الفهرس 
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Abstract
CHAPTER ELEVEN
SUMMARY AND CONCLUSIONS
The Paleozoic succession in southwestern Sinai overlies the Precambrain Basement Complex. This succession consists of the formations (from older to younger): Sarabit El-Khadim, Abu Hamata, Adediya, Um Bogma and Abu Thora.
The mineralized Lower Carboniferous Um Bogma Formation was studied in eight localities distributed in a vast area east of Wadi Baba wherein its exposures are relatively well developed. These localities are (from west to east) Talet Seliem, Abu Zarab, Abu Thor, Farsh Al- Azraq, Allouga, Lehyan, Wadi Sewig and Ramlet Himeyir. Extensive field studies on the Um Bogma Formation in these localities comprised the construction of geologic maps, the collection of a large number of spot and channel samples as well as carrying out radioactive field measurements.
The formation in the study area consists of three distinct members showing marked lateral variations in thickness and lithology. Throughout the study area, the lower member is made up of siltstone, sandstone, dolostone, dolomitic limestone, siltstone- hosting Fe- sulfides and siltstone and sandstone hosting Fe- Mn ore. The middle member consists of shale, claystone, mudstone, siltstone, sandstone, conglomerate, dolostone and dolomitic limestone. The upper member is made up of dolostone, dolomitic limestone, sandstone and conglomerate. Geographically, the lower member of Um Bogma Formation is composed mainly of argillaceous rocks interbedded with some carbonates in the western, central and southern sectors of the study area and argillaceous rocks in its eastern sector. The middle member is made up of interbeds of siltstone, claystone, dolostone and conglomerate in the most western sector of the area, siltstones and mudstone in its western and central sectors and siltstone, sandstone and dolostone in its southwestern sector and siltstone at the east. The upper member is composed of dolostone and dolomitic limestones throughout the study area except for its central sector (siltstone) and at the most east locality (sandstone and conglomerate). The rocks of the formation display some primary sedimentary structures such as bedding, lamination and flow structure. The secondary structures are represented by normal faults and some fissures filled with gypsum and carbonates.
The facies characteristics of the three members of Um Bogma Formation in the study area were determined based on their lithologic and petrographic attributes as well as the primary sedimentary structures. The lower member consists of four facies types; namely: mudstone- gibbsite- dolostone- dolomitic limestone, bedded laminated siltstone, dolostone- dolimitic limestone and sandstone. The middle member is made up of five facies types; namely: siltstone- claystone- dolostone, laminated siltstone- mudstone, laminated siltstone, siltstone- gibbsite- dolostone and siltstone- sandstone- dolostone. The upper member consists of three facies types; namely: dolostone, siltstone and sandstone- conglomerate.
The detailed petrographic and mineralogic characteristics of the studied Um Bogma rocks were determined using several laboratory techniques. The obtained results revealed that the argillaceous rocks are texturally classified as claystone, mudstone, sandy mudstone, siltstone, muddy sandy siltstone and sandy siltstone. The arenaceous rocks are represented by sandstone, muddy sandstone and gravelly muddy sandstone whereas the rudaceous rocks are muddy conglomerate and muddy sandy conglomerate. The argillaceous rocks are slightly calcareous, poorly- to very poorly- sorted and consist of variably highly ferruginous clayey matrices containing much silt- sized and some medium to coarse sand- sized quartz grains that are mainly angular to subrounded, monocrystalline and display unit extension. In many cases, the matrices are colour- and/or size- laminated. Fe oxides occur in the form of grain- coatings, pigments, minute floccules and globules, colloform aggregates, laminae, streaks, vug-, fracture- and intergranular pore- fillings and/or replacing parts of the rock matrix. The Fe- Mn ore in the lower member at Lehyan and Ramlet Himeyir localities consists of highly ferruginous, occasionally manganiferous silty and sandy matrices. On the other hand, the carbonate rocks in the three members are composed of groundmasses consisting of varying proportions of anhedral to subhedral micro- to macrocrystalline calcite and dolomite crystals together with some silt- and sand- sized quartz grains. These carbonates are petrographically classified as sparite, dolosparite, intra-sparite, dolo- intrasparite, dolo- macrosparite, dolo-biosparite, dolomitized intra- oosparite and dolomitized oo-bio-microsparite. Occasionally, the Um Bogma rocks are fossiliferous. They contain ooids, peloids, echinid fragments and unidentifiable fossil remains.
The rocks of Um Bogma Formation consist of large assemblages of essential and accessory minerals that are non-radioactive, radioactive and U-, Th-, REEs- and base metals- bearing. The argillaceous rocks of the lower member are composed of kaolinite, chlorite and traces of illite together with quartz, gibbsite, hematite, pyrite, dolomite, calcite, gypsum, anhydrite, magnetite, jarosite, cryptomelane and pyrolusite. Their heavy fractions contain uranophane, monazite, cuprosklodowskite, torbernite, kasolite, thorite, fergusonite, sklodowskite, zircon, xenotime, celestite, wolframite, coronadite, carlenite, jalpaite, zincite, vernadite, rutile, goethite, galena and barite, chalcopyrite, chalcocite, atacamite, azurite, bunsenite, brass, silver, gold together with unidentifiable base metals- bearing minerals. The argillaceous rocks of the middle member are composed of chlorite, kaolinite, phillpsite and traces of illite together with quartz, hematite, goethite, gibbsite, gypsum, anhydrite, halite, dolomite, calcite, ankerite, jarosite, pyrolousite, , cryptomelane and malachite. Their heavy fractions contain sklodowskite, torbernite, uranothorite, carnotite, uranophane, kasolite, thorite, thorianite, allanite, zircon, monazite, xenotime, baddeleyite, wolframite, laffosaite, willemite, pyrite, chalcopyrite, arsenopyrite, galena, atacamite, chalcocite, barite, celestite, celectobarite, molybdnite, vernadite, pyrochroite, cochromite, geogericksenite, grimaldite, bunsenite, quadratite, chorargyrite, brass and gold together with unidentifiable REEs- bearing minerals. Rocks of the upper member are composed of dolomite, calcite, quartz, hematite and pyrolusite, in addition to minor kaolinite, chlorite and traces of illite. Their heavy fractions contain uranophane, thorite, xenotime, allanite, monazite, zircon, columbite, molybdinite, cassiterrite, bunsenite, pyrolusite, vernadite, pyrite, chalcopyrite, barite, celestite, celestobarite and gypsum together with unidentifiable REEs- bearing minerals. Based on their forms, habits and modes of occurrences, the essential and accessory minerals in the Um Bogma rocks could be cassified into four groups; namely; primary allogenic, primary authigenic, secondary allogenic and secondary authigenic.
The geochemical characteristics of the studied rocks were investigated by determining the concentrations and behaviour of major and trace elements in addition to REEs and base metals. The obtained chemical data were normalized to those of the Upper Continental Crust (UCC), the Post-Archean Australian Average Shale (PAAS) and chondrites. Also, these data were statistically treated to calculate a number of significant chemical parameters and plotted on several variation diagrams. The obtained results revealed that most of the clastic rocks are variably enriched in Fe2O3 and MnO. The varying degrees of their depletion in MgO, Na2O, K2O, Al2O3, P2O5 and TiO2 are attributed to their extensive ferrugination. On the other hand, the carbonates are variably enriched in CaO, MgO and MnO. The Um Bogma rocks are generally characterized by the presence of high concentrations of trace elements as a result of the existence of a large number of their bearing minerals. Rocks of the middle member have the highest trace elements contents. The distributions of these elements among the different rock types indicate their affinities to specific lothologies. Their concentrations in ironstones, bauxite and argillaceous rocks are higher than those in the calcareous, arenaceous and rudaceous rocks. Zn and Y show more affinity to ironstones and bauxite. The light and heavy REEs are relatively more enriched in bauxite, U and Pb show more affinities to the argillaceous rocks whereas Cu is more related to the carbonates. These findings are in accordance with the distributions of these elements in the Um Bogma rocks of the different localities. Those in Allouga and, to a lesser extent, Talet Seliem and Abu Zarab localities are the most enriched in these elements. This implies that the concentrations of the trace elements in the rocks increase from east to the west in the study area. The middle member has the lowest averaged values of Th/U ratio as compared to those of the other two members indicating deposition under relatively more reducing conditions.
The varying uranium enrichment in the rocks of Um Bogma Formation is correlated with the occurrence of uranyl silicate minerals such as uranophane, sklodowskite, cuprosklodowskite, carnotite, uranothorite, kasolite and torbernite. The averaged values of U content in the rocks of all localities are much higher than that for the UCC. The association of these uranyl minerals with a set of alteration products (such as goethite, hematite and malachite) and the occurrence of Mn ore in some localities suggest that the study area was affected by high- and low- temperature hydrothermal activities. This fluids activity could have been started with the Red Sea extension that permitted the circulation of the hydrothermal solutions through the different types of fractures and the initiation of ore mineral precipitation under near-neutral conditions. The residual fluids continued to move upwards through the fractures and became neutral to slightly alkaline and, thus, suitable for the precipitation of uranyl silicates. The chemistry of these fluids was greatly influenced by their reaction with their host rocks. These fluids were responsible for the oxidation of sulfides and alteration of primary U- and REEs- bearing minerals. Uranium and REEs were released from the pre-existing accessory minerals such as monazite, xenotime, uranothorite and zircon. This is in accordance with the occurrence of surficial uranyl and REEs- enriched mineral phases. The co-existence of several uranyl silicate minerals in the Um Bogma rocks in the study area reflects variations in the activities of these elements.
Rocks of Um Bogma Formation in the study area show enrichment in the light and heavy REEs. Those of the lower and middle members in the most western Talet Seliem contain the highest concentrations of REEs. Also the rocks of the middle member in Abu Thor and Abu Zarab localities contain relatively high REEs concentrations whereas those in the central Allouga and Lehyan localities and the southern Farsh Al-Azraq locality have the lowest concentrations of these elements. Rocks of this member in the most eastern Ramlet Himeyir locality contain moderate concentrations of REEs. The upper member of the formation in all localities, except for Allouga, has the lowest concentrations of REEs. The non-significant correlation between the ΣREEs and Al2O3 indicates that the terrigenous clayey materials do not represent a major source for the REEs. On the other hand, bauxite is their most favorable host rock type.
All the reported REEs data share a mild but diagnostic depletion in the LREEs when normalized to the PAAS shale and UCC. This could be the result of the REEs characteristics of the detrital load, their interaction with seawater and/or a combination of these two effects. The rocks have super chrondritic Y/Ho ratios which are consistent with their LREE depletion.
All the analyzed samples showed negative Eu anomalies when normalized to the chonderite. Also, all the rocks show negative Ce anomalies when normalized to the chonderite except for those of the middle member in Talet Seliem locality and some samples of this member in Abu Thor, Abu Zarab and Lehyan localities, in addition to the lower member.
Statistical treatments of the chemical data for the studied rock samples showed that there are strong positive correlation between Th and each of Zr and Hf which strongly suggests that Th largely exists in the structure of zircon. Also, the positive correlations among the elements Th, Bi, V, Zr, Sn, Hf, Ta and Nb reflect their common source. Factor analysis of the obtained chemical data revealed that there are five factors that controlled the deposition of Um Bogma rocks. The terrestrial factor expresses about 34% of the total variance and indicates the influence of the terrestrial controls (positive loading) with negative loading with the marine controls on the depositional environments. The second factor accounts for 10% of the total variance and represents the redox-sensitive elements Mo, Cd, Ni and Zn. This factor shows positive loadings with Y, LREEs and HREEs, where xenotime is considered to be a good HREEs accumulator. The third factor represents the effect of diagenesis and shows positive loadings with the main partners Cu, Cr and Re. The ferrugination factor expresses about 7% of the total variance and loads positively with Fe2O3, MnO, Na2O, As and Ba. The marine sediments factor expresses about 5% of the total variance and displays positive loadings with Mg, P2O5, Te, Sr and Ca. The positive loading of P2O5 with Ca reflects a shallow marine depositional regime. The factor analysis of the REEs in the studied clastic rocks showed a distinct geochemical trend. This trend is strongly distinguished for LREEs and HREEs due to the presence of associations of REEs- bearing minerals (such as monazite, fergusonite and xenotime) in addition to other unidentifiable minerals.
Applying the enrichment factor revealed that although almost all the elements show distinict enrichment, however, most of them bear no significant potentiality. The Um Bogma rocks in Talet Seliem locality and the gibbsite mineralizations in the middle member at Lehyan locality contain high potential of REEs. Allouga locality contains the highest potential of uranium and copper. The lower member of Um Bogma Formation in Lehyan and Ramlet Himeyir localities contain the highest poteintial of MnO. The extension of the area is a very important factor controlling the potentiality. Abu Zarab locality contains high potential of some elements but their concentrations are distributed in a vast area and, thus, need big effort and cost for extraction. On the contrary, Wadi Sewig and Ramlet Hemiyir localities have limited extensions which makes them promising for extraction. Also, many of elements (such as Sb, Rb, Sr and Ga) are of insignificant. The heterogeneity in the concentrations dominates both in vertical and horizontal scales as well as with respect to lithology. Some lithologies seem to have preferential affinity for accumulating certain elements (such as gibbsite for concentrating REEs). The sporadic and disseminated spots or patches that have high content of specific element(s) (such as the high Cu content in sample L10.M) are of no economic importance.
Sequential extraction of elements from the studied Um Bogma rock samples showed that the detrital input (insoluble residue) and the elements related to the Fe, Mn- bearing solutions are the most responsible factors for accumulating the rare earth metals. The relatively weak electrostatic interactions have the smallest effect for accumulating these elements. The organic activity had a smaller effect in the Um Bogma rocks of the eastern sector of the study area as compared with those at the west.
The results of the laboratory radiometric examinations carried out on the Um Bogma rock samples are closely similar to those obtained by the field measurements. The studied samples show wide variations in their U, Th, Ra and K contents. The eU values vary from U.L.D to 20000 ppm, eTh content from U.L.D to 63 ppm, Ra from 2 to 930 ppm and potassium from U.L.D to 3.91 %. The activity concentrations of these radionuclides vary among the rocks in the various localities as a result of differences in the mineral and chemical and compositions.
In all localities (except for Allouga), the mean activity concentrations are of the order 232Th < 238U < 40K. The studied samples have higher averaged contents of uranium, thorium and potassium when compared with those for arenaceous and argillaceous rocks as reported by IAEA (1979) and Boyle (1982). Also, the total dose rates (35.72 to 120400.78 nGy/hr) are much higher than the world average value (57 nGy/hr). The outdoor gamma dose rates (0.0031 to 53 nGy/hr) are higher than the world average (60 nGy/hr) as given by UNSCEAR (2000). The Um Bogma rocks in the western and central sectors of the study area are more radioactive than those at the east that, however, have higher Th contents. This is mostly attributed to differences in the type and extent of diagenetic processes. Most of the rock samples have values of chemically analyzed uranium greater than those obtained by the radiometric measurements reflecting disequilibrium states characterized by the addition of uranium. This strongly suggests that the uranium mineralizations in the studied rocks are recent.
The sedimentary history of the studied Um Bogma rocks and their hosted mineralizations was assessed based on their geological, petrographic, mineralogic and geochemical characteristics. The clastic rocks are argillaceous and, much less commonly, arenaceous and rudaceous. Evidently, their detrital components were inherited from mixed sources. These are represented by felsic to mafic igneous rocks and, to a lesser extent, metamorphic and older sedimentary rocks in Post-Archean source areas. These parent rocks were moderately to strongly affected by chemical weathering and alteration under predominating tropical climatic conditions. The detritus were derived from different zones of weathering. The weathering products were transported (mainly mechanically) to the depositional environments by rivers and streams having low to moderate strength with a minor role played by winds.
Deposition of the Um Bogma sediments occurred in shoreline and, in a few cases, fluviatile environments. Those of the lower member were accumulated in lakes, lagoons and, in the most western Talet Seliem locality, tidal flats. In contrast, sediments of this member in the most eastern Ramlet Hemiyer locality were deposited in fluviatile environments represented mainly by river channels and alluvial fans. Similar to those of the lower member, sediments of the middle member were accumulated in lakes, lagoons and, in the southwestern Farsh Al-Azraq and the central Lehyan localities, tidal flats and tidal deltas. The transition from the middle to the upper members witnessed some changes in the depositional environments as a result of minor sea transgression. In most localities, sediments of the upper member were accumulated in tidal flats and tidal deltas. However, those in the central Allouga locality were laid down in lakes and lagoons whereas those in Wadi Sewig locality were deposited in fluviatile environments represented mainly by river channels and alluvial fans.
Geochemical evidences indicate that the depositional environments of the Um Bogma sediments were characterized by weakly stratified water columns. Deposition of sediments of the three members occurred under warm humid climatic conditions and witnessed fluctuations in the water salinity as a result of varying degrees of inflow of fresh waters.
Diagenesis played a major role in modifying the original textural and compositional characteristics of the studied Um Bogma rocks. Burial compaction effects are represented by the development of fissility and flow structures in the argillaceous rocks, pressure- solution grain contacts in sandstones and partial fracturing of the carbonate rocks. Cementation is the most prominent post- depositional process. It commenced during the burial stage but was most profound during epigenesis by the action of subsurface meteoric waters under occasional evaporitic conditions as well as the circulating subsurface waters. The cements consist of Fe oxides (mainly hematite) and, much less commonly, goethite, Mn- bearing minerals, gypsum, anhydrite, halite and barite. Replacement of parts of the rock matrices by Fe oxides and, rarely, Mn- bearing minerals, carbonates and silica was a rather common process. Alteration processes in the argillaceous rocks are represented primarily by the epigenetic oxidation of Fe- and Mn- bearing minerals that resulted in the formation of their oxides (mainly hematite) and hydroxides. In the carbonate rocks, alteration processes involved mainly the partial or complete dolomitization of the originally calcitic and/or aragonitic limestones. Most probably, this process took place during the mesogenetic (burial) stage by the action of circulating groundwaters having high Mg2+/Ca2+ ratios and/or during epigensis in the zone of mixing of meteoric groundwater and sea water that was developed as a result of minor sea regressions. Dissolution effects are more profound in the carbonates than in the argillaceous rocks. These effects resulted in the development of vugs, channels and other karstification features,.