Page 268 - Handbook of Gold Exploration and Evaluation

Sedimentation and detrital gold 237

(Dyson, 1990) reflect the environment of deposition. Attempts to reconstruct the
morphology and flow characteristics of ancient systems rely heavily upon the
application of empirical relationships derived from modern streams. Galloway
(1985) illustrates the geomorphic and sedimentary characteristics of bed-load,
mixed-load and suspended-load channel segments and their deposits in Fig.
4.24. Table 4.5 is a classification of alluvial channels by the same author.
Evolution of topography as produced by tectonic uplift and volcanism at
convergent plate boundaries is controlled by valley entrenchment and extension
in the headwaters, and by the rejuvenation of streams and migration of
weathering fronts through valley systems to the headwaters. The most active
stages of orogeny produce steep irregular slopes in the headwaters of streams
and the rapid downslope movement of large quantities of partly weathered and
unsorted rock material. No significant development of gold placers takes place
on slopes at this time. Only during protracted periods of sediment transport and
sorting, are the effects of tectonic adjustments reflected in changes in the base
level of erosion of streams and hence in the consistency of rate of erosion of
valleys. Conditions favourable for the progressive liberation of gold from source
rocks and its concentration in sites of preferred accumulation at the base of
sedimentary sequences are portrayed schematically in Fig. 4.25. The most
productive streams (in terms of placer concentration) are developed where the
gold-bearing veins are distributed over the whole of the catchment area.

4.5.1 Paystreak development in unglaciated terrain

The concept of a model for the formation of paystreaks in unglaciated terrain
provides a genetic scheme for the development of gold placer settings in
sequence to upper valleys and middle and lower stream settings. Sediment
transport is a function of topography and is thereby time related. In terrains of
high relief transportation down very steep valleys involves intervals of deposition
in settings of optimum concentration potential. The sites of paystreaks for
unglaciated terrain are basinal intervals controlled by recessive lithologies in the
valley reaches. Downstream flattening provides progressively longer periods of
time for sedimentation and sorting at each basinal interval and for the
concentration of progressively more finely sized gold. This system envisages:
· a single stage of downcutting with minor crustal compensation, but without
abrupt changes in either base level or climate
· development of sites of gold concentration in basal gravel and bedrock
structures during intervals of stillstand
· a virtual state of equilibrium between the inflow and outflow of sediment
across each section of the deposit
· gold concentrations that typically become centrally located along the valley
floor as the valley widens.