THE GEOMETRY OF THE NORTHERN LOBE OF THE BUSHVELD COMPLEX: IMPLICATIONS FOR INTRUSION VOLUMES AND FEEDER LOCATIONS

Determining the three-dimensional extent of the Northern Lobe of the world’s largest layered mafic intrusion, the Bushveld Complex, is critical for understanding the origin of the intrusion and aiding platinum group element (PGE) exploration. Exposures of the dense layered mafic portion of the Bushveld Complex produce positive gravity anomalies. Normally and reversely magnetized layered mafic rocks produce both high amplitude positive and negative magnetic anomalies, respectively. Coincident gravity and magnetic anomalies characterize much of the exposed Northern Lobe. Modeling of the gravity and magnetic data constrained by geologic mapping, rock properties, borehole stratigraphy, paleomagnetic data, electrical resistivity and various seismic estimates of crustal thickness help delineate the 3-dimensional extent of the Northern Lobe layered mafic intrusion. The models show that the Bushveld layered mafic rocks extend 145 km west of the exposures in the Northern Limb and 75 km west of the Villa Nora outcrop. The predominantly reversely magnetized northern portion of the complex is truncated by magnetic, highly mylonitized portion of the Palala shear zone. The layered mafic rocks continue south through the Thabazimbi-Murchison Lineament (TML), partially connecting with the Main Lobe of the Bushveld complex. The thickness of the layered mafic complex averages ~6000 m or ~3800 m if lower crustal mafic underplating (shear velocities of >4 km/s) is included in the models. The area covered by the layered mafic complex from the TML to the north is ~31000 km² and volumes range from 180,000-195,000 km³. For the models with mafic underplating, the intrusion volume of ~133000 km³ added to the volume of mafic underplating of ~177000 km³, yields a total mafic volume of ~310,000 km³ in the Northern Lobe region alone. The thickness is largest over the TML, center, and southern portion of the Northern Limb suggesting that these regions represent feeders for the magma that then flowed north and west.