Zhongzheng Yuan, PhD

Iron-oxide-apatite (IOA) deposits, also known as Kiruna-type deposits, are not only important sources of iron but can also contain substantial amounts of phosphorus, cobalt, vanadium and rare earth elements, which are critical for both traditional industries and emerging energy technologies (e.g., batteries and electric vehicles). Recently, the significance of molten salts (e.g., carbonate, sulfate, and phosphate) in the genesis of IOA deposits has been highlighted by the ubiquitous presence of iron-rich salt melt inclusions. However, several key questions remain unresolved due to the lack of relevant experimental data, such as the solubility of magnetite as a function of salt melt compositions and trace element partitioning data for magnetite and apatite equilibrated with molten salts. 

To address these issues, a series of high pressure and temperature experiments are being conducting using a piston-cylinder press. Based on the composition of salt melt inclusions reported in the literature, the starting materials include calcium carbonate (CaCO₃), calcium sulfate (CaSO₄), and trace element-doped magnetite, all encapsulated in platinum capsules. Initial experiments are being performed at 1 GPa and 1350 ˚C, focusing on the carbonate-rich side of carbonate-sulfate join, before expanding the composition space to include calcium phosphate and investigating the dependence of magnetite solubility on pressure and temperature. Phosphate bearing experiments are expected to also crystallize apatite, allowing element partitioning between apatite and salt melts to be studied. Run-products to-date consist of a quenched, iron-bearing melt phase and mineral phases, including magnetite and hematite. The textures and chemical compositions of the experimental products are being determined by scanning electron microscopy (SEM), electron probe micro-analysis (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS). Additionally, the trace element signatures of magnetite and apatite in natural samples from IOA deposits will be analyzed and compared with the experimental data. 

The outcome of these experiments aims to address the current knowledge gap regarding molten salts and provide new insights into the genetic model of IOA deposits, where molten salts potentially act as important ore-forming fluids.