Page 267 - Handbook of Gold Exploration and Evaluation

236 Handbook of gold exploration and evaluation

4.23 Surface of discontinuity and zone of deposition at confluence of two
streams.

solids. Textural variations result from changing velocities and stream com-
petency over the surfaces of the bars. High velocities and a coarse sediment load
in the deepest part of the channel give rise to bedded gravel horizons and cross
beds, which may, as meandering continues, create an underlying gold-bearing
lag zone. Large and medium sized cross beds in the mid-zone of a bar may also
carry anomalous gold values but are unlikely to be as rich in gold as basal lags
although there are exceptions. Individual layers are frequently separated by clay
`false bottoms' that represent overbank deposits, or deposits from ephemeral
bodies of standing water in lakes formed by a single base level drop.

Lag gravels
The highest gold values in any stream section are associated with lag gravels, so
called because they are the slowest moving constituents of the bed-load as it
moves downstream. Lag gravels may occur in any stream section that represents
a local region in which a temporal lag occurs between a change in flow and a
corresponding change in bed form. They are most extensive and continuous in
well-developed drainage channels in which the gravels are well graded and
where a general movement of much of the bed-load takes place at flood stage
without excessive scour. The nature of the facies produced by bar accretion is
particularly relevant to their gold content and its distribution. High-grade zones
(paystreaks) develop in lag gravels by the entrapment of gold in bedrock
structures along the valley floors.

4.5 Fluvial gold deposition

A normal approach to the study and exploitation of fluvial gold deposition lies in
the formulation of models establishing the relationships between accumulations
of heavy minerals, the dynamic conditions of transport and the direction of
sediment transport. Texture, and sedimentary structures, geometry and facies