Arabian Nubian Shield is made up of juvenile Neoproterozoic rocks that are part of a significant accretionary orogen. It was formed as island arcs in the late Tonian-Cryogenian period(1000 Ma-720 Ma), on the site of the Mozambique Ocean that was created by the breakup of Rodinia which is characterized by arc collisions, magmatism, volcanism, sedimentation, and orogeny.
The Shield contains hundreds of gold occurrences and evidence thousand years history of gold mining. The main deposit types are orogenic gold and gold associated with volcanogenic massive sulfides (VMS).
The known occurrences of gold-bearing Volcanogenic Massive Sulphide(VMS) deposits in the Nubian Shield are mainly confined to arc assemblages in the northern half of the Shield, from northern Ethiopia upwards. While the greenstone belts of western and southern Ethiopia do contain Neoproterozoic juvenile, submarine volcanic rocks, there are only limited indications of VMS mineralization in these areas. Why?
According to Peter Johnson 2017, the cause of this disparity is unclear, but it poses intriguing questions about whether it is due to an initial difference in the geological history of the north and south, which prevented the development of VMS deposits in the south, or if it is a result of the selective preservation of VMS deposits in the north due to the less intense exhumation and erosion of the volcanic-arc host rocks.
Walter Leopold Pohl Mobina Esmaeili Nikolaos Quintana Mas Solomon Rebso Abebe Sepideh Habibikhanbaghi Angel Eduardo Espinosa Borges
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VMS (Volcanogenic Massive Sulfide) deposits are associated with submarine volcanic activity and are often found in ancient volcanic terrains. The Arabian-Nubian Shield (ANS) is a geological formation in the northern part of East Africa, extending through parts of Egypt, Sudan, Eritrea, Ethiopia, Saudi Arabia, and Yemen. The distribution of VMS deposits in the ANS, with a concentration in the northern half compared to the southern half, may be influenced by several geological factors:
1. **Tectonic Setting:**
- The geological history and tectonic setting of the Arabian-Nubian Shield vary across its extent. The northern part of the shield is associated with a more complex history of subduction, collision, and volcanic activity. Tectonic settings that favor the creation of volcanic arcs and subduction zones are often conducive to VMS deposit formation.
2. **Magmatic Activity:**
- VMS deposits are typically associated with volcanic arcs and back-arc basins. The presence of extensive magmatic activity in the northern part of the ANS during specific geological periods may have provided favorable conditions for the formation of VMS deposits.
3. **Hydrothermal Systems:**
- The formation of VMS deposits involves the interaction of hot hydrothermal fluids with volcanic rocks. The availability of suitable hydrothermal systems, driven by volcanic and tectonic processes, is critical for the formation of these deposits.
4. **Metallogenic Belts:**
- VMS deposits often occur in metallogenic belts or districts. The distribution of these belts is influenced by the geological evolution of the region. In the northern half of the ANS, there may be more favorable metallogenic belts for VMS deposits compared to the southern half.
5. **Age of Rocks:**
- The age of volcanic and sedimentary rocks in different parts of the ANS can vary. The age of rocks influences the geological processes that occurred, and certain periods of volcanic activity may have been more conducive to VMS deposit formation in the northern half.
6. **Structural Controls:**
- The presence of specific geological structures, such as faults, shear zones, and volcanic conduits, can control the localization of VMS deposits. The structural geology of the northern ANS may have provided more favorable conditions for the formation and preservation of VMS deposits.
It's essential to note that the geological history of a region is highly complex, and the factors influencing the distribution of mineral deposits are multifaceted. The southern half of the Arabian-Nubian Shield, including parts of West and South Ethiopia, may have different geological processes and histories, resulting in variations in the types of mineral deposits found. Detailed geological mapping, exploration, and research are necessary to understand the specific factors influencing the absence of VMS deposits in certain regions within the ANS.