Geology of Sudan – The Arabian Nubian Shield.

Figure 1 (from Bierlein 2016; modified after Kroner et al, 1987)

The Arabian-Nubian Shield (Figure 1) comprises a collage of micro-terranes and arc belts, accreted together before becoming amalgamated with the Sahara metacraton during the end- Neoproterozoic Pan African collision between West and East Gondwana. This represents a extensive area of juvenile Neoproterozoic crustal accretion, analogous to older Palaeoproterozoic and Archaean accretionary orogens. Within Sudan, the Arabian-Nubian Shield occurs east of the Keraf-Ingessana-Kurmuk suture from the Red Sea Hills to the Ingessana Hills in Blue Nile state, covering an area of over 200,000 square kilometres. In the north, the Neoproterozoic volcano-sedimentary terrain extends west of the suture indicating a complex boundary or triple-point junction into the Atmur-Delgo belt and the Zalingei belt of the Sahara metacraton.

The Arabian-Nubian Shield has demonstrated high potential for orogenic gold and gold-rich VHMS witnessed by substantial known deposits in several countries including the Hassaï deposit in Sudan with current resources of c. 10 M oz gold equivalent. Together with the >10 M oz Sukari orogenic gold deposit in Egypt and substantial deposits in Saudi Arabia, Eritrea and Ethiopia, the Arabian-Nubian Shield has clear potential to be a world-class copper-gold province. The Red Sea Hills represents the stand-out area for mineral potential in Sudan, mainly polymetallic gold- rich VHMS and orogenic gold. Although the belt also hosts podiform chromite, magnetite skarn, and manganese deposits, the potential for large deposits of this type is considered limited. There is speculative potential for porphyry/IRG/skarn deposits and epithermal gold deposits, though preservation potential has been low especially for epithermal deposits. However, the recent discovery of the Jebel Ohier porphyry Cu deposit clearly demonstrates that this long-held paradigm is in need of revision. There are also A-type granite tantalum-niobium-tin mineralised systems and layered intrusions with anomalous PGM in Saudi Arabia.

The Sudan portion of the Arabian-Nubian Shield is almost unexplored by modern targeting techniques; considering its high prospectivity and (mostly artisanal) proven pedigree, this implies enormous potential for further discovery from systematic exploration.

Geology of Blocks 62 and 62b.

Figure 1

Blocks 62 and 62b are located in the Gebeit Terrane of the Arabian Nubian Shield (ANS). The ANS is an accretionary orogenic belt comprising a series of predominantly juvenile intra-oceanic island arcs, oceanic islands and micro-continental masses which extends along both sides of the Red Sea from Egypt in the NW, the Sinai Peninsula in the N, and Saudi Arabia in the NE, to Ethiopia and Yemen in the SW and SE, respectively (Johnson et al. 2011). The ANS evolved between 900 and 550 Ma in response to the closure of the Mozambique Ocean (800– 650 Ma) and the subsequent collision between East and West Gondwana (Stern 1994, 2002). The Red Sea Hills constitute the Sudanese and Egyptian parts of the ANS.

The tectonic setting of the volcanic rocks in the Block 62/b area is interpreted to be an oceanic arc, as confirmed by trace element geochemistry (Bierlein et al., 2016). This arc sequence comprises arc-related low-grade volcanosedimentary sequences and syn-tectonic igneous complexes of andesitic through to rhyo-dacitic composition. Amalgamation of the Gebeit Terrane with its neighboring occurred via northward subduction and was associated with ophiolite emplacement along suture zones at the margins of the terrane, though no ophiolites have been positively identified within the B62/b area. The major bounding structures to the SE of Block 62/b is the Nakasib suture. The Ariab Cu-Zn-Au VHMS deposits, including Hassai, occur within the Nakasib zone.

The NE-trending dextral Hamisana Shear Zone to the NW of Block 62 (Figure 1) does not have associated ophiolites but is a major structural boundary separating the Gebeit terrane from the Gabgaba terrain to the west. The NW-trending Oko Shear Zone to the east of Block 62 sinistrally offsets the Nakasib Suture (Figure 1). Between these crustal scale structures, Block 62 is traversed by a number of regional scale faults which are mainly NE trending, with two named faults being the Western Shear Zone and Eastern Shear Zone (Figure 1).

Figure 2

QMSD’s current hypothesis for the tectonic setting of Block62/b is shown below in Figure 2. The regional setting is that of an accreted oceanic arc interpreted to be similar in its makeup to the present-day configuration of the Indonesian island arc chain (inset to Figure 2). The large size of Block62/b allows it to encompass the entire deposit-generating mechanism associated with these subduction related arcs including fore arc, magmatic arc and back arc settings. Apart from the advanced exploration project of Jebel Ohier PCD, situated in the magmatic arc sequence, numerous other targets associated with the associated fertile tectonic environments have also been identified. A major goal of the ongoing regional exploration is to thoroughly test this tectonic hypothesis and continually adjust as required.