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The character of large titaniferous magnetite crystals in late pegmatitic carbonatite at Okorusu, north central Namibia



The character of large titaniferous magnetite crystals in late pegmatitic carbonatite at Okorusu, north central Namibia



Abstracts with Programs - Geological Society of America 32(7): 85



Field mapping and petrographic study at the Okorusu alkaline igneous rock-carbonatite ring dike complex in North Central Namibia has lead to the recognition of pegmatitic carbonatites and silico-carbonatite that are younger than the main fine-grained carbonatite intrusion and that traverse sodic fenites and cement breccias of sodic fenite. The pegmatitic carbonatites are white, coarse-grained, and composed of calcite, magnetite, apatite, biotite, aegirine-augite, pyrrhotite, and pyrite. Calcite crystals average about 9-12 cm, magnetite crystals are up to 12 cm, and aegirine-augite prisms may be as long as 30 cm in the pegmatitic carbonatites. The term pegmatite is used here in the sense of coarse crystal size only. Most of the pegmatitic carbonatite at Okorusu contains significant amounts of magnetite crystals. The magnetite crystals typically occur at the margins of the pegmatitic carbonatite bodies, where they average about 6 cm across. Magnetite crystals also are disseminated throughout most pegmatitic carbonatite bodies and those crystals average about 3 cm across. Magnetite crystals at the margins of the pegmatitic carbonatite bodies have grown inward from the contacts of the host sodic fenites. The temperature of deposition of the magnetite, as measured by the Fe-Ti-O geothermometer, is 475 degrees C. Although the magnetite has crystallized from a carbonatite magma, the magnetite textures are compatible with crystallization from a hydrothermal fluid. The textures may be explained by a deposition from a fluid that was transition from magmatic to hydrothermal conditions. Magnetite-bearing carbonatites are uncommon rocks and their presence at Okorusu provides an rare opportunity to further characterize those rocks and better understand their character and origin. Other carbonatite containing substantial magnetite include Palabora, South Africa, Kalkfield, Namibia, Jacupiranga, Brazil, and others. Fluorite ores at Okorusu replace the pegmatitic carbonatite leaving the magnetite as rims around the fluorite orebodies.

Accession: 020301462

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