Hassai-Ariab Cluster - Hassai North and South, Hadal Awatib SW, Hadal Awatib East, Hadayamet, Talaiderut, Oderuk, Baderuk, Adassedakh, Onur
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The Hassai (or Ariab) cluster of gold deposits are located in the Red Sea Hills desert in north-eastern Sudan, ~200 km west of Port Sudan and 500 km NE of Khartoum (#Location - Hassai: 18° 41' 40"N, 35° 23' 37"E).
Between 1977 and 1981, as part of a larger international cooperation agreement, French (BRGM) and Sudanese teams of geologists investigated 17 previously known gossans in the central part of the Red Sea Hills, or Ariab-Arbaat area. These gossans are the weathering products of massive sulphide deposits found at depth which are the source of most of the gold in the Ariab mining district. Work continued through the 1980s, firstly on the polymetallic potential of the massive sulphides, but from 1983 switched to the gold content of the gossans and the silica-kaolinite-(barite) rocks associated with those gossans. By February 1985, sufficient surface trenching, percussion drilling and diamond drilling had been undertaken at Hassai to justify a pilot plant, and in March 1987, the first gold was poured from that plant.
A feasibility study by BRGM was completed in May of 1990 and mine production commenced in 1991. Production was from a series of oxide pits which included Hassaï North and South, Hadal Awatib SW, Hadal Awatib East, Talaiderut, Oderuk, Baderuk, Hadayamet and Adassedakh. The main sources of gold was from the ores developed in the oxide zone above the volcanic hosted massive sulphide (VHMS) mineralisation that underlies 6 of the 18 mined-out open pits which exploited supergene oxide ores. By 2009, these mines had produced >80 t of gold from 18 open pits within a 25 km radius of the main Hassai camp, with gold recovered by heap leach. At that stage, only two pits were still in operation, Hassaï North and Hadal Awatib, and gold from the oxide zone was all but depleted, with most of the existing pits floored by massive sulphide.
In 2006, the Canadian company La Mancha had acquired a 40% interest in the operator, Ariab Mining Company which was then owned by the Sudanese Government (56%) and some private French shareholders (4%). In March 2011 La Manch prepared a Definitive Feasibility Study that proposed a two phase redevelopment plan. The first stage involved an upgrade of the existing heap leach plant to Carbon-In-Leach technology to retreat existing heap leach residue stockpiles and to treat sulphide ore. The second stage involved building a flotation plant to facilitate recovery of copper from the massive sulphide mineralisation below the current oxide open pits. The main massive sulphide deposits are at: Hassaï South, Hadal Awatib, Hadayamet, Taladeirut, Oderuk, Adassedakh and Onur.
Delays were experienced in the implementation of the CIL and flotation plant proposals and La Mancha sold its share of Ariab Mining Company to the Sudanese Government in 2015. The plant expansions were still not completed in 2020, although the CIL facility was apparently scheduled for completion by 2022.
The deposit lies within the broad, NE-SW trending ophiolite bearing Nakasib Suture that separates the Haya terrane in the south from the Gebeit terrane to the north, both of which are part of the Neoproterozoic the Arabian-Nubian Shield. For a description of the regional setting of the Shield and the Nakasib Suture, and its geology and distribtion of mineralisation, see the separate Arabian Nubian Shield Overview record.
The Nakasib Suture, also locally known as the 'Ariab Greenstone Belt', formed as a result of closing of an oceanic domain along a north-dipping subduction zone and collision between the Haya and Gebeit Terranes at ~750 Ma (Abdelsalam and Stern, 1993; Stern and Abdelsalam, 1998; Johnson et al., 2003). The Nappe emplacement from north to south (Adbelsalam 2010) is consistent with closure by northward subduction beneath the Gebeit Terrane. It resulted in arc-arc collision and the emplacement of greenschist-facies (locally metamorphosed to amphibolites-facies), SE-verging nappes during the early stages of the Nakasib deformation (Abdelsalam and Stern, 1993; Abdelsalam, 2010). Some oblique slip is inferred from flower structures associated with dextral displacement along the southern margin of the suture which imply some oblique slip (Schandelmeier et al., 1994; Johnson et al., 2003). This collision resulted in the accretion of a dislocated intra-oceanic island arc and back-arc basin complex to the Gebeit Terrane.
Five thrust separated stratigraphic slices have been recognised across the suture zone. These are from south to north (after Abdelsalam and Stern, 1993), the:
• Arbaat Volcanic Group, ~730 Ma rift-related volcanic rocks, mainly a basalt unit with subordinate felsic metavolcanic rocks and sandstones, intruded by diorite and gabbro. The unit is ~2 km in thickness;
• Salatib Group - early passive margin volcanic rocks, predominantly andesitic and dacitic lavas along with pyroclastic and sedimentary rocks.;
• Meritri Group - the lower section of which contains alluvial and submarine fans of an incipient ocean basin rocks, while its upper part may represent a platform of clastic sedimentary sequence with carbonate intercalations, marking the opening of a mature ocean basin. It is composed of four formations, which are from oldest to youngest: i). conglomerate, which is ~2 km thick; ii). lithic wacke with minor intercalated felsic volcanic layers and limestone beds, totalling ~2.5 km in thickness; iii). intercalated limestone, red sandstone and felsic tuff that together are ~200 m thick; and iv). felsic volcanic rocks and clastic sediments that form a sequence with a cumulative thickness of ~600 m.
• Nakasib Ophiolite, a dismembered ophiolitic sequence of mafic and ultramafic rocks that occurs as six disconnected lenses of gabbro and serpentinite distributed along the length of the suture;
• Shalhout Group - comprising arc-related volcanic and sedimentary rocks, and comprises felsic and intermediate volcanic rocks, including sodium-rich rhyolitic and rhyodacitic lavas, tuffs and domes, and fine-grained wackes, as well as ignimbrite and felsic tuffs.
In the immediate deposit area, 5 units are recognised (Plyley et al., 2009), namely :
• Unit A, which is the basalt of the Arbaat Volcanic Group intruded by diorite and gabbro;
• Unit B, andesitic and dacitic lavas and pyroclastic rocks of the Salatib Group;
• Unit C, sodium-rich rhyolitic and rhyodacitic lavas, tuffs and domes, most likely the volcanic facies of the Meritri Group. It has been subdivided into subunits: C1, which is very restricted in size and is interbedded with Unit B; and C2, that is regionally extensive and overlies Unit B;
• Unit D, which predominantly comprises the main massive sulphide gold deposits, but also includes intermediate-mafic lavas and overlies the felsic volcanic rocks of Unit C. It ranges from 10 to 100 m in thickness and is discontinuous.
• Unit E which conformably overlies Unit D and is a thick complex of green schist facies sedimentary rocks.
Several generations of plutonic rocks intrude this sequence, some of which are coeval with the volcano-sedimentary units, although most of are late- to post-tectonic.
The volcanic rocks occur in the core of a major ENE-WSW trending anticlinorium that dips to the NW beneath the sedimentary rocks. Two phases of Neoproterozoic folding have been recognised: i). which produced bedding-parallel foliation, and ii). a subtle phase perpendicular to the first that created interference patterns. Three major fault sets, ENE, SE and NNE trending, cut all the units in the area. These and later structural elements were formed during four deformational events, as follows (Plyley et al., 2009):
• D1 - syn-peak metamorphism, producing the main bedding parallel schistosity and dextral shearing along NE trending zones as well as the first folding to form NE-SW trending isoclinal folds through NW-SE directed compression, accompanying collision of the Haya and Gebeit terranes;
• D2 - a mainly folding phase, resulting in NNW-SSE and north-south interference folds, doming, kink bands, weak axial planes schistosity and crenulation. This is interpreted to have accompanied the collision of the more distal terranes of the shield;
• D3 - that occurred under more brittle-ductile conditions, with NW-SE sinistral shearing, axial planes schistosity and crenulation. This event is interpreted to have accompanied the main final collision and accretion of the Nubian-Arabian Shield and deformed the VHMS mineralised bodies;
• D4 - brittle regime deformation, with faults reactivation, minor folding, and the Cenozoic Red Sea rifting.
The best age estimate for deposition of the Hassai district VHMS and barite deposits is 702 ±15 Ma, n=8, based on lead isotope data for nine samples from different deposits (Plyley et al., 2009).
Production to 2010 was predominantly from supergene mineralisation deposits overlying the VHMS deposits, occurring as gold in gossans and in quartz-kaolinite-barite rocks. The latter style was created by enrichment of the gold-bearing volcanogenic massive sulphides by weathering and is composed of fine-grained free gold between gangue minerals or under iron oxides. The gossans were generally between 700 and 3000 m in length and between 3 m and 50 m in thickness. The supergene profile comprises a near surface gold-silver rich oxide gossan-silica barite zone; a gold ±silver horizon that has been subjected to leaching bleaching acid leaching; a supergene copper sulphide-enriched zone; the underlying copper-zinc-gold hypogene massive sulphide body. The base of oxidation is generally at 90 to 100 m below surface.
Where oxide-quartz-kaolinite-barite ore is developed, a generalised profile commences with the hypogene massive sulphide containing 70 to 100% pyrite with chalcopyrite and sphalerite below the base of oxidation. The uppermost section of the primary sulphide body, immediately below oxidation, contains enrichments of barite, quartz and gypsum. The core of the overlying steeply dipping oxidised massive sulphide body passes upward from powdery or sugary white 'silica-barite rock' (SBR) to rocky SBR, which at a depth of 50 to 60 m becomes oxidised SBR and then closer to surface melds into the residual brecciated gossan cap, recognised as a zone of barite crystals. Between the surface and base of oxidation, the SBR passes outward, in both the hanging and footwall, into a zone of leached foliated rock with lenses of jarosite and gypsum, then to a halo of foliated gossan and peripheral zones of iron-stained clay altered schist. At surface, the cap of residual brecciated gossan overlies much of these peripheral zones that sandwich the oxidised massive sulphide band (after schematic diagram in Plyley et al., 2009 and Bosc et al., 2012).
Some small deposits and prospects (e.g., Ganaet, Rawai, Hamim) occur as gold bearing barite lenses without any underlying massive sulphide or overlying gossan, although the mineralisation shares some of the features of the SBR deposits. The barite lenses do not extend to depth below the base of oxidation. They occur within highly deformed corridors, with mylonite fabrics and green chloritic alteration. Gold is found in late brittle faults that parallel the fabric and has an apparent association with magnetite (Plyley et al., 2009; Bosc et al., 2012).
Most of the massive sulphide mineralisation occurs as tabular bodies that range from 0.3 to >25 m in thickness. In the Hadal Awatib deposit, massive sulphide mineralisation persists over a strike length of >2500 m. Sulphides are typically massive, fine grained, banded and locally brecciated, and are predominantly composed of pyrite, sphalerite and chalcopyrite, with lesser pyrrohotite, galena, tetrahedrite-freibergite and arsenopyrite. Hydrothermal alteration within the host rocks includes proximal silicification and more distal chloritisation, sericitisation and locally, carbonitization (Abu Fatima, 2006; Plyley et al., 2009). The primary massive sulphides of Ariab are mainly pyritiferous, locally enriched in Cu and Zn in the form of chalcopyrite and sphalerite. They are characterized by very low Pb contents, and contents of 0.3 to 1.5 g/t Au.
The more significant massive sulphide deposits are:
• Hassaï South is located near the center of the Ariab district. It is ~1400 m long, 5 to 35 m thick and persists for to at least 550 m with a dip of ~60 to 70°S. Geophysical data suggests it continues to a depth of >700 m. It initially had an oxide resource of 7.7 t of contained gold in SBR ore grading 7 g/t Au. This resource was mined to a depth of ~100 m with mineralisation occurring over widths varying from 4 to 35 m, underlain by supergene mineralisation grading >5% copper at the base of the oxide pit. These supergene zones were underlain by Indicated + Inferred Mineral Resources of 42.1 Mt of hypogene massive sulphide ore averaging 1.25% Cu, 0.29% Zn, 1.47 g/t Au (Bosc et al., 2012). The supergene and hypogene massive sulphide comprise a single lens with minor chloritic partings. The host succession/alteration assemblage comprises chloritic volcaniclastic and siliciclastic rocks composed of chlorite-quartz ±sericite ±carbonate ±iron oxide ±pyrite ±epidote ±incipient garnet that have been foliated to form phyllite and schist. Minor lapilli tuffs are also present. Thin diorite dykes occupy minor north-south offset cross-faults with displacements of 0.1 to 1 m (Barrie et al., 2016).
The deposit lies on the south-dipping southern limb of an east-west trending and south-dipping anticline, whilst the smaller Hassai North oxide gold deposit is 1 km to the north on the northern limb. The rocks have a predominant east-west strike and dip at 60 to 70°S (Barrie et al., 2016).
The oxide gold zone (SBR, silica-barite rock) contains native gold, electrum, anglesite, cerrusite, smithsonite, calaverite, petzite, tellurides of lead, silver and bismuth, chalcocite, tenorite, malachite, azurite chrysocolla, native copper, within a gangue of kaolinite, gypsum, anhydrite, barite, jarosite, chalcedony, iron-titanium oxides, epidote, chlorite and sericite (Barrie et al., 2016).
The hypogene and supergene massive sulphide are composed of pyrite, chalcopyrite, sphalerite, marcasite, galena, clausthalite, tetrahedrite, tennantite, friebergite, arsenopyrite, polybasite, anglesite, altaite, tetradymite, tellurobismuthite, hessite, petzite, calaverite, electrum gold, cobaltite, magnetite, pyrrhotite, molybdenite, bismuthinite, mackinawite, bornite, covellite, digenite, cubanite, enargite and several other minerals (Barrie et al., 2016).
• Hadal Awatib, the largest of the known deposits, is located in the northern part of the Ariab mining district. It has been structurally dislocated into the Hadal Awatib East, West and North lenses. It is >3100 m long, from 10 to 100 m thick, and has a drill tested down dip extent of 400 m, although geophysical data indicate it persists to at least 800 m, with a 65 to 90°N dip.
The Hadal Awatib VHMS-oxide gold deposit is exploited by seven open pit mines developed on a single, giant VHMS system. The underlying massive sulphide has been estimated to contain a total resource of 68.8 Mt @ 1.22 % Cu, 0.74 % Zn, 1.10 g/t Au (Bosc et al., 2012). The host lithology/alteration succession comprises: i). footwall silicified and sericitised, massive rhyolite and rhyolite breccia; ii). chloritised basalt-andesite flows, tuffs and agglomerates; and iii). hanging wall felsic tuff, intermediate to felsic volcaniclastic rocks and fine-grained siliciclastic rocks. Massive and semi-massive sulphides are interspersed with chloritic tuffaceous partings that commonly contain stringer pyrite-chalcopyrite mineralisation. The hypogene sulphides are pyrite, chalcopyrite and low-iron sphalerite, with minor galena and sparse tennantite, hessite, cobaltite and electrum. Oxide gold is developed to depths of 80 to 120 m, whilst supergene Cu is developed over the interval from 100 to 130 m, underlain by the hypogene sulphides (Barrie et al., 2016).
• Hadayamet - comprising a cluster of multiple lenses that have an overall strike of 70° and dip of 70°N. The overall dimensions of the deposit are 700 m long, with sulphide lenses up to 300 m in length, and sulphides developed over a width od 130 m and vertical extent of 250m.
Production, Reserves and Resources
Historic production between 1991 and 2014 amounted to 14.535 Mt @ an average grade of 6.83 g/t Au. The mined grade peaked at 15.5 g/t Au in 1993 and progressively decreased to 10.62 g/t in 1999, 6.04 g/t in 2004, to 3.49 g/t in 2010 and 3.00 g/t Au in 2014. Recovery averaged 79%, being 94% in 1993 to 70% in 2010 (Louit et al., 2011) to 71% in 2014 (Abu Fatima, 2015).
Bosc et al. (2012) produced an NI 43-101 compliant Mineral Resources for the Hassaï South, Hadal Awatib East and Hadaymet deposits, for both the remaining oxides and sulphide ore as follows:
Indicated Resource - 80.8 Mt @ 1.26 g/t Au, 1.12% Cu;
Inferred Resource - 37.5 Mt @ 1.17 g/t Au, 1.09% Cu.
Sulphide cut-off 0.5% CuEq., and oxide cut-off Au 0.4 g/t (Hadal Awatib East) and 0.7 g/t (Hassaï South).
CIL Ore Reserves at the Hassai Project in heap leach tailings, and unmined oxide and quartz-kaolinite-barite ores (Bosc et al., 2012):
Proved Reserve - 2.456 Mt @ 2.15 g/t Au;
Probable Reserve - 11.204 Mt @ 2.30 g/t Au.
Sections of this summary are drawn from: "Bosc, R., Tamlyn, N. and Kachrillo, J.J., 2012 - The Hassai Mine project, VMS resources update, Red Sea State, Sudan; and NI 43-101 Technical Report prepared for La Mancha Resources Inc., Montreal, Quebec, Canada, by Arethuse Geology, 227p."
The most recent source geological information used to prepare this summary was dated: 2012.
This description is a summary from published sources, the chief of which are listed below.
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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.|
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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