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Geofísica internacional
versión On-line ISSN 2954-436Xversión impresa ISSN 0016-7169
Geofís. Intl vol.44 no.1 Ciudad de México ene./mar. 2005
Articles
Mineralogy and geochemistry of sulfide-bearing tailings from silver mines in the Taxco, Mexico area to evaluate their potential environmental impact
1 Escuela Regional de Ciencias de la Tierra, Universidad Autónoma de Guerrero. Ap. Post. 197, Taxco, Gro.
2 Universidad Tecnológica de Tulancingo, Camino a Ahuehuetitla No. 301, Tulancingo, Hgo.
3 Instituto de Ciencias Básicas e Ingenierías, Universidad Autónoma del Estado de Hidalgo, Cd. Universitaria, Pachuca, Hgo.
4 Departamento de Geología, Universidad de Sonora. Rosales y Blvd. Encinas, Hermosillo, Son.
Six sulfide-bearing, flotation tailings dams and one waste-rock dump from epithermal Pb-Zn-Ag-Cu deposits in Taxco, Guerrero have been mineralogically and geochemically characterized to evaluate their potential environmental impact. Primary phases are dominated by quartz and rock fragments with scarce feldspar, barite and ferromagnesian. Calcite is found in the less oxidized tailings. Primary sulfides are pyrite and sphalerite with some pyrrothite, galena, chalcopyrite, Ag-sulfosalts and arsenopyrite. In oxidized zones, sulfides are low or absent. Secondary pilases are widespread and include gypsum, chalcocite, keoheite, nimite, hetaerolite, jarosite, smithsonite, bernalite, epsonite, rozenite, pickeringite, hexahydrite, bassanite, boyleite, hematite, scorodite, cryptocrystalline and amorphous precipitates and poorly crystallized Fe-oxyhydroxides. EDS analysis suggests that adsorption is important in the retention of metals.
All Taxco tailings and waste-rock dumps show high concentrations of total Ag (9.5 74.2 mg/kg), Cd (1.0-780 mg/kg), Cu (71.8-1320 mg/kg), Fe (2.49-25.1%), Mn (18.6-13 800 mg/kg), Pb (780-43 700 mg/kg), V (2.0-127 mg/kg), Zn (380->10 000 mg/ kg) and As (19.0-11 800 mg/kg) exceeding the regional background concentrations in crop soils. Contents of water-soluble metals are heterogeneous but contain significant amounts of toxic metals (e.g. Cd = <0.005-19.2 mg/L; Cu = <0.025-63 mg/L; Fe = >0.025-1105 mg/L; Pb = <0.025-0.655 mg/L; As = <0.010-1.79 mg/L), particularly from highly oxidized zones. The release is primarily controlled by pH. Sulfide oxidation and generation of AMD are coupled by dissolution, neutralization via calcite, reprecipitation and adsorption.
KEY WORDS: Taxco; tailings; sulfide; heavy metals; acid mine drainage; bioavailability; toxicity
Seis depósitos de jales y un terrero resultantes de la explotación de los depósitos epitermales de Pb-Zn-Ag-Cu de la región de Taxco, Guerrero han sido caracterizados mineralógicamente (microscopía óptica, MEB y XRD) y geoquímicamente (EDS, metales totales y solubles en agua) para estimar su potencial para liberar metales al medio ambiente. Las fases primarias están dominadas por el cuarzo y los fragmentos de roca con escasos feldespatos, barita y ferromagnesianos. La calcita sólo ha sido reconocida en los jales menos oxidados. Los sulfuros primarios incluyen pirita y esfalerita con escasa pirrotita, galena, calcopirita, sulfosales de plata y arsenopirita, los cuales sólo se conservan en los depósitos no oxidados. En los jales oxidados, los sulfuros están ausentes o aparecen como relictos. Las fases secundarias están ampliamente distribuidas e incluyen al yeso, calcocita, keoheita, nimita, hetaerolita, jarosita, smithsonita, bernalita, epsonita, rozenita, pickeringita, hexahidrita, hematita, escorodita así como precipitados amorfos y criptocristalinos y oxihidróxidos de hierro pobremente cristalizados. Análisis por EDS indican que la adsorción en las fases secundarias es un importante proceso que influye en la retención de metales.
Todos los depósitos de jales y terreros de Taxco se caracterizan por presentar altas concentraciones totales de Ag (9.5-74.2 mg/kg), Cd (1.0-780 mg/kg), Cu (71.8-1320 mg/kg), Fe (2.49-25.1%), Mn (18.6-13 800 mg/kg), Pb (780-43 700 mg/kg), V (2.0 127 mg/kg), Zn (380->10 000 mg/kg) and As (19.0-11 800 mg/kg) y todos se encuentran por encima de los niveles regionales de fondo de los suelos de cultivo. Los contenidos de metales solubles en agua indican que los jales son muy heterogéneos y que liberan cantidades importantes de metales tóxicos, (e.g. Cd = <0.005-19.2 mg/L; Cu = <0.025-63 mg/L; Fe = >0.025-1105 mg/L; Pb = <0.025-0.655 mg/L; As = <0.010-1.79 mg/L) particularmente los niveles oxidados, y que la liberación de estos metales está controlada esencialmente por el pH. Los datos indican que la oxidación de los sulfuros y la generación de AMD están acompañados por procesos de disolución de otras fases, de neutralización por la calcita, la reprecipitación de fases secundarias y la adsorción, y que estos procesos son los principales responsables de la movilidad y disponibilidad de metales en los desechos mineros de Taxco.
PALABRAS CLAVE: Taxco; jales; sulfuros; metales pesados; drenaje ácido de mina; biodisponibilidad; toxicidad
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Received: December 11, 2002; Accepted: February 04, 2004