GENESIS OF CARBONATITE-RELATED FLUORSPAR DEPOSITS: OKORUSU FLUORSPAR MINES, NORTH-CENTRAL NAMIBIA

Carbonatites in alkaline igneous complexes may form the hosts for important fluorspar deposits. The fluorspar deposits associated with the early Cretaceous alkaline igneous-carbonatite complex in north-central Namibia are the world’s largest carbonatite-related fluorspar deposits. Similar carbonatite-related fluorspar have been mined at Amba Dongar, India and Mato Preto, Brazil. Deep exploration in the Southern Illinois fluorspar district has discovered significant fluorspar reserves that have the mineralogical characteristics of carbonatite-related fluorspar deposits. The very productive Kenya fluorspar deposits along the East African Rift zone in southern Kenya also have the mineralogical characteristics of carbonatite-related fluorspar deposits.

Carbonatites commonly contain significant amounts of fluorine. At the currently active Oldoinyo Lengai carbonatite volcano in Tanzania, for example, the carbonatite contains about 5% fluorine. Fluids can redistribute the fluorine and deposit it as fluorspar that replaces reactive rocks within and in the vicinity of a complex.

At Okorusu fluorspar is mined from four open pits: A, B, C, and D. Exploration has outline additional fluorspar deposits elsewhere in the igneous complex. By-product magnetite from the carbonatite is marketed in the nearby cement industry. Exploration for rare earth minerals has occurred recently in the complex. Synchycite, the rare earth fluorcarbonate, has been identified in polished sections of the Okorusu carbonatite.

Regionally metamorphosed late Precambrian Damaran Series rocks have been intensely metasomatized to fenites in the vicinity of the Okorusu complex. Fenite is the principal rock type associated with the fluorspar mines. It is a fine-grained, hard, dense sodic fenite. The fenite at Okorusu has been mined, crushed and used for road aggregate locally.

The genesis of the Okorusu fluorspar deposits at Okorusu is fluorine contained in carbonatites at depth. Heated ground waters or hydrothermal fluids with temperatures in the range of 160-128oC leached the fluorite and deposited it as fluorite by the replacement of reactive carbonate rocks. A total of 84% of the fluorspar deposits at Okorusu replaced carbonatite in the igneous complex and 15% replace late Precambrian marble associated with the complex.