Academics bemoan lack of clarity on China

By James Sean Dickson
Published: Monday, 30 November 2015

Universities lack data on Chinese deposits; Geological origin of Bayan Obo disputed while classifications in southern China uncertain.

The British Columbia Geological Survey (BCGS) Symposium on Strategic and Critical Minerals in Victoria, Canada, in November was told how, in Western circles of geological academia, understanding of China’s rare earths industry deposits and industry remains as elusive as ever.

Anthony Williams-Jones, a hydrothermal mineral deposit expert at McGill University, Quebec, told IM that the science and geology of China’s rare earths deposits is limited, even in the case of the largest rare earths mine in the world, Bayan Obo, located in Inner Mongolia.

"The origin of the Bayan Obo deposit remains controversial, almost certainly because the deposit is closed to foreign researchers and Chinese researchers have very limited access to it," Williams-Jones said.

The academic knowledge and data covering ion adsorption-type rare earth deposits in the southern provinces of China is yet more incomplete.

"Understanding of the origin of the so-called ion adsorption clay deposits is even worse, partly because of the lack of access and also because there are few reliable data on these deposits," Williams-Jones told IM.

"It is not even clear that they are correctly named, and there is at least the possibility that some of all of the southern China rare earth ores were concentrated as secondary minerals, rather than being adsorbed onto the surfaces of clay minerals," he added.

Southern China’s rare earths deposits remain the largest source for heavy rare earth elements, which are typically scarcer in rare earths mines for geological reasons.

Williams-Jones said that the uncertainty over their depositional origin needs to be resolved "if they are ever to be mined responsibly".

Explorers: Look to limestones

While the exploration fervour for rare earths has cooled off substantially in the last two years owing to low prices and weak investor interest, publicly funded academia is still on the lookout for new mineral deposits.

Williams-Jones believes that exploration companies would be best placed to base their searches for new rare earths deposits in limestone and marble provinces, which also have strong expressions of alkaline and carbonatite-type magmatism.

"Hydrothermal processing is a natural geological process which follows the rules of separation chemistry," he said.

Williams-Jones explained that scientific studies indicate that rare earths are typically transported as chloride complexes in high-temperature, metal-pregnant fluids which comprise the source material for rare earth mineralisation, correcting earlier assumptions that transport occurred with rare earths mobile as fluorides.

High temperatures of between 200-400°C are required, alongside low fluid pH values, to maintain the solubility of rare earths within geological fluids.

As limestone and marble can act as a natural buffer, both by lowering the temperature of source material via heat conduction and as a strong pH neutraliser, deposits formed when soluble rare earths meet with the two rock types are likely to result in a high grade concentration of the metal group.

The high grade would be a product of the metals being "instantly dumped" at the contact point, McGill’s Williams-Jones said.

He told IM that prospectors looking for additional geological resources of rare earths should identify limestone and marble provinces with nearby carbonatite and alkaline igneous intrusion material.

Carbonatites and alkaline igneous rocks are indicative of highly mineral-enriched fluids, owing to their probable deep mantle genesis.

Williams-Jones explained that the higher solubility of cerium and lanthanum at lower temperatures and higher pH values also facilitated the presence of heavy rare earth-biased mineralisation at sites like Northern Minerals Ltd’s Browns Range project in Australia’s Northern Territory. In this instance, less soluble heavy rare earth elements were deposited at the project site, whereas cerium and lanthanum, among other light rare earths, remained mobile.

"The cerium and lanthanum ions simply pass onwards further away from the primary deposit," Williams-Jones told IM. "In this regard, geochemical studies can identify the location of potential deposits, whereby lanthanum and cerium concentrations form a distal indicator, which will give way to heavier rare earths concentrations closer to the deposits."