Volatile evolution of ore-forming magmas in the Aegean constrained by apatite major and trace element geochemistry

Apatite is a common accessory mineral that incorporates major magmatic volatiles and can therefore record the volatile evolution of magmas. Constraining the timing of volatile saturation and fluid exsolution is crucial for understanding porphyry and epithermal deposit formation, as exsolved hydrothermal fluids control the transport and concentration of metals such as Cu, Au, Ag and Mo. Therefore, the composition of apatite can provide important insights into the ore-forming potential of magmas.

Numerous porphyry and epithermal systems related to subduction zone magmatism occur throughout the eastern Aegean region. Ongoing subduction of the African plate beneath the Eurasian plate since ~30 Ma with associated slab rollback has caused a southward migration of the subduction zone. This migration allows spatial and temporal variations in the volatile evolution of arc magmatism to be traced over a distance of ~300 km, from the early magmatism in Western Thrace (NE Greece) to the currently active South Aegean Volcanic Arc. Apatite chemistry from volcanic and plutonic rocks across several Aegean localities (Western Thrace, Limnos, Chalkidiki, Samos, Aegina and Milos) reveals substantial variations in halogen contents (Cl = 0.06–3.07 wt%, F = 0.22–3.67 wt%), reflecting differences in the volatile evolution of these magmas.

In Western Thrace, strong negative Eu anomalies in apatites from the most primitive samples (whole-rock SiO₂ < 53 wt%) indicate early plagioclase fractionation, consistent with relatively dry initial magma compositions. During differentiation, increasing Sr/Y ratios likely reflect amphibole fractionation, implying an increase in melt H₂O contents. Variations in apatite halogen and S contents further indicate that early fluid saturation, shallow magma intrusion depths and sustained degassing appear to favour the formation of porphyry mineralized systems (e.g., at Maronia). In contrast, later fluid saturation and more explosive eruption mechanisms may inhibit extensive mineralisation, as observed, for example, in nearby shoshonitic volcanic rocks of the Petrota graben. Despite similar whole-rock geochemical signatures at these nearby sites, apatite chemistry records differences in the timing of volatile saturation and fluid exsolution, reflecting distinct conditions of magma storage and ascent.