Page 240 - Handbook of Gold Exploration and Evaluation

Sedimentation and detrital gold 211

4.2.5 Background to sedimentation

Theoretical aspects of solids-fluid flow are mainly concerned with the ideal
conception of frictionless, incompressible fluids. In practice the rheology of
many of the fluids encountered changes according to variations in drag forces
due to viscosity. The mass density of water in proportion to the mass density of
the solids and their concentration by weight is increased by addition of dissolved
or suspended solids. Due to its content of dissolved salts seawater has a higher
mass density than fresh water.
The three principal fluids in alluvial gold settings and their approximate
densities are:
· fresh water w ˆ 1:00 gcm ÿ3
· seawater sa ˆ 1:25 gcm ÿ3
· air a ˆ 1:25 kgcm ÿ3 (dry air at sea level).
Air, broadly speaking, has an average unconfined pressure at the surface of the
2
Earth of about 1 kg/cm . However, because it is a mixture of gases, it has the
attribute of filling any container and can readily be compressed into a smaller
volume. The air pressure within the container then rises above that of the
surrounding air. Otherwise it has certain similarities to water by exhibiting a
definite viscosity.
The analytical framework and equations of fluid motion are based upon
investigation of one-dimensional flow in an `ideal' hence frictionless fluid. In
such conditions, stream flow is said to be either `steady' or `unsteady' at any
point in the fluid, depending upon whether the velocity vector changes or does
not change in either magnitude or direction with time. `Uniformity' of flow refers
to the lack of variation of the velocity vector with distance along a streamline.
Non-uniform flow is such that conditions involving the velocity vector vary from
place to place at any instant. In all cases the condition of steady, uniform flow,
even in laboratory scale apparatus, can be regarded only in the statistical sense.
Flow in natural stream channels takes place in a `real' fluid (water) which,
being essentially turbulent in nature, is both unsteady and non-uniform. The
flow is invariably `unsteady' because the magnitude or direction of velocity or
both varies with time. It is `non-uniform' because the velocity, measured from
one point to another in the direction of flow, changes with every change in
boundary geometry. At flood time, the stages of flow change instantly as the
waves and surges pass by and the rate of sediment transport past any one section
varies accordingly.

Viscous flow

Viscosity is the property of a fluid that gives rise to an internal shear stress
opposing change in the shape or arrangement of the elements of the fluid during
flow, and is the degree to which this property exists in a particular fluid. In