The formation and detection of metal dispersion halos in organic lake sediments

Abstract

The application of organic-rich lake sediment (gyttja) to exploration geochemistry is discussed. Gyttja was collected from some 42 lake sites in the Red Lake-Uchi Lake volcanic-sedimentary sequences. The existance of free sulphide ion in most gyttja is confirmed and theoretical calculations are used to define the approximate concentrations of sulphide ion which determine whether the accumulation of Cu + , Cu 2+ , Zn 2+ and Fe 2+ is by metal sulphide precipitation, organic complexing or both. These calculations show that in the sediments collected in this study, the accumulation of copper will probably be by metal sulphide formation, whereas for zinc, organic complexing and possibly sulphide precipitation are likely mechanisms. It is shown that the background concentrations of copper and zinc on a dry matter basis increase with the content of organic matter in the sediment and it is further shown that the quantity of interstitial water in the sediment increases linearly with the organic content. Also it is demonstrated that the organic matter in the sediment is dispersed in the interstitial water in approximately the same concentration irrespective of the sediment composition. On the assumption that copper sulphide colloids are dispersed in the interstitial water of the sediment, it is suggested that the copper content of gyttja should be expressed as μg/g interstitial water for meaningful interpretation. Similarly, the zinc content should be expressed relative to the organic matter content. The ability of reagents to selectively extract only chemically dispersed metals and not silicate lattice metals from gyttja is discussed. Both the analytical procedures and the interpretations are applied to the samples collected.