A NEW ("OLD") TYPE OF ZN-ORE RESOURCE: THE "CALAMINE" OF SW SARDINIA (ITALY)

_{The Iglesiente mining district in SW Sardinia, Italy, is one of the classic areas where primary carbonate-hosted Zn-Pb sulfide ores are associated with a relatively thick secondary oxidation zone consisting of Zn-(hydroxy-)carbonates and silicates, the so called "Calamine". These ores were the principal source of Zn before the development of sulfide flotation techniques. The extent of the capping oxidized ore zones, reaching down to several hundreds of meters below the surface, is generally independent from oscillations of the present-day water table. The base of the oxidation profile in various uplifted blocks delimited by post-Variscan faults, can be both elevated above or submerged below the recent water table. The economic minerals are smithsonite, hydrozincite and hemimorphite. We have conducted a combined mineralogical and stable isotopic study on smithsonites and hydrozincites from various mines throughout the district, as well as on recent Ca-carbonates of meteoric origin in the same area. The} d¹⁸O values of smithsonites are very homogeneous, regardless of generation, position and mine (26.5±1.5 ‰ VSMOW). This points to a relatively constant isotope composition of the oxidation fluid: the corresponding formation temperature should be ranging around 28±5°C. Considering the karstic environment of smithsonite formation in SW Sardinia, this temperature seems quite high, although not completely impossible, due to heating during sulfide oxidation. The carbon isotopic compositions of secondary Zn carbonates display considerable variations of more than 8 permil (d¹³C_VPDB from -2 to -10). This large range indicates participation, during sulfide oxidation, of variable amounts of reduced organic and marine carbonate (from Cambrian host rocks) carbon. This isotopic variation is paired by a similar variation in crystal morphologies of the smithsonites, the latter reflecting different environments of formation, with regard to water table oscillations in karstic environments (upper to lower vadose, to epiphreatic). The same range in d¹³C isotopic values is displayed by the calcites associated to Zn-carbonates and by recent speleothemes.