Page 279 - Handbook of Gold Exploration and Evaluation

Sedimentation and detrital gold 245

size that they can no longer settle faster than the sediment with which they are
associated. Deposition tends to be unpredictable because of local changes in
flow resistance and stream energy.
Because of the variable nature of stream sediments and processes, it is clear
that no single facies model can be used to describe sedimentation in a fluvial
setting. Other local conditions that may change the character of streams include
bed roughness, variations in discharge, obstructions caused by falling trees along
river banks and differences in channel geometry due to changing bank or
bedrock lithologies. In this respect, Dyson (1990) has extensively reviewed
literature describing fluvial facies models and sites of gold placer deposition and
warns against adopting too rigid an assignment of one or other of the models to
any fluvial placer deposit.

4.5.2 Glacial deposition

As discussed in Chapter 3, 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 inter-
glacials. In response to the formation of ice sheets rapid atmospheric cooling and
ensuing cold climates increased mass wasting on slopes but decreased fluvial
transport in valleys, thus producing burial of many of the Tertiary placers. For
many Cainozoic placers glacial erosion then resulted in the development of
discontinuous valley margin paystreaks, which were either buried by renewed
mass wasting on slopes or dispersed and reconcentrated in other settings, e.g., by
shallow marine processes on beaches and platform areas. Existing channel
sediments derived from downcutting the rivers contained reworked gravels in
which the gold was typically redistributed in a much-diluted form. Only
remnants of palaeo-drainages now remain as terraces around valley walls.
In tracing the evolving pattern of secondary placer development over time,
data from primitive placer environments can be introduced into the basic model
in order to build up and finally elucidate the geological history of a promising
area and hence its resource potential. Glaciation and the high rate of sediment
formation by freeze-thaw processing follows long periods of deep chemical
weathering in tropic and sub-tropic environments. Processes of erosion, trans-
port and deposition are reactivated in direct response to renewed tectonism and
climatic change. A pause in uplift or tilting of the strata brings a variety of
changes such as the generation of elevated paystreaks within an aggrading
fluvial system and a tendency for the superposition of drainage, and stranding of
pre-uplift rivers and flood plains on uplifted plateaux. Each environmental
system thus produces some unique features; every environmental change in
some way modifies the existing forms. Changes include the glacial transporta-
tion or telescoping of pre-existing placers in very steep valley segments, and
secondary reconcentration in low gradient intervals during cyclical periglacial