The Tongon gold mine and associated mining permit is owned by Société des Mines de Tongon SA (Tongon), in which Barrick has an 89.7% interest, the State of Côte d’lvoire 10%, and 0.3% is held by Ivorian investors.
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Summary:
Tongon was identified as a skarn gold deposit in 2013 and well-developed spatial and temporal mineral zonation patterns, typical of gold skarns, are evident in both the SZ and NZ. Zonation is classified based on garnet: pyroxene ratios, chemistry of the major calc-silicate phases, the skarn colour and grain size, and the degree of retrograde replacement (epidote-clinozosite, prehnite-pumpellyite).
The Tongon mine is located within the north northeasterly to northeasterly trending Birimian Senoufo greenstone belt, which extends from Kani in the south, northward into Burkina Faso for a distance of greater than 200 km. Basement granitoid-gneiss terranes flank the belt to the west and east.
The Senoufo belt comprises a western basaltic sequence, a central andesitic volcanic volcaniclastic package, and an eastern back-arc basin infill sequence of meta-sedimentary units consisting of finegrained siliciclastic rocks including shales, carbonaceous shales, greywackes and quartz-mica schists. Felsic-intermediate calc-alkaline plutons, of various generations and sizes, intrude the volcanicsedimentary packages.
Gold mineralisation occurs in two zones, namely Tongon North and Tongon South. Both zones are characterized by a sequence of volcaniclastic rocks, ranging from coarse grained polymictic breccias, to fine grained tuffs intercalated with carbonaceous sedimentary units. Immediately to the west of the orebodies, a granodiorite-diorite pluton intrudes the volcaniclastic sequence and appears to postdate mineralisation. Late porphyritic and gabbroic dykes are seen crosscutting all lithologies.
The Senoufo belt is largely controlled by a series of northnortheasterly to northeasterly striking thrust faults. A 3 km wide zone of east northeast striking transfer faults cut across the Senoufo belt in the vicinity of Tongon. These transfer faults link thrust faults to the north and south. The Tongon orebodies themselves are located in and adjacent to a kilometre-scale jog in one of these transfer faults. A post mineralisation granodiorite-diorite pluton on the immediate northwestern side of Tongon was also emplaced in this jog.
An earlier phase of intense calc-silicate alteration was followed by a silica ankerite±biotite±chlorite±prehnite alteration event coincident with sulphide and gold mineralisation. The Tongon South orebody is characterized by multiple, structurally controlled, north to northeasterly trending, shallow to moderately northwest dipping lodes, which extend for approximately 1.5 km along strike, and which may represent a thrust duplex. The shears appear to terminate along the contact with the diorite pluton. These structures control both main stages of hydrothermal alteration as well as gold mineralisation. Individual lodes mainly vary in thickness from less than 5 to 20 m.
Mineralisation at both deposits consists of disseminated pyrrhotite, pyrite, arsenopyrite, löllingite and minor chalcopyrite. High-grade zones at Tongon South are associated with elevated tungsten (W) concentrations in the form of scheelite. The majority of the gold at Tongon is associated with arsenopyrite as small inclusions (generally <10 µm) and rarer colloidal and lattice bound gold. Silver contents of native gold grains average approximately 10 wt%.