Metallogenic inception of the Arabian-Nubian Shield: Daero Paulos porphyry copper prospect, Eritrea

Highlights

• Oldest dated mineralization in the Arabian-Nubian Shield

• Formation in an embryonic intra-oceanic island arc

• Porphyry copper mineralization spanned ~200 m.yr. of Shield development.

Abstract

The Daero Paulos porphyry copper prospect, in the Asmara region of the central Eritrean highlands, is part of the Nakfa tectono-stratigraphic terrane, which, along with four other major and several minor terranes that constitute the Neoproterozoic Arabian-Nubian Shield, is characterized by juvenile, intra-oceanic island-arc crust. Alteration and mineralization at Daero Paulos occupy a north-northeast-trending corridor of multiphase, rhyolitic to dacitic intrusions emplaced into mafic and intermediate volcanic and subvolcanic rocks and derivative volcaniclastic strata. New U-Pb geochronological data for the volcanic rocks in the region confirm a >860-Ma age for the Neoproterozoic (Tonian) host sequence. The porphyry copper alteration evolved from early epidote-chlorite-albite through sericite-chlorite to late quartz-sericite. Local advanced argillic alteration is the erosional remnant of the lithocap that formerly dominated the shallow parts of the Daero Paulos system. A post-mineralization, Alaskan-type pyroxenite-gabbro body intruded the porphyry copper corridor, and is interpreted as the feeder for mafic dikes along the periphery of the porphyry copper system. Re-Os ages of 881.0 ± 5.5 and 864.2 ± 3.6 Ma for molybdenite from veinlets at Daero Paulos constrain an early Tonian timing for porphyry copper formation during early opening of the Mozambique Ocean after Rodinian supercontinent break-up. The ages also imply a rapid transition from initial rifting to the start of subduction and consequent establishment of an intra-oceanic island arc in what is now the Nakfa terrane. The molybdenite ages show that Daero Paulos is currently the oldest dated mineralization in the Arabian-Nubian Shield, and ~60–200 and, possibly, ~250 m.yr. older than porphyry copper mineralization elsewhere in the Shield. However, volcanogenic massive sulfide (VMS) mineralization could have been forming at broadly the same time, albeit not genetically associated with Daero Paulos. Preservation of such shallowly formed Neoproterozoic mineralization is mainly attributed to burial beneath thick post-amalgamation volcano-sedimentary basinal cover, collision-generated thrust sheets, and later Phanerozoic strata.

Introduction

The Arabian-Nubian Shield, a tectonic collage of intra-oceanic island arcs, microcontinental fragments, and volcano-sedimentary basins, is considered as one of the world's largest tracts of juvenile Neoproterozoic crust (Stern, 1994; Johnson, 2014; Fig. 1). Its tectono-magmatic evolution between ~870 and 560 Ma records a complete Wilson orogenic cycle, including establishment of intra-oceanic magmatic arcs (~870–615 Ma) in the Mozambique Ocean following Rodinia breakup, multiple arc and microcontinent accretion events (780–600 Ma), and final orogenic extensional collapse and sedimentation in post-amalgamation basins (600–560 Ma; Stern, 1994; Johnson and Woldehaimanot, 2003; Stern and Johnson, 2010; Johnson et al., 2011). Final suturing of East and West Gondwana to form the East African Orogen (EAO; Fig. 1b) at ~560 Ma resulted in conversion of the Arabian-Nubian accretionary collage into the stable Arabian-Nubian Shield, which is divisible into at least 15 tectono-stratigraphic terranes using ophiolite remnants, shear zones, and other criteria (Johnson and Woldehaimanot, 2003; Fig. 1c).

The Arabian-Nubian Shield is well known for hosting numerous volcanogenic massive sulfide (VMS), orogenic gold, and rare-metal deposits (Sillitoe, 1979; Klemm et al., 2001; Botros, 2002; Sangster and Abdulhay, 2005; Küster, 2009; Barrie et al., 2016; Johnson et al., 2017), although epithermal gold, reduced intrusion-related gold, carbonate-replacement base metal, iron oxide-copper-gold, and porphyry copper mineralization is also present (Huckerby et al., 1983; Jaques et al., 1999; Doebrich et al., 2004, Doebrich et al., 2007; Sangster and Abdulhay, 2005; Bierlein et al., 2016a, Bierlein et al., 2016b, Bierlein et al., 2020; Perelló et al., 2020a; Fig. 1a). Recent definition of the large, low-grade Jebel Ohier porphyry copper deposit in northeastern Sudan (Fig. 1a) has highlighted the potential for this mineralization type throughout the Arabian-Nubian Shield (Bierlein et al., 2016a, Bierlein et al., 2020).

This paper describes porphyry copper mineralization at Daero Paulos, a modest prospect (informal mineral inventory of 40 million tonnes averaging 0.14% Cu and 0.1 g/t Au) near Asmara in the Eritrean highlands (Figs. 1a and 2a), and provides direct evidence for its age based on Re-Os dating of molybdenite associated with chalcopyrite in porphyry-style veinlets. Following a brief overview of the geological setting of the Eritrean highlands and the study area, the main features of the Daero Paulos prospect are described (Fig. 3a). The paper then discusses the metallogenic position and timing of porphyry copper formation in the Arabian-Nubian Shield, supported by new U-Pb and Re-Os ages for the Mibari and Wadi Al Ghuathayra prospects in Saudi Arabia (Fig. 1a). Final considerations, employing new ages from the Emba Derho deposit in the Asmara district of Eritrea, address the relative timing of porphyry copper and other Neoproterozoic mineral deposit types in the Arabian-Nubian Shield and the likely reasons for their preservation.