A combined in situ oxygen, silicon isotopic and fluid inclusion study of a chert sample from Onverwacht Group (3.35 Ga, South Africa): New constraints on fluid circulation

Abstract

Cherts have been extensively used as a proxy of Precambrian environmental conditions of the Earth. Cherts from the Onverwacht group (3.5–3.3 Ga, South Africa) are considered as well-preserved and have been used as a paleo-oceanic thermometer, especially by using the oxygen isotope composition of the chert. Nevertheless, there are several lines of evidence (size and number of the quartz veins) that fluids circulated through these cherts after their deposition. These cherts are composed mainly of microquartz with large quartz veins (~ 100 μm) and iron carbonates as ankerites. A combined study of oxygen, silicon and fluid inclusions study have been performed on one chert sample from the Onverwacht group in order to assess the effect of these fluid circulations on the chert oxygen and silicon isotopic compositions. In-situ oxygen and silicon isotopic compositions on microquartz and vein quartz were measured at high spatial resolution using a multicollector Cameca 1270 ion probe (CRPG-CNRS, Nancy). Moreover, fluid inclusions present in the quartz veins are considered to determine the origin and conditions of the fluid circulation. Three types of primary fluid inclusions, carbonic (Lc), aqueous carbonic fluids (Lc-w) and aqueous fluid (Lw) have been observed. The temperatures of this fluid circulation were determined by microthermometry and using quartz–carbonate pair thermometry at between + 200 °C and + 300 °C. Fluid pressures range from 100 MPa to 30 MPa, which correspond to depths ranging from 4 to 1 km. This evolution could be related to an exhumation associated with a dilution and cooling process. This fluid circulation could be associated to the gold-bearing fluid responsible of the Sheba–Fairview ore system dated at 3074 ± 6 Ma. The results of the isotopic study show that these fluid circulations homogenised the oxygen isotope composition but not the entire silicon isotope composition of the microquartz. The δ 18O value calculated for the fluid show that it equilibrated with carbonate rocks and is similar to the composition of the gold-bearing fluid. The silicon isotope composition of the microquartz allows determination of the initial δ 30Si of the microquartz precursor. This precursor has been precipitated from a hydrothermal fluid. Hence, a combined δ 18O, δ 30Si and fluid inclusion study allows the conditions of the fluid circulation to be constrained and the origins of chert to be determined.