A GEOCHEMICAL STUDY IN THE TAILINGS DEPOSIT DOS CARLOS, PACHUCA-REAL DEL MONTE MINING DISTRICT, CENTRAL MEXICO

In this work we provide a detailed geochemical study in a tailings deposit to estimate whether such deposits represent a serious environmental danger for the population. The tailings deposit under consideration is known as Dos Carlos, and belongs to the mining district Pachuca-Real del Monte in the state of Hidalgo, central Mexico. Siliceous minerals with low contents of sulfides (pyrite, sphalerite and galena) have been exploited, and as a result, tailings were deposited during several years in urban areas and on alluvial channels, remaining exposed to atmospheric conditions. We selected two profiles called Profile A and B, located at opposite sides in the deposit, and sampled vertically, from top to base. In Profile A, 35 samples were taken along a depth of 5.32 m, while from profile B were taken 10 samples along a depth of 7 m. These samples were immersed in distilled water for a period of two to four weeks to determine through the leachates, the concentration of pH, electrical conductivity, oxidation-reduction potential (Eh), NH₄⁺, free and total Cl, Cr VI, Cu, NO₃^-, NO₂^-, dissolved oxygen, HPO₄^2-, SiO₂ and Zn. Mineralogical analysis were also performed using X-ray diffraction in 14 samples. Finally, we conducted a hydrogeochemical model, and calculated the geoaccumulation index. The most representative values were: moisture content (1-15%), pH (6.83-8.18), Eh (182-270 mV), for both profiles, suggesting a reducing environment to moderately reducing. The chemical composition indicates that no ion exceeds the limits set by Mexican standard for drinking water and potentially toxic elements. The predominant mineral was quartz, followed by orthoclase, kaolinite and calcite. Based on the physico-chemical and mineralogical composition of the tailings, the hydrogeochemical model indicates that the phases of Ag, Cu, Cd, Mn, Ni, Pb and Zn are dissolved, whereas the phases of Fe and some varieties of minerals containing Pb and Zn precipitate in the tailings leachate. Besides, the contamination level was estimated throughout the geoaccumulation index, which indicates the reservoir is not contaminated with respect to Cu, Cr and Zn. Therefore, it is suggested that the deposit Dos Carlos is not a potential source of contamination in the short term for the inhabitants of Pachuca.