Deposits Formed by Sedimentary and Surficial Processes - Metals and Society: An Introduction to Economic Geology - page 122

is highly prized, or stays locked up in the vaults of central banks. Gold is also
a “crisis metal”, a refuge that investors seek in times of economic turbulence.
During 2008-2009, as the price of ferrous and base metals plunged, the price
of gold approached record heights. The high values have been maintained
during the following years of global uncertainty.
How can we balance the positive and negative aspects of gold mining?
Discuss the geological, economic and ethical aspects of this activity.
Although we have chosen to describe the deposits of the Witwatersrand basin in
this chapter of deposits in sedimentary rocks, the origin of these deposits is
controversial. There are two competing hypotheses for the origin of gold, the placer
model and the hydrothermal model . Arguments for and against each are
summarized in Table 5.1 .
According to the placer model, detrital grains of gold and were transported into
the basin and deposited in the matrix of the conglomerates. Figure 5.2 shows
rounded grains of detrital pyrite in the Witwatersrand conglomerate. Under the
modern oxygen-rich atmosphere these grains would be replaced by iron oxides and
could not survive in fluvial conditions. Their presence is taken as evidence for more
reducing conditions in the later Archean.
According to the hydrothermal model, hot H 2 O-CO 2 fluids, perhaps derived
from dehydration of metavolcanic rocks beneath the basin, flowed along the
permeable conglomeritic horizons and deposited gold and other minerals in the
pore space of these sediments. Proponents of both schools agree that hydrothermal
fluids did indeed flow through the conglomerates on one or more occasions: the
hydrothermal school believes that most or all of the gold was introduced in these
fluids, whereas the placer school believes that these fluids caused relatively minor
recrystallization and redistribution of gold of detrital origin. Because of this remo-
bilization, the latter model is commonly referred to as the “modified placer model”,
as in Table 5.1 . Both groups have developed numerous lines of evidence and yet,
despite many decades of research and argument, the issue is still not resolved. Just
as with another major South African ore type - the chromite and PGE deposits of
the Bushveld (Chap. 3) - neither the fame of the deposits, nor their economic
importance, nor long years of research, have been able to resolve outstanding
questions surrounding the origin of the ores!
5.2.2 Beach Sands
Most of the world's supplies of titanium and zirconium come from concentrations
of heavy minerals in beach sands. These elements are known “high-technology” or
“space-age” metals because their high strength-to-weight ratio makes them very