Bisha Cluster - Bisha Main, Hambok, Harena, Bisha Northwest, Ashelli
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The Bisha precious and base metal-rich volcanogenic massive sulphide (VHMS) deposits are located some 150 km west of Asmara, in north-western Eritrea, East Africa, some 43 km southwest of the regional town of Akurdat, 50 km north of Barentu and 200 km west of the Red Sea coast (#Location: 15° 30' 46"N, 37° 30' 8"E).
Bisha Main is the principal deposit of a cluster within a 20 km radius, which includes Hambok, Harena, Bisha Northwest and Ashelli, as described separately below.
Exploration in the Bisha area in late 1996 by Ophir Ventures, a private Canadian company, located the surface exposure of the Bisha Main deposit. After purchasing an option over the Bisha prospect from Ophir Ventures, Nevsun Resources Ltd was granted a prospecting licence over the area in May 1998. Nevsun discovered the Bisha Main deposit in November 2002 by diamond drilling coincident geophysical and geochemical anomalies associated with a prominent gossan that locally had highly anomalous gold values. While delineation drilling of the Bisha Main deposit continued, exploration and external drilling led to the discovery of the Bisha Northwest and Harena deposits, ~2 km NNW and ~10 km SSW of Bisha Main respectively.
The Bisha Northwest discovery followed the recognition of altered felsic volcanic rocks with a coincident strong airborne EM response to the north of the Bisha Main area in 2002. Drilling programs in 2003 intersected significant Cu, Au and Ag in massive and stringer sulphide mineralisation. Followup drilling in 2004-06 and 2012-13 outlined VHMS mineralisation over a strike length of at least 800 m. Harena was discovered in 2005 by drilling of a gossan zone within felsic volcanic rocks. It had a small open pit reserve which was mined for oxide gold-silver ore in 2012-13. The ownership of the project had been incorporated as the Bisha Mining Share Company (BMSC - Nevsun Resources 60%, Eritrean Government 40%) in 2006.
The Bisha Main CIL plant was commissioned in October 2010, and commercial gold production commenced by February 2011. Construction of a copper flotation circuit was completed in June 2013 and began processing supergene copper ore in September 2013. A zinc flotation circuit was commissioned in 2016, and the first shipment of zinc concentrate was later in the same year. Concentrates are transported ~350 km by sealed road to the Red Sea port of Massawa.
Sanu Resources Ltd. identified a number of gold anomalies, as well as, a large gossan (siliceous breccia) at Hambok during a regional stream‐sediment sampling program in 1998. Follow-up of the gossans and associated anomalies from 2003 ultimately led to discovery of the Hambok massive sulphide deposit in a drill intersection in 2006. Hambok is ~6 km WSW of Harena. Work during the same period found the significant massive sulphide prospects at Aderat/Ankereit and Ashelli, located ~4 km north and ~10 km west of Hambok respectively. In October 2012, BMSC acquired the Sanu Resources titles. Nevsun Resources was subsequently purchased by Zijin Mining Group Company Limited in December, 2018.
The Bisha District deposits lie within 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 Tonian-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.
The Arabian-Nubian Shield in Eritrea is divided into a number of north- to NE-trending Proterozoic terranes, which are separated by major crustal sutures. The terranes from west to east are:
i). Haya Terrane, which comprises a NE-trending volcano-sedimentary belt that includes intra-oceanic arc-related volcanic rocks (Kroner
et al., 1987). Within the central part of the terrane there are intensely folded high-grade rocks which include paragneiss and quartzite (Abdelsalam and Stern, 1993). These are intruded by syn-tectonic granitoids of arc affinity (Klemenic and Poole 1988). Some of these have been dated, yielding ages of 870 to 830 Ma (U/Pb zircon; Kroner et al., 1991).
ii). Barka Terrane - which comprises metasediments that have been subjected to polyphase deformation. The terrane is bounded to the east by the Barka River Fault/Suture, which is a 200 km long structure that follows the north-northeast-trending course of the Barka River valley. This terrane appears to represent the eastern margin of the Haya Terrane in Sudan. It is composed of upper amphibolite to hornblende-granulite facies orthogneiss, amphibolite, marble, pelitic schist and orthoquartzite, intruded by a swarm of east-west felsic dykes. It likely represents the margin of the Haya Terrane that was deeply underthrust below the Nakfa Terrane and subsequently exposed during post-orogenic rebound and relaxation.
iii). Hager/Tokar Terrane - which is composed of ultramafics, and olistostrome sediments within a volcano-sedimentary layered sequence is east of the Barka Suture and considered to be a possible ophiolite sequence. The Hagar Terrane was thrust into contact with the Nakfa Terrane to the east and separates it from the Barka/Haya terranes to the NW and would seem to represent a broad section of the Barka-Adobha Suture complex.
iv). Nakfa (or Nacfa) Terrane - which comprises a low-grade metamorphosed suite of calc-alkaline volcanics and sediments. The Nakfa Terrane volcanics are considered to represent a back-arc island arc to the Hagar Terrane. The Bisha deposit is located within the Nakfa Terrane. The terrane is regarded to be a western extension of the Asir Composite Terrane of Saudi Arabia that hosts numerous gold and base metal deposits in several north-south-trending belts (eg. the Wadi-Bidah and Wadi Schwas Mining Districts). The Bisha District deposits are located towards the western margin of the Nakfa Terrane, in an ~300 km long, NNE trending strip of more intense shearing, folding and metamorphism known as the Augaro-Adobha Belt. This belt, which is up to 50 km wide, is highly deformed with double plunging sheath folds and planar foliations containing shallow plunging stretching lineations. Its northern and central sections are dominated by a north-trending sheath fold, with an axial trace extending some 120 km. Mylonitic fabrics are common, dominated by sinistral strike-slip shear (Ghebreab, 1996). The juxtaposition of these high-grade rocks with the low-grade volcano-sedimentary sequences in central Eritrea to the SE occurred across a west-dipping, low-angle, extensional mylonite zone, and has been assigned to orogenic collapse. Extension resulted in exhumation and cooling of these high-grade rocks between 586 and 572 Ma, and 576 and 563 Ma (Ghebreab et al., 2005).
v). Arig Terrane - is part of a dislocated, narrow, linear, north-south trending belt known as the "Western Ophiolite Suture Zone" which extends north for many hundreds of kilometres from Western Ethiopia, continues north along the Barka River valley and further northwards into Sudan and thence into the Red Sea. This terrane lies along the Red Sea coast to the east of the Nacfa Terrane.
Nakfa Terrane, Augaro-Adobha Belt volcanic and sedimentary rocks that host the Bisha-Hambok cluster of deposits have been metamorphosed to upper greenschist to lower amphibolite facies, commonly increasing in grade toward gabbro and regional granitic contacts, and, broadly from the south to north. Andalusite porphyroblasts are common in argillic sedimentary rocks, whilst garnet, diopside and actinolite are found in silicate bands within the carbonate units to the west. The Barka Terrane to the west is composed of metasedimentary rocks that have been subjected to strong polyphase deformation. The boundary between the Nakfa and Barka terranes follows the NNE trending Barka River, a few tens of kilometres to the west of the Hambok deposit (Chisholm, 2003).
The Bisha-Hambok District, the Neoproterozoic volcanic and sedimentary stratigraphy of the Augaro-Adobha Belt comprises:
• A lower sedimentary sequence of < 300 m in thickness, commencing with fine-grained siliciclastic rocks, including psammite and quartzite, followed by carbonates, including siliceous iron formation;
• A volcanic sequence of mafic to felsic lapilli and ash crystal lapilli tuffs with intercalated minor mafic flows and hyaloclastite that totals ~5000 m in thickness. Rhyolites are the dominant volcanic rock type, mostly tuffs, with minor blocky flows and agglomerates, with lesser, ~5% dacites. Other volcanic rocks include tholeiitic basalts. This volcanic sequence is the host to all of the VHMS deposits of the district, which occur towards the top of the pile. In general, the massive sulphide mineralisation divides the volcanic sequence into an upper, predominantly felsic volcanic suite and a lower bimodal volcanic succession. The lower bimodal volcanic suite, which is capped by the stratabound mineralised unit, comprises felsic lapilli and ash crystal lapilli tuffs with lesser, but significant intercalated mafic flows and hyaloclastite, exposed mainly to the east of the main mineralised zones. The lower section of this succession is cut by Neoproterozoic granite and syenite intrusions and minor mafic dykes/sills which are texturally and chemically distinct from the other felsic strata. These intrusions and the lower sedimentary sequence separate the volcanic sequence from the voluminous Bisha Gabbroic Complex.
• An uppermost sequence of fine-grained volcaniclastic and siliciclastic rocks and minor tuffs that are >2400 m thick cap the felsic suite of the volcanic succession.
The Bisha Gabbroic Complex, that is ~5 km east of Bisha Main, is a large, partly layered, tholeiitic gabbro to gabbro-norite intrusion that forms high hills. It is ~25 km long, a maximum width of ~12 km, has a NNE-SSW orientation, and is interpreted to be coeval or near coeval with the enclosing sequence. That sequence generally strikes NNE, with moderate to steep dips to either the east or west. The Bisha Gabbroic Complex broadly forms a north-plunging antiform that appears overturned, with dips that are generally steep to the east. Volcanic and sedimentary strata were apparently thrust from the WSW against the complex that acted as a buttress, leading to the formation of a nappe-like structure, with internal antiforms and synforms on a scale of hundreds of metres.
The sequence from the underlying Bisha Gabbroic Complex contact to the top of the overlying sedimentary facies totals around 7000 m in thickness, with volcanic rocks comprise around 50% of the stratigraphic section.
The stratigraphy and tectonic fabric of the district have been disrupted, at least locally, by late-stage brittle faulting. Numerous west over east thrust panels have dissected the sequence in the western Nakfa Terrane into north and NNE trending elongate blocks with strata generally trending NNE with moderate to steep dips to the east and west.
Within the individual deposits of the district, footwall alteration is typically composed of pervasive quartz + chlorite alteration of tuffs, that can extend for tens of metres below massive sulphide units. Immediately below the massive sulphides, there is usually a thin <3 m thick zone of silicification and K feldspar replacement (Chisholm et al., 2003) which is more variable in both intensity and thickness compared to the chlorite alteration, and in some cases, is entirely absent. Alteration in the hanging wall is typically pervasive quartz + muscovite alteration of tuffs, which may extend for tens of metres above massive sulphide units. Less common alteration assemblages include carbonate, epidote and albite wich range from weak and patchy to strong/intense and pervasive (SRK Consulting, 2017).
The Bisha Main deposit is composed of four massive sulphide lenses, distributed over a 1200 m north-south strike length. The lenses vary in thickness from as little as 2.5 m, up to as much as 70 m, and extend to a depth of 600 m below surface. They comprise a southern and a northern zone. The main section of the southern zone strikes at ~345° for ~600 m, can be trace down a dip of 65 to 70°W for 500 m and plunges 45°S. The main section of the northern zone strikes due north and dips steeply to the west, with strike and dip lengths of some 500 m and 100 m respectively. Two smaller massive sulphide pods, interpreted as folded limbs of the two main zones, are found in the hanging walls of both the southern and northern zones. The two eastern lens are almost continuous along the full strike length, while the two lenses on the western limb only occupy approximately half of the of the limb. Metal zoning within the massive sulphides appears to indicate an upward transition from Cu-rich to Zn-rich to barren pyrite mineralisation confirming the interpretation that the sequence is right-way-up and west-facing.
The massive sulphide lenses are typically hosted by strongly foliated rhyolite and rhyolite breccia units interspersed with dark grey, thinly bedded mudstone and polymicitc breccia. The rhyolites are interpreted to represent thick submarine lava/domes with coherent cores and quench-fragmented margins. The immediate host rocks are variably altered felsic lapilli and lapilli-ash tuffs, crystal tuffs and minor felsic dykes within this sequence. Massive and coarse felsic fragmental rocks are common to the west of the massive sulphide units.
The massive sulphide mineralisation at Bisha Main and Bisha Northwest is interpreted to have formed as sub seafloor replacement and impregnation rather that sulphide accumulation on the seafloor because: i).the footwall is strongly to intensely altered, whilst the hanging wall is also strongly altered; ii).the massive sulphides are gradational into 'stringer' sulphides both above and below; and iii).the massive sulphide occurs within thick felsic/rhyolite units that do not represent the seafloor (SRK Consulting, 2017).
The felsic lithologies within the volcanic sequence forms both the immediate host to the mineralisation and the stratigraphic hangingwall, and are exposed to the west and southwest of the mineralised zones, and grade upward into a sequence of generally fine-grained volcaniclastic/siliciclastic rocks and then to sediments to the west.
The precious metals-enriched massive sulphide deposits are part of a tightly and complexly folded, intensely foliated sequence. The Bisha orebody has been subjected to strong deformation and has a complex history of at least four phases of deformation. The two hanging wall lenses are interpreted to be on the western limbs of shallowly SSW plunging open D2 folds that were the result of east-west compression associated with the 700 to 640 Ma East African Orogeny (Johnson et al., 2011). These folds were subjected to further deformation during D3 under NE-SW compression. Whilst the main ore zones are generally tabular, in detail, ore lenses may be folded around south-plunging structures with amplitudes of as much as tens of metres.
The Bisha Main Zone mineralisation comprises a precious metal-rich (Au, Ag) oxide zone and base metal-rich (Cu, Zn, Pb) massive sulphide lenses as a result of a deeply weathering profile that is complex and locally dependant on lithology and acidic groundwater generated from oxidation of the massive sulphides.
Four principal zones of mineralisation have been defined within the Bisha Main Zone, as follows:
i). Surface gossan and near-surface Au-Ag rich oxide zone. The gossan occurs as small hills with abundant black to brown pebbles and boulders varying from highly siliceous and somewhat ferruginous to a massive goethite-hematite-jarosite to 100% hematite. Weathering and gossan development is typically developed for 30 to 35 m below surface, although it reaches depths of 60 to 70 m in the fold axis. Locally, original rock textures are preserved. A breccia unit is developed within the oxide domain with higher gold grades (in the range of 1 to 9 g/t Au) than the surrounding oxide material. It flanks the gossan and appears to be a product of oxidation, lateritic weathering and desegregation of the original rock and is mostly composed of quartz breccia or silicified fragments within oxidized material. The underlying oxide zone (or saprolite) is 5 to 30 m thick. It comprises brown, free-digging limonite, goethite and hematite, devoid of sulphide, but is enriched in Au, Pb, Ba and Mo and depleted in Cu, Zn, Cd and Co, carrying ~6 g/t Au;
ii). A Au (±Ag) horizon that has been subjected to extreme acid leach. Oxidation of the massive sulphides generated strong acid solutions that have progressively destroyed the sulphides and host rock to produce a layer of extremely acid-leached material composed of clay and silica separating the Oxide and Supergene zones. The vertical thickness of this horizon varies from 0.5 to 6 m, averaging 3 m. It has high gold and silver values and is usually devoid of significant base metals. The gold and silver grades range from 0.55 g/t to 7.75 g/t Au and 7.8 g/t to 167 g/t Ag respectively. This zone may be subdivided into an upper 0 to 15 m thick 'Soap Zone' which is a bleached, white, light yellow or light-brown zone which consists of clay, quartz, barite and possibly anglesite, underlain by the 0.5 to 1.5 m thick 'Acid Zone' composed of dark grey to black powdery clay, with enriched Au-Ag±galena with quartz and minor to moderate pyrite. The transition to the underlying supergene zone may be occupied by 0.5 to 5 m of fine grained, unconsolidated recrystallised pyrite sand that is Cu poor, but has elevated Ag and averages ~10 g/t Au. Cu from this Zone has been remobilised and deposited in the Supergene Zone;
iii). A 0.5 to 15 m thick supergene Cu-enriched, Ag-poor sulphide horizon which occurs between depths of 35 and 65 m and is characterised by the development of covellite and some chalcocite at the base of the Acid and Oxide Domains. Sooty secondary sulphides coat and replace primary sulphides. Sphalerite and chalcopyrite are replaced by chalcocite, covellite, digenite and native copper (Ashley, 2013). Copper and gold grades are in the range 0.76 to 4.55%, averaging ~4.1% Cu and 0.38 g/t to 0.95 g/t Au. Some Cu and Ba from the hypogene zone are interpreted to have migrated upwards into the supergene zone along steeply dipping basement structures;
iv). An underlying (Zn-Cu), hypogene massive sulphide unit, starting at a vertical depth of 60 to 70 m. The main massive sulphide lenses are composed of disseminated, semi-massive, massive, banded and sulphide clasts within chloritised volcanics.
Resource figures published in October 2006 (Nevsun Resources Ltd website), based on 347 diamond drill holes, were:
Measured + indicated resources
Oxide ore (0.5 g/t Au cut-off) - 4.80 Mt @ 7.02 g/t Au, 30.2 g/t Ag, 0.09% Cu, 0.08% Zn
Supergene ore (0.5% Cu cut-off) - 7.50 Mt @ 0.72 g/t Au, 32.3 g/t Ag, 3.96% Cu, 0.12% Zn
Primary Zinc ore (2% Zn cut-off) - 8.58 Mt @ 0.76 g/t Au, 59.5 g/t Ag, 1.06% Cu, 9.19% Zn
Primary Cu-Zn ore (2% Zn cut-off) - 1.66 Mt @ 0.75 g/t Au, 31.4 g/t Ag, 0.79% Cu, 3.09% Zn
Hangingwall Cu ore (0.5% Cu, <2% Zn cut-off) - 4.77 Mt @ 0.67 g/t Au, 33.2 g/t Ag, 1.15% Cu, 1.01% Zn
Total measured + indicated resources - 27.29 Mt.
Oxide ore (0.5 g/t Au cut-off) - 0.06 Mt @ 2.85 g/t Au, 17.5 g/t Ag, 0.03% Cu, 0.02% Zn
Supergene ore (0.5% Cu cut-off) - 0.21 Mt @ 0.48 g/t Au, 21.1 g/t Ag, 1.94% Cu, 0.03% Zn
Primary Zinc ore (2% Zn cut-off) - 6.83 Mt @ 0.65 g/t Au, 53.3 g/t Ag, 0.83% Cu, 8.42% Zn
Primary Cu-Zn ore (2% Zn cut-off) - 0.51 Mt @ 0.62 g/t Au, 36.5 g/t Ag, 1.02% Cu, 3.29% Zn
Hangingwall Cu ore (0.5% Cu, <2% Zn cut-off) - 4.15 Mt @ 0.68 g/t Au, 37.3 g/t Ag, 0.99% Cu, 0.87% Zn
Total Inferred resource - 11.73 Mt.
Proven + probable reserves (included in resources)
Oxide ore (0.5 g/t Au cut-off) - 4.02 Mt @ 7.99 g/t Au, 32.85 g/t Ag
Supergene ore (0.5% Cu cut-off) - 6.35 Mt @ 0.83 g/t Au, 36.0 g/t Ag, 4.40% Cu
Primary ore (2% Zn cut-off) - 9.71 Mt @ 0.76 g/t Au, 54.0 g/t Ag, 1.14% Cu, 7.21% Zn
Total proven + probable reserves - 20.079 Mt.
Remaining Mineral Resources and Ore Reserves at Bisha Main as at 31 December, 2016 were (SRK Consulting NI 43-101 Technical Report, 2017):
Measured + Indicated Mineral Resources - 34.91 Mt @ 0.6 g/t Au, 33 g/t Ag, 1.02% Cu, 4.18% Zn;
Inferred Mineral Resources - 33.97 Mt @ 0.8 g/t Au, 25 g/t Ag, 1.01% Cu, 4.74% Zn;
Proved + Probable Ore Reserves, Supergene sulphides - 12 Mt @ 0.71 g/t Au, 17 g/t Ag, 2.57% Cu;
Hypogene sulphides - 7.351 Mt @ 0.74 g/t Au, 50 g/t Ag, 1.14% Cu, 6.98% Zn.
NOTE: Mineral Resources are inclusive of Ore Reserves.
The Harena massive sulphide deposit is located ~10 km SSW of Bisha Main at the same contact between intermediate volcanic rocks of the lower bimodal succession, and the overlying felsic volcanic suite at the top of the volcanic sequence described previously. Massive sulphides occur as an up to 60 m thick tabular body that strikes 40° and dips ~60°NW. It plunges shallowly to the SW and has been drilled over a strike length of ~900 m.
The hosts to mineralisation are the bimodal suite of basalts and rhyolite-dacite volcanic rocks within a stratigraphic succession that comprises:
• A lower unit composed of rhyolite and dacite tuffs that have been subjected to an intense proximal chlorite-sericite-sillimanite alteration episode in the immediate footwall of the massive sulphide, and a laterally distal silica-sericite±biotite assemblage;
• At least two stratigraphically distinct massive sulphide units with associated stringer mineralisation on the stratigraphically lower southeastern side;
• An upper unit in the hanging wall of the massive sulphide composed of intercalated felsic and fine to medium grained, plagioclase-phyric mafic rocks, with a distinctive felsic quartz breccia unit developed along the length of the mineralisation. The mafic rocks have a moderate silica-chlorite biotite alteration mineralogy;
• An upper sequence of graphitic mudstone and greywacke.
The rocks weakly to moderately schistose, with small scale folds commonly being observed in the hanging wall. The hypogene sulphide mineralisation is mainly composed of massive sulphides with subordinate semi-massive sulphide and volcanic lithologies. They range from 0.4 to 100 m in thickness, averaging ~19.8 m, and predominantly comprise fine to medium-grained subhedral to anhedral pyrite with interstitial and separate bands of sphalerite and chalcopyrite. The massive sulphides have a tabular to lensoidal shape, with the thicker mineralisation forming elongated shallowly SW-plunging shoots. Each massive sulphide unit is zoned from a Cu-Au-Ag rich base to a Zn-Ba rich top. Associated stringer mineralisation in the footwall of the massive sulphides occurs in about half the drill holes, and where present it is up to 58 m thick, averaging 6.6 m. The full mineralised package of massive sulphides and stringer ore averages 23 m in thickness.
Weathering has produced 45 to 50 m thick surface oxide profile composed of gossan overlaying a very thin secondary supergene horizon, which grades into a primary massive sulphide horizon at depth. This oxide layer is depleted in Cu and Zn compared to the hypogene mineralisation, but has elevated Au and Ag. The entire Neoproterozoic sequence is almost completely masked by Quaternary alluvium and colluvium up to 10 m thick.
Remaining Mineral Resources at Harema as at 31 December, 2016 were (SRK Consulting NI 43-101 Technical Report, 2017):
Indicated Mineral Resources, open pit - 3.95 Mt @ 0.6 g/t Au, 28 g/t Ag, 0.87% Cu, 3.16% Zn;
Inferred Mineral Resources, open pit oxide - 0.12 Mt @ 2 g/t Au, 20 g/t Ag;
Inferred Mineral Resources, open pit sulphide - 1.92 Mt @ 0.6 g/t Au, 28 g/t Ag, 0.87% Cu, 2.19% Zn;
Inferred Mineral Resources, underground sulphide - 23.02 Mt @ 0.8 g/t Au, 30 g/t Ag, 0.93% Cu, 4.96% Zn.
The Northwest deposit is ~2 km NNW of Bisha Main, and is composed of a cluster of poly-metallic massive sulphide lodes that dip from 70°NW to sub-vertical and are distributed over a NE striking interval of 800 m. The deposit is thickest, at >85 m in the centre, tapering to widths of <8 m at its extremities. It has the form of a downward tapering wedge that extends to a maximum depth of 250 m below surface, although a stringer vein network extends to 350 m.
The deposit has been subdivided into three domains:
i). Northern Main Lode, which is the largest, a copper-dominated massive- to semi-massive sulphide body in which zinc grades increase northwards;
ii). Southern Lode, that is zinc-dominated, and is a discontinuous massive- and semi-massive sulphide body in which pyrite is dominant;
iii). Eastern Lode - a separate, poorly drill defined (in 2017) narrow lode on the footwall side of the deposit. that carries appreciable copper and zinc grades.
Bisha Northwest has a gold oxide cap and supergene copper zone which are less well endowed in gold, silver, and copper compared to Bisha Main, reflecting the overall lower grade of the underlying hypogene sulphide mineralisation.
Unmined Mineral Resources at Bisha Northwest as at 31 December, 2016 were (SRK Consulting NI 43-101 Technical Report, 2017):
Indicated Mineral Resources, supergene - 1.02 Mt @ 0.2 g/t Au, 10 g/t Ag, 1.47% Cu;
Indicated Mineral Resources, hypogene - 2.53 Mt @ 0.3 g/t Au, 13 g/t Ag, 1.04% Cu, 1.08% Zn;
Inferred Mineral Resources, oxide - 0.50 Mt @ 3.7 g/t Au, 18 g/t Ag;
Inferred Mineral Resources, supergene - 0.10 Mt @ 3.7 g/t Au, 19 g/t Ag, 0.8% Cu.
Inferred Mineral Resources, hypogene - 0.10 Mt @ 2.9 g/t Au, 15 g/t Ag, 0.9% Cu, 0.9% Zn.
The Hambok deposit is ~15 km SW and ~6 km WSW of Bisha Main and Harena respectively.
The stratigraphy, as mapped within the Hambok area is as follows (after Giroux and Barrie, 2009), from the base:
Mafic and intermediate volcanic rocks – a >5 km thick suite of mafic flows and hyaloclastites with intercalated intermediate and felsic tuff horizons. Intermediate volcanics predominate to the north around Bisha, while more mafic varieties predominate to the south of Hambok;;
Felsic volcanic sequence, that is up to 5 km thick, largely composed of tuffs with local flow and dome complexes;
Marble - composed of dolomitic marbles with calc-silicate bands and layers that is only tens of metres thick in the west, but thickening to the east;
Siliclastic Sedimentary rocks - at least 5 km of mainly graphitic and pyritic argillites, with minor siltstones, chert, juvenile arenites and carbonate horizons. This unit forms a cap to the bimodal volcanic sequence;
Gabbro, Pyroxenite, Granophyre, Granodiorite - that may be co-magmatic differentiates, or differentiates from the same magma sequence.
The Hambok deposit lies on the eastern flank of a broad, complex, NNE-SSW anticlinorium cored by basaltic rocks. The Bisha Gabbroic Complex mafic intrusive forms the core of the next significant regional anticline to the east of Hambok, whilst a subordinate, unnamed gabbro diorite pyroxenite intrusive complex and ultramafic and granitic intrusions core western the anticline on whose flanks Hambok is located. The smaller Ashelli and Mai Melih massive sulphide prospects appear to occupy the western flank of the latter anticlinorium. The Bisha Main deposit lies on the eastern limb of the syncline separating the two main anticlines.
Semi-regionally, the Hambok deposit is found at a similar stratigraphic position to Bisha Main, Harema and Ashelli, near the contact between the mafic to intermediate suite and the overlying felsic unit. It occurs within a sequence of chloritic, volcaniclastic rocks with lesser, massive basaltic to andesitic lavas and felsic tuff. A 300 to 600 m thick dyke or sill like granitoid body that is >30 km in length and which trends NNE, parallel to the regional fold axes, cuts the volcanic sequence a few tens of metres east of the deposit. This intrusion is an equigranular biotite granite, with local biotite hornblende syenite and granodiorite phases and has tectonic overprinted cataclastic margins that have reactivated intrusive contacts.
The main Hambok massive sulphide body is a homoclinal lenticular body dipping steeply to the east, with some shallowing of dip to the south. It is mostly masked by alluvium, with the only surface expression being several 1 to 3 m thick gossan lenses up to 30 m long at the northern end of the mineralisation, and one small, ~0.75 m diameter outcrop in a shallow watercourse toward the southern end. Extensive outcrops of 'siliceous breccia' are interpreted to represent fault zones which bound the deposit at depth, and have been silicified in the oxidising environment above the base of oxidation. The oxide zone mineralisation is narrow, generally <5 m in true thickness, with only low or erratic precious metal grades.
The unweathered chlorite altered volcaniclastic rocks that enclose the Hambok deposit dip subparallel to the sulphide lenses in the immediate deposit area and into the footwall, but dip at progressively shallower angles into the hanging wall. As such, these hanging wall rocks, which consist of basalt and basaltic volcaniclastic rocks, fine grained rhyolitic tuff, coarse volcaniclastic rocks of intermediate composition and granitic sills, appear to thicken down dip.
The massive sulphide deposit has a strike length of >1000 m but is thickest in a central ~750 m long section. It is up to 75 m thick at its best developed, but thins to between 4 and 13 m on its northern margin, whilst to the south, mineralisation becomes progressively more disseminated, enclosing a 10 to 12 m thickness of the main massive sulphide near its southern extremity. It extends for >350 m down dip and >325m vertically, although it is thickest at ~300 m depth. It has an overall disc shaped geometry and comprises a series of lenses, separated in part by thin tuff horizons. The thin edge of the deposit cuts the surface, expanding downward. The massive sulphide is enclosed by a 1 to several tens of metres thick envelope of chloritic tuffs, in both the hanging- and footwall, that appear more highly strained than adjacent rocks. Local stringer sulphide vein mineralisation occurs within the footwall chlorite envelope. Chlorite in the alteration envelope is commonly accompanied by magnetite, which is also present and locally abundant in tuffs in the stratigraphic hanging wall to the massive sulphides. Itoccurs as fine and heavy disseminations, and as mm to cm scale wispy layers intercalated with tuffs. The thicker accumulations have 10 to 30% magnetite over 10 to 30 cm intervals. Sericite occurs as very pale, greenish cream to white, fine grained aggregates interstitial to pyrite within the massive sulphides and is a common constituent of the massive sulphide mineralisation. The colour is close to that of sphalerite making the minerals difficult to distinguish. The hanging wall mafic and felsic volcanic rocks are variably sericite altered, producing pale, greenish cream to white replacement of feldspar phenocrysts in felsic volcanic rocks and in the groundmass in mafic rocks.
The major minerals within the massive sulphides are pyrite, sphalerite, chalcopyrite and magnetite with minor and accessory galena, tennantite and digenite and trace altaite (PbTe), rucklidgeite ((Bi,Pb)3Te4) tellurobismuthite (Bi2Te3) and volynskite (AgBiTe2). Gangue minerals include quartz, sericite, Mg-chlorite, siderite and feldspar, with minor and accessory siderite, zincian-siderite, manganoan and ferroan dolomite, clinozoisite, biotite, anhydrite (or gypsum), barite and talc, and trace rutile, and an Fe-Zn phosphate mineral. Pyrrhotite and arsenopyrite appear to be absent. Silver occurs in Pb and Bi-Te bearing minerals, including galena, altaite, rucklidgeite and volynskite. Pyrite is dominant, occurring in a variety of textures, from fine grained, banded, massive sulphides to coarse breccias of finer grained massive pyrite clasts. Economic mineralisation consists of disseminated sphalerite and chalcopyrite. The sphalerite is fine grained, cream coloured and difficult to distinguish from groundmass sericite. Compositional layering is present only where the zinc grade is over 5%. Chalcopyrite is generally fine grained and interstitial to pyrite, with concentrations of up to 5 vol.%.
Cu and Zn have a weak zonation, with Cu mineralisation increasing down-dip and to the west of the massive sulphide, whilst Zn has a tendency to be enriched up-dip and to the east. The best metal grades occur at the top, bottom and edges of the thickest accumulation of sulphides with the thick pyrite core of the massive sulphide mineralisation only poorly mineralised.
Preliminary Mineral Resource at Hambok as at January, 2009 at a 2% Zn equiv. cut-off were (Giroux and Barrie, 2009):
Indicated Mineral Resources - 10.7 Mt @ 0.2 g/t Au, 7.1 g/t Ag, 1.04% Cu, 2.21% Zn;
Inferred Mineral Resources - 15.9 Mt @ 0.2 g/t Au, 6.15 g/t Ag, 0.93% Cu, 1.77% Zn.
Unmined Mineral Resources at Hambok as at 31 December, 2016 were (SRK Consulting NI 43-101 Technical Report, 2017):
Indicated Mineral Resources, hypogene - 6.86 Mt @ 0.2 g/t Au, 10 g/t Ag, 1.14% Cu, 1.86% Zn;
Inferred Mineral Resources, oxide - 20 Kt @ 1.57 g/t Au, 17 g/t Ag.
Ashelli (or Asheli)
The Ashelli massive sulphide deposit is located ~10 km west of Hambok, hosted within the felsic volcanic sequence detailed in the Hambok description above. Various lithologies outcrop in the area, generally with a north-south strike and dipping steeply to the west. The lowest unit is composed of a series of mafic flows with strong carbonate and moderate chlorite alteration. This unit, which thickens to the south and tapers to the north, is cut by several felsic and mafic dykes. It is overlain by a series of felsic flows which have undergone moderate to strong sericite and chlorite alteration and are locally strongly foliated. This felsic suite includes the massive sulphide mineralisation, magnetite rich siliceous chemical sediments and intermediate lava flows. The Ashelli deposit is a tabular, steeply north-plunging body that is ~400 m long by 100 m wide and up to 30 m thick, composed of pyrite, sphalerite and chalcopyrite. The top of the deposit is 60 m below surface. The host felsic suite is overlain by a siliceous chert horizon with associated thin marble beds followed by a thick sequence of finely laminated graphitic sediments and mudstone. This unit is strongly foliated and contains a graphitic horizon with coarse box works after oxidised pyrite (SRK Consulting, 2017).
Unmined Mineral Resources at Ashelli as at 31 December, 2016 were (SRK Consulting NI 43-101 Technical Report, 2017):
Inferred Mineral Resources, low grade - 1.677 Mt @ 0.36 g/t Au, 28 g/t Ag, 1.9% Cu, 5.2% Zn, 0.05% Pb;
Inferred Mineral Resources, high grade - 0.723 Mt @ 0.39 g/t Au, 33 g/t Ag, 1.9% Cu, 16.6% Zn, 0.14% Pb;
TOTAL Mineral Resources - 2.40 Mt @ 0.37 g/t Au, 30 g/t Ag, 1.9% Cu, 8.6% Zn, 0.08% Pb.
This summary is largely drawn from "SRK Consulting, 2017 - Independent Technical Report, 2016 Resources and Reserves Update Bisha Mine, Eritrea; An NI 43-101 Technical Report, prepared by SRK Consulting (Canada) Inc., for Nevsun Resources Ltd., 313p." and
"Giroux, G.H. and Barrie, C.T., 2009 - Hambok Deposit, Mogoraib Exploration License, Gash-Barka District, Western Eritrea, Preliminary Resource Assessment; an NI 43-101 Technical Report prepared for Sanu Resources, 108p."
The most recent source geological information used to prepare this summary was dated: 2016.
Record last updated: 4/3/2020
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.
Barrie C T, Nielsen F W and Aussant C H, 2007 - The Bisha Volcanic-Associated Massive Sulfide Deposit, Western Nakfa Terrane, Eritrea: in Econ. Geol. v102 pp 717-738|
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.|
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|
Giroux, G. H. and Barrie, C.T., 2009 - Hambok Deposit, Mogoraib Exploration License, Gash-Barka District, Western Eritrea 43-101 Technical Report and Preliminary Resource Assessment: in NI 43-101 Technical Report, 108p.|
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.|
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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