Jurassic-Early Cretaceous Magmatic Arc Maturation and Ore Formation of the Central Tethyan Metallogenic Belt: Evidence from the Gedabek Mining District, Lesser Caucasus, Azerbaijan

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

Tethyan Subduction to Post-Subduction Magmatic Evolution and Pulsed Porphyry Cu-Mo Deposit Emplacement in the Southernmost Lesser Caucasus

The composite Meghri-Ordubad and Bargushat plutons and associated porphyry Cu-Mo deposits of the Zangezur-Ordubad region, southernmost Lesser Caucasus, were emplaced during a long-lasting, stationary Tertiary magmatic evolution. All magmas have a subduction-related signature. Radiogenic isotopes reveal a mantle-dominated magmatism, with the mantle component becoming more predominant with time. Trace element geochemistry indicates progressive thickening of the crust from the Eocene to the Miocene. Eocene calcalkaline, normal arc (non-adakitic) magmatic activity was related to subduction of the Neotethys beneath Eurasia, and magmatism was dominated by fluid-related enrichment. The Eocenece magmatic stage generated the first pulse of porphyry Cu-Mo deposits. During collision to post-collision evolution, magmatism evolved to shoshonitic during the Oligocene, and calc-alkaline to high-K calc-alkaline and adakitic during the Late Oligocene and Mio-Pliocene. The younger magmatic evolution involved a mantle source metasomatised by a sedimentary component. The second stage of porphyry Cu-Mo deposits is associated to the collision to postcollision Oligo-Miocene adakitic magmatism. Increasing mantle contribution with time was favoured by mantle flow through a slab window/tear opened as a consequence of progressive indentation and deformation of the Eurasian margin by the Gondwana-derived South Armenian block during final closure of the Neotethys

Gold deposits of the Lesser Caucasus: products of successive Mesozoic and Cenozoic geodynamic settings

Gold deposits were formed during two different geodynamic evolution stages of the Lesser Caucasus, starting with Mesozoic arc construction along the Eurasian margin, and followed by Cenozoic subduction-related to post-collision magmatism and tectonics during final Arabia- Eurasia convergence and accretion. Gold deposits are of the low-, and intermediate- to high-sulfidation type, with the latter ones being associated with porphyry deposits. Some deposits could be analogous to transitional VMS-epithermal systems in the Late Cretaceous Bolnisi district and during nascent Jurassic arc evolution along the Eurasian margin, but require further investigation

Long-lived, stationary magmatism and pulsed porphyry systems during Tethyan subduction to post-collision evolution in the southernmost Lesser Caucasus, Armenia and Nakhitchevan

The composite Meghri-Ordubad and Bargushat plutons of the Zangezur-Ordubad region in the southernmost Lesser Caucasus consist of successive Eocene to Pliocene magmatic pulses, and host two stages of porphyry Cu-Mo deposits. New high-precision TIMS U-Pb zircon ages confirm the magmatic sequence recognized by previous Rb-Sr isochron and whole-rock K-Ar dating. A 44.03 ± 0.02 Ma-old granite and a 48.99 ± 0.07 Ma-old granodiorite belong to an initial Eocene magmatic pulse, which is coeval with the first stage of porphyry Cu-Mo formation at Agarak, Hanqasar, Aygedzor and Dastakert. A subsequent Oligocene magmatic pulse was constrained by U-Pb zircon ages at 31.82 ± 0.02 Ma and 33.49 ± 0.02 Ma for a monzonite and a gabbro, and a late Miocene porphyritic granodioritic and granitic pulse yielded ages between 22.46 ± 0.02 Ma and 22.22 ± 0.01 Ma, respectively. The Oligo-Miocene magmatic evolution broadly coincides with the second porphyry-Cu-Mo ore deposit stage, including the major Kadjaran deposit at 26-27 Ma. Primitive mantle-normalized spider diagrams with negative Nb, Ta and Ti anomalies support a subduction-like nature for all Cenozoic magmatic rocks. Eocene magmatic rocks have a normal arc, calc-alkaline to high-K calc-alkaline composition, early Oligocene magmatic rocks a high-K calc-alkaline to shoshonitic composition, and late Oligocene to Mio-Pliocene rocks are adakitic and have a calc-alkaline to high-K calc-alkaline composition. Ragiogenic isotopes reveal a mantle-dominated magmatic source, with the mantle component becoming more predominant during the Neogene. Trace element ratio and concentration patterns (Dy/Yb, Sr/Y, La/Yb, Eu/Eu*, Y contents) correlate with the age of the magmatic rocks. They reveal combined amphibole and plagioclase fractionation during the Eocene and the early Oligocene, and amphibole fractionation in the absence of plagioclase during the late Oligocene and the Mio-Pliocene, consistent with Eocene to Pliocene progressive thickening of the crust or increasing pressure of magma differentiation. Characteristic trace element and isotope systematics (Ba vs. Nb/Y, Th/Yb vs. Ba/La, 206Pb/204Pb vs. Th/Nb, Th/Nb vs. δ18O, REE) indicate that Eocene magmatism was dominated by fluid-mobile components, whereas Oligocene and Mio-Pliocene magmatism was dominated by a depleted mantle, compositionally modified by subducted sediments. A two-stage magmatic and metallogenic evolution is proposed for the Zangezur-Ordubad region. Eocene normal arc, calc-alkaline to high-K calc-alkaline magmatism was coeval with extensive Eocene magmatism in Iran attributed to Neotethys subduction. Eocene subduction resulted in the emplacement of small tonnage porphyry Cu-Mo deposits. Subsequent Oligocene and Miocene high-K calc-alkaline and shoshonitic to adakitic magmatism, and the second porphyry Cu-Mo deposit stage coincided with Arabia-Eurasia collision to post-collision tectonics. Magmatism and ore formation are linked to asthenospheric upwelling along translithospheric, transpressional regional faults between the Gondwana-derived South Armenian block and the Eurasian margin, resulting in decompression melting of lithospheric mantle, metasomatised by sediment components during the previous Eocene subduction event