Geochemistry of dolerite intrusions of the southeastern Main Karoo Basin, South Africa: Magma evolution, evidence for thermogenic gas sequestration, and potential implications for the early Toarcian Oceanic Anoxic Event

Abstract

The igneous intrusions of the Karoo Large Igneous Province (LIP) provide critical information on the magma plumbing system, which served as source rock maturation agent for organic-rich sedimentary rocks of the Main Karoo Basin as well as pathway and barrier to subsurface fluid flow. Here, we present results from the research borehole KWV-01 drilled in the Eastern Cape Province of South Africa. The intersected succession, spanning the Dwyka to the lower Beaufort Group, provides representative samples of the Karoo LIP magma plumbing system.Using whole-rock and Sr-Nd isotope analyses, our data suggest that primary melts were derived from a sub-lithospheric mantle source and acquired a subduction signature through interaction with a metasomatised lithospheric mantle. Three magma types identified within this study show a geochemical overlap with previously reported data from the Golden Valley Sill Complex, ca. 220 km to the NW of the analysed borehole. The results of MELTS and EC-RAFC modelling on the three magma types reveal that their compositional differences are related to variable degrees of fractional crystallisation and contamination by lower crustal rocks and/or the sedimentary host rocks of the Main Karoo Basin.Our results indicate that at least one of the sills acted as fractured reservoir, thereby providing evidence that igneous intrusions are important for CO2 sequestration. We quantified the implications of this process for the Karoo LIP-related Toarcian Oceanic Anoxic Event using the thermogenic gas budget produced by state-of-the-art computer simulations and tested various end-member scenarios for partial thermogenic gas sequestration. In agreement with recent studies, we show that all the flat parts of the Karoo sills acted as sequestration reservoirs. Our results have general implications for LIP volcanism in organic-rich basins, suggesting that a good first-order approximation of thermogenic gas emission is half the amount mobilised in the contact aureoles of LIP plumbing systems.