Page 246 - Handbook of Gold Exploration and Evaluation

Sedimentation and detrital gold 217

4.3.1 Channel networks

The basic pattern of drainage is established and channel networks develop most
rapidly during early stages of denudation. Subsequent changes are due largely to
headward erosion and expansion of tributaries. Effectively, the networks are
open systems in which the inputs are water, energy and sediment. The outputs
are losses of energy expended in friction, erosion and transport; sediment
deposition in flood plains and deltas; and water discharged into lakes and
oceans. Channels aggrade as a result of net sediment deposition when changes in
channel geometry are inadequate to cope with increased sediment loadings. The
same channel beds erode and downcutting occurs when the capacity of the
stream to transport sediments is greater than needed to deal with the actual
sediment input. Network efficiency is determined by how quickly the channels
adjust to any changing flow conditions.
Individual channels are descriptive of the nature of the underlying rocks and
may be interpreted to assess the texture of a landscape and to identify features
pointing to differences in rock type and structure associated with possible
metallogenic settings. The most productive channel systems (in terms of placer
concentration) are developed where exposed primary gold-bearing veins are
controlled structurally in relation to basin development. Figure 4.12 illustrates
the effect of multi-source vs. single-source provenances on the possible extent of
placer deposits formed in main trunk streams. Major alluvial goldfields such as
those in California and Australia are all associated with drainage systems having
extensive and closely spaced networks of tributaries in individually confined
basin settings. Parameters relating to drainage density and stream frequency are
interrelated features; the density of a drainage network is the total length of

4.12 Effects of single-source and multi-source gold provenances on patterns
of paystreak development.