The Precambrian Rocks

The Precambrian rocks are found in the western part of Saudi Arabia, which constitute a geologically interesting and significant terrane called the Arabian Shield. The Shield is exposed from southern Jordan to Yemen, but its larger areal extent is found in Saudi Arabia. The length of the Arabian Plate is estimated to be roughly 2,600 km from the north to the south and about 3,000 km from the east to the west. The Arabian Plate is bordered to the west by the Red Sea Rift and the Dead Sea Transform Fault, to the east by the Zagros subduction zone and the Owen Fracture Zone, to the north by the East Anatolian Fault Zone, and to the south by the Aden Rift.

The Arabian Shield originated from the suturing of an orogenic belt that evolved 850-550 million years ago, which was situated at the northern end (current coordinates) of the East African Orogen. This Orogen comprises a wide belt of metamorphic and deformed rocks, and was formed from the convergence and suturing of the crustal blocks of the East and West Gondwana.

Besides the Khada’ subterrane, which is believed to be a block of a continental crust that existed within the structurally complex Afif and the Dawadmi terranes, the Shield consists mainly of young rocks of magmatic origin, which evolved from the Tonian to the Ediacaran Periods. The tectonic terranes are composed of highly diverse rock packages with chemical fingerprints relating to overlapping tectonic settings that are indicative of both island arcs and continental margin systems with associated continental and marine sedimentary rocks. The ophiolite sequences in the Shield delineate the boundaries between these tectonic terranes. The rocks of the Arabian Shield underwent metamorphism in the greenschist and the amphibolite facies during several successive episodes of deformation. The Shield is divided now into several terranes and subterranes, which are, from the north to the south: the Madyan, Holaya, Hail, Hejaz, Afif, Dawadmi, Rayn, Assir, and Tathlith terranes and the Khada’ subterranean.

The exposed rocks of the Shield are bounded by the Phanerozoic sedimentary rocks in the central and eastern parts of Saudi Arabia, and in the northern, eastern, and southern parts by the sedimentary and volcanic rocks that were deposited along the Red Sea Basin. The basin was formed coeval with the separation of the Arabian Peninsula from northeast Africa about 25 million years ago, creating the Arabian Plate and initiating the opening of the Red Sea and the Gulf of Aqaba. The uplift of the rift ridges led to the formation of the sloping topography of the Arabian Plate, which was largely the responsible factor in the erosion of the Phanerozoic rocks and the exposure of the Precambrian basement or the Arabian Shield.

The Precambrian rocks are likely of similar origin and structure as those of the Shield. They underlie the Phanerozoic rocks in the central part of Saudi Arabia as the crystalline basement and reappear as small outcrops in Oman.

The igneous rocks of the Arabian Shield are mostly Neoproterozoic rock assemblages of oceanic volcanic arc system, and in a larger extent, of volcanic and sedimentary rocks that were deposited incongruent to the arc sequences with extensive granite intrusives.

The Arabian Shield is of fundamental importance to the study of the geologic history of the Earth. It is one of the largest areas on Earth with preserved Neoproterozoic young crust that was formed directly from magma. It is well exposed, and its rock assemblages had undergone moderate metamorphism and deformation. The Shield serves as an excellent example of a large young crust that had evolved within 300 million years from an 8-km long oceanic crust to about 45-km long continental crust. As such, the Arabian Shield is a unique natural laboratory for the study of the rich geologic history of the Earth during the Neoproterozoic Era. It keeps records of very significant events about the major periods of cooling and glaciation, the intensified oxygenation event of the Earth’s atmosphere and oceans, the sudden changes or deviations in the abundance of strontium, carbon, and oxygen isotopes in the atmosphere and oceans, and the emergence of multicellular life forms.

Today, the rock assemblages of the Shield serve as important primary sources of metallic minerals in Saudi Arabia, such as the rich gold deposits being mined in Mahd Adh-Dhahab, Bulghah, and other places; the copper deposits being mined through an underground mine in Jabal Sayid; and other precious metal ore deposits, like the rare earth metals (REMs), tin, tungsten, and uranium from granitic hosts.

The oldest Shield rocks in Saudi Arabia are dated 1.8 to 2.6 billion years ago. They consist of granite, gneiss, and schist, and crop out in a limited area in the eastern edge of the Shield, although they may be previously distributed extensively as evidenced by the distinct isotopic signatures preserved in the less widespread rocks. The ages of volcanic arc rock assemblages range from the Tonian and middle Cryogenian (850-700 million years ago) in the southern and western areas of the Shield, to late Cryogenian and Ediacaran (˃690-615 million years ago) in its eastern proximities. These rocks were formed from magmatic differentiation related to the Neoproterozoic subduction in the Mozambique Ocean. They are relatively younger rocks with positive primary ratios(?) and neodymium model ages that are close to their crystallization ages. This indicates that the magma that crystallized to form these rocks separated from the mantle shortly before its emplacement. The arc assemblages, on the other hand, converged and were amalgamated along the sutures and shear zones, forming a younger crust onto which sedimentary and volcanic sequences were deposited.  A large part of the Shield was subjected to granitic intrusion during the late Cryogenian and Ediacaran, which strengthened the Shield and resulted in the development of its current form and structure. The compositions of the granitic rocks vary from calc-alkaline series, which dominate those in the arc assemblages and the middle to late Cryogenian intrusives to the relatively more abundant alkaline series, represented by the Ediacaran intrusive rocks.

The Arabian Shield rocks are suitable hosts for metallic ore mineralization due to their geologic history and their evolution and location on the margins of the converging plates during the 300 million years of the formation and growth of this Earth’s crust. Due to these favorable conditions for mineralization, the Arabian Shield rocks were mineralized with gold, copper, zinc, lead, iron, nickel, REMs, and other mineral deposits associated with active tectonic margins and progressively evolving orogenic systems.

The mineral resources of the Arabian Shield have been historically exploited for more than 3,000 years. Until now, gold mining in the Shield areas is very much active. Polymetallic massive sulfides of volcanic origins and deposits formed by thermal solutions are also now being explored. The occurrences of mineral deposits in the Shield tend to be confined into mineral belts because of the geologic, tectonic, and structural characteristics of the Shield.