Luiz Eduardo Faria Coura Filho

The Morro Redondo artisanal mine (garimpo) explores an LCT pegmatite. The operation focuses on semi-precious tourmaline varieties, including shorl and elbaite (rubellite, melanite, and indicolite), and spodumene and beryl as by-products. Even though the deposit is economic for elbaite, can this pegmatite also be economic for lithium? This studied focused on deposit mapping, petrography and geochemistry to define its prospectivity. The pegmatite body is vertical, discordant, and with a sharp contact with its quartz-biotite schist host of the Chapada Acauã Formation, the final phase of the Macaúbas Group. pegmatite is characterized by strong horizontal and vertical mineral zoning, which are presented here. External zone: fine crystals (1-8 cm) of quartz (graphic texture), Ca-rich albite, K-feldspar, biotite, muscovite (or rarer mica sub-species), tourmaline (vermiform and intergrown with quartz), and almandine. Intermediate zone: coarse crystals (~12cm) of albite, tourmaline, and quartz, the latter in intergrowth and overgrowth textures with the tourmalines, although apatite also shows intergrowth with tourmaline sometimes. Accessory beryl can also occur in this zone. Transition zone: coarse crystals (up to 60 cm) of muscovite, quartz, and albite in smaller quantities. Inner zone: megacrystals (up to a meter) of quartz, albite, spodumene, and lepidolite, the latter often replacing spodumene in a distinctive texture. There is also a distinct zone at the south extension of the body, where spodumene concentrations decrease and crystallization of primary lepidolite increases as the main Li host in the pegmatite. The pegmatite extends for 720 m and its inner zone for 397 m. From its mineralogy, zonation, texture, and type of contact, one may conclude that (1) its characteristic mineral zonation tracks the evolution of the body; (2) despite the occurrence of spodumene and lepidolite, the body is not an economically viable Li deposit; (3) possible magmatic mixing or geochemical disturbance occurred during the crystallization of the pegmatite; and (4) body depth and distance from the source were crucial factors for gemological diversity and mineral zoning.