Deposits Formed by Sedimentary and Surficial Processes - Metals and Society: An Introduction to Economic Geology - page 116

Table 5.3 Arguments for and against the hydrothermal and placer models for Witwatersrand gold
deposits
Hydrothermal model Modified placer model

Gold is late in the paragenetic sequence and is
associated with zones of hydrothermal alteration

Coexistence of rounded gold nuggets and
hydrothermal gold

Rounded pyrite grains and uraninite are of
postdepositional hydrothermal origin

Pyrite morphology, crystallography and
zonation patterns indicate detrital origin

Gold, and uranium, are closely associated with
pyrobitumen that was remobilized during
metamorphism

The association relates only to
hydrothermally remobilized gold and
uranium

Permeable conglomerate beds channelized the flow
of hydrothermal fluids

Strong sedimentary control on gold
distribution

Gold was deposited after sediment deposition Re-Os ages of gold are older than the

sedimentation

Gold was deposited during peak metamorphism Metamorphism remobilized pre-existing

detrital gold

Lack of suitable source of placer gold Surrounding granites and greenstones were

the gold source

Modified from Frimmel et al. ( 2005 )

rocks and carried along in rivers or ocean currents. Gold, of course, is very soft but
it is also highly malleable and ductile, which makes it durable in the sedimentary
transportation (Table 5.3 ).
In addition to being resistant in the surface setting, most of the ore minerals in
placer deposits are significantly denser than other minerals that are transported by
sedimentary processes. It is this characteristic that allows these minerals to be
sorted out from detrital minerals or rock fragments that make up most of the
sediment load and become concentrated in ore bodies. Table 5.2 shows some
examples: diamond is only slightly denser than detrital minerals like quartz or
feldspar, and only its extreme hardness allows it to accumulate. Gold is the counter
example, its density being six times greater than that of common rock-forming
minerals. Because of the high density contrast, sorting by river currents allows even
small particles of gold to separate efficiently from other detrital grains, to produce
placer ore bodies that contain gold concentrations that are greater by a factor of
several 1,000 than concentrations in normal continental crust. Sorting of moder-
ately dense minerals like zircon and rutile is less efficient and placer deposits of
these minerals are far less enriched, containing concentrations of Zr and Ti that are
normally only 10-100 times greater than in normal crustal rocks.

5.2.1 Gold Placers

Placer gold deposits have produced two thirds of all the gold that has ever been
mined. The easily won gold in the fluvial placers of California, Australia and
elsewhere was mined out very rapidly in the gold rushes, usually over a period of