200 YEARS OF MINING ACTIVITIES AT LA-PAZ SAN-LUIS-POTOSI MEXICO - CONSEQUENCES FOR ENVIRONMENT AND GEOCHEMICAL-EXPLORATION

The Ag-Pb-Zn-Cu-Au mining district of Santa Maria de La Pat has been e xtensively exploited for approximately 200 years. Consequences of thes e activities are several deposits of tailings with high As and heavy m etal concentrations, which are completely unstable. The climate is sem iarid and as the dumps have no protective cover, material from the dum ps is dispersed by strong winds. It is also washed out during seasonal ly heavy rainfalls. By these processes approximately 100 km(2) of surr ounding have been contaminated by dump material. The As and heavy meta l content of the soils was determined as well as their level in crops (Zea Maize) from agricultural lands in the vicinity of the dumps. In t he direction of prevailing winds concentrations up to 1000 ppm Zn, 400 ppm Pb, 16 ppm Cd, 550 ppm Cu and 300 As have been detected in top so ils. Using fuzzy cluster analysis the different contamination sources could be identified. Grains of corn from contaminated sites showed no critical concentrations, but leaves which are also used for fodder, ha ve As-concentrations up to 20 ppm.