Metamorphism and gold mineralization of the Kenticha-Katawicha area: Adola belt, southern Ethiopia

摘要

I present geological and mineral chemistry results aiming at understanding the relationship between metamorphism and gold mineralization in the N-trending Neoproterozoic Kenticha-Katawicha area in the Adola belt of southern Ethiopia. The Kenticha-Katawicha area comprises low-grade metamorphic rocks including pelitic schists, marble, graphite schists and Fe-Mn quartzites (Kenticha marine metasedimentary rocks) and serpentinites and talc-tremolite-chlorite schists with podiform chromites (Kenticha ophiolite). Lenses of high-grade amphibolite schist are also locally present within the Kenticha-Katawicha area. These rocks are sandwiched between high-grade para- and ortho-gneisses, migmatites, schists and amphibolites. Post-orogenic granites intrude the metamorphic sequences. Mineral assemblages and textural data obtained from the high-grade rocks indicate that the dominant metamorphic condition is of upper amphibolite facies. Garnet-biotite (garnet-core-matrix-biotite) and amphibole-plagioclase geothermometry gave estimates for peak metamorphic temperatures of 630-650℃ and pressure of 7 kbar for the high-grade rocks. On the other hand, the low-grade rocks recrys-tallized in the greenschist facies conditions. The estimated peak T-P values for the high-grade rocks suggest a burial to ～25 km depth and a clockwise P— T path is deduced from mineral thermometry and the textural and paragenetic relations. This combined with the geochemistry of the magmatic rocks and clockwise P-T is consistent with a collision setting in which rocks of the calc-alkaline, volcanic-arc, and oceanic fragments are assembled. In the study area, gold occurs in quartz veins and veinlets in lenses of biotite schists within ultramafic rocks that are confined to shear zones. Fluid inclusion studies of gold quartz veins and veinlets indicate a H_2O- and CO_2-rich fluid with low salinity (< 5 wt% NaCl equivalent). The gold mineralization in the area is post-peak regional metamorphism and is associated with the low-grade wall-rock alteration.