Page 105 - Geochemical Remote Sensing of The Sub-Surface

82 S.M. Hamilton
GEOCHEMICAL TRANSPORT MECHANISMS

Transport processes resulting from the chemical weathering of mineralisation involve
the dispersal of elements down gradients of one kind or another, including chemical,
temperature, piezometric (both gaseous and aqueous) and electrochemical gradients.
These mechanisms have been invoked as components of the numerous transport and
dispersion concepts that have been used by researchers to account for soil geochemical
anomalies overlying mineralisation. The models used in the past can be grouped broadly
into those that rely on: (1) diffusion; (2) advective groundwater transport; (3) gaseous
transport; and (4) electrochemical transport. Combinations of these are also possible.

Diffusion

Diffusion along concentration gradients has been named as a possible cause for
observed soil anomalies over mineralisation (Govett and Chork, 1977; Smee, 1979,
1983). Smee (1979) calculated diffusion rates and total diffusion distances for a number
of ions through glaciolacustrine clay for an 8000-year period. The calculations were
based on measured parameters or reasonable approximations. None of the ions was
calculated to travel more than 10 m, with the exception of H § which would have
travelled 28 m. Smee (1979) concluded that diffusion was too slow to account for soil
anomalies in Quaternary glacial units thicker than 5 m, whereas anomalies spatially
related to mineralisation have been noted overlying thicknesses of up to 45 m of young
glacial sediments.
Hydrogen ions and other ions are likely to be the product of oxidation of
mineralisation, which requires the downward diffusion of oxygen. This would decrease
the speed of the process by limiting the formation of ions at the bedrock surface and
thereby limiting the development of a concentration gradient between bedrock and
ground surface. This further precludes chemical diffusion as a major contributor to the
formation of selective leach anomalies in thick, young, exotic overburden. Diffusion
becomes a more likely transport mechanism with increasing age and/or decreasing
thickness of the overlying material.

Advective groundwater transport

Bolviken and Logn (1975) and Smee (1983) include groundwater transport as a
possible mechanism for element dispersion from mineralisation. Webber (1975) pointed
to the much higher potential migration rate of groundwater as compared with that of
diffusion or electrochemical transport and concluded that advective groundwater
transport is likely to be the most important dispersal mechanism.