Camborne homing in on heavy rare earths
Examinations of an unusual carbonatite deposit in Malawi could help mining companies track down heavy rare earth elements. Simon Frost reports.
Rare earth elements (REEs) may not be as scarce as the name suggests, but China has the monopoly on their supply. Now pivotal research undertaken in Malawi and led by the Camborne School of Mines, UK, could help to make REEs a viable prospect worldwide, as their demand in applications as diverse as consumer electronics, power generation and anti-fraud technologies continues to grow.
While relatively abundant, REEs are typically distributed in low concentrations, making them difficult to extract cost-effectively. In recent years, China has become the dominant force in the supply of REEs, with typical estimates ranging between 90–95% of the global market share (see Materials World, February 2015, page 50). The USA, once the global leader in REE supply and capable of fulfilling all its domestic needs, now imports 87% of its REEs from China, owing to unmatchable low costs.
As part of global efforts to resolve this supply problem, a new study led by Dr Sam Broom-Fendley at Camborne examines the conditions under which REEs – in particular, the scarcer heavy rare earths (HREEs) – are concentrated in carbonatite rock formations at the Songwe Hill Rare Earth Project in Malawi. HREEs such as dysprosium, europium and terbium are increasingly being used in lighting, anti-fraud and safety technologies.
‘Our focus is on developing a new sustainable source of light and heavy rare earths outside China,’ said William Dawes, CEO of Mkango Resources Ltd, Canada, which owns the project. Canada has lofty ambitions in the rare earth market, its Government announcing in 2014 a target of securing 20% of global supply by 2018. Songwe Hill, which hosts an unusually high concentration of HREEs, is one of several projects in development worldwide that could begin to address the geopolitical imbalance of their supply.
‘The work involved two field seasons in Malawi, where we investigated core drilled by our industrial collaborator, Mkango Resources Ltd, and looked at the relations of different rock units in the field,’ Broom-Fendley told Materials World. ‘We took representative samples of the different rock units and looked at thin sections of these samples using a petrographic microscope, as well as electron beam techniques such as cathodoluminescence and scanning electron microscopy.’
The mineral apatite, they found, is key to the high HREE content of the Songwe Hill deposit compared to similar carbonatite host rock elsewhere. Broom-Fendley explained that the luminescence of apatite is highly susceptible to change when there are small variations in its structure, making cathodoluminescence a particularly useful tool for identifying apatite and recognising variations in mineral composition that could indicate richness in HREEs.
In the study REE Minerals at the Songwe Hill Carbonatite, Malawi: HREE-Enrichment in Late-Stage Apatite, published in Ore Geology Reviews, Broom-Fendley and his co-authors explain that few examples of HREE-enriched carbonatite are known, and even these are ‘typically minor occurrences forming in late-stage fluid-rich environments’.
The Songwe deposit, however, has the highest HREE concentration of any known carbonatite apatite, and the researchers believe they have determined a likely cause – the presence of hot fluids. ‘At Songwe, there is distinct evidence for hydrothermal activity in the shape and chemistry of apatite and clear textural evidence for the later formation of the light rare earth element mineral synchysite-(cerium),’ Broom-Fendley explained. This rare combination of both light and heavy REEs creates a well-balanced deposit that could be particularly useful as a supply for the growing magnetics industry.
‘Combining the “paragenetic history” [the order of crystallisation] with our observations at other intrusions, and through comparison with experimental work in other institutes, we concluded that a hydrothermal model best fits the evidence available,’ he said.
HREE-enriched apatite was also found to be fine-grained and elongated in a “stringer-like” shape. Identifying these characteristics would mean that mining companies could quickly and inexpensively evaluate the HREE potential of a carbonatite REE deposit using microscopy. If found to meet these criteria, the company could then move on to cathodoluminescence and, if HREE-enriched apatite is still evidenced, the more expensive laser ablation to assess the chemistry of the apatite. This would prevent time and money being wasted in pursuit of HREEs, as the most expensive methods would only be employed once a high likelihood of their presence has been established.
An unknown fluid
While the study points towards a hydrothermal model for HREE enrichment, the composition of the fluid responsible remains unknown. ‘Based on the mineralogy, fluid-inclusion composition and recent work, we suggest a chloride complex is responsible for transporting REE,’ said Broom-Fendley. ‘However, there is still a great deal of scope for other complexing agents, including fluoride. Both chloride and fluoride are capable of fractionating the REE, owing to the different stabilities of
light and heavy rare earth element complexes. We also suggest carbonate as a potential transporting agent, although experimental data for carbonate complexation are still limited.’
Broom-Fendley is now looking into both the composition of the fluid and temperature of the process, and building a more general model for carbonatite REE enrichment, while colleagues are working on the processing of carbonatite deposits for efficient extraction of both light and heavy REEs. Work continues, but recognising the common accessory mineral apatite as a likely indicator of HREE enrichment may be a significant milestone in the pursuit of technologically important materials.