THE RECOGNITION OF MARBLE REPLACEMENT IN THE GENESIS OF THE OKORUSU CARBONATITE-RELATED FLUORSPAR DEPOSITS, NORTH-CENTRAL NAMIBIA

The fluorspar deposits at Okorusu, Namibia are related to a late Cretaceous alkaline igneous-carbonatite complex with a diameter of about 6 kilometers. The ore deposits occur along the southwest margin of the complex where late Precambrian Damoran metasedimentary quartzites, marbles, and biotite schists have been partially fenitized. The fluorite deposits most commonly have preferentially replaced the abundant calcite in carbonatite bodies that are intrusive into the fenites. Pyroxene carbonatite, pegmatite carbonatite, and calciocarbonatite are the most abundant of the recently recognized carbonatite rock types that are replaced by the fluorite ores. Goethite pseudomorphs after coarse crystals of altered diopside pyroxene, titaniferous magnetite, and pyrrhotite also are common in the fluorite ores that replaced carbonatites. Recent exposures in both open pits (A and B pits) have revealed that some ore runs or pods have formed by replacement of marble. The main ore run, the A run, in the B pit has formed at the contact between pyroxene carbonatite and marble and has replaced both rock types. A small satellite ore pod at the west end of the A pit also has formed by the replacement of marble. The C orebody, which has received minimal mining to date but that is scheduled for substantial mining, appears to have formed entirely by the replacement of marble. Fluorite ores that have formed by marble replacement differ in several respects from the more common ores that formed by replacement of carbonatite. They are much finer-grained, devoid of deleterious apatite, lack geothite pseudomorphs after pyroxene, magnetite, and pyrrhotite, and they are not characterized by magnetite anomalies. The fluorite-depositing fluids preferentially replaced carbonatite and marble because of the easily soluble character of the calcite. The dissolved calcite also provided much or most of the calcium for the formation of the calcium fluoride, fluorite.