Geology and Geochemistry of the Negash Pluton and their Metallogenic Significance, Central Tigrai

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Addis Ababa University


The Negash area is located in Central Tigrai, Northern Ethiopia. The area consists of metamorphic rocks th at are cut by some granitoid intrusions. The present investigation was directed to one of such intrusions, the Negash granitoid stock. Geological mapping at a scale of I: 50 000, and geochemical studies led to the assessment of the petro logy, geochemistry and tectonic setting of both the intrusion and the country rocks and the economic potential of the area. The meramorphic rocks are mainly low - grade metavolcanic rocks with a wide compositional variation from tholeiitic to calc - alkaline mafic to felsic composition formed in an island arc environment. There are also minor intercalations of metased imentary rocks. Hornblende - hornfels facies rocks are found in proximi ty to the intrusion. The Negash granitoid body is unde fo rmed, discordant, circular stock that consists of diorites, quartz diorites, tonalites, monzonites and granodiorites with minor aplitic, grani tic and pegmatitic dikes. Its western boundary lies along a fault line (the Suluh Fault) that forms the Suluh River valley. The intrusion is dominantly of mafic and intermediate composition rocks, even though the felsic rocks crop out extensively at the top most part of the intrusion. Geochemical and petrological data suggest that the intrusive rocks were diffe rentiated from a mafic magma by fract ional crystallization and assimilation with fractional crystallization (AFC). Hornblende was the main fractionating mineral phase. This process resulted in relatively dry residual magma, as can be evidenced from the scarc ity of pegmatites and related mineralization. Preliminary geochemical survey of the Negash granitoid stock show no significant anomalies except for Zn and Sn. The country rocks show anomalies for several elements including Au, Zn, U, Sb, As and Ba. This indicates that the intrusive magma did not play an important role in generating mineralization in the area but it furnished the heat which triggered fluid circulation in the country rocks. The geochemical anomalies for most of the elements are concentrated along the western boundary of the , intrusion, the Suluh fault line. The fault presumably provided the ease through which the metasomatic fluids enriched the area with metals. Further geological and geochemical investigations should be directed to this zone.