|Biological Monitoring: Signals from the Environment (GTZ, 1991)|
|Concept for a biological monitoring study:|
|2. Study area: the central Andean plateau|
The Andes are rich in ore deposits of many kinds. Major deposits are centered in the mountainous country of southem Peru, Bolivia and northern Argentina (PUTZER 1967, 1976) (Fig. 6). The Chilean ore deposits are located on the western slopes of the Andes.
The number of mines currently in operation in Bolivia is estimated by AHLFELD (1965) at 2200, although only 2% of these can be regarded as large operations. In Bolivia today, most of the large-scale mining operations are owned by the state mining corporation (Corporacion Minera de Bolivia, COMIBOL). The total number of mines has probably declined in the last 20 years, since many veins have already been exhausted. In Argentina as well, the mining operations of major economic importance are also state owned. Many small and frequently unprofitable mines are operated by private owners, with production rates that are subject to great fluctuations (GTZ 1983). New developments in the field of ore prospecting indicate the presence of uranium deposits on the Altiplano which could feasibly be exploited (PARDO 1981).
In descending order of importance, the following metals are extracted on the Altiplano: tin, zinc, silver, tungsten, antimony, lead and copper; all other metals account for only an insignificant proportion of the extracted ores (Fig. 7).
In order to evaluate the economic value of a mine and decide on the most appropriate processing technique, the most important criterion is the ore content of the extracted material. As a rule, this declines at increasing depths below the surface and thus with the amount of time during which a mine has been operated. When mining of tin first began in Bolivia (in 1905), only ores with tin concentrations of at least 12% were subjected to further processing. By the time between the two World Wars, the tin content had dropped to 4-6%, and since 1960 in general only low-grade ores with 0.5-0.6% of tin are extracted (AHLFELD 1965). It can be reasonably assumed that the metal content of the waste and tailings of the ore dressing processes in each case has decreased proportionally. Many old mine dumps and mill-tailing piles, originally regarded as worthless, are now again regarded as valuable, especially since the treatment methods have also been improved.
During recent years, the decline in productivity has spawned intensive, internationally supported prospecting activities which have also led to the discovery of rich mineral deposits in many locations, with the consequence that new mines are currently being established.
The extracted low-grade ores are dressed prior to transport in order to concentrate their valuable constituents into products of smaller bulk. This process, in which the various different ores are separated from one another and as much worthless material (gangue) is collected out as possible, is energy-intensive and therefore also very cost-intensive in the Andean plateau region, which has little wood and no coal deposits or petroleum reservoirs.
The roasting, flotation and filtration methods used result in discharge of contaminants into the atmosphere (often liberating considerable amounts of airborne dust), and contaminants find their way from waste dumps into the soil and aquatic systems, also being blown into the air once the sludge has dried.
Since the tin deposits of the "Bolivian tin province" are associated with considerable amounts of sulfides and arsenic, these compounds can be emitted into the atmosphere during the course of the separation processes, in part in gaseous form, and in part as particulates. Very little information is contained in the literature on the composition and amounts of emanated contaminants and their impacts on the terrestrial and aquatic systems of the Andean plateau. The only exception we were able to find is the description given by MILLONES (1982) on water contamination (fish mortality in the Mantaro river) and the deleterious effects caused by atmospheric contaminants on 700,000 ha of pastureland in the vicinity of the tin mine and ore mill at Oroya, Peru.