Northern Territory, NT, Australia

Main commodities: U Au
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The Jabiluka uranium deposit is located approximately 230 km east of Darwin in the Alligator Rivers uranium field of the Northern Territory, Australia, some 25 km north of the Ranger mine (#Location: 12° 30'S, 132° 55'E).

The Jabiluka deposits are located in the north-eastern part of the Paleoproterozoic Pine Creek Geosyncline which overlies Archaean basement. The Paleoproterozoic comprises the 2470 to 1800 Ma basement Nanambu Complex granitic gneiss and biotite schists. Regionally, this complex is unconformably overlain by the Paleoproterozoic Kakadu Group conglomerate, sandstone and arkose, although in the Alligator Rivers area, the overlying Cahill Formation lies directly on the Nanambu Complex. In the Jabiluka area, the Cahill Formation comprises three units, i). a lower unit composed of carbonate, generally dolomite or magnesite; ii). a middle unit which hosts the bulk of the mineralisation which consists of graphitic pelitic schist, semi-pelitic schist and minor carbonate; and iii). the upper unit which is more psammitic and includes quartz-muscovite schist and amphibolite.

The Cahill Formation is conformably overlain by the dominantly clastic Mourlangie Schist psammitic schist, carbonates and quartzite. All of these rocks are cut by the 1870 Ma Zamu dolerite prior to regional deformation and metamorphism of the Pine Creek Geosyncline between 1870 and 1800 Ma. Four phases of deformation are recorded, the most intense being D2 which produced isoclinal recumbent folds with a flat lying foliation sub-parallel to lithological layering and low angle thrust faults. F2 folds were refolded by D2 and D3. These deformations were followed by three post-orogenic igneous events, prior to deposition of the main Mesoproterozoic sequence, namely: i). the felsic volcanics and tuffs of the 1760 Ma Edith River Volcanics in the south; ii). the 1750 Ma granitic pegmatites throughout the Alligator Rivers area; and iii). the 1690 Ma Oenpelli Dolerite.

Following a period of erosion, the preceding rocks were unconformably overlain by Mesoproterozoic Kombolgie Formation of the McArthur Basin which is preserved as a flat lying sequence of sandstone with minor shale and conglomerate and two volcanic units. The lower and thicker of the volcanic units is the up to 250 m thick, predominantly basaltic Nungbalgarri Volcanic Member which is about 300 m above the base of the formation and has been dated at ~1650 Ma. Post Kombolgie structure is characterised by gentle warping and NW striking strike slip faults, associated with reactivation of earlier low angle reverse faults which follow preferred layers in the Paleoproterozoic and in places follow the unconformity and pass into the overlying sandstones. The Kombolgie Formation is cut by 1370 basic and 1320 Ma phonolitic dykes and overlain by the sandstones and conglomerates of the Cretaceous Bathurst Island Formation.

Within the immediate Jabiluka area, the host Cahill Formation is locally exposed as a window through the Kombolgie Formation hosting the Jabiluka I deposit, truncated to the west by down-faulted Kombolgie sandstones across the north-south trending Rowntree Fault. To the east the Jabiluka I mineralisation fades out, but reappears 500 m further east by Jabiluka II which is entirely concealed below 20 to 200 m of Kombolgie Formation. The Cahill formation dips to the south from near horizontal to near vertical below the unconformity in the deposit area and when the stratigraphy compared to the regional sequence appears to be overturned, interpreted to represent the lower limb of a recumbent fold.

The nearest Nanambu Complex rocks are quartz-feldspar-biotite gneisses 3 km to the south of the deposit, which are directly overlain by the Kombolgie Formation. Within 600 m of the deposit the Complex rocks are faulted out against carbonates of the Cahill Formation below the unconformity.

Jabiluka I and II are unconformity style deposit hosted by carbonaceous schists and brecciated, bedded carbonates of the Paleoproterozoic Cahill Formation. In detail the deposits are contained within a set of up to nine bands of alternating mineralised and barren schists, which have been variously interpreted as a stratigraphic sequence, or structural repetition of a simpler succession by folding or faulting. The mineralised intervals are characterised by brecciation and chloritisation and contain quartz, sericite and graphite, while the barren intervals tend to contain coarse shiny muscovite flakes, and are often porphyroblastics schists with muscovite after garnet. Outside of the ore zone these barren and mineralised bands are apparently equivalent to a thicker sequence of bedded carbonate which towards the ore thins drastically or is replaced by cryptocrystalline silica, hematite and apatite ('cherts'). Fragments of this chert are found in chlorite cemented breccia in the ore zone.

In the immediate deposit area the Kombolgie Formation comprises a alternating medium- and coarse-grained (locally to conglomerate) quartz sandstone unit with an onlapping unconformable contact above the Cahill Formation basement ridge. Hematite and chlorite at the unconformity have been attributed to a pre-Kombolgie regolith development or to post-Kombolgie alteration. Hematite and chlorite alteration is extensively developed within the Kombolgie Formation.

The mineralisation at Jabiluka I is confined to a single unit within the Cahill Formation, the 'main mine sequence', while at Jabiluka II around 75% of the resource is within the same unit. Ore is also found within the overlying 'upper graphite sequence' (separated from the 'main mine sequence' by the barren 'hangingwall sequence'), and in the 'lower mine sequence 1' and 'lower mine sequence 2', separated from each other and the overlying 'main mine sequence' by barren bands. The 'main mine sequence' is higher grade at Jabiluka II with 0.51% U3O8 compared to the overall geological average of 0.39% U3O8 for the deposit. Ore is almost entirely below the unconformity, within the Cahill Formation.

The primary mineralisation is uraninite, with minor coffinite, brannerite and organo-uranium minerals. It occurs in three main forms: i). in breccias, representing the bulk of the resource, where uraninite commonly occurs as infillings occupying voids and cementing clasts; ii). in veins adjacent to the breccias which cut across schistosity; and iii). as fine grained, low grade uraninite disseminations in schistose host rocks adjacent to the breccias and veins. Sulphides generally only amount to a few percent of the hosts, although up to 10% pyrite can occur in graphitic zones. Galena and chalcopyrite with quartz and/or dolomite is found in fractures cutting uraninite veins.

Mineralisation related alteration forms a zone parallel to the unconformity with the overlying Kombolgie Formation and decreases intensity downwards and is characterised by chlorite and quartz with Mg marble in the Cahill Formation also and is dated at 1600 to 1300 Ma. An outer, ealry phase zone is apparently represented by iron rich chlorite and anatase replacing metamorphic biotite and white mica after feldspar. Within a few metres of the unconformity, or adjacent to breccias, more intense clay-sized magnesian-aluminium rich chlorite and phengitic (celadonite) mica is developed. Veins of chlorite and white mica are also found up to 300 m above the unconformity within the Kombolgie Formation.

Gold mineralisation is found as a discrete zone in the western half of Jabiluka II in association with graphitic horizons accompanied by high grade uranium (averaging 0.8% U
3O8). Like the uranium the bulk of the gold (52%) is hosted by the the 'main mine sequence', lesser amounts in the 'upper graphite sequence' (17%), the 'lower mine sequence 1' and 'lower mine sequence 2', while 22% is in the 'barren' 'hangingwall schist' without accompanying uranium. The gold occurs as 10 to 100 µm native gold in uraninite and in cross-cutting veinlets. Un-economic traces of Pd correlate with Au and U.

Jabiluka II has an E-W strike length of 1000 m and a down dip extent of 500 m.

Resource and reserves in 1986 (Hancock, et al., 1990) were:
  Jabiluka I - 1.3 Mt @ 0.25% U
  Jabiluka II - 52 Mt @ 0.39% U
  Jabiluka II includes - 1.1 Mt @ 10.7 g/t Au
  Jabiluka II reserves included 207 000 t U

In December 2008, the reserves and resources at Jabiluka were (ERA Media Release, Jan. 2009):
    Proved + probable reserves (0.20% U
3O8 cut-off) - 13.8 Mt @ 0.49% U3O8 = 67 700 tonnes of U3O8 plus, in addition to the reserves,
    Measured + indicated + inferred resources (0.20% U
3O8 cut-off) - 15.44 Mt @ 0.48% U3O8 = 73 940 t U3O8.

The most recent source geological information used to prepare this summary was dated: 1982.    
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:
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
Gustafson L B, Curtis L W  1983 - Post-Kombolgie metasomatism at Jabiluka, Northern Territory, Australia, and its significance in the formation of high-grade Uranium mineralization in Lower Proterozoic rocks: in    Econ. Geol.   v78 pp 26-56
Hancock M C, Maas R, Wilde A R  1990 - Jabiluka Uranium-Gold deposits: in Hughes F E (Ed.), 1990 Geology of the Mineral Deposits of Australia & Papua New Guinea The AusIMM, Melbourne   Mono 14, v1 pp 785-793
Ludwig K R, Grauch R I, Nutt C J, Nash J T, Frishman D, Simmons K R  1987 - Age of Uranium mineralization at the Jabiluka and Ranger deposits, Northern Territory, Australia: new U-Pb isotope evidence: in    Econ. Geol.   v82 pp 857-874
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, Thomas D, Marlatt J and Drever G  2005 - Re-evaluation of the petrogenesis of the Proterozoic Jabiluka unconformity-related uranium deposit, Northern Territory, Australia: in    Mineralium Deposita   v40 pp 257-288
Rowntree J C, Mosher D V  1975 - Jabiluka uranium deposits: in Knight C L, (Ed.), 1975 Economic Geology of Australia & Papua New Guinea The AusIMM, Melbourne   Mono 5 pp 321-326
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

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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