The geology of the Lega Dembi gold deposit, southern Ethiopia: implications for Pan-African gold exploration |
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Authors: | A Y Billay A F M Kisters F M Meyer J Schneider |
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Institution: | Institut für Mineralogie und Lagerst?ttenlehre, RWTH Aachen, Wüllnerstr. 2, 52056 Aachen, Germany (e-mail: kisters@rwth-aachen.de), DE Institut für Geowissenschaften und Lithosph?renforschung, Justus-Liebig Universit?t Giessen, Senckenbergstr. 3, 35390 Giessen, Germany, DE
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Abstract: | The Lega Dembi deposit is the largest gold producer in Ethiopia. It is situated in late-Precambrian metamorphosed sediments
of the N-S trending, volcano-sedimentary Megado belt, which forms part of the late-Proterozoic Adola granite-greenstone terrane
in southern Ethiopia. The lode-gold mineralization occurs in a N-S trending, steep westerly dipping quartz-vein system that
follows the structural contact between underlying feldspathic gneisses and the volcanosedimentary sequence of the Megado belt.
This contact also marks the northernmost extension of the regional-scale, sinistral strike-slip Lega Dembi-Aflata shear zone.
Mineralization and intense quartz-veining is best developed in graphite-rich sediments within an area not more than 80 m away
from this tectonic contact. Hydrothermal wall-rock alteration includes actinolite/tremolite-biotite-calcite-sericite and chlorite-calcite-epidote
assemblages. Gold occurs preferentially in the sericite alteration zone, where it is closely associated and intergrown with
galena. The variable deformation of the gold-quartz veins suggests a syn-kinematic timing for the gold mineralization during
transcurrent shearing in a dilational segment of the shear zone. In addition to the structural control, lithological control
on gold deposition is indicated by the almost exclusive occurrence of the gold mineralization in graphite-rich metasediments.
This close relationship suggests that gold precipitation was the result of chemical reduction of regional ore-bearing fluids.
Temperature conditions of mineralization are constrained by the actinolite-biotite alteration assemblage and by arsenopyrite
chemistry, which indicate that ore deposition occurred at or close to peak metamorphic conditions at upper-greenschist to
lower-amphibolite metamorphic grades. Rb-Sr dating of sericite indicates an age of about 545 Ma. for hydrothermal alteration
and, thus, for gold mineralization. The style of gold mineralization, structural pattern and lithological assemblages at Lega
Dembi are very similar to lode-gold deposits most commonly reported from Archaean granite-greenstone terranes. These similarities
may open new perspectives for the exploration of lode-gold deposits, which has previously primarily focused on Archaean greenstone
belts rather than Proterozoic or even Phanerozoic meta-volcanosedimentary belts.
Received: 26 July 1996 / Accepted: 8 January 1997 |
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