Page 229 - Handbook of Gold Exploration and Evaluation

200 Handbook of gold exploration and evaluation

4.3 Increasing specific surface area to volume relationships.

when split up into one million 0.01 cm/side cubes is 1:600. For 0.0001 cm/side
clay particles, the ratio is 1:60 000 and is enormously large for very minute
particles (colloids). Suspensions of particles whose settling properties are
significantly affected by the slow settling of finely divided particles are loosely
defined as slimes in placer technology. The particle size at which this occurs
appears to be less than about 100 m for quartz density sediments.
Local sorting is a function of distance of transport and Plumley (1948)
demonstrated the variable rate of degradation of different sediment types in
natural stream settings by recording the downstream size changes of sediment in
600 samples of terrace gravels from Battle Creek in the Black Hills of South
Dakota. Figure 4.4 plots the results of these measurements. Chert, as the most
resistant of the minerals present in the gravel, was used as the standard of
reference in assessing the degrees of lithological change. The chert ratio R c is
defined as:
R c ˆ C=X ‡ C 4.2
where X is the percentage of other rocks and C is the percentage of chert in the
sample. The higher the chert content the more complete is the removal of other
rock types.

Gold grain-size modification

As shown in Chapter 1, corrosion (chemical weathering) of gold grains in an
alluvial setting may sometimes increase the size of gold particles, either geo-
chemically by supergene enrichment or electro-chemically by the overprinting