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The Syama and Tabakoroni goldfields, Mali
The syama and tabakoroni goldfields, Mali Ibrahima Ballo, Kim A.A. Hein, Boukary Guindo, Lassana Sanogo, Yalema Ouologuem, Gaoussou Daou, Amadou Traore PII: DOI: Reference:
S0169-1368(15)30128-1 doi: 10.1016/j.oregeorev.2015.10.019 OREGEO 1649
To appear in:
Ore Geology Reviews
Received date: Revised date: Accepted date:
28 May 2015 13 October 2015 14 October 2015
Please cite this article as: Ballo, Ibrahima, Hein, Kim A.A., Guindo, Boukary, Sanogo, Lassana, Ouologuem, Yalema, Daou, Gaoussou, Traore, Amadou, The syama and tabakoroni goldfields, Mali, Ore Geology Reviews (2015), doi: 10.1016/j.oregeorev.2015.10.019
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ACCEPTED MANUSCRIPT The Syama and Tabakoroni goldfields, Mali Ibrahima BALLOa; Kim A.A. HEINb; Boukary GUINDOa; Lassana SANOGOa; Yalema
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OUOLOGUEMa; Gaoussou DAOUa; Amadou TRAOREa
a. Societe Des Mines de Syama (SOMISY SA) Rue 50, Porte 688; BP-E 1582 Bamako,
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Badalabougou, Mali: [email protected]/ Tel: +223 66 75 35 44/79 15 38 49 b. Corresponding Author, School of Geosciences; University of the Witwatersrand, Johannesburg,
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South Africa; Tel +27-11 7176623; Fax +27-11-7176579; Email: [email protected]
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Abstract
Gold deposits in the Syama and Tabakoroni goldfields in southern Mali occur along a north-northeast trending mineralised litho-structural corridor that trends for approximately 40 km. The deposits are
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interpreted to have formed during a craton-wide metallogenic event during the Eburnean orogeny. In
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the Syama goldfield, gold mineralisation in 9 deposits is hosted in the hanging-wall of the SyamaBananso Shear Zone in basalt, greywacke, argillite, lamprophyre, and black shale. Gold is currently mined primarily from the oxidised-weathered zone of the ore bodies. In the Syama deposit,
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mineralisation hosted in altered basalt is associated with an intense ankerite-quartz-pyrite stockwork vein systems, whereas disseminated style mineralisation is also present in greywackes. In contrast, the Tellem deposit is hosted in quartz-porphyry rocks.
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In the Tabakoroni goldfield, gold mineralisation is hosted in quartz veins in tertiary splay shears of the Syama-Bananso Shear Zone. The Tabakoroni orebody is associated with quartz, carbonate and graphite (stylolite) veins, with pyrite and lesser amounts of arsenopyrite. There are four main styles of gold mineralisation including silica-sulphide lodes in carbonaceous fault zones, stylolitic quartz reefs in fault zones, quartz-Fe-carbonate-sulphide lodes in mafic volcanics, and quartz-sulphide stockwork veins in silicified sediments and porphyry dykes. The several deposit styles in the goldfield thus present a number of potential exploration targets spatially associated with the regional Syama-Bananso Shear Zone and generally classified as orogenic shear-hosted gold deposits.
Keywords: West African Craton; Birimian; Syama; Tabakoroni; orogenic gold
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ACCEPTED MANUSCRIPT 1. Introduction
The Syama goldfield in southeastern Mali (Figure 1) includes the Syama gold mine and several associated satellite gold deposits including the Syama extension, Alpha, BA01, Tellem, Basso, Quartz
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Vein Hill, Chert Ridge and A21 deposits (Figure 2; Table 1). The Tabakoroni goldfield is the southern
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extension of the main Syama litho-structural corridor (Figures 1, 3). The goldfields are situated
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approximately 280 km southeast of Bamako and 70 km south-southwest the regional capital of Sikasso. They are hosted in a north-northeast trending, steeply west-dipping Bagoe volcanosedimentary belt (after Kušnίr, 1999), which is the northwestern extension of the Bondiali-Diaoula belt in Cote d’Ivoire (Bessoles, 1977). The Bagoe belt is crosscut by the Syama-Bananaso Shear Zone
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in the Syama and Tabakorni goldfields, and marks the regional boundary between the KadianaMadinani terrane to the west and the Kadiolo terrane to the east (Figure 1).
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In the Syama goldfield, gold is primarily mined from the oxidised-weathered zone, but hard rock sulphide mineralisation occurs at depth. The total mineral resource reported for the Syama goldfield is 38.27 Mt including 13.28 Mt at 2.89 g/t Au for the Syama main open deposit for 1237
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contained koz Au, 10.69 Mt at 2.34g/t Au for the satellite deposits for 805 contained koz Au, and 14.3
Ltd, 2014; Table 1).
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Mt at 2.4g/t Au for the Syama underground deposits for 1103 contained koz Au (Resolute Mining
Gold mineralisation is hosted in the hanging-wall of the Syama-Bananaso Shear Zone (Figures
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1, 2a, 2b) in tholeiitic basalt (plagioclase pyroxene), lithic (grey)wacke, argillite, lamprophyre and black shale. Ankerite, quartz, sulphide and albite veinlets are associated with gold mineralisation, and zones of massive fine-grained silica and pyrite correlate with higher gold grade zones. In comparison, the Tabakoroni goldfield is part of the Finkolo Exploitation Permit, located in
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the district of Kadiolo, immediately adjacent to the international border of Mali with Cote d’Ivoire, approximately 40km west of Kadiolo and 15km south of Fourou (Table 2). The total mineral resource reported by Resolute Mining Limited (December 2013) for the Tabakoroni goldfield is 9.96Mt @ 2.59g/t Au for 830 contained koz Au (Table 2). Gold mineralisation is hosted Tabakoroni shear zone which is a tertiary play shear off the Syama-Bananaso Shear Zone. Gold mineralisation occurs in quartz veins and intense silica alteration, with varying amounts of pyrite, carbonate and sericite. Mineralised quartz veins range from a few centimetres to metres in width.
2. Exploration History
Gold mining in Mali has a long history with a peak in the 14th century when Emperor Kanku Moussa transported 8 tons of gold on his pilgrimage to Mecca. Additionally, gold has been exploited in ancient Mali since the 5th century by artisanal miners. In the Syama region, the BRGM (Bureau de Recherche et Geologie Miniere) and SONAREM (Societe Nationale de Recherche Miniere) 2
ACCEPTED MANUSCRIPT undertook region geological studies along the Bagoe river from 1960 to 1974 (Olson et al 1992), which was followed by a regional sampling program (1000m x 200m grid) over 25,000 km2 initiated by the United Nations Development Program (UNDP) and the Direction Nationale de la Geologie et des Mines (DNGM). By 1986, the United Nations reported an oxide resource of 1.5Mt @ 1.67g/t
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calculated from exploration pits (Kušnίr et al., 1986).
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Subsequently, BHP Minerals International defined several targets in the Syama permit and
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BHP Minerals concluded that the Syama belt was under-explored both within the exploitation leases and at the regional scale.
In 1996, Randgold Resources Ltd acquired all BHP’s properties in Mali and took over management responsibilities of Syama. In 1997, a RAB drill program was conducted north of Alpha
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pit and confirmed the extension of mineralisation. In 1998, a successful study of the oxide potential north of Syama delineated 12 targets, and an RC drill program was completed at A21, BA01 and
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north of Syama mine in April 1999. This program returned a geological resource of 52,000 oz at A21 and 24,000 at BA01.
Resolute Mining Limited acquired the Syama project in April 2004 after completing a positive
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12-month pre-feasibility study in joint-venture with Randgold Resources from April 2003. In 2005, Resolute Mining Limited acquired an 80% interest in the project through its equity in Societe des
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Mines de Syama (SOMISY S.A.). The Malian Government holds a 20% interest in SOMISY. After a long process of construction, processing of ore commenced in August 2008.
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With regard to Tabakoroni on the Finkolo exploration permit, the permit was granted to Bagoe International Corporation in July 2001. An agreement was signed with Etruscan Resources Inc. in June 2002 allowing Etruscan to explore the tenement. In July 2004, following the decree No 04
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1407/MMEE-SG, the permit was transferred to Etruscan Resources Mali SARL with 50% of its area; the permit was granted for a period of 30 years for the exploitation of Group II minerals. In August 2013, the permit was transferred to Société des Mines de Finkolo SA (SOMIFI SA) under Decree transfer No 2013-667/PM-RM, which is a Joint Venture partnership between Resolute Mining Limited and Government of Mali. Resolute Mining Limited is the current owner and continues with active exploration.
3. Regional Geology
The Syama and Tabakoroni goldfields are situated in the Baoulé-Mossi domain of the West African craton in the southwest of Mali. Kušnίr (1999) recognised four belts of Birimian rocks in southern Mali; the easternmost he termed the Bagoe Belt which hosts the Syama and Tabakoroni goldfields. Bentley et al. (2000) described the Bagoe Belt in terms of two terranes: the KadianaMadinani Terrane in the west and Kadiolo Terrane in the east (Figure 1). The Bagoe Belt consists of
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ACCEPTED MANUSCRIPT sedimentary schists and metamorphosed greywackes, with subordinate basalt and intermediate volcanics and chert interflow sedimentary rocks. The Syama goldfield straddles the boundary between the Kadiana–Madinani terrain and the Kadiolo terrain (Figure 1); the boundary is defined by the north-northeast trending Syama-Bananso
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Shear Zone (Girard et al., 1998; Standing, 2005). Geophysical data shows the shear zone is 64 km
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long and 4 km wide from combined magnetic and gravity data, and has a clearly defined western
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branch that trends northerly (Yossi, 2015). The Syama-Bananso Shear Zone is described by Milési et al. (1989), Girard et al. (1998), SIGAFRIQUE (2000) and Randgold (2008) as a sinistral strike-slip shear along its main branch; the western branch corresponds to a fault or minor splay (Yossi, 2015). The Kadiana-Madinani terrain is dominated by lithic greywackes of the Sikoro Formation and a
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narrow belt of interbedded tholeiitic basalt and argillite locally known as the Syama Formation. The sequence is intruded by a series of dykes of andesitic/dioritic composition, or lamprophyres (Mason,
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2007). In contrast, the Kadiolo terrain is made up of (Tarkwa-like) polymictic conglomerate and sandstone that are sourced from the Kadiana-Madinani terrain and were deposited in a late to syntectonic basin (Standing, 2007).
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Gold mineralisation at Syama and Tabakoroni is similar to many other Palaeoproterozoic gold deposits in the Baoulé-Mossi domain of the West African Craton and formed during a craton wide
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gold metallogenic event late in the Eburnean Orogeny (Lawrence et al., 2013a,b; Treloar et al., 2014; Masurel et al., in press; Feybesse et al., 2006; Markwitz et al, in press). The deposits are similar to
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other deposits hosted by mafic volcanic rocks such as the Yaoure gold deposit in Cote d’Ivoire in terms of tectonic setting, wall-rock alteration, form of mineralisation, and associated structures (Rossi et al., 2014). Ar-Ar dating of sericite, fuchsite and plagioclase indicated that hydrothermal activity at
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Syama took place at 2100 ± 40 Ma (Hanssen et al., 1997).
4. Local Geology
The stratigraphy of the Syama gold mine was originally defined by Randgold Resources Ltd (Harbidge, 2001) and updated as mining continued, and new drilling data and mapping became available. The geology and structure of the Syama mine is described and discussed by Olsen et al (1992) and summarised herein. The metamorphic grade for the Syama goldfield is defined by the mineral assemblage chlorite, epidote, calcite, quartz ± actinolite ± biotite which corresponds with low to upper greenschist facies metamorphism; while at Tabakoroni the metamorphic grade is defined by the chlorite, epidote, calcite, quartz which is lower greenschist facies. Tourmaline and accessory biotite occurs locally in hornfels around intrusions. The region presents three stratigraphic sequences from west to east, namely,
The Sikoro Formation (Kadiana-Madinani terrain), with basalt with interbedded greywacke, argillite siltstone, chert and graphitic shales. The formation dips steeply to the west and not mineralised at Syama. It separates into two segments north of Syama. However, to the south of 4
ACCEPTED MANUSCRIPT Bia Chert hills near the Tabakoroni gold deposit, the formation is overlain by younger cover sequences.
The Syama Formation (Kadiana-Madinani terrain) hosts the Syama gold mine and is composed of rocks type similar to the Sikoro Formation. The Syama Formation is mineralised in the
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hangingwall of the Syama-Bananso Shear Zone and is crosscut by lamprophyre, feldspar
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porphyry and quartz feldspar porphyry dykes. Standing (2007) reported that the geochemistry of
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samples of lamprophyres were higher in Cr content relative to basalt, with high Mg and LREE, and lower Fe; they are notably altered to sericite-ankerite-fuchsite. The feldspar porphyry and quartz feldspar porphyry dykes are typically 2 to 10m wide, and strongly albitic with fuchsite altered ferro-magnesium phenocrysts.
The Ngolopene Group Formation (Kadiolo terrain) in the eastern footwall is essentially
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dominated by the polymictic conglomerate and is not mineralised (Olsen et al., 1992). It
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comprises clasts from the Syama Formation with inclusion of arkose, sandstone, siltstone and rare mafic volcanics clasts.
Gold mineralisation in the A21 deposit (1900 x 90 metres) is associated with quartz veining and
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a thrust contact between greywacke and basalt. The mineralized zones narrow at depth. Gold mineralisation in the Quartz Vein Hill deposit (500 x 90 metres); is associated
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with shallow-dipping quartz veins located in two zones. The mineralisation does not extend to depth, but is generally concentrated in the upper 25m of the profile and above the base of oxidation. Intense silicification and a series of quartz stockwork veins are observed parallel to foliation, together with sulphide mineralisation.
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The host lithologies of the satellite deposits include:
Two styles of gold mineralisation occur at the Chert Ridge open pit (330 x 50 metres). Style 1 is related to narrow shallow-dipping quartz veins that trend N200-N220, while the second style forms breccias veins at the contacts between basalt/lamprophyre and lamprophyre/sedimentary rocks that trend southwest. In general, north and northeast trending thrust faults bound type 2 gold mineralisation. At the BA01 deposit (750 x 80 metres), gold mineralisation is located in basalts above the hanging-wall thrust, adjacent to the contact between sedimentary rocks and basalt. It is also distributed between the footwall thrust and the hanging-wall thrust, and adjacent to the contact between sedimentary rocks and basalt, or within the basalts. Gold mineralisation at the Syama extension (500 x 60 metres) occurs at the contact between volcaniclastic sedimentary rocks and basalt. The mineralized zone forms a sigmoidal jog and trends N010° (sub-vertical dip in the south), to N030° where the dip of the deposit flattens and it thickens, to N010° in the north.
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ACCEPTED MANUSCRIPT Gold mineralisation at the Alpha mine (400 x 80 metres) is host in altered basalt and lamprophyre adjacent to a conglomerate contact. Gold mineralisation at the Basso mine (700 x 100 metres) is developed in the western contact between sedimentary rocks and basalt, and hangingwall and footwall basalt and sedimentary
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rocks.
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Gold mineralisation at the Tellem deposit (4200 x 50 metres) is associated with a north-south
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trending feldspar porphyry dyke at the contact with sedimentary rocks.
5. Structural Geology
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The Syama gold mine is hosted in a 500m to 2000m thick isoclinal fold that trends northnortheasterly and dips 60-70° west. The Syama ore body is open down plunge to the northeast: a
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diamond drilling program intersected sulphide mineralisation at depths of more than 500m below the surface. Hangingwall and footwall thrusts bound the main ore zone with vertical linking structures joining the two thrusts (Olsen et al., 1992; McCuaig, 2005). Also north-northwest trending subvertical
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structures in the footwall of the main ore zone and late faults influence the geometry of the orebody. Ore shoots are controlled by the influence of isoclinal folding of a preferential lithology, late faulting
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and possible duplexing within the orebody by vertical link faults. (Standing, 2007) The structural architecture of the Tabakoroni deposit is described in several company reports by
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Standing (2005; 2006). In summary, the deposit is hosted in the Tabakoroni Shear Zone is developed in carbonaceous shale and siltstone at the margin of the Syama Formation (Standing, 2006). The shear zone is estimated to be 2km long and 5-25m wide with complex intrafolial folds within the shear zone
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and a penetrative shear cleavage. Displacement is dextral strike-slip with later sinistral reactivation. In the shear zone gold mineralisation is developed as a pyritic and carbonaceous fault gouge, with quartz-carbonate stockwork veins and pyrite disseminations developed in the wall rocks. Quartz lodes are developed along the length of the shear zone with narrow breccia and intense Fe carbonatearsenopyrite-pyrite replacement assemblage of wall rocks. Additionally, megascopic isoclinal folds were recognised by Standing (2005) with moderate fold plunges to the north-northeast and a northeast trending moderately west dipping foliation.
6. Mineralisation Styles
The Syama gold deposit is hosted in the hangingwall of the Syama-Bananso Shear Zone. Ankerite-quartz-sulphide veinlets with selvages of fine-grained sulphide (pyrite) are the primary minerals associated with gold (Olsen et al., 1992). The presence of albite veinlets is a good indicator
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ACCEPTED MANUSCRIPT for gold mineralisation and zones of massive fine-grained silica and pyrite correlate with higher gold grades. Visible gold is rare at Syama. In contrast, gold mineralisation at Tabakoroni is hosted within the Tabakoroni Main Shear Zone, which is a splay shear off the Syama-Bananso Shear Zone. Gold mineralisation is best
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developed in breccia zones and zones of strong quartz, carbonate and graphite veining and
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silicification/carbonate alteration, with pyrite and lesser arsenopyrite as the dominant sulphides.
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However, the nature of gold mineralisation at Tabakoroni is strongly influenced by the chemistry and rheology of the host rock, and there are four main styles of gold mineralisation, namely, (1) silicasulphide lode in carbonaceous fault zones, (2) stylolitic quartz reef in fault zones, (3) quartz-Fecarbonate-sulphide lodes in mafic volcanics, and (4) quartz-sulphide stockworks in silicified
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sedimentary rocks and porphyry dykes.
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7. Ore Body Characteristics
The Syama orebody is characterised by a variety of breccias. Gold mineralisation is associated
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with an intense ankerite-quartz-pyrite stockwork and is hosted in altered basalt, with zones of disseminated pyrite and stockwork veins in greywacke. The orebody dips steeply west within the
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Syama-Bananso Shear Zone, but is occasionally overturned to the east. A wide variety of geophysical data was collected from airborne to ground surveys. Airborne
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magnetic, electromagnetic (EM) and radiometric surveys have been completed with success. The VTEM seem to be the best EM method, resolving well the shear zone and the contact between the Syama and Ngolopene Group formations with conductivity contours. In terms of ground surveys,
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induced polarization (IP) was the main method used to aid in delineation of local anomalies. A number of ground IP surveys were completed with gradient and pole-dipole arrays. Due to the conductivity highs of the central part of the shear, only pole-dipole arrays were adequate to provide a significant chargeability setting with IP. Geophysically, the Syama ore body is non-magnetic, with moderate chargeability (7mV/V to17 mV/V) and resistivity (200Ωm to 850Ωm) values at the contact between mafic units and sediments in the hangingwall of an isoclinal fold. The Syama deposits are very rich in potassium, but poor in thorium and uranium. The Tabakoroni orebody is associated with quartz, carbonate and graphite (stylolites) veins, with pyrite and lesser arsenopyrite. The Tabakoroni ore body is physically non-magnetic with strong chargeability due to the present of sulphites, and strong conductivity because of graphite. Poor in thorium and uranium, the Tabakoroni deposits are very rich in potassium.
8. Alteration
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ACCEPTED MANUSCRIPT In the Syama mine there are wide zones of carbonate alteration with the mineral assemblage of ankerite ± sericite ± chlorite ± albite. In general the rocks are moderately bleached. In highly mineralized zones, albite increases in quality and is often salmon pink in colour due to the addition of fine grained ankerite. Silicification is often intense in narrow zones adjacent to the ore bodies. These
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zones consist of grey to black silica. They are barren and often enclosed by breccia and ankerite
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alteration (Figure 4a). At depth, basalt sequences are haematised. These rocks are known as red eyes
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basalt and are not associated with gold mineralisation (Figure 4b). In the satellite pits, alteration is restricted to narrow enveloping zones of ankerite-sericite.
At Tabakoroni several different metamorphic and hydrothermal alteration styles are developed in the country and host rocks. Most are unrelated to gold mineralisation, or have had some role in
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preparing the rocks both chemically and physically for the auriferous fluids that arrived late in the evolution of the deposit. They include epidote-plagioclase-quartz-carbonate alteration forming wall
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rock selvedges in dolerite and pillowed lavas. Chlorite-calcite alteration occurs throughout the mafic sequences. Sericite-quartz-carbonate-pyrite alteration overprints biotite ± magnetite alteration in feldspar porphyry intrusions. In general, the Syama formation is strongly silicified. Fe-carbonate -
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quartz - leucoxene - arsenopyrite - pyrite alteration is also present and associated with quartz lodes
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hosted in the Tabakoroni shear zone.
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9. Discussion and Conclusions
The Syama gold deposit and associated satellite pits and the Tabakoroni orebody are typical Palaeoproterozoic orogenic gold deposits that formed during the 2.1 Ga Eburnean orogeny (Béziat et
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al., 2008; Feybesse et al., 2006) along or proximal to shear zones in metamorphosed volcanic and sedimentary units of the Birimian Supergroup sequences (Milési et al., 1991, Tshibubudze et al., 2015) including those at Obuasi (Fougerouse et al., this volume), Essakane (Tshibubudze and Hein, this volume), Gounkoto (Lambert-Smith et al., this volume) and others. With its associated satellite deposits, the Syama gold deposit is hosted in an overturned and structurally layered sequence of basalt, greywacke-argillite-black shales, and a lamprophyre intrusion, and hosts in veins in the Syama-Bananso shear zone. The Tellem deposit is hosted in quartz-veins in a feldspar porphyry dyke. Gold mineralisation at Syama is characterized by a dolomite ± ankerite/silica/pyrite and native gold. Alteration in the mine sequence is asymmetrically zoned from the main lamprophyre intrusive complex in the northern sector of the open pit out toward the hanging-wall. Gold grades are generally found to be highest in areas where pyrite occurs as blebs in the host rocks or within strongly dolomite veined intervals, or where fine disseminated pyrite occurs within the immediate vein selvedge. The Syama ore body is open down plunge to the northeast: a diamond drilling program intersected sulphide mineralisation at depths of more than 500m below the surface.
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ACCEPTED MANUSCRIPT The Tabakoroni orebody is a 2km long and 5-25m wide gold deposit contained in the Tabakoroni Main Shear Zone (a tertiary splay of the Syama-Bananso Shear Zone) and hosted in basalt, andesitic dykes, greywacke, siltstone, chert and graphitic argillite. Gold mineralisation is best developed in breccia zones and zones of strong quartz, carbonate and graphitic veins. The orebody is dominated by
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quartz-carbonate alteration, with pyrite and lesser arsenopyrite as the dominant sulphides.
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Acknowledgements
We thank Resolute Mining Limited SOMISY SA Exploration for its availability to provide the data and permission to publish. The manuscript was greatly improved by two anonymous reviewers.
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D., Toure, S., and Morel, B., 1989. Les mineralisations auriferes de l’Afrique de l’Ouest. Chron. Recherche Miniere, no. 497, pp 3-98. Olsen S.F., Diakite K., Ott, L., Guindo, A., Ford, C.R.B., Winer, N., Hanssen, E., Bradley, R., Pohl,
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D., 1992. Regional setting, structure, and descriptive geology of the Middle Proterozoic Syama gold deposit, Mali, West Africa. Economic Geology 87(2), 310-331. Randgold Resources, 2008. Delivering profits, growth and new opportunities. Presentation to the Denver Gold Forum, September, 2008, 27 slides. Resolute Mining Ltd, March 2014. Pre-Feasibility Study Boosts Ore Reserves at Syama by 54% to 3.15 Million Ounces, 33p. Rossi, M.E., Hallewell, M., Brown, P., January 2014; Technical report and mineral resource estimates for AMRA MINING, 183p. SIGAFRIQUE, 2000. SIGAFRIQUE mapping program. Standing, J., 2005. Geological and structural mapping in the Syama – Finkolo prospect area, Mali, West Africa. Confidential Report by Jigsaw Geoscience Pty Ltd to Resolute Mining Limited, April 2005, 29p. Standing, J., 2006. Structural geology of the Tabakoroni prospect, and detailed mapping in the quartz vein hill-chert ridge-A21 pit area, Syama-Finkolo project, Mali, West Africa. Fluid Focus Pty Ltd, A.C.N. 094 880 627, 25p. 10
ACCEPTED MANUSCRIPT Standing, J., 2007. Report and recommendations from the geological campaign - Syama- Mali. Confidential Report to Resolute Mining Limited by Jigsaw Geoscience Pty Ltd, March 2007, 99p. Treloar, P.J., Lawrence, D.M., Senghor, D., Boyce, A., Harbidge, P., 2014. The Massawa gold
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deposit, Eastern Senegal, West Africa: an orogenic gold deposit sourced from magmatically
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derived fluids? Geological Society, London, Special Publications 393.
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Tshibubudze, A., Hein, K.A.A., this volume. Gold mineralization in the Essakane goldfield in Burkina Faso, West African craton. Ore Geology Reviews
Yossi, M., 2015. 3D geophysical modelling used for structural interpretation in the southern Mali and northeast of Guinea, West Africa. Unpublished MSc thesis, University of the Witwatersrand,
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Johannesburg, 159p.
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ACCEPTED MANUSCRIPT List of Figures
Figure 1: Location map for the Syama and Tabaorni goldfeilds in southeastern Mali. The goldfields are hosted in a north-northeast trending, steeply west-dipping greenstone belt (Bagoe belt) that marks
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a regional boundary between the Kadiana-Madinani terrane to the west and the Kadiolo terrane to the
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east.The Bagoe Belt consists of sedimentary schists and metamorphosed greywackes, with
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subordinate basalt and intermediate volcanics and chert interflow sediments.
Figure 2: Regional geology of the Syama goldfields showing the location of the Syama extension, Alpha, BA01, Tellem, Basso, Quartz Vein Hill, Chert Ridge and A21 deposits. The gold deposits are
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variously hosted at the contact of metasedimentary rocks and basaltic units.
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Figure 3: Regional geology of the Tabakoroni goldfield in the southeast of Mali. The Tabakoroni deposit trends north-northwest and is hosted at the sheared contact between basalt and shale units.
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Figure 4:
A. Photograph of intense silicification zone with grey to black silica and ankerite alteration.
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B. Photograph of the red eyes basalt in diamond drill core.
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C. Photograph of typical Syama ore with grade higher than 3g/t Au.
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ACCEPTED MANUSCRIPT List of Tables
Table 1: Summary table for gold deposits in the Syama goldfield including general location, deposit
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status and type and resource information.
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general location, deposit status and type and resource information.
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Table 2: Summary table for the Tabakoroni gold deposits in the Tabakoroni goldfield including
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Commodity of exploitation Longitude Latitude
Syama goldfield includes the Syama mine and the Syama extension, Alpha, BA01, Tellem, Basso, Quartz Vein Hill, Chert Ridge, A21 deposits Gold -6.0703 10.7917
Geographic location
District of Fourou, Kadiolo area, in the region of Sikasso.
Geological location
West African craton; Leo-Man shield; southeastern Mali; between the Kadiana-Madinani terrane to the west and the Kadiolo terrane. The Syama gold mine was owned and operated by BHP Utah in 1990-1996), Randgold Resources in1996-2001 and Resolute Mining Ltd from late 2008. The goldfield is currently owned and operated by the Societe Des Mines de Syama (SOMISY SA) Mali, which is a joint venture partnership between Resolute Mining Limited and the Malian government. Gold in breccia, intense ankerite-quartz-pyrite stockwork, and sheeted veinlet sets in altered basaltic host rocks with zones of disseminated pyrite and weak stockworks in greywacke. The total mineral resource reported for the Syama goldfield is 38.27 Mt including 13.28 Mt at 2.89 g/t Au for the Syama main open deposit for 1237 contained koz Au, 10.69 Mt at 2.34g/t Au for the satellite deposits for 805 contained koz Au, and 14.3 Mt at 2.4g/t Au for the Syama underground deposits for 1103 contained koz Au (Resolute Mining Ltd, December 2014).
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Economic value Grade Tonnages Past production
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TABLE 1
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ACCEPTED MANUSCRIPT Tabakoroni gold deposit Gold -6.164 10.520
Geographic location
District of Kadiolo, in the region of Sikasso. Immediately adjacent to the Cote d’Ivoire border, approximately 40km west of Kadiolo and 15km south of Fourou
Geological location
West African craton; Leo-Man shield; southeastern Mali; between the Kadiana-Madinani terrane to the west and the Kadiolo terrane. The Tabakoroni gold mine was owned by Resolute Mining LTD and Etruscan Ressources until 2012, but currently owned by Societe des Mine de Finkolo (SOMIFI SA) Mali, which is a joint venture partnership between Resolute Mining Limited and the Malian government. Mining is currently scheduled to start in 2016. Shear hosted orogenic deposits Total reported mineral resource for the Tabakoroni goldfield is 9.96Mt @ 2.59g/t Au for 830 koz Au (Resolute Mining Ltd, December 2013).
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Deposit type Economic Value
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Table 2
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Figure 1
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Sikoro Fm
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Ngopelene Fm
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Syama Fm
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ACCEPTED MANUSCRIPT Highlights
Syama and Tabakoroni goldfield in West Africa host deposits that are spatially associated with shears.
The deposits occur at the sheared contact of basalt and metasedimentary units.
Gold is primarily mined from an upper oxidised-weathered zone that overlies a sulphide ore
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zone.
The several deposit styles present a number of potential exploration targets.
The deposits represent an example of orogenic shear-hosted gold deposits in west Africa
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S0169-1368(15)30128-1 doi: 10.1016/j.oregeorev.2015.10.019 OREGEO 1649
To appear in:
Ore Geology Reviews
Received date: Revised date: Accepted date:
28 May 2015 13 October 2015 14 October 2015
Please cite this article as: Ballo, Ibrahima, Hein, Kim A.A., Guindo, Boukary, Sanogo, Lassana, Ouologuem, Yalema, Daou, Gaoussou, Traore, Amadou, The syama and tabakoroni goldfields, Mali, Ore Geology Reviews (2015), doi: 10.1016/j.oregeorev.2015.10.019
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ACCEPTED MANUSCRIPT The Syama and Tabakoroni goldfields, Mali Ibrahima BALLOa; Kim A.A. HEINb; Boukary GUINDOa; Lassana SANOGOa; Yalema
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OUOLOGUEMa; Gaoussou DAOUa; Amadou TRAOREa
a. Societe Des Mines de Syama (SOMISY SA) Rue 50, Porte 688; BP-E 1582 Bamako,
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Badalabougou, Mali: [email protected]/ Tel: +223 66 75 35 44/79 15 38 49 b. Corresponding Author, School of Geosciences; University of the Witwatersrand, Johannesburg,
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South Africa; Tel +27-11 7176623; Fax +27-11-7176579; Email: [email protected]
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Abstract
Gold deposits in the Syama and Tabakoroni goldfields in southern Mali occur along a north-northeast trending mineralised litho-structural corridor that trends for approximately 40 km. The deposits are
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interpreted to have formed during a craton-wide metallogenic event during the Eburnean orogeny. In
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the Syama goldfield, gold mineralisation in 9 deposits is hosted in the hanging-wall of the SyamaBananso Shear Zone in basalt, greywacke, argillite, lamprophyre, and black shale. Gold is currently mined primarily from the oxidised-weathered zone of the ore bodies. In the Syama deposit,
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mineralisation hosted in altered basalt is associated with an intense ankerite-quartz-pyrite stockwork vein systems, whereas disseminated style mineralisation is also present in greywackes. In contrast, the Tellem deposit is hosted in quartz-porphyry rocks.
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In the Tabakoroni goldfield, gold mineralisation is hosted in quartz veins in tertiary splay shears of the Syama-Bananso Shear Zone. The Tabakoroni orebody is associated with quartz, carbonate and graphite (stylolite) veins, with pyrite and lesser amounts of arsenopyrite. There are four main styles of gold mineralisation including silica-sulphide lodes in carbonaceous fault zones, stylolitic quartz reefs in fault zones, quartz-Fe-carbonate-sulphide lodes in mafic volcanics, and quartz-sulphide stockwork veins in silicified sediments and porphyry dykes. The several deposit styles in the goldfield thus present a number of potential exploration targets spatially associated with the regional Syama-Bananso Shear Zone and generally classified as orogenic shear-hosted gold deposits.
Keywords: West African Craton; Birimian; Syama; Tabakoroni; orogenic gold
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ACCEPTED MANUSCRIPT 1. Introduction
The Syama goldfield in southeastern Mali (Figure 1) includes the Syama gold mine and several associated satellite gold deposits including the Syama extension, Alpha, BA01, Tellem, Basso, Quartz
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Vein Hill, Chert Ridge and A21 deposits (Figure 2; Table 1). The Tabakoroni goldfield is the southern
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extension of the main Syama litho-structural corridor (Figures 1, 3). The goldfields are situated
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approximately 280 km southeast of Bamako and 70 km south-southwest the regional capital of Sikasso. They are hosted in a north-northeast trending, steeply west-dipping Bagoe volcanosedimentary belt (after Kušnίr, 1999), which is the northwestern extension of the Bondiali-Diaoula belt in Cote d’Ivoire (Bessoles, 1977). The Bagoe belt is crosscut by the Syama-Bananaso Shear Zone
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in the Syama and Tabakorni goldfields, and marks the regional boundary between the KadianaMadinani terrane to the west and the Kadiolo terrane to the east (Figure 1).
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In the Syama goldfield, gold is primarily mined from the oxidised-weathered zone, but hard rock sulphide mineralisation occurs at depth. The total mineral resource reported for the Syama goldfield is 38.27 Mt including 13.28 Mt at 2.89 g/t Au for the Syama main open deposit for 1237
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contained koz Au, 10.69 Mt at 2.34g/t Au for the satellite deposits for 805 contained koz Au, and 14.3
Ltd, 2014; Table 1).
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Mt at 2.4g/t Au for the Syama underground deposits for 1103 contained koz Au (Resolute Mining
Gold mineralisation is hosted in the hanging-wall of the Syama-Bananaso Shear Zone (Figures
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1, 2a, 2b) in tholeiitic basalt (plagioclase pyroxene), lithic (grey)wacke, argillite, lamprophyre and black shale. Ankerite, quartz, sulphide and albite veinlets are associated with gold mineralisation, and zones of massive fine-grained silica and pyrite correlate with higher gold grade zones. In comparison, the Tabakoroni goldfield is part of the Finkolo Exploitation Permit, located in
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the district of Kadiolo, immediately adjacent to the international border of Mali with Cote d’Ivoire, approximately 40km west of Kadiolo and 15km south of Fourou (Table 2). The total mineral resource reported by Resolute Mining Limited (December 2013) for the Tabakoroni goldfield is 9.96Mt @ 2.59g/t Au for 830 contained koz Au (Table 2). Gold mineralisation is hosted Tabakoroni shear zone which is a tertiary play shear off the Syama-Bananaso Shear Zone. Gold mineralisation occurs in quartz veins and intense silica alteration, with varying amounts of pyrite, carbonate and sericite. Mineralised quartz veins range from a few centimetres to metres in width.
2. Exploration History
Gold mining in Mali has a long history with a peak in the 14th century when Emperor Kanku Moussa transported 8 tons of gold on his pilgrimage to Mecca. Additionally, gold has been exploited in ancient Mali since the 5th century by artisanal miners. In the Syama region, the BRGM (Bureau de Recherche et Geologie Miniere) and SONAREM (Societe Nationale de Recherche Miniere) 2
ACCEPTED MANUSCRIPT undertook region geological studies along the Bagoe river from 1960 to 1974 (Olson et al 1992), which was followed by a regional sampling program (1000m x 200m grid) over 25,000 km2 initiated by the United Nations Development Program (UNDP) and the Direction Nationale de la Geologie et des Mines (DNGM). By 1986, the United Nations reported an oxide resource of 1.5Mt @ 1.67g/t
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calculated from exploration pits (Kušnίr et al., 1986).
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Subsequently, BHP Minerals International defined several targets in the Syama permit and
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BHP Minerals concluded that the Syama belt was under-explored both within the exploitation leases and at the regional scale.
In 1996, Randgold Resources Ltd acquired all BHP’s properties in Mali and took over management responsibilities of Syama. In 1997, a RAB drill program was conducted north of Alpha
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pit and confirmed the extension of mineralisation. In 1998, a successful study of the oxide potential north of Syama delineated 12 targets, and an RC drill program was completed at A21, BA01 and
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north of Syama mine in April 1999. This program returned a geological resource of 52,000 oz at A21 and 24,000 at BA01.
Resolute Mining Limited acquired the Syama project in April 2004 after completing a positive
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12-month pre-feasibility study in joint-venture with Randgold Resources from April 2003. In 2005, Resolute Mining Limited acquired an 80% interest in the project through its equity in Societe des
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Mines de Syama (SOMISY S.A.). The Malian Government holds a 20% interest in SOMISY. After a long process of construction, processing of ore commenced in August 2008.
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With regard to Tabakoroni on the Finkolo exploration permit, the permit was granted to Bagoe International Corporation in July 2001. An agreement was signed with Etruscan Resources Inc. in June 2002 allowing Etruscan to explore the tenement. In July 2004, following the decree No 04
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1407/MMEE-SG, the permit was transferred to Etruscan Resources Mali SARL with 50% of its area; the permit was granted for a period of 30 years for the exploitation of Group II minerals. In August 2013, the permit was transferred to Société des Mines de Finkolo SA (SOMIFI SA) under Decree transfer No 2013-667/PM-RM, which is a Joint Venture partnership between Resolute Mining Limited and Government of Mali. Resolute Mining Limited is the current owner and continues with active exploration.
3. Regional Geology
The Syama and Tabakoroni goldfields are situated in the Baoulé-Mossi domain of the West African craton in the southwest of Mali. Kušnίr (1999) recognised four belts of Birimian rocks in southern Mali; the easternmost he termed the Bagoe Belt which hosts the Syama and Tabakoroni goldfields. Bentley et al. (2000) described the Bagoe Belt in terms of two terranes: the KadianaMadinani Terrane in the west and Kadiolo Terrane in the east (Figure 1). The Bagoe Belt consists of
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ACCEPTED MANUSCRIPT sedimentary schists and metamorphosed greywackes, with subordinate basalt and intermediate volcanics and chert interflow sedimentary rocks. The Syama goldfield straddles the boundary between the Kadiana–Madinani terrain and the Kadiolo terrain (Figure 1); the boundary is defined by the north-northeast trending Syama-Bananso
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Shear Zone (Girard et al., 1998; Standing, 2005). Geophysical data shows the shear zone is 64 km
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long and 4 km wide from combined magnetic and gravity data, and has a clearly defined western
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branch that trends northerly (Yossi, 2015). The Syama-Bananso Shear Zone is described by Milési et al. (1989), Girard et al. (1998), SIGAFRIQUE (2000) and Randgold (2008) as a sinistral strike-slip shear along its main branch; the western branch corresponds to a fault or minor splay (Yossi, 2015). The Kadiana-Madinani terrain is dominated by lithic greywackes of the Sikoro Formation and a
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narrow belt of interbedded tholeiitic basalt and argillite locally known as the Syama Formation. The sequence is intruded by a series of dykes of andesitic/dioritic composition, or lamprophyres (Mason,
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2007). In contrast, the Kadiolo terrain is made up of (Tarkwa-like) polymictic conglomerate and sandstone that are sourced from the Kadiana-Madinani terrain and were deposited in a late to syntectonic basin (Standing, 2007).
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Gold mineralisation at Syama and Tabakoroni is similar to many other Palaeoproterozoic gold deposits in the Baoulé-Mossi domain of the West African Craton and formed during a craton wide
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gold metallogenic event late in the Eburnean Orogeny (Lawrence et al., 2013a,b; Treloar et al., 2014; Masurel et al., in press; Feybesse et al., 2006; Markwitz et al, in press). The deposits are similar to
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other deposits hosted by mafic volcanic rocks such as the Yaoure gold deposit in Cote d’Ivoire in terms of tectonic setting, wall-rock alteration, form of mineralisation, and associated structures (Rossi et al., 2014). Ar-Ar dating of sericite, fuchsite and plagioclase indicated that hydrothermal activity at
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Syama took place at 2100 ± 40 Ma (Hanssen et al., 1997).
4. Local Geology
The stratigraphy of the Syama gold mine was originally defined by Randgold Resources Ltd (Harbidge, 2001) and updated as mining continued, and new drilling data and mapping became available. The geology and structure of the Syama mine is described and discussed by Olsen et al (1992) and summarised herein. The metamorphic grade for the Syama goldfield is defined by the mineral assemblage chlorite, epidote, calcite, quartz ± actinolite ± biotite which corresponds with low to upper greenschist facies metamorphism; while at Tabakoroni the metamorphic grade is defined by the chlorite, epidote, calcite, quartz which is lower greenschist facies. Tourmaline and accessory biotite occurs locally in hornfels around intrusions. The region presents three stratigraphic sequences from west to east, namely,
The Sikoro Formation (Kadiana-Madinani terrain), with basalt with interbedded greywacke, argillite siltstone, chert and graphitic shales. The formation dips steeply to the west and not mineralised at Syama. It separates into two segments north of Syama. However, to the south of 4
ACCEPTED MANUSCRIPT Bia Chert hills near the Tabakoroni gold deposit, the formation is overlain by younger cover sequences.
The Syama Formation (Kadiana-Madinani terrain) hosts the Syama gold mine and is composed of rocks type similar to the Sikoro Formation. The Syama Formation is mineralised in the
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hangingwall of the Syama-Bananso Shear Zone and is crosscut by lamprophyre, feldspar
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porphyry and quartz feldspar porphyry dykes. Standing (2007) reported that the geochemistry of
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samples of lamprophyres were higher in Cr content relative to basalt, with high Mg and LREE, and lower Fe; they are notably altered to sericite-ankerite-fuchsite. The feldspar porphyry and quartz feldspar porphyry dykes are typically 2 to 10m wide, and strongly albitic with fuchsite altered ferro-magnesium phenocrysts.
The Ngolopene Group Formation (Kadiolo terrain) in the eastern footwall is essentially
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dominated by the polymictic conglomerate and is not mineralised (Olsen et al., 1992). It
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comprises clasts from the Syama Formation with inclusion of arkose, sandstone, siltstone and rare mafic volcanics clasts.
Gold mineralisation in the A21 deposit (1900 x 90 metres) is associated with quartz veining and
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a thrust contact between greywacke and basalt. The mineralized zones narrow at depth. Gold mineralisation in the Quartz Vein Hill deposit (500 x 90 metres); is associated
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with shallow-dipping quartz veins located in two zones. The mineralisation does not extend to depth, but is generally concentrated in the upper 25m of the profile and above the base of oxidation. Intense silicification and a series of quartz stockwork veins are observed parallel to foliation, together with sulphide mineralisation.
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The host lithologies of the satellite deposits include:
Two styles of gold mineralisation occur at the Chert Ridge open pit (330 x 50 metres). Style 1 is related to narrow shallow-dipping quartz veins that trend N200-N220, while the second style forms breccias veins at the contacts between basalt/lamprophyre and lamprophyre/sedimentary rocks that trend southwest. In general, north and northeast trending thrust faults bound type 2 gold mineralisation. At the BA01 deposit (750 x 80 metres), gold mineralisation is located in basalts above the hanging-wall thrust, adjacent to the contact between sedimentary rocks and basalt. It is also distributed between the footwall thrust and the hanging-wall thrust, and adjacent to the contact between sedimentary rocks and basalt, or within the basalts. Gold mineralisation at the Syama extension (500 x 60 metres) occurs at the contact between volcaniclastic sedimentary rocks and basalt. The mineralized zone forms a sigmoidal jog and trends N010° (sub-vertical dip in the south), to N030° where the dip of the deposit flattens and it thickens, to N010° in the north.
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Gold mineralisation at the Tellem deposit (4200 x 50 metres) is associated with a north-south
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5. Structural Geology
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The Syama gold mine is hosted in a 500m to 2000m thick isoclinal fold that trends northnortheasterly and dips 60-70° west. The Syama ore body is open down plunge to the northeast: a
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diamond drilling program intersected sulphide mineralisation at depths of more than 500m below the surface. Hangingwall and footwall thrusts bound the main ore zone with vertical linking structures joining the two thrusts (Olsen et al., 1992; McCuaig, 2005). Also north-northwest trending subvertical
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structures in the footwall of the main ore zone and late faults influence the geometry of the orebody. Ore shoots are controlled by the influence of isoclinal folding of a preferential lithology, late faulting
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and possible duplexing within the orebody by vertical link faults. (Standing, 2007) The structural architecture of the Tabakoroni deposit is described in several company reports by
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Standing (2005; 2006). In summary, the deposit is hosted in the Tabakoroni Shear Zone is developed in carbonaceous shale and siltstone at the margin of the Syama Formation (Standing, 2006). The shear zone is estimated to be 2km long and 5-25m wide with complex intrafolial folds within the shear zone
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and a penetrative shear cleavage. Displacement is dextral strike-slip with later sinistral reactivation. In the shear zone gold mineralisation is developed as a pyritic and carbonaceous fault gouge, with quartz-carbonate stockwork veins and pyrite disseminations developed in the wall rocks. Quartz lodes are developed along the length of the shear zone with narrow breccia and intense Fe carbonatearsenopyrite-pyrite replacement assemblage of wall rocks. Additionally, megascopic isoclinal folds were recognised by Standing (2005) with moderate fold plunges to the north-northeast and a northeast trending moderately west dipping foliation.
6. Mineralisation Styles
The Syama gold deposit is hosted in the hangingwall of the Syama-Bananso Shear Zone. Ankerite-quartz-sulphide veinlets with selvages of fine-grained sulphide (pyrite) are the primary minerals associated with gold (Olsen et al., 1992). The presence of albite veinlets is a good indicator
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ACCEPTED MANUSCRIPT for gold mineralisation and zones of massive fine-grained silica and pyrite correlate with higher gold grades. Visible gold is rare at Syama. In contrast, gold mineralisation at Tabakoroni is hosted within the Tabakoroni Main Shear Zone, which is a splay shear off the Syama-Bananso Shear Zone. Gold mineralisation is best
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developed in breccia zones and zones of strong quartz, carbonate and graphite veining and
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silicification/carbonate alteration, with pyrite and lesser arsenopyrite as the dominant sulphides.
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However, the nature of gold mineralisation at Tabakoroni is strongly influenced by the chemistry and rheology of the host rock, and there are four main styles of gold mineralisation, namely, (1) silicasulphide lode in carbonaceous fault zones, (2) stylolitic quartz reef in fault zones, (3) quartz-Fecarbonate-sulphide lodes in mafic volcanics, and (4) quartz-sulphide stockworks in silicified
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sedimentary rocks and porphyry dykes.
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7. Ore Body Characteristics
The Syama orebody is characterised by a variety of breccias. Gold mineralisation is associated
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with an intense ankerite-quartz-pyrite stockwork and is hosted in altered basalt, with zones of disseminated pyrite and stockwork veins in greywacke. The orebody dips steeply west within the
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Syama-Bananso Shear Zone, but is occasionally overturned to the east. A wide variety of geophysical data was collected from airborne to ground surveys. Airborne
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magnetic, electromagnetic (EM) and radiometric surveys have been completed with success. The VTEM seem to be the best EM method, resolving well the shear zone and the contact between the Syama and Ngolopene Group formations with conductivity contours. In terms of ground surveys,
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induced polarization (IP) was the main method used to aid in delineation of local anomalies. A number of ground IP surveys were completed with gradient and pole-dipole arrays. Due to the conductivity highs of the central part of the shear, only pole-dipole arrays were adequate to provide a significant chargeability setting with IP. Geophysically, the Syama ore body is non-magnetic, with moderate chargeability (7mV/V to17 mV/V) and resistivity (200Ωm to 850Ωm) values at the contact between mafic units and sediments in the hangingwall of an isoclinal fold. The Syama deposits are very rich in potassium, but poor in thorium and uranium. The Tabakoroni orebody is associated with quartz, carbonate and graphite (stylolites) veins, with pyrite and lesser arsenopyrite. The Tabakoroni ore body is physically non-magnetic with strong chargeability due to the present of sulphites, and strong conductivity because of graphite. Poor in thorium and uranium, the Tabakoroni deposits are very rich in potassium.
8. Alteration
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ACCEPTED MANUSCRIPT In the Syama mine there are wide zones of carbonate alteration with the mineral assemblage of ankerite ± sericite ± chlorite ± albite. In general the rocks are moderately bleached. In highly mineralized zones, albite increases in quality and is often salmon pink in colour due to the addition of fine grained ankerite. Silicification is often intense in narrow zones adjacent to the ore bodies. These
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zones consist of grey to black silica. They are barren and often enclosed by breccia and ankerite
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alteration (Figure 4a). At depth, basalt sequences are haematised. These rocks are known as red eyes
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basalt and are not associated with gold mineralisation (Figure 4b). In the satellite pits, alteration is restricted to narrow enveloping zones of ankerite-sericite.
At Tabakoroni several different metamorphic and hydrothermal alteration styles are developed in the country and host rocks. Most are unrelated to gold mineralisation, or have had some role in
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preparing the rocks both chemically and physically for the auriferous fluids that arrived late in the evolution of the deposit. They include epidote-plagioclase-quartz-carbonate alteration forming wall
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rock selvedges in dolerite and pillowed lavas. Chlorite-calcite alteration occurs throughout the mafic sequences. Sericite-quartz-carbonate-pyrite alteration overprints biotite ± magnetite alteration in feldspar porphyry intrusions. In general, the Syama formation is strongly silicified. Fe-carbonate -
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9. Discussion and Conclusions
The Syama gold deposit and associated satellite pits and the Tabakoroni orebody are typical Palaeoproterozoic orogenic gold deposits that formed during the 2.1 Ga Eburnean orogeny (Béziat et
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al., 2008; Feybesse et al., 2006) along or proximal to shear zones in metamorphosed volcanic and sedimentary units of the Birimian Supergroup sequences (Milési et al., 1991, Tshibubudze et al., 2015) including those at Obuasi (Fougerouse et al., this volume), Essakane (Tshibubudze and Hein, this volume), Gounkoto (Lambert-Smith et al., this volume) and others. With its associated satellite deposits, the Syama gold deposit is hosted in an overturned and structurally layered sequence of basalt, greywacke-argillite-black shales, and a lamprophyre intrusion, and hosts in veins in the Syama-Bananso shear zone. The Tellem deposit is hosted in quartz-veins in a feldspar porphyry dyke. Gold mineralisation at Syama is characterized by a dolomite ± ankerite/silica/pyrite and native gold. Alteration in the mine sequence is asymmetrically zoned from the main lamprophyre intrusive complex in the northern sector of the open pit out toward the hanging-wall. Gold grades are generally found to be highest in areas where pyrite occurs as blebs in the host rocks or within strongly dolomite veined intervals, or where fine disseminated pyrite occurs within the immediate vein selvedge. The Syama ore body is open down plunge to the northeast: a diamond drilling program intersected sulphide mineralisation at depths of more than 500m below the surface.
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ACCEPTED MANUSCRIPT The Tabakoroni orebody is a 2km long and 5-25m wide gold deposit contained in the Tabakoroni Main Shear Zone (a tertiary splay of the Syama-Bananso Shear Zone) and hosted in basalt, andesitic dykes, greywacke, siltstone, chert and graphitic argillite. Gold mineralisation is best developed in breccia zones and zones of strong quartz, carbonate and graphitic veins. The orebody is dominated by
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quartz-carbonate alteration, with pyrite and lesser arsenopyrite as the dominant sulphides.
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Acknowledgements
We thank Resolute Mining Limited SOMISY SA Exploration for its availability to provide the data and permission to publish. The manuscript was greatly improved by two anonymous reviewers.
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ACCEPTED MANUSCRIPT List of Figures
Figure 1: Location map for the Syama and Tabaorni goldfeilds in southeastern Mali. The goldfields are hosted in a north-northeast trending, steeply west-dipping greenstone belt (Bagoe belt) that marks
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a regional boundary between the Kadiana-Madinani terrane to the west and the Kadiolo terrane to the
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east.The Bagoe Belt consists of sedimentary schists and metamorphosed greywackes, with
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subordinate basalt and intermediate volcanics and chert interflow sediments.
Figure 2: Regional geology of the Syama goldfields showing the location of the Syama extension, Alpha, BA01, Tellem, Basso, Quartz Vein Hill, Chert Ridge and A21 deposits. The gold deposits are
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variously hosted at the contact of metasedimentary rocks and basaltic units.
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Figure 3: Regional geology of the Tabakoroni goldfield in the southeast of Mali. The Tabakoroni deposit trends north-northwest and is hosted at the sheared contact between basalt and shale units.
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Figure 4:
A. Photograph of intense silicification zone with grey to black silica and ankerite alteration.
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B. Photograph of the red eyes basalt in diamond drill core.
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C. Photograph of typical Syama ore with grade higher than 3g/t Au.
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Table 1: Summary table for gold deposits in the Syama goldfield including general location, deposit
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status and type and resource information.
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general location, deposit status and type and resource information.
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Table 2: Summary table for the Tabakoroni gold deposits in the Tabakoroni goldfield including
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Commodity of exploitation Longitude Latitude
Syama goldfield includes the Syama mine and the Syama extension, Alpha, BA01, Tellem, Basso, Quartz Vein Hill, Chert Ridge, A21 deposits Gold -6.0703 10.7917
Geographic location
District of Fourou, Kadiolo area, in the region of Sikasso.
Geological location
West African craton; Leo-Man shield; southeastern Mali; between the Kadiana-Madinani terrane to the west and the Kadiolo terrane. The Syama gold mine was owned and operated by BHP Utah in 1990-1996), Randgold Resources in1996-2001 and Resolute Mining Ltd from late 2008. The goldfield is currently owned and operated by the Societe Des Mines de Syama (SOMISY SA) Mali, which is a joint venture partnership between Resolute Mining Limited and the Malian government. Gold in breccia, intense ankerite-quartz-pyrite stockwork, and sheeted veinlet sets in altered basaltic host rocks with zones of disseminated pyrite and weak stockworks in greywacke. The total mineral resource reported for the Syama goldfield is 38.27 Mt including 13.28 Mt at 2.89 g/t Au for the Syama main open deposit for 1237 contained koz Au, 10.69 Mt at 2.34g/t Au for the satellite deposits for 805 contained koz Au, and 14.3 Mt at 2.4g/t Au for the Syama underground deposits for 1103 contained koz Au (Resolute Mining Ltd, December 2014).
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ACCEPTED MANUSCRIPT Tabakoroni gold deposit Gold -6.164 10.520
Geographic location
District of Kadiolo, in the region of Sikasso. Immediately adjacent to the Cote d’Ivoire border, approximately 40km west of Kadiolo and 15km south of Fourou
Geological location
West African craton; Leo-Man shield; southeastern Mali; between the Kadiana-Madinani terrane to the west and the Kadiolo terrane. The Tabakoroni gold mine was owned by Resolute Mining LTD and Etruscan Ressources until 2012, but currently owned by Societe des Mine de Finkolo (SOMIFI SA) Mali, which is a joint venture partnership between Resolute Mining Limited and the Malian government. Mining is currently scheduled to start in 2016. Shear hosted orogenic deposits Total reported mineral resource for the Tabakoroni goldfield is 9.96Mt @ 2.59g/t Au for 830 koz Au (Resolute Mining Ltd, December 2013).
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Deposit name Commodity of exploitation Longitude Latitude
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Table 2
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Sikoro Fm
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Syama Fm
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Syama Fm Ngopelene Fm
Figure 2
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Figure 3
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ACCEPTED MANUSCRIPT Highlights
Syama and Tabakoroni goldfield in West Africa host deposits that are spatially associated with shears.
The deposits occur at the sheared contact of basalt and metasedimentary units.
Gold is primarily mined from an upper oxidised-weathered zone that overlies a sulphide ore
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zone.
The several deposit styles present a number of potential exploration targets.
The deposits represent an example of orogenic shear-hosted gold deposits in west Africa
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