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Drawing on existing analytical techniques,
this project developed a reliable way to detect minute amounts of
gold dissolved in water from streams, springs and boreholes. This
has proved to be a valuable addition to previous ways of locating
gold deposits, especially as it can pinpoint the source of the gold
to within a few hundred metres.
Fieldwork in Sardinia and the UK has shown that the new technique
agrees with previous prospecting methods and can also find gold in
areas where other methods are less useful. The results suggest than
an integrated multidisciplinary methodology is an efficient prospecting
tool for gold. A new gold mine in Sardinia, due to start production
in early 1997, is commercial proof that the new techniques work.
To the distress of health-conscious consumers,
modern analytical techniques are revealing a growing list of undesirable
substances in our drinking water. Alongside the nitrates and pesticides,
however, is gold, and a new way of tracking gold in streams and
boreholes is already helping to pinpoint commercial quantities of
this valuable natural resource where it was previously unknown.
The value of the techniques developed by the partners in project
PL 119 will become obvious to the outside world in April 1997, when
production starts at a new gold mine near Furtei, in southern Sardinia.
In fact, Progemisa SpA, a natural resources management company funded
by the Sardinian regional government and an industrial partner in
this project, has set up a new company, Sardinian Gold Mining SpA,
with two Australian partners, for the exploitation of Sardinian
gold deposits. During its operational life the mine is expected
to produce several million dollars' worth of gold for its Australian
and Sardinian owners. In northern Sardinia, near Osilo, another
gold deposit is in an advanced exploration stage and the feasibility
study should be completed in 1997. The ore at Furtei and Osilo was
also located with the help of the new water-analysis techniques.
Looking for the invisible...
The story starts in Sardinia at the end of the 1980s, when earth
scientists discovered outcrops of hidrotermally altered andesite,
a rock often associated with gold. For centuries Sardinians have
mined lead, zinc, copper and silver, but not gold. Now the scientists
realised that the island might also have deposits of 'invisible
gold' - ore containing such tiny particles of the metal that they
cannot be seen even under a microscope. A tonne of 'invisible gold'
ore may contain only two or three grammes of gold, but this is still
enough for commercial mining if the deposits are large.
Especially interested in the discoveries were two local organisations,
PROGEMISA and the University of Cagliari. PROGEMISA, a company part-funded
by the Sardinian regional government, is involved in research and
the management of natural resources including metal ores. Researchers
from PROGEMISA and the university decided to develop a novel technique
to help them look for the gold.
Prospectors have traditionally relied on analysing samples of rock,
soil and stream sediments to locate gold deposits. Geological maps,
geophysical prospecting techniques, and more recently satellite
images, show the areas where gold-bearing rock is most likely to
be found. The Sardinian researchers reasoned that analysing water
from streams and boreholes might give them an extra tool to help
in the search. They started this project to gain help and experience
from other European experts in geochemistry.
...and analysing the scarce
Gold is almost, but not quite, insoluble in water. As it leaches
through gold-bearing rock, groundwater picks up gold in solution
and suspension and this can be detected using the most sensitive
analytical techniques currently available. In practice, the amount
of gold found in the water was in the range 0.4-10 nanogrammes per
litre. 10 ng/l is equivalent to a grain of gold measuring half a
millimetre across in a 25-metre swimming pool full containing 100,000
litres of water.
The researchers used three techniques to measure the gold in their
water samples. At the University of Cagliari they used an ion-exchange
resin followed by extraction with an organic solvent to concentrate
the solution, which they then analysed using a graphite furnace
atomic absorption spectrometer (GF-AAS). To check their results
the Sardinians enlisted the help of two other institutions: the
University of Antwerp and the British Geological Survey.
The British Geological Survey used a different preconcentration
procedure followed by inductively-coupled plasma mass spectrometry
(ICP-MS). The researchers at the University of Antwerp took a different
tack; instead of measuring the dissolved gold, they filtered the
water samples and studied the resulting sediment using neutron activation
analysis (INAA).
Prospectors had tried looking for dissolved gold before but with
little success. The key to this project was threefold: highly sensitive
analytical techniques that register even trace amounts of gold;
analysis of the suspended matter, which had not been done previously,
and which can sometimes detect gold even when it cannot be found
in solution; and the use of other elements such as antimony and
arsenic as clues. These elements often accompany gold deposits,
and can be present in groundwater in higher concentrations than
the gold itself. They thus form 'pathfinders' showing where gold
is likely to be found. Because the new analyses reveal a wide range
of minerals other than gold, they are known collectively as hydrogeochemical
techniques.
Sifting through the evidence
The partners carried out field studies in two areas of Sardinia
and five areas of the UK, in Wales and Scotland. The new analyses
showed gold everywhere it had been expected and in several unexpected
places too. The three different types of water analysis agreed both
with one another and with conventional analyses of rocks and sediments,
confirming that hydrogeochemistry is a useful addition to the prospector's
armoury.
The researchers found that dissolved gold does not travel very far
from its source, in most cases less than 500 metres. This means
that water analysis is especially useful when used in conjunction
with other prospecting techniques that give a wider picture. The
ideal approach, say the researchers, is to start prospecting using
satellite images and geophysical instruments such as magnetrometers,
which can be used from an aeroplane. Once this has located the broad
areas likely to contain gold, conventional analysis of rocks and
sediments will confirm whether gold is actually present. If it is,
water analysis can pinpoint the places to dig.
Hydrogeochemistry depends on having water to study, so it may be
of limited use in very dry areas. Southern Sardinia, for example,
is an arid landscape and the surveyors around Furtei could find
only seven sources of water - three streams, two springs, a well
and a borehole - in an area of 16 square kilometres. Nevertheless,
five out of the seven samples contained gold, a discovery which
helped to confirm the decision to continue prospecting.
In wet climates large amounts of surface water can dilute the evidence,
making dissolved gold harder to detect. Again, the hydrogeochemistry
proved promising when the researchers found gold in the water around
Dolgellau, where gold was mined commercially in the past and a single
mine survives. The Dolgellau gold, which is 'visible', seems to
be reluctant to dissolve compared to the 'invisible' gold in Sardinia,
the researchers noted. Further surveys in Wales and the highlands
of Scotland found more gold, though probably not in commercial quantities.
Papers published on the new techniques have attracted considerable
academic interest, though as yet there is no evidence of a commercial
benefit beyond the opening of the Furtei mine. Small deposits and
concern about the environmental impact of mining, especially open-cast
mines handling large amounts of low-grade ore, will probably limit
the future of gold mining in Europe. On the other hand, PROGEMISA
discovered a lot of gold prospects in Sardinia using the exploration
methodologies tested within the framework of this project. These
results confirm Sardinia as a new gold province and open valuable
new perspectives in the Sardinian mining industry, while traditional
mining activities are closing down.
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