Evaporites and brines – geological,hydrological and chemical aspects of resource estimation

Abstract

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.