Structural control, fluid inclusions and cathodoluminescence studies of birimian gold-bearing quartz vein systems in the paleoproterozoic mako belt, southeastern Senegal
The Paleoproterozoic Mako Belt in eastern Senegal is characterised by gold-bearing quartz veins. These are superimposed on the
regional Birimian structural complex consisting of volcano-sedimentary and intrusive rocks in an Early Proterozoic greenstone belt.
In the Mako Belt the multi-stage deformed quartz veins and hydrothermally altered protoliths are characterised by distinctive
microstructures and CL properties. The high-grade Au-bearing quartz veins contain one or more generations of hydrothermal quartz.
Brecciation of the veins is indicated by cemented fragments of zoned quartz. Gold in the mineralised zones occurs either as
inclusions in pyrite or as native gold.
Mineralisation typically occurs in vein systems which are brecciated, laminated, and/or show crack-seal textures. Microstructures
indicate syn- to late-tectonic mineralisation. Au-mineralised veins contain quartz, carbonate, muscovite, fuchsite, tourmaline, and
chlorite. Fluid inclusion studies reveal early, highly saline aqueous inclusions followed by main-stage aqueous-carbonic inclusions
of low salinity and finally aqueous low-salinity inclusions.
Petrographic observation and microthermometric data suggest that fluid inclusions in main stage quartz were trapped after phase
separation from a heterogeneous H2O-CO2 fluid. Gold deposition occurs over a temperature range of 220 to 320°C and pressures
of 1.4 to 2.75 kbar (~5 to 10 km depth). The late-stage quartz carbonate vein corresponds to pressures of 0.75 to 1.25 kbar and
depths of 3.0 to 4.5 km. A fluid pressure drop due to fracture failure is likely to have triggered Au precipitation. The Mako Belt Au
deposits are comparable to a large number of orogenic lode gold deposits in the West African Craton.
Auteur(s) : M. Gueye, A.M. Van Den Kerkhof, U.F. Hein, M. Diène, A. Mücke & S. Siegesmund
Pages : 20
Année de publication : 2013
Revue : South African Journal Of Geology
N° de volume : 116.2
Type : Article
Mise en ligne par : DIENE Mahamadane