Factors controlling heavy-metal dispersion in mining areas: the case of Vigonzano (northern Italy), a Fe-Cu sulfide deposit associated with ophiolitic rocks

Abstract.

Heavy-metal dispersion around the Vigonzano copper mine has been investigated by the analysis of waste dump material, surface soils, stream sediments and stream waters. Factors controlling their distribution have been investigated by means of mineralogical analyses, grain-size separation, and sequential extractions. Sheet silicates (chlorite and talc) and quartz dominate the mineralogy of the waste dump which is characterized by goethite, a product of sulfide weathering. Smectite, chlorite and talc are abundant in the <2 µm fraction. Chromium, Ni, Co and Cu have high enrichment levels within the waste area, but with the exception of Cu, are related to the occurrence of barren ultramafic rocks. Cu and, to a lesser extent, also Zn derive from ore minerals and are associated with their weathering products. Acid drainage has been observed but it is restricted to the waste area. Dissolved metal concentrations are locally high (e.g. 63 mg/l for Cu) in surface runoff from the waste area, but do not severely affect the surroundings because of precipitation of amorphous Cu-Al sulfate, related to an alkaline geochemical barrier. The limited impact of the mine is also due to the geological setting of this type of mineralization (Cyprus type), characterized by the association of the ore with mafic and ultramafic rock because the latter are characterized by alkaline drainage.