Geology and Geochemistry of the Negash Pluton and their Metallogenic Significance, Central Tigrai
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Date
1997-01
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Addis Ababa University
Abstract
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
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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.
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Geology