Volcanogenic Massive Sulphide–Oxide Gold Deposits of the Nubian Shield in Northeast Africa

Abstract

The 0.85–0.55 Ga Arabian-Nubian Shield of northeast Africa comprises an assemblage of volcanic arc and sub-arc strata that are fertile for volcanogenic massive sulphide (VMS) and overlying oxide gold deposits. At least fifty VMS deposits are known in the Nubian Shield west of the Red Sea, many of which are clustered in districts. The most important districts, including Ariab in NE Sudan, Bisha in western Eritrea, and Asmara in eastern Eritrea, have seen significant exploration and development recently. The Ariab VMS–oxide gold district contains over 20 oxide gold deposits that have been mined since 1991 (>2 Moz Au recovered), with several having large Cu–Au (+ Zn + Ag) sulphide resources below the oxide gold resources, including Hadal Awatib East (67.8 Mt), Hassai South (42.1 Mt), and Hadayamet (8.4 Mt). The Hadal Awatib East VMS resource comprises only one third of a giant sulphide lens that extends for more than 3.1 km continuously along strike, making it among the most extensive VMS deposits known globally: only Kidd Creek, Ontario (3.1 km to depth), and Neves Corvo, Portugal (3.4 km length), are comparable in size. Mining began in 2011 in the Bisha VMS district, following the discovery of five deposits with resources of >70 Mt. The main Bisha VMS–oxide gold deposit was discovered in 2003 when the first drill program tested a large hematite gossan that extended along strike for over 300 m. Subsequent discoveries at Harena, Bisha Northwest, Hambok, and Yakob Dewar have added significantly to the resource base. In the Asmara district, VMS deposits and occurrences were known for decades at Adi Nefas, Debarwa, and Emba Derho; however, exploration in the past decade has significantly increased known resources, particularly at Emba Derho (85 Mt). Many of the VMS deposits in the Nubian Shield have been subjected to strong, near-surface oxidation and supergene processes, probably since the early Tertiary, during a period of rapid uplift and concomitant deflation associated with opening of the Red Sea. This secondary alteration has produced unusual profiles consisting of horizontally stratified, gold-bearing oxide above supergene copper, and primary Cu–Zn–Au–Ag massive sulphides below. At Hadal Awatib East, Hassai South, and Bisha, a 10-fold enrichment in gold exists from primary VMS to oxide gold, and a 3.5-fold enrichment in copper from primary massive sulphide to the supergene zone. Above the redox front, the deposits are commonly surrounded by an incomplete halo of secondary copper oxide. The significant, near-surface gold and copper enrichments make these deposits particularly attractive targets for mineral exploration and development.