Page 296 - Handbook of Gold Exploration and Evaluation

262 Handbook of gold exploration and evaluation

Guinea and damming of the Bulolo Valley, gold-bearing tributaries of the
Bulolo River included Big Wau Creek, Koranga and Namie Creeks and Edie
Creek. The formation of a lake created lacustrine deltaic conditions, which led to
formation of the Bulolo placer. Dredgeable ground in this deposit extended 6.4
km downstream to the junction of the Bulolo River with the Watut River and 4.3
km along the Watut. Production statistics show a recovery of 2.13 million oz. of
3
fine gold from some 207 million m of gravel between 1931 and 1967. A typical
cross-section of one of the shallower valley sections is shown in Fig. 4.38.
Although good sample values were obtained in drill samples down to 90 m in
depth (Fisher, 1935), dredging was constrained to about half that depth because
of limited dredger capabilities. The presence of the water table near to the
surface precluded dry stripping.

Shallow marine placers

The sediments of shallow marine placers are derived from source rocks either on
the land or below present sea levels along continental shelves. Gold-bearing
gravels derived from on-shore provenances may reach the sea front for further
sorting only where the source rocks occur near the coast and are drained by steep
gradient streams, or where transportation is by ice flow or glacial telescoping.
Gold grains deposited glacially at shorelines comprise all sizes from coarse to
fine. The larger particles remain close to the shore but are less well sorted than
gold in stream placers and rely upon wave action for further upgrading. The very
fine particles are carried out to sea by wave action. Initial gold grades need not
be high, but grain size is important. Small isolated occurrences of gold found on
the beaches at Yamba in NSW, Australia between 1870 and 1885 could not be
recovered economically but they led to the mining of much larger concentrations
of heavy minerals (rutile, zircon, ilmenite, etc.) with gold as a by-product.
Beach placer concentrations are formed at the base of frontal dunes on open
beaches and in natural traps as provided by headlands and other barriers to the
flow of longshore currents. The movement of the sea gradually sorts the beach
sands, directing the finer particles into deep water and the coarser materials
towards the shore. Because of its high density, gold becomes concentrated along
with the other heavy minerals and coarser sediments. The final distribution of
values is influenced by the differential sedimentation rates of the particles and
by the strength and direction of the wind, waves and ocean currents.
Present evidence suggests that shelf areas were exposed to atmospheric
weathering for only brief periods of time during Pleistocene interglacial
intervals. The most recent exposure may have occupied less than 25,000 years
and earlier interglacial intervals were probably of similar short duration. During
the course of the Holocene Marine Transgression, which commenced about
10,000 years ago, sea level rose in a series of oscillations from a low of minus
130±160 m up to its present level.