ASSESSMENT OF THE PRECISION AND ACCURACY OF LEAD-ISOTOPE RATIOS MEASURED BY TIMS FOR GEOCHEMICAL APPLICATIONS - EXAMPLE OF MASSIVE SULFIDE DEPOSITS (RIO-TINTO, SPAIN)

The true reproducibility of lead-isotope measurements by thermal-ioniz ation 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 s amples from Rio Tinto sulphide orebodies in the Spanish part of the SI PB. Independent lead-isotope analyses were made in the UM2 laboratory (Montpellier), using a VG Sector 54 mass spectrometer, and at BRGM (Or leans) using a Finnigan Mat 262 mass spectrometer. Internal precision and reproducibility of the isotope measurements were calculated for le ad-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-20 7/Pb-204 and 0.22% for Pb-208/Pb-204 while the internal precision is o n average 0.01% for each isotope ratio (2 sigma(m)). This indicates th at the usually adopted errors of between 0.10% and 0.25% are appropria te for geological samples. TIMS has been the most common method for me asuring U/Pb ratios by isotopic dilution, but since a few years ICP-MS has also been used for such determinations. We thus checked the repro ducibility of U/Pb-ratio measurements by ICP-MS, in order to apply thi s method to sulphide samples, Independent measurements (2 to 7) for ea ch of 20 analysed samples showed that the measured average reproducibi lity for U/Pb ratios is better than 5%. This method is thus suitable f or determination of U/Pb ratios in sulphide samples and most other geo logical materials. Lead analyses for the Rio Tints deposit were made o f the pyritic and orebodies, and the stringers. This deposit, one of t he biggest massive sulphides in the world, is remarkably homogeneous f rom a lead-isotope viewpoint, and no difference can be seen between py ritic and polymetallic orebodies. The isotope composition of the depos it can be considered as the average composition of the South Iberian c rust during the Devonian-Early Carboniferous period of crustal fusion. (C) 1998 Elsevier Science B.V.