Page 155 - Handbook of Gold Exploration and Evaluation

Geology of gold ore deposits 133

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1,000 km and has yielded large quantities of placer gold for hundreds of years
with no evidence or record of underground workings.
With decreasing relief the stability of slope material increases in response to
the decreasing amount and size of the material in transport. Slopes then tend to
be provided with increasing levels of plant growth and in regions of heavy sub-
tropical rainfall a profusion of tree and plant roots shatter the near-surface rocks
thereby increasing infiltration and decreasing surface run-off. The crumbling
effects of plant growth and biological weathering processes increase contact
between rock and circulating water and expose the surface rocks to deeper and
more extensive alteration. Ultimately, the topography will be so lowered in level
that only a low undulating land surface remains and sediments of all types may
overlie a deeply weathered regolith. In such regoliths, the chemical mobilities of
elements are determined by the stability of their primary host minerals, the
presence or absence of secondary host minerals and the changing weathering
environments to which they have been subjected. Differentiation of the remnants
of weathered ore zones from the rest of the landscape is made more difficult by
the continued action of other processes, which continually modify the landscape.