Low-grade to extremely high-grade ignimbrites and associated deposits of the uppermost Schrikkloof Formation in the Paleoproterozoic Rooiberg Group, Bushveld large igneous province, South Africa

Abstract

The volcanic rocks of the Schrikkloof Formation of the Paleoproterozoic Rooiberg Group provide crucial information on the eruptive and depositional processes preceding or concurrent with the intrusive activity of the Bushveld large igneous province of South Africa. A study area near the town of Modimolle (formerly Nylstroom), approximately 190 km north of Johannesburg, was chosen due to the relatively good exposure. The results of this study show that all rocks within the study area, ranging in composition from rhyolites to dacites, have been affected in various ways by circulating hydrothermal fluids, the presence of which is demonstrated by existing hydrothermal breccias and fumarolic features. The volcanic rocks are classified into three lithofacies types, which can be interpreted as low-grade, high-grade, and extremely high-grade welded ignimbrites. The ignimbrite-forming eruptions, which may have been characterized by low columns, were possibly fed by fissures (potentially related to one or more as yet unidentified calderas) associated with a reactivation of the Thabazimbi-Murchison Lineament, a prominent suture zone in the north of South Africa that has been reactivated several times since the Archean. A decline in volcanic activity may be seen near the top of the volcanic succession, which is coupled with a continual increase in clastic sedimentation. This demonstrates a gradational relationship between the volcanic-dominated Schrikkloof Formation and the overlying clastic-dominated Swaershoek Formation of the Waterberg Group. Therefore, the findings of this study may have profound implications for the Paleoproterozoic stratigraphy of South Africa, which hitherto considers the Swaershoek Formation to have an unconformable relationship with the Rooiberg Group below. Furthermore, the evidence provided in this article may aid in the identification and interpretation of other high-grade ignimbrites encountered around the world.