Trending Shear Zones Research Articles

Evidencia de una deformación rotacional varisca a partir de braquiópodos distorsionados (Meseta, Marruecos)

En 2021-2022, las obras de construcción de la carretera costera R322, que conecta la Kasbah de Oudayas (Rabat) con la localidad costera de Bouznika, han hecho posible estudiar esquistos con braquiópodos (Spirifer sp.) del Viseense sup. en la playa de Val d’Or, 12 km al suroeste de Rabat. El análisis estructural de las muestras recogidas indica una deformación de los fósiles en el plano XY del clivaje pizarroso (S1), afectados por pliegues recumbentes de vergencia oeste y por una foliación de crenulación (S2) asociada a los pliegues. Los fósiles de Spirifer sp. son triangulares, con forma de mariposa, y tienen el interés de mostrar simetría bilateral en el estado no deformado, con dos direcciones perpendiculares en T (seno o línea de simetría y charnela). Después de la deformación, estos ejes se cruzan con un ángulo agudo / obtuso que indica una deformación angular. Las medidas de los ángulos de cizalla nos llevan a reconstruir el círculo de Mohr para ángulos en el estado deformado, con objeto de obtener la forma de la elipse de deformación finita y determinar los mecanismos responsables de la distorsión utilizando el diagrama de forma de las elipses. Este método permite determinar la relación axial de la elipse de ladeformación finita mediante los círculos de Mohr. Los resultados se comparan con los obtenidos por el método de Wellman y el ábaco de Breddin. Finalmente, se propone un modelo estructural que explica las deformaciones dúctiles observadas como desarrolladas dentro zonas de cizalla dextras de dirección N-S.

The heterogeneous crustal architecture of the Falkland Plateau Basin

AbstractContinental break‐up can be oftentimes associated with intracontinental wrenching that can lead to the generation of transform margins and transform marginal plateaus. The wrenching phase can be accompanied by complicated processes, which result in heterogeneous structural and crustal architectures. This makes understanding the evolution of such tectonic settings challenging. The Falkland Plateau is an example of a transform marginal plateau where regional wrenching accompanied the incipient stages of Gondwanan continental break‐up to result in a mosaic of crustal types underlying its largest basin: the Falkland Plateau Basin (FPB). The uncertainties in crustal boundaries have led to several models for the evolution of the plateau, which hinder the development of a reliable plate reconstruction of Southern Gondwana. We integrate seismic reflection, gravity and magnetic data to propose an updated crustal architecture of the FPB. The results show that extended continental crust underlies the basin in the west and north. The eastern and central parts consist of a juxtaposition of intruded and underplated continental crust which transitions southwards to a thick oceanic domain. The basin is crosscut by three main NE–SW trending shear zones which facilitated the development of the contrasting crustal and structural domains interpreted across the plateau. This integrated reassessment of the FPB provides new insights into the tectonic evolution of the plateau, the deformation associated with wrenching and transform margin formation and our understanding of the tectono‐stratigraphic evolution of such areas.

Remote sensing and geochemical investigations of sulfide-bearing metavolcanic and gabbroic rocks (Egypt): Constraints on host-rock petrogenesis and sulfide genesis

Field geology, geochemical analyses and multi-sensor remotely sensed data of Landsat-8, ASTER and Sentinel-2 were used to detect various types of hydrothermal alteration and sulfide-gold mineralization in Neoproterozoic metavolcanic and gabbroic rocks from Darhib and El-Beida areas in the South Eastern Desert (SED) of Egypt. We also used RADAR data of Sentinel-1 using PCI Geomatica software to decipher the structural lineaments that control this mineralization. Different styles of disseminated to massive sulfide deposits include: 1) undeformed magmatic disseminated ores in Darhib gabbros, metavolcanic rocks and metabasic dykes; 2) deformed (remobilized) massive ores mostly of a magmatic-hydrothermal origin in Darhib tremolite talc rocks along E-W shear zones; 3) hydrothermal sulfide ores along El-Beida NW–SE shear zones. Cu-Zn massive ores represent the main sulfides in the Darhib silicified tremolite-talc rocks and meta-andesites, and occur as bands, veins and patches along Darhib E-W shear zones. Discontinuous occurrences of sulfide ores-bearing tremolite-talc rocks indicate that major shear zones act as structural controls on mineralization. Sulfide ores consist chiefly of chalcopyrite, sphalerite, pyrite and galena as well as minor covellite and bornite. These ores are highly concentrated along E-W striking Darhib shear zones and resulted from granite-derived hydrothermal fluids that form/and or accumulate sulfide minerals from the sulfide-bearing metavolcanic rocks around shear zones. Two stages of hydrothermal alterations are recorded in the Darhib talc mine including pre- (talcification, amphibolitization and carbonatization) and syn- (silicification and chloritization) sulfide ore alterations. Disseminated sulfide mineralization hosted in Darhib gabbros, metavolcanic rocks and metabasic dykes is of a magmatic origin, similar to Cu-Ni sulfide deposits hosted in gabbroic varieties in worldwide mafic–ultramafic intrusions. On the other hand, El-Beida gold-sulfide-bearing quartz and quartz carbonate veins are hosted in ferruginated and silicified metavolcanics/ metavolcaniclastic rocks along NW–SE shear zones (Najd Fault System trend). These are spatially associated with regional convergent structures (high-angle convergent wrench structures), suggesting orogenic gold type mineralization. The E-W with minor WNW-ESE shear zones in Darhib and NW-SE trending shear zones in El- Beida structurally controlled sulfides and gold mineralization in the SED of Egypt. El-Beida sulfide mineralization occurs as disseminated ores that consist mainly of pyrite, chalcopyrite, covelite and chalcocite with occasional occurrence of gold. The spatial occurrence of native gold in ferrigenous alteration zones indicates the role of iron in gold precipitation through buffering of gold-bearing complexes. Mineralogical and geochemical data of Darhib and El-Beida sulfide-bearing rocks indicate that they crystallized from depleted mantle sources in a back-arc basin setting, starting with the generation of arc-like volcanic rocks in a nascent BAB stage followed by the emplacement of plutonic equivalent of MORB/OIB-like rocks such as gabbroic intrusions in a mature BAB.

A tectonic model for the juxtaposition of granulite- and amphibolite-facies rocks in the Eburnean orogenic belt (Sassandra-Cavally domain, Côte d’Ivoire)

The Sassandra-Cavally (SASCA) domain (SW Côte d’Ivoire) marks the transition between the Archean Kenema-Man craton and the Paleoproterozoic (Rhyacian) Baoule-Mossi domain. It is characterized by the tectonic juxtaposition of granulite-facies and amphibolite-facies rocks. Migmatitic grey gneisses, garnet–cordierite–sillimanite migmatitic paragneisses and garnet–staurolite-bearing micaschists reached peak pressure conditions ranging from ∼ 6.6 kbar at 620 °C to ∼ 10 kbar at 820 °C. These conditions are associated with the first recorded deformation D1and correspond to a Barrovian geothermal gradient of ∼ 25 °C/km. Subsequent exhumation, associated with a second deformation D2, was marked by decompression followed by cooling along apparent geothermal gradients of ∼ 40 °C/km. A D3deformation phase is marked by folding and local transposition of the regional S1/S2foliation into E-W trending shear zones. LA-ICP-MS U–Pb dating of monazite, which displays complex internal structures, reveals four age groups correlated to textural position of monazite grains and analytical points: (1) rare relictual zones yield dates at the Archean-Paleoproterozoic transition (ca. 2400–2600 Ma); (2) a cluster of dates centered at ca. 2037 Ma on grains aligned along the S2foliation of the migmatitic grey gneiss, attributed to D2; (3) a cluster of dates centered at ca. 2000 Ma, and (4) dates spreading from ca. 1978 to 1913 Ma, documented for the first time in the West African Craton monazites. The ages of the latter two groups are similar to the ones identified in the Guiana Shield, and could be attributed to a disturbance by fluids, to a periodic opening of U–Pb system or to an episodic crystallization of monazite during slow cooling lasting several tens of Myrs. These data allow to propose a model for the tectonic evolution of the SASCA domain at the contact between the Rhyacian Baoule-Mossi domain and the Archean Kenema-Man nucleus whereby crustal thickening is achieved by crustal-scale folding and is followed by and concomitant with lateral flow of the thickened partially molten crust accommodated by regional transcurrent shear zones. This combination of crustal thickening controlled by tectonic forces and gravity-driven flow leads to the juxtaposition of granulite- and amphibolite-facies rocks.

Hiekkapohja hydrothermal system – ore mineral, lithogeochemical and paleomagnetic evidence from the Paleoproterozoic Central Finland Granitoid Complex

The Paleoproterozoic Svecofennian Central Finland Granitoid Complex (CFGC) has been regarded as an area of low mineralisation potential. The Hiekkapohja area, 20km north-east of the town of Jyväskylä, host a concentration of variable metalliferous showings. Samples from mineralised boulders and outcrops display variable combinations of anomalously high concentrations of Cu, Mo, Zn, Pb, W, Pb, Ag, As, and Au. The area is composed mainly of peraluminous and ferroan granitoids. The dominant porphyritic Hiekkapohja granodiorite (~1.88 Ga) is cross-cut by the equigranular Soimavuori granite of similar age. The porphyritic Lehesvuori granite on the western side of the study area represents marginally older (~1.89 Ga) magmatism. The paragenetic sequence of the ore minerals shows that the Hiekkapohja area has been affected by at least two separate stages of hydrothermal activity. The first mineralisation stage was widespread, crystallising typically chalcopyrite, pyrrhotite, sphalerite, galena, arsenopyrite, magnetite and Ag-bearing minerals. After the first stage, a low temperature oxidising phase formed hematite and marcasite. The second mineralisation stage enclosed low temperature minerals, such as marcasite and native Ag and Ag-minerals, as inclusions inside chalcopyrite, pyrite, pyrrhotite, sphalerite, and arsenopyrite. The mineralised samples typically display signs of K-metasomatism and less commonly signs of propylitic alteration. During the second mineralisation stage the fluid flow was controlled by the dominant 120°–135° trending shear zones. Both the hydrothermal activity and the regional geology indicate that porphyry type ore forming processes have occurred in the Hiekkapohja area. Paleoproterozoic resetting of the remanent magnetisation is further evidence for the role of the hydrothermal system.

A newly recognised 1860–1840 Ma tectono-magmatic domain in the North Australia Craton: Insights from the Tennant Region, East Tennant area, and the Murphy Inlier

New U-Pb monazite ages from amphibolite-facies metasedimentary rocks from the Tennant Region, Murphy Inlier and intervening East Tennant area, together with existing data, reveal the presence of an 1860–1840 Ma tectono-magmatic domain over 600 km long towards the centre of the North Australian Craton. In-situ ion probe U-Pb dating of biotite-hosted monazite in amphibolite-facies schist in the Tennant Region yielded an age of 1858 ± 7 Ma, which is attributed to north–south shortening (D1) at 1860–1855 Ma. Existing data indicate that D1 was associated with east–west trending, upright folds and mostly low-grade, regional metamorphism (M1) in the Tennant Region and the Murphy Inlier. The D1 event preceded voluminous and widespread felsic magmatism between 1855 and 1845 Ma. This included the emplacement of the Tennant Creek Supersuite, as well as the Yungkulungu Formation and equivalent stratigraphy, in the Tennant Region and in East Tennant, and the Nicholson Granite and Cliffdale Volcanics in the Murphy Inlier. Newly determined monazite ages from amphibolite-facies schist from the East Tennant area and the Murphy Inlier constrain a second episode of deformation and metamorphism (D2/M2) to ∼ 1845 Ma, coincident with the cessation of widespread magmatism. D2 is characterised by regional southeast to northeast trending shear zones. Phase equilibria modelling reveals that peak pressure–temperature (P–T) conditions during M2 in the East Tennant area were 2.8–3.3 kbar and 655–680 °C, indicating an extremely high apparent geothermal gradient (>190 °C/kbar) that was likely influenced by the preceding magmatism. Existing data indicate that D2 also affected the Tennant Region, where it coincided with significant Cu-Au-Bi mineralisation, albeit at significantly lower P–T conditions (sub-greenschist facies) than in the East Tennant area. The development of the 1860–1840 Ma tectono-magmatic domain, extending from west of the Tennant Region to east of the Murphy Inlier, marks an intermediate step in the migration of tectonism in the North Australia Craton, from the Arnhem Province in the north at 1880–1860 Ma to the Aileron and Tanami provinces in the south by ca. 1830 Ma.

Carboniferous-Permian tectono-metamorphic evolution of the Pelvoux Massif (External Crystalline Massif, Western Alps), with discussion on flow kinematics of the Eastern-Variscan Shear Zone

Based on new structural, petrological and U-Th-Pb geochronological data, a reappraisal of the Variscan tectono-metamorphic history of the Pelvoux Massif (External Crystalline Massif, French Alps) is proposed with the aim to understand the flow pattern and kinematics of the Variscan partially molten crust and the Eastern Variscan Shear Zone. The Pelvoux Massif consists of high-grade metamorphic rocks of middle to lower crust, mostly migmatites, that record a prominent syn-metamorphic deformation event (D2) characterized by a pervasive NE-SW striking, steeply dipping, S2 foliation, and a network of anastomosed NS and NW-SE trending shear zones, the kinematics of which indicates a sinistral transpression. Relics of an early syn-metamorphic event (D1/M1) related to crustal thickening and top-to-the-east nappe stacking are also reported. Both the D1 and D2 features are interpreted as reflecting a NW-SE shortening event, firstly marked by dominant nappe stacking, and secondly overprinted by a sinistral transpression that started at peak metamorphism with the onset of crustal partial melting at ca. 650 °C during the late Visean (ca. 335–330 Ma). Ongoing sinistral D2 transpression in the partially molten middle-lower crust of the Pelvoux involved strain partitioning between C and C’ shear zones and horizontal longitudinal flow in the range 330–300 Ma. Along the anatectic front, vertical shortening and top-to-the-NW shearing (D3) is coeval with D2 and argue for southeastward motion of the partially molten crust. The contemporaneity between NW-SE directed transpressional flow and vertical shortening is supported by our radiometric data of D2 and D3 and attests for strain partitioning between the suprastructure and infrastructure during horizontal crustal flow under transpressive regime. The exhumation of deep-seated rocks during sinistral transpression followed a near isothermal (ca. 700 °C) evolution down to pressure of ca. 0.5 GPa in the period 325–306 Ma. The sinistral transpression recorded in the Pelvoux Massif might corresponds to an antithetic shear zone coeval with the dextral East-Variscan Shear Zone, proposed for this part of the Variscan orogen.

Geology of late-Variscan Sàrrabus pluton (south-eastern Sardinia, Italy)

ABSTRACT This paper deals with the geological mapping of the late-Variscan Sàrrabus pluton, (south-eastern Sardinia), a shallow multiple and composite igneous complex dominated by several generations of granodiorites, metaluminous and peraluminous granites and repeated pulses of mantle-derived mafic magmas. The map has been compiled based on geological surveys at 1:10,000 and 1:5,000 scales, assisted by in situ gamma-ray spectrometry and detailed petrographic investigations. Granite-related ore deposits have been also reported. The emplacement age of the pluton can be constrained by U/Pb dating on zircons of Cala Regina granodiorite, yielding an age of 286 ± 9 Ma. The resulting scenario documents a bimodal magmatism controlled by an EW trending shear zone, followed by the shallower emplacement of several pulses of independent granite magmas.

Remote sensing and structural studies of the Central Asir Shear Zone, Western Arabian Shield: Implications for the late Neoproterozoic E-W Gondwana assembly

The Central Asir Shear Zone (CASZ) is a N- to NNE-oriented inhomogeneous strain zone parallel to the Nabitah shear belt in southern Saudi Arabia. It is ~200 km long and 20 km wide, and traverses the Asir terrane. The deformed terrains in the vicinity of the CASZ have experienced a prolonged history involving three successive phases of deformation: arc-collision, dextral transpression and sinistral strike-slip faulting. Remote-sensing data analysis and field investigation provide insights into the geological and structural features along the CASZ. Image transformation and enhancement reveal faults and folds, and discriminate between granitoids, metavolcanics, gabbros and younger (Ablah Group) sediments. The obtained evidence points to polyphase deformation of the area, characterized by narrow zones of high strain, folding and shearing enclosing slices of less deformed rocks. Lineament analysis shows predominantly NNE, NNW, N and NE trends of folds and shear zones, and NW, NE and E fracture trends. The CASZ has a >100 m.y. long deformation history, including a change from dextral to sinistral shearing. Both shearing senses are found on the older N-S and younger NW-SE fault systems. N-S oriented shearing was accompanied by intrusion of granitoids and localized hydrothermal alteration along N to NNE trending shear zones. Strain analysis shows low strain in the Ablah basin sediments, consistent with Ablah deposition after the tectonic climax in the region.

Late orogenic gold mineralization in the western domain of the Karagwe-Ankole Belt (Central Africa): Auriferous quartz veins from the Byumba deposit (Rwanda)

Gold mineralization in the KAB (Karagwe-Ankole Belt) of Central Africa dominantly occurs as quartz lode and ferruginous breccia systems. Primary gold mineralization in the KAB is possibly linked to a deformational event in the early-Neoproterozoic which is accompanied by shear movements and post compressional magmatism. The Byumba deposit is chosen due to its position in the KAB Mesoproterozoic metasedimentary stratigraphy (Akanyaru Supergroup), interesting structural trend (NW trending shear zone) and its closeness to magmatic intrusions. Explorative drillings from the Byumba deposit in Rwanda were logged and sampled for further petrographic investigation, in situ Rb-Sr geochronology (Laser Ablation Inductively-Coupled-Plasma Triple Quadrupole Mass Spectrometry) and Micro X-ray fluorescence (µXRF) major and trace element mapping. Distinct phases of folding and shear deformation are present at Byumba and three main phases of quartz veining are identified (pre-, syn-, post-folding). The first quartz vein generation is layer-parallel and folded along the bedding, indicative of a pre-folding origin (V1 generation). A second generation of quartz veins (V2a) is concentrated in fold hinges and related to the primary folding of the layers. Cleavage parallel and often boudinaged V2b quartz veins are emplaced during late-stage folding. Post-folding (V3) massive or sigmoidal quartz veins, crosscut the folds and cleavage. Element mapping of the gold by using µXRF shows that Chlorite-rich quartz veins (V3) host the primary gold mineralization in the form of sub-micron gold and of small disseminated blebs. Cores additionally recorded a supergene enrichment phase of gold at reduction zone boundaries. In situ Rb-Sr geochronology on white mica of the crosscutting post-folding quartz veins (V3) point towards an Early-Neoproterozoic age of ∼900 Ma. The gold mineralization at Byumba is interpreted to be related to the regional early-Neoproterozoic compressional deformation event based on 1) the post-folding timing of the auriferous veins, 2) the observation of shear-related textures both in the host-rock and the veins, and 3) the location of the Byumba deposit within a regional shear structure. This association with compressional shearing points towards an orogenic origin for gold mineralization. However, a magmatic-hydrothermal influence from contemporaneous granitic magmatism cannot be excluded.

Geochemical characterization of mineralized albitite from Paleoproterozoic Bhukia IOCG‐IOA deposit of Aravalli‐Delhi Fold Belt, Rajasthan, western India: Genetic linkage to the gold (±Cu ± U) mineralization

Paleoproterozoic Bhukia gold deposit is located within the Debari Group of Aravalli Supergroup, Rajasthan, northwestern India. The major lithounits that are associated with gold–graphite (±Cu ± U) mineralization exposed in Bhukia are albitite, carbonates, and quartzite which have undergone upper greenschist to amphibolite facies metamorphism. Ore mineralization viz. pyrrhotite, arsenopyrite, chalcopyrite, pyrite, graphite, magnetite, and goethite in the study area are associated with NNW–SSE trending shear zone. Nuggets of gold are identified in the thick gossan zones that occurs parallel to the shear zone. The gold–sulfide lodes of the Bhukia deposits are largely confined either to marble or albitized marble and pure monomineralic albitite. The mineralization that is associated with the albitite, follows the boundaries of albite grains which confirms the role of these weak zones for the mobilization of mineralizing fluid and concentration of metal ions. Hence, geochemical studies of albitite would help to evaluate the petrogenesis, likely magmatic affinity, tectonic setting, controls of the mineralizing fluid, source, and their linkage to the gold (±Cu ± U) metallogeny. Bhukia albitite is characterized by Na–Al–rich and K‐poor compositions along with metaluminous nature. Geochemical analysis of various key elements suggests that albitite is the product of low‐K calc‐alkaline magma series with variation in Nb/Ta ratio indicating their fractionation during the melt–rock interaction within the shear zone. Geochemical signatures suggest the role of upper continental crust in channelizing the mineralized fluids that generated albitite with felsic to intermediate composition in and around Bhukia deciphered from their low Nb/Y ratios. Lower concentration of compatible elements (Ni and Cr) and higher concentration of incompatible elements (Th, La, and Nd) with SiO2 suggest that evolution of albitite rock is related to the fractional crystallization process. Zr/Hf ratios of 9–35 with the average of 18 suggest that albitite formed under magmatic–hydrothermal transitional environment. Rare Earth Elements (REE) patterns of albitite samples are moderately fractionated showing Light Rare Earth Elements (LREE)‐enrichment with a rather steep slope and exhibit distinct negative Eu anomalies (0.42–0.71, average 0.57). The enrichment of LREE in comparison to the Heavy Rare Earth Elements (HREE) suggests a more evolved source, i.e., process of melting at shallow environments. Tectonic setting of albitite samples suggest their origin in a volcanic arc region that underwent rifting. Detailed geochemical characterization unravelled complex evolutionary history of albitite in the Bhukia area, which attests derivation of the parental melt possibly from an enriched SCLM (Sub‐continental lithospheric mantle) source similar to most of the IOCG‐IOA deposits worldwide.

Structural features and age of gold mineralization in the Troia Massif, Borborema Province, NE Brazil: A Paleoproterozoic (∼2029 Ma) hypozonal orogenic gold deposit overprinted by the late Neoproterozoic Brasiliano/Pan–African orogeny

Hypozonal orogenic gold mineralization has been recently recognized in the Troia Massif, an Archean/Paleoproterozoic basement inlier within the Neoproterozoic Borborema Province, NE Brazil. The mineralization occurs in association with quartz veins hosted by the amphibolite facies Paleoproterozoic Serra das Pipocas greenstone belt. The main mineralized areas are located near parallel to lithological contacts, siting on shear zones established at the metavolcanic and metasedimentary boundary. Four deformation events are recognized: (1) Dn was responsible for the early Sn foliation, parallel to the bedding (So) of the greenstone units. (2) Dn+1 is characterized by a pervasive, southeasterly–dipping Sn+1 foliation that is axial–planar to several asymmetric, tight to isoclinal and recumbent folds. (3) Dn+2 represents a strike–slip NE–trending shear zones, and (4) the late Dn+3 event is characterized by ductile–brittle deformation. Evolution from Dn+1 to Dn+3 is interpreted as a progressive deformation, with the maximum compressive stress in the WNW–ESE direction. The main stage of gold mineralization is associated with the Dn event, and comprised by deformed quartz veins and associated high–temperature calc–silicate alteration (diopside, hornblende, titanite) and albitization of the host rocks. Lower temperature gold (±Te, Ag) occurs along late stage brittle structures (Dn+3). The U–Pb titanite age of 2029 ± 28 Ma for the calc–silicate alteration of the early gold mineralization is similar to the age of many Transamazonian/Eburnean gold mineralizations elsewhere. However, the strong Pb loss of titanite grains defines a 574 ± 7 Ma lower intercept age, indicating that early gold mineralization was affected (remobilized) by Neoproterozoic deformational events and metamorphism (Brasiliano/Pan–African orogeny). This is in agreement with the U–Pb zircon age of 575 ± 3 Ma obtained for the syn–tectonic Guaribas dikes, which bracketed the age of Dn+3. Therefore, as Dn+1, Dn+2 and Dn+3 evolved by progressive deformation; they are all of Neoproterozoic age. This two–stage gold mineralization is here interpreted as product of crustal exhumation, with (1) early hypozonal orogenic gold mineralization occurring shortly after the high–grade Paleoproterozoic metamorphism and first exhumation processes of the greenstone pile; and (2) late gold mineralization occurring at shallow levels (second exhumation process), associated with the Neoproterozoic Brasiliano/Pan–African orogeny.

The Sukari Neoproterozoic granitoids, Eastern Desert, Egypt: Petrological and structural implications

The Sukari granitoid pluton is situated in the southern central Eastern Desert of Egypt and is considered as one of the best examples of the gold–bearing granites in the Arabian Nubian Shield (ANS). It consists of coarse– to medium–grained alkali–feldspar granite, syenogranite and monzogranite intruded the volcano–sedimentary sequences and metagabbro–diorite complex. Geochemically, these granitoids are metaluminous to peraluminous in character. They are characterized by positive Rb, K, Zr and Pb and negative Ba, Sr, P, Nb and Ti anomalies and exhibit similar trace element patterns. Moreover, they show pronounced LREE–enriched patterns and negative Eu anomalies that indicate plagioclase fractionation. The studied rocks are typical of within plate granites generated in tensional environment. They belong to ferroan/A–type granites, particularly A2–type suggesting post-collisional setting. Their high SiO2, Al2O3 and low MgO contents along with the low Ce/Pb ratios reflect derivation from melting crustal source.The major structures of the Sukari area encompass Akbar Wahid, Golden Boy, Buthinae and Kaolinitic Faults. The area is characterized by NNE-NE trending shear zone and WNW-EW stress orientation. The mineralized structures contain quartz veins trending mostly EW, NS and NNW; joints with NS, EW, NNE and NNW trends; faults with NNW, NNE, NS and EW trends; and breccia with NNW trend. The kinematic indicators are represented by mylonites, quartz fish and augen structures. Mineralization in Sukari area found as quartz veins, stockworks and brecciated veins.

Iron-copper mineralization associated with metagabbro in Mirawa Village, Kurdistan region, northeastern Iraq

Mirawa iron-copper mineralization in Mirawa village, Kurdistan region, occurs as a zone of 15 m thick within the metagabbro of the Mawat Ophiolite. It is hosted by brecciatedamphibolite facies gabbroic rocks with a northwest–southeast trending shear zone. Amphibole, magnetite, plagioclase, quartz, epidote, chlorite, clinopyroxene and chalcopyriterepresent the mineral constitutes of metagabbro. Metamorphic conditions have been estimated from the green metamorphic amphibole replacing primary magmatic clinopyroxene (M1, 500 < T < 560 °C; 4.3 < P < 5.2 kbar) and retrograde actinolite (white amphibole) replacing the green amphibole due to continuous decreasing pressure and temperature (M2, 415 < T < 480 °C; 1.9 < P < 2.3 kbar). Occurrence of quartz with ironcopper mineralization hydrothermal fluids played a significant role during metamorphism. Chlorite and epidote were formed at the late stage of metamorphism. Iron-copper mineralization occurred in two paragenetic stages. Coarse grains of magnetite, chalcopyrite and pyrite formed during the first stage of mineralization representing syngenetic mineralization. Magnetite is the main Fe-ore mineral and it is characterized by large euhedral to subhedral grains with ilmenite exsolution lamellae. The second stage is epigenetic where mineralization is characterized by the formation of secondary minerals, replacing the primary minerals. Hematite replaces primary magnetite along the rim. Chalcopyrite, pyrite and magnetite were replaced by goethite whereas covellite overgrows chalcopyrite forming a corona texture. Fine-grained idomorphic aggregates of magnetite are dominant in the second stage and seem to be formed from iron liberated from pyroxene and other iron-bearing minerals. Microstructural orientation of the coarse-grained magnetite indicates a NW-SE direction for the maximum stretching of strain ellipse.

Shear-Controlled Gold Mineralization of G. R. Halli Area of Chitradurga Schist Belt, Dharwar Craton: Insights from Fluid Inclusion Study

Gold mineralization at G. R. Halli is located along the Central shear zone of Chitradurga schist belt, extending from west of Gonur, through east of G. R. Halli and C. K. Halli to east of Honnemardi, roughly parallel to stratigraphic units. The NNW-SSE trending shear zone has a width of 0.5 to 1.5 km shows extensive carbonatization of metabasalts and the associated lithologies confined to NNW-WNW trending arcuate brittle-ductile zone. The sheared and silicified contact zones between carbonaceous argillite and schistose metabasalt form the potential sites for localization of mineralization. The gold is associated with sulphides mainly pyrite, arsenopyrite, galena, sphalerite, minor chalcopyrite. Textural relationship indicates two stage sulphide mineral assemblages co-relatable with two stage fluid ascents having temperature of homogenization between 125°C and 256°C. It is a typical epigenetic lode gold system, which got affected by later deformation.

Magma mixing and gold mineralization in the Maevatanana gold deposit, Madagascar

The Maevatanana gold deposit in Madagascar is hosted by Archean metamorphic rocks in quartz–sulfide veins that are structurally controlled by NNW–SSE trending shear zones. Fluid inclusion data show that the trapping conditions in quartz range from 0.87 to 2.58kbar at temperatures of 269–362°C. Laser Raman spectroscopy confirms that these inclusions consist of CO2, SO2, and H2O. The δ34S values of the pyrites range from 1.7‰ to 3.6‰, with an average of 2.25‰, supporting a magmatic origin. Noble gases (He, Ne, Ar, Ke, Xe) are chemically inert, thus will not be involved in chemical reactions during geological processes. Also due to the low concentration of He in the atmosphere and the low solubility of He in aqueous fluids, the atmosphere-derived He is unlikely to significantly affect He abundances and isotopic ratios of crustal fluids, ensures that He production should have the typical crust 3He/4He ratios. The 3He/4He ratios of fluid inclusions in pyrite from the deposit range from 0.06 to 0.12Ra, while the 40Ar/36Ar ratios range from 6631 to 11441. We infer that the ore-forming fluids could have been exsolved from a granitic magma. The oxygen and hydrogen isotope compositions of the ore-forming fluids (1.5‰≤δ18OH2O≤7.8‰; –72‰≤δD≤–117‰) indicate they were derived from a granitic magma. Four pyrite samples from the gold deposit yield a precise Re–Os isochron age of 534±13Ma. Given that the post-collisional granites in northern and central Madagascar were derived by melting of sub-continental lithospheric mantle and formed between 537 and 522Ma, we can state that the gold metallogenesis was coeval with the crystallization age of these parental magmas. These data could be accounted for the formation of the Maevatanana gold deposit. First, the shear zones hosting the deposit formed around 2.5Ga, when the Madagascan micro-continental blocks collided with other continental blocks, triggering large-scale tectono-magmatic activity and forming NNW–SSE trending shear zones. The gold mineralization at Maevatanana is coeval with the crystallization age of the Cambrian post-collisional A-type granitoid plutons in northern and central Madagascar, implying that this deposit is associated with extensional collapse of the East African Orogen. This extension in turn induced asthenospheric upwelling, melting of sub-continental lithospheric mantle. These magmas underplated the lower crust, generating voluminous granitic magmas by partial melting of the lower crust. The mixing magma during tectono-thermal reactivation of the East African Orogen produced large volumes of volatiles that extracted gold from the granitic magma and produced Au–S complexes (e.g., Au(HSO3)2−). The shear zones, which were then placed under extensional collapse of the East African Orogen in the Cambrian, formed favorable pathways for the magmatic ore-forming fluids. These fluids then precipitated gold-sulfides that form the Maevatanana gold deposit.

Hoggar geochronology: a historical review of published isotopic data

A dataset of more than 400 isotopic ages on the Hoggar Shield, published from 1963 to 2017, was obtained by increasingly precise isotopic dating techniques and low-temperature thermochronology. Data were arranged by eras and terranes and classified in two categories “before 1980” and “after 1980”. They illustrate the protracted geological history of the Hoggar Shield. The first continental nuclei were formed 3.5–2.5 Ga ago during the Archean, with high-grade metamorphic and associated magmatic episodes. A second group of continental terranes was created 2.40–1.75 Ga ago during the Paleoproterozoic, with Eburnean orogenic episodes marked by reworking of older Archean terranes associated with juvenile terranes. After the 1.80–0.90 Ga long period of quiescence, the 870–540 Ma Neoproterozoic times were characterized by Pan-African episodes, with early overthrusting of eclogitic nappes and late strike-slip movements along north-south trending shear zones, high-grade metamorphism and anatexis, emplacement of large granitoid batholiths followed by complexes of the Taourirt igneous suite. Cambrian hydrothermal activity evidences either a slow cooling process, or more likely discrete thermal pulses. After scarce Carboniferous mafic magmatism, the Mesozoic and the beginning of the Cenozoic constituted a period of quiescence marked by subsidence and burial after the Early Cretaceous. Low-temperature chronology records episodes of alternating subsidence and exhumation. Widespread Eocene exhumation predated volcanic activity beginning in the Late Eocene and continuing until recent times, in association with Africa–Europe convergence processes.

Soil-gas radon anomalies in three study areas of Central-Northern Calabria (Southern Italy)

Soil-gas radon concentrations and exhalation rates have generally been observed to be anomalously high along active faults in many parts of the world. The soil-gas method is based on the principle that faults and fractures in rocks are highly permeable pathways along which gases can migrate upward from deep crust and mantle to soil cover, retaining their source signatures. The present study summarizes the influence of fault zones on anomalous radon concentrations in soil by integrated geophysical and geo-structural analyses in three study areas of Central-Northern Calabria (Southern Italy). Soil-gas radon surveys have been carried out by means of an alpha scintillation counting system, at 12,509 locations between 2002 and 2004. A geostatistical approach has been used to estimate the spatial distribution of soil radon concentrations. Relations among soil-gas distribution and geo-structural features have been evaluated by ordinary multi-Gaussian kriging. Highest soil radon concentrations (ca. 90 kBq m−3) have been measured in the Rossanese sector. In the three study areas, no appreciable differences can be noticed among lithotypes, with the highest concentration values (ca. 89 kBq m−3) measured in alluvial deposit and in clay. Measurements of soil-gas radon reveal anomalies clearly connected to the tectonic structures. Increased signals are linearly distributed along regional WNW–ESE trending shear zones, with main pathways of concentration also recognizable along the E–W fault system in the Rossanese sector, the N–S fault system in the Crati Graben and the Catanzaro Trough, and the NE–SW fault system in the Catanzaro Trough. The distribution of epicentres of historical earthquakes occurred between 1184 and 2001 confirms the recent activity of the same fault systems. Soil-gas radon concentrations generally increase, as expected, with decreasing distance to the faults.

Ore minerals and geochemical characterization of the Dungash gold deposit, South Eastern Desert, Egypt

The Dungash historic gold mine is located in the South Eastern Desert of Egypt. The gold-bearing quartz veins are hosted by the metavolcanic and metavolcaniclastic rocks along an ENE–WSW trending shear zone. Alteration types recorded in the wall rocks are sericitization, silicification, carbonatization, chloritization, sulfidization, ferruginization, and listwanitization. The ore mineral assemblage comprises arsenopyrite, pyrite, native gold, pyrrhotite, sphalerite, chalcopyrite, and galena. The primary sulfide mineral assemblage formed during a hypogene hydrothermal stage whereas anglesite and goethite occur as secondary supergene phases. Microthermometric fluid inclusion analysis revealed that the auriferous quartz precipitated from a moderately saline (5 to 11.22 wt% NaClequiv) solution at temperatures above the recorded homogenization temperatures (Th), which range from 380 to 177 °C. The minimum pressures of trapping are between 350 and 400 bars. The fluid evolution during mineralization is explained by mixing of a magmatic fluid with meteoric waters. Initially, the high temperature and moderately saline magmatic fluid dominated and progressively became diluted with meteoric waters. Highest gold content is recorded in the carbonatized zone and the quartz veins. However, gold content in the carbonatized zone of the footwall exceeds several times its content in the quartz veins and the carbonatized zone of the hanging wall.

Aquifer recharge, depletion, and connectivity: Inferences from GRACE, land surface models, and geochemical and geophysical data

Data from the Gravity Recovery and Climate Experiment (GRACE) and outputs of the CLM4.5 model were used to estimate recharge and depletion rates for large aquifers, investigate the connectivity of an aquifer’s subbasins, and identify barriers and preferred pathways for groundwater flow within an aquifer system. The Nubian Sandstone Aquifer System and its subbasins (Dakhla, Northern Sudan Platform, and Kufra) in northeast Africa were used for demonstration purposes, and findings were tested and verified against geological, geophysical, remote sensing, geochronologic, and geochemical data. There are four major findings. (1) The average annual precipitation data over recharge areas in the southern Kufra section and the Northern Sudan Platform subbasin were estimated at 54.8 km 3 , and 32.8 km 3 , respectively, and knowing the annual extraction rates over these two areas (∼0.40 ± 0.20 km 3 ), recharge rates were estimated at 0.78 ± 0.49 km 3 /yr and 1.44 ± 0.42 km 3 /yr, respectively. (2) GRACE-derived groundwater depletion rates over the Dakhla subbasin and the Northern Kufra section were estimated at 4.44 ± 0.42 km 3 /yr and 0.48 ± 0.32 km 3 /yr, respectively. (3) The observed depletion in the southern parts of the Dakhla subbasin is apparently caused by the presence of the east-west−trending Uweinat-Aswan basement uplift, which impedes the south-to-north groundwater flow and hence reduces replenishment from recharge areas in the south. (4) A major northeast-southwest−trending shear zone (Pelusium shear system) is apparently providing a preferred groundwater flow pathway from the Kufra to the Dakhla subbasin. Our integrated approach provides a replicable and cost-effective model for better understanding of the hydrogeologic setting of large aquifers worldwide and for optimum management of these groundwater resources.