Neoproterozoic Ophiolites of the Arabian-Nubian Shield

Characterization of talc deposits in ultramafic rocks of Gebel El Maiyit and its economic feasibility

A comprehensive, multiscale investigation, integrating remote sensing, mineralogy, whole rock chemistry, Electron Microprobe (EMP), and stable isotopes (oxygen-18O and carbon-13C), was undertaken to assess the feasibility of talc deposits and their host serpentinite at Gebel El-Maiyit in the Eastern Desert of Egypt. Sentinel 2 remote sensing images were applied to discriminate talc from serpentinites followed by geochemical study of serpentinites using RO`/SiO2 ratios, AFM diagram and MgO versus SiO2 relationship indicates a peridotite origin formed at low temperature Alpine type. Our study revealed that talc deposit has a varied mineralogical composition and according to the predominant talc and gangue minerals three main types have been distinguished: 1- pure talc, 2- tremolite talc and 3- chlorite talc. Paragenetically, talc is derived from serpentine minerals, tremolite and chlorite. The latter is formed at about 231 °C. The chemical data of talc deposit reveals that the summation of talc components (SiO2 + MgO + H2O) is 92.68%, while that of impurity oxides (Al2O3 + CaO + Fe2O3 + FeO) is 5.56%. The carbon13C) and oxygen18O) contents of pure magnesite revealed that the pure phase of Gebel El-Maiyit was formed at low temperature (around 100 °C) while magnesite contained in talc carbonate rock was formed at high temperature (140-175 °C). In terms of source fluids, the metamorphic and /or magmatic water was supposed to be the main fluids which are circulated during the hydrothermal alteration. Although S and P are very minor components in all the talc ore types of the considered area and do not affect their industrial use. Copper (Cu) was not detected. Iron (Fe) and manganese (Mn) concentrations are significantly high, necessitating treatment to reduce these elements for the ore to be suitable as an electrical insulator. Arsenic (As) levels are consistently below 5 ppm, indicating the ore's potential use in the cosmetic industry without further processing.

Utilizing remote sensing and field data for geological mapping and polyphase deformation analysis of Um Laseifa ophiolites, Eastern Desert, Egypt

The Wadi Um Laseifa area, located in the Central Eastern Desert of Egypt, encompasses a range of Neoproterozoic rock units, including ophiolitic mélange, island arc assemblage, and granitic intrusions as well as Miocene clastic deposits. The current research attempts to analyze the structural and lithological characteristics of this area by integrating data from multisource remote sensing (Sentinel 2, Planetscope and hyperspectral PRISMA), along with field and structural relationships, geometrical analysis of structural readings, and petrographic studies. Applying various techniques of remote sensing, such as false color composite (FCC), principal component analysis (PCA), and Minimum noise fraction (MNF), enabled the identification of the structural features over various scales besides accurate lithological discrimination. Data analyses have discriminated the intricate Neoproterozoic rocks into ophiolitic mélange that includes serpentinites, meta-pyroxenites, metagabbro, chert and mélange matrix, island arc assemblage comprising metavolcanics, metavolcano-sedimentary rocks and hornblende schist, and monzogranite and granodiorite intrusions. These rocks have been affected by a thrust stack of three major faults striking NW-SE to NNW-SSE and dipping steeply to the SW. There are two prominent folds represented by a major anticline affecting the island arc metavolcano-sedimentary rocks and a major syncline affecting the ophiolitic rocks. Both folds possess axial planes striking NW-SE and gently plunging NW fold axes. The area is also intersected by E-W or ENE-WSW strike-slip faults, along with major NW-SE normal faults that controlled the distribution of the Miocene clastic deposits. Geometrical analysis has identified three ductile deformation phases: D1 is marked by NW-SE isoclinal folds; D2 produced NW-SE major folds and thrust faults that are coaxial with D1; and D3 led to the formation of NE-SW open folds. The multisource remote sensing analysis that has been carried out in this work illustrated the efficacy of the employed methodology in conducting thorough geological analyses and strongly advocates for its application in analogous studies in arid environments.

Development of the Arabian-Nubian Shield along the Marsa Alam-Idfu transect, Central-Eastern Desert, Egypt: geochemical implementation of zircon U-Pb geochronology

The magmatic complex along the Marsa Alam-Idfu transect, Central-Eastern Desert of Egypt, represents the northern segment of the Arabian-Nubian Shield (ANS), which developed within the framework of the East African Orogen. The basement rocks of the Arabian-Nubian Shield have been developed through three distinct phases of magmatic activity: the island-arc, the syn-orogenic, and the post-orogenic phases. Transitioning of the magmatic phases from the syn-orogenic to the post-orogenic, identifies changing the tectonic regime from a compressional to an extensional setting. The scarcity of comprehensive regional geochronological data that rely on precise isochron methods, such as the zircon U-Pb technique, could limit the comprehensive understanding of this region's geological and tectonic history. That would raise a number of uncertainties ranging from the timing of the different magmatic activities and timing of changes in the tectonic regime to the existence of the pre-Pan-African crust in the CED. Our study provides new insights into the aforementioned uncertainties through zircon U-Pb dating of different rock units along the Marsa Alam-Idfu transect, CED, Egypt. The resulting ages ranged from 729 ± 3 Ma to 570 ± 2 Ma, constraining the temporal evolution of the ANS in the studied region into (1) the island-arc phase, represented by a metamorphic sample with an age of 729 ± 3 Ma. (2) the syn-orogenic phase, represented by calc-alkaline and alkaline granitic samples with ages ranging from 699 ± 4 Ma to 646 ± 2 Ma. These two phases indicate initiation of the compressional subduction regime in the CED since 729 ± 3 Ma and being dominated till 646 ± 2 Ma. (3) the post-orogenic phase, represented by metavolcanics, volcanic rocks, and alkaline plutonic samples with ages ranging from 623 ± 3 Ma to 570 ± 2 Ma. This phase suggests dominance of the compressional-to-extensional tectonic transition setting from 623 ± 3 Ma to 600 ± 1 Ma along with the Dokhan volcanism and activation of post-collision tensional regime activated at 582 ± 3 Ma. Our findings discourage the proposed dominance of the island-arc and syn-orogenic phases in the CED and the classical restriction of older magmatic activity to calc-alkaline granitic rocks and younger magmatic activity to alkaline granitic rocks. Additionally, we identified evidence of local magmatic sources by dating five grains with Mesoproterozoic (pre-Arabian-Nubian Shield) xenocrysts with ages ranging from 1549 ± 4 to 1095 ± 25 Ma.

The importance of continents, oceans and plate tectonics for the evolution of complex life: implications for finding extraterrestrial civilizations

Within the uncertainties of involved astronomical and biological parameters, the Drake Equation typically predicts that there should be many exoplanets in our galaxy hosting active, communicative civilizations (ACCs). These optimistic calculations are however not supported by evidence, which is often referred to as the Fermi Paradox. Here, we elaborate on this long-standing enigma by showing the importance of planetary tectonic style for biological evolution. We summarize growing evidence that a prolonged transition from Mesoproterozoic active single lid tectonics (1.6 to 1.0 Ga) to modern plate tectonics occurred in the Neoproterozoic Era (1.0 to 0.541 Ga), which dramatically accelerated emergence and evolution of complex species. We further suggest that both continents and oceans are required for ACCs because early evolution of simple life must happen in water but late evolution of advanced life capable of creating technology must happen on land. We resolve the Fermi Paradox (1) by adding two additional terms to the Drake Equation: foc (the fraction of habitable exoplanets with significant continents and oceans) and fpt (the fraction of habitable exoplanets with significant continents and oceans that have had plate tectonics operating for at least 0.5 Ga); and (2) by demonstrating that the product of foc and fpt is very small (< 0.00003-0.002). We propose that the lack of evidence for ACCs reflects the scarcity of long-lived plate tectonics and/or continents and oceans on exoplanets with primitive life.

Exploring gold mineralization in altered ultramafic rocks in south Abu Marawat, Eastern Desert, Egypt

Gold mining is an important strategic sector. The search for mineral reserves is moving deeper as more accessible shallow resources are discovered. Geophysical techniques are now being employed more frequently in mineral exploration because they are quick and can provide crucial subsurface information for discovering potential metal deposits, particularly in high-relief and inaccessible places. The potential for gold in a large-scale gold mining (LSGM) locality in the South Abu Marawat area is investigated using a geological field investigation that includes rock sampling, structural measurements, detailed petrography, reconnaissance geochemistry, and thin section analysis, integrated with various transformation filters of surface magnetic data (analytic signal, normalized source strength, tilt angle), contact occurrence density maps, and tomographic modelling for the subsurface magnetic susceptibilities. The benefits of remote sensing (RS) and its technology in mapping detailed rock differentiation, and characterizing physical objects on the land surface using various spatial, and spectral resolution datasets are integrated. Both aeromagnetic and measured land magnetic profiles are used to investigate the area's present geological conditions and possible future mining localities. Results indicate that gold mineralization in the study area is linked to the altered ultramafic zones that are associated with faulting and shearing and characterized by a low magnetic susceptibility anomaly.

Petrogenesis of Neoproterozoic Ultramafic Rocks, Wadi Ibib–Wadi Shani, South Eastern Desert, Egypt: Constraints from Whole Rock and Mineral Chemistry

This contribution deals with new geology, petrography, and bulk-rock/mineral chemistry of the poorly studied ultramafics of Wadi Ibib–Wadi Shani (WI–WS) district, South Eastern Desert, Egypt. These ultramafics are dismembered ophiolitic rocks that can be subdivided into serpentinites and serpentinized peridotites. Primary minerals such as olivine and pyroxene are absent in serpentinites, but relics of them occur in serpentinized peridotites. Pseudomorph after olivine is indicated by common hourglass textures with less mesh, whilst schistose bastites reflect a pyroxene pseudomorph. Chromite can be subdivided into Cr-spinel and Al-spinel. Cr-spinel ranges from chromite to magnesochromite in composition, whereas Al-spinel belongs to the spinel field. Cr-spinel includes YCr (Cr/(Cr+Al+Fe+3), YAl (Al/(Al+Cr+Fe+3), and YFe+3 (Fe+3/(Fe+3+Al+Cr), similar to forearc peridotite, whilst Al-spinel is more similar to abyssal peridotite, and may be formed during inanition of subduction processes in proto forearc environments. The main secondary minerals are tremolite, talc, and chlorite—which is subdivided into pycnochlorite and diabantite—and their temperature ranges from 174 to 224 °C. The examined rocks had undergone high partial melting degrees (>25%), as indicated by the Cr# of their unaltered cores (Cr-spinel, >0.6), whole rocks (Al2O3, SiO2, CaO, and MgO), trace and REEs, depleted Na2O, Al2O3, and Cr2O3 of clinopyroxene, and high forsterite content ((Fo = 100 Mg/Mg + Fe), av. 95.23 mol%), consistent with forearc settings.

Neoproterozoic Tectonic Events of Egypt

The Egyptian Nubian Shield (ENS) represents the northwestern part of the Arabian‐Nubian Shield and the northern extension of the East African Orogen. The ENS is regarded as being formed due to northward‐directed escape tectonics. It is characterized by mild accretion and suture zones dominated by major strike‐slip zones with a commonly sinistral sense of movement; some shear zones display a dextral sense of shear. It is dominated by gneisses and migmatites in the south, arc volcaniclastic metasediments and highly dismembered ophiolites in the central parts, whereas its northern part is dominated by late‐ to post‐tectonic granitoids. In southern Sinai, the Neoproterozoic rocks are grouped into four complexes, namely Feiran–Solaf, Sa'al–Zaghra, Kid and Taba. The ENS ophiolites were formed between 730–750 Ma, mainly in a supra‐subduction zone setting. The ENS has undergone a Neoproterozoic deformation history involving three successive phases: (1) Early N–S shortening phase (D1), (2) Syn‐accretionary phase (D2) and (3) Post‐accretionary phase (D3). The initial island‐arc stage (780–730 Ma) is a N–S shortening phase initiated by collision between the Eastern Desert tectonic terrane to the north with both the Gebeit and Gabgaba terranes to the south (830–720 Ma). During the arc‐splitting and back‐arc spreading stage (730–620), voluminous syn‐tectonic granitoids intruded into the ENS (750–610 Ma). The E–W‐directed compressional/transpressional phase (620–450 Ma) led to the overall uplift of the central part of the ENS and consequently the development and exhumation of the core complexes in oblique convergent zones. The E–W intense shortening deformation resulted also in the formation of NW‐ and NE‐striking sinistral and dextral strike‐slip shear zones, respectively. The latest periods of the E–W‐directed compressional/transpressional regime were characterized by deposition of the molasse‐type Hammamat Sediments unconformably over the Dokhan Volcanics, or interbedded with them. The combined thrusting, folding and sinistral‐reverse shearing structures have been interpreted to resulted from the E–W‐directed compressional/transpressional phase in response to the oblique shortening of the Arabian‐Nubian Shield between East and West Gondwana.

Mariana-type ophiolites constrain the establishment of modern plate tectonic regime during Gondwana assembly

Initiation of Mariana-type oceanic subduction zones requires rheologically strong oceanic lithosphere, which developed through secular cooling of Earth's mantle. Here, we report a 518 Ma Mariana-type subduction initiation ophiolite from northern Tibet, which, along with compilation of similar ophiolites through Earth history, argues for the establishment of the modern plate tectonic regime by the early Cambrian. The ophiolite was formed during the subduction initiation of the Proto-Tethys Ocean that coincided with slab roll-back along the southern and western Gondwana margins at ca. 530-520 Ma. This global tectonic re-organization and the establishment of modern plate tectonic regime was likely controlled by secular cooling of the Earth, and facilitated by enhanced lubrication of subduction zones by sediments derived from widespread surface erosion of the extensive mountain ranges formed during Gondwana assembly. This time also corresponds to extreme events recorded in climate and surface proxies that herald formation of the contemporary Earth.

Origin of arc magmatic signature: A temperature-dependent process for trace element (re)-mobilization in subduction zones

Serpentinite is a major carrier of fluid-mobile elements in subduction zones, which influences the geochemical signature of arc magmatism (e.g. high abundances of Li, Ba, Sr, B, As, Mo and Pb). Based on results from Neoproterozoic serpentinites in the Arabian-Nubian Shield, we herein report the role of antigorite in the transportation of fluid-mobile elements (FME) and light rare earth elements (LREE) from the subducted slab to arc-related magma during subduction. The serpentinites contain two generations of antigorites: the older generation is coarse-grained, formed at a temperature range of 165-250 °C and is enriched in Li, Rb, Ba and Cs, whereas the younger generation is finer-grained, formed at higher temperature conditions (425-475 °C) and has high concentrations of B, As, Sb, Mo, Pb, Sr and LREE. Magnesite, on the other hand, remains stable at sub-arc depths beyond the stability field of both antigorites, and represents a potential reservoir of FME and LREE for deeper mantle melts. Magnesite has high FME and LREE absorbing capacity (over 50-60%) higher than serpentine phases. Temperature is the main controlling factor for stability of these minerals and therefore the release of these elements from subducted slabs into arc magmatism. As the liberation of these elements varies along the length of the slab, the resulting cross-arc geochemical variation trend can help to determine the subduction polarity of ancient arcs.

Genesis of the Koka Gold Deposit in Northwest Eritrea, NE Africa: Constraints from Fluid Inclusions and C–H–O–S Isotopes

: The Koka gold deposit is located in the Elababu shear zone between the Nakfa terrane and the Adobha Abiy terrane, NW Eritrea. Based on a paragenetic study, two main stages of gold mineralization were identified in the Koka gold deposit: (1) an early stage of pyrite–chalcopyrite–sphalerite–galena–gold–quartz vein; and (2) a second stage of pyrite–quartz veins. NaCl-aqueous inclusions, CO2-rich inclusions, and three-phase CO2–H2O inclusions occur in the quartz veins at Koka. The ore-bearing quartz veins formed at 268 °C from NaCl–CO2–H2O(–CH4) fluids averaging 5 wt% NaCl eq. The ore-forming mechanisms include fluid immiscibility during stage I, and mixing with meteoric water during stage II. Oxygen, hydrogen, and carbon isotopes suggest that the ore-forming fluids originated as mixtures of metamorphic water and magmatic water, whereas the sulfur isotope suggests an igneous origin. The features of geology and ore-forming fluid at the Koka deposit are similar to those of orogenic gold deposits, suggesting that the Koka deposit might be an orogenic gold deposit related to granite.

Accretionary orogens through Earth history

Accretionary orogens form at intraoceanic and continental margin convergent plate boundaries. They include the supra-subduction zone forearc, magmatic arc and back-arc components. Accretionary orogens can be grouped into retreating and advancing types, based on their kinematic framework and resulting geological character. Retreating orogens (e.g. modern western Pacific) are undergoing long-term extension in response to the site of subduction of the lower plate retreating with respect to the overriding plate and are characterized by back-arc basins. Advancing orogens (e.g. Andes) develop in an environment in which the overriding plate is advancing towards the downgoing plate, resulting in the development of foreland fold and thrust belts and crustal thickening. Cratonization of accretionary orogens occurs during continuing plate convergence and requires transient coupling across the plate boundary with strain concentrated in zones of mechanical and thermal weakening such as the magmatic arc and back-arc region. Potential driving mechanisms for coupling include accretion of buoyant lithosphere (terrane accretion), flat-slab subduction, and rapid absolute upper plate motion overriding the downgoing plate. Accretionary orogens have been active throughout Earth history, extending back until at least 3.2 Ga, and potentially earlier, and provide an important constraint on the initiation of horizontal motion of lithospheric plates on Earth. They have been responsible for major growth of the continental lithosphere through the addition of juvenile magmatic products but are also major sites of consumption and reworking of continental crust through time, through sediment subduction and subduction erosion. It is probable that the rates of crustal growth and destruction are roughly equal, implying that net growth since the Archaean is effectively zero.