Zincian dolomite: A peculiar dedolomitization case?

A peculiar dedolomitization phenomenon is associated with the supergene alteration of Zn-Pb sulfide ores, resulting in the precipitation of newly formed carbonate phases. In addition to the deposition of calcite and several metal carbonates, this phenomenon results in a widespread replacement of host-rock dolomites by zincian dolomite phases. Dolomite samples have been collected in the oxidation zone of the mining districts of southwestern Sardinia (Italy), Yanque (Peru), and Jabali (Yemen), and the results compared with data sets from Upper Silesia (Poland) and Namibia. In all districts Zn dolomite (as much as 20% ZnO) replaces the previous dolomite phases through fractures and along crystal growth zones, and smithsonite (ZnCO3) may be high in Mg (to 15% MgO). Zn dolomites are produced by the reaction of metal-carrying and O-rich meteoric fluids with preexisting dolomite bodies hosting Zn sulfide ores, subjected to active weathering. Their occurrence in the oxidation zone of the sulfide deposits corroborates their supergene origin. The ample extent of the Zn dolomite replacement bodies, underestimated so far, is important for the exploration of nonsulfide Zn ores, because it may lead to an incorrect evaluation of the extractable metallic resources calculated from the assay data.