Apatite as record of multi-stage evolution and ore-forming processes in carbonatites and derived regoliths

Apatite is a ubiquitous accessory phase in igneous rocks and a prevalent phase in carbonatites, where it is commonly present throughout their entire evolutionary history. It is typically enriched in light rare earth elements (REE) and can attain high modal proportions, especially in carbonatite-derived lateritic deposits. This study investigates the textural features and apatite geochemistry in a Paleoproterozoic complex in the peri-cratonic terrains of the Reguibat Shield to elucidate the crystallization environment, petrogenetic evolution and potential REE redistribution and phosphate ore grade enhancement. Three distinct apatite populations were identified within the regolith and underlying carbonatite protolith. A primary large group of apatite consists of ovoid to pill-like crystals, reflecting the primary igneous carbonatite system with typical light REE enrichment and high Sr and Ba concentrations. A secondary generation of apatite shows evidence of hydrothermal overprinting and occurs in two distinct mineral associations. Apatites accompanied by carbonates, clinohumite, and serpentine minerals exhibit low Sr, REE, and Th contents with a negative Eu anomaly, indicating early hydrothermal reworking in a reduced environment. Whereas, light REE-depleted apatites associated with monazite, barite, and siderite point to a subsequent hydrothermal stage under more oxidizing conditions. The intimate association between monazite and apatite grains highlights fluid-mediated apatite alteration and subsequent monazite nucleation. The composition of apatite and monazite grains in the regolith mantle mirrors their composition in the underlying carbonatite, reflecting their strong genetic link. Besides, the third apatite group consists of secondary carbonate-bearing apatites cementing primary ovoid grains in the regolith horizons. The secondary apatites are depleted in REE, Sr, and Na with CO₃^2- uptake in the PO₄^3- site, along with F enrichment, which acts as a balancing element. Despite abnormal levels of certain deleterious elements, such as Zn and Cd, these apatites display very low concentrations of As and Th, enhancing their potential for industrial applications. The secondary apatite formation reflects the late-stage supergene processes, which triggered the leaching, recrystallization and lithification of various mineral phases, resulting in the formation of variably sized apatite-rich horizons reaching locally mineable grades.