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Adola Gold Field - Lega Dembi, Sakaro |
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Ethiopia |
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The Lega Dembi and Sakaro gold deposits are located ~365 km south of Addis Ababa in southern Ethiopia. Lega Dembi is ~6 km SSW of the town of Shakiso, and Sakaro is a further 4 km to the SW.
(#Location: Lega Dembi - 5° 43' 1"N, 38° 53' 38"E).
The deposits are situated within the South Ethiopia Terrane in the southwestern section of the Arabian-Nubian Shield, specifically within the Adola granite-greenstone belt that covers an area of approximately 5000 km2.
Numerous gold occurrences, both primary and placers, have been known historically from various regions in Ethiopia. Placer gold was found the in Adola belt 1936 and to 1996, had produced >55 t of placer gold. This figure does not include production by illegal artisanal miners. Placer production started to decline in the 1950s and by 1978 had been substantially reduced. Two hard rock gold occurrences were discovered by artisanal miners in 1975 at Sakaro and Dermi Dama. Subsequently, an intensive and detailed exploration program carried out in the Adola area by EMRDC (Ethiopian Mineral Resources Development Corporation) during the late 1970s resulted in the discovery in 1979 of the Lega Dembi and Sakaro primary gold deposits. The government owned EMRDC mined the Lega Dembi deposit, initally as an open pit before it was sold to the Saudi backed MIDROC Gold in 1997. MIDROC continued mining by open pit before going underground. Reserves at Lega Dembi were exhausted in 2018 and the mine was closed. EMRDC had mined alluvial and eluvial gold from Sakaro from 1986, with 895.7 kg being recorded by 1991. MIDROC Gold commenced mining at Sakara as an underground operation in 2009 (Girma, 1993; Billay et al., 1997; Tadesse, 1999).
Regional Setting
For a description of the regional setting of the Shield and its geology and distribtion of mineralisation, see the separate Arabian Nubian Shield Overview record.
The Adola greenstone belt comprises two linear, north-south trending blocks of metamorphosed supracrustal rock. Each is ~5 to 15 km wide and the two are ~10km apart. They are the:
• Megado volcanosedimentary belt in the west, composed of ultramafic and tholeiitic mafic volcanic rocks and intrusives that are intercalated with predominantly arkose, feldspathic quartzite, quartzite and pelite, together with subordinate polymictic conglomerate and greywacke. Small pod- or lens-like, mainly tonalitic bodies intrude the mafic rocks (Gilboy 1970; Chater 1971; Bisrat 1993; Ghenzebu et al., 1994; Worku and Schandelmeier 1996). The eastern margin of this belt is tectonic, regionally known as the Lega Dembi- Aflata shear zone (Worku and Yifa 1989; Worku and Schandelmeier 1996). The western margin is marked by the development of gneissose tonalite which has primary intrusive contacts with the supracrustal assemblages.
• Kenticha ultramafic belt in the east, dominated by ultramafic rocks, with subordinate amphibolites and sedimentary rocks, occurring as biotite schists and minor graphitic schists and marbles (Gilboy 1970; Chater 1971).
The two belts are separated and flanked by a gneissic rocks comprising para- and orthogneisses, including monotonous quartzo-feldspathic biotite gneisses with subordinate muscovite-quartz schists, staurolite-garnet- biotite schists, impure marbles and amphibolites (Gilboy 1970; Chater 1971; Kozyrev et al., 1985; Ghebreab 1989; Worku and Yifa 1989). Large tonalite bodies intrude the gneissic terrane. Gneissose granites are confined to the gneissic terrain, whilst post-tectonic granites occur marginal to the greenstone belts and within the gneissic terrane. A tonalite intrusion to the west of the Megado belt has been dated at ~765 Ma (~U-Pb; EIGS, quoted by Gichile and Fyson 1993), whilst a Neoproterzoic age is also inferred for the gneissic basement which is correlated with gneisses of the Mozambique belt (Kazmin et al., 1978; Gass 1977, and references therein; Vail 1976, 1983). Ages of 680 to 630 Ma (Rb-Sr whole rock) for the gneisses and syntectonic granites are interpreted to represent the age of M2 metamorphism (Gilboy 1970; Chater 1971). Post-tectonic granites have been dated at 550 to 500 Ma (whole-rock Rb-Sr; Gilboy 1970; Chater 1971).
Structure
Both the Lega Dembi and Sakaro deposits are on or near the eastern margin of the Megado volcanosedimentary belt.
Five main deformations have been recognised within the Megado belt and surrounding gneissic terrane, as follows:
D1 - an early gneissosity-forming event in the gneissic terrane, expressed as early folds in the Megado belt, related to a compressive thrust event;
D2 - that produced regional north-south trending upright folds that are the dominant the structural grain of the region, and are associated with an east-west contractional event (Worku and Schandelmeier 1996);
D3 - strike-slip shearing along the contacts between the greenstone and the gneissic domains, resulting from a NW-SE directed transpressional event (e.g. Beraki 1995; Worku and Schandelmeier 1996);
D4 - responsible for east-west trending, upright, moderate to steep easterly and/or westerly plunging folds, interpreted by some authors to be the result of the D3 transpressional shearing;
D5 - late, brittle NW-SE and NE-SW to east-west trending faults which disrupt the north-south trending granite-greenstone terrane.
The greenstones of the Adola granite-greenstone belt are interpreted to have originated as suture-related ophiolites, that were thrust over the gneissic domain. These ophiolites were later refolded by major north trending folds and subsequently modified by strike-slip shearing (Kazmin 1976; De Wit and Chewata 1981; Beraki et al. 1989; Beraki 1995; Worku and Schandelmeier 1996). Alternatively they are interpreted to be related to the inversion of intra-continental rift by the same structural events (e.g., Worku and Yifa 1989; Ghebreab 1989; Amenti et al., 1992).
LEGA DEMBI
Geology and Structure
The deeply weathered, poorly exposed host sequence at Lega Dembi dips steeply to the west. The lithostratigraphic sequence can be subdivided into quartzo-feldspathic, biotite gneisses and amphibolites of the gneissic domain in the east, and the Megado belt volcanosedimentary sequence to the west. In the immediate Lega Dembi area the succession comprises ultramafic schists and various meta-sedimentary lithologies. Ultramafic talc schists with mylonitic fabrics are commonly developed along the 40 to 70°W dipping contact between the gneissic domain and supracrustal sequence. This contact is tectonic and generally sharp, although bands of talc schists are locally interleaved with the gneisses. The width of the talc schist varies along the length of the four interconnected pits, which are, from north to south, the Upper Lega Dembi, Northern, Central and Southern. In the Northern pit, the talc schist is <5 m, locally pinching out, but progressively thickens to the south where it reaches a maximum thickness of ~180 m to the south of the Southern pit.
The meta-sedimentary succession has been subdivided into a lower leucocratic muscovite-quartz-plagioclase schist, overlain by laminated, dark-greyish, graphite-rich, locally graded feldspathic arenites and quartzwackes. These sedimentary rocks are composed of an assemblage of quartz, biotite, muscovite and plagioclase, with accessory rutile, epidote, graphite, tourmaline and chlorite. The latter overgrows the main foliation. Kyanite locally occurs in proximity to quartz veins. The sedimentary sequence has a maximum thickness of ~280 m in the Central Pit (the second from the south), but progressively thins to <20 m in the northernmost Upper Lega Dembi pit, where it is buttressed between a massive meta-gabbro to the west and quartzo-feldspathic gneisses in the east. This massive meta-gabbro and associated minor amphibolites, form the western margin of the Lega Dembi deposit. These mafic bodies form a north-south ridge, paralleling the structural grain of the Megado belt, and are locally intruded by stringer- and pod-like tonalite bodies that have penetrative planar and linear fabrics, and are folded together with the amphibolites on a metre scale.
The Lega Dembi deposit is located within a large, north-south trending F2 fold that has deformed the Megado belt. The fold closure is south of the deposit where the ultramafic talc schists are at their thickest, although there are insufficient indicators of whether it is a syn- or antiform. Whilst F2 folds regionally predominantly plunge shallowly to the north and south, mineral and mineral stretching lineations within the deposit area, together with mesoscale fold axes, suggest a steep westerly plunge for the main F2 fold at Lega Dembi. A regionally developed, upright, north-south trending S2 fabric is axial planar to the F2 folds. This fabric intensifies to become a pervasive mylonitic foliation towards the eastern structural margin of the Megado belt, with transposition of the coplanar S2 foliation into the mylonitic shear foliation (S3) to become S2/3. This composite foliation is further accentuated by foliation-parallel quartz veins.
Shear sense indicators in the meta-sediments indicate both a normal and a reverse dip-slip, but consistently show sinistral strike-slip. Similarly, structural elements show a prominent sinistral strike-slip component along the contact between the gneissic basement and Neoproterozoic volcano-sedimentary sequence. This deformation is progressive, with the S2/3 veins and the foliation-parallel quartz veins being displaced by the sinistral strike-slip shearing, while the latter is transected by the north-south S2/3 fabric. The upright F4 folds refold the composite S2/3 fabric, the large north-south trending folds, as well as the ductile strike-slip structures. An axial-planar S4 foliation is expressed by the growth of biotite at a high angle to the north-south trending S2/3 transposition fabric. A set of NW-SE and east-west brittle and brittle-ductile D5 faults affects the Lega Dembi deposit in its northern and southern parts, disrupting the continuity of the mineralisation.
Both the gneissic basement and Neoproterozoic volcano-sedimentary sequence of the greenstone belt contain a variety of moderate to steep westerly plunging linear fabrics. These include: i). a mineral stretching lineation of quartz-feldspar aggregates in the gneisses, ii). a mineral lineation defined by the preferred growth of minerals such as amphiboles and tourmaline in the supracrustals, together with kyanite in proximity to quartz veins, and iii). an intersection lineation between S0 and the S2 and S3 foliations in the greenstones, respectively. The mylonitic foliations close to the contact between the basement gneisses and the greenstones are characterised by a steep westerly plunging mineral stretching lineation and by quartz-rodding.
Mineralisation
Gold mineralisation at Lega Dembi is concentrated in strongly foliated meta-sedimentary rocks along the sheared contact between the basement quartzo-feldspathic gneisses and the volcano-sedimentary sequence on the eastern margin of the Megado belt. Mineralisation extends over a north-south strike extent of ~2 km, parallel to the S2/3 fabric and lithological layering. It has a maximum width of ~140 m in the central parts of the deposit (the Northern pit), but gradually tapers to <20 m in the north, corresponding to the northward pinching of the meta-sedimentary unit. In the northern two pits mineralised zones are connected along strike by a continuous development of mineralised quartz veins, the other segments of the deposit are separated by barren intervals as a result of offsets along E-W trending D5 faults.
The main mineralisation is restricted to within 80 m of the contact between the basement gneisses and the volcano-sedimentary succession. It is characterised by three main composite quartz vein systems that are each up to 10 m in thickness, the Eastern, Central and Western veins, although thin foliation-parallel quartz veinlets occur throughout the metasedimentary succession.
The principal vein system in the Northern pit has a strike extent of ~250 m and semicontinuously persists down-dip for >350 m (Billay et al., 1997). The Eastern vein is hosted by muscovite-quartz-plagioclase schist, whilst the Central and the Western veins are within laminated, graphite-bearing arkose and quartzwacke hosts.
The amount of graphite within the meta-sedimentary rocks increases from ≤0.1 to 0.4 wt.% in proximity to the main vein system, up to 1.5 wt.% in some of the less altered wall rocks external to the mineralised zone.
Four sets of quartz veins have been differentiated:
i). Type 1 - massive to laminated quartz veins that are intensely deformed, and are the most abundant and principal hosts to gold mineralisation. They are either parallel to the S2/3 foliation, or form tight to isoclinal folds with S2/3 foliation parallel axial planes. The quartz veins commonly pinch and swell and are boudinaged, both down-dip and along strike. Although they may occur in isolation, veins are typically close spaced, with gradual transitions from laminated or ribbon-textured to massive quartz, and are up to 5 m in thickness. The three main vein systems show gradual transitions along strike from massive quartz veins to a more stringer-style towards their lateral terminations, although massive veining may recur.
ii). Type 2 - thin, commonly <1 cm thick veinlets that are discordant to S2/3, are variably folded, and are subordinate to Type 1 veins. They are folded at variable degrees and/or sheared parallel to the S2/3 foliation.
iii). Type 3 - breccia quartz veins, that although only locally developed, extensively brecciate wall rocks. Wall rock clasts are angular, intensely foliated and folded, and contain sulphide mineralisation. They range from minute inclusions to rafts that are up to 0.5 m across. The wall rocks have been folded prior to brecciation which is in turn pre-mineralisation. These veins are described as disaggregation breccias, with a progressive textural progression from their margins, where the wall rocks are largely intact, to their centres, where clasts are isolated and rotated rafts cemented by a massive, milky quartz matrix.
iv). Type 4 - largely undeformed, distinct cross-cutting veins which cut the S2/3 fabric. Although apparently undeformed, microscopic deformation is indicated by the undulose extinction of quartz and the formation of quartz subgrains. Tese veins are subordinate in significance and are predominantly <5 cm thick. Some strike parallel to the S2/3 foliation, but dip at shallower angles of 35 to 40°W, whilst others dip at steep-to-moderate angles to the east.
Sulphides mainly occur as fine disseminations in quartz veins and in wall rocks, or parallel to the S2/3 foliation, in boudin necks or in cross-cutting fractures. The main gold- and sulphide mineralisation occurs in association with Type 1 veins. Sulphides in the quartz veins comprise, in decreasing order of abundance: chalcopyrite, galena, pyrrhotite and pyrite, with minor to rare sphalerite, gersdorfite, arsenopyrite, bournonite, molybdenite, tellurides, silver-tetrahedrite and gold. Microscopic gold occurs along sericite alteration lamellae and wall-rock scepta within quartz veins, and are commonly <0.25 mm, mainly <0.05 mm, and elongated parallel to S2/3 foliation. Gold is typically spatially associated and intergrown with galena, and is also commonly associated with chalcopyrite, pyrrhotite and the tellurides hessite and stuetzite. Inclusions of gold in pyrrhotite, galena, tellurides and chalcopyrite are rare. The fineness of gold ranges from 783 to 904, averaging 820 (Billay et al., 1997).
A pyrrhotite-gersdorfite assemblage dominates in the biotite-actinolite alteration bordering Type 1 quartz veins, with minor pyrite, chalcopyrite, arsenopyrite, pentlandite and niccolite. No microscopically visible gold has been observed in the alteration assemblage adjacent to the veins (Billay et al., 1997).
Less-altered wall rocks between the quartz-vein systems and outside of the main mineralised zone contain up to 5 vol.% sulphides, mainly pyrrhotite and minor chalcopyrite and arsenopyrite. Rare sphalerite, gersdorfite and molybdenite are found adjacent to foliation- parallel quartz veinlets (Billay et al., 1997).
Types 2 and 3 veins are characterised by a pyrite/pyrrhotite-chalcopyrite assemblage, with minor galena and rare microscopic gold. Occasional globular textures of pyrite possibly indicate open-space filling. Feathery textured pyrite results from the filling of foliation planes in the intensely foliated wall rocks adjacent to the quartz veins. Type 4 veins are commonly devoid of any gold- and sulphide mineralisation, although pyrite, chalcopyrite and pyrrhotite occur in the biotite-actinolite alteration haloes enveloping Type 4 veins (Billay et al., 1997).
Alteration
Alteration is broadly zoned on a deposit scale, although the pattern is confused by the intense and multiple quartz-veining and associated alteration overprint, as well as variations in hostrock lithologies. The broad zonation is as follows as the main quartz veining is approached:
• chlorite-carbonate-epidote, which occurs between the Western and the Central veins, and is distal to the vein system in wall rocks, giving the rocks a distinct greenish coloration. It may also occur as foliation-parallel anastomosing bands. Minor chlorite is also developed after muscovite within the quartz veins.
• actinolite/tremolite-biotite-calcite, which is pervasive and the most characteristic alteration at Lega Dembi. It is developed within 15 to 20 m of the main quartz veining. At least two main stages of this alteration can be differentiated:
Stage 1 - commonly characterised by development of an S2/3 foliation-parallel actinolite/tremolite-biotite-calcite assemblage, which may also occur as strongly folded discrete, light greenish veins and veinlets of this same mineralogy. This alteration is locally developed as massive, up to 5 m wide pervasive zones flanking the main quartz vein systems. Locally, tourmaline defines a steep westerly-plunging mineral lineation.
Stage 2 - principally of actinolite-tremolite which overgrows the stage 1 assemblage and the S2/3 foliation, occurring as rosettes or discordant pockets and veinlets. This alteration type also occurs along the margins of type 4 veins.
• sericite, the innermost alteration product which occurs in close proximity to the massive quartz veins. Within the quartz veins, fuchsite and sericite occur as thin, commonly <3 mm thick, foliation-parallel bands and laminae, whilst minor calcite may also be locally preserved. In the wall rocks complete to partial sericite replacement of feldspar is common. Biotite sometimes accompanies sericite on vein margins.
SAKARO
The Sakaro deposit is located some 4 km SW of Lega Dembi. Gold mineralisation is hosted in an en-echelon quartz-vein system developed in graphitic metapelites parallel to the contact between massive amphibolites and metasediments, the latter being correlated stratigraphically with the meta-sediments of the Lega Dembi area. The ore is hosted by gold-rich and sulphide-poor quartz veins that are from 1 up to 9.3 m thick and occupy fault planes, follow foliation and rock contacts in the Neoproterozoic low-grade metamorphic host rocks. The veins, which strike NE and dip at 45 to 80°NW, form a mineralised zone that is ~760 m in strike length and up to at least 150 m in vertical extent. The veins are mainly composed of quartz, containing partings, streaks and fragments of the altered wall rocks, particularly graphite. Veins No. 1 and 2 were the richest shoots known in 1993, with average grades of 3.4 and 5.7 g/t Au respectively.
Within each vein, three distinct hypogene mineralised zones have been recognised, from vein margin to centre, involving marcasite-chalcopyrite in the peripheries → galena-chalcopyrite-marcasite → sphalerite-high galena-chalcopyrite in the centre. Gold is present in all three zones, although it is strongly enriched in the sphalerite-high galena-chalcopyrite section in the vein cores, followed by the hanging wall marcasite-chalcopyrite zone, and least in footwall zone of the same sulphide assemblage. There is an outward metal zonation from Au-Ag-Pb within the veins to Ag-Pb-Cu on the vein margins and proximal wall rock to as much as 3 m from the vein. This passes into a halo in the wall rocks enclosing the veins comprising an inner wolframite-scheelite-Fe sulphides (W-As-Cu) zone and an outer marcasite-pyrrhotite-chalcopyrite (Fe-Co-Ni-Mo-Cu) periphery extending to as much as 20 m from the vein. This halo tapers downward. The mineralogy suggests the vein development was the result of multiple episodes of vein opening and filling characterised by wolframite-scheelite-quartz, sulphide-gold-quartz and disulphide-carbonate stages. The generalised paragenetic sequence is an early wolframite + scheelite stage, followed by an early sulphide event involving arsenopyrite + pyrrhotite + gold, then a late sulphide stage with chalcopyrite + sphalerite + galena + gold, and a disulphide-carbonate stage of marcasite + melnicovite-pyrite + siderite. (Girma, 1993).
Production, Reserves and Resources
Selected statistics for the Lega Dembi - Sakarao district are as follows:
Placer gold production 1936 to 1996 (excluding artisanal mining) - >55 t of gold (Billay et al., 1997);
Endowment at Lega Dembi (Johnson et al., 2017):
Open Pit - ~66 t of gold @ 3.7 g/t Au, which equates to ~17.8 Mt of ore;
Underground - ~11 t of gold @ 3.6 g/t Au, which equates to ~3 Mt of ore;
Underground resource at Sakaro - ~20 t of gold (MIDROC Newsletter June 2014);
Inferred resource at East Sakaro - 2 Mt @ 9.8 g/t Au for ~19.6 t of gold (MIDROC Gold website, viewed 2020)
The most recent source geological information used to prepare this decription was dated: 1999.
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
Lega Dembi
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Selected References:
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Billay, A.Y., Kisters, A.F.M., Meyer, F.M. and Schneider, J., 1997 - The geology of the Lega Dembi gold deposit, southern Ethiopia: implications for Pan-African gold exploration: in Mineralium Deposita v.32, pp. 491-504
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Girma, M., 1993 - Mineralogical and geochemical studies on the Sakaro primary gold deposit (Sidamo, Southern Ethiopia): zonation in ore bodies and host rocks: in A thesis presented to the School of Graduate Studies, Addis Ababa University, 159p.
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Johnson, P.R., Zoheir, B.A., Ghebreab, W., Stern, R.J., Barrie, C.T. and Hamer, R.D., 2017 - Gold-bearing volcanogenic massive sulfides and orogenic-gold deposits in the Nubian Shield: in S. Afr. J. Geol. v.120, pp. 63-76.
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Tadesse, S., 1999 - Geology and Gold Mineralization in the Pan-African Rocks of the Adola Area, Southern Ethiopia: in Gondwana Research v.2, pp. 439-447.
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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