NATURAL VS. ANTHROPOGENIC HEAVY METAL SOURCES, GUANAJUATO MINING DISTRICT, CENTRAL MEXICO

The Guanajuato Mining District, located in central Mexico, has been exploited for silver and gold from quartz and calcite rich epithermal veins for more than 400 years. There are some 150 millions of tons of mine waste material scattered in a area of some 100 km2 around the city of Guanajuato. Most of the historic tailings piles undertake strong eolic and hydrologic erosion, besides the natural leaching during the rainy season (which can bring strong thunder storms and flash flows). There is public concern about possible contamination of the local aquifer with heavy metals (Fe, Mn, Zn, As and Se) derived from the mining tailings. We present here, a general description of the most important historic tailings piles, and their possible environmental problems associated. However, experimental and field data indicate strong geochemical evidences that most of the mine waste material have very low possibilities for generation acid mine drainage (AMD) due to the high carbonates/sulfides ratio (12:1), and very low potential for leaching heavy metals into the groundwater system. Experimental and modeled geochemical data also indicate that natural processes, like metal adsorption onto Fe-oxy-hydroxides surfaces (>FeO-OH) control the mobility of dissolved metals in the environment. Finally, the strong possibility that some anomalous concentrations of Zn in groundwater wells are due to natural dissolution processes (natural background values) of deeper levels of the epithermal veins is investigated.