Hervé Rezeau

Temporal and genetic link between incremental pluton assembly and pulsed porphyry Cu-Mo formation in accretionary orogens

Economically important porphyry Cu-Mo deposits (PCDs) are generally hosted by upper-crustal plutons of variable chemical compositions related to distinct geodynamic settings. The absolute timing and duration of pluton assembly and PCD formation are critical to understanding the genetic relationship between these interrelated processes. Here, we present new comprehensive zircon U-Pb and molybdenite Re-Os ages that tightly constrain the timing and duration of pluton assembly and the age of mineralization in one of the largest ore-bearing plutons of the central Tethyan metallogenic belt, the Meghri-Ordubad pluton, southern Armenia and Nakhitchevan, Lesser Caucasus. This composite pluton was incrementally assembled during three compositionally distinct magmatic episodes over ∼30 m.y., comprising Middle Eocene (48.9–43.1 Ma) calc-alkaline subduction-related magmatism lasting 5.8 ± 0.8 m.y., followed by postsubduction Late Eocene–Middle Oligocene (37.8–28.1 Ma) shoshonitic magmatism over 9.7 ± 0.9 m.y., and Late Oligocene–Early Miocene (26.6–21.2 Ma) adakitic magmatism consisting of shoshonitic dikes and high-K calc-alkaline granodioritic magmas emplaced over 5.4 ± 0.4 m.y. Despite the distinct geodynamic settings and magma compositions, each intrusive suite culminated in the formation of variably sized PCDs, including the giant Oligocene Kadjaran porphyry Cu-Mo deposit associated with high-Sr/Y shoshonitic magmas. Complementary in situ zircon hafnium (eHfzircon = +8 to +11.3) and oxygen (δ18Ozircon = +4.6‰ to +6.0‰) isotope data support a mantle-dominated magma source with limited crustal contribution and/or cannibalization of young and juvenile lower-crustal cumulates. We conclude that, independent of geodynamic setting and magma composition, long-lived (5–10 m.y.) incremental mantle-derived magmatism is a prerequisite to form fertile magmatic-hydrothermal systems, and especially giant PCDs.

Trace element diffusion and incorporation in quartz during heating experiments

Heating of quartz crystals in order to study melt and high-temperature fluid inclusions is a common practice to constrain major physical and chemical parameters of magmatic and hydrothermal processes. Diffusion and modification of trace element content in quartz and its hosted melt inclusions have been investigated through step-heating experiments of both matrix-free quartz crystals and quartz crystals associated with sulfides and other minerals using a Linkam TS1500 stage. Magmatic and hydrothermal quartz were successively analyzed after each heating step for Cu, Al, and Ti using electron probe micro-analyzer. After the last heating step, quartz crystals and their hosted melt inclusions were analyzed by laser ablation inductively coupled plasma mass spectrometry and compared to unheated samples. Heated samples reveal modification of Cu, Li, Na, and B contents in quartz and modification of Cu, Li, Ag, and K concentrations in melt inclusions. Our results show that different mechanisms of Cu, Li, and Na incorporation occur in magmatic and hydrothermal quartz. Heated magmatic quartz records only small, up to a few ppm, enrichment in Cu and Na, mostly substituting for Li. By contrast, heated hydrothermal quartz shows enrichment up to several hundreds of ppm in Cu, Li, and Na, which substitute for originally present H. This study reveals that the composition of both quartz and its hosted melt inclusions may be significantly modified upon heating experiments, leading to erroneous quantification of elemental concentrations. In addition, each quartz crystal also becomes significantly enriched in Cu in the sub-surface layer during heating. We propose that sub-surface Cu enrichment is a direct indication of Cu diffusion in quartz externally sourced from both the surrounding sulfides as well as the copper pins belonging to the heating device. Our study shows that the chemical compositions of both heated quartz and its hosted inclusions must be interpreted with great caution to avoid misleading geological interpretations.

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.