Assay Smarts: Keeping the Chemistry Connected for Better Exploration

Late fall 2010, while working in a small Yukon office cleaning data and writing assessment reports, was when I first encountered 4-acid digestion for early-stage mineral exploration. My client, fresh from a conversation with a PGeo advocating the method, asked me about its merits. Having transitioned from working on diamond projects to gold projects over the past few years, I was more familiar with traditional methods like aqua regia and fire assays.

The immediate question that struck me was: Why use a method that doesn’t analyze for gold on a gold project? The answer, as I later realized, lies in the marketing strategy behind geochemical analysis and highlights a fundamental incongruity in current exploration practices.

I was recently reminded of the 4-acid conundrum after reading a news release from a junior explorer who re-assayed core samples using 4-acid digestion for multi-elements and fire assay for gold, then tried to compare these results with historical data that used aqua regia and fire assay. Their goal was to “verify” the historical data for use in a future Mineral Resource Estimate.

Comparing different assay methods is not an uncommon scenario in the industry, but it immediately raises a red flag: why is an assay method that doesn’t include gold so popular on gold projects and can assay results from fundamentally different digestion methods be directly compared for verification purposes?

A quartz sulfide vein from Northwestern British Columbia - photo credit Diana Benz

What is 4-Acid Digestion?

4-acid digestion is considered a comprehensive technique in geochemical analysis, providing “near-total” digestion by dissolving nearly all minerals except for some highly resistant phases such as barite, chromite, and zircon. It employs a combination of hydrochloric, nitric, perchloric, and hydrofluoric acids to break down most minerals, including silicates, which are often resistant to weaker methods like aqua regia.

Despite its power, even this method does not fully dissolve certain minerals, and some elements - including gold - may be volatilized or only partially recovered. As a result, 4-acid digestion is not used to obtain accurate determinations of gold.

It does provide valuable data for many other elements, but it also cannot be used to obtain accurate values for the rare earth elements, tantalum, niobium, arsenic, antimony, tin, mercury, chromium, and uranium – where arsenic, antimony, mercury, and sometimes tin are important pathfinder or associated elements for certain types of gold deposits.

The Incongruity: Gold Projects without Gold Assays

Here’s the paradox: 4-acid digestion does not directly analyze for gold. While it excels at providing a broad suite of major and trace elements, gold is not in the list of available analytes. Often, explorers will pair this method with a separate fire assay technique for gold, and it’s not uncommon to see early-stage gold projects relying on multi-element data from 4-acid digestion with gold analyses from the fire assay method.

As a prospector and data scientist, this raises a critical concern. Ideally, gold is evaluated in context with other elements - particularly in early mineral exploration where understanding the provenance of gold in soils and rocks, and the geochemical halos of associated elements, is essential for vectoring toward economic mineralization.

Geochemical mineral exploration is not just about the absolute value of a commodity element; it’s about contrast (background versus anomaly), element associations, and the subtle patterns that emerge from multi-element relationships.

Analytical Nuances: Volatility, Matrix Effects, and Data Integrity

When selecting an assay method, it’s important to recognize that some elements - especially those present in trace amounts or are easily volatilized, like arsenic, antimony, and caesium - can be lost during sample digestion. Both aqua regia and 4-acid digestion are subject to these effects, though the extent depends on the specific element and the sample composition. Additionally, the chemical environment created during digestion can introduce matrix effects, sometimes interfering with the accurate measurement of certain trace elements, particularly in complex multi-element systems like those found in copper-gold or silver-gold deposits.

These analytical nuances are well understood in geochemical labs and are typically managed through careful method validation and the use of internal certified reference materials. For elements prone to volatility or matrix interference, complementary analytical techniques - such as Instrumental Neutron Activation Analysis (INAA), fusion methods like fire assay (for gold and Platinum Group Elements), or X-ray Fluorescence (XRF) for major elements - may be considered to ensure robust and meaningful data. For mineralogical or micro-scale geochemical studies, Scanning Electron Microscopy (SEM) or Laser Ablation ICP-MS (LA-ICP-MS) can provide valuable additional context. Where appropriate, these complementary techniques can improve the quality and reliability of the data, but some are often more expensive.

How Gold Gets Disconnected from Its Geochemical Context

A critical but often overlooked issue is the disconnection of gold from its associated elements when different assay methods are used. Fire assay, aqua regia, and INAA are all valid techniques for gold determination, but if multi-element data are obtained from 4-acid digestion while gold comes from a different assay method, the resulting datasets are incompatible and not suitable for direct comparisons.

This incompatibility arises because each assay method dissolves or analyses (e.g., INAA) a different portion of the sample and can target different minerals, fundamentally altering the ratios between elements. This is important because assay data are typically reported as ratios—parts per million, parts per billion, or percentages — rather than absolute (raw) quantities.

In ratio form, assay results are expressed as a proportion of an element relative to the whole sample, while absolute quantities reflect the actual amount present, independent of the sample’s total size. This further complicates direct comparisons when different methods are used and, as a result, gold values may not be directly comparable to other elements measured using a different technique, and the geochemical associations can be distorted.

For example, 4-acid digestion may liberate copper effectively, but does not determine gold, while fire assay or INAA targets gold more effectively. Directly associating gold values from one assay method with multi-element data from another is scientifically unsound, as the ratios and associations may no longer reflect natural geochemical relationships, but rather the artifacts of different extraction chemistries. This becomes especially problematic when the multi-element data from the 4-acid digestion are used as pathfinders for gold, defeating the purpose of trying to find and use associated elements for mineral exploration.

This disconnect between different assay methods undermines:

• Geochemical Vectoring: The ability to use pathfinder elements is compromised.

• Anomaly Detection: Background and anomaly contrasts become less reliable, as the baseline for each element is set by a different assay method.

• Artificial Intelligence and Machine Learning: Multivariate analyses, including clustering and predictive modelling, are weakened because the input data do not reflect consistent or comparable assay conditions.

The Impact on Data Science and Historical Records

The practice of using 4-acid digestion with or without fire assay, and often reporting gold as a stand-alone value, has deeper implications than many realize. We are creating a gap in the historical geochemical record. Future geologists, prospectors, and data scientists will look back and find gold values without the critical context of its associated pathfinder elements. This disconnect not only hampers traditional geochemical interpretation but also undermines the effectiveness of modern machine learning approaches, which often rely on multivariate relationships to identify patterns and predict mineralization.

 When gold is reported without its geochemical context, it becomes just another number. It is fundamentally disconnected from the other elements that make exploration science so powerful. This limits the ability to train robust machine learning models, as the essential relationships between gold and its pathfinders are missing from the dataset. In effect, we risk creating a generation of “orphaned” gold values, with diminished scientific utility.

 Building Better Early-Stage Exploration Databases

The solution is clear: early-stage exploration databases should be developed using assay methods that allow direct, meaningful comparisons among commodity elements and their pathfinders. This requires selecting compatible digestion and analysis protocols - such as aqua regia digestion for select multi-element suites - so that copper, gold, and associated elements are measured from the same digested sample matrix. This approach preserves the integrity of the geochemical ratios and associations, which is essential for effective vectoring, anomaly detection, and multivariate analysis. While total digestion techniques like fire assay and near-total digestion like 4-acid provide the most comprehensive gold or copper values, they are separate processes and do not allow for direct multi-element association.

Assay Method Selection in Copper-Gold Porphyry Exploration

In copper-gold systems - including both alkalic and calc-alkalic porphyry deposits - copper is typically hosted in sulfide minerals (e.g., chalcopyrite, bornite, chalcocite, and in calc-alkaline systems, abundant pyrite and minor arsenopyrite), even when the host rock is silicate-rich. Aqua regia digestion is highly effective at dissolving these sulfides, making it an appropriate and reliable method for early-stage geochemical exploration and vectoring in these systems. This allows for the direct measurement of copper, gold, and pathfinder elements from the same digestion method, which is crucial for interpreting geochemical gradients and associations - such as the gold/copper ratio - that are central to porphyry exploration.

Aqua regia will generally provide accurate copper and gold values for early-stage work, as long as these metals are present as sulfides or as fine, liberated gold.

Refractory minerals (e.g., gold in arsenopyrite, pyrite, or encapsulated in unbroken sulfide grains) are common in some systems, but these minerals are generally not refractory to acid digestion. Aqua regia can dissolve most sulfide-hosted copper and gold, except where gold is extremely finely locked or encapsulated within unreactive minerals, or present as tellurides or within silicate lattices - situations that are rare in most porphyry systems and tend to be more relevant to advanced resource estimation or specific deposit types (e.g., some orogenic gold systems).

For most alkalic and calc-alkalic porphyry copper-gold systems, aqua regia is suitable for early-stage exploration and preserves the essential multi-element associations needed for effective targeting and vectoring.

Assay Methods for Copper-Gold Systems: Abilities, Limitations, and Uses

A summary of common assay methods for copper-gold systems

 Notes on aqua regia:

• Aqua regia is highly effective for both copper and gold when they are in sulfide minerals, regardless of whether the rock is weathered or unweathered.

• Recovery is limited if metals are present in refractory forms (e.g., encapsulated in silicates, tellurides, or as extremely fine inclusions).

• The method preserves the direct association between Cu, Au, and pathfinders, which is critical for geochemical vectoring and multivariate analysis.

• For total metal accounting or in the rare case of significant refractory mineralogy, more aggressive digestion (4-acid, fusion, fire assay) is required.

Key Takeaways for Assay Selection

• Aqua regia is suitable for any sample type where copper and gold are hosted in sulfide phases — not just soils and weathered rocks, but also unweathered ores. Its main limitation is with rare refractory or encapsulated forms of gold or copper, which are uncommon in most porphyry and epithermal deposits.

• 4-acid digestion has become a cornerstone of early mineral exploration for a wide range of deposit types, providing near-total, detailed multi-element geochemistry and enabling more sophisticated exploration strategies. However, in gold projects, it is essential that gold is not treated as an afterthought. For robust scientific and financial decision-making, gold must be analyzed in context — using compatible methods alongside supporting geochemical data to accurately interpret its significance.

• As the industry evolves, it is important to focus not just on high gold numbers, but on building geochemical datasets that are reliable, interpretable, and future-proof.

If you’re working with legacy data, creating new data, exploring greenfield terrains, or blending AI with real-world geology — I’d love to connect.

If your AI/ML team is growing and you’re interested in adding expertise in geoscience and data science, I’d love to chat.

 #GeologicalDataScience #MineralExploration #Prospectivity #Geochemistry #Gold #ArtificialIntelligence #DataDrivenDiscovery #MachineLearning About the Author:

Dr. Diana Benz has 28 years of experience in the mineral exploration industry searching for diamonds and metals in a range of roles: from heavy minerals lab technician to till sampler, rig geologist, project manager and business owner/lead consultant. She has a Bachelor of Science in General Biology, a Master of Science in Earth Sciences researching diamond indicator mineral geochemistry, and a PhD in Natural Resources and Environmental Studies using geochemical multivariate statistical analysis techniques to interpret biogeochemical data for metallic mineral exploration. Diana has conducted fieldwork in Canada (BC, NWT, YT and ON) as well as in Greenland. She has also been involved, remotely through a BC-based office, in mineral exploration projects in South America, Africa, Eurasia, Australia and the Middle East. Diana owns Takom Exploration Ltd., a boutique geological and environmental firm focused on mineral exploration in BC and the Yukon. She currently holds 3 mineral properties in central BC.