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This reappraisal of the mineral potential of the Saudi Arabian Shield presents a geologic and metallogenic GIS of the Arabian Shield and Peninsula, updated data on the potential subsurface mineral resources and an evaluation of the mineral exploration work aimed at providing a tool for the development of Saudi Arabia's mining policy.
The Arabian Shield is a part of a larger geologic ensemble, the Arabian-Nubian Shield, that extends 2200 km N-S x 1200 km E-W and underlies several countries, mainly Egypt, Eritrea, Ethiopia, Saudi Arabia, Somalia, Sudan and Yemen . These different areas, accreted during the Late Proterozoic, share a very similar geologic evolution (accretion of volcanic island arcs) with many mineral deposits formed by similar processes (epithermal, mesothermal, VMS gold deposits, and so on…). The geology (in a broad sense) and mineral deposits have been studied in very different ways during the past decades. The resulting information is very heterogeneous and only rarely has it been synthesized. In particular, synthesis documents for mineral exploration purposes are very scarce, and often only available in the form of paper maps and not in a more modern version, namely a Geographic Information System (GIS) and attached databases. The GIS is a modern and efficient method of valorizing the abundant data that has been collected over several decades. It provides an opportunity to re-examine the Shield from a geologic and metallogenic standpoint and also to develop a sophisticated mineral exploration tool. A first stage in the achievement of this program involved a geologic and metallogenic synthesis (under ArcView™) at the scale of the Arabian-Nubian Shield (1:1,000,000). Following this preliminary stage, we focused on the Arabian Shield itself, and reevaluated and compiled, under ArcView™ and MapInfo™ software, several layers of information provided by BRGM, the DMMR (Deputy Ministry of Mineral Resources) and other providers. This reevaluation took into account many aspects, such as geology, structures, geochemistry, aeromagnetics, geochronology, paleosurfaces, surficial formations…that we crossed against a mineralized occurrence database containing not only known information on the occurrences, but also a new reappraisal of the metallogenic data and a quantitative evaluation of the regional- and local-scale geologic, geophysical and geochemical exploration work that has been carried out. The crossanalysis of these different information layers using SynArcR, a software tool developed by BRGM and ESRI, allowed us to produce synthetic documents and thematic maps to help in the selection of new exploration targets and adapted methodologies.
The report is subdivided into six main parts. Part 1, after a short presentation of the study objectives and methodologies, gives a short historical account of the mining and exploration work carried out to date in Saudi Arabia. Part 2 provides details of the different information layers used within this multicriteria analysis : geography, geochronology, geochemistry, aeromagnetics, geology and structural evolution, surficial formations, metallogeny and exploration. Part 3, after a presentation of the geologic history of the Arabian Shield discusses the metallogeny of the Arabian Shield. Part 4 exposes a set of short- and long-term recommendations to stimulate a renewal of the mineral exploration in Saudi Arabia. Part 5 provides the bibliographic references. Part 6, corresponds to the appendices. It presents several new thematic maps (aeromagnetic, surficial formations, structural) that are included within this report and also the data structure of the attached CD-Rom.
Table of contents and site map Abstract Foreword Part 1 - Introduction
Part 2 - GIS - Data synthesis
Part 3 - Geotectonic and metallogenic setting of the Arabian Shield
Part 4 - Potential for mining and recommendations for future exploration trends Part 5 - References Part 6 - Appendices
Introduction
1.1 Specification of problem and objective of the project Saudi Arabia is located in a part of the world that has an ancient history of mining; base and precious metals were once intensley mined, as shown by the many vestiges remaining today. Several decades of activity by the DMMR and the BRGM and USGS Missions were devoted simultaneously to a geologic survey of the Shield, to an inventory of the mineral occcurrences (essentially through rediscovering the old workings), and to developing the most important of them. The result is two active mines, several mining projects at various stages of development, and several promising indications at the evaluation stage. Mineral exploration, which (with hindsight) was doubtless too focused in the past, is now in search of new paths. While continuing to develop the known occurrences, it is now necessary to stand back, consider broader sectors on a more systematic basis, look for more discreet signs of mineralization, and envisage new types of mineralization trap. The preparation for such a change in strategy requires gitological reflection, using modern tools to cross the acquired geologic and metallogenic data, along with a quantitative and qualitative assessment of the earlier exploration work.
The main objective of this project is thus to provide a modern metallogenic vision of the Arabian Shield that can be used as a conceptual and factual basis for updating the mineral exploration approach in Saudi Arabia.
1.2. - Methodology
The approach used to attain the project objective was to construct a GIS based on metallogenic and geologic databases.
A GIS (Geographic Information System) covers three aspects :
In addition, through the interface of a GIS it is possible to restitute all or part of a map and print the result on a plotter. This is a method of generating second-stage derived maps showing, for example, only the major gold occurrences, or combining a type of occurrence (such as copper for example) with selected structural features and rock types. The GIS is thus a powerful tool to help decide where further exploration or development efforts would be interesting (figure I-1).
Creating the GIS of the Arabian Shield and Peninsula required the synthesis, reevaluation, introduction and analysis of several layers of information including :
The origin and type of data used for each layer of information is detailed in Part 2 - GIS - Data synthesis.
Figure I-1 - Underlying philosophy of the project. Digital synthesis and reevaluation of different information layers within a GIS enables the generation of prospective maps and the selection of new exploration targets and adapted exploration methodologies.
1.3 - Geography, Geology and Mineral Occurrences of the Arabian Shield: an introduction
1.3.1 - Natural regions
The Arabian Shield (figures I-2, I-3) underlies much of the western half of the Arabian peninsula. To the west, it is bounded by the Red Sea and the Red Sea coastal plain, a rather flat region where the Shield rocks are mainly covered by later alluvial and beach deposits. East of the coastal plain, a steep escarpment rises to an altitude to more than 3000 m in the south and slightly less in the north. The Shield here forms a gentle eastward sloping plateau that drops to 1000 m where it encounters the Phanerozoic deposits of eastern Arabia. The morphology of the Shield area itself is one of steep hills surrounded by sandy flats in which occasional floods occur after rainstorms.
Figure I-2 - Digital elevation map of the Arabian Peninsula
1.3.2 - geologic frame Before the opening of the Red Sea, 30-25 Ma ago (Camp and Roobol, 1992), the Arabian Shield (650,000 km2) formed part of a larger geologic ensemble (Stern, 1994), the Arabian-Nubian Shield that now extends 2200 km N-S x 1200 km E-W and underlies several countries, mainly Egypt, Eritrea, Ethiopia, Saudi Arabia, Somalia, Sudan and Yemen . The Arabian-Nubian Shield consists primarily of Neoproterozoic juvenile crust and represents an area of suturing between East and West Gondwana before the Paleozoic. It formed through the accretion of numerous, mainly inter-oceanic, island arcs along ophiolite-lined suture zones and gneissic fault zones (Kröner, 1985; Stoeser and Camp, 1985; Vail, 1985; Pallister and others, 1987; Quick, 1991; Windley and others, 1996; Johnson, 1998; Al Saleh and others, 1998) between 900 Ma and 550 Ma when the Mozambique ocean closed (Stern, 1994). These "terranes" are now separated by major, mainly N-S and NE-SW, suture zones lined by serpentinized ultramafic rocks (ophiolites and tectonic slices) and by major NW-SE faults known as the Najd Faults. Terrane analysis, in terms of lithostratigraphy and ages of suture zones, fault zones, overlapping basins and stitching plutons, provides a tool that helps constrain the geologic history of the Arabian Shield. The more than 500 U-Pb, Rb-Sr, Sm-Nd, K-Ar and Ar-Ar age determinations available for the Arabian Shield, both previously published and obtained within the framework of this project, were reevaluated on analytical grounds. They span a very long time interval from 2340 to 340 my, with three main maxima at 590-580 Ma, 640-630 Ma, 730-720 Ma (Figure I-4). Ages greater than 910 Ma result from the recycling of old continental crust in the eastern terranes. Ages obtained on ophiolitic and volcanic-arc material are between 910 and 680 Ma, with the volcanic material from that period being older in the south (Asir terrane) than in the north (Midyan terrane) and east (Ar Rayn terrane).
Figure I-4 - Chronology of events in the Arabian Shield. Histogram showing the distribution of all available ages and their analytical confidence of the determination: 1 = acceptable, 2 = acceptable with restrictions (little available data, high MSWD, etc…), 3 = not acceptable (MSWD too high, or not enough data, or distant samples, etc…), 4 = not enough published data for reevaluation.
The final accretion of the different island arcs caused strong tectonic deformation during what is known as the Panafrican orogenic period or cratonization. Except for some suture zones characterized by mainly subvertical lineation, most Panafrican structures correspond to chains that either have a northwest-southeast (left-lateral transpression: Najd Faults) or north-south to northeast-southwest (right-lateral transpression) strike. According to most published models of the Arabian Shield (Moore, 1979; Camp, 1984; Johnson and Vranas, 1984; Stoeser and Camp, 1985) a continuous N-S orogenic belt (the Nabitah Belt) resulted from the final frontal collision of volcanic-arcs. This belt was subsequently displaced by northwest trending left-lateral strike-slip faults (Najd Faults). New field and laboratory data, along with a new aeromagnetic map, shows that the Nabitah Fault zone and Najd Faults are part of the same cycle and that they formed contemporaneously during the final transpressional left-lateral accretion of the terranes. Major molasse basins were also formed at this time in response to the accelerated erosion accompanying this mountain building. The Panafrican orogeny was followed by the intrusion of a large number of orogenic and post-orogenic granitic plutons. The last deformation is characterized by left-lateral strike-slip movement along the "Najd" faults. This event created the Jibalah pull-apart basins, with thick sedimentary deposits along the major older faults and associated with major acid volcanism, as witnessed by the presence of rhyolites. After the cratonization event, a generalized immersion of the Shield followed a period of erosion as witnessed by the thick sandstone deposits at the base of the Phanerozoic succession. Today's aspect of the Shield is largely due to the opening of the Red Sea during the Tertiary, with uplift of the western margin of the Arabian Shield, strong erosion of the Proterozoic and overlying Phanerozoic rocks, and extensive basaltic volcanism of the "Harrats".
1.3.3 - Mineralizations and geodynamic context
Based on this general geodynamic evolution, the different types of mineralization found in Saudi Arabia can be associated with specific structural settings:
1.3.4 - The Mineral Occurrences Documentation System (MODS) Database
The presence of thousands of ancient workings, generally marked by excavations, tailings, slag piles, and village ruins, are scattered throughout the Arabian Shield (Sabir, 1991), highlights the existence of an early, large-scale mining activity in Saudi Arabia. Most of the workings were quite small and the ancient production is difficult to evaluate. However, based on the volume and grade of many tailings and slag piles, it can be assumed that around 5000 tons of copper, and 30 to 50 tons of gold were extracted from the main mining centers over the centuries. These ancient workings, most of which have been rediscovered since the 1960s, are a precious heritage for modern exploration because they are excellent surface indicators of metal concentrations; The two active mines , the current mining projects, as well as most of the significant mineral occurrences identified, were initially targeted from old workings. Since 1971, the DMMR has recorded all known Saudi Arabian mineral occurrencesin a database. Each occurrence is recorded under a sequential number, with a descriptive file, location, type of occurrence or mineralized body, condition of discovery, list of reference reports, chemical analyses, etc.). Today, the databank contains over 5300 entries. This database was summarized by Beziat and others (1994) who attached all geographically related MODS numbers to a principal MODS number. It is this database that has been reevaluated in terms of metallogenical favorability and mineral exploration.
The work underlying this project was funded by the Research division of BRGM with partial support from the French Ministry of Foreign Affairs and from the Deputy Ministry of Mineral Resources (Saudi Arabia). We thank the Saudi Arabian Ministry of Petroleum and Mineral Resources and the Saudi Geological Survey (SGS) for permission to use their data. This work would not have been possible without the strong assistance of Dr M.A. Tawfiq (President of the SGS, Acting) and Mr F. Le Lann (Director, BRGM Saudi Arabia) in making the data available and for their logistical support. P. Bernard, J.M. Eberlé, and J.M. Leistel provided material support in Saudi Arabia, and we thank H.M. Kluyver and P.A.D. Skipwith for their translation and editing assistance
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