounded to ovoid, millimetric to centimetric remnants of primary ore suspended in a matrix of white, fine grained illite and disseminated hematite. This mineralisation overlaps the primary ore zone, and is mainly found within the fault breccia zone in the upper parts of the deposit. The main uranium mineral is coffinite, forming rims to cubes of uraninite, and sometimes completely replacing it.

Weathered ore is accompanied by illite, minor kaolinite, anatase, and hematite/goethite alteration in the wallrocks. The uranium occurs as fine grained yellow minerals which include sklodowskite, rutherfordine, kasolite and curite, with associated digenite, covellite, native copper and marcasite after primary sulphides. Torbenite and autunite (phosphates) occupy open fractures and frequently accompany orange goethite.

The deposit was mined in 4 months and 11 days starting in October 1979 with the removal of 2.33 Mt of overburden, 564 437 t of ore @ 1.86% U
3O8 and 157 000 t of low grade 0.05% U3O8 material.   This was stockpiled and treated between June 1980 and June 1988.   A total of 10 858 t of U3O8 was produced.

For detail see the reference(s) listed below.

The most recent source geological information used to prepare this summary was dated: 1990.    
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:
Anthony P J  1975 - Nabarlek uranium deposit: in Knight C L, (Ed.), 1975 Economic Geology of Australia & Papua New Guinea The AusIMM, Melbourne   Mono 5 pp 304-308
Beaufort D, Patrier P, Laverret E, Bruneton P and Mondy J  2005 - Clay Alteration Associated with Proterozoic Unconformity-Type Uranium Deposits in the East Alligator Rivers Uranium Field, Northern Territory, Australia: in    Econ. Geol.   v100 pp 515-536
Derome D, Cuney M, Cathelineau M, Fabre C, Dubessy J, Bruneton P, Hubert A  2003 - A detailed fluid inclusion study in silicified breccias from the Kombolgie sandstones (Northern Territory, Australia): inferences for the genesis of middle-Proterozoic unconformity-type uranium deposits: in    J. of Geochemical Exploration   v80 pp 259-275
Ewers G R, Ferguson J, Donnelly T H  1983 - The Nabarlek Uranium deposit, Northern Territory, Australia: some petrologic and geochemical constraints on genesis: in    Econ. Geol.   v78 pp 823-837
Maas R  1989 - Nd-Sr isotope constraints on the age and origin of unconformity-type Uranium deposits in the Alligator Rivers Uranium Field, Northern Territory, Australia: in    Econ. Geol.   v84 pp 64-90
Polito P A, Kyser T K, Marlatt J, Alexandre P, Bajwah Z, Drever G,  2004 - Significance of Alteration Assemblages for the Origin and Evolution of the Proterozoic Nabarlek Unconformity-Related Uranium Deposit, Northern Territory, Australia: in    Econ. Geol.   v99 pp 113-139
Wilde A R, Mernagh T P, Bloom M S, Hoffmann C F  1989 - Fluid inclusion evidence on the origin of some Australian unconformity-related Uranium deposits: in    Econ. Geol.   v84 pp 1627-1642
Wilde A R, Noakes J S  1990 - Nabarlek Uranium deposit: in Hughes F E (Ed.), 1990 Geology of the Mineral Deposits of Australia & Papua New Guinea The AusIMM, Melbourne   Mono 14, v1 pp 779-783
Wilde A R, Wall V J  1987 - Geology of the Nabarlek Uranium deposit, Northern Territory, Australia: in    Econ. Geol.   v82 pp 1152-1168

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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