Abstract

The Tongon deposit in northern Côte d’Ivoire is the first reported occurrence of Au skarn mineralization hosted in Paleoproterozoic greenstone rocks of the West African craton. Tongon is an unusually large skarn (3.8 Moz at 2.5 g/t Au) replacing noncarbonate host rocks. The deposit is hosted exclusively in basaltic-andesitic crystal tuffs of the Senoufo greenstone belt. All other rock types in the local stratigraphy, including carbonaceous shales, siltstones, and dacitic crystal and lapilli tuffs, are devoid of mineralization. This observation implies a strong bulk compositional control on the distribution of the skarn. At the district scale, the Tongon hydrothermal system is confined to a corridor of ENE-striking orogen-oblique transfer faults, which link orogen-parallel N-to NNE-striking reverse faults to the north and south. Ferroan A-type granites and lamprophyric dikes also occur along these orogen-oblique faults. To date, no Au skarn mineralization has been discovered outside the corridor of ENE-striking faults elsewhere in the Senoufo belt.

The alteration haloes surrounding the deposit formed during three main stages: stage 1, early/distal potassic (biotite-K-feldspar-actinolite) metasomatism; stage 2, prograde clinopyroxene ± grossularitic garnet skarn; and stage 3, retrograde skarn consisting of epidote-clinozoisite-prehnite-albite and slightly later quartz-sulfide-Au. Garnets contain high Al and Ti, and the skarns have elevated Ni and Cr, reflecting the mafic composition of the host rocks. Proximal to peripheral mineral zonation patterns are defined by decreases in the ratio of garnet to pyroxene, increasing Fe in pyroxene, higher Au concentrations, and an increase in the intensity of retrograde alteration, seen as a change from white-to green-colored skarn. Spatially, gold-rich stage 3 alteration coincides with stage 2 intermediate diopside-rich skarn and distal hedenbergite-ferroactinolite skarn, whereas the proximal garnet-bearing skarn typically has much lower Au grades.

Fluid inclusions studies in the prograde skarn assemblages show primary, high temperature (>400°C), moderately saline (8–14 wt % NaCl equiv) H2O-NaCl-CaCl2 inclusions, with pressures estimated at 2 ± 0.5 kbar. The source of these fluids remains unclear, as plutonic rocks immediately to the west of Tongon intrude the skarn. U-Pb dating of hydrothermal titanite indicates the Au skarn formed between 2139 ± 21 and 2128 ± 21 Ma, 20 to 70 m.y. before the major episode of orogenic Au mineralization in southwest Ghana and western Mali. Dating provides further evidence that Au mineralization within the Paleoproterozoic rocks of West Africa craton occurred over several tens of millions of years through a variety of different processes.

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