Halogen analysis

Halogen analysis pre-treatment

Vegetation, soil and sediment samples for super trace halogen analysis require pre-treatment to remove the organic component from samples. This is achieved by ashing the sample under controlled conditions. Organics will interfere with the measurement of some halogens. Even with the volatilisation of the halogens during ashing, the ashed samples generally report concentrations within the detection limits due to the concentrating effect of ashing. Analysis of halogens is not quantitative so is applicable only as an exploration tool where the comparison of relative concentrations is useful.

Choice of preparation method

ALS method HAL-PREP01 is used to pre-treat soil and sediment samples for halogen analysis to remove organic material. Vegetation samples are ashed at 475ᵒC for 24 hours using method VEG-ASH01. Both the pre- and post-ashing sample weights are reported for vegetation. The average ash yield is 2-4% for species commonly used in exploration surveys.

Halogen analysis

Routine, cost-effective analysis of halogens has previously been difficult. Recent analytical technique 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. This data is 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 (NAA) methods are offered. Please contact ALS for more information about halogen analysis.

Methods - ME-HAL01™ and ME-HAL01a™

Halogen analytical methods use a water leach and measurement on a combination of inductively coupled plasma-mass spectrometry and ion chromatography. For the analysis of pre-treated vegetation samples method ME-HAL01a™ should be used. For the analysis of soils or sediments method ME-HAL01™ should be used. These halogen methods do not report quantitative values and are applicable for exploration applications only.

Evidence for halogens in exploration

Historically, published investigations into halogens in ore deposits were dominated by Russian research described by Trofimov and Rychkov (2004, translated from Russian version published in 1994) on iodine and bromine associations with mineral deposits. More recent work on the use of fluorine, chlorine, bromine, and iodine, has investigated till soils (Dunn et al., 2007), and vegetation (Dunn et al., 2007; Dunn and Heberlein, 2020). These works have identified a relationship between mineralisation and halogen concentrations.

Because halogens can be highly soluble and mobile in water, they can be transported far away from where they are leached from alteration minerals associated with ore deposits. This can produce a dispersion halo both larger than that visible in alteration minerals, and with the ability to form in post-mineralisation cover sequences that overlay target lithologies. Direct detection by geochemical methods through transported cover requires that elements can move through the cover sequences to form surface anomalies. The high solubility also permits uptake and concentration in organic phases by vegetation (Dunn et al., 2007). Dunn et al., (2007) noted that the halogen element associated with each deposit is variable and therefore the analyses of all four non-radiogenic halogens is recommended.

References & further reading

Dunn, C.E. and Heberlein, D.R., 2020. Geochemical investigation of halogens in spruce treetops and integration with existing multi-element data from the Blackwater region and TREK project area, central British Columbia (NTS 093C, F); in Geoscience BC Summary of Activities 2019: Minerals, Geoscience BC, Report 2020-01, pp. 101–108.

Dunn, C.E., Cook, S.J., and Hall, G.E.M., 2007. Halogens in surface exploration geochemistry: evaluation and development of methods for detecting buried mineral deposits. Geoscience BC, Report 2007-10, 62 pages.

Hannington, M.D., Kjarsgaard, I.M., Galley, A.G., and Taylor, B., 2003. Mineral-chemical studies of metamorphosed hydrothermal alteration in the Kristineberg volcanogenic massive sulfide district, Sweden. Mineralium Deposita, issue 38, pp. 423-442.

Trofimov, N.N., and Rychkov, A.I., 2004. Iodine and bromine: Geochemical indicators of deep ore deposits. Colorado Mountain Publishing House. Originally published 1994 in Russian. Wilkinson, J.J., 2001. Fluid inclusions in hydrothermal ore deposits. Lithos. Vol. 55. pp. 229-272.

Yardley, B.W.D., Banks, D.A., and Bottrell, S.H., 1993. Post-Metamorphic Gold-Quartz Veins from N.W. Italy: The Composition and Origin of the Ore Fluid. Mineralogical Magazine. Vol. 57, pp. 407-422.