Page 224 - Handbook of Gold Exploration and Evaluation

4

Sedimentation and detrital gold

Processes of entrainment, transport, sorting and deposition in natural stream
channels depend as much on the physical properties of the sediment as they do on
the hydraulic characteristics of the flow. Hydraulic properties are typical of the
individual particle, of particle distribution and of the sediment in bulk. Individual
particles are irregularly shaped and of diverse size and distribution. Fluvial
drainage systems are described briefly in respect of the effects of various shapes
and patterns of stream channels on sediment transport and deposition.
Discussions of bed-load and suspended-load as the two most distinct modes of
transport serve as an introduction to practical aspects of fluvial gold placer
formation. The initial development of gold paystreaks takes place in stream
channels during a single stage of downcutting and before tectonic and/or base
level change can produce entrenchment, recycling and reconcentration. The
effects of Quaternary tectonism and associated climatic adjustments are reflected
in changes in the base level of erosion, hence of the consistency of rate of erosion
of valleys. Quaternary adjusted placers of economic significance are preserved as
deep leads under outpourings of basaltic lava or are reconstituted in other forms
in fluvial, fluvio-glacial, fluvio-aeolean and shallow marine settings.

4.1 Sediment characteristics
Sediment comprises solid particles and grains of rock material that have been
eroded from their parent bodies in a depositional environment. Quartz and other
silicate minerals, as the most common and durable constituents of sediment at all
stages of transport may survive throughout several erosional cycles. Less stable
minerals such as calcite, pyroxene and sulphide minerals (pyrite, arsenopyrite,
chalcopyrite, etc.) weather rapidly and their presence in an alluvial train usually
denotes a very close source. The wide variability of gold shape and density is a
major factor in predicting the behaviour of very small quantities of gold grains
in transport with very large quantities of stream sediments.
Physical properties of sedimentary particles usually reflect many of the
parent qualities of grain size, shape and density, modified according to the