Page 182 - Handbook of Gold Exploration and Evaluation

158 Handbook of gold exploration and evaluation

Present understanding of the effects of climatic change on the burial,
dispersal, recycling and re-working of pre-existing placers relates mainly to the
erosion and recycling of Tertiary placers during rapid changes in the Pleisto-
cene. The best-known gold placers in North America were formed in Eocene
times when global climatic conditions were humid and very hot; tropic and sub-
tropic rain-forests cloaked most of the northern hemisphere and prolonged
periods of deep weathering and erosion led to the large-scale development of
alluvial goldfields. Down cutting of streams due to tectonic uplift was extensive
and pay streaks were formed characteristically in valley centre positions as lags
at the base of gravels and other bedrock features. Volcanic activity along the
edge of the Pacific volcano belts then produced a cover of rhyolitic tuffs,
andesitic breccias and basalt lavas to depths of up to 500 m. Renewed uplift and
tilting in late Pliocene±early Pleistocene times caused streams to cut deeply into
the volcanic rocks. In response to the formation of ice sheets, rapid atmospheric
cooling and ensuing cold climates increased mass-wasting on slopes and
decreased fluvial transport in valleys, thus producing burial of many of the
alluvial goldfields.

Palaeoclimatology
The older the rocks, the sparser their distribution and the more problems are
involved in predicting the patterns of climate of their times. For example, the
Precambrian Eon (+4.8±0.6 Ma) includes almost 90% of geological time, yet
about 80% of Precambrian rocks have been removed by erosion, the remainder
have either been extensively altered by metamorphic processes or lie buried
beneath younger sediment or volcanics. It was probably not until about the end
of the Archaean Eon that the Earth's climate first experienced the cooling effects
of large continental masses. However, large cratons continued to evolve through
to the Proterozoic Era and this may have been the approximate stage of division
of the Earth's climate into both warm and cold periods. The extremes of climate
are represented by ice ages on the one hand and by warm equable climates on
the other. The range of average annual temperatures between ice ages and
sustained interglacial intervals appears to be about 10±12 ëC.
Investigation of ancient world climates is based upon evidence of geo-
chemical changes in the properties of the atmosphere, spatial and temporal
variations of latitude, altitude and location of land and sea, influence of oceanic-
atmospheric circulation on the global energy balance and evolution of plant and
animal life. The breakup of Pangaea, which commenced in the Mesozoic era,
was accompanied by the creation of major mountain chains throughout most of
the world. Initially, because of the distance of most of its terrain from an ocean,
arid conditions prevailed over most of the land. Changes then occurred gradually
as rifting divided the land and seawater inundated low-lying areas of individual
continental masses. Rivers and lakes formed in previously arid areas and sea