Unearthing the Xade Complex, Botswana

Abstract

The Xade Complex, situated in central Botswana, is completely buried under younger cover. It was identified during the first regional aeromagnetic survey of the country in 1976. This paper presents the results of an exploration venture which has been in progress since 2005. The Complex, as previously published, is evidenced by coincident magnetic and gravity anomalies, occurring on the margins of the Kaapvaal and Zimbabwe Cratons. It is shown here that the Complex comprises two lobes - a southern lobe (SL), and an hitherto unidentified northern lobe (NL). Forward modelling of both data sets over the SL indicates it to be a lopolithic feature. The NL is deeply buried in the northwest beneath the Neoproterozoic Passarge Basin, but its southern and eastern margins partially suboutcrop beneath Karoo sediments. Inversion depths range from 250 to 900 m beneath Kalahari and Karoo sediments, and well in excess of this beneath the Passarge Basin. One recent and two historic boreholes in the SL reveal a sequence of basaltic lavas with subordinate gabbro. A further single historic and two recent boreholes drilled into the NL margins reveal a package of heterogeneous gabbronorites. An U-Pb zircon age of 1109.0±1.3 Ma has been published for a gabbro unit, which is coeval with the Umkondo Igneous Province. The combined extent of both lobes of the Xade Complex is approximately one-third the size of the Bushveld Complex, making it a very large differentiated magmatic system, with Ni, Cu and PGE potential.