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
"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
"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
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
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
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."