THE CHARACTER OF THE FLUORSPAR DEPOSITS AT OKORUSU, NAMIBIA: MINERALOGY, PARAGENSIS, AND ORE CONTROLS

The fluorite ored deposits at Okorusu, Namibia occur within and adjacent to highly fenitized host rocks along the south and southwest margin of the Cretaceous Okorusu alkaline igneous-carbonatite complex that intruded Precambrian Damara Sequence quartzites, marbles, and schists. Sparce exposures of carbonatite probably reflect the presence of a ring-like intrusion of carbonatite at depth beneath the one kilometer-wide area of fenitization. Most of the fluorite orebodies have formed by the replacement of small intrusions of carbonatite into the brittle highly brecciated fenites. Early fluorite introduction formed replacement ores, whereas slightly later fluorite deposition occurred as well-formed crystals deposited in open spaces and those euhedral crystals are highly prized as mineral specimens. Two open pits, A and B, have received extensive mining, and a third open pit, C, is currently being developed. All of the ore pods in the A pit, except for one, formed by the replacement of carbonatite bodies. Similarly, the fluorite ores in the B pit have formed dominantly by the replacement of carbonatite, but fluorite ore replacement of marble has recently been exposed on the lowest two levels, 1650 and 1640, of mining in the B pit. Examination of drill core, together with geophysical evidence, suggests the the fluorite ores in the C pit orebody have been formed by the replacement of marble. The Okorusu fluorspar ores consist dominantly of fluorite. Smaller amounts of calcite, apatite, diopsite, magnetite, ilmenite, ulvöspinel, and pyrrhotite are present as remnants from the replaced carbonatites. Small amounts of aegirine and potash feldspar are present in the fluorite ores as remnants of locally replaced fenite. Synchysite and quartz were introduced into the carbonatites by an early hydrothermal event prior to their replacement by fluorite. Pyrrhotite was altered to secondary magnetite, hematite, marcasite, and pyrite during that hydrothermal event. Pyroxene, magnetite, and the iron sulfides were altered to goethite during the introduction of the oxidizing fluorite-depositing fluids. Minor amounts of post-fluorite stalactitic calcite, quartz, manganite, and goethite were deposited locally in vugs on top of the latest fluorite crystals.