| Summary: | Archean greenstones represent a large percentage of worlds total gold endowment and are actively mined on every continent barring Antarctica. Greenstone-hosted gold deposits often have complex deformation and alteration histories, and a general deposit model remains controversial. The aim of this thesis is to improve our understanding of gold mineralisation in Archean greenstone belts, based on a comprehensive case study of the world-class Geita Hill deposit in Tanzania. Geita Hill is one of the largest gold deposits within the Geita Greenstone Belt in north-western Tanzania and has been mined as an open pit since 2002. The deposit is hosted within a greenschist facies metamorphosed and complexly deformed sedimentary package dominated by ironstone and intruded by diorite dykes. The gold mineralisation is spatially associated with the Geita Hill Shear Zone which, is a NE-trending, moderately west dipping deformation zone of discontinuous shear fractures. Detailed structural studies have defined a deformation history for the deposit, providing an opportunity for an in-depth study of the hydrothermal alteration and fluids associated with gold mineralisation. The first component of this thesis builds a paragenetic framework for the Geita Hill deposit. The regional metamorphism is characterised by biotite + chlorite + actinolite + K-feldspar + magnetite ± pyrrhotite ± pyrite indicating upper greenschist facies metamorphism. The gold-related alteration overprints the regional metamorphism and is characterised by silicification and sulfidation fronts that end within one meter of the mineralised zone. Locally, the silicification and sulfidation of the wall rock occurs along a series of mineralised quartz veins which have a sub-vertical dip and tend E-W. Paleostress analysis of the mineralised shear fractures of the Geita Hill Shear Zone suggests vertical maximum compressive stress (σ₁) and northerly extensional stress (σ₃) consistent with the orientation of the mineralised quartz veins and indicating N-S extension. The ...
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