Brittle Deformation Events Research Articles

Relics of ophiolite-bearing accretionary wedges in NE Brazil and NW Africa: Connecting threads of western Gondwana´s ocean during Neoproterozoic times

Neoproterozoic breakup of Rodinia resulted in the formation of several oceanic realms between dispersing cratons, which were later consumed during the assembly of Gondwana. In its western portion, the interior orogenic belts of Gondwana formed during the Brasiliano-Pan African Orogeny in the late Neoproterozoic-early Cambrian. Available geophysical, structural and petrological data suggest that the complex network of shear zones that once connected the Borborema province (NE Brazil), Tuareg shield (Hoggar) and Central African domain (NW Africa) likely represent ancient sutures that mark collisional episodes between Archean-Paleoproterozoic paleocontinents such as Amazonian-West African and São Francisco-Congo. Mafic, ultramafic and sedimentary sequences associated with this set of structures respresent dismembered ophiolite slices interpreted as oceanic remnants (sensu lato) that were emplaced during the late stages of the Gondwana assembly. For instance, the composite Transbrasiliano-Khandi-In-Tedeini-Silet shear system crosscuts rock assemblages preserving a complex history of oceanic-crust-transition development (Novo Oriente complex) in association with primitive to evolved magmatic arcs and UHP rocks both in the Borborema province and NW Africa. In the central Borborema province, preserved ophiolitic slices are strongly overprinted by ductile and brittle deformation events, but partially preserved MORB-like amphibolites are akin to subduction-related-types that crystallized in early- and late Neoproterozoic times docked via terrane accretion and dispersed by strike-slip shear zones. In the southern Borborema province, an example of a Neoproterozoic ophiolitic assemblage is the Monte Orebe complex, that encompasses T-MORB mafic rocks, ultramafic lenses, and exhalative sedimentary rocks akin to early to late stages of oceanic basin spreading, emplaced during convergent plate motions between the Pernambuco-Alagoas superterrane and the São Francisco craton. Correlative units are found in Cameroon, including the strongly hydrotermalized ultramafic rocks of the Lomié and Boumnyebel complexes, that are structurally controlled by top-to-the-south verging nappes found in the N-NW margin of the Congo craton. In all scenarios, the ophiolitic complexes are related to intra-oceanic and continental magmatic arcs as well as to geophysical signatures comparable to Phanerozoic suture zones. Although strongly dismembered, scrapped off Neoproterozoic oceanic crust partially preserved within the major belts of western Gondwana demonstrate the role of accretion-collisional orogenesis during its assembly.

Repeated brittle reactivations of a pre-existing plastic shear zone: combined K–Ar and 40Ar–39Ar geochronology of the long-lived (>700 Ma) Himdalen–Ørje Deformation Zone, SE Norway

AbstractBrittle reactivation of plastic shear zones is frequently observed in geologically old terranes. To better understand such deformation zones, we have studied the >700 Ma long structural history of the Himdalen–Ørje Deformation Zone (HØDZ) in SE Norway by K–Ar and 40Ar–39Ar geochronology, and structural characterization. Several generations of mylonites make up the ductile part of HØDZ, the Ørje Shear Zone. A 40Ar–39Ar white mica plateau age of 908.6 ± 7.0 Ma constrains the timing of extensional reactivation of the Ørje mylonite. The mylonite is extensively reworked during brittle deformation events by the Himdalen Fault. 40Ar–39Ar plateau ages of 375.0 ± 22.7 Ma and 351.7 ± 4.4 Ma from pseudotachylite veins and K–Ar ages of authigenic illite in fault gouge at c. 380 Ma are interpreted to date initial brittle deformation, possibly associated with the Variscan orogeny. Major brittle deformation during the Early–Mid Permian Oslo Rift is documented by a 40Ar–39Ar pseudotachylite plateau age of 294.6 ± 5.2 Ma and a K–Ar fault gouge age of c. 270 Ma. The last datable faulting event is constrained by the finest size fraction in three separate gouges at c. 200 Ma. The study demonstrates that multiple geologically significant K–Ar ages can be constrained from fault gouges within the same fault core by combining careful field sampling, structural characterization, detailed mineralogy and illite crystallinity analysis. We suggest that initial localization of brittle strain along plastic shear zones is controlled by mechanical anisotropy of parallel-oriented, throughgoing phyllosilicate-rich foliation planes within the mylonitic fabric.

Fault zone architecture and lithology-dependent deformation mechanisms of the Himalayan frontal fold-thrust belt: Insights from the Nahan Thrust, India

Brittle shallow crustal faults typically develop a complex fault zone architecture with distinct structural domains that display diverse microstructures, mineralogy, and deformation mechanisms. The development of such domains is typically controlled by the strength and composition of the protoliths, physical conditions of deformation, fluid ingress, and diachronous fault growth in response to stress accumulation and co-seismic slip. Herein, we studied the microstructure-mineralogy-kinematics of fault rocks in the Nahan Thrust, in the vicinity of the Main Frontal Thrust that represents a tectonically active zone in the Himalayan orogen. The Nahan Thrust is characterized by alternating red and gray gouge layers, and a single black gouge layer. Our results from electron microscopy and X-ray diffractometry indicate that the protolith of the red gouge layers is argillaceous sandstone, whereas that of the gray and black gouge layers is sandstone. Microstructures suggest an initially distributed deformation (aseismic creep), followed by a protracted brittle deformation event, and a later aseismic creep stage. The brittle stage is marked by progressive localization of stress, fracture development, cataclasis, frictional sliding, and seismic slips. The black gouge layer acted as the principal slip zone and exhibited ultrafine bands of micrometer-scale slip zones with vapor escape structures and clay clast aggregates, indicating seismic faulting and frictional heating during seismic slips. The preferential seismic rupture nucleation in the black gouge layer indicates a strong lithological dependence on seismic slip in the Nahan Thrust. We also conclude that heterogeneity within the Nahan Thrust resulted from primary lithological variations of the protoliths.

Brittle Deformation History Based on the Analyses of Dikes and Faults within Sedimentary Rocks on Geoje Island, SE Korea

Kinematic analyses of magmatic intrusions and faults can provide useful information on stress conditions and chronological relationships between dike emplacement and brittle deformation events. We studied structures in rocks exposed on a coastal platform in Geoje Island off the southern Korean Peninsula because of its well-developed dikes and faults. The geology of the study area includes the Cretaceous Seongpo-ri Formation, which is composed mostly of shale, sandstone, and hornfels intruded by magmatic dikes. Most of the dikes are developed along pre-existing structural features (faults and fractures), indicating that their emplacements were structurally controlled. Because dikes commonly open along the direction of the minimum principal stress, the direction of this stress can be obtained from dike geometry and orientation through the matching of piercing points on either side of a dike. In addition, the deformed dikes can give information regarding later deformation. On the basis of the kinematic analyses, we identified five deformation events in the study area, which are kinematically related to changes of the regional maximum principal stress. Results indicate that the structures in the study area have been controlled predominantly by episodes of reactivation of the NNE-trending Yangsan strike-slip fault, located to the northeast of the study area, under different stress regimes. In a wider tectonic context, the brittle deformation of the rocks of Geoje Island was probably induced by interactions among the Philippine Sea, Pacific, and Eurasian plates, including changes in subduction parameters with respect to the latter two plates over time.

The role of intracratonic crustal reworking in the tectonic compartmentalization of the Juruena Terrane, SW Amazonas Craton: the Roosevelt-Guariba Transpressive Belt

The multiscale structural analysis carried out in the NNW sector of the Juruena Terrane, SE of the State of Amazonas combines structural mapping, microtectonic analysis and aeromagnetic data analysis. It shows a complex structural and metamorphic history composed of successive deformation events, responsible by the formation of its tectonic framework. Event D2 (1520-1460 Ma) is responsible for the main structure of the NNW sector of the Juruena Terrane, and it generates a dextral transpressive shear zone NW-SE, called Roosevelt-Guariba Transpressive Belt (RGTB). This is an intracontinental high-temperature, medium-pressure tectonothermal event, compatible with the conditions of the upper amphibolite to granulite facies. The RGTB controls the tectonic compartmentation of the region, particularly affecting the Granitic Gneiss Domain (1760-1750 Ma). Other sectors, such as the Supracrustal Rock Domain (1800-1730 Ma) and the Pre-Juruena geological-structural association (>1830 Ma), are subordinately affected. Event D2 overlaps the regional structure E-W (ENE-WSW) attributed to Event D1 (1690-1630 Ma), which is restricted to the deep basement in the NNW sector, but dominant in the south central sector of the Juruena Terrane. There are also two late brittle-ductile to brittle deformation events (events D3 and D4). Event D3 (1320-1300 Ma) is represented by NE-SW cataclastic and shear zones, commonly serving as conduits for hydrothermal fluids relative to gold mineralizations. Event D4 is constituted by normal faults and fractures that reactivate RGTB shear zones, limiting Neoproterozoic and Paleozoic-Mesozoic sedimentary exposures. The tectonic structural model proposed in this study reinforces the interpretation that the Juruena Terrane evolved from a foreland basin, which was formed during the Juruena Orogen and whose basement is the Tapajos Crust. This basin was preserved from the tectono-thermal effects of the end of the Statherian (D1), and it was only during event D2, after emplacement of the RGTB, that it underwent high-grade deformation and metamorphism, whose driving force is attributed to the propagation of stresses in pericratonic regions, associated with Calymmian orogens. The successive events are related to the cratonization of the Juruena Terrane, represented by late crust reactivation (D3) and neotectonic events (D4).

In-situ calcite U-Pb geochronology of hydrothermal veins in Thailand: New constraints on Indosinian and Cenozoic deformation

U-Pb dating of calcite veins allows direct dating of brittle deformation events. Here, we apply this method to hydrothermal calcite veins in a fold-and-thrust belt and a large scale strike-slip fault zone in central and western Thailand, in an attempt to shed new light on the regional upper crustal deformation history. Calcite U-Pb dates for the Khao Khwang Fold and Thrust Belt (KKFTB) of 221 ± 7 Ma and 216 ± 3 Ma demonstrate that calcite precipitated during tectonic activity associated with stage II of the Indosinian Orogeny (Late Triassic – Early Jurassic). One additional sample from the KKFTB suggests that the Indosinian calcite has locally been overprinted by a Cenozoic fluid event with a different chemistry. For the Three Pagodas Fault Zone (TPFZ), our calcite U-Pb results suggest a complex, protracted history of Cenozoic brittle deformation. Petrographic information combined with contrasting redox-sensitive trace elemental signatures suggest that the vein arrays in the TPFZ precipitated during two distinct events of brittle deformation at ~48 and ~23 Ma. These dates are interpreted in the context of far-field brittle deformation related to the India-Eurasia collision. The presented calcite U-Pb dates are in excellent agreement with published age constraints on the deformation history of Thailand, demonstrating the utility of the method to decipher complex brittle deformation histories. The paper further illustrates some of the complexities in relation to calcite U-Pb dating and provides suggestions for untangling complex datasets that could be applied to future studies on the deformation history of Thailand and other regions.

Remote detection and interpretation of structural style of the Zuru schist belt, northwest Nigeria

Mapping of the major structures in the Zuru schist belt and preliminary evaluation of distribution of the locations of gold mineralization relative to these structures was achieved using remote sensing and field data. On the basis of interpretation of remote sensing and field data, four deformational episodes are identified. The first two deformational episodes are ductile in nature, with the first resulting in the development of tight to isoclinal folds with E-W trending axial planes. These folds are refolded during the second folding event, producing open to tight and tight to isoclinal folds with N-S to NE-SW trending axial planes. These folding events are then followed by two brittle deformation events, the first leading to the development of NE-SW strike-slip faults with the second producing NW-SE dip-slip faults. Mineralized veins in the area are found to occur in and around major structures, notably the brittle NE-trending strike-slip faults and ductile E-W and N-S to NE-SW trending folds. However, further studies using geostatistical methods is required to quantitatively ascertain role of the structures on gold mineralization and their distribution within the area.

Sidi M'Barek: a representative example of the Moroccan massive sulfide deposits

Abstract In addition to a Zn-rich lens representing the northern prolongation of the Draa Sfar volcanogenic massive sulfide deposit, the Sidi M'Barek deposit includes Cu-rich lenses in which gold is being recovered profitably as a by-product. The Cu-rich mineralization is composed of numerous structurally controlled veins whose morphology suits that of a lode rather than that of a stratiform orebody. Although the Cu-rich and Zn-rich orebodies are dominated by pyrrhotite, pyrrhotite is coarser grained in the Cu-rich lenses and mineralization is polyphase. Pyrrhotite is deformed and recrystallized; it is cross-cut and replaced by pyrite infilling veins. Pyrite also crystallizes along the S 1 cleavage and in pressure shadows around bioclasts in the disseminated ore. Chalcopyrite veins postdate both the pyrrhotite and pyrite phases. The concentration of Co and Au is higher in the Cu-rich lenses relative to the Zn-rich lenses, which have higher concentrations of Pb, Ag and As. Pyrite and pyrrhotite have distinct sulfur and lead isotopic signatures. We propose a polygenetic model in which a pre-existing syngenetic pyrrhotite-rich mineralization is remobilized in shear zones during a major ductile deformation event allowing deposition of pyrite. The shear zones were reopened during a late ductile–brittle deformation event allowing deposition of chalcopyrite.

The nature and age of Mesoproterozoic strike-slip faulting based on Re–Os geochronology of syntectonic copper mineralization, Assynt Terrane, NW Scotland

In ancient basement regions such as the Lewisian Complex, NW Scotland, the ages of brittle deformation events are commonly poorly constrained owing to a lack of datable fills. An array of NW–SE sinistral and antithetic east–west dextral faults related to a regionally recognized episode of brittle shearing cut Neoarchean gneisses and c. 2.25 Ga quartz–pyrite veins close to the trace of an unexposed, regional-scale NW–SE fault. Copper–iron mineralization occurs at an intersection between an antithetic dextral fault and an older c. 2.25 Ga quartz vein. Optical microscopy, SEM and XRD analyses reveal an array of intergrown, co-genetic copper–iron sulfides, hematite and barite. Complex millimetre-thick zoned alteration rims rich in epidote occur at contacts between the sulfides and gneisses. Rhenium–osmium copper–iron sulfide geochronology yields an age of c . 1.55 Ga for the hydrothermal mineralization event associated with faulting. Fault movements demonstrably overlap with mineralization based on the asymmetric fibrous growth forms of these minerals within local dextral shears, which acted as local channelways for mineralizing fluids during and after faulting. We tentatively propose that this regionally recognized strike-slip faulting, previously termed the ‘Late Laxfordian’, should be referred to as the ‘Assyntian’ to distinguish it from kinematically distinct Laxfordian events.

Fracture network characterization using 1D and 2D data of the Mórágy Granite body, southern Hungary

A disposal system for low- and medium-level nuclear waste in Hungary is being constructed inside the fractured rock body of the Lower Carboniferous Mórágy Granite. Previous studies proved that the granitoid massif is rather heterogeneous in terms of lithological composition, brittle structure and hydrodynamic behaviour. A significant part of the body consists of monzogranite, while other portions are more mafic in composition and are monzonites. As a result of at least three significant brittle deformation events, the area is at present crosscut by wide shear zones that separate intensively fractured zones and poorly deformed domains among them. Due to late mineralization processes, some of these fractured zones are totally sealed and cannot conduct fluids, while others are excellent migration pathways. The spatial distribution of these two types nevertheless does not show any systematics. Hydrodynamic behaviour clearly reflects this heterogeneous picture; in some places, hydraulic jumps as great as 25 m at compartment boundaries can be detected.In this study, the fracture network of the Mórágy Granite body is evaluated from a geometric aspect using datasets measured at a wide range of scales. 2D digitized images of a hand specimen, one large (20 × 60 m) and 12 smaller subvertical wall rocks (outcrops) and 120 images from tunnel faces representing the ground level of the underground repository site were analysed. Moreover, 1D data from 13 wells that all penetrate the granitoid massif were studied. Based on measured geometric data (spatial position, length, orientation, and aperture) fracture networks are simulated to study connectivity relations and for computing the fractured porosity and permeability at different scales. The results prove the scale-invariant geometry of the fracture system. Geostatistical calculations indicate that measurable fracture geometry parameters behave as regionalized variables and so can be extended spatially. Estimated localities of connected subsystems fit very well with fault zones mapped previously. Moreover, the spatial position of regimes of different hydrodynamic behaviours can be explained by connectivity relations both regionally and within wells.

Absolute ages of multiple generations of brittle structures by U-Pb dating of calcite

Abstract Direct dating of brittle structures is challenging, especially absolute dating of diagenesis followed by a series of superimposed brittle deformation events. We report 22 calcite U-Pb ages from tectonites and carbonate host rocks that date 3 diagenetic and 6 brittle deformation events. Results show that U-Pb dating of calcite fibers from these structures is compatible with overprinting relationships. Ages indicate that diagenesis occurred between 147 ± 6 Ma and 103 ± 34 Ma, and was followed by top-to-the-south, layer-parallel shearing due to ophiolite obduction at 84 ± 5 Ma (2σ errors). Sheared top-to-the-northeast, layer-parallel veins were dated as 64 ± 4 Ma and are interpreted to have developed during postobduction exhumation. After this event, a series of strike-slip structures, which crosscut and reactivated older faults due to northwest-southeast horizontal shortening, were dated as 55 ± 22 Ma and 43 ± 6 Ma. Eight ages from strike-slip faults and thrusts resulting from northeast-southwest shortening range from 40.6 ± 0.5 Ma to 16.1 ± 0.2 Ma. The youngest ages are from minor overprinting fibers ranging in age between 7.5 ± 0.9 Ma and 1.6 ± 0.6 Ma. Our results show that U-Pb dating of calcite fibers can be successfully used to constrain a complicated succession of brittle deformation structures that encompasses two orogenies and an intervening extension period.

Rodingites from the Xigaze ophiolite, southern Tibet – new insights into the processes of rodingitization

The Xigaze ophiolite represents one of the most prominent occurrences of Neo-Tethyan ocean remnants in the central part of the Yarlung-Zangbo suture zone, southern Tibet, which marks the collision of the Indian and Eurasian plates during the Jurassic and Cretaceous. Abundant veins and bands of rodingite, which are spatially related to gabbro and diabase dykes and intermingled with serpentinized mantle peridotite, were found in the ophiolite sequence, extending from Jiding to Luqu, Bairang and Baigang. The gabbro as well as diabase formed in a back-arc basin that developed by subduction of the Tethys ocean in response to the convergence of India and Eurasia. Both rodingite and gabbro/diabase show the same geochronological and geochemical behavior, thus demonstrating the genetic relationship of these lithological types with oceanic lithosphere. Rodingitization can be seen as a Ca-involving metasomatic process that is genetically linked to serpentinization; the Xigaze rodingites typically show a gain in Ca and a loss of Na and K. The mineral assemblages of the rodingites are highly variable and can be divided into two groups. Group I rodingite, where prehnite is absent, consists of the subgroups clinozoisite rodingite, garnet–diopside rodingite and garnet–vesuvianite rodingite. The grossular-rich (hydro) garnet may contain up to 13.7 wt% TiO 2 , essentially as morimotoite component (23 mol%). In group II rodingite, prehnite is present and vesuvianite absent; subgroups are prehnite rodingite and prehnite–garnet rodingite. Group I rodingites mainly occur in deformed schistose serpentinites and have experienced a more intense rodingitization in a SiO 2 undersaturated environment than those of group II. Group II rodingites occur along extensional fractures in serpentinized peridotite. Their setting is SiO 2 saturated and reflects a later metasomatic fluid/rock interaction that occurred at shallow crustal levels. Group I rodingites presumably formed within the oceanic crust and were later involved in a subduction-zone environment at high pressures in the early stages of the geodynamic evolution of the Xigaze ophiolite, with a final late-stage imprint at shallow crustal levels. Group II rodingites are interpreted to have formed during late-stage ductile and brittle deformation events at shallow crustal levels, when the uplift process of the mantle section was nearly completed. Rodingites may thus represent an important chronological marker within ophiolites, which may help to evaluate the sequence of processes such as early magmatic intrusions, rodingitization, serpentinization, late basic intrusions, shallow crustal level alterations.

Structural analysis of tin-bearing mineralization in Bou El Jaj district (Hercynian Central Massif, Morocco)

The aim of this paper is to characterize the geological setting of tin-bearing mineralization at the Bou El Jaj (BLJ) sector, located in the NE termination of the Moroccan Central Massif, South of Meknes city, along the NE-SW-striking shear zone. The main tin mineralization corresponds to a NNE-SSW altered corridor of tourmaline, about 10 to 12 km long, from BLJ to Achmmach mount. The geological structures are affected by three ductile deformation phases D1, (E-W shortening), D2 (NW-SE shortening), and D3 (N-S shortening), overprinting folds, and one brittle deformation event D4 (NW-SE shortening), which was synchronous with alteration and mineralization. The tourmaline-altered sediments occur in two parallel veins, about 2.5 km long and 200 m wide for each one, and are controlled by structures such as bedding, main cleavage, thrusts, and joints. Tin mineralization as cassiterite is always associated with tourmaline alteration and is controlled by the different structures.

Exhumation of a migmatite complex along a transpressive shear zone: inferences from the Variscan Juzbado–Penalva do Castelo Shear Zone (Central Iberian Zone)

High-grade metamorphic rocks associated with S-type granites are recorded in the Central Iberian Zone, Iberian Variscides. Though most of these occur as inferred metamorphic core complexes affiliated with detachment faults, others, such as the Figueira de Castelo Rodrigo–Lumbrales Anatectic Complex, crop out between low-grade metamorphic rocks separated by steeply-dipping strike-slip shear zones, such as the Juzbado–Penalva do Castelo Shear Zone. Our structural analysis has been able to constrain two major deformation stages during the Variscan D 3 : (a) D 3a ductile deformation event, with clear sinistral kinematic criteria; and (b) D 3b thrusting ductile–brittle deformation event. The petrological investigation confirmed the jump in metamorphic grade between the host rocks and the anatectic complex. P–T estimates on calc-silicate rocks interlayered with the metapelites of the anatectic complex provided minimum metamorphic peak conditions of T = 761 ± 50°C and P = 5.0 ± 1.0 kbar. However, petrological modelling results show that P–T conditions must have exceeded T > 800°C. Both structural and geothermobarometric data support a two-step model for the exhumation of the Anatectic Complex, including a 5 – 8 km vertical exhumation along a 65 – 100 km horizontal displacement due to a simple shear-dominated transpression mechanism during the Variscan D 3 events. Supplementary material: The petrology data, mineral chemistry analyses of the calc-silicate units and EPMA analytical conditions, and the P–T modelling methodology can be found at https://doi.org/10.6084/m9.figshare.c.3785648

Evidence for brittle deformation events at eclogite-facies P-T conditions (example of the Mt. Emilius klippe, Western Alps)

Eclogitic rocks are crucial for the understanding of tectonic processes as they provide key constraints on both the P-T-t evolutions and the deformation modes sustained by rocks in subduction zones. Here we focus on eclogitised and deformed mafic bodies that are exposed within granulites from the continental basement slice of the Mt. Emilius klippe (Western Alps, Italy). These eclogites exhibit highly deformed garnetite and clinopyroxenite layers. In some places, these deformed rocks (up to mylonitic grade) can be found as clasts within meter-thick brecciated fault rocks that formed close to the lawsonite-eclogite facies peak P-T conditions. Garnet-rich layers are dominated by brittle features, whereas deformation within clinopyroxene-rich layers is accommodated by both creep and fracturing. We present a petro-structural study of these eclogites, that allows to track the brittle deformation history associated with chemical evolution. Based on these data, we propose a new tectono-metamorphic model for these rocks, related to the alpine eclogitic stage. This model is consistent with the coexistence of both ductile and brittle features that developed at similar P-T conditions (i.e., at P~2.15–2.40GPa and T~500–550°C), and closely associated with fluid circulations. Our study demonstrates that crustal material, buried along the subduction interface at HP-LT conditions, can record several successive brittle events in places where deformation is classically envisioned as ductile. We suggest, based on our observations, that strain-rate increase along plate interface shear zones may trigger fracturing and fluid infiltration which in turn enables brittle-ductile instabilities along these deformation networks.

Középső-triász dolomitok képződésének története és töréses deformációja a Szegedi-medence területén

A Pannon-medence aljzatának egyik részmedencéje a Szegedi-medence, mely szénhidrogén-földtani szempontból hazánk egyik kiemelt fontosságú területe. A prekainozoos aljzat legelterjedtebb mezozoos képződményei a középső-triász dolomitok, melyek fontos szerepet töltenek be a terület szénhidrogén-rendszerének felépítésében. Jelen tanulmány célja, hogy az évtizedek óta nem vizsgált képződmények képződéséről és deformációtörténetéről új, korszerű leírást nyújtson. Értelmezésünket általános petrográfiai leírásra, fluoreszcens és katódlumineszcens vizsgálati módszerekre, valamint a területről szóló szakirodalomra alapozzuk.A vizsgált anisusi képződmények változatos partszegélyi, sekélytengeri környezetben ülepedtek le. Ezek a leülepedést követő süllyedés következtében, a sekély és közepes betemetődés után, a kora-krétára juthattak a mély betemetődési diagenezis zónájába. Ezen zónákban a képződmények feltehetően többfázisú, részben szövetmegőrző, részben szövet -romboló dolomitosodási folyamatokon mentek keresztül. A petrográfiai alapon elkülönített dolomit típusok közülközepes vagy mély betemetődési környezetet jeleznek a porfirotópos és cukorszövetű dolomitok, valamint a póruskitöltő nyeregdolomit, de ezek szerepe alárendelt a korábbi dolomitosodási folyamatokhoz viszonyítva. A középső-miocént megelőzően a kőzeteket többször is töréses deformáció érte, melynek hatására breccsa, ezt követően pedig üreg- és repedéskitöltő nyeregdolomit képződött. A nyeregdolomit kristályai több esetben kőolajtartalmú fluidumzárványokat csapdáztak. A dolomittestek felszínközeli helyzetét középső-miocén, abráziósparti kör -nyezetben leülepedett fedőkőzetek jelzik, továbbá dedolomitosodás is kapcsolódhat ehhez a szakaszhoz. A Pannon-medence kialakulásával párhuzamos süllyedés és eltemetődés hatására a képződmények ismét nagy mélységbe kerültek, mely során újabb töréses deformáció és cementáció folyt. A legfiatalabb repedéskitöltő ásványegyüttest fluoreszcens tulajdonságú dolomit, valamint pirit és szilárd bitumen alkotja.A vizsgált dolomitok többfázisú diagenezis- és deformációtörténetéhez többfázisú szénhidrogén-migrációs események kapcsolhatóak, melyek pontosabb megismerése és elhelyezése a medence fejlődéstörténetében kulcsfontosságú. Az erősen átalakult nyeregdolomit tartalmú minták további vizsgálata rezervoárgeológiai szempontból a jövőben szintén fontos feladat.

Diagenetic analysis of tetrapod from the Upper Triassic, Puesto Viejo Group, Argentina

The Puesto Viejo Group crops out in the San Rafael Block, southwest Mendoza, Argentina. This group is an important unit for the knowledge theTriassic faunas. It is composed of basal Quebrada de los Fósiles Formation (QF) overlying by the Río Seco de la Quebrada Formation (RSQ). In this study, we report tetrapod remains from the RSQ Formation that includes, until now, exclusively therapsids. Non-mammalian therapsids predominated in terrestrial ecosystems during the Late Paleozoic and Early Mesozoic. Although cynodonts and dicynodonts have been studied from several perspectives, the relation between taxa and sedimentary context has been scarcely documented. This study is aimed to elucidate the diagenesis of tetrapods from the Puesto Viejo Group and its relation to depositional environment. Diagenetic features of therapsid bones from the RSQ Formation were analyzed through thin sections to interpret the degree of taphonomic alteration. Diagenetic processes inferred include substitution; fracturing; brittle deformation and different permineralization events. Combined analyses through scanning electron microscopy; energy-dispersive X-ray spectrometry (SEM-EDX); X-ray diffractometry (XRD) and petrographic studies reveals the substitution of hydroxyapatite by francolite. The presence of fluorine in some samples suggests a link between the elemental composition and depositional environments: floodplain and crevasse splay. Permineralization stages include infilling of vascular canals, trabeculae and fractures with iron oxides and iron carbonate minerals during the burial history. This contribution represents a relevant approach to elucidate the relation between diagenesis on tetrapods and their paleoenvironments, as exemplified in this case study from the Puesto Viejo Group during the Upper Triassic.

GEOLGICAL STRUCTURE MAPPING OF THE BENTONG-RAUB SUTURE ZONE, PENINSULAR MALAYSIA USING PALSAR REMOTE SENSING DATA

Abstract. The Bentong-Raub Suture Zone (BRSZ) of peninsular Malaysia was selected as case study to evaluate the capability of the Phased Array type L-band Synthetic Aperture Radar (PALSAR) satellite remote sensing data for structural geology mapping in tropical environments. The structural elements in the BRSZ were enhanced using multi-polarization configuration of PALSAR data at a regional scale. Adaptive local sigma and directional filters were applied to PALSAR data for detailed structural mapping. Numerous tectonic lineaments with consistent variation in trend, length and density were detected in the study area. Structural analysis of the BRSZ reveals that two distinct parts can be defined, a western part affected mainly by ductile fabrics in the Cameron Highlands and an eastern part affected mainly by brittle deformation in the BRSZ. Ductile deformation indicates several generation of folding in the Cameron Highlands. Several faults, joints and fractures represent brittle deformation events in the BRSZ. The results of this study demonstrate the usefulness of PALSAR satellite remote sensing data for mapping geological structures in tropical environments.

A 3-D Geological and Structural Synthesis of the Leinster Area of the Agnew-Wiluna Belt, Yilgarn Craton, Western Australia, with Special Reference to the Volcanological Setting of Komatiite-Associated Nickel Sulfide Deposits

Research Article| March 01, 2015 A 3-D Geological and Structural Synthesis of the Leinster Area of the Agnew-Wiluna Belt, Yilgarn Craton, Western Australia, with Special Reference to the Volcanological Setting of Komatiite-Associated Nickel Sulfide Deposits Caroline S. Perring Caroline S. Perring † BHP Billiton Nickel West, 125 St George’s Terrace, Perth, WA 6000, Australia †E-mail, Caroline.Perring@bhpbilliton.com Search for other works by this author on: GSW Google Scholar Economic Geology (2015) 110 (2): 469–503. https://doi.org/10.2113/econgeo.110.2.469 Article history received: 06 Mar 2014 accepted: 04 Aug 2014 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Caroline S. Perring; A 3-D Geological and Structural Synthesis of the Leinster Area of the Agnew-Wiluna Belt, Yilgarn Craton, Western Australia, with Special Reference to the Volcanological Setting of Komatiite-Associated Nickel Sulfide Deposits. Economic Geology 2015;; 110 (2): 469–503. doi: https://doi.org/10.2113/econgeo.110.2.469 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyEconomic Geology Search Advanced Search Abstract The Leinster area is situated toward the southern end of the Agnew-Wiluna Belt in the Yilgarn craton of Western Australia and hosts the giant Perseverance komatiite-associated nickel sulfide deposit. At in excess of 1.2 Mt contained Ni, this is the biggest type 1 (basal sulfide-rich accumulation) deposit in the world, and it is associated with a number of smaller deposits that extend over a strike length of 20 km adjacent to the Perseverance fault on the eastern margin of the belt. In broad terms, the greenstone sequence comprises an eastern, rift-proximal komatiite-dacite sequence and a western and more marginal basalt-dominated sequence with minor komatiite.The komatiites display a wide range of compositions and textures, from highly magnesian, olivine meso- to adcumulate rocks composed of olivine with forsterite contents of up to 94.5% to sequences of thin, spinifex-textured komatiite flow units. This compositional range indicates considerable variation in volcanic facies, from rift-proximal to distal. Loci of focused flow (termed pathways), indicated by thickened cord-like bodies of more olivine rich rock, occur at all scales and are intimately associated with the Ni sulfide mineralization. The pathways are linked by thinner sequences of lower-MgO rocks, either layered olivine orthocumulate rocks or sequences of thin spinifex-textured flow units. Some of the lower-MgO komatiite sequences appear to emanate from the tops of pathways, in the manner of breakout flows in tube-fed basaltic terrains. Variably sulfidic black shales and cherts and barren exhalative iron sulfide horizons are intimately interlayered with the komatiitic rocks.The overprinting effects of regional deformation and metamorphism, spanning at least 80 m.y., are considerable. At least eight local deformation events have been recognized by previous workers. These have been used to constrain the 3-D model and can be linked to craton-wide structural schemes. Some of the major faults are clearly accretionary and reflect reactivation of underlying basin-forming structures. Early north-over-south thrusting was followed by E-W–directed basin closure and the development of NNW-trending open regional folds. Extension accompanied batholithic granitoid intrusion and reactivated rift-parallel NNW-trending faults, producing the major strike-slip fault systems that divide the greenstone succession into a series of structural and stratigraphic domains. The ensuing orogenic collapse resulted in overturning of the stratigraphy to the east. Subsequent brittle deformation events have produced fault sets in a variety of orientations that can be related to ongoing shortening with a variable principal stress orientation.There is a complex interplay of structural and volcanological controls on the formation and subsequent deformation of the Ni sulfide deposits in this ancient terrane. Three-dimensional modeling has furthered a detailed understanding of the geologic and structural evolution of the belt, allowed the identification of komatiite pathway positions and their plunges, and provided quantitative data on the magnitude and direction of fault offsets of different generations. The modeling exercise has shown that it is possible to strip away the effects of overprinting metamorphism and deformation to reveal some of the primary controls on komatiite volcanism and the location of Ni sulfide deposits, even in those portions of greenstone belts that are highly deformed and have undergone amphibolite facies metamorphism. This is illustrated with case studies of the Perseverance, Venus, Rocky’s Reward, Harmony, and Sir Lancelot deposits.Three-dimensional modeling has the capacity to integrate geologic, geochemical, and geophysical datasets in a way that fully utilizes the results of past exploration programs and produces a premium geologic product that can be used to guide further exploration. The recent discovery of the Venus deposit is, in part, a tribute to the power of this approach. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Formation and Deformation of Pyrite and Implications for Gold Mineralization in the El Callao District, Venezuela

The El Callao mining district is the most important gold-producing region in Venezuela. It is hosted in the Paleoproterozoic Guasipati-El Callao greenstone belt, which forms part of the Guayana craton, the Venezuelan extension of the Guiana Shield of South America. It consists of volcanic and volcanosedimentary sequences that were affected by several deformation events, particularly localized shear zones. The Colombia mine, the largest active mine in the district, produces 4 tonnes (t) (128,600 oz) of gold annually with reserves estimated at 740 t (24 Moz) and grades of up to 60 g/t. Gold mineralization is concentrated within a vein network in the Colombia corridor, a shear fracture-hosted mesh of interconnected quartz-ankerite-albite veins enclosing fragments of altered metabasaltic host rocks. Gold occurs mostly in the metabasaltic fragments and is spatially associated with pyrite, in which it occurs as invisible gold, micron-sized native gold inclusions, and filling fractures. Based on optical and scanning electron microscopy-backscattered electron observations, two types of pyrite are recognized: a simple-zoned pyrite and a less common oscillatory-zoned pyrite. Both types consist of a mineral inclusion-rich core and a clearer rim; however, in oscillatory-zoned pyrite, the latter is composed of complex rhythmic overgrowths of alternating As-rich and As-poor bands. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analysis and elemental mapping reveal the presence of invisible gold in all generations of pyrite. The highest concentrations (5–23 ppm Au) are found in oscillatory-zoned pyrite rims, which correlate with the highest As concentrations (16,000–23,000 ppm). In As-poor bands, Au (up to 1.5 ppm) and As (300–6,000 ppm) concentrations decrease by about an order of magnitude. Copper, Bi, Te, Sb, Pb, and Ag always occur with invisible gold, particularly in pyrite cores, suggesting that at least part of the gold occurs in sulfosalt nanoparticles of these metals and metalloids. Visible native gold grains occur as small inclusions throughout core and rim of both pyrite types, as well as in fractures within it. In both occurrences, chalcopyrite, sphalerite, tellurobismuthite, ankerite, albite, and chlorite accompany native gold, and gold fineness ranges between 900 and 930. At an early stage of vein mesh formation, pyrite formed in the metabasaltic fragments at the expense of ankerite, which, in turn, resulted from alteration of Fe-Ti oxides. Gold, together with other chalcophile elements, was incorporated within the structure of pyrite, most likely by destabilization of metal sulfide complexes during ankerite replacement. Subsequent cyclic reactivations of the shear zone caused development of pressure shadows around pyrite, generating local and repeated decreases in pressure, which triggered local boiling of the hydrothermal fluid, as evidenced by the presence of primary fluid inclusions containing immiscible liquid-rich and vapor-rich aqueous-carbonic fluids. This process was responsible for a number of physical-chemical changes in the liquid, all of which contributed to the formation of the As- and Au-rich overgrowths in pyrite: (1) removal of H 2 O into the vapor phase, inducing saturation of dissolved metals in the remaining liquid; (2) an increase in pH due to partition of H 2 S and CO 2 into the vapor, thus decreasing the solubility of sulfide minerals; and (3) an adiabatic decrease in temperature, lowering the solubility of As and Au in the liquid. Waning of this process restored precipitation of As-poor pyrite, until the onset of a new cycle. Because pressure drops are more significant adjacent to open spaces, oscillatory-zoned pyrite probably crystallized near newly formed veins whereas simple-zoned pyrite formed away from them. Previously formed pyrite underwent fracturing during reactivation of the deformation, especially through the brittle deformation events that postdated shearing, resulting in local pulverization of pyrite. This newly created porosity facilitated fluid circulation and remobilization of structurally bound gold, as well as of other chalcophile elements (Ag, Cu, Bi, Te, Pb, and Sb), which reprecipitated together with pyrite in the form of native gold, sulfides, and tellurides, either as small inclusions or as larger grains within fractures. This remobilization process facilitates the exceptionally high gold tenor found in the deposit, where the Colombia corridor is intersected by the Santa Maria fault.