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Asmara District - Debarwa, Emba Derho, Adi Nefas, Kodatu, Gupo, Adi Rassi |
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Eritrea |
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Au Cu Zn Ag
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Super Porphyry Cu and Au
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The Asmara District volcanic hosted massive sulphide (VHMS) copper, zinc, silver and gold deposits are located in close proximity to, Asmara the capital of Eritrea, within 15 km to the NNW and 30 km to the SSW. Individual VHMS deposits and occurrences described below include
Debarwa,
Emba Derho,
Adi Nefas and
Kodatu.
The district also includes low sulphidation epithermal gold at
Gupo and
Adi Rassi in close proximity, accompanied by identical alteration and in the same host units, although these deposits are also described in the literature (e.g., Barrie et al., 2016), as orogenic.
The exposed gossans of the Debarwa deposit in particular were the focus of the earliest drilling activity both pre- and post-Eritrean Independence in 1993. It was mined briefly in the 1980s until closure was forced by military conflict during Eritrea’s prolonged struggle for independence from Ethiopia. Following the cessation of renewed border hostilities between 1998 and 2000, exploration of the Asmara district began again. Considerable systematic exploration had taken place centred on the gossans at Emba Derho, with numerous significant drill intersections. However, no coherent resource could be delineated until Sunridge Gold Corporation tested a gravity anomaly several hundred metres to the NE of the gossan in 2006 and intersected 203 m of massive sulphides @ 1.77 % Zn, 0.41 % Cu. The subsequent drill program eventually delineated a resource totaling 85 Mt in oxide, supergene, Cu-rich sulphide and Zn-rich sulphide zones. This resource, complemented by additional tonnages from other deposits of the district, led to Sunridge Gold Corp being able to complete a positive feasibility study in 2012. In 2015 an operation that encompassed the VHMS Debarwa Cu-Au, Adi Nefas Zn-Cu-Au and Emba Derho Cu-Zn-Au, and the Gupo orogenic Au deposit was planned to commence in late 2016. This operation involved open pit mining at Debarwa, Emba Derho and Gupo, whilst Adi Nefas was planned as an underground mine. Staged development comprised an initial campaign of direct shipping ore from Debarwa, followed by gold heap-leaching of near surface gold ore, followed, in turn, by mining of supergene copper ores from both Debarwa and Emba Derho, to be processed at a central flotation plant at Emba Derho. Copper concentrate and by-product gold and silver were to be shipped via the Port of Massawa. The final stage involved hypogene copper and zinc ores from Debarwa, Adi Nefas and Emba Derho, processed at the Emba Derho flotation plant. In 2015 Sunridge Gold Corp. sold its 60% interest in the management company Asmara Mining Share Company to the Chinese state owned Sichuan Road and Bridge Mining and Investment Development Corp. Ltd. The remaining 40% is owned by the Eritrean Government instrumentality Eritrea National Mining Corporation (Enamco).
Whilst the VHMS deposits of the Bisha and Hassai/Ariab districts elsewhere in Eritrea and Sudan have deep, well developed gold-rich oxide profiles, most of the Asmara VHMS deposits lack deep oxidation. The exception is Debarwa. This reflects the proximity of the Asmara District deposits to the pre-Miocene peneplain that extended across much of northeast Africa and the Arabian Peninsula. Although there has been >2 km of uplift around Asmara, erosion and deflation of the land surface since the Miocene have been minimal. It remains a plateau that still retains a shallow, unconformable cover of Miocene tholeiitic basalts that were emplaced during the initial opening of the Red Sea and shields the underlying Neoproterozoic rocks from post-Miocene oxidation.
The Asmara District deposits lie within the Nakfa Terrane in the western or Nubian half of the Arabian-Nubian Shield. The Shield is postulated to be the northern extension of the Mozambique Precambrian Belt which is composed of poly-deformed Pan-African reworked Archaean and Paleoproterozoic gneissic terranes, overlain by, and in thrust contact with Pan-African juvenile oceanic volcano-sedimentary and ophiolite assemblages. The Arabian-Nubian Shield is almost entirely composed of juvenile, late Tonian to Cryogenian supra-subduction ophiolites and island arc rocks, intruded by syn-, late- and post-tectonic Late Tonian to Ediacaran mafic to felsic intrusions. NOTE: Tonian = 1000 to 850 Ma; Cryogenian = 850 to 635 Ma; Ediacaran = 835 to 541 Ma.
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.
For more detail on the tectonic divisions recognised in Eritrea, see the separate Bisha Cluster record.
The Asmara District deposits are localised in the >200 km long, sinistral Asmara-Nakfa Belt, a complex network of ductile to brittle-ductile transpressional shear zones and folds that is several tens of kilometres in width in eastern Eritrea. To the north, this structure merges with the NNE trending Augaro-Adobha Belt which encompasses the Bisha-Hambok cluster of VHMS deposits. Over much of its length, the Asmara-Nakfa Belt strikes north-south to NNW, cutting across the eastern Nakfa Terrane. However, in the vicinity of Asmara, over the interval where the main VHMS deposits are located, it curves to trend NNE-SSW, parallel to the Augaro-Adobha Belt (Ghebreab et al., 2009; Fritz et al., 2013; Johnson et al., 2017). The same belt also embraces significant Orogenic Gold deposits (e.g., Gupo).
The Neoproterozoic rocks of the Asmara District are generally east-facing, tightly folded about northerly trending fold axes, and predominantly dip at 45 to 85°E or SE. They are primarily a bimodal sequence of volcanic and volcaniclastic rocks that are overlain to the east by a metasedimentary succession. They are preserved at or below lower greenschist facies metamorphism and have a well-developed foliation that parallels the regional NNE structural-stratigraphic grain. The foliation trend changes to the north of Asmara in the vicinity of a number of syn- to post-tectonic intrusions of variable composition and age of emplacement. All of these rocks are overlain by up to more than 200 m of flat-lying Tertiary olivine basalts that overlie a well-developed palaeo-weathering horizon. This old weathering surface locally includes thick laterites that overlie extensive saprolites.
Three mineralised trends are observed within the district, all oriented NNE. These are from west to east, the i). Emba Derho trend that includes the Emba Derho deposit and smaller Dairo Paulos and Woki Duba occurrences; ii). Debarwa-Adi Nefas trend that extends >25 km south and 5 km north of Asmara and includes the Debarwa, Shiketi, Adi Lamza, Adi Nefas deposits and Adi Adieto occurrence; iii). Adi Rassi-Kodatu trend to the east of Asmara. The second and third of these trends are partly defined by a chert/chemical sedimentary unit, whilst the Emba Derho trend becomes less-well defined to the SW where many of the metal occurrences are hosted within granitoids.
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Emba Derho (#Location: 15° 24' 56"N, 38° 52' 51"E)
The Emba Derho VHMS deposit is located ~15 km NNW of Asmara within Neoproterozoic metavolcanic and metasedimentary rocks that are cut by granitoid plutons. The deposit is reflected at surface by a prominently outcropping gossan developed over an area of ~800 x 220 m, outcropping as a tightly folded unit with NW oriented fold axial planes and steeply dipping limbs. The gossan has been subjected to at least two phases of folding and has a 'W' shape, open to the NW. These gossans have been known since at least the 1970s when first tested by the Ethiopian Geological Survey with six drill holes. They comprise oxidised and supergene acid-leached felsic tuffaceous rocks and flows, weathered massive to semi-massive sulphides and orange-brown weathering 1 to 2 m thick rhyolite dykes/sills. The gossans are surrounded by relatively poor exposure of typically well foliated acid-leached, predominantly fine tuffaceous rocks of both mafic and felsic composition. These are cut by strongly weathered post-deformation granitic dykes of various compositions.
The footwall sequence below the massive sulphide bands comprises blue quartz-phyric rhyolite flows, flow breccia and associated felsic fragmental tuffaceous rocks that are locally altered to sericite-chlorite schists. These are overlain by stacked layers of semi-massive to massive sulphide that each range 5 up to 40 m in thickness, separated by numerous tuffaceous and volcanic flow partings and one barite layer. All of these rocks are cut by various post-deformation felsic dykes that are typically 1 to 5 m thick. The hanging wall sequence is composed of pillow basalt and pillow breccia units that have been subjected to significant epidote-silica alteration. The altered mafic volcanics units just above the massive sulphides contain several manganiferous, siliceous chemical sedimentary units, whilst a thin sill of altered and deformed coarse-grained pyroxenite occurs within the mafic volcanic flows. The entire sequence generally dips steeply to the north.
The deposit is zoned as follows:
• Gossan/Oxide Zone - a dense, dark red-brown gossan composed of hematite, limonite, goethite and locally magnetite that was largely derived from surface weathering of massive sulphides. It occurs as discontinuous, commonly folded layers that may be up to several metres or more in thickness. Outside of the main gossan zone, as detailed above, it is locally found as centimetre to tens of cm scale layers and veins, predominantly within acid-altered host rocks. It has also been locally remobilised to form 'ferricrete' caps with very delicate textures. Oxidation typically extends to the water table, 20 to 30 m below the surface.
• Copper-enriched supergene zone - which occurs as fine to medium-grained, vuggy, locally sandy, massive pyrite with interstitial covellite, digenite and minor bornite. This zone is found at or below the water table. Zinc is depleted relative to the hypogene mineralisation.
• Copper-rich massive sulphide hypogene zone - which is well defined in the northern part of the deposit and comprises medium to coarse-grained massive pyrite and pyrrhotite with interstitial chalcopyrite and magnetite, with massive bands, stringers and blebs of chalcopyrite.
• Zinc-rich massive sulphide hypogene zone - which is well developed in the southern and western part of the deposit and comprises fine to medium-grained massive pyrite with sphalerite occurring as interstitial disseminations and as thin bands and laminae within the pyrite. At least three types of sphalerite are recognised, namely a rusty-brown (Fe-rich, high temperature phase), honey-yellow and whitish-grey (lower temperature phase) sphalerite.
• Pyritic massive sulphide hypogene zone - massive sulphides that occurs immediately below the chert and silica-rich chemical sediments and mafic flows of the hanging wall sequence, representing the stratigraphic top of the massive sulphide event. Sulphides are predominantly fine to medium-grained massive pyrite with fine-grained disseminated magnetite and very minor chalcopyrite and sphalerite.
The host sequence and mineralisation are cut by at least five felsic intrusive phases, the most prominent of which is a suite of fine- to medium-grained post-kinematic leucocratic rocks, ranging from tonalite to granite in composition. These are interpreted to be related to a high-level intrusion of similar composition exposed to the north of the deposit. Dykes generally parallel the NW structural-stratigraphic grain of the host sequence they intrude, although NNE trends are common in the NE. An earlier, much less voluminous suite of post-kinematic diorite dykes is also mapped, generally trending to the north.
Pre-mining feasibility study NI 43-101 compliant Mineral Resources at Emba Derho as at May, 2013 were (Senior et al., 2013):
Measured + Indicated Mineral Resources
Gold oxide (0.5 g/t Au cut-off) - 1.74 Mt @ 1.06 g/t Au, 4.3 g/t Ag, 0.07% Cu, 0.04% Zn;
Copper supergene (0.5% Cu cut-off) - 1.64 Mt @ 0.17 g/t Au, 12.2 g/t Ag, 0.94% Cu, 0.38% Zn;
Copper hypogene (0.3% Cu cut-off) - 49.8 Mt @ 0.17 g/t Au, 7.7 g/t Ag, 0.83% Cu, 0.93% Zn;
Zinc hypogene (<0.3% Cu; >1% Zn cut-off) - 16.8 Mt @ 0.31 g/t Au, 9.9 g/t Ag, 0.14% Cu, 2.80% Zn;
TOTAL Tonnage - 70 Mt
Inferred Mineral Resources
Copper hypogene (0.3% Cu cut-off) - 13.28 Mt @ 0.25 g/t Au, 10 g/t Ag, 0.87% Cu, 0.89% Zn;
Zinc hypogene (<0.3% Cu; >1% Zn cut-off) - 1.77 Mt @ 0.39 g/t Au, 11 g/t Ag, 0.20% Cu, 1.94% Zn.
TOTAL Tonnage - 15.05 Mt
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Debarwa
The Debarwa deposit is located ~30 km SSW of the centre of Asmara. It is hosted by an overturned, moderately to steeply dipping sequence of variably and intensely altered, bimodal, submarine, low-K tholeiitic basaltic and rhyolitic volcanic rocks together with minor chemical sedimentary rocks (Mauritsen and Arafine, 1998). The immediate host to mineralisation are altered felsic rocks, underlain by mafic rocks that locally mark the stratigraphic footwall. Mafic rocks predominate within the 6 km long NNE trending belt that hosts the Debarwa deposit and Shiketi Gossan.
The massive sulphide mineralisation is expressed at surface by a gossan with a strike length of ~1.2 km which marks the crest of a sharp, west facing ridge flanking the Gual Mereb River. There are two main zones of massive sulphides:
i). the larger ~830 m long Debarwa Main lens - which comprises at least three sub-parallel mineralised horizons, the best developed, thickest and most continuous of which is the most westerly. It dips at ~50 to 60°W, is ~8 to 30 m thick and has been traced to a depth of ~250 m below surface. Massive sulphides are confined to this western horizon and vary from <1 to ~22 m in thickness. The overlying supergene and oxide zones are up to 50 m in thickness.
ii). the smaller Debarwa South lens, which is ~285 m long, with a massive sulphide zone that steepens from ~35 to 45°W in the north to around 60°W in the south. It is thinner, lower in grade, and not as continuous as the Debarwa Main zone, but has been traced to ~250 m below the surface.
Mineralisation at Debarwa is divided into:
• Gossan/Oxide Zone - from which base metals have been predominantly leached over a vertical interval of ~80 m below the highest points to between 35 and 50 m below the floor of the Gual Mereb River valley. The Debarwa gossans are largely composed of iron oxides and hydroxides (hematite, limonite, goethite and minor jarosite), silica and remnant clay, and vary from deep brick red to black. They may include a variety of lithologies, from siliceous botryoidal limonite-hematite, to jasperoid, to greenish impure barite layers or beds. The gossan above Debarwa Main have oxides that typically contain erratic but generally high gold grades of from 0.4 to 14 g/t Au, commonly >4 g/t Au over widths of between 7 and 17 m. Silver values range from 0.4 and 183 g/t, typically >15 g/t, while Cu and Zn values are relatively insignificant.
• Transition Zone - which is ~10 to 15 m thick and marks the gradual change from the oxide to supergene zones corresponding to the interval of water table fluctuation. This zone is the most enriched in gold and silver and most depleted in base metals particularly copper and zinc. An example intersection was from 32 to 40 m depth (8 m) averaging 39.1 g/t Au, 519.5 g/t Ag, 0.07% Cu and 0.02% Zn.
• Supergene Zone - characterised by extreme enrichment of copper derived from acid leaching of metal from the Oxide Zone, deposited as an assemblage that includes digenite, chalcocite, tenorite, covellite and bornite. These secondary sulphides replace and form coatings around hypogene sulphides such as chalcopyrite, bornite and pyrite and crystallise in the voids left by sphalerite which is unstable in acid conditions. Supergene grades are higher in the Debarwa Main zone with typical supergene intersections ranging from between 2 and 26 m, with grades varying from 0.9 to 32% Cu. Precious metals range from 0.5 to 4 g/t Au (typically between 1.5 and 3 g/t Au), and 16 to 144 g/t Ag (typically 30 to 80 g/t Au). Zinc is usually very low, having been almost completely stripped.
• Hypogene Zone - which is preserved below the supergene zone, at depths of >65 to 90 m below the valley floor of the Mereb and Gual Mereb rivers. The predominant sulphides are pyrite and chalcopyrite, with common bornite and sphalerite, in massive, semi-massive, and stringer vein zones that range up to 15 m in thickness. Typical drill intersections of primary zone mineralisation at Debarwa Main carry between 2.0 and 9% Cu (typically 2 to 4%), 0.5 and 7 g/t Au (typically <2 g/t), 6 and 150 g/t Ag, and 1 to 12% zinc (typically 2 to 3%).
Pre-mining feasibility study NI 43-101 compliant Mineral Resources at Debarwa as at May, 2013 were (Senior et al., 2013):
Measured + Indicated Mineral Resources
Gold oxide (0.5 g/t Au cut-off) - 0.374 Mt @ 1.47 g/t Au, 6 g/t Ag, 0.06% Cu, 0.04% Zn;
Copper transition (0.5% Cu cut-off) - 0.72 Mt @ 2.85 g/t Au, 27 g/t Ag, 0.08% Cu, 0.05% Zn;
Copper supergene (0.5% Cu cut-off) - 1.389 Mt @ 1.40 g/t Au, 33 g/t Ag, 5.15% Cu, 0.07% Zn;
Copper hypogene (0.5% Cu cut-off) - 0.774 Mt @ 1.30 g/t Au, 29 g/t Ag, 2.34% Cu, 3.92% Zn;
Zinc hypogene (<0.5% Cu; >2% Zn cut-off) - 1.24 Mt @ 0.31 g/t Au, 22 g/t Ag, 3.05% Cu, 2.80% Zn.
TOTAL Tonnage - 3.312 Mt
Inferred Mineral Resources
Gold oxide (0.5 g/t Au cut-off) - 0.239 Mt @ 1.1 g/t Au, 5 g/t Ag, 0.10% Cu, 0.10% Zn;
Copper transition (0.5% Cu cut-off) - 0.138 Mt @ 1.4 g/t Au, 22 g/t Ag, 0.10% Cu, 0.00% Zn;
Copper supergene (0.5% Cu cut-off) - 0.144 Mt @ 0.6 g/t Au, 31 g/t Ag, 2.7% Cu, 0.07% Zn;
Copper hypogene (0.5% Cu cut-off) - 0.154 Mt @ 2.6 g/t Au, 41 g/t Ag, 1.2% Cu, 3.60% Zn;
Zinc hypogene (<0.5% Cu; >2% Zn cut-off) - 6 Kt @ 1.1 g/t Au, 21 g/t Ag, 0.40% Cu, 3.30% Zn.
TOTAL Tonnage - 0.681 Mt
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Adi Nefas
The Adi Nefas deposit is located ~10 km north of the centre of Asmara and ~8 km SE of the Emba Derho deposit. It is a VHMS deposit hosted by a Neoproterozoic bimodal sequence of mafic and felsic volcanic rocks, intruded by later quartz porphyry sills and dykes. Basalts in the footwall, or western side of the deposit, are strongly epidotised and are locally epidosites. The sequence on the eastern, hanging wall, side of the massive to semi-massive sulphides mainly comprise pillow basalts and foliated equivalents, intruded by minor post-mineral quartz-porphyry dykes and sills. The pillowed basalts and associated mafic metavolcanic rocks are followed by undifferentiated tuffaceous sedimentary rocks and more pillow basalts containing silicate-magnetite chemical sediment lenses. The succession may be summarised as follows, upwards (after Barrie, 2004):
• Basalt - >300 m of pillowed basalt with pillow breccia and moderate to intense quartz-epidote alteration;
• Dacite - heterolithic tuff/fragmental and ash lapilli tuff that has been altered to sericite and chlorite schist containing 2 to 10% disseminated pyrite. This part of the sequence, which is ~200 m thick, is intruded by multiple quartz porphyry dykes and sills.
• Adi Nefas massive to semi-massive sulphides
• Basaltic tuff with minor dacite lapilli tuff, that is ~200 m thick, and altered to chloritic schist with a few quartz porphyry sill/dyke intrusions.
• Basalt - >250 m of pillowed basalt with pillow breccia, locally containing amygdules. This part of the sequence has been subjected to quartz-epidote alteration. The interval contains one or more silicate-magnetite chemical sediment lenses and a few thinner quartz porphyry sill/dykes.
The altered part of the sequence has accommodated later or coeval strain to a greater degree forming schists, such that the strain intensity increases from both the footwall and hanging wall toward the sulphide lenses. The sequence and main structural grain trend NNE and the stratigraphy is subvertical to steeply east dipping. The deposit occurs within a tight, moderately northerly plunging, westerly-verging antiform, with the more felsic rocks in the fold core and the massive sulphide lenses on the eastern limb of the fold (Zeremariam, 2005).
The Adi Nefas gossan averages some 10 m in thickness and is well-exposed over a ~700 m interval but is sporadically mapped over a strike length of ~2 km. It comprises a silica, hematite and goethite-rich assemblage that represent the surface expression of the massive sulphide unit and the immediate semi-massive to dense disseminated sulphide-rich host rocks. The Adi Nefas deposit has a vertical zonation due to weathering. The underlying deposit is an elongate NNE trending, steeply east dipping, massive sulphide lens ranging from 5 to 20 m in thickness and is predominantly hosted within a hydrothermally altered felsic quartz-sericite-chlorite-pyrite schist that is, in turn, flanked stratigraphically above and below by altered metabasaltic rocks. The more heavily altered section of the felsic sequence ranges from 25 to 60 m in thickness.
The upper oxide and underlying transition zones are leached and strongly depleted in Cu and Zn relative to the hypogene sulphide mineralisation, accompanied by a slight enrichment in Au. There is, however, a slight increase in the tenor of base metal with depth in these zones. These upper zones are typically developed from surface to the water table which is generally at a depth of 20 to 30 m below surface. Below this, a typically 20 to 40 m thick supergene zone contains significantly enriched Cu and Au and slightly enriched Ag relative to the hypogene sulphide mineralisation. Zinc is still depleted relative to the hypogene zone. Mineralisation within the hypogene sulphide zone is more zinc rich compared to the other deposits of the Asmara District making it a zinc-copper-silver-gold VHMS deposit. In all other respects it is very similar in style to the Debarwa deposit.
Pre-mining feasibility study NI 43-101 compliant Mineral Resources at Adi Nefas as at May, 2013 were (Senior et al., 2013):
Indicated Mineral Resources
Hypogene sulphides (2% Zn cut-off) - 1.841 Mt @ 3.31 g/t Au, 115 g/t Ag, 1.78% Cu, 10.05% Zn.
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Gupo
The Gupo epithermal (?) vein gold deposit is located ~1.5 km south of the Adi Nefas massive sulphide mineralisation, and appears to lie on the same stratigraphic horizon, although mineralisation in the former is structurally controlled and related to quartz veining. The rocks hosting the Gupo deposit are highly deformed and strongly foliated mafic flows and mafic tuffs, intercalated and cross cut by thin layers of quartz-phyric felsic flows and subordinate dykes. The mafic volcanic rocks have been pervasively chlorite altered throughout, with locally epidote development. However within the shear zone that host the mineralised veins, this alteration changes to a combination of sericite, pyrite and carbonates.
The highest grade gold mineralisation is found within crystalline, coarse-grained pyrite in quartz veins, with lower grades in medium to coarse grained, euhedral to sub-euhedral pyrite hosted by a sericite alteration selvage. This alteration halo varies from few centimetres up to several metres in width, depending of the thickness of the quartz veins, width of the shear zone and the porosity of the host rocks. The quartz veining occurs as a complicated network of stockworks that pinch and swell, within the shear zone.
The Gupo gold deposit has been defined over ~1.6 km of strike length and a width of 10 to 20 m, divided into two zones, Gupo North and Gupo South. The two zones are separated by a 400 m barren interval interpreted to be an offset across a late stage normal fault associated with the uplift of the terranes of the Arabian-Nubian Shield during the opening of the Red Sea. The Gupo North zone splits into eastern and western sub-zones half way to the south, probably representing the root system of the gold mineralisation uplifted by the normal faulting.
Pre-mining feasibility study NI 43-101 compliant Mineral Resources at Gupo as at May, 2013 were (Senior et al., 2013):
Indicated Mineral Resources at a 0.5 g/t Au cut-off - 0.9518 Mt @ 1.53 g/t Au;
Inferred Mineral Resources at a 0.5 g/t Au cut-off - 1.8086 Mt @ 1.83 g/t Au.
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Adi Rassi
The Adi Rassi deposit is developed in the southern section of the easternmost of the three mineralised trends, the Adi Rassi-Kodatu trend in the Asmara District, 30 km south of Asmara.
The rocks in the Adi Rossi area are a tight to isoclinally folded, and west-overturned Neoproterozoic volcano-sedimentary sequence, with later NNE trending shear zones overprinting and obscuring the folding style. Tertiary basaltic dykes cut the host Neoproterozoic basement rocks. The succession is as follows:
Volcano-sedimentary Lithologies - which are the predominant host rock to mineralisation and comprise mafic volcanic flows and tuffs, which include pillowed and hyaloclastite basalt with thin interbeds of volcanic breccia, lava flows and felsic rocks, as well as lesser lenses of feldspathic tuffaceous greywacke. Highly siliceous non-magnetic chemical sediments are interbedded within the mafic flows to the east of the deposit area. This succession is cut by a series intrusives, mainly dykes:
• Quartz Diorite Porphyry Dykes, which intruded the package of mafic volcanic rocks. They are generally narrow, with cm to metre-scale, and are medium-grained and granular-textured with local quartz phenocrysts;
• Sub-Volcanic Andesite found to the west of the deposit area where they form a structural hanging wall to the mineralised mafic volcanics. They have a massive texture and a dacitic-andesite to andesitic composition and are feldspar-phyric. The contact zone with the mafic volcanic rocks to their east contains rafts of mafic volcanics within tens of metres of the contact. They are thought to post-date mineralisation.
• Felsic Dykes, which also cut the mafic volcanic sequence and are more prevalent within the southern part of the deposit. They are fine grained, have very few feldspar phenocrysts with or without visible quartz, and where fresh are usually massive-textured. They do not contain any sulphide mineralisation, but are occasionally sheared and deformed.
• Tertiary Dykes - NNE trending basaltic dykes that cut all other rocks.
The volcanic sequence has undergone upper greenschist to lower amphibolite facies metamorphism, as evidenced by the presence of chlorite,
fine grained amphibole and local garnet in the mafic rocks (Greig, 2004). The sequence is cut by a prominent NNE trending planar penetrative foliation that dips moderately to steeply to the west. This structural grain is offset by dextral ENE-trending and steeply dipping faulting. The structural history of the district indicates two main styles of deformation: i). compression and related tight folds and penetrative foliations accompanied by brittle-ductile deformation, followed by ii). orogenic collapse-related brittle deformation.
Adi Rossi is ~8 km SE of Debarwa. It differs from the VHMS deposits of the Asmara District in that it is associated with a major, ~3 km long, NNE trending shear zone that dips steeply to the west. The Cu and Au mineralisation is hosted by strongly foliated and distorted altered mafic volcanic tuffs. This alteration has been defined over a NNE strike length of 450 m, width of 40 to 100 m, averaging ~80 m, and vertical extent of at least 360 m. This alteration is typically pervasive chlorite-silica alteration of mafic volcanic rocks with later carbonate veinlets and fracture filling veinlets. The thin felsic tuff interbeds within the mafic volcanic unit are sericite-chlorite altered. The altered and mineralised zone dips steeply west and is offset by a dextral ENE-trending and steeply dipping post-mineralisation fault zone.
During the 1930s Italian interests mined Adi Rassi by excavating 10 to 20 m deep pits along the shear zone to mine oxide copper, most of which was focused in the mineralised South Hill zone.
The Adi Rassi mineralisation is mainly hosted in strongly foliated, brecciated and distorted chlorite altered mafic volcanic tuff and flows. Copper, gold and silver mineralisation is associated with foliation planes, breccia zones and frequently in quartz veins. At least, two generations of quartz veins have been recognised: i). NNE trending, steeply dipping veins parallel to the shear zones/faults; ii). a later set of shallow dipping cross-cutting veins. The mineralised breccias are dominantly a mafic volcanic clast-supported variety, with the matrix being composed of intensely chlorite-altered fine grained rock-flour. The clasts are generally very angular.
A poorly developed mixed zone of oxide and supergene copper mineralisation is developed near-surface, generally to depth of <50 m. Supergene Cu minerals are malachite, chalcocite, covellite and cuprite. The supergene zone is poorly developed as a result of the very low pyrite content of the hypogene sulphide mineralisation.
Below the oxide-supergene zone, the hypogene sulphide mineralisation has a total sulphide content of from <1 to 4 vol.%. The sulphides are dominated by chalcopyrite, with very little pyrite. Chalcopyrite occurs as coarse blebby cement fill within the matrix of the breccias and as irregular fracture-filling veinlets within the quartz veins.
Pre-mining feasibility study NI 43-101 compliant Mineral Resources at Adi Rossi as at May, 2013 were (Senior et al., 2013):
Inferred Mineral Resources, sulphide ore, open pit, at a 0.21% Cu equiv. cut-off - 14.94 Mt @ 0.54% Cu, 0.30 g/t Au, 1.5 g/t Ag.
Inferred Mineral Resources, sulphide ore, underground, at a 0.72% Cu equiv. cut-off - 0.84 Mt @ 0.67% Cu, 0.89 g/t Au, 1.5 g/t Ag.
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Kodatu
The Kodatu deposit is developed in the northern section of the easternmost of the three mineralised trends in the Asmara District, the Adi Rassi-Kodatu trend, and is 11 km south of Asmara.
The rocks in the Kodatu area are a tight to isoclinally folded and west-overturned Neoproterozoic volcano-sedimentary sequence, with NNE aligned late shear zones overprinting and obscuring the folding style. Tertiary basaltic dykes cut the host Neoproterozoic basement. The volcanic sequence is dominated by felsic flows and tuffs in the stratigraphic footwall to mineralisation, whilst pillowed mafic volcanic flows form the stratigraphic hangingwall. The basement sequence is overlain by up to 200 m of flat-lying Tertiary olivine basalts, deposited over a well-developed palaeo-weathering horizon below which locally thick laterites have developed above an extensive and thick pre-Tertiary saprolite. The succession is as follows:
Footwall Lithologies - mainly undifferentiated felsic flows and tuffs, comprising,
• Quartz-phyric rhyolite-dacite flow or sub-flow, which when fresh, it is medium to coarse quartz-phyric, greenish, chlorite-altered and poorly foliated; when strongly saprolitised, it has no indication of fragments or clasts;
• Undifferentiated Felsic Tuffs and Flows, including rhyolitic lava flows, felsic pyroclastic (tuffs and flows) that occur adjacent to the massive sulphide lenses and marks the stratigraphic footwall. It is well-foliated with local tuffaceous clasts. Where directly overlain by undifferentiated meta-basalt and meta-andesitic rocks, it is characterized by the development of stringers, and alteration zones. It is commonly altered to sericite-quartz-pyrite;
• Felsic Sub-intrusive, a cohesive unit without fragments that is fine to medium-grained with poorly developed flow banding and/or bedding;
• Quartz-Sericite-Pyrite-Kaolinite-altered rocks, which occurs within the oxide to supergene transition at the contact between the stratigraphic footwall and the gossan/massive sulphides, and is intensely altered and acid-leached rock. It may have originally been a felsic flow, and is a reddish yellow to brownish yellow friable rock, sand and gravel.
Mineralised Zones - comprising chemical sedimentary rocks and/or massive sulphides/gossan,
• Siliceous and Magnetic Chert commonly up to 30 m of jasperoid with pyrite or magnetite-rich lenses and small clasts. Four such horizons are recognised, the first and second are on the eastern side of the deposit area and outcrop ~1 to 1.5 km stratigraphically above the gossan. The third is exposed 200 to 300 m east of the gossan. The fourth occurs in the northern part of the deposit and is associated with mineralisation within the gossan. Each extends over a kilometer scale strike lenght as discontinuous or detached lenses or beds.
• Gossan after Massive Sulphide - a group of oxidised massive sulphide horizons extend intermittently over a strike interval of 1350 m with a maximum width of ~30 m, bounded by a felsic and a pillowed mafic flow stratigraphic footwall and hanging wall respectively. Gossan is best developed on the southern and northern sections of the deposit, with an only weakly mineralised central, faulted zone.
Hanging Wall Rocks
• Mafic Flows are the principal lithology in the stratigraphic hanging wall of the deposit, comprising pillowed, epidote-chlorite altered rocks with some interbedded magnetic, silica-rich chemical sedimentary lenses. Where sheared, the pillows are obliterated and the rock becomes a chlorite schist. Locally, quartz-filled amygdales are evident, and laterally the mafic flows may grade into agglomeratic and mafic tuffs. Highly siliceous non-magnetic cherts are locally interbedded within the mafic flows.
• Heterolithic Pyroclastics, which includes heterolitic felsic pyroclastic deposits, lapilli tuffs and clastic interbeds, all of which have been altered and/or lateritised. It includes clasts of gossan, mafic flows and felsic rock fragments. Siliceous chemical sedimentary rocks occur as thin lenses and thicker layers in the felsic pyroclastics of the unit.
Intrusions and Dykes
• Quartz Porphyritic Rhyolite, which occurs as a distinctive sub-volcanic unit characterised by quartz phenocrysts in a medium-grained groundmass. It is cut by a second generation of finer grained felsic dykes.
• Mafic Dykes, syn- to post-deformation, late-stage dykes that cut the meta-basalts along shear zones, faults and lithological
contacts. They are medium-grained, massive and range from <1 to 5 m in thickness. A NE trending, second mafic dyke phase is hornblende-phyric, fine to medium-grained and is intruded sub-parallel to the bedding planes.
• Felsic Dykes - the meta-basalt of the hanging wall sequence is cut by a few felsic dykes that are fine-grained with a few phenocrysts of
feldspar with or without quartz.
• Quartz veins are developed throughout the deposit area and appear to include two sets that are predominantly associated with shear zones/faults and other weaknesses. Early vein sets that are concordant to the foliation are cut by shear zones and a second vein set developed parallel to the shear planes.
• Tertiary dykes - basaltic in composition, trending NNE and cutting all other rock types including those that are lateritised and weathered.
Mineralisation at Kodatu occurs in two styles:
Volcanic hosted massive sulphides is hosted within an overturned succession of variably and intensely altered, submarine, bimodal, low-K tholeiitic basalt-rhyolite along with locally interbedded volcaniclastic rocks and cherts (Mauritsen and Arafaine, 1998). The immediate hosts to mineralisation are altered felsic rocks, with mafic rocks locally marking the immediate stratigraphic foot-wall. These felsic rocks also predominate within the NNE trending 15 km corridor of prospective stratigraphy between Kodadu to the Adi Rassi deposit. The Kodadu gossan, has been mapped over a strike length of ~1.2 km, enclosed within an alteration halo that is from 5 to 30 m wide and has a strike length of ~1350 m. Alteration is most intense immediately adjacent to the massive sulphides, particularly in the stratigraphic footwall lithologies immediately to the west, where it occurs as pervasive sericite and silicification, commonly with pyrite, forming an inner of quartz-sericite pyrite envelope. This is, in turn, surrounded by an outer envelope of quartz-chlorite±pyrite alteration. Mafic volcanic rocks in the stratigraphic hanging wall are extensively chlorite-, epidote- and variably carbonate-altered, with pillow rims being richer in chlorite, and the cores of the pillows richer in epidote. Argillic (clay) alteration with lesser kaolinite and sericite and the development of iron oxides e.g., hematite, goethite and limonite, are common in the oxide zone. There is strong acid leaching at the contact between the transition zone sulphides and the stratigraphic footwall felsic rocks below the oxide zone with the development of sulphate and clay minerals (alunite, barite and kaolinite).
The mineralisation trends NE-SW and dips at 40 to 50°W. Variably mineralised gossan is recognised in two main zones: i). a larger zone in the south that is ~350 m long and ii). a smaller 240 m long lens to the north. These are separated by narrow and intermittent developments of mineralisation. The gossan is generally leached of Cu and Zn, but is enriched in Au and Ag. The vertical transition from gossan to massive sulphides is occupied by a zone of acid-leaching that produced extremely friable and Au-Ag enriched material. The underlying hypogene sulphide mineralisation includes chalcopyrite, sphalerite and pyrite with a total sulphide content that varies from <5% in stringer style mineralisation, to 100 vol.% in massive pyrite dominated sulphide mineralisation. No supergene Cu zone has been intersected in drilling, and if present will only be of limited in extent:
Vein hosted structurally controlled vein mineralisation which is associated with two sub-parallel NE-SW striking and 60 to 70°NW dipping shear zones. Each has strike dimensions of ~400 m and varies from 5 to 50 m in width. The shear zones are ~100 to 250 m apart. The westernmost is largely barren, with discontinuous gold mineralisation and a best intersection of 1.19 g/t Au over 1 m. The eastern shear zone hosts more continuous gold mineralisation with intersections that range from 9 to 24 m downhole intervals. The mineralisation is mainly hosted by strongly foliated, brecciated and deformed quartz-sericite-pyrite altered felsic volcanic tuff and flows, with gold mineralisation occurring on foliation planes, in quartz veins and breccia zones. This veining is described in the literature as orogenic (e.g., Barrie et al., 2016), but may well be low sulphidation epithermal related to the same event as the VHMS mineralisation.
Pre-mining feasibility study NI 43-101 compliant Mineral Resources at Kodatu as at May, 2013 were (Senior et al., 2013):
Inferred Mineral Resources, oxide ore, open pit, at a 0.45 g/t Au cut-off - 0.99 Mt @ 1.24 g/t Au, 1.61 g/t Ag.
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This summary is largely drawn from "Senior, N., Finch, A., Ross, A.F., Rees, S.D. and Martin, C.J., 2013 - Asmara Project Feasibility Study; An NI 43-101 Technical Report prepared by SENET Pty Ltd. for Sunridge Gold Corp. 264p."
"Thomas, D.G. and Martin, C.J., 2013 - The Adi Rassi Deposit Eritrea, East Africa; An NI 43-101 Technical Report prepared by Fladgate Exploration Consulting Corporation for Sunridge Gold Corp. 98p."
"Thomas, D.G. and Martin, C.J., 2013 - The Kodadu Deposit Eritrea, East Africa; An NI 43-101 Technical Report prepared by Fladgate Exploration Consulting Corporation for Sunridge Gold Corp. 111p."
The most recent source geological information used to prepare this decription was dated: 2017.
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.
Emba Derho
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Selected References:
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Barrie, C.T., Abu Fatima, M. and Hamer, R.D., 2016 - Volcanogenic Massive Sulphide-Oxide Gold Deposits of the Nubian Shield in Northeast Africa: in Bouabdellah, M. and Slack, J.F. (eds.), 2016 Mineral Deposits of North Africa, Mineral Resource Reviews, Springer International Publishing Switzerland DOI 10.1007/978-3-319-31733-5_17, pp. 417-435.
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Ghebreab, W., Greiling, R.O. and Solomon, S., 2009 - Structural setting of Neoproterozoic mineralization, Asmara district, Eritrea: in J. of African Earth Sciences v.55, pp. 219-235
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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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