In many environments, if surface materials host geochemical signals from nearby mineralisation, at least part of the signal may have been transported chemically from source to the (near) surface. The ability of surface materials to retain these signals can depend on many variables. However, pH, the relative concentration of water leachable salts, and the ability of a sample to neutralise acid might be considered as master variables with a major influence on how the concentration of pathfinder elements will present.
pH determinations are performed under controlled laboratory conditions. Sample material is mixed with deionised water prior to determination with a calibrated glass (pH) electrode. Method variations are offered for continuity with previous work.
The available pH methods consider different sample aliquots (5g, 20g, 10g), different sample to water ratios (1:10, 1:1, paste) and different sample-water reaction times (1 minute, 1 hour, 1 hour).
|OA-ELE03||pH||0.1-14||pH on 1:10 sample to water ratio. 5g sample|
|OA-ELE05||Soil pH||0.1-14||Soil pH on 1:1 sample to water ratio. 20g sample|
|OA-ELE07||Paste pH||0.1-14||Paste pH on 10g sample saturated with water.|
Electrical conductivity (EC) determinations are performed under controlled laboratory conditions. Sample material is mixed with deionised water prior to determination with a calibrated conductivity electrode. Method variations are offered for continuity with previous work.
The available EC methods consider different sample aliquots (5g or 20g), different sample to water ratios (1:10 or 1:1) and different sample-water reaction times (1 minute or 1 hour).
|OA-ELE04||Conductivity||1-100,000μS/cm||Specific conductivity on 1:10 sample to water ratio. 5g sample|
|OA-ELE06||Soil Conductivity||1-100,000μS/cm||Soil conductivity on 1:1 sample to water ratio. 20g sample|
The ability of a sample to resist or buffer addition of acid can be used to infer potential roles for a material in mine waste disposal. Neutralisation capacity may also be an effective indicator of the ability of a sample to resist pH variations that can cause a sample to lose a mineralisation signal that it may have acquired.
Neutralisation methods are available as add-on methods to the standard soil and paste pH determination methods (OA-ELE05 & OA-ELE07).
|OA-ELE05AP||Soil pH and soil acid neutralisation||0.1-14||Add on to Soil pH. Addition of HCl and pH re-measured.|
|OA-ELE07AP||Paste pH and soil acid neutralisation||0.1-14||Add on to Paste pH. Addition of HCl to paste and pH remeasured.|
In the near-surface environment there are many processes that can occur to influence both pH and EC. Electrical conductivity is a useful proxy for the amount of easily (water) leachable salts that are present in a sample. Salts may be concentrated and brought to the near surface through groundwater and soil water capillary action. Multiple saturation-evaporation events can result in salt accumulations, and the EC of a sample can help put this into perspective. Samples could be sorted or normalised/levelled on the basis of their EC, so that pathfinder element anomalies can be better compared between different samples. Likewise, soil pH can vary in response to formation phenomena that drive acid production or neutralisation. Sulphide oxidation, the production of organic acids from decaying organic material that is incorporated into soil and felsic protolith can all contribute to hydrogen ion generation (lower pH). Soils that develop from carbonate or ultramafic protoliths can buffer soil pH to neutral-alkaline conditions. Knowledge of sample pH can allow the pathfinder element response of different samples to be interpreted with greater insight. For example, a sample with a low pH may be able to support a higher concentration of a pathfinder element typically present as an oxyanion (such as Mo) than a sample with a neutral or alkaline pH. Conversely, for pathfinder elements that are mobilised as cations, alkaline pH conditions can tend to promote higher concentrations. The concentration of a pathfinder element in a sample may be a function of its local availability plus the capacity of the sample to retain it. Furthermore, pH can be an indicator of oxidising pyrite associated with mineralisation. The ability of H + ions to diffuse means that pH anomalies can be larger (spatially) than many pathfinder elements. Consequently, mapping pH can be a useful exploration tool in its own right.
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These methods are able to be conducted on all soils, sediments and pulps.