Fold Hinges Research Articles

Microtectonics and fluid evolution in the Katanga Supergroup (Tenke Fungurume Mining District): Evidence of pre- to post-orogenic Cu-Co mineralization in the Central Africa Copperbelt

The sediment-hosted stratiform copper-cobalt deposits in the Central African Copperbelt (CACB) are renowned for their resource potential, encompassing Cu-Co (Ni, U) and Cu-Zn-Pb (Ag, Ge, Mo, Cd) mineralization. Microstructural and microthermometric analyses of carbonate-quartz veins from the Tenke Fungurume mining district, situated in the central part of the Katanga Copperbelt (KCB), have delineated a pre- to post-folding paragenesis of vein formation and hypogene Cu-Co ore mineralization. The mineralization primarily consists of chalcopyrite, chalcocite, carrollite, and bornite.Pre-folding veins comprise bedding-normal veins related to the extension stage, occasionally linked with mullion development during the following compressional stage, as well as bedding-parallel beef veins. Syn-folding mineralization manifests as saddle reef veins within fold hinge zones, fracture-filling veins aligned with tangential longitudinal strain in hinge zones of folded bedding-parallel veins, thin veinlets forming parallel to an axial-planar and shear-band cleavage, and bedding-parallel shear veinlets with oblique fiber orientations. The post-folding Cu-Co mineralization consists of veins that cross-cut earlier veins and folds.Microthermometric analysis of fluid inclusion assemblages from the pre-, syn-, as well as post-folding veins reveals the presence of different fluid types. A high-temperature (180–340 °C) and high-salinity (27–52.5 wt% NaCl + (KCl) eq.) fluid was responsible for the Cu-Co mineralization during the pre- to syn-folding stages. A hypersaline fluid (27.0–42.1 wt% NaCl eq.) with a broad range in temperature (45–387 °C) circulated during the post-folding stage of the Lufilian orogeny. A fluid exhibiting low to moderate homogenization temperatures (28–215 °C) and low to moderate salinity (3.4–23.0 wt% NaCl eq.) is related to fluid circulation during the post-Lufilian epoch. The wide range and variations in salinity of these three fluid types associated with Cu-Co mineralization provide evidence for a multistage fluid flow and a complex hydrothermal system linked to the deposition of Cu-Co ores within the Tenke Fungurume Mining District and the broader Katanga basin. This hydrothermal system has operated during diagenesis, the basin development stage to the Lufilian orogeny and the subsequent post-Lufilian period.

Pulsating orogenesis and Neoproterozoic crustal architecture in the western part of Aravalli-Delhi Mobile belt, India: new evidence from structural, metamorphic, and U-Pb zircon geochronological data

ABSTRACT Structural, metamorphic, and geochronological study of the Delhi Supergroup rocks near the western margin of the Aravalli-Delhi Mobile Belt (ADMB) elucidate the Neoproterozoic tectonics of the South Delhi Fold Belt (SDFB) and its implication for the evolution of the Greater Indian Landmass (GIL). In this study, we establish three deformational events in the SDFB. While the D1 event produced isoclinal, reclined F1 folds synchronous with prograde amphibolite facies metamorphism (~5 kbar, 650°C), D2 formed outcrop to map-scale upright F2 folds. D3 comprised ‘partitioned transpression’ along two subvertical shear zones. Oblique dextral-reverse movement (D3a) along one shear zone formed an outcrop-scale positive flower structure, while sinistral-reverse movement along the other rotated and steepened the hinges of the F2 folds (D3b). U-Pb zircon dating of post-tectonic (post-D2, but pre-D3) and syntectonic (syn-D3) granite intrusives suggests >844 Ma age for D2 and ca. 820 Ma for D3. An older magmatic event (ca. 990 Ma) is recorded from sheared pink granites (pre- to syn-D1 intrusion). We suggest that the amalgamation of the ADMB and the Marwar Craton was initiated by the Grenville-age (ca. 1000 Ma) subduction-collision, while the younger events (ca. 844 Ma and 820 Ma) mark the final stitching of the ADMB – Marwar Craton along the Phulad-Ranakpur palaeo-suture zone to form the GIL. The distribution of early (ca. 1000–900 Ma) and late (ca. 800–700 Ma) Tonian magmatic and metamorphic ages in the SDFB suggest that the final growth of the GIL took place through multiple stages of oblique collision in a pulsating orogenesis through the Early Neoproterozoic.

Sulfide Remobilization in the Metamorphosed Kayad Sedimentary Exhalative Zn-Pb Deposit, Western India: Evidence from Mode of Occurrence, Texture, Hydrothermal Alteration Features, and Trace Element Chemistry

Abstract The Kayad Zn-Pb deposit, situated within the Proterozoic Aravalli-Delhi fold belt in western India, is primarily characterized by sphalerite and galena along with pyrrhotite and chalcopyrite. The mineralization occurs as disseminated ores in quartzite, disseminated/laminated and massive ores in quartz-mica schist, and in pegmatite and quartz veins. The laminated ores conform to the regional schistosity and folding, whereas the massive Zn-Pb ores postdate the pervasive tectonic fabric, accumulating at the fold hinges. The massive ore is characterized by durchbewegung texture, discrete blebs of galena and chalcopyrite in a sphalerite matrix with low interfacial angles, and discrete intergrowths of sulfides and sulfosalts such as pyrargyrite, gudmundite, Ag-tetrahedrite, and breithauptite. Geochemical analyses of sulfides reveal microinclusions of sulfosalts comprising Ag, Sb, Cu, Tl, and As, which are regarded as low-melting chalcophile elements (LMCEs). Hydrothermal alteration is insignificant in the laminated and massive ores but prominent around Fe-Cu ± Zn-Pb and Zn-Pb ± Fe-Cu veins. The alteration assemblages in these veins evince a pervasive K + Na ± Fe alteration, later overprinted by a subsidiary Ca ± Na alteration. We interpret the laminated/disseminated ores to be of syndiagenetic sedimentary-exhalative (SedEx) origin formed within an euxinic basin. Conversely, the textural features, mineralogical composition, lack of associated hydrothermal alterations, and evident structural influence on the emplacement of the massive ores suggest they have been remobilized both via plastic flow and by sulfide partial melting. Temperature estimates of up to 650°C, derived from Ti-in-biotite geothermometry of the metamorphosed host rocks, indicate lower-middle amphibolite facies conditions during regional metamorphism. The initiation of melting at these temperatures was promoted by the desulfurization of pyrite to pyrrhotite in quartz-mica schist, aided by melting point depression due to the presence of LMCEs like Ag, Sb, and As.

The role of polyphase deformation in the formation of iron deposits and gold mineralization in the paleoproterozoic nyong complex greenstone belt (Southwestern Cameroon)

The Toko-Nlonkeng area is situated in the Nyong Complex of the northwestern Congo craton. In this study, airborne geophysical data (radiometric and magnetic) and structural data (outcrops and core samples) were used to investigate the relationship between structural evolution, iron formation (IFs)-hosted iron ore deposits, and gold mineralization in the Paleoproterozoic Nyong complex greenstone belt in southwestern Cameroon. Magnetic and radiometric trends, combined with field data, show general NE-SW, NW-SE, and E-W directions that correlate with the main gold-bearing structures. The study area has undergone four (04) deformation phases relating to polycyclic metamorphic and magmatic events. The D1 and D2 phases are the result of progressive ductile and transpressional deformation; D3 is a brittle-ductile deformation; and D4 is essentially brittle with various types of fracture networks late to post-orogenic. The D3 deformation phase is characterized by constrictive deformation with C3 shears and meso‑ to mega-P3-folds, and the main stress axes show φσ2 > φσ1> φσ3, reflecting a shearing tectonic regime with a maximum E-W shortening. This polyphase deformation is derived from the Eburnean/Trans-Amazonian deformation related to the collision between the Congo and São Francisco cratons. The main strain in the structural style of the Nyong Complex appears to be transpressive tectonics. This study reveals that iron ore and gold mineralization were mostly found on the hinges of folds and confirms that folding shear zones controlled the iron ore and gold target in the Nyong Complex and Northeast Brazil. The study proposes an approach to optimize future exploration activity based on the integration of the data.

Bioinspired Soft Electrostatic Accordion-Fold Actuators.

Increasing interests have been directed toward the exploitation of origami techniques in developing biomimetic soft robots. There is a need for effective design solutions to exploit the properties of origami structure with simplified assembly and improved robotic mobility. In this study, inspired by human long-standing jumps, we present a soft electrostatically driven legged accordion fold actuator made by turning a flat paper into hollow polyhedron structure with a spring like rear and capable of electrostatic pad-assisted steering and carrying loads. Without the need for integration of external actuators, the actuator is composed of the electrostatic origami actuator itself supported by a single-fold leg with fast response, easy fabrication process, and low cost. Initiated by periodic deformation around the folding hinges caused by alternating current voltage and ground reaction forces, the actuators exhibit a unique jump-slide movement outperforming other existing soft electrostatic actuators/robots in terms of relative speed. We examined the effect of different geometric and external factors on the relative speed and highlighted the significance of body scale and short-edge panels as the elastic elements, as well as operating at resonance frequency in producing effective performances. Theoretical locomotion models and finite element analysis were carried out to interpret the working principle and validate experimental results.

Exploring for structurally concealed Carlin-type mineralization: A case study from the northern Shoshone Range, Nevada, USA

This study integrates results from previous studies with new reconnaissance-scale mapping of rock types, structures, and hydrothermal alteration at Goat Ridge and Tenabo to provide a district-scale cross section and palinspastic restoration of an area in Nevada with high potential for Carlin-type mineralization in the footwall of a dismembered thrust fault. Reinterpretation of legacy drill data in light of recent advances in the understanding of Cenozoic normal faulting in the area with the expected stratigraphy of the Roberts Mountains autochthon allows for deciphering whether previous drill holes piercements of faults are the Roberts Mountains thrust or are tilted normal faults that down-dropped the deep-water siliciclastic rocks of Roberts Mountains allochthon onto the coeval carbonate rocks of the Roberts Mountains autochthon. A cross-sectional reconstruction shows that ∼11.1 km (120 %) of extension was accomplished along one set of faults that initiated at 60-70°and tilted 40°E. Previously underappreciated Mesozoic folds are present, which likely formed with horizontal fold hinges. Because of Cenozoic normal faulting and associated tilting, the hinge lines currently plunge at moderate angles and are offset as they step across each crosscutting fault. The anticlinal fold hinges produced the modern exposures of lower plate rocks in the range. The Eocene paleogeologic map that results from a plan-view structural restoration shows that the hinge lines of the original folds trended northwest-southeast in the southern half of the range but turned northward toward the northern end of the range.Mineralization in the range includes silver-dominated polymetallic veins/lodes, weak porphyry-type, Carlin-type, and siliciclastic-hosted gold deposits of uncertain origin. Porphyry-style and silver-dominated polymetallic vein mineralization is associated with ∼39–40 Ma granodiorites, with the silver-dominated polymetallic veins interpreted as the distal veins around weakly mineralized and/or deeply buried porphyry system(s), probably of the quartz monzonitic-granitic porphyry Mo-Cu subclass of porphyry molybdenum deposits. Crosscutting relationships demonstrate that at least one of “upper plate” gold deposits is Eocene, but a Miocene age cannot be ruled out for the other gold deposits.

Natural fracture patterns at Swift Reservoir anticline, NW Montana: the influence of structural position and lithology from multiple observation scales

Abstract. Subsurface datasets typically lack the resolution or coverage to adequately sample fracture networks in 3D, and fracture properties are typically extrapolated from available data (e.g. seismic data or wellbore image logs). Here we assess the applicability of extrapolating fracture properties (orientation, length, and intensity) across observation scales in deformed, mechanically layered carbonate rocks. Data derived from high-resolution field images, medium-resolution digital outcrop data, and relatively low-resolution satellite imagery at Swift Reservoir anticline, NW Montana are leveraged to (i) assess interacting structural and stratigraphic controls on fracture development, and (ii) compare estimated fracture properties derived from multiple observation scales. We show that hinge-parallel and hinge-perpendicular fractures (i) make up the majority of fractures at the site; (ii) are consistently oriented with respect to the fold hinge, despite along-strike variability in the fold hinge orientation; and (iii) exhibit systematic increases in intensity towards the anticline hinge. These fractures are interpreted as having formed during folding. Other fractures recorded at the site exhibit inconsistent orientations, show no systematic trends in fracture intensity, and are interpreted as being unrelated to fold formation. Fracture orientation data exhibit the greatest agreement across observation scales at hinge and forelimb positions, where hinge-parallel and hinge-perpendicular fracture sets are well developed, and little agreement on the anticline backlimb, where fracture orientations are less predictable and more dispersed. This indicates that the scaling of fracture properties at Swift Reservoir anticline is spatially variable and partly dependent on structural position. Our results suggest that accurate prediction and extrapolation of natural fracture properties in contractional settings requires the assessment of structural position, lithologic variability, and spatially variable fracture scaling relationships, as well as consideration of the deformation history before and after folding.

Geological characteristics and models of fault-fold-fracture body in deep tight sandstone of the second member of Upper Triassic Xujiahe Formation in Xinchang structural belt of Sichuan Basin, SW China

In the second member of the Upper Triassic Xujiahe Formation (T3x2) in the Xinchang area, western Sichuan Basin, only a low percent of reserves has been recovered, and the geological model of gas reservoir sweet spot remains unclear. Based on a large number of core, field outcrop, test and logging-seismic data, the T3x2 gas reservoir in the Xinchang area is examined. The concept of fault-fold-fracture body (FFFB) is proposed, and its types are recognized. The main factors controlling fracture development are identified, and the geological models of FFFB are established. FFFB refers to faults, folds and associated fractures reservoirs. According to the characteristics and genesis, FFFBs can be divided into three types: fault-fracture body, fold-fracture body, and fault-fold body. In the hanging wall of the fault, the closer to the fault, the more developed the effective fractures; the greater the fold amplitude and the closer to the fold hinge plane, the more developed the effective fractures. Two types of geological models of FFFB are established: fault-fold fracture, and matrix storage and permeability. The former can be divided into two subtypes: network fracture, and single structural fracture, and the later can be divided into three subtypes: bedding fracture, low permeability pore, and extremely low permeability pore. The process for evaluating favorable FFFB zones was formed to define favorable development targets and support the well deployment for purpose of high production. The study results provide a reference for the exploration and development of deep tight sandstone oil and gas reservoirs in China.

Synsedimentary slump folding: Examples and consequences of an under-recognized process in epicratonic basins

Small-scale synsedimentary folds in the Eagle Ford Formation in southwest Texas formed during deposition in a shallow epicratonic basin on the North American craton. These folds formed by slumping (i.e., gravity-driven deformation) and are characterized by (i) plastic deformation of limestone beds, (ii) discontinuous bedding and thickness changes of limestone beds, (iii) horizontal compactional fabric at high angle to folded bedding, (iv) low-angle or recumbent axial surfaces, and (v) erosion of slumped intervals and/or onlapping by resumed sedimentation. Exposures of the Eagle Ford near the northwest margin of the Maverick Basin reveal slump folds in 1–2 m thick zones with observed lateral extents of 10's to 100's of meters to >1 km – actual extents are likely much larger. Curvilinear fold hinges changing trend by >90° over short distances, indicating markedly non-cylindrical fold geometries. Bed thickness changes reflect flowage of unlithified sediment during slumping, and subsequent compaction. Previous descriptions of slump folds in the Eagle Ford Formation have focused on examples from the hydrodynamic facies of the basal portion, but slump folds are also present in pelagic facies of the upper Eagle Ford. Slump folding locally represents significant shortening (e.g., 50%) and thickening (e.g., 100%) of slumped intervals, with up-dip absence of the same interval represented by missing section, bed terminations, boudinage, and/or extensional faulting. Repeated slumping can result in significant cumulative updip thinning and downdip thickening along depositional slopes. Our experience working unconventional reservoirs in a variety of North American epicratonic basins indicates that slump folding is common and underrepresented in the literature. Criteria for recognizing slumping in the subsurface include a combination of discontinuous and contorted beds, thickening into fold hinges, compactional fabric postdating folds, and inverted or repeated section. Natural fracture networks may be less regular than in similar but flat-lying beds. Occurrence of slump folded intervals adds complexity and unpredictability to unconventional reservoirs, with implications for reservoir quality, drilling, and optimal stimulation during completion.

The Dugald River-type, shear zone hosted, Zn-Pb-Ag mineralisation, Mount Isa Inlier, Australia

The Dugald River Zn-Pb-Ag mine is situated in the Mount Isa Inlier, a globally significant base metal province. Zn-Pb deposits in the Mount Isa Inlier are stratabound with four main genetic models, including SEDEX-style, remobilised SEDEX, epigenetic and Broken Hill-type mineralisation applied to interpret their formation. We propose that the Zn-Pb-Ag mineralisation at Dugald River represents a unique, shear zone hosted deposit type that formed through a series of successive deformation events during the Paleoproterozoic Isan Orogeny that concentrated the mineralisation within the Dugald River Shear Zone during two main mineralising phases. The first phase of mineralisation occurred during regional D2 shortening, which is associated with the formation of large-scale F2 folds and a regionally penetrative S2 fabric. During this phase, progressive tightening of upright F2 folds resulted in several sets of secondary space accommodating quartz-carbonate veins that were progressively rotated into parallelism with the pervasive, steep, W-dipping S2 cleavage. The quartz-carbonate veins were coevally replaced by sulphides, which migrated to extensional sites (boudin necks and fold hinges) in tight folds. Thereby creating a sulphide-rich horizon within a developing high strain zone, which during D4 developed into the Dugald River Shear Zone. The second phase of mineralisation occurred during the regional D4 transpressional deformation event and resulted in significant metal enrichment and the current geometry of the ore bodies. The significant enrichment of the mineralisation during D4 resulted from further fold tightening within the high strain zone, which resulted in the attenuation and dismembering of folds and produced a transposed fabric (S4). The sulphide veins were transposed into parallelism with S4 forming sulphide-rich planar ore textures. Strain partitioning at the contact between the ductile deforming sulphide horizon and the brittle deforming slates resulted in the development of an anastomosing shear zone, known as the Dugald River Shear Zone. A right-handed releasing bend in the shear zone produced a dilational jog and a thick, high-grade ore body. The mobilisation of sulphides within the dilational jog involved fragmentation of sulphides and wall rock, brecciation, rotation and rolling of fragments, and the formation of durchbewegung texture.

Post-collisional magmatic-hydrothermal mineralization in the São Gabriel Terrane (southern Brazil): Insights from mineral chemistry, stable isotopes, and sulfide trace-elements of the Caçapava do Sul skarns

Prograde and retrograde magnesian skarns associated with the Caçapava do Sul Granitic Complex intrusion (central Dom Feliciano Belt, southern Brazil) host Cu–Fe (Mo–Au) mineralization. Vein and contact prograde skarns contain disseminated pyrite, pyrrhotite, molybdenite, and chalcopyrite formed from the interaction of mafic and felsic intrusions and dolomitic marbles at ca. 578 Ma. The prograde skarn mineralization is mainly localized in fold hinges and pinch-and-swell structures. Retrograde skarns host massive chalcopyrite and pyrite with chlorite and calcite associated with brittle structures at ca. 557 Ma. In this contribution, we provide mineral chemistry analyses of magmatic and skarn assemblages, calcite stable isotope analyses, and trace-element characterization of pyrite and chalcopyrite to assess ore-forming processes involved in the formation of the skarns. Amphibole-plagioclase geothermobarometry in granodiorite and monzonite apophyses of the Caçapava do Sul Granitic Complex yielded crystallization pressures between 4 and 5 kbar and temperatures from 680 to 750 °C. Biotite and amphibole composition of granodiorite, monzonite, and garnet-leucogranite apophyses suggest a trend of decreasing oxygen fugacity from early to late intrusions. Calcite-dolomite solvus thermometry in prograde skarns indicates temperatures between 580 and 630 °C whereas chlorite thermometry in retrograde skarn yields lower temperatures between 280 and 310 °C. Calcite from the retrograde skarns shows values of approximately −5.5‰ for δ13CVPDB and ∼9.0‰ for δ18OSMOW, indicating a major contribution of magmatic fluids during the retrograde stage whereas previous studies indicate a mixed igneous-sedimentary character of the calcite in prograde skarns. Our findings propose a multiphase magmatic-hydrothermal origin for the Cu–Fe (Mo–Au) skarn mineralization hosted by the Passo Feio Metamorphic Complex. The prograde and retrograde magnesian skarns of Caçapava do Sul were formed in the same age interval (578 - 557 Ma) as many other magmatic-hydrothermal base and precious metals mineralization in the São Gabriel Terrane. Hence, we suggest that the studied skarns belong to a multi-intrusive Cu-porphyry system in the São Gabriel Terrane formed during the post-collisional stage of the Dom Feliciano Belt.

Reconstruction of petrophysical zoning of the Blagodatnoye gold deposit in the Yenisei Ridge: Geodynamic and physical-chemical aspect

Research subject. Petrophysical zoning of the Blagodatnoye gold-sulfide deposit in the Yenisei Ridge. Aim. To determine indicative petrophysical characteristics of the products of the main occurrence stages and to develop an evolutionary petrophysical model of the investigated deposit.Materials and methods. Physical fields were studied by the methods of magnetic and electrical exploration and gamma-spectrometry. The petromagnetic heterogeneity and mineralogical-geo chemical features of formation of polymetamorphic complexes, metasomatites and ores were studied by a neutron activation analysis of the content of rare earth and radioactive elements, petrochemical x-ray fluorescence analysis, as well as by an electron-probe microanalysis of pyrite.Results. The syncollisional fold-overthrust fault (785 Ma) of the preparatory stage provided structural control over the ore-bearing mineral-forming system. The signs of zone dislocation metamorphism include geophysical anomalies: magnetic and natural electrical anomalies due to pyrrhotite and graphite mineralization of cleavage zones on fold limbs, and specific electrical resistance from silicification zones in fold hinges. The metasomatism of the pre-ore (753 Ma) and ore (698 Ma) stages took place under rifting conditions. Pre-ore quartz-muscovite and chlorite metasomatites with carbon mineralization and supra-background Au concentrations were formed under the action of reducing reaction solutions; they remained unaltered in the non-productive part of the deposit. These formations are characterized by elevated concentrations of radioactive elements and natural electrochemical polarizability. During the ore stage, Au was concentrated by fluids with hydro-carbonate-sulfide composition under the violation of the strike-slip kinematics, which caused significant petrophysical transformations of the productive part of the deposit. Early carbon metasomatites in the sub-ore and root sections of the ore bodies were depleted in terms of U, at the same time as retaining their electrochemical activity. Uranium accumulated in the upper horizons of the productive part, whose rocks lost their polarizability due to scattered carbonate mineralization. Magnetic pyrrhotite crystallized as part of sulfides with a regular increase in its proportion in the root sections of the ore bodies. At the final stage (368 Ma), the mineralized zone was broken into a series of blocks with unequal vertical displacements and levels of erosional truncation by upcasts. This led to the exposure of various-depth sections with contrast petrophysical characteristics.Conclusions. The Blagodatnoye deposit was formed in four stages: preparatory, two ore-generating and final. The petrophysical features of the products of each stage formed the basis for the developed evolutionary petrophysical model, which will be tested on the materials of geophysical studies of the Yenisei Ridge territories.

Late Carboniferous Schlingen in the Gotthard nappe (Central Alps) and their relation to the Variscan evolution

Pre‐Mesozoic basements of the Alpine belt commonly contain kilometre‐scale folds with steeply inclined axial planes and fold axes, which are named “Schlingen” folds. The structural evolution of Schlingen folds and their geodynamic significance for the Variscan evolution are unclear. To close this gap, this study investigates a well-preserved Schlingen structure in the Gotthard nappe (Central Swiss Alps). This Schlingen fold evolved by a combination of shearing and folding under amphibolite-facies conditions. Detailed digital field mapping, coupled with petrographical and structural investigations, reveal local synkinematic migmatisation in the fold hinges parallel to axial planes. Zircon crystals from leucosomes in the fold hinges have magmatic cores that yield an age of 449 ± 3 Ma, and rims with a range of dates from ~ 270 to 330 Ma (main cluster at 315 ± 4 Ma). We ascribe this late Carboniferous age to peak metamorphic conditions of the late Variscan Schlingen phase. Moreover, we describe for the first time post-Schlingen, but pre-Alpine transpressional deformation. The investigated Schlingen fold is discussed with respect to comparable structures of the wider Alpine realm, located in the most southern part of the Variscan belt. We propose that Schlingen formation concurred with the crustal-scale transpressional tectonics. This scenario separates, at least in a structural sense, the Southern Variscides from more northern parts (also Gondwana derived), where Schlingen folds are absent.

Combined X-Ray Computed Tomography and X-Ray Fluorescence Drill Core Scanning for 3-D Rock and Ore Characterization: Implications for the Lovisa Stratiform Zn-Pb Deposit and Its Structural Setting, Bergslagen, Sweden

Abstract We present the results of a pilot study that integrates automated drill core scanning technology based on simultaneous X-ray computed tomography (XCT) and X-ray fluorescence (XRF) analyses to provide high-spatial-resolution (<0.2 mm) information on 3-D rock textures and structures, chemical composition, and density. Testing of its applicability for mineral exploration and research was performed by scanning and analyzing 1,500 m of drill core from the Paleoproterozoic Lovisa stratiform Zn-Pb sulfide deposit, which is part of a larger mineral system also including Cu-Co and Fe-(rare earth element) mineralization, hosted by the highly strained West Bergslagen boundary zone in south-central Sweden. The obtained scanning data complements data derived from structural field mapping, drill core logs, and chemical analysis as well as from multiscale 3-D geologic modeling at Lovisa. Data integration reveals macro- and mesoscopic folding of S0/S1 by asymmetric steeply SE-plunging F2 folds and N-striking vertical F3 folds. Stretching lineations, measured directly from the scanning imagery, trend parallel to F2 fold hinges and modeled ore shoots at the nearby Håkansboda Cu-Co and Stråssa and Blanka Fe deposits. The textural character of the Lovisa ore zones is revealed in 3-D by XCT-XRF scanning and highlight remobilization of Zn and Pb from primary layering into ductile and brittle structures. The downhole bulk geochemical trends seen in scanning and traditional assay data are generally comparable but with systematic variations for some elements due to currently unresolved XRF spectral overlaps (e.g., Co and Fe). The 3-D deformation pattern at Lovisa is explained by D2 sinistral transpression along the West Bergslagen boundary zone in response to regional north-south crustal shortening at ca. 1.84–1.81 Ga. Local refolding was caused by D3 regional east-west crustal shortening resulting in dextral transpression along the West Bergslagen boundary zone, presumably at ca. 1.80–1.76 Ga. Based on polyphase ore textures and modeled ore shoots aligned to F2 fold hinges, we postulate that D2 and D3 transpressive deformation exerted both a strong control on ore remobilization and the resulting orebody geometries at Lovisa and neighboring deposits within the West Bergslagen boundary zone. We conclude that the combined XCT-XRF drill core scanning technique provides a valuable tool for 3-D ore and rock characterization, generating continuous downhole data sets, with the potential for increasing precision and efficiency in mineral exploration and mining.

Early Neoproterozoic Tectonics in the Godhra–Chhota Udepur Sector: Evidence for Two-Stage Accretion in the Great Indian Proterozoic Fold Belt

Abstract The Great Indian Proterozoic Fold Belt (GIPFOB) is a curviplanar highly-tectonized zone of Precambrian crystalline rocks. In the GIPFOB, the N/NNE-striking western arm (the Aravalli Delhi Fold Belt, ADFB) and the E-striking southern arm consisting of the Chottanagpur Gneiss Complex (CGC) and the central/southern domains of the Satpura Mobile Belt (SMB) converge at the Godhra-Chhota Udepur sector. To investigate the tectonics of the sector, we combine the results of analyses of mesoscale and regional structures, U-Pb (zircon) geochronology, and monazite chemical dating to constrain the convergence. The sector is dominated by an ensemble of shallow-dipping granitoid mylonites (D2 deformation) and recumbently folded anatectic granulite-facies basement gneisses interleaved with allochthonous greenschist/epidote-amphibolite facies supracrustal rocks thrust top-to-the-south. The shallow-dipping carapace is traversed by a network of E-striking steep-dipping shear zones with sinistral and N-down kinematics (D3 deformation). The D3 shear zone hosted granitoids exhibit E-striking suprasolidus deformation fabrics and chessboard microstructures. In the shallow-dipping carapace, the partly overlapping stretching lineations associated with D2-D3 deformations share low-angle obliquities with the W/WNW plunging hinges of D2 recumbent folds and the upright/moderately-inclined D3 folds in the basement gneisses and the supracrustal rocks. The transition from thrust-dominated (D2) to wrench-dominated (D3) deformation involved flipping of Y and Z strain axes for similar orientations of orogen-parallel stretching caused by N-S shortening. U-Pb LA-ICP-MS (zircon) and monazite chemical dates suggest the D2-D3 deformation and felsic plutonism occurred at 0.95–0.90 Ga, the pre-D2 high-grade metamorphism in the anatectic gneisses at 1.7–1.6 Ga. The 0.95–0.90 Ga structures in the Godhra-Chhota Udepur are identical to those in CGC-SMB in the southern arm and terminate the N/NNE-striking structures in the ADFB. We suggest the GIPFOB comprises two Early Neoproterozoic accretion zones, e.g., the western arm (ADFB) and the younger (GC-SMB-CGC) southern arm.

Regional and local controls of hydrothermal fluid flow and gold mineralization in the Sheba and Fairview mines, Barberton Greenstone Belt, South Africa

High-grade gold-quartz-sulphide lodes of the Sheba-Fairview Complex (SFC) of gold mines in the Barberton Greenstone Belt of South Africa describe a wide range of geometries, orientations, controls, and a number of different and often contradicting models have been postulated to account for their formation. This paper aims to present a holistic model that describes the focussing of a regional-scale fluid flow into narrower, mineralized structures during the late-stage refolding and tightening of the first-order folds that host the mineralization.The auriferous reefs of the SFC are hosted by well-bedded, low-grade metamorphic, clastic sedimentary rocks of the Paleoarchaean Fig Tree and Moodies groups that are preserved in two regional-scale, strongly curved and refolded synclinal structures along the northern margin of the belt. The majority (85 %) of presently and historically mined orebodies occur in the broad hinge zone of the refolded synclines. This suggests the regional-scale fluid redistribution from fold limbs into the first-order fold hinge following regional hydraulic gradients during refolding. Ubiquitous bedding-parallel quartz (-carbonate-sulphide) veins and associated alteration underline the significance of bedding anisotropies and bedding-parallel flexural slip for regional-scale fluid flow during refolding. The focussing of these fluids and economic-grade gold mineralization occurs preferentially along contacts between lithologies with pronounced rheological contrasts. Lithological contacts combine the effects of strain localization in weaker units and the formation of fracture permeabilities in competent units across these contacts. Examples include kilometer-scale hinge-parallel ore shoots that are controlled by undulations (jogs) along low-displacement slip horizons between retrogressed ultramafic rocks and competent metasediments. In contrast, low-angle thrusts or steep reverse faults form accommodation structures related to the late-stage tightening of the first-order folds. These structures transect bedding at high angles and are, thus, ideally orientated to collect fluid from the regional system of bedding-parallel fluid flow.Mineralization in the SFC may be explained by the spatial and temporal coincidence of (1) late-stage NW-SE shortening and refolding, of (2) a well-bedded, anisotropic wall-rock sequence, associated with (3) regional-scale fluid redistribution along bedding planes during flexural slip, leading to (4) fluid focussing into either jog geometries or bedding discordant structures. The combination of these factors is unique to the SFC and accounts for the location of the main Barberton gold mines along the northwestern margin of the greenstone belt.

Evidence for a 1750–1710 Ma orogenic event, the Wonga Orogeny, in the Mount Isa Inlier, Australia: Implications for the tectonic evolution of the North Australian Craton and Nuna Supercontinent

In this contribution we propose that Mount Isa Inlier was affected by a new orogeny, the Wonga Orogeny, which occured between 1750 and 1710 Ma. Evidence for this event is principally given by structural field data and geochronology from this study and that documented in Spence et al. (2021). In this study, field relationships and geochronology within the Duchess Belt of the Mary Kathleen Domain reveals two main deformation events during the Wonga Orogeny, here D1 and D2. The former produced predominantly upright, tight, east–west trending folds whereas the latter produced upright to overturned, north–south trending folds. D1 can be constrained between 1741 ± 6 Ma to 1736 ± 3 Ma, based on the U-Pb magmatic zircon ages of the Revenue granite which intrudes prior to or early during D1 and the intrusion of the Overlander granite which has not been affected by D1. The age of the D2 event can be constrained between 1736 ± 3 Ma and 1722 ± 7 Ma based on the U-Pb magmatic zircon ages of the Overlander granite which intrudes prior to or early during D2 and the Myubee granite which intrudes syn-D2 – within the fold hinge of an F2 anticline. These timing relationships coupled with F2 folds associated with mineral lineations (L2) defined by biotite and hornblende, as well as syn-D2garnet, kyanite and sillimanite porphyroblastssuggest that amphibolite-facies metamorphism, deformation andcontemporaneous magmatism defined by the Wonga and Burstall Suite magma’s all occured between 1750 and 1710 Ma. These characteristics typically define orogenesis and is here, accordingly proposed as a third orogenic event within the Mount Isa Inlier. These findings have implications on the tectonic evolution of the North Australian Craton and Nuna Supercontinent.

Fold-Related Fracture Distribution in Neogene, Triassic, and Jurassic Sandstone Outcrops, Northern Margin of the Tarim Basin, China: Guides to Deformation in Ultradeep Tight Sandstone Reservoirs

Abstract Ultradeep (6-8 km depth) low-porosity sandstone oil and gas reservoirs, in the Kuqa thrust belt of Tarim Basin, are an important natural gas supply source of China. Both opening and shear-mode fractures are extremely necessary reservoir elements for gas production in these rocks but are challenging to characterize with sparse core and well log observations. Here, we use outcrops of some of the same units from four exposed folds to describe fracture types and patterns. These four folds have a range of shapes that are representative of folds at depth. At the Dongq, Kuqa, Misib, and Tuger anticlines, we analyzed fracture type, cross-cutting and abutting relations, density, spatial arrangement, kinematic aperture, orientation, and mineral fill. One-dimensional inventories, field sketches, photographs, and LiDAR imagery documented fracture patterns. Most dips of all fractures shown everywhere on the fold are greater than 60 degrees (over 50%). Fracture kinematic apertures are 3 mm to ~6 mm, and fracture density is 0.5 traces/m to ~1.5 traces/m. All fractures are divided into shear mode (or small faults) and opening mode. Shear-mode fractures, with high dips that strike N-S, mutually crosswise arranged with intersection angles of 30-60 degrees, are found extensively on the flanks of these folds. In contrast, most opening-mode fractures strike E-W, are arranged parallel to each other, and are localized in fold hinges. Besides, exposed folds (reservoir analogues, figure 1) in a proximal (hinterland, Tuger and Misib) position have fracture abundance distributions and aperture size patterns compatible with fold-related fracture development whereas distal (basinward, Kuqa and Dongq) folds lack this correlation, but patterns in distal folds might be explained by overprinted effects of lithological heterogeneity on fracture abundance or the effects of nearby faults. The positive correlation between fold-related strain and fracture spatial distribution in the northern (proximal) folds permitted inference of fracture patterns in deep wells.

Geometric mechanics of folded kirigami structures with tunable bandgap

We study the geometric mechanics and tunable band structures of a recently developed new class of folded kirigami structures through experiments, theoretical modeling, and numerical simulation. The folded kirigami structures with square and triangular cut patterns are constructed by replacing the point hinge in conventional kirigami sheets with a 3D folding hinge. We find that the folded design can effectively overcome the polarization constraint in the conventional kirigami sheets without folds. Specially, as the creases continue to fold from 0° to 180°, the folded design achieves a unique polarization switch, i.e., the structure expands first and then shrinks to be even smaller than that before folding. Geometric mechanics models are developed to predict how the geometry of the folding hinges determines both the shape changes and structural responses, including nominal strains, polarization switch, Poisson’s ratio, folding rate, surface porosity, and structural stiffness. The models are validated through related experiments. We find that the observed polarization switch corresponds to both the peak nominal strains and stiffness singularity in the structures. Lastly, we numerically explore its shape change induced tunable phononic bandgap structures. We find that for special designs with polarization switch, it leads to symmetric bandgap structures changing with the folding angle. This work could find potential applications in designing kirigami metamaterials, shape-morphing materials, and phononic materials with tunable band structures.

Lineament mapping for a part of the Central Sulaiman Fold\u2013Thrust Belt (SFTB), Pakistan

This study aims to analyze the lineaments using the field data and a remote sensing approach, to describe their relationship with the folds, faults, and regional tectonic stress of the central Sulaiman Fold-Thrust Belt. Joint data, from nine anticlines, has been collected using the scanline method and classified into three sets (JS1, JS2, and JS3) based on their geometrical relationships. Lineaments extracted from the 30 m digital elevation model have been classified subsequently into three lineament sets (LS1, LS2, and LS3) based on the azimuths from the corresponding joint sets. A very high correlation coefficient (rs = 0.97), between the azimuths of the field joints and the remotely sensed lineaments, has been observed which validates that the lineaments are the regional representation of the local field joints. The geometrical relationship of the lineament sets with the fold hinges indicates that the older LS1 and LS2 are strongly related to the regional folding episode, while the younger LS3 is a result of local shears. The chronological interpretation of the deformational events responsible for the lineament sets is constrained by the presence of the Kingri Fault, which induces a strike-slip component within the study area. Furthermore, the controls on the joint and lineament sets, established using multivariate statistics to decipher the effects of lithological and structural contrasts on the lineament density, reveal that an increase in the lineament density can be attributed to the competence and thickness of the rock units as well as the variable local stresses across the different folds. Based on the orientations of these lineament sets, the cumulative direction of the compressive event in the NW-SE direction (310–320) coincides with the regional stress direction of the SFTB.