Uranium and Thorium Geology

Basin- and surface-related uranium mineral systems

Australia is host to the world’s largest resource of easily recoverable uranium, and is a leading exporter of uranium worldwide. Australia’s uranium deposits are hosted in a variety of deposit styles, most importantly the giant Olympic Dam uranium-bearing iron oxide Cu-Au deposit in South Australia, the Ranger unconformity-style deposits in the Northern Territory, the Yeelirrie calcrete-hosted deposit in Western Australia, and the sandstone-hosted deposits of the Frome Embayment, South Australia. Although the geological setting and precise uranium mineralisation process is highly variable, there are sufficient commonalities between deposit styles that they may be conveniently categorised into three families of uranium mineral systems (Skirrow et al., 2009), with some deposits containing characteristics of multiple families of uranium systems, and thus termed hybrid-type deposits. The Olympic Dam deposit is an example of a hybrid-type system.

The basin- and surface-related family of deposits is related to processes occurring at the earth’s surface and in sedimentary basins, and encompass calcrete-, sandstone- and unconformity-style deposits. The behaviour of uranium in these systems is strongly controlled by variations in oxidation state (redox). Uranium has two dominant ions, U4+ which is reduced, and U6+ which is oxidised.

Uranium ions carrying a +4 charge are highly immobile, and therefore the uranium will not move from where it is located under typical conditions. On the other hand, uranium with a charge of +6 (oxidised uranium) is highly mobile, and will dissolve readily in fluids which are themselves oxidised, such as meteoric water.

In sandstone-hosted systems, such as those in the Frome Embayment, uranium is leached from uranium-rich source rocks (eg. granites) by oxidised fluids. The uranium is then transported within the oxidised fluid into a permeable aquifer (eg. a sandstone unit) within a basin. Uranium is deposited when the uranium-bearing oxidised groundwaters encounter reduced material, which may be in-situ, such as organic matter within the aquifer, or may be mobile, such as oil, gas or H₂S seeps (Jaireth et al., 2008). Bacteria may play an important role in uranium reduction in some cases.

Unconformity uranium deposits, such as Ranger, also occur in sedimentary basin settings. These deposits are formed via similar processes to the sandstone-hosted systems described above, but the mineralising fluid is circulated much deeper and therefore is heated to higher temperatures. Uranium mineralisation occurs at the interface between contrasting oxidised and reduced geological units on either side of an unconformity.

Magmatic and magmatic-hydrothermal uranium mineral systems

The magmatic-hydrothermal deposit style is fundamentally related to igneous rocks and processes. Prominent examples of this deposit style include the Rössing deposit in Namibia. Australia contains some deposits corresponding to the magmatic-hydrothermal family, such as the Crocker Well deposit in South Australia, but is presently under-represented in this deposit style. This is despite the widespread distribution of uranium-rich igneous rocks throughout Australia (Lambert et al., 2005; Schofield, 2009).

Uranium mineralisation in magmatic-hydrothermal systems may be related to either volcanic or intrusive igneous rocks. Igneous chemistry plays a major role in these systems. In systems involving only the igneous rock itself ('orthomagmatic' systems), uranium is concentrated in the magma as it evolves via fractional crystallisation (the removal of crystals from the magma as they form). Eventually, uranium-rich minerals (such as uraninite) may form when uranium saturation occurs. High concentrations of elements such as fluorine and peralkalinity (high sodium and potassium) prevent uranium-bearing minerals from crystallising early at low concentrations. Magmatic-hydrothermal deposits behave in a similar way, although in these examples the action of a magmatic fluid involved, which can increase the grade of uranium mineralisation.

Metamorphic-related uranium mineral systems

As with magmatic-related systems, deposits formed via metamorphic processes are rare in Australia. The fluids involved in these deposits may be derived from metamorphic processes, such as dehydration reactions, or may include fluids which have been equilibrated with metamorphic rocks. Uranium deposits which are associated with metasomatism are included within the metamorphic-related family of deposits. In Australia, the most prominent example of this type of deposit is the Valhalla deposit in Queensland.

Thorium Geology

Thorium is a naturally occurring radioactive element which is found in the Earth mainly in oxides, silicates, carbonates and phosphates. Thorium exists almost entirely as ²³²Th which has a half-life of 14 050 million years. From its natural state, ²³²Th decays through a number of stages to eventually form ²⁰⁸Pb, which is stable.The main difference is that thorium is far less mobile than uranium in oxidising surface conditions (Mernagh and Miezitis, 2008). Most thorium resources are found in heavy mineral sands such as those in the Murray Basin, which are derived from the erosion of older rocks, where thorium is contained primarily in monazite grains.

Thorium is also concentrated in igneous processes. Carbonatites, such as the Mount Weld carbonatite in Western Australia are known to have anomalously high thorium contents. Alkaline igneous complexes also contain enrichments in thorium relative to average igneous rocks. Other granitic bodies and pegmatites may also have high thorium.

Another important category is thorium-bearing veins and lodes, the most prominent example of which in Australia is the Nolans Bore rare earth element-P-U(-Th-F) deposit in the Northern Territory. Mineralisation is associated with multiple generations of fluorapatite formation. Analyses of fluorapatite grains reveal that they contain an average of 0.233 per cent thorium (Hussey, 2003), making the Nolans Bore deposit a world-class thorium resource. Concentrations of thorium may also be found in iron oxide Cu-Au, skarn and phosphate deposits, and in coal and peat accumulations.

References

Hussey, KJ 2003, ‘Rare earth element mineralisation in the eastern Arunta Region’, Northern Territory Geological Survey Report, 2003-004. 20p.

Jaireth, S, McKay, A and Lambert, I 2008, ‘Association of large sandstone uranium deposits with hydrocarbons’, AusGeo News, 89:8-12.

Lambert, I, Jaireth, S, McKay, A and Miezitis, Y 2005, ‘Why Australia has so much uranium, AusGeo News, 80:7-10.

Mernagh, TP and Miezitis, Y 2008, ‘A review of the geochemical processes controlling the distribution of thorium in the earth’s crust and Australia’s thorium resources’, Geoscience Australia Record 2008/05, 48p.

Schofield, A 2010, ‘Uranium content of igneous rocks of Australia 1:5 000 000 maps – Explanatory notes and discussion’, Geoscience Australia Record 2009/17, 20p.

Skirrow, RG, Jaireth, S, Huston, DL, Bastrakov, EN, Schofield, A, van der Wielen, SE and Barnicoat, AC 2009, ‘Uranium mineral systems: Processes, exploration criteria and a new deposit framework’, Geoscience Australia Record 2009/20, 44p.