Page 230 - Handbook of Gold Exploration and Evaluation

Sedimentation and detrital gold 201

4.4 Changes in sediment lithology in the terrace gravels of Battle Creek, Black
Hills, South Dakota (after Plumley, 1948).

of secondary films of gold on grain surfaces. Size increases may also occur
mechanically by impact and melding together of gold grains during transport or
by cementation of new grains by new gold. Samples must be sufficiently large at
every significant stage of deposition to ensure the recovery of representative
samples from all of the material to be evaluated.
Usually however, gold grain modification takes place by largely destructive
processes, e.g. physical deformation and surficial wear. Detrital gold grains are
degraded differently in different geomorphic settings (regolith, glacial, fluvial
and aeolean) depending largely upon the habit of the original grain. Any
prediction of rate of wear with distance of travel is highly speculative without
adequate knowledge of hinterland geology. Physical deformation includes
cracking and rounding of equant grains, compacting, pinching and folding of flat
grains, and folding and rolling of wire gold to form cigar-shaped particles.
Figure 4.5 describes some repetitious gold grain shapes from Kasongan,
Kalimantan, Indonesia. Shapes (a) and (b) make up 70% of the suite, and (e), (f),
(g), (h) 20%; the others 10%.
While the type of gross deformation and surficial wear of a particle of gold is
generally predetermined by grain morphology, the nature and energy of the
environment should also be considered:
· Fluvial regimes grade from high energy boulder-pool action with high impact
forces in the upper reaches of streams, to low energy-low gradient action and
mainly abrasive forces acting in lower channel sections.
· Under aeolean transport conditions deformation occurs by both impact and
abrasion under high-energy conditions.
· In a glacial setting comminution ranges from mainly abrasive due to high-
pressure contact with wall rock and basement, to mainly polishing during low-
pressure englacial transport; sub-glacial transport takes place in channels under