The Tongon gold deposit, northern Cote d'Ivoire: an example of Palaeoproterozoic Au skarn mineralisation.

The Tongon deposit in northern Côte d’Ivoire is the first reported occurrence of Au skarn mineralisation hosted in Palaeoproterozoic 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. The alteration haloes surrounding the deposit formed during three main stages. Stage 2 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-coloured 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. U-Pb dating of hydrothermal titanite indicates the Au skarn formed between 2 139 ± 21 and 2 128 ± 21 Ma, 20 to 70 m.y. before the major episode of orogenic Au mineralisation in southwest Ghana and western Mali. Dating provides further evidence that Au mineralisation within the Palaeoproterozoic rocks of West Africa craton occurred over several tens of millions of years through a variety of different processes.
The Tongon deposit in northern Côte d’Ivoire is the first reported occurrence of Au skarn mineralisation hosted in Palaeoproterozoic 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. The alteration haloes surrounding the deposit formed during three main stages. Stage 2 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-coloured 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. U-Pb dating of hydrothermal titanite indicates the Au skarn formed between 2 139 ± 21 and 2 128 ± 21 Ma, 20 to 70 m.y. before the major episode of orogenic Au mineralisation in southwest Ghana and western Mali. Dating provides further evidence that Au mineralisation within the Palaeoproterozoic rocks of West Africa craton occurred over several tens of millions of years through a variety of different processes.