Stable isotope geochemistry of carbonate minerals in supergene oxidation zones of Zn-Pb deposits
Ore Geology Reviews 33(2): 117-133
The O-18/O-16 and C-13/C-12 ratios of carbonate minerals formed by supergene oxidation of Zn-Pb deposits and submarine alteration of ancient slags were studied in order to constrain isotope fractionation factors for smithsonite, cerussite, and phosgenite, and to characterize conditions of nonsulfide ore formation. We present new isotope data of carbonate-hosted ores from the lglesiente district (Sardinia, Italy) and Vila Ruiva (Portugal), as well as from the silicate-hosted ores of the Broken Hill district, New South Wales (Australia) and Freihung (Germany) and review previously published isotope data. The temperature dependence of oxygen isotope fractionation between Pb and Zn carbonate minerals and water below 240 degrees C for cerussite and below 100 degrees C for smithsonite and phosgenite can be expressed as|10001n alpha(cerussite-water) = 2.29(10(6)/T-2)-3.56|10001n alpha(smithsonite-water) = 3.10(10(6)/T-2)-3.50|10001n alpha(phosgenite-water) = 2.55(10(6)/T-2)-3.50|with Tin Kelvin. The carbon isotope fractionation between smithsonite, phosgenite, hydrozincite and calcite is less than about 2 parts per thousand, while cerussites are strongly depleted in C-13 by about 10 parts per thousand as compared to the former minerals. Oxygen isotope variations of individual carbonate minerals within a deposit are relatively small indicating constant formation temperatures and a single, meteoric fluid source. Average formation temperatures of the studied deposits are calculated at 20 +/- 5 degrees C using the estimated isotope compositions of local paleometeoric waters. We observe a linear relationship between the isotope values of estimated paleometcoric waters and supergene carbonates. Thus, base metal carbonate minerals from supergene deposits may provide paleoclimatic information. The carbon isotope values are in most carbonate-hosted deposits highly variable (more than 10 parts per thousand) suggesting at least two isotopically distinct carbon sources. The isotopically light component can be related to oxidation of C3 plants (soil-derived carbon) and/or microbes that take active part in sulfide oxidation, while the isotopically heavy component originates from carbonate carbon from wall rocks and/or atmospheric carbon. The C-13-enriched component is less dominant in silicate-hosted deposits. (C) 2007 Elsevier B.V. All rights reserved.