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Gold occurrence Maljavr is the first Archean conglomerate-hosted gold mineralization found in the Fennoscandian Shield. Gold-mineralized metasomatic rocks form a set of lenses within a 10 m thick linear zone, conformable to the bedding of host conglomerates. The lenses are up to 10 m long and up to 1 m thick and they clearly exhibit three alteration envelopes: the rock in the central part consists of garnet and quartz or garnet-only; biotite, garnet, and quartz make the intermediate biotite–garnet envelope; hornblende, hedenbergite, and quartz are the principal rock-forming minerals in the outer zone of the lenses. All metasomatic rocks contain sulfide mineralization up to 15–20 vol.% and up to 0.6 g/t Au. The main ore mineral is pyrrhotite, and the minor minerals are arsenopyrite, chalcopyrite, pentlandite, löllingite, and troilite. The age of zircon from biotite gneiss in the zone of alteration is 2664 ± 18 Ma, this is considered as the time of formation of lenses of metasomatic rocks. Biotite gneiss-conglomerate and metasomatic rocks were later intruded by tourmaline granite pegmatite 2508 ± 7 Ma. The injection of pegmatite caused re-crystallization of sulfides (mainly arsenopyrite and löllingite) and redistribution of gold. Visible gold in association with Bi minerals native bismuth, ehrigite, maldonite, bismuthinite, joseite-B, and hedleyite was found in inclusions in recrystallized arsenopyrite and löllingite. Au content in the rocks with recrystallized arsenopyrite and löllingite is >1 g/t, up to 30 g/t in hand samples. The 2508 Ma pegmatite is interpreted as synchronous with formation of gold mineralization in its present form. The linkage of gold mineralization with pegmatite and geochemical association Au-As-Se-Te-Bi in the mineralized rocks agree with characteristics of intrusion-related gold deposits worldwide. Biotite gneiss–metaconglomerate, hosting the mineralized altered rocks, was the probable primary source of arsenic and gold for mineralization. [ABSTRACT FROM AUTHOR]

Three variably old groups of banded iron formation (BIF) are known in the Kostomuksha Greenstone Belt (KGB) of the Karelian Craton. This paper deals with the earliest of them, Mesoarchean (2.87–2.81 Ga) BIF-1. BIF-1 occurs among the komatiite–basalt unit of the KGB. BIF-1 consists mainly of quartz and magnetite, with varying amounts of amphibole, biotite, and garnet; the variations of SiO2 (48.3–58.6 wt %) and (21.34–33.82 wt %) suggest that the rocks are BIF. BIF-1 of the KGB, as well as most Archean BIFs, contain high concentration, display a contrasting positive Eu anomaly, lack of Ce anomaly, and the depletion of LREE relative to HREE. However, they differ from other BIFs in the higher Al2O3, TiO2, MgO, K2O, Cr, Ni, Zr, Ba, Cu and Zn concentrations. BIF-1 was formed in a marine basin at an anoxic atmosphere due to hydrothermal fluids, the proportion of which varies from 20 to 80%, and a terrigenous component derived mainly from basalts, komatiites, and dacites of host rocks. Mesoarchean BIF-1 of the KGB was accumulated in a small rift structures within an oceanic volcanic plateau, the formation of which is associated with the influence of a mantle plume on the oceanic lithosphere. [ABSTRACT FROM AUTHOR]

The target of this study is the Takanen greenstone belt (TGB), a small 11-km-long and east-west oriented supracrustal belt in the northern Lentua complex of the Western Karelia Subprovince (eastern Finland). We present a new geological interpretation of the TGB based on lithology, geochemistry, stratigraphy, and two new zircon U-Pb age determinations and Lu-Hf-in-zircon data. The TGB forms a ≥800-m-thick synclinal sequence composed of intercalated mafic-komatiitic and felsic-intermediate volcanic and volcaniclastic rocks (lavas, tuffs, and tuffites) crosscut by granitoid dykes, and metamorphosed in lower amphibolite facies with few primary volcanic textures visible. The felsic-intermediate rocks are calc-alkaline and split into strongly and weakly REE-fractionated groups. The mafic-ultramafic rocks are tholeiitic with flat REE-patterns, but some samples show LREE-enrichment. Massive Fe-sulfide layers (≤1 m) are associated with volcaniclastic rocks. Komatiites are Al-undepleted and comprise differentiated (peridotite-gabbro) and undifferentiated (dunite/serpentinite) cumulates and thin undifferentiated flows. Channel-facies komatiitic cumulates are found stratigraphically above Fe-sulfide-rich units and have potential for komatiite-hosted Ni-deposits. Our new isotope data from two felsic-intermediate volcanic rock samples shows that the TGBs stratigraphically lowermost and uppermost units are 2.96 Ga and 2.71 Ga old, respectively. Based on Hf isotope data, the older 2.96 Ga sample contains reworked crustal material, possibly originating from ~3.2 Ga crust. In contrast, the 2.71 Ga sample is isotopically juvenile (near-chondritic), indicating an episode of juvenile crust formation in the northern Lentua Complex. No unconformity is found between the stratigraphically upper and lower parts of the TGB, but the ~250 Ma age difference (and geochemical indicators) suggests they may have formed in different tectonic settings. The lower part of TGB stratigraphy is correlated with the ~2.94 Ga Luoma Formation in the nearby Suomussalmi greenstone belt. The 2.71 Ga volcanic unit does not have a known equivalent in greenstone belts in Finland, but a corollary is found in Russia, and a tentative correlation to a possible 2.75–2.70 Ga arc-system in Takanen–Khedozero-Bolshozero–Ilomantsi is suggested.

Reactivation of pre-existing fractures is attributed to fluid pressure conditions and orientation of fractures with respect to the tectonic stress field. In most cases, multiple fluid pressure events reactivate fracture networks. However, it is difficult to perceive and quantify the number of such fluid pressure events directly from a heterogeneous vein data distribution. In the present study, we undertake a statistical approach, well known and extensively used for fuzzy clustering of planar data to approximate the minimum number of fluid pressure events which formed/reactivated fractures. We have also combined the deterministic method for quantifying the minimum number of such fluid pressure events with the traditional method of scaling 3D Mohr circles to determine the absolute fluid pressure magnitudes. The method has been applied to published data of the Gadag-Chitradurga greenstone belt, a prominent Archean greenstone belt of the western Dharwar Craton, south India, to understand the mechanism behind shallow crustal emplacement of auriferous quartz veins within a transtensional tectonic regime. Further, we provide field evidence of fracture reactivation and estimate their reactivation potential, which play a significant role in the spatial variation of fluid pressure along the greenstone belt. Identifying data clusters using Bingham statistics for Pf determination. [ABSTRACT FROM AUTHOR]

The Bundelkhand craton in India preserves important records of archean geological evolution, where several ultramafic rocks belonging to the Babina Greenstone Belt (BGB) occur as isolated and oval shaped bodies. These rocks are composed of olivine, orthopyroxene, amphiboles, and serpentine along with accessory mineral phases like chromian spinel and ilmenite. Here we present the major and trace element geochemistry of these ultramafic rocks that are characterised by low SiO2 (45.16–49.00 wt%), high MgO (24.41–29.15 wt%) and moderate Fe2O3 (5.82–9.95 wt%) with high Ni (1164–1674 ppm), Cr (1532–3477 ppm) and Cu (14.7–39.5 ppm) suggesting primary magmatic nature. The rocks show low rare earth element (REE) content (ΣREE 2.1–3.5 ppm) with depleted LREE pattern and flat to slightly fractionated HREE pattern similar to abyssal peridotite signature. The Nb/Yb ratio ranges between 0.01 to 0.20 (average = 0.03), similar to that of N-MORB, suggesting magma derivation from a depleted mantle source, further substantiated by the Th/Yb vs. Ta/Yb plot. Trace elements like Ta and Pb show positive spikes, whereas La, Nb, Pr and Ce show depleted nature. The rocks generally have low platinum group elements (PGE) content (

Gold occurrence Maljavr is the first Archean conglomerate-hosted gold mineralization found in the Fennoscandian Shield. Gold-mineralized metasomatic rocks form a set of lenses within a 10 m thick linear zone, conformable to the bedding of host conglomerates. The lenses are up to 10 m long and up to 1 m thick and they clearly exhibit three alteration envelopes: the rock in the central part consists of garnet and quartz or garnet-only; biotite, garnet, and quartz make the intermediate biotite–garnet envelope; hornblende, hedenbergite, and quartz are the principal rock-forming minerals in the outer zone of the lenses. All metasomatic rocks contain sulfide mineralization up to 15–20 vol.% and up to 0.6 g/t Au. The main ore mineral is pyrrhotite, and the minor minerals are arsenopyrite, chalcopyrite, pentlandite, löllingite, and troilite. The age of zircon from biotite gneiss in the zone of alteration is 2664 ± 18 Ma, this is considered as the time of formation of lenses of metasomatic rocks. Biotite gneiss-conglomerate and metasomatic rocks were later intruded by tourmaline granite pegmatite 2508 ± 7 Ma. The injection of pegmatite caused re-crystallization of sulfides (mainly arsenopyrite and löllingite) and redistribution of gold. Visible gold in association with Bi minerals native bismuth, ehrigite, maldonite, bismuthinite, joseite-B, and hedleyite was found in inclusions in recrystallized arsenopyrite and löllingite. Au content in the rocks with recrystallized arsenopyrite and löllingite is >1 g/t, up to 30 g/t in hand samples. The 2508 Ma pegmatite is interpreted as synchronous with formation of gold mineralization in its present form. The linkage of gold mineralization with pegmatite and geochemical association Au-As-Se-Te-Bi in the mineralized rocks agree with characteristics of intrusion-related gold deposits worldwide. Biotite gneiss–metaconglomerate, hosting the mineralized altered rocks, was the probable primary source of arsenic and gold for mineralization.

A gold deposit in the Tanzanian Archean greenstone belt belongs to the vein-type altered rock deposit. The results of process mineralogy research show that the ore contains 5. 17 g/t of gold, 0.43 % of arsenic, and 2.50% of sulfur, and the main metallic minerals are pyrite, arsenopyrite, magnetite, and siderite. The gangue minerals are mainly quartz, feldspar, carbonate, chlorite, biotite, and clay minerals. The main gold mineral is native gold, and the main gold-bearing minerals are pyrite, arsenopyrite, and quartz. The gold mineral distribution is mainly related to encased gold and intergranular gold, with contents of 49 % and 47 %, respectively, and there is less fissure gold. The particle size of gold mineral is very fine, mainly composed of micro and fine-grained gold, accounting for 92.5 %. Based on the properties of the ore, exploration and testing of the ore-dressing process were carried out. The results show that the gold recovery rate is low when using a single process such as gravity separation, all-sliming cyanide leaching, and flotation. To improve the flotation index, experiments were carried out to optimize the reagent regime including grinding fineness, collector, activator and frother, and the process structure, and it was determined to use the flotation + tailings leaching process. The results show that under the optimal conditions, the gold grade of gold concentrates in the closed-circuit flotation process is 44.70 g/t, the gold recovery rate is 83.56 %; the cyanidation gold operational recovery rate in the flotation tailings is 34.41 %, and the total gold recovery rate is 89.22 %. [ABSTRACT FROM AUTHOR]

Whether Archean tectonics were horizontally or vertically dominated is controversially discussed because arguments bear on the kinematics and thermal state of the Archean mantle and constrain the mode of formation of the earliest continental crust. Highly deformed strata of Archean greenstone belts figure prominently in this debate because they record long periods of time and multiple deformation phases. Among the best‐preserved greenstone belts counts the Barberton Greenstone Belt (BGB) of southern Africa. Geological mapping of part of the southern BGB in Eswatini (Swaziland), combined with U‐Pb zircon dating, shows that the region preserves a tightly re‐folded imbricate thrust stack in which metavolcanic and ‐volcaniclastic strata of the Onverwacht Group, deposited at 3.34–3.29 Ga, have been thrust on top of ca. 3.22 Ga siliciclastic strata of the Moodies Group. The structurally highest element, the Malolotsha Syncline, forms a tectonic klippe of substantial size and is >1,450 m thick. Forward modeling of a balanced cross section indicates that this thrust stack was part of a northwestward‐verging orogen along the southern margin of the BGB and records a minimum horizontal displacement of 33 km perpendicular to its present‐day faulted, ductily strained and multiply metamorphosed margin. Because conglomerate clasts indicate a significantly higher degree of prolate strain which extends further into the BGB than at its northern margin, late‐stage tectonic architecture of the BGB may be highly asymmetrical. Our study documents that the BGB, and perhaps other Archean greenstone belts, preserves a complex array of both vertically‐ and horizontally‐dominated deformation styles that have interfered with each other at small regional and short temporal scales. Plain Language Summary: Worldwide, only a few regions exist where ancient rock strata document how earth cooled, surface strata deformed, and continents grew. It is debated whether vertical movements dominated (akin to a lava lamp) and when major horizontal motions (as they dominate Earth today) began; certainly, there was also overlap between these regimes. Radiometric age dating of zircons extracted from strata along the southern margin of one of the best‐preserved ancient regions in southern Africa, the Barberton Greenstone Belt in Eswatini, show that older strata were thrust there over younger strata for at least 33 km distance subhorizontally. Then they were shingled, and then folded. The results show that even at a time when Earth's oldest continents were just forming, significant horizontal displacements existed already. Key Points: U‐Pb zircon dating and geological mapping confirm a folded thrust‐stack along part of the southern margin of the Archean Barberton Greenstone Belt (BGB)Forward modeling of a balanced cross‐section indicates >33 km of horizontal shortening toward the northwestVertically‐ and horizontally‐dominated tectonics interfered with each other in the BGB and may have done so in other Archean greenstone belts as well [ABSTRACT FROM AUTHOR]

The Pitangui Synclinorium is located in the northwestern portion of the Quadrilátero Ferrífero mineral province, São Francisco Craton, in southeast Brazil. It corresponds to a NW-SE-trending synformal structure composed of an Archean greenstone belt sequence, which is covered by Paleoproterozoic metasedimentary rocks to Neoproterozoic sedimentary rocks, and surrounded by Archean granite-gneiss complexes. This study reports the first detailed 1:25.000 scale geological mapping in the Pitangui Synclinorium and provides cartographic, stratigraphic and petrographic improvements, allowing the formalization of its lithostratigraphic units and unraveling the sedimentary and volcanic depositional environments. The Archean Pitangui greenstone belt occurs in most parts of the homonymous synclinorium, and is subdivided into the Pitangui Group and the Antimes Formation. The metavolcano-sedimentary sequence of the Pitangui Group comprises basal mafic/ultramafic metavolcanic rocks with subordinate felsic/intermediate metavolcanic and metavolcaniclastic rocks of the Rio Pará Formation, suggesting a bimodal volcanism. It is followed by volcanogenic and chemical metasedimentary rocks of the Rio São João Formation, and pelitic to sandy metasedimentary rocks of the Onça do Pitangui Formation deposited in subaqueous environments mainly corresponding to submarine fan systems. The occurrence of clastic rocks increases towards the top of the sequence. The Antimes Formation comprises quartzites and metaconglomerates related to fluvial and fan delta depositional environments. The Pitangui greenstone belt is overlapped, in the central and northwest regions of the study area, by the Fazenda Tapera Formation, a Paleoproterozoic siliciclastic package containing arkosic metasandstones, metagreywackes and metarhythmites deposited in a shallow marine environment. Subordinated tuff layers occur as ash/fall deposits from distal volcanism. Neoproterozoic clastic-chemical sedimentary rocks of the Bambuí Group cover the northern portions of the synclinorium, and are mainly composed of diamictites, limestones, marls and rhythmites from the Carrancas (lacustrine or restricted marine), Sete Lagoas (shallow marine) and Serra de Santa Helena formations (deep water grading to prodelta and delta front).

The Kurrat–Girar–Badwar greenstone sequence on the southern part of Bundelkhand Craton consists of a volcano-sedimentary association that has undergone lower greenschist facies metamorphism. The present work is a detailed field account of this greenstone sequence's lithologies and geochemistry of the Dhasan metabasalts. The litho-assemblage of the Kurrat–Girar–Badwar greenstone belt consists of well-preserved volcanic rocks of basalt–basaltic–andesite composition (2.98 Ga), Banded Iron Formations (BIFs), Quartz Pebble Conglomerates (QPCs), fuchsite quartzites and minor shales, schists, sandstones, quartzites, etc. Based on field observation, the assemblage is divided into four formations, viz., Manpura Formation, Badwar Formation, Girar Formation and Dhasan Formation, respectively, from oldest to youngest. The mafic (metavolcanic) rocks of the Dhasan Formation are classified as tholeiitic basalt–basaltic andesite from major and trace element geochemistry. They have enriched LREE pattern {(La/Yb)N = 2.2–4.5} and negative Nb and Ti anomaly with negligible exogenic crustal contamination. Geochemical signatures point to the formation of most of the Dhasan metabasalts from a parental source similar to the primitive mantle, which has undergone 10–30 degrees of partial melting. Source heterogeneity is evident from Nb content (Nb = 4.04–17.27 ppm). Mantle source petrogenesis hints at source contamination of an initial Depleted Mantle Morb (DMM) by fluids from subduction zone giving an enriched signature to the metabasalts. From overall geochemical signature, we suggest the formation of Dhasan metabasalts in a sub-arc mantle in an intra-oceanic arc-backarc regime. Research highlights: We present a lithostratigraphic column for Kurrat–Girar–Badwar Greenstone Belt, Bundelkhand Craton, India We report Dhasan metabasaltic rocks, their geochemical signatures and petrogenesis. We infer back-arc basin magmatism in a subduction-extension regime for the Dhasan metabasalts, Bundelkhand Craton. [ABSTRACT FROM AUTHOR]

This study aims to delineate concealed subsurface linear structures and their relationship with mineralisation in the Iramba-Sekenke greenstone belt. Results reveal three different types of linear structures and their concealed continuations, which were previously not identified. These structures include dykes trending in NE-SW and NS; shear zones with NW-SE trending and faults that trend in NE-SW, NW-SE and N –S. The faults and shear zones, which were previously shown to terminate at the northwestern end of the Iramba Plateau due to sediment cover, have been revealed to continue into Wembere depression. Moreover, the aeromagnetic and gravity results show that some faults are intruded by dykes in the same trends, which were not previously identified. The geological relationship between these linear structures shows that the youngest structures are the western dolerite dyke trending N –S and the oldest is the NE-SW Wembere-Iramba Plateau fault. The delineation of geological structures from magnetic and gravity data using different analysis techniques reveals that the mining pits are located close to geological structures trending in NW-SE, NE-SW and N –S directions. The integration of the findings from CET grid analysis techniques, together with magnetic maps and geological information reveals a good correlation between known occurrences of minerals and promising exploration target areas. The good correlation between the artisanal mining pits with the location of the potential exploration targets is consistent with the location of lineaments and provides future promising prospects for mineral exploration. The correlation suggests that mineralisation and ore deposits in the study area are respectively structurally controlled and hosted in deformed rocks. This study is potentially useful as a guide for the delineation of concealed subsurface linear structures and integration of structural geometry and geophysical methods for exploration of hidden mineral deposits in the Iramba-Sekenke greenstone belt. • New geological linear structures have been extracted trending in NW–SE and NE–SW. • The faults and shear zones have been revealed to continue into Wembere depression. • Some faults, previously not identified, are intruded by dykes in the same trend. • Potential exploration targets correlate with the location of geological lineaments. • Mineralisation are structurally controlled and ore deposits hosted in deformed rocks. [ABSTRACT FROM AUTHOR]

Palaeo‐Mesoarchean Banded Iron formations (BIFs) of central Bundelkhand greenstone belts occur from Babina (in the west) to Mahoba (in the east) via Mauranipur along with the E–W trending of the Bundelkhand Tectonic Zone (BTZ). In the present study, we report petrological, geochemical, and mineralogical data of BIFs from the Babina, Mauranipur, and Mahoba greenstone belts of central Bundelkhand Greenstone Complex to address their source characteristics and deposition environment. The lithological association of BIFs in the Babina and Mauranipur greenstone belts consists of pillowed basic–ultrabasic volcanics and metasediments, whereas in the Mahoba greenstone belt, BIFs and quartzite occur within basic volcanics. These lithological associations collectively suggest that the entire succession was deposited in a marine environment. Based on geological setting, they are classified as Algoma‐type. The main mineral assemblages of the Babina and Mauranipur BIFs are composed of magnetite, quartz, ±garnet, hornblende, chlorite, and apatite, whereas the Mahoba BIFs along with magnetite and quartz, poised of two distinct mineral assemblages; the first one is Opx–Cpx–Grt and the other one is Grt–Hbl. The preliminary P–T study of these BIFs implies that they had gone through amphibolite‐ to granulite‐facies metamorphism. The geochemical characteristics show that BIFs from the Babina and Mauranipur greenstone belts are dominated by higher quartz and magnetite (total content = 90.63–99.95 wt.%), whereas the Mahoba greenstone belt is characterized by lesser quartz and magnetite components (total content = 86.68–94.58 wt.%). The PAAS (Post Archean Average of Australian Sediments)‐normalized REE patterns display significant positive Eu anomaly (Eu/Eu* up to 2.50) with well‐represented negative Ce anomalies (Ce/Ce* = 0.27–1.20). The REE and trace element ratios of BIFs from the Babina and Mauranipur greenstone belts suggest that they were formed by high‐T hydrothermal fluid activity without or very less continental input. Moreover, the Mahoba greenstone belt BIFs are formed by low‐T hydrothermal fluid activity with substantial continental input. Their chemical variation suggests the shallowing of the proto‐basin from the Babina in the west to Mahoba in the east. Tectonic discrimination plots imply the BIFs of the Babina, Mauranipur, and Mahoba greenstone belts of the central Bundelkhand Greenstone Complex are formed in a back‐arc tectonic setting. [ABSTRACT FROM AUTHOR]

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

The paper presents data acquired on epigenetic processes in rocks of the Meso- to Neoarchean greenstone complex of the Bundelkhand Craton of the Indian Shield. The study was focused on felsic and mafic metavolcanic and mafic–ultramafic rocks of the Central Bundelkhand Greenstone Complex (CBGC). The rocks contain metamorphic mineral assemblages produced during an earlier high-temperature [Grt–Prg–Pl; Cpx–Prg–Pl; Cpx–Grt–Pl; Cpx–Prg (± Ksp, Qz, Mag)] and later retrograde [Grt–Pl–Hbl; Grt–Chl–(Hbl–Act); Grt–(Zo–Czo)–(Hbl–Act); Grt–Chl–Pl (± Ksp, Qz, Mag, Ms)] episodes of metamorphism. The retrograde metamorphism was associated with the origin of zoned Ep and Qz–Ep (±Grt, Act) veins, patches, stripes, and small bodies of epidosites and actinolites. The mineral assemblages of the final episodes of the rock transformations contain prehnite and pumpellyite (Prh–Pmp–Alb; Prh–Pmp–Grs; Pmp–Grs–Alb; Prh–Cc–Act; and Prh–Grs–Cc). The composition of the garnet and amphibole was determined to broadly vary (Alm–Sps to Grs and Prg–Ts to Act and Cum, Gru, respectively). The garnet is contrastingly zoned (comprises three zones), with the boundaries of the zones marking drastic changes in the crystallization parameters (episodes of transformations). The identified sequence of the epigenetic events was as follows: amphibolite-facies metamorphism under elevated pressures (580–680°C at 7.2–10 kbar), with a clockwise P-T-t path and retrogression to the greenschist facies → Ca(Mn) metasomatism → metamorphism to the prehnite–pumpellyite facies. The metamorphic evolution of the CBGC rocks in the Bundelkhand Craton is in good agreement with the geodynamic model of cratonic crustal growth, with Archean subduction–accretion processes and Paleoproterozoic intraplate rifting. [ABSTRACT FROM AUTHOR]

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

The relative and absolute timing of orogenic gold deposits in complex structural settings are active and challenging topics of research, especially in Precambrian greenstone belts. The Suurikuusikko gold deposit in Central Lapland Greenstone Belt is currently the largest primary gold producing deposit in Europe, located on a slight bend of the strike-slip Kiistala shear zone (KiSZ). Gold is refractory and locked inside arsenopyrite and pyrite. In this study, different structural features were investigated along the KiSZ from the recently stripped Etelä pit, which is the southern extension of the Suurikuusikko ore body. Our data source ranges from aeromagnetic to high-resolution aerial images, X-ray computed tomography scans of selected rock samples and regional geological and geophysical datasets. The KiSZ has recorded five discrete deformation phases, spanning between ca. 1.92 and 1.76 Ga. The refractory gold at the Suurikuusikko deposit formed during E-W contraction related to the D1 thrusting phase. This was followed by a N-S shortening event (D2), where most of the strain was taken up by the northern and southern thrusts. Tectonic vectors then switched from N-S to NE-SW and, as a result, dextral strike-slip regime (D3) commenced along the KiSZ. This event exsolved invisible gold from the sulfides and remobilized it along with fractures. A near-orthogonal switch of the regional stress regime from NE-SW to NW-SE flipped the kinematics of the KiSZ from dextral to sinistral (D4). The last deformation phase (D5) produced widespread veining under E-W contraction and secured gold mineralization at the Iso-Kuotko gold deposit within the KiSZ. [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]

An exceptionally well‐exposed part of the Flin Flon Greenstone Belt (Manitoba/Saskatchewan) is used to characterize the mineral assemblage evolution associated with prehnite–pumpellyite through amphibolite facies metamorphism of basalts. Data from these rocks are combined with a large literature data set to assess the ability of current thermodynamic models to reproduce natural patterns, evaluate the use of metabasic rocks at these grades to estimate pressure–temperature (P–T) conditions of metamorphism, and to comment on the metamorphic devolatilization that occurs. At Flin Flon, five major isograds (actinolite‐in, prehnite‐ and pumpellyite‐out, hornblende‐in, oligoclase‐in, and actinolite‐out) collectively represent passage from prehnite–pumpellyite to lower amphibolite facies conditions. The evolution in mineral assemblages occurs in two narrow (~1,000 m) zones: the prehnite–pumpellyite to greenschist facies (PP‐GS) transition and greenschist to amphibolite facies (GS‐AM) transition. Across the GS‐AM transition, significant increases in the hornblende and oligoclase proportions occur at the expense of actinolite, albite, chlorite, and titanite, whereas there is little change in the proportions of epidote. The majority of this mineral transformation occurs above the oligoclase‐in isograd within the hornblende–actinolite–oligoclase zone. Comparison with thermodynamic modelling results suggests data set 5 (DS5) of Holland and Powell (1998, Journal of Metamorphic Geology, 16(3):309–343) and associated activity–composition (a–x) models is generally successful in reproducing natural observations, whereas data set 6 (DS6) (Holland & Powell, 2011, Journal of Metamorphic Geology, 29(3):333–383) and associated a–x models fail to reproduce the observed mineral isograds and compositions. When the data from Flin Flon are combined with data from the literature, two main pressure‐sensitive facies series for metabasites are revealed, based on prograde passage below or above a hornblende–albite bathograd at ~3.3 kbar: a low‐pressure 'actinolite–oligoclase type' facies series, characterized by the appearance of oligoclase before hornblende, and a moderate‐ to high‐pressure 'hornblende–albite type' facies series, characterized by the appearance of hornblende before oligoclase. Concerning the PP‐GS transition, the mineral assemblage evolution in Flin Flon suggests it occurs over a small zone (<1,000 m), in which assemblages containing true transitional assemblages (prehnite and/or pumpellyite coexisting with actinolite) are rare. This contrasts with thermodynamic modelling, using either DS5 or DS6, which predicts a wide PP‐GS transition involving the progressive appearance of epidote and actinolite and disappearance of pumpellyite and prehnite. Patterns of mineral assemblages and thermodynamic modelling suggest a useful bathograd ('CHEPPAQ bathograd'), separating prehnite–pumpellyite‐bearing assemblages at low pressures and pumpellyite–actinolite‐bearing assemblages at higher pressures, occurs at ~2.3 to 2.6 kbar. Observations from the Flin Flon sequence suggests devolatilization across the GS‐AM transition (average: ~1.8 wt% H2O) occurs over a very narrow interval within the actinolite–hornblende–oligoclase zone, associated with the loss of >75% of the total chlorite. By contrast, modelling of the GS‐AM transition zone predicts more progressive dehydration of ~2 wt% H2O over a >50°C interval. Observations from the field suggest devolatilization across the PP‐GS transition occurs over a very narrow interval given the rarity of transitional assemblages. Modelling suggests fluid release of 1.0–1.4 wt% resulting from prehnite breakdown over a ~10°C interval. This fluid may not be entirely lost from the rock package due to involvement in the hydration of igneous mineralogy across the PP‐GS transition as observed in the Flin Flon sequence. [ABSTRACT FROM AUTHOR]

The Barberton Greenstone Belt (BGB) has been classically thought to be a first-order synclinal structure, therefore constraining vertically-dominated models of early Earth lithosphere (de-)formation. Geological mapping, strain analysis, U-Pb zircon age dating, and Raman spectroscopy of carbonaceous matter (RSCM) of the Ntaba Mhlope Syncline (NMS) and the Mkomozane Homocline (MH) close to the southeastern linear margin of the BGB in Eswatini assess this interpretation by constraining the type and timing of the dominant late-phase deformation in this region. Geologic mapping shows that the NMS is a steeply doubly plunging, highly strained syncline composed of basal Moodies conglomerates overlain by sandstones and subordinate shales and siltstones. Deformation of conglomerate clasts is markedly constrictional and of uniform orientation throughout the syncline, suggesting that regional deformation occurred in zones of ductile strain. RSCM indicates maximum peak paleotemperatures of ca. 500 °C, somewhat elevated compared to temperatures measured throughout the BGB. Shortening may have occurred contemporaneously with lower crustal extension. • The Moodies age Ntaba Mhlope Syncline, a steeply doubly plunging, highly strained syncline and the adjacent Mkomozane Homocline formed by late-stage deformation along the southeastern margin of the Barberton Greenstone Belt. • Northwestward-directed shortening resulted in strongly prolate, transport-parallel strain of conglomerate clasts and tight folding. • Northwest-directed shortening in the region may have been contemporaneous with northeast-directed extension documented along the southwestern margin of the BGB. [ABSTRACT FROM AUTHOR]

• Metamorphosed komatiites and basalts from the Badampahar greenstone belt. • Geochemistry reveals a genetic connection among the volcanic rocks. • Al-depleted and -undepleted komatiites generated at different depths and melting degrees. • Melt evolved from Al-undepleted komatiite to komatiitic basalt to tholeiitic basalt. • Such evolution ensued by assimilation and fractional crystallization processes. The ultramafic–mafic volcanic rocks of Archean greenstone belts are important archives for lithospheric and asthenospheric processes of the early Earth. Despite decades of research on this context, many issues still remain unsolved. For example, the process of komatiite magma genesis and the genetic relationship among komatiites, komatiitic basalts and tholeiitic basalts in Archean greenstone belts are not clearly understood. The metavolcanic rocks of the Badampahar greenstone belt (BGB), Singhbhum Craton are studied by major–trace element geochemistry to address the said problems and better understand the evolution of melts in Archean lithosphere. Our research suggests that the protoliths of the metavolcanic rocks were komatiites (both Al –depleted and –undepleted), komatiitic basalts and tholeiitic basalts. The Al–heavy rare earth element (HREE) depleted komatiites were formed by moderate degree mantle melting at a higher depth and the Al–HREE undepleted komatiites are products of moderate to high degree mantle melting at a shallower depth. The melting–assimilation–fractional crystallization modelling result shows that komatiitic basalts were generated from Al–undepleted komatiites, and tholeiitic basalts were generated from evolved komatiitic basalts by assimilation and fractional crystallization processes. The older age limit of the BGB is determined to be 3.25 Ga. and the basement of sedimentation and volcanism was composed of plutonic felsic rocks. [ABSTRACT FROM AUTHOR]

Highlights • The rock analyzed at the Venera 8 landing site is thought to be silicic. • Fractional crystallization of Venusian basalt can produce the theoretical Venera 8 rock. • It is possible that sialic crust was encountered at the Venera 8 landing site. Abstract The surface rock composition measured by gamma (γ)-ray spectrometry at the Venera 8 landing site has anomalously high Th (6.5 ± 2.2 ppm) and U (2.2 ± 0.7 ppm) concentrations with respect to the material analyzed at other landing sites (Vega 1, Vega 2, Venera 9, Venera 10). A calculated bulk rock composition of Venera 8, constrained by the measured Th, U and K 2 O (4.0 ± 1.2 wt%) contents, is similar to silicic to intermediate rocks (diorite/granodiorite) that are typical of terrestrial convergent margins (magnesian, calc-alkalic). In this study, major and trace elemental modeling is applied in order to determine if the calculated whole rock composition of Venera 8 can be derived from a parental magma composition similar to Venusian basalt. The modeling results indicate that polybaric fractional crystallization of a hydrous (H 2 O = 0.4 wt%) and relatively oxidizing (ΔFMQ + 0.7) parental composition similar to Venera 14 basalt can yield residual silicic liquids that match the calculated Venera 8 whole rock composition. The measured Th and U concentrations can also be reproduced within the data uncertainty. Although Venus lacks modern Earth-style plate tectonics, magnesian, calc-alkalic compositions are common within Archean greenstone belts and some rift settings (Haida Gwaii). Consequently, it is possible that the Venera 8 probe encountered a fragment of crust that resembles a terrestrial greenstone belt. [ABSTRACT FROM AUTHOR]

Este trabalho apresenta os resultados da cartografia geológica da Folha Arraias, Tocantins, enfocando as características geológicas e geoquímicas do Grupo Riachão do Ouro (GRO). Os principais objetivos que nortearam este trabalho foram o estudo geológico, petrográfico e geoquímico das rochas metavulcânicas e metassedimentares do GRO. Os resultados permitiram a individualização de uma sequência vulcano- sedimentar tipo Greentone belt de idade máxima Paleoproterozoico (Sideriano), distribuídas em faixas estreitas, concordantes com a estruturação regional NE, e circundadas por granitóides das suítes cálcio-alcalina alto K (monzogranitos) e suite cálcio-alcalina baixo K (tonalitos e granodioritos metaluminosos e peraluminosos), distribuídas nas faixas Serrinha e Conceição. Há predomínio de rochas metavulcânicas, anfibolitos e rochas metaultramáficas na faixa Serrinha, que representam a Formação Córrego Paiol, correspondente à unidade basal do GRO. Na faixa Conceição foram individualizadas tanto rochas metavulcânicas como metassedimentares químicas (BIFs, metacherts) e detríticas (quartzitos e metaconglomerados). O magmatismo máfico- ultramáfico apresenta assinatura de basaltos toleíticos alto Fe - Mg, e komatiítico (mais primitivo - MORB), desenvolvidos em ambiente de arco magmático do tipo arco de ilha em zona de subducção de margem continental ativa. As rochas deste conjunto vulcano- sedimentar (anfibolito, metabasalto, metapiroxenito, metagabro) foram deformadas e metamorfisadas na fácies anfibolito (hornblenda e granada) a xisto verde, enquanto que o pacote metassedimentar apresenta paragênese metamórfica cianita-sillimanita da fácies anfibolito alto. Neste conjunto foram mapeadas importantes mineralizações auríferas associadas a alterações hidrotermais pervasivas, com direção geral N-NW. A compartimentação geotectônica enquadra esta região de estudo no contexto da borda ocidental do Cráton São Francisco, que representam núcleos cratônicos remanescentes do Protocontinente Columbia.