Geochemical digests are the backbone of many geochemical analyses, providing critical insights into the composition of rocks, soils, and sediments. Selecting the right digest for your project is essential, as it dictates the elements you can analyze and the interpretability of your data. In this edition we are exploring the more common digests, weak and sequential leaches, and some best practices to keep in mind.

Common Geochemical Digests

1. Aqua Regia

• Composition: A 3:1 mixture of hydrochloric acid (HCl) and nitric acid (HNO3).
• Use Case: Partial digest that targets metals in sulfides, carbonates, and oxides but does not fully dissolve silicates.
• Applications: Effective for soil geochemistry and exploration for pathfinder elements, especially in areas with secondary dispersion halos.
• Limitations: Not suitable for total digestion of silicate-hosted elements.

2. 4-Acid Digest

• Composition: A mixture of hydrofluoric acid (HF), perchloric acid (HClO4), nitric acid (HNO3), and hydrochloric acid (HCl).
• Use Case: Near-total digest capable of breaking down silicates and some refractory minerals.
• Applications: Ideal for most rock and drill core analyses to understand the bulk composition of samples.
• Limitations: Some elements (e.g., Zr) may still remain in resistant mineral phases.

3. Lithium Borate Fusion

• Process: Samples are fused with lithium borate flux, producing a glass bead that is dissolved in acid.
• Use Case: Total digest that captures all elements, including refractory minerals.
• Applications: Required for whole-rock analyses and critical in studies involving major and trace elements.
• Limitations: Time-consuming and more expensive than acid digests.

​

Weak Leaches

Weak leaches, such as ammonium acetate, MMI, or cyanide leaches, are designed to target specific geochemical fractions. These are widely used in soil and sediment geochemistry to identify mobile metals associated with secondary dispersion or adsorption onto clay and organic matter. They are especially valuable in undercover exploration where subtle anomalies may signal buried mineralization.

1. Applications:

• Identifying pathfinder elements in soil geochemistry.
• Targeting bioavailable or exchangeable fractions in environmental studies.

2. Limitations:

• Results are highly matrix-dependent and may not correlate directly with total metal concentrations.
• Work with a geochemist before deploying… just because you read a paper where a weak leach worked, does not mean that it is fit-for-purpose for your project.

​Sequential Leaches for Geometallurgy

Sequential leaching is an essential tool in geometallurgy, offering insights into the deportment and extractability of metals in complex ore systems. By using a series of increasingly aggressive leaching steps, geologists and metallurgists can partition metals into distinct mineralogical or chemical phases.

Example: Sequential Copper Leaches in Porphyry Systems

1. Sulfuric Acid Soluble Copper: this step targets copper oxide minerals such as malachite, azurite, chrysocolla, and portions of cuprite and tenorite; in other words, the acid-soluble copper fraction.
2. Cyanide Soluble Copper: the residue from the sulfuric acid leach is treated with 10% sodium cyanide to dissolve secondary copper minerals such as chalcocite, covellite, and portions of bornite and chalcopyrite, isolating the cyanide-soluble copper fraction.
3. Residual Copper: the remaining residue undergoes four-acid digestion (i.e., nitric, perchloric, hydrofluoric, and hydrochloric acids) to dissolve primary copper sulfides and refractory minerals to quantify the residual copper that could not be extracted in the previous steps.

These steps help in understanding:

• Copper Deportment: the distribution of copper among different mineralogical hosts.
• Metallurgical Recovery: which copper phases are likely to be recoverable through heap leaching, flotation, or other processes.
• Ore Variability: spatial changes in mineralogy that affect processing strategies.

Sequential leaches are also used for other metals (e.g., Au, Ni, REE) to inform metallurgical processes and optimize recovery.

​We appreciate you sticking with us so far! To close out this incredibly brief discussion on digests, we would like invoke the “our blog, our soapbox” rule in the hopes that we can eradicate two horrible practices in 2025.

The Perils of Switching Labs Mid-Project: Soapbox #1

Not all digests are created equal. Each laboratory has its own proprietary twist to the “standard” formulas for digests. Switching labs mid-project because the accounting department found a better rate can result in legacy dataset issues. Differences in digestion efficiency, detection limits, and reporting formats can create inconsistencies that complicate data interpretation. It is crucial to maintain consistency in laboratory selection throughout a project to ensure data comparability and reliability.

Say No to Aqua Regia for Drill Core: Soapbox #2

Aqua regia is a good tool for soil and sediments geochemistry but it has no place in drill core analyses. Its inability to digest silicates means you are leaving significant portions of your sample unaccounted for, leading to incomplete or misleading data. Using aqua regia on drill core may save money initially, but it undermines data quality, rendering your interpretations unreliable.workflows. If you’re curious about optimizing your lab practices, let’s connect!