Bierlein, and 9 co-authors, reported in 2019 that new SHRIMP U-Pb data from dioritic to granodioritic synmineral intrusions associated with the Jebel Ohier porphyry copper deposit (mineral inventory, including NI43-101-compliant total inferred and indicated resources, of 593 million tonnes [Mt] at 0.33% Cu and 0.05 ppm Au, for 1.953 Mt of contained Cu and 933,600 oz of Au at 0.15% Cu cutoff) in the Red Sea Hills of northeastern Sudan have bracketed the age of mineralization to ca. 816 to 812 Ma. This age range, as well as constraints from new and existing lithogeochemical data, is consistent with the deposit’s formation from a productive parental magma source during the early stages in the evolution of an intra-Mozambique Ocean island arc.
The Jebel Ohier porphyry copper deposit bears many similarities to well-documented Phanerozoic analogues elsewhere in terms of:
1) the mapped style and zonation of hydrothermal alteration (i.e., proximal K-silicate–dominated, to sericitic, to distal propylitic alteration),
2) the occurrence of intense Cu-bearing A- and B-type vein stockwork, as well as sulfide-only C-type veins, anhydrite veins, and younger, peripheral D-type veins, and
3) the geochemical fingerprint of the associated porphyry, which is akin to those of ore-related Tertiary porphyries in the Escondida area in northern Chile.
The multi-phase intrusion hosting the Jebel Ohier porphyry copper deposit has been intruded by several generations of mafic to felsic post-mineralization dikes and voluminous plutons, with a peak in magmatic activity coinciding with the suturing of the Gebeit terrane at ca. 724 Ma. In spite of, or perhaps because of, the occurrence of extensive post-mineralization magmatism, and regardless of subsequent deformation, regional metamorphism, uplift, and erosion, the deposit has remained remarkably intact. The discovery of a relatively ancient, yet well-preserved porphyry copper deposit in the Neoproterozoic Arabian-Nubian Shield has major implications for the exploration potential of this resource-rich geologic terrain.
The authors concluded that the Jebel Ohier is an exceptionally well preserved Neoproterozoic porphyry copper deposit situated in the Red Sea Hills of northeastern Sudan. The deposit is hosted by a laterally extensive (>5 × 3 km), fine- to medium-grained, calc-alkaline dioritic intrusion, which, in turn, has been intruded by a series of syn- to post-mineralization diorite to granodiorite porphyry dikes. Several intermediate to felsic dike-like intrusions crosscut the porphyry copper deposit prior to the emplacement of a voluminous DGC.
Mapped hydrothermal alteration associated with the porphyry copper deposit at Jebel Ohier and a spatial zonation pattern that transitions from proximal K-silicate-dominated, to sericitic, and out to distal propylitic alteration are akin to that documented in many analogous settings elsewhere. Mineralization is associated with stockwork development, with three distinct centers of intense A-type veining recognized to date. Highest copper grades correlate well with A-vein abundance, with chalcopyrite (±bornite) mineralization present within A- and B-type quartz veins, as well as sulfide-only C-type veins, and as disseminated sulfides in proximal halos to these veins. Locally, magmatic-hydrothermal breccias are important from a Cu grade viewpoint.
Although the deposit has been affected by both brittle and ductile deformation, overprinted by regional-scale greenschist facies metamorphism, and subjected to repeated periods of uplift and partial erosion, the pos-mineralization evolution of Jebel Ohier has done relatively little to obscure, let alone obliterate, the original deposit. The emplacement of a series of postmineralization dikes and the DGC might have played a major role in preserving the relatively ancient porphyry deposit. Four samples from intrusive units hosting mineralization at Jebel Ohier, including three samples from syn-mineralization intrusions, yielded a weighted mean age range of ca. 816 to 812 Ma.
When viewed in the context of available geochronologic data sets for the Arabian-Nubian Shield, the new age data for Jebel Ohier suggest that it formed from a productive magma during the early stages of an immature intra-Mozambique Ocean island arc. A fifth sample demonstrates that emplacement of the DGC did not occur until some 90 m.y. later, possibly during the final suturing of the Gebeit and the Haya terranes. Jebel Ohier is a rare example of a Precambrian porphyry copper deposit and represents the first confirmed example of this globally important source of copper in the ArabianNubian Shield. Although its preservation was perhaps aided by several serendipitous circumstances, there is no reason to believe that Jebel Ohier remains the only porphyry deposit preserved in what is essentially an ancient equivalent of the southwest Pacific region, an assemblage dominated by intraoceanic island arcs and known for hosting some of the world’s largest porphyry deposits. As such, the discovery of Jebel Ohier has significant ramifications for porphyry Cu exploration both elsewhere in the Red Sea Hills and in other parts of the Arabian-Nubian Shield. Moreover, geochemical analysis demonstrates that the petrogenesis of fertile Neoproterozoic magmas capable of producing a potentially economic porphyry Cu deposit may be akin to the petrogenetic makeup of far more recent (i.e., Tertiary) ore-productive porphyries.
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