Petrology, geochemistry, and mineralogy of pyroxene and pegmatitic carbonatite and the associated fluorspar deposit at Okorusu alkaline igneous carbonatite complex, Namibia

Field observations, petrography, electron microprobe analysis (EPMA), geothermometry, and geochemical analyses (ICP-MS and XRF) of the fluorspar deposit associated with the Late Cretaceous alkaline igneous-carbonatite complex at Okorusu (Namibia) have identified two previously unrecognized types of carbonatites, namely pyroxene and pegmatitic carbonatites. These carbonatites exhibit interesting textural characteristics, with the pyroxene carbonatite exhibiting occasional bands of diopside alternating with coarse-grained calcite-rich bands, and pegmatitic carbonatite having the same mineralogy but much coarser texture. Both types of carbonatites are closely spatially associated. Ülvospinel ex-solution lamellae were recognized in magnetite crystals within the pyroxene and pegmatitic carbonatite. Stable isotope determinations for calcite crystals separated from pegmatitic carbonatites, pyroxene carbonatites, and marbles indicate that the carbonatites are primary in origin.; Cathodoluminescence microscopy (CL) and emission spectrography of the carbonatites indicated that the carbonate mineral is almost entirely calcite rather than dolomite, and there are at least two generations of calcite. CL study of fenites, which are metasomatised Precambrian metasedimentary rocks, intruded by carbonatites reveal that fenitization is mostly incipient, marked by the introduction of Fe3+ activated feldspars. Geothermometric determination from EPMA of apatite and biotite in pyroxene carbonatite provided a range of 537–409°C except in one sample which clearly indicated later hydrothermal alteration. The temperature range is similar to a previous titaniferous magnetite-ilmenite temperature determination and is interpreted to represent magmatic crystallization. Trace element patterns of carbonatites are largely consistent with the results of previous studies, although phosphorus values are anomalously high because of apatite.; As one of only two producing carbonatite-related fluorspar deposits in the world, the host rock-ore relationships of the Okorusu fluorspar deposit were identified and characterized. The recognition of certain fluorite ore textures, together with replacement remnants of carbonatite in fluorite ores, has shown that the fluorite orebodies have formed largely by the replacement of the pyroxene and pegmatitic carbonatite. Fluid inclusion geothermometry of purple and green vug-filling shows that fluorite crystallized over a temperature range of 168–144°C (uncorrected for pressure) and from fluids with a salinity of 1.57–4.96 wt% NaCl eq. Beneficiation problems, such as the presence of phosphate, rare earth elements and silica in the final fluorspar concentrate, have resulted from those replacements.