Summary:
The deposits at the Sabodala-Massawa Complex are classified as orogenic (mesothermal) gold deposits.
At the Sabodala-Massawa Complex, all of the defined mineral resources are within the Sabodala and Massawa exploitation permit areas. The permit areas are transected by two prominent, first order shear zones, the Main Transcurrent Shear Zone (“MTZ”) and the Sabodala-Sofia Zone (“SSZ”) both trending north-northeast. Existing deposits and exploration targets are closely associated with these first order structures.
Within the Sabodala licence, lithologies generally trend north-northeast to northeast with steep dips. The sequence is dominated by mafic volcanics, with intercalated interflow sediment horizons.
On the Massawa licence, the stratigraphy is dominated by a package of volcaniclastic rocks to the west, and a package of greywackes to the east. Bedding typically strikes to the north-northeast with a steep dip of between 75° to 80° toward the west. Several igneous rocks including sills of gabbro, felsic intrusions, and feldspar (and/or quartzfeldspar) porphyries intrude this dominantly clastic sequence.
The mineralisation is often associated with quartz shear veins, extension vein arrays, shear zones, and disseminated sulphides. Mineralisation is typically associated with greenschist metamorphic grade and vein dominated styles. The typical mineralogy of the gold-bearing mineralisation is quartz-carbonate ± albite ± K-feldspar veins with up to 10% (pyrite ± arsenopyrite ± base metals) sulphides. Alteration assemblages are typically dominated by iron-rich carbonate, albite, chlorite, scheelite, fuchsite and tourmaline. High grades are more commonly associated with high strain environments, and with the presence of arsenopyrite. The continuity of the gold grade is associated with alteration style, deformation intensity, and the presence of intrusive contacts. Gold is often hosted in brecciated zones, along with extensional and shear veins. Typically, moderate to strong silica-carbonate alteration and sulphides are present.
Sabodala Lot A Mining Licence
Lithologies generally trend north-northeast to northeast with steep dips, although local variations are apparent. Lower greenschist grade metamorphic assemblages affect lithologies. Mafic volcanic rocks dominate in the sequence and interflow sediment horizons occur locally in the mafic volcanic sequence, with the most prominent being a cherty horizon referred to as mylonite by Painter, 2005. Other interflow units comprise narrow carbonaceous (graphitic) mudstone-siltstone horizons, which locally are often exploited by shear zones.
Principal structures on the Sabodala Mining Licence form a steeply west-northwest dipping, north-northeast trending shear zone network, which has previously been referred to as the Sabodala Shear Zone (SSZ). The north-northeast trending shear zones at Sabodala likely represent first and second order structures of regional scale to first order features such as the Main Transcurrent Shear Zone (MTS), while the northwest trending shear zones may be third order features that accommodate strain between these higher order features (Rhys, 2009).
Two dominant foliations have been recognised: a locally intense (S1) foliation that trends east-west to northeast, and a north-northeast trending steep northwest dipping foliation (S2). The foliation is inhomogeneous and large areas in the massive mafic volcanics, gabbro and felsic intrusions often lack foliation or are only weakly foliated. Field relationships indicate that the gold mineralisation at Sabodala and other deposits in the region is likely to be coeval with later stages of shear zone development.
There are several alteration assemblages spatially associated with gold mineralisation. The principal alteration assemblages are silica-hematite alteration, albite-quartz alteration, and sericite-carbonate-quartz alteration. Alteration style varies with proximity to the mineralisation. The silica-hematite alteration assemblage is commonly fairly restricted with alteration of particular units or pre-existing structures, whereas the albite-quartz alteration assemblage is the most widely distributed alteration style. Overprinting both of these is the proximal sericitecarbonate-quartz alteration assemblage.
Sabodala Lot B Mining Licence
The Sabodala Lot B Mining Licence lies approximately 25 km northeast of the Sabodala processing plant along the transition between the Mako and Diale-Dalema Belts. It was originally part of the Sonkounkou Exploration Permit.
The Sabodala Lot B Mining Licence is dominated by a moderate to steep, southeast dipping, northeast trending sequence of turbiditic sandstone, siltstone and mudstone which is locally overturned and southeast-facing. The sedimentary package hosts veins of undetermined thickness and is intruded or bound by various sill-like intrusions to the east and west, including granodiorite. The mineralisation hosting sediments have been affected by upper greenschist grade metamorphic conditions, defined by aluminosilicate porphyroblasts and biotite, probably in the thermal aureole of the surrounding intrusions. The host rocks contain a slaty foliation which strikes parallel, but which dips variably with respect to bedding.
The main gold mineralisation deposit is Gora deposit with quartz shear veins which trend north-northeast with moderate to steep east-southeast dips. The two principal veins are 50 m to 100 m apart, defining a veining corridor at least 100 m wide. Veining extends for at least 700 m along strike.
Massawa Mining Licence
The most continuous structures identified within the Licence are the Massawa-Delya Structure (MDS, also referred to as the MTS) and the Sofia-Sabodala Structure (SSZ, also referred to as the Sabodala Shear Zone). The MDS is consistently northeast-striking while the SSZ varies from northeast-striking in the south to north-striking in the north. Both structures vary along-strike from lithology-parallel to lithology-oblique and locally have minor splays, stepovers and bends, particularly the SSZ.
Intense brecciation is intersected in both the Central Zone (CZ) and Northern Zone (NZ) of the Massawa deposit and also at Samina. In the CZ, hydrothermal breccias occur at the edges of a gabbro which had earlier intruded the system. Local occurrences of this facies are also seen in the footwall, within the greywacke and related to brittle-ductile shear zones. These rocks are associated with strong brittle-ductile fabrics (fracture stockworks), an intense silica-sericitecarbonate alteration and quartz veins and abundant sulphides hosting high-grade mineralisation.
Two phases of mineralisation are noted at the Massawa deposits:
1) Early disseminated pyrite and arsenopyrite-rich mineralisation in which gold generally occurs as inclusions and solid solution within arsenopyrite and arsenian pyrite. Mineralised Phase 1 arsenopyrite and pyrite are accompanied by minor stibnite, tetrahedrite, galena, and cobaltite, generally as inclusions in pyrite, indicating an overall Phase 1 metal association of As, Au, Pb, Ni, Co, Sb, Cu, and Sn. Metallogenic test work indicates gold associated with Phase 1 mineralisation is refractory.
2) Later gold mineralisation associated with quartz-stibnite veins that locally host free gold. Other Phase 2 sulphide minerals include relatively small amounts of arsenopyrite compared with that present in the Phase 1 assemblage, traces of tetrahedrite, aurostibite, and a variety of secondary Pb, Sb, ±Ag minerals, indicating an overall Phase 2 metal association of Sb, Au, As, Pb, Ag, ±Cu, ±Sn. The disseminated pyrite is commonly found along the SSC and is non-refractory.
The Bakana Corridor hosts gold that is transitional between non-refractory and refractory. The Main Transcurrent Shear (MTS) Zone, which hosts the Massawa, Samina and Delya deposits commonly host refractory styles of mineralisation.