An 8 cm gold nugget (Care Enga, Papua New Guinea). © BRGM - Urbain de Cayeux

Research worth gold dust in French Guiana

08.09.2016
To help fight illegal gold production in French Guiana, the BRGM has developed a method that produces a physicochemical "identity card" for each gold sample, so that its geographical provenance and method of production can be determined.

In  French Guiana, illegal  gold  panning is having disastrous impacts on  people and the  environment. The method of production used by  illegal  panners  involves a mercury-based amalgamation process, and is causing severe pollution in forests and rivers,  while  competition between some 10 000 "garimpeiros", many of whom are undocumented workers from neighbouring countries, frequently flares up into  violence.

"Illegal gold production in Guiana is estimated at about 10 tonnes a year, as compared to one to two tonnes of legal production", says the BRGM's Laurent Bailly, who heads the European TAO project on analytical tracing of gold in Guiana, conducted with the WWF.

But curbing these illegal activities, which involve hundreds of small  production sites  (204 identified in 2015 by the  "Harpie" military operation against illegal gold panning), is not a simple matter.

"The solution we decided to work on," says L. Bailly, "involves tracing the gold coming onto the market to identify the method of production, but  also its geographical origin, by investigating the mineralogical and chemical characteristics that are specific to each deposit."

Shape, colour and mineral inclusions in gold samples from Guiana seen under a microscope. A physicochemical "identity card" for each sample can be determined by combining different methods of analysis. © BRGM - PIERRE VASSAL

Shape, colour and mineral inclusions in gold samples from Guiana seen under a microscope. A physicochemical "identity card" for each sample can be determined by combining different methods of analysis. © BRGM - Pierre Vassal

Several complementary methods

The work  undertaken in Guiana is the  first  of its kind. The research involved applying a series  of physicochemical analysis techniques to 30 samples from seven known deposits in Guiana and 4 samples from neighbouring Suriname produced by mercury amalgamation.

As L. Bailly explains, "we investigated the  morphological and  chemical  characteristics of  the nuggets, using a series of methods from microscope observations of their shape, colour and any mineral inclusions to isotopic analyses, with each method providing different clues. These analyses enabled us to determine a physicochemical "identity card" for each sample, which  gives the  "signature" of the  mining area they came  from."

The method also  produced a characterisation of the  production method used,  which  is of major importance to fight illegal gold production.

"The shape of the grains is different depending on whether they are from primary or alluvial deposits or produced by amalgamation. Grains taken from close to their primary source have  relatively flat surfaces on which the imprints of associated minerals can be seen. They become more rounded when carried away by a stream. When a mercury-based amalgamation process has been used, the chemical reaction produces a characteristically cauliflower-shaped deformation."

Sample with the characteristic cauliflower shape of gold grains amalgamated with mercury. © BRGM

Sample with the characteristic cauliflower shape of gold grains amalgamated with mercury. © BRGM

Work is still in the early stages  and the methods need to be refined. However, the  results so far are very  promising and  the  next few years could see the  creation of a reference databank in  Guiana. This would make the legal situation more secure by ensuring full traceability of products coming onto the  market, thus providing better guarantees as to their origin.

Analysing gold samples from French Guiana. © BRGM

Analysing gold samples from French Guiana. © BRGM

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