Page 42 - Handbook of Gold Exploration and Evaluation

Nature and history of gold 23

1.6 (a) Sketch of polished section of gold grain showing two different
consecutive generations of rims. The high-grade film around the upper surface
of x has a lower Ag content (3.96 avg. wt %) than the one on its lower surface,
or on the upper surface of y (9.92 avg. wt %). At the contact between x and y.
(b) Sequential diagram representing four major stages (1 to 4) of the evolution
of a gold particle in a placer: (i) side view, (ii) front view, and (iii) bottom view.
At stage 4iii the shape of the gold particle is similar (upside down) to that of the
grain in (a) (from Giusti, 1986).

grain is irregular in shape at the moment in which it is introduced into the placer,
the morphological evolution presented in the sketch is even more likely to occur.
Sketches of detrital gold grains recovered from Olipai flood sediments in the
Lakekamu Embayment, Papua New Guinea by the author of this text are shown
in Fig. 1.7.
Some geologists suggest that biochemical corrosion of gold by plant life is
responsible for the entrapment of gold by chemical or biochemical means both
inside and outside hyphae of plants or by both. Observation of `hair roots' within
openings in gold grains in some geochemical soil anomalies is thought to be