Evaluation of rare earth element deposits

摘要

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 operation at Mt Weld, Western Australia has commenced production. One older operation at Mountain Pass, USA, has re-opened. Chinese production dominates the world rare earth industry. 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. Cut-off grades used to report resources for most REE deposits are unrealistically low and significantly less than those used by the only two Western operations. These cut-offs result from attaching notional values on the basis of available metal prices and unrealistically low production and realization costs. The Mt Weld deposit was put into production after a 30 year exploration history and was only successfully drilled after 1991 once the regolith hosting the mineralisation had been de-watered. This enabled the recovery of samples that had not suffered the loss of fines. Its first reported resource estimates in 2002 subsequently achieved close reconciliations within a few percent of actually mined material. Ordinary Kriging was used with no need to resort to more complicated or advanced methods.