Page 276 - Handbook of Gold Exploration and Evaluation

242 Handbook of gold exploration and evaluation

prospecting groups. They do, however, represent the first stages of placer gold
concentration and can provide valuable geochemical information of their source,
and of the possible size and value of larger concentrations further downstream.

Middle and lower stream settings

In contrast to conditions that promote net erosion in the upper reaches of
streams, the middle and lower tracts are regions of net deposition in which the
stream widens and flattens and high-energy flow and degradation gives way to
low-energy flow and aggregation. During periods of high discharge additional
energy is directed against the banks and the valley continues to widen as more
material is eroded and added to the bed-load. The abundance of sediment and the
high and sporadic nature of discharge cause the channel to be rapidly choked
with sediment. This results in lowering of the relative water level as sediment
builds up across the valley floor. The channel gradient is thereby decreased and
multiple connected anastomosing channels (braids) create a net-like formation,
with small islands (braid bars) located centrally within the net (Fig. 4.26).

Braiding
Interrelated parameters of a stream are its width, depth, velocity, slope and
transporting power. Individual changes occur and alluvial fans develop as a
function of discharge (total volume of water flowing through the channel in a
given space of time) and the quantity and nature (e.g., size) of sediment in
transport. Alluvial fans are typical features of stream channel sediments where
braided rivers debauch out violently from narrowly incised channels into a wide
valley or plain. `Sieve deposits' may appear as coarse gravel lobes on the fans
where the source supply contains relatively little sand, silt or mud. In arid

4.26 Main features of braided stream deposits.