Assessment of the precision and accuracy of lead-isotope ratios measured by TIMS for geochemical applications: example of massive sulphide deposits (Rio Tinto, Spain)

The true reproducibility of lead-isotope measurements by thermal-ionization mass-spectrometer (TIMS) was assessed on both the international standard (NBS 982) and sulphide samples from the South Iberian Pyrite Belt (SIPB). Lead-isotope analyses were made on 21 pyrite and galena samples from Rio Tinto sulphide orebodies in the Spanish part of the SIPB. Independent lead-isotope analyses were made in the UM2 laboratory (Montpellier), using a VG Sector 54 mass spectrometer, and at BRGM (Orleans) using a Finnigan Mat 262 mass spectrometer. Internal precision and reproducibility of the isotope measurements were calculated for lead-isotope standards (NBS 982), and for pyrite and galena samples. The reproducibility (2 sigma) is 0.12% for Pb-206/Pb-204, 0.16% for Pb-207/Pb-204 and 0.22% for Pb-208/Pb-204 while the internal precision is on average 0.01% for each isotope ratio (2 sigma(m)). This indicates that the usually adopted errors of between 0.10% and 0.25% are appropriate for geological samples. TIMS has been the most common method for measuring U/Pb ratios by isotopic dilution, but since a few years ICP-MS has also been used for such determinations. We thus checked the reproducibility of U/Pb-ratio measurements by ICP-MS, in order to apply this method to sulphide samples, Independent measurements (2 to 7) for each of 20 analysed samples showed that the measured average reproducibility for U/Pb ratios is better than 5%. This method is thus suitable for determination of U/Pb ratios in sulphide samples and most other geological materials. Lead analyses for the Rio Tints deposit were made of the pyritic and orebodies, and the stringers. This deposit, one of the biggest massive sulphides in the world, is remarkably homogeneous from a lead-isotope viewpoint, and no difference can be seen between pyritic and polymetallic orebodies. The isotope composition of the deposit can be considered as the average composition of the South Iberian crust during the Devonian-Early Carboniferous period of crustal fusion. (C) 1998 Elsevier Science B.V.