Introduction - Metals and Society: An Introduction to Economic Geology - page 17

Also important is the geological situation. The largest Ni deposit we know of is
in the centre of the Earth. The core contains some 10 19 tons of Ni metal but it is
course totally inaccessible (except for the heroes of American movies). The depth
of a deposit has a major influence on the cost of mining. A shallow deposit can be
exploited in an open-pit mine, which is far cheaper than the alternative, an under-
ground mine, that must be developed if the deposit is deeper. Friable and soft
sedimentary ores are easier to mine and process than ores in hard magmatic rocks.
And finally a continuous and compact ore body is far easier to mine than an ore
body that is disrupted by faulting or other geological factors. Two platinum deposits
in southern Africa provide an interesting example. Those in the Bushveld Complex
in South Africa are near-continuous reefs that make the mining operation predict-
able and efficient, but deposits in another intrusion, the Great Dyke in Zimbabwe,
although of similar grade to the Bushveld deposits, are so irregular and disrupted by
faulting that mining had proved very difficult. And then the destabilization of the
country's economy by the present government has made the operation even more
hazardous.
1.5.4 Technical, Economical and Political Factors
As illustrated by the examples discussed above, economic and diplomatic issues
may strongly influence the viability of a deposit in some cases increasing its value,
in other cases detracting from it. The role of technology, on the other hand, is
generally positive. Only through improvements in the techniques used to mine and
process ore have we been able to extract metals from deposits with lower and lower
grades. One example of this tendency is the decrease in the copper grade discussed
at the start of the chapter. Another striking example is the reprocessing of gold ores
in Western Australia. The ores of the Coolgardie-Kalgoorlie region were first
discovered in 1893 and initially only alluvial gold was exploited. Underground
mining soon followed and in the early part of the twentieth century, vast waste
dumps from underground mining littered the surroundings of the growing
boomtowns. In the following century these dumps have been reprocessed three or
four separate times and each time gold that had previously been discarded was
recovered. The process was driven by sudden increases in the price of gold, notably
with the abandonment of the gold standard in 1971 and the more recent hike in the
gold price associated with the metals boom at the start of this century. But coupled
with these economic pressures were technological advances that allowed the
recovery of gold that was unattainable using earlier techniques. The most recent
involves in-situ leaching in which fluids, commonly containing gold-eating bacte-
ria, are allowed to percolate through the waste dumps. Other advances include the
development of more efficient mining methods, as best expressed in the vast open
cast mines that exploit large, low-grade, near-surface deposits of copper, gold, iron
and other metals.