Page 275 - Handbook of Gold Exploration and Evaluation

Sedimentation and detrital gold 241

Headwater tracts

Valley forms as produced by slope and channel processes operating on the
various substrates are dependent upon the local lithology and structures.
Headwater tracts are regions of erosion in which detritus is swept downstream
at relatively high velocity during periods of intense precipitation and run-off.
Drainage patterns reflect the manner of dissection of the surface rocks and the
ability of the environment to cope efficiently with intermittent run off and
surges of rock waste into the channel system. Rivers cut deep narrow gorges
through bedrock structures and ground surfaces that are typically irregular in
profile and variable in their resistance to wear. Marked changes in gradient,
which may be as steep as 1:5 and even more are represented by the
development of waterfalls, ponds and rapids where streams flow over rock
formations of varying resistance to wear. Waterfalls, which create sharp breaks
in the longitudinal profile, are characterised by the development of deep plunge
pools below the falls. The mechanism of undercutting and erosion of the
riverbed beneath the plunging stream of turbulent water undermines the falls,
gradually transforming them into rapids as nick-points advance up the valley.
The flowing stream abrades the rapids and degradation gives way to
aggradation as the profile gradually flattens.
The stream load comprises clusters of partly weathered gold-bearing detritus
and slope materials that are mobilised and fall into the channel during periods of
intense precipitation and thaw. Particles in transport tend to settle selectively out
of the flow according to size. Bed-load movement is climatically controlled and
any significant movement of large masses of rock debris in the upper reaches of
stream channels occurs only at high flood stage. Boulders and other large frag-
ments tend to vibrate in place at high flow velocities without forward movement,
and are only gradually reduced in size by natural attrition to more transportable
proportions. The coarsest gold grains tend to become trapped behind or under
rock bars and boulders or lodge in cracks and potholes in the channel floor.
Grooves and any ridging in the bedrock collect gold just as efficiently when
alligned in the direction of flow as when the flow is at right-angles, e.g. bedding
of the country rock. Very finely divided gold grains are carried out of the system
in turbulent suspension along with the clay and silts. The remaining gold is
caught up for a time in patches of sand and gravel that gather into unstable and
loosely sorted fractions on flatter sections of the bed or behind rock bars and
other transient features which disturb the pattern of flow. Materials deposited
during falling-stream stage are re-entrained at rising-flood stage.
Prior to tectonic and climatic change deposits in the headwaters of streams
are typically thin and discontinuous because of the ephemeral nature of the flow
and the steep and irregular gradients over which flow takes place. Such deposits,
though often quite rich and transient in nature, may not be of immediate
commercial importance except as possibly bonanza-type discoveries for small