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

Abstract

The Princeton mine is one of several mesothermal gold deposits found in the Barberton Greenstone Belt. The host rocks of the ore bodies are carbonaceous and pyritic shales of the Fig Tree Group (∼3.4 to 3.2 Ga in age). The methothermal gold mineralization has occurred during the latest thrusting and deformation events at ∼3.1 Ga. Micro-analyses of sulfur isotopes of pyrite were performed on four representative samples collected from the Princeton mine. The δ 34S values were determined in situ on individual pyrite crystals or aggregates of fine-grained pyrite by the laser microprobe method (58 analyses), and also on aggregates of separated pyrite crystals by the conventional Cu 2O combustion method (11 analyses). Trace element concentrations of pyrite were also determined using an electron microprobe. 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 pyrite and pyrite laminae concordant to sedimentary bedding. Their textures suggest that the fine-grained pyrite was formed during sedimentation of the carbonaceous shale at 3.4 to 3.2 Ga. The δ 34S values of the fine-grained pyrite range from −0.8 to +4.4‰ within a micro-scale area (1 mm×2 mm) (29 laser microprobe analyses), suggesting that the microbial 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 previously popular theory that the Archean ocean was H 2S-rich and pyrite in Archean sedimentary rocks was formed by an inorganic process. Coarse-grained pyrite is associated with auriferous quartz veins and often contains gold grains. Trace element concentrations of coarse-grained pyrite, such as As concentrations, are different from these of fine-grained pyrite. The petrographic features suggest that the most coarse-grained pyrite was directly precipitated from high temperature hydrothermal fluids. Coarse-grained pyrite has homogeneous δ 34S values, ranging from +1.1 to +3.6‰ (29 laser microprobe analyses), suggesting that source of sulfur for the coarse-grained pyrite was reduced sulfur species in hydrothermal fluids.