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Selenium analysis

Routine selenium analysis

Until recently, the routine and effective application of selenium to mineral exploration has been hampered by high detection limits and interferences during measurement with ICP-MS. The average crustal abundance of selenium is just 0.05 ppm (Taylor and McLennan, 1983) but soils can vary dramatically depending on the rock type parent material (Levesque, 1974). 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 analysis which allows for much lower detection levels. The routine analysis of selenium in soils provides a new element in the exploration toolbox for sulphide mineralisation and sandstone-hosted U ore deposits.

Selenium in soils

Trace measurement of Se in soils and sediments holds information for both exploration applications and environmental modelling.

Low detection Se methods

ALS offers a standalone Se method for soils and sediments using aqua regia digestion and ICP-MS measurement, Se-MS46. Low detection Se is also offered as part of the element suite in the multi-element aqua regia method ME-MS41L™ and four acid method ME-MS61L™.

Code	Analyte	Range (ppm)	Description
Se-MS46	Se	0.003-100	Aqua regia digestion and ICP-MS analysis. 25g sample

Where is Selenium found?

The size of the Se^2-ion is very close to that of the S^2- ion, which along with the charge, allows selenium to substitute for sulphur in minerals, particularly sulphides. The substitution into sulphide minerals results in selenium anomalies associated with sulphide-bearing mineralisation. Under the low temperature, oxidising conditions of near-surface weathering, sulphur is more mobile than selenium (Howard, 1977). Selenium tends to be immobilised through adsorption onto hydrous Fe-Mn oxides (Queffurus and Barnes, 2015). This variable mobility at surface weathering conditions will move sulphur away from the location of sulphide mineral breakdown, but selenium will be retained proximal to mineral weathering. Selenium has also been found to be associated with sandstone-hosted U ore (Davidson, 1963; Rose et al 1979) making it a potential pathfinder for this mineralisation style.

Selenium is also an essential trace element for all living organisms but can become toxic at higher concentrations. Levels of selenium in soils and water are stipulated by government regulation but vary to a large degree with geology. A baseline characterised at the exploration phase can be used to provide regulators with information on pre-existing regional background concentrations.

References & further reading

Davidson, D.F., 1963. Selenium in some oxidized sandstone-type Uranium deposits. Geological Survey Bulletin. 1162-C.
Howard, J.H., 1977. Geochemistry of selenium: formation of ferroselite and selenium behavior in the vicinity of oxidizing sulfide and uranium deposits. Geochim. Cosmochim. Acta. Number 41, pp. 1665-1678.
Lévesque, M., 1974. Selenium distribution in Canadian soil profiles. Canadian Journal Soil Science. Number 54, pp. 63-68
Queffurus, M. and Barnes, S. 2015. A review of sulfur to selenium ratios in magmatic nickel-copper and platinum-group element deposits. Ore Geology Reviews. Number 69. Pp. 301-324.
Rose, A.W., Hawkes, H.E., and Webb, J.S., 1979. Geochemistry in mineral exploration. Second Edition. Academic Press.