Applied Earth Science: Transactions of the Institutions of Mining and Metallurgy

Sandstone-type uranium deposits contain approximately 28% of the world uranium resources. Many of these deposits are located below the water table in weakly lithified or non-consolidated sands, and therefore they can be exploited using in situ leach (ISL) technology. Such technology is based on dissolving uranium minerals directly in their host rocks (in situ) by reactive solutions that are injected through drill holes and then pumping the dissolved solution to the surface through some discharge drill holes. Uranium grade is determined by down-hole geophysics, in particular the prompt fission neutrons, or PFN, technique, coupled with sampling and assaying of the drill core. The drill grid which is used in Kazakhstan for definition of ISL uranium resources are as follows: (i) Measured: average 50×100 m (range from 25×50 to 50×100 m); (ii) Indicated: average 50×200 m (range from 50×100 to 50×200 m); and (iii) Inferred: average 50×400 m (range from 50×400 to 100×800 m). Estimation and reporting of uranium resources for ISL projects differ from hard rock mining projects in the need for quantitative estimation of the geotechnical and hydrogeological parameters which are specific for ISL technologies. The main parameters which need to be considered are as follows: (i) grade and geometry of mineralisation are estimated with accuracy sufficient for supporting the remote mining; (ii) if grade is estimated using the gamma logging technique secular disequilibrium should be studied and reported; (iii) hydrogeological confinement of the mineralised horizon; (iv) permeability of the mineralised horizon; (v) composition of the host rocks, in particular the carbonate content, in order to estimate if uranium mineralisation is amenable to dissolution by acid or alkaline solutions; (vi) groundwater flow; (vii) aquifer salinity; and (viii) rate of the in-situ dissolution of the uranium minerals. Hydrogeological and geotechnical information is obtained by testing the drill core samples and in the field, using the pump tests and the down-hole piezometers. Modifying factors for conversion resources to reserves are verified and corrected using field leach tests of uranium. This test is a strict requirement for feasibility studies of the ISL uranium projects in Kazakhstan.

Mineral sand deposits being economically exploited around the world fall into three main deposit styles; palaeo shorelines and marine placers, dunal and aeolian deposits, and alluvial deposits. Resource estimation for mineral sands follows standard estimation principles as applied to commodities throughout the Mineral Resource sector. There are, however, characteristics of mineral sands that require a slightly different approach and more emphasis on the geometallurgical characterisation to support the evaluation, Resource estimation and eventual reporting of the Mineral Resource estimate. Drilling is the main exploration and Resource definition tool used in the mineral sands industry and aside from logging and sampling, observation of hardness and induration attributes are critical aspects of drilling mineral sands deposits. Assaying is conducted on drill samples for heavy mineral (HM) content and for various particle size ranges and then further assaying or mineralogical assessment is undertaken on the HM to determine valuable heavy mineral components (typically zircon, ilmenite, rutile). The estimation of mineral sands resources requires a sound geological model before the determination of any geometallurgy based sub-domains within the mineralisation. Understanding the mineralogy and quality characteristics of deposits influences the confidence in subsequent Mineral Resource and Ore Reserve estimation and will in turn assist the Competent Person to select the appropriate cut-off-grade for Mineral Resource reporting.

Modern salt lakes are complex systems which are exploited for a variety of minerals, including solid and dissolved phases (brines). Phases commonly exploited include lithium, potassium and boron minerals, mainly from brines, and solid minerals including halite and gypsum. Resource estimation and successful exploitation of either solid minerals or brines require good understanding of the complex interactions within these systems. This includes careful definition of climate, geology, hydrogeology (hydrology), fluid flow dynamics and lastly chemical ion concentrations. Therefore estimation involves multidisciplinary team work. Brines are mobile resources which change through time, sometimes seasonally, so estimators must understand the climatic, hydrological and geological factors governing flows of both water and salts into and out of the system. Commercial extraction of brines involves brine collection (usually by pumping from wells) at rates that make the extraction economic, so brine resources should include only extractable brines, which are those in accessible aquifers available for pumping. Estimators must take into account potential limits on sustainable extraction rates when assessing the prospects for eventual economic extraction to determine whether a resource is present. Drilling on salt lakes can present significant challenges in safe access, and in recovery of uncontaminated samples of host rocks, which are often unconsolidated, and of brines. The merits of various methods of drilling and sampling are discussed. We recommend a combination of different sampling methods and the use of downhole geophysics to assist in detailed geological interpretation. Well-designed pump testing will provide hydrogeological parameters and enable derivation of the specific yields of all targeted aquifers, which are required to estimate total extractable brine resources. Reliable repeatable chemical analyses of brine samples are hard to achieve, and filtering, minimising storage times, and the choice of a reliable laboratory experienced in brine analyses will all assist in this objective. Stringent quality control is required, and this should include check analyses at one or more independent laboratories. Resampling of the same sample sites will measure seasonal and other fluctuations. Resource reporting must include discussion of all material factors and assumptions, and include discussion of limiting extraction rates, and risks to the sustainable extraction of the resource, which may include climatic risks and risk from neighbouring production. All resources need to have reasonable prospects for economic extraction, so the estimator must be familiar with potential exploitation methods (which include dredging or conventional mining for solid minerals, and for brines pumping from wells, brine recovery via ditches, evaporation and chemical separation) and the factors that affect the cost or feasibility of these methods. The potential for extraction to affect the remaining resource (e.g. by brine pressure reduction, increasingly dilute inflows into a brine resource, dissolution or enhanced crystallisation of solid minerals) must be considered. Resource classification must take into account the definitions of the higher confidence (Indicated and Measured) resources to be able to support mine planning, and we consider these classifications require a hydrological model, based on at least preliminary field testing for indicated, and a geological model and knowledge of chemistry across the resource volume.

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.

This paper presents procedures and technical parameters used for resource estimation of bauxite deposits including the non-consolidated pisolitic bauxite (Weipa) and the intensely lithified types (Gove, Sangaredi, Az Zabira). Bauxite resources are usually estimated by drilling and sampling drill holes at 0·25 to 0·5 m intervals. Short sampling intervals are necessary for accurate estimation of the mineralisation contacts; this is despite the broad drilling grid which varies from 125 m centres for definition of the measured resources to 500 m centres used for inferred resources. Samples of non-consolidated bauxite are often beneficiated by sieving and removing the barren fine grained material before chemical assays. Consolidated bauxite ores are not beneficiated and processed in a conventional manner. However, in both cases, the overall precision error (CV%) of the samples, which is measured and monitored using field duplicates, does not exceed 10% (Al₂O₃ – 5%; SiO₂ – 10%, Fe₂O₃ – 7%, LOI – 10%). Bauxite density is preferably measured using the sand replacement method which is a formally certified technique for measuring bauxite density at the Australian deposits. However, a more recent approach is to use Sonic drilling to collect intact samples of the bauxite ore. Bauxite grade is estimated using conventional geostatistical techniques, most commonly by Ordinary Kriging. However, the method is applied after geometry of the bauxite seam is flattened using an equal thickness unfolding method, or, in some cases, using top flattening approach. Direct estimation of the bauxite grades without flattening their bodies produces incorrect estimates due to excessive smoothing of the estimated grades.

Brockman 4 is a large iron ore deposit hosted by the Brockman Iron Formation in the Pilbara region and is mined by Rio Tinto Iron Ore. Drilling is mainly by the reverse circulation method and samples are taken at 2 m intervals using a rotary cone splitter or historically by riffle splitter. Samples are geologically logged, assayed by X-ray fluorescence for iron, silica, alumina, phosphorus, manganese, sulphur, titanium oxide, calcium oxide, magnesium oxide, and 19 other trace elements; and measured for loss on ignition by thermogravimetric analysis. Mineralisation is estimated by ordinary kriging with unfolding and waste by inverse distance interpolation. The model is validated, reconciled with historic production, classified according to the 2012 Edition of the JORC code and stored in a SQL Server database.