Conference Agenda

Ore formation in porphyry copper systems is controlled by degassing of metal-rich volatiles from crystallizing magma reservoirs. Recent geochemical studies indicate that mineralization is associated with super-hydrous magmas, but the causal link between high water contents of the magma and the chemical enrichment potential of the overlying magmatic-hydrothermal system remains unclear. Here, we present new insights into the interplay of physicochemical processes during magma and volatile evolution from homogeneous cooling of convecting low-crystallinity magmas to radial cooling of magma mushes after crystal lock-up. With the help of a coupled numerical model, we show that super-hydrous magmas can undergo different degassing stages with contrasting release rates. In combination with chemical fluid-melt partitioning of metals, this evolution can develop an increased potential to form high-grade deposits in the porphyry or epithermal environments, providing a new view on the ore formation as a specific combination of physical and chemical parameters. The described dynamics naturally evolves into a stage of initial fluid outburst during magma crystallization at the transition to tube-flow mush at intermediate crystallinities that may explain the formation of hydrothermal breccias and/or intense stockwork veining commonly described to predate the main mineralizing stage in porphyry copper deposits. The presented model thus indicates that such events may not require external triggers such as injections of new magma batches, mega-earthquakes or volcano flank collapses, but can rather be triggered internally by self-organization due to the interplay of non-linear magma and fluid properties.

The western Thrace region in NE Greece is part of the western Tethyan metallogenic belt and hosts numerous porphyry deposits enriched in base and precious metals (e.g., Cu, Au, Mo, Re). The related magmatism becomes progressively younger over a distance of ~40 km from Leptokarya (32.4 Ma) and Kassiteres (32.0 Ma) to Maronia (29.6–29.8 Ma) as a result of southwestward migration of the Aegean subduction zone. Here, we combine apatite volatile and trace element data with pyroxene-amphibole thermobarometry and plagioclase hygrometry to constrain the effects of magma storage conditions and volatile evolution on ore formation.

Hydrous and oxidized magmas in western Thrace ascended fast to upper crustal levels (1–5 kbar, 700–1000 °C), lacking evidence for deep crustal storage conditions. Subsequent formation of porphyry deposits occurred during variable volatile evolution in different magmatic centres. At Maronia, early volatile saturation (~55 wt.% SiO₂) enhanced by high H₂O contents (6–10 wt.%) led to porphyry mineralisation. By contrast, in the older magmatic centres of Leptokarya and Kassiteres volatile saturation was reached later (~66 wt.% SiO₂) during the evolution of comparatively less hydrous melts (3–8 wt.% H₂O). We suggest that rapid slab-rollback of the subduction zone led to the formation of several short-lived magmatic centres, each associated with individual porphyry systems. The limited duration of magmatic activity at each centre may have restricted the size of the deposits compared to long-lived porphyry systems, as known in other parts of the Tethyan metallogenic belt (e.g., Iran, Tibet).

Arsenic and antimony are ubiquitous in all kinds of gold deposits and are indicative for Au, Ag, Cu and Zn crustal deposits. Understanding the enrichment and association of these metalloids with critical metals requires better constraints on their extraction from magma during degassing and transport mechanism to shallower settings.

While studies of shallow crustal conditions provide valuable speciation data for As and Sb [1,2], they cannot be easily extrapolated to magmatic settings, and the fluid/melt partitioning studies cover a narrow range of conditions [3,4]. Thus, magmatic speciation and partitioning of As and Sb requires further investigation.

To determine the fluid/melt partition coefficients of As and Sb and their speciation in magmatic systems, we apply a multi-technique approach that combines in situ XAS analysis at BM16/BM30 beamlines of the ESRF with synthetic fluid inclusion (SFI) studies. Here we will discuss the effects of temperature, pressure, and major ligands on the distribution and speciation of As and Sb during magma degassing and their budget in magmatic volatiles.

[1] Testemale et al. (2011). DOI: 10.1127/0935-1221/2011/0023-2104

[2] Olsen et al. (2019). DOI: 10.1021/acsearthspacechem.8b00211

[3] Simon et al. (2007). DOI: 10.1016/j.gca.2007.01.005

[4] Fu et al. (2020). DOI: 10.1016/j.gca.2020.08.029