Trace Elements in Apatite as Genetic Indicators of the Evate Apatite-Magnetite Deposit, NE Mozambique

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.

The role of nanocerianite (CeO2) in the stability of Ce carbonates at low-hydrothermal conditions

“…The results of this study open a new path to understanding the mineralogy and chemistry of several rare earth deposits. Naturally formed cerianite has been found: (1) in association with bastnasite/monazite; [31][32][33][34][35][36] (2) as a secondary mineral, as a result of hydrothermally-altered carbonate/apatite weathering; 2,31,37,38 (3) replacing early crystallised ancylite and uorocarbonates; 40 (4) as an overgrowth on Ce-depleted monazite. 40 Our experiments explain these ndings in natural deposits and are consistent with the decarbonation processes of rare earth carbonates due to the crystallisation of Ce-carbonates under low-hydrothermal conditions (35-205 °C).…”

“…[31][32][33][34][35][36] It is identied as a late-stage secondary mineral. 2,31,37,38 For example, A. Mahmoud and Williams-Jones 31 suggested that cerianite is the product of the weathering of minerals containing light REEs such as monazite and bastnasite in their studied granitic batches from Egypt. Similarly, Zaitsev et al 39 found that cerianite formed during the alteration of apatite as a result of carbonate weathering precipitated from groundwater.…”

Ilaps – python software for data reduction and imaging with LA-ICP-MS

“…To demonstrate the usefulness of Ilaps application a range of samples were analysed, such as bulk analysis of archaeological samples, 18,19 imaging of platinum in tumour tissue of a mouse 20 and other biological samples, [21][22][23] geological samples 24,25 or imaging of the corrosion of Ni-based alloy. 26 In addition to Ilaps being used for many transdisciplinary collaborations, it also offers a way to easily handle data from other instruments, such as laser-induced breakdown spectroscopy (LIBS), 27 just by simple conversion of raw data to t the importing format.…”