1247 Hydrothermal ore-forming processes Abstract. A drilling program in NW Botswana based on anomalies detected by geophysical prospection allowed discovering the structure of the Proterozoic basement and several styles of mineralizations. A riftogenic Proterozoic basin trending NW-SE was filled with sedimentary series, mainly black shales and conglomerates in the depocenter and conglomerates, quartzites and limestones at the margins. Opening of the basin resulted in mantle rising and basaltic vulcanism. SEDEX-type Cu-Pb-Zn-Ni-Co-Ag sulphide deposits accumulated in black shales. During a compressive stage, tectonic inversion produced reactivation of the above mentioned normal faults as inverse and the emplacement of ophiolitic sequences. Movement of basinal hydrothermal fluids during this stage produced the removilization of sulphides along with As-Au-Bi-Mo- REE-Th-U. These filled the Pb-Zn-Ni-Co-Ag-As-Au-Bi- Mo-REE-Th-U veins in the sheared zones and developed IOCG deposits replacing the margins of the basin. The ensemble suffered folding and regional metamorphism in amphibolite facies. Post-tectonic granites also have vein polymetallic mineralization that could correspond to an Intrusion-Related Gold Deposits model; they produce skarns on the hosting limestones. The studied skarns are poor in metals, but the context can be favourable for Au- bearing skarns. Keywords. Botswana, IOCG, magnetite, skarn, shear, ophiolite, rift, tectonic inversion, heritage, SEDEX, IRGD. 1 Introduction Many areas in Botswana are covered by large extensions of alluvial sediments associated with the Okavango delta. Therefore, no much information was available about the geology of the Precambrain basement of the area. During exploration for hidden kimberlites, Tsodilo Resources Ltd found huge geophysical anomalies extending tens of kilometres. Drilling on these areas allowed establishing the geology of the basement and opened an interesting window to the exploration of an unsuspected area. The aim of this work is to give a first draw of the metallogenetic events registered in the area. 2 Geographic and geological setting The Tsodilo Hills are located at the NW of Botswana, 200 km NW far of the Maun City (Fig. 1), close to the Okavango delta. Most of the area is covered by recent sediments (e.g. Kalahari formation, thick soils, calcretes and alluvial materials of the Okavango River). Therefore, information on the underlying formations can only be achieved by applying geophysical methods and diamantine drilling. New data obtained by Tsodilo Company suggest that the area has an Archaean gneissic basement unconformably overlain by series of neoproterozoic rocks affected by the PanAfrican orogeny. Thrusting caused by that event can be observed to be roughly parallel to the Okavango River, approximately NNW-SSE. These thrusts seem to define an undescribed Upper Proterozoic sedimentary basin with an elongate shape, oriented approximately NNW-SSE. This basin is roughly parallel to the Okavango River. The Okavango River currently forms a delta in an endorheic basin close to the Maun city. This endorreic basin is recent in age, and is controlled by extensional faults roughly NW-SE. Lacustrine sediments and evaporites are currently being deposited in it. Figure 1. Overview of the study area. Proterozoic metallogeny in the NW border of Botswana Sergi Cardona, Lisard Torró, Marc Campeny, Eloi Andreu, Joan Carles Melgarejo, Joaquín A. Proenza Dpt. Cristal·lografia, Mineralogia i Dipòsits Minerals, Universitat de Barcelona. C/ Martí i Franquès s/n 08028 Barcelona, Catalonia, Spain Mike C.J. De Witt, James M. Bruchs Tsodilo Resources Limited. Canada Trust Tower. BCE Place. 161 Bay Street, P.O. Box 508 Toronto, ON M5J 2S1 (Canada) John F. Lewis Department of Earth and Environmental Sciences, George Washington University, Washington, DC 20052, USA