Sulfur isotope evidence for the origin of 3.4 to 3.1 Ga pyrite at the Princeton gold mine, Barberton Greenstone Belt, South Africa

The Princeton mine is one of several mesothermal gold deposits found in the Barberton Greenstone Belt. The host rocks of the ore bodies are carbonaceo us and pyritic shales of the Fig Tree Group (similar to 3.4 to 3.2 Ga in ag e). The methothermal gold mineralization has occurred during the latest thr usting and deformation events at similar to 3.1 Ga. Microanalyses of sulfur isotopes of pyrite were performed on four representative samples collected from the Princeton mine. The delta(34)S values were determined in situ on individual pyrite crystals or aggregates of fine-grained pyrite by the lase r microprobe method (58 analyses), and also on aggregates of separated pyri te crystals by the conventional Cu2O combustion method (11 analyses). Trace element concentrations of pyrite were also determined using an electron mi croprobe. Based on their grain sizes and chemical compositions, the pyrite samples examined are divided into two groups: (1)fine-grained pyrite and (2 ) coarse-grained pyrite. The fine-grained group includes disseminated pyrit e and pyrite laminae concordant to sedimentary bedding. Their textures sugg est that the fine-grained pyrite was formed during sedimentation of the car bonaceous shale at 3.4 to 3.2 Ga. The delta(34)S values of the fine-grained pyrite range from -0.8 to +4.4 parts per thousand within a micro-scale are a (1 mm x 2 mm) (29 laser microprobe analyses), suggesting that the microbi al reduction of seawater sulfate was responsible for the formation of fine- grained pyrite. This further suggests that the 3.4 to 3.2 Ga ocean already contained an appreciable amount of sulfate, which is in contrast to the pre viously popular theory that the Archean ocean was H2S-rich and pyrite in Ar chean sedimentary rocks was formed by an inorganic process. Coarse-grained pyrite is associated with auriferous quartz veins and often contains gold g rains. Trace element concentrations of coarse-grained pyrite, such as As co ncentrations, are different from these of fine-grained pyrite. The petrogra phic features suggest that the most coarse-grained pyrite was directly prec ipitated from high temperature hydrothermal fluids. Coarse-grained pyrite h as homogeneous delta(34)S values, ranging from +1.1 to +3.6 parts per thous and (29 laser microprobe analyses), suggesting that source of sulfur for th e coarse-grained pyrite was reduced sulfur species in hydrothermal fluids. (C) 1999 Elsevier Science B.V. All rights reserved.