Fluorspar Deposits at Okorusu, North-Central Namibia: Economic Importance, History of Mining and Geology
The fluorspar mine at Okorusu, Namibia operated principally from 1988 to 2014 and was a very significant producer of fluorspar used in the chemical industry. It also produced at times several by-products including ground fenite for local road material and magnetite for the cement industry. Together with South Africa the two countries produced as much as 300,000 tons of fluorspar concentrate per year that amounted to 20% of the western world's fluorspar consumption. During 2015 the Okorusu mine has been under care and maintenance. Okorusu is a carbonatite-related fluorspar mine. Carbonatite is a calcite-rich rock originated in a volcano and derived from the earth's upper mantle. Fluorine-rich fluids developed late in the history of the volcano and fluorine in those fluids combined with calcium in the carbonatites and country-rock marbles to form the fluorspar orebodies. Similar carbonate-related fluorspar ore deposits occur at Amba Dongar in west-central India and at Mato Preto in southern Brazil. Fluorspar deposits at the Kenya Fluorspar mine at Kimwarer in western Kenya and at Hicks Dome in southern Illinois have the geochemical characteristics of carbonatite-related fluorspar deposits. Several separate orebodies have been mined at Okorusu. The main orebodies were A, B, and D. Mining began at the A deposit. Subsequently both A and B orebodies were mined together. Most recently, significant mining of the D and E orebodies occurred along with continued mining at the B orebody and with minor additional mining at the A orebody. The A and D orebodies owe their genesis mainly to the replacement of carbonatite. The B and C deposits were formed partly by carbonatite replacement and partly by replacement of host rock marble. The carbonatite-replacement fluorspar ores are characterized by elevated deleterious phosphorus contents, elevated amounts of trace rare earth elements, goethite pseudomorphs of magnetite, pyroxene, and pyrrhotite crystals and by the local presence of replacement remnants of those carbonatite rocks. Carbonatite-replacement fluorspar orebodies transition into unreplaced carbonatite at their margins. Marble-replacement ores are characterized by elevated amounts of silica banded textures, and gradation into unreplaced marbles at their margins. The recognition that much of the fluorspar ore at Okorusu had formed as a replacement of carbonatite provided an exploration tool for use in the search for additional fluorspar orebodies. Because the carbonatites at Okorusu contain significant quantities of magnetite they have strong magnetic properties and magnetic anomalies are characteristic of orebodies replacing carbonatite because they contain significant amounts of unreplaced magnetite. Unfortunately, magnetic anomalies also occur for carbonatites that have not been reached by the fluorspar-depositing fluids and especially for syenites in the intrusive complex. The temperatures of deposition and salinity of the fluorspar-depositing fluids can be measured using small microscopic fluid inclusions within the fluorite. Fluid inclusion analysis of Okorusu fluorite shows that the main purple and green fluorites crystalized at temperatures mostly from 166 to 144°C, and that later yellow fluorite was deposited from 132 to 128°C. The salinity ranged 5-1.5 wt percent NaCl equivalent. The salinity is significantly lower than for Mississippi Valley-type fluorspar ores elsewhere and the temperatures are mostly lower than for epithermal ore fluids. The Okorusu fluorite mine was the world's largest carbonatite-related fluorspar mine for many years. It serves today as the best example of the characteristics of carbonatiterelated fluorspar ores against which fluorspar ore deposits elsewhere in the world can be compared and evaluated for their possible genetic connection to carbonatite intrusions.
R. D. Hagni, "Fluorspar Deposits at Okorusu, North-Central Namibia: Economic Importance, History of Mining and Geology," Handbook on Africa: Challenges and Issues of the 21st Century, pp. 149-182, Nova Science Publishers, Inc., Jan 2016.
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