Sponsored by: Excelsior Mining Corporation
Tectonics and Mineral Deposits of the Arabian-Nubian Shield: Base-Metals and Gold in Earth's Largest Mineralized Neoproterozoic Crustal Block
by Peter R. Johnson, Retired Consultant
The Arabian-Nubian Shield (ANS) is Earth’s largest block of juvenile Neoproterozoic crust and its largest repository of Neoproterozoic metallic minerals. As is evidenced by innumerable ancient workings, hundreds of smelting sites, 14C dating of charcoal, and the ruins of mine camps, the ANS has been the source of gold, copper, and lead for more than 7000 years up to, and including, the present. Seventeen mines are currently operating in the region, producing: (1) gold from orogenic- and epithermal deposits; (2) copper, zinc, and gold from VMS deposits; (3) gold from weathered caps above VMS deposits; and (4) tantalum from pegmatite in Ethiopia, the world’s 7th largest producer (40 tons in 2014). Exploration is underway in many parts of the shield, and thousands of artisanal workings produce gold amounting, in 2014, to 62 tonnes in Sudan and 8 tonnes Au in Ethiopia. Other types of occurrences that are sub-economic, are merely at the exploration stage, or are hypothetical targets include BIF, Nb-Th-U, Sn-W, PGM, Cr, porphyry copper, Fe(oxide)-(Cu-Au), and sedimentary-hosted Cu and Pb. This range of deposit types and the value of current metal production are the direct results of the rock types and tectonic evolution of the ANS. This presentation focuses on VMS, orogenic gold, and epithermal mineralization.
The Arabian-Nubian Shield is a Neoproterozoic accretionary orogen that developed in the Mozambique Ocean between ~870 and 550 Ma, bracketed by the break-up of Rodinia and assembly of Gondwana. By the end of the Neoproterozoic, the ANS was the northern (present-day coordinates) part of a larger orogenic belt referred to as the East African Orogen (EAO). At its origin, the EAO formed the axial region between convergent blocks of eastern and western Gondwana and was one of the Neoproterozoic orogenic belts that intertwined through Greater Gondwana. The EAO is now dispersed in the Nubian, Arabian, Indian, and Antarctic Plates. The ANS is exposed in the Nubian and Arabian Plates, divided by the Red Sea, which began to open ~25 M. It extends ~3500 km N-S from Jordan and Israel to Kenya, and ~1500 km W-E from the Rive Nile in Egypt and Sudan to the central part of the Arabian Peninsula, underlying an area of ~2.7 km2, and representing a crustal block about 27 times larger than Arizona.
Because of its origin in the Mozambique Ocean, the ANS has a distinctive juvenile character, in as much as most of the volcanic and igneous rocks of the shield have crystallization ages close to their Nd model ages, indicative of their formation from material newly extracted from a depleted mantle. Over 50% of the ANS rocks are volcanic-arc assemblages formed at convergent margins and composed of tholeiitic to calc-alkaline volcanic rocks, volcaniclastic and sedimentary rocks, and vast amounts of TTG-type intrusions. The arcs range between ~870-600 Ma, but most are Tonian and Cryogenian. They are characteristically intraoceanic arcs, apart from some that originated at ocean-continent boundaries, and tectonically include fore-arc basins and ophiolites, volcanic edifices and subvolcanic mafic-intermediate intrusions, and back-arc basins. Accretion of the arcs resulted in the build-up of composite crustal blocks, or tectonostratigraphic terranes, and the formation of ophiolite-decorated fold and thrust belts, or sutures, between the terranes. Major accretionary and suturing events occurred at 780-760 Ma, 740-715 Ma, 680-640 Ma, and 620-605 Ma. The remaining rocks of the ANS include late Cryogenian-Ediacaran volcanosedimentary successions deposited in post-amalgamation basins unconformable on newly accreted arcs, vast amounts of granitoids, and minor layered gabbro. The post-amalgamation basins range in age from ~660-550 Ma and granitoids were emplaced in pulses between ~630 and 540 Ma during a tectonic transition in the shield from orogenic, subduction settings to anorogenic, within-plate settings. Chemically, the granitoids are transitional between calc-alkaline and alkaline magmatism. Between ~650 Ma and 550 Ma, the ANS was affected by major transpressional shearing associated with final E-W convergence of Gondwana blocks, north-directed tectonic escape, and orogenic collapse. By 540 Ma, the entire region was stable continental crust: it formed a shelf at the margin of the proto-Tethys ocean, was flanked to the north by Cadomian and Amorican blocks, and passed southward into the Transgondwanan Mountains.
The most abundant mineral-deposit types in the ANS are VMS and orogenic gold, but epithermal mineralization, exemplified by Mahd adh Dhahab, is highly valuable at the few locations where it is found. Because of varying gold contents, some VMS deposits are referred to in the literature as gold deposits, others as Cu-Zn-Pb deposits. VMS deposits are present at many locations between northern Ethiopia, northern Egypt, and north-central Saudi Arabia. Most VMS deposits in the ANS are examples of convergent-margin polymetallic sulfide mineralization, but rare deposits appear to have formed at spreading centers or in back-arcs. The deposits are hosted by bimodal (basaltic to rhyolitic) submarine volcanic rocks, as well as volcaniclastic and sedimentary rocks. They comprise lenses of massive to semimassive sulfides of iron, copper, zinc, and lead, and have variable amounts of telluride. The VMS deposits range in size from < 1 to >24 Mt, and nineteen in the Arabian shield, for example, have a median size of 1.6 Mt at a median grade of 3.2% (Cu+Zn+Pb). The largest deposit in the Arabian Shield is Jabal Sayid, which recently started mining (24.4 Mt @ 2.53 5% Cu+Zn+Pb). Bisha, the largest deposit known in the Nubian Shield, was discovered in Eritrea in 1998. Development started by mining its gold-enriched weathered cap but is now transitioning to working the primary massive sulfide zone. The current (February, 2015) indicated resource comprises 22 Mt grading 0.7 g/t Au, of which the sulfide zone grades 1 % Cu, 5.7 % Zn, 0.76 g/t Au, 45 g/t Ag. Although they are not well dated, the oldest VMS deposits in the ANS appear to be in Eritrea (~850-720 Ma) and the youngest (~690-600 Ma) in the eastern Arabian Shield.
Gold-enriched weathered caps (oxide gold) are a distinctive feature of many ANS VMS deposits. The degree of gold enrichment is particularly intense in the Nubian Shield, commonly making occurrences of oxide gold an especially valuable and intently sought ore type. At Bisha, for example, the oxide zone contains as much as ~7 g/t Au, representing a ten-fold increase in gold grade from the primary sulfides. Grades in the oxide zone in the Ariab Mineral District (Sudan) reach 5-10 g/t Au, whereas the primary sulfides have grades of ~1-1.5 g/t Au.
Orogenic-gold occurrences are largely the product of the late Cryogenian-Ediacaran transpressional shearing event that affected all parts of the ANS. Concomitant metamorphism and(or) intrusion of granitoid stocks and plutons led to elevated temperatures and the resulting hydrothermal activity focused along extensional shears was favorable for epigenetic mineralization. The orogenic gold deposits typically consist of gold-bearing quartz±carbonate vein systems concentrated in dilatant-extensional en-echelon fractures in sheared and strongly altered volcanic and volcaniclastic rocks, small felsic intrusions, and ultramafic rocks. The quartz-vein arrays may exhibit both reverse and normal senses of shear and, in places, cut through or structurally overprint VMS deposits. They are characterized by native gold or gold in sulfides and tellurides. As is the case with its VMS deposits, the ages of orogenic gold formation in the Nubian Shield are not well established, but available data suggest that orogenic gold formation postdated VMS formation by as much as 100-150 million years. Arsenopyrite in quartz veins at Fawakhir in the Nubian Shield, interpreted as the most likely chronologic indicator of mineralization, yields a Re-Os age of 601±17 Ma. The Ad Duwayhi deposit in the Arabian Shield is estimated to be ~650 Ma and Sukhaybarat <620 Ma.
The preeminent epithermal gold deposit in the ANS is Madh adh Dhahab – meaning Cradle of Gold – is a polymetallic Zn-Cu-Au-Ag deposit. Mahd adh Dhahab is operated by the Saudi Arabian Mining Company (Ma’aden) and in 2014 produced 34,280 oz gold from 200,710 Mt ore grading 5.95 g/t Au. The deposit consists of fault-related quartz veins and stockwork in andesitic tuffs, flows, and agglomerate. Veins range from hairline fractures to stockworks as much as 10 m across, but the majority are 0.1-1 m. The veins are composed of quartz, chlorite, potassium feldspar, calcite, and sulfides. Several generations of quartz are evident, with banded, cockade, and cockscomb textures indicative of multiphase injection and incremental open-space filling under conditions of dilation. Primary sulfides include sphalerite, chalcopyrite, galena, and pyrite. In places, sulfides dominate and form pyrite-sphalerite veins. Gold and silver are present as gold-silver tellurides (hessite, petzite, and rare sylvanite), and gold occurs in native form as inclusions in sphalerite, along grain boundaries, and as small grains in quartz. The current size of the deposit is not published but, as of 2007, the mine had a reserve of 1.2 Mt grading 8.7 g/t Au. Production is in the form of concentrates grading ~150 g/t Au, 1,500 g/t Ag, 19% Cu, and 12% Zn, and as silver-gold doré obtained by cyanide leaching and electrolysis of slurry from the flotation cells.
The rock types of the ANS (bimodal volcanic assemblages, TTG complexes, and calc-alkaline to alkaline granitoids), their origins at convergent margins, and the pervasive development of shear zones that commonly form the site of increased hydrothermal activity in and around granitoid intrusions are geologic features highly favorable for base-metal sulfide and gold mineralization. Historically, attention focused on gold in quartz veins, a deposit type now known to belong to the class of orogenic gold, with lesser exploitation of copper and lead from gossans and sulfides at what are now known to be VMS. These deposit types – VMS and orogenic gold – are the primary focus of exploration today and are being successfully mined at localities throughout the region. Many successful exploration programs targeted ancient working in areas of outcropping gold-bearing quartz veins or gossans, but most modern mines have been developed by following geologic clues in addition to signs of previous mining. There is every reason to expect that a strategy of continuing to follow the favorable lithologies and structures so abundant in the ANS will lead to further successful development of the metal-mining sector in the region.
Bio: Peter R. Johnson, a graduate of London University (B.Sc) and Calcutta University (D. Phil), has spent most of his career as a geologist working on the Neoproterozoic rocks of the Arabian Shield in western Saudi Arabia. He started his career as an exploration geologist with Rio Tinto Zinc Corp., initially working on a copper-porphyry project in Wales, UK, then as Research Geologist at the corporation head office in London, and as manager of an exploration program in Norway. In 1977, he joined the Riofinex (RTZ) Geological Mission in Saudi Arabia, as a member of exploration teams looking for VMS, gold, and Sn-W. This work introduced him to the regional geology and tectonics of the Arabian Shield, which he pursued in the 1980s by compiling a new geologic map of the shield for the Saudi Arabian Deputy Ministry of Mineral Resources and helping to complete 250.000-scale quadrangle maps and reports for the U.S. Geological Survey Saudi Arabian Mission. In the 1990s, he was a Technical Expert helping with USGS Mission projects on the Arabian Shield, and in 2000, he became a Technical Advisor to the newly established Saudi Geological Survey. While with SGS, he was responsible for compiling a digital geologic map of the Arabian Shield, which gave him the opportunity to do extensive field work resolving stratigraphic and structural problems in the region. He took part in SHRIMP dating campaigns on the shield and helped with interpretations of aeromagnetic and gravity data. Dr. Johnson is the author of more than 100 reports and papers on the Arabian-Nubian Shield, is the co-author of a book on the geology of the Saudi Arabian Shield, and is currently involved with the international team preparing a new map of Gondwana and a review of gold in the Nubian Shield for presentation at the 35th International Geological Congress in Cape Town, 2016.