The Trompsburg Complex, South Africa: A preliminary three dimensional model

The 1915 ± 6 Ma Trompsburg Complex is a large layered mafic intrusion, measuring approximately 2500 km2 in area. Seven boreholes were drilled in the 1940s to constrain a strong gravimetric and magnetic anomaly. These approximately 2 km deep boreholes intersected up to 760 m of gabbro-troctolite–anorthosite containing up to 19 massive magnetite layers.Physical property analyses including density, magnetic susceptibility, intensity of magnetization, electrical resistivity and induced polarization were performed on specimens from these cores. Gridded aeromagnetic and Bouguer gravity data, combined with the available borehole information were used to construct a three dimensional polygonal model of the Trompsburg Complex. This was then used to calculate a forward three dimensional gravity and magnetic model using an algorithm that computes the gravity and magnetic fields for polyhedrons. Unfortunately the boreholes were confined to the northwestern part of the Complex and as a result the rest of the model is not as well constrained as in this area. The model is saucer-shaped at the top, with a large feeder to the south-east. The extremely high values of the Bouguer gravity anomaly reaching a maximum of almost 93 mGal with respect to the regional suggests a substantial thickness of more than 6000 m for the Complex reaching up to 16,000 m at the feeder.