Neoarchean Mantle-derived Magmatism within the Repulse Bay Block, Melville Peninsula, Nunavut: Implications for Archean Crustal Extraction and Cratonization

SUMMARYThe Repulse Bay block (RBb) of the southern Melville Peninsula, Nunavut, lies within the Rae craton and exposes a large (50,000 km2) area of middle to lower crust. The block is composed of ca. 2.86 Ga and 2.73–2.71 Ga tonalite-trondhjemite-granodiorite (TTG) and granitic gneiss that was derived from an older 3.25 and 3.10 Ga crustal substrate. This period of crustal generation was followed by the emplacement of ca. 2.69–2.66 Ga enderbite, charnockite, and granitoid intrusions with entrained websterite xenoliths. These voluminous batholith-scale bodies (dehydrated and hydrated intrusions), and the associated websterite xenoliths, have similar whole rock geochemical properties, including fractionated light rare earth element (LREE)–heavy (H)REE whole rock patterns and negative Nb, Ti, and Ta anomalies. Dehydrated intrusions and websterite xenoliths also contain similar mineralogy (two pyroxene, biotite, interstitial amphibole) and similar pyroxene trace element compositions. Based on geochemical and mineralogical properties, the two lithologies are interpreted to be related by fractional crystallization, and to be the product of a magmatic cumulate processes. Reworking of the crust in a ca. 2.72 Ga subduction zone setting was followed by ca. 2.69 Ga upwelling of the asthenospheric mantle and the intrusion of massif-type granitoid plutons. Based on a dramatic increase in FeO, Zr, Hf, and LREE content of the most evolved granitoid components from the 2.69–2.66 Ga cumulate intrusion, we propose that those granitoid plutons were in part derived from a metasomatized mantle source enriched by fluids from the subducting oceanic slab that underwent further hybridization (via assimilation) with the crust. Large-scale, mantle-derived Neoarchean sanukitoid-type magmatism played a role in the development of a depleted lower crust and residual sub-continental lithospheric mantle, a crucial element in the preservation of the RBb.RÉSUMÉLe bloc de Repulse Bay (RBb) dans le sud de la péninsule de Melville, au Nunavut, est situé dans le craton de Rae et expose une large zone (50 000 km2) de croûte moyenne à inférieur. Ce bloc est composé de tonalite-trondhjémite-granodiorite (TTG) daté à ca. 2,86 Ga et 2,73–2,71 Ga, et de gneiss granitique dérivé d’un substrat crustal plus ancien daté à 3,25 Ga et 3,10 Ga. Cette période de croissance crustale a été suivie par la mise en place entre ca. 2,69 et 2,66 Ga d’intrusions d’enderbite, charnockite et de granitoïde incluant des xénolites d’entraînement de websterite. Ces intrusions de taille batholitique (intrusions déshydratées et hydratées) ainsi que les xénolites d’entraînement de websterite associés, ont des propriétés géochimiques sur roche totale semblables notamment leurs profils de fractionnement des terres rares légers (LREE) et des terres rares lourds (HREE) ainsi que leurs anomalies négatives en Nb, Ti et Ta. Les intrusions déshydratées et les xénolites de websterite ont aussi des minéralogies similaires (deux pyroxènes, biotite, amphibole interstitielle) ainsi que des compositions semblables en éléments traces de leurs pyroxènes. Étant donné leurs propriétés géochimiques et minéralogiques, ces deux lithologies sont interprétées comme provenant d’une cristallisation fractionnée, et comme étant le produit de processus d'accumulations magmatiques. Le remaniement de la croûte dans un contexte de subduction vers ca. 2,72 Ga, a été suivi vers ca. 2,69 Ga d’une remontée du manteau asthénosphérique et de l’intrusion de granitoïdes de type massif. D'après l’importante augmentation en FeO, Zr, Hf et LREE dans les granitoïdes les plus évolués du magmatisme ayant pris place entre ca. 2,69 Ga et 2,66 Ga, nous proposons que ces plutons aient été en partie dérivés d’une source mantélique métasomatisée enrichies par des fluides d’une plaque océanique en subduction et qui a subi une hybridation supplémentaire (par assimilation) avec la croûte. Le magmatisme néo-archéen de type sanukitoïde, dérivé du manteau et de grande échelle, a joué un rôle dans le développement d’une croûte inférieure et d’un manteau lithosphérique continental résiduel appauvri, un élément déterminant pour la préservation du RBb

In situ multiple sulfur isotope analysis by SIMS of pyrite, chalcopyrite, pyrrhotite, and pentlandite to refine magmatic ore genetic models

With growing interest in the application of in situ multiple sulfur isotope analysis to a variety of mineral systems, we report here the development of a suite of sulfur isotope standards for distribution relevant to magmatic, magmatic-hydrothermal, and hydrothermal ore systems. These materials include Sierra pyrite (FeS2), Nifty-b chalcopyrite (CuFeS2), Alexo pyrrhotite (Fe(1 −x)S), and VMSO pentlandite ((Fe,Ni)9S8) that have been chemically characterized by electron microprobe analysis, isotopically characterized for δ33S, δ34S, and δ36S by fluorination gas-source mass spectrometry, and tested for homogeneity at the micro-scale by secondary ion mass spectrometry. Beam-sample interaction as a function of crystallographic orientation is determined to have no effect on δ34S and Δ33S isotopic measurements of pentlandite. These new findings provided the basis for a case study on the genesis of the Long-Victor nickel-sulfide deposit located in the world class Kambalda nickel camp in the southern Kalgoorlie Terrane of Western Australia. Results demonstrate that precise multiple sulfur isotope analyses from magmatic pentlandite, pyrrhotite and chalcopyrite can better constrain genetic models related to ore-forming processes. Data indicate that pentlandite, pyrrhotite and chalcopyrite are in isotopic equilibrium and display similar Δ33S values + 0.2‰.This isotopic equilibrium unequivocally fingerprints the isotopic signature of the magmatic assemblage. The three sulfide phases show slightly variable δ34S values (δ34Schalcopyrite = 2.9 ± 0.3‰, δ34Spentlandite = 3.1 ± 0.2‰, and δ34Spyrrhotite = 3.9 ± 0.5‰), which are indicative of natural fractionation. Careful in situ multiple sulfur isotope analysis of multiple sulfide phases is able to capture the subtle isotopic variability of the magmatic sulfide assemblage, which may help resolve the nature of the ore-forming process. Hence, this SIMS-based approach discriminates the magmatic sulfur isotope signature from that recorded in metamorphic- and alteration-related sulfides, which may not be resolved during bulk rock fluorination analysis. The results indicate that, unlike the giant dunite-hosted komatiite systems that thermo-mechanically assimilated volcanogenic massive sulfides proximal to vents and display negative Δ33S values, the Kambalda ores formed in relatively distal environments assimilating abyssal sulfidic shales

Actively forming Kuroko-type volcanic-hosted massive sulfide (VHMS) mineralization at Iheya North, Okinawa Trough, Japan

Modern seafloor hydrothermal systems provide important insights into the formation and discovery of ancient volcanic-hosted massive sulfide (VHMS) deposits. In 2010, Integrated Ocean Drilling Program (IODP) Expedition 331 drilled five sites in the Iheya North hydrothermal field in the middle Okinawa Trough back-arc basin, Japan. Hydrothermal alteration and sulfide mineralization is hosted in a geologically complex, mixed sequence of coarse pumiceous volcaniclastic and fine hemipelagic sediments, overlying a dacitic to rhyolitic volcanic substrate. At site C0016, located adjacent to the foot of the actively venting North Big Chimney massive sulfide mound, massive sphalerite-(pyrite-chalcopyrite ± galena)-rich sulfides were intersected (to 30.2% Zn, 12.3% Pb, 2.68% Cu, 33.1 ppm Ag and 0.07 ppm Au) that strongly resemble the black ore of the Miocene-age Kuroko deposits of Japan. Sulfide mineralization shows clear evidence of formation through a combination of surface detrital and subsurface chemical processes, with at least some sphalerite precipitating into void space in the rock. Volcanic rocks beneath massive sulfides exhibit quartz-muscovite/illite and quartz-Mg-chlorite alteration reminiscent of VHMS proximal footwall alteration associated with Kuroko-type deposits, characterized by increasing MgO, Fe/Zn and Cu/Zn with depth. Recovered felsic footwall rocks are of FII to FIII affinity with well-developed negative Eu anomalies, consistent with VHMS-hosting felsic rocks in Phanerozoic ensialic arc/back-arc settings worldwide. Site C0013, ∼100 m east of North Big Chimney, represents a likely location of recent high temperature discharge, preserved as surficial coarse-grained sulfidic sediments (43.2% Zn, 4.4% Pb, 5.4% Cu, 42 ppm Ag and 0.02 ppm Au) containing high concentrations of As, Cd, Mo, Sb, and W. Near surface hydrothermal alteration is dominated by kaolinite and muscovite with locally abundant native sulfur, indicative of acidic hydrothermal fluids. Alteration grades to Mg-chlorite dominated assemblages at depths of >5 mbsf (metres below sea floor). Late coarse-grained anhydrite veining overprints earlier alteration and is interpreted to have precipitated from down welling seawater as hydrothermal activity waned. At site C0014, ∼350 m farther east, hydrothermal assemblages are characterized by illite/montmorillonite, with Mg-chlorite present at depths below ∼30 mbsf. Recovered lithologies from distal, recharge site C0017 are unaltered, with low MgO, FeO and base metal concentrations. Mineralization and alteration assemblages are consistent with the Iheya North system representing a modern analogue for Kuroko-type VHMS mineralization. Fluid flow is focussed laterally along pumiceous volcaniclastic strata (compartmentalized between impermeable hemipelagic sediments), and vertically along faults. The abundance of Fe-poor sphalerite and Mg-rich chlorite (clinochlore/penninite) is consistent with the lower Fe budget, temperature and higher oxidation state of felsic volcanic-hosted hydrothermal systems worldwide compared to Mid Ocean Ridge black smoker systems

Lithology, geochemistry and geochronology of the Aillik Group and foliated granitic intrusions: implications on the formaton and early evolution of the Aillik domain, Makkovik province, Labrador

The Makkovik Province of eastern Labrador is part of an accretionary orogenic belt that formed during the Paleoproterozoic Makkovikian orogeny. The Aillik domain represents one of three domains that make up the Makkovik Province and is composed of the Aillik Group, a package of Paleoproterozoic bi-modal volcano-sedimentary rocks, and abundant variably deformed Paleoproterozoic intrusive suites. The Aillik Group has experienced several phases of deformation and has been metamorphosed to lower amphibolite facies during the Makkovikian orogeny. Two areas, Middle Head and Pomaidluk Point, are the focus of this project and are used as case studies to assess and examine the Aillik Group with respect to the objectives as outlined below. Middle Head is dominated by arkosic sandstone, felsic tuff, rhyolite and basalt; whereas, Pomiadluk Point is composed primarily of felsic tuff and polymictic conglomerate with lesser preserved rhyolite and basalt. This study consists of detailed bedrock mapping in conjunction with: insitu SHRIMP U-Pb zircon geochronology, insitu LA-MC-ICPMS Hf isotopic geochemistry, major and trace element geochemistry, and whole rock Nd isotope geochemistry. These methods are used to: 1) constrain the timing of volcanism within the Aillik Group, 2) determine the source of magmatism, 3) resolve the overall tectonic setting in which the Aillik Group was deposited, and 4) briefly investigate the subsequent evolution of the Aillik domain. -- U-Pb SHRIMP zircon geochronology on felsic tuff samples yields magmatic ages that range from ca. 1852 at Middle Head to ca. 1854 - 1862 Ma at Pomiadluk Point. These U-Pb ages indicate that sections of the Aillik Group occurring 27 km from one another were deposited contemporaneously, and that felsic volcanism continued to as late as ca. 1852 Ma. A foliated Paleoproterozoic granite from Middle Head yields an age of 1805 ±4 Ma, which further constrains the timing of deformation within the Aillik Group as continuing past its emplacement. One population of inherited zircons occurs between 1880 and 1920 Ma and is interpreted to be xenocrystic in nature. -- Initial εHfs in zircon from a ca. 1852 Ma felsic tuff and ca. 1805 Ma granite at Middle Head range uniformly from -1.6 to -5.0 with felsic crustal extraction ages of ca. 2.4 to 2.6 Ga. In contrast, two felsic tuff samples at Pomiadluk Point with magmatic ages of ca. 1854 and ca. 1861 Ma have initial εHfs values in zircon that range from -4.8 to -11.9 in 18 of 20 grains analyzed, giving (felsic) crust formation ages of 2.6 to 3.0 Ga. A third sample from Pomiadluk Point, a ca. 1862 Ma foliated felsic tuff that outcrops between two conglomerate beds, contains magmatic zircons with initial εHfs that range from +2.1 to -1.6, and crust formation ages of 2.3 to 2.5 Ga. None of the felsic volcanic rocks analyzed from the Aillik Group show Hf-isotope evidence of derivation from the North Atlantic Craton as the model ages are too young. Additionally, none of the samples demonstrate their formation on a truly juvenile, 1.9-2.0 Ga crust with short residence times (<100 Ma), as might be expected for an intra-oceanic island arc origin. Inherited zircon grains (1880 to 1920 Ma) demonstrate a similar Hf isotopic signature and depleted mantle model ages as the magmatic grains. -- Trace and REE geochemistry demonstrate that felsic volcanic rocks of the Aillik Group as well as temporally distinct deformed granitic intrusions are 'A-type' in nature. Felsic volcanic rocks and a deformed monzogranite demonstrate a range in Nd isotopic signatures (εNd(T)= -1.1 to -5.0), which reflects partial melting of a heterogeneous felsic crust. Based on geochemical signatures, mafic volcanic rocks can be classified into two groups. Group A basalts have geochemical signatures that demonstrate flat rare-earth element pattern, consistent with melting of a depleted mantle source, and are composed of primary plagioclase and clinopyroxene and metamorphic amphibole and biotite. Group B basalts and mafic tuff are chemically more evolved, and composed of primary plagioclase and metamorphic amphibole and magnetite. The two different REE patterns seen in mafic volcanic rocks are interpreted to reflect a variable amount of crustal contamination. Furthermore, Group A basalts demonstrate systematically more elevated εNd(T) signatures (+2.8 to +4.3) than Group B basalts and mafic tuff (-3.5 to +2.2). Based on mixing models, mafic magmas of the Aillik Group are determined to have formed by mixing of the depleted mantle with a small to moderately significant amount (5 to 35%) of the felsic volcanic rocks of the Aillik Group. -- The combination of early clastic sedimentation, bimodal volcanism and geochemical signatures of felsic and mafic melts suggests that the ca. 1883 to 1852 Ma Aillik Group formed in a continental back-arc setting, forming from a crust that had an age range of at least 700 Ma, including both Paleoproterozic and Late Archean components. The similar Hf isotopic signatures of the Cross Lake granite, the felsic volcanic rocks of the Aillik Group, and the xenocrysts in the felsic volcanic rocks of the Aillik Group confirms that the Aillik domain was generated from the same basement over 115 million years

Actively forming Kuroko-type VMS mineralization at Iheya North, Okinawa Trough, Japan: new geochemical, petrographic and δ34S isotope results

Irish Geological Research Meeting 2016, NUIG, Galway, Ireland, 19-21 February 2016In 2010, Integrated Ocean Drilling Program (IODP) Expedition 331 drilled five sites in the Iheya North hydrothermal field in the central Okinawa Trough back - arc basin, Japan. Hydrothermal alteratio n and sulfide mineralization is hosted in a geologically complex, mixed sequence of coarse pumiceous volcaniclastic and fine hemipelagic sediments, overlying a dacitic to rhyolitic volcanic substrate

The bimodal fluid evolution of the Nimbus Zn-Ag deposit: an Archean VHMS with epithermal characteristics

SGA Quebec 2017, Quebec City, Canada, 20-23 August 2017The Nimbus Zn-Ag VHMS deposit represents an exceptional mineralised occurrence in the Yilgarn Craton. While other VHMS systems in the craton are restricted to paleo-rift zones, Nimbus is associated to a plume-related stratigraphy (Hollis et al., 2017). Furthermore, the epithermal characteristics resulting from low temperature and shallow water conditions allowed the development of an unusual mineralisation dominated by Ag-rich sulfosalts. In this study we take advantage of state-of-the-art in-situ techniques to investigate the fluid evolution of this peculiar VHMS system. From the trace element compositions and S-isotope signatures we suggest that Nimbus experienced a bimodal fluid evolution consisting of (i) an initial intense interaction between deep-magmatic fluids and seawater that developed barren pyritic lenses, and (ii) a subsequent closure of the hydrothermal system during which the Agrich ore formed sourcing sulfur almost entirely from a deepmagmatic source

Post-Collisional Alkaline Magmatism as Gateway for Metal and Sulfur Enrichment of the Continental Lower Crust

Mafic and ultramafic magmas that intrude into the lower crust can preserve evidence for metal and sulfur transfer from the lithospheric mantle into the lower continental crust. Here we focus on a series of ultramafic, alkaline pipes in the Ivrea Zone (NW Italy), which exposes deeply buried (6-11 kbar), migmatitic metasedimentary rocks intruded by voluminous basaltic magmas of the Mafic Complex, a major crustal underplating event precisely dated via U/Pb CA-IDTIMS on zircon at 286.8 ± 0.4 Ma. The ultramafic pipes postdate the Mafic Complex and from 100 to 300 m wide cumulate-rich conduits. They are hydrated and carbonated, have unusually high incompatible element concentrations and contain blebby and semi-massive Ni-Cu-PGE sulfide mineralisation. The sulfides occur as coarse intergranular nodules ( \u3e 10 mm) and as small intragranular blebs ( \u3c 1 mm) hosted in olivine, and have homogeneous, mantle-like δ34S (+1.35 ± 0.25‰). This homogeneity suggests that the pipes reached sulfide supersaturation without addition of crustal sulfur, and that the δ34S signature is representative of the continental lithospheric mantle. One of the pipes, the 249 Ma Valmaggia pipe, carries a very distinctive Sr-Nd-Hf-Pb isotopic composition in its core (87Sr/86Sr 0.70250, εNd-18, εHf-18, 206Pb/204Pb 16.0, 207Pb/204Pb 15.16, 208Pb/204Pb 35.87), very different from the margin of this pipe and from other pipes that have higher 87Sr/86Sr, εNd and 206Pb/204Pb. The unusual isotopic composition of the Valmaggia pipe requires a source with long-term (2500–1500 million years) U-, Th- and Rb-depletion and LREE enrichment. Such compositions are found in Late Archean/Early Proterozoic granulites and lower crustal xenoliths. We suggest that the unusual isotopic composition of the Valmaggia pipe reflects contamination of the mantle source of the pipe with a crustal component that is neither represented in the local Paleozoic crust nor in the isotopically anomalous hydrated mantle inferred as the source of the large-volume mafic underplate that formed the Mafic Complex. During post-collisional gravitational collapse of the Variscan Orogen, this source produced the alkaline, metal (Ni, Cu, PGE)- and volatile (H2O, CO2, S)-rich mafic–ultramafic magma that formed the deep-crustal intrusion at Valmaggia. U/Pb dating of other chemically and geologically comparable pipes in the area shows that this process was active over at least 40 Ma. The Ivrea pipes illustrate how the lower continental crust can be fertilised with mantle-derived metals and volatiles, which are available for later remobilisation into upper-crustal ore systems. World-class mineral deposits along the margins of lithospheric blocks may thus be the result of both favourable crustal architecture (focussing of magmas and fluids) and localised volatile and metal enrichment of the lower crust related to mantle-derived hydrous metasomatism

Sulfur isotope composition of metasomatised mantle xenoliths from the Bultfontein kimberlite (Kimberley, South Africa) : contribution from subducted sediments and the effect of sulfide alteration on S isotope systematics

Sulfur isotopes are a powerful geochemical tracer in high-temperature processes, but have rarely been applied to the study of mantle metasomatism. In addition, there are very limited S isotope data on sub-continental lithospheric mantle (SCLM) material. For cratonic regions, these data are restricted to sulfide inclusions in diamonds. To provide new constraints on the S isotope composition of the SCLM and on the source(s) of mantle metasomatic fluids beneath the diamondiferous Kimberley region (South Africa), we investigated the S isotope systematics of five metasomatised mantle xenoliths from the Bultfontein kimberlite. Pentlandite and chalcopyrite in these xenoliths were analysed by in situ secondary-ion mass spectrometry (SIMS), with bulk-rock material measured by gas source isotope ratio mass spectrometry techniques. Based on previous studies, the xenoliths experienced different types of metasomatism to one another at distinct times (~180 and ~90-80 Ma). Contained pentlandite grains show variable alteration to heazlewoodite (i.e. Ni sulfide) + magnetite. The in situ S isotope analyses of pentlandite exhibit a relatively restricted range between -5.9 and -1.4‰δ³⁴S (compared to VCDT), with no statistically meaningful differences between samples. Chalcopyrite only occurs in one sample and shows δ³⁴S values between -5.4 and -1.0‰. The bulk-rock Ssulfide isotope analyses vary between -3.4 and +0.8‰δ³⁴S. Importantly, the only sample hosting dominantly fresh sulfides shows a bulk-rock δ³⁴S value consistent with the mean value for the sulfides, whereas the other samples exhibit higher bulk ³⁴S/³²S ratios. The differences between bulk-rock and average in situ δ³⁴S values are directly correlated with the degree of sulfide alteration. This evidence indicates that the elevated ³⁴S/³²S ratios in the bulk samples are not due to the introduction of heavy S (commonly as sulfates) and are best explained by isotopic fractionation coupled with the removal of light S during serpentinisation, when pentlandite is altered to S-poor mixtures of heazlewoodite and magnetite. Available bulk Ssulfide isotopic data for SCLM peridotite xenoliths are dominated by positive δ³⁴S values, which contrasts with the negative values of the sulfides. These results imply that the mantle S isotope values from bulk peridotite samples are commonly modified by isotopic fractionation during serpentinisation. Therefore, the S isotopic composition of the SCLM may require revision. The limited isotopic variability shown by sulfides in the Bultfontein mantle xenoliths is probably due to intermittent tapping of a mantle source with a relatively restricted S isotope composition. While the asthenospheric mantle (δ³⁴S ≤ -1.4‰) is a viable candidate, δ³⁴S values as low as -5.9‰ require input from recycled crustal material, possibly represented by the sulfur reservoir with negative δ³⁴S signature that is missing in the >500 Ma sedimentary record and could have been subducted and stored in the Earth's mantle.11 page(s