Selenium in soils

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 sulfur in minerals, particularly sulfides.

The substitution into sulfide minerals results in selenium anomalies associated with sulfide-bearing mineralisation.  Under the low temperature, oxidizing conditions of near-surface weathering, sulfur is more mobile than selenium (Howard, 1977).  Selenium tends to be immobilized through adsorption onto hydrous Fe-Mn oxides (Queffurus and Barnes, 2015).  This variable mobility at surface weathering conditions will move sulfur away from the location of sulfide 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.

The average crustal abundance of selenium is just 0.05ppm (Taylor and McLennan, 1983) while 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 characterized.   Proprietary ALS technology reduces the interference on selenium during analyses which allows for much lower detection levels.   The routine analyses of selenium in soils is now another element in the exploration toolbox for sulfide mineralization and sandstone-hosted U ore deposits.

Levels of selenium in soils and water are stipulated by government regulation but vary to a large degree with geology.  A good baseline characterized at the exploration phase can be used to provide regulators with information on pre-existing regional background.