RELATIONSHIP BETWEEN BASALT ALTERATION, CHERT FORMATION AND TECTONOSTRATIGRAPHY AT BARBERTON MT. LAND, SOUTH AFRICA

The basalts and cherts of the Mesoarchean Barberton Greenstone Belt in South Africa have been the focus of continuous study. Here we report the δ¹⁸O of basalts sampled along 18 measured sections from the Mendon, Kromberg and Hooggenog Complexes. The pillow basalts are enriched in ¹⁸O suggesting alteration by ambient seawater but the details of the alteration pattern may be obscured due to extensive metamorphism and tectonism but regional metamorphism has not erased the isotopic record. The lavas are commonly silicified and are intralayered between prominent chert horizons that systematically decrease in δ¹⁸O from 20‰ from the Mendon Complex to 13.3‰ at the Middle Marker, the base of the Hooggenog complex. All the cherts in the Hooggenog Complex are silicified volcanoclastic rocks. The δ¹⁸O of the basalt in some exposures is proportional to the degree of silicification extrapolating to the overlying chert value implying in those cases that the silicification of basalt was by the same fluid that formed the chert. Correlation of δ¹⁸O and SiO₂ is not observed in sections containing dykes, or enveloped by shear zones. In the lower parts of the Hooggenog such an extrapolation of δ¹⁸O vs. SiO₂ leads to higher δ¹⁸O end member than the actual chert. The δ¹⁸O of major cherts within the Hooggenog Complex translate to temperatures of about 110° at top to about 180° C at the Middle Marker, if the cherts formed in the presence of seawater derived fluids. The combined thickness of the volcanic pile and associated intrusive rocks of the Hooggenog is 4 to 4.5 km, and thus the implied thermal gradient seems too low to be the ambient geothermal gradient. The basalts under the higher δ¹⁸O cherts, which are also higher in the sections, may be silicified by the same fluid at the same time whereas the basalts associated with lower δ¹⁸O cherts, lower in the section, where silicified by a higher δ¹⁸O fluids or at lower temperature. These observations indicate a complex and dynamic relationship between the alteration of basalt and formation of these cherts.