URBAN TAILINGS AND THEIR ENVIRONMENTAL IMPLICATIONS: THE CASE OF PACHUCA (CENTRAL MEXICO)

The mining activity in the State of Hidalgo exists from more than 500 years and it remains still in exploitation. The past mining activities in Pachuca (capital of the State) originated several tailings deposits dispersed around the urban zone. The current activity allows the extraction of Ag and Au, as the main base metals. This mining residue has been deposited superficially in several places of the city of Pachuca; one of these places, called Dos Carlos, was selected for this study. This site represents an environmental risk, because of it is in direct contact with atmospheric conditions (rain and oxygen), favoring the oxidation of the material, and, consequently, the possible formation of acid mine drainage (AMD), as well as the aeolian dispersion of very fine particles that can be dangerous for human health. The tailings consist of sand-size material exposing a brown color, and represent a volume of approximately 21.5 million tons. The results obtained indicate a pH of 7.53 in solid material, whereas the content of base metals is of Ag (25 gt^-1), Au (0.25 gt^-1), and the weight % consists of SiO₂ (65), Al₂O₃ (5.0), Fe (2.8), Mn (0.32), Zn (0.273), and Pb (0.112). The mineralogical phases in tailings consist of quartz, albite, copper sulfide, silver, berlinite, orthoclase, K-jarosite and natrojarosite. The presence of these minerals could represent a potential buffer of acidity (e.g. quartz, albite, orthoclase, K-jarosite), whereas other minerals such as copper sulfide, berlinite and natrojarosite, could represent the factor that contributes to the formation of acidity (for their contents of S and Fe). By the other hand, the high content of Si in tailings, as well as their particle size (which varies from 75 and 38 micrometers), represent a high risk to the population by inhalation, due that it can provoke respiratory and pulmonary diseases. According to the results reported in this research, it can be concluded that tailings could be toxic to the environment, mainly due to the content of metals such as Pb, as well as for being in contact with atmospheric conditions. For these reasons, it is important to suggest some alternatives of control and/or remediation, and to reduce the environmental risk that this material is producing.