الفهرس 
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Abstract
Huge quantities of practically untapped talc-carbonate rocks are distributed in Central Eastern Desert (CED) and Southern Eastern Desert (SED) of Egypt. These unexploited natural deposits could serve as an important potential source of magnesia which is used in a broad spectrum of strategic industries such as refractory ceramics and electrical insulators.
This study deals with the talc-carbonate rocks of Barramiya and Wadi Atalla districts. The huge talc- carbonate masses at Barramiya area and Wadi Atalla are the largest outcrops in the CED. The objective is to characterizing and evaluating the tremendous talc-carbonate piles in both districts as well as preparing detailed geologic maps on adequate scales and estimating their reserves.
Evaluation of these characteristic rocks as planned has been achieved throughout two complementary steps: the first addresses the geological, mineralogical and geochemical characteristics and the second evaluates their suitability for some industrial applications.
In general, tremendous talc-carbonate piles which could serve as potential magnesia source are distributed in Egyptian Eastern Desert in intimate association with the ophiolitic serpentinites. Ophiolitic serpentinites represent obducted oceanic crust onto island-arc assemblages. The distribution of talc-carbonate deposits is confined either along regional faults that cut their ultramafic-hosts or at the contacts between the ultramafic rocks and the siliceous country rocks. The deposits located along fault system that cut the ultramafic host are generally magnesite-rich, whereas those located within serpentinites sheets; periphery and boarders of serpentinite slices are typically talc-rich.
The main serpentinite outcrops and their derivates talc-carbonates of the CED are scattered as elongated masses (ranges) along with both Mersa Alam – Edfu and Quseir – Qift highways. Both localities are easily accessible via excellent asphaltic roads, whereas Barramiya district is approximately midway between Mersa Alam (on the Red Sea coast) and Idfu (on the Nile valley) and Wadi Atalla is located along Quseir – Qift highway.
Detailed geologic studies of the huge talc-carbonate masses in both Barramiya and Wadi Atalla districts were carried out. These include the preparation of detailed geologic maps and reserve estimations using remote sensing and GIS techniques. Representative geologic and technologic samples were collected from both localities.
Regarding remote sensing and GIS techniques, the band ratioing is applied to Landsat-7(ETM+) images acquired in the year 2000. Enhancement process was carried out using ancillary data such as digitized topographic (1: 50.000) and geologic maps of 1: 250.000 (EGSMA/BGS, 1992a and b) of the studied areas. These tools were used in refinement process of the obtained false colour composite maps in conjunction with other ground truth data, i.e. previous geologic excursions to the considered areas including the field trips relating to the current work.
Consequently, detailed geologic maps of Barramiya and Wadi Atalla districts were successfully yielded from RGB band ratio images. In which, the different Late Proterozoic rocks were reasonably discriminated from each other. In this context, the talc-carbonate rock unit is reasonably discriminated from the serpentinites by using band ratios (7/5, 7/4, and 4/2).
After the separation of talc-carbonate rocks as separate layers, reserve calculations were accomplished for both localities, whereas about 30,903,766 and 13,705,955 tons are given as estimated reserves for Barramiya and Wadi Atalla districts, respectively.
Petrographically, the serpentine minerals are mainly antigorite with subordinate content of lizardite. Antigorite occurs as aggregates of fibro-lamellar and/or arborescent structures. These aggregates are randomly or preferentially oriented bundles. Every gradation from serpentine to talc-carbonate assemblage is observed.
Regarding the studied talc-carbonate rocks, talc and magnesite represent the main constituents whereas dolomite, serpentine and quartz are common subordinates. The accessories are opaques, calcite, chlorite and phlogopite. Based on proportional distribution of the main constituents (talc and magnesite), the studied Barramiya talc-carbonate rocks are classified into the following varieties and sub-varieties (assemblages):
I. Talc-carbonate variety A (TCA): This petrographic variety is almost set up from talc and magnesite in roughly equal proportions by weight and could be further subdivided into:
I.1. TCA1: magnesite + talc (M>T)
1.2. TCA2: talc + magnesite (T>M) ± serpentine ± dolomite
II. Talc-carbonate variety B (TCB): talc is the main constituent in this rock type and characterized by the assemblage: talc ± magnesite ± serpentine ± dolomite
III. Talc-carbonate variety C (TCC): talc is the main constituent with frequent contents of dolomite, serpentine and quartz and could further be subdivided into the following assemblages:
III.1. TCC1: talc + dolomite ± serpentine ± magnesite
III.2. TCC2: talc + serpentine ± dolomite ± magnesite ± chlorite
III.3. TCC3: talc + quartz ± dolomite ± magnesite
Similarly, the talc carbonate rocks of Wadi Atalla are distinguished into three assemblages (varieties) according to the main mineralogical composition. These assemblages are:
I. TCA1: magnesite + Talc (M>T)
II. TCA2: talc + magnesite (T>M) ± serpentine
III. TCB: talc ± magnesite ± serpentine ± dolomite
Regarding the studied talc-carbonate rocks, the evolution from serpentinite precursors to talc was accompanied by slight enrichment in SiO2 and simultaneous depletion in all major contents. Of the trace elements, the transformation was accompanied by slight enrichment in Ba and depletion in Sr, Cr, Ni and Co.
Similarly, the transformation of serpentinite to magnesite-rich talc-carbonates is characterized by depletion of all major elements, except MnO. This was accompanied by more depletion of Sr, Cr, Ni & Co and slight enrichment in Zn.
The chemical composition of studied talc-carbonate rocks in general is consistent with the recorded petrographic assemblages. In terms of the main element constituents (SiO2 – MgO – CaO, all as mol %), the studied talc-carbonate rocks exhibit consistent evolution courses with the recorded petrographic assemblages, i.e. the assemblages consistently plot in accord to the composition of the predominant phase. Involvement of any phase in the process slightly shifts the trend towards the respective apex (for example, involvement of dolomite shift the composition towards CaO apex).
Evaluation of the studied talc-carbonate rocks on the H2O – MgO – SiO2 diagram which depicts the evolution of the anhydrous harzburgite during the serpentinization, substantiated the proposed harzburgite protolith.
The studied fissure-fed talc-carbonate deposits are the product of in situ metasomatic alteration of their country rocks. As stated earlier, they are confined to specific structurally-controlled environment (along major faults that cut the serpentinite slices or at thrust contacts between obducted serpentinites and other mélange components). In such environment, fluid-rock interaction predominates in addition to solid-solid reaction (net-transfer) and complex mixed volatile reactions and fluids.
Harzburgite protolith is advocated on the basis of the recorded bastite and in the light of their evolution to antigorite + talc assemblage, an original Fo / En ratio < 0.7 is postulated. CO2 in addition to H2O and SiO2 aqueous played important roles in the evolution of the studied talc-carbonate rocks. The involved SiO2 aqueous phase is attributed to the subsequent granitic magmas that invaded the studied districts and evolved to develop huge quartz lodes as plugs and veins of stockwork patterns in the studied areas.
At Barramiya district, the country rocks of the studied talc-carbonates lack a proper carbonate-bearing phase, instead they embrace graphite schist as a major constituent implying a reducing environment. In such depositional environment, the CO2 in metamorphic fluids is controlled mainly by graphite-water equilibrium and the fluids along the C – H2O tie line were essentially H2O-enriched with low CO2 (∂ CO2) activity.
With decreasing pressure, the ∂ CO2 increased in graphite-buffered fluids. Magnesite formation at expense of antigorite will shift the graphite-saturation towards highly reduced H2O-CH4-H2 fluids, giving rise to increase the siderite component in magnesite at expense of magnetite.
At 2 kbar, antigorite is stable only at low ∂ CO2 and the antigorite – magnesite assemblages coexist with talc-dolomite assemblages only at temperature below about 480 ºC with fluid composition of about 0.05 XCO2. The upper temperature stability limit for the antigorite – talc – magnesite assemblages at 2 kbar is about 490 ºC with maximum X CO2 of about 0.13.
However, the following metamorphic conditions could tentatively be assumed for the studied talc-carbonate assemblages (antigorite – talc – magnesite): temperature of about 490 ºC and a maximum XCO2 of about 0.13; hence, X H2O of about 0.87 could be reasonably expected.
Evolution of the studied talc-carbonate rocks were evolved as: serpentine → talc + carbonate and serpentine → talc → talc + carbonate. These two characteristic evolution trends evolved via: I) following the steep enrichment of CO2 content, irrespective to silica, giving rise to the formation of assemblage characterized by carbonate > talc, and II) increasing Si, irrespective to the CO2 component in initial stages to produce talc and then with decreasing silica and increasing CO2, the trend changed its course along the talc-magnesite tie line to produce assemblage characterized by significant mixture of carbonate and talc components.
The last part of the thesis is confined on studying suitability of a representative talc carbonate rock sample for the production of refractory shaped cordierite (Mg2Al2Si5O18) ceramics. This sample was made by mixing fifteen (15) field samples, collected in the preceding parts from the talc carbonate rocks, existing within the serpentinite rocks of El-Barramiya and Wadi-Attalh areas, Central Eastern Desert. The selected 15 lump samples represent the TCA1 and TCA2 talc-carbonate assemblages, that selection based on their preliminary chemical composition to contain maximum MgO with minimum calcarious and ferruginous minerals, i.e. minimum CaO and Fe2O3.
In order to achieve the main target of this part, twelve cordierite raw batches were designed, prepared and processed up to firing, on a lab scale, by using fine powders of the representative talc carbonate (magnesite) rock sample, with partial and full replacement by a high-grade Egyptian talc rock sample. These rocks were applied as main sources for magnesia (MgO) and silica (SiO2). Other materials were also applied, including Sinai kaolin and plastic Aswan clay as sources for Al2O3 and SiO2 as well as calcined natural bauxite and synthetic α-alumina (corundum) as additional sources of Al2O3. This enabled in studying effect of variation in chemical and mineral composition of the raw batches as well as firing conditions on phase composition, microstructure and physical properties of their cordierite-bearing fired-bodies, i.e. co-clinkers. The dense co-clinkers with maximum cordierite content were selected for processing refractory, shaped and volume-stable cordierite briquettes. These briquettes were classified into low, medium- and high- refractory grades, according to their physico- and thermo- mechanical properties. In addition, the three grades were recommended for application at their adequate service conditions as kiln-furniture for the different ceramic-tunnel kilns.
According to the obtained results and its scientific discussion, the following conclusions were derived:
1- Three raw batches, based on the talc-carbonate and/or pure-talc powders were selected for processing dense refractory cordierite co-clinkers after firing up to 1300-1400oC, according to their maximum cordierite content (95-100%).
2- The co-clinker prepared from the talc-carbonate, Sinai-kaolin and calcined-alumina raw batch was recommended for processing cordierite products with low refractory grade after firing for long time at only 1275oC, due mainly to its relatively high content of the fluxing Fe2O3, TiO2 and CaO impurity oxides.
3- Due to the medium level of these impurity oxides in the co-clinker prepared from 1 : 1 talc-carbonate : pure-talc -based co-clinker, with Sinai-kaolin and calcined-alumina, it is recommended for processing cordierite bodies with medium refractory-grade after firing up to 1350oC.
4- The co-clinker, based on pure-talc as well as Sinai-kaolin, Aswan-clay and calcined-alumina, containing the minimum level of the impurity oxides is recommended for manufacturing cordierite bodies with high refractory-grade after firing also at 1350oC.
5- The talc-carbonate-based cordierite products (C2) can be recommended for safe application as kiln furniture and car-top lining for the tunnel-kilns used in firing clay building bricks up to (1000-1100oC) as a maximum service temperature.
6- The cordierite products processed from the pure talc and/or impure talc-carbonate (C1 and C3) can be recommended for safe application at higher maximum service temperatures (1000-1200oC) as kiln furniture and car-top lining in firing other heavy-clay products.