Page 179 - Handbook of Gold Exploration and Evaluation

156 Handbook of gold exploration and evaluation

placer formation. Arid and sometimes desert conditions, which result from the
dehydration of air streams that continue into the interior, are not conducive to
placer formation.

3.2.3 Climate

A statement of the climate of a designated region includes measurements of the
above components, summarised over long periods of time. Account is taken of
averages and departures of means of values obtained and of probabilities that
extreme climatic conditions will recur at roughly predictable, e.g., 50- and 100-
year intervals. Such a statement is an essential part of any evaluation exercise
and requires careful analysis of both short- and long-term effects of these
extremes. Climatic variables include air and ground temperatures, as measures
of available heat energy; diurnal and seasonal transfer of temperature through
local winds and oceanic circulation patterns; and types and rates of precipitation.
Based upon these three sets of variables, climates may be broadly classified
within three morphogenic zones: glacial-periglacial, desert, and humid tropic.
There are many sub-divisions of climate within these groupings. Many geo-
graphers favour the Koppen Geiger Kohn system, which classifies five principal
and eight sub-groups on the basis of meteorological records; equivalents for the 13
climatic types are given in Table 3.5. Geologists usually prefer a simpler
classification in which the term `selva' is used to describe an equatorial rain-forest
environment; maritime refers to continental margins, and moderate to temperate
climates to conditions in regions between torrid and frigid zones. These climates
are discussed specifically in this book in relation to the geomorphic history of
regions of residual and placer gold deposits. Of particular importance are the
effects of climatic change and extremes of change on stresses that produce
intensive mass wasting and glaciation on the one hand, and fluvial transport,
sorting and deposition on the other. Either directly or indirectly and regardless of
age, these changes have all played significant roles in the formation, modification,
reconstitution and/or dispersal of detrital gold accumulations. The nature and
extent of adjustments after secular decay is examined in Chapter 4 in relation to
both long-term and short-term tectonic and climatic change.

Climatic cyclicity
Predictions of ancient patterns of climate are based mainly upon present-day
knowledge of atmospheric physics and the existing geological record is based
largely upon ages that are constrained by radiometric dating. Most of the earlier
evidence is fragmentary; it is only during the past 650 million years of Earth
history that abundant fossil evidence has shown a relatively even fluctuation of
climates between geologically short glacial-interglacial intervals and the long
intervening periods of ice-free poles and warm equable climates (Hay, 1987).