Localized Shear Zones Research Articles

Petrology and mineralogy of the La Peña igneous complex, Mendoza, Argentina: An alkaline occurrence in the Miocene magmatism of the Southern Central Andes

The La Peña alkaline igneous complex (LPC) is located in the Precordillera (32°41′34″ S - 68°59′48″ W) of Mendoza province, Argentina, above the southern boundary of the present-day flat-slab segment. It is a 19 km2 and 5 km diameter subcircular massif emplaced during the Miocene (19 Ma) in the Silurian-Devonian Villavicencio Fm. The LPC is composed of several plutonic and subvolcanic intrusions represented by: a cumulate of clinopyroxenite intruded by mafic dikes and pegmatitic gabbroic dikes, isolated bodies of malignite, a central intrusive syenite that develops a wide magmatic breccia in the contact with clinopyroxenite, syenitic and trachytic porphyries, a system of radial and ring dikes of different compositions (trachyte, syenite, phonolite, alkaline lamprophyre, tephrite), and late mafic breccias. The main minerals that form the LPC, ordered according to their abundance, are: pyroxene (diopside, hedenbergite), calcium amphibole (pargasite, ferro-pargasite, potassic-ferro-pargasite, potassic-hastingsite, magnesio-hastingsite, hastingsite, potassic-ferro-ferri-sadanagaite), trioctahedral micas (annite-phlogopite series), plagioclase (bytownite to oligoclase), K-feldspar (sanidine and orthoclase), nepheline, sodalite, apatite group minerals (fluorapatite, hydroxylapatite), andradite, titanite, magnetite, spinel, ilmenite, and several Cu–Fe sulfides. Late hydrothermal minerals are represented by zeolites (scolecite, thomsonite-Ca), epidote, calcite and chlorite.The trace element patterns, coupled with published data on Sr-Nd-Pb isotopes, suggest that the primary magma of the LPC was generated in an initially depleted but later enriched lithospheric mantle formed mainly by a metasomatized spinel lherzolite, and that this magmatism has a subduction-related signature. The trace elements pattern of these alkaline rocks is similar to other Miocene calc-alkaline occurrences from the magmatic arc of the Southern Central Andes.Mineral and whole-rock chemical compositions support the interpretation that a first batch of a tephritic magma produced a cumulate of clinopyroxenite (clinopyroxene + magnetite + apatite) and a residual melt that crystallized as malignite at a shallow emplacement level (<5 km). Fractional crystallization in a deep chamber, coupled with rock assimilation, produced successive magma pulses that gave the composite central syenite, the syenitic and trachitic porphyries, and trachytic dikes. The latest rocks (phonolites) reflect an extreme fractionation with Ca, Al and K removal by feldspars. Mingling relationships between tephrites-basanites and trachytes-phonolites, plus compositional variations linked to the reabsorption surface observed in the K-feldspar from the phonolitic dikes, suggest recharge events with local mixing in the late-stages of the LPC evolution.The emplacement of the LPC melts at a shallow crustal level was favored by the aperture of extensional NNW-SSE fractures genetically linked to a local brittle shear zone, active at 18–19 Ma during a strong compressional event, that initiated the regime of flat-slab subduction in this part of the Andes.

Downdip variations in seismic reflection character: Implications for fault structure and seismogenic behavior in the Alaska subduction zone

AbstractSeismic reflection data collected offshore of Alaska Peninsula across the western edge of the Semidi segment show distinctive variations in reflection characteristics of the megathrust fault with depth, suggesting changes in structure that may relate to seismic behavior. From the trench to ~40 km landward, two parallel reflections are observed, which we interpret as the top and bottom of the subducted sediment section. From ~50 to 95 km from the trench, the plate interface appears as a thin (&lt;400 ms) reflection band. Deeper and farther landward, the plate interface transitions to a thicker (1–1.5 s) package of reflections, where it appears to intersect the fore‐arc mantle wedge based on our preferred interpretation of the continental Moho. Synthetic waveform modeling suggests that the thin reflection band is best explained by a single ~100 to 250 m thick low‐velocity zone, whereas the thick reflection band requires a 3 to 5 km thick zone of thin layers. The thin reflection band is located at the center of the 1938 Mw 8.2 Semidi earthquake rupture zone that now experiences little interplate seismicity. The thick reflection band starts at the downdip edge of the rupture zone, correlates with a dipping band of seismicity, and projects to the location of tremor at greater depth. We interpret the thin reflection band as a compacted sediment layer and/or localized shear zone. The thick reflection band could be caused by a wide deformation zone with branching faults and/or fluid‐rich layers, representing a broad transition from stick‐slip sliding to slow slip and tremor.

Evolution of permeability across the transition from brittle failure to cataclastic flow in porous siltstone

AbstractPorous sedimentary rocks fail in a variety of modes ranging from localized, brittle deformation to pervasive, cataclastic flow. To improve our understanding of this transition and its affect on fluid flow and permeability, we investigated the mechanical behavior of a siltstone unit within the Marcellus Formation, PA USA, characterized by an initial porosity ranging from 41 to 45%. We explored both hydrostatic loading paths (σ1 = σ2 = σ3) and triaxial loading paths (σ1 &gt; σ2 = σ3) while maintaining constant effective pressure (Pe = Pc – Pp). Samples were deformed with an axial displacement rate of 0.1 μm/s (strain rate of 2 × 10−6 s−1). Changes in pore water volume were monitored (drained conditions) to measure the evolution of porosity. Permeability was measured at several stages of each experiment. Under hydrostatic loading, we find the onset of macroscropic grain crushing (P*) at 39 MPa. Triaxial loading experiments show a transition from brittle behavior with shear localization and compaction to cataclastic‐flow as confining pressure increases. When samples fail by shear localization, permeability decreases abruptly without significant changes in porosity. Conversely, for cataclastic deformation, permeability reduction is associated with significant porosity reduction. Postexperiment observation of brittle samples show localized shear zones characterized by grain comminution. Our data show how zones of shear localization can act as barriers to fluid flow and thus modify the hydrological and mechanical properties of the surrounding rocks. Our results have important implications for deformation behavior and permeability evolution in sedimentary systems, and in particular where the stress field is influenced by injection or pumping.

Influence of high strain rate deformation on40Ar/39Ar mica ages from marble mylonites (Syros, Greece)

Interpreting isotopic ages as deformation ages when they are acquired from moderate-temperature metamorphic environments can be a challenging task. Syros Island (Cyclades, Greece) is famous for Eocene high-pressure metamorphic rocks reworked by localized Miocene greenschist-facies deformation. In this work, we investigate phengites from coarse-grained marbles, which experienced the high-pressure event, and phengites from fine-grained localized marble shear zones attributed to the low-grade Miocene deformation. Based on structural criteria, both events can be easily discriminated because of their opposing kinematics. Laser-heating 40Ar/39Ar analysis on phengite yielded a 40 ± 1.6 Ma age for the host rock and a 37 ± 1.3 Ma age for the shear zone. Both ages are statistically indistinguishable, consistent with the regional Eocene event, and not the Miocene deformation event responsible for the formation of the shear zone. Thermodynamic modeling indicates that the observed high-variance mineral assemblage is stable without compositional change along the pressure-temperature path followed by the rocks of Syros. Although the marble within the shear zone was deformed at extremely fast strain rates (10−10 s–1), we observed no intracrystalline deformation of phengite grains and no resetting in the isotopic system, because strain was mostly accommodated by calcite. Consequently, a high strain rate does not necessarily create deformation ages in rocks with high-variance assemblages, such as marble mylonites.

From symmetric necking to localized asymmetric shearing: The role of mechanical layering

We study the formation of localized shear zones during the layer-parallel extension of viscous multi-layers using two-dimensional numerical simulations based on the finite-difference method. For power-law viscous layers and a linear viscous embedding medium, the extended multi-layer develops boudins due to necking. For power-law viscous layers embedded in a power-law viscous medium, the extended multi-layer develops first distributed necks, and subsequently a localized shear zone with a vertical offset (with a size of several layer thicknesses) along the multi-layer. During the extension, the deformation style switches from distributed and symmetric necking to localized and asymmetric shearing. A localized shear zone develops in the viscous multi-layer although the rheology is everywhere strain-rate-hardening (power-law stress exponent >1) and no material softening and/or energy feedback mechanism (e.g., shear heating combined with a temperature-dependent viscosity) is applied. The shear localization is caused by structural softening because the formation of a localized shear zone decreases the bulk resistance and hence the work required to deform the multi-layer. A localized shear zone forms in the multi-layer when the distance between the stiff layers is approximately equal to or less than the layer thickness. The shear localization was observed in multi-layers with nine and with only three stiff layers.

Localization and delocalization of deformation in a bimineralic material

AbstractWe investigate how localization and delocalization of deformation occurs in a bimineralic material composed of a strong plagioclase and a weaker quartz phase. We perform numerical, meter‐scale shear experiments in which we vary the temperature and the ratio of the two mineral phases. Three micromechanical deformation fields are identified according to the mechanical behavior of the minerals at play (brittle or ductile when both phases are in the brittle or ductile regime, respectively, and semibrittle when one phase is in the brittle and the other in the ductile regime). Besides these micromechanical deformation fields, we identify three deformation types characterizing the degree of localization (type I: localized shear zone, type II: localized anastomosing shear zone, and type III: delocalized shear zone). Type I is expected in the brittle deformation field. In the semibrittle field, all deformation types can be observed depending on the amount of weak phase present. In the ductile field, deformation is dependent on the strength ratio between the two phases. For a low strength ratio, deformation of type III is always observed. For high‐strength ratios, deformation of type II can be observed for a moderate amount of weak phase. A small amount of weak phase (&lt;10%) reverses the mechanical behavior of the strong phase and leads to the formation of a narrow anastomosing shear zone (type II) where fully ductile (type III) behavior is expected. This highlights the importance of a bimineralic material for the deformation localization and overall large‐scale deformation processes.

Analysis of adhered contacts and boundary conditions of the secondary shear zone

Prediction of the tribological parameters controlling the tool wear is one of the most complex research axes in the metal cutting literature. The nature of the friction on the tool–chip interface is the main process which influences the distribution of stresses and temperatures which in turn activates the thermomechanical process governing tool wear. Under extreme conditions of temperature, strain rates and pressure, occurring especially in high speed machining, the adhered contact phenomenon is highly localized especially near to the tool tip and extremely non-linear due to strong influence of the secondary shear zone (SSZ) and the nature of bonds between asperities of tool and chip. In addition, the analysis based on post-mortem examinations of chips and worn tools, especially in hard metal cutting alloy and high speed machining, show that the adhered friction with intimate contact and no relative motion of the material chip on the interface, are the principal cause of appearance of the plastic deformation layers (SSZ). This localized shear zone plays a role of intensive heat sources interacting with the tool side and, in turn, activates diffusive and abrasive wear. In this work, a hybrid model combining analytical and numerical approaches is performed in order to solve the non-linear thermomechanical problem on the chip and predicts the nature of friction contact, i.e. fully sliding, sticking/sliding or fully sticking contact, and these for a given distribution of asperities, characterizing the ratio between the real and apparent contact areas Ar/An on the interface, and a given global friction coefficient μ¯, characterizing the ratio of the experimental cutting forces. The shear stress generated in the primary shear band, the energy produced in the secondary shear zone, the local friction coefficient and the friction energy produced on the sliding part of contact, the proportion of the sticking/sliding interfaces, and geometrical parameters of the chip, are obtained by analytical means. The analysis of this model is based on experimental data and applied for a large cutting speed (1ms−1≤V≤60ms−1) and able to give some correlation about the distribution of adhesions marks on the contact with respect to the distribution of the tribological parameters at the interface. This model can be used in improving tool wear prediction and the estimation of tool-life.

Cultivation of shear stress sensitive and tolerant microalgal species in a tubular photobioreactor equipped with a centrifugal pump.

The tolerance to shear stress of Tetraselmis suecica, Isochrysis galbana, Skeletonema costatum, and Chaetoceros muelleri was determined in shear cylinders. The shear tolerance of the microalgae species strongly depends on the strain. I. galbana, S. costatum, and C. muelleri exposed to shear stress between 1.2 and 5.4 Pa resulted in severe cell damage. T. suecica is not sensitive to stresses up to 80 Pa. The possibility to grow these algae in a tubular photobioreactor (PBR) using a centrifugal pump for recirculation of the algae suspension was studied. The shear stresses imposed on the algae in the circulation tubes and at the pressure side of the pump were 0.57 and 1.82 Pa, respectively. The shear stress tolerant T. suecica was successfully cultivated in the PBR. Growth of I. galbana, S. costatum, and C. muelleri in the tubular PBR was not observed, not even at the lowest pumping speed. For the latter shear sensitive strains, the encountered shear stress levels were in the order of magnitude of the determined maximum shear tolerance of the algae. An equation was used to simulate the effect of possible damage of microalgae caused by passages through local high shear zones in centrifugal pumps on the total algae culture in the PBR. This simulation shows that a culture of shear stress sensitive species is bound to collapse after only limited number of passages, confirming the importance of considering shear stress as a process parameter in future design of closed PBRs for microalgal cultivation.

Small quantity but large effect — How minor phases control strain localization in upper mantle shear zones

Low viscosity domains such as localized shear zones exert an important control on the geodynamics of the uppermost mantle. Grain size reduction and subsequent strain localization related to a switch from dislocation to diffusion creep, is one mechanism to form low viscosity domains. To sustain strain localization, the grain size of mantle minerals needs to be kept small over geological timescales. One way to keep olivine grain sizes small is by pinning of mobile grain boundaries during grain growth by other minerals (second phases). Detailed microstructural studies based on natural samples from three shear zones formed at different geodynamic settings allowed the derivation of the olivine grain-size dependence on the second-phase content. The polymineralic olivine grain-size evolution with increasing strain is similar in the three shear zones. If the second phases are to pin the mobile olivine grain boundary the phases need to be well mixed before grain growth. We suggest that melt–rock and metamorphic reactions are crucial for the initial phase mixing in mantle rocks. With ongoing deformation and increasing strain, grain boundary sliding combined with mass transfer processes and nucleation of grains promotes phase mixing resulting in fine-grained polymineralic mixtures that deform by diffusion creep. Strain localization due to the presence of volumetrically minor minerals in polymineralic mantle rocks is only important at high strain deformation (ultramylonites) at low temperatures (<~800°C). At smaller strain and stress conditions and/or higher temperatures other parameters like overall energy available to deform a given rock volume, the inheritance of mechanical anisotropies or the presence of water or melts needs to be considered to explain strain localization in the upper mantle.

The Moho beneath western Tibet: Shear zones and eclogitization in the lower crust

The Tibetan Plateau is formed by continuing convergence between Indian and Asian plates since ∼50 Ma, involving more than 1400 km of crustal shortening. New seismic data from western Tibet (the TW-80 experiment at 80°E) reveal segmentation of lower crustal structure by the major sutures, contradicting the idea of a mobile lower crust that flows laterally in response to stress variations. Significant changes in crustal structure and Moho depth occur at the mapped major tectonic boundaries, suggesting that zones of localized shear on sub-vertical planes extend through the crust and into the upper mantle. Converted waves originating at the Moho and at a shallower discontinuity are interpreted to define a partially eclogitized layer that extends 200 km north of the Indus–Yarlung Suture Zone, beneath the entire Lhasa block at depths of between 50 and 70 km. This layer is thinner and shallower to the north of the Shiquanhe Fault which separates the northern Lhasa block from the southern part, and the degree of eclogitization is interpreted to increase northward. The segmentation of the Tibetan crust is compatible with a shortening deformation rather than shear on horizontal planes. Unless the Indian-plate mantle lithosphere plunges steeply into the mantle beneath the Indus–Yarlung suture, leaving Indian-plate crust accreted to the southern margin of Tibet, then it too must have experienced a similar shortening deformation.

Electrical resistivity cross-section across the Garhwal Himalaya: Proxy to fluid-seismicity linkage

Magnetotelluric (MT) measurements along a profile cutting across the Garhwal Himalaya of India are inverted to obtain 2-D electrical resistivity structures of the Himalayan wedge and of the underthrusting Indian plate. The imaged resistivity cross-section is dominated by a low-angle north-east dipping intra-crustal high conducting layer (IC-HCL) with an average thickness of 5km. At transition from the Lesser Himalaya to the Higher Himalaya, the IC-HCL is marked by a ramp structure across which its top jumps from a depth of 8km to 13km. High conductivity of the layer is caused by pounding of upward propagating metamorphic fluids trapped by tectonically induced neutral buoyancy. In compression regime of the Himalaya, the mechanical weakening effects of the fluids counteract the fault-normal stresses, thereby facilitating thrust-type earthquakes on a plane imaged as the top of the IC-HCL. It is suggested that in the Himalaya collision belt, like the active subduction zone, the active seismic plane forming seat of large and great earthquakes is located a few kilometers above the top of the down-going plate. In this tectonic setting, the high conductance ramp symbolizes a block of low shear strength and high strain, which under the deviatoric stresses release accentuated stresses into the brittle crust, thereby generating small but more frequent earthquakes in the narrow Himalayan Seismic Belt. In response to either the co-seismic pumping or the stress transfer during inter-seismic period, the upward infiltration of fluid fluxes into the over pressurized zones sufficiently reduces the shear strength of local thrusts and shear zones, turning these into locales of concentrated seismicity.

Upper–greenschist facies intragrain deformation of albite in mylonitic meta–pegmatite and the influence of crystallographic anisotropy on microstructure formation

We studied the deformation behaviour of albite from Permian meta–pegmatite in Cretaceous upper–greenschist facies shear zones from the Austroalpine Matsch Unit in the Eastern Alps (Italy). Sodium–feldspars from these rocks provide excellent natural examples for studying mechanisms of intragranular deformation under mid–crustal conditions in grains with different angular relations between their (010) planes and the kinematic frame. The studied rocks were deformed at c. 500 °C in localized shear zones with well characterized top–W shear kinematics supposedly during the Cretaceous upper–greenschist facies tectonometamorphic event. Microstructural and chemical data suggest that crystallographic anisotropies in albite exert a strong control on microstructure formation and that albite primarily deformed by a combination of brittle fracturing, dissolution–precipitation and incipient crystal plasticity as a function of the orientation of the crystallographic anisotropy relative to the supposed shortening direction. Dissolution along discontinuities forming stylolites perpendicular to the shortening direction is associated with the precipitation of fine–grained albite with some compositional variability (Ab96–98 and Ab89–91) in cracks. New albite precipitates form aggregates with straight segments of high angle grain boundaries, nearly 120° dihedral angles and only a poor or no orientation relation to the hosting clast. Intragranular kinking is related to continuous lattice rotation of up to 15° by a misorientation axis close to albite [100] and the formation of subgrain boundaries with maximum misorientations of 7°. Synthetic microshear zones supposedly nucleated on pre–existing cracks, and are associated with formation of subgrain boundaries in shortening quadrants and cracks together with precipitates of potassium feldspar in extensional quadrants adjacent to the microshear zone. New microstructural and textural data from mylonitic Permian meta–pegmatites document various closely linked crystal plastic and brittle deformation mechanisms and highlight the role of crystallographic anisotropies and their orientation with respect to the kinematic frame in microstructure formation.

Strain rate dependent calcite microfabric evolution – An experiment carried out by nature

A flanking structure developed along a secondary shear zone in calcite marbles, on Syros (Cyclades, Greece), provides a natural laboratory for directly studying the effects of strain rate variations on calcite deformation at identical pressure and temperature conditions. The presence and rotation of a fracture during progressive deformation caused extreme variations in finite strain and strain rate, forming a localized ductile shear zone that shows different microstructures and textures. Textures and the degree of intracrystalline deformation were measured by electron backscattered diffraction. Marbles from the host rocks and the shear zone, which deformed at various strain rates, display crystal-preferred orientation, suggesting that the calcite preferentially deformed by intracrystalline-plastic deformation. Increasing strain rate results in a switch from subgrain rotation to bulging recrystallization in the dislocation-creep regime. With increasing strain rate, we observe in fine-grained (3 μm) ultramylonitic zones a change in deformation regime from grain-size insensitive to grain-size sensitive. Paleowattmeter and the paleopiezometer suggest strain rates for the localized shear zone around 10−10 s−1 and for the marble host rock around 10−12 s−1. We conclude that varying natural strain rates can have a first-order effect on the microstructures and textures that developed under the same metamorphic conditions.

Granite-hosted molybdenite mineralization from Archean Bundelkhand craton-molybdenite characterization, host rock mineralogy, petrology, and fluid inclusion characteristics of Mo-bearing quartz

The dominantly high-K, moderate to high SiO2 containing, variably fractionated, volcanic-arc granitoids (± sheared) from parts of Bundelkhand craton, northcentral India are observed to contain molybdenite (Mo) in widely separated 23 locations in the form of specks, pockets, clots and stringers along with quartz ± pyrite ± arsenopyrite ± chalcopyrite ± bornite ± covellite ± galena ± sphalerite and in invisible form as well. The molybdenite mineralization is predominantly associated with Bundelkhand Tectonic Zone, Raksa Shear Zone, and localized shear zones. The incidence of molybdenite is also observed within sheared quartz and tonalite–trondhjemite–granodiorite (TTG) gneisses. The fluid inclusion data show the presence of bi-phase (H2O–CO2), hypersaline and moderate temperature (100°–300°C) primary stretched fluid inclusions suggesting a possible hydrothermal origin for the Mo-bearing quartz occurring within variably deformed different granitoids variants of Archean Bundelkhand craton.

Position-dependent shear-induced austenite–martensite transformation in double-notched TRIP and dual-phase steel samples

While earlier studies on transformation-induced-plasticity (TRIP) steels focused on the determination of the austenite-to-martensite decomposition in uniform deformation or thermal fields, the current research focuses on the determination of the local retained austenite-to-martensite transformation behaviour in an inhomogeneous yet carefully controlled shear-loaded region of double-notched TRIP and dual-phase (DP) steel samples. A detailed powder analysis has been performed to simultaneously monitor the evolution of the phase fraction and the changes in average carbon concentration of metastable austenite together with the local strain components in the constituent phases as a function of the macroscopic stress and location with respect to the shear band. The metastable retained austenite shows a mechanically induced martensitic transformation in the localized shear zone, which is accompanied by an increase in average carbon concentration of the remaining austenite due to a preferred transformation of the austenite grains with the lowest carbon concentration. At the later deformation stages the geometry of the shear test samples results in the development of an additional tensile component. The experimental strain field within the probed sample area is in good agreement with finite element calculations. The strain development observed in the low-alloyed TRIP steel with metastable austenite is compared with that of steels with the same chemical composition containing either no austenite (a DP grade) or stable retained austenite (a TRIP grade produced at a long bainitic holding time). The transformation of metastable austenite under shear is a complex interplay between the local microstructure and the evolving strain fields.

Failure characteristics of three-body model composed of rock and coal with different strength and stiffness

Four different types of three-body model composed of rock and coal with different strength and stiffness were established in order to study the failure characteristics of compound model such as roof-coal-floor. Through stress analysis of the element with variable strength and stiffness extracted from the strong-weak interface, the tri-axial compressive strength of the weak body and strong body near the interface as well as the areas away from the contact surface was found. Then, on the basis of three-dimensional fast Lagrangian method of continua and strain softening constitutive model composed of Coulomb-Mohr shear failure with tensile cut-off, stress and strain relationship of the four three-body combined models were analyzed under different confining pressures by numerical simulation. Finally, the different features of local shear zones and plastic failure areas of the four different models and their development trend with increasing confining pressure were discussed. The results show that additional stresses are derived due to the lateral deformation constraints near the strong-weak interface area, which results in the strength increasing in weak body and strength decreasing in strong body. The weakly consolidated soft rock and coal cementation exhibit significant strain softening behavior and bear compound tension-shear failure under uni-axial compression. With the increase of confining pressure, the tensile failure disappears from the model, and the failure type of composed model changes to local shear failure with different number of shearing bands and plastic failure zones. This work shows important guiding significance for the mechanism study of seismic, rock burst, and coal bump.

Effects of multiaxial cyclic loading conditions on the evolution of porous defects

Multiaxial loading conditions are one of the important parameters in estimating the lifetime of structure both in high and low cycle fatigue ([1 3]). In order to understand the coupling between the macroscopic multiaxial loading and the microscopic defects, we propose to investigate the evolution of an elasto-plastic porous material up to failure under low cycle fatigue conditions. The analysis is performed numerically, using finite elements, on a periodic 3D unit-cell under the assumption of finite strains and subjected to various stress triaxialities, translated as ratios between deviatoric, hydrostatic stress and Lode angles. The present discussion introduces several novel factors in the analysis: (i) 3D geometry in cyclic loading (ii) finite strains (iii) free evolving void shape (iiii) different hardening laws. That one of the important factors is the void shape and that its evolution during cyclic loading depends on its multiaxiality. Moreover, these factors will equally influence the apparent macroscopic hardening or softening of the material and the initiation of localized shear zones at the microscopic level. The Lode angle has a significant impact on the evolution of the aspect ratios and the ellipsoidicity of the pores, but has only a weak influence on the evolution of macroscopic variables such as the stress or the porosity. As a consequence, the results show that multiaxiality of the loading have an important on the evolution and growth of defects, pores in the present case problem, but are less important in the definition of the yield surface.

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.

Metasomatic hydration of the Oeyama forearc peridotites: Tectonic implications

In contrast to the widely recognized aspects of serpentinization, initial stages of hydration and tectonic processes of unserpentinized peridotites are still unclear, but have important implications for understanding the lithospheric architecture of supra-subduction zones. This study provides petrological evidence from the Oeyama ophiolite, SW Japan, of the effects of high-temperature metasomatic hydration immediately before the cooling and ductile deformation of forearc peridotites. Key findings in this study are: 1) complex association of high-temperature metasomatic minerals: tremolitic amphibole, cummingtonite, phlogopite, chlorite, olivine and orthopyroxene in veins and in mylonites; 2) the systematic variation in Si and Na+K contents of the tremolitic amphibole, corresponding to its mode of occurrence and mineral association; and 3) the presence of thin (<0.7mm) veins of fine-grained olivine accompanied by a narrow diffusion zone of the host primary olivine. On the basis of petrography and mineral chemistry, the temporal sequence of hydration and deformation of the Oeyama ophiolite is considered as follows: 1) infiltration of slab-derived fluids, causing decomposition of primary pyroxene and chemical modification of primary olivine, 2) metasomatic formation of variable modal amounts of amphibole, phlogopite, chlorite, vein-forming olivine and secondary orthopyroxene at 650–750°C; 3) early-stage mylonitization of the hydrous peridotites in localized shear zones; and 4) syntectonic serpentinization at 400–600°C to form serpentinite mylonites. Paragenesis and amphibole compositions suggest comparable temperature conditions for metasomatism and early-stage mylonitization. Mylonitization occurred exclusively in hydrous peridotites, and the peridotite mylonites were preferentially overprinted by syntectonic serpentinization. Diffusion profiles of olivine cut by a vein suggest rapid cooling immediately after the metasomatic fluid infiltration. From these observations and calculations, it is concluded that the exhumation of the forearc peridotites was closely related to the infiltration of high-temperature metasomatic fluids and hydration occurred under a wide range of temperature conditions.

Genesis of Neoproterozoic Au-bearing volcanogenic sulfides and quartz veins in the Ar Rjum goldfield, Saudi Arabia

The Ar Rjum goldfield is an example of late Neoproterozoic Au mineralization that is hosted by submarine arc assemblage and syn-anorogenic intrusive rocks. Apart from ancient workings, recent exploration in the goldfield defined three main targets along 3km N–S corridor (Um Na'am, Ghazal and Wasema), and indicated that Wasema alone hosts 11.8Mt @ 2.5g/t Au. The majority of gold and sulfide mineralization is confined to diorite, where gold content increases with shearing, pyrite–sericite–carbonate alteration and development stockworks of quartz–carbonate–pyrite veins and stringers. Generally, the concentration of gold increases in the diorite samples that experienced variable degrees of hydrothermal alterations near local shear zones. Anomalous gold content (up to 11.76g/t) in some metachert is the result of the remobilization of volcanogenic lattice-bound (refractory) Au into free Au due to post-metamorphic hydrothermal alterations. The chemistry of pyrite from the mineralized veins and stringers indicates considerable amounts of gold that reaches ~0.3wt.%.Chlorite that co-exists with pyrite in the hydrothermally altered metavolcanics is mostly sheridanite with up to ~25wt.% FeOt and minor amounts of ripidolite. Chlorite geothermometry suggests that two temperature ranges affecting the area. The first temperature range (290–334°C) is consistent with regional greenschist facies metamorphism, and the second (306–355°C) is interpreted to be related to recrystallization-submarine hydrothermal alteration related to the gold mineralization. Stable isotope (δ34S, δ18O and δ13C) data suggest an original volcanogenic arc signature that has been slightly modified by low-grade metamorphism, and finally by the late interaction of hydrothermal fluids. Ore evolution model for the Ar Rjum goldfield includes seafloor sulfide alteration, several deformation episodes and intrusive effects, and in this context the ore resulted from the reduction of seawater sulfates. The gold-rich veins interpreted as orogenic lode deposits are confined to localized shear zones in a syn-orogenic diorite.