7th International Conference on the Geology of the Arab World, Cairo University, Feb. 2004, P. 1-7

DISCOVERY OF Fe-Cr-Ni SPECKS WITHIN QARET EL-HANASH BRECCIA OF THE LIBYAN GLASS AREA, SOUTH WESTERN EGYPT

Ahmed El-Kammar1, Ibtsam Arafa1, Khiert A. Soliman2 and Aly Barakat2

1- Department of Geology, Cairo University, Egypt.2- The Egyptian Geological Survey and Mining Authority

ABSTRACTPetrographical studies of sandstone breccia from Qaret el-Hanash Hill, on the south-eastern side of the

Libyan glass area, demonstrate the presence of Fe-Cr-Ni particles and dispersed specks within a matrix of deformed quartz and other phases. Microanalysis of these specks manifests that they consist essentially of native Fe, Cr and Ni. The composition of these specks fits well with the established composition of Fe-Cr-Ni particles reported from the Ries Crater and has been considered as an indication of carbonaceous chondrite impactor body. The occurrence of this phase within the matrix of Qaret el Hanash breccia may favour the assumption of hypervelocity meteorite impact on the Libyan glass area.

INTRODUCTIONThe Libyan glass is a very unusual natural material composed almost entirely of silica (>96% SiO2), first

reported by Mr. P.A. Clayton in December 29, 1932 (Clayton and Spencer 1934). It occurs as fragments of various sizes, ranging from few mm up to 50 cm in maximum diameters, on an oval area between latitudes 25° 02ˉ - 26° 12ˉ N and longitudes 25° 24ˉ - 25° 55ˉ E, south-west Egypt (Fig. 1). Barakat, et al. (1997) reported that the main concentration of the Libyan glass fragments occurs on the area represented by Saad Formation (Late Cretaceous sandstone), hidden partly by a sand sheet and extensive field of sand dunes (Fig. 2). Fission-track analyses suggest an age of 28.5 m.a. (e.g., Bigazzi and de Michele, 1997).

The origin of the Libyan glass is a matter of discussion and controversy. Several hypotheses have been introduced to explain its formation. Both terrestrial and extraterrestrial origins were suggested as well as high and low temperature formation processes. However, there is a general tendency to consider it as a result of fusion of the sandstone country rock by the heat generated from impact of a large heavenly body, in particular a carbonaceous chondrite (e.g., Diemer, 1997).

Identification of hypervelocity meteorite impact features, such as, megascopic breccia, and shock deformation features within the quartz of the sandstone country rock of the area has been delayed for long time. This has encouraged some of the workers on this problem to connect between the formation of the Libyan glass and the supposed meteoritic craters in Libya (Koeberl, 1997 and Abate, et al. 1999).

In this regard, it is worthy to mention that most studies were devoted to the Libyan glass itself ignoring the geology of the hosting area. Giegengack and Underwood (1997) are of the opinion that the area of the Libyan glass was not yet sufficiently studied. They stated that the length orientation and height of the dunes make east-west travel extremely difficult for conventional desert vehicles, except by looping around the southern terminations of the dunes. Because of the difficulty of reaching the remote site with sufficient fuel and water for more than a day or so of study and exploration, the dimensions of the strewn field of the Libyan glass have not yet been certainly established.

Recent studies report additional evidence on impact, such as the PDFs within quartz of the sandstone in the Libyan glass area (Barakat, 1998), central uplift, megascopic breccias and post impact phases of mineralization represented by iron deposit (Barakat, 2001, see, Fig. 2). Meanwhile, a diamondiferous material (Barakat,1999) was found in association with the glass fragments and a relatively high Ir content (1.6-2.2 ppb) was reported in the sandstone breccias of the study area (Barakat, 2004). This supports a hypervelocity meteorite impact event on the area

The present work documents a discovery of native Fe-Cr-Ni phase within the matrix of Qaret el Hanash breccia that occurs at the southeastern side of the Libyan glass area. This confirms the assumption of hypervelocity meteorite impact event, in particular a carbonaceous chondrite.

QARET EL-HANASH BRECCIA

There are several isolated sandstone hills representing prominent topographic features around latitude 25° 04ˉ N and longitude 25° 55ˉ E, on the southeastern corner of the Libyan glass area (Fig. 3). The name Qaret el-Hanash is called after the highest hill that rises about 96 m above the surrounding desert level. During April 1994 expedition, the field party (under the leadership of Dr. Vincenzo de Michele, Natural History Museum, Milano, Italy), was obliged to avoid crossing the sand dunes and move in-between them. This problem provided

Ahmed El-Kammar et al.

excellent oppertunity to the group, who was able to visit the northwestern side of Qaret el-Hanash (the side facing the southeastern corner of the Libyan glass area). Traces of an eroded zone of megascopic sandstone breccia, of about 2-m thickness, have been recognized. The breccia occurs within the wall of Qaret el Hanash only on the side facing the Libyan glass area. It appears fresher than the enclosing sandstone country rock, while its matrix appears as dull greyish-black material. It is different from the ferruginous matrix reported in the sandstone of the area. For these reasons some attention has been given to the samples of this breccia.

Fig. 1: Location map of the Libyan glass area 1n Egypt

Fig. 2: Map of the Libyan glass area.

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Discovery of Fe-Cr-Ni Specks Within Qaret El-Hanash Breccia

Fig. 3: Part of the Qaret el Hanash hill (Photo courtesy of Dr. Armin Lieberam)

ANALYTICAL METHODSSamples from Qaret el-Hanash sandstone breccia were investigated petrographically using polarising

and ore microscopes. Thin-polished slices were investigated by scanning electron microscope model JEOL 5600 SEM with NORAN Vantage EDS. Acceleration voltage was 15 kV, at the Geology Department, Rand Afrikaans University (RAU), Johannesburg, Republic of South Africa. A scanning electron microscope, model JEDL JSM-840, with ANID EDS System, at the School of Biology, Witwatersrand University Johannesburg was also used. Some samples were also investigated by scanning electron microscope equipped with EDAX at the Central Laboratories of the Geological Survey of Egypt and the Nuclear Material Authority, Cairo, Egypt.

PETROGRAPHY OF QARET EL HANASH BRECCIAThe rock consists of various fragments of local sandstone, i.e., from the area of the glass distribution

itself. Fragments of igneous source have never been recorded. The rock fragments range from fraction of mm up to 4-cm in diameter, in the collected specimens. The rock fragments vary in colour from creamy white to brownish-red. They are angular to subrounded and embedded in a dull greyish-black matrix (Fig. 4).

The microscopic investigation and scanning electron microanalysis confirmed the above-mentioned observation and showed that the matrix consists of shattered and fragmented quartz grains of various sizes (Figs. 5 & 6). In addition to quartz, the matrix contains many other phases, such as; glass (Fig. 7), zircon, clay minerals, wollastonite, ilmenite, Mg-ilmenite, rutile and Fe-Cr-Ni specks.

Fig 4: The flat surface of the Qaret el Hanash breccia.

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Fig.5: Photomicrograph of Qaret el Hanash breccia showing shattered and fragmented quartz grains in a matrix of finer quartz admixed with other phases (width of the field= 1.0 mm).

Fig. 6: SEM image and EDS analysis of quartz and some other phases within the matrix of Qaret el Hanash breccia.

Fig. 7: SEM image and EDS analysis of quartz (spot 1) and glass of worm like structure (spot 2) in the matrix of Qaret el Hanash breccia.

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Discovery of Fe-Cr-Ni Specks Within Qaret El-Hanash Breccia

THE Fe-Cr-Ni SPECKS There are several metallic specks of various sizes ranging from 1 micron up to about 10 micron,

dispersed in the matrix of Qaret el Hanash breccia (Fig. 8). They invade some of the quartz grains (Fig. 9). These specks are mainly of irregular outlines and some of them show clear fissures. They are associated with glass and intercalated in some cases with halite. EDAX analyses of such specks indicate that they consist of native Fe, Cr and Ni, in addition to subordinate Si and Ca (Fig. 10). The average composition of this phase, calculated as oxides, is as follows; FeO = 70.00, Cr2O3 = 16.92, NiO = 7.68, CaO = 4.06 and SiO2 = 1.33 wt%. This composition fits well with the composition of the Fe-Cr-Ni particles detected within the compressed zone of the Ries crater (El Goresy and Chao, 1976). Finer particles of similar appearance have also been noticed by the petrographical microscope through the fractures of some of the quartz grains.

DISCUSSION

The petrographical and chemical studies indicate that Qaret el Hanash breccia represents a sandstone breccia, and it contains no fragments of igneous rocks. The breccia contains mineral phases that support meteorite impact effect such as glass, meteoritic material and Mg-ilmenite. Chemical analysis of samples from this breccia indicates relatively high concentration of Ir (1.6-2.1 ppb, Barakat, 2004). On these grounds, it can be suggested that the breccia is possibly imparted by meteoritic material. The detection of native Fe-Cr-Ni phase within the matrix of this breccia is unambiguous evidence of incorporation of meteoritic material. This phase, according to El Goresy and Chao, (op. cit) indicates that the impactor is a carbonaceous chondrite. This is in agreement with Diemer (op. cit.), who expected that the Libyan glass may be formed as a result of the impact of a carbonaceous chondrite. The present phase needs more investigation to verify whether it penetrates the quartz grains of the breccia or occurs only in the matrix. The results could help in determining the terminology of Qaret el Hanash breccia. If this phase occurs only in the matrix it may indicate that the breccia represents fallback breccia containing particles of the condensed vapour of the impacting body. Otherwise it may represent part of the crater wall and the Fe-Cr-Ni particles were injected by the force of the impact. These new discoveries may end debate about the presence of meteorite impact effect on the Libyan glass area and open discussion on the possible local and regional effect of the event, in particular the extinction of several Oligocene mammalian species from North Africa.

Fig. 8: Photomicrograph showing finer particles of the Fe-Cr-Ni phase dispersed within the matrix of Qaret el Hanash breccia (Reflected light, width of the field 0.01 mm).

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Fig. 9: SEM image of the matrix of Qaret el Hanash breccia showing one of the Fe-Cr-Ni specks.

Fig. 10: EDAX spectrum of one spot made on the Fe-Cr-Ni speck of Fig. 9. Notice the absence of oxygen Spectrum

ACKNOWLEDGEMENTS The authors thank Dr. Vincenzo de Michele, Natural History Museum, Milano Italy, and Mr. Giancarlo Negro, the leaders of the field parties to the Libyan glass area as well as, Dr. Serra Romano and Mr. Benito Piacenza for their kind help during the fieldwork. Grateful acknowledgements are due to Prof. Wolf Uwe Reimold, School of Geoscience, University of Witwatersrand, Johannesburg, South Africa and Dr. Willie Oldewage, Rand Afrikaans University, Johannesburg, South Africa for their kind helps.

REFERENCES

Abate, B. Koeberl, C. Kruger, F.J. and Underwood, J.R. Jr. (1999): BP and Oasis impact structures, Libya, and their relation to Libyan Desert glass. In Dressler B.O. and Sharpton V.I (eds.): Meteorite impacts and planetary Evolution II. Boulder, Colorado, Geol. Soc. America. Special Paper 339, pp. 177-192.

Barakat, A.A. (l998): Silica glass: A mystery in a mysterious land. In Wael T. Abed: The Other Egypt" travels in non-man’s Land". American University Press, Cairo, pp. 76-79.

Barakat A.A. (l999): Diamondiferous material from the Libyan glass area southwestern Egypt (Abstract). The first International Conference on the Geology of Africa, Nov. 23-25, 1999, Assiut University, Egypt, p.14.

Barakat A.A. (2001): Meteorite impact signs in the Libyan glass area, southwestern Egypt. (Abstract), the Second International Conference on the Geology of Africa, October. 28-30, 2001, Assiut University, Egypt,VI-22, pp.93-94.

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Discovery of Fe-Cr-Ni Specks Within Qaret El-Hanash Breccia

Barakat A.A. (2004): Meteorite impact effects in the Libyan glass area, south-western Egypt. Ph. D Thesis, Cairo University, 161 p.

Barakat A.A., de Michele V., Negro G., Piacenza B. and Serra R. (1997): Some new data on the distribution of Libyan Desert glass (Great Sand Sea Egypt). Sahara, special issue “Silica “96” Meeting”, July 18, 1996,pp. 29-36.

Bigazzi G. and Michele V. (1997): New fission-track ages of Libyan Desert glass. Sahara, special issue “Silica“96” Meeting” July 18, 1996,pp. 49-57.

Clayton, P.A. and Spencer, L.J. (1934): Silica glass from the Libyan Desert. Min. Mag. 23,pp. 501-508.Diemer E. (1997): Libyan Desert glass: an impactite state of the art in July 1996. Sahara, special issue “Silica

96”Meeting” July 18, 1996, pp.95-109.El Goresy A. and Chao E.C.T. (1976): Evidence of the impacting body of the Ries crater- The discovery of Fe-Cr-

Ni veilets below the crater bottom. Earth and Planet. Sci. Letters,v. 30,pp. 330--340. Giegengack R. and Underwood J.R. Jr (1997): Origin of Libyan Desert glass: some stratigraphic considerations.

Sahara, special issue “Silica 96” Meeting” July 18, 1996,pp. 37-39.Koeberl C. (1997): Libyan Desert Glass: geochemical Composition and origin. Sahara, special issue “Silica 96”

Meeting” July 18,1996,pp. 121-131.

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