Roman and modern slag at S. Domingos mine (IPB, Portugal): Compositional features and implications for their long-term stability and potential reuse

S. Domingos ores have been exploited for Cu and S from antiquity to 1966. Slag is composed of Fe-, Ca-, Zn- and Pb-rich silicate glass, fayalite, hedenbergite, Fe-spinel, and blebs of Fe-, Cu-, Zn- (and Pb-)sulphides, minor sulphosalts and Cu-Zn-Pb-iss with evidence for low-T exsolution and reactions. Silicates and oxides often crystallised far from equilibrium out of undercooled melts. Weathering leads to Fe-(hydro)oxide + digenite ± Fe-, Cu- and Pb-sulphate, Ca, Zn and Pb loss from glass and acid drainage. The slag is an excellent aggregate raw material but its chemical instability calls for preliminary processing, which might lead to economic metal recovery.