Geochemical Implications of Biotite and Amphibole in the Thermo- Barometric Conditions and the Petrogenic Relationships of Plutonic Rocks in the Bougouni Area, Southern Mali

Why It Matters

Understanding the pressure‑temperature‑fO₂ conditions of Bougouni plutons clarifies the tectonic setting that generated lithium‑rich pegmatites, guiding future exploration in West Africa’s Birimian belt.

Key Takeaways

• Plutonic rocks formed at 1.7–7 kbar, 780–977 °C.
• High fO₂ (ΔNNO –1.2 to 2.3) and 2.9–8 % H₂O.
• Mineral chemistry shows calc‑alkaline, metaluminous to slightly peraluminous composition.
• Subduction‑related, near‑collisional setting linked to lithium pegmatite formation.

Pulse Analysis

The Birimian terrane of West Africa hosts some of the world’s most promising lithium pegmatites, yet the tectonic processes that concentrated this critical metal remain debated. By focusing on the Bougouni Pegmatite Province, researchers have added a high‑resolution geochemical layer to regional models, linking the lithology of underlying Paleoproterozoic plutonic bodies to the overlying pegmatitic veins. This approach mirrors recent work in the Canadian Shield, where deep‑crustal magmatic signatures have been used to pinpoint lithium‑rich districts, underscoring the global relevance of such petrological investigations.

Geothermobarometric analysis of amphibole, biotite, and feldspar compositions reveals that Bougouni plutons crystallized under pressures of 1.7–7 kbar and temperatures between 780 °C and 977 °C, with elevated oxygen fugacity (ΔNNO –1.2 to 2.3) and water contents of up to 8 wt %. These conditions are characteristic of subduction‑related, near‑collisional magmatism, suggesting that the magmas originated from a hydrated mantle wedge modified by slab-derived fluids. The calc‑alkaline, metaluminous to slightly peraluminous chemistry further supports a type I magma series, which typically exhibits low sulfur affinity—an important factor for preserving lithium in late‑stage pegmatitic fluids.

For mineral explorers, the study offers a practical template: high‑fO₂, water‑rich mafic minerals can serve as proxies for locating lithium‑bearing pegmatites in similar Precambrian shields. The identified pressure‑temperature window aligns with zones of crustal thickening often associated with gold and base‑metal deposits, hinting at multi‑commodity potential. Future work that integrates isotopic dating and fluid inclusion studies could refine the timing of magmatic events, enabling more precise targeting of lithium resources across the broader West African Belt.