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Assimilation and prolonged suspension of crust-derived sulfide liquid in komatiites are essential to form Ni-rich mineralisation. Evaluating the spatial relationship between komatiite-hosted Ni mineralisation and crustal S sources may thus provide insights into mechanisms of transport, metal enrichment and deposition of assimilated sulfide liquid. This study applied facies analysis and S isotopes to sulfides in Ni-mineralised komatiites and stratigraphically underlying bimodal volcanic-volcaniclastic and sedimentary rocks, which formed during rifting in the Agnew-Wiluna Greenstone Belt, Western Australia. The results revealed a lateral variation from rift-distal sedimentary sulfides, through sulfidic BIF, to rift-proximal VMS-style sulfides, the latter of which was predominantly assimilated by komatiites. Both crustal and komatiite-hosted sulfides were overprinted by granite-related skarn alteration during later basin inversion. Spatial S isotopes correlation revealed that Ni mineralisation in komatiites predominantly formed < 5 km from their crustal S sources, excluding long lateral transport as the main metal enrichment mechanism. Rather, metal enrichment likely happened through multiple cycles of sulfide entrapment and entrainment in lava flow vortices that formed in the wake of topographic steps represented by syn-rift faults. These faults were the main loci for pre-existing crustal weaknesses, hydrothermal fluid circulation, and VMS-style sulfide deposition, which were subsequently utilised by komatiites for enhanced thermo-mechanical erosion and crustal sulfide assimilation. This study shows that proximity to the syn-rift faults was the dominant control on the formation of komatiite-hosted Ni–sulfide mineralisation, regardless of substrate lithology. The S isotope signatures of crustal sulfides may be used as a proxy to identify syn-rift faults in highly deformed terranes. [ABSTRACT FROM AUTHOR]

The late Archean Western Dharwar Craton supergroup has volcanic and sedimentary rocks deposited during 2900–2600 Ma underlain by the Sargur Group basement of 3.36–3.2 Ga granitic gneiss and older supracrustal rocks. The western part of the craton, known as the western province (Dharwar Foreland), comprises four major schist belts (Western Ghats-Bababudhan-Shimoga-Chitradurga). A regional gravity and magnetic (Total Field) survey was done in the Shimoga region. The geophysical data of the study area reveal the subsurface extension and behaviour of the exposed rocks of the area. The Euler deconvolution was used to define the depth of the anomaly sources. Euler depth solution for the spherical body (structural index 2), window size = 10, and depth tolerance = 5% depicts the depth of the various sources. The Euler depth solutions cluster over the hook-shaped schist rock of the Medur group (at the northern part of the area) reveals that the body's depth varies from 1 to 4 km in the central part, and at the northern flank, it goes up to 7 km. The solution also brought out the depth of the anomalous body near Hithala, within 4 km. The 2D forward depth modelling of the anomalous zone brings out the involvement of the tectonic forces in the generation of this anomalous body. The more precise depth from the model has been estimated between 2 and 3 km. The model clearly shows the relationship between schist rocks and the basement. However, the high-gravity anomaly has been modelled as the manganese bearing horizon, which has come to optimum depth because of folding and faulting in the region, thus providing a favourable zone for manganese deposition. [ABSTRACT FROM AUTHOR]

The greenstone belts in the Middle Dniprо Domain of the Ukrainian Shield comprise sedimentary-volcanogenic rocks of the Konka and Bilozerka Groups separated by a unconformity. The Konka Group is composed mainly of volcanic rocks varying in composition from ultrabasic to felsic composition with subordinate sedimentary rocks, while the Bilozerka Group is composed of sedimentary rocks with a small amount of felsic volcanic rocks. Three associations of ferruginous-siliceous rocks have been identified in the greenstone rocks of the Middle Dniprо Domain. These are the ferruginous-siliceous-mafic and ferruginous-siliceous-keratophyre (schist-jaspilite-tholeiite) associations in the Konka Group and the transitional ferruginous-siliceous-shale to ferruginous-siliceous-keratophyre association in the Bilozerka Group, which many researchers compare with the ferruginous-siliceous formations of Kryvyi Rih Group. The rocks of the early ferruginous-siliceous-mafic association (Konka Group) are found among the volcanogenic rocks with a komatiite-tholeiitic association. These rocks are intruded by 3.08-3.0 Ga-old plagioclase granites of the Sura Complex. In the younger ferruginous-siliceous-keratophyre (schist-jaspilite-tholeiite) association (Bilozerka Group), ferruginous-siliceous deposits are interlayered with sedimentary and tuffaceous rocks and reach a thickness of up to 350 m. The Chortomlyk iron ore deposit, which is located in the Chortomlyk Greenstone Belt, is confined to this rock association. The ages of the iron-bearing rocks has not been constrained to date. We dated 60 detrital zircon grains from quartz-amphibole schists sampled at a section of sedimentary-volcanogenic rocks of the Chortomlyk deposit. Fifty-three analyzed crystals yielded an age of 3.08-3.12 Ga; six crystals were dated at 3.08-3.06 Ga, and one crystal at 3.18 Ga. They all have low U contents (1.4-20 ppm), while the Th/U ratio falls in the range of 0.3-0.9, indicating a likely magmatic origin. Sedimentary-volcanogenic rocks of the Chortomlyk iron ore deposit could been formed on a marine shelf near uplifted highs of plagioclase granites of the Sura Complex (dated at 3.08-3.06 Ga), or near volcanoes of the same age. Thus, the sedimentary-volcanogenic rocks of the Chortomlyk iron deposit are younger than the rocks of the Konka Group. Similar detrital zircon ages were previously obtained from the ferruginous-siliceous association of the Bilozerka Group in the Bilozerka Greenstone Belt (3.05 Ga) and the Vysokopillya Greenstone Belt (3.06 Ga). Thus, the ferruginous-siliceous-mafic association formed before the plagioclase granites of the Sura Complex, while the ferruginous-siliceous-keratophyre (schist-jaspilite-tholeiite) association was formed after the emplacement of the plagioclase granites. [ABSTRACT FROM AUTHOR]

The nitrogen isotopic composition of organic matter is controlled by metabolic activity and redox speciation and has therefore largely been used to uncover the early evolution of life and ocean oxygenation. Specifically, positive δ15N values found in well‐preserved sedimentary rocks are often interpreted as reflecting the stability of a nitrate pool sustained by water column partial oxygenation. This study adds much‐needed data to the sparse Paleoarchean record, providing carbon and nitrogen concentrations and isotopic compositions for more than fifty samples from the 3.4 Ga Buck Reef Chert sedimentary deposit (BRC, Barberton Greenstone Belt). In the overall anoxic and ferruginous conditions of the BRC depositional environment, these samples yield positive δ15N values up to +6.1‰. We argue that without a stable pool of nitrates, these values are best explained by non‐quantitative oxidation of ammonium via the Feammox pathway, a metabolic co‐cycling between iron and nitrogen through the oxidation of ammonium in the presence of iron oxides. Our data contribute to the understanding of how the nitrogen cycle operated under reducing, anoxic, and ferruginous conditions, which are relevant to most of the Archean. Most importantly, they invite to carefully consider the meaning of positive δ15N signatures in Archean sediments. [ABSTRACT FROM AUTHOR]

This study presents multiple sulphur isotope (32S, 33S, 34S, 36S) data on pyrites from silicified volcano‐sedimentary rocks of the Paleoarchean Onverwacht Group of the Barberton greenstone belt, South Africa. These rocks include seafloor cherts and felsic conglomerates that were deposited in shallow marine environments preserving a record of atmospheric and biogeochemical conditions on the early Earth. A strong variation in mass independent sulphur isotope fractionation (MIF‐S) anomalies is found in the cherts, with Δ33S ranging between −0.26‰ and 3.42‰. We explore possible depositional and preservational factors that could explain some of this variation seen in MIF‐S. Evidence for microbial activity is recorded by the c. 3.45 Ga Hooggenoeg Formation Chert (HC4) preserving a contribution of microbial sulphate reduction (−Δ33S and –δ34S), and a c. 3.33 Ga Kromberg Formation Chert (KC5) recording a possible contribution of microbial elemental sulphur disproportionation (+Δ33S and –δ34S). Pyrites from a rhyo‐dacitic conglomerate of the Noisy Formation do not plot along a previously proposed global Felsic Volcanic Array, and this excludes short‐lived pulses of intense felsic volcanic gas emissions as the dominant control on Archean MIF‐S. Rather, we suggest that the MIF‐S signals measured reflect dilution during marine deposition, early diagenetic modification, and mixing with volcanic/hydrothermal S sources. Given the expanded stratigraphic interval (3.47–3.22 Ga) now sampled from across the Barberton Supergroup, we conclude that large MIF‐S exceeding >4‰ is atypical of Paleoarchean near‐surface environments on the Kaapvaal Craton. [ABSTRACT FROM AUTHOR]

Precambrian greenstone belts are prospective terrains for orogenic Au deposits worldwide, but the sources of Au, base metals, metalloids, and ligands enriched within the deposits are still debated. Metamorphic devolatilization is a key mechanism for generating Au-rich hydrothermal fluids, but the respective role of the metavolcanic and metasedimentary rocks present within these belts in releasing ore-forming elements is still not fully understood. The Central Lapland Greenstone Belt (CLGB), Finland, one of the largest Paleoproterozoic greenstone belts, hosts numerous orogenic Au deposits and is composed of variably metamorphosed volcanic and sedimentary rocks. Characterization of element behavior during prograde metamorphism highlights that (1) metavolcanic rocks release significant Au, As, Sn, Te, and possibly S; (2) metasedimentary rocks release significant S, C, Cu, As, Se, Mo, Sn, Sb, Te, and U, but limited Au; and (3) metakomatiite releases C and possibly Au. Throughout the CLGB metamorphic evolution, two main stages are identified for metal mobilization: (1) prograde metamorphism at ~ 1.92–1.86 Ga, promoting the formation of typical orogenic Au deposits and (2) late orogenic evolution between ~ 1.83 and 1.76 Ga, promoting the formation of both typical and atypical orogenic Au deposits. The complex lithologic diversity, tectonic evolution, and metamorphic history of the CLGB highlight that metal mobilization can occur at different stages of an orogenic cycle and from different sources, stressing the necessity to consider the complete dynamic and long-lasting evolution of orogenic belts when investigating the source of Au, ligands, metals, and metalloids in orogenic Au deposits. [ABSTRACT FROM AUTHOR]

• Hadean mantle was involved into formation-reworking process of NCC from 3.8Ga until 2.45Ga. • Early basalts can derive from a common 4.4Ga mantle formed shortly after lunar impact. • Eoarchean 142Nd heterogeneity seems analytical, rather than truly heterogeneous. • Hadean depleted domains can remain convectively isolated for 1.7 billion years. The 146Sm-142Nd systematics of Archean rocks have been extensively studied to decipher chronological and compositional constraints on Hadean differentiation processes. However, the paucity of 142Nd data on Meso- and Neoarchean mantle-derived rocks relative to earlier periods contributes to uncertainty about the ultimate fate of primordial silicate reservoirs in the convective system of the early Earth. Here we present 146,147Sm-142,143Nd and 176Lu-176Hf zircon data for Mesoarchean (3.0–3.1 Ga) to Paleoproterozoic (2.45 Ga) granitoids, amphibolites and sedimentary rocks representing the main lithological units of the eastern North China Craton (NCC). The analyzed samples have well-resolved positive 142Nd anomalies, suggesting that the Hadean materials have been involved into the generation and evolution of the Eoarchean-Neoarchean continental crust of the eastern NCC. The model age of differentiation (4. 44 − 0.07 + 0.05 Ga) and the 147Sm/144Nd source ratio (0.211 ± 0.002) estimated from the 3.1 Ga Caozhuang amphibolites are identical to those of the 3.8–3.6 Ga rocks from the North Atlantic Craton and the 2.7 Ga pyroxenites from the Abitibi Greenstone Belt, suggesting that these mantle-derived rocks originated from a common, moderately depleted reservoir that differentiated in the wake of the Moon-forming impact and evolved in closed-system until at least 2.7 Ga. These observations contradict the commonly accepted notion of progressive homogenization of 142Nd anomalies in the Archean due to crustal recycling and convective mixing, and instead suggest that primordial differentiation produced a long-lived, depleted reservoir that remained largely isolated from convective mixing on a timescale of several billion years. [ABSTRACT FROM AUTHOR]

The collaborative federal–provincial Southern Indian Lake project in north-central Manitoba covered an area of more than 3500 km2 of the Trans-Hudson orogen. Regional-scale geological mapping, sampling, and lithogeochemical, isotopic, and geochronological studies resulted in the identification of distinct assemblages of supracrustal rocks and varied episodes of plutonism. A granodiorite gneiss dated at ca. 2520 Ma is interpreted to represent the basement of the Southern Indian domain and is considered a separate crustal domain, named the Partridge Breast block. The Churchill River assemblage is composed of juvenile pillow basalt with intervening clastic sedimentary rocks, possibly a reflection of plume magmatism related to initial rifting of the Hearne craton margin. The Pukatawakan Bay assemblage consists mainly of massive to pillowed, juvenile metabasaltic rocks and associated basinal metasedimentary rocks. The Partridge Breast Lake assemblage is dominated by continental-arc volcanic and volcaniclastic rocks associated with basinal metasedimentary rocks. The Strawberry Island assemblage, consisting of arenite and polymictic conglomerate, is interpreted to have been deposited in a foreland-basin basin or intra-orogen pull-apart basin environment. The Whyme Bay assemblage is characterized by fluvial-alluvial orogenic sediments and is temporally linked to the Sickle Group rocks in the Lynn Lake greenstone belt. Granitoid rocks, dominantly monzogranite and granodiorite, range in age from ca. 1890 to 1830 Ma and occur throughout the Southern Indian domain, and intermediate and mafic intrusions of similar ages are also present. In this paper, we integrate these new data into a tectonic framework for the Southern Indian domain of the Trans-Hudson orogen in Manitoba. [ABSTRACT FROM AUTHOR]

One of the major challenges in early Earth geology is the interpretation of the nature of the crust and tectonic processes due to the limited exposures of Archean rocks. This question is predominantly addressed by numerical modeling, structural geology, geochemical analyses, and petrological approaches. Here we report on the reconstruction of one of the oldest, well-preserved volcano-sedimentary sequences on Earth, the 3.28-3.22 Ga Fig Tree Group in the Barberton Greenstone Belt, South Africa, based on geochronology, provenance, and stratigraphy to provide new constraints on the nature of tectonic processes in the Archean. The Fig Tree basin was asymmetric and the onset of deposition varied across the greenstone belt. The Fig Tree Group is now preserved in east-west oriented bands of fault-bounded structural belts with those preserved in the southern parts of the greenstone belt showing an onset of deposition at 3.28 Ga, those in the center at 3.26 Ga, and those in the north at 3.24 Ga. Stratigraphically, the rocks display a general up-section trend from deeper to shallower-water deposition and/or from finer- to coarser-grained sedimentary rocks. Associated with this up-section stratigraphic trend, the sedimentary rocks show a change in provenance from more regionally similar to more locally variable, and an increase in felsic volcanic activity, especially toward the closure of Fig Tree deposition. The data is consistent with formation of the Fig Tree Group in a compressional tectonic setting by deposition in a foreland basin that experienced progressive accretion of crustal terranes onto a northward prograding fold-and-thrust belt. [ABSTRACT FROM AUTHOR]

Bedrock geological mapping indicates that diverse granitoid intrusions occur closely in space and time in the Paleoproterozoic Lynn Lake greenstone belt of the Trans-Hudson Orogen, Manitoba, Canada. These intrusions display distinct geochemical characteristics of I-type, A-type and adakite-like granitoids as revealed by their respective alkalinity, alumina saturation state, and trace element concentrations and ratios. The ca. 1879 Ma I-type granitoid pluton intruded the 1892 to 1884 Ma supracrustal package of volcanic, volcaniclastic and minor sedimentary rocks that constitute the greenstone belt, which was cut by 1873 Ma A-type granite and then by 1854 Ma adakite-like quartz diorite intrusions. Why the geochemical affinities of these granitoids change so rapidly (within ∼25 Myr) in orogenic belts, however, is not as well understood. Here we propose using Sm Nd isotope data and trace element modeling that the compositional variety of the granitoid intrusions likely reflects the mineral assemblages of their source rocks rather than magmatic fractionation. They may have been formed by partial melting of the disparate sources at different depths to respond to the change in tectonic regimes. The timing of the granitoid intrusions tracks a rapid geodynamic transition from subduction to intra-arc extension and to slab break-off due to terrane accretion and collision. This records a crucial piece of history in geodynamic evolution of the Trans-Hudson Orogen, which to date has been overlooked largely in many other orogens. • I-type, A-type and adakite-like granitoids occur in the Lynn Lake greenstone belt. • The granitoids record a rapid transition in tectonic setting from arc, intra-arc extensional and slab break-off. • Their compositions are correlative to their source regions and geodynamic regimes. [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 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]

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]

In this study, the influence of source area weathering and provenance on the stream sediment geochemistry as well as the environmental impacts of selected potentially toxic trace elements (PTE) were evaluated. Four components derived from the R-mode factor analysis of additive logratio-transformed data pointed to the combined influence of weathering of granitoids, sedimentary rocks and greenstone belts and gold mineralisation on the stream geochemistry. Although 13 PTE were enriched in the majority of the samples, only five elements (As, Cr, Cu, Pb and Ni) were likely to have adverse biological effects. Based on the geochemical maps, the areas susceptible to produce adverse biological effects due to As enrichments are solely drained by the greenstone belts, whereas enrichment of Pb was pronounced in areas of high population densities. The linear regression between enrichment factor (EF) and adverse effect indices (AEI) indicated that in order for Pb to pose threats to the benthic organisms, EF should be 4.5, whereas EF for sediment toxicity are 1 for Cu, Cr and Ni and 2 for As. Consequently, Cu, Cr, Ni, As and, to a lesser degree, Pb pose serious environmental threats to benthic organisms in southwestern Burkina Faso. The stream sediment geochemical data of this study constitute a baseline for assessing future environmental risks. [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]

Archean komatiites form an important host rock for Ni-Cu sulfide deposits in the world. In this study, we examined Archean komatiites and komatiitic basalts from four areas in the Vedlozero-Segozero greenstone belt and two areas in the Tikshozero greenstone belt, Russian Karelia, to constrain their Ni-Cu sulfide mineralization potential using chalcophile element geochemistry. The platinum-group element (PGE) concentrations in these Munro-type komatiitic rocks are at a level of other similar S-undersaturated komatiites and komatiitic basalts globally, with Pt and Pd concentrations in the range of 5–20 ppb and Pd/Ir varying from < 10 (komatiites) to > 15 (komatiitic basalts and basalts). Generally, the metals of the iridium-group elements (IPGEs; Ir, Ru, Os) show a compatible behavior, decreasing in abundance with decreasing MgO, whereas the metals of the palladium group (PPGEs; Pt, Pd, Rh) exhibit an incompatible behavior. The poor correlation between Ir and MgO suggests that olivine fractional crystallization is not the main control on the behavior of IPGE. In contrast, Ir, Ru, and Os show positive correlations with Cr, which is consistent with the compatible behavior of IPGEs in chromite or during the co-precipitation of chromite and platinum-group minerals (PGMs). Palladium, Cu, and Au have been variably mobile during alteration and metamorphism whereas Pt appears to have been less mobile. Some samples from the Khizovaara area show low (Pt/Ti)N ratios, low Ni, and high La/Sm and La/Nb, suggesting localized sulfide saturation in response to crustal contamination. However, the potential of the Russian Karelian greenstone belts for Ni-Cu sulfide mineralization is considered relatively low because most samples show neither enrichment nor depletion of chalcophile elements, and the paucity of dynamic lava channel environments, as indicated by the scarcity of olivine-rich adcumulates. In addition, there appears to be a lack of exposed sulfidic sedimentary rocks in the region. [ABSTRACT FROM AUTHOR]

The Palaeoproterozoic greenstone belts of Fennoscandia are metamorphosed and deformed volcanic and sedimentary rocks that formed in basins with a high base-metal ore potential. One of these, the Repparfjord Tectonic Window (RTW), is exposed in the Caledonides of northern Norway and contains several sediment-hosted Cu deposits including Nussir and Ulveryggen. The RTW is composed of mafic metavolcanic rocks (metabasalts, volcanoclastic metabreccia and metatuffite) intercalated with carbonatesiliciclastic sedimentary rocks (dolomitic marble, metasandstone to metapelite). This succession was deformed and metamorphosed up to greenschist to lower amphibolite facies during the Svecofennian Orogeny (c. 1.84 Ga). The Cu-mineralisation at the Nussir deposit is hosted by a dolomitic marble. It occurs mostly in the form of quartz-carbonate veins with chalcopyrite, bornite, chalcocite and covellite as the main ore minerals. In contrast, the Ulveryggen mineralisation is predominantly disseminated within a metasiliciclastic succession and dominated by chalcopyrite, bornite, chalcocite, covellite and neodigenite. Mineralogical, geochemical, stable isotope and fluid-inclusion studies provide insights into the evolution of the Cu-bearing fluids. A wide range in homogenisation temperatures (135-350°C at the Nussir deposit and 102-520°C at the Ulveryggen deposit) and fluid-inclusion salinities (from 0.35 up to 36 wt.% NaCl equivalents) suggest an evolving system with brines developed by subsurface evaporite dissolution. Fluid-inclusion and Cu-sphalerite geothermometry data constrain the temperature-pressure conditions of the Cu mineralisation in the Nussir deposit at 330-340°C and 1.1-2.7 kbars. High salinities at relatively high temperatures within the ore-bearing fluids imply that Cu was transported predominantly by Cuchloride complexes. The interaction of ore-bearing fluids with carbonate-rich host lithologies is proposed as the main mechanism for deposition of the Cu mineralisation at Nussir. In contrast, at the Ulveryggen deposit the mineralisation was mostly controlled by dilution and cooling when ore-bearing fluids mixed with groundwaters. Locally, reaction of Cu-bearing fluids with sediment-hosted pyrite might also have triggered copper precipitation. Similar ranges of ẟ13C (- 0.9 to + 2.9‰ V-PDB) and ẟ18O (- 18.3 to - 15.9‰ V-PDB) values in carbonates from ore-bearing veins and underlying host dolomitic marbles reflect a carbonate rock-buffered system without a significant contribution of magmatic or hydrothermal CO². [ABSTRACT FROM AUTHOR]

Gold production in Fairview Mine in the Barberton Greenstone Belt, South Africa, is focused on high-grade, steeply plunging ore shoots contained within a gently undulating system of spaced, low-displacement, broadly bedding-parallel faults, and shear zones, collectively referred to as the Main Reef Complex. This shear zone system is developed in metaturbiditic sedimentary rocks of the Mesoarchean Fig Tree Group on the steep western limb of the regional (D3), refolded, and distinctly arcuate Ulundi Syncline. Shear zones are preferentially localized along lithological contacts with pronounced rheological contrasts, particularly pronounced along the limbs of tightly infolded ultramafic schist against Fig Tree Group metasedimentary rocks. The main ore shoot of the MRC at Fairview shows a steep southeasterly plunge that can be traced for > 2000 m down-plunge, parallel to the fold hinge (F3b) that refolds the Ulundi Syncline. Fabrics and structures in the ore shoot record combined top-to-the NW thrust sense and dextral strike-slip kinematics and strongly constrictional strains interpreted to indicate dextral transpression and associated steep extrusion of the rocks during progressive NW-SE (D3) shortening. A combination of structural and lithological controls govern ore-shoot formation and gold mineralization. The main ore shoot corresponds to dilational jog geometry that developed during dextral transpression, bedding-parallel shearing consistent with the flexural-slip refolding (F3b) of the Ulundi Syncline. High-grade pockets correlate with sheared, commonly dismembered, graphite-rich and sulfide-mineralized shale units, testifying to the preferential fluid focusing and strain localization into incompetent shale units during deformation. Cross-cutting relationships with other auriferous reefs from the Fairview Mine complex indicate a late timing of the MRC-type mineralization, underlining the complex fluid focusing and utilization of differently orientated structures with different kinematics during progressive deformation and either episodic or protracted fluid flow events. [ABSTRACT FROM AUTHOR]

• Extraterrestrial organic matter is detected by EPR in 3.33 Ga sediments. • It is associated with Ni-Cr-rich ferrite "cosmic" spinel nanoparticles. • A challenge for the research for organic traces of extinct life in Mars. Electron paramagnetic resonance (EPR) analysis of carbonaceous, volcanic, tidal sediments from the 3.33 Ga-old Josefsdal Chert (Kromberg Formation, Barberton Greenstone Belt), documents the presence of two types of insoluble organic matter (IOM): (1) IOM similar to that previously found in Archean cherts from numerous other sedimentary rocks in the world and of purported biogenic origin; (2) anomalous IOM localized in a 2 mm-thick sedimentary horizon. Detailed analysis by continuous-wave-EPR and pulse-EPR reveals that IOM in this layer is similar to the insoluble component of the hydrogenated organic matter in carbonaceous chondrites, suggesting that this narrow sedimentary horizon has preserved organic matter of extraterrestrial origin. This conclusion is supported by the presence in this thin layer of another anomalous EPR signal at g = 3 attributed to Ni-Cr-Al ferrite spinel nanoparticles, which are known to form during atmospheric entry of cosmic objects. From this EPR analysis, it was deduced that the anomalous sedimentary layer originates from deposition, in a nearshore environment, of a cloud of tiny dust particles originating from a flux of micrometeorites falling through the oxygen-poor Archean atmosphere. [ABSTRACT FROM AUTHOR]

• Archean barite in the Barberton Greenstone Belt was deposited along unconformities. • Barium was sourced from cold springs venting to the surface through fractures. • Sulfate was sourced from photolysis reactions in the atmosphere. • Deposition involved Ba-bearing spring water contacting sulfate-bearing meteoric water. • Barite geochemistry carries little information on composition of Archean sea water. Small sedimentary barite deposits are widespread in the 3.258–3.225 Ga Fig Tree Group, Barberton Greenstone Belt (BGB), South Africa. This report explores the stratigraphy of these units, the types of barite present, their depositional settings, and the controls on sedimentation in the Barite Valley area. Five morphological types of barite are recognized: masses of irregularly layered bladed barite termed barite plugs, mounded barite, barite splays, flat-layered bladed barite, and barite sand. In all areas studied, barite deposition was closely associated with active faults and fractures in the underlying rock sequences that are inferred to have been conduits by which barium-bearing fluids made their way to the surface. The barite deposits are also associated with unconformities below, within, and/or above the barite layers. Associated sedimentary rocks show evidence of deposition within intertidal, fan delta, and shallow water to subaerial settings. Previous sulfur isotopic studies have shown that the sulfate originated by photolysis in the atmosphere. We infer that the barite was deposited where barium-bearing, shallow subsurface waters vented to the land surface via mostly small, cool springs and interacted directly with sulfate-bearing meteoric waters, probably often rainwater, triggering barite precipitation. Plug barite marks spring vents, mounds developed in pools, and the outflow systems were characterized by layered bladed barite with increasing barite sand toward more distal settings. Associated thin jasper beds are thought to have formed by oxidation of sedimentary pyrite and siderite by meteoric waters carrying photolysis-produced oxidants, probably H 2 O 2 and perhaps O 2. The Barite Valley barite deposits do not appear to have developed in open marine or evaporitic settings and their geochemistry may carry little information on the composition of Archean sea water or nature of the underlying crust. [ABSTRACT FROM AUTHOR]

Highlights • Sediments were derived largely from shallow erosion of the BGB itself. • Deposition occurred in small-scale systems adjacent to local uplifts. • Onset of deposition was likely diachronous across the greenstone belt. • Adjacent structural belts show different evolution in provenance and lithology. • Structural belts may not have formed in spatial proximity to one another. Abstract Sedimentary rocks provide a valuable record of non-uniformitarian Paleoarchean tectonic processes. The rocks of the 3.28–3.23 Ga Mapepe Formation mark the initiation of orogenesis in the Barberton Greenstone Belt, South Africa. However, reconstruction of the Mapepe basin(s) is challenging because the rocks occur in fault-bounded structural belts that are characterized by lateral heterogeneity and whose correlation remains uncertain. We have studied the petrography, geochemistry, and detrital zircon geochronology from outcrop and drill core samples from two adjacent structural belts, the Manzimnyama Syncline and the Eastern Barite Valley area, to evaluate the timing of basin formation, correlate between these two adjacent areas, and provide constraints on the character of tectonic uplift. In the Eastern Barite Valley area sedimentation was initiated at about 3260 Ma. The section includes a lower, deep-water unit of mainly mudstone overlain by a shoaling-upward sequence of fan-delta sandstone and conglomerate. The sandstone throughout the formation was derived by erosion of local uplifts of the underlying greenstone-belt sequence except at the top of the section where sedimentation was dominated by dacitic volcaniclastic sediments dated at 3239 Ma. In contrast, rocks of the Mapepe Formation in the Manzimnyama Syncline area, 1–2 km south of the Eastern Barite Valley area, record the initiation of sedimentation at about 3277 Ma with deposition of a rhyolitic tuffaceous unit followed by persistent deep-water sedimentation of banded iron formation, banded ferruginous chert, and turbiditic sandstone. Siliciclastic sediments are immature and were derived by erosion of older supracrustal greenstone-belt rocks. No Fig Tree age volcanism younger than 3260 Ma is recorded in this belt. Overall, Mapepe sedimentary rocks record the formation of numerous local uplifts of underlying supracrustal rocks with periodic volcanic activity, and the dispersal of debris to form mainly small fan-deltas flanked by deep-water systems. The contrasting ages, lithologies, depositional settings, and provenance of the sedimentary rocks in the now adjacent Eastern Barite Valley and Manzimnyama Syncline belts suggest that they were deposited in either different parts of a large, complex Fig Tree basin or represent separate basins that have been juxtaposed during post-Fig Tree tectonism and shortening. [ABSTRACT FROM AUTHOR]

Documenting evidence of fossil microbial life on early Earth is made difficult by the paucity of suitable Archean sedimentary rocks, their common metamorphic overprint, the small outcrop areas, and the small size of the objects of interest. Although a large number of putative microfossils dating back as far as 3700 Ma have been described, the syngeneity and biogenicity of many occurrences is debated, and some of the proposed fossils have been found to be either contaminants or abiotic artefacts. The ~3200 Ma Moodies Group of the Barberton Greenstone Belt (BGB), South Africa, contains locally abundant and remarkably well-preserved microbial mats which show indirect evidence of photosynthetic activity. They also contain microstructures which strongly resemble remains of microbial cells. Detailed morphological and geochemical analyses, however, show that these structures mostly represent fragments of volcanic tephra. Our study demonstrates that opaque microstructures within microbial mats can potentially be misidentified as microfossils even when a strict protocol is followed. It also posits the question to which degree volcanic air-borne fertilization contributed to the remarkable growth rate, high mechanical tenacity and wide extent of these oldest tidal microbial mats in siliciclastic environments. [ABSTRACT FROM AUTHOR]

[Display omitted] • Garnet-hosted xenotime and gold are dated at ca. 1827 Ma • Early gold veins are metamorphosed to amphibolite facies (505–689 °C) • New ages for gold mineralization also post-date metamorphism by up to 80 Myr • Post-orogenic gold mineralization is coeval with biotite cooling ages (ca. 1746 Ma) • Exhumation of the Trans-Hudson orogen triggered gold mineralization Metamorphic devolatilization is thought to be the critical process that can liberate gold from amphibolite facies source rocks to form orogenic gold deposits. However, a growing number of studies demonstrate that orogenic gold deposits are the product of multiple hydrothermal stages over 10 s to 100 s of Myr rather than one metallogenic event. Herein we report new geochronology results for orogenic gold deposits hosted within the Paleoproterozoic Lynn Lake greenstone belt (Manitoba, Canada) to address the knowledge gap. Garnet-hosted xenotime crystals, which occur in a folded pyrite veinlet with gold, yield a weighted average 207Pb/206Pb date at 1827 ± 8 Ma. This new and more precise age demonstrates that the earliest auriferous fluids at the MacLellan gold deposit coincided with the rapid burial of ca. 1836 Ma sedimentary rocks before regional metamorphism. New garnet-biotite geothermometry results further demonstrate that the earliest auriferous veins were preserved despite being metamorphosed to amphibolite facies conditions (505–689 °C). The peak metamorphic mineral assemblage is overprinted by: (1) the main xenotime- and gold-bearing deformation fabric (S 2 ; 1790–1827 Ma); (2) an overprinting xenotime- and monazite-bearing S 3 fabric that is also associated with gold-rich arsenopyrite (1769–1790 Ma); and (3) chlorite-hosted xenotime and gold (1747 ± 12 Ma) that are coeval with the oldest U-Pb apatite dates (1691–1740 Ma) and a prominent mode of 40Ar/39Ar biotite dates at ca. 1746 Ma. We suggest that the post-orogenic gold events are incompatible with the standard devolatilization model because they post-date subduction and peak-metamorphism by up to 80 Myr. Instead, the new geochronological results suggest that gold deposition at the MacLellan deposit was linked to burial and the subsequent exhumation of chemically reactive trap rocks through favourable temperature windows that allowed gold transport as sulphide complexes. Exhumation and far-field tectonics are interpreted as the most likely drivers of post-orogenic auriferous fluids rather than regional metamorphism and active subduction. This revised mineral systems model provides an explanation for the high prospectivity of high- and low-temperature mineral assemblages and suggests that mineral exploration should focus on the earliest ore-forming structures that are reactivated during multiple stages of orogenesis. [ABSTRACT FROM AUTHOR]

The Piché Structural Complex (PSC) extends over 150 km within the Cadillac – Larder Lake Fault Zone (CLLFZ), a gold-endowed, east-trending, and high-strain corridor located along the southern edge of the Archean Abitibi Subprovince. The PSC consists of discontinuous units of volcanic rocks (<1 km thick) that host multiple gold deposits. It is spatially associated with molasse-type Timiskaming sedimentary basins. This study describes and interprets the origin of structures and lithologies within the poorly understood PSC to unravel the tectonic evolution of the CLLFZ. Field mapping, chemical analyses, as well as interpretations of cross-sections from drill-hole data, were used to interpret the geometry and structure of the PSC. The PSC is subdivided into six homogeneous fault-bounded segments or slivers. These slivers consist mostly of ultramafic to intermediate volcanic rocks and include some felsic volcanic flows and intrusions. Volcanic facies, chemical compositions, and isotopic ages confirm that these slivers are derived from the early volcanic units of the southern Abitibi greenstone belt, which are located north of the CLLFZ. Cross-cutting relationships between volcanic rocks of the PSC and the Timiskaming-aged intrusions suggest that the slivers were inserted into the CLLFZ during the early stages of the accretion-related deformation (<2686 Ma) and prior to Timiskaming sedimentation and ductile deformation (>2676 Ma). The abundant ultramafic rocks located within the CLLFZ may have focused strain, thereby facilitating the nucleation of the fault as well as the displacements along this crustal-scale structure. [ABSTRACT FROM AUTHOR]

Many countries and some Brazilian regions have defined the guideline values for metals in soils. However, the local geological features may be so heterogeneous that global or even regional guideline values cannot be applied. The Greenstone Belts are worldwide geological formations of vast extension, containing mineralization of various metals (e.g., Au, Cr, Ni, and Ag). Natural concentrations of soils must be known to correctly assess the impact of mining. We studied the soils of the Rio Itapicuru Greenstone Belt (RIGB), of Paleoproterozoic age, sampling at 24 sites (0–0.20 m) in the areas not or minimally human impacted, equally distributed in the three units of the RIGB: Volcanic Mafic Unit (VMU), Volcanic Felsic Unit (VFU), and Volcano-clastic Sedimentary Unit (SU). The natural pseudo-total concentrations of Cr, Ni, Cu, Zn, Pb, Fe, and Mn were obtained by acid digestion (EPA3050b) both in the soil and the particle-size fractions (sand and clay + silt). The concentrations of metals in RIGB soils, especially Cr and Ni, are generally higher than those reported for other regions of Brazil or other countries. Even the sedimentary soils have relatively high metal values, naturally contaminated by the VMU of the RIGB; a potential impact on Mesozoic and Cenozoic sedimentary rocks located near the study region is highly expected. Metals are concentrated (80%) in the fine particle-size fraction, implying an easy availability through surface transport (wind and runoff). We introduced a new index, called the Fe-independent accumulation factor – AF -Fe , which reveals that 90–98% of the dynamics of the trace metals is associated with the iron geochemical cycle. We primarily conclude that determining the guideline values for different soil classes in variable geological/geochemical environment and under semiarid climate is meaningless: the concentration of metals in soils is clearly more related to the source material than to the pedogenesis processes. [ABSTRACT FROM AUTHOR]

Hafnium and oxygen isotopic compositions measured in-situ on U-Pb dated zircon from Archaean sedimentary successions belonging to the 2.9–2.8 Ga Belingwean/Bulawayan groups and previously undated Sebakwian Group are used to characterize the crustal evolution of the Zimbabwe Craton prior to 3.0 Ga. Microstructural and compositional criteria were used to minimize effects arising from Pb loss due to metamorphic overprinting and interaction with low-temperature fluids. 207 Pb/ 206 Pb age spectra (concordance >90%) reveal prominent peaks at 3.8, 3.6, 3.5, and 3.35 Ga, corresponding to documented geological events, both globally and within the Zimbabwe Craton. Zircon δ 18 O values from +4 to +10‰ point to both derivation from magmas in equilibrium with mantle oxygen and the incorporation of material that had previously interacted with water in near-surface environments. In ε Hf -time space, 3.8–3.6 Ga grains define an array consistent with reworking of a mafic reservoir ( 176 Lu/ 177 Hf ∼0.015) that separated from chondritic mantle at ∼3.9 Ga. Crustal domains formed after 3.6 Ga depict a more complex evolution, involving contribution from chondritic mantle sources and, to a lesser extent, reworking of pre-existing crust. Protracted remelting was not accompanied by significant mantle depletion prior to 3.35 Ga. This implies that early crust production in the Zimbabwe Craton did not cause complementary enriched and depleted reservoirs that were tapped by later magmas, possibly because the volume of crust extracted and stabilised was too small to influence (asthenospheric) mantle isotopic evolution. Growth of continental crust through pulsed emplacement of juvenile (chondritic mantle-derived) melts, into and onto the existing cratonic nucleus, however, involved formation of complementary depleted subcontinental lithospheric mantle since the early Archaean, indicative of strongly coupled evolutionary histories of both reservoirs, with limited evidence for recycling and lateral accretion of arc-related crustal blocks until 3.35 Ga. [ABSTRACT FROM AUTHOR]

[Display omitted] • Meta-conglomerate-hosted gold characteristics in the Kibali gold district are investigated. • Meta-conglomerate-hosted gold has high Au/Ag, larger size, and less sulphide association. • The component of paleoplacer gold is considered within the meta -conglomeratic unit from the KCD area. The giant Karagba-Chauffeur-Durba (KCD) gold deposit and its peripheral gold satellite deposits are located within the Kibali gold district in the Neoarchaean Moto Greenstone Belt in the northeast of the Congo Craton, Democratic Republic of Congo. The gold mineralisation within the KCD area occurs within altered chemical sedimentary rocks such as banded iron formation (BIF) and chert, as well as in meta -carbonaceous argillite, meta -sandstones and poorly-sorted pebble meta -conglomerates. To elucidate and characterise the controls on gold mineralisation in the siliciclastic rocks of this sedimentary sequence dating from approximately 2630 to 2625 Ma, the present study critically considers an extensive data set comprising borehole logs, petrographic observations, gold assay data, and mineral chemistry. Detailed high magnification petrographic analyses of polished sections enabled evaluation of 3824 gold grains from meta -conglomerate and siliciclastic sedimentary host rock specimens. These were compared to an additional 985 gold grains from deformed quartz-carbonate (ACSA) altered rocks and BIF host rock types, representing end-members for a purely hydrothermal style of gold mineralisation. These analyses revealed that most of the gold grains in the meta -conglomerates occur within the gangue mineral assemblage in the form of free gold generally distal to any sulphide minerals, with a second population associated with the sulphides, mainly pyrite, followed by arsenopyrite and lesser pyrrhotite. The gold grains in the meta -conglomerates show larger average sizes (433 µm2) than in the grains in the ACSA (311 µm2) and BIF (180 µm2) host rock types. A large dataset of assayed gold results of 9,632,470 data from 3953 drill holes highlights that the ACSA-altered rocks and the chemical sedimentary rocks (chert and BIF) are the main hosts for gold, showing average grades of 3.9, 2.5 and 2.4 g/t when a 0.5 g/t cut-off is applied, respectively. Conversely, meta -conglomerates have a relatively low Au content (1.84 g/t), yet this dataset has an extensive set of outlying values, with 86 data points reflecting grades over 100 g/t. Scanning Electron Microscope (SEM-EDS/WDS) performed on 397 gold grains from meta -conglomerates and deformed quartz-carbonate (ACSA) altered rocks shows mainly Au-Ag alloys, dominated by native gold with averages of 85.46 wt% of Au and 9.76 wt% of Ag in the meta -conglomerates and 86.56 wt% of Au and 12.84 wt% of Ag within the ACSA. Minor amounts of Au-Ag alloys electrum and Au-Hg-Ag alloys amalgams are present predominantly at Gorumbwa and KCD deposits. Our analysis of textural features, mineral chemistry, morphology, assay results, and mineral associations suggests that there are significant differences between the purely hydrothermal mineralisation and the mineralisation hosted by the meta -conglomeratic units. We suggest that the pre-enrichment of these meta -conglomerates by placer processes has played a role in enhancing the fertility for gold mineralisation in the KCD area. This has far-reaching implications for the existing ore-genesis models and exploration paradigms employed in the KCD area. This work presents new opportunities for exploration by highlighting the significance of understanding the stratigraphy and identifying sub-basins that may have undergone early placer enrichments. To achieve this, the early sedimentary environment should be thoroughly investigated. [ABSTRACT FROM AUTHOR]

• Red Lake Greenstone Belt contains Earth's oldest known, thick, carbonate platform. • Outcrops and multiple cored drill holes provide excellent stratigraphic control. • Interlayering of lithofacies suggests short to long term relative sea level changes. • Precipitation rates of all types of chemical sediments varied temporally. • Archean seawater REE chemistry is similar in shallow and deep areas of the shelf. The > 200 m thick Red Lake carbonate platform of the 2.940-2.925 Ga Ball Assemblage (Red Lake Greenstone Belt, Superior Province, Canada) is the oldest known carbonate platform preserved on Earth. This study examined surface outcrops and multiple industry cored drill holes comprising a diverse assemblage of chemical and siliciclastic sedimentary rocks of the deeper water lithofacies associated with the shallow water carbonates. The offshore lithologies, underlying, interlayered with, and overlying the mainly stromatolitic carbonates, consist of alternating packages of sandstone, siltstone, carbonaceous slate, pyritic carbonaceous slate, chert, and oxide facies iron formation (OFIF). These interlayer with carbonate in transitional slope environments, which consist in places of carbonate with magnetite laminae above slumped carbonate debris in iron formation and/or carbonaceous slate. Chert and OFIF were deposited further offshore in a suboxic environment, with lithology dependent on concentrations of seawater Fe(II) and silicic acid. The iron sulfides were deposited in a reducing, anoxic, and organic-rich bottom water and muddy pore water environment not conducive to Fe hydroxide or chert precipitation. Carbonaceous mud was the background sediment, accumulating during intervals when the precipitation of chert, Fe hydroxides, or massive layers of Fe sulfides was inhibited. The interlayering of differing assemblages of chemical and siliciclastic sedimentary rocks indicates temporal and spatial fluctuations in the chemistry of local paleo-seawater and variable sedimentation rates on the margins of the Red Lake carbonate platform. Post-Archean Australian Shale (PAAS)- normalized rare earth element (REE) systematics of carbonate and OFIF show positive La and Gd anomalies, super-chondritic Y/Ho ratios, and depleted light rare earth elements (LREEs) relative to heavy rare earth elements (HREEs), similar to modern seawater, while the positive Eu anomalies in the carbonate and OFIF samples reflect hydrothermally influenced Archean seawater. The chemical sedimentary rocks exhibit enrichment of redox-sensitive elements (i.e., Cr, Mo, V, and U) compared to their siliciclastic associates. This, combined with concentrations of MnO up to 6.3 wt% and positive Ce anomalies in the iron formation directly below the carbonate platform, all indicate the likely presence of some O 2 ca 2.93 Ga. [ABSTRACT FROM AUTHOR]

The low-grade Contendas-Mirante meta-volcano-sedimentary belt (CMB) was previously interpreted as an Archean greenstone belt with the uppermost formation deposited in the Paleoproterozoic. New zircon U-Pb LA-ICP-MS ages on sedimentary groups and granite sheets intrusive in the belt constrain the timing of basin development. Phyllites of the Lower Group and meta-arenites of the Upper Group yielded similar young zircon populations at ca. 2080 Ma, which are interpreted as the maximum deposition age for the entire sequence. The unique exception is the 3273–3338 Ma zircon-bearing quartzite that occurs in spatial association with the 3300 Ma rhyolite. A granite sheet intrusive into metagraywacke of the Lower Group constrains the minimum deposition age of 2045 ± 26 Ma for the CMB. Altogether four main age clusters of detrital zircon grains were recognized, namely 2075–2200 Ma, 2200–2440 Ma, 2500–2770 Ma, 3270–3380 Ma. These age intervals match the ages of igneous and metamorphic rocks in the different terranes that comprise the São Francisco craton. Maximum deposition age for both groups indicates that sedimentation occurred immediately prior the high-grade metamorphism in the adjacent Archean-Paleoproterozoic Jequié Block and Itabuna-Salvador-Curaçá Orogen. These observations, along with coarsening upwards in the sediments, indicate that the CMB represents a foreland basin developed along the eastern margin of the Gavião Block during plate convergence. Paleoarchean basement rocks were reactivated and emplaced into the CMB during basin inversion and is timely correlated with the granitic intrusion. Recognition of older supracrustal fragments is ambiguous, one main example is the 3273–3338 Ma zircon-bearing quartzite, which correlates with the northern Jacobina basin along the Contendas-Mirante lineament (e.g., Bahia gold belt) and may represent part of an Archean intracontinental rift sequence. [ABSTRACT FROM AUTHOR]

We constrained the geological framework over polydeformed Paleoproterozoic Sonakhan Greenstone Belt and addressed the tectonic evolution of Singhora basin in the fringes of Bastar Craton, central India by utilizing aeromagnetic data interpretation, 2.5D forward modelling and 3D magnetic susceptibility inversions. The Sonakhan Greenstone Belt exposes volcano-sedimentary sequences of the Sonakhan Group within NNW-SSE to NW-SE trending linear belts surrounded by granite gneisses, which are unconformably overlain by sedimentary rocks of Chhattisgarh Basin. The orientations of aeromagnetic anomalies are coincident with geological trends and appear to correlate with lithology and geologic structure. Regional magnetic anomalies and lineaments reveal both NNW-SSE and NE-SW trends. Prominent E-W trending linear, high amplitude magnetic anomalies are interpreted as the Trans-Chhattisgarh Aeromagnetic Lineament (TCAL). NW-SE trending aeromagnetic signatures related to Sonakhan Greenstone Belt extends below the Singhora sedimentary rocks and forms the basement in the west. The analysis suggests that TCAL is a block fault with northern block down-thrown and affected the basement rocks comprising the Sonakhan Greenstone Belt and Samblapur Granitoids. The episode of faulting represented by the TCAL is pre-Singhora sedimentation and played a vital role in basin evolution. The basement configuration image generated by estimates of depth to magnetic basement suggests a complex pattern of NNE-SSW to NE-SW trending depressions separated by a linear N-S trending basement ridge. It is inferred from the 3D magnetic susceptibility inversion that the thickness of sediments is more towards the eastern basin margin and the N-S ridge is a manifestation of post sedimentary faulting. Results of 2.5D modelling of a WNW-ESE profile across the Singhora Basin combined with results from 3D inversion suggest suggests the basin subsidence was controlled by NE-SW trending regional faults in an active system. The basin geometry evolved by E-W block faulting overprinted by NE-SW trending pre- to syn-depositional normal faults generating NE-SW depression, which are affected by N-S trend post-sedimentary faulting. Though the present work relates the basin evolution with the initiation of rift basin, it warrants further work to establish the deformation within the basin pertaining to the proximal thrust and uplift along the craton fringe. [ABSTRACT FROM AUTHOR]

Depth profiling techniques for U-Pb geochronology and REE geochemistry were applied to unpolished, metasomatized Archean zircon from metasedimentary rock samples within the North Caribou greenstone belt, Western Superior Province, Canada. Samples were selected in order to investigate the isotopic and chemical effects of fluid interaction on zircon at temperatures between 300 and 700 °C. Zircon rims are < 5 μm thick and have 207 Pb/ 206 Pb ages between ca. 2788–2667 Ma, with two-thirds of rim ages ranging between 2754 and 2735 Ma, which is > 100 Myr younger than primary crystallization (interior) ages. These dates correspond to regional tectonothermal episodes including potassic alteration, suggesting that metamorphism and regional hydrothermal fluid flow were synchronous. Similarly, REE geochemical analyses on the outer margins of the same unpolished zircon reveal that rims are characterized by variable LREE concentrations, high Hf, and low Th/U ratios, indicating that elements were mobile during fluid-zircon interaction. Unpolished zircon crystal faces were also mapped with a LA-ICP-MS technique to expose their 2-dimensional trace element variability. The patterns reveal a dendritic network or a patchy mosaic form, which is perhaps promoted by the presence of fractures, adjacent mineralogy or variations in the crystallinity of the zircon. The integration of these microanalytical techniques into the study of zircon can provide insight into the timing of low- to moderate-temperature history of rocks that do not contain other common chronometers but have witnessed significant tectonothermal events. [ABSTRACT FROM AUTHOR]

The Tasiast gold deposits are hosted within Mesoarchean rocks of the Aouéouat greenstone belt, Mauritania. The Tasiast Mine consists of two deposits hosted within distinctly different rock types, both situated within the hanging wall of the west-vergent Tasiast thrust. The Piment deposits are hosted within metasedimentary rocks including metaturbidites and banded iron formation where the main mineral association consists of magnetite-quartz-pyrrhotite ± actinolite ± garnet ± biotite. Gold is associated with silica flooding and sulphide replacement of magnetite in the turbidites and in the banded iron formation units. The West Branch deposit is hosted within meta-igneous rocks, mainly diorites and quartz diorites that lie stratigraphically below host rocks of the Piment deposits. Most of the gold mineralisation at West Branch is hosted by quartz–carbonate veins within the sheared and hydrothermally altered meta-diorites that constitute the Greenschist Zone. At Tasiast, gold mineralisation has been defined over a strike length > 10 km and to vertical depths of 740 m. All of the significant mineralised bodies defined to date dip moderately to steeply (45° to 70°) to the east and have a south–southeasterly plunge. Gold deposits on the Tasiast trend are associated with second order shear zones that are splays cutting the hanging wall block of the Tasiast thrust. An age of 2839 ± 36 Ma obtained from the hydrothermal overgrowth on zircons from a quartz vein is interpreted to represent the age of mineralisation. [ABSTRACT FROM AUTHOR]

This contribution reports on three significant gold occurrences in southern Mali, the Syama and Tabakoroni deposits and the Tellem prospect, located in what is locally known as the Syama corridor. They are part of the Syama and Tabakoroni goldfields, which are the object of a twin paper in this issue (Ballo et al., this volume). They occur in the northern part of the Bagoe greenstone belt, which, in the study area, consists of a suite of Paleoproterozoic metamorphosed volcanic and sedimentary rocks. The sediments include conglomerate, black shale, sandstone and greywacke. The volcanic rocks have basaltic, andesitic and dacitic compositions. The basalts have flat rare earth element (REE) patterns, no Nb anomaly, and very low Zr/Y, whereas the latter two rock types are enriched in light REE, depleted in Nb, Ta and Zr/Y > 7. All rock units are crosscut by spessartite-type lamprophyre sills. Even though the three gold occurrences are aligned along the same major structure, each one has distinct geological features including (i) the nature of the dominant host rock in the mineralized zone (a basaltic volcanic sequence at Syama, a diorite pluton at Tellem and a felsic volcanic sequence at Tabakoroni); (ii) the structural characteristics of the zone (highly mineralized breccias for Syama and vein stockwork for Tabakoroni and Tellem); and (iii) the main sulfides present (pyrite in association with arsenopyrite at Tabakoroni and Tellem and pyrite alone at Syama). At Tabakoroni and Tellem, gold mineralization occurred mostly during D4 Sy , whereas at Syama, a large part of the visible gold occurs as infill in fractures in breccia zones, likely related to a brittle, late E–W phase of shortening (D5 Sy ). [ABSTRACT FROM AUTHOR]

The roots of the tectonically uplifted Archean continental crustal blocks containing various greenstone-granite terranes are the preserved crustal lithologies exposed in various Archean cratons of the world. The trace element geochemical signatures of some of the preserved lithologies of these magmatic belts reflect the primary melt compositions in the mantle which help in understanding the petrogenesis of the rocks and the involvement of subduction zone processes, if recognized, in the generation of these melts. The Dharwar craton (DC), one of the major cratons of the world located in the Indian subcontinent, is made up of different tectonic blocks amalgamated along various suture zones. Here we review the current understanding about the division of the DC, the polarity of subduction of the different blocks and the petrogenesis of the magmatic rocks of the greenstone belts, and present a synthesis based on the trace element characteristics of the primary basalts. Our study validates the heterogeneous nature of the Archean mantle during the Dharwar crustal evolution. Collation of geophysical, structural, lithological, geochemical and geochronological data imply that the Dharwar craton was accreted by the juxtaposition of at least three major cratonic blocks with noticeably distinct tectonic and magmatic histories. The continued preservation of siliciclastic sedimentary rocks in the greenstone belts of the western Dharwar craton combined with the dominant presence of the >3.0 Ga old gneissic crust, as compared to the central and eastern cratons, could be significant in constraining the involvement of continental crustal detritus in the generation of the magmatic rocks of the greenstone belts. In the central and eastern Dharwar cratons, granitoid crust older than 3.0 Ga was probably destroyed to different extents for the generation of magmatic rocks. The existence of such an older crust is implied by the available Nd model ages. The scattered records of bimodality in the greenstone belt rocks of the central and eastern Dharwar cratons attest for the presence of heterogeneous mantle during Neoarchean. Subduction processes had become predominant by Neoarchean as evidenced from the chemical composition of primary basalts of the greenstone belts of the Dharwar craton. In all likelihood, the polarity of subduction was directed westward with the WDC acting as the foreland and the amalgamation of distinct cratonic blocks were along suture zones represented scatteredly by and as greenstone belts. • Mantle heterogeneity and subduction-zone processes generated Dharwar primary basalts. • Dharwar craton is a collage of tectonic blocks amalgamated by subduction-accretion. • The polarity of subduction was directed westward. [ABSTRACT FROM AUTHOR]

The paper presents new geological and geochemical data on the Perseverance komatiite complex (PK) that hosts the Perseverance Ni-sulphide deposit and the volcanic and sedimentary rocks forming the footwall of the PK. Using the 3D reconstruction of the PK and its' footwall volcano-sedimentary succession the heterogeneity of the komatiite channel has been characterised on the basis of spatial distribution of the geochemical petrogenetic indicators. The study shows that the komatiite facies proximal to the mineralised bodies and the main pathway are expressed in the 3D models as the segments with anomalously high MgO concentrations coupled with low contents of Al 2 O 3 , TiO 2 and CaO (Fig - abstract). The geochemical indicators of komatiite fertility, Ni/Cr and Ni/Ti ratios, are commonly in the range of 1 to 3 for Ni/Cr and 2 to 5 for Ni/Ti within the main pathway. The Ni/Cr exceeds 5 and Ni/Ti 20 in the mineralised parts of the channel. In the peripheral zone komatiites these values commonly drop to less than 1, corresponding to barren komatiites. Similarly large variations are exhibited by the ratios of mantle normalized Pt and Pd to Al and Ti over short distances from 1 to 25. Closely spaced high and low values of these ratios hinder their use for discriminating fertile from barren komatiites. However, within the komatiite main lava pathway they can be used as geochemical pathfinders for magmatic sulphides. Reconstruction of the structurally complex komatiite sequences can be facilitated by using lithogeochemical correlation of the host volcano-sedimentary successions and particularly tracing geochemically distinctive marker units. For these purposes a special reference in this study was made to the geochemical systematics of rocks that make up footwall succession of the PK complex. It was shown that amphibolites of the host sequence can be geochemically subdivided into different types, including mafic volcaniclastics (mafic-1) and intrusive gabbro (mafic-2). The mafic volcaniclastics are strongly enriched in LREE, possibly due to presence of sedimentary epiclastic material or reflecting magma contamination by the crustal material during ascent through the crust. Additionally, they have notably high-Th contents, allowing them to be classified as having high-Th geochemical affinities. The second group (mafic-2), interpreted as coherent in origin, are characterised by flat REE patterns and lower overall abundances of the rare earths and Zr corresponding to a tholeiitic composition. Contents of Th and TiO 2 of these rocks are clustered in the two narrow intervals of compositions, corresponding to low-Th and intermediate-Th basalts, which form different amphibolite bodies. Mafic volcanics intercalate with the felsic volcanics, and quartz-feldspar-biotite-amphibole (QFBA) schists. It is shown that QFBA schist composition corresponds to metamorphosed clastic sedimentary and volcaniclastic rocks of mixed composition, which contain both clastic material derived from an Archaean provenance and volcaniclastics material derived from mafic and felsic tuffs. In conclusion, abundances and ratios of the trace elements in composition of the mafic rocks are matching to that of the different parts of the Agnew Wiluna greenstone belt and can be used for overall stratigraphic correlations. [Display omitted] • The paper presents new geological and geochemical data on the Perseverance komatiite complex (PK) that hosts the Perseverance Ni-sulphide deposit and the volcano-sedimentary succession forming the footwall of the PK. • The 3D reconstruction of the PK and its' footwall volcano-sedimentary succession has been undertaken that has allowed to enhance the heterogeneity of the komatiite channel on the basis of spatial distribution of the geochemical petrogenetic indicators. • The study shows that the komatiite facies proximal to the mineralised bodies and the main pathway are expressed in the geochemical 3D models as the segments with anomalously high MgO concentrations coupled with the anomalous values of the geochemical indicators of komatiite fertility, including Ni/Cr, Ni/Ti ratios and the ratios of mantle normalized Pt and Pd to Al and Ti. • Reconstruction of the structurally complex komatiite sequences can be facilitated by using lithogeochemical correlation of the host volcano-sedimentary successions and particularly tracing geochemically distinctive marker units. It was shown that amphibolites of the host sequence can be geochemically subdivided into different types, including mafic volcaniclastics (mafic-1) and intrusive gabbro (mafic-2). • Mafic volcanics intercalate with the felsic volcanics, and quartz-feldspar-biotite-amphibole (QFBA) schists. It is shown that QFBA schist composition corresponds to metamorphosed clastic sedimentary and volcaniclastic rocks of mixed composition, which contain both clastic material derived from an Archaean provenance and volcaniclastics material derived from mafic and felsic tuffs. The paper presents new geological and geochemical data on the Perseverance komatiite complex (PKC) that hosts the Perseverance Ni-sulphide deposit and the volcanic and sedimentary rocks forming the footwall of the PKC. Using the 3D reconstruction of the PKC and its' footwall volcano-sedimentary succession the heterogeneity of the komatiite channel has been characterised on the basis of spatial distribution of the geochemical petrogenetic indicators. The study shows that the komatiite facies proximal to the mineralised bodies and the main pathway are expressed in the 3D models as the segments with anomalously high MgO concentrations coupled with low contents of Al 2 O 3 , TiO 2 and CaO. The geochemical indicators of komatiite fertility, Ni/Cr and Ni/Ti ratios, commonly are in the range of 1 to 3 for Ni/Cr and 2 to 5 for Ni/Ti within the main pathway, The Ni/Cr exceeds 5 and Ni/Ti 20 in the mineralised parts of the channel. In the peripheral zone komatiites these values commonly drop to less than 1, corresponding to barren komatiites. Similarly large variations are exhibited by the ratios of mantle normalized Pt and Pd to Al and Ti over short distances from 1 to 25. Closely spaced high and low values of these ratios hinder their use for discriminating fertile from barren komatiites. However, within the komatiite main lava pathway they can be used as geochemical pathfinders for magmatic sulphides. Reconstruction of the structurally complex komatiite sequences can be facilitated by using lithogeochemical correlation of the host volcano-sedimentary successions and particularly tracing geochemically distinctive marker units. For these purposes a special reference in this study was made to the geochemical systematics of rocks that make up footwall succession of the PKC complex. It was shown that amphibolites of the host sequence can be geochemically subdivided into different types, including mafic volcaniclastic rocks (mafic-1) and intrusive mafic rocks, apparently metagabbro sills (mafic-2). The mafic volcaniclastic rocks are strongly enriched in LREE, possibly due to presence of sedimentary epiclastic material or reflecting magma contamination by the crustal material during ascent through the crust. Additionally, they have notably high-Th contents, allowing them to be classified as having high-Th geochemical affinities. The second group (mafic-2), interpreted as coherent in origin, are characterised by flat REE patterns and lower overall abundances of the rare earths and Zr corresponding to a tholeiitic composition. Contents of Th and TiO 2 of these rocks are clustered in the two narrow intervals of compositions, corresponding to low-Th and intermediate-Th basalts, which form different amphibolite bodies. Mafic volcanics intercalate with the felsic volcanics, and quartz-feldspar-biotite-amphibole (QFBA) schists. It is shown that QFBA schist composition corresponds to metamorphosed clastic sedimentary and volcaniclastic rocks of mixed composition, which contain both clastic materials derived from an Archaean provenance and volcaniclastics material derived from mafic and felsic tuffs. The different geochemical affinities of the mafic rocks distributed in the footwall succession of the PKC apparently suggests different magma sources and, possibly, can also reflect the position of the rocks within the mantle plume structure. In conclusion, abundances, and ratios of the trace elements in composition of the mafic rocks matches that of the different parts of the Agnew Wiluna greenstone belt and can be used for overall stratigraphic correlations. [ABSTRACT FROM AUTHOR]

The Kuhmo greenstone belt in eastern Finland is one of the most studied areas of the Karelia Province. A single-grain zircon U–Pb study of andesitic, trachyandesitic and dacitic volcanic rocks from the belt demonstrates multiple widely spaced ages of the rocks. The results show that the volcanic activity took place as two major episodes at ca. 2847–2836 Ma and ca. 2799–2792 Ma, both containing komatiitic members as well as volcanic-sedimentary units. Based on the geochronological results combined with previously published data, we propose a chronostratigraphic interpretation for the Kuhmo greenstone belt, dividing it into three units: the Nuolikangas and Siivikkovaara volcanic units, and the sedimentary Ronkaperä unit. The results support the previous interpretation that age period of 2.80–2.79 Ga was an important crustal forming episode in the Kuhmo area. The zircon populations in the uppermost sedimentary rocks contain >3.0 Ga zircon grains that are older than the volcanic rocks found in the greenstone belt. The youngest detrital zircon populations, ca. 2.73 Ga and 2.70 Ga in a conglomerate and a quartz sandstone, respectively, imply that they were deposited at least ca. 60–90 Ma after the last volcanic phase. Based on the detrital zircon record, it can be interpreted that during the deposition of the detritus of the sedimentary rocks belonging to the Ronkaperä unit, the Karelia subprovinces were likely juxtaposed together and acted as a source provenance for the detritus. [ABSTRACT FROM AUTHOR]

The Archaean Tipasjärvi greenstone belt is a small area (ca. 5 × 25 km) located within the Karelia Province in Central Fennoscandian Shield, Finland. The belt forms the southernmost part of the larger Archaean Suomussalmi–Kuhmo–Tipasjärvi greenstone complex. The present study results for this site are based on zircon grains of 12 metavolcanic and metasedimentary rock samples that were studied with the single-grain secondary ion mass spectrometry (SIMS) U–Pb method. Our data indicate that the volcanism in the belt took place over three separate episodes: ca. 2.84 Ga, 2.82 Ga and 2.80 Ga. The oldest volcanic rocks are older than the oldest dated tonalitic pluton adjacent to the belt. The main geochemical characteristics of the samples reveal that the felsic volcanic rocks of the different age groups resembled each other. However, contrasting trace element geochemical characteristics between the different age groups are also distinguishable. The detrital zircon record agrees with previous observations indicating that the sediments accumulated at least 50 Myr after the formation of the youngest volcanic rocks. During sediment deposition, older crustal material (> 3.0 Ga) was exposed within the source area. The geochronological results in this study enable us to update the chronostratigraphic interpretation of the Tipasjärvi greenstone belt and to compare these results to those from other parts of the Archaean Suomussalmi–Kuhmo–Tipasjärvi greenstone complex and adjacent tonalitic plutons. [ABSTRACT FROM AUTHOR]

The Lamego orogenic gold deposit (1.3 Mt measured resources at 7.26 g/t Au) is located at the south-western end of the 5 km-long Cuiabá–Lamego trend, Quadrilátero Ferrífero region, Brazil. Both Archean orogenic gold deposits are situated in the Rio das Velhas greenstone belt with the lithological succession at Lamego consisting of metamorphosed (greenschist facies) mafic volcanic rock, chert and banded iron formation (BIF), and carbonaceous and micaceous pelites. The Lamego fold, which controls the Lamego deposit, is the most visible structure related to the D 1 –D 2 event, with a perimeter of about 4.8 km and a maximum width of 450 m. The fold is defined by the layering of the mafic unit with minor BIF and large exposures of carbonaceous and micaceous pelites. It is a rootless, reclined, isoclinal, cylindrical fold with an axial trace striking northwest–southeast dipping 20° to 30°. The hinge zone is thickened and the limbs are thinned, with the limbs dipping 20° to 30° to the SE. Orebodies consist of the Lamego BIF, where gold-mineralized zones are related to iron-rich bands, and associated silicification zones. Replacement-style mineralization is associated with sulfide bands; mainly pyrite, As-rich pyrite, and arsenopyrite. Two structural generations, G 1 and G 2 , are recognized and encompass a set of structural elements. The G 1 structural generation developed in a progressive deformation event and resulted in structures oriented from NE to SW and dipping to the SE. Structures pertaining to the G 2 structural generation are oriented N–S dipping to the W. The four major high-grade gold orebodies are Carruagem, Queimada, Arco da Velha, and Cabeça de Pedra. Their gold grade shows a spheroidal pattern and a distribution that varies along the S 1–2 foliation. These lenses represent the hinge zone of F 2 reclined folds with the plunge of the orebodies controlled by the F 2 fold axes. The lower-grade gold lenses are controlled by pinch and swell, and locally expressed quartz boudins developed during D 1 –D 2 . They have two orthogonal directions, one to the NW–SE and the other to the NE–SW, thereby defining chocolate-tablet style boudinage. Hydrothermal monazite grains in a mineralized mafic volcanic rock indicate that mineralization formed at 2730 ± 42 Ma (U–Pb SHRIMP). Younger monazite dated at 2387 ± 46 Ma, and xenotime dated at 518.5 ± 9 Ma suggest Siderian and Cambrian imprint in the Lamego deposit area. The Cambrian age reflects the late stages of the Brasiliano orogenic cycle, which is expressed in the Lamego deposit by the S 3 crenulation cleavage (trend NS and dip steeply to the E). Importantly, the Cambrian structural modification is responsible for the present geometry of the Lamego orebodies, but is not associated with hydrothermal alteration; it has not introduced any new gold and also has not caused remobilization of the orebodies or ore minerals. [ABSTRACT FROM AUTHOR]

Nyankanga is the largest gold deposit in the Geita Greenstone Belt of the northern Tanzania Craton. The deposit is hosted within an Archean volcano-sedimentary package dominated by ironstones and intruded by a large diorite complex, the Nyankanga Intrusive Complex. The supracrustal package is now included within the intrusive complex as roof pendants. The ironstone fragments contain evidence of multiple folding events that occurred prior to intrusion. The supracrustal package and Nyankanga Intrusive Complex are cut by a series of NE–SW trending, moderately NW dipping fault zones with a dominant reverse component of movement but showing multiple reactivation events with both oblique and normal movement components. The deposit is cut by a series of NW trending strike slip faults and ~ E–W trending late normal faults. The Nyankanga Fault Zone is a major NW dipping deformation zone developed mainly along the ironstone–diorite contacts that is mineralised over its entire length. The gold mineralization is hosted within the damage zone associated with Nyankanga Fault Zone by both diorite and ironstone with higher grades typically occurring in ironstone. Ore shoots dip more steeply than the Nyankanga Fault Zone. The mineralization is associated with sulfidation fronts and replacement textures in ironstones and is mostly contained as disseminated sulphides in diorite. The close spatial relationship between gold mineralization and the ironstone/diorite contact suggests that the reaction between the mineralising fluid and iron rich lithotypes played an important role in precipitating gold. Intense brecciation and veining, mainly in the footwall of Nyankanga Fault Zone, indicates that the fault zone increased permeability and allowed the access of mineralising fluids. The steeper dip of the ore shoots is consistent with mineralization during normal reactivation of the Nyankanga Fault Zone. [ABSTRACT FROM AUTHOR]

The time of deposition of the rocks of the Karasjok Greenstone Belt, the most continuous and longest, linear Precambrian terrane in northern Norway, remains poorly dated. The belt is composed of several volcanic and sedimentary formations including a continuous unit of calcite and dolomite marbles. Most rocks have been strongly deformed and altered under amphibolite-facies metamorphism. Carbon isotope chemostratigraphy has been applied to provide an apparent depositional age of carbonate units and associated sedimentary and volcanic rocks. Thirty-eight samples representing marbles in four different locations in the northern part of the belt show high δ13Ccarb values, hence recording the Palaeoproterozoic Lomagundi-Jatuli isotopic event. The least-altered δ13Ccarb values suggest the presence of two isotopically different assemblages (+8 to +9‰ and +12 to +14‰) whose apparent time of deposition can be constrained between 2220 and 2140 Ma, and between 2220 and 2110 Ma, respectively. [ABSTRACT FROM AUTHOR]

We present data on the composition of metasedimentary rocks from the greenstone belt of the Onot terrane (Sharyzhalgay uplift) and results of U–Pb dating (SHRIMP II) and Lu–Hf isotope study of detrital zircon from garnet–staurolite schists. The metasedimentary rocks of the Onot greenstone belt are dominated by garnet- and staurolite-bearing schists alternating with amphibolites (metabasalts) in the upper part of the section. Compositionally the protoliths of garnet–staurolite schists correspond to sedimentary rocks, ranging from siltstone to pelitic mudstone. The trace-element characteristics of the garnet–staurolite schists indicate that the terrigenous material was derived from three different rock types, such as tonalite–trondhjemite plagiogneisses (elevated Gd/Yb ratios), mafic rocks (elevated Cr/Th ratios and reduced Th/Sc ratios), and felsic igneous rocks formed by crustal melting (the presence of a Eu minimum), which agrees with the set of potential source rocks from the Onot terrane. The age of predominant detrital zircon reflects the erosion of mainly Neoarchean igneous rocks; this fact, combined with the poor rounding of zircon and tectonically active sedimentation conditions accompanied by mafic volcanism, suggests that the probably depositional age is ca. 2.7 Ga. Older source rocks (2.80–3.35 Ga) contributed to the sediment deposition along with the Neoarchean ones. According to the Hf isotope composition of detrital zircon from the garnet–staurolite schists, the source provenances had different crustal prehistories. The source provenances include Paleoarchean and juvenile Neoarchean crust and rocks formed by the mixing of melts from ancient and juvenile crustal sources. [ABSTRACT FROM AUTHOR]

The Paleoproterozoic Mako Belt in eastern Senegal is characterised by gold-bearing quartz veins. These are superimposed on the regional Birimian structural complex consisting of volcano-sedimentary and intrusive rocks in an Early Proterozoic greenstone belt. In the Mako Belt the multi-stage deformed quartz veins and hydrothermally altered protoliths are characterised by distinctive microstructures and CL properties. The high-grade Au-bearing quartz veins contain one or more generations of hydrothermal quartz. Brecciation of the veins is indicated by cemented fragments of zoned quartz. Gold in the mineralised zones occurs either as inclusions in pyrite or as native gold. Mineralisation typically occurs in vein systems which are brecciated, laminated, and/or show crack-seal textures. Microstructures indicate syn- to late-tectonic mineralisation. Au-mineralised veins contain quartz, carbonate, muscovite, fuchsite, tourmaline, and chlorite. Fluid inclusion studies reveal early, highly saline aqueous inclusions followed by main-stage aqueous-carbonic inclusions of low salinity and finally aqueous low-salinity inclusions. Petrographic observation and microthermometric data suggest that fluid inclusions in main stage quartz were trapped after phase separation from a heterogeneous H2O-CO2 fluid. Gold deposition occurs over a temperature range of 220 to 320°C and pressures of 1.4 to 2.75 kbar (~5 to 10 km depth). The late-stage quartz carbonate vein corresponds to pressures of 0.75 to 1.25 kbar and depths of 3.0 to 4.5 km. A fluid pressure drop due to fracture failure is likely to have triggered Au precipitation. The Mako Belt Au deposits are comparable to a large number of orogenic lode gold deposits in the West African Craton. [ABSTRACT FROM AUTHOR]

An introduction is presented in which the editor discusses articles in the issue on topics including Greenstone belts, Granites, and Archaean geodynamics.

• The Pipestone Lake anorthosite complex consists of anorthosite, gabbro and leucogabbro. • The Pipestone Lake anorthosite complex formed in a continental back-arc setting. • The anorthosite complex and associated greenstone belt represent an Archean suture zone. • The anorthosite complex and greenstone belt were intruded by Neoarchean arc granitoids. • The Cross Lake-Pipestone Lake terrain originated at a Neoarchean convergent plate margin. The Neoarchean Cross Lake-Pipestone Lake terrain, northwestern Superior Province, consists of the Cross Lake greenstone belt, the Pipestone Lake anorthosite complex, and multiple generations of granitoids. The Cross Lake greenstone belt includes an assemblage of basalt, picrite, gabbro, and sedimentary rocks (Pipestone Lake Group) that formed between 2790 Ma and 2760 Ma, and an assemblage of polymictic conglomerate, sandstone, and siltstone (Cross Lake Group) that were deposited between 2704 Ma and 2691 Ma. The Pipestone Lake anorthosite complex is composed of gabbro, megacrystic anorthosite and leucogabbro, and mafic dykes. The Pipestone Lake Group and the Pipestone Lake anorthosite complex are intruded by ca 2760 Ma and younger granitoids. The trace element systematics of the granitoids suggest that they originated in a volcanic-arc to syn-collisional tectonic setting, whereas the trace element signatures of the basalt, picrite, gabbro, anorthosite and leucogabbro association are consistent with a back-arc setting. Whole-rock initial εNd values, zircon initial εHf values and 207Pb/206Pb zircon ages for the granitoids and the anorthosite complex are consistent with long-term depleted mantle sources and variable degrees of crustal contamination during their interaction with Mesoarchean to Paleoarchean continental crust. Zircon trace element data indicate that the granodiorite and the anorthosite complex formed in a continental crust to oceanic crust transition zone at an arc-related setting. On the basis of the field relationships, and trace element and isotopic data, we suggest that the Pipestone Lake anorthosite complex and the spatially and temporally associated Pipestone Lake Group formed in a continental back-arc setting. During and after the closure of the back-arc basin, both the Pipestone Lake Group and the anorthosite complex were intruded by arc-derived granitoids. Accordingly, we interpret the association of the Pipestone Lake Group and Pipestone Lake anorthosite complex as a Neoarchean suture zone. Following the collision and closure of the back-arc basin, the region was dissected by strike-slip faults resulting in the formation of pull-apart basins and deposition of the Cross Lake Group as Timiskaming-type sedimentary rocks. [ABSTRACT FROM AUTHOR]

The Julie greenstone belt of NW Ghana is characterized by a predominately volcanic succession of basalt with minor rhyolite and a mainly sedimentary succession of siltstone, shale and greywacke. The first unit is correlated with the Sefwi Group, and the second with the Kumasi Group of the Birimian Supergroup. The volcano-sedimentary succession is separated by a sheared unconformable contact from clastic sedimentary rocks, including sandstones and conglomerates, correlated with the Tarkwaian Group of Ghana. The supracrustal rocks are intruded by various granitoids. Two major deformational events have been identified. A regional N–S shortening event (D 1) affected rocks of the Birimian Supergroup and associated granitoids. A later NW-SE shortening event (D 2) affected both the Birimian and Tarkwaian units. Rocks have undergone regional greenschist to amphibolite facies metamorphism. Peak metamorphism took place during D 2. Several samples of igneous and sedimentary rocks were subjected to U–Pb zircon dating and Lu–Hf isotope analyses. All zircons provided Paleoproterozoic ages in the range of 2112 ± 5 to 2337 ± 11 Ma, which is consistent with existing zircon ages from volcanic and plutonic rocks in various parts of Ghana, and other Birimian and Tarkwaian terrains within the West African Craton. Rhyolite of the Sefwi Group has an age of 2182 ± 3 Ma and marks a period of explosive felsic volcanism. Detrital zircon ages from greywacke of the Kumasi Group yield a maximum depositional age of 2158 ± 4 Ma. This age compares well to a maximum depositional age of 2154 ± 2 Ma defined in southern Ghana. Zircon ages from Tarkwaian sandstones indicate a maximum depositional age of 2135 ± 5 Ma, which is in accord with similar depositional ages for the Tarkwaian in SW Ghana and Houndé in Burkina-Faso. Thus the Tarkwaian rocks from the Julie greenstone belt area are correlative to those in SW Ghana and elsewhere. Detrital zircon ages point to a derivation from erosion of felsic volcanic and plutonic rocks within the Paleoproterozoic terrain. Detrital zircon ages of Birimian and Tarkwaian rocks show a ∼70% overlap. This suggest that the Tarkwaian detritus may have been derived from reworking of older Birimian rocks. Alternatively, both groups sampled a similar source but were deposited at different periods. Dated granitoids include a 2209 ± 5 Ma phase and a post-Tarkwaian intrusion of 2112 ± 5 Ma in age. The older granitoid is in sheared contact with basalt, and both share similar D 1 structures. The ϵ Hf values for zircon grains from analyzed rocks are in the range of −1.78 to +6.37, in agreement with most Hf isotopic signatures currently available from the Paleoproterozoic Baoulé-Mossi Domain, and indicate crust components with apparent mantle derivation ages averaging c. 2300 Ma and ranging from c. 2100 to c. 2500 Ma. • The Julie Belt in northern Ghana includes Birimian greenstones and Tarkwaian clastic sediments intruded by granitoids. • Greenstones range in age from 2170 ± 6 Ma to 2323 ± 10 Ma and granitoids yield ages of 2209 ± 5 Ma and 2112 ± 5 Ma. • Tarkwaian sandstones record a maximum depositional age of 2135 ± 5 Ma. • ϵ Hf values for zircon grains range from −1.78 to +6.37, and T DM model ages range from c. 2.1 to 2.5 Ga. [ABSTRACT FROM AUTHOR]

The Mesoarchaean Pongola Supergroup is an exceptionally well-preserved succession of volcanic and sedimentary rocks that extends for 270 km close to the eastern margin of the Kaapvaal Craton in South Africa and Swaziland. It represents a volcano- sedimentary continental or epicontinental deposit that is one of the most extensive coherent Archaean terrains in the world, and the oldest of this extent. Its characteristics are unique amongst supracrustal terrains of this age and are unlike those of other Archaean greenstone belts. Its unique nature is that it marks the transition in southern African crustal development from preceding early greenstone belts, such as observed in the Barberton and Nondweni greenstone belts, to late Archaean basin formation on stable continental crest. The volcano-sedimentary succession of the Pongola Supergroup is divided into the lower, predominantly volcanic, Nsuze Group, which contains subsidiary but nonetheless important sedimentary formations, and the upper, dominantly sedimentary, Mozaan Group, which also contains several volcanic successions of highly variable forms and thicknesses. A basement high separates the Pongola Supergroup into a Northern Domain, which is relatively undeformed, and the Southern Domain, which is strongly deformed and folded. The focus of this paper is on the occurrence, field relations (with detailed maps), description and correlation of the volcanic rocks in both the Nsuze and Mozaan Groups in all areas of the Pongola Supergroup, focusing on 11 key areas in South Africa and Swaziland. The compositional range from basalt to rhyolite is remarkable, and this paper focuses on the distribution of different magma types and styles of eruption throughout the belt, including the first record of ultramafic compositions in the Pongola Supergroup. Ultramafic pyroclastic rocks in the Southern Domain contain highly vesicular fragments that may explain the absence of lava flows of the same composition. Volcanic styles transgress the entire length of the belt, with subaerial sheet flows and minor pillow development in the southern Nkandla region through to a possible continental marginal setting with both subaqueous and subaerial volcanism in the south of the Northern Domain and, finally, into a bimodal suite of felsic and mafic to intermediate volcanic rocks in the northernmost regions. Felsic ignimbrite volcanic rocks with abundant lithophysae are present in the central region. A 770 m-thick section of volcanic rocks that includes andesite and dacite compositions as well as fragmental and volcaniclastic units, including an ultramafic volcaniclastic deposit, is observed in a borehole section of the Tobolsk Formation of the Mozaan Group, that is completely overlain by Karoo sediments in the east. Cyclical episodes of volcanism, many of which were of an explosive nature, were interspersed with periods of rapidly deposited coarse-grained sediments in alluvial fans and braided rivers close to the continental margin. Carbonate formation, together with the development of stromatolites, took place during periods of volcanic quiescence, or reduced volcanic activity, in highly protected surrounds and in intertidal zones. Synchronous volcanism, sedimentation and deep erosion gave rise to the marked lateral variations in both subaerial and subaqueous deposits and marked lithologic variability. The largely volcaniclastic (with a high proportion of ultramafic component) Ndikwe Formation in the Southern Domain is enigmatic with contrasting suggested settings of an early phase of Pongola formation, or a later stage nappe structure, with a possible Mozaan Group time equivalence. No single tectonic setting, on modern day analogues, can explain all features of the Pongola volcano-sedimentary belt. Current evidence for the Nsuze Group is most consistent with a continental arc migrating into a continental rifted environment to the north. The overlying Mozaan Group is inferred to have formed in a cratonic basin following thermal relaxation after the preceding cycles of voluminous volcanism. Dedication: This paper is dedicated to the late Professor Don Hunter and Professor Peter Matthews of the University of Natal (now the University of KwaZulu-Natal) who provided the inspiration and insight to unravel the complexities of the Pongola rocks over a period of more than 30 years. [ABSTRACT FROM AUTHOR]

Abstract: Map, geochemical, and geochronological data are used to develop a new stratigraphic scheme and unravel the Archean tectonic evolution of the Murchison Domain of the Yilgarn Craton. Greenstones are divided into four groups: (1) c. 2960–2935Ma Mount Gibson Group of mafic and felsic volcanic and volcaniclastic rocks, in the southern part of the domain; (2) the widespread 2825–2805Ma Norie Group of mafic volcanic rocks, felsic volcaniclastic sandstones and banded iron-formation; (3) 2800–2735Ma Polelle Group of mafic-ultramafic volcanic rocks, intermediate to felsic volcanic and volcaniclastic sedimentary rocks, and banded iron-formation; (4) the 2735–2700Ma Glen Group of coarse clastic sedimentary rocks, komatiitic basalt, and minor rhyolite. Younger groups each have an unconformable relationship with older, underlying, greenstones, whereas the base of the Mount Gibson Group is intruded by younger granites. Very large layered mafic–ultramafic complexes of the Meeline and Boodanoo suites (e.g. Windimurra Igneous Complex) accompanied eruption of the Norie Group during crustal extension at 2825–2805Ma. Less voluminous mafic-ultramafic intrusive suites accompanied eruption of the Polelle and Glen groups. Common c. 2950Ma xenocrystic zircons in these rocks, combined with similar-age detrital zircons in 2820–2720Ma greenstones, implies autochthonous development of post 2820Ma greenstones on older crust. Greenstone belt volcanism was accompanied by widespread intrusion of syn-volcanic plutons and outlasted by 110Ma of widespread and voluminous granitic magmatism, from 2720 to 2600Ma, including 2640–2600Ma post-tectonic granites. All granites are crustal melts, indicating an extremely long period of crustal melting and thus an external thermal input, with or without the effects of thermal blanketing from newly erupted greenstones. Deformation consists of four events, including two early periods of greenstone tilting (D1 =2930–2825Ma; D2 =2735Ma) – possibly associated with crustal extension – and two later (c. 2680–2640Ma) periods of deformation resulting in tight to isoclinal folding of greenstones. D3 structures include steeply-plunging, east–west trending folds of greenstones and open domes of granitic rocks, which formed during a period of inferred partial convective overturn of dense greenstone upper crust and partially molten granitic middle crust at c. 2675Ma. Overprinting D4 structures developed in response to strong east–west compression, resulting in broad, splayed, north-northeast striking dextral shear zones, upright, north- to north-northeast trending folds, and minor north-northwest striking sinistral shear zones. Gold mineralization tends to be focussed in regions of D4 dextral shear and/or low-pressure domains in fold interference structures. Much of the late history of the domain, from 2720 to 2630Ma, is similar and contemporaneous with events that also affected the Eastern Goldfield Superterrane (EGS) of the craton. Shared events include komatiitic-basaltic volcanism at c. 2720Ma, followed by widespread felsic magmatism (2690–2660Ma), early deformation at 2675Ma, shear-hosted gold mineralisation at 2660–2630Ma, and post-tectonic granites at c. 2630Ma. In addition, the whole craton experienced a period of mafic-ultramafic magmatism (komatiitic–basaltic volcanic rocks, layered mafic-ultramafic complexes, and gabbros) at c. 2810Ma, indicating a shared early history. These findings, together with the low overall metamorphic grade (prehnite-pumpellyite to upper greenschist facies), lack of evidence for significant thrusting, and lack of passive margin/foreland basin/accretionary prism successions suggest that a re-evaluation of subduction-accretion tectonic models for craton development is warranted. [Copyright &y& Elsevier]

Abstract: The Kuetsjärvi Sedimentary Formation (KSF) records the global Paleoproterozoic (ca. 2200–2100Ma) positive δ13C excursion of sedimentary carbonates. This event is here called as the Lomagundi-Jatuli isotopic excursion. In this study, lateral δ13Ccarb variation in the KSF and local basinal factors amplifying global δ13Ccarb value were investigated and a secular δ13C curve for the KSF was constructed. Sedimentary carbonate samples from a new drillcore (Drillcore 5A) from the KSF were analysed for δ13C, δ18O and selected major and trace elements. The δ13C values of the samples vary from 5 to 8‰ (VPDB) and the δ18O values from −18 to −10‰ (VPDB). Most of the samples have retained their primary C isotopic composition, as they do not show correlation between the Mn/Sr ratios and the δ13C and δ18O values. The new results were compared to those previously obtained from another drillcore (Drillcore X) from the KSF (ca. 25km along strike from Drillcore 5A). Both cores show a similar kind of generally upward decreasing trend in δ13C, excluding some negative and positive spikes in Drillcore X and samples close to a contact with a Fe-picrite dyke in Drillcore 5A. A secular δ13C curve of the KSF was constructed based on the least altered δ13C data from the cores 5A and X. The secular δ13C curve of the KSF may represent the latest part of the Lomagundi-Jatuli isotopic excursion. [Copyright &y& Elsevier]

Abstract: Mafic igneous rocks of the Nuvvuagittuq Greenstone Belt (NGB) crystallized before 3.8Ga and possibly as early as 4.3Ga, potentially making the belt the oldest known supracrustal sequence on Earth. However, detrital zircons from a rare quartz-biotite schist in the NGB yield significantly younger ages of ≈3.77Ga or less. These appear to be inconsistent with the ages of the mafic igneous rocks, as the quartz-biotite schist has been interpreted as a metaconglomerate, formed by the dismantling of preexisting lithologies. In order to assess this genetic interpretation, we performed a sulfur and oxygen isotope study of the quartz-biotite schist. Sulfide grains found in quartz clasts and the matrix show significant mass-independent fractionation of sulfur isotopes (+0.2‰≤Δ33S‰≤+1.0‰; mean Δ33S=+0.5±0.1‰). Secondary sulfides from crosscutting veins do not show mass-independent fractionation of sulfur isotopes (−0.1‰≤Δ33S≤+0.3‰; mean Δ33S=+0.1±0.1‰). Oxygen isotope compositions of quartz from clasts, matrix and a fine-grained lens are highly enriched in 18O (16.9‰≤δ18O≤26.7‰). Non-zero Δ33S values indicate a surficial origin for sulfur, probably the Eoarchean atmosphere, while high δ18O values suggest a low-temperature (65±18°C) origin for the quartz, likely as chemical precipitation of a chert precursor from Si-saturated seawater. Therefore, the coupled S- and O-isotope measurements show that primary isotopic signatures characteristic of surficial environments survived the protracted metamorphic history of the NGB, and suggest that the quartz-biotite schist contains material that originated as chemical metasediments. The near mono-mineralic compositions of the clasts (quartz) and their shared 18O-enrichment suggest that they had a common protolith, which was deposited prior to the formation of the schist, and subsequently reworked. Whether the quartz-biotite schist represents a metaconglomerate or a structural melange, it preserves remnants of some of the oldest chemical sediments on Earth. [Copyright &y& Elsevier]