Extension of the Archaean Madibe-Kraaipan granite-greenstone terrane in southeast Botswana: Constraints from gravity and magnetic data

Abstract

The main goal of this study is to use suitably processed potential field data between longitude 24°E to 26°E and latitude 25°S to 26.5°S to gain a better geological and structural understanding of the extension of the Madibe-Kraaipan granite-greenstone terrane in southeast Botswana. Specifically, 150 new gravity measurements at 2–4 km intervals are reduced and later merged with existing gravity data in Botswana and South Africa for an integrated crustal interpretation with regional aeromagnetic data.Gravity and aeromagnetic anomalies of the region present partly coincident medium to high amplitude regions alternating with low zones. Analysis of the data revealed the existence of relatively narrow NS trending rocks of dense and high magnetic intensity extending to the village of Mmathethe, corresponding to the northern extent of the Kraaipan greenstone belt. Much of the north-central area forms a broad magnetic low and gravity high implying Kraaipan metavolcanic rocks are more extensively developed in this area than previously recognized, under a blanket of Kalahari sediments that are ∼55 m thick as indicated by borehole data. The whole area lies within an ENE-trending Pre-Transvaal dyke swarm visible on the regional aeromagnetic data and much clearer on (proprietary) high resolution aeromagnetic data. The derivative and analytic signal techniques applied for both gravity and magnetic data spatially map the greenstone belt and multiple granite plutons very well.Depth estimates obtained by the 3D Euler method in combination with two-dimensional power spectrum technique locate the high magnetic intensity horizon at around 4.0 km. The depths were further confirmed by gravity model results along two profiles across the granite-greenstone terrane in Botswana and South Africa in a W-E direction. The models show generally steep-sided bodies of comparable width with a maximum depth extent of 4.7 km for the greenstones and 4.4 km for the younger plutons. The distribution and configuration of the greenstones and plutons suggest assembly of this Kaapvaal craton western domain by plate tectonic processes. Episodic intrusion of granitoids into Kraaipan greenstone successions led to deformation of the belt with significance to Au and Au-PGE mineralization. The mapped structures could be important for groundwater exploration in this dry farming terrane.