The nature of orthomagmatic, carbonatitic fluids precipitating REE,Sr-rich fluorite: fluid-inclusion evidence from the Okorusu fluorite deposit, Namibia

Abstract

The Okorusu fluorite deposit occurs in Pan-African carbonaceous rocks of the Damara Sequence in northern Namibia. The epigenetic mineralization formed from fluids expelled from the Cretaceous Okorusu carbonatite, which is part of the Damaraland igneous province related to the South Atlantic oceanic opening. The fluorites contain abundant high-field-strength elements (∑REE=80–285 ppm, Sr=1900–2500 ppm, Y=8–50 ppm). They have 87Sr/ 86Sr 127 Ma values of 0.70458–0.70459, which are close to those of the Okorusu carbonatite. Primary and secondary aqueous fluid inclusions in the fluorites contain 3 wt.% NaCl equiv. at most, and homogenization temperatures are around 120 °C. The immediate country rocks of the fluorite mineralization contain a highly varied H 2O–CO 2 fluid-inclusion population trapped in quartz which we, based on their distribution and composition, consider to represent an orthomagmatic fluid responsible for the fluorite mineralization. Cathodoluminescence imaging of the quartz delineates the pathways for carbonatitic fluid movement and suggests a single fluid infiltration event. The fluid was trapped under conditions of extreme heterogeneity to form an inclusion population consisting of all volumetric proportions between an aqueous brine, a CH 4-bearing CO 2 phase and solid phases. Based on energy-dispersive X-ray analysis of opened inclusions and laser Raman spectrometry of unopened inclusions, the following solid phases have been identified: halite, sylvite, nahcolite, K-feldspar, a Ba,Ca oxide/hydroxide, possibly zharchikite AlF(OH) 2, an LREE hydroxide/fluoride (?), galena and cerussite, fluorapatite, cryolite, burbankite, pyrite, fluorite, barite and kyanite (?), amongst others. Crush–leach analysis applied to the inclusion-bearing quartz yielded 87Rb/ 86Sr=0.15059 and 87Sr/ 86Sr=0.70495 for the fluid leachate, 87Rb/ 86Sr=0.65141 and 87Sr/ 86Sr=0.70551 for the residual quartz, and 87Rb/ 86Sr=0.22752 and 87Sr/ 86Sr=0.70501 for the bulk quartz (all values calculated for T=127 Ma). The regression line for these data points represents a mixing line between the Damaran host rock and a Cretaceous fluid. At 87Rb/ 86Sr=0, the regression line yields 87Sr/ 86Sr 127 Ma=0.70478, which is only slightly more radiogenic than recorded for the fluorites and associated Okorusu carbonatite. Crush analysis of the bulk quartz and normalization of the raw data to 100 wt.% at CO 2=20 wt.% and H 2O=20 wt.%, yielded the following chemical composition for the carbonatitic fluid: Na 2O=21.1 wt.%, K 2O=8.0 wt.%, CaO=5.5 wt.%, FeO (total)=3.2 wt.%, F=3.0 wt.%, Cl=10.8 wt.%, ∑REE=3 wt.%, Ba=32,754 ppm, Sr=11,484 ppm, Zr=137 ppm, Y=462 ppm, Th=444, U=15.9 ppm and Pb=1260 ppm. It is concluded that the trapped fluid-inclusion population represents a sample of the carbonatitic fluid with only minor crustal contamination, which reacted with crustal carbonate rocks to precipitate fluorite. The characteristic features of this fluid are its high-alkali and comparably low-Ca contents resulting in (Na+K)/Ca=5.7, a high F content (Cl/F=3.6) and high-field-strength element concentrations in the percentage range. As a consequence, we infer that the low-temperature and low-salinity inclusions in fluorite are unlikely to represent the fluids responsible for the primary mineralization for carbonatite-associated fluorite deposits, as exemplified by the Okorusu samples.