Abstract
The studied cassiterite-rich greisens of Mayo Darlé are hosted in granites located on the NE slope of the Guengue hill. They have three various structural forms including massive, vein, and stockwork. This study aims to describe all the different structural forms of greisen and also explains the main processes of greisen deposition related to the tin mineralization. The petrographic study shows that the massive form presents two facies: the massive greisens are rich and barren in cassiterite respectively; the stockworks and veins are all mineralized. The mineralogical composition recorded for each of the forms are Quartz + Muscovite + Cassiterite + Zinnwaldite + Plagioclase for the cassiterite-bearing greisen; Quartz + Muscovite + Tourmaline + Chlorite for the barren greisen and Quartz + Muscovite + Cassiterite + Tourmaline + Chlorite for the veins and stockworks. This mineralogy could indicate a varied chemical nature of the hydrothermal/magmatic fluids and the host granite. Also, the presence of minerals such as tourmaline, zinnwaldite and chlorite can equally indicate that there were three main stages in the granitic process: (i) tourmalinization, (ii) lepidolitization, and (iii) chloritization. The greisens have a chemical composition rich in SiO2 (over 71 %) and Al2O3 (5.23–43 %). The chondrite-normalized REE pattern shows a negative Eu anomaly (0.0005–0.15) resulting from the disappearance of feldspars and biotite from the geochemical system, which is gradually transformed to quartz and chlorite respectively during leaching. The ratios of Rb/Sr, Zr/Hf, and Nb/Ta indicates an M-type tetrad effect, which also characterizes the variable values of the Eu anomaly. From a mining point of view, the areas most suitable for further prospection would be those hosting the massive, cassiterite-rich greisens. The significant occurrence of tourmaline and zinnwaldite in the studied greisens could indicate a possible Sn–W–Li polymetallic deposit.
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