Electrochemical mass-transport in overburden: a new model to account for the formation of selective leach geochemical anomalies in glacial terrain

Abstract

Recent studies have reported the presence of geochemical anomalies spatially related to mineralisation but which overlie thick glacial overburden. Existing models developed to account for similar anomalies over thin overburden do not adequately explain their presence in thick, young overburden, and to date no new model has been advanced that does. Problems with the existing models are discussed and a new theory is advanced that proposes electrochemically induced mass transport to account for the presence of geochemical anomalies over thick glacial drift. An upward increasing redox gradient exists in most surficial geological materials. Sub-vertical electronic conductors such as graphite or metallic sulphides in bedrock can provide a `short-circuit' route across this redox field between reducing agents abundant at depth and oxidising agents abundant in shallower areas. As electrons move up the conductor, oxidising agents in overlying overburden are consumed and a negative redox anomaly develops above the conductor relative to surrounding overburden. High redox gradients in this area induce the rapid migration of reduced anions away from the top of the conductor resulting in the development of a redox-front between reduced and oxidised areas. This front continues to migrate outward and upward at a quantifiable rate that far exceeds that of chemical diffusion until it encounters a continuous source of oxidising agents. The final result may be the propagation of the redox anisotropy to ground surface and the development of a permanently reduced `column' between bedrock mineralisation and surface. Higher redox gradients between mineralisation and the ground surface at the edges of the `column' relative to its centre result in the focusing of ionic current at the edges. `Rabbit-ear' anomalies in surface materials could result from both upward movement of reduced anions to the flanks and inward movement of oxidised cations. Depletions in soil media for iron and manganese and co-precipitating elements are expected in the centre of the column due to their higher mobility in reduced environments.