Geochemistry and mineralogy of mine-waste material from a “skarn-type” deposit in central Mexico: Modeling geochemical controls of metals in the surface environment

Abstract

Several metal sulfide ore deposits (epithermal veins and skarn-type deposits) occur in the central east portion of Mexico (Sierra Madre Oriental). Historically, some of these deposits have been exploited since the 1500s and 1600s. Currently, there are millions of tons of mine waste materials with high content of potentially toxic heavy metals. This work focuses on geochemistry and mineralogy and modeling of redox and acid–base reactions of mine waste material from the Xichú mining area (Aurora Mine), Guanajuato state. The processed ore material at Xichú (circa 1million tons) consisted of pyrite, arsenopyrite, galena, sphalerite, chalcopyrite, and minerals with Ag and some Au. The mine waste material contains considerable amounts of As, Fe, Pb, Zn, and Cu, in different mineral phases mostly, Fe-oxides and oxyhydroxides, sulfo-salts and traces of primary sulfides. Potential metal liberation and mobility pose a threat to the local population. Redox reactions locally produce pH around 2 in leachates with high metal content. Secondary mineralogy includes goethite, hematite, ferrihydrite, jarosite group minerals, gypsum and minor carbonates, among others. The geochemical dynamics is controlled by the seasonal rains (from June to September), along with high evaporation rates. These factors produce minor acid mine drainage and metal leaching. Redox and dissolution–precipitation reactions control the mobility of heavy metals into the surrounding environment. Arsenic and heavy metal content in tailings is up to 37% Fe, 62gr/Kg of As, 2.4gr/Kg of Cu, 13gr/Kg of Zn and 17gr/Kg of Pb. Sequential extraction experiments indicated that most of the As is bounded to Fe oxyhydroxides; Zn and Cu are mostly bounded to soluble phases and Pb is mostly bounded to sulfide. Mineralogical changes are controlled seasonally mainly by dissolution and precipitation of secondary phases: jarosite, gypsum, goethite and hematite and minor metal carbonates. Locally, some highly evaporated ponds have low pH (between 2 and 3) and very high concentrations of As and metals. The Geochemist's Workbench (Act2 module) software was used to model the redox conditions for the dissolution–precipitation reaction controlling liberation and mobility of As and metals into the environment. It seems that most of the As and metals are involved in a dissolution–precipitation cycle in the tailings piles and very few are released into the environment.