Abstract This overview illustrates the processes controlling magma fertility in the formation of porphyry Cu-Au deposits. Magma fertility means all magmatic parameters (e.g., metal and volatile contents, magma and fluid volumes) that might result in higher amounts of metals, which are exsolvable from the magma. Mantle source processes seem to play a fundamental role in the enrichment of primary melts with H 2 O, S, and Cl, all essential ingredients to form porphyry deposits, but do not have a particular role in Cu enrichment. Cu-rich porphyry Cu-Au deposits (i.e., with Au/Cu ~4 X 10 –6 ) are associated with large magmatic volumes accumulated in the lower thick crust of continental arcs during long-lived periods of compression in a synsubduction environment. Mineralization occurs after such accumulations have reached significant volumes and is the result of the transfer of hydrous magmas from deep to shallower crustal levels, probably favored by tectonic stress changes. Au-rich porphyry Cu-Au deposits (i.e., with Au/Cu ~80 X 10 –6 ) are associated with magmatic systems that have evolved at overall shallower crustal levels and for this reason can be found in geodynamic settings characterized by thinner crust (e.g., island arcs with intermediate crust thickness) and/or in variably extensional settings occurring above a slab-metasomatized mantle (postsubduction setting, extensional synsubduction setting). The six largest Au-rich porphyry Cu-Au deposits (>~1,300 tonnes Au) are associated with variably alkaline magmas, which are typical of postsubduction and/or extensional settings, suggesting a petrogenetic control on the Au-rich nature of these deposits.

Abstract The Jurassic to Early Cretaceous magmatic and metallogenic evolution of the Lesser Caucasus and Eastern Pontides segment of the Central Tethyan orogenic belt is still poorly understood. This study reports an investigation about the link between ore formation and magmatic evolution in the major Gedabek porphyry-epithermal mining district, which is located in the Somkheto-Karabagh belt, Azerbaijan. Long-lasting magmatic arc evolution of ~50 m.y., from the Middle Jurassic to the Early Cretaceous, is supported by new U-Pb zircon ages between 164.3 ± 0.7 and 125.1 ± 0.5 Ma. Middle Jurassic magmatic rocks have a dominantly tholeiitic to transitional and primitive island-arc composition, whereas Late Jurassic to Early Cretaceous magmatic rocks are calc-alkaline to shoshonitic and have mature island-arc compositions. Radiogenic isotopes document a higher mantle contribution during petrogenesis of the Late Jurassic-Early Cretaceous magmatic rocks. The combined data document progressive magmatic arc maturation and crustal thickening from the Middle Jurassic to the Early Cretaceous, accompanied by slab roll-back and asthenospheric upwelling. This evolution is shared by other areas of the Somkheto-Karabagh belt and its southern extension in the Kapan block, which also host porphyry-epithermal mining districts. Muscovite and K-feldspar from a porphyry Cu-related potassic alteration assemblage at the Gedabek deposit (overprinted by a younger intermediate- to high-sulfidation epithermal system) have yielded 40Ar/39Ar ages between 140.1 ± 1.0 and 136.3 ± 0.9 Ma. Together with a previous Re-Os molybdenite age, they document formation of the porphyry-epithermal systems at the end of the long magmatic arc maturation of the Gedabek district. Although ore-forming events were diachronous along the arc, the relative timing of magmatic evolution and ore formation at Gedabek is shared by the other Late Jurassic to Early Cretaceous mining districts of the Somkheto-Karabagh belt and the Kapan block. Our study demonstrates that long arc maturation and crustal thickening has taken place along the southern Eurasian margin from a Middle Jurassic nascent arc to an Early Cretaceous evolved arc. This evolution is in line with the essential prerequisites for the genesis of porphyry-epithermal systems in orogenic belts. It also provides evidence that Middle Jurassic to Early Cretaceous magmatic fertile systems and porphyry-epithermal centers have been preserved in this belt.