Geochemistry of Archean Radioactive Quartz Pebble Conglomerates and Quartzites from western margin of Singhbhum-Orissa Craton,eastern India: Implications on Paleo-weathering,provenance and tectonic setting

Lenses of radioactive Quartz Pebble Conglomerates (QPC) and associated quartzites are exposed along western margin of Archean Bonai Granite in Singhbhum-Orissa Craton, eastern India intermittently spreading over a strike length of 8–10 km. QPCs are radioactive while quartzites are mostly non-radioactive in nature. The purpose of the research is to investigate and characterize the radioactive QPC and quartzites geochemically to decipher their paleo-weathering conditions, provenance characteristics and possible tectonic setting of deposition. Geochemical data suggest moderate to high chemical weathering conditions in the provenance areas of QPC and quartzites. Major, trace and REE data indicate predominantly felsic to partly mafic-ultramafic sources for the deposition of radioactive Quartz Pebble Conglomerates from the surrounding Archean terrain.Elevated values of Th, U, Pb, La, Ce, Y and low Sc with high critical trace elemental ratios of Th/Sc, La/Sc, Th/Cr and Zr/Sc in radioactive QPC indicate their derivation from felsic igneous source. Low concentration of Th and Sc in quartzites compared to QPC and their variable Th/Sc ratios indicate both felsic and mafic sources for quartzites, albeit their preferential felsic affiliation. Higher Cr/Th ratios in quartzites (18.4), moderate Cr/Th in QPC (5.42), low to moderate Y/Ni in QPC (0.36–12.4) and quartzite (0.29–1.88), along with Au ranging from 30 to 1527 ppb, Pt up to 188 ppb and 682 ppb in QPC and quartzites respectively point towards some contribution from mafic-ultramafic source as well. REE patterns and in particular negative Eu anomalies for both QPC and quartzites further support their derivation from felsic rocks and could possibly linked to some of the phases of Archean Singhbhum Granite and Bonai Granite. Granitic to pegmatitic source for QPC is also revealed by the presence of rounded to sub-rounded monazite, zircon and thorian-uraninite grains in their matrix. Study indicates that QPC and quartzites were deposited in a passive margin tectonic setting developed during Archean between a span of 3.3 and 3.16 Ga along the western margin of Bonai Granite when the reducing condition was prevalent as indicated by their low Th/U ratios (<4.0) and presence of detrital grains of uraninite and pyrite in QPC. Radioactive QPC from western margin of Archean Singhbhum-Orissa Craton bears broad resemblance with QPC from Witwatersrand basin of South Africa and Elliot Lake, Canada and thus appears to be ideal sites for exploring QPC hosted U (+Au-PGE) mineralization in the analogous areas.     

New constraints for the source characteristics,deposition and age of the 2.1–1.9 Ga metasedimentary cover at the western margin of the Karelian Province

The Paleoproterozoic cover sequence at the 100–150 km wide western margin of the Archean Karelian Province is dominated by deep water Lower and Upper Kaleva metasediments. We present here an interpretation of Sm–Nd isotope and geochemical data on 36 samples, TIMS multi-grain U–Pb zircon analyses on nine samples, and ca. 100 SIMS analysis of detrital zircon grains from four Upper Kaleva and one Lower Kaleva samples.The Lower Kaleva is characterized by autochthonous–parautochthonous, lithologically heterogeneous metaturbidites showing common enrichment in quartz. All the analysed detrital zircons are of a local Neoarchean source but t_DM variation up to 2.4 Ga combined with geochemical data indicate abundant mixing of Paleoproterozoic mafic material, presumably from 2.1 Ga plateau lavas and dykes, in most of the Lower Kaleva samples.The Upper Kaleva is dominantly allochthonous with tectonically enclosed fragments of ophiolite bodies, and it is characterized by lithological and geochemical-isotopic homogeneity. Geochemical, isotopic and detrital zircon data favour material derived from an orogenic domain, comprising both Archean and Proterozoic units, followed by effective mixing during the transport. The Archean zircon grains (25%) are mostly Neoarchean. The Paleoproterozoic grains lack zircons at 2.5–2.2 Ga and plot dominantly (92%) between 1.92 and 2.05 Ga. The indicated maximum deposition ages vary from 1.95–1.94 Ga to 1.92 Ga. The main source area proposed is the Himalaya-type Lapland-Kola orogen (now) in the northeast, which experienced mountain building and erosion at 1.95–1.91 Ga.The western margin of the Karelian Province shows evidence of rifting and lithosphere thinning from 2.1 to 1.95 Ga but it is still under debate whether the craton breakup occurred at 2.06 Ga in a volcanic or later at 1.95 Ga in a non-volcanic margin setting. One hypothesis is that the onset of collision in the northeast changed plate motion and lead to a new spreading within the pre-existing passive margin at 1.97–1.95 Ga. Thus, both a volcanic margin at 2.06 Ga and a non-volcanic margin at ca. 1.95 Ga could have been operated at the western margin of the Karelian Province.     

In-situ LA-ICP-MS trace elemental analyses of magnetite: The Bayan Obo Fe-REE-Nb deposit,North China

The Bayan Obo Fe-REE-Nb deposit in northern China is the world's largest light REE deposit, and also contains considerable amounts of iron and niobium metals. Although there are numerous studies on the REE mineralization, the origin of the Fe mineralization is not well known. Laser ablation (LA) ICP-MS is used to obtain trace elements of Fe oxides in order to better understand the process involved in the formation of magnetite and hematite associated with the formation of the giant REE deposit. There are banded, disseminated and massive Fe ores with variable amounts of magnetite and hematite at Bayan Obo. Magnetite and hematite from the same ores show similar REE patterns and have similar Mg, Ti, V, Mn, Co, Ni, Zn, Ga, Sn, and Ba contents, indicating a similar origin. Magnetite grains from the banded ores have Al + Mn and Ti + V contents similar to those of banded iron formations (BIF), whereas those from the disseminated and massive ores have Al + Mn and Ti + V contents similar to those of skarn deposits and other types of magmatic-hydrothermal deposits. Magnetite grains from the banded ores with a major gangue mineral of barite have the highest REE contents and show slight moderate REE enrichment, whereas those from other types of ores show light REE enrichment, indicating two stages of REE mineralization associated with Fe mineralization. The Bayan Obo deposit had multiple sources for Fe and REEs. It is likely that sedimentary carbonates provided original REEs and were metasomatized by REE-rich hydrothermal fluids to form the giant REE deposit.     

Age and origin of uranium mineralization in the Camie River deposit (Otish Basin,Québec,Canada)

The Camie River uranium deposit is located in the southeastern part of the Paleoproterozoic Otish Basin (Québec). The uranium mineralization consists of disseminated and vein uraninite and brannerite precipitated close to the unconformity between Paleoproterozoic fluviatile, pervasively altered, sandstones and conglomerates of the Matoush Formation and the underlying sulfide-bearing graphitic schists of the Archean Hippocampe greenstone belt. Diagenetic orange/pink feldspathic alteration of the Matoush Formation consists of authigenic albite cement partly replaced by later orthoclase cement, with the Na₂O content of clastic rocks increasing with depth. Basin-wide green muscovite alteration affected both the Matoush Formation and the top of the basement Tichegami Group. Uraninite with minor brannerite is mainly hosted by subvertical reverse faults in basement graphitic metapelites ± sulfides and overlying sandstones and conglomerates. Uranium mineralization is associated with chlorite veins and alteration with temperatures near 320 °C, that are paragenetically late relative to the diagenetic feldspathic and muscovite alterations. Re-Os geochronology of molybdenite intergrown with uraninite yields an age of 1724.0 ± 4.9 Ma, whereas uraninite yields an identical, although slightly discordant, 1724 ± 29 Ma SIMS U-Pb age. Uraninite has high concentrations in REE with flat REE spectra resembling those of uraninite formed from metamorphic fluids, rather than the bell-shaped patterns typical of unconformity-related uraninite. Paragenesis and geochronology therefore show that the uranium mineralization formed approximately 440 million years after intrusion of the Otish Gabbro dykes and sills at ∼2176 Ma, which constrains the minimum age for the sedimentary host rocks. The post-diagenetic stage of uraninite after feldspathic and muscovite alterations, the paragenetic sequence and the brannerite-uraninite assemblage, the relatively high temperature for the mineralizing event (∼320 °C) following the diagenetic Na- and K-dominated alteration, lack of evidence for brines typical of unconformity-related U deposits, the older age of the Otish Basin compared to worldwide basins hosting unconformity-related uranium deposits, the large age difference between basin fill and mineralization, the older age of the uranium oxide compared to ages for worldwide unconformity-related U deposits, and the flat REE spectra of uraninite do not support the previous interpretation that the Camie River deposit is an unconformity-associated uranium deposit. Rather, the evidence is more consistent with a PaleoProterozoic, higher-temperature hydrothermal event at 1724 Ma, whose origin remains speculative.     

New evidence for ~ 4.45 Ga terrestrial crust from zircon xenocrysts in Ordovician ignimbrite in the North Qinling Orogenic Belt,China

Evidence for the earliest known terrestrial crust comes predominantly from Jack Hills in Western Australia, where hafnium isotopic results from > 3.8 Ga detrital zircons indicate crustal precursors as old as ~ 4.4–4.5 Ga. We present evidence from magmatic cores in > 3.9 Ga xenocrystic zircons from a felsic volcanic rock in the North Qinling Orogenic Belt, China, of similar Hf crustal model ages up to 4.45 Ga. These lie on the same Lu/Hf trajectory as the least disturbed Jack Hills and Apollo 14 zircons, therefore providing only the second example of the earliest known generation of continental crust on Earth. In addition, the rims of two zircon grains record later growth at 3.7 Ga and, when combined with the fact that the grains are incorporated in Paleozoic volcanic rocks, imply long-lived crustal residence within the basement of the North China Craton. These results therefore establish the wider distribution and survival of the most ancient crustal material on the Earth and highlight the possibility for the further discovery of ancient crustal remnants.     

Detrital zircon U–Pb ages and Hf-isotope systematics from the Gadag Greenstone Belt: Archean crustal growth in the western Dharwar Craton,India

The Western Dharwar Craton in peninsular India comprises a typical Meso- to Neo-Archean granite-greenstone terrain. Detrital zircons from two metagreywackes in a late basin from the Gadag Greenstone Belt preserve at least eight age populations ranging in age from ca 3.34 to 2.55 Ga, and grains as old as ca 3.54 Ga. The zircon provenances for the two samples appear to be the same up to ca 3.25 Ga, with relatively juvenile ε_Hf values largely between zero and depleted mantle values. After 3.25 Ga, one sample has similar ε_Hf values whereas the other has only negative values indicative of Hf-evolution in a crustal environment. After ca 3.25 Ga the source regions for the two samples were distinctly different.The detrital zircons reflect the age and evolution of the upper crust of the Western Dharwar Craton. Modeling of Hf isotopic evolution of the detrital zircons suggests two major crust-forming events at ca. 3.6 and 3.36 Ga, and some indication of juvenile addition to the crust at ca 2.6 Ga. The maximum sedimentation age of the greywackes is constrained by the youngest detrital zircon population at 2547 ± 5 Ma. Gold mineralization in the belt is dated at 2522 ± 6 Ma and constrains greywacke sedimentation, deformation and metamorphism to a ca 25 my interval.     

A review of the genesis of the world class Bayan Obo Fe–REE–Nb deposits,Inner Mongolia,China: Multistage processes and outstanding questions

The Bayan Obo Fe–REE–Nb deposit is the world's largest rare earth element (REE) resource and with the increasing focus on critical metal resources has become a focus of global interest. The deposit is hosted in the Palaeoproterozoic Bayan Obo Group, mainly concentrated in the H8 dolomite marble. The ores consist of light REE enriched monazite and bastnäsite, with a wide array of other REE minerals. Niobium mineralisation is hosted primarily in aeschynite and pyrochlore, although there are a wide range of other Nb-minerals. The origin of the host dolomite and ore bodies has been a subject of intense debate. The host dolomite has been proposed to be both of sedimentary origin and an igneous carbonatite. Carbonatite dykes do occur widely in the area, and consideration of the textural, geochemical and isotopic composition of the dolomite suggests an origin via intrusion of magmatic carbonatite into meta-sedimentary marble, accompanied by metasomatism. The origin of the ore bodies is complex, indicated most strongly by an ~ 1 Ga range in radiometric age determinations. Compilation of available data suggests that the ores were originally formed around 1.3 Ga (Sm–Nd isochron ages; Th–Pb ages of zircon), close in time to the intrusion of the carbonatite dykes. The ores were subsequently subjected to several stages of deformation and hydrothermal overprint, culminating in deformation, metamorphism and fluid flow related to the Caledonian subduction of the Mongolian Plate under the North China Craton from ~ 450 to 420 Ma (Th–Pb ages of monazite). This stage resulted in the formation of the strong foliation (‘banding’) of the ore. The presence of undeformed veins with alkali mineral fills, and the overprinting of the foliation by Nb minerals suggest that secondary fluid flow events may also have contributed to the metal endowment of the deposits, as well as remobilising the original Fe and REE mineralisation. The alteration mineralogy and geochemistry of the ores are comparable to those of many REE mineralised carbonatites. Initial Nd isotope ratios at 450 Ma, however, suggest crustal sources for the metals. These conflicting lines of evidence can be reconciled if a (at least) two stage isotopic evolution is accepted for the deposits, with an original mantle-sourced, carbonatite-related metal accumulation forming around 1.3 Ga with εNd close to 0. The ore was remobilised, with associated re-equilibration of Th–Pb isotope systematics during deformation at ~ 450 Ma. A further stage of alkaline hydrothermal fluid was responsible for Nb mineralisation at this stage. The complex geological history, with multiple stages of alkaline, high field strength element-rich, metasomatic fluid flow, is probably the main reason for the exceptional metal endowment of the Bayan Obo area.     

Major,trace and platinum group element (PGE) geochemistry of Archean Iron Ore Group and Proterozoic Malangtoli metavolcanic rocks of Singhbhum Craton,Eastern India: Inferences on mantle melting and sulphur saturation history

The geological and metallogenic history of the Singhbhum Craton of eastern India is marked by several episodes of volcanism, plutonism, sedimentation and mineralization spanning from Paleoarchean to Mesoproterozoic in a dynamic tectonic milieu. Distinct signatures of this Archean-Proterozoic geodynamic process are preserved in discrete crustal provinces that constitute the Singhbhum Craton. Here we report new major, trace and PGE geochemical data from the ~ 3.4 Ga Iron Ore Group (IOG) volcanic rocks of the Jamda-Koira basin, a part of the BIF-bearing volcano-sedimentary sequences of the Noamundi-Jamda-Koira iron ore basin in the western part of Singhbhum Granite (SBG), and ~ 2.25 Ga metavolcanic rocks of Malangtoli. The IOG and Malangtoli volcanic rocks are porphyritic basalts and despite belonging to different ages, they exhibit similar mineralogical composition marked by clinopyroxene, plagioclase (present as both phenocryst and groundmass), opaques and volcanic glass (restricted to groundmass). The igneous mineralogy of these rocks has been overprinted by greenschist to lower amphibolite grade of metamorphism. The Malangtoli samples show low and high MgO compositional varieties. Immobile trace element compositions classify the IOG samples as andesite having a calc-alkaline composition, whereas the Malangtoli rocks correspond to basalt and andesite displaying a tholeiitic to calc-alkaline trend. The IOG basalts show low to moderate PGE contents marked by 26.23–68.35 ppb of ΣPGE, whereas the Malangtoli basalts display a moderate to high concentration of PGE (ΣPGE = 43.01–190.43 ppb). The studied samples have relatively enriched ΣPPGE ranging from 24.1–63.3 ppb (IOG) and 34–227.3 ppb (Malangtoli) against 2.2–4.1 ppb and 1.9–8.9 ppb ΣIPGE contents respectively. PPGE/IPGE ratios for IOG and Malangtoli samples range from 7.7–17.6 and 4.8–59.9. HFSE, REE and PGE compositions suggest a low degree (< 1 to 1%) of partial melting in the garnet lherzolite domain for the generation of IOG volcanic rocks. The parental magma of the Malangtoli basalts were generated by lower to higher degrees (3–< 10%) of mantle melting at depths corresponding to spinel to garnet lherzolite regime. Trace element (Zr/Nb, Th/Ta, Th/Nb, Ni/Cu) and PGE (Pd/Ir, Pd/Pt, Cu/Pd, Ni/Pd, Cu/Ir) ratios corroborate a sulphide saturated and PGE depleted character of IOG volcanic rocks that underwent crustal assimilation. In contrast, the high MgO Malangtoli basalts exhibit sulphide undersaturated, PGE undepleted nature devoid of crustal contamination whereas the low MgO Malangtoli basalts are sulphide saturated, PGE depleted and crustally contaminated. The IOG volcanic rocks correspond to intraoceanic arc with polygenetic crustal signatures, and show affinity towards arc-generated calc-alkaline basalts. The low- and high MgO basalts of Malangtoli are affiliated to transitional arc to rift-controlled back arc tectonic setting in a basinal environment that developed proximal to an active convergent margin.     

Uplift-denudation history of the Qinling orogen: Constrained from the detrital-zircon U–Pb geochronology

This paper presents a great number of detrital zircon U–Pb ages from the Middle Triassic to the Middle Jurassic sediments in the Jiyuan basin, southern North China. The results represent age spectra from 2.9 Ga to 216 Ma, with five peaks at 2.5 Ga, 1.9 Ga, 840 Ma, 440 Ma, and 270 Ma and two grains of ∼220 Ma. The ages of 2.5 Ga and 1.9 Ga are mainly derived from the Precambrian basement of the North China Block, whereas the others are typical characteristics of the Qinling orogenic belt. An important observation is that the Qinling-sourced detrital zircons become older as the strata get younger. Samples from the Middle Triassic to early Late Triassic strata are characterized by the age peak at 270 Ma, whereas the Late Late Triassic to Early Middle Jurassic samples are dominated by age peaks at 840 Ma and 440 Ma and minor grains within 800–650 Ma. Two grains of ∼220 Ma are preserved in the Late Middle Jurassic sample, which may be contributed by the Carnian deep plutons. These signatures indicate that the unroofing pattern of the Qinling orogenic belt developed by the denudation of materials from young covers to old basements and the Carnian deep plutons. Integrated with the data reported from the Hefei Basin, it is well-established that the intensity of unroofing increased from the Qinling to the Dabie orogen in the Early Jurassic, and the denudation timing of the ultra-high pressure (UHP) and high pressure (HP) rocks or Carnian plutons changed successively from the Early Jurassic in the Dabie to the Late Middle Jurassic in the Qinling orogen.     

Geologically constraining India in Columbia: The age,isotopic provenance and geochemistry of the protoliths of the Ongole Domain,Southern Eastern Ghats,India

The Ongole Domain in the southern Eastern Ghats Belt of India formed during the final stages of Columbia amalgamation at ca. 1600 Ma. Yet very little is known about the protolith ages, tectonic evolution or geographic affinity of the region. We present new detrital and igneous U–Pb–Hf zircon data and in-situ monazite data to further understand the tectonic evolution of this Columbia-forming orogen.Detrital zircon patterns from the metasedimentary rocks are dominated by major populations of Palaeoproterozoic grains (ca. 2460, 2320, 2260, 2200–2100, 2080–2010, 1980–1920, 1850 and 1750 Ma), and minor Archaean grains (ca. 2850, 2740, 2600 and 2550 Ma). Combined U–Pb ages and Lu–Hf zircon isotopic data suggest that the sedimentary protoliths were not sourced from the adjacent Dharwar Craton. Instead they were likely derived from East Antarctica, possibly the same source as parts of Proterozoic Australia. Magmatism occurred episodically between 1.64 and 1.57 Ga in the Ongole Domain, forming felsic orthopyroxene-bearing granitoids. Isotopically, the granitoids are evolved, producing εHf values between − 2 and − 12. The magmatism is interpreted to have been derived from the reworking of Archaean crust with only a minor juvenile input. Metamorphism between 1.68 and 1.60 Ga resulted in the partial to complete resetting of detrital zircon grains, as well as the growth of new metamorphic zircon at 1.67 and 1.63 Ga. In-situ monazite geochronology indicates metamorphism occurred between 1.68 and 1.59 Ga.The Ongole Domain is interpreted to represent part of an exotic terrane, which was transferred to proto-India in the late Palaeoproterozoic as part of a linear accretionary orogenic belt that may also have included south-west Baltica and south-eastern Laurentia. Given the isotopic, geological and geochemical similarities, the proposed exotic terrane is interpreted to be an extension of the Napier Complex, Antarctica, and may also have been connected to Proterozoic Australia (North Australian Craton and Gawler Craton).     

Peri-Gondwanan origin and early geodynamic history of NE Sicily: A zircon tale from the basement of the Peloritani Mountains

The ages of detrital zircon grains from one paragneiss and inherited zircon cores from two augen gneisses from the amphibolite facies basement of the Peloritani Mountains (southern Italy) measured by SHRIMP U–Pb constrain the previously unknown deposition age of the original sediments and help to elaborate a model for their provenance and subsequent evolution. The deposition age is latest Neoproterozoic to Cambrian (~ 545 Ma), bracketed by the combined ages of the youngest detrital/inherited zircon populations and of zircon from virtually coeval granitoids that intrude the metasediments. This is consistent with the subgreenschist facies Palaeozoic volcano–sedimentary sequences exposed in the southern Peloritani Mountains being the original cover rocks of the northern Peloritani late Neoproterozoic to early Cambrian basement. The age spectra of the detrital/inherited zircon grains show that the Neoproterozoic/Cambrian sediments were derived from the erosion of sources dominated by Neoproterozoic rocks with ages in the range of 0.85–0.54 Ga, with other main components aged 1.1–0.9 and ~ 2.7–2.4 Ga, and a minor one aged ~ 1.6 Ga, as typically found in peri-Gondwanan terranes. The presence of a large amount of Grenvillian-aged zircon contradicts previous models that propose a West African affinity for the Calabria–Peloritani Terrane, and the absence of 2.2–1.9 Ga Trans Amazonian/Tapajós–Parima/Eburnean zircon rules out an Amazonian provenance. The age spectra are more consistent with the basement sediments having an East African origin, similar to that of the early Palaeozoic sandstones in southern Israel and Jordan, part of a “provenance regionality” shared with other terranes currently located in the eastern Mediterranean area.     

Detrital zircon U–Pb geochronology,trace-element and Hf isotope geochemistry of the metasedimentary rocks in the Eastern Himalayan syntaxis: Tectonic and paleogeographic implications

The origin of the Greater Himalayan Sequence in the Himalaya and the paleogeographic position of the Lhasa terrane within Gondwanaland remain controversial. In the Eastern Himalayan syntaxis, the basement complexes of the northeastern Indian plate (Namche Barwa Complex) and the South Lhasa terrane (Nyingchi Complex) can be studied to explore these issues. Detrital zircons from the metasedimentary rocks in the Namche Barwa Complex and Nyingchi Complex yield similar U–Pb age spectra, with major age populations of 1.00–1.20 Ga, 1.30–1.45 Ga, 1.50–1.65 Ga and 1.70–1.80 Ga. The maximum depositional ages for their sedimentary protoliths are ~ 1.0 Ga based on the mean ages of the youngest three detrital zircons. Their minimum depositional ages are ~ 477 Ma for the Namche Barwa Complex and ~ 499 Ma for the Nyingchi Complex. Detrital zircons from the Namche Barwa Complex and Nyingchi Complex also display similar trace-element signatures and Hf isotopic composition, indicating that they were derived from common provenance. The trace-element signatures of 1.30–1.45 Ga detrital zircons indicate that the 1.3–1.5 Ga alkalic and mafic rocks belt in the southeastern India is a potential provenance. Most 1.50–1.65 Ga zircons have positive ε_Hf(t) values (+ 1.2 to + 9.0), and most 1.70–1.80 Ga zircons have negative ε_Hf(t) values (− 7.1 to − 1.9), which are compatible with those of the Paleo- to Mesoproterozoic orthogneisses in the Namche Barwa Complex. Provenance analysis indicates that the southern Indian Shield, South Lhasa terrane and probably Eastern Antarctica were the potential detrital sources. Combined with previous studies, our results suggest that: (1) the Namche Barwa Complex is the northeastern extension of the Greater Himalaya Sequence; (2) the metasedimentary rocks in the Namche Barwa Complex represent distal deposits of the northern Indian margin relative to the Lesser Himalaya; (3) the South Lhasa terrane was tectonically linked to northern India before the Cambrian.     

The Ranger uranium deposit,northern Australia: Timing constraints,regional and ore-related alteration,and genetic implications for unconformity-related mineralisation

The Ranger 1 unconformity-related uranium deposit in the Northern Territory of Australia is one of the world's largest uranium deposits and has ranked in the top two Australian producers of uranium in recent years. Mineralisation at the Ranger, Jabiluka and other major unconformity-related deposits in the Alligator Rivers Uranium Field (ARUF) occurs in Paleoproterozoic metamorphic basement rocks immediately beneath the unconformity with the Paleo- to Mesoproterozoic McArthur Basin.The sites of uranium mineralisation and associated alteration at the Ranger 1 deposit (Number 3 orebody) were fundamentally controlled by reactivated shear zones that were initiated during the regional Nimbuwah tectonothermal event. The timing of shearing at medium metamorphic grade was constrained by ion microprobe U–Pb dating of zircons in two pegmatites, one weakly foliated (1867.0 ± 3.5 Ma) and another that is unfoliated and cuts the shear fabric (1862.8 ± 3.4 Ma). The younger age of ~ 1863 Ma represents the minimum age of D1 shearing during the Nimbuwah event at the Ranger 1 deposit (Number 3 orebody). Titanite within veins of amphibole-plagioclase-apatite yielded an ion microprobe U–Pb age of 1845.4 ± 4.2 Ma, which represents a previously unrecognised hydrothermal event in the ARUF. Based on previous data, retrograde hydrothermal alteration during D2 reactivation of D1 shear zones is interpreted to have occurred at ~ 1800 Ma during the regional Shoobridge tectonothermal event.Detailed paragenetic observations supported by whole-rock geochemical data from the Ranger 1 deposit (Number 3 orebody) reveal a sequence of post-D2 hydrothermal events, as follows. (1) Intense magnesium-rich chlorite alteration and brecciation, focussed within schists of the Upper Mine Sequence in the Cahill Formation. (2) Silicification of Lower Mine Sequence carbonate rock units and overlying schist units, comprising quartz ± Mg-foitite (tourmaline) ± muscovite ± pyrite ± marcasite, and rare uraninite (early U1). (3) Formation of main stage uranium ore and heterolithic breccias including clasts of olivine–phyric dolerite, with breccia matrix composed of uraninite (U1), Mg-chlorite ± Mg-foitite and minor pyrite and chalcopyrite. (4) A second generation of uraninite (U2) veinlets with disordered graphitic carbon and quartz of hydrothermal origin. (5) Late-stage veinlets of massive uraninite (U3). As inferred in a previous study and confirmed herein, olivine–phyric dolerite dykes at Ranger are mineralised and chloritised, and are geochemically similar to the regional Oenpelli Dolerite. A maximum age for uranium mineralisation at the Ranger 1 deposit is therefore set by the age of the Oenpelli Dolerite (~ 1723 Ma).In-situ ion microprobe U–Pb analysis of texturally oldest U1 uraninite yielded a discordia array with a ²⁰⁶Pb/²³⁸U-²⁰⁷Pb/²³⁵U upper intercept age of 1688 ± 46 Ma. The oldest individual ion microprobe ²⁰⁷Pb–²⁰⁶Pb age is 1684 ± 7 Ma whereas the oldest age determined by in-situ electron microprobe chemical dating of U1 uraninite is ~ 1646 Ma. Another sample containing both U1 and U2 uraninite yielded discordant data with a ²⁰⁶Pb/²³⁸U–²⁰⁷Pb/²³⁵U upper intercept age of 1421 ± 68 Ma. When the ²⁰⁷Pb/²⁰⁶Pb ages are considered the data are suggestive of U2 uraninite formation and possible resetting of the U1 age between ~ 1420 Ma and ~ 1040 Ma. All ion microprobe analyses of U1 and U2 uraninite indicate variable and possibly repeated lead loss. In contrast ion microprobe U–Pb dating of the third generation of uraninite (U3) yielded several near-concordant analyses and a ²⁰⁶Pb/²³⁸U–²⁰⁷Pb/²³⁵U upper intercept age of 474 ± 6 Ma. This age is supported by electron microprobe chemical ages of U3 uraninite between 515 Ma and 385 Ma.The new results constrain the timing of initial uranium mineralisation at the Ranger 1 deposit (Number 3 orebody) to the period ~ 1720 Ma to ~ 1680 Ma, which just overlaps with a previous U–Pb age of 1737 ± 20 Ma for uraninite-rich whole-rock samples. Our results are consistent with individual laser-ICPMS ²⁰⁷Pb/²⁰⁶Pb and chemical ages of uraninite as old as 1690–1680 Ma reported from other deposits and prospects in the ARUF.Whole-rock geochemical data in this study of the Ranger 1 deposit (Number 3 orebody) and in other studies in the ARUF demonstrate that zones of intense chloritisation associated with uranium mineralisation experienced large metasomatic gains of Mg, U, Co, Ni, Cu and S and losses of Si, Na, Ca, Sr, Ba, K, Rb, Y and the light REE. More broadly in the ARUF, a regionally extensive illite–hematite ± kaolinite-bearing ‘paleoregolith’ zone in basement beneath the McArthur Basin exhibits depletion of about half of its uranium as well as major losses in Na, Sr, Pb, Ba and minor losses of Mg. These features together with new petrographic observations suggest this zone is a regional sub-McArthur Basin alteration zone produced by interaction with diagenetic or hydrothermal fluids of primary basinal origin, rather than representing a low-temperature paleo-weathering zone before the deposition of the McArthur Basin, as previously suggested.Based on these results and a synthesis of previous work, a new multi-stage model is proposed for the Ranger 1 ore-forming mineral system that may apply to other major unconformity-related uranium deposits in the ARUF and which may be used for targeting new deposits in the region. As in most recent models, oxidised diagenetic brines within the McArthur Basin are envisaged as crucial in mobilising uranium. However, a different architecture of fluid flow is proposed involving the sub-unconformity regional basement alteration zone as a preferential source of leached uranium. Possibly driven by convection during regional magmatism at ~ 1725–1705 Ma, oxidised basinal brines were drawn downwards and laterally through fault networks and fractures in the regional sub-unconformity alteration zone, leaching uranium from hematite-altered basement rocks. Simultaneously within deeper and lateral parts of the hydrothermal system, Mg-metasomatism produced chloritic alteration and brines with increased acidity and silica content (from the desilicification of the basement rock), analogous to processes described in sub-seafloor hydrothermal systems. Silicification occurred locally (e.g., Ranger deposit) within upflow zones of convective systems due to decreases in temperature and/or pressure of the brines and/or CO₂ generation during carbonate dissolution. Interruptions to convection during transient regional extensional or strike-slip tectonic events resulted in generalised lateral and downwards flow of fluids from the McArthur Basin through deepened zones of sub-unconformity alteration, transferring leached uranium into reactivated shear zones within the basement. The main stage of uraninite precipitation at the Ranger deposit and elsewhere in the ARUF is proposed to have occurred between ~ 1720 Ma and ~ 1680 Ma as a result of reduction of oxidised and evolved basin-derived ore fluids during reaction with pre-existing Fe^2 +-bearing minerals and/or mixing of the ore fluids with basement-reacted silica-rich brines.A second, volumetrically minor but locally high-grade, stage of uraninite mineralisation was associated with hydrothermal disordered carbon and quartz of presently unknown origin. Available data suggest formation between ~ 1420 Ma and ~ 1040 Ma. Almost a billion years later at ~ 475 Ma, fluids capable of mobilising uranium again resulted in uraninite (U3) deposition as sparse veinlets in the Ranger deposit, representing the first documentation of uranium mineralisation of this age in the region.     

Coupled U–Pb–Hf of detrital zircons of Cambrian sandstones from Morocco and Sardinia: Implications for provenance and Precambrian crustal evolution of North Africa

U–Pb–Hf of detrital zircons from diverse Cambrian units in Morocco and Sardinia were investigated in order to clarify the sandstone provenance and how it evolved with time, to assess whether the detrital spectra mirror basement crustal composition and whether they are a reliable pointer on the ancestry of peri-Gondwanan terranes. Coupled with Hf isotopes, the detrital age spectra allow a unique perspective on crustal growth and recycling in North Africa, much of which is concealed below Phanerozoic sediments.In Morocco, the detrital signal of Lower Cambrian arkose records local crustal evolution dominated by Ediacaran (0.54–0.63 Ga) and Late-Paleoproterozoic (1.9–2.2 Ga; Eburnian) igneous activity. A preponderance of the Neoproterozoic detrital zircons possess positive ε_Hf(t) values and their respective Hf model ages (T_DM) concentrate at 1.15 Ga. In contrast, rather than by Ediacaran, the Neoproterozoic detrital signal from the Moroccan Middle Cambrian quartz-rich sandstone is dominated by Cryogenian-aged detrital zircons peaking at 0.65 Ga alongside a noteworthy early Tonian (0.95 Ga) peak; a few Stenian-age (1.0–1.1 Ga) detrital zircons are also distinguished. The majority of the Neoproterozoic zircons displays negative ε_Hf(t), indicating the provenance migrated onto distal Pan-African terranes dominated by crustal reworking. Terranes such as the Tuareg Shield were a likely provenance. The detrital signal of quartz–arenites from the Lower and Middle Cambrian of SW Sardinia resembles the Moroccan Middle Cambrian, but 1.0–1.1 Ga as well as ~ 2.5 Ga detrital zircons are more common. Therefore, Cambrian Sardinia may have been fed from different sources possibly located farther to the east along the north Gondwana margin. 1.0–1.1 Ga detrital zircons abundant in Sardinia generally display negative ε_Hf(t) values while 0.99–0.95 Ga detrital zircons (abundant in Morocco) possess positive ε_Hf(t), attesting for two petrologically-different Grenvillian sources. A paucity of detrital zircons younger than 0.6 Ga is a remarkable feature of the detrital spectra of the Moroccan and Sardinian quartz-rich sandstones. It indicates that late Cadomian orogens fringing the northern margin of North Africa were low-lying by the time the Cambrian platform was deposited. About a quarter of the Neoproterozoic-aged detrital zircons in the quartz-rich sandstones of Morocco (and a double proportion in Sardinia) display positive ε_Hf(t) values indicating considerable juvenile crust addition in North Africa, likely via island arc magmatism. A substantial fraction of the remaining Neoproterozoic zircons which possess negative ε_Hf(t) values bears evidence for mixing of old crust with juvenile magmas, implying crustal growth in an Andean-type setting was also significant in this region.     

Paleo- to Eoarchean crustal evolution in eastern Hebei,North China Craton: New evidence from SHRIMP U–Pb dating and in-situ Hf isotopic study of detrital zircons from paragneisses

In the Caozhuang complex in eastern Hebei, North China Craton, the Paleo- to Eoarchean crustal evolution was earlier revealed by the preservation of detrital zircon grains older than (or as old as) 3.8 Ga in fuchsite-quartzite. In order to test if the Eoarchean antiquity is also preserved in rocks other than the fuchsite quartzite, we collected two paragneisses, a hornblende gneiss and a garnet–biotite gneiss, from Huangbaiyu village and dated their detrital zircon grains. The zircon dating of the hornblende gneiss yielded concordant ²⁰⁷Pb/²⁰⁶Pb ages ranging from 3684 to 3354 Ma. However, an older date of 3782 Ma with 18% discordancy was also obtained. Detrital zircon grains from the garnet–biotite gneiss gave a similar ²⁰⁷Pb/²⁰⁶Pb age range, from 3838 to 3342 Ma. The metamorphic domains of the zircon grains from both samples, including the strongly recrystallized cores and rims, recorded an overprinting metamorphism at ca. 2.5 Ga, which correlates with the most widespread tectono-thermal event in the North China Craton. In situ zircon Hf-isotope analyses on the dated zircon grains yielded a wide range of model ages (T_DM1) from 4.0 to 3.3 Ga with corresponding ε_Hf(T) from −36.0 to +4.8. This suggests that the evolution of the crustal segment in this area has involved multiple phases of juvenile crustal addition as well as recycling of older crustal rocks. The new geochronological results imply the presence of a significant amount of Eoarchean crustal fragments in the eastern Hebei area. The sedimentary protoliths of the paragneisses and other high-grade metamorphic rocks in the Caozhuang complex were probably deposited between 3.4 and 2.5 Ga.     

Detrital zircon U–Pb age and Hf isotope constrains on the generation and reworking of Precambrian continental crust in the Cathaysia Block,South China: A synthesis

The South China Block, consisting of the Yangtze and the Cathaysia blocks, is one of the largest Precambrian blocks in eastern Asia. However, the early history of the Cathaysia Block is poorly understood due largely to intensive and extensive reworking by Phanerozoic polyphase orogenesis and magmatism which strongly overprinted and obscured much of the Precambrian geological record. In this paper, we use the detrital zircon U–Pb age and Hf isotope datasets as an alternative approach to delineate the early history of the Cathaysia Block. Compilation of published 4041 Precambrian detrital zircon ages from a number of (meta)sedimentary samples and river sands exhibits a broad age spectrum, with three major peaks at ~ 2485 Ma, ~ 1853 Ma and ~ 970 Ma (counting for ~ 10%, ~ 16% and ~ 24% of all analyses, respectively), and four subordinate peaks at ~ 1426 Ma, ~ 1074 Ma, ~ 780 Ma and ~ 588 Ma. Five of seven detrital zircon age peaks are broadly coincident with the crystallisation ages of ~ 1.89–1.83 Ga, ~ 1.43 Ga, ~ 1.0–0.98 Ga and ~ 0.82–0.72 Ga for known igneous rocks exposed in Cathaysia, whereas, igneous rocks with ages of ~ 2.49 Ga and ~ 0.59 Ga have not yet been found. The Hf isotopic data from 1085 detrital zircons yield Hf model ages (T_DM^C) between ~ 4.19 Ga and ~ 0.81 Ga, and the calculated εHf(t) values between − 40.2 and 14.4. The Archean detrital zircons are exclusively oval in shape with complicated internal textures, indicating that they were sourced by long distance transportations and strong abrasion from an exotic Archean continent. In contrast, the majority of detrital zircons in age between ~ 1.9 and ~ 0.8 Ga are euhedral to subhedral crystals, indicative of local derivation by short distance transportations from their sources. The oldest crustal basement rocks in Cathaysia were most likely formed by generation of juvenile crust and reworking of recycled Archean components in Late Paleoproterozoic at ~ 1.9–1.8 Ga, rather than in the Archean as previously speculated. Reworking and recycling of the continental crust are likely the dominant processes for the crustal evolution of Cathaysia during the Mesoproterozoic to Neoproterozoic time, with an intervenient period of significant generation of juvenile crust at ~ 1.0 Ga.Precambrian crustal evolutions of the Cathaysia Block are genetically related to the supercontinent cycles. By comparing detrital zircon data from Cathaysia with those for other continents, and integrating multiple lines of geological evidence, we interpret the Cathaysia Block as an orogenic belt located between East Antarctica, Laurentia and Australia during the assembly of supercontinent Columbia/Nuna at ~ 1.9–1.8 Ga. The Cathaysia Block amalgamated with the Yangtze Block to form the united South China Block during the Sibao Orogeny at ~ 1.0–0.89 Ga. The Laurentia–Cathaysia–Yangtze–Australia–East Antarctica connection gives the best solution to the paleo-position of Cathaysia in supercontinent Rodinia. The significant amount of ~ 0.6–0.55 Ga detrital zircons in Cathaysia and West Yangtze have exclusively high crustal incubation time of > 300 Ma, indicating crystallisation from magmas generated dominantly by crustal reworking. This detrital zircon population compares well with the similar-aged zircon populations from a number of Gondwana-derived terranes including Tethyan Himalaya, High Himalaya, Qiangtang and Indochina. The united South China–Indochina continent was likely once an integral part of Gondwanaland, connected to northern India by a “Pan-African” collisional orogen.     

Constraints of Sr isotopic compositions of apatite and carbonates on the origin of Fe and Cu mineralizing fluids in the Lala Fe-Cu-(Mo,LREE) deposit,SW China

Numerous Fe-Cu deposits with mineralization styles similar to iron oxide-copper gold (IOCG) deposits form the Kangdian Fe-Cu metallogenic province, southwestern (SW) China. As one of the largest deposits in the region, the ~ 1.0 Ga Lala Fe-Cu deposit is hosted in a Paleoproterozoic volcanic-sedimentary succession named the Hekou Group which is alternately intruded by ~ 1.0 Ga doleritic plutons. This deposit has a paragenetic sequence evolving from Stage I of Na-alteration to Stage II of Fe mineralization, and finally to Stage III of Cu-(Mo, REE) mineralization, coeval with mafic-felsic intra-plate magmatism in the region. This study conducted in-situ Sr isotopic analyses on apatite and carbonate, aiming to resolve the long controversial issue regarding the origin of the Fe and Cu mineralizing fluids in the deposit. Apatite of Stage II has ⁸⁷Sr/⁸⁶Sr ratios varying from 0.71380 to 0.72733, much higher than those of synchronous igneous rocks in the region (0.7074 to 0.7091), but similar to the Paleoproterozoic host rocks (0.71368 to 0.71837 at ~ 1.0 Ga). This similarity indicates that radiogenic Sr of the Fe mineralizing fluid was dominantly sourced from the host rocks. Apatite and calcites of Stage III have ⁸⁷Sr/⁸⁶Sr ratios (0.75758–0.79293) much higher than apatite of Stage II and the host rocks but similar to the Archean basement rocks (as high as 0.80 at ~ 1.0 Ga) beneath the cover of the Yangtze Block, suggesting that the highly radiogenic Sr isotopic composition of the Cu mineralizing fluid was mainly inherited from the old basement rocks. In combination with previous C-O-S isotopic data indicating a magma-hydrothermal origin, it was suggested that the Fe mineralizing fluid was exsolved from a mafic magma that generated the ~ 1.0 Ga doleritic plutons, and inherited radiogenic Sr from the host rocks during fluid-rock interaction. By contrast, the Cu mineralizing fluid might have been sourced from another pulse of magmatic, Cu-Mo-REE- and CO₂-rich fluid which have once interacted with Archean basement rocks prior to mineralization. The source of such a Cu-Mo-REE-rich fluid was not well constrained in current study but was inferred to be exsolved from a hidden felsic magma. We propose that intrusions of the bimodal magmas in Kangdian are responsible for regional hydrothermal circulation which led to Fe-Cu-(Mo, REE) mineralization in the Kangdian province.     

Diagenetic alteration of a Mesozoic fluvial gold placer deposit,southern New Zealand

Gold paleoplacers become progressively more affected by diagenetic processes with age and burial. Mesozoic paleoplacer deposits in southern New Zealand display intermediate stages of diagenetic transformation compared to little-affected Late Cenozoic paleoplacers and strongly-affected Paleozoic and Precambrian paleoplacers. The Mesozoic (Cretaceous) diagenesis resulted in near-pervasive alteration, cementation and lithification of the paleoplacer. Lithic clasts and matrix have been extensively altered to illite, ferrous iron-bearing smectite-vermiculite, and kaolinite, and the cement consists mainly of clays and calcite. Diagenetic pyrite, marcasite, vivianite, and Mn oxide also contributed to cementation. Alteration occurred under near-surface (<500 m depth) conditions with groundwater that had circumneutral pH, high alkalinity, and elevated dissolved K, Mg and Ca. Detrital albite remained unaffected by alteration. Detrital gold has been variably dissolved and redeposited, with widespread formation of gold overgrowths on the 1–10 μm scales, with 1–3 wt% Ag. Gold mobility was driven by reduced sulphur complexes in the low redox, high pH diagenetic environment. The overgrowth gold locally contributed to cementation of fine clastic grains, and has intergrown with diagenetic clays and Mn oxide. Post-diagenetic oxidation of the paleoplacer deposit has transformed much of the pyrite to ferric oxyhydroxide and deposited some ferric oxyhydroxide coatings on gold. These oxidation processes have had only minor effects on gold mobility and textures. Hence, the low redox conditions of diagenetic gold mobility were distinctly different from those typically associated with oxidation-related supergene gold mobility. Diagenesis can affect economics of paleoplacer mining by hindering rock disaggregation during processing, coating gold particles with secondary minerals, and increasing the clay content of the deposit, all of which can lower the efficiency of gold recovery.     

SHRIMP data on zircons from the Wanzi series: Constraints on the rock formation time and implications of migmatization at 2.1–2.0 Ga in the Fuping Complex,North China Craton

In most of the rocks of the Fuping Complex in the Trans-North China Orogen, a large quantity of leucocratic veins, dykes and granitic intrusions are present as migmatites. The incongruent partial melting of biotite to hornblende suggests the water-fluxed anatexis of the migmatization of which the melt is manifested by the euhedral crystallization of some quartzofeldspathic minerals and presence of albite rim on earlier plagioclase or K-feldspar. Importantly, some melt may be segregated in the sillimanite gneisses or extracted and injected as faint dispersed melt into the competent metasedimentary quartzite of the Wanzi series rocks. The quartzite has plenty of zircon ages cluster at c 2.5 Ga and some at c 2.1–2.0 Ga, the former may be detrital zircon age, while the latter represents the age of migmatization or crystallization of the injected melt, not detrital zircon ages as conventionally believed. The data have constrained that the sedimentation time of the protolith of the quartzite of the Wanzi series was between c 2.5 Ga and c 2.1–2.0 Ga, not younger than c 2.1–2.0 Ga as once proposed. Therefore it must be cautious in applying detrital zircon ages in constraining the time of sedimentation if the rocks have undergone middle-high grade metamorphism, especially when migmatization was involved in the metamorphism process.     

Paleoproterozoic arc and ophiolitic rocks on the northwest-margin of the Trans-Hudson Orogen,Saskatchewan, Canada: their contribution to a revised tectonic framework for the orogen

A new lithotectonic framework for the northwestern Reindeer Zone of the Trans-Hudson Orogen divides rocks into five northwest- to north-dipping volcano-sedimentary assemblages: (1) at the structural base, the 1.92–1.87 Ga largely sedimentary Levesque Bay Assemblage (partly equivalent to former ‘MacLean Lake gneisses’), which lies within the confines of the Kisseynew Domain and is tectonically imbricated with metasedimentary rocks of the <1.85 Ga McLennan and Burntwood groups; (2) the turbiditic Duck Lake Assemblage, also located along the northern edge of the Kisseynew Domain; it contains detrital zircons ranging in age between 1.92 and 1.87 Ga; (3) the ?1.92 Ga mafic–ultramafic volcano-plutonic Lawrence Point Assemblage of the La Ronge Domain; (4) the ≥1.88 Ga felsic to intermediate volcano-plutonic Reed Lake Assemblage of the La Ronge Domain; and (5) the turbiditic Milton Island Assemblage of the Rottenstone Domain, which contains detrital zircons ranging in age between 2.83 and 1.86 Ga. The assemblages are intruded by a variety of 1.91–1.78 Ga mafic to felsic plutons.The Lawrence Point Assemblage is interpreted as a dismembered supra-subduction zone ophiolite. High-MgO refractory harzburgite (‘Group 1’ ultramafic rocks), at the structural base of the assemblage, is geochemically identical to the upper mantle section of selected supra-subduction zone ophiolites and mantle tectonites. Chromite and olivine compositions of the ‘Group 1’ ultramafic rocks are also comparable to those of ophiolitic harzburgite and mantle tectonite. Mafic metavolcanic rocks of the assemblage are classified as subalkaline tholeiitic basalts. Their trace element patterns and Hf, Ta, Th, Y, Nb, and La element ratios resemble those of modern back-arc basin basalts. The Reed Lake Assemblage represents a subduction-generated arc complex that was built on top of the Lawrence Point Assemblage; its mafic metavolcanic rocks are subalkaline basalts, with calc-alkaline trends, and elevated Th and Ce concentrations and negative Nb anomalies. Feldspar porphyry dykes intruding the Lawrence Point and Duck Lake assemblages constrain timing of Lawrence Point ophiolite emplacement onto the Duck Lake Assemblage to 1.86–1.84 Ga. The trace element geochemistry of the dykes suggests continued arc volcanism after ophiolite emplacement. Mafic metavolcanic rocks of the Levesque Bay Assemblage are geochemically similar to those of the Lawrence Point Assemblage. Other ultramafic rocks (peridotite to pyroxenite) are abundant in the Lawrence Point Assemblage, but have similar geochemistry to small ultramafic bodies intruding the Reed Lake, Duck Lake and Levesque Bay Assemblages. They represent a separate, later phase (?1.86 Ga) of ultramafic plutonism, which post-dates ophiolite emplacement.Timing of Lawrence Point ophiolite emplacement (between 1.86 and 1.84 Ga) and geochemistry of later felsic and mafic/ultramafic volcanism suggest that the Lawrence Point ophiolite and overlying Reed Lake arc assemblage were not accreted to the Hearne Craton prior to 1.86 Ga, but were first accreted to the Flin Flon–Glennie Complex after 1.86 Ga.