Akobo Minerals AB (publ) is a Scandinavian-based gold exploration and mining company operating in southwest Ethiopia. The company's Norwegian 100 % subsidiary, Abyssinia Resources Development AS, wholly owns 99.94% Etno Mining Plc, which holds a gold exploration permit covering 182 km2 and a large-scale mining license for 16 km2 in the Gambela region.
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Summary:
The Segele Gold Deposit is situated in a mafic to ultramafic complex within a sequence of metasedimentary to ultramafic rock units bounded by large plutons to the north and northeast. The wider host sequence is poorly understood and poorly defined, especially to the west, but likely continues well into South Sudan. On a regional scale the host sequence is overlain by younger basaltic rocks forming the massifs of the Ethiopian highland, but in several areas basement windows expose the older rocks. The Akobo area is situated in one such basement window.
The gold deposits in the Akobo area are typical Orogenic Gold Deposits and the area can be described as a regular greenstone-belt with both vein- and alteration-hosted gold deposits. The metamorphic grade in the Akobo area varies from greenschist to lower amphibolite facies and hence ductile to brittle-ductile deformation plays a key role in the formation of the gold deposits.
The mineralisation in Segele is hosted by altered ultramafic rocks ranging from meta-pyroxenites to meta-peridotites, the alteration is a calc silicate alteration that has primarily interacted with ultramafic rocks close to the contact with a underlaying larger meta-peridotite unit. In several places the alteration continues into the meta-peridotites and the gold grade carries through as well. The alteration minerals are primarily amphiboles, chlorite and orthopyroxenes.
This mineralised envelope takes the form of several small, stacked lenses that are strung out along a predictable structural trend straight to the north from the surface outcrops. The individual lenses seem to follow the regional foliation with an average dip of 55' towards 330 while the mineralised envelope plunges 45' towards 360°.
The Segele gold mineralisation has a very high nugget effect with abundant visible gold - it is not uncommon for gold grades to vary two orders of magnitude between assays of the same sample. Despite this the position of the deposit is very predictable down plunge.
The Segele deposit is dominated by metagabbro, serpentinite, a chloritic unit with coarse magnetite crystals, a strongly sheared talc-chlorite-tremolite-carbonate unit, and fine-grained magnetite bearing carbonate-talc unit with minor mafic and felsic dykes. The Segele area has undergone a multistage ductile-brittle deformation resulting in pinch as swell structures. All the units are strongly sheared and boudinaged which has resulted in complex, irregular and discontinuous geological units and mineralised zones. Gold mineralisation is usually associated with carbonate-talc-magnetite alteration zones either within, or along the margins of the ultramafic units. The mineralisation is controlled by northwest–southeast shear movement which has created local dilatational zones oriented in an east–west direction which favoured precipitation of gold in narrow zones and pockets of intense shearing within the ultramafic and overlying mafic units.
The Segele mafic-ultramafic complex can be divided into several distinct units separated by east- west ductile shear zones. These shear zones are not parallel to each other but seem to form a large up to several hundred metres thick-shear lens. The mineralisation is hosted in the footwall of the lower shear zone.
Upper mafic unit
This unit is bounded on the lower side by the upper shear zone, this zone strikes due west with a very shallow dip to the north (10° towards 270°). The upper mafics are only observed in the top sections of a few drill holes in the northern end of the Segele resource drilling; based on this they are comprised primarily of amphibolite, mafic rocks with or without porphyritic texture, and of minor amounts of gabbro. Only limited amounts of ultramafics, and no alteration associated with gold have been observed within the upper mafic unit.
Middle mafic-ultramafic unit
The middle unit is bounded above and below by ductile shear zones; the shear zones both strike to the west but the dips are different. The lower shearzone dips at about 45" while the upper zone is very flat laying at about 10° dip. The lithologies are made up of equal amounts of gabbro and other mafics; the gabbro tends to be more frequent in the lower half of the unit while the upper half is dominated by finer grained mafic and porphyritic mafic rocks. There are ultramafic rocks spread out through the entire unit without any clear link to other lithologies. In the gabbroic parts some alteration is present, forming so called metapyroxenites, but no gold has been found in these units. The metapyroxenites do not have any clear link to the shear zones but a general association with the ultramafics seems clear.
Host sequence
The host sequence is made up of major ultramafic bodies surrounded by gabbroic rocks with lesser amounts of amphibolite and plagioclase porphyritic mafic when compared to the upper mafic unit. The host sequence is bounded on the upper side by the lower shear zone and the mineralisation seems to be located in a splay structure in the footwall of the lower shear zone. The orientation of the mineralised zone is oblique to the shear zone and follows the trend of the regional foliation.
Dimensions
The Segele mineralisation is approximately 40 m wide (east–west) and extends approximately 200 m down plunge to depths of up to 140 m below the topographic surface. The mineralised lenses are typically between 2–5 m thick but can vary from 1 m to 20 m thick.
Akobo Project local geology
The Akobo Gold Project exploration and mining licences lie within primarily greenschist facies metamorphic supracrustal rocks, containing schists of both mafic and felsic volcanic origin associated with various metasediments as well as mafic and ultramafic rocks. In the Akobo basin to the south the rocks are relatively low-grade metasedimentary and metavolcanic rocks enclosing characteristic meta ultramafic lenses and surrounding well preserved plutons ranging from gabbro to granite.
The major rock types include mafic metavolcanics and metasediments, such as quartzite, marble, graphitic schist, quartz-mica and quartz-feldspar schist. These lithologies were intruded by plutons ranging in composition from mafic to felsic. Granitic and pegmatitic dykes are rare. Quartz veins of variable dimensions, ranging from centimetre to hundreds of meters occur in the area and commonly display a pinch and swell pattern concordant with the foliation of the enclosing rocks that trends northwest to southeast. The quartz veins occur as ridges of variable size and are commonly boudinaged and some veins occur as shear-hosted vein systems.
The Akobo domain is flanked along the southwest side, along the border between Ethiopia and South Sudan, by straight layered and mylonite type gneisses separated from the rest of the region by a zone of intense mylonitisation occurring within the Surma domain. Metamorphic grade is invariably amphibolite facies with gneissic layering trends persistently striking northwest and dipping mainly to southwest for the most part.