Comparison of organic geochemistry and metal enrichment in two black shales: Cambrian Alum Shale of Sweden and Devonian Chattanooga Shale of United States

In most black shales, such as the Chattanooga Shale and related shales of the eastern interior United States, increased metal and metalloid contents are generally related to increased organic carbon content, decreased sedimentation rate, organic matter type, or position in the basin. In areas where the stratigraphic equivalents of the Chattanooga Shale are deeply buried and and the organic material is thermally mature, metal contents are essentially the same as in unheated areas and correlate with organic C or S contents. This paradigm does not hold for the Cambrian Alum Shale Formation of Sweden where increased metal content does not necessarily correlate with organic matter content nor is metal enrichment necessarily related to land derived humic material because this organic matter is all of marine source. In southcentral Sweden the elements U, Mo, V, Ni, Zn, Cd and Pb are all enriched relative to average black shales but only U and Mo correlate to organic matter content. Tectonically disturbed and metamorphosed allochthonous samples of Alum Shale on the Caledonian front in western Sweden have even higher amounts for some metals (V, Ni, Zn and Ba) relative to the autochthonous shales in this area and those in southern Sweden.     

Isotope geochronology,geochemistry, and mineral chemistry of the U-bearing and barren granites from the Zhuguangshan complex,South China: Implications for petrogenesis and uranium mineralization

The Zhuguangshan complex carries some of the most important granite-hosted uranium deposits in South China. Here we investigate the Changjiang and Jiufeng granites which represent typical U-bearing and barren granites in the complex, using zircon U-Pb ages, whole-rock geochemistry, Sr-Nd isotopic and zircon Hf isotopic data, and mineral chemistry, to constrain the petrogenesis and uranium mineralization. LA-ICP-MS zircon U-Pb dating shows that both the Changjiang and Jiufeng granites were emplaced ca. 160 Ma. These rocks show high silica, weakly to strongly peraluminous compositions, enrichment in Rb, Th, and U, and depletion in Ba, Nb, Sr, P, and Ti. These features coupled with the high initial ⁸⁷Sr/⁸⁶Sr ratios, negative εNd(t) values and εHf(t) values, and the Paleoproterozoic two stage model ages of these two granites suggest that the two granites belong to S-type granites, and the parental magmas of the two granites were derived from the Paleoproterozoic metasedimentary rocks. However, the granitoids show different mineralogical characteristics. The biotite in the Changjiang granite belongs to siderophyllite, marking higher degree of chloritization, whereas the biotite in the Jiufeng granite is ferribiotite, characterized by only slight chloritization. Compared with the Jiufeng granite, the biotite in the Changjiang granite has lower crystallization temperature and oxygen fugacity, but higher F content, and the uraninite has higher UO₂ content but lower ThO₂ content, and stronger corrosion. The chemical ages of uraninites from both granites are (within error) consistent with the zircon U-Pb ages and are considered to represent the emplacement ages of granites. Chemical ages of pitchblende in the Changjiang granite yield 118 ± 8 Ma, 87 ± 4 Ma, and 68 ± 6 Ma, representing multiple episodes of hydrothermal events that are responsible for the precipitation of U ores in the Changjiang uranium ore field. Our study suggests that the degree of magma differentiation and physicochemical conditions of the magmatic-hydrothermal system are the key factors that control the different U contents of these two granites. The mineralogical characteristics of uraninite and biotite can be used to distinguish between U-bearing and barren granites, and serve as a potential tool for prospecting granite-hosted uranium deposits.     

Uraniferous bitumen nodules in the Talvivaara Ni–Zn–Cu–Co deposit (Finland): influence of metamorphism on uranium mineralization in black shales

In the central part of the Fennoscandian Shield, the Talvivaara Ni–Zn–Cu–Co deposit, hosted by Palaeoproterozoic metamorphosed black schists, contains low uranium concentrations ranging from 10 to 30 ppm. The Talvivaara black schists were deposited 2.0–1.9 Ga ago and underwent subsequent metamorphism during the 1.9–1.79 Ga Svecofennian orogeny. Anhedral uraninite crystals rimmed by bitumen constitute the main host of uranium. U–Pb secondary ion mass spectrometry dating indicates that uraninite crystals were formed between 1,878?±?17 and 1,871?±?43 Ma, during peak metamorphism. Rare earth element patterns and high Th content (average 6.38 wt%) in disseminated uraninite crystals indicate that U was concentrated during high temperature metamorphism (>400 °C). The formation of bitumen rims around uraninite may be explained by two distinct scenarios: (a) a transport of U coincident with the migration of hydrocarbons or (b) post-metamorphic formation of bitumen rims, through radiolytic polymerization of gaseous hydrocarbons at the contact with uraninite.     

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.     

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.     

Magmatic and hydrothermal behavior of uranium in syntectonic leucogranites: The uranium mineralization associated with the Hercynian Guérande granite (Armorican Massif,France)

Most of the hydrothermal uranium (U) deposits from the European Hercynian belt (EHB) are spatially associated with Carboniferous peraluminous leucogranites. In the southern part of the Armorican Massif (French part of the EHB), the Guérande peraluminous leucogranite was emplaced in an extensional deformation zone at ca. 310 Ma and is spatially associated with several U deposits and occurrences. The apical zone of the intrusion is structurally located below the Pen Ar Ran U deposit, a perigranitic vein-type deposit where mineralization occurs at the contact between black shales and Ordovician acid metavolcanics. In the Métairie-Neuve intragranitic deposit, uranium oxide-quartz veins crosscut the granite and a metasedimentary enclave.Airborne radiometric data and published trace element analyses on the Guérande leucogranite suggest significant uranium leaching at the apical zone of the intrusion. The primary U enrichment in the apical zone of the granite likely occurred during both fractional crystallization and the interaction with magmatic fluids. The low Th/U values (< 2) measured on the Guérande leucogranite likely favored the crystallization of magmatic uranium oxides. The oxygen isotope compositions of the Guérande leucogranite (δ¹⁸O_{whole rock} = 9.7–11.6‰ for deformed samples and δ¹⁸O_{whole rock} = 12.2–13.6‰ for other samples) indicate that the deformed facies of the apical zone underwent sub-solidus alteration at depth with oxidizing meteoric fluids. Fluid inclusion analyses on a quartz comb from a uranium oxide-quartz vein of the Pen Ar Ran deposit show evidence of low-salinity fluids (1–6 wt.% NaCl eq.), in good agreement with the contribution of meteoric fluids. Fluid trapping temperatures in the range of 250–350 °C suggest an elevated geothermal gradient, probably related to regional extension and the occurrence of magmatic activity in the environment close to the deposit at the time of its formation. U-Pb dating on uranium oxides from the Pen Ar Ran and Métairie-Neuve deposits reveals three different mineralizing events. The first event at 296.6 ± 2.6 Ma (Pen Ar Ran) is sub-synchronous with hydrothermal circulations and the emplacement of late leucogranitic dykes in the Guérande leucogranite. The two last mineralizing events occur at 286.6 ± 1.0 Ma (Métairie-Neuve) and 274.6 ± 0.9 Ma (Pen Ar Ran), respectively. Backscattered uranium oxide imaging combined with major elements and REE geochemistry suggest similar conditions of mineralization during the two Pen Ar Ran mineralizing events at ca. 300 Ma and ca. 275 Ma, arguing for different hydrothermal circulation phases in the granite and deposits. Apatite fission track dating reveals that the Guérande granite was still at depth and above 120 °C when these mineralizing events occurred, in agreement with the results obtained on fluid inclusions at Pen Ar Ran.Based on this comprehensive data set, we propose that the Guérande leucogranite is the main source for uranium in the Pen Ar Ran and Métairie-Neuve deposits. Sub-solidus alteration via surface-derived low-salinity oxidizing fluids likely promoted uranium leaching from magmatic uranium oxides within the leucogranite. The leached out uranium may then have been precipitated in the reducing environment represented by the surrounding black shales or graphitic quartzites. As similar mineralizing events occurred subsequently until ca. 275 Ma, meteoric oxidizing fluids likely percolated during the time when the Guérande leucogranite was still at depth. The age of the U mineralizing events in the Guérande region (300–275 Ma) is consistent with that obtained on other U deposits in the EHB and could suggest a similar mineralization condition, with long-term upper to middle crustal infiltration of meteoric fluids likely to have mobilized U from fertile peraluminous leucogranites during the Late Carboniferous to Permian crustal extension events.     

Shoshonite-latite series of the Eastern Transbaikalia: 40Ar/39Ar age,geochemistry, and Sr-Nd isotope composition of rocks from the Akatui volcano-plutonic association of the Aleksandrovskii Zavod depression

The paper presents new data on age, geochemistry, and Sr and Nd isotope composition of rocks from the Akatui massif and comagmatic rocks from the lower unit of the Kailas Formation (Akatui volcano-plutonic association), localized within the Aleksandrovskii Zavod depression. The amphibole ⁴⁰Ar/³⁹Ar age date the monzogabbro of the early phase of the Akatui massif at 154.8 ± 4.4 Ma; the monzonite of the main phase yields a ⁴⁰Ar/³⁹Ar age of 160.7 ± 3.9 Ma, and the shoshonite basalt of the lower unit of the Kailas Formation yields a ⁴⁰Ar/³⁹Ar age of 161.5 ± 1.7 Ma. The leading petrogenetic mechanism for the Akatui volcano-plutonic association is crystal fractional differentiation of melts with minor crustal contamination, which can be suggested from the mineralogical and petrographic features and geochemical and isotope characteristics of rocks. The geochemical data for the Akatui volcano-plutonic association show LILE, LREE, U, Th, and Pb enrichment with a characteristic depletion in high-field strength elements (HFSE), such as Nb and Ti. They are also depleted in P. Sr-Nd isotope data (⁸⁷Sr/⁸⁶Sr(₁6_{0 Ma}) = 0.70642-0.70688 and £Nd(160 _Ma) = − 0.6 to − 2.2) suggest an EMII-type mantle source and could also indicate a negligible degree of crustal contamination in the evolved melts.     

The effects of radioactive decay of uranium on elemental and isotopic ratios of Alum shale kerogen

The organic matter in the Alum Shale of Sweden is believed to have been affected post-depositionally by irradiation from the natural decay of U. Alum Shale kerogen H/C ratios are inversely proportional to the natural log of the U concentration, presumably as a result of the liberation of H by irradiation of the organic matter. Stable isotopic ratios of¹³C/¹²C in Alum Shale kerogen are directly proportional to the natural log of the U concentration. Experimental irradiation of Green River shale generated hydrocarbon gases 18% lighter than the parent organic matter, which demonstrates the possibility that irradiation induced generation of isotopically light gases could lower¹³C/¹²C ratios in parent organic matter. Irradiation may be a factor governing the relation between¹³C/¹²C ratios in the Alum Shales. Alum Shale O/C ratios generally increase with increasing U concentration and it is suggested that irradiation of organic matter may facilitate oxidation. The “Rock-Eval” maturity parameters “P.I.” and “Tmax” decrease with increasing U concentration. “P.I.” is presumed to decrease as a result of bitumen destruction or polymerization by irradiation.     

U–Pb detrital zircon data of the Rio Fuerte Formation (NW Mexico): Its peri-Gondwanan provenance and exotic nature in relation to southwestern North America

U–Pb detrital zircon studies in the Rio Fuerte Group, NW Mexico, establish its depositional tectonic setting and its exotic nature in relation to the North American craton. Two metasedimentary samples of the Rio Fuerte Formation yield major age clusters at 453–508 Ma, 547–579 Ma, 726–606 Ma, and sparse quantities of older zircons. The cumulative age plots are quite different from those arising from lower Paleozoic miogeoclinal rocks of southwestern North America and of Cordilleran Paleozoic exotic terranes such as Golconda and Robert Mountains. The relative age-probability plots are similar to some reported from the Mixteco terrane in southern Mexico and from some lower Paleozoic Gondwanan sequences, but they differ from those in the Gondwanan-affinity Oaxaca terrane. Major zircon age clusters indicate deposition in an intraoceanic basin located between a Late Ordovician magmatic arc and either a peri-Gondwanan terrane or northern Gondwanaland. The U–Pb magmatic ages of 151 ± 3 Ma from a granitic pluton and 155 ± 4 Ma from a granitic sill permit a revision of the stratigraphic and tectonic evolution of the Rio Fuerte Group. A regional metamorphism event predating the Late Jurassic magmatism is preliminarily ascribed to the Late Permian amalgamation of Laurentia and Gondwana. The Late Jurassic magmatism, deformation, and regional metamorphism are related to the Nevadan Orogeny.     

In situ U–Pb dating and trace element analysis of zircons in thin sections of eclogite: Refining constraints on the ultra high-pressure metamorphism of the Sulu terrane,China

In situ U–Pb dating and trace element analysis of zircons, combined with a textural relationship investigation in thin section, is a powerful tool to constrain the ultra high-pressure stage of high-grade metamorphism. Two types of zircon grains have been identified in thin sections of a retrograde eclogite from the main hole of the Chinese Continental Scientific Drill project in the Sulu UHP terrane. Type 1 zircon grains occur as inclusions in fresh garnet and omphacite, and Type 2 zircon grains were found in symplectite around omphacite. The fresh rims of Type 1 zircons and mantles of a few Type 2 zircons exhibit remarkably lower REE, Y, Nb and Ta contents than the inherited zircon cores, suggesting coeval growth with garnet, rutile and apatite during UHP metamorphism. These may have formed in the UHP metamorphism and survived retrograde metamorphism. The weighted average ²⁰⁶Pb/²³⁸U age of these zircon domains (230 ± 4 Ma, 2σ) agrees well with the published age of coesite-bearing zircon separates (230 ± 1 Ma, 2σ), suggesting that the peak UHP metamorphism in the Sulu terrane may have occurred at ~ 230 Ma.Zircon domains surrounded or cut across by symplectite could have been altered by retrograde metamorphism. Together, they provide a younger weighted average ²⁰⁶Pb/²³⁸U age of 209 ± 4 Ma (2σ). These retrograde zircon domains have similar REE compositions to the ~ 230 Ma UHP zircon domains. These observations imply that the ~ 209 Ma zircon domains could have formed by fluid activity-associated alterations in the amphibolite-facies metamorphism, which could have resulted in the complete loss of Pb but not REEs in these domains.     

Coupled uranium mineralisation and bacterial sulphate reduction for the genesis of the Baxingtu sandstone-hosted U deposit,SW Songliao Basin,NE China

The Baxingtu deposit is a typical redox front tabular-shaped uranium deposit hosted in sandstones of the Late Cretaceous Yaojia Formation deposited within a braided river environment during the post-rift stage of the Songliao Basin, in northeast China. This study proposes the first metallogenic model for the Baxingtu deposit and provides new data on genetic processes involved in the uranium mineralisation of sandstone-type deposits that were characterised through petrographic observations, whole-rock geochemistry, and geochemical and/or mineralogical study of iron disulphide, uranium minerals, Fe-Ti oxides (EPMA, LA-ICP-MS), and organic matter (REP). The δ³⁴S value has been measured in situ by SIMS on the different generations of iron disulphide.Within regional primary reduced sandstones, pre-ore uranium enrichment (U_mean = 7.6 ppm in whole rock) was identified on altered Fe-Ti oxides along with minor concentrations on organic matter (respectively 26.3% and 1.3% of the whole-rock U content), which together represent a significant source of uranium for the mineralisation. Additional pre-ore uranium concentrations may also be associated with clay minerals. Petrographic observations and REP data indicate that organic matter occurring in the host-sandstone is mainly inherited from land plants and corresponds to type III or type IV kerogens. Ore-stage iron disulphides largely occur as framboids and in replacement of organic matter or also as sub-idiomorphic to idiomorphic cement and crystal. Trace element signatures detected within framboids are likely indicative of formation mainly from a single event. Framboids and iron disulphide in replacement of organic matter have a light sulphur isotope signature characterised by δ³⁴S values from −72.0 to −6.2‰, suggesting that sulphur originated from bacterial sulphate reduction, which was mainly responsible for (1) the liberation of U from Fe-Ti oxides and organic matter, (2) the generation of ore-stage iron disulphides, (3) the bioreduction of uranium and (4) the production of a secondary H₂S-rich reducing barrier also involved in uranium reduction. Uranyl and sulphate ions were transported through the host sandstone by low-temperature oxygenated groundwater and U(IV) was precipitated at the redox interface as nano to microcrystals of pitchblende and coffinite, dominantly associated with bacterial substrate and as intergrowth with biogenic iron disulphide or directly associated with organic matter and residual Ti-Fe oxides. The uranium mineralisation does not replace ore-stage iron disulphides. Therefore, the combined mineralogical, geochemical, and isotopic characteristics of the Baxingtu tabular uranium deposit characterise dominantly biogenic processes for the genesis of the uranium mineralisation.     

Migration of brines in the basement rocks of the Athabasca Basin through microfracture networks (P-Patch U deposit,Canada)

The unconformity-type uranium deposits of the Athabasca Basin (Saskatchewan, Canada) are hosted near the unconformity between a middle Proterozoic intracratonic sedimentary basin and an Archean to Paleo-Proterozoic metamorphic and plutonic basement. These deposits, which are considered to be the richest U deposits in the world, are the result of massive basinal fluid migrations in the basement rocks.This study shows that basinal brines have strongly penetrated into the basement not only through faults and major pathways but also by way of dense networks of microfractures which favoured the percolation of fluids down to considerable depths (hundred metres below the unconformity) and their chemical modification (salinity increase) by interaction with basement lithologies. These processes are one of the major causes of uranium mobility within the basement rocks and the formation of unconformity-type mineralization.Microfracture networks, which opened during the basinal brine stage (ca. 1600–1400 Ma) are interpreted as sets of mode I cracks corresponding to a specific stage of deformation and occur as fluid inclusion planes after healing. The stress field at that stage (σ₁ = N130–150 °E, subvertical) partly reopened the earlier microcrack networks (σ₁ = N80–110 °E and N130–150 °E, subvertical) issued from the Trans-Hudson Orogeny late retrograde metamorphic stage (ca. 1795–1720 Ma). The circulation of the two types of fluids (carbonic and brines) occurs thus at two distinct events (Trans-Hudson Orogeny late retrograde metamorphism for carbonic fluids and maximal burial diagenesis for brines) but the same main microfissure geometry was used by the fluids. This demonstrates the existence of a similar stress field direction acting before and after the basin formation. Moreover, the brine circulations in the basement acted in a wider volume than the clay-rich alteration halo surrounding the U-ores, generally considered as the main envelope of fluid percolation outside the fault systems. The data on the chemistry of the fluids and on the geometry of their migration at various scales emphasise the fundamental role of the basement in the chemical evolution of highly saline brines linked to unconformity-related uranium mineralization in the Athabasca Basin.     

The Triassic age for oceanic eclogites in the Dabie orogen: Entrainment of oceanic fragments in the continental subduction

Low-temperature and high-pressure eclogites with an oceanic affinity in the western part of the Dabie orogen have been investigated with combined Lu–Hf and U–Pb geochronology. These eclogites formed over a range of temperatures (482–565 °C and 1.9–2.2 GPa). Three eclogites, which were sampled from the Gaoqiao country, yielded Lu–Hf ages of 240.7 ± 1.2 Ma, 243.3 ± 4.1 Ma and 238.3 ± 1.2 Ma, with a corresponding lower-intercept U–Pb zircon age of 232 ± 26 Ma. Despite the well-preserved prograde major- and trace-element zoning in garnets, these Lu–Hf ages mostly reflect the high-pressure eclogite-facies metamorphism instead of representing the early phase of garnet growth due to the occurrence of omphacite inclusions from core to rim and the shell effect. An upper-intercept zircon U–Pb age of 765 ± 24 Ma is defined for the Gaoqiao eclogite, which is consistent with the weighted-mean age of 768 ± 21 Ma for the country gneiss. However, the gneiss has not been subjected to successive high-pressure metamorphism. The new Triassic ages are likely an estimate of the involvement of oceanic fragments in the continental subduction.     

Genesis of the Yuanlingzhai porphyry molybdenum deposit,Jiangxi province,South China: Constraints from petrochemistry and geochronology

The newly discovered Yuanlingzhai porphyry molybdenum (Mo) deposit in southern Jiangxi province belongs to the group of Mo-only deposits in the Nanling region. The mineralization developed at contact zones between the Yuanlingzhai granite porphyry and Neoproterozoic metamorphic rocks of the Xunwu Formation. Precise LA–MC–ICPMS zircon U–Pb dating of the Yuanlingzhai porphyry, as well as the adjacent western Keshubei and eastern Keshubei granites, yielded ages of 165.49 ± 0.59 Ma, 159.68 ± 0.43 Ma, and 185.13 ± 0.52–195.14 ± 0.63 Ma, respectively. Molybdenite Re–Os isochron ages of the ores are 160 ± 1–162.7 ± 1.1 Ma, which is consistent with the age of large-scale W–Sn deposits in South China. The Yuanlingzhai porphyry is characterized by high K₂O, P₂O₅, and A/CNK (1.33–1.59), and low CaO and Na₂O. The rock shows relatively enriched LREE without significant Eu anomalies (Eu/Eu^* = 0.80–0.90). Geochemical and mineralogical characteristics indicate that the ore-hosting porphyry is a typical S-type granite generated from the partial melting of crustal material with only minor mantle contribution. Both Harker and evolutionary discrimination diagrams indicate that the Yuanlangzhai and western Keshubei granites are not products of co-magmatic evolution. The Keshubei granites and Xunwu Formation were not significant sources for the components in the porphyry mineralization, but the Yuanlangzhai granite may have supplied some ore-forming material. However, the main ore-forming material was carried by fluids from deep sources, as demonstrated by fluid inclusion and stable isotope data from the molybdenum deposit. The Mo porphyry deposit formed in an extensional setting, and was possibly associated with Jurassic subduction of the Izanagi Plate.     

In-situ SIMS uraninite U–Pb dating and genesis of the Xianshi granite-hosted uranium deposit,South China

The southeastern part of the Nanling metallogenic province, China is host to numerous granite-hosted vein-type hydrothermal uranium deposits. The geology and geochemistry of these deposits have been extensively studied. However, accurate and precise ages for the uranium mineralization are scarce because the uranium minerals in these deposits are usually fine grained, and may have formed in several stages. Therefore, the ages previously obtained by the bulk dating techniques are possibly a mixed age.The Xianshi uranium deposit, located in the southeastern part of the Guidong granite complex, is a major uranium deposit in South China. The uranium mineralization from this deposit is mainly fine grained uraninite in quartz or calcite veins which are spatially associated with the Cretaceous mantle-derived mafic dykes. Micro-Raman spectroscopy and X-ray diffraction analyses indicate that the dominant uranium mineral occurs as a rare form of uraninite (U₃O₇). Three distinct generations of uranium minerals have been identified based on petrographic and field relations. Stage 1 uraninite has the lowest UO₂ and highest PbO contents whereas Stage 3 uraninite has the highest UO₂ and lowest PbO contents.Uraninite from the Xianshi deposit was dated using an in-situ SIMS U–Pb dating technique. The results show three distinct age groups: 135 ± 4 Ma, 113 ± 2 Ma and 104 ± 2 Ma, which are in excellent agreement with the ages of three episodes of mantle-derived mafic dykes. Therefore, the Xianshi uranium deposit has experienced at least three hydrothermal events that are responsible for the deposition of uranium ores, which are genetically related to the emplacement of three sets of mafic dykes.     

Geochemistry of black shales from the Mesoproterozoic Srisailam Formation,Cuddapah basin,India: Implications for provenance,palaeoweathering, tectonics,and timing of Columbia breakup

The Mesoproterozoic Srisailam Formation, exposed along the northern part of the Cuddapah basin, India, comprises mainly medium- to fine-grained siliciclastics, and is devoid of any carbonate sediment. Preliminary sedimentological studies helped in recognizing fifteen distinct facies (five facies associations) in Chitrial outlier of the Srisailam Formation deposited in continental half-graben basin(s). Black shales (sensu lato) are minor components of the Srisailam Formation, and inferred to have deposited in deep lacustrine and prodelta facies of the half-graben(s). The black shales show restricted thickness (up to 29.0 m), and are characterized by overall high ‘black shale' to ‘total shale' ratio (>0.51). Their geochemical characteristics were studied to constrain provenance, palaeoclimate, and tectonic setting of deposition of the Srisailam Formation. Further, an attempt has been made to use the Srisailam black shales as proxy for constraining the timing of breakup of the supercontinent Columbia.The Srisailam black shales are geochemically quite distinct. At similar SiO₂ contents they are considerably different from PAAS. They are characterized by considerably lower ΣREE (Av. 136.0 ± 50.4 ppm) but a more conspicuous negative Eu-anomaly (Av. 0.34 ± 0.09) than PAAS. Al₂O₃/TiO₂ and TiO₂/Zr ratios coupled with Eu/Eu*, Gd_CN/Yb_CN, La/Sc, Th/Sc, and Th/Cr ratios suggest their derivation from granite and granodiorite. The CIA values (65–90, Av. 72 ± 9) as a whole indicate moderate chemical weathering under semiarid climate. Discriminating geochemical parameters indicate passive margin depositional setting. The combined sedimentological and geochemical characteristics reveal deposition of the Srisailam sediments in continental rift basin(s).Thick succession of black shales (with high CIA values) that deposited with shelf carbonates proxy for mantle superplume and supercontinent breakup events. The sedimentological characteristics and geochemical data of the Srisailam black shales plausibly exclude any large-scale breakup of Columbia during the interval (1400–1327 Ma) of deposition of the Srisailam Formation.     

Early to Middle Paleozoic tectonometamorphic evolution of the Hongseong area,central western Korean Peninsula: Tectonic implications

The migmatized gneiss (viz. Gwangcheon gneiss) of the southern Hongseong area, central–western Korean Peninsula underwent intermediate to high granulite-facies metamorphism (ca. 9.2–12.0 kbar and 770–870 °C) during Early Silurian to Early Devonian in age. Field and petrographic evidences from the Gwangcheon migmatite provide a clear indication of partial melting. Particularly the migmatized paragneiss is juxtaposed with arc-related doleritic metagabbro, which might be related to the heat source of the migmatization. The migmatized paragneiss has similar arc-related geochemical characteristics, such as depletions in Ta–Nb, Sr, P, and Ti and enrichments in large ion lithophile elements (LILE) caused by the partial melting effect related to the doleritic metagabbro. Sensitive high-resolution ion microprobe (SHRIMP) zircon U–Pb dating of the migmatized paragneiss yielded ages of 432 ± 6 Ma, 431 ± 10 Ma, 421 ± 3 Ma, and 403 ± 3 Ma. The U–Pb ages of inherited zircons from the migmatized paragneiss yielded an age spectra from Neoarchean to Early Paleozoic, with a dominant age population at Neoproterozoic. In contrast, new-growth and recrystallized equant zircons showed dominant concordant Early Silurian to Early Devonian ages that provide critical evidence on the timing of migmatization. These age results and the tectonic signatures are similar to those reported from the Central China Orogenic Belt in China, indicating their tectonic linkage at the northeastern margin of eastern Gondwana during Early to Middle Paleozoic in age.     

Palaeozoic polymetamorphism in the North Qinling orogenic belt,Central China: Insights from petrology and in situ titanite and zircon U–Pb geochronology

The Qinling orogenic belt experienced multiple phases of orogenesis during the Palaeozoic. Unraveling the timing and P–T conditions of these events is the key to understanding the convergence processes between the South China and the North China Blocks. The Songshugou Complex, located in the southern part of the North Qinling orogenic belt, has registered multistage metamorphism in Palaeozoic, and thus potentially provides insights into the tectonic evolution of the Qinling orogenic belt. In this study, three metabasic rocks (a garnet pyroxenite, a garnet amphibolite and a gneissic amphibolite) from the Songshugou Complex were selected for petrological study and zircon and titanite U–Pb dating. Our results show that the metabasic rocks experienced three metamorphic events during the Palaeozoic. The first metamorphic event (M1) is characterized by high pressure conditions. Two zircon grains in equilibrium with garnet and in absence of plagioclase were recognized from the garnet pyroxenite sample. They yielded Ti-in-zircon temperatures of 660–851 °C at ∼12.0 kbar and a weighted mean age of 498 ± 15 Ma, providing the constraints on the temperature and timing of prograde or peak metamorphism (M1-1). Zircons that are inequilibrium with garnet from the garnet pyroxenite and the garnet amphibolite gave U–Pb ages of 494 ± 9 Ma and 484 ± 4 Ma, and Ti-in-zircon temperatures of 793 ± 33 °C and 738 ± 18 °C, respectively. Thus, these zircons were formed on the retrograde amphibolite-facies conditions at ∼8.0 kbar (M1-2). Titanite inclusions were found in actinolite cores of zoned amphibole from the garnet amphibolite. They yielded a U–Pb age of ∼470 Ma and Zr-in-titanite temperature of 676 ± 23 °C at pressure of ∼7.0 kbar, suggesting that the amphibolite-facies retrogression perhaps persisted to ∼470 Ma.Weakly zoned zircons from the garnet amphibolite and inclusion-free titanites from the garnet pyroxenite gave consistent U–Pb ages of 418 ± 5 Ma and 423 ± 10 Ma, and Ti-in-zircon temperature of 742 ± 26 °C and Zr-in-titanite temperature of 764 ± 18 °C at ∼7.0 kbar, respectively. It is suggested that a heating event (M2) is registered by a subsequent phase of amphibolite-facies metamorphism. The ilmenite-bearing titanite crystals from the garnet pyroxenite yielded a U–Pb age of 352 ± 4 Ma, recording a late thermal event (M3).On the basis of combined petrological and geochronological results, we propose a revised tectonic model for the North Qinling orogeny in Palaeozoic. The high pressure granulites were formed by the northward subduction of the Shangdan oceanic slab and the arc-continent collision at ca. 500 Ma. Their exhumation happened at ca. 494–484 Ma as a result of slab breakoff. Subsequent amphibolite-facies metamorphism dated at ca. 440–420 Ma are coeval with the widespread magmatism in the North Qinling Terrane, which are likely caused by the reinitiation northward-subducted of Shangdan oceanic slab. At ca. 350 Ma, the North Qinling Terrane was likely affected by another thermal overprinting event.     

Relationship among titanium,rare earth elements,U–Pb ages and deformation microstructures in zircon: Implications for Ti-in-zircon thermometry

A zircon grain in an orthopyroxene–garnet–phlogopite–zircon–rutile-bearing xenolith from Udachnaya, Siberia, preserves a pattern of crystallographic misorientation and subgrain microstructure associated with crystal–plastic deformation. The zircon grain records significant variations in titanium (Ti) from 2.6 to 30 ppm that corresponds to a difference in calculated Ti-in-zircon temperatures of over several hundred degrees Celsius. The highest Ti concentration is measured at subgrain centres (30 ppm), and Ti is variably depleted at low-angle boundaries (down to 2.6 ppm). Variations in cathodoluminescence coincide with the deformation microstructure and indicate localised, differential enrichment of rare earth elements (REE) at low-angle boundaries. Variable enrichment of U and Th and systematic increase of Th/U from 1.61 to 3.52 occurs at low-angle boundaries. Individual SHRIMP-derived U–Pb ages from more deformed zones (mean age of 1799 ± 40, n = 22) are systematically younger than subgrain cores (mean age of 1851 ± 65 Ma, n = 7), and indicate that open system behaviour of Ti–Th–U occurred shortly after zircon growth, prior to the accumulation of significant radiogenic Pb. Modelling of trace-element diffusion distances for geologically reasonable thermal histories indicates that the observed variations are ~ 5 orders of magnitude greater than can be accounted for by volume diffusion. The data are best explained by enhanced diffusion of U, Th and Ti along deformation-related fast-diffusion pathways, such as dislocations and low-angle (< 5°) boundaries. These results indicate chemical exchange between zircon and the surrounding matrix and show that Ti-in-zircon thermometry and U–Pb geochronology from deformed zircon may not yield information relating to the conditions and timing of primary crystallisation.     

Is the HP–UHP Hong’an–Dabie–Sulu orogen a piercing point for offset on the Tan–Lu fault?

The Tan–Lu fault is a major strike-slip fault in eastern China that appears to offset the high-grade rocks of the Hong’an–Dabie–Sulu orogen left-laterally ∼540 km. We evaluate models for the collision between the South and North China blocks, published radiometric dates recording HP–UHP metamorphism and exhumation in the Hong’an–Dabie and Sulu terranes, and the timing of sinistral motion on the Tan–Lu fault to evaluate whether UHP rocks provide a piercing point for offset on the Tan–Lu fault. UHP metamorphism in Hong’an–Dabie was concurrent with Sulu based on U–Pb dating of coesite-bearing domains of zircon at 244 ± 5–226 ± 2 Ma for Hong’an–Dabie and 243 ± 4–225 ± 2 Ma for Sulu. Retrograde metamorphism began c. 220 Ma for both Hong’an–Dabie and Sulu, but retrograde zircon growth ended c. 214 Ma in Hong’an–Dabie and continued until c. 202 Ma in Sulu based on U–Pb dating of zircon domains external to coesite-bearing domains. Structures in Sulu are rotated 25° counter-clockwise from, but are broadly similar to, Hong’an–Dabie suggesting the two areas have a common Triassic orogenic history that pre-dates motion on the Tan–Lu fault, and that is consistent with paleomagnetic studies. We constructed a pre-Cretaceous restoration of the Hong’an–Dabie–Sulu belt that moves the Sulu terrane south, aligning the suture and the eclogite-facies isograd, and rotates Sulu c. 25° clockwise to re-align structures with Hong’an–Dabie. Our restoration is supported by published data and shows that the Hong’an–Dabie–Sulu orogen is a piercing point for post-collisional offset on the Tan–Lu fault and that these regions shared a common subduction–exhumation history. The Tan–Lu fault did not play a significant role in the Hong’an–Dabie–Sulu collision and likely developed later, in the Early Cretaceous.