Page 104 - Geochemical Remote Sensing of The Sub-Surface

Geochemical Remote Sensing of the Subsurface
Edited by M. Hale
Handbook of Exploration Geochemistry, Vol. 7 (G.J.S. Govett, Editor)
9 Elsevier Science B.V. All rights reserved 81
Chapter 3

SPONTANEOUS POTENTIALS AND ELECTROCHEMICAL CELLS

S.M. HAMILTON

INTRODUCTION

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.
A number of case studies have been published involving selective leaching of
samples taken over known mineralisation. It is apparent from this work that these
techniques have some capability to detect a geochemical response due to mineralisation
and other geological features through a significant thickness of rock or overburden.
What is less apparent, however, is the transport mechanism that moves elements to
surface from the source. Recent work in Quaternary glaciated terrain (Bajc, 1998;
Jackson, 1995; Hamilton and McClenaghan, 1998) has shown selective leach anomalies,
apparently related to bedrock features, above as much as 45 m of overburden sediments.
The young age (<10 Ka) and thickness of these deposits require that, at least in glacial
environments, a very fast transport mechanism operates.
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 mas~ transport and discusses the role of
natural geoelectrochemical processes in the formation of selective leach and
conventional geochemical soil anomalies.

This chapter is published with permission of the Senior Manager, Sedimentary Geoscience Section, Ontario
Geological Survey.