Geochemical Sampling and Testing: Methods for Mineral Exploration

Introduction: The Chemical Fingerprint of Mineral Deposits

Every mineral deposit leaves a chemical signature in the rocks, soils, sediments, and waters that surround it. Geochemical sampling is the science of detecting, measuring, and interpreting these chemical signatures to locate and evaluate mineral resources hidden beneath the Earth's surface. It is one of the most powerful and cost-effective tools available to the mineral exploration geologist, capable of identifying exploration targets across large areas before the expensive step of drilling is undertaken.

The principle is straightforward: mineral deposits concentrate certain elements to levels far above their normal abundance in the Earth's crust. As these deposits weather and erode over geological time, the concentrated elements disperse into the surrounding environment — carried by groundwater into soils, by streams into sediments, and by wind and gravity across the landscape. By systematically collecting and analysing samples from these media, geochemists can map the spatial distribution of element concentrations, identify anomalous zones where concentrations are significantly above background, and trace these anomalies back to their source — the buried mineral deposit.

In practice, geochemical exploration is a sophisticated discipline that requires careful sample design, rigorous field procedures, high-quality laboratory analysis, and expert data interpretation. This guide explains the principal geochemical sampling methods used in mineral exploration, the analytical techniques employed to determine element concentrations, the quality assurance and quality control (QA/QC) protocols that ensure data reliability, and the approaches used to interpret geochemical data and generate drill targets. The discussion is grounded in the geological context of Uganda, where ALOM Mining & Geohydro Services applies these techniques across a range of mineral exploration projects.

Types of Geochemical Sampling

Soil Sampling

Soil geochemistry is the most widely used geochemical exploration method, particularly in tropical environments like Uganda where thick soil cover often obscures the underlying bedrock geology. The method is based on the observation that elements released from weathering mineral deposits migrate upward and laterally through the soil profile, creating detectable anomalies in the overlying soil.

How it works: Soil samples are collected at regular intervals along survey lines or on a grid pattern. The typical sample depth is 20 to 40 centimetres below the surface — deep enough to reach the B-horizon soil, which is less affected by surface contamination and organic matter than the topsoil layer. Each sample weighs approximately 1 to 2 kilograms and is collected using a hand auger, mattock, or soil sampling probe. The sample location is recorded using a GPS, and field observations about the soil colour, texture, moisture, and presence of rock fragments are documented.

Sample preparation and analysis: At the laboratory, samples are dried, disaggregated, and sieved to isolate the fine fraction (typically minus-80 mesh or minus-180 micron), which concentrates the geochemically active clay minerals and iron-manganese oxide coatings that adsorb trace elements. The fine fraction is then analysed for a multi-element suite using techniques such as ICP-MS or ICP-OES following an appropriate digestion method.

Strengths and limitations: Soil sampling is highly effective for detecting near-surface mineralisation and for defining the lateral extent of geochemical anomalies. It is relatively inexpensive, can cover large areas efficiently, and provides spatially detailed data. However, its effectiveness diminishes where mineralisation is deeply buried, where the soil profile has been disturbed by erosion, colluvial transport, or human activity, or where thick laterite caps mask the underlying geochemical signal. In Uganda, the deep tropical weathering profile and widespread laterite development require careful consideration of soil sample design and interpretation.

Stream Sediment Sampling

Stream sediment sampling is a reconnaissance-scale technique used to survey large areas rapidly and cost-effectively. It exploits the natural drainage system: as water flows over mineralised ground, it erodes and transports mineral grains and dissolved elements downstream. By collecting sediment samples from active stream channels, geochemists can effectively sample the entire upstream catchment area in a single sample.

How it works: Samples are collected from the active channel of streams, ideally from fine-grained sediment accumulations behind boulders, at the inside of meanders, or in other low-energy depositional environments where fine material naturally concentrates. Each sample typically weighs 2 to 5 kilograms and is collected from the uppermost few centimetres of sediment. The sampling locations are chosen to characterise specific catchment areas, with sample spacing determined by the drainage density and the desired level of resolution.

Sample preparation and analysis: Stream sediment samples undergo similar preparation to soil samples — drying, sieving to a fine fraction, and multi-element analysis by ICP-MS or ICP-OES. In some programmes, a heavy mineral concentrate (panned concentrate) is also collected from the same location and analysed separately to identify indicator minerals associated with specific deposit types.

Strengths and limitations: Stream sediment sampling is exceptionally efficient for identifying mineralised catchments at the regional scale. A single sample integrates the geochemical signature of the entire upstream area, making it possible to cover hundreds or thousands of square kilometres with a manageable number of samples. The method is widely used in the early stages of exploration to prioritise areas for more detailed follow-up. Its limitations include the dilution of anomalies by barren material from upstream, the influence of secondary dispersion and hydrological conditions, and the potential for contamination from anthropogenic sources (artisanal mining, agriculture, settlements) along the stream course.

In Uganda, stream sediment surveys have been instrumental in identifying gold, tin, tungsten, and base metal anomalies across multiple geological domains and remain a standard tool in ALOM's regional exploration toolkit.

Rock Chip Sampling

Rock chip sampling involves collecting small pieces of rock from outcrops, road cuts, mine workings, trenches, or any other exposure of bedrock. Unlike soil and stream sediment sampling, which detect secondary dispersion halos, rock chip sampling provides a direct measure of the grade of mineralisation in the bedrock itself.

How it works: The geologist selects a representative section of the rock exposure and collects chips using a geological hammer or a portable rock saw. For channel sampling — a more controlled variant — a continuous channel of uniform width and depth is cut across the exposure, and all material from the channel is collected as a single sample. This provides a more representative and less biased measurement than grab sampling, where the geologist selects individual pieces of rock.

Types of rock samples:

Grab samples: Selective collection of individual rock pieces, often targeting visibly mineralised material. Useful for identifying the presence and type of mineralisation, but prone to selection bias and not representative of average grade.
Chip-channel samples: Continuous collection of chips along a line across the rock face. More representative than grab samples and suitable for preliminary grade estimation.
Channel samples: A continuous groove cut into the rock at a specified width and depth. The most representative surface sampling method for estimating true grade, particularly across veins, lodes, and stratiform mineralisation.

Strengths and limitations: Rock chip sampling provides direct geological and grade information from the bedrock. It is essential for ground-truthing geochemical and geophysical anomalies, for geological mapping purposes, and for preliminary grade assessment of exposed mineralisation. Its limitation is that it can only sample what is exposed at the surface — which, in deeply weathered tropical environments like much of Uganda, may represent a small fraction of the total mineralisation.

Trench Sampling

Trenching involves excavating shallow trenches (typically 1 to 3 metres deep) through the overburden to expose the underlying bedrock or the weathered zone immediately above it. Trench walls are then mapped, described, and systematically sampled by cutting continuous channels across the exposure.

How it works: Trenches are oriented perpendicular to the expected strike of the mineralised structure or formation to provide a cross-sectional view. They are excavated by hand (in soft ground) or by excavator (in harder material). Once excavated, one or both walls are cleaned and mapped in detail by a geologist, who records rock types, structures, alteration, and mineralisation. Channel samples are then cut at regular intervals (typically 1 to 2 metres) along the trench wall, with each sample representing a defined length and volume of rock.

Strengths and limitations: Trenching is one of the most cost-effective methods for testing geochemical anomalies and geophysical targets before committing to drilling. It provides continuous, representative samples across the full width of the mineralised zone and allows direct geological observation of the host rocks, structures, and mineralisation style. Trenching is particularly effective in Uganda's geological conditions, where deep weathering often extends mineralisation indicators close to the surface. However, it is limited by the depth of excavation — typically 3 metres maximum for hand trenching and 5 to 6 metres for machine trenching — and is not suitable for testing deeply buried targets.

Analytical Methods for Geochemical Analysis

The accuracy and reliability of geochemical data depend critically on the analytical methods used in the laboratory. The choice of method depends on the elements of interest, the required detection limits, the sample matrix, and the programme budget.

X-Ray Fluorescence (XRF)

XRF is a non-destructive analytical technique that determines the elemental composition of a sample by measuring the fluorescent X-rays emitted when the sample is irradiated with high-energy X-rays. It is widely used for major element analysis (silicon, aluminium, iron, calcium, magnesium, etc.) and for selected trace elements at moderate concentrations.

Advantages: Fast analysis times, relatively low cost per sample, non-destructive (the sample is preserved for re-analysis), and capable of analysing solid samples (pressed pellets or fused beads) as well as powders. Portable XRF (pXRF) instruments allow real-time, in-field analysis for rapid screening and decision-making during fieldwork.

Limitations: Detection limits are generally higher (less sensitive) than ICP-based methods, making XRF less suitable for ultra-trace analysis. Accuracy can be affected by matrix effects and sample preparation quality. Portable XRF instruments are valuable for screening but should not replace laboratory analysis for definitive results.

Inductively Coupled Plasma Mass Spectrometry (ICP-MS)

ICP-MS is the gold standard for multi-element trace analysis in geochemical exploration. The technique dissolves the sample in acid, nebulises the solution into an argon plasma at approximately 10,000 degrees Celsius, and measures the mass-to-charge ratio of the resulting ions to identify and quantify individual elements.

Advantages: Extremely low detection limits (parts per billion for most elements), simultaneous analysis of 40 or more elements in a single run, high accuracy and precision, and wide dynamic range. ICP-MS is the preferred method for pathfinder element analysis, multi-element geochemical surveys, and trace-level detection of indicator elements.

Limitations: Requires complete dissolution of the sample, which may not be achieved for all mineral phases with standard acid digestion methods. Some elements (notably gold, which occurs as discrete metallic particles) are poorly determined by standard ICP-MS and require specialised preparation methods.

Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES)

ICP-OES (also known as ICP-AES) uses the same plasma source as ICP-MS but measures the characteristic wavelengths of light emitted by excited atoms rather than their mass. It offers a good balance between sensitivity, speed, and cost.

Advantages: Robust, reliable, and well-suited for major and minor element analysis at concentrations above parts per million. Faster and less expensive per sample than ICP-MS for many applications.

Limitations: Higher detection limits than ICP-MS, making it less suitable for ultra-trace pathfinder element analysis.

Fire Assay

Fire assay is the definitive method for determining gold and platinum group element (PGE) concentrations in geological samples. The technique uses high-temperature fusion with lead oxide fluxes to collect precious metals into a lead button, which is then cupelled to remove the lead, leaving a precious metal bead that is dissolved and analysed by AAS or ICP.

Advantages: Fire assay is the industry-standard method for gold analysis because it processes a large sample charge (typically 30 to 50 grams), which is essential for overcoming the nugget effect — the uneven distribution of coarse gold particles that makes small-sample analytical methods unreliable for gold. It provides accurate and precise results at exploration and resource-grade concentrations.

Limitations: Destructive (the sample is consumed), relatively expensive per sample, and not suitable for multi-element analysis. Fire assay is typically used specifically for gold, platinum, and palladium, with other elements determined by ICP-MS or ICP-OES on a separate aliquot.

Quality Assurance and Quality Control (QA/QC)

Geochemical data are only as reliable as the systems put in place to ensure their quality. A robust QA/QC programme is essential for identifying and correcting errors in sample collection, preparation, and analysis. Without QA/QC, geochemical results cannot be trusted, and exploration decisions based on unreliable data can lead to wasted investment or, worse, missed discoveries.

Key Components of a QA/QC Programme

Certified Reference Materials (CRMs): Also known as standards, CRMs are samples with known, certified element concentrations prepared by independent organisations. CRMs are inserted into the sample stream at regular intervals (typically 1 in every 20 samples) and submitted to the laboratory under sample numbers that are indistinguishable from routine samples. When the laboratory results for CRMs are returned, they are compared against the certified values to assess analytical accuracy. Results outside acceptable tolerance limits (typically plus or minus 10% of the certified value) indicate a problem with the analysis that requires investigation and potentially re-analysis.
Blanks: Blank samples — material with negligible concentrations of the elements of interest — are inserted into the sample stream to detect contamination during sample preparation or analysis. Coarse blanks (barren rock chips) monitor contamination during crushing and pulverising, while fine blanks (certified blank pulps) monitor contamination during the analytical process. Any significant concentrations detected in blanks indicate cross-contamination, requiring corrective action.
Duplicates: Duplicate samples are collected or prepared to assess the precision (reproducibility) of the sampling and analytical process. Several types of duplicates serve different purposes:
- Field duplicates: A second sample collected from the same location to assess the variability introduced by the natural heterogeneity of the material being sampled.
- Preparation duplicates (coarse duplicates): A second split of the same coarse reject material submitted independently for preparation and analysis to assess the precision of the crushing and splitting process.
- Analytical duplicates (pulp duplicates): A second aliquot of the same pulverised sample submitted for re-analysis to assess the precision of the laboratory analytical process.
Umpire Laboratory Checks: A subset of samples (typically 5% of the total) is submitted to a second, independent laboratory for re-analysis. This external check provides an independent assessment of accuracy and identifies any systematic bias in the primary laboratory's results.

Implementing QA/QC in Uganda

In the Ugandan context, where most geochemical analysis is performed by accredited commercial laboratories in East Africa, Southern Africa, or internationally, QA/QC is particularly important because the distance between the field team and the laboratory means that errors cannot be quickly identified through direct observation. ALOM implements rigorous QA/QC protocols on all exploration programmes, with real-time monitoring of control sample results to ensure that data quality meets international standards before the data are used for geological interpretation and target generation.

Data Interpretation and Anomaly Identification

The ultimate purpose of geochemical sampling is to identify anomalies — zones where element concentrations are significantly elevated above the regional background — and to determine which anomalies are most likely to represent genuine mineralisation worthy of follow-up investigation.

Statistical Analysis

Geochemical data are first analysed statistically to characterise the background population (the range of concentrations in unmineralised material) and to define threshold values above which concentrations are considered anomalous. Common statistical approaches include:

Summary statistics: Mean, median, standard deviation, and percentile analysis for each element.
Probability plots: Graphical tools that identify distinct populations within a dataset and define natural threshold values.
Factor analysis and principal component analysis (PCA): Multivariate techniques that identify associations between elements, helping to distinguish geochemical signatures related to different mineralisation types, alteration systems, or lithological units.

Spatial Analysis and Mapping

Anomalous sample locations are plotted on maps and contoured to reveal the spatial extent and geometry of geochemical anomalies. Modern GIS software allows geochemical data to be integrated with geological maps, geophysical survey results, satellite imagery, and topographic data to build a comprehensive picture of exploration targets.

Key spatial analysis techniques include:

Point symbol maps: Each sample location is plotted with a symbol proportional to its element concentration, providing an immediate visual indication of anomalous areas.
Contour (isopleth) maps: Element concentrations are interpolated between sample points and contoured to show the spatial distribution of anomalies as continuous surfaces.
Multi-element overlay maps: Maps showing the coincidence of anomalies in multiple pathfinder elements, which strengthens the confidence that an anomaly represents genuine mineralisation.

Ranking and Prioritising Anomalies

Not all geochemical anomalies represent viable mineral deposits. Some may be caused by secondary dispersion from small, uneconomic occurrences; others by contamination, analytical errors, or natural geological variation. Anomaly prioritisation considers:

Anomaly contrast: The magnitude of the anomaly relative to background — stronger anomalies generally indicate larger or richer sources.
Anomaly coherence: Whether the anomaly is defined by multiple adjacent samples or a single isolated value. Coherent, multi-sample anomalies are more reliable than isolated spikes.
Multi-element association: Anomalies supported by multiple pathfinder elements in a geochemically logical association are more credible than single-element anomalies.
Geological context: Does the anomaly coincide with a favourable geological setting — a known mineralised structure, an appropriate host lithology, a geophysical target? Anomalies that are geologically plausible are prioritised over those in unfavourable settings.
Previous exploration results: Has the anomaly area been tested before? If so, what were the results? If not, it may represent an untested target.

The output of this interpretation process is a ranked list of exploration targets recommended for follow-up investigation, typically by more detailed soil sampling, trenching, or drilling. This systematic approach to target generation ensures that exploration investment is directed toward the most prospective areas and that the probability of discovery is maximised.

Geochemistry in Uganda's Geological Context

Uganda's tropical climate and deep weathering profile create both opportunities and challenges for geochemical exploration. On one hand, the intense weathering of mineral deposits produces broad, well-developed geochemical dispersion halos in soils and stream sediments that can be detected over considerable distances from the source. On the other hand, thick laterite caps, transported soils, and the leaching of certain elements from the weathering profile can obscure or modify the primary geochemical signature.

Successful geochemical exploration in Uganda requires:

Understanding the regolith: The thick weathering profile — which can extend 30 metres or more below the surface — is not simply overburden to be disregarded. It is a complex geochemical system in its own right, with distinct horizons that concentrate, dilute, or redistribute elements in predictable ways. Sampling the correct horizon within the regolith is critical for obtaining meaningful results.
Selecting appropriate elements: In addition to the primary target element (e.g., gold, copper, tin), the analysis should include pathfinder elements that are associated with the target mineralisation and that may disperse more widely or be more readily detected in the weathering environment. For gold exploration, for instance, arsenic, antimony, bismuth, and tungsten are commonly used pathfinders.
Adapting methods to local conditions: The choice between soil, stream sediment, and rock chip sampling — and the specific collection and preparation protocols used — should be tailored to the local geology, topography, vegetation, and weathering conditions. A method that works well in the semi-arid terrain of Karamoja may not be optimal in the densely vegetated, deeply weathered hills of southwestern Uganda.

At ALOM, our geochemists bring extensive experience in tropical geochemical exploration, enabling us to design and execute sampling programmes that account for Uganda's unique conditions and deliver reliable, interpretable data. Our mineral exploration services integrate geochemistry with geological mapping, geophysical surveys, and the full suite of exploration techniques described in our mineral exploration process guide.

Conclusion

Geochemical sampling and testing is a cornerstone of modern mineral exploration — a systematic, science-based approach to detecting the chemical fingerprints of buried mineral deposits. From the regional-scale efficiency of stream sediment surveys to the detailed spatial resolution of soil grids, from the direct information provided by rock chip and trench sampling to the analytical precision of ICP-MS and fire assay, the geochemical toolkit offers exploration geologists a range of methods suited to every stage and scale of investigation.

The value of geochemical exploration, however, is only as good as its execution. Proper sample design, rigorous field procedures, robust QA/QC protocols, and expert data interpretation are what separate geochemical programmes that generate genuine discoveries from those that produce meaningless numbers. In Uganda's challenging tropical environment, where deep weathering, laterite development, and transported overburden add layers of complexity, the experience and expertise of the exploration team are critical success factors.

ALOM Mining & Geohydro Services brings proven capability in geochemical exploration across Uganda's diverse geological terrains. Whether you are launching a regional reconnaissance programme, following up on a promising anomaly, or integrating geochemistry into a multi-disciplinary exploration campaign, our team has the technical expertise and local knowledge to deliver results. Contact us to discuss how our mineral exploration services can advance your project.