ALBERT

Description: 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 ( Hfzircon = +8 to +11.3) and oxygen ( 18 O zircon = +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.

Description: The Shagou vein-type Ag-Pb-Zn deposit in the Xiong’ershan district, southern margin of the North China craton, is hosted within amphibolite facies metamorphic rocks of the Late Archean to early Paleoproterozoic Taihua Group. The Ag-Pb-Zn veins are localized in NE- to NNE-trending brittle faults and typically display symmetrical zoning consisting of siderite, quartz + sphalerite, galena, and quartz + calcite from the margin toward the center of each vein. Ore-related hydrothermal alteration is well developed on both sides of the veins, dominated by silicification, sericitization, chloritization, and carbonatization. Sericite separates extracted from a major Ag-Pb-Zn vein yield a 40 Ar/ 39 Ar plateau age of 140.0 ± 1.0 Ma (1 ) and isochron age of 141.1 ± 1.6 Ma (1 ), indicating that mineralization occurred at the beginning of Early Cretaceous. Field and textural relationships indicate four hydrothermal stages marked by assemblages of quartz + siderite (stage I), quartz + sphalerite + ankerite (stage II), quartz + galena + silver minerals + ankerite (stage III), and quartz + calcite (stage IV), respectively. Silver minerals are abundant in all veins and are composed of, in paragenetic order, argentiferous tetrahedrite, polybasite, jalpaite, argentite, and native silver. These silver minerals commonly occur as replacements of galena, chalcopyrite, and other sulfides, or as fillings of microfractures in sulfides and quartz. Microthermometric measurements of primary fluid inclusions in quartz, carbonates, and sphalerite from various hydrothermal stages indicate that ore minerals were deposited at intermediate temperatures (267°–157°C) from aqueous-carbonic to aqueous fluids with moderate salinities (7.2–15.9 wt % NaCl equiv). Coexisting galena-sphalerite pair yields sulfur isotope equilibrium temperatures of 205° to 267°C, consistent with the overall homogenization temperatures of fluid inclusions. The microthermometric data also indicate that both fluid mixing and fluid-rock interaction were important mechanisms for ore precipitation. Carbonate minerals (siderite, ankerite, calcite) spanning the entire mineralization history have 13 C V-PDB values of –5.2 to –1.4 and 18 O V-SMOW of 10.9 to 15.0, corresponding to calculated values for the ore fluids of –6.5 to –1.8 and 1.4 to 5.4, respectively. 34 S V-CDT values of sulfide minerals (pyrite, sphalerite, galena) range from 1.1 to 5.5, consistent with a deep-seated sulfur source. Galena separates have 206 Pb/ 204 Pb ratios of 17.472 to 17.813, 207 Pb/ 204 Pb ratios of 15.411 to 15.498, and 208 Pb/ 204 Pb ratios of 38.178 to 38.506. The isotope data, together with geological and geochronological evidence, favor a primary metamorphic source for sulfur and other components in the ore fluids. A synthesis of available data suggests that the Shagou deposit is a typical vein-type Ag-Pb-Zn deposit that formed under an extensional geodynamic setting associated with tectonic reactivation of the North China craton during the late Mesozoic, a time period that is manifested by pervasive magmatism, widespread formation of metamorphic core complexes, and development of faulted basins throughout the eastern part of the craton. Metamorphic devolatilization of the Meso-Neoproterozoic marine sedimentary rocks previously subducted beneath the Xiong’ershan district, facilitated by extensive magmatism and elevated heat flow due to lithospheric extension, could have provided large amounts of ore fluids responsible for the Ag-Pb-Zn mineralization. The NE- to NNE-trending faults affiliated with the transcrustal Machaoying fault may have acted as pathways for the upward migration of deep-seated metamorphic fluids. Mixing of the metamorphically derived fluids with meteoric waters ultimately resulted in deposition of the Ag-Pb-Zn veins in brittle extensional structures.

Description: A detailed Re-Os molybdenite, pyrite, and chalcopyrite geochronology at five shear-hosted Au occurrences (Kenge, Mbenge, Porcupine, Konokono, and Dubwana) in the Lupa goldfield, southwestern Tanzania, is reported in this paper. Au occurrences within the Lupa goldfield share many geologic similarities with the orogenic Au deposit type and are situated within a Paleoproterozoic magmatic arc that intruded the Archean Tanzanian cratonic margin. Pyrite ± chalcopyrite ± molybdenite-bearing fault-fill veins and mylonitic shear zones crosscut granitic host rocks and are associated with the highest Au grades. Re-Os sulfide ages are deemed a suitable proxy to constrain the timing of Au based on the occurrence of Au-bearing minerals as inclusions within pyrite and chalcopyrite, whereas Au-bearing minerals filling pyrite fractures may represent a younger and undated metallogenic event. Molybdenite at Kenge occurs as ultrafine disseminations within fault-fill veins (1953 ± 6 Ma; n = 3) that possess nominally older weighted average Re-Os ages than molybdenite hosted by stylolite-like veins (1937 ± 8 Ma; n = 7). Both sample sets are ca. 70 m.y. older than a weighted average Re-Os pyrite age from the mylonitic shear zones at Kenge and Mbenge (1876 ± 10 Ma; n = 13), which contain fault-fill veins and record the timing of mylonitization. Molybdenite at Porcupine occurs as ultrafine disseminations within quartz veins and mylonitized granite samples (1886 ± 6 Ma; n = 4) that are broadly equivalent in age to weighted average Re-Os ages of molybdenite occurring as stylolite-like veins (1873 ± 5 Ma; n = 6) and pyrite within oblique-extension veins (1894 ± 45 Ma; n = 2). Weighted average Re-Os pyrite model ages at Konokono (1880 ± 14 Ma; n = 9) and Dubwana (1905 ± 25 Ma; n = 2) are also consistent with the ca. 1.88 Ga event observed at Kenge, Mbenge, and Porcupine. Gold occurrences in the Lupa goldfield therefore record a protracted hydrothermal history (1.95–1.87 Ga) comprising at least three temporally distinct hydrothermal events (ca. 1.95, 1.94, and 1.88 Ga), which are each represented in detail by a complex vein history that occurred at a time scale less than the resolution of the Re-Os method. The sampling of broadly contemporaneous sulfides from five shear zones suggest that mylonitic shear zones represented an interconnected network of midcrustal permeable fluid conduits at ca. 1.88 Ga that permitted the transportation and deposition of gold. Comparison between Re-Os sulfide and high-precision U-Pb zircon ages for the granitic host rocks provides unequivocal evidence for sulfidation concomitant with magmatism. However, the range of Re-Os ages argues against an intrusion-related deposit model whereby metallogenic fluids are solely derived from an individual intrusion. The regional ca. 1.88 Ga metallogenic event identified as part of this study occurred concurrently with eclogite facies metamorphism during the Ubendian orogenic cycle and provides one of Earth’s earliest temporal links between subduction zone processes and orogenic Au deposit formation during the Paleoproterozoic.

Description: Au mineralization in the western Lupa goldfield, southwestern Tanzania was associated with transpression and reverse sinistral slip along a network of steeply S dipping shear zones with non-Andersonian geometries. Slip was accommodated by: (1) frictional failure and sliding during emplacement of quartz ± Au-bearing veins; and (2) crystal plasticity and fluid-assisted diffusive mass transfer. The Kenge mineral system is situated along a NW-SE-trending shear zone and is characterized by ≤10-m thick, Au-bearing fault-fill veins hosted by well-developed phyllosilicate-rich mylonites. The broadly contemporaneous Porcupine mineral system is situated along an ENE-WSW– to E–W-trending shear zone, which is characterized by narrow, discontinuous mylonitic shear zone within a silicified and nonfoliated granitoid protolith. Au mineralization at Porcupine occurs within steeply dipping fault-fill and subhorizontal extension/oblique-extension veins. Three-dimensional frictional reactivation theory provides a self-consistent explanation for the different vein styles at Kenge and Porcupine and extends the classic fault valve model to the general case of oblique slip along multiple, arbitrarily oriented shear zones. Analysis of the differential stress required for frictional reactivation suggests the following: (1) the Kenge shear zone was intrinsically weaker than the Porcupine shear zone, consistent with the lack of well-developed mylonites at Porcupine; and (2) frictional reactivation of the Kenge shear zone occurred under suprahydrostatic but sublithostatic pore fluid pressures, whereas frictional reactivation of the Porcupine shear zone occurred under near-lithostatic fluid pressures. We hypothesize that near-lithostatic pore fluid pressures relieved effective normal stresses at grain-grain contacts, helping to preserve intragranular and fracture porosity at the Porcupine orebody. As such, these pore spaces may be important microstructural sites for Au mineralization. Low effective normal stresses can also explain the poorly developed phyllosilicate-rich mylonites and limited degree of shear zone weakening at Porcupine.

Description: The Caledonian orogenic belt of northern Britain hosts some significant quartz vein-hosted gold deposits. However, as in orogenic belts worldwide, the relationship between gold mineralization and regional tectonics, magmatism, and metamorphism is a matter of debate. This is primarily due to the absence of precise temporal constraints for the mineralization. Here we report high-precision 40 Ar/ 39 Ar and Re-Os ages for the largest known gold deposit at Curraghinalt (2.7 Moz) in Northern Ireland and use these ages to constrain the regional geologic setting of the gold mineralization and establish a genetic model. The gold resource is contained in a suite of quartz sulfide veins hosted by Neoproterozoic (Dalradian) metasediments, which have been thrust over an Ordovician island arc (Tyrone Igneous Complex). Previous studies recognized two generations of gold sulfide mineralization and we have identified a third in microshears that cut the veins. In the absence of precise geochronological data, mineralization ages from Ordovician to Carboniferous have been proposed. We have dated muscovite ( 40 Ar/ 39 Ar) in quartz vein-hosted clasts of Dalradian wall rock to 459.3 ± 3.4 Ma (all 40 Ar/ 39 Ar and Re-Os ages herein are reported at the 2 confidence level including all sources of uncertainty), an age that we interpret as representing the regional cooling path and which provides a maximum age constraint for all gold mineralization. This is consistent with the quartz veins postdating the end of main-stage deformation in the Grampian event of the Caledonian orogeny (ca. 465 Ma). Molybdenite (Re-Os) and sericite ( 40 Ar/ 39 Ar) from the newly identified gold-bearing microshears (third generation of gold mineralization) yield indistinguishable Re-Os models and 40 Ar/ 39 Ar ages, with a combined age of 455.8 ± 3.0 Ma. The radioisotope ages and field evidence temporally constrain gold mineralization at Curraghinalt to the lower Late Ordovician. Data show that the gold mineralization was emplaced during the Grampian event of the Caledonian orogeny. The ca. 10 Ma maximum possible mineralization interval (462.7–452.8 Ma) for all three episodes of gold emplacement is postpeak metamorphism and main deformation, coinciding with a period of rapid uplift and extensional tectonics following orogenic collapse. While previous studies have suggested the involvement of magmatic fluids in the deposition of the primary gold resource, the absence of magmatism throughout most of the mineralization interval and the nature of the geologic setting suggest that crustal orogenic fluids should also be considered. Overall Curraghinalt displays most of the characteristics of orogenic gold deposits but also some important differences, which may be explained by the geologic setting. The timing of mineralization at Curraghinalt broadly coincides with the shift from compressional to extensional tectonics. The extensional regime, rapid uplift, and a crustal profile comprising metasediments overlying a still hot island arc were ideal for creating large and long-lasting hydrothermal systems deriving heat, metals, and some of the fluids from the underlying arc.

Description: Zircon LA-ICP-MS and CA-TIMS and molybdenite Re-Os geochronology for the El Abra porphyry copper deposit (Chile) document a ~8.6 Ma protracted magmatic-hydrothermal history. Initial magmatic events occurred at ~45 Ma, and continued to ~36 Ma. An ~1.8 m.y. magmatic lull is recorded in the early stages of magmatism, which is characterized by equigranular rocks from the Pajonal suite. Magmatism from ~41 to ~36 Ma, corresponding to the El Abra Granodiorite Complex, is more recurrent, ending in a ~1.4 m.y. period of porphyritic intrusions, which are coeval with Cu-Mo mineralization. Late porphyritic units reveal subtle zircon inheritance likely associated with early magmatic pulses. The young stages of magmatism indicate system rejuvenation based on plagioclase phenocryst compositions, suggesting that the porphyry system is ultimately linked to a less evolved magma. Although there are multiple porphyry and veining events, molybdenite Re-Os ages define a focused mineralization episode from 36.34 to 36.18 Ma. Molybdenite Re-Os ages from nearby related prospects belonging to the El Abra cluster expand the mineralization period in the district up to ~1.0 m.y., suggesting a multistage, long-lived hydrothermal system. Integration of the molybdenite Re-Os dates and those from previous works at the El Abra and Toki clusters, accompanied by available tectonic reconstruction along the Domeyko fault system suggest the development, by late Eocene times, of a single, large (~30 km across) porphyry copper cluster, which was subsequently separated by an offset of ~35 km along the West fault.

Description: 〈span〉〈div〉Abstract〈/div〉Four porphyry Cu-Mo systems were investigated by Re-Os molybdenite geochronology to constrain their timing with respect to the geodynamic and magmatic evolution of the eastern Pontides, Turkey. Molybdenite from the Ispir-Ulutaş deposit yielded an Re-Os age of 131.0 ± 0.7 Ma, which is consistent with Early Cretaceous U-Pb laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) zircon ages of local calc-alkaline intrusions. It demonstrates that porphyry deposits were already formed during Early Cretaceous subduction of the Neotethys along the eastern Pontides, and that they can be correlated with porphyry Cu events in the adjacent Lesser Caucasus. Molybdenite Re-Os ages of 76.0 ± 0.4 and 75.7 ± 0.4 Ma at the Elbeyli prospect and 77.2 ± 1.0 Ma at the Emeksen prospect overlap with U-Pb LA-ICP-MS zircon ages of shoshonitic to high-K calc-alkaline intrusions in the region, which were emplaced during Late Cretaceous Neotethys subduction. A 50.7 ± 0.3 Ma molybdenite Re-Os age at the Güzelyayla deposit confirms porphyry Cu-Mo emplacement coeval with Eocene postcollisional, calc-alkaline adakitic magmatism of the eastern Pontides.An electron microprobe study of molybdenite samples, supplemented by data obtained during Re-Os dating, shows that the Eocene Güzelyayla deposit and the Late Cretaceous Emeksen prospect have the highest Re enrichment. Postcollisional melting of a thickened mafic lower continental crust and melting of a metasomatized lithospheric mantle with little to no interaction with upper crustal rocks may explain the Re enrichment at Güzelyayla and Emeksen, respectively.〈/span〉

Description: Owing to Rhenium (Re) having no known biological role, it is not fully understood how Re is concentrated in oil kerogens. A commonly held assumption is that Re is incorporated into decomposing biomass under reducing conditions. However, living macroalgae also concentrate Re to several orders of magnitude greater than that of seawater. This study uses Fucus vesiculosus to assess Re uptake and its subsequent localization in the biomass. It is demonstrated that the Re abundance varies within the macroalgae and that Re is not located in one specific structure. In F. vesiculosus , the uptake and tolerance of Re was evaluated via tip cultures grown in seawater of different Re(VII) compound concentrations (0–7450 ng g –1 ). A positive correlation is shown between the concentration of Re-doped seawater and the abundance of Re accumulated in the tips. However, significant differences between Re(VII) compounds are observed. Although the specific cell structures where the Re is localized is not known, our findings suggest that Re is not held within chloroplasts or cytoplasmic proteins. In addition, metabolically inactivated F. vesiculosus does not accumulate Re, which indicates that Re uptake is via syn-life bioadsorption/bioaccumulation and that macroalgae may provide a source for Re phytomining and/or bioremediation.