#1. Why hasn’t hydrogeochemistry been traditionally used for gold exploration?

Whereas many other commodities of interest are relatively more soluble, such as copper, uranium, and lithium, native gold (e.g. visible free-gold in quartz veins or gold nuggets in placer deposits) has been regarded as essentially not soluble in natural waters. In some important types of gold deposits, however, and in Carlin-type gold deposits (CTGDs) in particular, the gold and associated trace elements are intimately associated with soluble complex sulfides, such as pyrite and arsenopyrite.  When these complexes encounter groundwater, they react with the oxygen in the groundwater (oxidize) and release gold and associated trace elements into the groundwater, at which point the gold is briefly available in the groundwater to be sampled and analyzed, albeit still at ultra-trace concentrations best measured in parts per trillion (ppt) for gold.  Until recently, laboratory equipment did not exist to reliably and cost effectively analyze gold in groundwater at these levels.  With the latest analytical tools and new sample collection protocols, we can now reliably and repeatedly measure gold in groundwater down to 1 ppt, which provides the resolution to use hydrogeochemistry as an important exploration tool to discover new CTGDs.


#2. Nevada is a desert, right?  How much groundwater can there be in a desert?

Nevada receives from 4” to 14” of precipitation per year, much of it as snow in the mountains.  The Great Basin physiographic province centered over Nevada is a closed basin, with no rivers flowing back to the sea.  The precipitation that falls within the Basin runs down the exposed mountain ranges and percolates into the sand and gravel at the edges of the valley basins as groundwater, where it continues to move more slowly toward low points in the centres of the valley basins where the groundwater rises toward the surface, in some cases forming lakes, from which it evaporates back into the atmosphere.  While it may seem surprising when looking out across Nevada’s large, seemingly-dry, high desert valleys, in most of the valleys where we work, the sand and gravel covering the bedrock in the valley basins are saturated with groundwater to, on average, within 30 metres of the surface, a depth that is generally easily reachable by our groundwater sampling equipment.


#3. How do you collect samples of groundwater in a desert?

When we have an area of interest, the first samples we collect come from existing groundwater sampling locations, such as: irrigation wells, domestic wells, stock wells, windmills, and springs.  There are tens of thousands of such existing groundwater sampling opportunities across Nevada, and we have already collected thousands of groundwater samples from these sources, across many of Nevada’s most prospective valleys.  When we found areas where samples from existing locations provided indications of elevated gold and other pathfinder elements, as well as when we delineated prospective areas based on other geologic tools that may not possess a sufficient distribution of existing groundwater sampling locations, we used customized sampling equipment to complete purpose-drilled boreholes to collect groundwater samples at locations of our choosing.  This equipment allows us to collect samples from depths that often exceed 60 metres at a sample spacing suitable to respond to direct indications of gold (approximately one sample per square kilometre). For more information on our groundwater sampling program, see in Seeing Through Cover.


#4. Parts per trillion!?  Can you really obtain and believe information from such low levels, or is it just statistical noise?

Yes, we can obtain high-quality gold-in-groundwater samples and analyze them with high analytical certainty down to the part-per-trillion level; and yes, gold-in-groundwater concentrations at the part-per-trillion level are statistically significant and anomalous compared to the background levels found across Nevada.

It is important to acknowledge that when working with and making decisions based on such ultra-trace concentrations, that internal and lab quality assurance and quality control (QA/QC) processes are extremely important to ensure that we can believe analytical results before investing money to acquire ground, collect additional samples, conduct geophysical surveys, drill, etc.  Accordingly, we carefully monitor each batch of samples for inadvertent field contamination by including field blanks, as well as for lab contamination by including lab blanks.  We also include duplicates and high and low gold standards to make sure that the results we receive back from the lab are consistent both within a sample batch and over time.  In addition to our own QA/QC protocols, the independent labs we use to analyze our groundwater samples also insert their own sets of blanks, duplicates, and standards.  Having analyzed hundreds of standards, blanks, and duplicates over more than 10 years, we have strong support that part-per-trillion gold-in-groundwater analyses represent a reliable and robust geochemistry exploration tool.

We have collected and analyzed thousands of groundwater samples across Nevada, including several hundred samples from the groundwater surrounding Nevada’s large known gold deposits.  With this large dataset we can evaluate the distribution of gold in groundwater across Nevada to establish both the low-level background concentrations, as well as the high-level concentrations associated with known gold mineralization.  We use these goal posts to orient our statistical distributions of gold-in-groundwater to determine the thresholds of what we consider to be anomalous.

Generally, most samples show no to low detectable gold (at current detection limits), with about 80 percent of samples clustering within three times the median value, which is only slightly above the detection limit; and we consider gold at these levels to be background.  Once we see gold-in-groundwater concentrations one order of magnitude (10x) greater than background we consider the gold concentrations anomalous.  Generally, our projects are defined by gold-in-groundwater concentrations one to two orders of magnitude (10x-100x) greater than background.  At these levels, we are only responding to the top 5% percent of all the samples we have collected.  With multi-order-of-magnitude contrast against background, we consider our gold-in-groundwater anomalies to be high contrast, especially compared to other geochemical sampling mediums and techniques, such as soil and vegetation sampling, where anomalies are most often defined by less contrast with only single-digit multiples of background (e.g. 2 to 5 times background).

For additional context on gold-in-groundwater, including examples of deposit-scale characterization studies and other large groundwater sampling programs, see Parts per trillion gold in groundwater – can we believe it and what’s anomalous available on the Technical Resources page.


#5. Aren’t there other sources of gold that might present false readings other than a hidden, bedrock-sourced gold deposit, such as placer gold, enriched evaporative playa brines, or geothermal water coming from depth?

Yes, it is certainly possible that elevated concentrations of gold in groundwater may have been derived from other sources or processes, and it is important to recognize that not every gold-in-groundwater anomaly may be associated with a covered bedrock-sourced gold deposit.  Hydrogeochemistry is not a “silver bullet” to be blindly relied upon separate from other conventional geologic techniques.  Rather, hydrogeochemistry is an important additional tool for focusing good conventional geology on targets where we can demonstrate the presence of measurable indications of gold.  This tool provides an up-until-now missing scale of information to reduce Nevada’s vast covered valleys down to smaller discrete targets where we can justify spending the money to apply conventional geologic tools.  Exploration is about reducing risk and improving the odds.  With the addition of the gold-in-groundwater information, we systematically reduce the size of the areas we are exploring and increase the odds that there is a nearby gold deposit, thus accomplishing both objectives.


#6. Doesn’t groundwater move?  When you see gold in groundwater, how do you know it hasn’t traveled very far?

Yes, groundwater moves.  In basic terms, groundwater flows from areas of higher pressure (such as higher elevation) to areas of lower pressure (such as lower elevations) at different speeds depending on this pressure differential and the characteristics of the material it is flowing through (hydraulic conductivity).  In Nevada, groundwater predominantly starts as relatively fast flowing streams moving down from the bedrock exposed in the mountain ranges to the valley basins where it begins to move more gradually through the gravel-cover toward the central low spots in the basins.

While understanding the exact flow paths of Nevada’s groundwater can be challenging, gold is not easily held in solution and once gold has been introduced into groundwater, the gold is removed over relatively short distances by various organic and inorganic reactions with clay and other materials.  In other words, gold doesn’t travel very far in groundwater – once gold is introduced into groundwater it immediately begins to look for opportunities to leave the groundwater.  As a result, gold-in-groundwater concentrations associated with gold deposits rapidly decrease with distance, such that by the time we collect samples more than about 3-5 kilometres away from a gold deposit, we find that the gold-in-groundwater concentrations have generally fallen back down to low, background levels.  The important implication here for gold exploration is that the use of groundwater sampling has the potential to increase the footprint of a covered gold deposit to the size of a few square kilometres, giving us the scale of information we need to cost effectively reduce otherwise vast covered areas down to smaller targets conducive to the application of conventional exploration methods.  Additionally, by mapping the decrease in gold-in-groundwater concentration over distance at a target, an explorer can establish a vector, which can be used to point back to a bedrock source.


#7. Can gold-in-groundwater analyses provide information about the grade or size of a potential covered gold deposit?

Not currently, but perhaps in the future.  There are many variables that influence how groundwater interacts with a covered gold deposit and what sort of gold-in-groundwater concentration will result from such an interaction, including: the background hydrogeochemistry, the mineralized surface area available to interact with the groundwater, and the amount of residence time the groundwater has to interact with the mineralized material.  Additionally, there other variables that influence the gold-in-groundwater concentrations we see in our sampling, most importantly of which is how close to a deposit we are able to collect a sample.  Recognizing these many interrelated variables, we do not attempt to ascribe grade or size estimates to our gold-in-groundwater-defined targets.  Instead, we use this information to limit our exploration focus in Nevada to target areas where we can demonstrate the presence of gold in groundwater at concentrations at least one to two orders of magnitude (10x-100x) greater than background, which reduces our search space to the areas surrounding the top 5% of all samples.