The relation between shallow marine chert, barite and jaspilite deposition in the 3.2 Ga Mapepe Formation of the Barberton Greenstone Belt, South Africa.

Abstract

Normally rare barite deposits are quite common in the 3.260–3.226 Ga Fig Tree Group of the Barberton Greenstone Belt in South Africa. These barites in the Mapepe Formation of the Fig Tree Group were seemingly fed by chert dikes and are unusually rich in detrital minerals. Immediately below the barites jaspilite bands can be found containing oxidized iron. Barite is used for estimates of the atmospheric oxygen levels before 3.0 Ga and the sulphur inside the barites is extensively used to understand the sulphur cycle on the Early Earth. If the barites are to be used to answer these questions, it is paramount to understand the geological formation of the barites. The aim of this research is to determine the relationship between the barite, chert and jaspilite deposition in the Mapepe Formation and to uncover how the barite deposits formed. Barites are inert to instruments analysing trace elements and almost no trace element data exists on barites. In this research the first relatively complete and significant trace element data on barites are presented. Samples of the barites, cherts and jaspilites were taken from the Old Mine, Barite Valley and Puddingstone Hill sites. Bulk Laser-Ablation Inductively Coupled Plasma Mass Spectrometry (LAICP-MS) trace element analyses were done on pearls of the barites and cherts normally used for X-Ray Fluorescence (XRF). Moreover, microanalytical trace element analyses have been done on epoxy blocks of the cherts and jaspilites. To circumvent any possible BaO interference, barium was separated from the barite samples using column separation and the residue inside the columns analysed using Solution-ICP-MS. Lastly, cathodoluminescence, back scatter and secondary imaging was done on the zircons from the barites using the Electron Mirco Probe Analyzer (EMPA) and U-Pb dating was executed on these zircons using LA-ICP-MS. No evidence for a relation to sea water is found in the barites and the cherts. The barites show a relatively flat PAAS (Post Archean Average Shale) normalized REEY (Rare Earth Element Yttrium) pattern with small positive Eu anomalies in the Puddinstone Hill samples and some Barite Valley samples. The other Barite Valley samples and the Old Mine samples do not show a positive Eu anomaly. Similar REEY patterns can be found in the cherts with two additional types, one that is LREE depleted and one with positive REEY slope. The remaining trace elements are quite similar and demonstrate a relation between the barites and the cherts. Wall rock incorporation may have been a significant contributor of the REEY in the cherts. The observed REEY patterns for the cherts and the barites can be linked to the input of low temperature continental hydrothermal fluids and were deposited close to or on the continent. Chert dikes were likely the conduits supplying the Ba2+ to the surface where it reacted with photolytically produced SO4 2- supplied by meteoric fluids to form the barites. The difference in size of the positive Eu anomalies, likely indicates a difference in temperature and possibly composition of the hydrothermal fluids feeding the different sites. The U-Pb data of the zircons inside the barites is highly discordant and intersects the concordant line at ~3.25-(3.3) Ga and the origin. Only one concordant zircon with an age of 3.243 ± 0.016 Ga was found and the remaining zircons had experienced Pb loss due to recent weathering. Four other zircons deviate from the discordant line and intersect at ~3.45 Ga. The obtained ages are consistent with detrital zircons from the Fig Tree Group and had been sourced from TTG bodies from the Onverwacht Group and tuffs from the Mendon Formation and the Fig Tree Group itself. A large input of detritus to the barite deposits elevated the REEY patterns inside the barites. The geochemistry of the jaspilites is completely unrelated to the geochemistry of the barites and the cherts. The jaspilites have a clear sea water signature with a positive PAAS normalized REEY slope and a positive Eu anomaly and positive Y anomaly. The shallow slope of the REEY may indicate that the jaspilite bands were deposited from a mixture of marine and hydrothermal fluids on the sea floor. The oxidation of the jaspilites may have occurred either by microorganisms or by free oxygen in the lower Mapepe Formation.