Gold mineralisation in the Kirk Range area, southern Malawi
Thesis (PhD)--Stellenbosch University, 2020.
ENGLISH ABSTRACT: This study aims at understanding the characteristics and controls of gold mineralisation in the Kirk Range area, southern Malawi. Malawi’s mineral inventory is not known, particularly this is attributed to a general low resolution in the knowledge of Malawi’s geology, coupled with historical under-exploration for this important economic commodity. Regional scale interpretations are afforded by in-depth investigation of airborne geophysical data, which are then supported by ground geological mapping and by microscopic observations using optical and electron microscopy, and X-ray computed tomography (XCT). We further used petrography and mineral chemistry, zircon geochronology, PT estimates, fluid inclusion microthermometry and stable isotope studies. Gold is mainly hosted in biotite schists and is structurally controlled by the NE-SW trending ductile shear zones where it occurs as flakes ranging in size between 0.24 and 4 mm. The gold mineralisation is associated with smoky quartz carbonate veins. It is associated with a pyrite-chalcopyrite-sphalerite generation of sulphide, and to a lesser extent, with sericitised biotite schist wall rock. U–Pb ages for gold bearing-schists and proximal gneisses document two distinct events: an Irumide-age period of metamorphism and a Pan African period of metamorphism. The gneisses experienced both the Irumide and Pan African events while the biotite schist experienced only the Pan African deformation. Irumide metamorphism occurred at around 1066-991 Ma and Pan African metamorphism overprinted the Irumide and occurred at around 700-485 Ma. Based on a loose PT constraint, the rocks experienced peak metamorphic conditions of around 500-600ºC and 10-15 kbars. Mineralization occurred in the early stages of Pan African event before peak metamorphism likely under greenschist facies conditions. Later deformation to amphibolite facies conditions of metamorphism during the prolonged Pan African event led to the transposition of the original mineralization. Gold mineralisation occurred at temperature of around 222-300ºC and pressure of around 2.0–2.8 kbar. Minerals of economic interest were likely deposited by fluid immiscibility and fluid rock interaction.