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Gary R Byerly - One of the best experts on this subject based on the ideXlab platform.

  • Ironstone bodies of the barberton greenstone belt south africa products of a cenozoic hydrological system not archean hydrothermal vents
    Geological Society of America Bulletin, 2007
    Co-Authors: Donald R Lowe, Gary R Byerly
    Abstract:

    Irregular bodies of goethite and hematite, termed Ironstone pods , in the Barberton greenstone belt, South Africa, have been previously interpreted as the Earth9s most ancient submarine hydrothermal vent deposits and have yielded putative evidence about Archean hydrothermal systems, ocean composition and temperature, and early life. This report summarizes geologic, sedimentological, and petrographic evidence from three widely separated areas showing that the Ironstone was deposited on and directly below the modern ground surface by active groundwater and spring systems, probably during periods of higher rainfall in the Pleistocene. The deposits include a recently active spring terrace and outflow system containing iron oxide–encrusted microbial filaments, shallow subsurface groundwater areas showing Ironstone deposited between chert blocks and replacing chert, degraded spring and outflow deposits, and goethite-cemented Pleistocene landslide breccias and regolith. The predominance of goethite, a thermally unstable iron oxide; widely developed slope-parallel stratification; abundant primary open cavities, some with vertical goethite dripstone; and the lack of structural deformation collectively indicate that these bodies formed during relatively recent time. Veins and displaced masses of coarsely crystalline quartz show corrosion and represent relict Archean vein and cavity-fill quartz after iron oxide replacement of surrounding chert. Iron was sourced by Archean sideritic sedimentary units that hold up high ridges on which the Ironstone bodies occur. Siderite was dissolved by circulating groundwater, and the iron deposited as oxides within the shallow subsurface groundwater system and around springs where the Fe-rich water flowed onto the surface. These deposits represent a remarkable iron oxide–depositing Quaternary hydrologic system but provide no information about conditions or life on the early Earth.

  • stable isotope and rare earth element evidence for recent Ironstone pods within the archean barberton greenstone belt south africa
    Geochimica et Cosmochimica Acta, 2006
    Co-Authors: Michael T Hren, Donald R Lowe, Michael M Tice, Gary R Byerly, Page C Chamberlain
    Abstract:

    There is considerable debate about the mode and age of formation of large (up to � 200 m long) hematite and goethite Ironstone bodies within the 3.2 to 3.5 Ga Barberton greenstone belt. We examined oxygen and hydrogen isotopes and Rare Earth Element (REE) concentrations of goethite and hematite components of the Ironstones to determine whether these deposits reflect formation from sea-floor vents in the Archean ocean or from recent surface and shallow subsurface spring systems. Goethite d 18 O values range from � 0.7 to +1.0& and dD from � 125 to � 146&, which is consistent with formation from modern meteoric waters at 20 to 25 C. Hematite d 18 O values range from � 0.7 to � 2.0&, which is consistent with formation at low to moderate temperatures (40–55 C) from modern meteoric water. REE in the goethite and hematite are derived from the weathering of local sideritic Ironstones, silicified ultramafic rocks, sideritic black cherts, and local felsic volcanic rocks, falling along a mixing line between the Eu/Eu* and shale-normalized HREEAvg/LREEAvg values for the associated silicified ultramafic rocks and felsic volcanic rocks. Contrasting positive Ce/Ce* of 1.3 to 3.5 in hematite and negative Ce/Ce* of 0.2 to 0.9 in goethite provides evidence of oxidative scavenging of Ce on hematite surfaces during mineral precipitation. These isotopic and REE data, taken together, suggest that hematite and goethite Ironstone pods formed from relatively recent meteoric waters in shallow springs and/or subsurface warm springs.

  • Ironstone pods in the archean barberton greenstone belt south africa earth s oldest seafloor hydrothermal vents reinterpreted as quaternary subaerial springs comment and reply reply
    Geology, 2004
    Co-Authors: Donald R Lowe, Gary R Byerly
    Abstract:

    We respond to de Ronde et al. as per their numbered comments. 1. de Ronde et al. suggest that there are two populations of Ironstone pods in the Barberton greenstone belt: those on Farm Mendon that are Archean in age and composed primarily of hematite and all other Ironstone pods, which are young

Donald R Lowe - One of the best experts on this subject based on the ideXlab platform.

  • Ironstone bodies of the barberton greenstone belt south africa products of a cenozoic hydrological system not archean hydrothermal vents
    Geological Society of America Bulletin, 2007
    Co-Authors: Donald R Lowe, Gary R Byerly
    Abstract:

    Irregular bodies of goethite and hematite, termed Ironstone pods , in the Barberton greenstone belt, South Africa, have been previously interpreted as the Earth9s most ancient submarine hydrothermal vent deposits and have yielded putative evidence about Archean hydrothermal systems, ocean composition and temperature, and early life. This report summarizes geologic, sedimentological, and petrographic evidence from three widely separated areas showing that the Ironstone was deposited on and directly below the modern ground surface by active groundwater and spring systems, probably during periods of higher rainfall in the Pleistocene. The deposits include a recently active spring terrace and outflow system containing iron oxide–encrusted microbial filaments, shallow subsurface groundwater areas showing Ironstone deposited between chert blocks and replacing chert, degraded spring and outflow deposits, and goethite-cemented Pleistocene landslide breccias and regolith. The predominance of goethite, a thermally unstable iron oxide; widely developed slope-parallel stratification; abundant primary open cavities, some with vertical goethite dripstone; and the lack of structural deformation collectively indicate that these bodies formed during relatively recent time. Veins and displaced masses of coarsely crystalline quartz show corrosion and represent relict Archean vein and cavity-fill quartz after iron oxide replacement of surrounding chert. Iron was sourced by Archean sideritic sedimentary units that hold up high ridges on which the Ironstone bodies occur. Siderite was dissolved by circulating groundwater, and the iron deposited as oxides within the shallow subsurface groundwater system and around springs where the Fe-rich water flowed onto the surface. These deposits represent a remarkable iron oxide–depositing Quaternary hydrologic system but provide no information about conditions or life on the early Earth.

  • stable isotope and rare earth element evidence for recent Ironstone pods within the archean barberton greenstone belt south africa
    Geochimica et Cosmochimica Acta, 2006
    Co-Authors: Michael T Hren, Donald R Lowe, Michael M Tice, Gary R Byerly, Page C Chamberlain
    Abstract:

    There is considerable debate about the mode and age of formation of large (up to � 200 m long) hematite and goethite Ironstone bodies within the 3.2 to 3.5 Ga Barberton greenstone belt. We examined oxygen and hydrogen isotopes and Rare Earth Element (REE) concentrations of goethite and hematite components of the Ironstones to determine whether these deposits reflect formation from sea-floor vents in the Archean ocean or from recent surface and shallow subsurface spring systems. Goethite d 18 O values range from � 0.7 to +1.0& and dD from � 125 to � 146&, which is consistent with formation from modern meteoric waters at 20 to 25 C. Hematite d 18 O values range from � 0.7 to � 2.0&, which is consistent with formation at low to moderate temperatures (40–55 C) from modern meteoric water. REE in the goethite and hematite are derived from the weathering of local sideritic Ironstones, silicified ultramafic rocks, sideritic black cherts, and local felsic volcanic rocks, falling along a mixing line between the Eu/Eu* and shale-normalized HREEAvg/LREEAvg values for the associated silicified ultramafic rocks and felsic volcanic rocks. Contrasting positive Ce/Ce* of 1.3 to 3.5 in hematite and negative Ce/Ce* of 0.2 to 0.9 in goethite provides evidence of oxidative scavenging of Ce on hematite surfaces during mineral precipitation. These isotopic and REE data, taken together, suggest that hematite and goethite Ironstone pods formed from relatively recent meteoric waters in shallow springs and/or subsurface warm springs.

  • Ironstone pods in the archean barberton greenstone belt south africa earth s oldest seafloor hydrothermal vents reinterpreted as quaternary subaerial springs comment and reply reply
    Geology, 2004
    Co-Authors: Donald R Lowe, Gary R Byerly
    Abstract:

    We respond to de Ronde et al. as per their numbered comments. 1. de Ronde et al. suggest that there are two populations of Ironstone pods in the Barberton greenstone belt: those on Farm Mendon that are Archean in age and composed primarily of hematite and all other Ironstone pods, which are young

A M Afify - One of the best experts on this subject based on the ideXlab platform.

  • differentiation of Ironstone types by using rare earth elements and yttrium geochemistry a case study from the bahariya region egypt
    Ore Geology Reviews, 2018
    Co-Authors: A M Afify, M E Sanzmontero, J P Calvo
    Abstract:

    Abstract This paper deals with the geological and geochemical characterization of Ironstone deposits encountered in two different sedimentary successions (upper Cretaceous and lower Cenozoic) in northern Bahariya, Egypt. The Ironstones occur as uneconomic thin bands, lenses and concretionary beds in Cenomanian clastic rocks of the Bahariya Formation and as economic iron ore associated with Eocene carbonate depositional units. The Ironstones contain similar iron-bearing minerals, mainly goethite and hematite, which display a variety of fabrics, i.e. concretionary, massive, stromatolitic-like, oolitic, pisolitic, reniform aggregates, boxwork, liesegang, geode-like and brecciated. The iron-rich minerals preferentially replaced and/or cemented the primary and diagenetic (mainly dolomite) carbonates. Preservation of fabrics, sedimentary structures and thickness of the precursor carbonates is conspicuous. Whole-rock composition of the Cretaceous Ironstones shows lower Fe2O3 and MnO contents and relative enrichment in detrital-derived elements, namely Al, Zr and Nb, when compared with the Eocene Ironstones, the latter showing enrichment in Fe, Mn, Si, and Ba oxides as well as Cu, Zn, Ni and Sr trace elements. Total REE content in the Cretaceous Ironstones ranges widely from 70 to 348 ppm whilst the Eocene Ironstones show quite low content, mostly from 1.96 to 31 ppm. Post Archean Australian Shale (PAAS)-normalized patterns of the upper Cretaceous Ironstones display flat REE patterns, with small positive Eu and Pr, slightly negative Y and Ce anomalies which are close to unity and intermediate Nd concentrations (between 10 and 100 ppm). In contrast, PAAS-normalized REE + Y patterns of the Eocene Ironstones display LREE positive slope with enriched flat HREE trend as well as negative Ce anomaly, positive Y and Eu anomalies, and low Nd concentration (in general less than 10 ppm). Geochemical data along with sedimentary features of the upper Cretaceous and Eocene Ironstones in Bahariya point to different origins in the two Ironstone types, the former having been originated diagenetically whereas the latter were constrained by hydrothermal fluids. A syngenetic marine origin is ruled out for any of the studied Ironstones. Higher amount of detrital derived elements as well as higher REE concentration in the upper Cretaceous Ironstones than in the Eocene Ironstones suggest some contribution from the associated clastic sediments. The negative Ce anomaly determined in both types indicates anoxic iron-rich solution that passed to oxidized surface. The presence of positive Y and Eu anomalies in the Eocene Ironstones suggests that iron precipitation was favoured by hydrothermal reducing conditions, when slightly acidic fluids reached oxidizing alkaline waters.

  • Ironstone deposits hosted in eocene carbonates from bahariya egypt new perspective on cherty Ironstone occurrences
    Sedimentary Geology, 2015
    Co-Authors: A M Afify, M E Sanzmontero, J P Calvo
    Abstract:

    Abstract This paper gives new insight into the genesis of cherty Ironstone deposits. The research was centered on well-exposed, unique cherty Ironstone mineralization associated with Eocene carbonates from the northern part of the Bahariya Depression (Egypt). The economically important Ironstones occur in the Naqb Formation (Early Eocene), which is mainly formed of shallow marine carbonate deposits. Periods of lowstand sea-level caused extensive early dissolution (karstification) of the depositional carbonates and dolomitization associated with mixing zones of fresh and marine pore-water. In faulted areas, the Eocene carbonate deposits were transformed into cherty Ironstone with preservation of the precursor carbonate sedimentary features, i.e. skeletal and non-skeletal grain types, thickness, bedding, lateral and vertical sequential arrangement, and karst profiles. The ore deposits are composed of iron oxyhydroxides, mainly hematite and goethite, chert in the form of micro- to macro-quartz and chalcedony, various manganese minerals, barite, and a number of subordinate sulfate and clay minerals. Detailed petrographic analysis shows that quartz and iron oxides were coetaneous and selectively replaced carbonates, the coarse dolomite crystals having been preferentially transformed into quartz whereas the micro-crystalline carbonates were replaced by the iron oxyhydroxides. A number of petrographic, sedimentological and structural features including the presence of hydrothermal-mediated minerals (e.g., jacobsite), the geochemistry of the ore minerals as well as the structure-controlled location of the mineralization suggest a hydrothermal source for the ore-bearing fluids circulating through major faults and reflect their proximity to centers of magmatism. The proposed formation model can contribute to better understanding of the genetic mechanisms of formation of banded iron formations (BIFs) that were abundant during the Precambrian.

  • diagenetic origin of Ironstone crusts in the lower cenomanian bahariya formation bahariya depression western desert egypt
    Journal of African Earth Sciences, 2015
    Co-Authors: A M Afify, M E Sanzmontero, J P Calvo, H A Wanas
    Abstract:

    Abstract In this paper, a new interpretation of the Ironstone crusts of the Bahariya Formation as late diagenetic products is provided. The siliciclastic Lower Cenomanian Bahariya Formation outcropping in the northern part of the Bahariya Depression (Western Desert, Egypt) is subdivided into three informal units that are mainly composed of thinly laminated siltstone, cross-bedded and massive sandstone, fossiliferous sandstone/sandy limestone and variegated shale. Abundant Ironstone crusts occur preferentially within its lower and upper units but are absent in the middle unit. The Ironstone crusts show selective replacement of carbonate components, including calcretes, by iron oxyhydroxides. More permeable parts of the terrigenous beds such as burrow traces, subaerial exposure surfaces, concretionary features and soft-sediment deformation structures led to heterogeneous distribution of the iron oxyhydroxides. A variety of diagenetic minerals, where goethite and hematite are the main end-products, were characterized by mineralogical analysis (XRD), petrography and SEM observation, and geochemical determinations (EMPA). Other diagenetic minerals include Fe-dolomite/ankerite, siderite, manganese minerals, barite, silica, illite/smectite mixed-layer, and bitumen. These minerals are interpreted to be formed in different diagenetic stages. Some minerals, especially those formed during eodiagenesis, show features indicative of biogenic activity. During burial, dolomite and ankerite replaced preferentially the depositional carbonates and infilled secondary porosity as well. Also during mesodiagenesis, the decomposition of organic matter resulted in the formation of bitumen and created reducing conditions favorable for the mobilization of iron-rich fluids in divalent stage. Telodiagenesis of the Cenomanian Bahariya deposits took place during the Turonian–Santonian uplift of the region. This resulted in partial or total dissolution of Fe-dolomite and ankerite which was concomitant to iron oxyhydroxide precipitation upon mixing with shallow oxygenated water. Circulation of reducing iron-rich fluids through fractures and inter and intrastratal discontinuities is proposed as an alternative model to explain the controversial source of iron for the Ironstone crusts of the Bahariya Formation. The origin of iron-rich fluids is probably related to the basement rocks. The provided model relates the fluid movements through fractures and discontinuities with the preferential replacement of carbonates. This combination of processes is consistent with the heterogeneous geometries and the wide distribution of the Ironstones.

Walid Salama - One of the best experts on this subject based on the ideXlab platform.

  • facies analysis and palaeoclimatic significance of Ironstones formed during the eocene greenhouse
    Sedimentology, 2014
    Co-Authors: Walid Salama, Mortada El Aref, Reinhard Gaupp
    Abstract:

    Lower and middle Eocene Ironstone sequences of the Naqb and Qazzun formations from the north-east Bahariya Depression, Western Desert, Egypt, represent a proxy for early Palaeogene climate and sea-level changes. These sequences represent the only Palaeogene economic ooidal Ironstone record of the Southern Tethys. These Ironstone sequences rest unconformably on three structurally controlled Cenomanian palaeohighs (for example, the Gedida, Harra and Ghorabi mines) and formed on the inner ramp of a carbonate platform. These palaeohighs were exposed and subjected to subaerial lateritic weathering from the Cenomanian to early Eocene. The lower and middle Eocene Ironstone sequences consist of quiet water Ironstone facies overlain by higher energy Ironstone facies. The distribution of low-energy Ironstone facies is controlled by depositional relief. These deposits consist of lagoonal, burrow-mottled mud-Ironstone and laterally equivalent tidal flat, stromatolitic Ironstones. The agitated water Ironstone facies consist of shallow subtidal–intertidal nummulitic–ooidal–oncoidal and back-barrier storm-generated fossiliferous Ironstones. The formation of these marginal marine sequences was associated with major marine transgressive–regressive megacycles that separated by subaerial exposure and lateritic weathering. The formation of lateritic palaeosols with their characteristic dissolution and reprecipitation features, such as colloform texture and alveolar voids, implies periods of humid and warm climate followed major marine regressions. The formation of the lower to middle Eocene Ironstone succession and the associated lateritic palaeosols can be linked to the early Palaeogene global warming and eustatic sea-level changes. The reworking of the middle Eocene palaeosol and the deposition of the upper Eocene phosphate-rich glauconitic sandstones of the overlying Hamra Formation may record the initial stages of the palaeoclimatic transition from greenhouse to icehouse conditions.

  • paleoenvironmental significance of aluminum phosphate sulfate minerals in the upper cretaceous ooidal Ironstones e ne aswan area southern egypt
    International Journal of Earth Sciences, 2014
    Co-Authors: Walid Salama
    Abstract:

    Aluminum phosphate-sulfate (APS) minerals are present as small, disseminated crystals in the upper Cretaceous shallow marine ooidal Ironstones, E-NE Aswan area, southern Egypt. Their association with the Ironstones is considered as a proxy of subaerial weathering and post-diagenetic meteoric water alteration. The mineralogical composition of the ooidal Ironstones was investigated by optical and scanning electron microscopes, X-ray diffraction, Fourier transform infrared and Raman spectroscopy. The ooidal Ironstones are composed mainly of ooids and groundmass, both of which consist of a mixture of detrital (quartz) and diagenetic (fluorapatite, chamosite and pyrite) mineral assemblages. These mineral assemblages are destabilized under acidic and oxidizing, continental conditions. These conditions resulted from the oxidation of pyrite and probably organic matter under warm and humid, tropical climate followed the Santonian Sea regression and subaerial exposure. These pedogenic conditions promoted corrosion of quartz, dissolution of chamosite and apatite and hydrolysis of feldspars of the nearby exposed granitoids. The released Si, Al and Sr from quartz, chamosite and feldspars; Fe and S from pyrite and P, Ca and light rare earth elements (LREE) from apatite are reprecipitated as hematite, kaolinite, apatite and APS minerals from the pore fluids or along fractures. The paragenetic sequence and textural relationships of this post-diagenetic mineral assemblage indicate that hematite was formed by replacement of chamosite followed by formation of a secondary generation of pore filling chlorapatite and APS minerals and finally the precipitation of kaolinite in the remaining pore spaces. The formation of APS minerals and chlorapatite is simultaneous, but APS minerals are stable at shallow depths under acidic to neutral pH conditions, whereas chlorapatite is stable under alkaline pH conditions. Alkaline conditions were maintained at greater depths when the infiltrated acidic fluids reacted with chamosite. The APS minerals display a homogeneous chemical composition in all Ironstone locations in Aswan area, corresponding to a solid solution between crandallite (CaAl3(PO4)2(OH)5·H2O), goyazite (SrAl3(PO4)2(OH)5·H2O), svanbergite (SrAl3(PO4)(SO4)(OH)6) and woodhouseite (CaAl3(PO4)(SO4)(OH)6) end-members. The variations in the APS mineral chemistry (AB3(XO4)2(OH)6) are essentially due to variable substitutions of Sr and LREE for Ca at the A site and limited S for P at the X site. The spatial distribution of APS minerals and their composition in the ooidal Ironstones of Aswan area permitted to consider them as good tracers of physicochemical and paleoenvironmental changes, in particular those associated with subaerial exposure and pedogenesis. The post-diagenetic phosphatization and kaolinization of the Aswan Ironstones decrease their economic potentiality; thus, understanding paragenetic sequence and textural relationships is essential for the iron ore beneficiation.

  • identification of minerals and organic materials in middle eocene Ironstones from the bahariya depression in the western desert of egypt by means of micro raman spectroscopy
    Journal of Raman Spectroscopy, 2012
    Co-Authors: Valerian Ciobotă, Walid Salama, Reinhard Gaupp, Nicolae Tarcea, Petra Rosch, Mourtada El Aref, Jurgen Popp
    Abstract:

    The Middle Eocene Ironstones of the Bahariya Depression consist of four iron ore types: manganiferous mud-Ironstone, fossiliferous Ironstone, stromatolitic Ironstone and nummulitic–ooidal–oncoidal Ironstone. The upper surfaces of these sequences were subjected to subaerial weathering and a lateritic iron ore type was formed. The chemical composition of these Ironstone types was investigated by means of micro-Raman spectroscopy. Various closely related iron-containing and manganese-containing minerals were detected by means of the above-mentioned approach. The high spatial resolution and sensitivity of this method allowed us to identify minerals that could not be detected by other techniques. Well-preserved organic materials were observed in one type of Ironstones. Therefore, using Raman spectroscopy, we were able to provide evidence that the formation of some of the investigated rocks was biologically mediated. The application of Raman spectroscopy is considered a powerful technique for the identification of both organic and inorganic substances in the studied iron ore deposits. Copyright © 2011 John Wiley & Sons, Ltd.

J P Calvo - One of the best experts on this subject based on the ideXlab platform.

  • differentiation of Ironstone types by using rare earth elements and yttrium geochemistry a case study from the bahariya region egypt
    Ore Geology Reviews, 2018
    Co-Authors: A M Afify, M E Sanzmontero, J P Calvo
    Abstract:

    Abstract This paper deals with the geological and geochemical characterization of Ironstone deposits encountered in two different sedimentary successions (upper Cretaceous and lower Cenozoic) in northern Bahariya, Egypt. The Ironstones occur as uneconomic thin bands, lenses and concretionary beds in Cenomanian clastic rocks of the Bahariya Formation and as economic iron ore associated with Eocene carbonate depositional units. The Ironstones contain similar iron-bearing minerals, mainly goethite and hematite, which display a variety of fabrics, i.e. concretionary, massive, stromatolitic-like, oolitic, pisolitic, reniform aggregates, boxwork, liesegang, geode-like and brecciated. The iron-rich minerals preferentially replaced and/or cemented the primary and diagenetic (mainly dolomite) carbonates. Preservation of fabrics, sedimentary structures and thickness of the precursor carbonates is conspicuous. Whole-rock composition of the Cretaceous Ironstones shows lower Fe2O3 and MnO contents and relative enrichment in detrital-derived elements, namely Al, Zr and Nb, when compared with the Eocene Ironstones, the latter showing enrichment in Fe, Mn, Si, and Ba oxides as well as Cu, Zn, Ni and Sr trace elements. Total REE content in the Cretaceous Ironstones ranges widely from 70 to 348 ppm whilst the Eocene Ironstones show quite low content, mostly from 1.96 to 31 ppm. Post Archean Australian Shale (PAAS)-normalized patterns of the upper Cretaceous Ironstones display flat REE patterns, with small positive Eu and Pr, slightly negative Y and Ce anomalies which are close to unity and intermediate Nd concentrations (between 10 and 100 ppm). In contrast, PAAS-normalized REE + Y patterns of the Eocene Ironstones display LREE positive slope with enriched flat HREE trend as well as negative Ce anomaly, positive Y and Eu anomalies, and low Nd concentration (in general less than 10 ppm). Geochemical data along with sedimentary features of the upper Cretaceous and Eocene Ironstones in Bahariya point to different origins in the two Ironstone types, the former having been originated diagenetically whereas the latter were constrained by hydrothermal fluids. A syngenetic marine origin is ruled out for any of the studied Ironstones. Higher amount of detrital derived elements as well as higher REE concentration in the upper Cretaceous Ironstones than in the Eocene Ironstones suggest some contribution from the associated clastic sediments. The negative Ce anomaly determined in both types indicates anoxic iron-rich solution that passed to oxidized surface. The presence of positive Y and Eu anomalies in the Eocene Ironstones suggests that iron precipitation was favoured by hydrothermal reducing conditions, when slightly acidic fluids reached oxidizing alkaline waters.

  • Ironstone deposits hosted in eocene carbonates from bahariya egypt new perspective on cherty Ironstone occurrences
    Sedimentary Geology, 2015
    Co-Authors: A M Afify, M E Sanzmontero, J P Calvo
    Abstract:

    Abstract This paper gives new insight into the genesis of cherty Ironstone deposits. The research was centered on well-exposed, unique cherty Ironstone mineralization associated with Eocene carbonates from the northern part of the Bahariya Depression (Egypt). The economically important Ironstones occur in the Naqb Formation (Early Eocene), which is mainly formed of shallow marine carbonate deposits. Periods of lowstand sea-level caused extensive early dissolution (karstification) of the depositional carbonates and dolomitization associated with mixing zones of fresh and marine pore-water. In faulted areas, the Eocene carbonate deposits were transformed into cherty Ironstone with preservation of the precursor carbonate sedimentary features, i.e. skeletal and non-skeletal grain types, thickness, bedding, lateral and vertical sequential arrangement, and karst profiles. The ore deposits are composed of iron oxyhydroxides, mainly hematite and goethite, chert in the form of micro- to macro-quartz and chalcedony, various manganese minerals, barite, and a number of subordinate sulfate and clay minerals. Detailed petrographic analysis shows that quartz and iron oxides were coetaneous and selectively replaced carbonates, the coarse dolomite crystals having been preferentially transformed into quartz whereas the micro-crystalline carbonates were replaced by the iron oxyhydroxides. A number of petrographic, sedimentological and structural features including the presence of hydrothermal-mediated minerals (e.g., jacobsite), the geochemistry of the ore minerals as well as the structure-controlled location of the mineralization suggest a hydrothermal source for the ore-bearing fluids circulating through major faults and reflect their proximity to centers of magmatism. The proposed formation model can contribute to better understanding of the genetic mechanisms of formation of banded iron formations (BIFs) that were abundant during the Precambrian.

  • diagenetic origin of Ironstone crusts in the lower cenomanian bahariya formation bahariya depression western desert egypt
    Journal of African Earth Sciences, 2015
    Co-Authors: A M Afify, M E Sanzmontero, J P Calvo, H A Wanas
    Abstract:

    Abstract In this paper, a new interpretation of the Ironstone crusts of the Bahariya Formation as late diagenetic products is provided. The siliciclastic Lower Cenomanian Bahariya Formation outcropping in the northern part of the Bahariya Depression (Western Desert, Egypt) is subdivided into three informal units that are mainly composed of thinly laminated siltstone, cross-bedded and massive sandstone, fossiliferous sandstone/sandy limestone and variegated shale. Abundant Ironstone crusts occur preferentially within its lower and upper units but are absent in the middle unit. The Ironstone crusts show selective replacement of carbonate components, including calcretes, by iron oxyhydroxides. More permeable parts of the terrigenous beds such as burrow traces, subaerial exposure surfaces, concretionary features and soft-sediment deformation structures led to heterogeneous distribution of the iron oxyhydroxides. A variety of diagenetic minerals, where goethite and hematite are the main end-products, were characterized by mineralogical analysis (XRD), petrography and SEM observation, and geochemical determinations (EMPA). Other diagenetic minerals include Fe-dolomite/ankerite, siderite, manganese minerals, barite, silica, illite/smectite mixed-layer, and bitumen. These minerals are interpreted to be formed in different diagenetic stages. Some minerals, especially those formed during eodiagenesis, show features indicative of biogenic activity. During burial, dolomite and ankerite replaced preferentially the depositional carbonates and infilled secondary porosity as well. Also during mesodiagenesis, the decomposition of organic matter resulted in the formation of bitumen and created reducing conditions favorable for the mobilization of iron-rich fluids in divalent stage. Telodiagenesis of the Cenomanian Bahariya deposits took place during the Turonian–Santonian uplift of the region. This resulted in partial or total dissolution of Fe-dolomite and ankerite which was concomitant to iron oxyhydroxide precipitation upon mixing with shallow oxygenated water. Circulation of reducing iron-rich fluids through fractures and inter and intrastratal discontinuities is proposed as an alternative model to explain the controversial source of iron for the Ironstone crusts of the Bahariya Formation. The origin of iron-rich fluids is probably related to the basement rocks. The provided model relates the fluid movements through fractures and discontinuities with the preferential replacement of carbonates. This combination of processes is consistent with the heterogeneous geometries and the wide distribution of the Ironstones.