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The Neoarchean Qingyuan greenstone belt (QGB) is located at the northeastern margin of the North China Craton (NCC) and has experienced amphibolite- to granulite-facies metamorphism. The supracrustal rock assemblage is composed of biotite-hornblende (Bt-Hb) monzonitic gneiss, garnet-biotite-hornblende (Grt-Bt-Hb) monzonitic gneiss, Grt-Bt-Hb plagioclase gneiss, amphibolite and orthopyroxene-biotite-hornblende (Opx-Bt-Hb) plagioclase gneiss. This study involves thorough field and petrographic observations, whole-rock geochemistry and LA-ICP-MS zircon U‒Pb ages of these rocks to constrain the Neoarchean geodynamic setting of the QGB. Zircon U‒Pb dating indicates that these metavolcanic rocks formed during 2.57‒2.52 Ga and experienced subsequent regional metamorphism at 2.52‒2.47 Ga. Lithological and geochemical characteristics show that the QGB protoliths were tholeiitic to calc-alkaline basalts, andesites, dacites and rhyolites. The andesite-dacite-rhyolite assemblage exhibits high SiO2 and Mg# values, low Yb and Y contents, strongly fractionated chondrite-normalized REE patterns and depletion in Nb, Ta, Ti and P, resembling Phanerozoic adakites. These geochemical characteristics suggest an origin from the partial melting of a subducted slab interacting with mantle wedge material. The tholeiitic to calc-alkaline basalts exhibit slightly LREE-enriched chondrite-normalized REE patterns and negative Nb and Ti anomalies, like island arc basalts. These basalts might have originated by partial melting of a mantle source affected by metasomatism from subduction-derived fluids and melts. The lithological associations and geochemical characteristics imply that the Qingyuan greenstone belt developed in a continental margin arc setting. [ABSTRACT FROM AUTHOR]

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]

Paleoproterozoic (2.05 Ga) komatiites are widespread in the Central Lapland Greenstone Belt (CLGB), northern Finland. Close association with sulfur (S)-rich country rocks and spatiotemporal connection with the Cu-Ni(-PGE) deposits of Kevitsa and Sakatti make these komatiites interesting targets for sulfide deposit exploration. We provide whole-rock geochemical data from Sattasvaara komatiites and combine it with literature data to form a geochemical database for the CLGB komatiites. We construct a model for the komatiites from adiabatic melting of the mantle source to fractional crystallization at crustal conditions. Using MELTS, we calculate three parental melts (MgO = 20.6–25.7 wt%) in equilibrium with Fo92, Fo93, and Fo94 olivine for the CLGB komatiites. Based on REEBOX PRO simulations, these parental melts can form from a single mantle source by different pressures and degrees of melting when the potential temperature is 1575–1700 °C. We calculate ranges of S contents for the parental melts based on the different mantle melting conditions and degrees of melting. We use Magma Chamber Simulator to fractionally crystallize the parental melt at crustal conditions. These simulations reproduce the major element oxide, Ni, Cu, and S contents from our komatiite database. Simulated Ni contents in olivine are compatible with literature data from Kevitsa and Sakatti, hence providing a baseline to identify Ni-depleted olivine in CLGB komatiites and related intrusive rocks. We show that fractional crystallization of the komatiitic parental melt can form either Ni-rich or Cu-rich sulfide melt, depending on the initial Ni and S content of the parental melt. [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]

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]

Volcaniclastic deposits are cynosures for reconstructing volcanic archives and evaluating the tectonic imprints of the study area. This paper presents an elaborate geological field litholog, petrography and geochemistry of ~1.2 km thick sequence of the quartz-feldspar porphyry (QFP) and rhyolites in the Kadiri–Hindupur section of the Kadiri Greenstone belt from Eastern Dharwar Craton (EDC). The QFP exhibits primary stratification; the thickness of the beds varies from very thin to thick (3–8 cm) and shows large-scale tabular cross-bedding in the lower half of the section. The bedding surfaces of QFPs are marked by current crescent-shaped marks (long axis: 5–25 cm, short axis: 2–8 cm), speculated as possible remnants of volcanic bombs and pyroclastic debris during the explosive volcanism. Petrographically, these QFPs are characterised by spectacular blue opalescent quartz grains with embayed grain boundaries, anhedral feldspar with sieved texture, fractured plagioclase and polycrystalline groundmass suggesting a volcaniclastic origin. They are also endowed with significant post-depositional textures such as domino-listric faulting and sigma-shaped porphyroblasts. Geochemically, QFPs are unaltered with trachyte-trachy dacite compositions showing high-potash, calc-alkaline nature displaying shoschonitic affinity. The primitive mantle normalised trace and rare earth elements are marked by an enriched LREE-LILE pattern coupled by negative zircon–hafnium, niobium–tantalum and titanium anomalies affirming their common genetic link to a prominent island arc–back-arc system where explosive volcanism produced the felsic crystal tuffs that were deposited and preserved as volcaniclastic QFP. The associated blocks and bombs produced the current crescent-shaped marks preserved on the bedding surfaces of QFPs. Research highlights: Research Highlights Quartz-feldspar porphyry (~1.2 km thick) occurs in the Kadiri Greenstone Belt, Eastern Dharwar Craton. Blue opalescent quartz and deformed plagioclase porphyry indicate volcaniclastic origin. Enigmatic current crescent-shaped marks represent remnants of volcanic bombs and pyroclastic debris. The primitive mantle normalised REE diagram is characterised by LREE-LILE enrichment. They represent Neoarchean pyroclastic volcanism and sedimentation in an island arc–back-arc setup. [ABSTRACT FROM AUTHOR]

The results of studies of carbonaceous matter (CM) and the S isotopic composition of associated sulfides in metapelites of the Neoarchean banded iron formation of the Kostomuksha greenstone belt of Karelia (Karelian Craton of the Fennoscandinavian Shield) are presented. The petrographic observations show that the CM occurs inside and between silicates, in sulfides, or at the grain boundaries between the sulfide crystals and biotite or amphibole. The results of scanning electron and atomic force microscopy showed the presence of several CM types distinct in structure and C content. The analysis of Raman spectra of CM revealed the presence both of well-ordered graphite and weakly structured kerogene in samples. The isotopic composition of the total organic carbon is typical of biogenic processes. The δ13Corg values from –27.9 to –30.6‰ are consistent with the fixation of carbon by photo- or chemoautotrophs. The S isotopic composition of associated sulfides is characterized by a positive Δ33S anomaly (up to +0.94‰) and negative δ34S values (from –2.06 to –4.1‰). The positive Δ33S values indicate a genetic link with photochemical elemental sulfur (S8) from the atmosphere, whereas the negative δ34S values reflect the fractionation of isotopes during bacterial-related processes. Based on these observations, we suggest that the primary CM mostly has a biogenic origin. [ABSTRACT FROM AUTHOR]

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]

Shear zones are strain localization structures whose geometry and kinematics control the way how large crustal blocks are disposed and also provide important insights into the nature of pathways and deposition of ore forming fluids. Detailed geological field mapping, consolidated with meso- and micro-structural investigations, flow kinematics and finite strain analyses are fundamental tools to obtain information about the same. Formation, transport and precipitation of gold in major shear zone type gold deposits of the world, are also controlled by shearing activities. The Ramagiri-Penakacherla transcratonic shear zone system is characterized by NW-SE trending, 0.5 to 1 km wide shear zone, affecting both the Ramagiri-Penakacherla schist belt (RPSB) and surrounding granitoids. Sense of shearing is dominantly dextral and the main shear sense indicators include S-C fabric, down-dip stretching lineation, rotated quartz and feldspar porphyroclasts, drag folds, delta and sigma quartz porphyroclasts, bilaterally stretched quartz and feldspar porphyroclasts, and pinch-and-swell structures. Detailed kinematic and strain analyses were combined with vorticity estimates on rocks from these zone to document the patterns and geometry of deformation. The strain parameter measurements including strain ratio in the XZ principal plane of strain ellipsoid (RXZ), strain ellipsoid shape (K) and strain intensity (D) highlight a flattening strain shape of the strain ellipsoid. The shear zone is divided into three zones from its boundary to the center, depending on the amount of matrix and clast size viz. protomylonite, mesomylonite and ultramylonite. The quantitative kinematic analyses highlight that Wm varies between 0.774 to 0.89 for the protomylonite, 0.756 to 0.85 for the mesomylonite and 0.635 to 0.83 for the ultramylonite. This, in turn, indicate considerable proportion of pure shear, varying from 30% to 43% for the protomylonite, 35% to 45% for the mesomylonite and 38% to 56% for the ultramylonite (shear boundary to the center). Identification of pure shear component helped in explaining the varying width of the schist belt, along with its disappearance at punctuated intervals. Heterogeneous distribution of gold deposits in the RPSB is explained by better connectivity of the shear fractures in the mesomylonite and protomylonite zone (near the shear zone boundary) with lower strain than the ultramylonite zone (center of the shear zone), which facilitated better permeability for gold-bearing fluid pathways to flow and ultimate deposition. [ABSTRACT FROM AUTHOR]

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