Page 13 - Handbook of Gold Exploration and Evaluation

xii Preface

rejoin to form other and larger supercontinents. Many aspects of Earth's geology
and history are discussed, including the opening and closing of ocean basins,
origins of mountain ranges, geological structures, hydrothermal activity and
gold distribution in volcanic and tectonic settings. All past and present processes
relating to the modification and release of gold grains in the weathering
environment are shown to be paragenetically related to both primary and
secondary rock types of all ages, and to the local and regional geologic setting at
the time of their formation. Of particular relevance is recognition of a funda-
mental rock cycle in which all types of gold ores are seen as integrated parts of a
process in which the rocks are related to one another and can be transformed one
to the other.
Occurrences of detrital gold, saprolitic and lateritic-hosted gold ores are
integral parts of a weathering cycle in which the global energy balance is largely
dependent upon the influence of the Earth's oceanic-atmospheric heat circula-
tion. The effects of climatic change and extremes of change on stresses that
produce weathering and erosion have all played significant roles in the forma-
tion, sorting and deposition of secondary gold accumulations. Deep-seated
lateritic weathering became a topic of great interest to geochemists as well as to
geologists and new fields of gold exploration were opened up towards the close
of the 19th century. The long-term weathering of regolith has delineated
geochemical provinces and defined chalcophile corridors of strategic importance
to explorers.
The basic concepts of viscosity and boundary layer flows are examined as
they relate to sediment characteristics of size, shape and density and to sedi-
mentation in natural stream channels, and expressions are developed for
computing the relative magnitude of forces acting in solids/fluid flow.
Examination of the complex relationships of channel geometry leads naturally
to consideration of erosion, transport, sorting and deposition in the development
of alluvial gold concentrations. As a function of topography, sedimentation is
both space and time-related as demonstrated in the formulation of models, which
establish the relationships between accumulations of heavy minerals, the
dynamic conditions of transport and direction of sediment transport.
Ground geological surveys provide essential data for investigating the nature
and extent of surface and sub-surface ore horizons; both for open-cast mine
planning and treatment and for the exploration and development of any primary
orebodies that might be revealed as a result of the surface operations. Changes in
exploration methods have resulted from new theories of paragenetic relation-
ships and the development of remote sensing and geophysical techniques for
regional and sub-surface examination. From identification of the principal
sedimentary controls on the distribution of pay zones in some Alaskan gold
placers, the course is charted for further exploration of gold deposits in other
areas of Quaternary and pre-Quaternary glaciations. Geophysical surveys in
shallow offshore areas are typical of those used to investigate the extent of