Fathom Geophysics Newsletter 27

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Ore processing news: Researchers ID non-obvious hosts of Olympic Dam radionuclides

MINOR hosts of radioactive decay-related daughter elements collectively contribute significantly to the overall radium-226, lead-210, and polonium-210 budget at the Olympic Dam iron oxide-copper-gold (IOCG) deposit in South Australia's Gawler Craton.

A group of Adelaide-based researchers determined this by looking at nanoscale features within and around accessory mineral grains from intermediate metallurgical products of ore processing at Olympic Dam. [1]

Major host minerals of Olympic Dam radionuclides are the uranium-bearing minerals uraninite, coffinite and brannerite, and these are dealt with during leaching of concentrate and flotation tailings, the researchers said in their paper.

However, Olympic Dam mineralization also contains daughter products derived from uranium-235, uranium-238 and thorium-232 decay.

The distribution of the daughter radionuclides radium-226, lead-210 and polonium-210 throughout the deposit was poorly understood at the mineral-grain scale, as was the nature of their carriage during different stages of the ore processing circuit, they said.

They defined minor hosts of these daughter radionuclides as frequently occurring minerals possessing a low radionuclide content within and around their grains, together with rarely occurring minerals possessing a high radionuclide content within and around their grains.

The researchers focused on minor hosts with a view to helping improve the removal of radionuclides from Olympic Dam concentrates without sacrificing copper content.

"[E]fforts to further reduce radionuclide concentrations prior to smelting and refining are limited without a detailed understanding of specific mineral and non-mineral hosts," they said.

Nanoscale mineral grain mapping

Despite Olympic Dam's supergiant deposit size category, nanoscale phenomena have proven important there.

For instance, nanoscale inclusions of uranium-bearing minerals are almost ubiquitous throughout the deposit, the researchers noted in their paper.**

And because radionuclide distribution patterns in minor host minerals were also evident at the nanoscale, the researchers turned to semi-quantitative mineral grain mapping using the technique called nanoscale secondary ion mass spectrometry (nanoSIMS), which has low detection limits and sub-micron rastering resolution.

They also employed backscattered electron imaging and electron backscatter diffraction to help make sense of nanoSIMs mineral-grain mapping results.

They looked at individual grains and multi-grain agglomerates. They took samples from flotation concentrate, which is unleached material, as well as from concentrate leach discharge. They also looked at samples of solidified electric-furnace slag from a copper-recovery smelting step.

As part of the study's results, the researchers found relatively significant amounts of daughter radionuclides hosted internally within accessory minerals from the Olympic Dam deposit, either as part of the mineral's crystalline structure or as nanoscale inclusions.

Accessory minerals found playing host in this way included apatite, fluorite, hematite, rutile, zircon, rare earth element-bearing fluorocarbonates and phosphates.

Surface-hosting of radionuclides

But the researchers found an additional reservoir of radionuclides.

"One surprising fact resulting from nanoSIMS mapping — with major significance for radionuclide removal from final concentrates — is the nearly ubiquitous presence of radionuclides on grain surfaces, along grain boundaries, and within fractures and cleavage planes," they said.

The radionuclides were bound to the exterior of acid-leached samples taken from the concentrate leach discharge. In contrast, unleached samples from flotation concentrate showed little to no coating of mineral grains by surficial radionuclides.

This surface-hosting phenomenon may have been set in motion by the creation of insoluble radium-, lead-, and polonium-bearing sulfates during the acid leaching step, they said.

While surface-hosting could take place on any type of mineral, radionuclides tended to cluster at locations with high surface area, such as sites of topographic irregularity.

Electrostatic cling effects may also have influenced where surficial radionuclides ended up. Surfaces of molybdenite and covellite grains were found to be particularly favorable sites for deposition of secondary radionuclide-bearing materials. The researchers surmised that this was due to the pronounced micaceous cleavage of molybdenite and covellite creating large areas (at the microscopic scale) that were no longer isoelectric and could therefore draw in and anchor relatively large numbers of charged radionuclide-bearing nanoparticles.

If the theorized formation and behavior of these surface-hosted materials were validated, say by directly visualizing them, then this information might be useful in devising alternative radionuclide-removal strategies at Olympic Dam, the researchers said.

Alpha decay recoil-related redistribution

One process the researchers discussed as a potentially important phenomenon at the scale of the smallest radionuclide-bearing particles was recoil-related translation of daughter atoms.

The recoil taking place for an atom upon alpha-particle decay may, under the right conditions, be directed in such a way that the atom is expelled from its host mineral nanoparticle altogether.

Recoil wouldn't be a significant phenomenon for mineral grains of micron- or millimeter-size.

But for sub-micron particles, it's probable that nanoscale mechanisms such as decay recoil play a primary role in the mobility and distribution of radionuclides at Olympic Dam, they said.

References

[1] M. Rollog, N.J. Cook, P. Guagliardo, K. Ehrig and M. Kilburn (March 2020) "Radionuclide distributions in Olympic Dam copper concentrates: The significance of minor hosts, incorporation mechanisms, and the role of mineral surfaces", Minerals Engineering, 148, 106176.

** As well, previous research published a few years ago laid out how nanoscale features in Olympic Dam ore — bornite-chalcocite symplectites and lattice superstructuring — contributed to the conclusion that the deposit fell into the primary hypogene category. See: C.L. Ciobanu, N.J. Cook and K. Ehrig (2017) "Ore minerals down to the nanoscale: Cu-(Fe)-sulfides from the iron oxide copper gold deposit at Olympic Dam, South Australia", Ore Geology Reviews, 81, 4, 1218-1235.

About Fathom Geophysics

In early 2008, Amanda Buckingham and Daniel Core teamed up to start Fathom Geophysics. With their complementary skills and experience, Buckingham and Core bring with them fresh ideas, a solid background in geophysics theory and programming, and a thorough understanding of the limitations of data and the practicalities of mineral exploration.

Fathom Geophysics provides geophysical and geoscience data processing and targeting services to the minerals and petroleum exploration industries, from the regional scale through to the near-mine deposit scale. Among the data types we work on are: potential field data (gravity and magnetics), electrical data (induced polarization and electromagnetics), topographic data, seismic data, geochemical data, precipitation and lake-level time-lapse environmental data, and remotely-sensed (satellite) data such as Landsat and ASTER.

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