Hydrozincite and Brianyoungite precipitation in an abandoned Zn-Pb mine in the Spanish Pyrenees: implications for natural attenuation of heavy metals
Ponente: Max Giannetta (IDAEA)
Lugar: Departamento de Ing. Civil, D2 Planta Baja Aula 001
Título: Hydrozincite and Brianyoungite precipitation in an abandoned Zn-Pb mine in the Spanish Pyrenees; implications for natural attenuation of heavy
The Victoria Mine is located the Aran Valley, Spain, and was active until approximately 1950 when it was abandoned. The underground mine targeted mineral deposits comprised of mostly sphalerite (ZnS). Today the relict tunnels and shafts are exposed to air and flowing water prompting oxidative dissolution of the mineral deposits and microbial activity. A sampling campaign into one of the mining tunnels shows a trend of increasing dissolved metal concentrations (e.g. Cd change from 28 to 20 ppb) as a function of distance from the entrance (i.e. decreasing downgradient). White precipitates are prevalent throughout the tunnel galleries and preliminary XRD analysis results suggest that Hydrozincite and Brianyoungite are the predominant minerals. This is an indication of partial uptake of harmful metals (e.g. Cd, Ni, Li, Ti, Zn) into the crystal structure of the white precipitates, thus decreasing the mobility of the dissolved metals. The formation of these minerals may be a result of biomineralization, though further investigation is required. Moreover, microbially mediated processes (e.g. Fe oxidation and Mn oxidation) exhibited in the tunnel may contribute to the rate of ZnS dissolution. Eventually, this flowing water, still containing elevated metal concentrations, exits from the lowermost mining gallery and reconnects with the associated catchment system.
As a comparison, one of the abandoned Zn-Pb ore processing facilities in a neighboring catchment, known as Pontaut, was also sampled for dissolved metals. Rain water filters through the tailings dumps and flotation ponds before leaving the site. Here, there was no evidence of secondary white precipitates, and dissolved metal concentrations were elevated beyond that of the Victoria Mine tunnel (e.g. 127 ppb Cd).
Notably, the concentration of dissolved metals increases down gradient, contrary to the behavior of the Victoria Mine tunnel gallery. Preliminary geochemical modeling of the fluids at both sites suggest that the formation of Hydrozincite and Brianyoungite is less likely to occur at the processing facility due to lower alkalinity in comparison to the fluid in the mine. A result of this comparison between sites, we propose that the secondary mineral precipitation is helping to remediate some of the heavy metal contaminants. Both sites exhibit heavy-metal discharge concentrations above the MCL creating a risk for nearby plant and animal life. Two mushroom species in the Pontaut site, Amanita muscaria and Macrolepiota procera, were found to have dangerous Cd levels of 112 +/- 8 and 90 +/- 11 ppm, respectively. Eventually both systems flow into the Garonne River, however these concentrations are diluted to safe levels after mixing with the river.