Petrogenesis of the cenozoic Meghri-Ordubad pluton, southernmost Lesser Caucasus: implications for the formation of large porphyry Cu-Mo(-Au) systems during the Arabia-Eurasia accretionary orogenesis

This thesis composed of four chapters aims at establishing (1) a better general understanding between the magmatism and the formation of Cu-Mo(-Au) ore deposits, (2) a petrogenetic model for the composite Meghri-Ordubad pluton incrementally assembled over 30 m.y. and associated with ore-forming pulses. The multi-disciplinary approach allows to reveal a very complex magmatic-hydrothermal system operating over several tens of million years, which is very attractive to investigate the magmatic evolution in a geodynamic framework linked to the Alpine orogenesis and more specifically related to the closure of the Neotethys during the final convergence of the Arabian toward the Eurasian plate leading to a continental collision followed by a progressive post-collisional relaxation along the Turkish-Iranian plateau. Finally, the link with the formation of ore deposits allows us to assess optimal magmatic, tectonic and hydrothermal conditions leading to the formation of large economic resources

Formation of Archean batholith-hosted gold veins at the Lac Herbin deposit, Val-d'Or district, Canada: Mineralogical and fluid inclusion constraints

The Lac Herbin deposit consists of a network of mineralized, parallel steep-reverse faults within the synvolcanic Bourlamaque granodiorite batholith at Val-d'Or in the Archean Abitibi greenstone belt. There are two related quartz-tourmaline-carbonate fault-fill vein sets in the faults, which consist of subvertical fault-fill veins associated with subhorizontal veins. The paragenetic sequence is characterized by a main vein filling ore stage including quartz, tourmaline, carbonate, and pyrite-hosted gold, chalcopyrite, tellurides, pyrrhotite, and cubanite inclusions. Most of the gold is located in fractures in deformed pyrite and quartz in equilibrium with chalcopyrite and carbonates, with local pyrrhotite, sphalerite, galena, cobaltite, pyrite, or tellurides. Petrography and microthermometry on quartz from the main vein filling ore stage reveal the presence of three unrelated fluid inclusion types: (1) gold-bearing aqueous-carbonic inclusions arranged in three-dimensional intragranular clusters in quartz crystals responsible for the main vein filling stage, (2) barren high-temperature, aqueous, moderately saline inclusions observed in healed fractures, postdating the aqueous-carbonic inclusions, and considered as a remobilizing agent of earlier precipitated gold in late fractures, and (3) barren low-temperature, aqueous, high saline inclusions in healed fractures, similar to the crustal brines reported throughout the Canadian Shield and considered to be unrelated to the gold mineralization. At the Lac Herbin deposit, the aqueous-carbonic inclusions are interpreted to have formed first and to represent the gold-bearing fluid, which were generated contemporaneous with regional greenschist facies metamorphism. In contrast, the high-temperature aqueous fluid dissolved gold from the main vein filling ore stage transported and reprecipitated it in late fractures during a subsequent local thermal event

Mixing dry and wet magmas in the lower crust of a continental arc: new petrological insights from the Bear Valley Intrusive Suite, southern Sierra Nevada, California

Abstract Exposures of arc crustal sections represent rare opportunities to directly evaluate lower crustal magmatic processes and their link to arc products in the middle and upper crust. Within the southernmost Sierra Nevada batholith, the Bear Valley Intrusive Suite (BVIS) exposes a contemporaneously constructed ~ 30 km thick intrusive suite, and thus is ideal for this type of examination. Here we present detailed petrography and mineral major and trace element data for the BVIS. The deepest exposed portion of the BVIS (8–9 kbars) is composed of heterogeneous mafic igneous intrusions of olivine metagabbro, olivine-hornblende orthopyroxenite, olivine-bearing hornblende norite, hornblende norite, hornblende gabbronorite, hornblendite and hornblende gabbro. Shallower crustal intrusions (3–7 kbars) are comparatively homogeneous and dominated by hypersthene-bearing and hypersthene-free tonalites. Using amphibole-plagioclase geothermometry, we show that the mafic lower crustal intrusions crystallized over a wide temperature range from 850 to 1070 °C, highlighting mafic igneous fractionation during isobaric cooling in the lower crust of the Sierran arc, while tonalitic liquids were emplaced at temperatures < 800 °C in the middle and upper crust. Calculated trace element melt compositions in equilibrium with amphibole in lower crustal gabbros are similar to measured tonalite bulk compositions and support the generation of tonalites through fractionation of the observed gabbros. Further, petrography and mineral chemistry suggest multiple distinct crystallization sequences recorded in the different types of gabbro, requiring the presence of coexisting parental melts with contrasting compositions and H2O contents. Using available experimental data, we develop a model by which mixing of variably fractionated dry and wet magmas with similar viscosities followed by crystallization-differentiation in the deep crust to explain the formation of uniform tonalitic melts at shallower crustal levels in the BVIS. This process also explains the unusual predominance of orthopyroxene in the BVIS, and the limited aluminum enrichment compared to experimental differentiation sequences of hydrous basalts. Considering the similar geochemical characteristics of intermediate and felsic igneous rocks from the Sierra Nevada batholith and the Cascades, mixing magmas of variable H2O contents in the lower crust represents a viable petrological process to produce SiO2-rich liquids that may be more common than previously recognized

Magmatic sulphides in Quaternary Ecuadorian arc magmas

New petrographic and geochemical data on magmatic sulphide inclusions (MSIs) are presented and discussed for 15 Quaternary volcanic centers of the Ecuadorian frontal, main and back volcanic arc. MSIs occur mostly in Fe–Ti oxides (magnetite and/or magnetite–ilmenite pair) and to a lesser extent in silicate minerals (amphibole, plagioclase, and pyroxene). MSIs are present in all volcanic centers ranging in composition from basalt to dacite (SiO2 = 50–67 wt.%), indicating that sulphide saturation occurs at various stages of magmatic evolution and independently from the volcano location along the volcanic arc. MSIs also occur in dioritic, gabbroic and hornblenditic magmatic enclaves of the volcanic rocks. MSIs display variable sizes (1–30 μm) and shapes (globular, ellipsoidal, angular, irregular) and occur mostly as polymineralic inclusions composed of Fe-rich and Cu-poor (pyrrhotite) and Cu-rich (mostly chalcopyrite) phases. Aerial sulphide relative abundances range from 0.3 to 7 ppm in volcanic host rocks and from 13 to 24 ppm in magmatic enclaves. Electron microprobe anal- yses of MSIs indicate maximum metal contents of Cu = 65.7 wt.%, Fe = 65.2 wt.%, Ni = 10.1 wt.% for those hosted in the volcanic rocks and of Cu = 57.7 wt.%, Fe = 60.9 wt.%, Ni = 5.1 wt.%, for those hosted in magmatic enclaves. Relationships of the sulphide chemistry to the host whole rock chemistry show that with magmatic differentia- tion (e.g., increasing SiO2) the Cu and Ni content of sulphides decrease whereas the Fe and S contents increase. The opposite behavior is observed with the increase of Cu in the whole rock, because the latter is anti- correlated with the SiO2 whole rock content. Laser ablation ICP-MS analyses of MSIs returned maximum values of PGEs and noble metals of Pd = 30 ppm, Rh = 8.1 ppm, Ag = 92.8 ppm and Au = 0.6 ppm and Pd = 43 ppm, Rh = 22.6 ppm, Ag = 89 ppm and Au = 1 ppm for those hosted in volcanic rocks and magmatic enclaves, respectively. These PGE contents display a different range of values with respect to those in pre- viously investigated magmatic sulphides. MSIs that are Cu- and PGE/Cu-rich are found in less evolved rocks (i.e., lower SiO2 contents) that also display a lower amount of sulphide inclusions. Cu-rich sulphide phases (chalcopyrite ± bornite) are mostly hosted by magnetite, whereas PGE-rich ones consist of a Cu-poor phase (pyrrhotite) hosted by plagioclase. However, no systematic changes in the chemistry of the host silicate mineral are observed in coincidence with the occurrence of MSIs. We use the results of our study to draw some implications on Cu (and other chalcophile elements) behavior during arc magmatic processes potentially associated with the formation of porphyry-type deposits

Magmatic record along the Eastern Pontides-Lesser Caucasus orogenic belt: Implications for magma petrogenesis, regional tectonics and metallogeny

The present-day configuration of the Lesser Caucasus and Eastern Pontides regions has been shaped by multiple episodes of arc magmatism, opening and closure of oceanic domains, obductions and collisions from the Paleozoic (~350–310 Ma) to Cenozoic (~50–20 Ma). This fascinating magmatic and tectonic evolution along these two adjacent regions was also associated with an exceptional metal endowment in the form of polymetallic deposits, volcanogenic massive sulfide deposits, porphyry Cu-Mo deposits and epithermal Cu-Au deposits in the Mesozoic and Cenozoic. Several decades of research have significantly improved our understanding of these two regions, yet conflicting models exist for the tectono-magmatic evolution of this orogenic belt. Here we present new zircon Hf isotopic data (n = 575) for magmatic rocks formed in the Lesser Caucasus at Kapan (136–16 Ma, 51 Ma), Gedabek (138–158 Ma), Tsaghkuniats (530 Ma, 320 Ma, 136–153 Ma), Alaverdi (146–164 Ma) and Bolnisi (82–85 Ma, 52 Ma) districts, and in the Eastern Pontides at Elbeyli (74 Ma), Emeksen (78–79 Ma), Güzelyayla (80 Ma) and Ulutas-Ispir (128–129 Ma) districts. The new results are combined with a comprehensive compilation of igneous ages (n = 4560), zircon Hf isotopic compositions (n = 1466), bulk rock major and trace element (n = 1304–1647) and Sr and Nd isotopic compositions (n = 1079) for magmatic rocks formed along the Eastern Pontides and the Lesser Caucasus. These complementary datasets highlight three well-defined cycles of arc magmatism during the Mesozoic and Cenozoic between the two regions, which is characterized by comparable temporal and geochemical trends in the Middle Jurassic-Early Cretaceous and the Late Cretaceous cycles, whereas these trends are no longer similar in the Eocene-Miocene cycle. The new data also reveal that Carboniferous basement rocks in the Tsaghkuniats area have overlapping ages and zircon Hf data with Carboniferous basement in the northernmost Lesser Caucasus and Eastern Pontides. This may suggest the presence of a composite basement in the Lesser Caucacus made of southern Eurasian margin slivers rifted prior to Early Mesozoic and Gondwana-derived South Armenian block accreted to the southern Eurasian margin during the Late Mesozoic-Early Cenozoic. This regional synthesis of the geochemistry of arc magmatism across the Lesser Caucasus and Eastern Pontides over time offers new additional petrological constraints and allows to support our model for the geodynamic evolution of the region. Building on this regional geodynamic framework, the spatial and temporal distribution of ore deposits provides important pieces of information regarding key factors, such as long-lived arc magmatism and switch of tectonic stress regime, controlling arc magma fertility, which ultimately offers new perspectives for exploration opportunities in the middle section of the Tethyan metallogenic belt

The World-Class Kadjaran Mo-Cu-Porphyry Deposit, Southern Armenia, Lesser Caucasus: Structural Controls, Mineral Paragenesis and Fluid Evolution

This paper investigates the structural controls of the Kadjaran deposit, the evolution of hydrothermal fluids with variable salinity, temperature and pressure, based on a detailed field mapping, petrographic and fluid inclusion microthermometry. Detailed structural mapping and interpretation of stereonets compiling ore-bearing fractures within stockwork and vein zones allow us to define the main ore-controlling structures and main orientations of veins of the different mineralization stages of the Kadjaran deposit. The ore-enriched areas are zones sub-parallel to eastwest-oriented fractures. In particular, they are located in areas where they crosscut steeplydipping (70-85°) eastwest-, northsouth- and northeastoriented fractures. Five types of fluid inclusions were distinguished according to their nature, bubble size, and daughter mineral content. Cathodoluminescence (SEMCL) images reveal four generations of quartz. Molybdenite is associated with a dark luminescent quartz generation (Q2), which contains typical brine and aqueous-carbonic fluid inclusions, with some of them coexisting locally as boiling assemblages. Final homogenization of all brine inclusions occurs by halite dissolution. Dissolution of halite between 356±8 and 422±10°C in brine (B1) inclusions of quartz - molybdenite veins indicates salinities between about 42.6 to 50.7 wt % NaCl equiv. The vapor bubbles homogenized between 278±4°C and 327±5°C

Zircon Petrochronology of the Meghri-Ordubad Pluton, Lesser Caucasus: Fingerprinting Igneous Processes and Implications for the Exploration of Porphyry Cu-Mo Deposits

The trace element composition of zircon, especially in tandem with U-Pb geochronology, has become a powerful tool for tracing magmatic processes associated with the formation of porphyry copper deposits. However, the use of the redox-sensitive Eu and Ce anomalies as a potential mineral exploration proxy is controversial. This study presents a comprehensive, temporally constrained data set of zircon trace element compositions (n = 645) for three compositionally distinct magmatic series identified in the Meghri-Ordubad pluton, southernmost Lesser Caucasus. The 30 million years of Cenozoic magmatism in the Meghri-Ordubad pluton are associated with several ore-forming pulses leading to the formation of porphyry copper deposits and epithermal-style mineralization. Our zircon geochemical data constrain the thermal and chemical evolution of this complex intrusive suite and allow an evaluation of the usefulness of zircon as a mineral exploration proxy for porphyry copper deposits. Our results combined with Rayleigh fractionation modeling indicate that the trace element composition of zircon (Th/U, Hf, Ti, YbN/DyN, Eu anomalies) is influenced by the composition and the water concentration of the parental magma, as well as by co-crystallizing titanite and apatite. In contrast, the variations of Ce anomalies remain difficult to explain by magmatic processes and could rather be ascribed to relative fluctuations of the redox conditions. In the Meghri-Ordubad pluton, we do not observe any systematic patterns between the trace element composition in zircons and the different ore-forming pulses. This questions the reliability of using the trace element composition in zircon as an exploration mineral proxy, and it rather emphasizes that a good knowledge of the entire magmatic evolution of a metallogenic province is required

30 Myr of Cenozoic magmatism along the Tethyan margin during Arabia–Eurasia accretionary orogenesis (Meghri–Ordubad pluton, southernmost Lesser Caucasus)

Three magmatic series of substantially different ages and compositions were successively emplaced to form the composite Meghri-Ordubab pluton (MOP), southernmost Lesser Caucasus. The protracted incremental assembly during 30 Myr, from Middle Eocene to Early Miocene, renders this location particularly suitable to characterize the petrogenetic evolution of Cenozoic magmatism within the realm of the final stage of the Neotethyan subduction. Based on whole-rock geochemistry, two main transitions tightly constrained in time are recognized. The first transition from Middle Eocene medium-K calc-alkaline to Late Eocene - Middle Oligocene shoshonitic magmatism corresponds to a marked increase in LREE and MREE and more juvenile 87Sr/86Sr and 143Nd/144Nd ratios. The second transition to Late Oligocene - Early Miocene high-K calc-alkaline «adakite-like» magmatism is coeval to a marked increase in Mg#, and Ni and Cr contents together with a depletion in HREE. Although the three differentiation series are derived from lower to mid-crustal hydrous magma fractionation, temporal variations of the magmatic source conditions are required to explain the contrasting chemistry of the parental magmas over time. Medium-K calc-alkaline parental magmas were generated by garnet lherzolite high degree of partial melting (~15%). The shoshonitic and adakitic magmatic series represent magmas produced by garnet lherzolite low degree (1-5%) partial melting, but higher amount of residual garnet is required for the adakitic magmatism. The timing of the two main geochemical transitions in the MOP is correlated with a progressive evolution from compressional to extensional stress regime linked to (1) the final stage of the Neotethyan subduction and by the Arabia-Eurasia continental collision during the Eocene-Oligocene, and (2) the transition toward a post-collisional magmatism combined with a switch toward extensional tectonics during the Late Oligocene – Early Miocene

Pulsed Porphyry Cu-Mo Formation during Protracted Pluton Emplacement in Southern Armenia, Lesser Caucasus: the Potential Role of Crustal Melting for Ore Recycling

The southern part of the Lesser Caucasus records a long lasting geological and metallogenic evolution from the Jurassic to the Cenozoic. The composite Meghri-Ordubad pluton, which hosts porphyry Cu-Mo deposits, includes Eocene to Miocene intrusive rocks over an area of 1400 km2. Based on LA-ICP-MS UPb zircon dating of thirty samples, we document three main magmatic events: (1) mid-Eocene from 46 to 43 Ma, (2) Eocene-Oligocene from 38 to 31 Ma, and (3) Oligocene-early Miocene from 27 to 21 Ma. Based on Re-Os molybdenite dating, each magmatic event is associated with porphyry Cu-Mo (or Mo-Cu) formation, including the giant Kadjaran porphyry Mo-Cu deposit. We propose an evolution from subduction-related calcalkaline magmas to collisional alkaline magmas followed by bi-modal magmatism over time. Preliminary Nd and Sr isotopic compositions of the magmatic rocks broadly indicate an increasing mantle-derived component with younger magmatic events. Hf isotopic compositions of zircons range from +8 to +11 for Jurassic, Cretaceous, Eocene, Oligocene and Miocene rocks. Based on Hf isotopic composition mixing models involving a depleted mantle and Jurassic to Miocene crustal rocks, we propose that an important volume of crustal rocks can be re-melted. We suggest an ore recycling process by crustal melting leading to the metal enrichment of younger magmas, especially during ore-forming protracted pluton emplacement