Selective leach anomaly

Hamilton, S.M. 1998. Electrochemical mass transport in overburden a new model to account for the formation of selective leach geochemical anomalies in glacial terrain. Journal of Geochemical Exploration, 63, 155-172. 

Selective leach techniques have become popular in mineral exploration for the treatment of geochemical soil samples. Their popularity stems from the fact that they are considered to extract selectively a particular hydromorphically-transported component of metals in the sample and, as such, show better anomaly-to-background contrasts than do conventional strong acid digestions which dissolve most of the chemical matrix of the soil. 

During the weathering process, elements can disperse from source mineralisation by a variety of chemical processes. For reasons discussed below, electrochemical processes are increasingly thought to be the primary transport mechanism in environments of thick, young, exotic (i.e., transported) overburden. They are also likely to operate in other environments but their dominance as a transport mechanism is less certain. This chapter presents the principles behind electrochemical masj transport and discusses the role of natural geoelectrochemical processes in the formation of selective leach and conventional geochemical soil anomalies. 

All four of the mechanisms described above probably operate to some degree at every site where mineralisation is buried and it is possible that any one of them could dominate in selected environments. However, the first three are precluded as major contributors to transport in thick, water-saturated Quaternary glacial environments. Since selective leach anomalies are now commonly reported in glacial terrain, electrochemical processes are likely to dominate in at least this environment. 

One outcome of the migration of ions fi om one redox region into another should be a zonation of elements in relation to the reduced column. Element zonation is a reported feature of selective leach anomalies (Clark, 1996). Zonation could occur due to a variety of processes, the most important of which would be progressive deposition of redox-active species as they migrate into or out of the reduced column. The migration paths of reduced and oxidised ions are predictable provided the current flow patterns can be inferred and therefore, if the redox behaviour of a particular ion is known, the shape of anomalies can be inferred. 

In addition to HIE, the melts are probably enriched in volatiles, notably in water. Uranium, strontium, and lead may indeed be selectively enriched in water-rich melts/fluids (Gill, 1990 Keppler and Wyllie, 1990 Erenan et al., 1996 Keppler, 1995 You et al., 1996). This might account for the frequent occurrence of positive anomalies of strontium (and lead—not shown) on PM-normalized diagrams, and for the U/Th ratios systematically higher than PM values, both in whole rocks (Figures 16 and 17) and in acid-leached minerals (Figure 21). 

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