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The timing of emplacement of the oldest supracrustal rocks represented by Sargur Group is significant in understanding crustal evolution processes in the western Dharwar craton. The age of ultramafic-mafic rocks of the Sargur Group is not well-constrained as the komatiitic and komatiitic basaltic rocks have always given unreliable ages due to larger errors attributed to their alteration. New Sm-Nd isotope data from relatively less-altered layered intrusive ultramafic-mafic rocks of Nuggihalli and Holenarsipur greenstone belts of Sargur Group, western Dharwar craton give a much precise age of 2934 ± 88 Ma. This is the youngest age obtained for the ultramafic-mafic rock complexes of Sargur Group and is indistinguishable from the oldest ages reported for the overlying Dharwar Supergroup rocks. Sargur Group rocks might merely represent older ultramafic equivalents of the Dharwar Supergroup. A clear temporal distinction does not exist to support the stratigraphic classification of these two groups separately. However, the sub-contemporaneous to older ages obtained for the tonalite-trondhjemite-granodiorite gneisses that surround these greenstone belts would imply that the narrow belts tucked within the gneisses are younger, and therefore, the greenstone belts, whose dismembered roots and parts are exposed alongside the gneisses, could represent an Archaean analogue of an ophiolite suite. [ABSTRACT FROM AUTHOR]

• Devolatilized fluids from metabasalts at ∼550 °C have δ11B between −14.2 to −10.8‰. • Metapelites at similar temperatures release fluids with δ 11B between −5.7 to 0.0‰. • Tourmaline δ 11B from worldwide orogenic gold deposits match modelled δ 11B for metamorphic fluids. • Tourmaline δ 11B in Hutti and Kolar (−14.0 to −3.3‰) suggest precipitation from metamorphic fluids. • Li and V in tourmaline may be used as reliable source rock indicators. The source of the mineralizing fluid in Archean greenstone-hosted orogenic gold deposits is widely debated, with the available geochemical and isotope data interpreted as reflecting the involvement of metamorphogenic fluid produced by devolatilization of greenstone belt metaigneous/metasedimentary rocks or granite-derived magmatic-hydrothermal fluids or both. Orogenic gold deposits form in complex geologic environment involving multiple fluid sources, reflected in the large variation in the B-isotope composition of tourmaline (δ11B = −24.8‰ to +19.8‰). The δ11B distribution of tourmaline from world-wide orogenic gold deposits define two peaks, one at ca. −15‰, and the other between −5 and 0‰. In this study, we modelled the release of boron and associated B-isotope fractionation during prograde metamorphic dehydration of representative mafic and pelitic greenstone rocks using a mass balance approach and compared the results with the B-isotope composition of tourmalines measured globally from orogenic gold deposits. The results indicate that the boron content of greenstone belt metabasalts decreases from ∼26 ppm to ∼2.7 ppm by 530 °C while those in metapelites decrease from ∼60 ppm at 300 °C to ∼40 ppm by the terminal chlorite breakdown temperature of about 570 °C. In both mafic and pelitic assemblages, terminal chlorite breakdown reactions occurring during the greenschist to amphibolite facies transition (530–570 °C) release large amounts of 10B-rich fluids, which rapidly lower the δ11B of the metamorphic fluid. The δ11B of the fluids released from metabasalts and metapelites at these conditions are estimated to be about −14.2 to −10.8‰ and −5.7 to 0.0‰, respectively. Tourmalines precipitating from such metabasalt-derived fluid are expected to have δ11B between −18.0‰ and −12.4‰ while those crystallizing from metapelite-derived fluid will have δ11B between −9.5‰ and −1.6‰ for the temperature range of ore formation (300–550 °C) in orogenic gold deposits. Our calculated range of δ11B for the metabasalt- and metapelite-derived metamorphic fluids matches closely with the two peaks in the δ11B distribution of tourmaline from orogenic gold deposits. The δ11B of metabasalt-derived fluid is highly variable (+2‰ to −15‰ between 450 °C and 610 °C) and strongly dependent on the metamorphic grade attained by the greenstone belt lithologies. The majority of measured tourmaline δ11B values from orogenic gold deposits of all ages can potentially be explained by different peak metamorphic temperatures with a single fluid source from metabasalts, or by mixing between basalt-derived and pelite-derived metamorphic fluids, or both. Prior characterization of the metamorphic grade of the rocks and the temperature of mineralization are desirable before using the B-isotope composition of tourmaline for evaluating fluid source. The δ11B (−14.0‰ to −3.3‰) of tourmalines measured by us from the Hutti and Kolar gold deposits as a case study can be explained by our modelled results with the isotopic variations attributed to mixing between metapelite- and metabasalt-derived fluids. Low Li concentrations in the tourmalines of Hutti and Kolar (avg. 21.7 and 28.0 ppm in Hutti and Kolar, respectively) further support a metamorphic origin for the hydrothermal fluid. The relatively higher V contents of the Hutti tourmalines (avg. 962 ppm) vis-à-vis the Kolar tourmalines (avg. 615 ppm) is suggestive of greater pelitic input in Hutti. [ABSTRACT FROM AUTHOR]

Komatiites and sedimentary rocks sampled during the International Continental Drilling Program (BARB1-2–3-4–5) in the Barberton greenstone belt, South Africa, were analyzed for 146Sm-142Nd systematics. Resolved negative μ142Nd values (down to −7.7 ± 2.8) were identified in komatiites from the 3.48 Ga Komati Formation and this signature correlates with low Hf/Sm ratios measured in these samples. The negative μ142Nd point to a source with subchondritic Sm/Nd ratio which formed during the Hadean. No analytically resolvable 142Nd anomalies were measured in crustal detritus-rich, Si-rich, Ca-Fe-rich sediments and cherts from the Buck Reef (3.42 Ga) and the Fig Tree Group (3.23–3.28 Ga). Our new measurements are incorporated into a larger set of 147Sm-143Nd and 176Lu-176Hf data to better understand the 142,143Nd-176Hf isotope signatures in the mantle source at the time of komatiite crystallization. Our calculations show that the 142,143Nd-176Hf isotope signatures and Hf/Sm ratios cannot be produced by recycling into the komatiite source of detrital sediments like those sampled in the Barberton area. Only cherts have the required trace element characteristics – low Hf/Sm, radiogenic ε176Hf –but the trace element concentrations in the cherts are so low that unrealistic amounts of chert would need to be added. We propose a four-stage model for the formation of these rocks. Negative μ142Nd and low Hf/Sm ratios developed during the crystallization of a deep magma ocean soon after Earth accretion. The material that ultimately became the source of komatiites was a residual liquid produced by 50% crystallization leaving a bridgmanite/ferropericlase/Ca-perovskite cumulate. The Lu-Hf and Sm-Nd isotope systems were decoupled at this stage. After extinction of 146Sm around 4 Ga, parent/daughter ratios fractionated during a melt extraction event. With this model we explain the positive ε176Hf and slightly negative ε143Nd in these samples. The 3.55 Ga Schapenburg komatiites in another part of the Barberton belt share similar chemical signatures, supporting our model of fractionation in a deep magma ocean early in Earth's history. [ABSTRACT FROM AUTHOR]

The Neoarchean U-Th-Pb (SHRIMP-II) age of 2718 ± 7 Ma was established for the large copper–nickel Allarechka deposit located in the Kola–Norwegian region of the Fennoscandian shield. The deposit is associated with metamorphosed and deeply eroded volcano-plutonic complexes, which are the age and formational analogues of the Neoarchean greenstone belts. The compositional similarity of hyperbasites of the Allarechka ore district and host metavolcanic rocks of the Annama Formation with komatiite–tholeiitic metavolcanic rocks of the similar age of the Ura Guba–Kolmozero–Voron'ya greenstone belt suggests that they are ascribed to the komatiite–tholeiite volcanoplutonic series, the parental high-Fe komatiite melts of which were derived by partial melting of depleted upper mantle εNd(t) = +7.0 ± 0.4) at high PT parameters. The petrographic and geochemical composition of ore peridotites indicates that they are a cumulate formed by fractionation of Ol + Opx ± Pl, Mag from the primary high-Fe komatiite melt in a shallow (P ~ 4 kbar) magma chamber at a liquidus temperature of at least 1200°С. The absence of a significant crustal contamination of the hyperbasites by Mesoarchean TTG-complexes is proved by the trace and rare-earth element geochemistry, the Nd isotopic composition, as well as by the homogeneous morphology and isotopic composition of the accessory magmatic zircon. In contrast, Re-Os and sulfur isotopic systematics of sulfide ores indicate the contribution of not only mantle but likely crustal component in the evolution of sulfide melt. After separation from a primary mantle melt under subcrustal conditions, the silicate and sulfide melts evolved independently. Obtained data suggest that a local Ni, Cu, and PGE-rich mantle anomaly was formed through a plume–lithosphere interaction beneath the Kola–Norwegian Terrane (in its present-day outlines) of the Fennoscandian Shield at 2.75–1.9 Ga. This anomaly was a source of ore matter for three Early Precambrian copper–nickel ore epochs, in particular, for Neoarchean (~ 2750 Ma) deposits of the Allarechka ore field, Paleoproterozoic deposits (~ 2500 Ma) of the layered intrusions (e.g., Monchegorsk and Fedorova-Pana), and Pechenga group (~ 1900 Ma). [ABSTRACT FROM AUTHOR]

The paper summarizes major, trace, and Sm-Nd isotopic data on metabasites of the Urik greenstone belt (GB) (Sharyzhalgai uplift, southwestern Siberian craton). The Archean metabasalts (amphibolites and garnet amphibolites) of the supracrustal complex and sheeted bodies represent four geochemical types. The first type includes low-Ti amphibolites of the Urik area. Their characteristic features are a clear LREE depletion ((La/Sm)n = 0.8–1.1), the absent negative Nb anomaly, and positive εNd(t) values (up to +4.1). The second type corresponds to moderate-Ti amphibolites of the Urik area. They differ from the rocks of the first type in the higher (La/Sm)n ratio (1.0–1.5), Th, Zr, Nb, Y contents, and lower εNd(t) values (+0.4...–1.6), but, like the low-Ti amphibolites, they do not show Nb depletion. The third type includes amphibolites of the Tagna area, which show an increase in the concentrations of TiO2, LREE, Th, Zr, and Nb with a decrease in Mg# and have εNd(t) values from +1.3 to –0.5. They are characterized by the elevated (La/Sm)n ratio (1.1–2.3) and Zr and Nb contents and the presence of a clear Nb minimum. The fourth type includes all high-Ti amphibolites of sheeted bodies, which are enriched in LREE ((La/Sm)n = 1.9–3.4), Th, Zr, and Nb, have an increased (Gd/Yb)n (1.4–2.0), a sharp Nb depletion, and negative εNd(t) values. Geochemical modeling coupled with trace element and isotopic composition indicates the formation of the low- and moderate-Ti metabasalts of the Urik area from weakly depleted and nearly primitive mantle sources, respectively. Their protoliths were oceanic basalts. The metabasalts of the Tagna area were formed from a weakly depleted and hydrated mantle source contaminated with crustal material. The enrichment of the source with incompatible elements could be caused by sediment subduction shortly before basalt formation. The metabasalts of the fourth type originated from a long-lived enriched source such as subcontinental lithospheric mantle. They resemble the present-day intraplate continental basalts. None of the three dominant types of metabasalts of the Urik GB shows signatures of sharply depleted or sharply enriched mantle sources, which indicates a weak differentiation of the mantle by the Meso- Neoarchean boundary (~2.8 Ga). The formation of a subduction-modifed lithospheric mantle (hydrated mantle) has only occurred since ~ 2.8 Ga. Different compositional and isotopic characteristics of the metabasalts of the Urik GB suggest that the Bulun terrane was formed by accretion and tectonic juxtaposition of fragments of the oceanic crust, an oceanic island arc, and an older TTG continental crust at ~ 1.86 –1.82 Ga. [ABSTRACT FROM AUTHOR]

The sources of metals enriched in Archean orogenic gold deposits have long been debated. Metasedimentary rocks, which are generally accepted as the main metal source in Phanerozoic deposits, are less abundant in Archean greenstone belts and commonly discounted as a viable metal source for Archean deposits. We report ultralow-detection-limit gold and traceelement concentrations from a suite of metamorphosed sedimentary rocks from the Abitibi belt and Pontiac subprovince, Superior Province, Canada. Systematic decreases in the Au content with increasing metamorphic grade indicate that Au was mobilized during prograde metamorphism. Mass balance calculations show that over 10 t of Au, 30,000 t of As, and 600 t of Sb were mobilized from 1 km3 of Pontiac subprovince sedimentary rock metamorphosed to the sillimanite metamorphic zone. The total gold resource in orogenic gold deposits in the southern Abitibi belt (7500 t Au) is only 3% of the Au mobilized from the estimated total volume of high-metamorphic-grade Pontiac sedimentary rock in the region (25,000 km3), indicating that sedimentary rocks are a major contributor of metals to the orogenic gold deposits in the southern Abitibi belt. [ABSTRACT FROM AUTHOR]

Trace elements in native gold provide a "fingerprint" that tends to be unique to individual gold deposits. Fingerprinting can distinguish gold sources and potentially yield insights into geochemical processes operating during gold deposit formation. Native gold grains come from three historical gold ore deposits: Hollinger, McIntyre (quartz-veined ore), and Aunor near Timmins, Ontario, at the western end of the Porcupine gold camp and the southwestern part of the Abitibi greenstone belt. Laser ablation – inductively coupled plasma – mass spectrometry (LA–ICP–MS) trace element concentrations were determined on 20–25 µm wide, 300 µm long rastor trails in ∼60 native gold grains. Analyses used Ag as an internal standard with Ag and Au determined by a scanning electron microscope with an energy dispersive spectrometer. The London Bullion Market AuRM2 reference material served as the external standard for 21 trace element analytes (Al, As, Bi, Ca, Cr, Cu, Fe, Mg, Mn, Ni, Pb, Pd, Pt, Rh, Sb, Se, Si, Sn, Te, Ti, Zn; Se generally below detection in samples). Trace elements in native gold associate according to Goldschmidt's classification of elements strongly suggesting that element behaviour in native Au is not random. Such element behaviour suggests that samples from each Timmins deposit formed under similar but slightly variable geochemical conditions. Chalcophile and siderophile elements provide the most compelling fingerprints of the three ore deposits and appear to be mostly in solid solution in Au. Lithophile elements are not very useful for distinguishing these deposits and element concentrations may be controlled by microinclusions such as tourmaline. The deposits show low Ag contents, which is consistent with mesothermal Au. Hollinger and McIntyre deposits have similar trace element abundances with higher Ag, Pb, Bi, Sb, and Pd and generally low Cu; however, Cu concentrations in McIntyre are higher than in Hollinger. In contrast, Aunor shows comparatively low Ag, Bi, Sb, Pb, and Pd and higher Au/Ag consistent with higher hydrothermal fluid temperatures. Gold grain signatures reflect the chemical characteristics of the host rock superimposed on a chemical signature inherited from the mineralizing fluid. The association of Pb–Bi–Cu bearing phases such as galena and chalcopyrite with gold apparently enriched in these elements supports precipitation from hydrothermal fluids carrying the elements. [ABSTRACT FROM AUTHOR]

The Neoarchean potassic granite from the Hosadurga region adjacent to the Chitradurga greenstone belt (CGB), part of the western Dharwar Craton (WDC), is studied for its petrogenesis. The combined field, petrographic, whole‐rock elemental, and Nd–Sr isotope data of granite and associated Tonalite–Trondhjemite–Granodiorite (TTG) gneiss from this region are presented here. The granites are alkali‐calcic to calc‐alkalic in terms of modified alkali lime index (MALI) and are weakly peraluminous (molar Al2O3/(CaO + Na2O + K2O) = 1.0–1.1). The geochemical data indicate that these are formed by the melting of TTG gneiss at depths above the garnet stability field. Isotopic data (εNd = −2.9 to −7.6 at T = 2,600 Ma, ISr = 0.715–0.729, TDM > 3.0 Ga) also show the involvement of Archean crustal sources. Trace element modelling was carried out using non‐modal equilibrium melting equation, which constraints partial melting of TTG source to a low degree (11%) for the formation of this granite. This model could also produce a major element composition consistent with this granite. The upper bounds on the magmatic temperatures (724–785°C) assessed from zircon saturation thermometry suggest that fluid influx is required. TTG samples show variable and very high positive epsilon Nd values (1.5–8.9), higher than the contemporaneous mantle, suggesting re‐melting of mafic crust. The observed high Nd isotope ratios in TTG indicate involvement of a mafic source that formed as early as ~3.8 Ga, probably as a result of global differentiation of the early Earth. [ABSTRACT FROM AUTHOR]

Unravelling the petrogenesis and stratigraphy of Archean mafic lava flows is essential to our comprehension of the early geodynamic evolution and economic potential of greenstone belts. This study focuses on one of the oldest and thickest sequences of lava flows observed in the Neoarchean Abitibi Subprovince (greenstone belt), that is, the Obatogamau Formation. The undated formation extends for more than 100 km in an E–W direction and consists mostly of aphyric and feldspar megacryst-bearing basaltic-andesite lava flows. These lava flows are tholeiitic, mostly Fe-rich, and have nearly homogeneous chemistry. Petrogenetic modelling carried out using MELTS software points to limited magmatic differentiation as most samples of mafic lava flows did not reach Fe-Ti-oxide saturation. Zircon U-Pb dating establishes a crystallization age of 2726.2 ± 1.6 Ma for a felsic unit located at an intermediate stratigraphic position in the sequence of lava flows. Constraints from stratigraphically overlying volcanic units suggest that the Obatogamau Formation was likely emplaced rapidly, possibly within a few million years and as a consequence of frequent replenishment of shallow magma accumulations. High eruption rates are consistent with short episodes of volcanic quiescence deduced from field observations, indicating non-optimal conditions for volcanogenic massive sulfide systems. The pressure and temperature of peak metamorphism deduced from amphibole chemistry, however, points to favorable conditions for the release of metamorphic fluids. The study area may thus be prospective for orogenic gold mineralization, provided that fluids had access to a source of gold and that structural conduits allowed for the channeling of hydrothermal fluids. [ABSTRACT FROM AUTHOR]

The article analyzes the position of individual and average data points of the Archean and Early Proterozoic metaaleuropelites (Isua and Akilia associations, West Greenland; Beit Bridge Complex, Limpopo Province, South Africa; Moodies, Mozaan, and Pretoria groups, Barberton Mountain Land, South Africa; Kola Group, Baltic Shield, Russia; Nurmes paragneisses, Eastern Finland; Onot greenstone belt, southeastern Sayan region, Russia; Rampur Group, Lesser Himalayas, India; Kan metamorphic complex, eastern Sayan region, Russia; Ladoga Group, northern Ladoga region, Russia; Yenisei metamorphic complex, Russia; and others) in the (La/Yb)N–Eu/Eu* and (La/Yb)N–Th diagrams with classification fields constructed from data on the content of REE and Th in the fine grained clastic/aleuropelitic sediments near the estuary of different-category modern rivers, according to (Bayon et al., 2015). It has been shown that the vast majority of data points of metaaleuropelites are localized in fields 1 (particulates of major rivers in the world), 2 (particulates of rivers draining sedimentary substrates) and 4 (particulates of rivers flowing through volcanic rock terrains), as well as the zone overlapping fields 1–3 (particulates of rivers feeding on erosion products of the magmatic/metamorphic terrains). Data points of all examined objects with an age of more than 2.8 Ga in the (La/Yb)N–Eu/Eu* diagram are concentrated mainly in field 4. If all our assumptions are correct, these observations suggest the following conclusion: until the above-indicated time, large rivers and, most likely, rivers draining mainly sedimentary rocks did not exist, and processes of the recycling of fine-grained aluminosiliciclastics were reduced. [ABSTRACT FROM AUTHOR]

Characterization of terrestrial analog sites is critical for detection and determination of clay mineralogy in remote sensing studies of Mars aimed at geological, hydrological, and potentially biological investigations. In this study, we investigate a suite of hydrothermally altered early Archean rocks from the Barberton greenstone belt (BGB) of South Africa as potential petrological, mineralogical, and spectral analogs to hydrothermally altered metabasalts and mafic-ultramafic intrusions in the martian subsurface and impact craters. We present the first spectral imaging measurements on exceptionally well-preserved early Archean mafic-ultramafic rocks from the BGB, with the aim of studying their clay mineralogy and spectral signatures. Multiple spectral analyses were conducted on different sample textures (rock powders, crushed rocks, and rock slabs) appropriate for Mars rover and remote sensing exploration. Visible/near-infrared (VNIR) and mid-IR reflectance spectra were acquired on particulate samples, while VNIR spectral imaging data were collected on rock slabs. Mid-IR emission spectra were measured for the rock slabs and grains. Spectral features are compared from these different spectral techniques to identify the minerals present in the samples and compare macroscale vs. microscale detections. The measured spectra reveal absorption bands that correspond to clay mineralogy of the serpentine and chlorite mineral groups, consistent with petrographic observations, as well as magnetite, olivine, quartz, feldspar, and Al-phyllosilicate. The spectral data acquired in this study expand the reference spectra data set for remote sensing studies. The implications of this study are that rocks from early Archean greenstone belts, such as those of the BGB, serve as potential clay-bearing petrological analogs for hydrothermal environments on Mars. [ABSTRACT FROM AUTHOR]

The paper presents results of the first geochronologic study (LA-ICP-MS zircon method) of metasedimentary rocks that host the banded iron formation (BIF) and the metarhyolites (halleflinta) of the Gimoly Group in the Kostimuksha greenstone belt,Karelian Craton. Felsic magmatism that produced the rhyolite sills and dikes was dated at 2759 ± 19–2743 ± 15 Ma. The clearly predominant group of zircons in the schists (metagraywacke) has an isotope age of 2753 ±19 Ma, and scarce zircon grains of these rocks vary in age from 3.1 to 2.8 Ga. Geochemically, zircons from metarhyolites fall into three types (1) Eu-depleted and Ce-enriched, (2) Eu- and Ce-enriched, and (3) LREE- and MREE-enriched. The zircons of geochemical types (1) and (2) are also predominant in the sediments. The morphology and structure of zircon crystals in the rhyolites and sediments are also identical. The rhyolites were thus a major source of zircons for the metagraywacke, which alternates with BIF, while the Mesoarchean rocks played a subordinate role. The BIF of the Gimoly Group in the Kostomuksha Greenstone Belt was formed simultaneously with felsic volcanism at 2760–2740 Ma. [ABSTRACT FROM AUTHOR]

Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and electron microprobe analysis (EMPA) techniques were used to determine the mineralogy and chemistry of dominant sulphide and associated gold deposits at the Fairview Gold Mine of the Barberton Greenstone Belt (BGB). The rocks and ore mineral associations at the Fairview Mine indicate mesothermal (orogenic) conditions for the gold deposit. The whole rock chemistry of the Fairview Mine reflects calc-alkaline affinities, and felsic provenance is revealed by high SiO2, K2O, Al2O3, and TiO2, and low Fe2O3, MgO, Ni, and Cr contents. The mineral assemblages of the Fairview Mine rocks and ores indicate three phases of metamorphisms; regional prograde, thermal, and retrograde metamorphism, which caused zonation and heterogeneity of the ore minerals. The principal ore minerals in this deposit are pyrite, arsenopyrite, pyrrhotite, chalcopyrite, and rare sphalerite, as well as minor gold. The obtained results revealed that sulphides of the Fairview Mine are divided into two types (generations) varying in their morphology, association, and elemental composition. The old type 1 is porous, anhedral, heterogeneous, contains inclusions and rich in As, Ni, Co, and Au compared to type 2. Trace-element distribution and occurrence mode of gold at Fairview mine reflect two phases of gold mineralization. The first stage is associated with sulphides (mainly pyrite and arsenopyrite), whilst the second phase is free-lode hosted by silicates (mainly quartz). [ABSTRACT FROM AUTHOR]

[Display omitted] • Seabee Gold Operation that hosts Santoy and Fisher yield a similar ore genesis. • Fisher and Santoy deposits host multi-stage system of orogenic gold mineralization. • Early gold mineralization overprinted by main-stage orogenic mineralization. • Mineral chemistry of alteration mineral assemblages similar for Fisher and Santoy. The Seabee Gold Operation, located in northern Saskatchewan, Canada, hosts the Santoy and Seabee gold deposits, as well as the Fisher property; the latter is an important exploration target. This gold mineralization is hosted in rocks of the Pine Lake greenstone belt that is within the Glennie domain of the larger Paleoproterozoic Trans-Hudson orogen. Mineralization is focused within shear zones, including the Santoy shear system that extends south onto the Fisher property. The role of ore-adjacent intrusions, systematics of gold mineralization, and relationship between the mineralization of the properties are largely undefined. In this study, hydrothermally-altered samples from locations along the Santoy shear system, namely the Santoy 7, Santoy Gap, and the Fisher property, were analyzed to identify and compare the mineralogical and geochemical characteristics related to sulfide-gold mineralization and associated alteration. The altered host rocks primarily comprise upper greenschist to lower amphibolite-grade sheared metabasalt to chlorite-hornblende-biotite ± garnet schist, and intrusive rocks dominated by granodiorite and tonalite. These lithologies are cut by similar quartz vein generations and are associated with similar alteration assemblages and mineralization, including (i) syn -late-D 3 "distal" (∼0.4 to 3 m wide) sericite-biotite-chlorite-clinozoisite ± albite, titanite to vein-adjacent coarse-grained diopside-K-feldspar-albite-actinolite ± apatite, titanite calc-silicate alteration, associated with main-stage Py 2a, 2b -Cpy 2 -Po 1 -Au (syn -late-D 3) mineralization; (ii) remobilized, fracture-filling Py 3 -Po 2- Cpy 3 -Au-BiTel (late-D 3) mineralization within the calc-silicate assemblage; and (iii) early colloform Py 1a (pre- syn -D 3) and intrusion-hosted, inclusion-rich Py 1b (pre-D 3). Microprobe analyses reveal similar mineralogical trends between Fisher and Santoy alteration minerals typical of orogenic gold systems. Empirical geothermobarometric calculations of gold-associated vein-adjacent and vein-hosted chlorite and biotite exhibit orogenic-style formation conditions (∼320–330 °C, 3 kbar, ∼9 km depth). As such, these analyses suggest that the Fisher property and Santoy Mine Complex underwent inter-connected, multi-stage hydrothermal processes. These stages comprise (i) a potential early introduction of gold into the system closer to the timing of plutonism (Py 1b magmatic-hydrothermal; pre-D 1) or during the onset of gold mineralization (Py 1a ; orogenic; late-D 2 to early- syn -D 3) during the evolution of the Reindeer zone; (ii) orogenic main-stage mineralization (syn -late-D 3) concurrent with late- to post- collisional deformation throughout the Reindeer zone; and (iii) remobilization during continuous deformation (late-D 3) where elevated cyclical fluid fluxes and higher fracture density at Santoy produced higher-grade mineralization. [ABSTRACT FROM AUTHOR]

• Metabolic diversity in the lower platform facies of the Buck Reef Chert. • Exceptional preservation of Paleoarchean carbonaceous matter also on molecular level. • Different carbonaceous matter appearance (e.g., clots, laminae, blocky aggregates). • Rapid silicification supported carbonaceous matter preservation. Microbial life on Earth was well established in the Paleoarchean, but insight into early ecosystem diversity and thus, the complexity of the early biological carbon cycle is limited. Here we investigated four carbonaceous chert samples from the lower platform facies of the ca. 3.42-billion-year-old Buck Reef Chert, Barberton greenstone belt. The analysis on multiple scales revealed exceptionally well-preserved carbonaceous matter, even on molecular level (aliphatic and aromatic hydrocarbons), resulting from rapid silicification. Geochemical evidence from stable carbon and multiple sulfur isotopes supports the presence of different microbial metabolisms in the Paleoarchean ecosystem. The local biological carbon cycle was dominated by photoautotrophs, but autotrophic sulfate reducers and methane- or acetate-producing microbes were also present. In areas of microbial methane or acetate release, methanotrophs or acetotrophs contributed to the overall biomass. These results highlight the metabolic diversity in the lower platform environment of the Buck Reef Chert, and underline that an advanced biological carbon cycle already existed in the early Archean. [ABSTRACT FROM AUTHOR]

Whole-rock geochemistry yields better geological maps of the bedrock, since it allows, for example, visually similar lithologies to be distinguished. However, conventional laboratory geochemistry is not available in the field during a mapping campaign. Portable X-ray fluorescence (pXRF) analyzers can produce fit-for-purpose data rapidly, and constitute a useful complement to conventional geochemistry in a bedrock mapping project. The pXRF data can be employed (i) while still in the field, to orient the mapping campaign; (ii) to prepare a preliminary geological map back in the office; and (iii) to interpolate between conventional whole-rock analyses on the final map. The latter application requires a matrix-matched secondary calibration of the pXRF data to improve its accuracy. Attention must also be given to precision issues, as well as potential instrument drift and analytical interferences. This paper summarizes recent efforts to systematically integrate quantitative pXRF data in government mapping projects within the Province of Quebec (Canada). We first demonstrate how an Olympus Vanta series M pXRF analyzer was commissioned and calibrated. We then show how the pXRF data was systematically acquired and utilized during a 1:50,000 to 1:20,000 mapping campaign in a Precambrian greenstone belt setting (Abitibi Subprovince). Before the mapping began in 2021, volcanic rocks in the study area were supposed to consist of only two mafic tholeiitic formations, called Obatogamau and Bruneau. Integrating pXRF into this project meant that while still in the field and immediately afterwards during preparation of the preliminary map: (1) a marker horizon representing the top of the Obatogamau Formation was chemically identified and then traced laterally; (2) the felsic Waconichi Formation was recognized despite the high metamorphic grade and also traced laterally just above the marker horizon; (3) the Bruneau Formation was shown to be much more continuous than previously known and to have Mg-tholeiites at its base, like in the type locality; (4) the Blondeau Formation was shown to occur above the Bruneau Formation. Within the volumetrically dominant Obatogamau Formation, Fe-tholeiites were also distinguished from visually similar Mg-tholeiites, which allowed these subunits to be traced while still in the field, leading to a more detailed map. Later, after the conventional laboratory chemistry had been received, the final geological map was prepared, but it was very similar to the preliminary map, confirming the satisfactory performance of the pXRF method for this application. However, we do note some issues and limitations even with the latest generation of analyzers. • Portable X-ray fluorescence (pXRF) analyzers are very useful for bedrock mapping projects. • The pXRF data allows visually similar lithologies to be distinguished while in the field. • However a commissioning stage is required, and validation with conventional geochemistry. [ABSTRACT FROM AUTHOR]

We use offshore 2-3D seismic reflection and potential field data coupled with field work to investigate the control of lithospheric heterogeneity and shear zone rejuvenation on the mode of extension and the architectural segmentation of the southern Angola margins. The pre-rift basement is inherited from several Precambrian deformation events ended by the Pan-African/Brasiliano orogen that accreted four different age-dependent lithospheric domains with contrasted thickness, fabrics, thermal structure, and chemical composition. From north to south, the continental crust is composed of Neoproterozoic metapelites, Paleoproterozoic gneisses and migmatites and Archean greenstone belts pervasively intruded by TTG granitoids. The latter are compartmentalized by lithospheric-scale Archean – Paleoproterozoic shear zones with a general sub-vertical fabric. Upon extension, these pre-existing shear zones nucleated extensional deformation, focalized magmatism and accommodated different rates of extension and width of offshore rift domains. In the north, younger Neoproterozoic lithospheric domain deformed with a ductile behavior and multiple crustal detachments promoting lithospheric decoupling and a distributed extension, with late continental break-up. It favored the formation of a wide magma-poor margin where extension was accommodated by low-angle, seaward-dipping extensional faults and mantle-exhumation. To the south, Archean and Paleoproterozoic lithospheric domain with stronger and brittle strength profile promoted a more localized extensional deformation and favored an early lithospheric coupling and continental break-up. This ultimately led to the formation of a short margin with increased magmatic budget, where extension was accommodated by high-angle, landward-dipping extensional faults and mantle-exhumation. We demonstrate that the pre-rift heterogeneity of the lithosphere controlled the mode of extension during the rift and the along-strike variation of margin geometry. Moreover, the rejuvenation of Precambrian shear zones as extensional transverse fault zones also promoted the activation of lithospheric-scale thermal drains facilitating the emplacement of mantle-related melts into the upper crust. • The Precambrian lithosphere of South Angola is heterogeneous in terms of structural fabrics and rheology. • Limits between basement domains consist of large sub-vertical structures referred to lithospheric-scale shear zones. • Onshore inherited shear zones extend in the rifted margin domain and are rejuvenated, both by tectonics and magmatism. • Patterns of shear zone rejuvenation are change in fault trends, margin orientation, polarity, width and magmatic budget. • Brazil – Angola pre-rift basement reconstruction responsible for rift segmentation and magmatic budged upon extension. [ABSTRACT FROM AUTHOR]

[Display omitted] • Fractal and multifractal analyses were used to study four orogenic gold provinces. • Case study conducted on a key deposit at the local and micro scales. • Fractal dimension maps showed good correlation with known mineral occurrences. • Orogenic gold deposits showed multifractal behavior. • Multifractal indices help obtain information on gold deposits in multiple scales. Understanding the controls on the spatial distribution of mineral deposits is crucial for prospecting new occurrences. Various geological processes operate together at microscopic to macroscopic scales, generating mineral systems responsible for significant accumulations of economic minerals. This characteristic of functioning at various scales, accompanied by repetition of features over space during mineralization, indicates that these processes are self-similar, meaning they exhibit fractal geometry. Previous research has demonstrated that the combined action of processes involved in mineralization in such critical, nonlinear systems, produces objects with multifractal characteristics. Therefore, the spatial distribution of mineral deposits should be studied using appropriate methods for better understanding of their controls. In this context, this study investigated the multiscale geometric characteristics of worldclass orogenic gold systems in Archean greenstone belts using fractal and multifractal analyses. First, at the macroscopic-scale, regional-scale features were considered in the major greenstone belt provinces with orogenic gold deposits worldwide, namely: Abitibi (Canada), Norseman–Wiluna (Australia), Rio das Velhas (Brazil), and Central Lapland (Finland). Next, at an intermediate scale, local-scale features were examined in detail within the context of the Córrego do Sítio Lineament. Finally, at the microscopic scale, the focus was on the Córrego do Sítio orogenic gold deposit—a key deposit in the Rio das Velhas greenstone belt. Fractal dimensions of the spatial distributions of mineral deposits, magnetic lineaments, and their intersections were investigated at the macroscopic to local scales, as well as the spatial distribution of ore minerals at the microscopic scale, using the box-counting method. Maps of fractal dimensions of magnetic lineaments were generated and compared with the spatial distribution and tonnage of orogenic gold deposits at the regional scale. The same features were analyzed using the method of multifractal moments. The results at the three scales were analyzed and synthesized to identify meaningful patterns for understanding these orogenic gold systems. With regard to fractality, the orogenic gold deposits in all the investigated provinces exhibited unique spatial distributions at different scales, namely: less than 2–4 km, from 2 to 4 km to 10–30 km, and more than 10–30 km. The analysis of magnetic lineaments yielded a correlation between gradients of fractal dimensions and gold tonnages in the orogenic gold deposits. The multifractal analyses indicate multifractal geometries extending from the microscopic to the macroscopic scale. The calculated multifractal indices of Δα (singularity variation), τ"(1) (multifractality index), and Δf(α) (multifractal spectrum variation) showed direct relationships with endowment of orogenic gold in the studied provinces, as well as with differences in mineralization styles in thin sections. The findings demonstrate the effectiveness of the analyses used for improved characterization of the studied mineral systems; they also allow comparing similar systems at different scales and locations and they can be used to study other mineral systems with similar geological controls. [ABSTRACT FROM AUTHOR]

• Zircon petrochronological (U/Pb-O-Hf) constraints for the far-east Yilgarn Craton. • Relatively old (2840 – 2775 Ma) crustal block at the eastern edge of the craton. • Identification of the newly defined Dorothy Hills Block and Yamarna Rift. • Re-assessment of the nomenclature in the Eastern Goldfields Terrane (EGT). • The EGT formed via sustained, autochthonous crustal growth. The Archean Yilgarn Craton in Western Australia represents one of the key areas for investigating the mechanisms of crustal growth during Earth's infancy. The far-eastern margin of the Yilgarn Craton has, however, received limited scientific scrutiny. In this study, in-situ zircon petrochronology (U-Pb, O, and Lu-Hf isotopes) is used to constrain the formation and evolution of the easternmost Yamarna and Burtville regions as well as the encompassing Eastern Goldfields (Super)Terrane (EGT) of the Yilgarn Craton. The new data highlight the unequivocal existence of relatively old (ca. 2840 – 2775 Ma), compared to the EGT, and isotopically juvenile (εHf: +2.1 to +3.5) and mantle-like (δ18O: 5.2 to 5.9‰) crust at the eastern edge of the exposed craton. We refer to this crust as the 'Dorothy Hills Block'. The Dorothy Hills Block shows geochronological and isotopic similarities with the adjacent 'Burtville Block' and the Youanmi Terrane (>400 km further west), which suggests a shared history up to ca. 2720 Ma. In addition, linear greenstone belts characterised by younger magmatism (ca. 2720 – 2660 Ma) with a constant, mildly supra-chondritic Hf isotope composition (εHf: 0.0 to +3.5) are identified between the Burtville and Dorothy Hills Blocks. We refer to this region as the 'Yamarna Rift', by analogy to the better-understood 'Kalgoorlie-Kurnalpi Rift'. It is inferred that the Burtville and Dorothy Hills Blocks formed via peri-cratonic magmatism (ca. <2970 Ma), associated with mantle upwelling focused along the eastern edge of the ca. >3300 Ma Yilgarn proto-craton. The Yamarna and Kalgoorlie-Kurnalpi Rifts (ca. 2720 – 2660 Ma) overlie this extended older substrate and formed via intra-cratonic rifting broadly coeval with a major mantle upwelling event. The revised Meso- to Neoarchean crustal evolution model for the EGT, including new nomenclature, suggests a sustained (>200 Myr) and autochthonous geological development of the eastern Yilgarn Craton resulting from mantle upwelling events, potentially within a back-arc tectonic setting. Comparable crustal evolutionary trends are recorded in other Archean cratons (e.g., Superior Craton), which suggests that similar geodynamic regimes operated on a global scale in the Archean. [ABSTRACT FROM AUTHOR]

The Itareru tonalite is one of the Paleoproterozoic plutonic rock units that intruded the Serrinha block, an Archean-Paleoproterozoic crustal fragment of the São Francisco Craton (Bahia, Brazil). It is characterized by a ca. 80 km long, NW-SE elongated leaf-shaped body, mainly comprised of tonalite-diorite to granodiorite rocks with numerous dioritic enclaves. Magnetic studies were performed on 26 sites of the Itareru pluton (including country rocks). Anisotropy of low-field magnetic susceptibility (AMS) and anisotropy of anhysteretic remanent magnetization (AARM) are coaxial, indicating that all minerals have the same orientation, partially controlled by regional strain. Rock-magnetism measurements reveal that paramagnetic–matrix minerals contributed significantly to the magnetic susceptibility and AMS while magnetite grains in the range of 2–5 μm are responsible for AARM. Magnetic fabrics in the central region of the pluton are probably primary (magmatic), which was acquired when the rocks were crystallized, reflecting magma flow linked to a magma feeder zone (magma chamber). The moderated magnetic lineation plunges in the northern part of the pluton suggest that this region was close to the feeder zone. In contrast, the southern region should be far from it, since the magnetic lineations are horizontal to sub-horizontal. The Itareru tonalite emplacement was also controlled by regional strains related to the collision of the Paleoproterozoic Rio Itapicuru greenstone Belt with the Archean basement. • AMS and AARM are coaxial indicating that all minerals have the same orientation. • Magnetic fabrics in the central region of the pluton are probably magmatic acquired when the rocks were crystallized. • Magnetic fabrics in the central region of the pluton are related to magma flow. • The emplacement of the pluton was controlled by regional strains. • Itareru emplacement is related to the collision of the RIGB with the Archean basement. [ABSTRACT FROM AUTHOR]

The potential field (gravity & magnetic) data, along with results of available seismic and magnetotelluric studies over the Southern India Shield comprising the Dharwar craton (DC), Southern Granulite Terrane (SGT) and southern part of the Eastern Granulite Mobile Belt (EGMB), were synthesized. The study attempts to elucidate the geometry and continuity of greenstone belts/shear zones, isostatic condition and deeper crustal architecture. We observed a characteristic elevation-gravity relationship for each of the tectonic domains. The greenstone belts of the DC are characterized by N-S to NW-SE trending positive residual gravity anomalies and continue beneath the Cuddapah sediments. The basement of the Cuddapah basin is characterized by NW-SE trending greenstone belts with low-density intrusive rocks. The gravity anomaly suggests occurrence of a high-density rock unit occurs at mid-crustal depth beneath the 'Closepet granite' and 'Chitradurga schist belt'. The potential field signatures of the DC and SGT are distinctly different. The Cauvery Shear Zone is characterized by positive residual gravity and magnetic anomalies with ∼E-W to ENE-WSW trend that continues further east under sediment covers of the east coast. The residual gravity signatures and horizontal gravity gradient signatures suggest that the orogenic systems encompassing the Cauvery Shear Zone and the eastern part of the EGMB may be a part of a common structural belt or tectonic regime associated with the assembly of the Gondwana supercontinent. The potential field modelling reveals the presence of a thick, high-density material resting over the base of the crust in the entire stretch of the east-coast of India. (a) Gravity anomaly map, (b) magnetic (reduced to pole) anomaly map, (c) residual gravity map, (d) Isostatic anomaly map and (e) gravity-magnetic modelling. The residual gravity and horizontal gravity gradient signatures suggest that the orogenic systems encompassing the Cauvery Shear Zone and eastern part of the Eastern Granulite Mobile Belt may be a part of a common structural belt or tectonic regime associated with the assembly of the Gondwana supercontinent. The study also reveals magmatic under plating along east coast of India. [Display omitted] • Comprehensive potential field analysis of parts of Southern Indian Shield (SIS). • Outlines continuation and disposition of the greenstone belt and shear zones. • Continuation of structural belt from across different tectonic domains of the SIS. • The study reveals magmatic under plating along east coast of India. • Each geo-tectonic unit shows characteristic gravity-elevation relationship. [ABSTRACT FROM AUTHOR]

• The study proposes tools to improve environmental assessment of a mineral deposit. • XRF, mineral spectrometer, carbonate staining compose the preliminary protocol. • Application on two mining projects: a developed mine and an exploration project. • Protocol compared with traditional environmental assessment. Environmental assessment is generally performed when a mining project is at an advanced development stage, such as the prefeasibility study. It involves several laboratory tests such as detailed characterisation and geochemical mobility prediction tests, which require many samples considered representative of the deposit. Having early characterisation information could be useful to select the most relevant samples for the overall environmental assessment. The present study proposes a selection of non destructive and portable tools for improving the early environmental assessment of a mineral deposit. Three tools (x-ray fluorescence (XRF), mineral spectrometer, carbonate staining) were identified to form a preliminary environmental assessment methodology that can be performed directly on the exploration site. This methodology was applied to two different mining projects: i) a well developed mine, LaRonde zone 5, with available geochemical data in a volcanic-associated massive sulfide (VMS) context, and ii) an exploration project, O'Brien, with ongoing geochemical acquisition in an Au-quartz vein context. Both mine sites are in the south of the Blake River Group within the Abitibi greenstone belt, Quebec, Canada. The first site was used to validate the developed methodology, whereas the second acted as a test site to apply the methodology as a preliminary environmental assessment. The results on the first site confirmed that the selected tools provide geochemical and mineralogical data sets comparable to those of traditional testing methods. The developed methodology then enabled the classification of preliminary geo-environmental domains for the second site. Standard static and kinetic tests were performed on the second site samples to confront the preliminary assessment protocol. The proposed preliminary protocol made it possible to discriminate low-risk samples versus those with a potentially higher risk for the selection of subsequent environmental assessments. [ABSTRACT FROM AUTHOR]

An Erratum to this paper has been published: https://doi.org/10.1134/S1028334X23070152 [ABSTRACT FROM AUTHOR]

The paper presents newly acquired isotope-geochemical and U–Pb isotope zircon dating (SHRIMP) results on four posttectonic granitoid massifs in the southeastern part of the Karelian Granite–Greenstone Province (GGP) in the Fennoscandian Shield. The massifs are located near the Hautavaara Structure, in the southeastern part of the Mesoarchean (3.05–2.85 Ga) Vedlozero–Segozero Greenstone Belt, which is confined to the western margin of the Vodlozero crustal block with a Paleoarchean (TNdDM > 3.2 Ga) prehistory. All four massifs (Hautavaara, Chalka, Shuya, and Nyalmozero) were shown to have similar structural–tectonic settings, were emplaced nearly simultaneously (at 2745–2740 Ma), and display variations in the rock compositions that were predetermined by differences in the composition of the magma sources and the conditions of their derivation. The Hautavaara Massif in the central part of the structure and the Chalka Massif on its western margin are made up of moderately alkaline high-Mg granitoids (sanukitoids), whose initial diorite melts were derived by melting the lithospheric mantle metasomatized in an active-margin setting at 3.00–2.90 Ga. The Shuya granodiorites and Nyalmozero leucogranites, which are confined to the eastern flank of the structure, yield highly fractionated HREE patterns (Dyn/Ybn = 3.5 to 5.14), negative εNdT = –0.9 to –2.8, and were produced by melting a Mesoarchean crustal source at various depths. This source was similar to the 3.05- to 2.90-Ga felsic volcanics in the Hautavaara Structure. The Shuya granodiorites contain elevated Cr and Ni concentrations, suggesting that the melts were generated in the crust with the involvement of mafic magma, which was likely coeval with the primitive sanukitoids. The melting of the continental lithosphere at mantle and crustal levels in the Karelian GGP in the latest Neoarchean are thought to have occurred in an extensional environment during collapse of the collisional orogen, in accordance with the model (Laurent et al., 2014). [ABSTRACT FROM AUTHOR]

This paper focusses on obtaining a better understanding of the subsurface geology of the Chibougamau area, in the northeast of the Abitibi greenstone belt (Superior craton), using geophysical data collected along a 128 km long traverse with a rough southwest–northeast orientation. We have constructed two-dimensional (2D) models of the study area that are consistent with newly collected gravity data and high-resolution magnetic data sets. The initial models were constrained at depth by an interpretation of a new seismic section and at surface by the bedrock geology and known geometry of lithological units. The attributes of the model were constrained using petrophysical measurements so that the final model is compatible with all available geological and geophysical data. The potential-field data modelling resolved the geometry of plutons and magnetic bodies that are transparent on seismic sections. The new model is consistent with the known structural geology, such as open folding, and provides an improvement in estimating the size, shape, and depth of the Barlow and Chibougamau plutons. The Chibougamau pluton is known to be associated with Cu–Au magmatic-hydrothermal mineralisation and, as the volume and geometry of intrusive bodies is paramount to the exploration of such mineralisation, the modelling presented here provides a scientific foundation to exploration models focused on such mineralisation. [ABSTRACT FROM AUTHOR]

The nature of Earth's earliest crust and the processes by which it formed remain major issues in Precambrian geology. Due to the absence of a rock record older than ~4.02 Ga, the only direct record of the Hadean is from rare detrital zircon and that largely from a single area: the Jack Hills and Mount Narryer region of Western Australia. Here, we report on the geochemistry of Hadean detrital zircons as old as 4.15 Ga from the newly discovered Green Sandstone Bed in the Barberton greenstone belt, South Africa. We demonstrate that the U-Nb-Sc-Yb systematics of the majority of these Hadean zircons show a mantle affinity as seen in zircon from modern plume-type mantle environments and do not resemble zircon from modern continental or oceanic arcs. The zircon trace element compositions furthermore suggest magma compositions ranging from higher temperature, primitive to lower temperature, and more evolved tonalite-trondhjemitegranodiorite (TTG)-like magmas that experienced some reworking of hydrated crust. We propose that the Hadean parental magmas of the Green Sandstone Bed zircons formed from remelting of mafic, mantle-derived crust that experienced some hydrous input during melting but not from the processes seen in modern arc magmatism. [ABSTRACT FROM AUTHOR]

In comparison to the greenstone belt volcano-sedimentary successions, mature arenite dominated sedimentary deposits are very rarely preserved in Paleo-Mesoarchean rock record. Such arenitic deposits may be important for the recognition of extensive development of continental freeboard and onset of Phanerozoic style plate tectonics. The Keonjhar siliciclastics are ∼715-m thick mature arenite-dominated Mesoarchean succession that unconformably overlies Paleo-Mesoarchean granite-greenstone core of the Singhbhum craton. The succession is mapped on 1:50,000 scale, and is designated as the Keonjhar Quartzite. It is subdivided into conglomerate dominated lower unit as the Asurkhol Member and mature arenite dominated upper unit as the Kamalpur Member. A lensoid body of iron ore clast bearing conglomerate unconformably overlies the Keonjhar Quartzite, and is designated here as the Chamakpur Conglomerate. The succession is repeated by three westerly dipping reverse faults that have upthrown the deeper parts of the basinal transects against shallower parts. The Asurkhol Member represents subaerial part of a coastal alluvial fan. The Kamalpur Member represents shallow-shelf depositional setting. Detailed mapping and recognition of the fault slices led to the reconstruction of the paleogeographic setting. The present study also highlights the importance for the stratigraphic correlation of the Keonjhar Quartzite with underlying metabasics of Malangtaoli-Jagannathpur lava and the overlying western IOG succession. The internal stratigraphic classification proposed here is also significant from economic point of view with regard to the occurrences of uraniferous QPCs in the Asurkhol Member and detrital iron ore deposits in the Chamakpur Conglomerate. [ABSTRACT FROM AUTHOR]

The Oleninskoe gold deposit is located in the northwestern part of the Kolmozero–Voron'ya greenstone belt (Kola Peninsula), in a sequence of amphibolite with granite porphyry sills. The sills are genetically connected with a big granodiorite- and granite-porhyry massif, which hosts the Pellapahk Cu–Mo porphyry deposit. The geochemical association of the Oleninskoe includes As, Pb, Ag, Cu, Sb, and Au; the Au/Ag ratio is less than 0.2. The ore is of very complex mineral composition (more than 50 mineral names of intermetallic compounds, sulfides, and sulfosalts are defined), with widespread Ag and Sb minerals. Fluid inclusions in quartz are of high salinity with anomalous concentration of minor elements. All these characteristics show that the deposit formed in an Early Precambrian porphyry-epithermal system. Then the deposit underwent metamorphism (lower amphibolite facies) in the Neoarchean and Paleoproterozoic, indicating the absence of chlorite and a scarcity of carbonates in metasomatic rocks, replacement of pyrite with pyrrhotite, and signatures of sulfide anatexis in ore. This is the first finding of this genetic type of gold deposits in the northern part of the Fennoscandian Shield. [ABSTRACT FROM AUTHOR]

The early tectonic history of the Eastern Goldfields Terrane (EGT) is poorly understood, but in places ca 2800 Ma mafic–ultramafic sequences are conformably overlain by ca 2720–2670 Ma sequences (e.g. Leonora and Laverton districts), suggesting minimal early deformation. The first significant angular unconformities occur at the base of the ca 2670–2655 Ma late basins and indicate that deformation was contemporaneous with deposition of the late basins. These basins mark the end of the volcano-sedimentary record in the EGT and typically grade upwards from polymictic mafic-dominated conglomerates to more siliciclastic compositions with abundant granitic clasts. The clastic sequences record the uplift and exhumation of granite-cored domes. There is a distinct lack of clasts with internal deformation fabrics (e.g. schist or gneiss). The timing of late basin formation overlaps with the ca 2672–2660 Ma D1 event, which has been variously described as extensional or compressional. D1 structures comprise a bedding-parallel S1 that is axial planar to F1 folds. The late basins and early D1 fabrics are overprinted by upright north-trending F2 folds and a sub-vertical S2. A period of crustal thickening was achieved by autochthonous processes with deposition of greenstone sequences into local basins between ca 2720 and 2670 Ma. Partial convective overturn (or granite-up, greenstone-down tectonics) developed as a result of gravitational instability. As the large granite–gneiss bodies rose, solid-state extensional D1 shears developed around the granites with a radial pattern of L1 extension lineations. Areas of compression developed within the sinking greenstone sequences. A final phase of subsidence in the central parts of the greenstone belts produced depocentres (late basins). A shift from dominantly vertical tectonics to horizontal tectonics at ca 2655–2650 Ma is marked by the onset of east–west D2 compression and the change in granite compositions from high-Ca to low-Ca granites. Older ca 2800 Ma mafic–ultramafic sequences are conformably overlain by ca 2720–2670 Ma sequences suggesting minimal early deformation in the EGT. The first significant angular unconformities are at the base of the ca 2665 Ma late basin sequences. Late basin sequences record the uplift and exhumation of granite-cored domes and sedimentation is contemporaneous with the regional ca 2672–2660 Ma D1 event. The D1 event represents granite-up, greenstone-down tectonics (D1 event) that developed as a result of gravitational instability arising from autochthonous crustal thickening from ca 2720–2670 Ma. Granite-up, greenstone-down tectonics precludes an accretionary model for the development of the EGT. [ABSTRACT FROM AUTHOR]

A new 3D crustal-scale model has been created for the Kautokeino Greenstone Belt (KKGB) in northern Norway based on 3D density modelling and aeromagnetic data, integrated with qualitative interpretation of geophysical and geological data. Detailed geophysical analyses and their integration with an existing petrophysical database allowed us to identify and interconnect shallow and deeper structures throughout the study area. To have better control on the regional tectonic setting of the KKGB, the model area has been extended to include the northern part of Finland and Sweden. The new 3D density model reveals that the KKGB is broader than supposed previously. The 3D analysis indicates that the belt reaches depths of approximately 5-6 km and is emplaced as a highly deformed structure between the Archaean Jergul Gneiss Complex (JGC) to the east and the Rommaeno Gneiss Complex (RoGC) in Finland to the west. The Raiseatnu Gneiss Complex (RaGC) in the western part of the KKGB is characterised by a dominant NNW-SSE magnetic trend that is sub-parallel to the main trend of the KKGB. Moreover, the RaGC is similar to the KKGB in terms of the presence of a large number of sub-parallel magnetic anomalies. This gneiss complex shows mostly short-wavelength anomalies and some circular anomalies that differ from the more subdued and irregular anomaly pattern of the JGC and RoGC. In the new 3D model, the RaGC is assumed to constitute a migmatised part of the Kautokeino Greenstone Belt. At a large scale, this Proterozoic greenstone belt has some similarities to Archaean counterparts, where most of the deformation is caused by gravitational tectonics. This study shows that careful integration of geological and geophysical data can strongly improve the 3D understanding of the complex, poorly exposed, Precambrian terranes of the Finnmarksvidda region in Norway. [ABSTRACT FROM AUTHOR]

In this article, we present the textural, mineralogical, and geochemical studies of a suite of volcanic rocks and their economic mineral potential from the Kadiri Volcanic Province (KVP) – a greenstone belt in the Eastern Dharwar Craton (EDC) of southern peninsular India. The volcanic rocks are represented by primary pyroclastic deposits dominated by tuff breccia, minor lapilli tuff, and pyroclastic flows together with lava flows, namely, metabasalts, andesites, dacites, and rhyolites. Metabasalts and high‐Mg andesites in the KVP are associated with sporadic occurrences of disseminated‐type gold. The ancient works on gold are confined to the ductile‐brittle shear zones in en‐echelon pattern. The source of gold is likely to be from deep fractures (>200 km) at mantle depths that paved the magma to move upper crustal levels due to partial melting of peridotites which tapped the siderophile elements (Au–Ag) from deeper source. Some andesites and dacites contain atomic minerals [uranium, thorium, and rare earth element (REE)], which requires attention in future studies. Monazite occurring in such dacites is a major source of REE such as cerium (Ce), lanthanum (La), neodymium (Nd), and Yttrium (Y), contributing 50–60% of REEs. Presence of thorite (ThSiO4) in dacite, a metamict mineral, strongly radioactive containing upto 10% uranium (U). This study confirms multi‐metal mineralization in KVP, for example, gold, silver, atomic (U and Th), rare‐earths along with base metals. Emplacements of rhyolite and alkali granite appears to be synchronous with the major thermal event at 2.51 Ga of the Dharwar Craton. The diverse volcanic associations are formed in a subduction–accretion orogeny at ~2.7 Ga, coeval with the global accretion event and gold mineralization. Au–U–Th–REE abundances in the composite arc‐back arc system of Kolar‐Hutti‐Kadiri‐Jonnagiri greenstone belts of EDC are primarily attributed to devolatilization and melting of subducted oceanic slab, fluid‐fluxed metasomatism of mantle wedge and elemental cycling associated with different stages of subduction. [ABSTRACT FROM AUTHOR]

Limited taxonomic classification is possible for Archaean microbial mats and this is a fundamental limitation in constraining early ecosystems. Applying Fourier transform infrared spectroscopy (FTIR), a powerful tool for identifying vibrational motions attributable to specific functional groups, we characterized fossilized biopolymers in 3.5–3.3 Ga microbial mats from the Barberton greenstone belt (South Africa). Microbial mats from four Palaeoarchaean horizons exhibit significant differences in taxonomically informative aliphatic contents, despite high aromaticity. This reflects precursor biological heterogeneity since all horizons show equally exceptional preservation and underwent similar grades of metamorphism. Low methylene to end‐methyl (CH2/CH3) absorbance ratios in mats from the 3.472 Ga Middle Marker horizon signify short, highly branched n‐alkanes interpreted as isoprenoid chains forming archaeal membranes. Mats from the 3.45 Ga Hooggenoeg Chert H5c, 3.334 Ga Footbridge Chert, and 3.33 Ga Josefsdal Chert exhibit higher CH2/CH3 ratios suggesting mostly longer, unbranched fatty acids from bacterial lipid precursors. Absorbance ratios of end‐methyl to methylene (CH3/CH2) in Hooggenoeg, Josefsdal and Footbridge mats yield a range of values (0.20–0.80) suggesting mixed bacterial and archaeal architect communities based on comparison with modern examples. Higher (0.78–1.25) CH3/CH2 ratios in the Middle Marker mats identify Archaea. This exceptional preservation reflects early, rapid silicification preventing the alteration of biogeochemical signals inherited from biomass. Since silicification commenced during the lifetime of the microbial mat, FTIR signals estimate the affinities of the architect community and may be used in the reconstruction of Archaean ecosystems. Together, these results show that Bacteria and Archaea flourished together in Earth's earliest ecosystems. [ABSTRACT FROM AUTHOR]

Volcanic rocks in Archean and Paleoproterozoic greenstone belts are abundant and have been suggested as a potential Au source for orogenic Au deposits. The behavior of Au during metamorphism of these rocks is, however, poorly known. We present ultra-low-detection-limit Au analyses from a suite of variably metamorphosed rocks from the Archean La Grande subprovince, Canada, and the Paleoproterozoic Central Lapland greenstone belt, Finland. Both areas are well endowed in Au and have great potential for discovery of new orogenic Au deposits. The metavolcanic rocks in these belts are grouped into tholeiite and calc-alkaline magmatic series, for which the protolith Au contents are calculated using Au versus Zr/Y power-law regressions from greenschist facies samples. In the tholeiitic rocks, Au is compatible during magmatic processes and decreases with differentiation, whereas in the calc-alkaline rocks, Au is incompatible and increases with differentiation. Mass-variation calculations show that as much as 77% and 59% of the initial Au content is lost during progressive metamorphism to upper amphibolite facies conditions (>550 °C) in La Grande and Central Lapland respectively. This study highlights, first, that metavolcanic rocks release Au during metamorphism in Archean and Paleoproterozoic greenstone belts and are thus a good potential source rocks for orogenic Au deposits; second, that the Au fertility of the metavolcanic rocks is controlled by their mantle source and magmatic evolution; and third, that the metamorphic devolatilization model can be applied to Archean and Paleoproterozoic orogenic Au deposits. [ABSTRACT FROM AUTHOR]

The Flin Flon – Athapapuskow Lake area, situated in the Flin Flon Greenstone Belt, Manitoba, consists of ocean-floor and island-arc assemblages, deformed and metamorphosed during the Trans-Hudson Orogeny (∼1.86–1.69 Ga). A new map of metamorphic mineral assemblages and isograds has been compiled that reveals a largely coherent regional metamorphic sequence increasing in metamorphic grade from prehnite–pumpellyite to amphibolite facies. Regional metamorphism postdates most of the deformation within the area, with the exception of the reactivation of major block-bounding faults. The regional prograde sequence has been subdivided into 10 metamorphic zones, separated by 9 isograds, that describe the transition from prehnite–pumpellyite to greenschist to amphibolite facies. The formation of contact metamorphic aureoles, pre-dating regional metamorphism, record conditions up to amphibolite facies. Equilibrium phase diagrams for the island-arc (low-Mg) and ocean-floor (high-Mg) assemblages were calculated and allow for the evaluation of the modelling techniques and determination of pressure–temperature conditions. Discrepancies between the modelling predictions and natural observations occur due to (1) limitations in the thermodynamic models for some of the complex minerals (e.g., amphibole); and (2) metastable persistence of some minerals to higher grade due to sluggish reaction kinetics. Notwithstanding these discrepancies, the modelling suggests that metamorphosed mafic rocks in the Flin Flon – Athapapuskow Lake area reached about 430–480 °C and 3.0–4.5 kbar. Peak metamorphic conditions within contact aureoles that preceded regional metamorphism did not exceed 500 °C (at a pressure between 2.7 and 4.4 kbar). The metamorphic field gradient records a transition from 250–300 °C/1.5–2.3 kbar to 430–480 °C/3–4.5 kbar (100–150 °C/kbar), defining a geothermal gradient of approximately 25–31 °C/km. [ABSTRACT FROM AUTHOR]

This work presents new U–Pb ID-TIMS geochronological data obtained for the rocks of the metagabbro–diorite–tonalite complex of the Olondo fragment of the Tokko–Khani greenstone belt of the Aldan Shield, Siberian Craton. Wide development of ultramafic and mafic volcanics associated with felsic volcanics has been recognized only within this fragment. It was established that the protoliths of metagabbro–diorite and quartz metadiorite intruding metavolcanic and metasedimentary rocks were crystallized at 3002 ± 5 and 3005 ± 7 Ma, respectively. The obtained age estimates determine the upper age boundary of the formation of the Olondo supracrustal rocks (3012–2997 Ma). The Nd model ages of the Olondo metaandesites and metadacites are close to 3.0 Ga, which, to the first approximation, determines the lower age boundary of their formation. Thus, the sedimentary–volcanogenic sequence of the Olondo fragment are about 3.0 Ga old, and their formation, including superimposed structural and metamorphic transformations, occurred within a short period (a few million years). [ABSTRACT FROM AUTHOR]

Two high-resolution seismic reflection profiles acquired north and south of Chibougamau, located in the northeast of the Abitibi subprovince of Canada help understand historic volcanic-hosted massive sulfide (VMS) deposits and hydrothermal Cu-Au mineralization found there. Major faults crossed by the profiles include the Barlow fault in the north and the Doda fault and the Guercheville fault in the south, all targets of this study that seeks to determine spatial relationships with known metal endowment in the area. Common-offset DMO corrections and common-offset pre-stack time migrations (PSTM) were considered. Irregularities of the trace midpoint distribution resulting from the crooked geometry of both profiles and their relative contribution to DMO and PSTM method and seismic illumination were assessed in the context of the complex subsurface architecture of the area. To scrutinize this contribution, seismic images were generated for offset ranges of 0 -9 km using increments of 3 km. Migration of out-of-plane reflections used cross-dip element analysis to accurately estimate the fault dip. The seismic imaging shows the thickening of the upper crustal rocks near the fault zones along both profiles. In the north seismic reflection section, key geological structure identified include the Barlow fault and two diffraction sets imaged within the fault zone that represent potential targets for future exploration. The south seismic reflection section shows rather a complicated geometry of two fault systems. The Guercheville fault observed as a subhorizontal reflector connects to a steeply dipping reflector. The Doda fault dips subvertical in the shallow crust but as a steeply dipping reflection set at depth. Nearby gold showings suggest that these faults may help channel and concentrate mineralizing fluids. [ABSTRACT FROM AUTHOR]

Dharwar craton (DC), by far the largest geological domain in South Indian Shield, occupying about 0.5 million sq. km area, is well-studied terrain both for regional geoscientific aspects and as part of mineral exploration over several important blocks such as the greenstone belts, ultramafic complexes, granite-gneissic terrain and the Proterozoic sediments of Cuddapah basin. The re-look into regional gravity data offers several insights into nature of crust, sub-divisions within the craton, bedrock geology in the covered areas and mineral potentiality of this ancient and stable crust. The regional gravity profiles drawn across the south Indian region mainly suggest that the area can be divided into five domains as Western Dharwar craton (WDC), Central Dharwar craton (CDC), Eastern Dharwar craton (EDC 1), transitory zone of EDC (EDC 2) and Eastern Ghats mobile belt (EGMB) areas. The Bouguer gravity anomaly pattern also questions some of the earlier divisions like eastern margin of Chitradurga schist belt between the WDC and EDC and the boundary of DC with southern granulite terrain (SGT) as they do not restrict at these main boundaries. In this study, mainly four issues are addressed by qualitative and quantitative analysis of regional gravity data and those revealed significant inferences. (1) A distinct gravity character in central part of south Indian shield area occupying about 60, 000 sq. km, suggests that the transitory crustal block, faulted on both sides and uplifted. This area designated as central Dharwar craton (CDC) is characterized with schist belts having characters of both parts of western and eastern Dharwar craton. This inference also opens up the debate about the boundary between western and eastern parts of the craton. Another significant inference is the extension of major schist belts beneath both Deccan volcanic province (DVP) in northwestern part and Cuddapah basin (CB) in southeastern part. (2) Eastern Dharwar craton is reflected as two distinct domains of different gravity characters; one populated with number of circular gravity lows and a few linear gravity high closures indicative of plutonic and volcanic activity and another domain devoid of these intrusive younger granites or schist belts. (3) Large wave length gravity highs occupying thousands of sq.km area and those not relatable to surface geology in eastern Dharwar craton that may have significance for mineral exploration. (4) Gravity data was subjected to further processing like two dimensional modeling which have yielded insights into crustal architecture beneath the Dharwar craton, crustal scale lineaments, craton-mobile belt contact zone and younger intrusives. [ABSTRACT FROM AUTHOR]

The United States Geological Survey (USGS) Three-part Undiscovered Mineral Resource Assessment provides a framework for estimating undiscovered mineral endowment. Previous studies that applied the Three-part Assessment to estimate the undiscovered orogenic gold endowment of the Sandstone Greenstone Belt, Western Australia, have relied upon dated or expert-derived grade-tonnage models. Here, several assessments are conducted using local grade-tonnage models, comprising known orogenic gold deposits within the entire Yilgarn Block and several individual terranes with contrasting lithosphere- to terrane-scale characteristics. These models are generated through comprehensive review of historical exploration, resource and production data. Based on these models, the Sandstone Greenstone Belt is estimated to contain significant undiscovered gold mineralisation, with a median total endowment of between 166 and 298 t gold, and mean of 167–319 t gold. Although these updated grade-tonnage models provide an approximately 80 per cent variation in predicted gold endowment, it is still evident that the belt remains an under-explored region within the Yilgarn Block, Western Australia. [ABSTRACT FROM AUTHOR]

The discovery of komatiites, first in South Africa and then in many other Archean greenstone belts, with MgO concentrations of 20–30% and eruption temperatures of more than ∼1600 °C, showed that some parts of the mantle were hotter in the Archean than they are now. Since their discovery there have been many speculative proposals as to how such magmas can form. At present melt is produced by mantle upwelling, because the solidus temperature gradient of the mantle is steeper than that of isentropic decompression gradient at depths of less than 300 km. In contrast, in the lower half of the upper mantle the solidus gradient is shallower than the isentropic gradient, and, therefore, isentropic upwelling cannot generate melt. At the base of the upper mantle limited melting can occur, either in the thermal boundary layer at the base of the upper mantle, or in the upper part of the lower mantle where the solidus gradient is steeper than the isentropic gradient. In both cases melting can occur at depths of more than 600 km, where Ca perovskite, CaPv, is a stable phase on the solidus. A surprising feature of the partitioning between melt and solid CaPv is that most trace elements are compatible in the solid. Partitioning into CaPv can, therefore, account for the low concentrations of such elements in komatiites. The temperatures required to generate such magmas in plumes need be no more than ∼50 °C above those of Phanerozoic plumes. The presence of komatiites in the Archean, therefore, requires plume temperatures in the first half of the Earth's history to have been somewhat hotter than they are now, but does not constrain the average temperature of the Archean upper mantle. [ABSTRACT FROM AUTHOR]

The Neoarchean Shimoga greenstone belt (SGB) of western Dharwar Craton is dominated by ultramafic‐mafic‐felsic magmatic suite and sedimentary rocks. Here, we investigate the mineral chemistry and whole‐rock geochemistry of the Koppa–Herambapura mafic magmatic rocks (tholeiitic metagabbro and metabasalt) from the southern SGB in order to understand their petrogenesis and geodynamic implication. The geochemical features classify the Shimoga rocks as basaltic‐basaltic andesite with a sub‐alkaline tholeiitic signature. Both the rock types display uniform depletion in HFSE relative to LILE‐LREE, higher LILE/HFSE, and LREE/HFSE ratios (La/Nb = 1.30–8.07, Nb/Ta = 11.58–16.19, Zr/Sm = 10.24–29.77) and negative Nb–Ta, Zr–Hf, and Ti anomalies in multi‐element patterns, corresponding to magmatic arc rocks generated within intraoceanic subduction environment. Their Nb/Th (2–15), Zr/Sm (10–29), and Zr/Hf (44–233) ratios indicate a depleted to enriched mantle source, and the parental magma of the mafic rock suite was generated by low to medium degrees (~8–30%) of mantle melting at depths corresponding to spinel to garnet peridotite fields of arc‐back arc setting. Our field and analytical data suggest that the tholeiitic metagabbros were derived from depleted mantle source generated in an arc environment, whereas the metabasalts had a mantle source attributed to (a) development of juvenile back‐arc rift; (b) upwelling of MORB‐like mantle; and (c) flux‐induced melting in the compositional domain of spinel‐garnet peridotite. Based on these results, we conclude that the mafic rock suite from the southern SGB possibly marks one of the earliest Neoarchean arc‐back arc subduction in the western Dharwar Craton. [ABSTRACT FROM AUTHOR]

Most of the upper crustal fluid flows are strongly influenced by the pre-existing fractures/foliations in the rocks under a certain state of tectonic stress and fluid pressure condition. In the present study, we analyzed a wide range of crosscutting fractures that are filled with quartz veins of variable orientations and thicknesses, from the gold-bearing massive metabasalts (supracrustals) of the Chitradurga Schist Belt adjacent to the Chitradurga Shear Zone (CSZ), Western Dharwar Craton, southern India. The study involves the following steps: (1) analyzing the internal magnetic fabric, using anisotropy of magnetic susceptibility (AMS) studies, and determining strength of the host metabasalts, (2) quantifying the fluid pressure condition through lower hemisphere equal area projection of pole to veins by determining the driving pressure ratio (R′), stress ratio (ϕ), and susceptibility to fracturing, and (3) deciphering the paleostress condition using fault-slip analysis. We interpret the NNW–SSE to NW–SE (mean 337/69 ∘ NE) oriented magnetic fabric in the rocks of the region as having developed during regional D1/D2 deformation on account of NE–SW shortening. However, D3 deformation manifested by NW–SE to E–W shortening led to the sinistral movement along CSZ. As a consequence of this sinistral shearing, fractures with prominent orientations formed riedel shear components, with CSZ as the shear boundary. Subsequently, all the pre-existing fabrics along with the riedel shear components were reactivated and vein emplacement took place through episodic fluid pressure fluctuation from high to low Pf at shallow depth (∼ 2.4 km). However, NNW–SSE orientations were prone to reactivate under both high- and low- Pf conditions, thereby attaining maximum vein thickness along these orientations. The deduced paleostress from fault-slip analysis along with the kinematics of the fractures and veins are in good agreement with previously estimated regional tectonics. Thus, integrating multiple domains of studies helps in the logical interpretation of fluid flow conditions and vein emplacement mechanisms in the study area that has not been ventured before. [ABSTRACT FROM AUTHOR]

Low saline aqueous carbonic fluids are considered to be the ore forming solutions for orogenic lode gold deposits. Phase separation/fluid immiscibility of the ore fluid is quite common and is one of the major reasons for deposition of gold in these deposits. Abundant carbonic fluid inclusions have been observed in quartz grains of Hira-Buddnini Gold Deposit. Theoretical estimation indicates that more volume of H2O compared to CO2 is likely to be trapped in inclusions at different P-T conditions. Preferential loss of H2O from fluid inclusions during ductile deformation of quartz grains have been attributed as the suitable reason for abundance of carbonic fluid inclusions. [ABSTRACT FROM AUTHOR]

The Mesoarchean Kalva Rangan Durga Formation of the Shimoga greenstone belt in the western Dharwar Craton consists of orthoquartzite, ferruginous phyllite, banded ferruginous chert, and conglomerates. Thick beds of orthoquartzites of this formation are well exposed in Kalva Rangan Durga ridge, trending northeast to southwest for about 9 km, in the southwest part of the Honnali Dome. Quartzites are fine‐grained in nature and mostly composed of quartz grains and fuchsite mica. In the present study, we have collected quartzite samples and carried out geochemical, Sr, and Nd isotope analyses to decipher their paleo‐weathering conditions, provenance characteristics, and possible tectonic setting of deposition. Geochemical data indicate they are arkose to subarkose in nature. Moderate to intense chemical weathering of the source rocks of these quartzites during the Mesoarchean is suggested based on various weathering indices and ternary plots. Enriched LREE and flat to variably fractionated HREE patterns with negative Eu anomalies of quartzites indicate that they were mainly derived by weathering of tonalities, trondhjemite, and granodiorites (TTG) gneisses and granites. The relatively high concentration of Cr with variable Th/Sc and high Cr/Th ratios indicate both felsic and mafic sources for quartzites. Initial εNd values (t = 3.0 Ga) of quartzites range from −3.04 to +1.24 which further support their derivation from both older evolved continental crustal and juvenile sources. The presence of symmetrical ripple marks on the surface of quartzite indicates a shallow marine environment. [ABSTRACT FROM AUTHOR]

We present 40Ar/39Ar ages and boron isotopes of tourmalines from a quartz-tourmaline intrusion cross-cutting the Hadean detrital-zircon-bearing metasedimentary rocks from the Mt. Alfred and Brooking Hills localities of the Illaara Greenstone Belt, Western Australia. Concordant 40Ar/39Ar plateau ages on the tourmalines at Mt. Alfred give a weighted mean age of 2935 ± 9 Ma (2σ). These results are the first reported Archean 40Ar/39Ar ages directly obtained on tourmaline, and provide a minimum depositional age for this sequence, placing constraints on the depositional timing of detrital Hadean zircons to between 2935 ± 9 Ma and c.a. 3250 Ma (youngest detrital zircon age in sequence). This is the last known occurrence of abundant detrital Hadean zircons having been directly deposited in a sedimentary sequence, and thus suggests that most Hadean zircon sources have been exhausted by this time. The 2935 ± 9 Ma tourmalines occur within a quartz-tourmaline intrusion with associated stratiform layers and veins, inferred to have grown during a hydrothermal fluid circulation event. A younger generation of tourmalines from the Brooking Hills, 30 km south of Mt. Alfred yielded concordant 40Ar/39Ar plateau ages with a weighted mean age of 2624 ± 16 Ma. These tourmalines occur within and along the margins of post-kinematic quartz veins, parallel to the foliation fabric and marks the end stage of high temperature tectono-thermal events within the Illaara Greenstone Belt. The retention of 2935 ± 9 Ma tourmaline ages within the Illaara Greenstone Belt shows that the K/Ar system of tourmaline remained closed throughout repeated upper-greenschist metamorphic events (∼450 °C) between the ages of c.a. 2930 to 2630 Ma in contrast to muscovite recording plateau ages of about 2600 Ma. This is in agreement with an approximate Ar closure temperature between ∼534 and 628 °C experimentally determined in this study. [ABSTRACT FROM AUTHOR]

The Eastern Dharwar Craton (EDC) in southern India comprises several thin, linear N-S and NW-SE trending greenstone belts that hold rich economic mineral deposits, surrounded by gneisses and granites of different ages. Very long, linear, high-frequency, high-amplitude magnetic anomalies trending along the E-W, NE-SW, ENE-WSW, NW-SE and N-S directions associated with several sets of dike swarms, fault shears and fractures forming a rhombic pattern over the granite-gneissic terrain in the western part of EDC hindered the recognition of these thin, linear belts. The N-S strike of the belts in the low-magnetic latitudes posed another limitation to demarcate these zones. A comprehensive relook into the regional aero-magnetic data using the prevailing interpretation techniques through diverse approaches helped identify the feeble magnetic signatures due to known narrow zones and enabled us to map hitherto unknown belts, namely Kurnool and Lattavaram-Atmakuru. 2.5D GM-SYS modelling has been carried out to delineate the vertical disposition of known belts that reaffirmed the existence of newly inferred belts. Isolated magnetic anomalies are also modelled for source-body parameters. [ABSTRACT FROM AUTHOR]

On the basis of phylogenetic studies and laboratory cultures, it has been proposed that the ability of microbes to metabolize iron has emerged prior to the Archaea/Bacteria split. However, no unambiguous geochemical data supporting this claim have been put forward in rocks older than 2.7–2.5 giga years (Gyr). In the present work, we report in situ Fe and S isotope composition of pyrite from 3.28‐ to 3.26‐Gyr‐old cherts from the upper Mendon Formation, South Africa. We identified three populations of microscopic pyrites showing a wide range of Fe isotope compositions, which cluster around two δ56Fe values of −1.8‰ and +1‰. These three pyrite groups can also be distinguished based on the pyrite crystallinity and the S isotope mass‐independent signatures. One pyrite group displays poorly crystallized pyrite minerals with positive Δ33S values > +3‰, while the other groups display more variable and closer to 0‰ Δ33S values with recrystallized pyrite rims. It is worth to note that all the pyrite groups display positive Δ33S values in the pyrite core and similar trace element compositions. We therefore suggest that two of the pyrite groups have experienced late fluid circulations that have led to partial recrystallization and dilution of S isotope mass‐independent signature but not modification of the Fe isotope record. Considering the mineralogy and geochemistry of the pyrites and associated organic material, we conclude that this iron isotope systematic derives from microbial respiration of iron oxides during early diagenesis. Our data extend the geological record of dissimilatory iron reduction (DIR) back more than 560 million years (Myr) and confirm that micro‐organisms closely related to the last common ancestor had the ability to reduce Fe(III). [ABSTRACT FROM AUTHOR]

Data on the carbon and oxygen isotopic composition of carbonates from the rocks of the Giyani greenstone belt, Kaapvaal craton, South Africa are presented. This belt is immediately adjacent to the Hout River Shear Zone, along which the Southern Marginal Zone (SMZ) of the Limpopo granulite complex is overthrusted onto the Kaapvaal craton. The δ13C values are –2.1 and –2.2‰ for carbonates from two metapelite samples, –5.1‰ for a meta ultrabasite sample, and –7.1 and –7.7‰ for two metabasite samples, respectively. The δ18О values are 17.1 and 17.5‰ for carbonates from the metapelites, 14.3‰ for carbonates from the metaultrabasite, and 12.9 and 13.0‰ for carbonates from the metabasites. The comparison of the data obtained to the published data on the isotopic composition of carbonates from the rocks of the different greenstone belts indicates both hydrothermal (in the metabasites and the metaultrabasite) and sedimentary (in the metapelites) origins of the carbonate material in the rocks studied. Based on the comparison of these data to δ13C of magnesite that formed in ultrabasic granulites during the interaction of the Limpopo complex with the craton, the graphite and fluid inclusions in leucocratic garnet-bearing granitoids that transported fluids, and the carbon isotopic composition of graphite from metapelites of the SMZ, it is concluded that the carbon source was located outside the SMZ and represented a heterogeneous carbonate-bearing lithologies. It could be carbonate-bearing volcanogenic–sedimentary sequences of the greenstone belts of the Kaapvaal craton, which underwent prograde metamorphism during the interaction with the SMZ granulites. [ABSTRACT FROM AUTHOR]

The ~2.7‐Ga Taishan greenstone belt from the Eastern Block of the North China Craton has well‐exposed komatiite sections. The komatiite outcrops at Sujiagou have well‐preserved primary volcanic texture although they have experienced amphibolite‐facies metamorphism. Two komatiite lava flows are identified in the Sujiagou outcrops: "Flow Ι," with a thickness of 55 m, is characterized by poly‐sutured crust at the top, spinifex texture in the middle, and olivine cumulates at the flow bottom. "Flow ΙΙ" consists of abundant pahoehoe lava lobes with no interstitial materials. These volcanic characteristics, as well as the interbedding with terrestrial sediments, suggest that the Sujiagou komatiite erupted in a subaerial continental environment. The olivine–chromite geothermometer gives peak metamorphic temperatures from 684°C to 764°C. Phase equilibria modelling suggests that the mineral proportions and mineral compositions are consistent with a retrograde temperature of ~488°C, at which the komatiite would have low olivine Fo (79–89) and high bulk‐rock Mg# (83–90). Linear mixing of the in situ trace element compositions of the metamorphic olivine, augite, tremolite, serpentine, and chlorite from Sujiagou komatiite samples can reproduce the trace elements of representative komatiite compositions, suggesting that later metamorphism and fluid–rock interactions played key roles in trace element variations in the bulk‐rock komatiite samples. These results also imply that the "arc signature" of the Sujiagou komatiite may be an overprint from posteruption fluid alteration rather than a primary subduction geochemical feature. Therefore, the Sujiagou komatiites may belong to a part of the global plume‐related greenstone belts at ~2.7 Ga. [ABSTRACT FROM AUTHOR]