0361-0128/01/3351/731-18 $6.00 731 Introduction THE RECENT revival of commercial interest in nonsulfide zinc ores (Large, 2001; Hitzman et al., 2003), related to the devel- opment of solvent-extraction and electrowinning technology applied to their treatment, is renewing scientific research fo- cused on “zinc oxide” deposits throughout the world (e.g., An- gouran, Iran: Annels et al., 2003; Gilg et al., in press a; Beltana, Australia: Groves and Carman, 2003; Shaimerden, Kazakhstan: Boland et al., 2003; Skorpion, Namibia: Borg et al., 2003). In the Iglesiente area of southwest Sardinia, Italy, one of the oldest districts in the world mined for base metals, no- table occurrences of secondary nonsulfide Zn >> Pb ores (“calamine”) were found at the end of the nineteenth century by the Belgian company Vieille Montagne and exploited in several mines. Historically, the so-called “calamine” ores, a mixture of supergene zinc (with minor Pb) carbonates, hy- droxy carbonates, and silicates, capping the primary sulfide bodies (Boni et al., 1996), were the principal source of zinc from this district for more than 60 years. Their characteristics were described in several mine reports (e.g., Münch and Siebdrat, 1960) and their spatial distribution and mineralogy were investigated by several authors (Billows, 1941; Cavinato, 1952; Zuffardi, 1952). However, only limited data exist in the recent geologic literature about the Sardinian calamine de- posits (Moore, 1972; Bonifazi and Massacci, 1987). In Boni et al. (in press) and Aversa et al. (2002), the theme of calamine and of its relationship with primary sulfide ores was briefly The “Calamine” of Southwest Sardinia: Geology, Mineralogy, and Stable Isotope Geochemistry of Supergene Zn Mineralization MARIA BONI, † Dipartimento di Geofisica and Vulcanologia, Università di Napoli “Federico II,” Via Mezzocannone 8, 80134-Napoli, Italy H. ALBERT GILG, Fakultät Chemie, Technische Universität München, Lichtenbergstr. 4, 8574-Garching, Germany GASPARE AVERSA,* Dipartimento di Geofisica and Vulcanologia, Università di Napoli “Federico II,”Via Mezzocannone 8, 80134-Napoli, Italy AND GIUSEPPINA BALASSONE Dipartimento di Scienze della Terra, Università di Napoli “Federico II,” Via Mezzocannone 8, 80134-Napoli, Italy Abstract The mining district of southwest 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 containing Zn (hydroxy-)car- bonates and silicates, the so-called “calamine,” exploited until the 1970s. The extent of the capping oxidized ore zones, reaching deep below the surface, is generally independent of the present-day water table. The base of the oxidation profile containing nonsulfide Zn minerals in various uplifted blocks in the Iglesiente area can be both elevated above or submerged below the recent water table. The genesis of the ores is therefore con- sidered to be related to fossil, locally reactivated, oxidation phenomena. The mineralogy of the nonsulfide min- eralization is generally complex and consists of smithsonite, hydrozincite, and hemimorphite as the main eco- nomic minerals, accompanied by iron and manganese oxy-hydroxides and residual clays. This study places the secondary ores in the context of the tectonostratigraphic and climatic evolution of Sar- dinia and includes a petrographic and mineralogic study of the most abundant minerals, relating the mineral- ogy of secondary Zn and Pb carbonates to their stable C and O isotope geochemistry and constraining the ori- gin of the oxidizing fluids and the temperature of mineralization. The δ 18 O VSMOW values of smithsonite are homogeneous, regardless of crystal morphology, position, and mine location (avg. 27.4 ± 0.9‰). This homo- geneity points to a relatively uniform isotopic composition of the oxidation fluid and corresponding formation temperatures of 20° to 35°C. Considering the karstic environment of smithsonite formation in southwest Sar- dinia, this high temperature could be due to heat release during sulfide oxidation. The carbon isotope compo- sitions of secondary Zn carbonates display considerable variations of more than 9 per mil (δ 13 C VPDB from –0.6 to –10.4‰). This large range indicates participation of variable amounts of reduced organic and marine car- bonate carbon during sulfide oxidation. The isotopic variation can be related to a variation in crystal mor- phologies of smithsonite, reflecting different environments of formation with respect to water table oscillations in karstic environments (upper to lower vadose to epiphreatic). The same range in δ 13 C isotope values is dis- played by the calcite associated with Zn carbonates and by recent speleothems. The most reliable time span for the deposition of bulk calamine ore in southwest Sardinia ranges from mid- dle Eocene to Plio-Pleistocene, although further multiple reactivation of the weathering profiles, peaking within the warm interglacial periods of the Quaternary, cannot be excluded. Economic Geology Vol. 98, 2003, pp. 731–748 † Corresponding author, e-mail: boni@unina.it *Present address: Department of Geology, Rand Afrikaans University, P.O. Box 524, Auckland Park 2006, South Africa.