Evolution and Mineralization of the Arabian–Nubian Shield. Proceedings of a Symposium Convened by Ahmad M.S. Al-Shanti

184 Pages·1979·11.13 MB·English

by  S. A. Tahoun (Eds.)

KINGDOM OF SAUDI ARABIA
KING ABDULAZIZ UNIVERSITY
Mohammad O. Zubeir
President
Abdullah O. Nasseef
Vice President
INSTITUTE OF APPLIED GEOLOGY
Ahmad M. S. Al-Shanti
Dean
I. A. G. BULLETIN No. 3
EVOLUTION AND MINERALIZATION
OF THE
ARABIAN-NUBIAN SHIELD
Proceedings of a Symposium convened by
AHMAD M. S. AL-SHANTI
Volume 1
Published for the
INSTITUTE OF APPLIED GEOLOGY, KING ABDULAZIZ UNIVERSITY
JEDDAH, KINGDOM OF SAUDI ARABIA
by
PERGAMON PRESS
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Copyright @ 1979 Institute of Applied Geology, King Abdulaziz University, Jeddah
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First edition 1979
British Library Cataloguing in Publication Data
Evolution and mineralization of the Arabian - Nubian Shield. -(King Abdulaziz
University. Institute of Applied Geology. IAG bulletins; no. 3). Vol. 1
I. Geology - Near East - Congresses I. Al-Shanti, Ahmed M 11. Series 555.6
QE319.N4 79-40353
ISBN 0-08-024460-2
Printed in Great Britain by Hillman Printers (Frome) Ltd., Somerset
Bound by A. Wheaton & Co. Ltd., Exeter
FOREWORD
These volumes represent an important step in the bination of regional and topical reviews, reports on
scientific study of those parts of the Middle East completed research projects, and progress reports
which lie upon a Precambrian basement. They on current projects. This approach is clearly ap­
record the scientific proceedings of a major sym­ parent in the following pages.
posium held in Jeddah in February 1978 under the The symposium was convened by Dr. A. M. S.
auspices of the Institute of Applied Geology of Al-Shanti, Dean of the Institute of Applied
King Abdulaziz University. As such, they Geology, and organized by the staff of the Institute
represent the first occasion on which reviews and under his chairmanship. Funding was provided by
studies of a broad range of geological topics King Abdulaziz University. The cost in time, work
related to the Precambrian basements of Saudi and money of any scientific meeting such as this is
Arabia, Yemen, Egypt, Sudan, Ethiopia and Jordan great, and Dr. Al-Shanti and his team are to be
have been brought together in one place. thanked for their efforts.
The symposium itself was held between 6th and I believe that these volumes will stand as a basic
16th February 1978. There were four days of con­ reference on the geology and mineralization of the
ference sessions in Jeddah, followed by two Arabian-Nubian Shield for many years, and I hope
separate field excursions of six days duration. In that they will provide a platform for the develop­
all, there were some 320 participants from 25 ment of many further studies.
countries. Forty papers were orally presented and
discussed in the conference sessions, and about 60
ABDULLAH O. NASSEEF
full texts are given in these proceedings. The
scientific objective was, and is, to give a com­ Vice-President, King Abdulaziz University
vu
INTRODUCTION
The Arabian-Nubian Shield comprises the Pre- of the African Shield. The other school holds that
cambrian basement terrain underlying large areas the Arabian-Nubian Shield (if not a larger propor­
of the Middle East (mainly Saudi Arabia, Egypt, tion of the African Shield) is not ensialic, but has
Sudan, Yemen, Somalia and Ethiopia). The recent evolved on oceanic crust from the welding
intense interest and activity in searching for new together of a series of island arcs. For the resolu­
mineral deposits in this region has led to a wealth tion of this fundamental problem, the critical fac­
of geological investigations by diverse individuals tors appear to be: the nature of the ultrabasic
and groups from many parts of the world. In this belts in the shield (Are they remnants of oceanic
context, exchange of current ideas is difficult. It is crust?); the nature of the gneisses and granitic
also very important because exploration for rocks (Do any of them represent original con­
mineral resources depends so completely on basic tinental basement?); and the nature of the principal
geological knowledge, and particularly on tectonic volcanic and sedimentary units of the shield (Were
evolutionary models. It is, therefore, an ap­ they deposited in oceanic or continental environ­
propriate time to bring together those scientists ments?).
who are directly concerned with these problems in The proceedings of the symposium will appear
the Arabian-Nubian Shield. Such a meeting has in four volumes each containing about 15 papers
been arranged by the Institute of Applied Geology, grouped into several sections. It will be noted that
King Abdulaziz University. some papers answer some of the imposed ques­
The gross evolutionary style of the African tions pertaining to the geology of the Arabian-
Shield, and especially that of the Pan-African Nubian Shield while others impose a new set of
belts, remains one of the major unsolved geologi­ questions. Thus, the tectonic evolution, structure,
cal questions. The Arabian-Nubian portion of the and metallogenesis of the Arabian-Nubian Shield
African Shield is, therefore, critical in view of its are not any easier than before but they may be
excellent exposures and accessibility. There are more fully appreciated to initiate further research
currently two main schools of thought on the ori­ work.
gins of the complex terrains of the Arabian-
Nubian Shield. One school considers the shield to
have evolved as essentially intra-cratonic, ensialic AHMED M. S. AL-SHANTI
belts with a basement of older sialic material. This
Dean
is the kind of model generally suggested for most Institute of Applied Geology
vu
SYMPOSIUM PERSONNEL
Convenor: A. M. S. AL-SHANTI
Organizers: M. L. ABDEL-KHALEK, A. B. AL-
KHATIB, A. M. AL-SHANTI, A. N. BA-SAHEL, P.
L. BINDA, O. R. EL-MAHDY, A. A. A. HUSSEIN,
N. J. JACKSON, W. J. MACLEAN, J. G. MOLTZER,
F. I. OSMAN, C. R. RAMSAY, M. J. ROOBOL, S. A.
TAHOUN, and M. A. TOFIQ.
Field Excursion Leaders: A. M. AL-SARI, P. L.
BINDA, O. R. EL-MAHDY, N. J. JACKSON, M.
E. KUKANDY, C. R. RAMSAY, M. J. ROOBOL, J.
H. SCHELLEKENS, M. I. THEKAIR, and M. O.
ZUBEIR.
Editorial Board: A. M. AL-SHANTI, P. L. BINDA,
C. R. RAMSAY, and S. A. TAHOUN.
Referees: M. L. ABDEL-KHALEK, A. A. ABDEL-
MONEM, M. A. AL-GAZZAZ, A. N. BA-SAHEL, E.
Z. BASTA, P. L. BINDA, H. R. BLANK, C. B.
DUBOIS, O. R. EL-MAHDY, H. A. HAKIM, A. A.
A. HUSSEIN, N. J. JACKSON, M. M. KHUDA, W. J.
MACLEAN, J. G. MOLTZER, C. R. RAMSAY, M. J.
ROOBOL, J. WAKEFIELD, and A. J. WARDEN.
Arabie Abstracts: I. A. M. FARAG
Associate Editors: P. L. BINDA, P. G. COORAY, A.
A. A. HUSSEIN, N. J. JACKSON, M. M. KHUDA, C.
R. RAMSAY, and M. J. ROOBOL.
Editor of Volume 1: S. A. TAHOUN.
vu
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AN OVERVIEW OF THE GEOLOGY OF WESTERN ARABIA*
G. F. BROWN and R. O. JACKSON
U.S. Geological Survey, Reston, Virginia
U.S. Geological Survey, Jeddah, Saudi Arabia
Abstract—Serious study of the geology of western Saudi Arabia began in 1950 (1369 A.H.) at the request of King
Abdulaziz AI Saud. Reconnaissance mapping and photo-interpretation were combined with maps compiled by
international oil companies, and the resultant maps were published by 1963 at scales of 1:500,000 and 1:2,000,000.
The crystalline 36% of the Kingdom was mapped on the basis of petrologie units described mostly in field terms,
as it was recognized at an early stage that lithostratigraphic groups range from unmetamorphosed rocks to
greenschist and amphibolite facies.
♦Published with permission of Directorate General of Mineral Resources, Jeddah and Director, U.S. Geological Survey.
3
4 G. F. BROWN and R. O. JACKSON
During the past 15 years, more detailed mapping by many geologists has begun to elucidate structural
complexities, chronologic episodes, sedimentary and volcanogenic facies changes, and metallogenic epochs.
Computer-enhanced imagery from satellite signals supplied a new tool, supplementing aerial photographs and
standard color composites of satellite-derived scenes. Airborne magnetic and digital gamma-radiation surveys also
helped in interpreting the geology.
Lithostratigraphic sequences include, from lowermost parts upwards, the Baish, Bahah, Jiddah*, Ablah,
Hulayfah, Murdama, Shammar, and Jubaylah* Groups. These range in radiometrie age from about 1150 to 510 Ma.
The general chronological trend is from older basaltic (oceanic) suites, through the calc-alkalic suites, to alkalic
and peralkalic igneous rocks among the youngest suites. Ophiolitic belts suggest plate collision, especially in the
central and eastern reaches of the shield. Subduction dips eastward along the Idsas-Al-Amar fault at the eastern
edge of the shield but is reported to be predominantly westward in the central belt. Important metallogenic epochs
culminated at about 658 Ma (syngenetic) and about 550 Ma B.P. (epigenetic).
Red Sea-Gulf of Aden rifting began in early Tertiary without substantial marine invasion until the Middle
Miocene, which is also a metallogenic epoch. Ramping and en echelon graben faulting became somewhat skewed
as the peninsula rotated counterclockwise on a second Euler's fixed-axis pole beginning anew in the early Pliocene
and continuing to the present; the rotation is accompanied by outpourings of plateau-type basalt.
INTRODUCTION Survey-Arabian American Oil Company, 1963) was
published, the western part of Saudi Arabia was
Except for brief descriptions by early travelers,
mapped only in reconnaissance fashion; petrologie
the geology of western Arabia, particularly the
units were described mostly in field terms, and the
shield area, was largely unknown until recent
map was based largely on aerial photo-inter­
years. In 1950 (1369 A.H.), King Abdulaziz, realiz­
pretation. Although the map was planned only as a
ing the necessity of evaluating the natural
general guide for ore and water search, lithostrati­
resources of the Kingdom, particularly water sup­
graphic groups were recognized at an early stage to
plies, asked the U.S. Geological Survey (USGS) to
range from unmetamorphosed to greenschist and
make a study of western Arabia. Accordingly,
amphibolite metamorphic facies, and to change
aerial photographs were taken, first of a segment
lithologically in short distances along the strike of
of the Asir, later throughout the Hijaz, at the
the volcanogenic and related rocks. Because the
request of the late King Faisal, then Viceroy of the
resolution of such stratigraphie problems would
Hijaz, and fieldwork began.
require detailed work during many years, the units
Following the organization of the Directorate
were described in general terms on the map legend.
General of Mineral Resources within the Ministry
of Petroleum and Mineral Resources in 1954, much
time and effort have been spent in geological stu­ Geologic Divisions
dies in western Arabia. The purpose has been to
In the first published attempts at division of the
locate and evaluate raw materials other than
basement rock, Karpoff (1956, 1957) described two
petroleum. Much work has been and is being done
series—the Medina and the Wadi Fatima, in the
cooperatively with the Directorate by the USGS,
central part of the shield. We had begun our
the French Bureau de Recherches Géologiques et
reconnaissance study 5 years earlier at the Yemen
Minieres (BRGM), the Japanese Geological Sur­
border and had worked northward. In 1960 we
vey, and more recently, the Group Contractors and
summarized the results of our first 10 years of
Riofinex, mostly British Commonwealth geologists,
work (Brown and Jackson, 1960), extending our
all working with an expanding group of young
general rock assemblages into the areas described
Saudi Arabians who have completed or are com­
in more detail by Karpoff (1960), wherein we ten­
pleting their education abroad or at home. The
tatively recognized eight units exclusive of plu-
extensive good outcrops, the availability of all the
tonic rocks. Of these, six were equivalent to parts of
modern tools of exploration, including satellite
the older Medina Series of Karpoff, and two were
imagery, and the diverse cultural and scholastic
equivalent to and coordinate with his Wadi Fatima
backgrounds of the scores of geologists now stu­
Series. By 1963, after mapping seven quadrangles
dying the Hijaz has led to an outburst of geologic
(3°x4° at a scale of 1:500,000) and segments of
knowledge—and multiple theories.
three others—about 36% of the Kingdom—we
compiled a map at 1:2 million scale which was
RECONNAISSANCE MAPPING combined with the mapping of the Arabian
Americal Oil Company (ARAMCO) and various
At the time the Saudi-sponsored geologic map of
others into the composite map of the peninsula
the Arabian Peninsula (I-270A, U.S. Geological
(USGS-ARAMCO, 1963). By that time we recog­
nized 21 additional units among the basement
rocks, of which 17 were given rock-term names
* Editor's note: Jiddah/Jeddah and Jubaylah/J'Balah are used
interchangeably by different authors. without formal reference to geographic localities.
An Overview of the Geology of Western Arabia 5
Of the names based on geographic localities, two DETAILED MAPPING AND CURRENT CONCEPTS
were regional, namely the Halaban and Shammar.
During the past 15 years, more detailed mapping
The Halaban was named for Halaban Ridge and
by numerous geologists has begun to elucidate
the region around the Halaban Pass near the
structural complexities, chronologic episodes and
southern end of the ridge in the east-central part of
sequences, sedimentary and volcanogenic facies
the shield, and the Shammar was named for Jabal
changes, and metallogenic epochs. Computer-
Shammar, the regional name for the area around
enhanced imagery from satellite signals supplied a
Ha'il in the north-eastern part of the shield.
new tool that supplements aerial photographs and
Some explanation of the first use of these names
standard color composites of satellite-derived
is desirable. The first geologist to describe the beds
scenes. Airborne magnetic and digital gamma-
at Jabal Halaban was Bogue (1953) of the USGS.
radiation surveys also helped in interpreting the
He considered that the volcanic greenstone and
geology.
related rocks there were probably the same as the
"Shawaq volcanics" that he had earlier described Hali Supergroup
as being at Wadi Shawaq in the Hisma region
900 km N.W. of Halaban and near the north-west­ As a result of the more detailed mapping, some
ern edge of the shield. We felt that even though the formations of the USGS-ARAMCO map (1963)
lithology and metamorphism appeared to be were raised to group rank (Schmidt et al, 1973) to
similar, the units were separated by too great a include various formations named for specific
distance of unmapped terrain to justify correlation localities and type sections. Also, because many
until the areas were mapped in more detail. Ac­ more radiometrie ages have been determined and
cordingly, we (Brown and Jackson, 1960) chose the structural control is better understood, the
name "Halaban Andésite" for his "Shawaq vol­ sequence of groups has been changed somewhat.
canics"; we later used the term "Halaban For­ The older Wadi Lith Complex (USGS-ARAMCO,
mation", because we knew that rock types other 1963) included rocks of diverse ages and fragments
than andesitic greenstone are in the section and of other groups, so the name was abandoned.
that an assemblage of at least group rank would be Schistose beds originally designated the Hali
found if it were mapped in any detail. Schist were redefined because they were con­
sidered to be younger than the Baish Greenstone
Likewise the name "Shammar Rhyolite" was
and were renamed the Bahah (Schmidt et al,
used (Brown and Jackson, 1960) to designate the
1973). Later the name Hali Group was abandoned,
largely unmetamorphosed and silicic volcanic rocks
as the beds were believed to be more metamor­
associated with sedimentary rocks cropping out in
phosed parts of the Ablah Group (Hadley, 1975).
the northern part of the shield, in order to dis­
tinguish these younger rocks from older rhyolite As the name Hali Formation has not been
and related volcanogenic beds farther south. retained, it would seem desirable to designate
the Baish and Bahah Groups as units of a Hali
The six other names published in 1960 (Brown
Supergroup, inasmuch as they contain essentially
and Jackson, 1960) were, from oldest to youngest,
the same beds originally mapped as Hali and are
the Hali Schist, Baish Greenstone, and Wadi Lith
interbedded, at least locally (Greenwood et al.,
Complex below the Halaban; and the Murdama,
1978). The two groups have the following com­
Fatima, and Ablah Formations above the Halaban
mon components—pillow meta-basalt, marble,
and below the Shammar Rhyolite. Later we des­
chert, carbonaceous or graphitic schist, and some
cribed (USGS-ARAMCO, 1963) the Jiddah
meta-andesite, quite distinct from the overlying
Greenstone, which is intruded by Mecca Granite;
andesitic lavas and volcanogenic sedimentary
the Hibshi Formation, dominantly a clastic-vol­
rocks of the Jiddah and younger groups (Schmidt
canic formation that disconformably overlies a
et ai, 1973; Hadley, 1975a, b; Greenwood et a/.,
gray biotite granite and that underlies the Mur­
1975).
dama in the Jabal Shammar region; the Hadiyah
Slate in the north-western part of the shield and
Jiddah Group
the Silasia iron-bearing formation. Units were in­
dividually named because field information was The Jiddah Group, first described on the penin­
insufficient to warrant closer correlation over large sular map (USGS-ARAMCO, 1963) as the Jiddah
areas. Names of large wadis or coastal towns were Greenstone, and exposed in the foothills east of
used to avoid giving a specific type locality until Jiddah, includes andésite, diabase, red and gray
the units could be mapped in detail and best type slate, conglomerate, minor rhyolite, marble, and
sections and localities could be chosen. The Mur­ andésite porphyry all metamorphosed into green­
dama and Hibshi Formations are exceptions; both stone and a greenschist facies complex. The Jiddah
have excellent sections on the mountains of those Group was intruded by the Mecca Granite, which
names. has yielded ages from 720 to 1025 Ma B.P.; the
6 G. F. BROWN and R. O. JACKSON
younger K-Ar ages possibly represent an argon andésite, dacite, and minor basalt, mostly intrusive
loss due to metamorphism or to an epeirogenic and probably of a later epoch. The colors and
uplift and cooling of the craton (Aldrich et al, first-cycle sedimentary rocks, which together
1978). For this reason, the Jiddah Greenstone was with the stromatolitic structures, indicate a con­
considered to be among the oldest rocks of the tinental and shallow-marine deposition for these
shield and was placed beneath the Baish Green­ beds prior to the Ranyah episode of the Asir
stone in the geologic column. Later the unit was orogeny or orogenic cycle (Mozambiquian) and
redefined (Schmidt et al, 1973), and the name after the accumulation of the Jiddah Group
Jiddah Group was applied to andesitic (Greenwood et al, 1978).
agglomerate, breccia, tuff, dacite, basalt, pillow
lava and minor conglomerate, marble, and slate.
Halaban Supergroup
The overlying Halaban Group includes basal con­
glomerate containing pebbles and clasts of meta­ Besides suggesting that the lower more mafic
morphosed basaltic and andesitic rocks of the Baish, and marine groups be assigned to the Hali Super­
Bahah, and Jiddah Groups (Schmidt et al, 1973). group, we also propose that similar rank be ac­
More recent work extending northward along corded the Halaban, a name we earlier applied to
the coast indicates that meta-basalt, meta-andesite, dominantly calc-alkaline volcanic and quartz
and graphitic schists that were mapped as Jiddah dioritic intrusive rocks in the central and northern
as far north as Al-Wajh lithologically resemble the reaches of the shield (Brown and Jackson, 1960).
Hali Supergroup (Okumi et al, 1965; John Kemp These rocks are above an erosional surface below
(BRGM), per. com., 1973; H. W. Blodget and G. F. which are diorite-granodiorite-trondjhemite bath­
Brown, unpub. data, 1978). Concurrently, volcanic oliths intruded into the meta-basalt and related
and sedimentary beds in the south-eastern part of rocks of the Hali and Ablah metamorphic sedi­
the shield that were considered Jiddah Group by mentary strata and flows. The batholiths were
Schmidt et al (1973) have been re-evaluated by evidently emplaced more than 800 Ma B.P., ac­
Greenwood et al (1978); on the basis of chemical cording to isotopie ages in the southern shield, in
analyses as well as on the basis of structural rela­ the area 200 km S.E. of Al-Madinah, and at the
tions and metamorphism, they are shown to be northern edge of the shield near Khaybar (Green­
similar to the Hali Supergroup as it was originally wood et al, 1975; Anderson, 1977; Aldrich et al,
mapped in 1963 (USGS-ARAMCO, 1963). The 1978). The basal rocks above the erosional surface
most striking evidence of Hali affinity is the cherty are serpentinized ultramafic and related oceanic
and carbonaceous or graphitic schists that crop out lithospheric rocks resembling ophiolite; these
on both flanks of a gneiss in the Asir highlands. rocks form a discontinuous north-trending belt but
However, the Jiddah Group is mostly meta- are offset left laterally by the Najd Fault System
andesite in the greenschist facies, as are the (Brown, 1972; Frisch and Al-Shanti, 1977). The
metavolcanic rocks in the south-eastern part of the relationship is strikingly illustrated S.E. of Al-
shield. Both Hali and Jiddah Groups as well as Madinah where granodioritic gneiss radiometric-
some beds older or younger may be present there. ally dated at 915 Ma B.P. (K-Ar hornblende, Al­
drich et al, 1978) shows strong lineation discordant
with that of the serpentinized belt immediately to
Ablah Group
the east. The proposed Halaban Supergroup has
Originally used as a formation name (Brown and been studied extensively by the BRGM (Delfour,
Jackson, 1960), the Ablah was raised to a group of 1977) who have named the upper formations the
three formations (Greenwood, 1975). The Ablah Hulayfah Group. The Hulayfah Group crops out
Group extends southward discontinuously from above the lowermost Urd Group (ophiolitic) at the
the Ablah mine area in a narrow faulted belt in type locality of the Hulayfah Group. The calc-
which it is found as roof pendants in, or as grabens alkaline igneous rocks associated with the Halaban
between the diorite-granodiorite-trondjhemite as here defined are mostly diorite, quartz diorite,
batholiths of the Asir and Tihamat al Yemen andésite, and dacite and have yielded radiometrie
(Greenwood, 1975; Bayley, 1972; Anderson, 1977; dates of about 780 Ma to about 658 Ma B.P. The
Hadley, 1975b). Metamorphic rank increases younger age of 658 Ma B.P. is from a Rb-Sr
southward through greenschist to almandine-am- isochron of samples collected by J. Delfour from
phibolite and sillimanite facies. The group is drill holes at three localities in the uppermost
lithologically similar to the Fatima Series of formation (Nuqrah) of the Hulayfah Group (Fleck
Karpoff (1957) in that the sedimentary rocks are et al, 1976; Aldrich et al, 1978). Most of the gneiss
red and green, have basal conglomerate, and con­ domes seem to have been injected during the early
sist of arkose and graywacke, stromatolitic, limes­ part of this time span and then reactivated near the
tone or marble, and sills, flows, and tuffs of end of the period (Schmidt et al, this symposium).