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sur le développement des zones arides et semi-arides

Titre : Geology, mineralogy, geochemistry and alteration patterns of Banana zone copper-silver mineralisation , Ghanzi-Chobe belt, northwest-Botswana

Auteur : Kelepile, Tebogo

Université de soutenance : Botswana International University of Science & Technology (BIUST)

Grade  : MSc-Geology 2017

Résumé partiel
The Banana Zone Cu-Ag Prospect is within the SW-NE trending Ghanzi-Chobe Belt (GCB), which extends from the Namibia-Botswana border in the south to Shinamba Hills in the north near the Botswana-Zambia border. The Banana Zone is also part of the Ghanzi Project owned by Khoemacau Copper Mining and situated between the Ghanzi and Maun townships, in Botswana. The Banana Zone has inferred resource of over 225.4 million metric tonnes (Mt) grading 0.64% Cu and 8.1 g/t Ag. The GCB consists of deformed and metamorphosed (greenschist) metasedimentary rocks of the Ghanzi Group, that is commonly subdivided into four formations : the Kuke Formation, Ngwako Pan Formation, D’Kar Formation, and Mamuno Formation, in ascending stratigraphic order. In this study only two formations including the Ngwako Pan and the D’Kar Formations were investigated. The Ngwako Pan Formation is made up of continental siliciclastic sediments, mainly immature to submature sandstone with siltstone and shale intercalations, while the D’Kar Formation contains shallow marine to lacustrine grey-green and locally carbonaceous mudrocks (siltstone and mudstone/shale) with fine-grained sandstone and minor limestone to marlstone. The Cu-Ag mineralisation is hosted at the contact between hematite-rich Ngwako Pan and reduced organic matter-bearing D’Kar Formations. The presence of reduced organic matter is likely to have controlled Cu-Ag precipitation by providing a reducing environment required for the precipitation of sulphides. Petrographic and geochemical data were combined in order to decipher the petrogenesis of the Neoproterozoic clastic sedimentary succession of the Ngwako Pan and the D’Kar Formations associated with the Banana Zone Cu-Ag mineralisation. Sandstone of the Ngwako Pan and the D’Kar Formations is arkose and subarkose, composed of quartz (Q),feldspars (F), and lithic fragments (L). Moreover, geochemically the sandstone of the Ngwako Pan and the D’Kar Formations is considered as potassic and classified as arkose. Therefore, sandstone of both the Ngwako Pan and the D’Kar Formations is chemically and compositionally immature. On the other hand, mudrocks of the D’Kar Formation are finely laminated and are dominated by muscovite, sericite, chlorite, and quartz. The carbonate rocks of the D’Kar Formation are composed of pinkish-white crystalline limestone and yellowish grey or greyish black marlstone characterised by laminations that are marked by undulating marly and silty layers that give the rock a “zebra-skin” appearance. The marlstone exhibits variable degree of deformation from shearing to folding and recrystallisation.

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