Page 265 - Handbook of Gold Exploration and Evaluation

234 Handbook of gold exploration and evaluation

4.21 Flow separation across rock pools and bars on a streambed; formation of
vortices and scour.

these means, the concept of streamlines can be used to depict the division
between recirculating flow and external flow where a boundary layer separates
from a solid surface and enters the flow as a free shear layer. Re-attachment of
separation streamlines may occur in the body of the flow or at some point on the
solid surface downstream.
Practical examples of the preferential settling of gold particles across pools
and bars on a streambed are shown in Fig. 4.21. In this figure, a rock bar cutting
transversely across a stream illustrates the effects on separation of local pressure
gradients higher than the general gradient for the stream. The flow is not
disturbed upstream of the bar but increased pressure during an active river stage
leads to re-circulation of the flow and a slow moving vortex is developed as a
closed loop in the zone of separation. In the lee of the bar, the abrupt change in
flow velocity provides enhanced conditions for backward flow over a wider range
of flow rates. An eddy is formed where flow passes over a sharp edge and the
sudden fall in velocity causes the flow to re-circulate in a roller or closed loop.
The phenomenon of separation for low velocity flow passing across a transverse
slot or hollow in the surface of the bar is illustrated in the same figure. Deposition
of gold and other heavy minerals occur predominantly in the most active zones of
flow separation and paystreaks may occur on both sides of the bar.
Depositional sites described in Fig. 4.22 (a) and (b) show the influence on the
preferential settling of gold of different sized particles cropping out on a
streambed. In (a) the disturbance is small around a small pebble and in practice
the effects are similar to the settling of heavy minerals under aeolean flow
conditions where the sites of deposition are in flow shadows surrounding the
obstruction. In (b) the swirling of waters around boulders and other large objects
sets up eddies and velocity fluctuations affecting settling and entrapment; the
larger gold particles are caught under the edges of the boulders, the finer
particles are swept away to be deposited in less turbulent stream conditions
downstream.
Flow separation occurs at the confluence of two streams. Since no equality of
pressure exists along the surface of discontinuity separating the two streams, the
velocity of flow must differ on the two sides. The direction of flow is also
different on the two sides and between them these features result in an abrupt
longitudinal discontinuity in the velocity and one in the transverse section (Fig.