Hydrogeochemical baseline studies prior to gold mining: A case study in Sardinia (Italy)

An epithermal gold deposit is planned to be exploited in the Osilo area (North Sardinia). A detailed hydrogeochemical study was carried out to evaluate the water quality prior to mining, and to assess the potential impact of mining on the aquatic system. Forty-eight surface and ground waters were sampled at different seasonal conditions. A large set of chemical components was determined, taking into account the composition of ore and host rocks, as well as those components, such as cyanide, planned to be used in the mineral processing. Waters show different salinity and element concentrations depending on the composition of rocks drained, the time of circulation, the interaction with mineralised bodies, and climatic conditions. Most waters show relatively low concentrations of toxic or potentially harmful elements.Waters interacting with the ore deposit are generally non acidic, likely due to the low sulphidation system, the limited extension of the ore veins, the presence of calcite in the hydrothermal alteration assemblage, and the weathering of silicate rocks, which appears to contribute in buffering the acidity produced by the oxidation of sulphide minerals, especially pyrite. The nonacid environment does not favour the mobility of elements associated with the mineralisation, such as Cd, Co, Ni, Pb, Zn, and As. In addition, aqueous transport of these elements seems limited by the precipitation of ferrihydrite, which is able to reduce or remove these elements, particularly As, from solution. However, considering that exploitation will expose large quantities of mineralised materials to weathering, an adequate disposal of waste dumps and tailings needs to be carefully planned to prevent the potential dispersion of contaminants.