Ankita Mukherjee, PhD

The main point of contention regarding the genesis of magnetite-apatite deposits is what kind of fluids give rise to these rocks. There are multiple competing hypotheses for the formation of IOA deposits, with models that range from purely magmatic to purely hydrothermal. Some IOA rocks host carbonates and apatite, the latter being a home to significant concentrations of REEs. The understanding of how REEs partition between apatite and melt and calcite and melt is essential to comprehending how they develop. Calcite-apatite partitioning coefficients (DCc/Ap) have proven to be helpful to distinguish igneous from fluid-mediated processes in carbonatites. 

Some IOA rocks host carbonates and apatite, the latter being a home to significant concentrations of REEs. The understanding of how REEs partition between apatite and melt and calcite and melt is essential to comprehending how they develop. Calcite-apatite partitioning coefficients (DCc/Ap) have proven to be helpful to distinguish igneous from fluid-mediated processes in carbonatites. 

Calcite-apatite (Cc/Ap) partition coefficients (D) has been calculated for 15 IOA samples distributed globally from localities of Kiruna, Humboldt, Buena Vista, Iron Springs and Pea Ridge. The study conducted a detailed petrographic analysis of calcite-apatite minerals, using BSE, CL imaging, EPMA, and LA-ICP-MS to identify growth and alteration zones, P and Ca concentrations, and trace element content within the mineral pair.

The flat, slightly U-shaped pattern for La through Lu and the absolute values for DCc/Ap in some samples supported a magmatic origin of the carbonate minerals, while the other samples did not, suggesting a secondary recrystallized origin of the melt. The results of the IOAs are also compared with a set of carbonatites from Nooitgedacht, Kaapvaal Craton, South Africa which are representative of a magmatic origin and with Jumilla lamproite, which has evidences of secondary recrystallization of the minerals within them.

This research aims to deepen the understanding of the formation of IOA deposits, potentially resolving the longstanding debate over their origin. Additionally, it sheds light on the role of REE portioning in the minerals in IOAs. The demand for REEs increasing due to their critical role in high-tech industries such as electronics, clean energy, and electric vehicles. IOA deposits offer untapped REE sources. Understanding these processes could provide valuable insights into exploration strategies and economic potential.