Page 259 - Handbook of Gold Exploration and Evaluation

230 Handbook of gold exploration and evaluation

· `Wash load', i.e., those particles held in suspension by fluid momentum
transfer alone, i.e., by random eddy currents of turbulence having velocity
current normal to the bed greater than the terminal velocity of the particles
relative to the surrounding fluid.
· `True suspensions' of particles (e.g., clays) that remain in suspension from
surface to bedrock and only settle in stilled water over long periods of time;
such particles are distinguished from particles that are held in suspension in
direct proportion to the energy of the stream at each point in the flow.
The wash load makes up the greater part of the total stream load and in active
stream channels its constituent particles are typically absent from the coarse
bed-load sediments. The presence of clay-sized particles in pay gravels at the
base of a fluvial sequence is thought to be due to clay-bearing ground waters
seeping through the gravels and eventually filling the spaces between individual
clasts. Lag gravels, though originally laid down in turbulent conditions in the
virtual absence of very fine sediments, may thus present major desliming or
lithification problems when mined.

4.4.3 Sorting and deposition
Fundamentals of the transport mechanism relate to sedimentary processes
involving the development of paystreaks in natural stream channels. Sorting and
deposition are time related and placer formation occurs at various stages along
the sedimentary train where local flow conditions provide suitable conditions
and time for gold and other heavy minerals to settle out preferentially to the
lighter particles. At normal flow rates only the upper sediments are disturbed
and the pebbles act as riffles to trap and hold back particles of gold. As flood
stage approaches, the stream velocity increases and the lower parts of the bed-
load are disturbed thus allowing the coarser gold grains to settle towards the bed.
Where flow velocities exceed the critical velocities for incipient cavitation
absolute pressures approach or equal the vapour pressure of the fluid. Cavitation
is a special condition that may occur in the upper reaches of streams at flood
stage. Varying levels of flooding result in small-scale features that are enriched
superficially at several positions on bar surfaces by the deposition of skim, or
flood gold. Skim bars are transient features and tend to be removed by scouring
during each fresh flooding.

Sorting
Selective removal of some particles occurs at and above a certain critical
velocity below which no movement can take place while gold, being of higher
density and smaller in size than the particles with which it is associated, tends to
collect within spaces between particles exposed to the flow. Entrapment of gold