Abstract Volatile saturation influences the physicochemical behavior of magmas and is essential for the sequestration of metals in porphyry copper deposits. Tracking the evolution of volatile components (F, Cl, H2O, S) in arc systems is complicated by their mobility and tendency to rapidly re-equilibrate with late-stage melts. We demonstrate that accurate measurements of volatile concentrations in apatite offer a reliable method for identifying the occurrence of volatile saturation. Fluorine, Cl, S, and calculated OH concentrations in apatite obtained by scanning electron microscope–energy-dispersive X-ray spectroscopy and electron microprobe analysis were used to compare two end-member volcanic systems in the West Luzon Arc (Philippines): Pinatubo (a fluid-saturated analogue for porphyry copper deposits) and Taal (a barren and fluid-undersaturated comparator). Apatites from Pinatubo are S-rich (0.04–0.64 wt%) and show a progressive decrease in XCl/XOH (0.6–0.25) and an increase in XF/XCl (1.5–8) and XF/XOH (0.75–1.2) during crystallization. Modeling indicates that these changes result from efficient partitioning of Cl into a continuously saturated H2O-rich fluid, while high regions of S in apatite reflect episodic flushing by a separate S-rich flux. Little S is evident in apatites from Taal (<300 ppm), which show increasing XCl/XOH and XF/XOH together with constant XF/XCl during crystallization. This cannot be explained using an H2O-saturated model, and instead reflects fluid-undersaturated crystallization and cooling in a reduced and/or S-depleted system. Measured volatiles in apatite therefore effectively discriminate volatile-saturated and undersaturated magmatic systems, providing an important ‘fertility’ filter for porphyry exploration.
Subduction zones and their associated porphyry Cu-Mo deposits (PCDs) in continental settings are often considered in terms of Andean-type orogenies, with magmas generated in a compressional regime via fluid-fluxing of the metasomatized sub-arc mantle.PCDs are also found in regimes where subduction is still active, sluggish, or ceased altogether, but involve significant extension, often as the result of slab rollback, slab tears, and/or slab detachment [1, 2].This tectonic environment would have a limited role for fluid-fluxing; instead, magmas are generated from partial melts of metasomatized lithospheric mantle, melting of hydrous cumulates from earlier subduction, and/or asthenospheric upwelling.Where back-arc post-collisional extension is a major cause of magma generation, magmas range from calc-alkaline to shoshonitic.These magmas are commonly referred to as postsubduction, even though subduction can still be ongoing, if in a minor, limited way.In this post-subduction environment, magmas are an important source of mineralisation in porphyry Cu-Au deposits, which notably host significant amounts of critical elements such as Te, and PGE [2].The Aegean is host to multiple post-subduction magmatic centres and associated PCDs, in tandem with contemporary subduction-related magmatism.It has a well-established but complex tectonic and geodynamic evolution that spans ~35 Myr of magmatism related to subduction roll-back and subsequent extension, making it an excellent natural laboratory to study magma sources and petrogenesis behind post-subduction PCDs.The island of Lesvos in NE Aegean hosts porphyry Cu mineralisation and barren volcanism.This study reports new U-Pb/Hf data from a sample suite from Lesvos, including porphyry intrusions.The results show progression to more crustal signatures from the barren caldera volcanism, through host rock, the porphyry sample, to the late-stage lava dome.These results are contrary to the hypothesis that post-subduction mineralisation in the Aegean is sourced from mantle sources, with "barren" magmatism sourced from crustal sources [3].
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