Abstract
In the Adola Gold District of Southern Ethiopia the outline of the contacts that separate the greenstone belt and the gneissic terrain defines regional scale ductile to brittle-ductile shear zones. Possibly economic mineralization is found to be controlled by these major shear zones. Mapping out and documenting the movement senses of these major shear zones in detail, therefore, was vital for the prediction of the nature and orientation of the minor structures that are likely to host gold deposits. Structural information combined with identification and mapping of suitable host rocks such as the Lega Dembi schists, can provide a powerful tool in exploration for gold in the Adola area. Such techniques are not only important in mineralized areas such as the Lega Dembi Gold Mine, where some, but not all, mineralized structures have been discovered, but also in unexplored terrains. Mapping at a scale of 1:25,000 resulted in detection of several superposed shearing events with both sinistral and dextral senses of displacement. These shear zones are expressed as ductile, brittle-ductile and brittle types. Study of geometric compatibilities between the structural elements viz., shear zones, tectonic lineations and folds helped in locating the Lega Dembi Gold Mine within a grossly N-S trending major ductile transcurrent sinistral shear zone. This has subsequently been reactivated and overlapped by N-S trending mesoscale shear zones with similar trend but of opposed sense of displacement. The formation of the later dextral shear zones took place under similar ductile conditions within a single but extended phase of deformation, D 2, that generated the reversed motion. Generally D 3 E-W wrench faults with minor WNW-ESE/WSW-ENE trending conjugate pairs and mesoscale D 4N-S/NE-SW trending conjugate systems of brittle to brittle-ductile shear zones have been recognized. Mineralization is hosted by quartz veinlets and stringers that propagated along mesoscale D 2 ductile to brittle-ductile shear zones.
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