Mass-dependent and mass-independent sulfur isotope fractionation (δ34S and δ33S) from Brazilian Archean and Proterozoic sulfide deposits by laser ablation multi-collector ICP-MS

Mass-independent sulfur isotope fractionation (MIF) has been observed in rocks of the geological record older than about 2.45 Ga, a characteristic which is thought to be related to processes in the Neoarchean atmosphere. Samples recording a MIF effect therefore have to contain sulfur of the exogenic sulfur cycle, while endogenic sulfur should not show this effect. The sulfides analysed from six Brazilian deposits represent either exogenic or endogenic sulfur sources, with supposed ages ranging from about 1.9 to 2.7 Ga. Sulfur isotopes were analysed by in-situ laser ablation MC-ICP-MS. A range of experiments were conducted using international and in-house isotope standards, which were run under various conditions and set-ups. These include the reference materials IAEA-S1, IAEA-S3, NBS123 (sphalerite), NBS127 (barite), and in-house standards BSB-py (pyrite) and BSB-cpy (chalcopyrite). During six days of analysis, an internal precision of sulfide analyses of 0.10-0.15 parts per thousand (1 s) for delta S-34 and 0.40-0.60 parts per thousand (1 s) for delta S-33, and an accuracy of similar to 0.30 parts per thousand for delta S-34 and delta S-33 was achieved. The standard measurements define a delta S-33/delta S-34 relationship of delta S-33 = 0.513*delta S-34 + 0.149, with R-2 = 0.9997, which is close to the theoretical relationship for mass-dependent fractionation. The by far best analytical errors were obtained for natural pyrite, reaching a within-run precision of about 0.05-0.15 parts per thousand for delta S-34 (1 s), and 0.10-0.15 parts per thousand (1 s) for delta S-33 determinations. Including all precision and accuracy data, we arrive at the following 1 s error limits to which sulfur isotope analyses for MIF studies obtained with this method are reliable: 0.32, 0.34 and 0.46 parts per thousand for pyrite (delta S-34, delta S-33 and Delta S-33 determinations, respectively), 0.34, 0.42 and 0.54 parts per thousand for pyrrhotite, and 0.34, 0.50 and 0.58 parts per thousand for chalcopyrite. Pyrites and pyrrhotites from 2200 Ma old sediments and volcanics associated with the Pilar greenstone belt, Goias, have delta S-34 from - 15.0 to + 2.0 parts per thousand, and delta S-33 from -8.0 to +1.0 parts per thousand, with Delta S-33 being slightly but not significantly positive. Pyrites from supposed ca. 2060 Ma old clastic sediments of the Aguas Claras Formation (Carajas, Para), have delta S-34 = 9.5-12.5 parts per thousand and delta S-33 = 5.0-6.5 parts per thousand with Delta S-33 close to zero. Chalcopyrites from the Salobo and Sequerinha sulfide deposits, also Carajas, are genetically related to 2570 Ma old A-type alkaline granites with endogenic sources. They have delta S-34 = 1.6-2.9 parts per thousand and delta S-33 = 1.0-1.5 parts per thousand, and delta S-34 = 0.2-0.9 parts per thousand and delta S-33 = 0.65 parts per thousand, respectively, with Delta S-33 ranging from - 0.15 to 0.56. The only Delta S-33 values of this study which clearly suggest sulfur isotope MIF come from the mineralized banded iron-formations of the Quadrilatero Ferrifero, Minas Gerais. Pyrites from the Mina de Cuiaba deposit (2700 Ma) have delta S-34 = 3.8-5.1 parts per thousand and delta S-33 = 2.9-4.8 parts per thousand with Delta S-33 = 0.9-2.2 parts per thousand, while pyrites and chalcopyrites from Sao Bento (2720 Ma) have delta S-34 = 1.6-3.2 parts per thousand and delta S-33 = 1.5-2.8 parts per thousand with Delta S-33 = 0.2-1.5 parts per thousand. The sulfides represent an epigenetic mineralization, derived from supracrustal rocks making part of an exogenic sulfur cycle. The study shows, that in-situ laser ablation ICP-MS is sufficiently sensitive for detecting MIF effects in Neoarchean rocks. Whether or not such samples show MIF-related Delta S-33 values depends on the age and provenance of the sulfur. (C) 2012 Elsevier B.V. All rights reserved.