Osama R Sallam
email: orsallam@yahoo.com

Abdalla S Alshami

Abd Elhadi

A Abbas

Guibal Eric

E Guibal

Gold and Base Metals Associated with Uranium Mineralization in some Lower Carboniferous Rock Units at Wadi Abu El Mogheirat area, Southwestern Sinai, Egypt

Keywords: gibbsite, rare earth elements, Um Bogma

est destinée au dépôt

Introduction

Most of the gold deposits and mineralization in Egypt are located in the Eastern Desert area [START_REF] Botros | Gold in Egypt: Does the future get worse or better?[END_REF], which was extensively exploited in the Pharaonic times. In central Easter Desert of Egypt gold is associated to quartz/ carbonate veins; the hydrothermal activity explains the metamorphic source of gold ore deposits in relation with regional shear setting [START_REF] Zoheir | Greenstone-hosted lode-gold mineralization at Dungash mine, Eastern Desert, Egypt[END_REF]. Remote sensing has been applied to identify the connections between several gold-bearing formations in this area; this allowed correlating surface structural elements, hydrothermal effects and lithological characteristics between different ore deposits [START_REF] Gabr | Prospecting for new gold-bearing alteration zones at El-Hoteib area, South Eastern Desert, Egypt, using remote sensing data analysis[END_REF]. Chemical analysis also demonstrated the association of gold with zinc in many analyzed samples. In this area, recent studies have shown the impact of carbonate alteration of ophiolithic ores; under hydrothermal conditions the remobilization of gold and sulfur caused the formation of gold deposit [START_REF] Boskabadi | Carbonate alteration of ophiolitic rocks in the Arabian-Nubian Shield of Egypt: Sources and compositions of the carbonating fluid and implications for the formation of Au deposits[END_REF]. These hydrothermal alterations may explain not only the formation of these gold deposits but also its association to other base metals and calcareous mineralization [START_REF] Helmy | Metal and fluid sources in a potential world-class gold deposit: El-Sid mine, Egypt[END_REF][START_REF] Abdelnasser | Mineral chemistry and geochemical behavior of hydrothermal alterations associated with mafic intrusive-related Au deposits at the Atud area, Central Eastern Desert, Egypt[END_REF].

Recently, the intensive geological exploration of Sinai area allowed identifying some promising anomalies for polymetallic ore deposits that contain not only base metals but also uranium and gold. The explored area (Wadi Abu El Mogheirat) is located close to Abu Zeneima (about 40 km in the ESE direction, Fig. 1a, this is like a "mirror" zone to the Central Eastern desert on the other side of the Gulf of Suez. Similarities may exist between the characteristics and origin of these different deposits belonging to the Arabian-Nubian shield (Bentor, 1985), although it is commonly accepted that Sinai is not a goldmining area. The main cover in this area corresponds to Paleozoic sediments with concentrations of coal, copper, manganese, kaolin, glass sands, rare earth elements (REEs) and uranium. More recently, traces of thorium and gold have also been identified. The East Abu Zeneima area is made up of the Precambrian younger granites, overlain nonconformably by the Paleozoic succession.

The Paleozoic outcrops of Southwestern Sinai have been documented since the early 20th century: [START_REF] Barron | The Topography and Geology of the Peninsula of Sinai (Wester Portion)[END_REF] reported three different series: (a) the Lower Sandstone Series (LSS), (b) the Carboniferous Limestone Series (CLS) (which was called the Um Bogma Formation by [START_REF] Weissbrod | The Paleozoic of Israel and Adjacent Countries. Part II: The Paleozoic outcrops in southwestern Sinai and their correlation with those of southern Israel[END_REF] and (c) the Upper Sandstone Series (USS). The Lower Sandstone Series was subdivided by [START_REF] Soliman | Petrology of Carboniferous and sandstone in West Central Sinai, Egypt[END_REF] into the Sarabit El Khadim, Abu Hamata and Adedia formations, also splitting the Upper Sandstone Series into the El-Hashash, Magharet El-Maiah and Abu Zarab formations.

Geological Setting

The Wadi Abu El Mogheirat area is covered by rock exposures ranging from Precambrian to Paleozoic in age (Fig. 1b). Quartz diorites is the main component of the Precambrian rocks in this area, the Paleozoic sequences being subdivided into seven formations (arranged from oldest to youngest) as follows: Sarabit El Khadim Formation (Cambrian), Abu Hamata and Adedia formations (Cambro-Ordovician), Um Bogma, El Hashash, Magharet El Maiah and Abu Zarab formations (Lower Carboniferous).

Sarabit El Khadim Formation: Cross-bedded sandstones with a conglomerate layer, alternating with sandstones possessing a pinkish to brownish color, nonconformable with the underlying basement rocks.

Abu Hamata Formation: Conformably overlies and underlies the Sarabit El Khadim and Adedia formations, respectively. It is easily distinguished by its characteristic greenish color and lithology, which can be used as a marker in the field.

Adedia Formation: Made up of coarse-to fine-grained, pink to brown in color, hard ferruginous sandstones and siltstones. The copper mineralization is recorded in the upper part of this formation. Also, there are Mn-Fe veinlets, in addition to some radioactive anomalous recorded in the Adedia Formation.

Um Bogma Formation: In the study area, the Um Bogma Formation unconformably overlies the Adedia Formation and underlies the El Hashash Formation. It is regarded as the most important Paleozoic rock unit in the study area, due to its content of uranium, Mn-Fe ore deposits and secondary copper mineralization. The formation exhibits a thickness of about 7 m and is classified from base to top as follows: The lower member rests unconformably on the Adedia Formation with a distinct contact, due to its different lithology. It is enriched by manganese and iron ores. This member exhibits three different lithological facies, including a) Mn-Fe ore (Fig. 2a), ferromanganese siltstones and silty shale facies with black, blackish-brown and reddish-brown colors; b) sandy dolostone facies, identifiable from its thick beds, pink color and occasional horizontal lamination; c) black carbonaceous shale, siltstone facies which is considered the most important facies for the uranium mineralization. The middle member is a yellow bed that consists mainly of marl and dolostone with some intercalated shale. The middle member is characterized through the distribution of highly radioactive anomalies, in addition to evaporite minerals such as gypsum, anhydrite and halite in fibrous and platy habits in the form of parallel veinlet's and/or intersection with bedding planes. Also, pockets of secondary copper mineralization, white and black gibbsite (Fig. 2b) were observed within the middle member of Um Bogma Formation with high radioactive anomalous. The upper member is composed of yellow, pink, and grayish crystalline dolostone and some sandstones.

El Hashash Formation: This consists of brownish, cross-laminated sandstones, intercalated with thin beds of shale and siltstone. Siltstones vary in color from white, yellow, red to violet (Fig. 2c).

Magharet El-Maiah Formation: It unconformably underlies the Abu Zarab Formation and overlies on the El Hashash Formation (Fig. 2d). The Magharet El-Maiah Formation consists of carbonaceous shale, sandstones, kaolin and claystone. There is a lateral variation in the lithology of this formation, i.e., the kaolin beds change laterally into carbonaceous shale in two horizons. Exploratory old mines for kaolin and coal were excavated parallel to the formation. It can be easily identified in the field through its distinctive physical characteristics.

Abu Zarab Formation: This formation consists of white, semi friable sandstones with siltstones and shale. It is considered to be the main source of glass sand ore in Sinai. The Abu Zarab Formation is locally capped by basaltic sheets.

Generally, the Um Bogma, El Hashash, Magharet El-Maiah and Abu Zarab formations are the only ones exposed at the Wadi Abu El Mogheirat. Wadi Abu El Mogheirat is affected by normal and horst faults (Fig. 2e and f) trending NW, with most of the observed radioactivity and mineralization being concentrated along these fault planes, which reveal the structural control of the mineralization. The lithology, structure and

Materials and Methods

Field radiometric measurements were performed using a handheld gamma-ray spectrometer (Model Rs-230 BGO Super-Spec, Radiation Detection Systems AB, Backehagen, Sweden) (Fig. 2h) for the determination of eU (ppm), eTh (ppm) and K (%). The frequency for acquiring radiometric measurements was set to every 30 s.

Inductively coupled plasma optical emission spectrometry (720 ICP-OES Agilent Technologies, Santa Clara, CA, USA) was utilized for measuring uranium, trace and rare earth elements (REEs) (promethium was not analyzed). Samples were analyzed at the Central Laboratory Sector of the Egyptian Mineral Resources Authority (EMRA).

The mineral samples (crushed and sieved to 200 Mesh) were digested in teflon crucibles using 0.5 g of ground sample with 3 ml of perchloric acid, 5 ml of nitric acid and 15 ml of hydrofluoric acid. The mixture was covered with a glass lens and heated for 3 hours until digestion had been completed. In a second step, the glass lens was removed to evaporate silica tetrafluoride until complete evaporation. 5 ml of concentrated HCl was added to dissolve the solid residue; after drying through heating the solid was completely dissolved with 50 ml HCl aqueous solution (1:1 w/w); the volume was adjusted to 100 mL with distilled water, the concentration of metal ions being analyzed by ICP-QES (720 ICP-OES, Agilent Technologies, Santa Clara, CA, USA).

The determination of gold content required a further specific procedure; fire assay analysis was carried out at EMRA. 50 g samples (crushed and sieved to 200 Mesh) were fired in the presence of alkali fusion agents. More specifically, the ground sample was added to a mixture of (lead oxide), borax (sodium borate), sodium carbonate, flour, silica and silver in a ceramic crucible. The mixture was melted at 1000°C for 90 min; this step was followed by the cupellation of lead/gold/silver alloy at 900°C for 60 min. The resulting alloy (Ag/Au) was then dissolved with concentrated nitric acid and aqua regia under heat, in order to dissolve the gold; the solution was further analyzed by atomic absorption spectrometry (using a Savant AA spectrometer, GBC Scientific Instruments, Braeside, Australia).

The mineralogical composition of the samples required a preliminary conditioning of the samples. 3 to 5 kg of selected samples were crushed and sieved to collect a 0.074-0.5 mm (35-200 mesh) fraction. The sieved fraction was submitted to two separation steps: (a) densitometry separation using bromoform (Sp.G.: 2.85-2.89) (Quinif et al., 2006) and (b) magnetic separation using a Frantz Isodynamic Magnetic Separator (S.G. Frantz Co., Inc, Tully town, PA, USA). For magnetic separation, the side slope was set at 5° and the forward slope at 20°, under an amperage of 0.5 A [START_REF] Flinter | The magnetic separation of some alluvial minerals in Malaya[END_REF].

Separated fractions were hand-picked using binocular microscope (Meiji, Japan), then analyzed with an Environmental Scanning Electron Microscope ESEM (XL30-ESEM, Philips, FEI, Thermo Fisher Scientific, Hillsboro, OR, USA) associated with an attached energy dispersive X-ray spectrometer (EDX) unit. Analysis was operated with the following parameters: 25-30 kV accelerating voltage, 1-2 mm beam diameter and 60-120 s counting time. Polished sections were specifically analyzed using a Quanta FEG200 (FEI-France, Thermo Fisher Scientific, Merignac, France) coupled with an Oxford Inca 350 EDX micro analyzer (Oxford Instruments France, Saclay, France) at the C2MA at IMT-Mines Ales.

Results and Discussion

Radioactivity

The radioactivity was recorded in several areas in Sinai. The most notable result was recorded in the Paleozoic rocks, specifically in the Um Bogma Formation. Most of the radioactive anomalies are concentrated with a definite stratigraphic horizon among them; the most significant uranium occurrences are located in the middle member of the Um Bogma Formation in the study area.

According to the radiometric measurements of the anomaly locations in the study area (Table 1) most radioactive anomalies are concentrated in the middle member of the Um Bogma Formation. More specifically, the gibbsitic marl shows the highest eU contents ranging between 310 ppm and 360 ppm with an average of 333.6 ppm, while varying from 227.3 ppm to 280 ppm with an average of 261.6 ppm in the black gibbsite. Siltstones of the El Hashash Formation show low eU contents relative to the Um Bogma Formation, ranging between 23.7 ppm and 44.8 ppm. The black shale of the Magharet El-Maiah Formation shows much lower eU contents, varying from 17.5 ppm to 39.6 ppm with an average of 30.9 ppm.

When uranium content was measured chemically for the studied anomalous facies (Table 1), the gibbsitic marl of the Um Bogma Formation showed the highest uranium content ranged between 412 ppm and 717.4 ppm with an average of 698.3 ppm, while the black gibbsite varied from 138.6 ppm to 149.8 ppm with an average of 144 ppm. Siltstones of the El Hashash Formation had as high uranium content, ranging between 310 ppm and 340 ppm with an average of 321.2 ppm, while the black shale of Magharet El-Maiah varied from 50 ppm to 60 ppm with an average of 54.6 ppm.

The eU/eTh ratio is a very important geochemical index for U migrating in or out, being approximately constant within the same rock type or geological unit in a relatively closed environment. The Clark value for eU/eTh in the sedimentary rocks is equal to 1 [START_REF] Clark | Section 24: Abundances of uranium, thorium and potassium[END_REF]. The eU/eTh ratio of the studied anomalous facies is much higher than the Clark value, in which the average ratio reaches 7.0 and 13.3 for the gibbsitic marl and the black gibbsite of the Um Bogma Formation respectively, while reaching 2.3 and 2.4 for the siltstones of the El Hashash and the black shale of the Magharet El-Maiah formations respectively. This indicates that the studied anomalous facies have experienced inward migration (leaching) of uranium from the surrounding country rock units, which is finally concentrated in fractures and fault planes. The presence of clay and iron minerals in addition to the organic matter play an important factor in the capture of uranium and the deposition of that metal in the fractures [START_REF] Baioumy | Mineralogy and geochemistry of the sedimentary kaolin deposits from Sinai, Egypt: Implications for control by the source rocks[END_REF]. The alteration of rocks contributes to U mobility and leads to the concentration of radioelements in fractures and faults [START_REF] El Aassy | Effect of alteration processes on the distribution of radionuclides in uraniferous sedimentary rocks and their environmental impact, southwestern Sinai, Egypt[END_REF]. The hydrothermal conditions strongly control the migration of heavy metals and the deposition of Mn-Fe mineralization [START_REF] Al-Ateeq | Geochemistry and genesis of base metal-rich Mn-Fe mineralization in volcaniclastic sediments, Asfar Thwelil Area, Saudi Arabia[END_REF].

The D-factor represents the ratio of the chemically measured uranium content to the radiometrically measured uranium (D: U/eU). When this factor is more or less than unity; it indicates that the system is in a disequilibrium state, which could be due to the addition or removal of U [START_REF] Hansink | Equilibrium analysis of sandstone rollfront uranium deposits[END_REF].

The chemical analysis of samples systematically shows higher values than the radiometric measurements: the Dfactor is around 1.73 for gibbsitic marl (in the Um Bogma Formation), 10.09 for siltstones of the El Hashash Formation and 1.9 for the black shale of the Magharet El- Maiah Formation. These indicate an addition of recent uranium, indicating that these facies are in a state of disequilibrium. A completely different trend is observed when considering black gibbsite from the middle Um Bogma Formation (deeper layers); indeed, in these rocks, the D-factor remains close to 0.55: uranium was leached from these layers, probably transported away and finally deposited in the fractures of surrounding formations. Similar phenomena have been reported for U mineralization in Eastern Desert deposits [START_REF] Dawood | Useries isotopic composition of kasolite associated with aplitepegmatite at Jabal Sayid, Hijaz region, Kingdom of Saudi Arabia[END_REF]. Figures 34show the correlation of eU, eTh and U (chemical measurements) with the K (%) content in the mineral samples. The evolution of the Clark coefficient and D-factors with K (%) content is also presented on Fig. 56. The grouping of data is also correlated to the corresponding geological formations and lithofacies. The radiometric measurements can be easily correlated in the different lithofacies and the content of K (%) hardly changes with the relative values of eU and eTh (Fig. 4).

There is no continuity in terms of U content between the different geological layers: Figure 5 shows that the relative richness in U decreases in the following pattern: gibbsite marl (middle Um Bogma Formation) > siltstones (El Hashash Formation) > black gibbsite (middle Um Bogma Formation) > black shale (Magharet El-Maiah Formation). The Clark coefficient (i.e., eU/eTh) shows a progressive decrease from black gibbsite to black shale and siltstone lithofacies (lower layers to higher layers); this means that the deeper layers were more affected by the alteration of U -bearing rocks (probably due to epigenetic effects). The Dfactor varies in the opposite direction: the upper layers are more significantly affected by the mobility of uranium (leaching and subsequent deposition in fractures and faults). Two main hypotheses can be proposed for explaining this selective localization of uranium: (a) the epigenetic deposition of radioelements during lithification and (b) the leaching of the radioelements (in a second step), followed by their transport and re-precipitation in fractures and faults. The secondary ascending hydrothermal solutions transport the radioactive minerals to deposit primarily along fractures and faults.

Geochemistry 4.2.1 Base metals and uranium/thorium

The chemical analysis of the selected samples (#1-8) collected in four different lithofacies (belonging to three geological formations) is summarized in Table 2 (Figs. 78). Gibbsitic marl and black gibbsite from the middle Um Bogma Formation show high enrichment in Co, Cu, Mn, Ni, Pb and Zn (levels exceeding 1000 ppm, with even more than 10000 ppm for Mn). Mn-Fe ore constitutes an important fraction of rocks in the Um Bogma Formation [START_REF] Kora | Lithostratigraphy of the manganese-bearing Um-Bogma Formation, West-Central Sinai, Egypt[END_REF][START_REF] Shaaban | Multiple dolomitization events of the Lower Carboniferous Um Bogma Formation, west central Sinai, Egypt[END_REF][START_REF] Khalifa | Assessment of hazardous mine waste transport in west central Sinai, using remote sensing and GIS approaches: A case study of Um Bogma area, Egypt[END_REF][START_REF] Rabeh | Tracing the manganese ore accumulations in Sinai Peninsula, Egypt, using magnetic method[END_REF].

The metal content in black gibbsite is generally higher than in gibbsitic marl, with the exception of Zn (a similar order of magnitude) along with Co and Cr (higher content in gibbsitic marl) (Fig. 7). For siltstones (El Hashash Formation) and black shale (Magharet El-Maiah Formation) the levels of metal content are much lower (below 450 ppm), with the exception of Nb and Zr (only in black shale, with levels of 300-450 ppm). It is noteworthy that metal content is generally higher in the black shale than in the siltstones of these geological Fig. 3. Correlation between K content of selected samples and both eU and eTh radiometric measurements (and geological formations or lithofacies). formations.

Thorium presence is limited in superficial layers (siltstones and black shale) with concentrations lower than 14 ppm; in contrast, the middle Um Bogma Formation shows much higher content (i.e., 154-178 ppm for gibbsitic marl and up to 335-357 ppm for black gibbsite lithofacies). Uranium (Table 1) is also predominantly detected in the gibbsitic marl (703-717 ppm) when compared to the black gibbsite (i.e., 139-150 ppm). However, contrary to Th distribution, substantial levels of U are detected in the El Hashash siltstones (i.e., 310-340 ppm) and to a lower extent in the Magharet El-Maiah black shale (i.e., 50-60 ppm). This is indicative of the presence of secondary mineralization due to alteration, metal migration and local sedimentation/precipitation [START_REF] Abu-Zeid | Mineral genesis and radioactivity of the upper part of the Adediya Formation, southwestern Sinai, Egypt[END_REF].

Rare earth elements (REES)

The presence of rare earth elements requires particular attention (Table 3, Fig. 9a-c). The lithofacies can be ranked according to their total rare earth content: siltstones The contents are in the same order of magnitude in the two lithofacies of this geological formation. The case of La is remarkable: gibbsitic marl contains between 58 and 66 ppm of lanthanum, while its content decreases below 2 ppm in black gibbsite. The REEs can be classified in two groups: light REEs (LREEs) with atomic number below 63 (i.e., up to and including samarium) and heavy REEs (HREEs) with atomic number higher than 62 (i.e., beginning with Eu). This separation remains debatable in the literature. In most cases, the LRREs/HREEs ratio varies between 5.7 and 8.7, except for black gibbsite where it ranges between 14.6 and 17.8. This confirms the anomalies of the mineral fingerprint of this lithofacies within this specific geological formation.

The REEs distribution is frequently normalized using models associated with both chondritic abundance [START_REF] Schmitt | Rare-earth, yttrium and scandium abundances in meteoritic and terrestrial matter-II[END_REF] (Fig. 9a) and North American Shale Composite abundance (NASC) (Gromet et al., 1984, Fig. 9b;Haskin et al., 1968, Fig. 9c). These simulations are summarized in Figs. 9a-c for selected lithofacies from the different geological formations. The profiles are relatively stable, with regard to the normalized values of the REEs; the curves roughly follow the same trend independent of the type of geological formation.

The normalized profile against chondrite (Fig. 9a) shows a substantial enrichment of the REEs. The main anomalies are identified for holmium and cerium. Indeed, in siltstones and black shale, negative anomalies are observed for Ho, while positive anomalies can be seen for gibbsitic marl and black gibbsite (a similar positive anomaly, but to a lesser extent, is observed for praseodymium). Cerium anomalies are generally associated with laterization effects and oxidative/reductive conditions during the degradation of primary rocks [START_REF] Al-Khirbash | Rare earth element mobility during laterization of mafic rocks of the Oman ophiolite[END_REF]. The impact of oxidative conditions on REE distribution can be assessed by the effect of microbial activity in altered zones [START_REF] Censi | Authigenic phase formation and microbial activity control Zr, Hf, and rare earth element distributions in deep-sea brine sediments[END_REF].

The concentration of rare earth elements (such as uranium) into preferential geological layers was correlated to lithological characteristics such as porosity and permeability: the presence of impermeable stratigraphic levels contributes to the accumulation of mobile metals [START_REF] Abu-Zeid | Mineral genesis and radioactivity of the upper part of the Adediya Formation, southwestern Sinai, Egypt[END_REF].

Gold

Many studies on gold exploration in Egypt have focused on the Eastern Desert; on the other hand, Sinai is not considered a promising area for Au exploitation. The early exploration programs of the Geological Survey of Egypt concluded that gold in Sinai was essentially confined to quartz veins and carbonated ultramafics (appearing as listwaenite) (Khalid and Oweiss, 1995b). In Pharaonic times, gold was extracted in the Eastern Desert of Egypt from quartz veins and placers. In some specific local areas, small-tonnage mines exploited gold, which occurred as dispersed grains or was associated with sulfides in quartz veins along shear planes at Wadi Kid [START_REF] Surour | Dahab stream sediments, southeastern Sinai, Egypt: A potential source of gold, magnetite and zircon[END_REF], where relatively high levels of gold (up to 16.5 ppm) appeared in altered rocks (Khalid and Oweiss, 1995a). Minerals bearing Ag and Au (under the form of uytenbogaardtite and furutobeite) were identified in the lower member of the Um Bogma Formation in the El Sheik Soliman area (to the south of the study area) [START_REF] Schmitt | Rare-earth, yttrium and scandium abundances in meteoritic and terrestrial matter-II[END_REF]; (b) rare earth element pattern of the anomalous samples normalized with NASC [START_REF] Haskin | Relative and Absolute Terrestrial Abundances of the Rare Earths[END_REF]); (c) rare earth elements pattern of the anomalous samples normalized with NASC [START_REF] Gromet | The North-American shale composite -its compilation, major and trace-element charcateristics[END_REF]. [START_REF] Sallam | Occurrence of silver-gold mineralization associated with uranium bearing minerals and base metal sulphide, El Sheik Soliman area, South Sinai, Egypt[END_REF]. It is commonly accepted that sedimentary basins that contain paleo-placers may have been formed under tectonic regimes that existed only during the Proterozoic era [START_REF] Mitchell | Nature and dating of karstic filling in the Hainaut Province (Belgium). Karstic, geodynamic and paleogeographic implications[END_REF], such as the gold bearing Taba conglomerate [START_REF] Hassan | Preliminary record of the Proterozoic-Phanerozoic paleoplacer gold in Gebel Ghazalani area: Chemo-stratigraphic evidence closing debate about Paleozoic sedimentation sequence, east Sinai, Egypt[END_REF].

Table 4 summarizes the evaluation of gold traces in the samples. Gold is either undetectable (in the black shale of the Magharet El-Maiah Formation) or occurs only at a very low content (below 1 ppm in the siltstones of the El Hashash Formation) in the upper geological layers; a few higher contents are measured in the samples collected in the middle Um Bogma Formation, especially in the gibbsitic marl (10.4 ppm vs. 2.8 ppm in black gibbsite lithofacies). This is consistent with the levels (i.e., around 9.7 ppm) reported by [START_REF] Alshami | Iinfra-cambrian placer gold-uraniferous Paleozoic sediments, southwestern Sinai, Egypt[END_REF], who observed a progressive decrease in the gold content of Um Bogma mineralization, correlated with the time of deposition during the Cambrian -Ordovician sedimentary period [START_REF] Alshami | Iinfra-cambrian placer gold-uraniferous Paleozoic sediments, southwestern Sinai, Egypt[END_REF]. The levels of gold are consistent with those reported in volcanic and subvolcanic rocks of the Wadi Abu Khushayba area (Jordan) [START_REF] Al-Hwaiti | Epithermal gold mineralization at Wadi Abu Khushayba, southwestern Jordan[END_REF]. The same order of magnitude was also found for non-altered rocks and altered rocks samples collected in the El-Hoteib area (in the south eastern Desert, Egypt) [START_REF] Gabr | Prospecting for new gold-bearing alteration zones at El-Hoteib area, South Eastern Desert, Egypt, using remote sensing data analysis[END_REF]. The alteration phenomena were furthermore associated with the formation of low-grade gold deposits in the central Eastern Desert (Egypt): gold content varied between 1.9 and 4.9 ppm in quartz veins and decreased to 1.4-2.8 ppm in listwaenite alteration rocks [START_REF] Salem | Exploration of new gold occurrences in the alteration zones at the Barramiya district, central Eastern Desert of Egypt using ASTER data and geological studies[END_REF].

Lithofacies-main characteristics of metal distributions

In the gibbsitic marl the REEs represent about 629-900 ppm with about 87% of LREEs (including up to 89% for Ce) (Table 4): there is a strong enrichment of LREEs in this geological formation (the L/H ratio increases up to 14 -18 compared to 6-9 for other lithofacies). Figures 9a-c show that these REEs are almost equally enriched when considering the normalized plots against chondrite and North American Shale Composite (NASC). Figure 7 shows that the samples contain high levels of Mn (above 12000 ppm); this is logically associated with the Fe-Mn ore component of Um Bogma mineralization. There are also significant amounts of Zn (up to 7000 ppm) and Co (close to 4900 ppm); other high level traces consist of Pb, Ni and Cu. Lower levels of base metals were identified: essentially Ba (around 200 ppm), V (close to 100 ppm) and Th (around 160 ppm, consistent with U-enrichment and radiometric measurements, see above). This is also the geological formation where the gold level is the highest (around 10 ppm) and the levels of As are relatively low (12-17 ppm).

The metal contents in the black gibbsite layer were consistent with those found in the gibbsitic marl though usually higher; especially for Mn (up to 14000 ppm), Ba (550 ppm), Th (up to 350 ppm), V (remarkably increased to 500 ppm) and Nb (up to 120 ppm). Gold content decreased to 2.8 ppm, while arsenic increased to 52-57 ppm: this means that there is no direct correlation between Au and As; in other words, Au mineralization is probably not associated with the occurrence of arsenopyrite. In this lithofacies, the REE content increased up to 1800 ppm, with LREEs representing up to 94% of total REEs (and Ce up to 86%). The positive Ce anomaly is confirmed by Figs. 10a-c, while La shows a substantial deviation from the general trend, with a negative anomaly.

The metal contents in siltstones were remarkably low, when compared with the levels observed in the other lithofacies: REEs represented less than 90 ppm, with base metals being consistently below 100 ppm. This is depicted in Fig. 9a-c: normalized plots (against both chondrite and NASC) show much lower enrichment in REEs compared to gibbsitic marl and black gibbsite lithofacies. The holmium anomaly is detected, but with a dramatic decrease in content (in fact undetectable in one of the two samples). Abu-zeid et al., (2017) correlated the highest radiometric activity and content of REEs in highly porous and permeable sandstones compared with the siltstones with their porous and permeable structure.

The black shale lithofacies shows an intermediary trend: total REE content reaches up to 250 ppm (about 89% of LREES, including 46% of Ce). In this lithofacies, the normalized plots (Figs. 9a-c) allow the identification of the negative Ho anomaly, while the enrichment factor is remarkably stable for the other REEs. The base metal contents are also higher compared to the siltstone layer, but is significantly lower than for the samples collected in the Um Bogma Formation. The most representative trace elements are Ba (around 130 ppm), Zn (around 200 ppm) and Pb (in the range 140-210 ppm). However, the most surprising observation is the high level of neodymium (Nd) in this geological layer (in the range 400-440 ppm).

The enrichment of REEs (and especially LREEs) can be explained by the carbonaceous nature of the mineralogical structure of the Um Bogma Formation. This is associated with the deposition of organic matter under continental and fluviomarine conditions, an effect that is due to the direct adsorption of metal ions on to this organic matter, or is due to the indirect effects on the physico-chemical conditions that promote the reduction or precipitation of metal ions into the sediments. This can be followed by further migration and re-deposition due to variations in physico-chemical conditions and permeability properties. The accumulation of organic matter in sediments is frequently associated with the enrichment of rocks with sulfur and iron [START_REF] Shata | Mineralogy and geochemistry of the Mo-U-REE bearing carboniferous shale in Um Bogma area, Southwest Sinai, Egypt[END_REF].

Field observations and laboratory characterization of samples confirm that the middle member of the Um Bogma Formation can be considered as a potential source for gold, associated with other exploitable metals that can be used as secondary resources, such as U, Th, Ce, Zn, Co, Cu, Mn, Pb and V.

In general, the probable origin of the gold, uranium, base metals and REE is related to the epigenetic concept, in which the secondary ascending hydrothermal solutions transport the metals through the network of fractures and the change of physico-chemical environment causes the deposition of metals along the fractures and faults. This is in addition to the leaching concept, in which the Au, U, base metals and REE in the surrounding rock units has been transported by means of weathering, especially circulating water, until it is finally captured and deposited by the organic matter in addition to the iron oxides present in the studied facies.

Mineralogy

The samples collected from the gibbsitic marl layer are characterized by the highest relative contents of U and Au. They were further characterized by mineralogical observation on polished sections and analysis using ESEM and EDX analysis. elements significantly present in the samples: Fe, Au, Cu, Ag, Ni, S, Mn, Zr, Co, Ba, Ce, Pb, Y, P, Dy, Zn, As and Th. In the sedimentary succession of Um Bogma and Adedia formations several important mineral assemblage associations were reported including: coffinite, marcasite, zincochromite, sodium meta-autunite and pucherite, in addition to proper Mn-Fe structural deposit (El Akeed et al., 2014). Most of the minerals analyzed at the surface of the polished sections are associated to a relatively high content of C (ranging between 38 and 59 wt%): this is directly associated to the carbonaceous nature of the sedimentary rocks. Figure 11a shows a different trend in the identification of Au/Ag/Cu mineralization: C content does not exceed 3.5 wt%, while iron oxide predominates (Fe content close to 44 wt%).

Uranium minerals in sedimentary rocks can exist in different forms; they are associated with oxides, oxyhydroxides, carbonates, silicates, phosphates, arsenates, vanadates, molybdates and sulfates [START_REF] Alshami | Structural and lithologic control of uranium and copper mineralization in the Um Bogma environs, southwestern Sinai, Egypt[END_REF][START_REF] Shata | Role of epigenetic processes in the enrichment of mineralized dolostones of the Um Bogma formation, Soutwestern Sinai, Egypt[END_REF][START_REF] Shata | Mineralogy and geochemistry of the Mo-U-REE bearing carboniferous shale in Um Bogma area, Southwest Sinai, Egypt[END_REF][START_REF] Shata | REE and uranium mobilization in the karst bauxites of um Bogma area, Southwest Sinai, Egypt[END_REF]. They are frequently found in Paleozoic sediments of southwestern Sinai in Mn-Fe ores, claystones, siltstones, gibbsites, dolostones, marls and shales of the Um Bogma Formation.

The combination of geological and lithological data, SEM and EDX observations, as well as chemical analyses, allows the identification of the main minerals associated with these valuable metals. Figure 13 shows some examples of these minerals, including the results of their EDX analysis. Uranothorite (Fig. 13a) is a uraniumthorium silicate, characterized by a yellow to yellowish brown color, with a crystallographic structure corresponding to subhedral-to-unhedral shapes with rounded corners. Uranium and thorium contents approach 15.4% and 55.7% (wt%), respectively. Impurities are also observed such as Y, Zr, Fe, Al, Ca, V and P. This is consistent with the conventional contents reported in the literature [START_REF] Heinrich | Mineralogy and Geology of Radioactive Raw Materials[END_REF]. Celestite (strontium sulfate, Fig. 13b) occurs as prismatic crystals, the EDX analysis showing the presence of Cs, K, Fe and Al impurities.

Fig. 13c shows a zircon crystal (zirconium silicate); yellowish to reddish-brown colored, with a crystallographic structure with characteristics between euhedral and subhedral. It is noteworthy that the EDX analysis shows the presence of Fe, Ca, K, Al elements and some indices of Hf and U. The metamictization of zircon crystals is associated with the presence of U and/or Th within the crystal structure. High levels of uranium may cause the destruction of zircon structure due to energy dissipation during the decay of the radioactive elements. In the present case, the level of uranium was low enough for the crystallized structure of zircon to be maintained. Figure 13d permits the identification of a relatively rare form of oxychloride of copper (i.e., atacamite), which is usually formed during the oxidation or weathering of primary copper minerals under arid climates.

Fig. 13e confirms the presence of barium in the Um Bogma samples: barite (barium sulfate) appears as white, rounded, pitted and grooved grains; it is noteworthy that strontium also appears as an impurity. Figure 13f shows an yttrium phosphate crystal (yellow to brownish yellow colored grains): xenotime is a typical mineral appearing in sedimentary rocks during diagenetic phases. The EDX analysis shows that many other REEs are associated with Y and that impurities are presents, such as Fe, Si, Al, Pb and Ca. The presence of high base metal contents in mineral samples collected in the Um Bogma Formation at Wadi El Sahu and Ramlet Hemiyir (localized respectively at the north and northeast of the Wadi Abu El Mogheirat area) has been correlated with xenotime (and with Mn-Fe ore deposits, to a lesser extent) [START_REF] El Kammar | Rare earth elements and crystal chemistry of xenotime bearing sediments of Um Bogma area, southwestern Sinai, Egypt[END_REF]. Figure 13g shows a rutile crystal (TiO 2 ), characterized by an euhedral structure; and reddish brown to deep brown color. Some indices of Al, Si and Cl are also identified. Rutile is usually generated under high temperatures and pressures in metamorphic and igneous rocks. This means that the different minerals identified in the Um Bogma Formation reflect a wide range of genetic conditions: the deposit is characterized by a mix of different rocks, with successive metal migration and deposition under changing environments.

Conclusion

The Wadi Abu El Mogheirat area is affected by normal fault trending NW-SE; most of the radioactive anomalies and mineralization indices are specifically concentrated along the planes of these faults. Radiometric field measurements showed that radioactive anomalies are concentrated in the Um Bogma Formation: the highest eU measurements were assigned to the gibbsitic marl (i.e., 360 ppm). The ore deposit is in disequilibrium, as shown by the higher levels of uranium in ore samples (i.e., 703-717 ppm) compared to the radiometric measurements.

The enrichment of the deposit in base metals follows the same trend with higher mineralization of metals such as Co, Cu, Mn, Ni, Pb, Zn and V in the middle Um Bogma Formation. REEs are also significantly enriched: black gibbsite contains the highest REE content (up to 1800 ppm mainly consisting of LRREs; i.e., about 94%, including Ce that represents about 86% of total REEs). Gibbsitic marl is less enriched: the content in REEs is close to 900-930 ppm (87% of LREEs, including 59% Ce). The carbonaceous rocks (rich in organic matter) in the Um Bogma Formation facilitate the direct binding of metal ions or the physico-chemical deposition of these metals.

The analysis of gibbsitic marl reveals surprisingly high levels of gold (i.e., 10.5 ppm); lower levels were detected in the black gibbsite (i.e., 2.8 ppm), while other lithofacies show negligible amounts of gold. The gold content sharply decreases in layers according to the time of deposition during the Carboniferous sedimentation era.

The probable origins of the gold, uranium, base metals and REE are related to many concepts. The epigenetic concept in which the secondary ascending hydrothermal solutions transport the metals through the network of fractures and the change in physico-chemical environment causes the deposition of metals along the fractures and faults. Furthermore, it is not possible to reject the contribution of the leaching concept; in which the Au, U, base metals and REE in the surrounding rock units has been transported by means of weathering, especially circulating water, until it is finally captured and deposited by the organic matter, in addition to the iron oxides present in the studied facies.

This preliminary study of ore deposits at Wadi Abu El Mogheirat clearly shows promising indices for the presence of valuable metals (precious metals, rare earth elements, zinc, neodymium, uranium and thorium). The most interesting lithofacies in this regard are associated with the middle Um Bogma Formation: more specifically the black gibbsite and gibbsitic marl. This 10 m-thick layer is characterized by a soft, friable structure; this is covered by a variable thickness of the semi-hard layer of the El Hashash Formation, which contains much lower contents of valuable metals. Although supplementary investigations are necessary to evaluate more precisely the spatial heterogeneity of metal distribution in the promising rock units, these first indications clearly justify the continuation of the investigation of this promising mining area.

Fig. 1 .

 1 Fig. 1. (a) Landsat image showing location of the study area; (b) geological map of the Abu El Mogheirat area.

Fig. 2 .

 2 Fig. 2. Wadi Abu El Mogheirat; (a) Mn-Fe ore, lower Um Bogma Fm.; (b) pocket of secondary copper mineralization with white and black gibbsite, middle Um Bogma Fm.; (c) siltstones vary in color from white, yellow, red to violet, El Hashash Fm.; (d) Magharet El-Maiah Fm. underlines Abu Zarab Fm.; (e) normal fault.; and (f) horst faults.

Fig. 4 .

 4 Fig. 4. Correlation between K content of selected samples and U chemical measurements (and geological formations or lithofacies).

Fig. 5 .

 5 Fig. 5. Correlation between K content of selected samples and the Clark coefficient (eU/eTh) (and geological formations or lithofacies).

Fig. 6 .

 6 Fig. 6. Correlation between K content of selected samples and the D-factor (U/eU) (and geological formations or lithofacies).

  (El Hashash Formation) [38-90 ppm] << black shale (Magharet El-Maiah Formation) [248-252 ppm] << gibbsitic marl [899-929 ppm] << black gibbsite [1776-1824 ppm] (Um Bogma Formation). The greatest part of these REEs content is cerium: around 42% -46% for Magharet El-Maiah and El Hashash formations, respectively, around 59% for gibbsitic marl and up to 86% for black gibbsite of the Um Bogma Formation. Among the most representative REEs in the samples collected in the middle Um Bogma Formation, apart from Ce, are: Nd (in the range 56-136 ppm), Pr (in the range 40-64 ppm), Gd (in the range 36-48 ppm), Dy (19-28 ppm) and Sm (15-27 ppm).

Fig. 7 .

 7 Fig. 7. Metal content in samples collected from the middle member of the Um Bogma Formation (black gibbsite and gibbsitic marl) (trace elements).

Fig. 8 .

 8 Fig. 8. Metal content in samples collected from the El Hashash Formation (siltstones) and the Magharet El-Maiah Formation (black shale).

Fig. 9 .

 9 Fig. 9. (a) Rare earth element distribution pattern of the anomalous samples normalized to chondrite (Schmitt et al., 1964); (b) rare earth element pattern of the anomalous samples normalized with NASC[START_REF] Haskin | Relative and Absolute Terrestrial Abundances of the Rare Earths[END_REF]); (c) rare earth elements pattern of the anomalous samples normalized with NASC[START_REF] Gromet | The North-American shale composite -its compilation, major and trace-element charcateristics[END_REF].

Fig. 10 .

 10 Fig. 10. EDX and BSE image images showing: (a) Au/Cu/Ag, (b) Ni, (c) Zr rich mineral objects (spots identified by arrows).

  Fig. 11. EDX and BSE image images showing: (a) Ba, (b) Ce, (c) Pb rich mineral objects (spots identified by arrows).

Fig. 12 .

 12 Fig. 12. EDX and BSE image images showing: (a) Y, Dy and S, (b) Y, Mn, Zn and As, (c) Th, Mn, Fe, Zn and S rich mineral objects (spots identified by arrows).

Fig. 13 .

 13 Fig. 13. EDX image showing: (a) uranothorite, (b) celestite, (c) zircon, (d) cupper, (e) barite, (f) xenotime, (g) rutile, (representative of metal resources), Wadi Abu El Mogheirat area.

Table 1 Radiometric and chemical measurements of the anomalous samples, Wadi Abu El Mogheirat area

 1

	Age Fm. Lithofacies	eU (ppm)	U (ppm)	eTh (ppm)	K (%)	eU/ eTh	U/ eU
		Magharet El-Maiah	Black shale Max. Min.	33.5 60.0 12.1 0.3 30.1 53.2 9.7 0.9 25.7 55.7 11.3 1.5 39.2 50.0 19.2 0.3 17.5 51.5 13.1 0.1 39.6 57.3 16.8 1.2 39.6 60.0 19.2 1.5 17.5 50.0 9.7 0.1	2.8 3.1 2.3 2.0 1.3 2.4 3.1 1.3	1.8 1.8 2.2 1.3 2.9 1.4 2.9 1.3
			Average 30.9 54.6 13.7 0.7	2.3	1.9
				26.2 340.0 17.2 2.3	1.5	12.97
				33.5 325.2 25.3 1.9	1.3	9.71
	Lower Carboniferous	El Hashash Middle Um Bogma	37.9 327.6 15.1 1.7 35.2 310.0 11.7 2.1 23.7 315.1 9.2 3.2 44.8 311.3 13.0 1.0 44.8 340.0 25.3 3.2 23.7 310.0 9.2 1.0 Average 33.55 321.2 15.25 2.03 Siltstones Max. Min. 360.0 717.4 42.5 1.3 345.6 412.0 50.3 0.9 320.4 615.2 47.8 1.8 310.5 508.6 41.5 0.6 310.0 503.0 53.2 1.1 Gibbsite marl 355.2 716.9 56.5 1.2 Max. 360.0 717.4 56.5 1.8 Min. 310.0 412.0 41.5 0.6 Average 333.6 698.3 48.6 1.2 280.0 149.8 22.8 0.9 277.3 146.5 19.5 0.7 250.9 139.1 17.3 1.8 260.7 144.7 28.4 0.4 Black gibbsite 227.3 138.6 15.6 1.3	2.5 3.0 2.6 3.4 3.4 1.3 2.4 8.5 6.9 6.7 7.5 5.8 6.3 8.5 5.8 7.0 12.3 14.2 14.5 9.2 14.6	8.64 8.81 13.30 6.95 12.97 6.95 10.06 1.99 1.19 1.92 1.64 1.62 2.02 2.02 1.19 1.73 0.51 0.53 0.55 0.55 0.61
				273.4 145.3 18.2 1.7	15.0	0.53
			Max.	280.0 149.8 28.4 1.8	15.0	0.61
			Min.	250.9 138.6 15.6 0.4	9.2	0.53
			Average 261.6 144.0 20.3 1.1	13.3	0.55

Table 2 Chemical analysis of trace elements (ppm) from the anomalous samples, Wadi Abu El Mogheirat area

 2

	Formation		Middle Um Bogma		El Hashash	Magharet El-Maiah
	Rock type Gibbsitic marl Black gibbsite Siltstones	Black shale
	Sample #	1	3	2	4	5	7	6	8
	As	16.5	12.5	52	57	12.5 5.3	31	12.1
	Cd	57	68	107	u.d.
	Cr	145	168	64	52	122 9.2	72	69
	Cu	1346 1329 2072 2055 2.18 u.d. 14.2 16.1
	Mn	12926 12874 15976 15873 45.7 16.1 29.2 14.9
	Ni	1005 1011 1949 1953 0.26 u.d.	5.8	1.1
	Pb	724	718	2141 2128	1	11	140	256
	V	83	92	492	495	56	9.1	111	12
	Zn	7158 7164 7072 7061 43.1 38.9 194.0 210.6
	Th	178	154	357	335	u.d. u.d.	4.2	14.0
	Nb	40	43	120	118	228	28	400	440
	Ta	20	19	52	47	u.d. u.d.	8	8
	Zr	120	111	170	168	128	80	250	290
	Ba	206	211	560	549	88	u.d. 140	128
	u.d.: under detection limit.

u.d. u.d. u.d. u.d. Co 4980 4899 2822 2853 u.d. u.d. u.d. u.d.

Table 3 Chemical analysis of rare earth elements (ppm) from the anomalous samples, Wadi Abu El Mogheirat area

 3

	Formation	Middle Um Bogma	El Hashash	Magharet El-Maiah
	Rock type Gibbsitic marl Black gibbsite	Siltstones	Black shale
	Sample #	1	3	2	4	5	7	6	8
	La	65.9	58	1.65	1	18.7	7.9	44.7 46.8
	Ce	543.8 533.7 1562 1543 37.9	16.9 115.7 112.9
	Pr	43.14 39.7 53.7 63.7	4.5	2.13 13.1 10.8
	Nd	132.5 135.6 75.4 55.8	13.3	5.7	42.6 36.18
	Sm	26.5 21.3	15	17.5	2.3	0.66 9.64 9.53
	Dy	25.28 19.2 28.16 19.7	2.3	0.67	4.6	5.07
	Er	12.8	13	10.5 11.3	2.9	0.56 5.44 6.12
	Eu	7.5	6.5	6.8	7.1	0.5	0.19	1.7	1.39
	Gd	46	48.1 47.4 35.8	4	1.8	8.1	6.18
	Ho	2.75 2.21	2.2	1.7 0.0044 u.d. 0.106 0.086
	Lu	1	0.9	3.2	2.1	0.58	0.28 0.688 3.92
	Tb	7.5	6.6	7.2	7	1.44	0.4	2.2	2.43
	Tm	4	2	u.d.	u.d.	u.d.	u.d.	u.d.	u.d.
	Yb	10.5 11.8 11.7	9.7	1.7	0.5	3	3.3
	LREEs 811.8 788.3 1707 1681 76.7 33.29 225.7 216.2
	HREEs 117.3 110.3 117.2 94.4	13.4	4.4	25.8 28.5
	ΣREEs 929.2 898.6 1824 1776 90.1	37.7 251.6 244.7
	L/H	6.9	7.2	14.6 17.8	5.7	7.6	8.7	7.6

u.d.: under detection limit.

Table 4 Gold concentrations (ppm) in different lithofacies in the Wadi Abu El Mogheirat area

 4

	Formation	Lithofacies	Semple #	Au (ppm)
	Magharet El-Maiah	Black shale	6	u.d.
	El Hashash	Siltstones	5	0.6
	Middle Um Bogma	Black gibbsite Gibbsitic marl	2 1	2.8 10.4
	u.d.: under detection limit.

Acknowledgements

The authors acknowledge the support of the IMHOTEP project MetalValor (funded by the French Government though the Institut Français d'Egypte and by the Egyptian Academy of Scientific Research). The authors also acknowledge the support of the Nuclear Materials Authority (NMA), Cairo, Egypt, for the generous use of their field and laboratory facilities during the preparation of this work.


K Coulié

H Aziza

W Rahajandraibe
email: wenceslas.rahajandraibe@im2np.fr

Investigation of Single Event Effects in a Resistive RAM memory array by SPICE level simulation

Keywords: OxRAM, RRAM, SEE, SPICE simulation. I

Emerging non-volatile memories, based on resistive switching mechanisms and known as Resistive Random Access Memory (RRAM), are attractive candidates to overcome power, cost and integration density limitations of conventional memories. Moreover, RRAM has exhibited very good tolerance to radiation. In this context, this paper proposes to investigate Single Event Effects in RRAM memory arrays. The decoding circuitry of the memory array, including bit line and source line drivers is targeted. Currents generated by an ionizing particle crossing the memory array are first injected at specific nodes of the memory circuit. Their impact is evaluated by extracting the resistance state of each cell of the memory array before and after the ionizing particle strike. Worst cases scenarios are studied in order to point out the most sensitive configurations able to induce SEE.

INTRODUCTION

Among the most promising technologies to meet the ITRS requirements for non-volatile memories, Resistive RAM (RRAM) [START_REF] Baek | Multi-layer cross-point binary oxide resistive memory (OxRAM) for post-NAND storage application[END_REF][START_REF] Butcher | Proton-based total-dose irradiation effects on Cu/HfO2:Cu/Pt RRAM devices[END_REF][START_REF] Aziza | Bipolar OxRAM memory array reliability evaluation based on fault injection[END_REF][START_REF] Bocquet | Self-consistent physical modeling of set/reset operations in unipolar resistive-switching memories[END_REF] are attractive candidates, due to their appealing performances. Indeed, compared to conventional floating gate technologies, they gather fast write/read operations, low power consumption, CMOS compatibility and high endurance. It is worth noting that in literature, the acronym RRAM is often used to refer to both Oxide Random Access Memory (OxRAM) and Conductive-Bridging Random Access Memory (CBRAM). In the rest of the paper, OxRAM technology will be considered. As radiation effects are nowadays recognized as a real reliability concern, it is mandatory to evaluate the hardness of these particular devices against radiations. Studies exist about the specific radiation constraint [START_REF] Gerardin | Present and Future Non-Volatile Memories for Space[END_REF][START_REF] Bi | Total ionizing dose and single event effects of 1 Mb HfO2-based resistive-random-access memory[END_REF][START_REF] Wang | The heavy ion radiation effects on the Pt/HfO2/Ti resistive switching memory[END_REF][START_REF] Tosson | 1T2R: A novel memory cell design to resolve single-event upset in RRAM arrays[END_REF][START_REF] Wang | Highly Stable Radiation-Hardened Resistive-Switching Memory[END_REF], but available works often address dose effects in the resistive switching element. Alternatively, RRAMs elementary cells based on hafnium oxide have demonstrated a tolerance against various kinds of radiations [START_REF] Bi | The impact of X-ray and proton irradiation on HfO2/Hfbased bipolar resistive memories[END_REF][START_REF] He | SuperiorTID hardness in TiN/HfO2/TiN RRAMs after proton radiation[END_REF][START_REF] Weeden-Wright | TID and displacement damage resilienceof 1T1R HfO2/Hf resistive memories[END_REF][START_REF] Yuan | γ-ray irradiation effects onTiN/HfOx/Pt resistive random access memory devices[END_REF][START_REF] Fang | Total ionizing dose effect ofγ-ray radiation onthe switching characteristics and filament stability of HfOxresistiverandom access memory[END_REF][START_REF] Petzold | Heavy Ion Radiation Effects on HafniumOxide-Based Resistive RandomAccess Memory[END_REF]. In OxRAM memories, MIM (Metal-Insulator-Metal) structure is used as a single-bit memory cell, while the semiconductor transistor subsystem enables performing all the peripheral operations (input/output, coding/decoding, line driving, and sense amplification). In the complementary study presented in this paper, as in previous investigations [START_REF] Castellani-Coulié | Analysis of Single Event Effects in an OxRRAM cell by using SPICE level simulations[END_REF][START_REF] Castellani-Coulié | SPICE level analysis of Single Event Effects in an OxRRAM cell[END_REF], the case of an ion crossing the MIM (Metal-Insulator-Metal) structure of an OxRAM cell is not considered. However, considering the insulator thickness and the probability of occurrence of such an event, we can expect that the impact on the global SER should be low if only SEE are considered (no combined effects with dose).

The aim of the paper is to perform a complementary analysis of the ones presented in [START_REF] Castellani-Coulié | Analysis of Single Event Effects in an OxRRAM cell by using SPICE level simulations[END_REF][START_REF] Castellani-Coulié | SPICE level analysis of Single Event Effects in an OxRRAM cell[END_REF], which focus on Single Event Effects (SEE) in the close proximity of the OxRAM cell (i.e., selection transistors). In these papers, the studied OxRAM cell has shown low sensitivity to SEE. A threshold effect was pointed out and quantified in terms of current features. It was shown that the only configurations inducing effects were simultaneous impacts in a NMOS and a PMOS transistors, one being connected to the Bottom Electrode (BE) node and the other to the Top Electrode (TE) node of the OxRAM cell. Then it was observed that the sensitive NMOS should be physically close to the sensitive PMOS to induce SEE occurrence. The occurrence of such a case would probably be low but possible. Indeed, depending on the considered technology and on the memory array, the distance between two sensitive transistors can vary from tens to hundreds of microns. So, only a particle crossing the structure (i.e., a particle with a long range), could impact the OxRAM memory array behavior. Note that the simulations were performed for a specific structure corresponding to the worst case for single event radiation effects. However, in order to improve integration density and scalability in emerging technologies, 3D stacking technology is adopted where NMOS and PMOS transistors could be physically close. In that case, the OxRAM technology sensitivity to SEE could increase.The remainder of this paper is organized as follows. Section

II. RRAM TECHNOLOGY

An OxRAM memory cell consists of two metallic electrodes that sandwich a thin dielectric layer serving as a permanent storage medium. Its resistance may be switched between '0' and '1' states. The OxRAM Metal-Insulator-Metal (MIM) structure can be easily integrated in the Back-End Of Line (BEOL) on top of the CMOS subsystem [START_REF] Bi | Total ionizing dose and single event effects of 1 Mb HfO2-based resistive-random-access memory[END_REF][START_REF] Wang | The heavy ion radiation effects on the Pt/HfO2/Ti resistive switching memory[END_REF][START_REF] Tosson | 1T2R: A novel memory cell design to resolve single-event upset in RRAM arrays[END_REF][START_REF] Wang | Highly Stable Radiation-Hardened Resistive-Switching Memory[END_REF][START_REF] Bi | The impact of X-ray and proton irradiation on HfO2/Hfbased bipolar resistive memories[END_REF]. Oxide-based resistive random access memory (so-called OxRAM) elements present a lot of interesting features like high integration density, high-speed operations (write/erase/read) and satisfactory reliability performances in terms of retention and cycling. Fig. 1b shows the basic 1T-1R memory cell, where one MOS transistor is connected in series with an OxRAM cell. Fig. 1c presents a typical 1T-1R OxRAM I-V characteristic in logarithmic scale. Based on the I-V curve, the memory cell operation can be seen as follows: after an initial electro-FORMING (FMG) step [START_REF] Fieback | Device-Aware Test: A New Test Approach Towards DPPB Level[END_REF], the memory element can be reversibly switched between the Low Resistance State (LRS) and the High Resistance State (HRS). Resistive switching corresponds to an abrupt change between the HRS and the LRS. The resistance change is triggered by applying specific biases across the 1T-1R cell, i.e., VSET to switch to LRS and VRST to switch to HRS. In the 1T-1R configuration, the transistor controls the amount of current flowing through the cell according to its gate voltage bias. The maximum current allowed by the select transistor is called the compliance current and is referred to as IC in Fig. 1c. IC controls the LRS resistance value in the SET state as well as the maximal RESET (RST) current Ireset.

III. SEE IN THE OXRAM MEMORY ARRAY

Fig. 2 presents the OxRAM memory array, studied in this work, which is composed of a word line driver to select the active row (WLX), a bit line driver to select the active bit lines (BLX) during a FORM/SET and READ operations, and a source line driver to select a specific source line (SLX) during RST. Sense amplifiers are used to convert the cell current into a logical value at the circuit output during a READ operation. Memory cells are usually grouped into 8, 16 or 32 bits to form a memory word (dashed line). During the READ operations, the sense amplifier compares the cell current to a strictly controlled reference current.

The whole structure is simulated using Eldo simulator at transistor level (SPICE simulation). The effect of a particle crossing the structure is simulated by adding a parasitic current source at the impacted node(s). The current value is negative when a NMOS transistor is struck and positive when a PMOS transistor is struck. The current source comes from the realistic simulation of a particle crossing a silicon volume using TCAD device simulation tools (Synopsis ©). The effect of the ion strike is simulated using the Heavy Ion module of Synopsis [START_REF]TCAD Sentaurus user's manual[END_REF], considering an electron-hole pair column centered on the ion track axis. The Linear Energy Transfer is defined as the energy lost by the particle, by unit of length and varies along the track depending on the initial energy of the particles [START_REF] Akkerman | Ion and electron trackstructure and its effects in silicon: model and calculation[END_REF]. An actual variation of the LET was integrated in our simulations, based on the value given by SRIM tables [21]. Two kinds of ions are simulated: Alpha and Aluminum ions. The simulated Alpha particle is supposed to cross the device with an initial energy of 0.6MeV, which corresponds to a range of 2.3µm in the silicon volume. It corresponds to the end of an Alpha particle path of 1.47MeV, generated by the initial interaction of a thermal neutron with boron-10 (Alpha contamination). The case of a more energetic particle is also considered. It is a 50MeV Aluminum which could be produced by the interaction of fast protons with silicon [START_REF] Vasiliy | Evaluation of LET Spectra Produced by High Energy Protons in Si[END_REF]. Two geometries are simulated for the two ions: crossing horizontally Thus, SEE is evaluated by injecting and simulating sequentially parasitic current source using the electrical simulator. Current sources, reflecting SEE, are injected at specific nodes of the memory array presented in Fig. 2. The goal is to determine which current source causes logical failures and which one does not but causes an important shift of OxRAM resistances ("analog" faults). To evaluate the impact of each source on the memory circuit, the simulation process is automated. Currents are injected sequentially and the circuit responses are automatically extracted.

As the aim of this study is to get information about the worstcase configuration, the configurations able to induce a change in the initial state will be the only ones studied here. The investigation focuses on the most sensitive peripheral circuits to demonstrate the impact of a particle strike on the steady-state of the transistors directly involved to program the RRAM value. Among possible states (RST, SET, READ, WRITE, STBY), the standby operation is the longer state in terms of duration. Then the probability for a radiation event to occur during standby is the highest and this configuration is particularly explored here.

1) Fault simulation strategy

We implemented the memory circuit presented in Fig. 2 using a 0.13μm High Voltage CMOS technology offering a 3.3 V supply voltage. This technology was targeted because both, measurements and model calibration, were available for the 130 nm technology. Moreover, transistors withstanding high voltages are available in the 130nm technology. The latter are required during the FORMING operation achieved at 3.3V. To verify the operation of our design scheme, SPICE simulations are performed using the Eldo simulator. Various methodologies have been developed since several years to get reliable results about SEE sensitivity using SPICE simulations [START_REF] Peczalski | Physical SEU Model for Circuit Simulation[END_REF][START_REF] Pouget | SPICE Modeling of the Transient Response of Irradiated MOSFETS[END_REF][START_REF] Douin | Influence of Laser Pulse Duration in Single Event Upset Testing[END_REF][START_REF] Godlewski | Electrical modeling of the effect of beam profile for pulsed laser fault injection[END_REF][START_REF] Fulkerson | Prediction of SOI Single-Event Effect Using a Simple Physics-Based SPICE Model[END_REF]. Using this kind of simulation decreases considerably the simulation time. Indeed, SPICE simulations run at least 100 times quicker than TCAD simulations. It allows a very quick analysis even if the accuracy is limited for the study of radiations effects [START_REF] Castellani-Coulie | Circuit Effect on Collection Mechanisms Involved in Single Event Phenomena: Application to the Response of a NMOS Transistor in a 90 nm SRAM Cell[END_REF][START_REF] Castellani-Coulié | Analysis of SEU parameters for the study of SRAM cells reliability under radiation[END_REF]. It is particularly convenient when mixed-mode simulation is used to simulate the peripheral circuits at SPICE level and the impacted cell at TCAD level [START_REF] Castellani-Coulié | Development of a CMOS Oscillator Concept for Particle Detection and Tracking[END_REF].

Table I presents typical voltage levels applied to the memory cells during FMG, RESET, SET, READ and STBY operations. These voltage levels are provided by the Bit Line driver s (BL), the Source Line drivers (SL) and the Word Line (WL) drivers of Fig. 4. Transient simulation results are presented in Fig. 4 considering the voltage levels presented in Table I. One cell of the memory is addressed (Cell_37) by activating WL3, BL7 and SL3. The cell current Icell (XIC37) and the final cell resistance (Rcell) are extracted after FMG (∼30 kΩ), RST (∼30 kΩ) and SET (∼45 kΩ) operations. In this study, the investigation is focused on the peripheral circuit of one memory point (XR) depicted in Fig. 5.

As shown, three logic gates are involved in the memory configuration, namely the bit line (BL), the word line (WL) and the source line (SL) drivers.

In order to better understand the electrical mechanism managing the bias voltage of each terminal (BL, WL and SL), the transistor level schematic of each CMOS driver is depicted in Figs 6. The schematic diagram of one memory point is presented in Fig. 7 where only the transistors directly connected to the memory nodes are shown. As we can state, each terminal (BL, WL and SL) are faced with PMOS and NMOS transistors that are used to set the good operating voltage, according to the chosen configuration, as summarized in Table I.

Particle strike is supposed to cause a transient error on the nominal value of the bias voltage at each node (BL, WL and SL) directly connected to the RRAM memory. These specific nodes are considered as sensitive nodes regarding fault simulation due to particle crossing. Current spikes resulting of particle strikes are injected at these sensitive nodes and the state of each memory cell of the array is evaluated. Based on the considered memory array architecture, the only operating stage subject to SEE errors, leading to resistance drift is the STBY stage (Table II). In this configuration, the electrical values of the RRAM terminals (BL, SL and WL) are respectively high impedance (HI), low (L) and high or Low (H/L). To well understand the STBY configuration, we simulated the electrical operation of each driver (Fig. 8) focusing on the BL driver with HI value (depicted as a controlled switch in the Fig. 5). The BL driver is in fact a CMOS tri-state gate. When the EN terminal is set, the gate is on and the output voltage at the terminal Z (BL) is the same as the input one (IN). From an electrical point of view, these values correspond to VDD=2.5V (H) or GND=0V (L). However, when the gate is off, the voltage value at Z terminal is around VDD/2 depending on the voltage value at the input. This value corresponds to a metastable state (HI). This configuration is associated with the STBY state. Fig. 9. shows the impact of a particle striking the peripheral transistor NBL of Fig 6, during the STBY mode. This is done by injecting a very short current pic (ISEU) at the Z node of the BL driver. We can see in Fig. 9 that the voltage value at node Z switches to VDD (stuck at VDD) leading to a fault on the memory configuration.

Several scenarios are possible depending on the path and the energy of the particle. We will focus in this paper only on the particles striking the peripheral transistors (PBL; NBL), (PWL; NWL) and (PSL; NSL) of Fig 7 . Indeed, these transistors act directly on the potential difference between the memory terminals. The next section are dedicated to the worst case scenario identification.

2) Worst cases scenarios simulation

The worst-case scenarios are related to a particle strike impacting both the Bit Line voltages and Source Line voltages in STBY mode. Thus, extensive simulations are performed to assess the robustness of the memory array versus particles striking at these specific locations. Two configurations are studied. The first one considers a memory cell at LRS (Low Resistance State: 48 kΩ). The second one considers a memory cell at HRS (High Resistance State: 400 kΩ). Fig. 10 shows the impact of a particle (at 50 us) on a HRS cell in STBY mode. The simultaneous impact of the particle on BL and SL nodes induces a voltage difference across the cell of the memory array connected to the impacted drivers. Then, a programming current through the cell is triggered and a change of the cell resistance from HRS to LRS is detected. This event is named Single Event Upset (SEU) as the memory state is switched with respect to the OxRAM nominal values. For each simulated configuration in this work, a PMOS is simultaneously impacted with a NMOS. Then a negative current is injected when a NMOS is supposed to be struck, and a positive current for a PMOS. Fig. 11 shows the impact of the Al hor. particle on a LRS cell in STBY mode. In this configuration, a shift of memory cell resistance towards lower LRS levels is observed. This resistance shift improves the LRS. However, going deeper in the LRS state can lead to programming issues during the next RESET operation. Indeed, if the LRS resistance value is too low, resetting the cell to HRS can be difficult or even impossible [START_REF] Fieback | Device-Aware Test: A New Test Approach Towards DPPB Level[END_REF]. Then a stuck at fault will occur.

Table III summarizes the particle impacts on the cell resistance when the cell is in an initial HRS or LRS state. Resistance shift, compared to the resistance nominal value before the strike, or radiative event (SEU or stuck-at-fault) are captured and presented. The analysis was conducted for the different current sources correlated with Alpha and Aluminum particles and presented in Section III. From this table, we can conclude that the SEU will occur only in the HRS state and for a particle striking simultaneously a NMOS connected to the BL node and a PMOS connected to the SL node. The opposite configuration does not induce errors but resistance shifts. Concerning the initially LRS state, for each configuration the LRS state is reinforced, potentially leading to stuck-at-faults depending on the value of the resistance shift (Stuck?). The resistance shift is then related to the injected current and the occurrence of stuck-atfaults would depend on the threshold of the RRAM cell. The resistance shift seems to be linked to the peak value of the generated currents. From Tab. III and from Fig. 3, we can note that the resistance shift is 28% for the Alpha particles, 39% for the Aluminum in the diagonal configuration and 45% for the Aluminum in the horizontal configuration. This clearly follows the trend of the current peaks presented in Fig. 3. This peak is linked to the deposited energy. Then the stuck-at-fault occurrence would depend on the energy deposited by the particle in the sensitive transistors surrounding the RRAM. This paper provides a complementary analysis of Single Event Effect in a RRAM memory cell. As it was shown in [START_REF] Castellani-Coulié | SPICE level analysis of Single Event Effects in an OxRRAM cell[END_REF], some specific configurations can induce effects in this kind of device, particularly when striking simultaneously a NMOS and a PMOS transistors, in standby operation. The configurations simulated here show that SEU could occur in RRAM cells for some specific topologies. This is the case if a NMOS connected to BL node is close enough to the PMOS connected to the SL node to be crossed by a single particle when initially at HRS. By avoiding this topology, SEU could be reduced. Other configurations, related to the LRS, can induce stuck-at-fault which could be more difficult to address than SEU because the cell then remains permanently in LRS.

Finally, the study was performed on a 130nm technology. At the transistor level, downscaling should induce a shrinking of the drain/source junctions and should make them closer. As transistors for smaller technology nodes are known to be more sensitive to ionizing particles, the SEE rate should increase. Moreover, at the memory matrix level, the shrunk transistors should be closer, facilitating the crossing of a single particle through several sensitive transistors and making the scenario studied in this paper more probable.

Fig. 1 .

 1 (a) TEM cross-section of an OxRAM device (b) symbol view of a 1T-1R cell (c) OxRAM I-V characteristic in log scale.

  presents the OxRAM technology. In Section III, SEE is analyzed at the memory array level. Section IV presents simulation results showing the effect of SEE. Finally, Section V concludes this paper.

Fig. 2 .

 2 Fig. 2. Elementary memory array architecture used for simulations

Figure 6 .

 6 Figure 6. Sensitive node location at the memory array driver level

Fig. 4 .

 4 Fig. 4. Transient simulation results after FMG, RST and SET operations.

Fig. 5 .

 5 Fig. 5. Schematic diagram of one memory point and its peripheral circuit.

Fig. 7 . 9 :

 79 Fig. 7. Schematic diagram of one memory cell and its peripheral circuit at the transistor level. Fig. 9: Simulation of SEU strike on the BL driver (passgate).

Fig. 8 :

 8 Fig. 8: Electrical value simulated at the output of the BL driver (BL node)

Fig. 10 .Fig. 11 .

 1011 Fig. 10. Transient simulation results for a particle Al hor. striking a cell in HRS state

TABLE I

 I

	.	OPERATING VOLTAGE LEVELS (H:2.5V, HV:3.3V, L:0V)
		FMG	RST	SET	READ	STBY
	BL	HV	L	H	HI	HI
	SL	L	H	H	H	L
	WL	H	H	H	H	H/L

TABLE II

 II

		.	PARTICLE STRIKE IMPACT
		FMG	RST	SET	READ	STBY
	Impact	NO	NO	NO	NO	YES

TABLE III .

 III WORST CASE SCENARIOS RESULTS IN STBY MODE V. CONCLUSION