Mineralogical and geochemical study of element mobility at the sulfide-rich Excelsior waste rock dump from the polymetallic Zn–Pb–(Ag–Bi–Cu) deposit, Cerro de Pasco, Peru

Abstract

We present a mineralogical and geochemical study of the sulfide-rich waste rock dump Excelsior (size: 94 ha) originating from the Cerro de Pasco mine (altitude: 4300 m a.s.l.), Central Andes, Peru. The aims of this study were: to characterise (1) the secondary mineral assemblage and (2) the acid mine drainage (AMD) from this waste rock dump. (3) This information was used to create a model of the element transport in and out of the waste rock dump under the highly variable mountain climate with high precipitation during the wet season and high evaporation during the dry season. The main ore minerals found in the polymetallic Pb–Zn–(Ag–Bi–Cu) deposit are pyrite, sphalerite, galena and enargite with minor finds of tennantite, covellite and cerrusite. Gangue was dominantly quartz ± hematite ± siderite ± muscovite. The waste rocks had a high potential of acid generation with low neutralization potential (estimated > 60 wt.% pyrite, < 5 wt.% calcite/dolomite) with an already acidic environment (average paste – pH 2.8). Gypsum, different types of jarosite and a variety of Fe-sulfates (e.g. melanterite (Fe(SO 4)·7H 2O) and rozenite (Fe(SO 4)·4H 2O)) were the dominant secondary minerals. Less frequent secondary minerals were: Fe(III)-hydroxides (schwertmannite (Fe 16O 16(OH) 12(SO 4) 2) , fibroferrite (Fe(SO 4)(OH)·5H 2O)), and Mg-, Mn- and Zn-sulfates (e.g. starkeyite (Mg(SO 4)·4H 2O), mallardite (Mn(SO 4)·7H 2O), goslarite (Zn(SO 4)·7H 2O), respectively). For the primary mineral assemblage, X-ray fluorescence analyses displayed average concentrations of the heavy metals: Cu 0.1 wt.%, Zn 1.1 wt.%, Pb 1.2 wt.% and Cd 40 mg/kg and of the metalloid As 0.15 wt.%. In secondary minerals an average enrichment of Cu: 0.8 wt.%, Zn: 2.9 wt.%, As: 0.27 wt.% and Cd: 71 mg/kg was observed. Effects of rain events on the waste rocks were simulated by water-leach tests of (1) solid samples from the top, representing waste rocks leached by infiltrating rainwater and (2) solid samples around the base, representing waste rocks affected by outcropping AMD generated in the waste rock dump. The leachates showed stored acidity (pH 1.2–5.6 of the leachates) and high solubility of secondary minerals (average electrical conductivity: 9.7 mS/cm and average concentrations for Fe: 928 mg/L, Zn: 315 mg/L, Cu: 7.5 mg/L, As: 4.1 mg/L, Cd: 1.59 mg/L, and Pb: 0.31 mg/L). The AMD output at springs at the base of the waste rock dump is chemically similar to the leachates (acidic pH (2.78–5.10), oxidizing E h (319–684 mV)), but higher charged with maximum concentrations for Fe: 5640 mg/L, Zn: 3000 mg/L, Cu: 161 mg/L, As: 8.0 mg/L, Cd: 6.6 mg/L, and with higher electrical conductivity (19–26 mS/cm). The data suggest that AMD formation is strongly controlled by the local climate. During the dry winter season, high evaporation of outcropping pore solutions and subsequently precipitation of efflorescent salts resulted in a heavy metal-enrichment at the base of the Excelsior waste rock dump. During the wet summer season, rain events caused the dissolution of most efflorescent salts, removed the enrichment at the base and resulted in a washout of acid solutions rich in Fe, Mn, Zn, Cu, Cd, As and S.