Page 277 - Handbook of Gold Exploration and Evaluation

Sedimentation and detrital gold 243

climates, detritus in fault-bounded areas is moved by infrequent flooding and
accumulates in fan-like structures, which coalesce at the base of mountain
ranges to form extensive sloping plains or bajadas (Spearing, 1974).
Intermittent torrential rainfall in humid climates gives rise to sequential mud-
debris flows in intermontain basins and coastal areas. Mud/debris fans are
developed comprising mud layers inter-bedded with layers of sand and gravel
with occasional very large boulders. Traction current activity during periods of
relative tectonic calm establishes a partial upgrading of gold in streams that
traverse the fan surface. Depositional processes are typical of braided stream
sediment facies in which steadily decreasing levels of stream competence are
reflected in a transition from mainly coarse to mainly fine sediment sizing away
from the apex of the fan. As the fans grow, the larger channels divide into
networks of distributaries, and sites of deposition change from one side of the
fan to the other.
Braid bars and islands are built up by lateral and vertical accretion and are
predominantly lenticular in shape although otherwise of varying dimensions.
With continual working and reworking in changing flow conditions, longitudinal
bars migrate in the direction of least pressure by eroding sediment from the
upstream ends of bar the and depositing it in the lee of the bars downstream.
New bars are created as others disappear but ultimately, all of the material is
moved downstream. A steady reduction in particle size makes for better sorting
and gradually decreasing stream velocity results in the grading of each sedi-
mentary unit from coarse to fine upward. Although occasionally bars become
stable for a time when silt deposited during flooding is covered by vegetation the
structures are essentially transient.

Meandering
A not very well understood feature of fluvial channels is alternation between
braiding and meandering and conditions that influence the accretion of braid and
point bars and their style of mineralisation. In general, braided rivers differ from
meandering rivers by having steeper gradients and a coarser sediment load.
However, frequent alternations from braiding to meandering occur in streams
traversing alluvial fans where gradients change rapidly; in some cases a
steepening gradient may lead to braiding in an otherwise meandering stream
section. Gradation from braiding to meandering may also occur locally in a
valley that flattens and widens sufficiently for braided streams to meander
freely. Such streams are typically low gradient with moderate sediment loading
of mixed size range and moderate fluctuations in discharge.
Historically, it was thought that meanders were initiated by such factors as
rotation of the Earth, obstructions in streambeds and the Coriolis effect.
Although these factors may contribute to the development of meandering
channels the control of meandering appears to be related mathematically to the