Page 289 - Handbook of Gold Exploration and Evaluation

Sedimentation and detrital gold 255

the distribution of values at any one time may be exceedingly complex.
Differences in sedimentary behaviour stem from changing base levels and
variations in stream power. The cyclical inflow of sediments may have been
different for different reasons and may not have been derived proportionately
from the same sources or at the same rate. During epeirogenic uplift the same
valley may be filled and scoured many times. In a typical sequence:
· Tributaries are rejuvenated as the weathering front moves up the valley.
· Surges of sediment brought down by the tributaries choke up the main
channel, which becomes braided.
· Paystreaks are developed in the channel lag.
· Renewed deep scouring takes place at the valley outlet and the sequence is
repeated until equilibrium is reached at the reduced base level.

River terraces

Terrace deposits are remnants of alluvial valley fill that now exist as bench-like
landforms in which streams have incised their way into the underlying rocks.
The deposits follow the course of the streams, approximating their gradients and
containing remnants of earlier placers deposited on the valley floors during
periods of tectonic stillstand characteristic of a temporary stay in downcutting.
Prior to a further strong tectonic uplift, terraces at each erosional level tend to
reflect a particular stage of stratification and sorting of gold grains within the
stratigraphic section.
In most areas affected by pulsatory epeirogenic uplift in Eastern Australia
and in the Pacific and Southeast Asia, the depositional sequence of placer gold
deposits is generally as follows:
· Recent deposits are found within presently active streams.
· Pleistocene gravels occur in terraces rising some 4 m to 10 m above present
stream levels.
· Pliocene gravels are represented in terraces at levels of 10 m to 60 m or so
above present stream levels.
Pliocene gravels and boulder beds are usually thicker and vary more widely in
depth than Pleistocene gravels. This denotes a more extensive range of pulsatory
uplift during the Pliocene, a longer period of weathering and a correspondingly
greater supply of both sediment and gold.

The Turon model
In the Turon Valley, NSW, Australia, pre-Miocene river gravel carried gold
derived from Pleistocene times. A post-Miocene uplift left these rivers and
outwash gravels stranded high in the Miocene peneplain. Some gold from these
gravels was fed into the ensuing Pliocene gravels below. A further uplift at the