Petrogenesis of gold-bearing listvenites from the carbonatized mantle section of the Neoproterozoic Ess ophiolite, Western Arabian Shield, Saudi Arabia

The variably serpentinized mantle peridotites of the Late Neoproterozoic Ess ophiolite (Western Saudi Arabia) are highly altered along shear zones and thrust planes to form erosion-resistant listvenites. The listvenites are distinguished petrographically and geochemically into three types: carbonate, silica-carbonate and silica (birbirite) listvenites. Geochemical analyses are consistent with expectations from petrography: carbonate listvenite is low in SiO₂ content but high in MgO, Fe₂O₃, and CaO relative to silica-carbonate and birbirite, which is remarkably high in SiO₂ at the expense of all other components. The total REE contents are low in silica-carbonate and carbonate listvenites but highly enriched in birbirite, with a large positive Eu anomaly. The host serpentinites have all the characteristics typically associated with highly depleted mantle harzburgite protoliths in supra-subduction fore-arc settings: bulk compositions are low in Al₂O₃ and CaO with high Mg# [molar Mg/(Mg + Fe)], relict Cr-spinel has high Cr# [molar Cr/(Cr + Al)] and low TiO₂, and relict olivine has high Mg# and NiO content. The Cr-spinel relics are also found in the listvenites; those in serpentinite and carbonate listvenites have significantly higher Mg# than those in silica-carbonate and birbirite, suggesting re-equilibration of Cr-spinel in the later phases of listvenitization. The varieties of listvenite capture successive stages of fluid-mediated replacement reactions. The carbonate listvenite appears to have developed syn-contemporaneously with serpentinization, whereas silica-carbonate listvenite and birbirite formed later. The listvenite formation resulted in leaching and removal of some components accompanied by deposition of others in the solid products, notably CO₃, SiO₂, REE (especially Eu), Au, Zn, As, Sb and K. Our data show that listvenitization concentrated gold at sub-economic to economic grades; measured gold concentrations in the host serpentinite are 0.5–1.7 ng/g, versus 4–2569 ng/g in carbonate listvenite, 43–3117 ng/g in silica-carbonate listvenite and 5–281 ng/g in birbirite. The listvenite deposits in the Jabal Ess area merit further exploration for gold

The common origin and alteration history of the hypabyssal and volcanic phases of the Wadi Tarr albitite complex, southern Sinai, Egypt

New data and interpretations are presented for the igneous albitites of the Wadi Tarr area, southern Sinai, Egypt. The albitite masses are isolated in outcrop from any granitic intrusions and have intrusive contacts against the country rocks without any structural control. They have marginal zones of breccias with jigsaw-fit angular clasts suggesting explosive, in-situ formation. The albitites are of two types: the western, medium-grained, hypabyssal albitite and the eastern, fine-grained porphyritic albitite. The field relations suggest emplacement at different levels in a magmatic cupola: the hypabyssal texture and steeply dipping slope of the upper contact of the western albitite imply deeper emplacement whereas the gently dipping contacts and porphyritic texture of the eastern albitite masses indicate that they define the probable location of the cupola apex. Both types of albitites consist of albite (92–97%) with minor amounts of quartz, K-feldspar and biotite. The accessory minerals include Fe-oxides, augite, sulphides, zircon, rutile, xenotime, titanite, allanite and monazite. The whole-rock compositions of the hypabyssal and porphyritic albitites are closely related, but the porphyritic type has lower abundances of Sr, Ba, Y, Nb, Th and Zr. We show that the hypabyssal and porphyritic albitites have a common petrogenetic origin, most likely as late-stage cumulates from a fractionating, strongly alkaline A-type magma, consistent with the compositions of the mafic minerals. The source magma was probably a tephritic liquid; we use MELTS models to show that only a sufficiently alkaline magma follows a differentiation path that both avoids quartz saturation and encounters the alkali feldspar solvus, reaching a residual liquid in equilibrium with highly sodic feldspar. Although the MELTS results show a chemically consistent means of forming igneous albitite, they are incomplete in that physical segregation mechanisms are still required to isolate the albite from mafic minerals and or a low-temperature aqueous alteration stage is needed to leach K from the feldspar. Alteration surrounding the Wadi Tarr albitites is extensive and dominated by alkali metasomatism similar to fenitization. Alteration in the marginal breccia zone of the albitite is dominated by precipitation of amphibole and carbonate in veins and in the breccia matrix, whereas the volcanic country rocks show replacement of feldspars by sericite, carbonate and epidote as well as vein carbonate. The altered volcanic country rocks show lower concentrations of Fe_2O_3, Sr, Cu, Pb, Ba and Ce, accompanied by higher concentrations of Na2O and MgO compared to unaltered equivalent samples

Petrogenesis of gold-bearing listvenites from the carbonatized mantle section of the Neoproterozoic Ess ophiolite, Western Arabian Shield, Saudi Arabia

The variably serpentinized mantle peridotites of the Late Neoproterozoic Ess ophiolite (Western Saudi Arabia) are highly altered along shear zones and thrust planes to form erosion-resistant listvenites. The listvenites are distinguished petrographically and geochemically into three types: carbonate, silica-carbonate and silica (birbirite) listvenites. Geochemical analyses are consistent with expectations from petrography: carbonate listvenite is low in SiO₂ content but high in MgO, Fe₂O₃, and CaO relative to silica-carbonate and birbirite, which is remarkably high in SiO₂ at the expense of all other components. The total REE contents are low in silica-carbonate and carbonate listvenites but highly enriched in birbirite, with a large positive Eu anomaly. The host serpentinites have all the characteristics typically associated with highly depleted mantle harzburgite protoliths in supra-subduction fore-arc settings: bulk compositions are low in Al₂O₃ and CaO with high Mg# [molar Mg/(Mg + Fe)], relict Cr-spinel has high Cr# [molar Cr/(Cr + Al)] and low TiO₂, and relict olivine has high Mg# and NiO content. The Cr-spinel relics are also found in the listvenites; those in serpentinite and carbonate listvenites have significantly higher Mg# than those in silica-carbonate and birbirite, suggesting re-equilibration of Cr-spinel in the later phases of listvenitization. The varieties of listvenite capture successive stages of fluid-mediated replacement reactions. The carbonate listvenite appears to have developed syn-contemporaneously with serpentinization, whereas silica-carbonate listvenite and birbirite formed later. The listvenite formation resulted in leaching and removal of some components accompanied by deposition of others in the solid products, notably CO₃, SiO₂, REE (especially Eu), Au, Zn, As, Sb and K. Our data show that listvenitization concentrated gold at sub-economic to economic grades; measured gold concentrations in the host serpentinite are 0.5–1.7 ng/g, versus 4–2569 ng/g in carbonate listvenite, 43–3117 ng/g in silica-carbonate listvenite and 5–281 ng/g in birbirite. The listvenite deposits in the Jabal Ess area merit further exploration for gold

Evaluation of metals that are potentially toxic to agricultural surface soils, using statistical analysis, in northwestern Saudi Arabia

© 2015, Springer-Verlag Berlin Heidelberg. Heavy metals in agricultural soils enter the food chain when taken up by plants. The main purpose of this work is to determine metal contamination in agricultural farms in northwestern Saudi Arabia. Fifty surface soil samples were collected from agricultural areas. The study focuses on the geochemical behavior of As, Cd, Co, Cr, Cu, Hg, Pb and Zn, and determines the enrichment factor and geoaccumulation index. Multivariate statistical analysis, including principle component analysis and cluster analysis, is also applied to the acquired data. The study shows considerable variation in the concentrations of the analyzed metals in the studied soil samples. This variation in concentration is attributed to the intensity of agricultural activities and, possibly, to nearby fossil fuel combustion activities, as well as to traffic flows from highways and local roads. Multivariate analysis suggests that As, Cd, Hg and Pb are associated with anthropogenic activities, whereas Co, Cr, Cu and Zn are mainly controlled by geogenic activities. Hg and Pb show the maximum concentration in the analyzed samples as compared to the background concentration

The late Neoproterozoic Dahanib mafic-ultramafic intrusion, South eastern Desert, Egypt: Is it an Alaskan-type or a layered intrusion?

In Egypt, mafic-ultramafic complexes have been classified into three major types: incomplete ophiolite sequences; Alaskan-type intrusions, concentrically-zoned bodies formed in a subduction arc environment; and layered intrusions, vertically-zoned bodies intruded in post-collisional tectonic environments and rift-related bodies associated with the opening of the Red Sea. We present new field work, geochemical data, mineral chemistry and interpretations for the late Neoproterozoic Dahanib mafic-ultramafic intrusion in the South Eastern Desert of Egypt (northernmost Arabian–Nubian Shield, ANS). The Dahanib intrusion shows no evidence of metamorphism or deformation, with excellent preservation of intrusive contacts, well-preserved textures and primary mineralogy. Field relations indicate that it is younger than the surrounding metamorphic rocks and syn-tectonic granitoids. The intrusion is composed of a basal suite of ultramafic rocks (dunite, lherzolite, wehrlite and pyroxenite) and an overlying suite of mafic rocks (olivine gabbronorite, gabbronorite and anorthosite). It displays evident layering of modal abundance, visible directly in outcrop, as well as cryptic layering discernible through changes in mineral compositions. The western and eastern lobes of the Dahanib intrusion occur in the form of a lopolith with readily correlated layers, especially in the upper mafic unit. The present-day dip of the layering decreases from the ultramafic units into the mafic sequence. Structural and compositional relations show that the ultramafic units are cumulates from a high-Mg tholeiitic parent magma emplaced at deep crustal levels and evolved via fractional crystallization rather than any kind of residual mantle sequence. Fo content of olivine and Mg# of pyroxenes display a systematic decrease from ultramafic to mafic rocks, well-correlated with whole-rock Mg#. Spinels in ultramafic samples vary from Cr-rich to Al-rich and have Mg# and Fe^(3+)# similar to spinels from typical stratiform complexes and clearly different from those found in ophiolitic and Alaskan-type complexes. Although the mafic and ultramafic units are clearly related and can be derived from common parent magma, they were not emplaced coevally; rather, they represent different pulses of magma. The Dahanib mafic–ultramafic intrusion does not display any features that convincingly identify it as a typical Alaskan-type body, particularly the lack of clinopyroxenite and hornblendite, rarity of primary hornblende, and the notable abundance of orthopyroxene and plagioclase in its rocks. Our results confirm that it is more akin to a layered mafic-ultramafic intrusion with a multistage evolution. It was emplaced into a stable post-orogenic cratonic setting, with a trace element signature indicating contamination of the mantle source by previous subduction events

Genesis and geodynamic evolution of serpentinized ultramafics and associated magnesite deposits in the Al-Wask ophiolite, Arabian Shield, Saudi Arabia

Situated along the Yanbu Suture Zone, the Al-Wask ophiolite is one of the largest and best-preserved ophiolite sequences in the Proterozoic Arabian shield. A mantle section of serpentinized ultramafics is structurally overlain by a crustal section of gabbros and pillow lavas. The whole ophiolite sequence is capped by pelagic sedimentary cover, and tectonically emplaced over a metamorphosed island-arc volcano-sedimentary succession. The Al-Wask ultramafic rocks are strongly deformed, metamorphosed, and altered by carbonatization and silicification. Samples dominated by antigorite indicate upper greenschist to lower amphibolite facies peak metamorphic grade, whereas samples dominated by lizardite and magnesite preserve lower grade conditions that we interpret as a cooling path buffered to low CO₂ activity by the increasing stability of magnesite with decreasing temperature. Nearly all the primary silicate minerals have been replaced by serpentine minerals, leaving only relics of primary olivine and chromian spinel. Petrographic observation of relict olivine and spinel and of mesh and bastite textures in the serpentines suggest that the peridotite protoliths were mainly harzburgite with minor dunite. Whole-rock compositions of serpentinites show low CaO ( 60) and low TiO₂ (≤0.2 wt. %) of spinel and high forsterite contents (90–92) of associated olivine indicate residual mantle that underwent extensive partial melt extraction. The whole-rock and mineral chemistry of the serpentinized ultramafic rocks are both consistent with extracted melt fractions from ∼32 to 38 percent. This extent of melting is typical of fore-arc supra-subduction zone settings, which is the most likely tectonic environment for formation and preservation of the Al-Wask ophiolite. Two types of magnesite deposits can be distinguished in the Al-Wask mantle section: an early generation of massive magnesite and a later generation of magnesite veins. Hence the Al-Wask ophiolite underwent multiple stages of carbonatization, likely involving different sources of CO₂-bearing fluids. The massive magnesite likely formed at relatively high temperature during cooling from peak metamorphic condition from CO₂-bearing fluid probably derived from decomposition of subducted carbonates. Using thermodynamic calculations in the simple MgO-SiO₂-H₂O-CO₂ system, we constrain the path of the reaction boundary where lizardite and magnesite can coexist at equilibrium. On the other hand, the cryptocrystalline magnesite veins fill tectonic fractures and likely formed at low temperature and shallow levels, after serpentinization and ophiolite obduction

The late Neoproterozoic Dahanib mafic-ultramafic intrusion, South eastern Desert, Egypt: Is it an Alaskan-type or a layered intrusion?

In Egypt, mafic-ultramafic complexes have been classified into three major types: incomplete ophiolite sequences; Alaskan-type intrusions, concentrically-zoned bodies formed in a subduction arc environment; and layered intrusions, vertically-zoned bodies intruded in post-collisional tectonic environments and rift-related bodies associated with the opening of the Red Sea. We present new field work, geochemical data, mineral chemistry and interpretations for the late Neoproterozoic Dahanib mafic-ultramafic intrusion in the South Eastern Desert of Egypt (northernmost Arabian–Nubian Shield, ANS). The Dahanib intrusion shows no evidence of metamorphism or deformation, with excellent preservation of intrusive contacts, well-preserved textures and primary mineralogy. Field relations indicate that it is younger than the surrounding metamorphic rocks and syn-tectonic granitoids. The intrusion is composed of a basal suite of ultramafic rocks (dunite, lherzolite, wehrlite and pyroxenite) and an overlying suite of mafic rocks (olivine gabbronorite, gabbronorite and anorthosite). It displays evident layering of modal abundance, visible directly in outcrop, as well as cryptic layering discernible through changes in mineral compositions. The western and eastern lobes of the Dahanib intrusion occur in the form of a lopolith with readily correlated layers, especially in the upper mafic unit. The present-day dip of the layering decreases from the ultramafic units into the mafic sequence. Structural and compositional relations show that the ultramafic units are cumulates from a high-Mg tholeiitic parent magma emplaced at deep crustal levels and evolved via fractional crystallization rather than any kind of residual mantle sequence. Fo content of olivine and Mg# of pyroxenes display a systematic decrease from ultramafic to mafic rocks, well-correlated with whole-rock Mg#. Spinels in ultramafic samples vary from Cr-rich to Al-rich and have Mg# and Fe^(3+)# similar to spinels from typical stratiform complexes and clearly different from those found in ophiolitic and Alaskan-type complexes. Although the mafic and ultramafic units are clearly related and can be derived from common parent magma, they were not emplaced coevally; rather, they represent different pulses of magma. The Dahanib mafic–ultramafic intrusion does not display any features that convincingly identify it as a typical Alaskan-type body, particularly the lack of clinopyroxenite and hornblendite, rarity of primary hornblende, and the notable abundance of orthopyroxene and plagioclase in its rocks. Our results confirm that it is more akin to a layered mafic-ultramafic intrusion with a multistage evolution. It was emplaced into a stable post-orogenic cratonic setting, with a trace element signature indicating contamination of the mantle source by previous subduction events

Genesis and geodynamic evolution of serpentinized ultramafics and associated magnesite deposits in the Al-Wask ophiolite, Arabian Shield, Saudi Arabia

Situated along the Yanbu Suture Zone, the Al-Wask ophiolite is one of the largest and best-preserved ophiolite sequences in the Proterozoic Arabian shield. A mantle section of serpentinized ultramafics is structurally overlain by a crustal section of gabbros and pillow lavas. The whole ophiolite sequence is capped by pelagic sedimentary cover, and tectonically emplaced over a metamorphosed island-arc volcano-sedimentary succession. The Al-Wask ultramafic rocks are strongly deformed, metamorphosed, and altered by carbonatization and silicification. Samples dominated by antigorite indicate upper greenschist to lower amphibolite facies peak metamorphic grade, whereas samples dominated by lizardite and magnesite preserve lower grade conditions that we interpret as a cooling path buffered to low CO₂ activity by the increasing stability of magnesite with decreasing temperature. Nearly all the primary silicate minerals have been replaced by serpentine minerals, leaving only relics of primary olivine and chromian spinel. Petrographic observation of relict olivine and spinel and of mesh and bastite textures in the serpentines suggest that the peridotite protoliths were mainly harzburgite with minor dunite. Whole-rock compositions of serpentinites show low CaO ( 60) and low TiO₂ (≤0.2 wt. %) of spinel and high forsterite contents (90–92) of associated olivine indicate residual mantle that underwent extensive partial melt extraction. The whole-rock and mineral chemistry of the serpentinized ultramafic rocks are both consistent with extracted melt fractions from ∼32 to 38 percent. This extent of melting is typical of fore-arc supra-subduction zone settings, which is the most likely tectonic environment for formation and preservation of the Al-Wask ophiolite. Two types of magnesite deposits can be distinguished in the Al-Wask mantle section: an early generation of massive magnesite and a later generation of magnesite veins. Hence the Al-Wask ophiolite underwent multiple stages of carbonatization, likely involving different sources of CO₂-bearing fluids. The massive magnesite likely formed at relatively high temperature during cooling from peak metamorphic condition from CO₂-bearing fluid probably derived from decomposition of subducted carbonates. Using thermodynamic calculations in the simple MgO-SiO₂-H₂O-CO₂ system, we constrain the path of the reaction boundary where lizardite and magnesite can coexist at equilibrium. On the other hand, the cryptocrystalline magnesite veins fill tectonic fractures and likely formed at low temperature and shallow levels, after serpentinization and ophiolite obduction

Multiple Stages of Carbonation and Element Redistribution during Formation of Ultramafic-Hosted Magnesite in Neoproterozoic Ophiolites of the Arabian-Nubian Shield, Egypt

We present a study of the serpentinized peridotites of the Ghadir-Mohagar-Ambaut area, Egypt. They represent the mantle section of a dismembered ophiolite, tectonically emplaced over a volcanosedimentary succession of island arc assemblages. The serpentinites are variably metamorphosed from greenschist to lower-amphibolite facies, metasomatized, and altered, including development of talc-carbonate and quartz-carbonate rocks, especially along shear zones and fault planes. Nevertheless, some samples contain relics of primary chromian spinel, olivine, and pyroxenes. Relict textures and whole-rock compositions (Mg#[molar Mg/(Mg+Fe^(2+))]=0.92–0.93, with low Al_2O_3 and CaO contents) both suggest harzburgite protoliths. The high Mg# and Ni contents of relict olivine and the high Cr# (molar Cr/(Cr+Al)molar Cr/(Cr+Al)) of fresh chromian spinel cores indicate that the protoliths experienced high degrees of partial melt extraction (∼34%–39%), well beyond the limit for exhaustion of clinopyroxene from the residue and consistent with formation in a forearc suprasubduction zone environment. The serpentinized ultramafic rocks in the study area are divided into massive serpentinite, serpentinite-hosted magnesite masses, and magnesite-filled veins. The carbonation and formation of magnesite ores took place through two metasomatic stages; the first is represented by the magnesite masses and associated with deep-seated metasomatism and alteration during serpentinization, whereas the second, vein-forming stage postdates serpentinization and occurred during obduction of the ophiolite. The differences in chemical composition between massive serpentinite and serpentinite-hosted magnesite masses suggest leaching of some elements and enrichment of others during carbonation; MgO, Cr, and Ni are depleted, whereas Fe_2O_3, CaO, MnO, Nb, Ba, Cu, Pb, Sr, and Zn are enriched in the serpentinite-hosted magnesite masses, relative to the host massive serpentinite

Multiple Stages of Carbonation and Element Redistribution during Formation of Ultramafic-Hosted Magnesite in Neoproterozoic Ophiolites of the Arabian-Nubian Shield, Egypt

We present a study of the serpentinized peridotites of the Ghadir-Mohagar-Ambaut area, Egypt. They represent the mantle section of a dismembered ophiolite, tectonically emplaced over a volcanosedimentary succession of island arc assemblages. The serpentinites are variably metamorphosed from greenschist to lower-amphibolite facies, metasomatized, and altered, including development of talc-carbonate and quartz-carbonate rocks, especially along shear zones and fault planes. Nevertheless, some samples contain relics of primary chromian spinel, olivine, and pyroxenes. Relict textures and whole-rock compositions (Mg#[molar Mg/(Mg+Fe^(2+))]=0.92–0.93, with low Al_2O_3 and CaO contents) both suggest harzburgite protoliths. The high Mg# and Ni contents of relict olivine and the high Cr# (molar Cr/(Cr+Al)molar Cr/(Cr+Al)) of fresh chromian spinel cores indicate that the protoliths experienced high degrees of partial melt extraction (∼34%–39%), well beyond the limit for exhaustion of clinopyroxene from the residue and consistent with formation in a forearc suprasubduction zone environment. The serpentinized ultramafic rocks in the study area are divided into massive serpentinite, serpentinite-hosted magnesite masses, and magnesite-filled veins. The carbonation and formation of magnesite ores took place through two metasomatic stages; the first is represented by the magnesite masses and associated with deep-seated metasomatism and alteration during serpentinization, whereas the second, vein-forming stage postdates serpentinization and occurred during obduction of the ophiolite. The differences in chemical composition between massive serpentinite and serpentinite-hosted magnesite masses suggest leaching of some elements and enrichment of others during carbonation; MgO, Cr, and Ni are depleted, whereas Fe_2O_3, CaO, MnO, Nb, Ba, Cu, Pb, Sr, and Zn are enriched in the serpentinite-hosted magnesite masses, relative to the host massive serpentinite