THE ALKALINE IGNEOUS-CARBONATITE COMPLEX AND FLUORSPAR DEPOSITS AT OKORUSU, NORTH-CENTRAL NAMIBIA

The Okorusu Alkaline Igneous-Carbonatite Complex is located about 50 km north of Otjiwarongo in North-Central Namibia. The complex was intruded during early Cretaceous into late Precambrian Damaran Series metasedimentary rocks. It is nearly circular in plan view with a diameter of about 8 km. Coarse-grained nepheline syenites and foyaites are exposed in low hills near the northern edge of the complex. Early alkalie-rich fluids pervasively fenitized the metasedimentary rocks along the southern margin of the complex forming an east-west ridge of resistant hills that include Okorusu Mountain. The fenites were subsequently brecciated and intruded by several carbonatites, especially medium-grained iron-rich diopside pyroxene carbonitite and very coarse-grained pegmatitic carbonatite. In addition to predominant calcite, the carbonatites contain titaniferous vanadiferous magnetite crystals and diopside crystals as large as one-third meter and hexagonal pyrrhotite crystals as long as one meter.

For the past two decades, Okoruru has been the leading carbonatite-related fluorspar producer in the world. Fluorspar has been mined from five separate ore deposits in open pits A, B, C, D, and E. The deposits formed principally by the replacement of carbonatite as shown by local unreplaced remnants of carbonatite in the fluorspar ores, goethite pseudomorphs in fluorspar ores after carbonatite magnetite, diopside, and pyrrhotite crystals, transitions of the ores into carbonatite, and by elevated phosphorus contents resulting from carbonatite apatite crystals that were incompletely replaced by fluorite. Locally, marbles also are replaced by fluorite to form fluorspar ores that are distinguished from carbonatite-replacement fluorspar ores by their finer grain size and lack of phosphorus contents. Fluid inclusions in the fluorite crystals indicate that the fluorspar ores were deposited from 166 to 128^oC from fluids of low salinity with less than 5% NaCl equivalent.

The genesis of the fluorspar ores is interpreted to have resulted from deeply circulated ground waters that dissolved fluorine from carbonatite at depth. The fluorine in those ore fluids combined with calcium released during the replacement of calcite in carbonatite and marbles at the sites of the fluorspar ore deposition.