Page 184 - Handbook of Gold Exploration and Evaluation

160 Handbook of gold exploration and evaluation

aridity. Thus, while a general correspondence exists between orbital variations
and the waxing and waning of glaciers, the mechanisms of climatic change
remain elusive.
Short-term patterns of climatic change are associated with glacial and
deglacial stages of waxing and waning of ice sheets and alpine type glaciers and,
to a lesser extent, the warmer more equable climates of interglacials. For all such
conditions local aberrations are due to a variety of possible causes, e.g. blotting
out of sunlight by outpourings of volcanic ash, changes in albedo, changes in
oceanic circulation and disturbances of the Earth's motion. Cretaceous mean
global temperatures were probably similar in the tropics to those of today (around
32 ëC) but mean annual polar temperatures could not have been lower than the
freezing point of fresh water, or more than 12 ëC higher than today (Hay, 1987).
Glacial action could not have been significant except at high altitudes.
Successive episodes of glacial and fluvial reconstruction during Pleistocene
ice ages and humid interglacial intervals profoundly affected both local and
global weathering patterns. Events leading to Pleistocene ice ages were probably
set in motion about 20 million years ago with the Alpine orogeny which created
mountain ranges stretching from the Pyrenees and Alps in Europe through the
Caucasus to the Himalayas in Asia, and the Rocky Mountains and Appalachians
in North America. Glacial erosion resulted in the development of discontinuous
valley margin pay streaks, which were either buried by renewed mass-wasting
on slopes or dispersed and reconcentrated by shallow marine processes on
beaches and platform areas. Holocene climatic fluctuations provided at least
seven individual intervals of erosion representing separate episodes of placer
reconstruction in both warm and cold climatic conditions.
Much less is known of the older ice ages. Extremes of climate and climatic
change are represented by alternations of ice ages and interglacial intervals that
are cyclical over a wide range of geological timescales. Ancient glaciations
dating back to the Precambrian Huronian (2.0±2.5 billion years ago) indicate an
extensive period of glaciation shortly after the transition from Archaean to
Proterozoic time. Interglacial intervals appear to extend throughout the next 1±
1.5 billion years although there may have been some glacial episodes during this
period that have not yet been identified. Glaciations in the Late Proterozoic range
in age from 1.0 to 0.6 billion years. The widespread distribution of till from these
deposits suggests that Late Proterozoic global temperatures could have been quite
low. Eyles and Eyles (1992) broadly demonstrate the intermittent recording of the
Earth's glaciations in Fig. 3.13.

Depositional effects of climatic change

Possible effects of different processes acting upon alluvial gold deposition at
ground surfaces during change from one set of environmental conditions to
another are summarised as follows: