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Industrial Processes

Turning water into gold

   
 
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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.

 

 

Project Title:  
Development of geochemical, geophysical and remote sensing methods for the prospecting of gold deposits, with particular attention to the role of hydrogeochemistry

Programmes:
Industrial and Materials Technologies (BRITE-EURAM/CRAFT/SMT)

Contract Reference: PL-119

Cordis DatabaseFor more information on this project,
go to the CORDIS Database Record

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