SAP 2.76/WP.130
Industrial Activities Branch
Working Paper
Small-scale gold mining:
Examples from Bolivia, Philippines & Zimbabwe
Edited by Norman S. Jennings
1. Small-scale gold mining at San Simón, Bolivia
Thomas Hentschel; Diógenes Roque; Evelyn Taucer (3) (4)
Bolivia is traditionally a mining country. Since the times of the Incas and later the Spaniards, mining has been one of the main economic activities in the country. Silver, gold, tin, tungsten, antimony and lead were traditionally the principal metals produced by the Bolivian mining industry. Nowadays, however, gold, zinc and silver predominate.
The extent of small-scale mining in Bolivia is significant. Since the reduction in production by the state mining corporation Corporación Minera de Bolivia (COMIBOL) in 1985, small-scale mining has attracted a large number of miners, creating employment and ensuring their livelihood. Today about 100,000 persons work as small-scale miners, and about 500,000 people (including families) therefore depend on it for their needs. This number is extremely high when compared to the population of the country (7 million), making Bolivia one of the most important countries as far as small-scale mining is concerned.
This situation is reflected in the foreign aid that the country receives. Presently there are two international cooperation programmes that support small-scale mining:
This study focuses on the San Simón highlands where there are groups of small-scale mines. The production of gold in San Simón, using traditional methods, has many problems that are typical of small-scale mining -- legal; social; economic; technical; and environmental.
Legislation on environmental issues is relatively recent in Bolivia. The Environment Law was promulgated in 1992 and its regulations came into force in 1996. This law stipulates that all activities, construction works or projects that are underway or are to commence must obtain the necessary environment licence for which certain requirements have to be met.
In 1997 the Environment Regulation on Mining Activities was passed whereby, in the framework of the Environment Law, specific aspects of mining activities are covered in relation to the environment.
In San Simón, however, all mining activity is carried out without the required environmental permit -- from exploration by concession holders to gold extraction by the mining association Sociedad Minería de San Simón de Mategúa. The activities of the association have had devastating repercussions on the ecosystem and on the health of the miners themselves, mainly because of the excessive use of mercury during gold processing.
Environmental contamination is due to almost 500 small-scale miners who cause about 15 tonnes of mercury to be emitted into the ecosystem every year -- a world record rate of mercury emission. This situation led MEDMIN to initiate activities with small-scale miners in the highlands of San Simón. This study describes the general situation, focusing on technical aspects and methodologies to reduce the emission of mercury.
The San Simón plateau is about 700 metres above sea level on the eastern side of Bolivia in the Itenez Province, south-east of the department of Beni at a latitude of 13 and 30' south and a longitude of 62 and 10' (Figure 1). The closest points of reference are the river Itenez o Guaporé in the north and east that forms part of the Amazon basin and borders with Brazil; the National Reserve Park and the Noel Kempff Mercado National Park in the south-east; and the forest Bosque Permanente de Producción de Bajo Paragúa in the west. The latter three are protected areas under the Sistema Nacional de Areas Protegidas. The main access is by air from Santa Cruz or other cities in Beni by small aircraft.
Figure 1. Location of San Simón project
Access by land is only possible during the dry season. During most of the year the surrounding areas of the San Simón mountain range are flooded and the only way to reach the mines is by fixed-wing aircraft from the city of Santa Cruz. The distance by land from La Paz to San Simón is almost 1,700 km or three days of travelling under good conditions. From Beni it is 35 km by road following the Itenez river.
A characteristic of the Itenez Province in the department of Beni is its sparse population. There are only a few communities in the San Simón highlands, the more important being Cafetal, Remanzo and Piso Firme housing about 1,200 families. The population density is less than 1 per km.
The San Simón highlands are part of the ecological zone known as Sabanas del Cerrado -- plains that are surrounded by lower lying lands with rain forests. Sabanas del Cerrado is a special ecological zone in Bolivia that forms a small part of the Matto Grosso plateau. The scattered plains are small and are one of the least explored areas in the country as regards flora and fauna. The weather is warm with a temperature of 28-35C, but falling to below 12C in winter.
The Government pays little attention to the provision of services in this region, especially those related to sanitation and public health. Malaria is the main health problem and treatment, prophylactics or mosquito control have to be obtained from Brazil. The only state institution that is present in the zone is the Bolivian navy with a base and a harbour master's office on the Itenez river.
Some settlements have been created around the mining activities, the most significant being the San Simón camp that houses Bolivians and Brazilians alike, engaged in mining and trade. Small-scale agriculture is also carried out, with small quantities of rice, corn, cassava, banana, beans and vegetables being harvested. Some wild fruits, such as chestnuts, are also obtained. Livestock farming is limited to beef. Apart from mining there is no other industrial activity. Many illegal drug-related activities are carried out here and in nearby areas.
Gold mining in Bolivia has been carried out since the time of the Incas. During the colonial period, however, gold was far less important than silver (e.g. Cerro Rico in Potosí). Also, from the last century to the mid-1980s, Bolivia was renowned for its tin production. Only in the last 20 years or so has gold mining been gaining momentum within the mining sector and the economy as a whole.
A significant part of this gold production comes from small-scale mines, organized as miners' cooperatives. Gold miners' cooperatives have existed since the 1954 revolution. In the 1980s cooperatives exploiting alluvial gold in the department of La Paz (Tipuani river) boomed. Since 1985, when the tin price collapsed, small-scale gold mining has been expanding throughout the country, especially where primary gold deposits are found, mainly organized in cooperatives. They draw their members from the alluvial gold cooperatives formed in the 1980s as well as from the bankrupt state-owned mining corporation COMIBOL. Currently small-scale gold mining is an employer of first resort for many miners. These cooperatives are usually established legally, i.e. they possess their own mining concessions. However, as the miners' cooperatives do not pay taxes and generally do not abide by environmental regulations, they are characterized as being informal.
There are currently only two large gold mining operations in Bolivia -- the Inti Raymi Corporation, operating in the traditionally mined area of the Andean region; and the Puquio Norte mine operating in the east of the country where San Simón is located (the first formal gold mining operation). Table 1 shows official gold production from formal "mines".
Table 1. Gold production from formal mines, 1990-97
(kg fine gold)
| 1990 | 1991 | 1992 | 1993 | 1994 | 1995 | 1996 | 1997 |
| 1 513 | 1 560 | 1 675 | 6 563 | 9 731 | 10 529 | 9 573 | 10 681 |
| Source: National Institute for Statistics (1998). | |||||||
In addition, about 10,000 kg of gold is produced each year from small-scale mines, a quantity that has been remained steady for the last few years. The greater part of this production is informal, by miners' cooperatives and by individual miners using traditional methods.
The gold deposits in the San Simón highlands come from to the Precambrian Brazilian Shield in eastern Bolivia. The highlands consist of Proterozoic meta sediments. San Simón is an extensive area with enriched quartz and gold mineralization and thus a deposit with attractive mining prospects. Four separate zones with mineral deposits have been identified that represent three specific types of mineralization:
The gold veins are present in sedimentary rock and the gold is usually surrounded with quartz gangue mineral.
In 1742 the Jesuits founded the San Simón mission and a year later the Santa Rosa mission. The latter is presently in Brazilian territory. Gold production in this area has taken place since then, mainly in the San Simón highlands which are still being exploited. There are currently several exploration concessions, within which Bolivian and Brazilian small-scale miners are producing gold. When exploration began in the early 1990s, the number of informal miners started to increase progressively.
At first, only locals worked in mining, but over the years more and more miners migrated from other regions of the country, especially those with experience in gold mining and traders. Today the San Simón highlands have been "invaded" by about 500 miners. The total population on the mountain is about 1,000, including merchants, relatives, etc.
In the San Simón region there are several legally established concession holders who are organized in the Bolivian prospecting and exploration corporation Excalibur Holdings S.A. Once they had pinpointed the potential places for exploration in the San Simón region, and after having arranged the legal aspects, Excalibur prepared an exploration project and signed an exploration agreement with the concession holders. Following basic geological prospecting, a new project was prepared and finance for exploration was sought overseas. An agreement was signed with the Canadian exploration corporation Eaglecrest Ltd which named Excalibur as its representative in Bolivia and charged it with the administration of exploration. To date approximately $4 million has been invested in the exploration of San Simón. But, due to financial problems (notably the fall on the Vancouver stock exchange after the Bre-X scandal in Indonesia), exploration is currently limited to minor activities.
As mentioned, the great influx of small-scale miners to the San Simón highlands began at the onset of this decade when the corporation had its first highly positive results from its exploration activities. With the help mainly of the "invading" cooperatives from other gold mining regions an unsuccessful attempt to organize three miners' cooperatives in the San Simón region was made. Next, the small-scale miners created a mining association, Sociedad Minera San Simón de Mategua (henceforth called Society) which has three independent regional sections -- Remanso section; Cafetal Itenez section; and Puerto Villazón section. The Society, which is operating de facto and is not legally registered, can best be described as an informal organization of illegal miners. The main objective is to ensure the presence of the small-scale miners in San Simón through negotiations with Excalibur.
Currently about 250 miners, processing plant owners and merchants are members of the Society; the remaining miners are mostly Brazilians who are in Bolivia illegally. No labour laws are adhered to; each miner is self-employed. Thus both social security and occupational health and safety are non-existent. There are no contracts between the miners and the Society; the legal minimum wage in the country is not applied; nor are the legally established hours of work. Gold is traded informally, including by barter. Gold buyers, who arrive from Santa Cruz and some Brazilian cities, pay 1-2% less than the international price.
The miners are exclusively male. Women and children often work in the processing plants and are therefore directly involved in the amalgamation process, normally unaware of the dangers arising from contact with mercury.
As mentioned above, there is no control or supervision of legislation exercised by the national, departmental, or municipal Bolivian authorities. San Simón falls under the jurisdiction of the Baures municipality, one hour by aircraft from San Simón.
In order to facilitate its work in the region Excalibur has initiated dialogue with the invaders through their Society and an agreement was signed in 1996. The highlights are:
To date Excalibur has fully complied with the agreement; the Society only partially. Nonetheless, good relations of mutual respect prevail and ensure the coexistence of both parties. More problematic at times are the relations between the miners of the Society and the illegal Brazilian miners; and there have been several confrontations with fatal consequences.
Technical aspectsDue to the informal nature of the mining and processing operations, it is difficult accurately to calculate gold production in San Simón, even more so to provide statistical data because of the special nature of work relations in the production process -- some are proprietors of mines from where the mineral is extracted; others are owners of the processing plants. Between them relations are off and on). Neither is trading an indicator of production since as it is completely informal. However, following an assessment of the capacity and operation of the equipment in some processing plants and the average head grade and rate of gold recovery MEDMIN was able to make a reasonable estimate of gold production (table 2).
Table 2. Gold production at San Simón
| Plant capacity | 30 tph (18 plants of 1.5-6 tph) |
| Annual operation time | 1 280 hours (8 hours/day; 20 days/month; 8 months/year |
| Average head grade | 20 gm/t |
| Recovery rate | 50% |
| Annual gold production | 384 kg |
The alluvium washed down from the outcrops of gold-bearing quartz veins in the San Simón highlands also holds secondary gold deposits. In the recent past some secondary deposits were exploited hydraulically, using pressurized water to remove the material; a suction pump to send the gold pulp to the wash pan; and rudimentary sluices for recovering the gold. However, alluvial gold production is not common.
Mining is presently being carried out in stratabound quartz stockwork zones within altered quartzites on the San Simón and Guarayos plateaux, wholly managed by the San Simón Mining Society (with its Sections: Remanso with 150 members; San Simón with 50 members; and Cafetal-Itenez with 50 members) in accordance with an agreement with Excalibur that administers most of the San Simón concessions.
Mining is carried out in teams working as production units, each being the owner of what they produce. Operations are completely empirical, without any technical criteria, guided only by the instincts and limited experience of the miners. A clause in the agreement with Excalibur stipulates that the miners can only work to a depth of 30 metres below the upper surface of the plateau. Compliance with this restriction involves considering preparing the mine adequately and also determining the most appropriate mining method. In general, mining is carried out underground along ramps and galleries, following the quartz veins. To crack the veins pneumatic drills and explosives are used. The ore is transported in wheelbarrows from the work face through horizontal galleries to the foot of a ramp. From here it is hand-carried to wooden carts (sledge type) on wooden rails on the ramp to be lifted to the surface using a pulley. The contents of the wooden carts are tipped into a wooden box at the mine entrance, from where it is later carried to trucks to be transported to the processing plants.
Ore processing and the environment
The grains of gold contained in this deposit are fine (generally less than 0.5 mm). To process the extracted material there are about 18 plants, their capacity ranging from 1.5-6 tonnes per hour. Only a few teams possess their own plants. In general they are the property of small investors, the majority Brazilian (mostly illegal), who charge a percentage of the value of the gold produced for their use.
The technology is entirely Brazilian and comprises:
Ninety per cent of the operators are also Brazilian. Figure 2 shows flow sheets of the San Simón plants.
A matter for concern that has a direct impact on the environment is the irresponsible and indiscriminate use of mercury to recover the gold. The fine grain size in this deposit has led to the erroneous belief that the only way to recover the gold is by amalgamation with mercury during the milling stage. For this reason mercury is generally added directly into the hammer mills with the ore. At speeds of 1,500 rpm or more the mercury atomizes. The fervedor (a container that is driven by pressurized water) is placed at the outlet of the mill to catch the amalgam. For every tonne of material fed into the mill, 400-500 gm of mercury are used. But only 6-8% of it is retained on the plates or in centrifuges in the form of amalgam; over 90% is lost in the tailings as atomized mercury and flakes of amalgam, the latter due to the apparently low specific weight. Needless to say, much gold is also lost in this manner -- a fact that MEDMIN was able to demonstrate convincingly to the miners.
Assuming that: total plant capacity at San Simón is 30 tph; that they operate eight hours per day, 20 days a month, 8 months of the year; an average of 400 gm of mercury is used for every tonne of material treated; that 10% of the mercury used is captured: the amount of mercury entering the environment is 1,920 kg per month (15.36 tonnes per year). These figures are worrying.
Taking into account the use of 15.36 tonnes of mercury per year (recapturing only 10%) and the estimated annual production of 384 kg of gold; for each kilogram of gold produced, 36 kg of mercury is emitted to the atmosphere. This figure is surely one of the highest in the world.
Only one plant, the largest in San Simón, uses retorts to separate gold from the amalgam and recover the mercury. The rest burn their amalgam in the open, directly exposing the operator to mercury vapour and, indirectly, the inhabitants of the surrounding area too.
Most of the processing plants are small and, because of the milling system used (small hammer mills), they are to all intents and purposes mobile plants, easily moved to places where water is available (water is extremely scarce during the dry season). The plants are therefore set up near ponds or streams so as to have enough water for processing. All the plants dispose of their tailings in the small river beds and streams that eventually flow into the Itenez river. As fish is a main component in the diet in the region, the potential health risks of irresponsible mercury use in San Simón are considerable.
Figure 2. Gold treatment plant at San Simón: a) 1.5tph; b) 5tph
The negative impact on the environment due to the expansion of mining is highly disturbing, especially because of its proximity to three protected areas: the Bosque Permanente de Producción de Bajo Paragúa; the National Reserve Park; and the Noel Kempff Mercado National Park. Moreover, its proximity to the Itenez o Guaporé river on the border with Brazil that drains into the Amazon basin has an affect beyond Bolivia's borders. But the whole region, including the existing protected areas, still has not been studied sufficiently as regards its diverse biosphere, due both to difficult access and to the risks caused by drug trafficking.
Apart from mercury contamination, there are several direct and indirect environmental effects from mining, including the destruction of the countryside, water pollution and silting, oil spills, the accumulation of solid waste from mining and the camps, deforestation, and the hunting of wild animals.
As in all small-scale mining operations, the means to prevent accidents or ensure occupational health are minimal. Often the miners lack basic equipment such as protective helmets, boots or gloves. There is no artificial ventilation and the miners use battery-operated flashlights to illuminate their underground tunnels.
Bone fractures, principally of arms and legs caused by cave-ins are frequent, but there are no statistics. Since 1993 three fatal accidents have occurred, all because of tunnels caving in. The most common illness in the region is malaria. The Bolivian programmes for malaria prevention have never reached the San Simón region. The only medical post with a nurse was set up by Excalibur, to which the miners have access.
From the first visit by MEDMIN to the gold-rich San Simón region, the alarming problem caused by the irrational use of mercury was evident. Sensitizing and awareness-raising campaigns among the small-scale miners were initiated. Simultaneously, technical/environmental investigations were undertaken in the processing plants to develop a technical solution that could improve the separation of gold with the minimum use of mercury. A comparative test was carried out between the system used in San Simón (mill-amalgamation, fervedor and amalgamation plates) and one proposed by MEDMIN without using mercury in the circuit (6 metres of sluices covered with 2 m of Nomad carpeting and 4 m of Cabello de Negro carpeting) (figure 3).
Figure 3. Comparative flowsheets: a) MEDMIN; b) San Simón.
The results (table 3) show conclusively that at least the same amount of gold can be recovered without using mercury as with it. In some cases higher recovery rates can be achieved. Also, this process leads to a saving of about 10% in operating costs (mercury is expensive in San Simón, about $30 per kg) when compared with the average of 400 gm of mercury used per tonne of ore treated in the San Simón system.
Table 3. Comparison of recovery methods
| MEDMIN system | San Simón system | |
| Mill | H2 hammers | H2 hammers |
| Capacity | 3 tonnes | 3 tonnes |
| Degree of milling | -2mm | -2mm |
| Gold recovery system | Sluices/carpets | Mercury in mill/plates |
| Mercury used | 50 gm | 2 kg |
| Mercury recaptured | 50 gm (100%) | 100 gm (5%) |
| Gold recovered | 28.8 gm | 28 gm |
The final size of the milled material in all the San Simón plants is too big for maximum gold recovery, as illustrated in the ore size and the assay of gold from different fractions of a tailing sample that was systematically obtained in a test performed using the MEDMIN processing system (table 4).
Table 4. Gold recovery test results
| Fraction (mm) | Weight % | Weight % cumulative | Gold content (gm/t) | Fine units | Distribution (%) |
| +2 | 6.4 | 100.0 | 2.31 | 14.8 | 2.4 |
| -2 +1 | 22.1 | 93.6 | 3.30 | 72.9 | 11.8 |
| -1 +0.5 | 27.4 | 71.5 | 8.06 | 221.0 | 35.9 |
| -0.5 +0.25 | 19.6 | 44.1 | 6.87 | 134.8 | 21.9 |
| -0.25 +0 | 24.4 | 24.4 | 7.05 | 172.3 | 28.0 |
| Head grade | 100.0 | 615.9 | 100.0 |
20 mg of liberated fine gold was contained in a 5.6 kg analysed sample. This is equivalent to 3.6 gm/t of tailings; thus the true content of gold in the tailings reaches 9.7 gm/t.
Since the amount of non-liberated gold that is trapped in the ore is too high, milling should be to a finer particle size to obtain greater liberation of gold.
With the calculated tailing grade, the tonnage treated and the gold recovered, the metallurgical balance of the test with the MEDMIN processing system was determined (table 5) and shows that the gold content in the treated material is 19.33 gm/t, considered to be quite good but the rate of recovery of gold (only 49.7%) is too low -- largely due to to insufficient liberation because the fragments are too coarse.
Table 5. Metallurgical balance
| Product | Weight (t) | Gold content (gm/t) | Fine gold (gm) | Recuperation (%) |
| Physical gold recovered | 28.8 | 49.6 | ||
| Tailings | 3.0 | 9.7 | 29.2 | 50.3 |
| Calc. head grade | 3.0 | 19.3 | 58.0 | 100.0 |
It is clear, therefore, that the rate of gold recovery in the processing plants can easily be increased without the use of mercury in open circuits. An agreement was undersigned between MEDMIN, Excalibur and the Mining Society of San Simón to tackle the mercury problem together. MEDMIN installed a pilot plant to experiment with various treatment alternatives and identify the behaviour of the different materials in the zone. The plant was inaugurated in May 1998 in the presence of national and regional authorities. Figure 4 is a flow sheet of the following procedures in the pilot plant:
Table 6 shows the metallurgical balance of the pilot plant.
Table 6. Metal balance of MEDMIN pilot plant
| Stage | Gold recovery (%) |
| Spiral concentrator | 51.0 |
| nomad carpet | 5.7 |
| C. Negro carpet | 3.8 |
| Total recovery | 60.6 |
| Tailings | 39.4 |
| Calc. head grade | 100.0 |
It is worth noting that about 80% of the gold is recovered in the spiral concentrator. A gold recovery of 60% is acceptable. It can be increased using finer milling -- down to 65 mesh (0.2 mm) in a ball mill. The fact that almost all the mercury used was recovered (table 7) is possibly the most outstanding aspect of the technological changes in San Simón.
Figure 4. San Simón pilot plant
Table 7. Mercury balance
| Mercury used (gm; %) | 200.0 | 100.0 |
| Mercury recovered (gm; %) | 197.2 | 98.6 |
| Mercury lost (gm; %) | 2.8 | 1.4 |
Complementary information, covering the consumption of steel in the hammer mills and the consumption of diesel in the plants, is as follows:
Mill capacity 1.6 t/h
Hammer consumption 0.97 kg/t mineral
Diesel consumption 4.0 l/t mineral
The MEDMIN plant has demonstrated an increase in gold recovery of at least 10%. Also, there is a further 10% in cost savings from using far less mercury and recovering almost 100% of it. Since the milling system is the same as that used in traditional processing plants, the largest investment is in the spiral concentrator ($1,500) but it is recouped in a short time.
The above results have convinced those involved to continue with action to solve the problem of mercury emission and increase gold production. The following steps are foreseen:
The gains from the increased efficiency of the "new technology" are used to defray part of the cost of MEDMIN's technical assistance, the other part is shared between the small-scale miners and Excalibur.
Figure 5. San Simón future pilot plant
This study shows that improvements in environmental protection in small-scale mining can be made even under very harsh conditions. It is hoped that this experience in Bolivia will be beneficial also for other countries. The following points should be borne in mind when small-scale mining projects are undertaken:
References
Ahlfeld, F., Schneider-Scherbina, A.: "Los yacimientos Minerales y de Hidrocarburos de Bolivia", in Ministry of Mines and Petroleum, Bulletin, No. 5 (La Paz, 1964).
Cabrera, G.A.: Al Mar por las Hidrovias y Corredores de la Integración Sudamericana (Cochabamba, 1993).
Eaglecrest Exploration Ltd.: http://www.canvest.com/eaglecrest/
Marconi, M., Ed.: Conservación de la Diversidad Biológica en Bolivia, (La Paz, 1992).
MEDMIN-COSUDE, Ed.: Manejo ambiental en la pequeña minería (La Paz, 1998).
Priester, M., Hentschel, T., Benthin, B.: Tools for mining, techniques and processes for small-scale mining (Wiesbaden, Braunschweig, 1993).
