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Koka, Zara |
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Eritrea |
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Super Porphyry Cu and Au
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The Koka gold deposit is located in northern Eritrea, ~115 km NNE of the Bisha VHMS deposit and 165 km NNW of Asmara (#Location: 16° 29' 53"N, 37° 58' 16"E).
Artisanal working are known over the Koka area since at least the mid 1990s. The 400 km2 Zara Prospecting License was granted to Dragon Mining Limited in 1998, which was reduced to 4 x 50 km2 Exploration Licenses in 2000. Dragon entered into a joint venture with Sub-Sahara Resources NL in 2003, with the latter taking over management of exploration. The first drilling at Koka commenced in August 2005. This involved a systematic diamond drilling program covering the known strike extent of artisanal workings. The titles had been reduced to 147 km2 in November 2008, and in August 2009, Sub-Sahara merged with Chalice Gold Mines Limited. By June 2010 a Probable Reserve of 4.6 Mt @ 5.1 g/t Au has been estimated at Koka and a feasibility study completed. Subsequently Chalice acquired Dragon's interest, while in 2011 the state owned Eritrean National Mining Corporation ENAMCO took a 40% share (30% equity, 10% free carried) of the Zara Operation. In 2012 Chalice sold its 60% interest to the Chinese SFECO Group, a subsidiary of Shanghai Construction Group Co. Mining had commenced by early 2016.
Regional setting
The Koka deposit and satellite lie within the western or Nubian section of the Arabian Nubian Shield in northeastern Africa. 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.
As described in more detail in the Bisha Cluster record, the tectonic framework in northern Eritrea is composed of the following tectonic elements, each separated by major, crustal scale, NE trending deformation zones. They are, from NW to SE, the:
i). Barka Terrane in the south, tapering northward, composed mainly of upper amphibolite to granulite metasedimentary and mafic gneiss complexes. It appears to represent the eastern, more highly metamorphosed margin of the Haya Terrane in Sudan.
ii). Hagar Terrane in the north, tapering southwards where it occurs to the east of the Barka Terrane, while in the north it is east of the Haya Terrane. It is dominated by ultramafics, and olistostrome sediments within a volcano-sedimentary layered sequence of oceanic affinity supra-subduction mafic and felsic volcanic rocks.
iii). Nakfa Terrane, composed of greenschist facies volcano-sedimentary and syn- to post-collision granitoid rocks. Its eastern margin adjoining the Hagar Terrane is occupied by a NNE trending strip of more intense shearing, folding and metamorphism known as the up to 50 km wide Augaro-Adobha (or Adobha-Abiy) Belt which is principally composed of highly deformed ophiolites and post-accretionary basinal sediments, which are imbricated by the regional Elababu Shear Zone in the east and the Baden Shear Zone to the west
Regional structures within the Neoproterozoic volcano-sedimentary and granitoid rocks of Eritrea include NNW trending brittle-ductile shear zones and strike-slip faults, low angle thrust faults, folds and local macroscopic en-echelon quartz veins and tension gashes that have overall trends of NNE to NNW. Ductile strike-slip shear zones are the most prominently developed and can be traced for several kms in length and several to several tens of metres in width. The dominant structures in northern Eritrea are the sinistral NNE Augaro-Adobha Belt, and the dextral NNW Arig Belt/Terrane (or Asmara-Nakfa Belt) to the east. These trends, which are zones of transpressional strike-slip shear, embrace the important mineral deposits (e.g., Koka and the Bisha Cluster in the former and the Asmatra District Cluster in the latter). Semi-brittle shear zones commonly developed synchronously along the axial planes of isoclinal folds within greenschist metamorphic rocks, whilst syn- to late-tectonic granitoid magmatic rocks intrude along ductile shear zones as rigid elliptical bodies. These magmatic rocks are dominantly granite, granodiorite and diorite, accompanied by fine-grained dolerite and quartz porphyry.
Geology and Structure
The Koka and satellite deposits are all located within flexures in the Elababu Shear Zone which marks the eastern contact of the Augaro-Adobha Belt with the rest of the Nakfa Terrane. To the west of the shear zone, in the footwall, there are predominantly siliciclastic meta-sedimentary and metabasaltic lithologies, together with minor meta-chemical sedimentary rocks, basalt and syn-tectonic granitoids of the Augaro-Adobha Belt. The meta-sedimentary rocks comprise tuffaceous greywackes, siltstones and shales with minor mafic intrusive rocks. This sequence is isoclinally folded. To the east of the shear zone,
in the hanging wall, the Nakfa Terrane is represented by a meta-volcanic and meta-volcaniclastic sequence that has been metamorphosed to greenschist facies, together with post-tectonic granitoids. These rocks are more massive, principally intermediate and acidic, pyroclastic rocks and intrusions of microgranite and micrographic microgranite together with minor rhyolite and dacite.
The Elababu Shear Zone is the dominant structure in the area, controlling the emplacement of both ore and granitoid intrusions. It is characterised by nappes/thrust faults, striking at 10 to 20° in low grade metamorphosed volcanic-sedimentary rocks, and upright folds with limbs dipping at 75 to 90°E. The gold-bearing quartz veins commonly occur as networks, with branching and converging features, locally expanded to form lens-shaped bodies. Granite within the shear zone is strongly mylonitised, with sericite defining the main foliation, anastomosing around rotated K feldspar porphyroclasts.
Koka
The main Koka deposit is >650 m in strike length and persists for >165 m depth below surface. It is hosted within or on the contact of an elongate, lensoid, nearly vertically dipping body of relatively fine to medium grained, grey Koka microgranite. This microgranitic host is dominantly composed of plagioclase and quartz with subordinate K feldspar, overprinted by an alteration assemblage that is predominantly sericite, micro-granular albite and quartz. It has been strongly silicified and due to its relative competence, has been brecciated. The host intrusion is bounded immediately to the east by greenschist facies metamorphic rocks after intermediate to felsic volcanic and pyroclastic rocks and post-tectonic granitoids of the Nakfa Terrane, and to the west by a sequence of siliciclastic meta-sedimentary and meta-volcanic rocks, including tuffaceous greywacke, sandy mudstone, shales and mafic meta-basaltic flows, with associated syn-tectonic granitoid rocks of the Augaro-Adobha Belt.
The deposit area is dominated by NNE trending shear zones that comprise a series of asymmetric, overturned, isocline folds and thrust faults, particularly evident in the fine grained volcanic and sedimentary wall rocks. High-angle thrust faults are the main controlling structures for the
ore-bearing hydrothermal activity in the deposit area. Mineralisation is preferentially developed within the relatively competent, 851.2±1.9 Ma (Zhou et al., 2019) Koka microganite which has undergone post-magmatic deformation and brittle fracturing related to these structures. This tectonic-induced brittle fracturing was the focus for the deposition of stockworks of quartz veins. The mineralised zone includes both quartz veins and disseminations in wall-rock alteration halos and is developed in two zone, as follows:
• The quartz veining type mineralisation which is characterised by intense associated alteration, is best developed within the main mineralised zone which is an ~10 m wide zone within the microgranite, between 50 and 80 m above its footwall contact. Individual stockwork veins have varying widths, but are generally <2 m thick. Gold is mainly present in its native form within the quartz veins, accompanied by pyrite, chalcopyrite, galena and sphalerite. This zone is enriched in Ca, Mg, K and Fe. Gangue minerals are predominantly quartz, calcite and sericite. The alterations types closely related to gold mineralisation are silicification, sericite, pyrite and carbonates, which are superimposed on each other and are generally zoned. Silicification coincides with the main mineralised veins, whilst sericite and pyrite form a peripheral selvage. Carbonate alteration often occurs in the surrounding wall rock mainly as veins, either accompanying other alteration assemblages or separately. The hydrothermal fluid is interpreted to have been composed mainly of H20 and CO2, with minor dissolved S, Zn, Pb, Cu, Au and possibly Sb. The hangingwall contact of the main mineralised zone is taken as the first appearance of unaltered, pinkish, potassic feldspar-bearing microgranite, which is, in turn, cut by later basaltic intrusives.
• The wall-rock alteration type mineralisation is predominantly developed in a second mineralised zone along the contact zone between the Koka microgranite and footwall metavolcano-sedimentary rocks. This contact is the conduit for a zone of intense carbonation and sericitisation up to 20 m wide. Within this zone, the meta-sedimentary rocks behaved competently, compared to the metavolcanic and meta-volcaniclastic sequence, resulting in brecciation and and multiple phases of quartz veining, although mineralisation is still predominantly within the microgranite. This zone is also enriched in Ca, Mg, K and Fe. The footwall contact of the mineralised Koka microgranite hosting gold mineralisation is the contact with the meta-sedimentary rock. No anomalous gold mineralisation has been intersected in any of the footwall rocks (Reidy 2012).
Thin fracture networks were sealed by fine-grained foliated sericite±carbonate (dolomite), and are appear as thin pale yellowish green fracture fillings in white host rock which underwent selective pervasive replacement by an assemblage of albite, minor sericite, dolomite and pyrite ±inclusions of sphalerite and galena, and trace leucoxene. Primary quartz and zircon survived, whilst primary K feldspar remained in pale pinkish cream rocks that have undergone less intense alteration. Rare small grains of gold occur adjacent to veins.
Microscopic investigations indicate two main mineralisation events occurred in the Koka gold deposit:
• Stage I which is characterised by quartz-sulphide-gold veins, mainly containing pyrite, chalcopyrite, galena and sphalerite. Gold is xenomorphic granular and is generally distributed between pyrite and quartz. Pyrite mainly occurs as clusters of hypautomorphic-cubic crystals with large particle size variation. Galena is xenomorphic granular and has a distinct triangular cleavage. Chalcopyrite and sphalerite have solid solution structures, although aggregates are also present, and occasionally single grains.
• Stage II is characterised by the development of milky white quartz veins with low sulphides, predominantly chalcopyrite and fine pyrite.
Limited post-vein deformation has generated shadowy strain extinction in vein quartz and thin micro-cracks in vein pyrite. It has also caused local remobilisation of galena, sphalerite and trace gold for short distances along some micro-cracks.
Zhao et al., 2019 note that NaCl-aqueous inclusions, CO2-rich inclusions, and three-phase CO2-H2O inclusions occur in the quartz veins at Koka. They estimate the ore-bearing quartz veins formed at 268°C from NaCl-CO2-H2O(-CH4) fluids averaging 5 wt.% NaCl equiv. The conclude that the ore-forming mechanisms included fluid immiscibility during Stage I, and mixing with meteoric water during Stage II. Oxygen, hydrogen, and carbon isotopes suggest that the ore-forming fluids originated as mixtures of metamorphic and magmatic water, whereas the sulphur isotope suggests an igneous origin.
Koka South
Koka South is a south extension of the main Koka deposit, occurring as Koka-style quartz stockwork mineralisation in altered mixed microgranite and porphyry intrusive body which shows various degrees of brecciation and quartz stockworking. Mineralisation has been delineated over a strike length of >250 m (Reidy 2012). The mineralisation is invariably accompanied by accessory amounts of galena and sphalerite which are increasingly recognized as critical pathfinder minerals at Koka.
Other Mineralisation
Deep penetration Induced Polarisation surveys over the north-south Koka-Konate corridor revealed numerous strong resistivity targets along a 7.5 km long strike length, interpreted to reflect strong silicification associated with gold mineralisation. These are mostly coincident with soil gold anomalies and in some cases with artisanal workings as well as including the Koka deposit. Other areas of artisanal workings are known at Zara North, ~10 km north of Koka.
Reserves and Resources
As of June 2012, the following JORC compliant estimates were published (Chalice Gold Mines release 2012):
Probable Ore Reserve - 4.6 Mt @ 5.1 g/t Au for 23.46 t of contained gold;
Indicated Mineral Resource at a 1.2 g/t Au cut-off - 5.0 Mt @ 5.3 g/t Au for 26.5 t of contained gold.
This summary is mainly drawn from Zhou et al., 2019 (see below) and Reidy, P., 2012 - Independent Technical Valuation of Mineral Assets, Zara Gold Project, Eritrea; an Independent Technical Report prepared by CSA Global Pty Ltd for Chalice Gold Mines Limited, 37p.
The most recent source geological information used to prepare this decription was dated: 2019.
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.
Koka
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Selected References:
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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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Woldehaimanot, B., 2000 - Tectonic setting and geochemical characterisation of Neoproterozoic volcanics and granitoids from the Adobha Belt, northern Eritrea: in J. of African Earth Sciences v.30, pp. 817-831.
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Zhao, K., Yao, H., Wang, J., Ghebsha, F.G., Xiang, W. and Xiong, Y.-Q., 2019 - Genesis of the Koka Gold Deposit in Northwest Eritrea, NE Africa: Constraints from Fluid Inclusions and C-H-O-S Isotopes: in Minerals (MDPI), v.9, 20p. doi:10.3390/min9040201
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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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