Abstract
The Mwanesi Greenstone Belt (MGB) occupies the central part of the Zimbabwe Craton and trends in a NNE direction. The MGB consists of greenstones (intercalated with banded iron formations, BIF) of the Bulawayan Supergroup. The MGB is described as a doubly plunging NNE-trending syncline, with BIF units defining fold closures at both the northern and southern ends of the belt. Gold mineralisation is hosted in quartz veins in the supracrustal rocks and adjacent granite gneisses. The MGB is one of the least studied greenstone belts of the Zimbabwe Craton. The age of the greenstone belt and adjacent granites, and the link between deformation and gold mineralisation in terms of the timing and controls of mineralisation are unknown. The main structure of the MGB is only known from colonial mapping programs in the 1950s. This study focused on the structural architecture and gold mineralisation of the south-eastern MGB and adjacent granite gneisses. New data from lithological and structural mapping, zircon U-Pb and 40Ar/39Ar geochronology are reported.
The lower structural units of the south-eastern MGB are composed of mafic rocks (locally pillowed and amygdaloidal, and variolitic pyroclastic breccia), minor felsic volcanic rocks, intercalated with thin-bedded and thin-laminated metasedimentary rocks, traditionally referred to as the Lower Greenstone Series (LGS). These are overlain by phyllites and BIFs (i.e., the Lower Sedimentary Series, LSS). The LGS and LSS are intruded by mafic rocks and are overlain by locally pillowed basaltic rocks of the Middle Greenstone Series (MGS). The stratigraphy of the south-eastern MGB is similar to other Lower Bulawayan greenstone belts of the Zimbabwe Craton. The MGB is underlain to the east by various granite gneisses, some in intrusive contact with the MGB. The granite gneisses are transected by a km-wide N-S-striking sinistral shear zone (the Mhou Shear Zone, MSZ).
Two events of magmatism constrained from LA-(MC-Q)-ICP-MS zircon U-Pb dating are recorded in the granite gneisses at ~ 3243 Ma and ~ 2770-2700 Ma episode. An indicative crystallisation age of ~ 2871 Ma for the felsic volcanic rock of the LGS agrees with the age of the Bulawayan Supergroup. The crystallisation of the felsic volcanic rock was coeval with the emplacement of the ~ 2852 Ma porphyritic leucogranite gneiss to the west of the MSZ. The basement of the MGB was probably ensialic as revealed by ~ 3.0-3.7 Ga zircons from the felsic volcanic rock of the LGS interpreted as sourced from the Sebakwe Protocraton.
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Structural mapping coupled with 40Ar/39Ar dating of syn-deformational micas reveals the following three deformation events: (1) ~ 2688-2617 Ma dextral strike-slip shearing D1 event, which occurred under lower to middle greenschist facies, is characterised by shallowly SW- to NW- and NNE-dipping planar fabrics in the supracrustal rocks and underlying granite gneisses. Recumbent folds observed in the supracrustal rocks only are interpreted as progressive folds formed by layer parallel shortening with a significant component of subhorizontal shearing. (2) D2 is an upper greenschist to lower amphibolite facies wrench-dominated localised sinistral transpression in the granite gneisses, which formed medium to high-grade mylonites of the MSZ. This event was constrained at ~ 2588-2541 Ma. D2 operated under NW-SE (or NNW-SSE) contraction and is likely responsible for forming the MGB syncline. Lastly, D3 formed steeply NE-dipping fold axial planar cleavage in the supracrustal rocks and granite gneisses (and F3 box folds in the MSZ). D3 structures formed under lower greenschist facies in a brittle-ductile regime.
Two important hydrothermal events forming quartz veins are recognised in the study area, particularly in the adjacent granite gneisses. The first event formed up to five m-thick syn-D1 quartz veins in the granite gneisses to the west of the MSZ. Their timing and associated alteration halos are constrained at ~ 2685-2634 Ma, overlapping with the D1 age ~ 2688-2617 Ma, also giving an age for the gold mineralisation. The second event formed up to 60 m-thick late syn-D2 quartz veins in the MSZ, which are not mineralised. The grades in mineralised veins are generally low (< 0.2 g/t).
Evidence from this study suggests the observed structures are best explained by modern-type tectonics. This inference is evidenced by shallow fabrics (S1) in the underlying granite gneisses and supracrustal rocks, inferred detachment separating the underlying granite gneisses from the supracrustal rocks, wrench tectonics in the MSZ, and steeply dipping fold axial planar cleavage S3 and F3 box folds which reflect the horizontal contraction of the crust.
In conclusion, this study shows a very strong link between deformation and hydrothermal activity forming quartz veins and orogenic gold mineralisation. The study significantly contributes toward a better understanding of the Archaean tectonics of the Zimbabwe Craton.