Abstract
The Evate deposit is a Neoproterozoic (~590 Ma) magnetite-apatite-carbonate body emplaced parallel to foliation of the Monapo granulite complex in NE Mozambique. A complicated history of the deposit is recorded in apatite textures visualized in cathodoluminescence (CL) images. In spite of different solid and fluid inclusions, mineral assemblages, and the CL textures, electron probe microanalyses indicate relatively consistent apatite compositions corresponding to fluorapatite (XF = 0.51–0.73, XOH = 0.21–0.47, XCl = 0.02–0.06) with limited belovite- and cesanite-type substitutions. Laser ablation inductively coupled plasma mass spectrometric analyses show that apatites from unaltered magnetite-forsterite-spinel ores are depleted in Y, REE, Ba, and Sr compared to apatites from carbonate-anhydrite ores. Hydrothermally overprinted apatites with complex patchy domain CL textures are enriched in Y-REE in greenish-grey zones, Fe-U-Th in blue zones, and Mn-Sr-Ba in brown domains. Observed CL-emissions in the Evate apatites result from very subtle variations in REE, Mn, and U contents controlled by the variability of redox conditions. The decreased Th:U ratio in the hydrothermally overprinted apatites reflects the oxidation and partial removal of U4+ from the apatite structure during the interaction with oxidizing aqueous fluids capable of transporting U6+. Flat, LREE (La-Sm)-enriched chondrite-normalized patterns with Eu/Eu* = 0.7–1.4 and Ce/Ce* = 0.9–1.5, together with concentrations of diagnostic trace elements (Sr, Mn, Y, REE) are consistent with apatites from magmatic carbonatites and phoscorites. This study corroborates that the Evate deposit is a post-collisional orogenic carbonatite genetically linked with mafic plutonic rocks intruding the Monapo granulite complex after granulite-facies metamorphism, and later overprinted by intensive hydrothermalism. The Evate apatite is peculiar in retaining its pristine magmatic signature despite the extensive hydrothermal-metasomatic alteration accompanied by dissolution-reprecipitation.
Highlights
The Evate deposit in NE Mozambique is the largest undeveloped apatite deposit in southeast Africa
Different CL-induced colors and zoning types have been observed in investigated apatites
The major element composition of fluorapatite from Evate is similar to carbonatites worldwide, including phoscorites and sövite-ijolite rocks, with Ca contents scattered within the range from 37.0 to 39.8 wt. % and the median value of 38.8 wt. %
Summary
The Evate deposit in NE Mozambique is the largest undeveloped apatite deposit in southeast Africa. The Evate could be affiliated with the magnetite-group Kiruna-type iron oxide-apatite (IOA) deposits such as those in the Chilean Iron Belt [7] or Mongolia [8]. This link does not seem to be well-supported by increased Ti and Mg contents in the Fe-oxide assemblage [9]. Possible genetic relationship with continental rift-bound carbonatites [10,11,12] is invalidated by the age of magnetite-spinel-olivine-apatite ores coeval with the retrograde stage of the Neoproterozoic granulite-facies metamorphism [13]
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