Regional Greenschist Research Articles

Tectonics of the Basement of the Kara-Bogaz Arch (Turan Plate)

The Kara-Bogaz geoblock is interpreted by many researchers as a structure underlain by the Precambrian sialic crust, incorporated in the young (epi-Hercynian) Turan Plate. The paper presents the results of a detailed study of the material composition of igneous and metamorphic rocks making up the basement of the Kara-Bogaz Arch and recovered by deep boreholes. To subdivide and correlate the sections, we employed data of geophysical borehole surveys, including all types of logging. Microscopic examination of rocks was also performed to determine their composition, genesis, and degree of postsedimentation alterations. Data on absolute ages of rocks and paleontological data were also employed. The results of the work of our predecessors have been studied and critically analyzed. It is established that there are no direct indications of the continental crust older than Paleozoic in the basement of the Near-Kara-Bogaz region. The metamorphic units of the Kara-Bogaz Arch are represented by primary sedimentary and volcanosedimentary deposits that were altered at the stage of regional greenschist metamorphism. These rocks are intruded by granitoid bodies corresponding to the final stages of Hercynian tectogenesis. In parts spatially close to intrusions, the degree of secondary alterations in metamorphic rocks increases due to the thermal effect. Amphibolites of the Kara-Bogaz Arch are metamorphosed igneous rocks, which are closely associated with primary terrigenous deposits and have undergone subsequent metamorphosis (greenschist facies regional metamorphism). Gneisses of the discussed region refer to the marginal facies of granitoid plutons that formed as a result of metasomatic reworking of the host strata (protomagmatic gneissic banding). We think that the Kara-Bogaz Arch is a Hercynian megaanticlinorium of the young platform, and this does not exclude the possibility of fragments of more ancient crust in the structure of this arch. The sedimentary–metamorphic and volcanosedimentary rocks making it up and intruded by multiple granitoid bodies are units of the active margin.

Geology, zircon U–Pb dating and εHf data for the Julie greenstone belt and associated rocks in NW Ghana: Implications for Birimian-to-Tarkwaian correlation and crustal evolution

The Julie greenstone belt of NW Ghana is characterized by a predominately volcanic succession of basalt with minor rhyolite and a mainly sedimentary succession of siltstone, shale and greywacke. The first unit is correlated with the Sefwi Group, and the second with the Kumasi Group of the Birimian Supergroup. The volcano-sedimentary succession is separated by a sheared unconformable contact from clastic sedimentary rocks, including sandstones and conglomerates, correlated with the Tarkwaian Group of Ghana. The supracrustal rocks are intruded by various granitoids. Two major deformational events have been identified. A regional N–S shortening event (D1) affected rocks of the Birimian Supergroup and associated granitoids. A later NW-SE shortening event (D2) affected both the Birimian and Tarkwaian units. Rocks have undergone regional greenschist to amphibolite facies metamorphism. Peak metamorphism took place during D2.Several samples of igneous and sedimentary rocks were subjected to U–Pb zircon dating and Lu–Hf isotope analyses. All zircons provided Paleoproterozoic ages in the range of 2112 ± 5 to 2337 ± 11 Ma, which is consistent with existing zircon ages from volcanic and plutonic rocks in various parts of Ghana, and other Birimian and Tarkwaian terrains within the West African Craton. Rhyolite of the Sefwi Group has an age of 2182 ± 3 Ma and marks a period of explosive felsic volcanism. Detrital zircon ages from greywacke of the Kumasi Group yield a maximum depositional age of 2158 ± 4 Ma. This age compares well to a maximum depositional age of 2154 ± 2 Ma defined in southern Ghana. Zircon ages from Tarkwaian sandstones indicate a maximum depositional age of 2135 ± 5 Ma, which is in accord with similar depositional ages for the Tarkwaian in SW Ghana and Houndé in Burkina-Faso. Thus the Tarkwaian rocks from the Julie greenstone belt area are correlative to those in SW Ghana and elsewhere. Detrital zircon ages point to a derivation from erosion of felsic volcanic and plutonic rocks within the Paleoproterozoic terrain. Detrital zircon ages of Birimian and Tarkwaian rocks show a ∼70% overlap. This suggest that the Tarkwaian detritus may have been derived from reworking of older Birimian rocks. Alternatively, both groups sampled a similar source but were deposited at different periods. Dated granitoids include a 2209 ± 5 Ma phase and a post-Tarkwaian intrusion of 2112 ± 5 Ma in age. The older granitoid is in sheared contact with basalt, and both share similar D1 structures.The ϵHf values for zircon grains from analyzed rocks are in the range of −1.78 to +6.37, in agreement with most Hf isotopic signatures currently available from the Paleoproterozoic Baoulé-Mossi Domain, and indicate crust components with apparent mantle derivation ages averaging c. 2300 Ma and ranging from c. 2100 to c. 2500 Ma.

40Ar behaviour and exhumation dynamics in a subduction channel from multi-scale 40Ar/39Ar systematics in phengite

The behaviour of the phengite 40Ar/39Ar system has been explored at different scales on Sifnos and Syros, two key islands of the Upper Cycladic Blueschist system (UCB). These islands expose well-preserved eclogites and blueschists that were exhumed via extensional collapse of the Aegean subduction/accretionary wedge during the Eocene and Miocene. Both islands record the switch from convergence to extension via a nappe-scale thermal-kinematic reworking marked by a sharp (~2 km thick) metamorphic transition from preserved high-pressure low-temperature (HP/LT) eclogites and blueschists at the top of the stack (peak P-T conditions of 22 ± 2 kbar and 550 ± 25 °C), to retrogressed greenschists at the base (overprinted at P-T conditions of 12–10 kbar and 500-450 °C). Extensive mapping of regional- to grain-scale 40Ar/39Ar age variations along this gradient reveals that phengite ages consistently decrease from 51 Ma down to 18 Ma towards the base of the stack, in parallel with progressive strain localization during exhumation and reworking by the regional greenschist ductile overprint. Homogeneous grain-scale Ar distributions are recorded in the poorly deformed topmost HP samples as Eocene (51–37 Ma) plateau ages and homogeneous in situ UV ablation ages, and by Miocene (20–18 Ma) plateaus in the intensely deformed basal greenschist samples. The homogeneous eclogite and blueschist ages are interpreted to record protracted crystallization in the subduction channel between 51 and 37 Ma, while the homogeneous basal ages likely record complete resetting and/or neocrystallization at 20–18 Ma. All intermediate samples across the metamorphic transition display variably complex age spectra and discordant in situ ages. Sub-grain scale Ar disequilibrium is ubiquitous in the form of within-sample and within-grain variations, with in situ (≪1 mm) variations locally encompassing almost the entire age span recorded regionally (up to 26 Myr). These variations occur mostly independent of microstructures and overlapping textural relationships. We interpret the bulk downward younging and associated internal discordance as the result of competing mechanisms of 40Ar retention and partial resetting operating during the regional greenschist thermo-kinematic overprint at the sub-crystal level. Grain-scale variations in solid-state Ar retention properties, fluid-phase dissolution/precipitation creep, and shear localization resulted in a mosaic of subdomains composed of partially reset to fully preserved relics of early HP clasts coexisting with neo-crystallized domains formed during metamorphic re-equilibration. This domain-like Ar behaviour operated largely below the spatial resolution of the UV probe (<50 μm), irrespective of phengite compositional variations, indicating that structural and thermal-kinetic crystallographic processes driving 40Ar and stoichiometric recombinations in phengite are coupled at a more elementary level, possibly the K-interlayer proper. A major outcome of this study is that it is clearly not possible to properly interpret 40Ar/39Ar phengite ages from HP/LT settings without integrating the information at all scales, from the crystal proper to the regional relationships. On Syros and Sifnos, the interplay between thermal-induced volume diffusion and strain localization is temporally and spatially related to the exhumation dynamics at the scale of the UCB, but these appear locally decoupled at the microscale. Such a complex behaviour is likely to be prevalent in other HP/LT successions exhumed elsewhere.

Structural evolution and U/Pb zircon age of the Xambioá gneiss dome, contributions to the Araguaia fold belt tectonic history

The Araguaia belt in central Brazil is an important key to re-access ancient geological settings, as it records the Gondwana agglutination and the final stages of an oceanic basin closure. Small basement inliers occur along transversal position in the Araguaia belt and could compose the extension of Amazonian craton. The structural analysis of the Xambioá gneiss dome region shows that three main deformational events were responsible for the actual scenario. The first deformational event was a NW-SE non-coaxial compression event (Dn), which led to the formation of ductile structures, such as open and slightly open folds (Fn), from a previous Sn-1 structures, axial plane foliation and spaced foliation (Sn), in the mantling metasedimentary sequence, and the mylonitic foliation (Sn) in dome rocks. This event was associated with a thrust system developed under regional greenschist to local upper amphibolite facies. The second deformational event Dn+1 was characterized by refolding with Sn transposition, followed by asymmetric Fn+1 folds and transposed foliation (Sn+1), and local stretching patterns forming boudins and Ln+1 lineations. The third deformational event Dn+2 is represented by WNW-ESE sinistral transcurrent faults (Sn+2) which sectioned the Dn structures and formed shear zones of different scales. U–Pb zircon ages in gneiss and in a migmatite of the Xambioá dome indicate Mesoarchean (~2.93 Ga) crystallization ages. U–Pb zircon provenance ages from metasedimentary rocks bordering the Xambioá dome show significant contribution of Archean to Neoproterozoic sources. The consistent loss of Pb in the dome rocks evidences tectono-metamorphic events presumably connected to the lithosphere ocean closure and continental collision responsible for the Araguaia Belt edification during Neoproterozoic-Cambrian period.

The late Neoproterozoic Pan-African low-grade metamorphic ophiolitic and island-arc assemblages at Gebel Zabara area, Central Eastern Desert, Egypt: Petrogenesis and remote sensing - Based geologic mapping

The late Neoproterozoic Pan-African basement rocks of the Gebel Zabara area, Central Eastern Desert of Egypt is of particular interest where the medium-grade Hafafit gneisses juxtapose the low-grade ophiolitic and island-arc assemblages along major structural contacts. The area comprises important mineral deposits including gold, base metal vein-type deposits as well as emerald and talc. In the current work, various image processing algorithms including Principal Component Analyses (PCA), Minimum Noise Fraction (MNF) and Decorrelation Stretch (DS) were proposed and applied on Landsat-8, ASTER and Sentinel-2 data to delineate and discriminate the different rock units. Several ASTER mineral indices are successfully used in the mapping process to detect the alteration zones, which are mainly confined to the NNW-SSE and N-S trending structures. The petrogenesis studies for the different rock unites in the study area indicate that the ophiolitic assemblage include serpentinite-talc carbonate, metagabbros complex and metabasalts. Serpentinite rocks represent the hydrated low temperature form of a harzburgite precursor, while the talcose serpentinites evolved via SiO2 aqueou metasomatic alteration. Geochemically, the ophiolitic rocks exhibit N-MORB tholeiitic affinity and imply a back-arc setting. The island-arc rock assemblages encompass calc-alkaline meta-andesite, metadacite and metarhyolite, which imply a normal continental island-arc setting, i.e. less mature than the proper active continental margin. Apart from serpentinites, the ophiolitic and island-arc assemblages experienced regional greenschist facies metamorphism. The ophiolitic metagabbros exhibit the metamorphic assemblage: actinolite + albite (An < 5%)/oligoclase (An 19–20%) + chlorite + epidote + quartz. The coexistence of albite and oligoclase may indicate either: (1) disequilibria association, or (2) a possible greenschist-amphibolite facies transition. The P-T metamorphic condition of the metagabbros was constrained at about 5 kbar and 430 to 452°C.

The Cedrolina Chromitite, Goiás State, Brazil: A Metamorphic Puzzle

The Cedrolina chromitite body (Goiás-Brazil) is concordantly emplaced within talc-chlorite schists that correspond to the poly-metamorphic product of ultramafic rocks inserted in the Pilar de Goiás Greenstone Belt (Central Brazil). The chromite ore displays a nodular structure consisting of rounded and ellipsoidal orbs (up to 1.5 cm in size), often strongly deformed and fractured, immersed in a matrix of silicates (mainly chlorite and talc). Chromite is characterized by high Cr# (0.80–0.86), high Fe2+# (0.70–0.94), and low TiO2 (av. = 0.18 wt %) consistent with variation trends of spinels from metamorphic rocks. The chromitite contains a large suite of accessory phases, but only irarsite and laurite are believed to be relicts of the original igneous assemblage, whereas most accessory minerals are thought to be related to hydrothermal fluids that emanated from a nearby felsic intrusion, metamorphism and weathering. Rutile is one of the most abundant accessory minerals described, showing an unusually high Cr2O3 content (up to 39,200 ppm of Cr) and commonly forming large anhedral grains (&gt;100 µm) that fill fractures (within chromite nodules and in the matrix) or contain micro-inclusions of chromite. Using a trace element geothermometer, the rutile crystallization temperature is estimated at 550–600 °C (at 0.4–0.6 GPa), which is in agreement with P and T conditions proposed for the regional greenschist to low amphibolite facies metamorphic peak of the area. Textural, morphological, and compositional evidence confirm that rutile did not crystallize at high temperatures simultaneously with the host chromitite, but as a secondary metamorphic mineral. Rutile may have been formed as a metamorphic overgrowth product following deformation and regional metamorphic events, filling fractures and incorporating chromite fragments. High Cr contents in rutile very likely are due to Cr remobilization from Cr-spinel during metamorphism and suggest that Ti was remobilized to form rutile. This would imply that the magmatic composition of chromite had originally higher Ti content, pointing to a stratiform origin. Another possible interpretation is that the Ti-enrichment was caused by external metasomatic fluids which lead to crystallization of rutile. If this was the case, the Cedrolina chromitites could be classified as podiform, possibly representing a sliver of tectonically dismembered Paleoproterozoic upper mantle. However, the strong metamorphic overprint that affected the studied chromitites makes it extremely difficult to establish which of the above processes were active, if not both (and to what extent), and, therefore, the chromitite’s original geodynamic setting.

Mineralogy and Metal Zoning of the Cambrian Zn-Pb-Cu-Ag-Au Lemarchant Volcanogenic Massive Sulfide (VMS) Deposit, Newfoundland

The Lemarchant deposit is a Cambrian bimodal felsic Zn-Pb-Cu-Ag-Au volcanogenic massive sulfide (VMS) deposit located in the Central Mobile Belt, Newfoundland, Canada. Despite regional greenschist metamorphism and faulting, primary mineralogy and mineral textures are well preserved. The deposit is a type example of a precious metal-enriched VMS deposit in the Appalachians. Mineralization consists of a stratiform massive sulfide zone that lies at the contact between a rhyolitic footwall and a basaltic hanging wall. A stringer sulfide zone that is hosted in footwall rhyolite breccia underlies the massive sulfide lens. The stratiform sulfide zone contains massive baryte that is heterogeneously replaced by sphalerite and pyrite, lesser galena, and trace chalcopyrite. The stringer zone contains chalcopyrite, pyrite, and lesser sphalerite and galena. Sphalerite ranges in color from white (low-Fe) to red (high-Fe). The palest sphalerite (<1 mol.% FeS) occurs in the baryte-rich stratiform zone and is associated with early stage exhalative mineralization, intermediate sulfidation epithermal suite minerals (tetrahedrite, bornite, colusite, Ag-bearing gold, covellite) and sulfide minerals enriched in the epithermal trace element suite (Au, Ag, As, Bi, Co, Cr, In, Mo, Ni, Sb, Sn, Te). Darker sphalerite (4.7–13.6 mol.% FeS) in the stratiform zone overprints early stage mineralization and occurs with chalcopyrite; high-Fe sphalerite is also present in the stringer zone. Early exhalative/epithermal-type VMS mineralization was deposited from low temperature (150–250 °C), oxidized, acidic to near-neutral hydrothermal fluids with high sulfur activity. Early mineralization was likely deposited in relatively shallow water ( 300 °C), less oxidized, near-neutral hydrothermal fluids, likely in deeper water (>1500 mbsl). Abundant epithermal suite minerals and epithermal trace element suite-enriched sulfides at Lemarchant suggest a direct magmatic contribution to the hydrothermal fluid.

Formation of Archean batholith-hosted gold veins at the Lac Herbin deposit, Val-d’Or district, Canada: Mineralogical and fluid inclusion constraints

The Lac Herbin deposit consists of a network of mineralized, parallel steep-reverse faults within the synvolcanic Bourlamaque granodiorite batholith at Val-d’Or in the Archean Abitibi greenstone belt. There are two related quartz-tourmaline-carbonate fault-fill vein sets in the faults, which consist of subvertical fault-fill veins associated with subhorizontal veins. The paragenetic sequence is characterized by a main vein filling ore stage including quartz, tourmaline, carbonate, and pyrite-hosted gold, chalcopyrite, tellurides, pyrrhotite, and cubanite inclusions. Most of the gold is located in fractures in deformed pyrite and quartz in equilibrium with chalcopyrite and carbonates, with local pyrrhotite, sphalerite, galena, cobaltite, pyrite, or tellurides. Petrography and microthermometry on quartz from the main vein filling ore stage reveal the presence of three unrelated fluid inclusion types: (1) gold-bearing aqueous-carbonic inclusions arranged in three-dimensional intragranular clusters in quartz crystals responsible for the main vein filling stage, (2) barren high-temperature, aqueous, moderately saline inclusions observed in healed fractures, postdating the aqueous-carbonic inclusions, and considered as a remobilizing agent of earlier precipitated gold in late fractures, and (3) barren low-temperature, aqueous, high saline inclusions in healed fractures, similar to the crustal brines reported throughout the Canadian Shield and considered to be unrelated to the gold mineralization. At the Lac Herbin deposit, the aqueous-carbonic inclusions are interpreted to have formed first and to represent the gold-bearing fluid, which were generated contemporaneous with regional greenschist facies metamorphism. In contrast, the high-temperature aqueous fluid dissolved gold from the main vein filling ore stage transported and reprecipitated it in late fractures during a subsequent local thermal event.

Closed-system behaviour of the Re–Os isotope system recorded in primary and secondary platinum-group mineral assemblages: Evidence from a mantle chromitite at Harold's Grave (Shetland Ophiolite Сomplex, Scotland)

This study evaluates in detail the mineral chemistry, whole-rock and mineral separate Os-isotope compositions of distinct platinum-group mineral (PGM) assemblages in an isolated chromitite pod at Harold's Grave which occurs in mantle tectonite in the Shetland Ophiolite Complex (SOC), Scotland. This was the first ophiolite sequence worldwide that was shown to contain ppm levels of all six platinum-group elements (PGE) in podiform chromitite, including the contrasting type localities found here and at Cliff. At Harold's Grave the primary PGM assemblage is composed mainly of laurite and/or Os-rich iridium and formed early together with chromite, whereas the secondary PGM assemblage dominated by laurite, Os-rich laurite, irarsite, native osmium and Ru-bearing pentlandite is likely to reflect processes including in-situ serpentinization, alteration during emplacement and regional greenschist metamorphism. The osmium isotope data define a restricted range of ‘unradiogenic’ 187Os/188Os values for coexisting laurite and Os-rich alloy pairs from ‘primary’ PGM assemblage (0.12473–0.12488) and similar ‘unradiogenic’ 187Os/188Os values for both ‘primary’ and ‘secondary’ PGM assemblages (0.1242±0.0008 and 0.1245±0.0006, respectively), which closely match the bulk 187Os/188Os value of their host chromitite (0.1240±0.0006). The unprecedented isotopic similarity between primary or secondary PGM assemblages and chromitite we report suggests that the osmium isotope budget of chromitite is largely controlled by the contained laurite and Os-rich alloy. This demonstrates that closed system behaviour of the Re–Os isotope system is possible, even during complex postmagmatic hydrothermal and/or metamorphic events. The preserved mantle Os-isotope signatures provide further support for an Enstatite Chondrite Reservoir (ECR) model for the convective upper mantle and are consistent with origin of the complex as a Caledonian ophiolite formed in a supra-subduction zone setting shortly before obduction.

Reflectance spectroscopy and alteration assemblages at the Leven Star gold deposit, Victoria, Australia

Hylogger-2 reflectance spectroscopy data from two drill holes at the Leven Star gold deposit, provides an insight into the hydrothermal alteration associated with vein sets and faults that host gold mineralisation. The metasedimentary host rock is a biotite-rich hornfels, which overprints a regional greenschist facies metamorphism. Gold mineralisation is associated with quartz (±carbonate) stockwork-breccia zones. HyLogging short-wave infrared reflectance spectra detected carbonates, chlorites, epidote (propylitic: carbonatisation, chloritisation, epidote), muscovite (phyllic/sericitic) and kaolinite, illite, ammonium alunite, montmorillonite and dickite (intermediate argillic alteration) assemblages that were confirmed using petrographic observations, drill core and geochemical data. The gold occurs within a sulfidation halo with carbonate phases (ankerite–siderite–dolomite) in structurally controlled veins and faults, which also host a late Sb–As–Bi–Cu–W assemblage, attributed to an intrusion-related style of mineralisation superimposed on a hydrothermal orogenic gold event.

U–Pb, 39Ar/40Ar geochronology of the metamorphosed volcanic rocks of the Bainaimiao Group in central Inner Mongolia and its implications for ore genesis and geodynamic setting

The greenschist facies volcano-sedimentary sequence of the Bainaimiao Group in central Inner Mongolia hosts the Bainaimiao Cu–Mo–Au deposit and the Bainaimiao lode gold deposit. But the ore classification and genetic process of the Bainaimiao Cu–Mo–Au deposit is still disputed. In this study we have selected the interbedded metamorphosed volcanic rocks of the Bainaimiao Group for LA-ICP-MS zircon U–Pb analysis. 17 analyses form a tight cluster on the concordia and yield a weighted mean 206Pb/238U age of 465±1Ma (95% confidence level, MSWD=0.95, n=17), which represents the best estimate of the crystallization age of the volcanic rocks of the Bainaimiao Group. Biotite from the greenschist of the Bainaimiao Group yielded a well-defined plateau with an age of 429.2±4.1Ma. The inverse isochron age is 429.1±4.2Ma with an elevated (40Ar/36Ar)i ratio of 299.9±22.7, which represents the time of regional greenschist facies metamorphism. Previous studies showed that the Re–Os isochron age of the molybdenite from the Bainaimiao Cu–Mo–Au deposit is 445.0±3.4Ma and the mineralized granodiorite porphyry yielded a weighted mean 206Pb/238U age of 445±6Ma. These data indicate that the Bainaimiao volcanic rocks were formed at about 465Ma and the porphyry Cu–Mo–Au mineralization took place at about 445Ma.Volcanic rock emplacement is 20Ma older than the Cu–Mo–Au mineralization indicates it is a porphyry deposit rather than VMS deposit. The dynamic setting for the porphyry mineralization is the Paleo-Asian Oceanic plate subduction along the northern margin of the North China Craton in early Paleozoic. Subsequently, the collision between the Bainaimiao arc and the North China Craton led to regional metamorphism and deformation at about 430Ma and the old porphyry system may have been overprinted during this metamorphic process.

Genesis of Neoproterozoic Au-bearing volcanogenic sulfides and quartz veins in the Ar Rjum goldfield, Saudi Arabia

The Ar Rjum goldfield is an example of late Neoproterozoic Au mineralization that is hosted by submarine arc assemblage and syn-anorogenic intrusive rocks. Apart from ancient workings, recent exploration in the goldfield defined three main targets along 3km N–S corridor (Um Na'am, Ghazal and Wasema), and indicated that Wasema alone hosts 11.8Mt @ 2.5g/t Au. The majority of gold and sulfide mineralization is confined to diorite, where gold content increases with shearing, pyrite–sericite–carbonate alteration and development stockworks of quartz–carbonate–pyrite veins and stringers. Generally, the concentration of gold increases in the diorite samples that experienced variable degrees of hydrothermal alterations near local shear zones. Anomalous gold content (up to 11.76g/t) in some metachert is the result of the remobilization of volcanogenic lattice-bound (refractory) Au into free Au due to post-metamorphic hydrothermal alterations. The chemistry of pyrite from the mineralized veins and stringers indicates considerable amounts of gold that reaches ~0.3wt.%.Chlorite that co-exists with pyrite in the hydrothermally altered metavolcanics is mostly sheridanite with up to ~25wt.% FeOt and minor amounts of ripidolite. Chlorite geothermometry suggests that two temperature ranges affecting the area. The first temperature range (290–334°C) is consistent with regional greenschist facies metamorphism, and the second (306–355°C) is interpreted to be related to recrystallization-submarine hydrothermal alteration related to the gold mineralization. Stable isotope (δ34S, δ18O and δ13C) data suggest an original volcanogenic arc signature that has been slightly modified by low-grade metamorphism, and finally by the late interaction of hydrothermal fluids. Ore evolution model for the Ar Rjum goldfield includes seafloor sulfide alteration, several deformation episodes and intrusive effects, and in this context the ore resulted from the reduction of seawater sulfates. The gold-rich veins interpreted as orogenic lode deposits are confined to localized shear zones in a syn-orogenic diorite.

Geological characteristics and metallogenesis of the shilu Fe-ore deposit in Hainan Province, South China

The Shilu Fe-ore deposit, a large-scale hematite-rich ore deposit in Hainan Province, South China, contains proven reserves of over 460Mt Fe-ore at 51% FeO, 4.07Mt Co-ore at 0.29% and 6.65Mt Cu-ore at 1.18%. The deposit is largely hosted within the Meso- to Neoproterozoic (ca. 0.85–1.30Ga) Shilu Group, a suite of metamorphosed neritic siliciclastic rocks and sedimentary carbonates, in which original sedimentary textures and structures are elegantly preserved. The Shilu Group, which hosts banded iron formations (BIFs), locally called amphibolitic itabirites, comprises six lithostratigraphic sequences. Fe-rich ores mainly reside in pyroxene–amphibole rocks of the middle sixth sequence, whereas the Co–Cu ores are located about 30–60m below the Fe-ore horizons and are primarily hosted by banded or impure dolostones of the lower sixth sequence. These ores are present mainly as lens-like or sheet-like bodies trending along troughs and/or transitional zones from limbs to troughs of the NW–SE trending synclinorium, and generally show banded or massive structures with lepidoblastic, cryptocrystalline or microcrystalline textures. Occasional oolitic and blastopsammitic textures are present, indicating an original sedimentary source for the Shilu Fe-polymetallic ores. At least two stages of deformation and associated metamorphism can be recognized. D1 deformation corresponds to a NW–SE trending synclinorium with associated regional greenschist facies metamorphism. D2 deformation records an early NW–SE orientated contraction (D2a), followed by doming with apparent skarnization (D2b) and finally transitional to a nearly E–W orientated extension (D2c), resulting in various amounts of folding, faulting and mineral recrystallization.Radiogenic isotopic analyses conducted on ores and host rocks record three metamorphic ages, including two Sm–Nd isochron ages of 830±16Ma and 212.9±6.6Ma, and a 40Ar–39Ar plateau age of 132.08±1.57Ma corresponding to D1, D2b, and D2c, respectively. Combined with previous U–Pb SHRIMP ages from ca. 960Ma to ca. 1300Ma and isotopic geochemical data (Xu et al., 2009), as well as other regional metamorphic events, an updated four-stage metallogenic model is proposed here: (1) deposition of the BIF-type ore source horizons between ca. 830 and 960Ma, (2) formation of a metamorphic sedimentary-type ore deposit from ca. 830–360Ma, (3) refinement of the deposit due to D2 deformation between ca. 250 and 210Ma and (4) superposed mineralizing stage formed by magma-related hydrothermal fluids between ca. 130 and 90Ma. The Shilu Fe-ore deposit can thus be better regarded as a structurally and hydrothermally reworked and re-enriched BIF-type ore deposit.

First U–Pb SHRIMP zircon and 40Ar/39Ar ages of metarhyolites from the Afyon–Bolkardag Zone, SW Turkey: Implications for the rifting and closure of the Neo-Tethys

The Afyon–Bolkardag Zone (ABZ) is one of the most important metamorphic zones in Turkey. This zone consists of metasedimentary and metavolcanic rocks, which have undergone regional greenschist facies metamorphism. The metavolcanic rocks are dominated by rhyolite, dacite, and trachyandesite. In the Ilgin (Konya) area, the low grade metavolcanics were originally rhyolitic lavas and tuffs. We present new precise Sensitive High-Resolution Ion MicroProbe (SHRIMP) 206Pb–238U age analyses of zircons and 40Ar/39Ar age data of phengite from metarhyolites that enable a more precise reconstruction of the ABZ. SHRIMP analyses of euhedral or sub-euhedral magmatic zircons from two metarhyolite samples yield weighted means 230±2Ma and 229±2Ma. These ages are interpreted as formation age of rhyolitic volcanics, implying they were formed in the Late Triassic time (Carnian). We suggest that these Upper Triassic magmatic rocks were formed in an extensional tectonic setting on the northern active margin of Gondwana, which led to the development of the northern branch of the Neo-Tethys. 40Ar/39Ar dating of two phengite samples from metarhyolites gave plateau ages of between 63.7±0.1Ma and 62.6±0.1Ma. These dates are interpreted as the metamorphic age of these volcanic rocks, and as the age of metamorphism that affected the ABZ. Consequently, these earliest Tertiary ages also constrain the timing of the closure of the Neo-Tethys.

Juvenile Paleoproterozoic crust evolution during the Eburnean orogeny (∼2.2–2.0 Ga), western Burkina Faso

We have investigated three greenstone belts (Boromo, Houndé, and Banfora) and associated granitoid terrains, which form part of the Eburnean orogen, situated in western Burkina Faso. These belts expose tholeiitic basalts (basal unit) followed by calc-alkaline intermediate predominantly effusive volcanic and sedimentary suites geochemically reminiscent of present-day volcanic island arc environments. The basal mafic unit probably corresponds to a juvenile arc crust or oceanic plateau. It contains unusual megacrystic tholeiitic basalts, allowing us to correlate the western margin of the Boromo belt with the eastern margin of the Houndé belt. These two N–S trending belt-parallel tholeiitic units are interpreted as limbs of a crustal scale anticline, intruded and partially obliterated by tonalite–trondhjemite–granodiorite (TTG) and granite intrusions. Three deformation events (D1–D3) can be distinguished in western Burkina Faso. The first deformation phase (D1) operated under an E–W to WNW-oriented compression. Regional greenschist to lower amphibolite facies metamorphism and intense folding characterize early-Eburnean deformation phases, during which time the crust was thickened by lateral shortening of volcanic island arcs and concomitant magma input. The crustal-scale antiform between the Boromo and Houndé belts is attributed to the D1 event. Shallow water detrital Tarkwaian-type sediments were deposited during the late D1 event within the Houndé belt, in a belt-parallel basin extending for 400 km. The subsequent D2 phase overprints the structural grain of the study area, and is best visible in airborne magnetic data. It is characterized by N to NE-trending transcurrent shear zones, which are considered preferred host structures for gold mineralization. We suggest that the newly formed and thickened crust reached the maximum thickness supportable by a weak and hot mantle during the D1 phase, and the pure shear dominated compressional regime switched to simple shear dominated transpression during the subsequent D2 phase. Granitoid diapirism played an important role at all stages of the Eburnean crustal growth processes in particular through early volume addition to the newly formed orogen and through later accommodating part of the lateral shortening. Pluton emplacement contributed to the greenstone belt structuration at local scales; however, the regional scale system geometry was controlled by coaxial shortening of the viscous volcanic units (basalts, gabbros, and andesites) of the greenstone belts, supported by coeval magma input. The last D3 deformation, which is either late-Eburnean or perhaps even Pan-African in age, is characterized by shallow N or S dipping minor thrust faults or an E-W trending steeply dipping spaced crenulation cleavage and kink folds, occurring mainly in highly anisotropic lithologies across the study region.

Origin of Witwatersrand gold: a metamorphic devolatilisation–hydrothermal replacement model

Goldfields extend for 300 km around the margin of the Archaean Witwatersrand Basin of South Africa associated with regional greenschist facies metamorphism and deformation. Metamorphic mineral assemblages involving pyrophyllite-chloritoid are associated with gold in all goldfields, reflecting low pressure and 300 to 400°C conditions, and indicating high geothermal gradients. The origin of Witwatersrand gold can be explained by metamorphic devolatilisation to generate auriferous fluids beneath and outside the Witwatersrand Basin, followed by passage of these fluids along large structures and into the Basin. In this model, generation of the fluid is a consequence of the transition to amphibolite facies of mafic rocks such as Archaean greenstone belts, as in . In a grain-by-grain scale devolatilisation process, the lower temperature minerals have released their H2O, CO2 and H2S to form a metamorphic fluid which at the time of its formation has already dissolved gold from these unenriched rocks (say at the ∼2 ppb Au level). A substantial volume of auriferous fluid is inferred given that regional metamorphism is on the scale of thousands of cubic kilometres, all evolving a few per cent of fluid. In the model, this auriferous fluid migrated from the immediate source region along grain boundaries, then in shear zones, and into the Witwatersrand Supergroup via the major thrust faults adjacent to all goldfields. Once in the Supergroup, fluid migration was via bedding planes and especially unconformity surfaces, along cross-cutting faults, shear zones and reef packages. The few thicker shale units and especially the overlying Archaean Ventersdorp basalt pile acted as barriers to fluid flow and facilitated fluid focussing. Deposition of gold from this auriferous fluid occurred where there was abundant carbon and/or iron. The distribution of these two elements was pre-determined by sedimentary distribution, and diagenetic migration. Many bedding features were preserved or mimicked during the hydrothermal replacement process because the volumetric strain in most of the sequence was low. Critical to the success of the hydrothermal process in producing gold deposits were the widespread local concentrations of C and Fe on unconformity surfaces, the reef packages, the major thrust faults adjacent to each goldfield, source rocks of greenstone-like character that were once at (sub)-greenschist facies grade with assemblages that included chlorite–calcite–pyrite, and a regional metamorphic event of high geothermal gradient. Moreover, critical to the exposure and preservation of the goldfields has been the limited erosion of the Basin since the hydrothermal deposition of the gold. The metamorphic devolatilisation–hydrothermal replacement model predicts many features of Witwatersrand ores, and can explain quantitatively the origin of its ∼100 000 t gold endowment. Calculations suggest that there is adequate gold at background (2 ppb) levels in inferred source rocks to form the necessary auriferous hydrothermal fluids; in contrast, there appears to have been insufficient particulate gold to form a significant placer accumulation. Without a viable source region for detrital gold, and indeed no plausible sorting mechanism to produce such deposits, it would be unwise to accept a placer model.

Paleoproterozoic structural evolution of the Man-Leo Shield (West Africa). Key structures for vertical to transcurrent tectonics

In the Man-Leo Shield, Paleoproterozoic (Birimian) belts crop out in nine countries of West Africa. Dominant domains include: (i) greenstone belts composed of plutono-volcanic, volcano-clastic and sedimentary rocks, deformed and weakly metamorphosed under regional greenschist facies conditions; (ii) widespread granitoid batholiths. The domains display a basin and dome-like architecture, and are overprinted by partitioned structures from successively shallower crustal depth. Analyses of key ductile and brittle structures has shown that the structural evolution of Man-Leo Shield is characterized by early vertical magmato-tectonics and subsequently, horizontal transcurrent tectonics with progression from ductile to brittle behavior. Basin and dome-like architectures, and the formation of an ubiquitous vertical foliation (MF) formed during emplacement of early amphibole-bearing (PAG) granite plutons at ca. 2.2 Ga by diapirism during NW–SE crustal shortening. Subsequent to a late stage of predominantly NW–SE shortening that created steeply-dipping mylonite zones (Mz1), transcurrent faults became predominant. The formation of transcurrent faults began transpressively, with development of N–S trending regional-scale mylonite zones (Mz1), and a steeply-plunging stretching lineation that probably formed during emplacement of PAG-type granitoids ca. 2.15 Ga. NNE–SSW transpressive sinistral horsetail faults and many NW–SE trending tension veins are interpreted to have formed at this stage. After cooling of the upper crust ca. 2.1 Ga, transcurrent faults became strike-slip in character with formation of dominantly NE–SW dextral faults (Mz2) and the passive emplacement of biotite (PBG) granitoids. Clockwise rotation of the extensional stress axis ( σ 3) from NNE–SSW trending to ENE–SSW trending assisted the propagation of dextral NE–SW and sinistral NW–SE extensional en echelon horsetail faults. WNW–ESE trending extension jogs (Egz) are interpreted to have been initiated under the same stress conditions. Displacements on strike-slip/transcurrent faults are interpreted as the product of rotation of rigid nuclei blocks producing faults’ re-activation. On the Man-Leo Shield Paleoproterozoic rocks are poorly exposed, but the tectonic model proposed in this study can help to shed light on the structural setting in areas of the shield which are poorly exposed, and in particular, why regional-scale structures do not display significant horizontal displacements. For practical use, key structural criteria can help to identify mylonite zones and transcurrent faults at different scales of investigation.

Collision-related granitic magmatism in the Granites–Tanami Orogen, Western Australia

The Palaeoproterozoic Granites–Tanami Orogen (GTO) is a significant auriferous province located in the poorly exposed southwestern part of the North Australian Craton (NAC). U-Pb data from Sensitive High Mass Resolution Ion Microprobe (SHRIMP) studies, Sm–Nd isotopic data, geochemistry and petrological studies all suggest that granitic rocks in the Western Australian portion of the GTO were emplaced around ca. 1795 Ma broadly synchronous with gold mineralization. These are variably fractionated, peraluminous, and derived from, or interacted with, ca. 2600 Ma upper crustal material. Deformation, metamorphism and magmatism around 1795 Ma in the GTO were synchronous with the Stafford Orogeny in the Arunta Orogen of central Australia. This deformation was the product of north- to northeast-directed compression within the GTO, and is associated with greenschist facies regional metamorphism. Amphibolite facies metamorphism is restricted to the immediate aureole of certain granitic rocks. This process is clearly a thermal overprint of the regional greenschist facies metamorphism, and indicates that the granitic rocks were emplaced during peak regional metamorphism or soon after. The granitic rocks are inferred to have formed in response to convergence and amalgamation of the GTO and the northern part of the Arunta Orogen along an accretionary margin delineated by the Willowra Lineament, which extends for thousands of kilometres in central and western Australia. Synchronous ca. 1795 Ma orogenic gold mineralization and syn-collisional granitic rocks have been identified across extensive regions throughout the NAC (e.g. the Pine Creek and Halls Creek orogens) suggesting that the Palaeoproterozoic assembly of Australia was marked by a widespread, significant metallogenic event at that time.

Geochronology of metarhyolites from the Kangbutiebao Formation in the Kelang basin, Altay Mountains, Xinjiang: Implications for the tectonic evolution and metallogeny

The Altay orogenic belt (AOB), situated in the middle part of the Central Asian Orogenic Belt (CAOB), is one of the most important metallogenic belts in China. The Kangbutiebao Formation is a Late Paleozoic stratigraphic unit that hosts many important iron and Pb–Zn deposits. The Kangbutiebao Formation consists of intercalated volcanic and sedimentary rocks that have undergone regional greenschist to lower amphibolite facies metamorphism, and mainly outcrops in three NW-trending fault-bounded volcano–sedimentary basins, including the Maizi, Kelang, and Chonghuer basins. SHRIMP analyses of zircons from three metarhyolites of the Kangbutiebao Fm. in the Kelang Basin yield weighted mean 206Pb/ 238U ages of 412.6 ± 3.5 Ma, 408.7 ± 5.3 Ma and 406.7 ± 4.3 Ma, respectively, which can be interpreted as the eruption age of the Kangbutiebao silicic volcanic rocks in the Kelang Basin. These ages indicate that the Kangbutiebao Formation was formed during the Late Silurian to Early Devonian. They also demonstrate that the deposits hosted in the Kangbutiebao Formation were formed after 412–407 Ma. They play a key role in understanding the Paleozoic tectonic evolution and metallogenesis of the southern margin of the Chinese AOB.

Petrography and geochemical behaviour of trace element, REE and precious metal signatures of sulphidic banded iron formations from the Chikkasiddavanahalli area, Chitradurga schist belt, India

The ∼1.2 km long and ∼250 m wide Chikkasiddavanahalli (C.S. Halli) hill range consists of mixed sulphidic-oxide banded iron formations (BIFs) and Fe-rich phyllites (±carbonaceous), which overlie carbonated schistose and massive meta volcanics. In stratigraphic succession, the lithologies represent the Ingaldhal Formation and are an integral part of the Chitradurga schist belt in the Western Dharwar Craton. The general strike at C.S. Halli varies from N–S to 340° with vertical to steep dips towards east and west. The sulphides, oxides and silicates exhibit intergrowth replacement textures developed during regional greenschist- and amphibolites- facies metamorphism. The BIFs show mesobands of recrystallised cherts and iron sulphides such as pyrite, arsenopyrite, and silicates such as subordinate grunerite, hornblende, chlorite, muscovite, actinolite and minor carbonates such as ankerite, calcite and magnesian siderite. Chemical data indicate depletion in Ti, Mn, Co, Cu, Cr and Ni in these iron formations. Most chondrite normalized REE patterns of the iron formation show moderate LREE and HREE enrichment coupled with strong positive Eu anomaly; the mineralized portions exhibit characteristic negative Ce and Eu anomalies (Eu/Eu∗ 0.21 to 3.00). The total REE abundance varies, correlates well with the iron contents of the BIFs, and similar to those exhibited by hydrothermal plumes [Chown, E.H., Dah, E.N., Mueller, W.G., 2000. The relation between iron formation and low temperature alteration in a Archean volcanic environment. Precambrian Research 101, 263–275]. Trace and REE data suggest that primary mantle-derived hydrothermal solutions were added to the pore fluids of sediments of the Chitradurga basin and supplied chemical constituents such as FeO, SiO 2 and REE. Oxidation of FeO to Fe 2O 3 was caused by the photosynthesis of primitive stromatolite-building cyanobacteria. Geochemical data suggest a model involving epigenetic gold mineralisation in close association with shear zone deformation, quartz-calcite vein activity and sulphidation in the mixed sulphide oxide facies BIF and associated iron phyllites in the C.S. Halli area, Western Dharwar Craton, India.