Evaluating Rare Earth Element Deposits

Abstract

There has been a significant growth in exploration activity for rare earth element (REE) deposits since the firming of prices began in 2003. Numerous deposits have been subject to detailed evaluation, though during this period only one new western-world operation at Mt Weld, Western Australia is in production. One older operation at Mountain Pass, USA, re-opened in 2012 but, due to low rare earth prices, shut down in 2015. Chinese production dominates the world rare earth industry, accounting for approximately 80 - 85% of the world’s annual production of ~125,000 tonnes, from numerous deposits in five provinces. It is reported that in excess of approximately 45,000 tonnes per year of total Chinese production is illegal. Resource estimation of REE presents no special difficulties provided care is taken to avoid over-domaining and definition of domains based on rigid grade-based criteria that are close to the lower reporting cut-off grades. These are likely to result in overstated grades and understated tonnages. Primary and supergene copper resources are natural analogies for the estimation of non-alluvial REE deposits. Rare earth deposits are typically hosted by carbonatite, alkali- intrusives and supergene-enriched mineralisation developed on REE-enhanced substrate. An important type of heavy-enriched REE mineralisation, first recognised in southern China, has been described as an Ionic Adsorption Deposit (“IAD”) and typically is hosted by secondary clays. REE in these deposits are relatively easy to recover using low cost leaching techniques based on lixiviants or eluants consisting of simple electrolytes such as ammonium sulphate and sodium chloride. Current practice in China is to use in-situ leaching (“ISL”), vat or tank leaching or heap leaching to extract the REE. The Mt Weld deposit, in Western Australia, was put into production after a 30 year exploration history and was only successfully drilled after 1991 once the regolith that hosts the mineralisation had been de-watered. This enabled the recovery of samples that had not suffered from the loss of fines. Its first reported resource estimates in 2002 achieved close reconciliations within a few percent of actual mined material. In general, cut-off grades used to report resources for many REE deposits of a similar style are unrealistically low and significantly less than those used by the only two recent Western operations. These cut-offs result from over-optimistic assumptions mainly relating to the use of:  notional values applied to all the REE  projected metal prices from periods of unstable price hikes and  unrealistic low costs associated with production and sales.