Groundwater Chemistry: The Missing Link
Knowing that gold deposits await discovery in Nevada’s valleys, the exploration industry has advanced several new exploration techniques to try to see through sand and gravel cover to the bedrock beneath. These techniques are largely based on surface sampling and geophysics.
Alluvial valley fill (sand and gravel) is made up of eroded material washed down from the mountains, and is by definition, transported material. Although understanding the chemistry and nature of alluvium though sampling soil, soil-gas, and vegetation can provide useful, relative geologic information, these methods are heavily influenced by the composition of transported material that may bear little or no direct connection to the bedrock below.
Similarly, while geophysical techniques can provide valuable information about relative changes in subsurface geology, they cannot provide direct information about the prospectiveness of bedrock or its chemistry. Surface and geophysical based exploration techniques work well when applied as part of multi-stage, project-scale exploration programs, but when used independently, offer only indirect, often ambiguous, solutions to exploration in covered areas and are not appropriate for large-scale, regional exploration aimed at evaluating large areas and identifying new prospective targets.
Nevada’s valleys are not only filled with alluvium, they are also saturated with groundwater. In contrast to alluvium, groundwater does interact directly with bedrock and its chemistry does relate directly to the bedrock it has encountered. Water is known as the “Universal Solvent”. As groundwater flows and interacts with bedrock, groundwater gradually dissolves bedrock and assumes chemistry representative of it. When groundwater flows near a gold deposit, it retains a unique chemical ‘memory’ or ‘scent’ of the encounter; specifically, groundwater that has encountered gold mineralization contains elevated concentrations of gold and other associated trace-elements. Once identified, distinctive groundwater chemistry can be followed back up stream to its bedrock source through careful sampling and analysis.
Because groundwater moves, its chemistry can provide information about a much larger area than any other type of geologic sampling medium. Moreover, because of the mixing and dilution that occurs with groundwater, a distinctive groundwater chemistry plume loses its concentration away from its source. As a result, Groundwater chemistry can not only provide a larger, easier to find, target than conventional exploration techniques, but with multiple samples, groundwater chemistry can also provide a vector that points back towards its source. Whereas a drill hole only collects a narrow cylinder of rock representative of a small area, a groundwater sample provides information about the bedrock across a large area. Groundwater chemistry can evaluate large areas with few samples and is well suited to large-scale, generative exploration across Nevada’s valleys.
With recent advances in environmental technology, groundwater is now relatively easy to sample and analyze. Groundwater chemistry is a logical source of geologic information, especially in covered settings; however, up until now, Nevada’s gold exploration industry has largely ignored it.
NGE has recognized the importance and value of groundwater chemistry as a new exploration technology and has established Nevada’s first and only groundwater chemistry exploration program.