Conductivity, pH & neutralisation

pH determinations on soils & geological materials

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.

Methods - OA-ELE03, OA-ELE05 & OA-ELE07

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).

Electrical conductivity determinations on soils & geological materials

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.

OA-ELE04 & OA-ELE06

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).

Acid neutralisation capacity

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.

Methods - OA-ELE05AP & OA-ELE07AP

Neutralisation methods are available as add-on methods to the standard soil and paste pH determination methods (OA-ELE05 & OA-ELE07).

Why bother with pH and EC in mineral exploration soil surveys?

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.

References & further reading

Bastrakov, E. N., Main, P., Wygralak, A., Wilford, J., Czarnota, K. and Khan, M., 2018. Northern Australia Geochemical Survey Data release 1 – Total (fine fraction) and MMI™element contents. Record 2018/06. Geoscience Australia, Canberra.

Dunn, C.E., 2007. Biogeochemistry in Mineral Exploration. Volume 9. Handbook of Exploration and Environmental Geochemistry. Ed: Hale, M. Elsevier, The Netherlands.

Govett, G.J.S., 1976. Detection of deeply buried and blind sulphide deposits by measurement of H+ and conductivity of closely spaced surface soil samples. Journal of Geochemical Exploration. Vol. 6. pp. 359-382.

Govett, G.J.S., and Dunlop, A.C.1984. Electrical techniques in deeply weathered terrain in Australia. Journal of Geochemical Exploration. Vol 21. pp. 311-331.

Hamilton, S.M, Cameron, E.M., McClenaghan, M.B. and Hall, G.E.M., 2004. Redox, pH and SP variation over mineralization in thick glacial overburden. Part I: methodologies and field Investigation at the Marsh Zone gold property. Geochemistry: Exploration, Environment, Analysis, Vol. 4. pp. 33-44.

Hamilton, S.M, Cameron, E.M., McClenaghan, M.B. and Hall, G.E.M., 2004. Redox, pH and SP variation over mineralization in thick glacial overburden. Part II: field investigation at Cross Lake VMS property. Geochemistry: Exploration, Environment, Analysis, Vol. 4. pp. 45-58.

Kelley, D.L., Hall, G.E., Closs, L.G., Hamilton, I.C., and McEwen, R.M., 2003. The use of partial extraction geochemistry for copper exploration in northern Chile. Geochemistry: Exploration, Environment, Analysis, Vol. 3, pp. 85-104.

Levinson, A. A., 1980. Introduction to Exploration Geochemistry, 2nd Edition, Applied Publishing Ltd. pp. 924.

Smee, B.W., 1983. Laboratory and field evidence in support of the electrogeochemically enhanced migration of ions through glaciolacustrine sediment. In: G.R. Parslow (Editor), Geochemical Exploration 1982. J. Geochem. Explor., 19: pp. 277-304.