Generative exploration

Aqua Regia Digestions

ALS offers aqua regia methods for small (0.4-0.5g) to large (25 and 50g) sample aliquots. Aqua regia is a useful digestion when mobile trace elements are under investigation, and you want to maximise geochemical contrast by leaving most silicate minerals in the sample relatively untouched. It is a partial decompsition using nitric and hydrochloric acid at relatively low temperature. The low temperature and relatively large aliquot size (compared to four acid methods) allow Hg and Au to be reported as part of the multi-element suite of elements.

What minerals are dissolved by aqua regia?

Aqua regia is effective at dissolving metal sulphides, most sulphates, carbonates, phosphates, organically bound metals, Au, Pt, Pd, tellurides, selenides and arsenides. Some silicates and alumino-silicate minerals are partially attacked but most remain undissolved so do not form part of the reported results.

Why is near-total gold recovery key in exploration?

Traditional aqua regia digestion can leave some of the gold trapped in silicate minerals, limiting anomaly identification. Au-NANO51™ combines aqua regia and hydrofluoric acid in the right ratio to achieve near-total dissolution of gold, ensuring that even gold encapsulated in silicates and sulphides is accurately measured. This improves the reliability of geochemical data and allows the viability of a deposit to be assessed with greater certainty.

Four-Acid Digestions

Four-acid decomposition is useful when exploring in residual terrain where soils have formed from the rocks of interest. A four-acid digestion utilises a combination or nitric, perchloric, and hydrofluoric acid with a final dissolution stage using hydrochloric acid. This digestion breaks down most minerals allowing for a “near-total” sample decomposition and subsequent analysis. Proprietary ALS techniques work to minimise the loss of elements generally considered volatile in a four-acid digestions including As, Se, Sb, Te, and Tl. Due to the relative high temperature of digestion and the small sample aliquot digested, Hg and Au cannot be reported from the same digestion.

What minerals are dissolved by a four-acid digestion?

Most silicate and oxide minerals are effectively broken down plus all of the mineral phases that are digested in aqua regia methods. Some of the most resistate minerals may not be fully broken down. These minerals include barite, chromite, columbite-tantalite, cassiterite, celestite, rutile, scheelite, wolframite, and zircon. If full quantification of elements of interest hosted in these minerals is required, a fusion method may be needed for a full sample decomposition.

Fusion Decomposition

Fusion decomposition is where a flux material is added to a pulverised sample and then heated to melt the sample. Common fluxes include lithium borate and sodium peroxide. The flux that is added to the sample melts at a lower temperature than the sample itself forming a liquid in which the powdered rock sample can decompose i.e. it effectively lowers the temperature of melting. The most common fusion method is fire assay but samples for multi-element geochemistry can also be prepared by fusion when elements of interest are hosted in resistate minerals.

Why are there different flux fusions?

When a flux is added to a sample the elements contained in that flux cannot themselves be determined in the sample. As well, different fluxes may be more suitable for different sample types for example, lithium borate flux is unable to effectively oxidise samples with high sulphide composition. For these reasons a sodium peroxide fusion maybe used when Li is an important element or samples contain more than 4% sulphides.

Hydrogeochemistry

Water that has interacted with rocks and soil may dissolve trace elements which are then transported with the water, potentially offering a larger footprint that can be diagnostic of the rocks and soil. Where the collection of traditional media such as soils is difficult or impossible such as in swamps, areas with significant transported cover and areas where invasive sampling is not possible, hydrogeochemistry provides a direct detection tool on the same scale as stream sediment sampling.

Which methods are suitable for my water samples?

The choice of method for water samples depends on the total dissolved solids in the sample. Super-trace methods such as Au-PATH14L™ and ME-MS14L™ are only able to be used for very low TDS samples. Add-on methods for anion concentrations and alkalinity can add great value to a multi-element method for both geochemical anomaly modelling and for reviewing the quality of results.

Biogeochemistry

Geochemical exploration in covered terrain requires the ability to detect mineralisation at depth, and separate the mineralisation signature from background lithological variation. Plants have long been recognised as a medium that can produce a surface geochemical response from covered lithologies by concentrating elements within their tissues.

Using plants for exploration

Careful selection of plant species, tissue type and growth age are important factors to consider as the geochemical response will vary with these factors. Another decision is whether to ash samples before analysis or not. Ashing is the process by which biogeochemical samples are heated at 475ᵒC to reduce their weight and pre-concentrate elements of interest. The other method of preparation is to mill (macerate) dried vegetation.

Ionic Leach™

When exploring in regions where prospective lithologies are overlain or covered by younger rocks or sediments, a secondary transported ion signal is more important than the geochemistry of the surface soils. Extensive research has identified that ions can be mobilised through thick transported cover by a variety of mechanisms. This mobilisation to surface soils and sediments allows direct detection of buried mineralisation from surface sampling.

What is Ionic Leach™?

The Ionic Leach™ method is designed to liberate the most loosely bound ions in a sample but will not digest the minerals onto which the ions have adsorbed. Complexing agents selectively extract and hold ionic species in the leach solution from a 50g sample. Reagents are added in at 1:1 ratio which eliminates the need for dilution prior to measurement. The method is designed to liberate ions that have been mobilised in the weathering environment and are loosely bound to the surface of grains.

Targeted Leaches

Some fractions of soil and sediments are more likely to adsorb or host elements of interest for exploration targeting. An example is adsorption and concentration of metal ions by organic matter in a soil which forms chelation complexes with metals. A range of targeted mineral leaches are offered that focus on digesting just parts of a sample. In the case of organic matter sodium pyrophosphate is used to liberate these organically bound heavy metals (ME-MS07).

Leach methods offered

In addition to leaches that target the organic component in a sample, other fractions that can be targeted are the water-soluble component, acetate soluble secondary minerals, carbonates, Mn oxides, and amorphous Fe oxides. A variety of targeted leaches applicable to mineralised samples are described in the Precious Metal Analysis and Base Metal Analysis section of this web page.

Halogen Analysis

Numerous lines of evidence suggest that halogens play an important role in ore deposit formation and thus can be indicative of the presence of ore systems where they are found in anomalously high concentrations. One line of evidence is that high concentrations of halogens are frequently found in fluid inclusions from some ore deposits. These fluid inclusions represent preserved bubbles of the mineralising fluids and show that halogens were a significant part of these fluids. Also, some halides such as Cl and F, are present in the crystal structure of minerals associated with mineralisation but not in locations distal to mineralisation. Halides are thought to be a key complexing agent for metals and therefore facilitate their transport in hydrothermal solutions during the formation of ore deposits.

Analysis in a variety of matrices

Routine, cost-effective analysis of halogens has previously been difficult. Recent analytical developments have driven significant improvements in the process, while reducing associated costs. The analysis of halogens by ALS method ME-HAL01™ represents an extractable component rather than the total concentration. These data are considered most applicable as an exploration tool in soils and vegetation where the comparison of relative concentrations is useful. Where analysis of the total concentration of Cl, Br and I are required in solid media, alternative neutron activation analysis methods are offered.

Selenium Analysis

Selenium is a chalcophile element with a similar size and charge to sulphur, which allows it to readily substitute into sulphide minerals. It is also less mobile than sulphur in oxidising conditions, like those experienced by most soils, which means it will be retained proximal to the location of sulphide oxidation. These characteristics make Se a powerful trace element for use in targeting of sulphide mineralisation.

Measurement of Selenium

Until recently, the routine and effective application of selenium to mineral exploration has been hampered by high detection limits and interferences during ICP-MS measurement. ALS offers an industry-leading detection limit of 0.003 ppm, well below average crustal abundance, allowing the true background to be characterised. Proprietary ALS technology reduces the interference on selenium during analyses which allows for this much lower detection level.

Conductivity, pH & Neutralisation

The mobilisation of ions in near-surface environments can be strongly controlled by pH. As such it is vital to quantify any variation in pH over a project area which may introduce variability in metal concentrations due to changing pH rather than due to lithology. The use of pH as a primary detection tool has also been postulated in surface environments where the break-down of sulphides generates increased acidity. Conductivity has also been used as a direct detection tool in regions where active seismicity results in hydraulic pumping of saline waters from depth up to surface.

Choice of method

The methods offered by ALS to determine conductivity, pH and the ability of a sample to neutralise acid have applications in mineral processing and environmental assessment in addition to exploration. The choice of method for environmental assessment applications may depend on government requirements for reporting. ALS client services can provide details of each method to ensure the applicable method is chosen.

pXRF for Exploration

During active exploration projects access to geochemical values can be required for quick decision making. ALS offers semi-quantitative pXRF analysis on samples directly after sample preparation. The application of pXRF on a pulverised or sieved sample produces a more reliable result than measurements taken on less homogenised samples. pXRF results are then available to the exploration geologist for drill hole planning or extension of soil grids while the traditional geochemical analyses are pending in the laboratory.

Advantages of laboratory based pXRF

The effective use of pXRF requires appropriate calibration, understanding of matrix interference effects, trained operators, quality checks with matrix matched reference materials, and detailed attention to sample representivity. Also, depending on the region, special licenses may be needed to transport and operate a pXRF. All these considerations take up a geologist’s time and distract from the task at hand, exploring for ore. By outsourcing pXRF readings to ALS the exploration geologist can be confident in the values reported without the day-to-day management of the collection of results.