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Angela, Pamela |
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Northern Territory, NT, Australia |
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U
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Super Porphyry Cu and Au
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The Angela and Pamela sandstone hosted uranium deposits are within Devonian to Carboniferous sediments of the Amadeus Basin located 25 km south of Alice Springs in the Northern Territory, Australia.
The east-west oriented intracratonic Amadeus Basin extends across the southern part of the Northern Territory and into Western Australia and is approximately 800 km long by up to 300 km wide, and contains up to 14 000 m of Neoproterozoic to Carboniferous sedimentary rocks. It is bounded to the north by the Paleoproterozoic Arunta Complex and to the south by the Musgrave Block.
On the southwestern margin of the basin, the basal Amadeus succession is also underlain by a 1080 to 1050 Ma rift succession Tjauwata Group, composed of bimodal volcanic rocks and intercalated clastic sediments. Sedimentation in the basin proper commenced in the early Neoproterozoic, with platformal quartz sandstone of the Heavitree Quartzite and its correlatives. These were succeeded by evaporites, carbonate rocks and red-beds of the Bitter Springs Formation and Pinyinna beds. Neoproterozoic deposition ceased in the southwest with the 570 to 530 Ma Petermann Orogeny. Uplift of the Arunta Region along the current northern margin of the basin took place during the DevonoÐCarboniferous Alice Springs Orogeny. Subsequently, up to 4000 m of non-marine molasse sediments belonging to the Pertnjara Group were accumulated in a foreland basin on the southern flank of this uplift.
The uppermost unit, the Pertnjara Group, comprises 7000 m of upper Devonian to lower Carboniferous terriginous sediments. These sediments were deposited in the foreland basin that developed at the front of a south-moving thrust wedge of the southern Arunta Province. The uppermost units in this sequence, the Undandita sandstone member (fine to coarse lithic sandstone to arkose with thin mudstone interbeds) and the overlying Brewer Conglomerate, which interfinger and represent a stratigrapic thickness of up to 3000 m, host the Angela and Pamela deposits and are the main hosts to uranium mineralisation within the basin. The Pertnjara Group has been deformed into gently double-plunging, east-trending, upright open folds, with uranium deposits and prospects known on the southern limb and eastern closure of one of these structure, the Missionary Syncline.
Most of the mineralisation is hosted by medium to coarse feldspathic lithic arenite with calcite cement of the Undandita Member. The sandstones of the Undandita Member are generally oxidised, but contain a reduced sedimentary wedge, sandwiched between regionally extensive upper and lower redox boundaries. The reduced facies retain the primary calcite cement deposited during burial, while the oxidised sandstone has had the calcite subsequently disolved and flushed by oxidising ground water. The uranium mineralisation has been deposited at both the upper and lower margins of the gently north-dipping reduced wedge, although the upper is usually richer.
On a regional scale, the redox boundaries are planar, but at prospect scale, they step across bedding to higher or lower stratigraphic levels. This stepping is controlled by permeability contrasts between different layers in the Undandita Member sandstone.
At Angela the higher-grade mineralisation is found at a 'Z' shaped step zone in the upper redox front where the front cuts across bedding and steps up sequence by 30 to 40 m. This step zone persists over a linear interval of 5700 m, plunging west at 9° with a width varying from 70 to 250 m, and may be related to a small fault, parallel to the axial plane of the Missionary Syncline. In detail, the mineralised zone consists of a series of stacked horizons, comprising one or more small uranium roll-fronts.
The primary uranium mineralisation comprises crystalline uraninite and pitchblende with minor coffinite as grain coatings, void linings and blebs. The bulk of the mineralisation is fine-grained or amorphous. Secondary uranium minerals occur both in the zone of oxidation and at depth, probably from decomposition of primary minerals and includes carnotite, autunite, tyuyamunite and metatyuyamunite. Gangue minerals are mainly fine-grained to amorphous haematite, which occurs as grain coatings and interstitial infills in the matrix, with magnetite, calcite, minor pyrite and organic material.
Resource figures at Angela in 1987 were:
Measured, using a 0.05% U3O8 and 2 m cutoff to a depth of 650 m - 4700 tonnes of U3O8 @ 0.13% U3O8.
Indicated, using a 0.05% U3O8 cutoff - 1950 tonnes of U3O8 @ 0.10% U3O8.
Wider spaced drilling in the deeper western extension of Angela and adjacent northern satellites gave an inferred resource of 3600 to 6000 t U3O8 @ 0.1 to 0.13% U3O8.
The NT Geological Survey (2006) reports the maximum total resource within Angela I to V as: 12 650 t of U3O8, grading 0.1% U3O8.
For detail consult the reference(s) listed below.
The most recent source geological information used to prepare this decription 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.
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Selected References:
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Borshoff J, Faris I 1990 - Angela and Pamela Uranium deposits: in Hughes F E (Ed.), 1990 Geology of the Mineral Deposits of Australia & Papua New Guinea The AusIMM, Melbourne Mono 14, v2 pp 1139-1142
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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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