Page 31 - Handbook of Gold Exploration and Evaluation

14 Handbook of gold exploration and evaluation

1.2 Two examples of gold grains in a primary state.

and associated minerals. Gold tends to be coarse grained when deposited in
quartz veins with only small amounts of pyrite, but may occur in an extremely
finely divided state in massive pyrite or other sulphide ores. Figure 1.2 shows
two examples of the many ways that gold may occur in the primary state.
Gold grains in alluvial settings are distinguished by contrasting morphologies
that reflect chemical and physical conditions in depositional environments that
include regolith, glacial, fluvial, aeolean and shallow marine. When liberated,
the type of gross deformation of a gold grain that the grain will undergo is
predetermined by its original morphology. Pounding of equant grains produces
flattening and cracking; flat grains are folded and pinched; elongated grains,
which are transported by rolling, tend to develop into cigar-shaped particles.
Lobate projections may represent gold grains that have been rounded by later
supergene processes involving dissolution and re-precipitation of gold with
limited fluvial or glacial transport. Table 1.6 summarises morphological and
source rock relationships of some Asian placers.

Surface texture
Surface texture is a product of dynamic physical and electrochemical processes
that control the modification and overprinting of secondary films or patinas of