Temporal controls on fluid–rock interaction in the evolution of the Abovyan iron oxide–apatite deposit, Armenia

Iron oxide–apatite (IOA) deposits are globally significant sources of Fe, P, and Rare Earth Elements (REEs). Although input of hydrothermal processes into their origin is acknowledged, direct temporal constraints on the duration and style of their hydrothermal evolution remain scarce. The Abovyan deposit in central Armenia, hosted by late Miocene andesites, is one of the youngest known IOA systems worldwide and the only example in the Lesser Caucasus, providing a rare opportunity to address this problem. We integrate in-situ U–Pb dating of apatite and monazite with electron microprobe and LA-ICP-MS analyses, Raman spectroscopy and textural observations on samples from a single continuous drill core (67–337 m). A total of 162 analyses define a narrow age window between 5.58 ± 0.08 and 4.98 ± 0.07 Ma. Pristine F-rich apatite from the top and base of the section yields the oldest ages, consistent with magmatic crystallization. Monazite confined to Cl-bearing apatite domains gives an intermediate age of 5.26 ± 0.06 Ma, demonstrating new hydrothermal crystallization. Younger ages in Cl-rich, inclusion-rich apatite of the upper–middle interval extend down to 4.98 ± 0.07 Ma and provide direct evidence that hydrothermal activity continued after monazite formation. The depth-dependent spread of apatite ages is a record of staged fluid–rock interaction. Incomplete replacement of early F-rich apatite by later Cl-rich generations is reflected in younger apparent (mixed) ages, which document a minimum duration of hydrothermal activity in the upper-middle part of the deposit for at least several hundred thousand years after magmatic crystallization. Altered apatite domains are depleted in REE and enriched in Cl, consistent with coupled dissolution–reprecipitation. These features closely parallel fluid–rock interaction in IOA systems such as Kiruna and El Laco, but Abovyan uniquely provides temporal control to demonstrate that such processes persisted over protracted timescales in a roof-focused, compartmentalized regime.