Brittle-ductile Shear Zones Research Articles

Chloritisation along the Thanewasna shear zone, Western Bastar Craton, Central India: Its genetic linkage to Cu–Au mineralisation

The Cu±Au mineralisation of Thanewasna, Bastar Craton, Central India, is hosted in quartz–chlorite veins traversing granitoid rocks. Mineralisation and associated alteration are controlled by a NW–SE trending brittle–ductile shear zone. Large scale alkali magmatism in the form of K-rich granite emplacement is found in the vicinity of the shear zone. The emplacement of this granite (1587±14Ma) along the northern shoulder of the Godavari rift produced a high geothermal gradient, resulting in intense chloritisation and mineralisation. Field and petrographic studies show that the chlorite formed in two possible modes, viz. a) replacement of ferromagnesian minerals of the host rock, and/or b) precipitated directly from hydrothermal solutions. Mineral chemistry (EPMA) coupled with SEM and XRD studies reveals that chamosite with minor clinochlore varieties of chlorite is commonly associated with Cu and Au mineralisation. Chlorite shows variation in Fe/(Fe+Mg) ratios from 0.54 to 0.60, 0.55 to 0.64 and 0.49 to 0.55 and AlIV from 1.88 to 2.40, 1.91 to 2.50 and 1.80 to 1.94apfu respectively at the surface and in the sub-surface mineralisation and host rock.Empirical and thermodynamic geothermometers have been used to determine the temperature of chlorite formation based on chemical composition. Six empirical methods, based on AlIV, AlVI and Fe/Fe+Mg ratio showed a temperature range of 187°C to 381°C. Temperatures of chlorite formation have also been determined by the thermodynamic solid solution method. The thermodynamic equations of three intra-crystalline equilibria at 2kbar pressure show that temperature ranges from 215° to 454°C for low Si contents (<2.8apfu) and 110°–133°C for higher Si contents (Si>2.8apfu). The thermodynamic method of Vidal et al. (2001) and (2005) was not so compatible for an estimation of temperature of hydrothermal Fe-rich chlorites associated with Cu–Au mineralisation at the Thanewasna copper deposit (TCD).The temperature of chlorite formation increases from the host rock (mean—201°C) to proximal mineralisation zone (mean—234°C) to distal K–Na alteration zone (mean—251°C). Integrating field and laboratory results suggested that TCD chlorite is part of a structurally controlled high sulphidation epithermal system. This characterization would help in the regional exploration for base and precious metals and the results are applicable for the entire Western Bastar Craton (WBC) of Central India and similar deposit elsewhere.

Shear Sense Analysis of the Higher Himalayan Crystalline Belt and Tectonics of the South Tibetan Detachment System, Alaknanda-Dhauli Ganga Valleys, Uttarakhand Himalaya

In the central parts of Uttarakhand Himalaya, more than 20 km thick homoclinally NE-dipping Higher Himalayan Crystalline (HHC) Belt is thrust over the Lesser Himalaya Sedimentary Belt along the Main Central Thrust (MCT), and is almost continuously exposed between Helang and Malari along the Alaknanda and Dhauli Ganga valleys. The upper contact of this belt with the Martoli Formation of the Tethyan Himalayan Sequence is demarcated by the South Tibetan Detachment System (STDS). The belt is ubiquitously marked by small-scale asymmetrical structures like S-C and S-C' foliation, porphyroclasts and porphyroblasts, mineral fishes, intrafolial folds, duplex structures, ductile-brittle shear zones, and asymmetric shear boudins. Sense of ductile to brittle- ductile shearing has been determined from these structures across the whole belt, the MCT and the STDS, and reveals two phases of shear deformation: (a) an older top-to-SW upwards phase throughout the HHC, having an overall thrust geometry (DS1), and (b) a younger superposed top-to-NE downwards phase with normal fault sense from the middle to upper parts (DS2). These shear senses provide invaluable constraints on various tectonic models currently in use for the evolution of the Himalayan metamorphics.

Geological characteristics and ore-forming process of the gold deposits in the western Qinling region, China

The western Qinling, belonging to the western part of the Qinling–Dabie–Sulu orogen between the North China Block and South China Block, is one of the most important gold regions in China. Isotopic dates suggest that the Mesozoic granitoids in the western Qinling region emplaced during the Middle-Late Triassic, and the deposits formed during the Late Triassic. Almost all gold deposits in the western Qinling region are classified as orogenic, Carlin-type, and Carlin-like gold deposits, and they are the products of Qinling Orogenesis caused by the final collision between the North China Block and the South China Block. The early subduction of the Mian-Lue oceanic crust and the latter collision between South Qinling Terrane and the South China Block along the Mian-Lue suture generated lithosphere-scale thermal anomalies to drive orogen-scale hydrothermal systems. The collision-related magmatism also provided heat source for regional ore-forming fluids in the Carlin-like gold deposits.Orogenic gold deposits such as Huachanggou, Liziyuan, and Baguamiao lie between the Shang-Dan and Mian-Lue sutures and are confined to WNW-trending brittle-ductile shear zones in Devonian and Carboniferous greenschist-facies metasedimentary rocks that were highly-deformed and regionally-metamorphosed. These deposits are typical orogenic gold deposits and formed within a Late Triassic age. The deposits show a close relationship between Au and Ag. Ores contain mainly microscopic gold, and minor electrum and visible gold, along with pyrite. The ore-forming fluids were main metamorphic fluids. Intensive tectonic movements caused by orogenesis created fluid-migrating channels for precipitation locations. Although some orogenic gold deposits occur adjacent to granitoids, mineralization is not synchronous with magmatism; that is, the granitoids have no genetic relations to orogenic gold deposits. As ore-forming fluids converged into dilated fractures during the extension stage of orogenesis, changes of physico-chemical conditions resulted in fluid immiscibility that played a key role in gold and sulfide deposition.The geochemical and mineralogical characteristics of the Carlin-type deposits in the western Qinling region are similar to those in the Carlin trend, Nevada, USA. Gold deposits such as La’erma and Jinlongshan occur mostly in the southeastern margin of the western Qinling regionic region whereas some deposits occur in its eastern part. These deposits are hosted in slightly metamorphosed Cambrian to Triassic sedimentary rocks, showing structurally- and stratigraphically-controlled features. The deposits mainly contain submicroscopic and microscopic gold in arsenian pyrite and arsenopyrite, with characteristic ore-forming elements of Au–As–Sb–Ba. The ore-forming fluids are early-stocked formation water and later-recharged meteoric water. Meteoric water apparently evolved in ore-forming fluids by circulation, indicating the extensional setting, and led to the deposition of Au and other elements in cool reactive permeable rocks at shallow levels, forming the disseminated ores.Carlin-like gold deposits occur between the Shang-Dan suture and the Fengxian-Zhen’an fault. The host rocks are mainly sedimentary rocks that underwent reconstruction through reworking by structural metamorphism. These deposits are structurally controlled by brittle-ductile shear zone and occur adjacent to granitoid plutons. The most important characteristic that differ to the orogenic and Carlin-type gold deposits is the genetic relationship with the synchronous magmatism. Gold occurs mainly as microscopic gold. Pyrite and arsenian pyrite can be recognized as gold-bearing minerals. The ore-forming fluids are main magmatic water mixed with metamorphic and/or formation water. Similar to orogenic gold deposits, fluid immiscibility caused the deposition of gold Carlin-like gold deposits.

Geology of the Brenner Pass-Fortezza transect, Italian Eastern Alps

We present a 1:30,000 geological map resulting from detailed geostructural surveys carried out along the Italian segment of the design corridor for the Brenner Pass railway base tunnel (BBT), extending from Fortezza (Italy) to Innsbruck (Austria). The map covers the southern part of the Austroalpine-Penninic collisional wedge, the Periadriatic Fault System, associated Oligocene igneous bodies (Periadriatic magmatism) and part of the Southalpine basement. The Penninic Zone in the western Tauern Window is represented by the double domal structure of the Europe-derived Tux and Venediger-Zillertal basement and cover nappe system, capped by the ophiolitic Glockner nappe. The overlying Austroalpine nappe system is here represented by the polymetamorphic Merano-Mules basement and minor cover sheets. The Southern Alps domain includes the Bressanone Granite and pre-granite quartz-phyllites. Four Alpine ductile deformation phases have been recognized, followed by ductile-brittle shear zones, and finally brittle deformations along faults with extensional and strike-slip kinematics. The Quaternary is characterized by glacial deposits, large gravitational mass movements and landslides.

Infrared microthermometric and noble gas isotope study of fluid inclusions in ore minerals at the Woxi orogenic Au–Sb–W deposit, western Hunan, South China

The Woxi Au–Sb–W deposit, hosted by the Neoproterozoic low-grade metamorphic clastic rocks, is located in a brittle-ductile shear zone within the Xuefengshan Range, South China. Orebodies are predominantly banded quartz veins, which are strictly controlled by bedding faults and display significant vertical extents (up to 2km) without obvious vertical metal zoning. Fluid inclusions hosted in quartz, scheelite, and stibnite from quartz–scheelite and quartz–sulfide–gold veins have been studied using conventional and infrared microscopy, respectively. Four types of fluid inclusions were identified based on petrography, including type I (two-phase, liquid-rich aqueous inclusions), type II (two- or three-phase, CO2-rich inclusions), type III (two-phase, vapour-rich aqueous inclusions), and type IV (single-phase aqueous inclusions). The fluid inclusions in ore minerals (scheelite and stibnite) and their coexisting quartz largely share similar characteristics in terms of their types, homogenization temperatures and salinities. This is consistent with the fact that these ore minerals are always intergrown with quartz. Microthermometric and laser Raman data indicate a low-to-moderate temperature (140–240°C), low salinity (<7.0wt.% NaCl equiv.), CO2-rich, N2-bearing aqueous ore-forming fluid. Such fluid is further identified as a deeply non-magmatic crustal fluid rather than a mantle-source fluid by the significantly low 3He/4He ratios (0.002–0.281Ra), and a small amount of meteoric water or host-rock-buffered fluid could be involved. W ore precipitation was probably associated with mixing between a deeply-originated crustal fluid and host-rock-buffered fluid based on the fluid inclusion features in scheelite and quartz-I. However, Au and Sb ore deposition probably resulted from boiling which was caused by the marked pressure drop. Geological features (such as banded structure and crack-sealing structure) also indicate that fluid pressure fluctuation induced by fault-valve mechanism occurred during ore precipitation. These characteristics of the ore-forming fluids in the Woxi deposit are in good agreement with the definition of orogenic gold deposits and the Woxi Au–Sb–W deposit is probably an atypical orogenic gold deposit for its unique ore-forming element association.

Structure of the Variscan metamorphic complexes in the central transect of the Posada-Asinara Line (SW Gallura region, Northern Sardinia, Italy)

This study represents the first detailed tectono-metamorphic map of the metamorphic complexes cropping out in the inner portion of the Variscan belt in north-central Sardinia Island (Italy). The Main Map is at a 1:30.000 scale and covers an area of c. 148 km2. It is based on 1: 10.000 scale classic field mapping and represents an overview of the lithological and structural complexities documented in the metasedimentary and migmatite domes cropping out along the central transect of the Posada-Asinara Line (PAL). The PAL is a crustal scale discontinuity that divides migmatites from the metasedimentary sequences affected by greenschist- to amphibolite-facies metamorphism. The map shows the orientations of the superimposed foliations, fold axes and mineral lineations on the basis of geometric crosscutting relationships and, for the first time, the location of ductile-brittle and brittle shear zones developed during the long-lived activity of the Posada-Asinara Line.

Applying a general triclinic transpression model to highly partitioned brittle-ductile shear zones: A case study from the Torcal de Antequera massif, external Betics, southern Spain

Oblique convergence and subsequent transpression kinematics can be considered as the general situation in most convergent and strike-slip tectonic boundaries. To better understand such settings, progressively more complex kinematic models have been proposed, which need to be tested against natural shear zones using standardized procedures that minimise subjectivism. In this work, a protocol to test a general triclinic transpression model is applied to the Torcal de Antequera massif (TAM), an essentially brittle shear zone. Our results, given as kinematic parameters of the transpressive flow (transpression obliquity, ϕ; extrusion obliquity, υ; and kinematic vorticity number, Wk), suggest that the bulk triclinic transpressive flow imposed on the TAM was partitioned into two different flow fields, following a general partitioning type. As such, one flow field produced narrow structural domains located at the limits of the TAM, where mainly dextral strike-slip simple-shear-dominated transpression took place (Outer domains, ODs). In contrast, the remaining part of the bulk flow produced pure-shear-dominated dextral triclinic transpression at the inner part of the TAM (Inner domain, ID). A graphical method relating internal (ϕ, Wk) to far-field (dip of the shear zone boundary, δ; angle of oblique convergence, α) transpression parameters is proposed to obtain the theoretical horizontal velocity vector (V→), which in the case of the TAM, ranges between 099 and 118. These results support the applicability of kinematic models of triclinic transpression to brittle-ductile shear zones and the potential utility of the proposed protocol.

Geology of the Riacho do Pontal iron oxide copper-gold (IOCG) prospect, Bahia, Brazil: hydrothermal alteration approached via hierarchical cluster analysis

The Riacho do Pontal prospect is situated on the border between the Borborema Province and the São Francisco Craton, in Bahia state. It comprises rocks polydeformed during the Neoproterozoic. The prospect area includes migmatites and gneissic rocks intruded by several sin- to post-tectonic granites. Structural analysis indicates a strong relationship between the development of ductile to brittle-ductile shear zones and associated hydrothermalism. The main tracts of high-strain rate are represented by the Riacho do Pontal (north) and Macururé (south) shear zones. Several copper occurrences have been mapped within the Riacho do Pontal prospect along secondary shear zones. In these areas, the gneissic rocks were affected by intense hydrothermal alteration. Hierarchical cluster analysis permitted the identification of the main hydrothermal mineral associations present in these rocks, which resulted from potassic (biotite) and sodic-calcic (amphibole-albite) alteration, in addition to silicification and iron alteration (hematite). These hydrothermal alteration types are similar to those typically found in iron oxide copper-gold deposits developed at intermediate crustal levels. Hematite-quartz-albite-chalcopyrite-pyrite hydrothermal breccias host the highest-grade copper ore (chalcopyrite-pyrite-chalcocite) zones. The spatial relationship between copper deposits and shear zones improves the metallogenic potential for copper of the Borborema Province and has important implications for mineral exploration in the region.

The role of silica redistribution in the evolution of slip instabilities along subduction interfaces: Constraints from the Kodiak accretionary complex, Alaska

Quartz veins from a subhorizontal shear zone in the Kodiak accretionary complex in southwest Alaska record cycles of cracking and sealing contemporaneous with simple shear, and analysis of the vein array in the context of silica dissolution, diffusion, and precipitation leads to the conclusion that the time needed to seal after a fracture event overlaps with the time scales associated with the recurrence of slip instabilities along active megathrusts. The central belt of the Kodiak accretionary complex is interpreted as a shear zone developed adjacent to a paleo-decollement that was active in the Late Cretaceous based on large strain magnitudes, recumbent isoclinal folds, and regional cleavage patterns that depict rotation of the steep cleavage that characterizes most of the Kodiak Formation into the horizontal fabrics of the central belt. Within this regional shear zone, crack-seal veins composed of quartz with phyllosilicate inclusion bands are spaced at less than a cm but en echelon vein arrays are spaced at 500 cm. These arrays indicate top-to-the southeast shear on southeast dipping brittle-ductile shear zones. Vein textures indicate cracking and sealing of thin veins in the matrix, but thicker veins in en echelon sets record initial cracking and sealing followed by periodic collapse of fracture porosity at a frequency that is lower than the frequency of crack-seal cycles. Silica depletion zones in the wall rock adjacent to veins indicate local transport of silica by diffusion. By combining the maximum time in which diffusion could dominate over advection with the time needed to precipitate the quartz associated with one crack-seal band, we show that a reasonable range of ΔP, or pressure difference between matrix and crack, can produce crack seal bands in less than 10 days. This time frame is less than the recurrence of earthquakes, and similar to the duration of some slow earthquakes in nature. Therefore, silica redistribution could play a role in modulating the frequency of plate boundary slip instabilities along convergent plate boundaries.

Influence of Rock Foliation on Fracture Permeability: The Case of Precambrian metamorphics (Purulia, Eastern India)

Influence of Rock Foliation on Fracture Permeability: The Case of Precambrian metamorphics(Purulia, Eastern India) Hydrological influence of foliation in variably fractured Precambrian metamorphic rocks is usually difficult to predict. As an exemplar, the Precambrian metamorphic exposed in and around the Balarampur town of Purulia district, West Bengal (India) were studied to find out the influence of rock foliation on fracture permeability using a hydrostructural domain-based model. These Precambrian metamorphic rocks, characterized by foliated fabric, resulted from the combination of (a) brittle granite gneisses rich in quartz and garnet, (b) an intense brittle-ductile shear zone, and (c) ductile metapelites with micaceous minerals. Pre- and post-monsoon depth-to-groundwater table contour maps, mode, coefficient of variance (Cv) and contour maps of frequencies and apertures of fractures demonstrate inter- and intra-domain variable qualitative fracture permeability’s.

The Hongliuhe fold-and-thrust belt: Evidence of terminal collision and suture-reactivation after the Early Permian in the Beishan orogenic collage, Northwest China

The Early Permian strata of the Hongliuhe Formation are located in the Beishan orogenic collage, a tectonically significant region between the Tianshan orogen and the Southern Mongolian accretionary system, of the Central Asian Orogenic Belt (also called the Altaids). Stratigraphic reconstruction of the formation shows a general fining upwards sequence of rhythmically interbedded sandstones and siltstones with thick basal successions of conglomerates, all unconformably overlying an earlier mid-Paleozoic volcanic assemblage. The clast lithotypes of conglomerates successively change from being polymictic metamorphic tectonites at the base to a nearly monomictic granitoids mid-section, illustrating the unroofing sequence of an arc-related provenance. The intra-oceanic Gongpoquan composite arc that comprises most of the central Beishan fits the clast-lithotype profile of the conglomerates, and is the most probable provenance. The Hongliuhe Formation stratigraphy experienced a thin-skinned fold-and-thrust belt style of deformation that has a consistent southward vergence. Deformation in the lower stratigraphic levels created thrust imbrication of formation-scale blocks, each separated by steep, north-over-south brittle–ductile shear zones. The blocks are internally deformed with large-scale folding, with one significant block containing a sheath fold 15km in breadth. The upper stratigraphic levels accommodated strain through low-angle thrust ramping, shear folding, imbricate fanning and duplexing. This indicates that the Hongliuhe Formation was syn-orogenically deposited in a progressively deepening terrestrial- to nearshore environment within the Gongpoquan composite arc. Our observations and data indicate that the basin overlies an earlier Paleozoic suture zone. This implies that the northern half of the Gongpoquan composite arc overrode the southern half in a fault system reactivation centered about this zone of weakness, and created a stratigraphic and structural response consistent with a subduction- or collision-driven fold-and-thrust belt. Its southward structural vergence is supporting evidence of a north-dipping subduction-related fault system under the northern Gongpoquan. The timing of both the genesis of the Hongliuhe Formation and this deformation event coincides with the subduction of the Liuyuan Ocean or docking of the Dunhuang block further south, at the southern margin of the Gongpoquan composite arc.

Microfabric diversity and grain shape analysis of fault rocks from the selected areas of the Western Tatra Mountains

Fault rocks in brittle and brittle-ductile shear zones played a key role in the evolution of the Western Tatra Mountains crystalline rocks (Poland–Slovakia). Microfabrics of these rocks, including grain shape analyses, were investigated in the six areas of the Western Tatra Mountains. Based on studies of thin sections, 14 types of fault rock microfabric are distinguished, according to the following criteria: (a) the presence and abundance levels of a cataclastic matrix and (b) the presence and form of a preferred orientation features. General tendencies observed in these areas indicate southwards increasing non-coaxial deformation as well as the domination of ultracataclasites or ultramylonites to phyllonites in areas with negative relief (e.g., sedlo Zabrat’ Pass, Dziurawa Przełęcz Pass). A model of shear zone evolution embracing following three stages is proposed: (1) deformation partitioning and block-controlled cataclastic flow, (2) matrix-controlled cataclastic flow, (3) selective leaching and deposition of silica, leading to the formation of softened and hardened deformation domains respectively. These microstructural observations were supported by statistical analyses of the grain shape indicators (compactness, isometry, ellipticity, solidity, convexity). Two trends of relationships between compactness and convexity were noted: the first, horizontal on the correlation diagrams, was interpreted as an effect of rapid cataclasis and then sericitization, the second, with a strongly negative correlation coefficient, was considered as an effect of long-term cataclastic flow. The different microfabric data and microstructural interpretations described in this paper are consistent with a new model of the tectonic history of the Western Tatra Mountains evolution, with an important role for a non-coaxial deformation during Alpine orogeny in brittle and brittle-ductile conditions.

Field and ASTER imagery data for the setting of gold mineralization in Western Allaqi–Heiani belt, Egypt: A case study from the Haimur deposit

Although associated with carbonatized/listvenitized ophiolites and thrust structures, the morphology and internal structures of the auriferous quartz veins in the Haimur deposit suggest mineralization concurrent with NE–SW dextral brittle–ductile shear zones. The latter are attributed to intense transpression regime and are associated with (N)NE-trending tight to isoclinals folds that deform the early accretionary structures.Image processing techniques applicable to the Advanced Space-borne Thermal Emission and Reflection Radiometer (ASTER) data are used for mapping structures and hydrothermal alteration associated with the Haimur deposit. The automated lineament extraction by LINE module on high resolution ASTER imagery provides efficient data for potential dilation loci. Emphasis is placedon reliability of mineral indicesextracted fromthe ASTER band ratios for identification of possibly mineralized alteration zones associated with NE-trending shear zones.Field and remote sensing data, together with the structural fabrics along the lode-associated shear zones clearly constrain on the genetic relationship between the Haimur gold deposit and post-accretionary transpression/shearing. We conclude that hydrothermal alteration zones that are confined to tightly enfolded ophiolites and transpressive shear zones along the Western Allaqi–Heiani belt are most potential targets for new exploration plans.

The Tam Ky-Phuoc Son Shear Zone in central Vietnam: Tectonic and metallogenic implications

The Tam Ky-Phuoc Son Shear Zone (TKPSSZ) is one of the most pronounced crustal structures in central Vietnam and juxtaposes high-grade rocks of the Kontum Massif against lower grade rocks of the Truong Son Fold Belt. An internal zone comprises highly strained rocks, including ultramafic–mafic tectonites, bounded by mylonitic zones that straddle tens to hundreds of kilometers. An external zone comprises mainly high-grade metamorphic complexes (southern flank), and weakly metamorphosed siliciclastic and carbonate rocks with subordinate greenstone (northern flank). Detailed structural analysis reveals that the shear zone is a multi-deformed terrane: D1 produced a regional high grade schistosity and mylonitization; D2 generated regional northwest–southeast trending fold arrays and brittle–ductile shear zones; D3 locally produced northeast-trending folds; brittle faulting occurred during D4–D5. U–Pb dating of zircon and monazite suggests that D1 involved metamorphism and felsic magmatism at ca. 430Ma, recording part of a regional collisional orogeny. Monazite and titanite growth at ca. 250–240Ma in basement rocks is synchronous with widespread syn- to post-D2 magmatism between ca. 260 and 245Ma suggesting a second major collisional event during the Indosinian Orogeny. D3 may have occurred as part of (or soon after) this Permo-Triassic event. Deformation during post-collisional stages, perhaps in response to extrusion/wedging and oroclinal rotation of terranes, led to post-D3 structures (D4–D5). The E–W trending TKPSSZ is here shown to be a continuation of the N–S trending Po Ko Shear Zone (PKSZ). Most of the significant lode gold occurrences in central Vietnam occur along this TKPSSZ–PKSZ structure and are associated with, and controlled by, D1 ductile to ductile–brittle high-strain zones. Mineralized later-stage structures support remobilization and reconcentration of ore during subsequent events at ca. 240Ma (Re–Os molybdenite age). The TKPSSZ–PKSZ D1 structure likely represents part of a paleosuture zone, marking the closure of an ancient ocean basin through terrane assembly in the Early Paleozoic.

The mineralizing fluid in the Piaba gold deposit, São Luís cratonic fragment (NW-Maranhão, Brazil) based on fluid inclusion studies in quartz veins

Piaba is the first gold mine to operate in the Sao Luis cratonic fragment, NW-Maranhao, northern Brazil. The geological setting comprises chiefly metavolcano-sedimentary sequences (Au- rizona Group) and subduction-related granitoids (Tromai Intrusi- ve Suite), formed in island arc between 2240 and 2150 Ma. Gold mineralization is hosted in a fine-grained granophyric granodiorite (Piaba Granophyre) and in a subvolcanic andesite of the Aurizona Group. The mineralized zone is confined within the limits of the Piaba fault (ENE-WSW-trending brittle-ductile shear zone) and con- sists of quartz veins and veinlets and accompanying hydrothermal haloes (chlorite + muscovite + carbonate + pyrite + chalcopyrite + gold) disposed in stockwork geometry. Petrographic, microthermo- metric and microRaman spectroscopic studies of quartz have defined two- and three-phase aqueous-carbonic fluid inclusions produced by heterogeneous trapping during phase separation, in addition to late aqueous fluids. The mineralizing solution is an aqueous-carbonic fluid composed of CO 2 (5 – 24 mol%), H 2 O (74 – 93 mol%), N 2 (≤ 1 mol%) and CH4 (≤ 1mol%). It presents low salinity (5.5 wt.% NaCl equivalent) and density of 0.96 – 0.99 g/cm 3 ). Ore deposition occurred at 267 – 302 oC and 1.25 – 2.08 kbars, corresponding to 4 – 7 km in depth, in agreement with the structural information. The P-T-X and reduced characteristics (log ƒO 2 -31.3 to -34.3) of the fluid, combined with host rock sulfidation, altogether indicate that gold has been transported as a sulfur complex and that ore deposition occurred in response to phase separation and lowering of the sulfur activity and ƒO 2 during fluid-rock interaction.

Stress analysis, post-orogenic extension and 3.01 Ga gold mineralisation in the Barberton Greenstone Belt, South Africa

Gold deposits in the Barberton Greenstone Belt (BGB) have formed along a common network of brittle–ductile shear zones that developed during extension and emplacement of felsic porphyry dykes at ∼3015Ma. At a deposit scale, mineralised shears occur in en-echelon arrays, distributed as Riedel, P- and anti-Riedel shears, with envelopes that connect groups of mine workings along NNW, ENE, NE and ESE trends. The dip of the mineralised shear envelopes is generally moderate to steep (50–80°). The existence of the directional alignment of gold deposits at the scale of the greenstone belt has been demonstrated with Fry analysis.Stress inversion for mineralised shear zones from workings near 28 mines using a Bingham axial distribution and an optimized right dihedron approach provide near identical results with σ1 vertical and σ3 horizontal NW. The stress regime is generally radial extensional, or pure extensional. At the scale of the greenstone belt, the mineralised shear envelopes comprise two sets of near-orthogonal trends, positioned symmetrically with respect the measured far field stress: 1. A NW and NE trending pair with the latter accommodating pure normal slip and the former both dextral or sinistral strike-slip; and 2. A NNW and ESE trending pair accommodating dextral transtensional and sinistral transtensional movements respectively.The fact that all mineralised shears developed systematically in response to a similar far field stress suggests that mineralisation formed during a single tectonic stage. 207Pb/206Pb ages of 3017.1±17.8Ma and 3008.8±16Ma from a syn-mineralisation, porphyry dyke near Golden Quarry indicates gold deposition at ∼3015Ma; i.e. ∼85Ma after the craton had stabilised. Older porphyry dykes intruded near Sheba and Consort mines have been dated at ∼3100Ma and ∼3260Ma respectively, and provide a minimum and maximum age constraint on accretionary deformation in the area.We propose that the main phase of gold enrichment in the BGB occurred after tectonic and thermal stabilisation of the crust at a time the craton was acting as a stable terrane. Mineralised brittle–ductile shear zones allowed ingress of deeply sourced fluids during cratonic extension. Mine camps display a cluster spacing of ∼12km, which suggests a regular distribution of controlling shears, possibly as a function of the elastic rigidity of the lithosphere. Rather than linking mineralisation to orogenic processes during greenstone belt accretion, we link gold mineralisation to the opening of fault-bounded intracratonic basins including the Pongola and Witwatersrand Basins, and concomitant igneous activity.

Hydraulic Significance of Fracture Correlated Lineaments in Precambrian Rocks in Purulia District, West Bengal

Abstract Filter analysis of lineaments in Precambrian metamorphic rocks was used to delineate fracture-correlated lineaments and hydraulically significant fractures. The unfiltered analysis technique fails to show correlation between major lineaments and fractures. Domain-based and discrete filtering techniques successfully identify fracture-correlated lineaments within the brittle-ductile shear zone in conjunction with fractures characterized by high fracture frequencies (&amp;gt;10/m). The locales of hydraulically significant fractures can thus be assessed if the geological controls governing the spatial distribution of fracture frequencies are computed using structural domain approach. The concurrence of fracture-correlated lineaments and hydraulically significant fractures within the brittle-ductile shear zone is evident.

Geochemistry and mineral chemistry of lode gold mineralisation, SE Egypt: implications for ore genesis and exploration

Orogenic, lode gold mineralisation in the South Eastern Desert of Egypt is related to quartz veins spatially and temporally associated with conjugate NW- and NE-trending brittle–ductile shear zones. These structures are assumed to be linked to a regional transpression deformation which occurred late in the tectonic evolution of the area. In the Betam deposit, gold is confined to quartz(±carbonate) veins cutting through tectonised metagabbro and metasedimentary rocks in the vicinity of small granite intrusions. The ore bodies contain ubiquitous pyrite and arsenopyrite, in addition to minor disseminated chalcopyrite, pyrrhotite, galena, tetrahedrite and rare gold/electrum. New ore microscopy and electron microprobe studies indicate that most free-milling Au is intimately associated with the late-paragenetic galena–tetrahedrite–chalcopyrite assemblage. An early Fe–As sulphide assemblage, however, shows minor traces of refractory gold. New mineralogical and geochemical data are used to better constrain on possible element dispersions for exploration uses. This study indicates that parameters that most consistently define primary dispersion of gold in the mine area include pervasive silicification, sericite and carbonate alteration. The trace element data of gold lodes reflect a systematic dispersion of gold and certain base metals. Low-cost, extensive exploration programs may use elevated concentrations of Ag, Sb, Cu and Pb as tracers for Au ore zones in the Betam mine area and surroundings.

Analysis of thrust shear zones in curve-shaped belts: Deformation mode and timing of the Olevano-Antrodoco-Sibillini thrust (Central/Northern Apennines of Italy)

This work reports the results of our analysis of the brittle-ductile shear zone associated with the Olevano-Antrodoco-Sibillini (OAS) curved-shape thrust in the Central/Northern Apennines of Italy. Its southern sector is characterized by NNW–SSE trending footwall anticlines, which also affect the OAS thrust surface. S tectonites in the NNE–SSW trending sector are crenulated by conjugate extensional surfaces (extensional crenulation cleavage). The NW–SE trending segment of the OAS thrust is characterized by SC tectonites developed along the thrust-related shear zone. Both the SC and S tectonites are consistent with a top-to-the N60–70°E direction of tectonic transport.The SC tectonites are associated with a simple-shear-dominated deformation (Wn = 0.86–0.98). On the other hand, the S tectonites and their related extensional shear planes document two stages of strain: (i) sub-simple shear (Wn = 0.34–0.50, related to OAS thrusting contemporaneous to growing incipient footwall anticlines, and (ii) pure-shear-dominated deformation (Wn = 0.17–0.00), subsequent to the OAS thrusting and caused by the definitive growth of footwall anticlines within an in-sequence deformation context.The present study proposes the analysis of shear zones to: (i) discriminate in-sequence against out-of-sequence evolution, and (ii) use as a tool to constrain the modes and timing of the curved belt's development.

Petrology, Geochemistry and Fluid Inclusion Studies of Cu-Au Mineralization in Paleoproterozoic Salumber-Ghatol Belt, Aravalli Supergroup, Rajasthan

Abstract The Salumber-Ghatol belt in Rajasthan, India, situated along southern margin of the Aravalli Craton, hosts a cluster of Cu-Au deposits in calcitic and dolomitic marbles that belong to Debari Group of the Paleo-mesoproterozoic Aravalli Supergroup. The Fe-Mn rich dolomitic marble of the Delwara Formation hosts Cu-Au-Fe-oxide mineralization at Ghagri and associated distal K-Fe-Mg rich altered rocks (cryptocrystalline microcline + magnesioriebeckite + magnetite + phlogopite) and proximal feldspathised carbonate rocks (medium grained albite + microcline + dolomite + magnetite). The calcitic marble of Mukandpura Formation hosts Dugocha Cu-Au deposit with development of distal graphite-tourmaline-bearing albitites and proximal albite-microcline-magnetite rocks. Calcite and dolomite carbonates of Bhukia region with development of albite-actinolite-bearing alteration assemblages host the largest of the Cu-Au deposits in this belt. The second generation folds and associated ductile-brittle shear zones of the multiply deformed events constitute conduits for the mineralizing fluids at all locations in this belt. Carbon isotope data for the dolomitic marbles of Ghagri area suggest mixed isotopic signatures: (i) normal marine values close to zero per mil (mean= -0.269 ‰) and (ii) enriched positive values (mean= + 4.04 ‰) akin to the 1.9 Ga global event of positive excursion. The calcitic marbles of Dugocha area have significantly depleted δ13C values that are interpreted to be the result of pervasive inter-mixing of isotopically distinct carbon derived from primary depositional carbonates, hydrothermally exchanged carbonates, with significant influence of the organic carbon. The petrographic distinction of calcitic and dolomitic marbles of Bhukia area is corroborated by δ13C values that show a bimodal distribution of near 0 per mil and depleted values of -3 per mil. Fluid inclusion studies suggest predominance of saline brines in the ore fluids. The δ34S values for all the three deposits are narrowly constrained between 10.4 and 15.1 ‰, which suggests a common metasedimentary source for sulfur. Identical chondrite-normalized REE patterns and primitive mantle-normalized multi-element patterns suggest a common Na- and/or K-source for the formation of Ghagri and Dugocha-Bhukia feldspathic rocks. Based on mineralization styles and attendant alteration patterns, it is argued that the mineral deposits of Salumber-Ghatol belt in the Aravalli Supergroup (i) represent sedimentary carbonate-hosted Cu-Au deposits belonging to the wider class of Proterozoic Iron-Ore-copper-Gold type (IOCG-like) mineral deposits; (ii) the fluids responsible for the mineralization are generated from common sub-crustal source and (iii) during mineralization, the influx of Na-K-rich fluids is so intense that it created ‘artificial’ hyper-saline conditions in the normal depositional basin that facilitated crystallization of scapolite, brucite, spadaite, povondraite and phlogopite bearing mineral assemblage.