What environmental impact does it have ?

How can sites be regenerated ?

French Guiana has considerable gold potential. The discovery of gold around 1850 resulted in a “gold rush” lasting nearly a century with intense gold mining activity across much of the territory. And then the “gold fever” subsided.

But since the beginning of the 1990s French Guiana has seen an increase in gold mining due to rising gold prices on international markets, the development of new mechanised techniques imported from Brazil, and the fact that data from the Mining Inventory carried out by the BRGM on public land was made available to the public in 1996.

What is alluvial mining?

Alluvial mining consists in extracting gold from primary deposits which are currently in the sedimentary deposits on riverbeds due to erosion and other phenomena altering rock formations. Mining the alluvial deposits in the major bed of the river involves diverting the water from the minor bed* (or creek) into a diversion channel. Unearthing the deposit involves the complete destruction of the forest in the alluvial valley and the removal of surface soil. This can sometimes involve removing several metres of soil before reaching the layers in which the gold is to be found.

The ore is then extracted by removing layers of terrain using “firehose” equipment fed by a motorised pump, and sometimes using hydraulic shovels to break up the material. The hydraulic pulp thus obtained is fed down a channel to a capture point where a new pump directs it into a gravimetric scrubbing station. This scrubbing station, sometimes called a washing table, is made directly on-site using wood cut up using a chainsaw, or else assembled from metallic parts in a workshop. In general a washing table has three parts:

- a nugget trap made up of a case that can vary in depth but running across the width of the table. This traps the heaviest particles of gold and allows the water carrying finer particles to flow through;

- a first sloping table made up of a chute covered with various metallic grilles and plastic mats. The slope of this table can be varied to control the speed with which the mixture flows according to the throughput from the gravel pump and the dilution ratio of the pulp. The shape of the metallic grilles means the vertical waves flow through the pulp to encourage suspended particles to be deposited and trapped by the mats;

- a second sloping table which is identical to the first except it dips the other way. This sudden change in slope and reversal of drainage interrupts the flow of material and thus helps the last gold particles in suspension to be deposited (knowing that the majority of the gold is deposited in the first section of the table)

The water used to wash the ore is flushed from the table towards a series of tailing ponds as the operations move along the valley. The system has to be perfectly watertight to avoid polluting the river with the suspended material when using this mode of operation.

The impact of alluvial mining

Alluvial mining has consequences on the surrounding environment, leading to:

- the destruction of stretches of the natural waterway and associated alluvial forest for the whole length of the mining operation (for mines in natural sectors not previously affected);

- soil erosion: the cleared areas lead to leaching and erosion along the banks of the creeks. Seen from the sky, the orange colour of the laterite contrasting with the green of the treetops is evidence of this erosion;

- the pollution of water on site and downstream: the leaching of the soil before it is stabilised by the return of plant growth damages the quality of the water and of the aquatic environment, due especially to the asphyxiation of plants and fish and the clogging up of the beds;

- the creation of expanses of mud: decantation in the tailing ponds creates thick muddy zones of quicksand where no machinery can venture. These muddy zones are sources of pollution if they are not stabilised by planting a new biotope* on what is in biological terms an unattractive substrate.

Experiments in site regeneration

It is true that nature is good at regenerating tropical forest zones, but it can nevertheless end up creating “blocking” environments where nothing can grow other than herbaceous plants. As for the zones of washed gravel, they sometimes never lose their lunar appearance as they accumulate heat and are unable to retain sufficient levels of humidity for seeds to germinate there.

What was needed was to come up with a large number of methods to restore mines and to test them out to see if they could regenerate mining sites (see the diagram on page 74 for a detailed explanation of the stages of regeneration). Following on from the joint efforts of mining professionals, the relevant public authorities, consultancies, and researchers, several pilot sites have been used as a reference point to set up a year-round monitoring scheme. Recently a Mining Nursery and Development Centre was set up to develop innovative techniques drawing on knowledge built up in French Guiana over nearly 10 years. A generation of plants has been grown at the centre on a former mining site in the region of Cacao, and they are robust enough to be subsequently planted on mines and quarries.

Reducing the ecological costs

Whilst the current extent of the wounds inflicted on the forest by mining in French Guiana is known, it is harder to assess how deep these wounds are and to what extent they can be healed. Is it possible for the washed, sorted soils to support vegetation that will be as varied as the original forest, with over 120 species per hectare? Are the diverted, channelled, creeks that have been clogged up with fine deposits irreversibly polluted and dead? An observatory was set up in 2008 by staff at the French National Forestry Office (Office National des Forêts) with the backing of specialised science laboratories (Hydreco and Ecofog) so as to answer these questions and better assess the long-term ecological cost of gold mining. A network of 10 sites which have not been active for several years was taken as a sample and minutely observed, confirming how important the regeneration phase is, but also bringing up some surprises.

The best surprise is that satisfactory water quality can return after four years in creeks where the minor bed* has been regenerated. This is shown by the presence of insect larvae that are normally sensitive to pollution and of a diverse and balanced fish population. But note that this is only possible when the river basin upstream has remained intact and is sufficiently large to “absorb” the effects of mining and to be able to recolonise the mined stretches once the pollution has disappeared. The heads of isolated creeks are condemned if the mining operations reach too far up. Another worrying thing is the latent threat of mercury that has been re-mobilised by the leaching of soil.1 It is still present in the flesh of the first carnivorous fish to return to the site, meaning they are unfit for human consumption.

Findings are also mixed on dry land. Whilst dense, stratified plant cover is to be found across most of the area within a relatively short space of time given the length of forest life cycles, this is merely concealing the problem, for only soils that have been carefully and progressively regenerated have any real chance of supporting what will eventually become forest. This is shown by the activity of microorganisms in the soil, which act as the foundation for any biotope* worthy of the name. The tallest and densest plant formations found on non-regenerated sites, comprising a cover of cannon wood (Cecropia spp.) invaded by a lianescent shrub (Mimosa pigra), are likely to end up being ecological dead-ends as the poorly managed soils show little sign of life and their scant reserves are in danger of being rapidly depleted. Palliative planting on these unhealthy soils can be highly effective. But this means of boosting nature and reactivating natural dynamics is only possible when careful regeneration has been integrated into the various phases of mining operation so as to ensure the soil is immediately replaced, thus retaining its fertility and limiting erosion.