Contact Metamorphism of Black Shales in the Thermal Aureole of a Dolerite Sill Within the Karoo Basin

Abstract

Recent interest in the shale gas potential of the Karoo Basin has highlighted the metamorphic effects that Jurassic dolerite intrusions may have had on the Lower Ecca Group black shales. We explore the use of metamorphic petrology and Ti in biotite geothermometry to constrain conductive heat flow modelling of a thermal aureole adjacent to two closely spaced, thin dolerite sills intruded into these black shales. Our numerical heat flow modelling shows that a 45 m thick sill may heat a ~100 °C host rock to 650–700 °C at the contacts, and to ~350 °C, 45 m away from the contact. Mineral phase equilibrium modelling constrains minimum temperatures of ~375 °C up to 13 m above the two sills and cooler temperatures farther away and below the sills. Titanium in biotite (TIB) geothermometry results are varied, suggesting disequilibrium conditions within the contact aureole.