Irena Peytcheva

Zircon crystallization and the lifetimes of ore-forming magmatic- hydrothermal systems

Magmatic-hydrothermal copper ore formation involves multiple pulses of subvolcanic porphyry intrusion, vein opening, and hydrothermal ore deposition. It is driven by larger subjacent magma reservoirs, acting as the source of fluid and ore-forming components. High-precision U-Pb ages of individual zircon crystals from porphyries immediately predating and postdating Cu-Au mineralization at Bingham Canyon (Utah, United States) and Bajo de la Alumbrera (northwestern Argentina) show a significant spread of reliably concordant ages. This demonstrates zircon crystal formation over a protracted period of ∼1 m.y., which is interpreted to record the lifetime of the magma reservoir from which porphyries and ore fluids were extracted. The youngest zircons in all pre-ore and post-ore intrusions overlap within a much shorter time interval of 0.32 m.y. at Bingham Canyon and 0.090 m.y. at Alumbrera; these youngest zircons of each intrusion are interpreted to bracket the maximum duration of porphyry emplacement and ore formation to short periods, consistent with thermal constraints. This study illustrates that age brackets based on individual magmatic zircon grains are geologically more informative than the calculation of means and standard deviations based on apparently normal age distributions in zircon populations.

The Elatsite porphyry copper deposit in the Panagyurishte ore district, Srednogorie zone, Bulgaria: U-Pb zircon geochronology and isotope-geochemical investigations of magmatism and ore genesis

Abstract Single zircons from several porphyry dykes bracketing the time of formation of the Elatsite porphyry Cu-Au deposit (Bulgaria) were dated by high-precision U-Pb isotope analysis, using thermal ionization mass spectrometry (TIMS). On the basis of cross-cutting relationships, and the mineralogy and geochemistry of igneous and altered rocks, five dyke units are distinguished. The earliest porphyry dyke is associated with, and overprinted by, the main stage of ore-related veining and potassic alteration. U-Pb analyses of zircons yield a mean 206Pb/238U age of 92.1 ± 0.3 Ma, interpreted to reflect the time of intrusion. Zircons of the latest ore forming dyke, crosscutting the main stage veins but still associated with minor potassic alteration and veining, give an intrusion age of 91.84 ± 0.3 Ma. Thus, ore mineralization is confined by individually dated igneous events, indicating that the entire time span for the ore-forming magmatism and high temperature hydrothermal activity extended over a maximum duration of 1.1 Ma, but probably much less. Zircon analyses of a late ore dyke cutting all ore veins and hosting pyrite as the only sulphide mineral give a concordant 206Pb/238U age of 91.42 ± 0.15 Ma. Based on a spatial relationships of the magnetite-bornite-chalcopyrite assemblage with coarse-grained hydrothermal biotite and K-feldspar, a Rb-Sr age of 90.55 ± 0.8 Ma is calculated using the two K-rich minerals. This age is interpreted as a closing date for the Rb-Sr system at T ≈ 300 °C consistent with published K-Ar data. Therefore the entire lifespan of the magmatic-hydrothermal system is estimated to have lasted about 1.2 Ma. Soon after, the Cretaceous complex was exposed by erosion, as shown by palaeontologically dated (Turonian; 91–88.5 Ma) sandstones containing fragments of porphyry dykes. Geochemical discrimination ratios suggest a mixed mantle and crustal source of the Cretaceous magma. Isotope analyses of Sr, Nd and Hf confirm the conclusion that all porphyry rocks within and around the Elatsite deposit originate from an enriched mantle source at Cretaceous times, with crustal contamination indicated by moderately radiogenic Pb.

LA-ICP-MS Pb–U dating of young zircons from the Kos–Nisyros volcanic centre, SE Aegean arc

Zircon Pb–U dating has become a key technique for answering many important questions in geosciences. This paper describes a new LA-ICP-MS approach. We show, using previously dated samples of a large quaternary rhyolitic eruption in the Kos–Nisyros volcanic centre (the 161 ka Kos Plateau Tuff), that the precision of our LA-ICP-MS method is as good as via SHRIMP, while ID-TIMS measurements confirm the accuracy. Gradational age distribution over >140 ka of the Kos zircons and the near-absence of inherited cores indicate near-continuous crystallisation in a growing magma reservoir with little input from wall rocks. Previously undated silicic eruptions from Nisyros volcano (Lower Pumice, Nikia Flow, Upper Pumice), which are stratigraphically constrained to have happened after the Kos Plateau Tuff, are dated to be younger than respectively 124 ± 35 ka, 111 ± 42 ka and 70 ± 24 ka. Samples younger than 1 Ma were corrected for initial thorium disequilibrium using a new formula that also accounts for disequilibrium in 230Th decay.

U–Pb dating of CA/non-CA treated zircons obtained by LA-ICP-MS and CA-TIMS techniques: impact for their geological interpretation

Chemical Abrasion Isotope-Dilution Thermal Ionization Mass Spectrometry (CA-ID-TIMS) is known as a high precision technique for resolving lead loss and improving the interpretation of U–Pb zircon age data. Here, we argue that combining CA with the widely applied Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) improves the precision and accuracy of zircon dates, while removing the substantial parts with lead loss, reducing data scatter, and providing meaningful geological interpretations. The samples are magmatic rocks chosen from different geological time periods (one Paleozoic, one Mesozoic and three Cenozoic). All zircon separates are analysed by LA-ICP-MS before and after CA, and age data are compared with CA-ID-TIMS 206Pb/238U dates that are considered as the most accurately obtainable age. All CA-treated zircon crystals show up to 50% less data scatter compared to the non-CA treated zircon grains and thus reduction of the calculated uncertainties is apparent. The obtained wt average LA-ICP-MS 206Pb/238U ages of the CA-treated zircon grains are up to 4–6% higher than those of the non-CA treated crystals, exceeding the analytical uncertainties of the LA-ICP-MS dating technique of 1–2%. The damaged crystal parts, caused by U-decay, with lead loss are removed, so that we can exclude younging from the possible geological scenarios. CA-LA-ICP-MS age data are in good agreement with the CA-ID-TIMS dates and suggest advantages of using CA-LA-ICP-MS in order to define accurate ages. The use of the CA technique for very young zircons (∼0.2 Ma, Kos rhyolitic tuff, Greece) seems optional; as the obtained mean 206Pb/238U ages of non-CA and CA treated zircons coincide within the uncertainty. The negligible time to produce the lattice damage (based on alpha decay or spontaneous fission) makes lead loss less important for age dating and data interpretation of very young zircons (<1 Ma).

Constraints on Variscan and Cimmerian magmatism and metamorphism in the Pontides (Yusufeli–Artvin area), NE Turkey from U–Pb dating and granite geochemistry

Abstract Metamorphic and igneous rocks exposed in NW-vergent thrust sheets and their autocthonous basement in the NE Pontides were dated by the U–Pb method using zircons, supported by geochemical data for granitic rocks. Two meta-sedimentary units (Narlık schist and Karadağ paragneiss) yielded detrital zircon populations of 0.50–0.65 and 0.9–1.1 Ga, suggesting an affinity with NE Africa (part of Gondwana). The youngest concordant zircon age is Ediacaran for the schist but Devonian for the paragneiss, bracketing the paragneiss depositional age as Mid-Devonian to Early Carboniferous. Metamorphic rims of zircon cores in the paragneiss gave Carboniferous ages (345–310 Ma). The zircon rim data indicate two Variscan metamorphic events (334 and 314 Ma) separated by a hiatus (320–325 Ma). Granite emplacement took place during early Carboniferous, Early Jurassic and Late Jurassic phases. The crystallization age of the early Carboniferous granites (c. 325 Ma) corresponds to a hiatus in the zircon age data that could reflect subduction slab break-off. The Variscan granitic rocks intruded a Gondwana-derived continental terrane that was loosely accreted to Eurasia during early–late Carboniferous time but remained isolated from Eurasian-derived terrigenous sediment. In contrast, the Jurassic granitic magmatism relates to later back-arc extension along the southern margin of Eurasia. Supplementary material: Full isotope data (8 tables) are available at www.geolsoc.org.uk/SUP18558

High-precision zircon U/Pb geochronology by ID-TIMS using new 1013 ohm resistors

Accessory mineral U–Pb geochronology by isotope dilution thermal ionization mass spectrometry (ID-TIMS) requires precise and accurate determinations of parent–daughter isotope ratios. The small sample size, particularly with respect to radiogenic Pb (Pb*), requires highly sensitive ion detection systems. Most studies therefore employ either secondary electron multipliers (SEMs) or Daly photomultipliers that provide low background noise and high sensitivity but have a limited linear range and require dynamic peak-hopping. We here evaluate the application of new 1013 ohm resistors in a Faraday cup amplifier feedback loop for the static collection of all Pb isotopes (sample and tracer) with 202,205,206,207,208Pb measured on Faraday cups and 204Pb measured in the axial SEM of a Thermo Scientific™ TRITON™ Plus TIMS instrument. We demonstrate long-term stability of the amplifier gain calibration using a secondary Nd standard and test short- and long-term stability and reproducibility of amplifier baselines. Accurate calibration of static detector arrays is demonstrated by repeated analyses of synthetic and natural U–Pb standards (ET100, Temora-2 and AUS_Z7_5) with variable Pb* (0.551 to 699 pg) and comparison with conventional dynamic ion counting data. Excellent agreement between the two detector systems for all analysed standards suggests that our static measurement routine with 1013 ohm resistors produces accurate and precise U–Pb isotopic data with superior external reproducibility. We anticipate that this new technique will push the frontiers of high-precision U–Pb geochronology and may represent a crucial advancement in the quest towards inter- and intra-laboratory reproducibility at the 0.01% level.

Tectonic, magmatic, and metallogenic evolution of the Late Cretaceous arc in the Carpathian‐Balkan orogen

The Apuseni – Banat – Timok – Srednogorie (ABTS) Late Cretaceous magmatic arc in the Carpathian –Balkan orogen formed on the European margin during closure of the Neotethys Ocean. It was subsequently deformed into a complex orocline by continental collisions. The Cu-Au mineralised arc consists of geologically distinct segments: the Apuseni, Banat, Timok, Panagyurishte and Eastern Srednogorie segments. New U-Pb zircon ages and geochemical whole rock data for the Banat and Apuseni segments are combined with previously published data to reconstruct the original arc geometry and better constrain its tectonic evolution. Trace element and isotopic signatures of the arc magmas indicate a subduction-enriched source in all segments and variable contamination by continental crust. The magmatic arc was active for 25 m.y. (~92-67 Ma). Across-arc age trends of progressively younger ages towards the inferred paleo-trench indicate gradual steepening of the subducting slab away from the upper plate European margin. This leads to asthenospheric corner flow in the overriding plate, which is recorded by decreasing 87Sr/86Sr (0.70577 to 0.70373) and increasing 143Nd/144Nd (0.51234 to 0.51264) ratios over time in some segments. The close spatial relationship between arc magmatism, large-scale shear zones and related strike-slip sedimentary basins in the Timok and Pangyurishte segments indicates mild transtension in these central segments of the restored arc. In contrast, the Eastern Srednogorie segment underwent strong orthogonal intra-arc extension. Segmental distribution of tectonic stress may account for the concentration of rich porphyry Cu deposits in the transtensional segments, where lower-crustal magma storage and fractionation favoured the evolution of volatile-rich magmas.

Megacrystic zircon with planar fractures in miaskite-type nepheline pegmatites formed at high pressures in the lower crust (Ivrea Zone, southern Alps, Switzerland)

Abstract Trace element, Hf, and O isotopic composition and U-Pb geochronological data are reported for zircon megacrysts found in miaskitic (zircon, biotite, plagioclase-bearing) nepheline syenite pegmatites from the Finero complex in the Northeastern part of the Ivrea-Verbano Zone, southern Alps. Zircon from these pegmatites was reported to reach up to 9 cm in length and is characterized by ~100 μm spaced planar fractures in different directions. Small volumes of these highly evolved alkaline melts intruded into the lower crust and were emplaced within amphibole peridotites and gabbros between 212.5 and 190 Ma. A zircon crystal of 1.5 cm size records a systematic core-to-rim younging of 4.5 Ma found by high-precision CA-ID-TIMS 206Pb/238U dating of fragments, and of 8.7 Ma detected by laser ablation ICP-MS spot dating. Volume diffusion at high temperatures was found to be insufficient to explain the observed within-grain scatter in dates, despite the fact that the planar fractures would act as fast diffusion pathways and thus reduce effective diffusion radii to 50 μm. The U-Pb system of zircon is therefore interpreted to reflect an episodic protracted growth history. These high-pressure miaskites probably formed by episodic, low-degree decompression melting of a metasomatically enriched mantle source and subsequent crystallization in the lower crust at volatile saturation with explosive volatile release, evidenced by their brecciated texture in the field and by the occurrence of planar fractures in zircon. They point to the existence of a long-lived period of heat advection in the deep crust by highly differentiated melts from enriched, lithospheric mantle.

Origin and evolution of the Havran Unit, Western Sakarya basement (NW Turkey): new LA-ICP-MS U-Pb dating of the metasedimentary-metagranitic rocks and possible affiliation to Avalonian microcontinent

One of the pre-Jurassic metamorphic basements of the Western Sakarya Zone (NW Turkey) is the Havran Unit which contains metasedimentary (i.e. the Kalabak formation) and metaintrusive rocks of Devonian age. An LA-ICP-MS U-Pb zircon dating study was conducted on 125 zircon grains obtained from a metasedimentary rock to better understand the origin and evolution of the Kalabak formation. Around 107 of 175 spot analyses are concordant with rates 90–110% and the zircon ages vary between 426 ± 5.2 and 3406.9 ± 195.7 Ma. Zircon populations cluster in ranges: 426 ± 5.2–535 ± 13 Ma (8.4%, Palaeozoic), 549 ± 14–999 ± 11 Ma (54.21%, Neoproterozoic), 1012 ± 200–1543 ± 50 Ma (6.54%, Mesoproterozoic), 1736 ± 123–2414 ± 58 Ma (18.7%, Palaeoproterozoic), and 2512 ± 30–3406.9 ± 195.7 Ma (12.15%, Archean). Dominating Neoproterozoic ages notate Cadomian-Avalonian terrains and Mesoproterozoic ages should mainly be related to Avalon terrains. The Karacabey metagranite consists of coarse-grained, equigranular, hypidiomorphic, and rarely foliated monzogranite and granodiorite. It is located to the north of the town of Karacabey in the eastern part of the Biga Peninsula. Main mineral assemblage is quartz, K-feldspar, plagioclase, and biotite. Zircon, apatite, monazite, magnetite, and sphene occur as accessory phases. Aluminum Saturation Index (ASI) values vary between 0.91 and 1.63, indicating the pluton as a metaluminous-peraluminous, I-type granite. The metagranite samples are plotted in the high-K-calcalkaline field on the SiO2 vs. K2O diagram. N-MORB-normalised spidergrams display a profound enrichment in the large-ion lithophile elements (LILE), light rare earth elements (LREE), and depletion in high field strength elements, suggesting that its genesis relates to hydrous melting of a mantle wedge in a subduction zone and/or melting of subduction zone-influenced source areas. Chondrite-normalised rare earth element spidergrams are indicative of the importance of plagioclase and amphibole fractionation. In tectonic discrimination diagrams, all of the metagranite samples fall into the volcanic arc granite and late post-collisional granite fields. U/Pb LA-ICP-MS zircon dating of the metagranite yielded an age of 400.3 ± 1.4 Ma (Early Devonian), interpreted as the age of igneous crystallisation. These age data show that the Karacabey metagranite is a member of the lower to mid Devonian granites, recently described from the Biga Peninsula. In light of these findings, we argue that the Havran Unit was an exotic terrain which possibly was once a part of far-travelled Avalon terrains and was attached to Sakarya Zone.

Geochemical and isotopic evidence for Carboniferous rifting: mafic dykes in the central Sanandaj-Sirjan zone (Dorud-Azna, West Iran)

Abstract In this paper, we present detailed field observations, chronological, geochemical and Sr–Nd isotopic data and discuss the petrogenetic aspects of two types of mafic dykes, of alkaline to subalkaline nature. The alkaline mafic dykes exhibit a cumulate to foliated texture and strike NW–SE, parallel to the main trend of the region. The 40Ar/39Ar amphibole age of 321.32 ± 0.55 Ma from an alkaline mafic dyke is interpreted as an indication of Carboniferous cooling through ca. 550 °C after intrusion of the dyke into the granitic Galeh-Doz orthogneiss and Amphibolite-Metagabbro units, the latter with Early Carboniferous amphibolite facies grade metamorphism and containing the Dare-Hedavand metagabbro with a similar Carboniferous age. The alkaline and subalkaline mafic dykes can be geochemically categorized into those with light REE-enriched patterns [(La/Yb)N = 8.32–9.28] and others with a rather flat REE pattern [(La/Yb)N = 1.16] and with a negative Nb anomaly. Together, the mafic dykes show oceanic island basalt to MORB geochemical signature, respectively. This is consistent, as well, with the (Tb/Yb)PM ratios. The alkaline mafic dykes were formed within an enriched mantle source at depths of ˃ 90 km, generating a suite of alkaline basalts. In comparison, the subalkaline mafic dykes were formed within more depleted mantle source at depths of ˂ 90 km. The subalkaline mafic dyke is characterized by 87Sr/86Sr ratio of 0.706 and positive ɛNd(t) value of + 0.77, whereas 87Sr/86Sr ratio of 0.708 and ɛNd(t) value of + 1.65 of the alkaline mafic dyke, consistent with the derivation from an enriched mantle source. There is no evidence that the mafic dykes were affected by significant crustal contamination during emplacement. Because of the similar age, the generation of magmas of alkaline mafic dykes and of the Dare-Hedavand metagabbro are assumed to reflect the same process of lithospheric or asthenospheric melting. Carboniferous back-arc rifting is the likely geodynamic setting of mafic dyke generation and emplacement. In contrast, the subalkaline mafic sill is likely related to the emplacement of the Jurassic Darijune gabbro.