Petrogenesis and Tectonic Setting of the Gold-Bearing Magmatic Rocks of the North-Western Melfi Massif, Central Chad

Abstract

This paper presents petrographic and geochemical data on magmatic rocks of the north-western Melfi massif (part of the Gera Massif at the center of Chad) in the Adamawa-Yadé domain. The study area consists of syenite and granite, all crosscut by minor dykes and quartz veins. Syenite and granite show porphyritic texture and are dominated by K-feldspar and plagioclase phenocrysts, biotite, a variable proportion of quartz, and often few amphiboles. These rocks are characterized by low Mg#, Ni and Co, high K2O, Na2O, LREE/HREE ratio, I-type geochemistry as well as positive Eu anomaly, suggesting an origin from partial melting of crustal rocks (here garnet-amphibolite) and metasomatized mantle-derived component with the input of sediments. The compositional evolution from syenites to granite raises the fractional crystallization process in their petrogenesis and involves the fractionation of amphibole, biotite, plagioclase, apatite, magnetite, and ilmenite. Studied samples are affected by the post-magmatic hydrothermal alteration with an assemblage made of epidote, biotite, sericite, and opaques minerals. In addition, they are characterized by considerable gold concentration (Au = 0.6-3.8 ppm) suggesting that the north-western part of the Melfi massif in the center of Chad is a preferential site of gold mineralization. The K-rich calc-alkaline to shoshonitic geochemistry and the pronounced Nb, Ta, and Ti negative anomalies of these rocks, combined with their high Nb/Ta (7.55-32.10), La/Nb (1.80-11.73), and La/Ta (21.53-376.60) ratios but low Rb (57.60-138.30 ppm) indicate that they are subduction-related or arc-related magmas belonging to the Great Central Gondwana Arc (GCGA) formed further to east-directed subduction direction.