Craton Margin Research Articles

Geochemical and zircon U-Pb-Hf isotopic study of volcanic rocks from the Yaolinghe Group, South Qinling orogenic belt, China: Constraints on the assembly and breakup of Rodinia

The tectonic mechanism responsible for the Neoproterozoic magma generation at the northern margin of the Yangtze craton is crucial to understanding the assembly and breakup of the Rodinia supercontinent. Here, we present a systematic study of the petrology, whole-rock geochemistry and geochronology of mafic and felsic volcanic rocks from the Yaolinghe Group, which outcrops mainly in the Shangnan area (north) and the Wudang area (south) of the South Qinling orogenic belt along the northern margin of Yangtze craton. Basalts from the Shangnan area are tholeiitic and calc-alkaline and indicated an arc environment. They have a LA-ICP-MS zircon U-Pb age of 847 ± 5 Ma with positive εHf(t) values of + 6.93 to + 8.79 and TDM1 model ages of 1130–1053 Ma. The rhyolites from this area exhibit geochemical characteristics similar to S-type granites and have a zircon U-Pb age of 841 ± 7 Ma, with a wide range of εHf(t) values of + 1.02 to −11.40 and TDM2 model ages of 2453–1674 Ma. This is different from the rhyolites from the Wudang area, which reveal geochemical affinities similar to A2-type granites, with a zircon U-Pb age of 788 ± 28 Ma, uniform negative εHf(t) values of −8.75 to −0.15 and TDM2 model ages of 2214–1663 Ma. Both rhyolites were sourced from partial melting of Paleoproterozoic crust with minor input of mantle material, but were formed in an arc setting and a post-collision extension setting, respectively. The 663–644 Ma mafic volcanic rocks from the Wudang area are enriched in Th, U, Nb and Ta, similar to oceanic island basalts and have positive zircon εHf(t) values ranging from + 9.24 to + 4.89 with TDM1 model ages peaking at 1018 Ma. They were derived from partial melting of an asthenospheric mantle source that had been modified by slab-derived melts and erupted during late Neoproterozoic continental rifting. Combining our results with existing data, we suggest that the South Qinling orogenic belt resulted from bidirectional subduction of the South Qinling Ocean between the Douling block and the Yangtze craton at ca. 950–750 Ma, accompanied by the formation of a trench-arc-basin system along the Douling block and an Andean-type continental arc belt at the northern margin of the Yangtze craton. After accretion, the amalgamated Douling block and Yangtze craton were affected by oceanic subduction of the Mirovoi Ocean at ca. 750–680 Ma, and subsequent continental rifting at ca. 680–620 Ma. The model further poses that the Yangtze craton was assembled to the periphery of the Rodinia supercontinent.

Ultramafic-alkaline-carbonatite Tajno intrusion in NE Poland: A new hypothesis about the massif formation and related mineralization

This manuscript presents results of the newest petrographic, mineralogical and bulk chemical, as well as H, C and O stable isotope study of carbonatites and associated silicate rocks from the Tajno Massif (NE Poland). The Tajno Intrusion is a Tournaisian-Visean ultramafic-alkaline-carbonatite body emplaced within the Paleoproterozoic rocks of the East European Craton (EEC). Carbonatites of the Tajno Massif can be subdivided into the calciocarbonatite (calcite), ferrocarbonatite (ankerite), and breccias with an ankerite-fluorite matrix. Due to location at the cratonic margin and abundance in the REE, Tajno classifies (Hou et al., 2015) as the carbonatite-associated REE deposit (CARD), and more precisely as the Dalucao-Style orebody (the breccia-hosted orebody). High Fe2O3 (13.8 wt%), MnO (2.1 wt%), total REE (6582 ppm), Sr (43895 ppm), Ba (6426 ppm), F (greater than10000 ppm) and CO2 contents points for the involvement of the slab – including pelagic metalliferous sediments – in the carbonatites formation. Spatial relations and Sr isotope composition ((87Sr/86Sr)i = 0.7043–0.7048; Wiszniewska et al., 2020) of alkali clinopyroxenite and syenite suggest that these are products of differentiation of the magma, generated by the initial melting of the SCLM due to influx of F-rich fluids from subducted marine sediments. Carbonatites Sr isotope composition ((87Sr/86Sr)i = 0.7037–0.7038), and Ba/Th (16–20620) and Nb/Y (0.01–6.25) ratios, link their origin with a more advanced melting of the SCLM, triggered by CO2-rich fluids from the subducted AOC and melts from sediments. The Tajno Massif – and coeval mafic-alkaline intrusions – age, high potassic composition, and location along the craton margin nearly parallel the Variscan deformation front, are suggesting Variscan subduction beneath the EEC. The oxygen isotope compositions of clinopyroxene (δ18O value = 5.2‰) and alkali feldspar (δ18O value = 5.7‰), from alkali clinopyroxenite and foid syenite, respectively, are consistent with mantle-derived magmas. Isotopic compositions of carbonatites and breccias (carbonate δ18O = 8.7‰ to 10.7‰; δ13C = -4.8‰ to −0.4‰) span from values of primary carbonatites to carbonatites affected by a fractionation or sedimentary contamination. The highest values (δ18O = 10.7‰; δ13C = -0.4‰) were reported for breccia cut by numerous veins confirming post-magmatic hydrothermal alteration. The lowest carbonate δ18O (9.3‰ to 10.7‰) and δ13C (−5.0‰ to −3.8‰) values are reported for veins in alkali clinopyroxenites, whereas the highest δ18O (11.2‰) and δ13C (−1.2‰ to −1.1‰) values are for veins in syenites and trachytes. Isotopic composition of veins suggests hydrothermal origin, and interaction with host mantle-derived rocks, as well as country rocks. In silicate rocks of the Tajno Massif, fluid influx leads to the development of Pb, Zn, Cu, Ag, Au sulfide mineralization-bearing stockwork vein system, with carbonate, silicate and fluorite infilling the veins. Bulk-rock contents of molybdenum (925 ppm), rhenium (905 ppb) and palladium (29 ppb) are notable. The Re-rich molybdenite association with galena, pyrite and Th-rich bastnäsite in carbonate veins is similar as in Mo deposits associated with carbonatites, implying the mantle source of Mo and Re.

Copper mobilization in the lower continental crust beneath cratonic margins, a Cu isotope perspective

Lower crustal sulfide-bearing cumulates beneath the convergent plate margins represent a major copper reservoir. However, how Cu is mobilized and migrated in the lower crust is still a matter of debate. Here, we analyzed the Cu elemental and isotopic compositions of well-characterized lower crustal xenoliths from the north margin of the North China Craton to decode Cu mobilization in the lower continental crust beneath cratonic margins. The mafic lower crustal rocks have experienced varying degrees of sulfide accumulation, with Cu contents ranging from 15 to 112 ppm. The majority of lower crustal xenoliths have variable and generally low (compared to the Bulk Silicate Earth) δ65Cu (the permil deviation of the 65Cu/63Cu ratio from the NIST 976 standard) values ranging from −3.17‰ to 0.13‰ (n = 24). Many intragranular and interstitial sulfide grains in these lower crustal xenoliths were partially dissolved and oxidized along the mineral fractures and boundaries. Oxidative dissolution of sulfide will preferentially release 65Cu into the liquids, leaving the residual sulfides enriched in the light isotope, implying that oxidized melt-rock interactions could account for the light Cu isotopic compositions of the lower crustal xenoliths. Sulfide accumulation and oxidative dissolution are responsible for the enrichment, mobilization, and transfer of Cu in the lower continental crust beneath cratonic margins.

Mineral systems: Their advantages in terms of developing holistic genetic models and for target generation in global mineral exploration

• Single deposit class mineral system model for orogenic gold deposits. • Application to conceptual exploration. • Multiple deposit class model for porphyry-high sulfidation-skarn Cu-Au (Mo, W, Ag). • Combined geodynamic and preservation factors for craton margin deposits. Since the concept of mineral systems was first proposed, there have been growing calls for their use in deriving holistic genetic models and in assisting in exploration targeting in a world of diminishing mineral discoveries. This is because the mineral systems approach provides a means of integrating information over a range of time and terrane scales using the broad critical components of Geodynamics, Fertility, Architecture, and Preservation. Although their adoption is limited, the value of such mineral systems for single deposit classes and closely related deposit groups is demonstrated and numerous disparate deposit classes are shown to occupy equivalent tectonic niches in terms of their Geodynamic and Preservation components. The value of a mineral system model for a single deposit class is demonstrated for orogenic gold deposits using the premise that their genesis is explained in terms of a single global model rather than a series of disparate local models. If all orogenic gold deposits define a coherent mineral system, there are only two realistic sources of auriferous fluid, based on their syn-mineralization geodynamic settings. These are from devolatilization of a subducted oceanic slab with its overlying sulfide-rich sedimentary package, or release from mantle lithosphere that was metasomatized and fertilized during a previous subduction event, particularly adjacent to craton margins. This orogenic gold mineral system can be applied to conceptual exploration by first identifying the required settings at Geodynamic to deposit scales. Within these settings, it is then possible to define the critical gold mineralization processes in the system: Fertility, Architecture, and Preservation. These among other parameters, dictate that the structures controlling ore fluid advection must be lithosphere-scale faults that can be identified in magneto-telluric surveys, and that amphibolite-facies metamorphic domains are prospective exploration search spaces. The porphyry-high sulfidation-skarn Cu-Au (Mo, W, Ag) deposit group is an example where the mineral system comprises several deposit classes that are commonly spatially and temporally associated. These occur in strike extensive, curvilinear volcanic, continental, island, and post-collisional arcs, where Geodynamics and province-scale Architecture are controlled by arc-parallel continental-scale faults and intersections of oblique crustal-scale accommodation structures, commonly in shallow subduction slab arc segments. Fertility of the arcs is indicated by geochemistry of volcanic components that reflect residence and replenishment of H 2 O-rich hybrid source magmas in upper crustal magma chambers. In terms of district-scale Architecture, the porphyry-related systems are sited within and above small finger-shaped porphyritic bodies that intrude the roof zones of oxidized granitic plutons. The mineral systems comprise vertically and laterally zoned orebodies, alteration envelopes, and metal ratios developed from long-lived, originally high salinity and low pH, evolving magmatic-hydrothermal fluids exsolved from the fertile porphyry intrusions. Preservation potential is limited because of high uplift and erosion rates in the host arcs, with most deposits restricted to the Cenozoic, although there are Mesozoic examples in post-collisional arcs. At district scale, the presence of lithocaps identifies systems at a suitable exhumation level for exploration, with a range of mapping, remote sensing spectral, and geophysical methodologies applicable to detect critical responses from extensive porphyry-related footprints. Craton and thick lithosphere margins represent Geodynamic settings with high Preservation factors for an incredibly large range of deposit classes enriched in Ag, Au, Bi, Co, Cu, Fe, Mo, Ni, P, Pb, PGE, REE, Sb, Sn, Te, W, and Zn, and including diamonds. These are sited within ∼100 km of the margins due to adjacent metasomatized and fertilized mantle lithosphere, development of lithosphere-scale fault arrays, or fertile marginal basins on these margins. Thus, although Fertility and Architecture components vary widely between component mineral systems, the combined Geodynamic and Preservation factors provide specific restricted exploration search spaces for a variety of deposit classes. This is shown in spectacular fashion for the North China and Yangtze cratons in eastern China where 66 diverse giant or world class deposits are sited within ∼100 km of their margins.

North-dipping relict subduction of the Paleo-Asian Ocean at the southeastern margin of the Mongolian Terrane: Study of two parallel deep seismic profiles

The final closure of the Paleo-Asian Ocean (PAO) and its tectonic characteristics have been debated for several decades owing to a lack of high-resolution information on the lithosphere structure. Scholars have been attempting to explain deep tectonic evolutionary processes while studying continental growth at the southern margin of the Mongolian Terrane. In a bid to provide a new interpretation of the deep structure with a higher resolution, we study two reprocessed deep seismic reflection profiles. We studied the northern part (210 km long) of the 630-km-long deep seismic reflection profile extending across the North China Craton (NCC) margin to the northern Sino-Mongolia border in the west; and a parallel profile (80 km long) in the east near the Sino-Mongolia border. Both profiles are characterized by consistently north-dipping layered reflections projecting from the lower crust to the upper mantle, with an estimated thickness of 3.6 to 6 km between adjacent reflections beneath the Uliastai and Hegenshan belts. Arched reflections are observed in the middle and lower crust; these may have been caused by later magmatic activities. In addition, the Moho reflection is observed to be fairly continuous and flat in most parts of these two profiles. The layered lower crust reflections and mantle reflections serve as important evidence that northward subduction occurred during the closure of the Paleo-Asian Ocean at the southeastern margin of the Mongolian Terrane. We propose a detailed model of the evolutionary processes from the early Paleozoic to early Mesozoic. The proposed model explains how these deep reflections were formed.

Reactivated Margin of the Western North China Craton in the Late Cretaceous: Constraints From Zircon (U‐Th)/He Thermochronology of Taibai Mountain

AbstractThe western North China Craton (NCC) entered a stage of craton thinning concentrated on its margin in the Cenozoic, but its Late‐Cretaceous conditions were previously deduced to be relatively quiescent. Therefore, whether the Late‐Cretaceous tectonic reactivation existed on the margin of this block is critical to understanding how the craton evolved into a thinned one. To investigate this possible margin reactivation in the Late Cretaceous, we apply zircon (U‐Th)/He thermochronology to Taibai Mountain on the southern margin of the western NCC. From an ∼3‐km‐high vertical profile with ages decreasing from 98 ± 5 Ma at the peak to 52 ± 7 Ma at the foot, the age‐elevation relationship suggests the rapid exhumation at ca. 78–71 Ma bracketed by two slow exhumation phases. This interpretation is confirmed by 1D half‐space modeling and QTQt inverse modeling, with the rapid exhumation stage having an exhumation rate of 0.1–0.6 km/Myr and a cooling rate of ∼10°C/Myr. Together with previously reported thermochronological data from the margin of the western NCC, the rapid exhumation at ca. 78–71 Ma implies a reactivated margin in the Late Cretaceous before widespread Cenozoic thinning of the craton margin. This Late‐Cretaceous reactivation likely resulted from WNW‐ESE extension expressed by thermo‐tectonic events, basin subsidence, and volcanism in East Asia. Excluding impacts from climate change, lithospheric delamination, and India subduction beneath Eurasia, we infer that the Late‐Cretaceous reactivation on the margin of the western NCC was driven by the far‐field effect of Izanagi Plate subduction beneath the Eurasian Plate during slab rollback of the subducted plate.

НОВЫЙ ВЗГЛЯД НА СТРАТИГРАФИЮ СЕЛЕМДЖИНСКОГО И ТОКУРСКОГО ТЕРРЕЙНОВ МОНГОЛО-ОХОТСКОГО ПОЯСА: РЕЗУЛЬТАТЫ U-Pb, Lu-Hf, Sm-Nd ИЗОТОПНЫХ ИССЛЕДОВАНИЙ

Results of U-Pb and Lu-Hf isotopic studies on detrital zircons from metasedimentary rocks in the eastern part of the Selemdzha and Tokur terranes of the Mongol-Okhotsk belt and Sm-Nd isotopic data for whole-rocks are in complete contradiction with existing ideas about the stratigraphy of the region under consideration and indicate the need to revise the traditional principles of mapping within the belt. Two types of deposits have been established, differing in Sm-Nd isotopic characteristics of whole-rocks and Lu-Hf isotopic composition of detrital zircons. Deposits of different types are involved in the formation of the two opposing accretion systems: in front of the Siberian Craton margin (type I) and the Amur superterrane (type II). Among type I deposits, discrete groups are distinguished that differ in the lower age limit: 553–498 Ma, Late Ediacaran-Cambrian; ~373 Ma, Upper Devonian; 333–327 Ma, Upper Mississippian; ~304 Ma, Pennsylvanian. The deposits of tectonic-stratigraphic units show a general pattern of younging from north to south. Thus, we observe a structure typical of accretionary wedge-shaped terranes with a rear part in the north and a frontal part in the south. Tectonic-stratigraphic units formed by type I deposits and previously considered part of the Selemdzha and Tokur terranes, apparently, are components of the Galam terrane of the accretionary wedge.

Regional geology and tectonic framework of the Southern Indian domain, Trans-Hudson orogen, Manitoba

The collaborative federal–provincial Southern Indian Lake project in north-central Manitoba covered an area of more than 3500 km2 of the Trans-Hudson orogen. Regional-scale geological mapping, sampling, and lithogeochemical, isotopic, and geochronological studies resulted in the identification of distinct assemblages of supracrustal rocks and varied episodes of plutonism. A granodiorite gneiss dated at ca. 2520 Ma is interpreted to represent the basement of the Southern Indian domain and is considered a separate crustal domain, named the Partridge Breast block. The Churchill River assemblage is composed of juvenile pillow basalt with intervening clastic sedimentary rocks, possibly a reflection of plume magmatism related to initial rifting of the Hearne craton margin. The Pukatawakan Bay assemblage consists mainly of massive to pillowed, juvenile metabasaltic rocks and associated basinal metasedimentary rocks. The Partridge Breast Lake assemblage is dominated by continental-arc volcanic and volcaniclastic rocks associated with basinal metasedimentary rocks. The Strawberry Island assemblage, consisting of arenite and polymictic conglomerate, is interpreted to have been deposited in a foreland-basin basin or intra-orogen pull-apart basin environment. The Whyme Bay assemblage is characterized by fluvial-alluvial orogenic sediments and is temporally linked to the Sickle Group rocks in the Lynn Lake greenstone belt. Granitoid rocks, dominantly monzogranite and granodiorite, range in age from ca. 1890 to 1830 Ma and occur throughout the Southern Indian domain, and intermediate and mafic intrusions of similar ages are also present. In this paper, we integrate these new data into a tectonic framework for the Southern Indian domain of the Trans-Hudson orogen in Manitoba.

MARBLE MÉLANGE: COMPOSITION VARIATIONS AND FORMATION MECHANISMS

The Olkhon terrane in the Western Baikal area accommodates four types of carbonate-silicate mixtures: injection (protrusion), metamorphic-boudinated, mingling, and tectonite marble mélange. The outcrops of injection mélange consist of a carbonate matrix with inclusions of native silicic rocks found in the immediate vicinities, commonly cover large areas and lack any distinct linearity in the map view. Mélange of the metamorphic boudinage type comprises diopsidite and tremilote-diopsidite fragments in a dolomitic or calcite-dolomitic matrix. Its origin is apparently due to tectonism and related metamorphism of quartz sandstones in Neoproterozoic strata on the passive margin of the Siberian craton. Mingling mélange appears as calcite marble or carbonate-silicate (calciphyre) veins with metadolerite and granite inclusions of different sizes. The veins formed by intrusion of carbonate and silicate melt batches and subsequent fragmentation of silicate rocks that crystallized earlier. Marble tectonites localized in narrow zones record the late phase of ductile marble injection.

Gravity Anomalies And Crustal Structure Of The Eastern Part Of The Verkhoyansk Fold-And-Thrust Belt, NE Russia: Evidence From The Junction Area Of The Adycha-Elga And Allakh-Yun Tectonic Zones

The results of geophysical studies of the junction area of the Adycha-Elga and Allakh-Yun tectonic zones of the Verkhoyansk fold-and-thrust belt located on the submerged eastern margin of the Siberian craton are presented. Three structural-mineral complexes are recognized: Archean-Paleoproterozoic, Mesoproterozoic-Middle Carboniferous, and Upper Carboniferous-Early Mesozoic. The Early Jurassic plume-related basaltic volcanism and suprasubduction Late Jurassic-Early Cretaceous granitoids, regional Brungadin and Suntar faults are identified. The goal of the research is to identify deep heterogeneities and clarify the structure of the Earth’s crust in the junction area of the Adycha-Elga and Allakh-Yun tectonic zones of the Verkhoyansk fold-and-thrust belt. The analysis of gravitational anomalies is carried out, their transformations are performed – distinguishing the medium and low-frequency components, the vertical derivative Vzz, and calculating the equivalent distribution of sources of density masses at depth. It is determined that the hidden granitoids of the Adycha-Elga tectonic zone are located mainly in linear zones of decompaction at a depth of about 3.5 km. In the Allakh-Yun zone, a large gravitational minimum has been identified, where it is assumed that there is a magma granitoid chamber occurring at a depth of about 9 km. The model of the deep structure of the territory is based on the analysis of materials on the reference seismic profile 3-DV with the use of gravimetric data and the regional structure of the territory. According to the results of the wave pattern interpretation, the thickness of the lithosphere varies from 41 to 44 km. The thickness of the Upper Carboniferous-Triassic terrigenous rocks is 8-12 km, Mesoproterozoic - Middle Carboniferous carbonate-terrigenous complex is up to 12 km. The Archean-Paleoproterozoic crystalline basement occurs at a depth of 19-21 km. The Conrad discontinuity is assumed at a depth of about 30 km. Intense deformations of the crystalline basement are recognized, and trans-crust faults are identified.

Ore Geology, RE–OS Isotope Geochemistry of the Au and Au-Sb Mineralizations, Kular–Nera Terrane, Northeast Asia: Implications for Time of Formation and Ore Genesis

The paper presents the first results of investigation of the Re–Os isotope system of native gold from the Malo-Tarynskoe, Khangalas, Bazovskoe, and chalcopyrite from the Dvoinoe orogenic gold deposits and stibnite from the Maltan Au-Sb depositin the Kular–Nera terrane, Northeast Asia. The deposits are spatially related to NW-trending lithospheric-scale major brittle faults or controlled by subsidiary faults and fracture zones. Such zones served as pathways for fluids rising from below the crust, and they have a long tectonic and reactivation history. The Kular–Nera terrane consists of Upper Permian, Triassic, and Lower Jurassic clastic sedimentary-rock sequences, metamorphosed to initial stages of greenschist facies. Magmatism is manifested by Kimmeridgian–Berriasian S- and I-types granitoids and mafic dikes of the Tas–Kystabyt magmatic belt. Re concentration in gold varies from 0.168 to 6.997 ppb, and that of osmium – from 0.068 to 1.443 ppb. Chalcopyrite from the Dvoinoe deposit occurrence contains 0.1522 ppb Re and 0.499 ppb Os. Stibnite from the Maltan Au-Sb depositoccurrence contains 0. 236 ppb Re and 0.903 ppb Os. The Re–Os ages of gold from the Malo-Tarynskoe (147.8 ± 3.8 Ma) and Bazovskoe (147.2 ± 1.8 Ma) and Khangalas (137.1 ± 7.6 Ma) orogenic deposits and the Maltan Au-Sb deposits (69.7±1.9 Ma) are determined. Malo-Tarynskoe and Bazovskoe represent the earliest known orogenic gold mineralization in the Kular–Nera terrane. The data obtained permit us to correlate the initiation of orogenic gold-ore systems with the completion of the formation at the end of the Late Jurassic Uyandina–Yasachnaya volcanic belt, crystallization and subsequent cooling in the Late Jurassic–early Early Cretaceous of granitoid massifs of the Tas-Kystabyt magmatic belt, and subduction–accretionary events at the northeastern active continental margin of the Siberian craton. Maltan Au-Sb deposit is related to completion of the formation of the Albian-Late Cretaceous Okhotsk–Chukotka volcano-plutonic belt. Contrasting mantle and/or crustal sources of ore-forming material are established. The osmium initial isotopic ratio in gold 187Os/188Os = 0.2210-0.4275 and antimonite (0,2543-0,2976) is typical for the ore-forming material from the fertile mantle reservoir, and for chalcopyrite (3.1904) – from the crust.

Development of Crustal-Scale Shear Zones at the Singhbhum Craton–Eastern Ghats Belt Boundary Region: A Critical Review of the Mesoarchean–Neoproterozoic Odyssey

Abstract A number of crustal-scale shear zones have developed along the southern margin of the Singhbhum Craton, in the boundary with the Neoarchean Rengali Province and the Meso-Neoproterozoic Eastern Ghats Belt. The cratonic part, evolved in a suprasubduction zone setting, bears imprints of late Mesoarchean orogenic episode (D1C) at ca. 3.1 Ga with folding and thrust imbrication of the cratonic rocks. The succeeding orogenic imprint is etched in the Neoarchean (~2.8 Ga) with development of the Sukinda thrust along the craton margin and thrust-related deformation of the rocks of the Rengali Province (D2C-D1R). The latter event remobilized cratonic fringe with development of a spectacular E-W trending transpressional belt in the Southern Iron Ore Group rocks cored by the Sukinda ultramafics. In the Eastern Ghats Belt, the major ultrahigh-temperature orogeny took place during the Grenvillian-age (~1.0-0.9 Ga) assembly of the supercontinent Rodinia. This belt eventually got juxtaposed against the expanded Singhbhum Craton in the end-Neoproterozoic time (~0.5 Ga) along the Kerajang Fault Zone. This latter event remobilized a large part of the Rengali Province (D2R) with development of an intraterrane transpressional belt bounded by the Barkot Shear Zone in the north. The northern fringe of the intruding Eastern Ghats Belt developed a complex network of strike-slip fault system under this impact, probably an outcome of tectonic activity along the Kuunga suture, which signifies the joining of greater India with East Antarctica. The present synthesis visualizes early development in the craton through formation of a typical orogenic sequence, imbricated in thrust piles, resulting from a ca. 3.1 Ga orogeny. Further cratonic expansion was achieved via repetitive accretion and remobilization, development of crustal-scale faults and transpressional belts at ca. 2.8 Ga and ca. 0.5 Ga, much in a similar fashion as documented along oblique convergent margins of all ages.

Sr-Nd isotope geochemistry and petrogenesis of ca. 2.26–2.25 Ga and ca. 2.08 Ga mafic dyke swarms from the Dharwar craton, India: Insights into their mantle sources and geodynamic implications

Whole-rock and Sr-Nd isotopic data on the N-S to NNE-SSW trending ca. 2.26–2.25 Ga Ippaguda-Dhiburahalli and the N-S, NW-SE to NE-SW trending ca. 2.08 Ga Devarabanda mafic dyke swarms of the Dharwar craton have been studied. The ca. 2.26–2.25 Ga dykes are linear in nature and contain markedly high SiO2 (53.27–55.85 wt%) and Mg# (60–67) with low TiO2 (0.56–0.61 wt%) contents. While the ca. 2.08 Ga dykes are emplaced in a radiating fashion and contain relatively low SiO2 (47.34–52.67 wt%) and Mg# (34–53) with high TiO2 (0.67–2.11 wt%) contents. The multi-element patterns for both the dyke swarms reflect enrichment in the large-ion lithophile elements (LILEs) and depletion in the high-field-strength elements (HFSEs) with prominent negative Nb-Ta and Ti anomalies. Furthermore, samples of the ca. 2.26–2.25 Ga swarm are relatively more enriched than the ca. 2.08 Ga samples, which is supported by the Nd-isotopic signature. εNd(t) values in the ca. 2.26–2.25 Ga samples vary between −4.5 and 2.4, whereas in the ca. 2.08 Ga dykes it varies between −0.6 and 0.2. The parental magmas are derived from a MORB-like depleted mantle source for both the dyke swarms. Moreover, the interaction between primary melt and the metasomatized sub-continental lithospheric mantle (SCLM) is suggested for the ca. 2.26–2.25 Ga swarm, whereas, the ca. 2.08 Ga swarm show imprints of crustal contamination. The trace element modelling suggests that the ca. 2.26–2.25 Ga swarm were derived from a relatively low (≤5%) degree of partial mantle melting as compared to the ca. 2.08 Ga swarm with a high (~10–15%) degree of partial mantle melting. A deeper mantle source is suggested for the ca. 2.26–2.25 Ga swarm (Garnet/Spinel >80/20), whereas the ca. 2.08 Ga swarm were derived from a relatively shallower mantle source (Garnet/Spinel ~40/60–80/20). The TDM model age for the ca. 2.26–2.25 Ga swarm is supportive of the Neoarchean convergence-related growth of the Dharwar craton, possibly created an enriched lithospheric mantle reservoir. Absence of OIB like plume component in the ca. 2.08 Ga swarm defines the plume centre not precisely below the cratonic lithosphere of the Dharwar craton but several hundred kilometers away from the cratonic margin.

Sedimentary architecture of a Middle Ordovician embayment in the Murzuq Basin (Libya)

The Hawaz Formation comprises a siliciclastic, shallow to transitional marginal marine succession, deposited on the north-western cratonic margin of Gondwana during the Middle Ordovician. This unit is well documented in the north central part of the Murzuq Basin, where it is often dramatically truncated by Late Ordovician glaciation unconformities, generating a major discontinuity, not only in the studied area but also across the whole Saharan Platform of North Africa. The Hawaz Formation is particularly relevant as one of the two major oil-bearing reservoirs in the Murzuq Basin. However, Late Ordovician erosion ensures that its present configuration bears very little relation to the original sedimentary architecture and, consequently, there is a need for a detailed large-scale correlation and sedimentary reconstruction of the Hawaz in order to improve subsurface management of this reservoir unit. The present study was developed from a previous sedimentological characterization of the Hawaz Formation, based on subsurface data provided by 35 wells. This sedimentological background provided the basis for the reconstruction of the sedimentary architecture of this unit by means of eight correlation panels oriented along both sedimentological dip (NNW-SSE) and strike (WSW to ENE). In addition, a series of Gross Depositional Environment (GDE) maps were also generated with the aim of providing insight into the lateral distribution of facies belts within the framework of a sequence stratigraphic-based reservoir zonation. The results of this study suggest that the Hawaz Formation was deposited in a relatively protected or embayed shoreline with multiple bays/estuaries as the main entry points for sediment into the basin, most likely influenced by the effects of pre-existing north-northwest to south-southeast Pan-African structures controlling local accommodation space and reactivated during Ordovician times. Correlation panels and GDE maps also show the Hawaz Formation to be an extensive and continuous reservoir across the studied area, deposited in a broadly extensive subtidal to intertidal paralic environment, with very few or possibly no modern sedimentary analogues.

Facies analysis and palaeoecology of the Jurassic Spiti Shale Formation in the Spiti area, Northern India

The Spiti Shale Formation is a widely distributed stratigraphic unit of the passive northern margin of the Indian craton, deposited between the Callovian and earliest Cretaceous. The siliciclastic strata are dominated by dark-grey to black argillaceous silt. As the formation has undergone intense tectonic stress involving folding and faulting, it is very difficult to document a complete section. In the type area, the Spiti Valley, six sections have been measured that document parts of the three informal members of the formation, the Lower, Middle, and Upper members. Despite its uniform appearance, eight facies/biofacies types could be distinguished, ranging from the anoxic shale facies, with ammonites and belemnites as the only faunal elements, to the dysoxic Malayomaorica and Bositra biofacies, and the oxic to anoxic offshore shelf facies, which are characterized by low-diversity macrobenthos associations. Other facies are the condensed glauconitic-phosphoritic mudrock facies and the Fe-oolitic siltstone facies (both characterized by sediment starvation), the aerated argillaceous silt–sandstone facies, and the tide-influenced nearshore shelf facies. The benthic macrofauna represents four bivalve-dominated associations all characterized by a very low to low species diversity. They are the Bositra buchii, the Australobuchia spitiensis, the Palaeonucula cuneiformis‒Pruvostiella hermanni‒Indogrammatodon egertonianus, and the Malayomaorica sp.‒Australobuchia spitiensis‒Retroceramus haasti association. Sediments and macrobenthic associations indicate that the Spiti Shale Formation represents outer to inner shelf environments, which for much of the time were subjected to upwelling and anoxic to dysoxic conditions. Distinct shallowing at the top characterizes the transition to the overlying Lower Cretaceous Giumal Formation.

Linking Gold Systems to the Crust-Mantle Evolution of Archean Crust in Central Brazil

The Goiás Archean Block (GAB) in central Brazil is an important gold district that hosts several world-class orogenic gold deposits. A better comprehension of the crustal, tectono-magmatic, and metallogenic settings of the GAB is essential to accurately define its geological evolution, evaluate Archean crustal growth models, and target gold deposits. We present an overview of gold systems, regional whole-rock Sm-Nd analyses that have been used to constrain the geological evolution of the GAB, and augment this with new in situ zircon U-Pb and Hf-O isotope data. The orogenic gold deposits show variable host rocks, structural settings, hydrothermal alteration, and ore mineralogy, but they represent epigenetic deposits formed during the same regional hydrothermal event. The overprinting of metamorphic assemblages by ore mineralogy suggests the hydrothermal event is post-peak metamorphism. The metamorphic grade of the host rocks is predominantly greenschist, locally reaching amphibolite facies. Isotope-time trends support a Mesoarchean origin of the GAB, with ocean opening at 3000–2900 Ma, and reworking at 2800–2700 Ma. Crustal growth was dominated by subduction processes via in situ magmatic additions along lithospheric discontinuities and craton margins. This promoted a crustal architecture composed of young, juvenile intra-cratonic terranes and old, long-lived reworked crustal margins. This framework provided pathways for magmatism and fluids that drove the gold endowment of the GAB.

Rhabdophane Th-Pb ages indicate reactivation of Mesoarchean structures in west Pilbara Craton during breakup of Greater India and Australia-Antarctica

Abstract Uranium-Th-Pb dating of phosphate minerals in very low-grade metasedimentary rocks from the Archean Pilbara Craton, Western Australia, has revealed a long history of deformation and fluid flow during the Paleoproterozoic. However, this technique has not detected evidence for fluid flow along craton margins during Phanerozoic rifting and breakup. We report the use of in situ Th-Pb geochronology of rhabdophane, a hydrous light rare earth element phosphate, to date fluid flow in shale from the 2.76 Ga Mount Roe Basalt from drill hole number 6 of the Archean Biosphere Drilling Program (ABDP6), northwestern Pilbara Craton. Thorium-Pb dating of rhabdophane in carbonaceous shale yields three main populations with weighted mean 208Pb/232Th ages of 152 ± 6 Ma, 132 ± 4 Ma, and 119 ± 4 Ma, which indicates phosphate growth up to 2.64 b.y. after deposition. The rhabdophane ages are coeval with three major breakup events in eastern Gondwana: separation of Southwest Borneo and Argoland from Australia (ca. 156–152 Ma), breakup of Greater India from Australia (ca. 140–135 Ma), and separation of Greater India/India from Antarctica (ca. 123 Ma). The proximity of drill hole ABDP6 to major Mesoarchean faults and shear zones on the craton margin, which are parallel to rift propagation and basin development, points to episodic reactivation of ancient crustal structures >2.8 b.y. after their formation. Our results also highlight the potential of rhabdophane as a U-Th-Pb geochronometer for dating low-temperature (<200 °C) fluid flow and hydrous alteration. The migration of Mesozoic fluids through Archean shales adds weight to questions about the origin of geochemical signals in ancient altered rocks and how to extract information about the early environment and biosphere.

Early Neoproterozoic Gold Deposits of the Alto Guaporé Province, Southwestern Amazon Craton, Western Brazil

Abstract The Alto Guaporé gold province, southwestern Amazon craton, contains gold deposits that have been mined since the beginning of the 18th century and these deposits, together, have modern-day, pre-mining gold resources of at least 1.8 Moz. The ore is associated with quartz vein systems along the southeastern part of the Aguapei belt, a ~35-km-wide and ~500-km-long, NNW-trending shear zone formed due to reactivation of a terrane-bounding suture. The Aguapei belt evolved by ca. 1150 to 1100 Ma rifting and deposition of siliciclastic sediments in an aulacogen basin, followed by deformation and low-grade metamorphism of the sedimentary sequences during 1100 to 900 Ma terrane collision along the craton margin. The deformation was characterized by a compressional regime until ca. 950 Ma and transition to a transpressional setting during the final 50 m.y. The gold deposits are hosted in a variety of structures that are second-order to the main Aguapei shear zone. The Ernesto and Pau-a-Pique deposits are located ~40 km apart and at jogs along the Aguapei belt. They are marginal to pre-ore igneous rocks, with Ernesto hosted in the basal part of the metasedimentary Fortuna Formation that overlies tonalite and Pau-a-Pique at the contact between metasedimentary rocks and diorite. Three deformational phases comprise the compressional (D1 to D2) to transpressional (D3) tectonic events. In the Pau-a-Pique deposit and the deeper level of the Ernesto deposit, the ore-bearing veins are bedding parallel and follow D2 strike-slip and reverse fault zones, respectively. However, the veins formed during D3 reactivation of the older structures by an array of oblique accommodation faults. In contrast, ores at shallower levels of Ernesto, both in discordant and bedding-parallel veins, are hosted within a ~20-m-thick rigid metaconglomerate with associated dilation due to the structural complexity as sedimentary rocks of the Aguapei Group were folded around the dome-shaped roof of the pre-ore tonalite. The ores in both deposits, as well as in many other deposits of the province, are characterized by disseminated and vein-hosted pyrite. Gold occurs mainly as inclusions in the pyrite, with other hydrothermal phases comprising muscovite, Fe-Ti oxides, and minor apatite, chalcopyrite, and galena. Fluid inclusion data, coupled with stable isotope geochemistry and geothermometry, indicate that gold precipitated from a low-salinity, CO2-rich fluid at ~300°C and ~2.5 kbar. The source for the fluid and gold was the interbedded pelites during devolatilization of the Aguapei Group sequence. The aqueous-carbonic fluid inclusions and the narrow range of δ18O values of quartz (12 ± 1‰) from many auriferous veins from the central part of the province represent a regional ore-forming fluid. The broad range of δD for hydrous minerals (–116 to –55‰) reflects influx of small amounts of meteoric water into the steeply dipping shear zones during postgold exhumation. The 40Ar/39Ar geochronology from hydrothermal muscovite indicates a widespread hydrothermal event along the belt between 928 and 920 Ma. Collectively, the geological, geochronological, and geochemical data suggest that metamorphic fluids migrated laterally into and then upward along the Aguapei belt and deposited gold in lower-order structures where strain gradients existed between lithounits. The province has many characteristics of large orogenic gold provinces worldwide and represents a highly prospective and underexplored target region for early Neoproterozoic gold, a time period that generally is not well endowed in gold ores.

U-Pb geochronology, geochemistry, and isotopic composition of granitoids across the Nolan-Zemlak domain boundary in the SW Rae craton, Laurentia: Evidence for a late Neoarchean suture reworked during Arrowsmith orogeny

The Nolan-Zemlak domain boundary in the southwestern Rae Province separates ca. 2.6 Ga granodioritic to granitic rocks (Nolan suite) of the Nolan lithotectonic domain to the north from an assemblage of ca. 2.52 Ga ultramafic-intermediate plutonic rocks (Zemlak suite) and ca. 2.3 Ga granitoids, and derived gneisses, of the Zemlak domain to the south, the latter representing part of the Taltson basement complex (TBC). The restriction of the Zemlak suite to south of the domain boundary suggests that its emplacement was structurally controlled, an inference supported by recognition of pre-2.52 Ga deformation in Nolan domain. This paper reports the results of U-Pb and Lu-Hf isotopic analysis of zircon, coupled with multi-element whole-rock geochemical analysis, for the Nolan and Zemlak suites, plus a younger suite of leucogranites that stitch the domain boundary. Both the Nolan and Zemlak suites have geochemical-isotopic signatures consistent with subduction-related magmatism, but the ca. 2.52 Ga Zemlak suite is distinguished by anomalously low Th concentrations, attributed to minimal sediment flux into the mantle wedge. Zircon rims from Nolan granites record ca. 2.52 Ga metamorphism ascribed to intrusion of the Zemlak suite. Regionally, these observations are best explained by the subduction-related accretion of the Zemlak terrane, part of a larger Paleo-Mesoarchean crustal block now embedded within the TBC, to a ‘proto-Rae’ cratonic core. A model is presented wherein circa 2.64–2.58 Ga (Nolan age) continental arc magmatism was generated via north-dipping subduction below the proto-Rae craton leading to eventual collision/accretion of the outboard Paleo-Mesoarchean block. Collisional suturing occurred by ca. 2.57 Ga, as recorded by both local and regional metamorphic ages. Following collision, subduction is inferred to have stepped outward, directed beneath the now amalgamated Nolan and Paleo-Mesoarchean (Zemlak/TBC) terrane with further arc magmatism represented by the ca. 2.52 Ga Zemlak suite. Thereafter, arc magmatism continued along the west-southwest margin of Rae craton from ca. 2.47 Ga to 2.4 Ga, likely the expression of Arrowsmith orogeny. Terminal collision ending the Arrowsmith orogenic cycle resulted in reactivation of the boundary zone and formation of the South Tazin Lake shear zone coeval with 2.37 Ga metamorphism and leucogranite emplacement.

Magmatic-hydrothermal origin for Carlin-type Au deposits: Evidences from in-situ S-Pb isotopic compositions of sulfides, the Chang’an Au deposit, southern Ailaoshan tectonic zone

The Chang’an Au deposit is a large Au deposit located in the south of the Ailaoshan-Red River tectonic zone, southwestern margin of the South China block. The deposit exhibit characters similar to Carlin-type Au deposits in Nevada and Dian-Qian-Gui “Golden Triangle”, including tectonic evolution (deformed and rifted cratonic margins), orebody controlled by lithology and structure and invisible Au ionically bound in arsenian pyrite and pyrite. Mineralization is controlled by a strike-slip fault and occurs as barren milky quartz veins, disseminated gold-bearing pyrite and arsenopyrite in the Early Paleozoic sandstone, siltstone and limestone. Gold is invisibly and ionically bound in arsenopyrite and pyrite or locally occurs as isolated native gold crystals (micron scale). Wall rock alteration includes sulfide, silicic, sericite and decarbonized alterations. Three hydrothermal mineralizing stages (I, II, III) can be identified: milky quartz-pyrite (I), quartz-sericite-pyrite-arsenopyrite (II) and dolomite-calcite-quartz (III). Different from those deposit in Dian-Qian-Gui, numerous plutons intruded in and around the mine. The syenite dikes are pyritized, thus, formed prior to the mineralization while lamprophyre dikes are unaltered and crosscut by the syenite dikes. The syenite dikes yielded zircon U − Pb ages of ca. 35.8 Ma and the lamprophyre yielded similar Ar-Ar age of ca. 36.6 Ma, largely constraining the age of Au mineralization. In situ S–Pb isotopic compositions for ore pyrite and arsenopyrite were measured by fs-LA-MC-ICP-MS. Sulfides from Chang’an show a uniform range from –6.4‰ to + 4.6‰, a typical characteristic of magmatic-derived S. Such sulfur isotopic compositions are similar to those of sulfides from Carlin-type Au deposits, but different from those of sulfides from orogenic-type deposit, which commonly exhibit a wide range of δ34S values. Pb isotopic ratios of pyrite and arsenopyrite are similar to those of the intrusive rocks in the deposit with more radiogenic Pb, suggesting that the metal is probably sourced from the coeval intrusive rocks and sedimentary wall rocks. Combined with the low Au concentrations of the sedimentary wall rocks, Au in the Chang’an deposit are probably sourced from the intrusive rocks. Differences between the Chang’an deposit and typical orogenic-type Au deposit include: (1) Orebodies in the Chang’an deposit exhibit transitional contact relationship with the host rocks and boundaries between them are defined by cut-off grade while these boundaries in typical orogenic-type deposits are clearly defined. (2) Sulfidized and decarbonized alterations are quite significant in the Chang’an deposit but commonly absent in typical orogenic-type deposits. (3) The Chang’an deposit have lower CO2 contents of ore-forming fluids and more shallow ore-forming depth relative to typical orogenic-type deposits. Based on the ore deposit geology, dating and isotopic data, the Chang’an mineralization is similar to typical Carlin-type gold deposit, formed by ore fluids originated from a deep magmatic S source in an post-collisional intracontinental setting at ca. 36 Ma.