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Amponsah Thesis 2015 é Résumé L’objectif de ce travail de thèse était de réaliser une étude structurale détaillée des minéralisations et des zones d’altération associées, de trois gisements d’or situés au Nord- Ouest du Ghana, sur la marge orientale du Craton Ouest Africain: Kunche et Bepkong, situés dans la ceinture de Wa-Lawra, et Julie situé dans la ceinture de Julie. Ces trois gisements présentent de multiples différences d’ordre géologique, structural, tectonique et géochimique, mais leur caractéristique commune est que leur minéralisation est associée à un métamorphisme de faciès schiste vert. A Julie, la minéralisation aurifère est encaissée dans des granitoïdes de composition tonalite- trondhjemite-granodiorite (TTG) alors qu’à Kunche et Bepkong elle est localisée au sein de formations sédimentaires volcanoclastiques et de schistes graphiteux fortement silicifiés. Cette minéralisation est associée à un réseau de veines souvent boudinées de quartz formées en relation avec une zone de cisaillement orientée Est-Ouest à Julie, mais N à NNO à Kunche et Bepkong, constituant un couloir de déformation de 0.5 à 3,5 km de longueur et de 20 à 300 m de puissance suivant les gisements. La paragenèse d’altération dominante de la zone minéralisée est à séricite, quartz, carbonate et sulfures, et suivant la nature de la roche hôte se rajouteront par exemple la tourmaline dans les granitoïdes et la chlorite dans les schistes ou les métavolcanites. A Julie, l’or est étroitement associé à la pyrite alors qu’à Kunche et Bepkong l’or est associé à l’arsénopyrite. Deux générations d’or sont distinguées ; la première correspond à de l’or invisible associé aux zones de croissance primaire des cristaux de pyrite à Julie et d’arsénopyrite à Bepkong, et de l’or visible tardif en inclusion et plus fréquemment en remplissage de fractures. Abstract The objective of this thesis was to perform a detailed structural, mineralization and associated alteration studies of three gold deposits located in northwest Ghana, on the eastern margin of the West African Craton. Thus, the Kunche and Bepkong deposits, located in the Wa-Lawra belt and Julie deposit located in the Julie belt. These three deposits have multiple differences interms of geological, structural, tectonic and geochemical characteristics but gold mineralization in these deposits are all associated with a green schist facies metamorphism. In Julie, the gold mineralization is hosted in granitoids with its composition akin to tonalite- granodiorite-trondhjemite (TTG) where as in the Kunche and Bepkong deposits, the gold mineralization are localized within volcaniclastic sedimentary formations and strongly silicified graphitic schists. The gold mineralization is associated with a network of quartz veins often boudinaged and formed in connection with a shear zones, oriented E-W in the Julie deposit and N to NNW at Kunche and Bepkong deposits. The dominant alteration in the mineralized zone is sericite, quartz, carbonate and sulfides with influences from the host rock. For example, the granitoid is influenced by tourmaline and chlorite in the schists or in themetavolcanic rocks. At Julie, gold is closely associated with pyrite whereas Kunche and Bepkong gold is associated with arsenopyrite. Two generations of gold are distinguished; the first corresponds to the invisible gold associated with primary growth areas in the pyrite crystals in Julie and arsenopyrite in Kunche and Bepkong, and late visible gold inclusions which are frequently found in fracture fillings. In Julie, the mineralizing fluid is rich in CO2, and has low to moderate salinity (NaCl-H2O- CO2 system), trapped in P / T conditions around 220 ° C and <1 kbar; whilst in Bepkong, the mineralizing fluid is associated with quartz are rich in CH4, with a low salinity (CH4- CO2-SO2-H2O system) which indicates that the visible trapping temperatures is around 320°C. Acknowledgement My sincerest gratitude goes to Azumah Resources Limited, the owners of Julie, Bepkong and Kunche gold deposits in NW Ghana, for allowing me to you use their deposits as the focus of my study and also financing the field work. This thesis would not have been possible without the support of the WAXI (West African Exploration Initiative) project. Many people near and afar supported me in immeasurable ways throughout this PhD. It is impossible to keep track of their generosity, hospitality and kindness. I am greatly indebted to Prof. Mark Walter Jessell for the confidence he had in me, proposing this PhD project and further went ahead to take me on as his student and nurturing my interest in metallogeny. I would like to thank Stefano Salvi, Didier Béziat, Lenka Baratoux and Luc Siebenaller for taking me on as their student and imparting some of their colossal knowledge in Geology on to me. I cannot thank them enough for their patience, excellent supervision, extraordinary insight, guidance and critical revision of my thesis. Special thanks goes to John Miller, Sylvain Block, Prosper Mackenzie Nude and the entire geological staff of Azumah Resources Limited for their insightful geological discussions about the various deposits I was dealing with. I will like to extent my gratitude to Nick Franey and Stephen Stone for taking the great risk to support and finance this work even with the downturn of global gold prices. I say “kudos” to Andrew Chubb for his valuable contributions towards my manuscripts. I would like to thank my family, especially my parents Peter Ofori Amponsah and Dorothy Akoi-Appiah, my sisters, my auntie Esther Akoi-Appiah and my in-laws Mr and Mrs Laud Mansfield Baddoo and their entire family for their immense support and prayers. I would also want to thank all the staff and workers of Azumah Resources Limited for their enormous support towards making this work a reality. Thanks to all my dear friends Manuel Dela Eli, Miriam Yahaya-Shiru, Wendy Annan, Kofi Eghan-Ekuban, Jennifer Agbetsoamedo, Emmanuel Abitty and Daniel Kwayisi for their support. Finally, I say ‘ayekoo’ to my dear and lovely wife, Mrs. Laudetta Amponsah for her steadfast love, support, patience and for providing me the motivation to complete this work. This thesis is dedicated to God almighty and my loving wife, Laudetta Awulaa Naa Lamiley Amponsah in appreciation of her love, patience and encouragement. Table of Contents Résumé ................................................................................................................................................... i Abstract ................................................................................................................................................. ii Acknowledgement ............................................................................................................................... iii Introduction ........................................................................................................................................... 1 Cadre général de la thèse ....................................................................................................................... 3 Géographie ............................................................................................................................................ 6 La zone d'étude et accessibilité Relief et réseau hydrographique Végétation, sols et climat Cartographie de terrain Background and Justification ............................................................................................................... 10 Geography ........................................................................................................................................... 12 Study area and Accessibility Relief and Drainage Vegetation, Soils and Climate Field work References ............................................................................................................................................ 14 Chapter 1 ............................................................................................................................................. 17 1.0 Overview of Orogenic Gold mineralization ................................................................................... 19 1.1 Introduction ................................................................................................................................... 19 1.2 Spatial and temporal distribution of Orogenic gold ...................................................................... 21 1.3 Control on relative and absolute timing ........................................................................................ 23 1.4 Global gold grade and tonnages associated with metamorphic gold .............................................. 26 1.5 Tectonic controls on Orogenic Gold mineralization ..................................................................... 28 1.6 Orogenic gold ore body geometry and ore body controls .............................................................. 30 1.7 Metamorphic grade ....................................................................................................................... 32 1.8 Hydrothermal alteration ................................................................................................................ 34 1.9 Sources of Gold ............................................................................................................................. 35 1.10 Fluid chemistry ...........................................................................................................................
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