GEOPHYSICAL INTERPRETATION OF THE NATURE AND EXTENT OF THE XADE MAFIC COMPLEX, BOTSWANA

Abstract

The Xade Mafic Complex, situated in central Botswana, was identified during the first regional aeromagnetic survey of the country in 1975–77. It is covered by sedimentary rocks of the Kalahari Group and Karoo Supergroup, including Karoo lavas, with a combined thickness varying from 220 m to 1000 m, extending to greater depths in the north beneath the Meso-proterozoic Passarge Basin. What has generally been viewed as the Xade Complex is evidenced by a high amplitude kidney-shaped zoned magnetic anomaly with two semi-linear anomalies extending to the northwest and northeast in a Y-shaped form. The complex is also marked by a coincident Bouguer gravity anomaly. Historical work has been limited, with only three diamond drill holes having been drilled, separately intersecting gabbroic rocks, weathered basalt grading into dolerite, and amygdaloidal lavas followed by Waterberg Group shales. An U-Pb zircon age of 1109.0 ± 1.3 Ma, which is coeval with the Umkondo Igneous episode, has been published for the gabbroic unit. This study has shown, for the first time, that the Xade Complex comprises two lobes of potentially different magmatic character, i.e. a Southern Lobe, which is the historically identified kidney-shaped zoned magnetic anomaly, as well as a hitherto unrecognized large Northern Lobe. The latter is mostly deeply buried in the north and northwest beneath the Passarge Basin, as evidenced by deep magnetic and gravity sources, but its southern and eastern margins partially suboutcrop beneath Karoo sediments, forming the Y-shaped anomalies north of the Southern Lobe. Forward modelling, of both magnetic and gravity data, indicates that the Southern Lobe is a lopolith with a depth extent of approximately 4 km, whereas the southeastern margin of the Northern Lobe has westerly dips consistent with deepening to the northwest. In addition to the historic boreholes, a recent borehole drilled into the Southern Lobe shows that it comprises a mafic volcanic sequence with subordinate gabbro. A further two boreholes drilled into the Northern Lobe margins indicate that it comprises a texturally heterogeneous and magmatically differentiated sequence of gabbroic rocks, with minor dioritic and monzonitic rocks. The present interpretation has also identified a dyke system associated with the Northern Lobe, which may represent either feeder or exit magmatic conduits. The interpretation further shows that the complex is located in a craton margin setting. The combined extent of both lobes is approximately one-third the size of the Bushveld Complex, making the Xade Mafic Complex the largest Meso-Proterozoic magmatic system in southern Africa. The craton margin setting, size, Meso-Proterozoic age and variability of plutonic and volcanic rocks hallmark the Xade Mafic Complex as being a very large dynamic magmatic system, with potential for Ni-Cu-PGE mineralization.