Page 154 - Handbook of Gold Exploration and Evaluation

132 Handbook of gold exploration and evaluation

and consumed during the same cycle of erosion. They are restricted to regolith
that is typically shallow and geologically short-lived. The total depth of
weathering and thickness of the residual regolith overlying an ore zone is
typically restricted to a relatively immature and transient surface layer of eluvial
rubble and a thin colluvial mantle of sandy-clay saprolitic subsoil, locally with
exposures of fresh bedrock.
The Strahler model of landmass denudation assumes that the average surface
elevation is reduced to one half every 15 million years as shown conceptually in
Figs 2.26 and 2.27. Within this scenario, an epithermal gold-vein system
emplaced in a zone 1 km below the ground surface would be subject to much
greater erosive forces than a mesothermal ore body emplaced at a depth of 5 km
below the surface. The epithermal orebody could be unroofed within a few
million years of uplift. The mesothermal orebody would probably not be
uncovered until at least 105 million years of uplift. It would also have a much
better chance of being deeply weathered chemically on flatter slopes prior to
mechanical erosion following renewed uplift. The Raigarh alluvial gold field in
Madya Pradesh, India, is an example of almost total erosion down to the lower
level of primary gold mineralisation. The Raigarh field occupies an area of about

2.27 Graph of increase in average surface elevation with time, as shown in Fig.
2.26. (From A.N. Strahler, Physical Geography, Harper and Row, Publishers.
Copyright by A.N. Strahler.)