Page 260 - Handbook of Gold Exploration and Evaluation

Sedimentation and detrital gold 231

grains takes place by these means when movement occurs by rolling and sliding
at low velocities. Grain size is a dominant feature and large sediment particles
are subject to preferential removal because of their projection into the flow.
Voids separating smaller particles will provide a large measure of protection
against movement of the gold until velocities rise sufficiently to disturb all of the
particles in the upper layers.
A typical sequence of bed forms develops with change in discharge.
Classified broadly as either low or high flow regimes these bed forms are related
to processes of erosion that occur in accordance with grain size and shape,
sediment loading, river stage and stream type when local critical entrainment
velocities are reached. Most interpretations are based on Fig. 4.20. Gold entrap-
ment occurs at the surface of the bed only in the low flow regime, which
includes all stages up to the formation of dunes. The upper flow regime is
associated with relatively low flow resistances and high gradient braiding, and is
represented typically in streams emerging from the confines of channel flow.
The division between low and high regimes in the above figure occurs during
transition from braiding to meandering.
The development of a rippled bed form and cross-bedded structures leads to
`dispersive' sorting and to rippled dunes as the main forms of transport, with
ripples tending to climb over the dunes. `Suspension' sorting is established when
the stream reaches its peak at velocities high enough to create anti-dunes, i.e.,
natural sand waves which travel upstream against the current. This occurs in the
upper reaches of streams when periodic flooding during periods of intense
precipitation and run-off disturb the bed load as a whole. Because of inertial
effects, the larger and denser particles lag behind the other sediments and settle
down into the bed to form concentrations in the lower gravel layers. The lighter,
flakier particles are hindered in their settling by rising inter-grain currents and
are carried further downstream.
In natural stream channels, the sorting mechanism relies upon differences in
the hydraulic behaviour of suspensions of a range of different particle sizes,
shapes and densities under conditions of unsteady and non-uniform flows that
vary both instantaneously and with time. Coarse gold is initially dispersed into
cracks and fissures in bedrock structures. Smaller and more transportable
particles are carried away in suspension following each base level adjustment.
Accumulations of winnowed gold accrue in depositional sites in bars and in
channels within the stratigraphic section downstream. Individual relationships
are categorised in terms of hydraulic equivalence, entrainment equivalence,
transport equivalence, and granular dispersion in shear flow.

Deposition
The preferential deposition of gold grains and other heavy minerals is usually
represented in illustrations of separated flow as a two-dimensional flow field. By