Sheet silicates as effective carriers of heavy metals in the ophiolitic mine area of Vigonzano (northern Italy)

In order to assess the role played by sheet silicates in controlling base metal distribution, the mineralogy of the <2 mu m fraction of waste material, surface soils, and stream sediments was investigated in the surroundings of a pyrite-chalcopyrite mine area in northern Italy. The results indicate that smectite is very abundant in the <2 mu m fraction of the tailings, and it exerts an effective control on the concentrations of Zn, Ni (and Cu). Normal soils and sediments are characterized by interstratified illite-smectite and by lower base metal concentrations. Away from the mine area, smectite decreases in the stream sediments, and chlorite becomes more important in controlling base metal distribution. Ni, Cr, Cu and Co increase with chlorite and talc, but are depleted in the <2 mu m fraction compared to coarser fractions in the stream sediments, because the sand-silt fraction of sediments concentrates ophiolitic fragments, variably enriched in ores. A mechanical dispersion of chlorite is probably the controlling factor. Zinc displays a systematic enrichment in the clay fraction of waste material and in the stream sediments but the main mineral carrier is not identified.