From skarn formation to supergene enrichment: Fluid evolution and gold remobilisation at Ity and Dahapleu, Western Ivory Coast

The Ity gold deposit (Ivory Coast) records two successive mineralising events: an early skarn-related hypogene stage, overprinted by supergene alteration under tropical weathering conditions ~2 Ga later. Gold at Ity occurs within skarns developed at contacts between carbonate-rich Birimian volcano-sedimentary rocks and felsic intrusions, whereas at the nearby Dahapleu prospect, mineralisation is structurally controlled within shear zones. Gold is present as native gold in pyrite and as Bi–Te–Au–Ag tellurides.

The hypogene Ity system reflects a long-lived thermal anomaly driving fluid circulation and metal deposition through successive favourable events—crustal exhumation, granite intrusion and skarn formation, followed by shear deformation and hydrothermal activity. Fluid inclusion data indicate thus that Ity was formed through a hybrid system: a mesothermal orogenic gold event (>350 °C, CO₂–CH₄ fluids) overprinting an earlier saline skarn stage. At Dahapleu, volatile-rich inclusions (CO₂, CO₂–CH₄, CO₂–N₂) reflect metamorphic fluids circulating through fault-controlled, convective systems. The Ity–Dahapleu system thus exhibits fluid characteristics typical of mesothermal orogenic deposits, albeit at higher temperatures than most Birimian gold systems, but the lower temperature stages yield to the Au-Bi-Te assemblage.

Subsequent tropical weathering and karst development during the Cenozoic generated residual gold enrichment within saprolite and laterite, forming the supergene ore blanket currently mined. Contrasting supergene behaviours between skarn- and diorite-derived ores result from marble dissolution, sulphide oxidation, and collapse brecciation. Kaolinite–goethite assemblages dominate lateritic zones, while smectite typifies saprolitic and marble-derived domains. Gold enrichment is accompanied by Cu, Bi, Mo, and W anomalies, highlighting selective metal mobility during supergene alteration.