— Daniel S. Weber, P.G.
M&A routinely characterizes hydrogeologic conditions in closed hydrologic basins — known as “salars” in South America — that feature porous and/or fractured-rock aquifers. Depending on a variety of conditions, brine aquifers can develop in these salars, where groundwater is enriched in economic grades of dissolved lithium, potassium, and boron . Unlike traditional, solid, hard-rock deposits, these enriched brines are liquid and vary in density and mobility. Their fluid nature presents a variety of unique hydrogeologic challenges for exploration, characterization, and ultimately for quantifying a deposit as a Mineral Resource and Mineral Reserve (MRMR) according to industry standards .
As an example, developing an MRMR estimate for a lithium brine deposit entails several steps, which include conceptualizing the occurrence of mineralized groundwater, exploring for elevated concentrations of dissolved lithium metal, and developing the mining method for its extraction. An accurate estimate hinges on understanding the spatial variability of fluid density and the dissolved mineral concentrations; such investigations should occur before, during, and after the mining period. Equally important is determining the hydraulic parameters of the host aquifer and the potential for freshwater sources to dilute the mineral deposit. Therefore, a number of variables are used to meet the definition of reasonable prospects of economic extraction of the brine mineral deposit — among them, brine volume and grade, aquifer geometry and hydraulic conductivity, key hydrogeologic units, effective porosity, specific yield, and overall projected extraction rates .
A deposit can be upgraded from Mineral Resource to Mineral Reserve, a process that entails quantifying critical elements and assessing “modifying factors” to demonstrate the project’s economic viability . Generally, this requires professionals from a variety of disciplines. It may involve conducting engineering studies of the mining and processing methods; assessing the environmental, social, and permitting aspects of the project; understanding the project’s legal requirements; and performing an economic analysis. From a hydrogeologic perspective, the work required to upgrade may include feasibility-level field studies and analyses that focus on extraction wells (or trenches, for shallower systems), which are generally the method used to mine brines. As such, the hydrogeologic perspective is a critical element in evaluating which portion of a Mineral Resource is economically extractable and can be defined as a Mineral Reserve.
The modeling methods used to estimate Mineral Resources and Mineral Reserves differ. Generally, we use analytical modeling methods to define a “Drainable Brine Mineral Resource.” This step is used to estimate a brine Mineral Resource according to industry terms of Measured, Indicated, and Inferred quantities based on confidence criteria such as:
- The geometry of the host aquifer as defined by a three-dimensional block model
- The effective porosity of host hydrogeologic units
- The grade or concentration of dissolved minerals in the brine
After evaluating the Drainable Brine Mineral Resource, we use numerical modeling methods to define an “Extractable Brine Mineral Resource” to move the project forward for a potential upgrade to Proven or Probable Brine Mineral Reserve status. This approach uses finite-difference or finite-element codes that are capable of simulating variable-density groundwater flow and transport for the method for extraction (such as a production wellfield) during the life of the mine. As illustrated below, numerical modeling can project lithium concentrations and, using sensitivity analyses, bracket the likely ore grade over time.
 Bradley, D., Munk, L.A., Jochens, H., Hynek, S., and Labay, K., 2013, A preliminary deposit model for lithium brines, U.S. Geological Survey Open-File Report 2013–1006
 Canadian Institute of Mining, Metallurgy and Petroleum (CIM), 2010, Definition Standards on Mineral Resources and Mineral Reserves, Resources and Reserves Definitions, Canadian Institute of Mining, Metallurgy and Petroleum, 27 November 2010
 Canadian Institute of Mining, Metallurgy and Petroleum, 2012, Best Practice Guidelines for Resource and Reserve Estimation for Lithium Brines
Daniel Weber, a hydrogeologist with M&A since 1986 and our operations manager in Denver, is a Registered Member of SME. Dan and other M&A hydrogeologists — including Mike Rosko, Ed Peacock, and Jon Whittier — are working on various projects that involve characterizing mineral-enriched brines in South and North America.