Relative timing of deformational,metamorphic and mineralisation events at the Mt. Todd (Yimuyn Manjerr) Mine,Pine Creek Inlier,Northern Territory,Australia

The Mt. Todd Mine (also known as the Yimuyn Manjerr Mine), located approximately 40 km northwest of the township of Katherine, in the Northern Territory, Australia, is host to several discrete ore bodies that strike NNE within a broad NE-trending corridor of gold mineralisation. The mine lies in the southern region of the Central Domain of the Pine Creek Inlier (PCI) and is hosted by a Palaeoproterozoic sequence of rocks termed the Burrell Creek Formation, which is dominated by greywacke, siltstone, sandstone and shale that exhibit sedimentary features akin to those of a river-dominant delta front to prodelta environment. The formation is conformably overlain by volcanoclastic and volcanolithic sedimentary rocks of the Tollis Formation (∼1890 Ma).Three deformation events are recognised in the Mt. Todd region, D1, D2 and D3. The earliest deformation, D1, is characterised by close to tight, NE to N to NW-trending asymmetric folds (F1), and a continuous axial–planar cleavage (S1). The deformation is associated with the development of conjugate buck–quartz veins and was preceded by the emplacement of the Yenberrie Leucogranite, which produced contact metamorphism of the sedimentary rocks of the Burrell Creek Formation to hornblende–hornfels facies (H1), with the development of cordierite porphyroblasts (type C1). D1 was coincident with peak regional metamorphism to greenschist facies.D2 is associated with westerly trending open folds (F2), and a spaced disjunctive to fracture cleavage (S2) in transection to the folds. It was preceded by the emplacement of the Tennysons Leucogranite of the Cullen Batholith (1835–1820 Ma), which produced contact metamorphism of the Yenberrie Leucogranite and the sedimentary rocks of the Burrell Creek and Tollis formations to hornblende–hornfels facies (H2), with the development of cordierite porphyroblasts (type C2).D3 is characterised by the reactivation of strike-slip faults (mostly sinistral), a steeply dipping Type S3-C type foliation, and mesoscopic en échelon folds (F3) that trend oblique to the faults in a left stepping (sinistral) array.The age of emplacement of the Tennysons Leucogranite, and the timing of D1 and D2 are broadly constrained by the age of emplacement of the Cullen Batholith at 1835–1800 Ma. D1 and D2 are correlated with deformation during the Maud Creek Event (∼1850 Ma), while D3 is correlated with deformation during the Shoobridge Event (∼1780 Ma). The age of the Yenberrie Leucogranite is constrained to the age of emplacement of granite batholiths at 1870–1860 Ma.A temporal and broad structural relationship exists between D2 structures, the Tennysons Leucogranite, and the several gold-bearing quartz–sulphide veins and lode systems of the Mt. Todd Mine. The systems appear to have formed after peak thermal metamorphism associated with the emplacement of the pluton at about 1825 Ma, and early in D2, prior to the development of the regional S2 fabric. W–Mo–Sn–Bi–Cu greisen-type mineralisation in the carapace of the Yenberrie Leucogranite of the Yenberrie Wolfram Field constitutes a discrete mineralising event that preceded the emplacement of the Tennysons Leucogranite.     

Evolution of the Mount Woods Inlier, northern Gawler Craton, Southern Australia: an integrated structural and aeromagnetic analysis

Structural mapping integrated with interpretation and forward modelling of aeromagnetic data form complimentary and powerful tools for regional structural analysis because both techniques focus on architecture and overprinting relationships. This approach is used to constrain the geometry and evolution of the sparsely exposed Mount Woods Inlier in the northern Gawler Craton. The Mount Woods Inlier records a history of poly-phase deformation, high-temperature metamorphism, and syn- and post-orogenic magmatism between ca. 1736 and 1584 Ma. The earliest deformation involved isoclinal folding, and the development of bedding parallel and axial planar gneissic foliation (S₁). This was accompanied by high-temperature, upper amphibolite to granulite facies metamorphism at ca. 1736 Ma. During subsequent north–south shortening (D₂), open to isoclinal south–southeast-oriented F₂ folds developed as the Palaeoproterozoic successions of the inlier were thrust over the Archaean nuclei of the Gawler Craton. The syn-D₂ Engenina Adamellite was emplaced at ca. 1692 Ma. The post-D₂ history involved shear zone development and localised folding, exhumation of metamorphic rocks, and deposition of clastic sediments prior to the emplacement of the ca. 1584 Ma Granite Balta Suite. The Mount Woods Inlier is interpreted as the northern continuation of the Kimban Orogen.     

Thrust-controlled, gold quartz-vein mineralisation at the Tom's Gully mine, Northern Territory, Australia

Metasedimentary and minor metavolcanic rocks of the Early Proterozoic Pine Creek Inlier rest unconformably on Late Archaean granitic basement. Three basin-wide, regional deformation events at ca.1885–1870 Ma are recognised: I) W- to NW-verging thrusts and recumbent folds (D₂), II) upright, open to tight, doubly-plunging, NNE- to NNW-trending folds (D₃), and III) open, upright, E-trending folds (D₄). In the centre of the Pine Creek Inlier, post-tectonic granites (1835–1820 Ma) are spatially, temporally and probably genetically associated with mesothermal gold-quartz vein deposits. The Tom's Gully deposit consists of a shallowly S-dipping quartz reef in graphitic shale and siltstone within the thermal aureole of the post-tectonic (1831 ± 6 Ma) Mt Bundey pluton. Gold mineralisation comprises two(?) SSW-plunging sulphidic ore-shoots which are intimately associated with brecciation and recrystallisation of early barren quartz. Where early quartz is absent from the thrust, gold mineralisation is not developed, indicating that this secondary brittle fracturing was essential to sulphide and gold deposition. The ore-shoots plunge parallel to the trend of D₃ fold axes. The reef is hosted by a D₂ thrust fault with transport to the NW. D₃ folds in the hangingwall and footwall decrease in amplitude toward the reef indicating that, during continued E-W compression, the thrust acted as a décollement zone. Field relationships and microstructural studies suggest that quartz and sulphide were deposited in a reactivated thrust during wrench shear along several NNE-trending faults associated with emplacement of the Mt Bundey pluton.     

Correlating multiple deformation events across the Mesoproterozoic NE Australia using foliation intersection axes (FIA) preserved within porphyroblasts

Foliation Intersection/Inflection Axes within porphyroblasts (FIAs) allow the chronological and kinematic linking of deformation episodes with associated metamorphism. Measurement of FIAs in the Mesoproterozoic Eastern Fold Belt (EFB) of the Mount Isa Inlier, NE Australia, has revealed phases of deformation and metamorphism that could not previously be distinguished from one another. Both the ‘asymmetry switch’ and ‘FitPitch’ FIA measurement techniques have been applied to key localities of polymetamorphosed and multiply deformed EFB, and they yielded the same result. These independent techniques have revealed (1) E–W trending structures that formed during N–S bulk shortening (D₁) and associated metamorphism (M₁) formed during a period of orogenesis (O₁) and N–S oriented structures that formed during E–W bulk shortening (D₂) and associated metamorphism (M₂) during a period of orogenesis (O₂), and (2) the crustal scale tectonic processes associated with polymetamorphism. Middle to upper amphibolite facies metamorphic conditions occurred during O₁ with crustal thickening followed by near-isothermal decompression leading to low-pressure/high-temperature (LP/HT) conditions with the emplacement of Williams and Naraku Batholiths around 1550 Ma. This was followed by a second period of middle-to upper-amphibolite facies metamorphism, O₂. This history not only correlates better across the EFB, but also with the tectono-metamorphic model recently proposed for the Mesoproterozoic Georgetown Inlier of the north Australian Craton.     

Geologic and U–Pb geochronologic evidence for early Paleozoic tectonism in the Dadeldhura thrust sheet, far-west Nepal Himalaya

The Dadeldhura thrust sheet inm western Nepal consists of Proterozoic–Lower Paleozoic sedimentary and plutonic rocks, and their metamorphic equivalents, that rest structurally on Proterozoic strata of the Lesser Himalayan sequence. Although regional metamorphism and ductile deformation were widespread during Tertiary thrust emplacement, relicts of early Paleozoic tectonism are preserved locally. New field and geochronologic studies, together with the findings of previous workers, indicate that this early Paleozoic tectonism included: (1) regional metamorphism to at least garnet grade, (2) regional folding of a thick metamorphic sequence into a broad east–west trending syncline, (3) outcrop-scale folding of metasedimentary rocks, (4) emplacement of Cambro–Ordovician granitic bodies during and after the metamorphism and deformation, (5) uplift and erosion of the metamorphic sequence, with garnet-grade rocks locally exposed at the surface, and (6) derivation of Ordovician conglomeratic sandstones from the early Paleozoic orogen. Similar records of metamorphism, deformation, and uplift/erosion have been found in other regions of the Himalaya, indicating that rocks of the Dadeldhura thrust sheet were originally involved in a regionally extensive orogenic system. Future tectonic models of Himalayan orogenesis must accommodate this early Paleozoic event.     

A geochronological framework for orogenic gold mineralisation in central Victoria, Australia

New ⁴⁰Ar/³⁹Ar geochronological data support, and significantly expand upon, preliminary age data that were interpreted to suggest an episodic and diachronous emplacement of gold across the western Lachlan fold belt, Australia. These geochronological data indicate that mineralisation in the central Victorian gold province occurred in response to episodic, eastward progressing deformation, metamorphism and exhumation associated with the formation of the western Lachlan fold belt. Initial gold formation throughout the Stawell and the Bendigo structural zones can be constrained to a broad interval of time between 455 and 435 Ma, with remobilisation of metals into new structures and/or new pulses of mineralisation occurring between 420 and 400 Ma, and again between 380 and 370 Ma, linked to episodic variations in the regional stress-field and during intrusion of felsic dykes and plutons. This separation of ages is incompatible with the view that gold emplacement in the western Lachlan fold belt was the result of a single, orogen-wide event during the Devonian. A distinct phase of gold mineralisation, characterised by elevated Cu, Mo, Sb or W, is associated with both Late Silurian to Early Devonian (~420 to 400 Ma) and Middle to Late Devonian (~380 to 370 Ma) magmatism, when crustal thickening and shortening during the ongoing consolidation of the western Lachlan Fold Belt led to extensive melt development in the lower crust and resulted in widespread magmatism throughout central Victoria. These ~420 to 400 Ma and ~380 to 370 Ma occurrences, best exemplified by the Wonga deposit in the Stawell structural zone and many of the Woods Point deposits in the Melbourne structural zone, but also evidenced by occurrences at Fosterville and Maldon in the Bendigo structural zone, clearly formed synchronous with, or post-date, the emplacement of plutons and dykes, and thus are spatially (if not genetically) related to melt generation at depth. This later, magmatic-associated and polymetallic type of gold mineralisation is economically subordinate to the earlier, metamorphic-associated type of gold deposition in the Stawell and Bendigo structural zones, but tends to be the dominant style in the Melbourne Zone. These new geochronological constraints, together with zircon U-Pb data from felsic intrusive rocks of known relationship to gold mineralisation, demonstrate that initial hydrothermal alteration associated with gold emplacement in the western Lachlan fold belt was metamorphic-related, predating the emplacement of granite plutons by as much as 80 million years. This timing differs from other important orogenic gold districts where gold deposition is closely associated spatially with felsic magmatism. The early introduction of metamorphically derived fluids well before magmatism may reflect variations in the timing of peak metamorphic conditions at different crustal levels in an accretionary prism undergoing simultaneous deformation and erosion. Consequently, no genetic link exists between the main phase(s) of gold mineralisation and magmatism in the central Victorian gold province. With the exception of formation of a minor magmatism-related and geochemically-distinct mineralisation style at about 420 to 400 Ma, and again at about 380 to 370 Ma, the apparent spatial relationship between gold mineralisation and felsic intrusions is merely the result of melts and fluids being channelised along the same structures.     

Ion microprobe baddeleyite and zircon ages for Late Archaean mafic dykes of the Pilbara Craton,Western Australia

Ion microprobe U–Th–Pb analyses of baddeleyite and zircon yield precise ages for several mafic intrusions in the Pilbara Craton of Western Australia. Baddeleyite was dated from four dolerite dykes of the north‐northeast‐trending Black Range swarm intruded into granitoid‐greenstone basement in the northern part of the craton. The mean ²⁰⁷Pb*/²⁰⁶Pb* age of 2772 ± 2 Ma, interpreted as an unambiguous age of emplacement for the dykes, is within error of previous ion microprobe U–Pb zircon ages for the Mt Roe flood basalts and confirms that the dykes acted as feeders to the volcanic rocks. The Sylvania Inlier, in the southeastern Pilbara Craton, also contains north‐northeast‐trending dykes that were correlated previously with the Black Range swarm. Based on concordant and discordant zircon analyses from samples of two dykes, the best estimate of the age of the Sylvania dykes is 2747 ± 4 Ma. The Sylvania dykes thus appear to be significantly younger than, and hence unrelated to, the Black Range swarm, but may have acted as feeders to younger volcanic units in the Fortescue Group such as the Kylena Formation.     

SHRIMP U–Pb zircon geochronological evidence for Neoarchean basement in western Arnhem Land, northern Australia

The Pine Creek Orogen, located on the exposed northern periphery of the North Australian Craton, comprises a thick succession of variably metamorphosed Palaeoproterozoic siliciclastic and carbonate sedimentary and volcanic rocks, which were extensively intruded by mafic and granitic rocks. Exposed Neoarchean basement is rare in the Pine Creek Orogen and the North Australian Craton in general. However, recent field mapping, in conjunction with new SHRIMP U–Pb zircon data for six granitic gneiss samples, have identified previously unrecognised Neoarchean crystalline crust in the Nimbuwah Domain, the eastern-most region of the Pine Creek Orogen. Four samples from the Myra Falls and Caramal Inliers, the Cobourg Peninsula, and the Kakadu region have magmatic crystallisation ages in the range 2527–2510 Ma. An additional sample, from northeast Myra Falls Inlier, yielded a magmatic crystallisation age of 2671 ± 3 Ma, the oldest exposed Archean basement yet recognised in the North Australian Craton. These results are consistent with previously determined magmatic ages for known outcropping and subcropping crystalline basement some 200 km to the west. A sixth sample yielded a magmatic crystallisation age of 2640 ± 4 Ma. The ca. 2670 Ma and ca. 2640 Ma samples have ca. 2500 Ma metamorphic zircon rims, consistent with metamorphism broadly coeval with emplacement of the volumetrically dominant ca. 2530–2510 Ma granites and granitic gneisses. Neoarchean zircon detritus, particularly in the ca. 2530–2510 Ma and ca. 2670–2640 Ma age span, are an almost ubiquitous feature of detrital zircon spectra of unconformably overlying metamorphosed Palaeoproterozoic strata of the Pine Creek Orogen, and of local post-tectonic Proterozoic sequences, consistent with this local provenance. Neoarchean zircon is also a common detrital component in Palaeoproterozoic sedimentary units across much of the North Australian Craton suggesting the existence of an extensive, if not contiguous, Neoarchean crystalline basement underlying not only a large part of the Pine Creek Orogen, but also much of the North Australian Craton.     

Extensional deformation along the late Precambrian-Cambrian Baltoscandian passive margin: the Sarektjåkkå Nappe,Swedish Caledonides

The structural evolution of a part of the late Precambrian Baltoscandian passive margin just before the inception of seafloor spreading is described, recording the change from deformation by faulting to dominantly magmatic extension of the crust. The allochthon of the Scandinavian Caledonides contains the imbricated passive margin of continental Baltica overlain by various exotic terranes. The Sarektjåkkå Nappe in the Seve Nappe Complex, which contains the outer parts of Baltica's passive margin, consists of sedimentary rocks, occurring as screens between Vendian (573±74 Ma) diabase dykes. These dykes constitute 70–80% of the nappe and locally form sheeted dyke complexes. The Sarektjåkkå Nappe largely escaped penetrative Caledonian deformation and preserves igneous, metamorphic and structural elements that are linked to the evolution of a pre-Caledonian rift to a passive continental margin. Extensional deformation before dyke emplacement is recorded by normal faults, pull-apart structures and folds. Unconformities, dykes affected by brittle deformation, and fluidization of sediments during dyke emplacement indicate close relations between the deposition of sediments, extensional deformation and dyke emplacement. The Sarektjåkkå Nappe is compared with other parts of the Baltica's passive margin and its tectonic evolution is discussed.     

The Arunta Inlier: a complex ensialic mobile belt in central Australia. Part 1: Stratigraphy,correlations and origin

The Arunta Inlier is a 200 000 km² region of mainly Precambrian metamorphosed sedimentary and igneous rock in central Australia. To the N it merges with similar rocks of lower metamorphic grade in the Tennant Creek Inlier, and to the NW it merges with schist and gneiss of The Granites‐Tanami Province. It is characterized by mafic and felsic meta‐igneous rocks, abundant silicic and aluminous metasediments and carbonate, and low‐ to medium‐pressure metamorphism. Hence, the Arunta Inlier is interpreted as a Proterozoic ensialic mobile belt floored by continental crust. The belt evolved over about 1500 Ma, and began with mafic and felsic volcanism and mafic intrusion in a latitudinal rift, followed by shale and limestone deposition, deformation, metamorphism and emergence. Flysch sedimentation and volcanism then continued in geosynclinal troughs flanking the ridge of meta‐igneous rocks, and were followed by platform deposition of thin shallow‐marine sediments, further deformation, and episodes of metamorphism and granite intrusion.     

The 2.40 Ga Ringvassøy mafic dykes, West Troms Basement Complex, Norway: The concluding act of early Palaeoproterozoic continental breakup

A north to northwest trending mafic dyke swarm of gabbronoritic and gabbroic composition makes up a significant part of the Archean basement on the island of Ringvassøy in northern Norway. U–Pb geochronology of zircon and baddeleyite in a gabbronorite provides an age of emplacement of 2403 ± 3 Ma. Metamict zircon in a plagioclase phyric dyke yield data that are discordant but consistent with the age of the gabbronoritic dyke. Titanite indicates a metamorphic overprint at 1768 ± 4 Ma. The two types of dyke show some distinct chemical characteristics. They are both tholeiitic, enriched in LREEs and LILE elements but depleted in HFS elements including Nb. Their Nd isotopic composition yields _Nd values of −1.5 to −1.8 for gabbronorites and −0.4 to +1.3 for the plagioclase phyric dykes. The chemical and isotopic constraints are typical of continental basalts.The Ringvassøy mafic dykes correlate broadly with a global Palaeoproterozoic magmatic event that formed extensive bimodal intrusive and extrusive suites in most Archaean cratons, including the northeastern Fennoscandian Shield. In detail, the 2403 ± 3 Ma Ringvassøy dykes postdated most episodes of magmatism at this time. On the regional scale there is a distinct trend from a 2505–2490 Ma suite present in the Kola Peninsula, over a second 2460–2440 Ma suite present both in Kola and further south in Karelia, to the 2403 Ma dykes on Ringvassøy. This variation suggests that the locus of maximum extension and magmatic activity may have been shifting with time.     

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.     

Tectonostratigraphic and geochronologic reappraisal constraining the growth and evolution of Singhbhum Archaean craton, eastern India

Reappraisal of field relationships between the different lithological ensembles supported by available geochronological data, and taking due note of the tectono-metamorphic, magmatic and sedimentation history helped to build up a coherent crustal evolutionary history of the Singhbhum Archaean craton, eastern India. The evolution of the earliest sialic crust, as the isotope ages suggest, was around 3700 Ma or even earlier. Deposition of the oldest, dominantly metasedimentary supracrustals, the Older Metamorphic Group (OMG), was initiated at around 3380 Ma, i.e. after a gap of about 320 million years. The closing of OMG basins synchronously with the emplacement of a granitoid phase was at ca.3285 Ma. No other fabric-forming ductile deformation and metamorphism associated with the development of foliation and mineral lineation is known in the rocks of the Singhbhum Archaean craton subsequent to this event. Formation of the succeeding geological ensembles including the deposition of BIF-bearing Iron Ore Group (IOG) and the emplacement of the post-IOG granitoids at ca.3100 Ma can be described as ??lsnon-orogenic?? event taking place during the phase of tectonic quiescence. Supracrustals like the Dhanjori and Simlipal mafic volcanics with intercalated beds of arenite evolved later during the phase of Plume outburst at around 2800 Ma. The end-Archaean intrusion of Newer Dolerite dykes in conjugate sets and the deposition of Kolhan Group in an N-S oriented basin during an E-W stress system mark the culmination of the Archaean crust-building activity in the Singhbhum Archaean craton.     

Late orogenic collapse and thermal doming in the northern Gondwana margin incorporated in the Variscan Chain: A case study from the Ozieri Metamorphic Complex, northern Sardinia, Italy

A branch of the South European Variscan chain is noticeably exposed in Sardinia. The early stage of collision between the Northern Gondwana margin and the Armorica Terrane Assemblage (ATA) generated syn-metamorphic folding and thrusting. The evidences of such deformation are well preserved in the nappe zone, a structural domain characterized by stacking of different tectonic units under metamorphism of Barrovian greenschist facies. A late, post-nappe, shortening, under retrograde metamorphic conditions, gave rise to wide, upright, N120–N160 trending antiforms that control the trend of the chain. The structural analysis of the Ozieri Metamorphic Complex (OMC) shows evidence of an important phase of late-Variscan extensional tectonics. Deformation results in, the formation of oppositely dipping, normal shear zones, which developed at upper and middle structural level along the limbs of major regional antiforms causing fabric reactivation, crustal thinning, and exhumation of the OMC core. Within the OMC, the activity of the shear zones was coeval with HT-LP metamorphism as suggests the occurrence of syn-kinematic cordierite + andalusite ± sillimanite + biotite. Whereas syntectonic dykes and a tonalite body in the deeper part of the OMC indicate that early emplacement of melt along shear zones and/or in the antiform hinges possibly supplied the heat for the anomalous thermal gradient and triggered the exhumation of a core complex-like structure.     

Two distinct Precambrian terranes in the Southern Prince Charles Mountains, East Antarctica: SHRIMP dating and geochemical constraints

The Southern Prince Charles Mountains (SPCM) are mostly occupied by the Archaean Ruker Terrane. The Lambert Terrane crops out in the northeastern part of the SPCM. New geochemical and zircon U–Pb SHRIMP ages for felsic orthogneisses and granitoids from both terranes are presented. Orthogneisses from the Ruker and Lambert terranes differ significantly in their major and trace-element compositions. Those from the Ruker Terrane comprise two distinct groups: rare Y-depleted and abundant Y-undepleted. U–Pb isotopic data provide evidence for tonalite−trondhjemite emplacement at 3392 ± 9 and 3377 ± 9 Ma, pre-tectonic granite emplacement at 3182 ± 9 Ma, metamorphism(?) at c. 3145 Ma, and thermal events at c. 1300(?) and 626 ± 51 Ma. The Lambert Terrane orthogneisses probably originated in a continental magmatic arc. Zircon dating shows a very different geological history: pre-tectonic granitoid emplacement at 2423 ± 18 Ma, metamorphism at 2065 ± 23 Ma, and syn-tectonic granitoid emplacement at 528 ± 6 Ma, syn-tectonic pegmatite emplacement at 495 ± 18 Ma. The Lambert Terrane can be correlated with neither the Meso- to Neoproterozoic Beaver Terrane in the Northern PCM, which differs in isotopic composition, nor with the Archaean Ruker Terrane, which differs in both granitoid chemical composition and the timing of major geological events. It represents a Palaeoproterozoic orogen which experienced strong tectonic re-activation in Pan-African times. The Lambert Terrane has some geochronological features in common with the Mawson Block, which comprises south Australia and some areas in East Antarctica.     

New radiometric dating of the dykes from the Hurd Peninsula, Livingston Island, South Shetland Islands

Fifteen new K–Ar ages in the range of 79–31 Ma are partially confirmed by three ⁴⁰Ar/³⁹Ar plateaus and isochron data of 64.9±0.4, 55.5±0.1 and 52.8±0.6 Ma. The new geochronological data reveal a much more detailed picture of the subduction imprint in the Hurd Peninsula. Using cutting relationships, the dyke emplacement history is divided into four episodes. The Late Cretaceous–Paleocene dykes in the range of 80–60 Ma are related to the main magmatism in Livingston Island and most likely reflect the final stages of subduction of the proto-Pacific oceanic crust. The Early Eocene dykes (56–52 Ma) fill the gap in volcanic activity 70–50 Ma ago. They are the only magmatic event manifested at this time in the region. The 45–42 Ma dykes may be related to the intrusion of the Barnard Point tonalite. Three samples of Oligocene age appear to represent the last igneous activities on the Hurd Peninsula prior to the opening of the Bransfield Strait.     

P-T Conditions of Metamorphism in the Neoproterozoic Rocks of the Negele Area, Southern Ethiopia

Neoproterozoic rocks constitute the Kenticha, Alghe and Bulbul litho-tectonic domains in the Negele area of southern Ethiopia. Structural features and fabrics in these rocks were developed during north-south folding (D1), thrusting (D2) and shearing (D3) deformation. From micro-structural inferences and fabric relationships in semi-pelitic schists/gneisses of the Kenticha and Alghe domains, three episodes of metamorphic mineral growths (M1, M2 and M3) are inferred to have accompanied the deformational events. Pressure-Temperature estimates on equilibrium garnet-plagioclase-biotite and garnet-biotite assemblages from semi-pelitic schists/gneisses of the two domains indicate metamorphic recrystallization at temperatures of 520–580°C and 590–640°C, and pressures of 4–5 kb and 6–7 kb in the Kenticha and Alghe domains, respectively. These results correspond to regional metamorphism at a depth of 16–20 km for the Kenticha and 22–25 km for the Alghe domains. The P-T results suggest that the protoliths to the rocks of the Kenticha and Alghe domains were subjected to metamorphism at different crustal depths. This implies exhumation of the Alghe gneissic rocks from intermediate crustal level (ca. 25 km) before juxtaposition with the Kenticha sequence along a north-south trending thrust at the present crustal level during the Neoproterozoic. The combined deformation, fabric and mineral growth data suggest that rocks in the Kenticha and Alghe domains evolved under similar tectono-metamorphic conditions, which resulted from crustal thickening and uplift followed by extension and orogenic collapse, exhumation and cooling before litho-tectonic domains coalesced and cratonized in the Neoproterozoic southern Ethiopian segment of the East African Orogen.     

Seismic evidence for preservation of the Archean Uchi granite–greenstone belt by crustal-scale extension

In the westernmost Superior Province of Canada, the east–west alignment of granite–greenstone belts and the adjacent, highly deformed gneiss belts led to the first proposals that plate tectonics existed before 2.5 Ga ago, with the belts thrust against one another by east–west-oriented subduction zones. Here, we present seismic reflection data, which demonstrate that in this region the present juxtaposition of the Uchi granite–greenstone belt and the North Caribou gneiss terrane occurred along a late southeast-dipping extensional shear zone that extends from the surface into the lower crust. The preservation of the Uchi belt and probably the English River metasedimentary belt is directly related to their dropping along extensional shear zones, which limited subsequent erosion. The relative lateral transport of these greenstone rocks implies that they were neither derived from the immediately underlying crust, nor preserved by vertical crustal movements as might occur in the absence of plate tectonics. Extension may have been associated with the emplacement of mantle-derived magmas at 2700 Ma, which has been linked to slab break-off or lithospheric delamination, making the extension approximately coeval with local gold mineralisation. Since crustal-scale faults can facilitate the circulation of gold-bearing fluids, we suggest that greenstone rocks preserved in the hanging walls of syn- to post-accretion extensional shear zones may preferentially host Archean lode-gold deposits. In the westernmost Superior Province, our seismic observations imply that some of the late structures in the well-developed belts defined by surface mapping arose through the collapse of a collage of laterally accreted terranes.     

Proterozoic evolution and tectonic setting of the northwest Paterson Orogen,Western Australia

The Proterozoic igneous, deformation and metamorphic histories of the Palaeoproterozoic Rudall Complex in the northwestern Paterson Orogen can be linked to those of the Arunta Inlier in central Australia, and in part with the Capricorn Orogen in central Western Australia. The similarities in deformation and metamorphic histories for these widely separated regions indicate a Palaeoproterozoic continent–continent collisional event between the Palaeoproterozoic West Australian and North Australian cratons between c. 1830 and 1765 Ma. In the Paterson Orogen this Palaeoproterozoic collisional event resulted in the Yapungku Orogeny, which included thrust stacking of clastic sedimentary and volcanic rocks, deposition of the protoliths for the c. 1790 Ma siliciclastic paragneiss succession contemporaneous with granitic intrusion, and metamorphism up to granulite facies. During this 65-million-year period, the Arunta Inlier and Capricorn Orogen were deformed, metamorphosed at medium to high grades and intruded by granitoids during the Strangways Orogeny in the Arunta Inlier and the Capricorn Orogeny in the Capricorn Orogen.The Neoproterozoic Tarcunyah, Throssell and Lamil groups are clastic sedimentary sequences that were deposited after 1070 Ma in the northwestern Paterson Orogen, and deformed by the Miles Orogeny before 678 Ma. The Miles Orogeny produced a northwesterly trending fold and fault system of tight to isoclinal upright and overturned folds and thrust faults. The orogeny may have been coincident with the c. 750–720 Ma Areyonga tectonic movement affecting the Arunta Inlier and the lower Neoproterozoic part of the Amadeus Basin in central Australia. At c. 550 Ma the Paterson Orogeny, which is most likely equivalent to the Petermann Orogeny in the Musgrave Complex of central Australia, deformed the northwestern Paterson Orogen and was preceded by local intrusion of granites.The similarities of styles and timing of deformation in the northwestern Paterson Orogen, Arunta Inlier and Capricorn Orogen indicate that these three regions were probably linked during most of the Proterozoic.     

Proterozoic igneous activity in the Tennant Creek region,Northern Territory,Australia, and its relationship to Cu-Au-Bi mineralisation

Proterozoic silicic magmatic activity in the Tennant Creek area includes a suite of pre- to syn-orogenic granitoids (i.e. the Tennant Creek Granite and compositionally similar porphyries) predominantly intruded during the period 1870–1830 Ma, a group of silicic volcanics and volcaniclastics (Flynn Subgroup) erupted shortly after the main D₁ deformation, and apparently anorogenic silicic intrusives represented by the Warrego Granite. Chemical and petrographic data indicate that the majority of the pre- to syn-orogenic granitoids and porphyries are I-type or infracrustal-derived, whereas the peraluminous muscovite-rich Warrego Granite has many characteristics of supracrustal-derived (S-type) granitoids. The Warrego Granite also appears to be moderately fractionated with relatively low Ba, Sr, Zr, K/Rb, and high Th, Nb, Be, Bi, Rb/Sr compared with the associated I-type granitoids, porphyries and volcanics. Fractionation of feldspar and minor zircon has also resulted in substantial relative enrichment in LREE together with a pronounced negative Eu anomaly in the Warrego Granite. Previous studies suggest that the Cu-Au-Bi mineralisation at Tennant Creek post-dates formation of the host ironstones, but the absolute timing is imprecisely known. Mass-balance calculations utilising background Au concentrations for potential source rocks suggest it is feasible for the Au in some of the deposits to have been leached by relatively high-temperature (250 to 300 °C) deep basinal brines, and deposited by reaction with the ironstones. However, the inferred hydrothermal leaching cells would need to have been very large unless the leaching process was very effective. An alternative preferred model is that the mineralising fluids were exsolved from incompatible element-enriched, fractionated granitic magma, mixed with ground water, and reacted with the ironstones to deposit the base and precious metals. On the basis of chemical similarity to other granitoids closely associated with mineralisation, the Warrego Granite seems the most likely source of the fluids. However, problematic constraints imposed by the apparent age of the mineralisation (1810 Ma), and an emplacement age (Rb-Sr) of 1670 Ma for the Warrego Granite indicate a need for additional geochronological studies.