Petrogenesis of a Neoproterozoic magmatic arc hosting porphyry Cu-Aumineralization at Jebel Ohier in the Gebeit Terrane, NE Sudan

F.P. Bierlein a,⁎, N. Reynolds b, D. Arne b, C. Bargmann c, S. McKeag c, W. Bullen a, H. Al-Athbah a,S. McKnight d, R. Maas e

a Qatar Mining, PO Box 20405, West Bay, Doha, Qatarb CSA Global Pty Ltd., PO Box 141, West Perth, WA 6872, Australiac QMSD Mining Co Ltd, PO Box 7828, Khartoum, Sudand School of Science and Engineering, Federation University, P.O. Box 663, Ballarat, Victoria 3353, Australiae School of Earth Sciences, University of Melbourne, Melbourne, VIC 3010, Australia

a b s t r a c ta r t i c l e i n f o

Article history:

Received 18 January 2016Accepted 11 May 2016Available online 16 May 2016

The ca. 730Ma porphyry Cu-Au deposit at Jebel Ohier in the Red Sea Hills of northeastern Sudan represents a rareexample of a preserved Neoproterozoic magmatic-hydrothermal systemwhich bears many similarities to majormineral-hosting (‘productive’) Tertiary–Cenozoic porphyries in circum-Pacific metallogenic belts. Petrographic,lithogeochemical and Sm-Nd isotope systematics confirm that the deposit formed in a supra-subduction settingand during the constructional stage of an evolving intra-oceanic magmatic arc. The calc-alkaline melts weresourced predominantly from juvenile reservoirs and received comparatively little input from continental-character material. Comparison with igneous rocks from barren intrusions elsewhere in the region point to theabsence of major crustal breaks but indicate that the ore-forming granodiorite-dacite porphyry complex atJebel Ohier is the result of ‘abnormal’ and prolongedmulti-phase arc plutonism. This process involved the forma-tion of relatively hydrous and oxidized melts via the fractionation of magmas, which possibly had ponded at thebottom of the thickening lithosphere for a protracted period prior to their ascent. The tectonic trigger for the em-placement of the productive pluton into a pre-existing volcanic edifice at Jebel Ohier remains unconstrained.Preservation of what is considered the first documented porphyry Cu-Au deposit in the NE African portion ofthe Arabian Nubian Shield can probably be related to the accretion of the magmatic arc to a stable continentalmargin within a fewmillion years of mineralization, thus enabling the deposit to escape excessive uplift, erosionand structural dismemberment.

© 2016 Elsevier B.V. All rights reserved.

Keywords:

NeoproterozoicArabian Nubian ShieldGebeit TerranePetrogenesisMetallogenyPorphyry copper deposits

1. Introduction

Porphyry copper deposits represent the world's principal source ofCu and contain about 65% of global Cu resources (e.g., Cooke et al.,2005; John et al., 2010; Sillitoe, 2010). Owing to their inherent relation-ship to active subduction–accretion processes, these deposits are diag-nostic for the occurrence of accretionary processes in space and timeand can be utilized to constrain the relative position and developmentalevolution of the hosting magmatic arc within a given plate margin sce-nario (e.g., Nelson, 1996). Vice versa, recognition of an evolving accre-tionary margin has profound metallogenic implications, which, inturn, govern the formulation of relevant conceptual explorationtargeting models.

Porphyry Cu-Au systems are typically related to the dehydration of asubducting oceanic slab and related high fluid flux into the overlyingmantle wedge (e.g., Kerrich et al., 2005). Melting of the resultantmetasomatized mantle enriched in incompatible elements gives riseto hydrous, metal-enriched basaltic magmas and the generation ofevolved, amphibole-bearing, high-level granitic magmas (e.g., Cookeet al., 2005; Richards, 2011; Sun et al., 2013). These high-level (b3 kmdepth), normally porphyritic intrusions exsolve hot, boiling saline min-eralizing fluids that fracture and alter the intrusion and its roof rocksand deposit Cu (±Mo) sulfides and Au in this permeable carapaceover time spans of 50,000 to 500,000 years (e.g., Seedorff et al., 2005;Correa et al., 2016). Porphyry Cu deposits in more primitive intra-oceanic arcs (e.g., SW Pacific: Garwin et al., 2005) tend to be enrichedin Au,whereas those inmore evolved (continentalmargin) accretionarysettings (e.g., North American Cordillera) may be enriched in Mo(e.g., Climax, Colorado), Sn (Bolivia), or W (New Brunswick, Canada;Seedorff et al., 2005).

Ore Geology Reviews 79 (2016) 133–154

⁎ Corresponding author.E-mail address: fbierlein@qatarmining.com (F.P. Bierlein).

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Although several discoveries have highlighted the occurrence ofporphyry Cu (±Mo ± Au) deposits in continent–continent collisionalsettings, such as in southern Tibet, Iran, and western Pakistan(e.g., Richards, 2009; Hou et al., 2011; Asadi et al., 2014), the vastmajor-ity of porphyry Cu deposits and their genetically linked high- and low-sulfidation epithermal Au–Ag deposits form at shallow crustal levelsduring the constructional stage of evolving magmatic arcs and back-arcs in accretionary margin settings (e.g., John et al., 2010, and refer-ences therein). These settings are almost invariably characterized byhigh uplift rates and as such, are highly susceptible to erosion(e.g., Cooke et al., 2005; Groves et al., 2005; Kesler and Wilkinson,2006). For this reason, the overwhelming majority of porphyry Cu de-posits globally are Mesozoic or younger in age and occur mainly in thecircum-Pacific and Tethys–Himalaya metallogenic belts, whereas por-phyry Cu deposits are regarded as rare in older terranes. Only a handfulof Paleozoic (e.g., Walshe et al., 1995;Wu et al., 2015; Porter, 2016) andPrecambrian examples have been identified to date (e.g., Gaál andIsohanni, 1979; Barley, 1982; Qui and Groves, 1999; Stein et al., 2004;Goodman et al., 2005; Bejgarn et al., 2013; Malenda et al., 2014; deOliveira et al., 2015). The preservation of these deposits is linked to geo-logically rapid accretion of their host arc terranes to stable continentalblocks; this may have prevented significant uplift and protected themineralized complexes frommajor structural disruption and erosive re-moval (Bierlein et al., 2009).

Recent and ongoing explorationwork by QatarMining Sudan Co Ltd.(“QMSD”) in the Gebeit Terrane in NE Sudan (Fig. 1) has identified

extensive Cu ± Au mineralization hosted by a multi-phase intrusive-extrusive igneous succession and centered on a topographic highnamed ‘Jebel Ohier’ by the local Beja tribe. The style of mineralization,presence of an extensive stockwork vein network over an area of atleast 2.5 by 0.6 kmwithin a broader, zoned potassic – propylitic –phyllic– argillic – advanced argillic altered system, and anore assemblage com-prising magnetite-pyrite-chalcopyrite-bornite (±gold, silver and tellu-rides) are all indicative of a porphyry copper-gold association (Arneet al., 2014; Arne, 2015; Bierlein et al., 2016), analogous to porphyryCu ± Au ± Mo deposits in Phanerozoic supra-subduction settingssuch as, for example, the SW Pacific (e.g., Garwin et al., 2005). The Cu-Au system at Jebel Ohier, which remains open laterally and at depth,contains wide zones of mineralization (grading between 0.2 and1.5wt% Cu) in the up to 75m (from surface) thick oxide zone of the sys-tem, and broad RC and diamond drill intervals of N0.5 wt% Cu and up to0.45 g/t Au from below the base of weathering to a depth of at least400 m.

Preliminary U-Pb age dating yields a maximum constraint of ca.730 Ma for the emplacement of the stockwork system into an older(ca. 800Ma) volcanic complex (Bierlein et al., 2016), but prior to region-al deformation and metamorphism at ca. 710–700 Ma (Abdelsalam,2010). The degree of preservation of the stockwork and lithocap alter-ation at Jebel Ohier is remarkable and raises questions as to (i) howthe deposit was preserved through the complex post-depositional his-tory of the Neoproterozoic Gebeit Terrane, (ii) whether there was any-thing ‘unusual’ about the specific tectonic make-up of themagmatic arc

o o

o

o

Fig. 1. Terranes and sutures of the Arabian-Nubian Shield showing predominant ages for arcmagmatism in each terrane (modified from Kröner and Stern, 2005), aswell as the location ofQMSD's Block 62 concession, major structural elements and principal localities (c.f. Fig. 2).

134 F.P. Bierlein et al. / Ore Geology Reviews 79 (2016) 133–154

hosting the Jebel Ohier Cu-Au deposit, and (iii) the possibility of quanti-fying the degree of ‘fertility’ of the causative pluton(s), which, in turn,would have a major bearing on the likely volume of the mineralizedmagmatic-hydrothermal system? Answers to these questions willallow more robust estimates of the size of the mineralization and assistwith further exploration formineral exploration elsewhere in theArabi-an Nubian Shield (ANS), a resource-rich region traditionally consideredto have extremely low potential for the preservation of porphyry-styleCu-Au mineralization.

2. Regional geology

The Neoproterozoic Arabian-Nubian Shield (ANS) extends alongboth sides of the Red Sea from Egypt in the NW, the Sinai Peninsula inthe N, and Saudi Arabia in the NE to Ethiopia and Yemen in the SWand SE, respectively (Fig. 1). In NE Sudan, the ANS underlies the RedSea Hills region. The evolution of the shield via the accretion of a seriesof predominantly juvenile intra-oceanic island arcs, oceanic islands andmicro-continental masses during the Pan-African tectono-thermalevent between 900 and 550 Ma is widely documented (e.g., Kröneret al., 1987; Stern, 1994, 2002; Abdelsalam et al., 2003; Abdelsalam,2010; Johnson et al., 2011). The geological record of the ANS covers acomplete orogenic cycle which commenced with the break-up ofRodinia (870–800 Ma), gave rise to the opening, inversion and closureof the Mozambique Ocean (800–650 Ma), and concluded with the ter-minal collision between East and West Gondwana between 650 and550 Ma (e.g., Johnson et al., 2011 and references therein).

Magmatism inmost of the accreted terranes in the ANS occurred be-tween 870 and 600Ma (Osman and El Kalioubi, 2014), recording almost300 m.y. of crustal growth. Hassan and Hashad (1990) and El-Bialy andOmar (2015) recognized three distinct igneous events in the northernANS in Egypt (850–800 Ma; 760–710 Ma; 630–610 Ma), withmagmatism during all three of these intervals dominated bysubduction-related calc-alkaline I-type granitoids, granodiorites, andgabbro-diorite complexes. In contrast, the final phase of magmatismfrom 610 to 580 Ma, whichmarks the collisional – lithospheric collapse– extension stage of the ANS, was dominated by the emplacement ofwithin-plate A-type granites and rhyolites, syenites, monzogranites,and mafic to felsic dikes into the newly formed continental crust (El-Bialy and Omar, 2015).

The geologic make-up of the Gebeit Terrane, a segment of juvenilecrust separated from geologically distinct settings by ophiolite-decorated suture zones, is dominated by arc-related low-gradevolcano-sedimentary sequences and syn-tectonic igneous complexes(Klemenic, 1985; Vail, 1985; Kröner et al., 1987; Reischmann andKröner, 1994). The oldest suite comprises an assemblage of subalkaline,calc-alkaline and tholeiitic, mostly subduction-related intrusives-extrusives, although some rocks with high Ti/V ratios (N20) may reflectan incipient back-arc basin in an immature, intra-oceanic island-arc en-vironment (Gaskell, 1985; Reischmann and Kröner, 1994; Johnson andWoldehaimanot, 2003). Whole rock Rb/Sr isochron ages of ca. 720 Mawere reported for volcanic and plutonic rocks from the terrane byFitches et al. (1983) and Almond and Ahmed (1987), with somewhatolder ages of about 830Ma obtained by Reischmann (1986) for volcanicrocks elsewhere in the Gebeit Terrane using Sm-Nd dating methods.Low initial 87Sr/86Sr ratios and Ndmodel ages similar to crystallizationages reported in Johnson andWoldehaimanot (2003) were interpretedby those authors to represent strong evidence of a juvenile oceanic set-ting, with strongly positive eNd(t) values between +6.1 and +8.4 in-dicative of depleted mantle sources. The terrane is separated fromgeologically distinct crustal segments by the Onib-Sol Hamed Suture(or ophiolite belt) to the north, the Red Sea rift to the east, and by theKeraf Suture to the west (Kröner et al., 1987). The southern boundaryof the Gebeit Terrane is the Nakasib Suture, a ca. 150 km-long NE-trending fold and thrust belt in the central part of the Red Sea Hillsthat formed as a result of the collision between the Haya and Gebeit

terranes at ca. 750 Ma (Abdelsalam and Stern, 1993; Fig. 1).The Nakasib Suture, in turn, is sinistrally offset by the broadly N-Soriented late-stage Oko Shear Zone (Abdelsalam, 2010). The Nakasibalso appears to be offset on the west by a north-south trending shearzone (Abdelsalam, 2010). The structurally deformed Haya Terrane,within the Nakasib suture deformation belt, contains the worldclass Ariab group of VMS deposits (e.g., Lissan and Bakheit, 2010;Fig. 1). From a metallogenic perspective, the Haya Terrane is markedby the presence of several VMS deposits and occurrences (e.g. AmuSamur, Tagoteb, Kafasai, Abu Mahmoud, Shulai) whereas the GebeitTerrane is marked by ancient and more recent artisanal gold minesexploiting orogenic gold mineralisation (Elsamani et al., 2001).The Hamisana Shear Zone (Figs. 1, 2) is a major N- to NE-trendingstructure separating the younger Gabgaba Terrane from the GebeitTerrane, but is not definitely an ophiolite-bearing suture (Millerand Dixon, 1992).

The area of study, QMSD's ‘Block 62’ (Fig. 2) is dominated by gabbro-ic to granodioritic intrusives and associated volcano-sedimentary rocksof the Nafirdeib Series (Reischmann and Kröner, 1994). The igneousand epiclastic rocks have been variably deformed under greenschistfacies conditions, as indicated by the occurrence of actinolite,chlorite, epidote and clinozoisite. In the vicinity of batholithicintrusions and locally within prominent shear zones, metamorphismincreases up to amphibolite grade, with incipient leucosomedevelopment in some andesitic units. Late-orogenic granitesunderlie extensive (low-topography) areas especially in the northand west of the block, with several post-collisional alkaline syeniteintrusions forming prominent peaks. Faulting is prominent, with nu-merous reverse, normal and strike slip faults, but the deposit appearsrelatively intact and appears to have escaped significant structuraldismemberment (Bierlein et al., 2016).

3. Samples and analytical methods

Between late 2014 and early 2015, 55 samples were collected fromigneous and volcanic units in the Jebel Ohier region and elsewhere inBlock 62, for the purpose of petrographically and petrogenetically char-acterizing the succession at Jebel Ohier, to ascertain the tectonicsetting(s) these units formed in, and to evaluate their potential forhosting porphyry Cu (+Au) mineralization (c.f. sample locationsshown in Fig. 2; Fig. 3).

In addition to thin section petrography, quantitative XRD analysiswas undertaken in order to accurately determine the mineralogicalcomposition of samples belonging to the various intrusive rocktypes and hydrothermally altered equivalents from within the min-eralized zone at Jebel Ohier. This method is particularly usefulwhere the fine-grained nature of the rock, and/or alteration effectsprevent optical assessment of the material under investigation. AllX-ray diffraction traces were obtained from the powdered samplesat Federation University (Ballarat, Australia) on a Siemens D501 dif-fractometer using Fe-filtered CoKα radiation. Operating conditionswere 40 kV/30 mA, with step scans of 0.02θ/2θ at 1°/2θ/min, fixed1° divergence and receiving slits, and a 0.15° scatter slit. Mineralphases present were identified by computer-aided searches of the2006 ICDD PDF4/Minerals sub-file. Quantitative XRD results wereobtained using SiroQuant™ version 3.0, and refinement of the mostsuitable mineral structures available in the current software packagedatabank. Results should be regarded as semi-quantitative;SiroQuant is a “whole pattern” Rietveld analysis technique. Toallow comparison, all phases identified in the samples as a grouphave been included in all refinements. Because of this, somemineralswill be recorded as present at low levels (background or noise contri-butions), when they may be absent. Therefore, results of b0.5 wt%should be considered as not detected. Representative QXRD analysesare summarized in Table 1.

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The 55 igneous sampleswere examined and cleaned, with slabs pre-pared for chemical analysis by jaw crushing (2–5 kg) and milling in atungsten-carbide mill. The pulverized samples were analyzed for 64

major and trace elements by ICP-OES and MS methods using a mixedacid digest (HNO3, HClO4, HF, HCl) at ALS Laboratories in Romania. Re-sults are summarized in Table 2.

a

b

Fig. 2. a: simplified interpreted geology of QMSD's Block 62 concession, showing major lithological units, structural elements and locations of samples investigated in thisstudy. Geology map draped over Landsat 7 ETM image (display bands 7, 4 and 2 in RGB color system). ‘GDDC’ = Granodiorite-Dacite Complex. Mafic units ‘1’ (high clay,high FeO, intermediate ferrous) and ‘2’ (intermediate clay, high FeO, intermediate ferrous) are distinguished on the basis of systematic compositional variations in clay-hosted Al-, Mg- and CO3-bearing hydroxides in ASTER data [SWIR (B5 + B7)/B6] from the Landsat 8 Operational Land Imager (OLI) sensor (e.g., http://landsat.gsfc.nasa.gov/?page_id=7195). b: Simplified geological map of the Jebel Ohier area showing major rock units, structural trends and extent of the stockwork zone (modified fromBierlein et al., 2016).

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Whole rock Sm-Nd isotope data were obtained at the University ofMelbourne (Maas et al. (2005). For this work, powders (30–50 mg)were spiked and dissolved at high pressure, followed by extraction of

Sm and Nd using EICHROM TRU- and LN-resin. Isotopic analyses werecarried out on a Nu Plasma MC-ICPMS. Further details are given inTable 3.

Fig. 3. images of representative igneous rock units investigated in this study; a) stockwork veins in tuffaceous dacite from Jebel Ohier porphyry complex;b) hornblende-biotite-bearing granodiorite porphyry, Jebel Ohier complex; c) late-stage dolerite dyke cutting stockwork zone at Jebel Ohier (FBJO-004); d) massivefeldspar-hornblende porphyry, Jebel Ohier regional (FBJO-015); e) massive, fine-grained andesite, Red and White Wadi; f) laminated-banded andesitic tuff, JebelOhier regional (FBJO-014); g) equigranular pink feldspar-muscovite granite, Jebel Ohier regional (FBJO-018); h) massive gabbro, Jebel Ohier regional (FBJO-030).Thin section photomicrographs (crossed polars) of a portion of i) sample FBJO-044 (pre-mineralization hornfelsed dacite; note andalusite clots); j) chlorite-alteredhornblende and sericite-altered plagioclase porphyritic grains in sample FBJO-043 (ore-forming granodiorite); k) sample FBJO-001 (sericite-chlorite altered syn-mineralization feldspar-hornblende porphyry; note dusty outer rims of plagioclase); l) sample FBJO-042 (quartz fragment in microcrystalline kaolinite and quartzmatrix of stockwork-hosting dacite); m) large phenocrysts of sericite-altered plagioclase and smaller greenish amphibole grains set in a lithoidal groundmass oflight brown-green augite, alkali feldspar, hornblende, quartz, magnetite and phene in sample FBJO-003 (post-mineralization gabbro), and n) partly haematitizeddacitic lapilli breccia (sample FBJO-023; plane polars). (For interpretation of the references to color in this figure legend, the reader is referred to the web versionof this article.)

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4. Results

4.1. Host rock petrography

The igneous complex at Jebel Ohier (samples ‘JO GDDC’ inTables 1 and 2) is made up of a multi-phase, mafic to intermediateintrusive-extrusive succession. The oldest unit recognized at JebelOhier is a medium-grained dacite, dated at ca. 800 Ma (Bierleinet al., 2016). The dacitic unit has undergone intense argillic and si-licification alteration at, or followed by, contact metamorphism tohornblende hornfels facies conditions. This is supported by thepresence of andalusite (e.g. sample FBJO-044) as clusters ofeuhedral rods and prisms up to 0.2 mm in length generally in asso-ciation with cryptocrystalline kaolinite and flakes of pyrophyllite.This pre-mineralization dacitic unit has been intruded by a porphy-ritic granodiorite (e.g. FBJO-043), which yields ca. 730 Ma zircon U-Pb ages (Bierlein et al., 2016) and is host to stockwork vein miner-alization. The high-level (sub-volcanic) granodiorite contains abun-dant zoned plagioclase grains, beta-quartz pseudomorphs, chlorite-altered biotite and hornblende that are set in a fine-grained, some-times granophyric groundmass of interstitial quartz, alkali feldspar,magnetite and clots of epidote. Plagioclase phenocrysts are com-plexly zoned with outer zones commonly altered to fine - crypto-crystalline sericite. Primary igneous minerals are strongly altered

to chlorite and other fine-grained secondary minerals. A youngerdacite (FBJO-042), which is associated with the ore-forming por-phyry complex and which hosts the majority of the stockwork sys-tem, yields a ca. 730 Ma age (Bierlein et al., 2016) and tends to beintensely silicified and kaolinized. Silicification and silica floodingas a function of stockwork abundance results in patchy, quartz-rich regions of variable grain size from microcrystalline to crypto-crystalline. Kaolinite is extremely fine grained with a highly disor-dered or poorly crystalline character (as indicated by XRD traces).Rare propylitic-altered and deformed lamprophyre dykes are ob-served to cut across the pre-ore dacite but have not been identifiedunequivocally within the main stockwork zone. Late syn-mineralization to post-mineralization phases at Jebel Ohier includedeformed Ca-plagioclase actinolite-chlorite-epidote-altered por-phyritic dykes, undeformed voluminous mafic hornblende feldsparporphyries and pyroxene-augite-plagioclase-magnetite gabbroicplugs, as well as relatively unaltered medium to coarse, evenly-grained mesocratic quartz diorites, and linear dolerite dykes of tho-leiitic association.

Similar lithologies are present in the wider JO area (~30 km radius,Fig. 2) and have been sampled for this study. These samples (labelled‘JO R’ in Tables 1 and 2) include pyroxene-plagioclase gabbros,feldspar-hornblende granodiorite porphyries, finely laminated andesit-ic and dacitic tuffs, pink feldspar-biotite micro-granites, late-stage

Fig. 3 (continued).

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Table 1

Representative quantitative XRD analyses for hydrothermal samples from the Jebel Ohier region and surrounding areas.

Sample FBJO-001 FBJO-002 FBJO-003 FBJO-004 FBJO-008 FBJO-010 FBJO-014 FBJO-017 FBJO-023 FBJO-042 FBJO-043 FBJO-044 FBJO-045

Phase (weight %)Plagioclase 44.1 51.2 54.9 45.6 38.9 56.8 12.3 33.2 39.1 2 48.5 1 3.4Amphibole 15.9 11.3 18.6 40.3 17.7 16.1 41.5Quartz 12.7 20.3 7.3 6.8 7.6 6.8 33.9 20.2 59.2 28.5 60.8 25.9Microcline 6.8 0.5 2.1 1.6 2.1 1.3 2.6 8.5Chlorite 6.5 4.2 3.1 0.9 5.9 3.2 13.3 2.4 12.4 8 28.9Epidote/clinozoisite 4.8 1 1.4 7.8 3.6 20.4 2.4 5.9 37.1Pyroxene/diopside 2 7 4.6 16.6 1Biotite 1.6 5.4 3 1.6 1.8 2.5 0.5 3 0.7Muscovite/sericite 13.2 4.8 10.5 0.8Andalusite 15.9Anatase 2.0Titanite 1.5 0.5 3.3 3.4 2.9 2.5 0.9Pyrite 0.5 0.2a 0.6 0.5Magnetite 0.5 1.6 2 2.8 0.4a 1.2 1.8Apatite 1 2.3Hematite 0.5 10.9 0.8Rutile/sphene 2.1 0.8 0.5Pyrophyllite 1.9Siderite 0.6Ankerite 0.9 0.7 0.4a 0.6 1Kaolinite 3 37.2 17.6Rock type Grano-diorite Quartz-diotite Gabbro Dolerite Diorite Gabbro Rhyolite tuff Hornbl. gabbro Andesitic tuff Silicified dacite Grano-diorite Dacite (hornfelsed) Lamprophyre

a NB: detection limit is ca. 0.5 wt%.

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quartz syenites and dolerite dykes. Further west, the ‘Central Massif’(Fig. 2) exposes mostly (hornfelsed) andesitic volcanics, undeformedgabbro, porphyritic granodiorite and pink-feldspar-biotite granites(samples ‘CM’ in Tables 1 and 2), as well as late-stage dolerites.

Analysis of remote sensing data from elsewhere in Block 62suggested that intrusions similar to those at Jebel Ohier are alsopresent in the area termed the ‘Western Shear Zone’ (Fig. 2).There, a complex interplay of igneous events, regional-scale

Table 2

Lithogeochemical data for 55 igneous rock samples from Block 62.

SAMPLE FBJO-001 FBJO-002 FBJO-003 FBJO-004 FBJO-005 FBJO-006 FBJO-007 FBJO-008 FBJO-009 FBJO-010 FBJO-011

association JO GDDC JO GDDC JO GDDC JO LSMD JO GDDC JO GDDC JO GDDC JO GDDC JO GDDC JO GDDC JO R

lat. (UTM) 724731 724920 725000 724700 723940 723860 723967 723873 723787 724484 724930

long. (UTM) 2189254 2188710 2188700 2188800 2188890 2189100 2189191 2190010 2190050 2189811 2187911

SiO2 (%) 56.4 66.5 54.3 52.5 53.4 51.2 56.2 57.2 66 54.4 58.6Al2O3 (%) 17.45 17.15 17.5 14.35 17.35 16.6 15.7 16.85 16.2 17.25 22Fe2O3 (%) 5.68 3.88 8.52 12.05 9.27 9.49 8.12 8.13 3.24 9.3 5.5CaO (%) 6.18 4 7.12 7.13 7.25 7.58 6.62 6.55 3.51 7 0.72MgO (%) 5.17 1.62 2.87 4.19 4 4.98 4.82 3.69 1.3 4.3 2.71Na2O (%) 4.29 4.79 4.04 3.55 3.75 3.24 3.42 3.69 4.77 3.8 0.84K2O (%) 0.81 2.25 1.56 1.69 1.79 1.48 1.84 1.91 2.16 2.11 4.42Cr2O3 (%) 0.03 0.01 0.01 0.01 0.01 0.02 0.02 0.01 0.01 0.01 0.01TiO2 (%) 0.61 0.46 1.46 2.04 1.71 1.43 0.96 1.23 0.39 1.56 0.62MnO (%) 0.09 0.06 0.13 0.18 0.14 0.15 0.14 0.12 0.05 0.16 0.06P2O5 (%) 0.18 0.14 0.48 0.53 0.54 0.43 0.22 0.39 0.15 0.53 0.24SrO (%) 0.07 0.08 0.06 0.04 0.05 0.05 0.04 0.06 0.09 0.06 0.03BaO (%) 0.05 0.08 0.06 0.06 0.08 0.05 0.04 0.07 0.08 0.07 0.1C (%) 0.02 0.03 0.03 0.01 0.06 0.03 0.18 0.03 0.05 0.02 0.01S (%) b0.01 0.01 0.01 0.01 0.02 0.01 0.02 0.02 0.01 0.01 0.01LOI (%) 2.46 0.87 1.2 1.07 1.62 1.84 2.6 1.72 0.78 1.34 5.38Total (%) 99.47 101.89 99.31 99.39 100.96 98.54 100.74 101.62 98.73 101.89 101.23

Ba (ppm) 393 669 549 513 693 461 358 577 737 609 899Ce (ppm) 21.1 23.5 46.6 48.1 44.7 39.1 31.6 41.5 21.4 52.3 28.1Cr (ppm) 170 40 30 50 60 90 120 80 30 60 50Cs (ppm) 0.48 0.38 0.67 0.52 0.7 0.62 0.29 0.73 0.49 1.11 5.57Dy (ppm) 1.56 1.22 5.72 7.67 5.82 5.26 3.61 4.31 1.07 5.68 1.58Er (ppm) 0.71 0.72 3.31 4.53 3.22 3.05 2.14 2.48 0.49 3.09 0.7Eu (ppm) 0.82 0.71 1.76 2.08 1.79 1.56 1.18 1.45 0.65 1.83 0.84Ga (ppm) 18.8 18.3 21.1 20.3 19.9 18.6 17.5 18.2 19.1 20 21.9Gd (ppm) 1.9 1.71 6.13 7.55 6.18 5.52 3.89 4.75 1.51 6.07 2.24Hf (ppm) 1.9 2.7 5.8 6.6 5.7 5 3.1 5.3 2.6 5.9 2.7Ho (ppm) 0.28 0.23 1.17 1.56 1.14 1.03 0.71 0.82 0.18 1.09 0.28La (ppm) 9.1 10.7 19.4 19.1 18.8 16.7 13.5 17.9 9.8 22.2 10.7Lu (ppm) 0.08 0.09 0.43 0.58 0.45 0.42 0.29 0.34 0.08 0.43 0.09Nb (ppm) 2.5 2.7 5.9 5.6 5.7 5.1 5.4 4.9 1.8 6.2 2.7Nd (ppm) 10.9 11.5 27.5 30.3 26.3 23.6 17.1 22.7 10.3 29.5 14.9Pr (ppm) 2.41 2.55 5.91 6.37 5.85 5.07 3.75 4.93 2.4 6.57 3.45Rb (ppm) 11.8 38.9 32.8 32.9 34 29.9 31.6 43.1 41.4 39.6 96.9Sm (ppm) 2.56 2.4 6.84 7.99 6.66 5.78 4.16 5.53 2 6.89 3.38Sr (ppm) 778 848 642 404 590 591 387 608 977 653 292Ta (ppm) 0.3 0.4 1.6 0.3 0.2 0.3 0.4 0.3 0.3 0.4 0.1Tb (ppm) 0.25 0.25 0.93 1.19 0.92 0.86 0.56 0.7 0.21 0.94 0.3Th (ppm) 0.98 1.73 2.71 2.61 2.62 2.17 2.53 3.38 1.82 3.19 0.96Tm (ppm) 0.11 0.1 0.46 0.61 0.47 0.41 0.28 0.34 0.08 0.45 0.12U (ppm) 0.38 1.06 1.05 0.98 1.03 0.93 0.96 1.42 1.01 1.3 0.41V (ppm) 145 63 213 347 255 243 184 192 59 219 77Y (ppm) 6.8 6.4 28.7 37.7 29 25.5 18.8 21.5 5 27.2 6.3Yb (ppm) 0.65 0.63 3.31 4.21 3.5 3.12 2.21 2.51 0.51 2.93 0.7Zr (ppm) 70 107 228 251 233 195 111 210 95 239 106As (ppm) 1.1 0.8 0.9 1.1 2.9 1.4 1.2 1.8 0.7 1.4 12.7Bi (ppm) 0.01 0.02 0.05 0.02 b0.01 b0.01 b0.01 0.03 0.03 b0.01 0.06Hg (ppm) b0.005 0.009 0.008 b0.005 b0.005 b0.005 0.005 0.006 b0.005 b0.005 b0.005In (ppm) 0.007 0.009 0.012 0.015 0.013 0.006 0.013 0.005 b0.005 0.007 0.006Sb (ppm) 0.05 b0.05 0.35 b0.05 0.11 b0.05 b0.05 0.17 b0.05 0.13 b0.05Se (ppm) b0.2 0.2 0.8 1.1 0.9 0.9 0.5 0.6 0.3 0.6 b0.2Te (ppm) 0.03 b0.01 0.01 b0.01 b0.01 b0.01 0.02 0.01 0.01 b0.01 0.01Tl (ppm) 0.03 0.04 0.08 0.07 0.05 0.03 0.03 0.04 0.07 0.17 0.05Co (ppm) 20 9 22 29 26 32 28 24 8 27 16Cu (ppm) 1700 6 112 160 136 117 28 82 23 69 2Ni (ppm) 65 11 23 24 39 54 47 42 6 43 20Pb (ppm) 10 6 5 6 4 7 6 8 8 4 5Sc (ppm) 14 5 18 30 21 23 21 17 5 20 11Zn (ppm) 158 38 88 112 91 91 84 79 51 105 62

b indicates below detection limit.JO GDDC= Jebel Ohier Granodiorite-Dacite Complex; JO LSMD= Jebel Ohier late-stage mafic dyke.JO R= greater Jebel Ohier region; RW= Red and White Wadi; WSZ= Shear Zone; CM= Central Massif.

140 F.P. Bierlein et al. / Ore Geology Reviews 79 (2016) 133–154

shearing and wrenching, metasomatism and post- deformationmagmatism resulted in a multi-phase succession of stronglymetamorphosed metavolcanics (including agglomerate and lapillituff) and sedimentary rocks (epiclastics) intruded by meta-

gabbros, hornblende-feldspar granodiorite porphyries, syenites,granites and dolerite dykes. Several representative samples(‘WSZ’ in Tables 1 and 2) from this zone were also included inthis study.

FBJO-012 FBJO-013 FBJO-014 FBJO-015 FBJO-016 FBJO-017 FBJO-018 FBJO-019 FBJO-020 FBJO-021 FBJO-022

JO R JO R JO R JO R JO R JO R JO R JO R JO R JO R JO R

724984 724943 724825 724683 724440 724983 726176 726318 726381 727273 725716

2187811 2187333 2185707 2184279 2183409 2181633 2180622 2179931 2179356 2174910 2177651

68.4 76.2 62.8 64.5 49.6 44.9 66.1 74.4 50.5 72.9 49.315.6 12.7 15.2 16.95 18.75 16.35 19.35 14.3 16.4 11.8 17.42.81 2.32 7.55 3.43 10.6 12.6 2.14 1.64 9.09 4.18 8.82.83 1.23 5.44 4.49 9.65 13.65 1.37 1.44 10.1 1.12 8.490.89 0.2 2.78 1.53 5.18 8.32 0.11 0.33 7.21 0.13 6.014.46 3.9 1.67 4.69 3.24 2.06 8.03 3.14 2.66 3.9 3.772.66 2.72 1.13 1.24 0.38 0.19 3.01 5.14 0.34 3.46 0.570.01 0.01 0.01 0.01 0.02 0.05 b0.01 b0.01 0.06 0.01 0.020.37 0.17 0.79 0.45 1.42 1.3 0.26 0.19 0.99 0.3 0.840.06 0.03 0.14 0.06 0.17 0.19 0.05 0.03 0.16 0.08 0.150.16 0.04 0.15 0.2 0.3 0.27 0.02 0.06 0.32 0.04 0.20.07 0.01 0.06 0.07 0.05 0.05 b0.01 0.01 0.04 b0.01 0.040.09 0.06 0.03 0.07 0.02 0.02 0.42 0.11 0.02 0.05 0.030.02 0.07 0.02 0.02 0.02 0.02 0.06 0.01 0.01 0.04 0.190.03 0.01 0.01 0.01 0.01 0.02 0.01 0.01 0.01 0.01 0.011.31 1.04 2.92 1.53 1.45 1 0.68 0.98 1 0.24 2.6799.72 100.63 100.67 99.22 100.83 100.95 101.54 101.77 98.89 98.21 98.29

862 553 295 570 204 145 4560 929 145.5 481 26136.2 34.4 22 22.6 22.8 15.7 20.5 46.5 22.3 66.9 17.220 40 30 30 80 300 10 10 360 20 900.79 0.23 0.84 0.15 0.37 0.06 0.18 1.42 0.36 0.26 0.351.71 4.01 3.75 1.04 4.62 3.75 1.76 1.96 3.22 12.6 2.890.87 2.67 2.42 0.47 2.64 1.98 1.44 1.22 1.89 8.2 1.910.83 0.58 1.03 0.7 1.43 1.28 2.51 0.64 1.03 2.26 0.9917.7 13.9 17.2 19 18.7 15.9 20.3 14.1 15.9 18.1 16.72.17 3.75 3.4 1.41 4.63 3.84 1.78 2.05 3.31 10.75 2.913.3 5.3 3.1 2.2 3.1 1 14.2 4.2 2.2 11.8 1.90.3 0.82 0.78 0.17 0.97 0.72 0.36 0.38 0.63 2.63 0.6117 14.5 9 10.5 8.7 5.2 10.8 25 9.4 26.6 6.80.13 0.45 0.36 0.06 0.38 0.23 0.31 0.19 0.28 1.21 0.263.1 4 3 1.5 2.4 1.2 2.9 4.3 2.7 8.5 1.816.7 18.1 12.9 10.4 15.5 12.4 9.1 17.3 13.3 39.7 10.64.23 4.19 2.59 2.55 3.06 2.18 2.13 4.8 2.81 8.72 2.1248.2 40 23.6 13.4 4.6 1.1 19.4 147.5 4.5 64 8.13.09 4.38 3.48 2.08 4.36 3.58 2.11 3.12 3.47 10.65 2.85720 116.5 619 730 576 519 89.2 161 506 63.9 4070.2 0.3 0.2 0.2 0.2 0.2 0.3 0.5 0.2 0.6 0.20.27 0.59 0.58 0.2 0.73 0.55 0.26 0.31 0.51 1.89 0.463.55 2.74 1.55 1.18 0.56 0.13 1.26 10.15 1.1 3.01 0.470.13 0.4 0.34 0.09 0.39 0.28 0.22 0.18 0.32 1.19 0.271.88 1.29 0.76 0.7 0.24 0.05 0.97 2.05 0.51 1.3 0.2438 5 163 63 228 374 5 16 210 5 2297.9 21.7 18.9 5 23.7 16.9 10.1 10.5 16.7 67.7 16.20.96 2.98 2.5 0.42 2.71 1.68 1.91 1.34 1.92 9.12 1.82119 174 112 86 124 32 875 140 90 440 700.4 2 2.4 0.8 0.6 0.6 0.6 1 0.5 1 0.90.02 b0.01 b0.01 b0.01 b0.01 b0.01 b0.01 0.01 b0.01 b0.01 b0.010.006 b0.005 0.005 0.005 b0.005 b0.005 b0.005 0.012 b0.005 b0.005 b0.0050.007 b0.005 0.007 0.007 0.005 0.012 0.014 0.005 b0.005 0.029 b0.005b0.05 b0.05 b0.05 b0.05 b0.05 b0.05 b0.05 b0.05 b0.05 b0.05 b0.05b0.2 0.2 0.4 b0.2 0.4 0.4 b0.2 b0.2 b0.2 1.5 0.30.02 b0.01 0.03 0.01 b0.01 b0.01 b0.01 b0.01 b0.01 b0.01 0.01b0.02 0.02 b0.02 0.02 b0.02 b0.02 b0.02 0.1 b0.02 b0.02 b0.025 1 15 9 36 47 1 3 37 1 3313 2 67 58 53 47 1 2 19 1 203 1 8 11 58 37 1 1 112 1 79 6 2 6 2 2 4 14 2 10 23 5 23 5 26 55 7 3 26 10 2856 23 81 57 83 90 45 30 63 122 70

(continued on next page)

141F.P. Bierlein et al. / Ore Geology Reviews 79 (2016) 133–154

4.2. Whole-rock compositions

The igneous rocks analyzed in this study have compositions that canbe defined as metaluminous to peraluminous (Fig. 4), and calc-alkaline

to shoshonitic (Fig. 5a). In the TAS diagram of Le Maitre (1989), theyplot mostly in the fields of sub-alkaline gabbro, diorite, granodioriteand granite (Fig. 5b). This classification is supported by immobiletrace element systematics (Winchester and Floyd, 1977), where almost

SAMPLE FBJO-023 FBJO-024 FBJO-025 FBJO-026 FBJO-027 FBJO-028 FBJO-029 FBJO-030 FBJO-031 FBJO-032 FBJO-033

association JO R JO R JO R JO R JO R JO R JO R JO R JO R JO R JO R

lat. (UTM) 725325 724876 724546 726303 718496 718146 716212 716145 720172 721840 725196

long. (UTM) 2185255 2185693 2185627 2187222 2191822 2191629 2194028 2194066 2197128 2197126 2195144

SiO2 (%) 50 73.3 68 68.3 66.6 74.3 73.9 49.5 57.7 70.4 57.6Al2O3 (%) 17.95 13.3 16.2 16.5 15.8 13.85 13.45 17.6 15.5 14 15.7Fe2O3 (%) 10.15 2.29 2.68 3.05 5.18 2.58 1.82 8.82 8.26 2.73 8.25CaO (%) 5.23 1.28 3.14 3.48 2.3 1.33 0.63 9.63 5.62 1.1 5.64MgO (%) 5.94 0.21 1.17 1.39 1.69 0.32 0.13 5.56 3.16 0.53 3Na2O (%) 3.56 5.34 4.82 4.78 5.07 5.09 3.91 3.32 3.28 4.78 3.38K2O (%) 0.49 1.95 1.98 2.35 2.64 1.95 4.7 0.97 2.54 3.56 2.64Cr2O3 (%) 0.01 0.01 0.01 0.01 0.01 0.01 b0.01 0.03 0.01 0.01 0.01TiO2 (%) 1.13 0.18 0.34 0.38 0.55 0.17 0.11 1.36 0.95 0.31 0.96MnO (%) 0.15 0.03 0.04 0.05 0.1 0.05 0.04 0.14 0.13 0.07 0.13P2O5 (%) 0.35 0.03 0.11 0.14 0.16 0.05 0.07 0.31 0.31 0.07 0.33SrO (%) 0.06 b0.01 0.09 0.09 0.02 0.02 b0.01 0.06 0.04 0.01 0.04BaO (%) 0.03 0.05 0.08 0.08 0.06 0.07 0.06 0.04 0.07 0.08 0.07C (%) 0.13 0.2 b0.01 0.03 0.08 0.16 0.01 0.05 0.07 0.05 0.02S (%) b0.01 0.01 b0.01 0.01 0.02 0.01 0.01 0.02 0.02 0.01 0.01LOI (%) 3.78 1.26 1.35 0.76 0.96 1 0.36 1.65 1.41 0.6 1.38Total (%) 98.83 99.23 100.01 101.36 101.14 100.79 99.18 98.99 98.98 98.25 99.13

Ba (ppm) 274 440 775 745 576 574 562 344 594 773 614Ce (ppm) 25.1 41.5 22 22.2 33.2 36.8 37.3 29.1 42.1 37.7 43.1Cr (ppm) 30 20 30 50 60 30 10 170 50 40 50Cs (ppm) 0.35 0.2 0.33 0.48 1.09 0.9 0.49 0.26 1.12 0.24 0.76Dy (ppm) 3.28 4.42 0.96 0.99 3.75 2.9 1.46 4.05 5.99 3.91 6Er (ppm) 1.85 2.86 0.36 0.43 2.28 1.85 0.8 2.16 3.81 2.67 3.73Eu (ppm) 1.3 0.68 0.59 0.61 0.92 0.53 0.47 1.42 1.38 0.75 1.48Ga (ppm) 19 15.4 19.1 19.6 17 13.7 13.8 18.2 17.3 15 18.5Gd (ppm) 3.68 4.22 1.33 1.58 3.51 2.64 1.89 4.25 6.29 3.86 6.18Hf (ppm) 1.9 5.7 2.6 2.3 6 4.8 4.4 3.2 5.7 6.4 5.8Ho (ppm) 0.6 0.88 0.16 0.18 0.75 0.59 0.28 0.76 1.22 0.8 1.19La (ppm) 10.4 18.5 10.2 10.5 14.1 16.8 19.1 12.1 16.8 16.8 18Lu (ppm) 0.24 0.45 0.04 0.06 0.38 0.34 0.13 0.28 0.56 0.43 0.58Nb (ppm) 3.5 4.3 1.5 1.9 4.6 4.8 2.8 6.5 4.7 5.1 4.9Nd (ppm) 16 21.4 10.6 10.4 16.8 16.5 14.7 17.3 26.1 18.8 27.3Pr (ppm) 3.2 4.94 2.51 2.42 3.99 4.14 3.92 3.71 5.62 4.4 5.68Rb (ppm) 12.6 32.6 25.4 40.2 56.2 38.1 81.4 13.5 48.4 71.4 56.6Sm (ppm) 4.01 4.94 2.04 1.99 4.16 3.56 2.53 4.52 6.85 4.21 6.74Sr (ppm) 681 112 867 876 265 236 60.4 613 385 149.5 377Ta (ppm) 0.2 0.3 0.2 0.2 0.4 1.1 0.4 0.4 0.3 0.5 0.4Tb (ppm) 0.51 0.66 0.19 0.21 0.61 0.43 0.26 0.65 0.96 0.61 0.98Th (ppm) 0.64 2.87 1.58 1.83 3.23 4.4 5.13 1.03 3.41 3.88 3.46Tm (ppm) 0.25 0.42 0.08 0.09 0.35 0.31 0.13 0.3 0.52 0.39 0.51U (ppm) 0.3 1.54 0.91 1.22 1.61 2.04 1.19 0.46 1.95 1.7 1.96V (ppm) 168 10 48 56 78 5 5 214 157 22 162Y (ppm) 15.1 23.9 4.4 4.4 20 16.6 7 18.9 31.1 21.5 31.9Yb (ppm) 1.69 3.23 0.42 0.42 2.45 2.33 0.96 2.07 3.87 2.87 3.84Zr (ppm) 75 185 93 89 225 157 153 131 203 224 198As (ppm) 10.5 0.6 0.8 0.9 1.4 1.3 0.6 1.4 2.5 1.1 11.4Bi (ppm) b0.01 b0.01 0.04 0.12 b0.01 b0.01 b0.01 b0.01 b0.01 0.12 0.01Hg (ppm) b0.005 b0.005 0.008 b0.005 b0.005 b0.005 b0.005 b0.005 b0.005 b0.005 0.005In (ppm) 0.008 b0.005 b0.005 b0.005 0.007 b0.005 b0.005 0.009 0.009 0.024 0.012Sb (ppm) b0.05 b0.05 b0.05 b0.05 0.11 0.14 b0.05 0.1 b0.05 0.1 0.25Se (ppm) 0.2 0.2 b0.2 b0.2 0.7 0.2 b0.2 0.4 0.7 0.6 0.8Te (ppm) b0.01 b0.01 b0.01 0.01 0.03 b0.01 b0.01 0.01 0.01 0.01 0.01Tl (ppm) b0.02 b0.02 b0.02 0.03 0.37 0.1 0.02 0.02 0.2 0.09 0.08Co (ppm) 31 1 7 8 10 2 1 30 19 4 18Cu (ppm) 1 4 32 21 36 8 1 49 21 9 22Ni (ppm) 12 1 9 9 3 1 1 48 8 1 9Pb (ppm) 2 2 4 6 7 6 10 3 7 5 10Sc (ppm) 20 5 4 5 10 2 2 26 21 6 21Zn (ppm) 100 45 45 50 56 31 20 83 100 35 97

Table 2 (continued)

142 F.P. Bierlein et al. / Ore Geology Reviews 79 (2016) 133–154

all of the samples plot within the extrusive equivalents of gabbroic-dioritic-granitic rocks (i.e., basalts, andesite, rhyodacite/dacite and rhy-olite; Fig. 6a), consistent with mapping and petrography. This suggeststhat widespread hydrothermal alteration has not produced major

changes in major element compositions, with the possible exceptionof Na that may have been lost during propylitic alteration of atleast some of the samples, possibly shifting the relevant data points inFig. 5.

FBJO-034 FBJO-035 FBJO-036 FBJO-037 FBJO-038 FBJO-039 FBJO-040 FBJO-041 FBJO-042 FBJO-043 FBJO-044

JO R JO LSMD JO R JO R JO R JO R JO R JO GDDC JO GDDC JO GDDC JO GDDC

726416 726417 724181 722306 723199 722787 721294 722946 722920 723396 723427

2194362 2194362 2195676 2186262 2185620 2185540 2185565 2188771 2189150 2189334 2189521

67.4 46.3 57.6 54.2 74.6 71.3 51.5 65.6 69.9 67 69.516.65 17.5 16.9 19.7 14.15 11.75 19.55 16.5 19.6 16.95 17.953.98 11.1 7.13 4.5 2.68 2.18 8.31 3.71 2.4 2.94 1.783.54 8.71 5.28 4.74 0.99 8.28 8.35 3.5 0.09 1.35 0.241.49 6.58 2.78 1.95 0.28 0.91 3.43 1.52 0.01 1.37 0.234.75 3.35 4.15 4.6 5.29 2.26 4.08 4.69 0.12 5.19 0.952.39 0.57 2.09 4.98 2.55 0.74 1.22 2.07 0.1 2.1 4.020.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 b0.010.39 1.61 0.85 0.73 0.19 0.35 1.34 0.43 0.54 0.39 0.450.06 0.17 0.13 0.07 0.03 0.1 0.11 0.05 b0.01 0.04 0.010.18 0.24 0.19 0.63 0.03 0.16 0.24 0.15 0.12 0.14 0.20.08 0.07 0.04 0.12 b0.01 0.04 0.07 0.1 0.07 0.07 0.050.08 0.06 0.05 0.06 0.07 0.07 0.04 0.07 0.01 0.07 0.090.04 0.16 0.04 0.21 0.04 0.27 0.05 0.02 0.01 0.01 0.020.02 0.03 0.02 0.02 0.01 0.03 0.01 0.01 0.03 0.01 0.030.7 3.03 1.56 2.58 0.86 2.01 1.05 0.67 7.39 1.54 2.7101.7 99.3 98.76 98.87 101.73 100.16 99.3 99.07 100.35 99.16 98.17

671 497 417 516 573 596 389 646 75.7 611 76021.8 24.5 32.6 31.8 42.5 24.3 23.8 20 13.3 21 7.130 20 30 40 40 80 40 50 20 40 201.05 0.47 0.49 1.03 0.97 0.49 0.49 0.78 0.03 0.62 1.691.1 4.26 4.18 2.54 5.08 2.06 3.17 1.2 0.94 1.29 0.890.63 2.2 2.53 1.45 3.17 1.21 1.89 0.61 0.45 0.59 0.540.57 1.43 1.13 1.3 0.92 0.57 1.45 0.67 1.02 0.67 0.5118 18.2 20.2 17.9 16.5 10.7 22 20.4 28.3 20 22.21.45 4.42 4.65 3.78 5.33 2.52 3.59 1.69 2 1.89 1.43 2.9 7.6 3.4 6.8 2.2 3.2 2.8 3.4 2.7 30.2 0.79 0.84 0.47 1.02 0.39 0.64 0.22 0.15 0.24 0.1710.5 9.5 14.4 14 19.3 13.8 11.1 10.3 5.5 11.1 3.20.08 0.29 0.37 0.19 0.48 0.2 0.22 0.07 0.06 0.08 0.071.9 7.6 2.4 2.6 4 1.3 1.5 1 1.7 1 1.510.2 15.6 19.3 18.8 23.4 11.4 14.2 10.3 9.5 10.4 5.52.34 3.04 4.45 4.42 5.63 2.88 3.4 2.58 1.95 2.7 1.1148.6 9.6 45.4 81.8 43.3 15.8 24.7 36.2 0.6 38.1 70.32.06 3.99 4.74 4.56 6 2.38 3.34 2.14 3.29 2.34 1.56786 692 489 1185 126 435 747 889 700 654 5440.3 0.6 0.3 0.2 0.3 0.2 0.3 0.2 0.3 0.2 0.20.2 0.68 0.69 0.47 0.8 0.31 0.49 0.21 0.18 0.23 0.172.28 0.75 4.17 2.32 3.51 1.94 1.59 1.91 0.22 2.38 0.850.11 0.33 0.39 0.24 0.49 0.19 0.26 0.1 0.09 0.11 0.090.92 0.28 1.98 1.38 1.81 0.88 0.74 1.16 0.28 1.3 0.3957 241 151 171 5 45 251 57 42 45 555.7 18.9 23.9 13.5 29.9 11.7 17.2 5.9 4.7 6.5 4.40.6 1.99 2.5 1.33 3.49 1.33 1.58 0.48 0.42 0.55 0.5598 113 265 123 210 77 107 98 113 91 1010.5 0.9 0.9 0.5 0.3 7.1 0.8 0.3 2.9 0.8 3.30.03 0.01 0.02 0.08 0.03 0.05 0.04 0.01 0.05 0.04 0.06b0.005 0.005 b0.005 b0.005 b0.005 0.007 0.009 b0.005 b0.005 b0.005 0.0060.005 0.008 0.017 0.009 0.007 0.007 0.011 0.005 b0.005 0.009 0.012b0.05 0.1 0.2 0.1 0.16 0.38 0.16 0.06 b0.05 0.07 b0.05b0.2 0.6 0.5 0.5 0.3 0.4 0.3 b0.2 1.3 0.3 0.4b0.01 0.01 b0.01 0.04 b0.01 0.02 b0.01 b0.01 0.03 b0.01 0.120.18 0.03 0.23 b0.02 0.02 b0.02 0.04 0.06 b0.02 0.02 0.098 47 19 10 2 9 25 7 1 8 114 84 64 137 2 14 55 6 73 143 889 70 10 23 1 3 37 11 8 9 18 4 6 10 7 14 5 8 22 10 415 25 17 6 5 7 17 5 1 5 553 102 71 41 54 39 69 46 2 60 16

(continued on next page)

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Their extensive compositional range notwithstanding, all felsicto intermediate samples plot entirely within the field for ‘volca-nic-arc’ granitoids in a discrimination diagram (Fig. 6b) fromPearce et al. (1984), though the majority of samples from the

Jebel Ohier North region and several samples from the otherquadrants also plot within the overlapping field for ‘post-colli-sional granitoids’. In the La-Yb-Th diagram of Pearce et al.(1984), the samples from within each region define a trend from

SAMPLE FBJO-045 FBRW01 FBWSZ01 FBWSZ02 FBWSZ03 FBWSZ04 FBCM01 FBCM02 FBCM03 FBCM04 FBCM05

association JO LSMD JO R WSZ WSZ WSZ WSZ CM CM LSMD CM CM CM

lat. (UTM) 723499 713719 663606 665277 665280 661804 697854 698716 698875 698542 699209

long. (UTM) 2189598 2184879 2182635 2183043 2183054 2189351 2186210 2186625 2187164 2189069 2190564

SiO2 (%) 51.5 50.4 69.2 73.5 49.4 65.1 57.2 54.8 64.9 44.4 75.1Al2O3 (%) 16.25 15.8 12.45 11.1 15.9 14.75 15.8 16.05 15.55 24.9 12.75Fe2O3 (%) 10.35 11 7.89 3.53 12.15 4.83 7 9.41 5.83 6.14 2.81CaO (%) 7.98 8.68 0.13 0.2 8.05 2.63 6.67 7.22 1.88 13.75 0.54MgO (%) 4.95 6.37 4.8 3.94 5.87 0.84 2.49 4.64 1.05 7 0.14Na2O (%) 1.36 3.33 0.05 0.11 2.94 4.04 4.37 3.44 4.69 1.35 4.87K2O (%) 0.04 0.69 2.61 2.5 0.84 4.16 0.93 1.63 3.46 0.14 4.42Cr2O3 (%) 0.01 0.02 0.01 b0.01 0.01 b0.01 0.02 0.01 0.01 0.04 0.01TiO2 (%) 0.77 1.32 0.28 0.27 0.76 0.64 1.14 0.83 0.75 0.21 0.16MnO (%) 0.25 0.17 0.29 0.17 0.23 0.07 0.08 0.14 0.11 0.09 0.07P2O5 (%) 0.5 0.2 0.06 0.12 0.36 0.18 0.34 0.3 0.21 0.05 0.04SrO (%) 0.09 0.04 b0.01 b0.01 0.07 0.03 0.06 0.08 0.03 0.07 b0.01BaO (%) b0.01 0.03 0.07 0.05 0.03 0.06 0.04 0.03 0.14 0.01 0.04C (%) 0.02 0.03 0.01 0.02 0.01 0.18 0.68 0.09 0.06 0.08 0.01S (%) 0.01 0.02 0.01 0.03 b0.01 b0.01 b0.01 0.01 0.01 0.04 0.01LOI (%) 4.33 1.64 3.5 3.22 1.6 1.05 3.68 1.54 1.33 2.46 0.24Total (%) 98.38 99.69 101.34 98.71 98.21 98.38 99.82 100.12 99.94 100.61 101.19

Ba (ppm) 37.6 194.5 569 383 232 518 352 320 1225 61.3 346Ce (ppm) 22.4 17.6 36.1 37.6 21.9 59.6 46 20.7 59.8 4.3 46.4Cr (ppm) 40 140 20 10 100 10 130 20 10 300 20Cs (ppm) 0.11 3.93 0.18 0.31 0.51 7.38 1.48 1.05 0.34 0.43 0.63Dy (ppm) 4.17 5.51 7.42 8.53 3.29 7.21 3.97 3.99 7.02 0.76 6.14Er (ppm) 2.31 3.07 5.13 5.24 1.89 4.52 2 2.43 3.95 0.36 4.16Eu (ppm) 1.27 1.44 0.5 0.94 1.09 1.06 1.43 1.2 2.12 0.4 0.41Ga (ppm) 19.4 16.6 18 14.7 17 17.6 19.1 18.1 19.8 16.5 17.3Gd (ppm) 4.64 4.62 5.25 7.57 3.81 7.21 4.72 3.9 7.49 0.76 5.66Hf (ppm) 2.5 3.1 6.9 6.1 1.8 11.4 5.4 2.4 8.7 0.5 9.3Ho (ppm) 0.77 1.03 1.55 1.66 0.59 1.44 0.75 0.88 1.48 0.15 1.2La (ppm) 9.8 7.3 10.5 14.8 9.3 25.5 20.3 8.5 26.5 2 23.8Lu (ppm) 0.33 0.43 0.88 0.78 0.26 0.66 0.25 0.39 0.6 0.05 0.59Nb (ppm) 1.1 1.4 7.5 6.7 1.6 6.4 4.1 1.5 7 0.2 5.8Nd (ppm) 15.6 13.5 17.9 28.7 15.2 35.1 26.2 14.5 35 2.6 28Pr (ppm) 3.32 2.59 3.72 5.87 3.04 7.51 6.33 3.06 8.53 0.61 6.32Rb (ppm) 1 27.3 48.8 62.3 26 127 28.2 47.2 76.7 2.4 69.2Sm (ppm) 4.39 3.87 4.36 6.26 3.92 7.48 5.89 3.88 8.18 0.78 5.69Sr (ppm) 820 320 12.1 13.7 598 240 587 734 364 719 42.9Ta (ppm) 0.7 0.1 0.5 0.8 0.2 0.5 0.3 0.1 0.4 0.1 0.5Tb (ppm) 0.67 0.7 0.95 1.29 0.46 1.08 0.67 0.67 1.23 0.11 0.86Th (ppm) 1.37 0.62 1.79 1.73 1.01 7.55 4.96 0.93 6.76 0.16 5.36Tm (ppm) 0.34 0.45 0.78 0.72 0.24 0.63 0.29 0.36 0.58 0.06 0.51U (ppm) 0.74 0.28 0.85 0.81 0.7 3.82 2.26 0.34 3.31 0.08 3.49V (ppm) 221 260 13 16 426 48 143 240 29 62 6Y (ppm) 22.5 26.9 44.1 43.6 17.2 41.7 21.2 23.9 41.9 3.5 33.7Yb (ppm) 2.37 2.75 5.56 5.19 1.87 4.25 1.81 2.21 3.9 0.29 3.84Zr (ppm) 82 96 230 218 53 370 250 99 402 16 343As (ppm) 1.8 1 0.8 0.4 1.8 1.8 0.4 1 0.9 3.3 1.6Bi (ppm) 0.03 0.03 0.01 0.01 0.02 0.31 0.06 0.01 0.02 0.03 0.06Hg (ppm) b0.005 b0.005 b0.005 0.007 b0.005 b0.005 b0.005 0.007 b0.005 b0.005 b0.005In (ppm) 0.015 0.007 0.025 0.021 0.011 0.043 0.035 0.01 0.016 b0.005 0.029Sb (ppm) 0.18 0.21 0.1 0.1 0.2 0.14 0.19 0.14 0.12 0.06 0.15Se (ppm) 0.3 0.8 0.9 1.3 0.5 1.3 0.5 0.5 0.8 0.4 0.9Te (ppm) b0.01 b0.01 0.01 0.01 0.04 0.02 b0.01 b0.01 b0.01 b0.01 b0.01Tl (ppm) 0.05 0.04 0.1 0.13 0.1 0.48 0.07 0.24 0.02 0.02 0.02Co (ppm) 47 32 5 1 34 6 20 26 4 34 b1Cu (ppm) 11150 36 b1 6 191 31 10 6 1 39 1Ni (ppm) 20 47 b1 1 44 b1 60 2 b1 77 b1Pb (ppm) 29 9 6 6 11 15 11 13 20 8 12Sc (ppm) 23 37 11 9 30 9 14 23 11 7 6Zn (ppm) 826 84 118 135 90 65 72 71 117 49 79

Table 2 (continued)

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primitive island arcs to continental margin arcs (Fig. 5c). Most ofthe samples display elemental compositions indicative of meltsderived from the sub-arc mantle wedge and with limited inputfrom continental-type sources (e.g., Manikyamba et al., 2015).The distinction from a mature continental arc setting for all sam-ples in the study area is further underlined by Rb/Zr over Nb con-centrations (Fig. 5d), with the majority of the rocks representativeof primitive-island and continental arcs (Jin, 1986).

On a primitive mantle-normalized element diagram (Fig. 7a),the samples are characterized by a general enrichment in low

field-strength (LFSE) and large ion lithophile elements (LILE),and generally conservative trends for incompatible high field-strength elements (HFSE). Notably, almost all samples displaynegative Th, Nb and Ti anomalies, while Pb and U are relativelyenriched. More scattered concentrations of Rb, Ba and K are prob-ably, at least to some extent, due to alteration but may also re-flect some mixing between the mafic rocks and dioritic-graniticmaterial during ascent. The relative importance of crustal assimi-lation over fractional crystallization in the evolution of some late-stage to post-collisional igneous units at Jebel Ohier and the

Fig. 4. Plot of alumina saturation of all samples investigated in this study. Molar ratios of CaO/(Na2O+ K2O+ CaO) compared to Al2O3/(Na2O+ K2O+ CaO) following Barton and Young(2002) and John et al. (2010). ‘GDRDC’ = Granodiorite-Dacite Complex; ‘LSMD’ = late-stage mafic dykes.

Table 3

Whole-rock Sm-Nd isotope data for representative igneous rocks associated with PCD mineralization and barren analogues.

Sample Sm ppm Nd ppm 147Sm/144Nd 143Nd/144Nd ±2se εNd TDM1 TDM2 Age t εNd

(Ga) (Ga) (Ga)a (initial)

FBJO-001 2.54 12.38 0.1237 0.512629 0.000008 −0.06 0.881 0.86 0.73 +6.7FBJO-003 6.61 29.58 0.1350 0.512684 0.000009 +1.01 0.85 0.70 +6.5FBJO-010 6.78 31.54 0.1299 0.512651 0.000009 +0.37 0.908 0.87 0.70 +6.3FBJO-043 2.23 11.74 0.1146 0.512552 0.000008 −1.56 0.920 0.91 0.73 +6.1FBJO-044 1.51 6.21 0.1471 0.512564 0.000010 −1.33 1.15 0.80 +3.7

Concentrations of Sm and Nd, and Sm/Nd ratios, by isotope dilution. All isotopic analyses bymulti-collector ICPMS; 143Nd/144Nd normalized to 146Nd/145Nd= 2.0719425 (equivalentto 146Nd/144Nd= 0.7219) and reported relative to a value of 0.511860 for the LaJolla Nd standard. Internal precisions (2se) as listed, external precision (2sd) ±0.000020; external pre-cision for 147Sm/144Nd ± 0.2%. Present-day CHUR has 147Sm/144Nd and 143Nd/144Nd ratios of 0.1960 and 0.512632, respectively (Bouvier et al., 2008). TDM1 is the depleted mantlemodel age, calculated using a modern depleted mantle (DM) with 147Sm/144Nd and 143Nd/144Nd ratios of 0.2129 and 0.513145, respectively. TDM2 is a 2-stage Nd model age with adefault 147Sm/144Nd ratio of 0.1100 for the first stage. λ147Sm 6.54E-12/yr. 147Sm/144Nd and 143Nd/144Nd for USGS basalt BCR-2 average 0.1383 ± 2 and 0.512642 ± 22 (all 2sd ofnumerous digestions).

a Ages based on U-Pb zircon dating for FBJO-001, -043 and -044; age estimates for FBJO-003 and -010 based on field relationships.

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genesis of more felsic end-members is indicated in a Th/Nb overZr diagram (from Nicolae and Saccani, 2003), where these co-magmatic pairs from the same region define steep increases inTh/Nb (Fig. 8).

Trace element compositions of five samples of the Jebel Ohiergranodiorite-dacite complex are very similar to those of igneoussamples from the surrounding regions with respect to LFSE andLILE, but differ significantly in terms of their accentuated depletionin HFSE and REE (Fig. 7a). Total rare earth element (REE) concen-trations vary significantly as a function of mineralogy in theserocks, but their patterns are coherent in most cases. All of the sam-ples display variable but consistent light REE enrichment, and over-all depletion in the middle and heavy REE (Fig. 7b). Rare earthelement patterns of samples from the Jebel Ohier granodiorite-dacite complex do not show the characteristic Eu depletions thatare so typical of upper crustal material and is observed in some ofthe samples from other parts of the Gebeit Terrane (Fig. 7b). Thisdifference is considered important as it suggests that the dacitesand granodiorites at Jebel Ohier formed at higher oxygen relativeto magmatic suites elsewhere in the region, that fractionation offeldspar (which leads to negative Eu anomalies in the developingmagma) played a less significant role, and that hornblende frac-tionation (which causes progressive depletion of middle and

heavy REE in the melt) may have been more important at JebelOhier than elsewhere in Block 62 (Ragland, 1989; Richards et al.,2001).

Five JO samples that encompass the lithological and age range of thepre- to post-mineralization volcano-intrusive complex have initial ɛNdvalues of+3.7 to+6.7 (Table 3). The ɛNd values of four of these samplesare indistinguishable within their errors (6.1 to 6.7). Nd model ages(TDM) for these four samples are similar to the geological ages(~0.86 Ga, Fig. 9) while sample FBJO-044 has a significantly oldermodel age (Fig. 9).

5. Discussion

5.1. A subduction setting for the Jebel Ohier complex

The wide compositional range observed for the samples studiedhere confirms field and petrographic evidence for strong diversityin magma composition in the JO region over a protracted period dur-ing the closure of theMozambique Ocean and subsequent collision ofEast and West Gondwana in the Late Proterozoic (e.g., Stern, 1994;Johnson et al., 2011). The overall juvenile nature of the igneousrocks is consistent with a geodynamic setting that was dominatedby intra-oceanic island arcs in an evolving accretionary setting

A

B

Fig. 5. A) K2O versus Si2O diagram (after Le Maitre, 2002); B) SiO2 vs Na2O-K2O diagram of Le Maitre (1989). ‘GDRDC’ = Granodiorite-Dacite Complex.

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with very limited, if any, contributions from thickened continentalcrust (e.g., Reischmann and Kröner, 1994; Fritz et al., 2013). Nb de-pletion is ubiquitous in all analyzed samples, again consistent withsimilar results from the Gebeit Terrane in Reischmann and Kröner(1994) who argued that low Nb contents (b10 ppm) in basaltsand basaltic andesites were strong evidence for subduction-related magmatism. This is because Nb is known to be preferential-ly retained in high-pressure stable phases such as amphiboles,titanite and rutile in the subducting plate (e.g., Pearce and Peate,1995). However, negative Nb anomalies, especially when coupledwith Ti depletion, can also result from crustal melting, crystalliza-tion processes or crustal assimilation (Kemp and Hawkesworth,2003). Assimilation of Th-enriched material is apparent from theTh/Nb trends, but crustal contamination of the parental magmasthat gave rise to the diorites and granodiorites at Jebel Ohier hadto be of limited extent, given the absence of significant Th enrich-ment in these samples, as well as generally high Ba/Rb ratioswhich are close to the primitive mantle estimate (Hofmann andWhite, 1983).

In more juvenile arc settings, which lack substantially thickenedcontinental-character supra-subduction components, a likely sourceof elevated Th (as well as Zr and Rb) is provided by the meltingincorporation of sediments from the subducting slab (e.g., El-Bialyand Omar, 2015). The moderately to highly fractionated REEand persistent significant Nb and Ti anomalies (in primitive

mantle-normalized plots) in the samples further support theirstrong arc-like geochemical affinity, whereby the degree of fraction-ation is linked to the lower degree of partial melting of the mantlesource as it extends from shallow to greater depth and passesthrough the garnet stability field at ca. 90 km (Pearce and Peate,1995).

A primitive arc setting of the JO Complex is supported by the Ndisotope results. Homogeneous initial ɛNd values at ca. +6.5(Table 3), calculated at 0.70–0.73 Ga (Bierlein et al., 2016), forthe Jebel Ohier intrusive phases indicate juvenile magma sourceseither in sub-arc mantle, or in young mafic-intermediate arccrust. This is consistent with Nd model ages around ~0.9 Ga(Table 3, Fig. 9). The JO isotopic data are similar to those ofReischmann and Kröner (1994) and Johnson and Woldehaimanot(2003) who reported strongly positive initial ɛNd for the GebeitTerrane. The older dacitic unit at Jebel Ohier, estimated to beabout 800 Ma, yields a lower ɛNd and older Nd model (+3.7 and1.15 Ga for sample FBJO-044). Based on major and trace elementdata, this unit cannot be distinguished from the younger intrusivephases but the distinct ɛNd suggests an older source or assimilationof older crust. This is supported by the presence of inherited zir-cons in sample FBJO-044.

Overall, the geochemical data presented here for suites of sur-face samples from the JO area, the Central Massif and the WesternShear Zone are regionally consistent and do not show systematic

A C

D

B

Fig. 6. A) Zr/TiO2 versus Nb/Y diagram (Winchester and Floyd, 1977). B) Rb versus Nb + Y discrimination diagram from Pearce et al. (1984). C) La/Yb over Th/Yb diagram (from Pearceet al., 1984). D) Rb/Zr over Nb diagram (Jin, 1986).

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variations of the type associated with strong variations in magmasource and/or tectonic setting. For the same reason it also seemsunlikely the JO region exposes distinct terranes separated bysutures.

5.2. Timing of magmatic activity at Jebel Ohier

The 800–700 Ma zircon age constraints reported in Bierleinet al. (2016) for the Jebel Ohier granodiorite-dacite complex

A

B

Fig. 7. A: primitive mantle-normalized element diagram showing the range of samples from the Jebel Ohier granodiorite-dacite complex (FBJO-001, -042, -043, -044)compared to the envelope of the same elements in igneous rocks from elsewhere in Block 62, and those of productive porphyries in northern Chile (from Richardset al., 2001). B: chondrite-normalized REE patterns of samples from the ore-forming granodiorite-dacite complex at Jebel Ohier (FBJO-001, -042, -043, -044) com-pared to the envelope of REE patterns of igneous samples from elsewhere in Block 62, and those of productive porphyries in northern Chile (from Richards et al.,2001).

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Fig. 8.Th/Nb over Zr diagram for igneous samples from the JebelOhier Granodiorite-Dacite Complex (GDDC), regional samples and late-stagemafic dykes (LSMD) from JebelOhier; arrowsindicate evolution of co-magmatic pairs where earlier mafic plutons have been intruded by post-collisional granites (from Nicolae and Saccani, 2003).

Fig. 9. Nd isotope evolution diagram for 5 samples analyzed for Sm-Nd in this study. Data from Table 3. For explanation, see text.

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agree well with the established framework for the occurrence andduration of magmatic events in the northern ANS. The geological,geochemical and isotopic data map a protracted evolution of themagmatic arc hosting the Jebel Ohier porphyry Cu-Au deposit.This may be important in the context of the model of Richardset al. (2001) who suggested that a prolonged and intense periodof ‘static’ arc magmatism preceded the eventual formation of sev-eral world-class Tertiary porphyry Cu deposits in the Andes ofnorthern Chile. Likewise, giant porphyry Cu deposits in, for exam-ple, the Melanesian Arc and northeastern New Guinea are alsointerpreted to have resulted from the occurrence of several cyclesof arc magmatism linked to protracted convergence, slab flatten-ing, and intermittent arc reversal (Cooke et al., 2005). The impor-tance of this process, whereby the axis of arc magmatism remainsrelatively static for a drawn-out period of time, lies in the abilityof a considerable volume of modified magma to develop in thelower crust, with associated heating, ascent, upper-crustal pondingand fractionation (e.g., Pitcher, 1997). The hybrid magmasresulting from these processes evolve to increasingly felsic andvolatile-rich compositions, which enables these magmas to captureand concentrate high volumes of volatile-transported metals(e.g., Hildreth, 1981; Richards, 2003).

5.3. Preservation of mineralization

Based on the tectonic setting and nature of the multi-phase por-phyritic intrusive succession, the observed alteration features andsulfide mineralization present at Jebel Ohier, there can be littledoubt that the deposit formed at shallow crustal levels in a nascentmagmatic island-arc setting broadly associated with the subductionof the Mozambique Ocean along a (long-lived) convergent platemargin. A detailed assessment of the tectono-structural evolutionof the study area is beyond the scope of this paper but it is clearthat porphyry Cu-Au mineralization at Jebel Ohier escaped thestrong uplift, erosion and structural dismemberment that is com-monly associated with porphyry-style ore deposits. We proposethat preservation of the Jebel Ohier complex was probably relatedto the relatively rapid accretion of the Gebeit Terrane to the stablecontinental margin of West Gondwana b20 m.y. after the depositformed (e.g., Abdelsalam and Stern, 1996). The emplacement ofgabbroic and dioritic intrusions into the Jebel Ohier granodiorite-dacite complex (e.g. samples FBJO-003, -009) may provide evi-dence for the occurrence of transient crustal extension in responseto (post-mineralization) stress relaxation following an extendedperiod of compression and volcanicity. Paleozoic analogues suchas the giant Au-rich Cadia deposit in the Lachlan Orogen of easternAustralia (e.g. Walshe et al., 1995; Cooke et al., 2005) escaped de-struction via a combination of post-depositional burial during ex-tension, immediate cessation of uplift due to accretion of thehosting magmatic arc to the advancing Proto-Australian continen-tal margin shortly after deposit formation, lack of a terminalcontinent-continent collision, and protection from structural dis-memberment in the absence of major subsequent far-field stressreorientation.

Relatively rapid uplift in response to the opening of the Red Sea sincethe Tertiary (e.g., Bosworth, 2015) is evident from the lack of a morecomprehensively developed supergene blanket at Jebel Ohier, significantsuperficial erosion and the pronounced topographic relief in the studyarea. It remains unconstrained as to howmuch of the shallower portionof the porphyry Cu-Au deposit has been removed since the Tertiary andwhether the system has been thrusted or tilted from its original position,but based on the observed vein abundances, alteration mineralogy andparagenesis, distribution of sulfides and minor-element geochemistry,and preservation of the lithocap with advanced argillic alteration, we es-timate that b1 km of the upper parts of the mineralized system at JebelOhier has been removed by erosion (e.g., Sillitoe, 2010).

5.4. ‘Fertility’ of the Jebel Ohier porphyry system — implications for

mineralization

From the perspective of mineral exploration, an important aspectof the current investigation relates to the mineral potential of theporphyry Cu-Au deposit at Jebel Ohier and whether the causativemagmatic system was capable of generating a major ‘productive’ Cu-Au deposit.

There is good evidence for a link of major porphyry Cu mineraliza-tion with ‘abnormal’ arc volcanism, which is typically triggered by a rel-atively sudden and pronounced change in the tectonic regime(e.g., Sillitoe, 1998; Kerrich et al., 2005; Richards et al., 2013; Correaet al., 2016). These triggers may involve crustal thickening, transientchanges in the angle of subduction, periodic extension, or topographicand thermal anomalies on the down-going slab (Cooke et al., 2005;Loucks, 2014). However, the preservation potential of the causative tec-tonic perturbations, much like that of the resulting porphyry-style oredeposits, is rather low and critical geological evidencewould be difficultto find in a Neoproterozoic porphyry system such as Jebel Ohier. Geo-chemical and isotopic fingerprinting of the igneous rocks associatedwith mineralization therefore remain as the only proxies still availableto investigate the metallogenic potential of the arc environment inwhich the deposit formed.

Given the wide geochemical range of intrusions hosting porphy-ry Cu deposits globally, extracting a set of petrogenetic featureswhich distinguish mineralized from barren or ‘non-productive’ plu-tons is a challenging task (see summary in John et al., 2010). Themost promising features identified so far are variations in REE pat-terns and - more generally - contrasts in the degree of chemicalevolution between fertile and barren intrusions of a given terrane(e.g., Richards et al., 2001; John et al., 2010; Sillitoe, 2010; Asadiet al., 2014). In almost all cases, the ore-forming intrusions tendto be calc-alkaline with high to moderate K2O, reflecting their deri-vation from hydrous and oxidized magmas which developed in re-sponse to slab dehydration and the concomitant transfer of water,sulfur, halogens, LILE (Rb, Cs, K, Ba, Sr) and metals into the mantlewedge (Richards, 2011). The key to this process is the involvementof a metasomatized, metal-enriched asthenospheric wedge in theformation of the fertile magma due to, for example, intermittentsteepening (or flattening) of the subduction angle, oceanic ridgesubduction, slab tear, or lithospheric thinning (e.g., Richards,2003, 2009; Vos et al., 2007; Cooke et al., 2005). The resultingmagmas are (initially) generated at relatively greater depths inthe garnet stability field and, as a result, form intrusive rocks withadakite-like signatures which are characterized by relatively highconcentrations of SiO2 (N56 wt%) and Al2O3 (N15 wt%), low concen-trations of MgO (b3 wt%), Y, and HREE (Y and Yb concentrations ofb18 and b1.9 ppm, respectively), high LILE concentrations, and Srconcentrations of N400 ppm (Defant and Drummond, 1990).

Changes in upper-plate stress, for example due to intermittent de-compression and stress relaxation, allows adakitic magmas, whichhave ponded near the base of the lithosphere, and accompanying as-thenospheric heat anomalies, to ascend into the upper crust, wherethey will undergo further fractionation, assimilation and differentiation(Hildreth, 1981; Pitcher, 1997; Richards et al., 2001; Hou et al., 2004;Cooke et al., 2005). A genetic link between adakitic magmas and por-phyry mineralization in continental margin–arc and post-subductionsettings has been inferred by several authors (e.g., Thiéblemont et al.,1997; Richards, 2003, 2011; Sun et al., 2012), although by no meansall mineralized porphyries are adakitic and the link remainscontroversial (Richards and Kerrich, 2007). This is because rockswith adakite-like compositions can also be produced by assimilationand fractional crystallization (AFC), the melting of delaminatedlower crust, or partial melting of the lower crust. We note thatigneous rocks from the Jebel Ohier region have highly variable Sr-Ysystematics (Fig. 10a), and many (but not all) would be classified

150 F.P. Bierlein et al. / Ore Geology Reviews 79 (2016) 133–154

as adakitic on this basis. Therefore, Sr/Y is not a useful fertility indi-cator in this case.

Another potentially useful discriminator between ‘productive’ and‘unproductive’ magmatic rocks is the Y/Mn ratio, which is inferred tobe controlled by involvement of hydrous phases during the early evolu-tion of the magma (Baldwin and Pearce, 1982; Richards et al., 2012).However, this ratio is not useful at JO. While most of the samples fromthe JO granodiorite-dacite complex plot within the field of ‘productive’

porphyries, some do not, and there are no consistent differences be-tween JO and regional samples (Fig. 10b). Themost consistent chemicaldifference between the magmatic rocks at JO and those in the widerarea (JO regional) are the REE patterns (Fig. 7b), notably the muchlower HREE concentrations and almost complete lack of Eu depletionsin the rocks associated with the JO deposit. The REE systematics atJebel Ohier resemble those of ore-related Tertiary porphyries from theEscondida area in northern Chile (Richards et al., 2001; Fig. 7b). The

A

B

Fig. 10. A) Sr/Y over to Y diagram for samples from the Jebel Ohier granodiorite-dacite complex (GDDC), the broader Jebel Ohier area, regional samples from elsewhere in Block 62, andlate-stagemafic dykes (LSMD). Fields of adakite and typical arc (i.e. ‘normal’ arc andesite, dacite and rhyolite) from Johnet al. (2010). B) Y vsMnO ratios of the same sample groupings as anindication of the perceived metal potential of the igneous rocks analyzed herein (discriminators from Baldwin and Pearce, 1982).

151F.P. Bierlein et al. / Ore Geology Reviews 79 (2016) 133–154

lack of Eu depletions and generally low middle to HREE in ‘productive’porphyries compared to other intermediate to felsic arc-related igneousrocks was attributed by Richards et al. (2012) to hornblende fraction-ation rather than plagioclase accumulation. Strong HREE fractionationand lack of pronounced Eu anomalies in all of the analyzed JO rocksalso suggests that plagioclase fractionation may have been suppresseddue to high water contents or a high magmatic oxidation state (e.g.,Richards et al., 2012). Rohrlach and Loucks (2005) showed that theore-forming dacitic magmas that produced the giant Tampakan high-sulfidation and porphyry Cu (-Au) deposits in southern Mindanao(Philippines) had accumulated approximately 8 wt% dissolved H2O byinheritance through several cycles of magma-chamber replenishmentand fractional crystallization. As illustrated by Loucks (2014), suchhigh dissolved H2O contents and/or higher total pressure will shift therelative positions of plagioclase and titanomagnetite and hornblendein the crystallization sequence to the point where hornblende produc-tion is advanced and enhanced at the expense of plagioclase production,which is delayed and diminished because hornblende consumesplagioclase-forming components of the melt. Furthermore, thehornblende-bearing granodiorite porphyries and dacites of the JebelOhier ore-forming porphyry complex are relatively depleted in mostof the LILE, Th, Nb, Y and P when compared to primitive mantle-normalized concentration of samples from elsewhere in the studyarea. These elements partition strongly into hornblende, but not intoplagioclase, and therefore undergo strong depletions from residualmelt as hornblende crystallizes (Loucks, 2014). In this regard, the ore-forming granodiorite-dacite porphyry complex at Jebel Ohier sharesthe same characteristics as most ore-related porphyries in thePeruvian-Chilean belt, those in China, the Mongolian orogenic belt, theSW Pacific region, SE Iran, the Western United States and northernMexico (John et al., 2010 and references therein).

None of the above mentioned features should be considered diagnos-tic in isolation, but when used in conjunction with other compositionalfeatures, they indicate that the porphyry system responsible for Cu ±Au mineralization at Jebel Ohier was derived from a distinct, calc-alkaline magma source characterized by high oxygen fugacity and rela-tively high water contents. Hydrous subduction-related magmas demon-strably are responsible for the vast majority of productive porphyriesglobally (e.g., Richards, 2003; Cooke et al., 2005; Sillitoe, 2010). As such,the chemical composition of the magmatic rocks associated with theJebel Ohier porphyry Cu ± Au deposit strongly supports the notion thatthe mineralized system is associated with a ‘productive’ pluton. Furtherdrilling is required to ascertain the magnitude of the (preserved) miner-alization at Jebel Ohier. As is so often the case, serendipity (in the formof a post-depositional history that ensured cessation of uplift and struc-tural dismemberment) might have played a significant role in the pres-ervation of the Jebel Ohier porphyry Cu-Au deposit. However, thechemical signatures of the magmatic rocks discussed above raisehopes that lithogeochemical criteria could provide a practical explora-tion guide for the purpose of targeting additional porphyry copper de-posits in permissive terrains elsewhere in Block 62 and the ANS, ageological region previously considered ‘too old’ by mostexplorationists and as such, unlikely to preserve significant porphyrycopper-gold mineralization.

6. Conclusions

Porphyry Cu-Au-forming intermediate magmatism at Jebel Ohierwas the result of supra-subduction processes in an intra-oceanic arc sys-tem without significant contributions from recycled continental-character sources. Protracted accretionary processes led to the forma-tion of progressively fractionated plutonism to form relatively hydrous,hornblende-bearingmelts. Thesewere injected into a significantly olderintrusive-extrusive complex after having undergone volatile saturation,which enabled the porphyritic granodiorite-dacitic melts to collect andconcentrate ore constituents. The tectonic trigger giving rise to the

ascent of the fertile magmas at Jebel Ohier remains obscure, but a fortu-itous conjunction of tectonic circumstances involving, possibly, 1) rapidaccretion of the arc system to a stable continental margin, 2) cessationof uplift, 3) burial due to post-collisional extension, and 4) protectionfrom post-depositional structural dismemberment, enabled theNeoproterozoic magmatic arc segment to remain intact and preserve alarge portion of the porphyry Cu-Au system. Petrogenetic fingerprintingprovides a reliable tool to distinguish the ‘anomalous’ plutonic-volcanicsuccession at Jebel Ohier from barren analogues and should be consid-ered when targeting metalliferous hornblende- or biotite-porphyritichypabyssal intrusions elsewhere in the ANS.

Acknowledgements

This study is published with the permission of QM and QMSD exec-utive management, and the Sudanese Ministry of Minerals. Technicaland support staff at QMSD are thanked for their invaluable assistancein the office and during field work. Many discussions with I. Davies(QM) have helped to clarify many aspects of this study. P Kumar (QM)is thanked for assistance with drafting. We are grateful to CSA Globalstaff (especially W Potma and C. Brauhart) for their input into ongoingwork at Jebel Ohier. Comments by H Frimmel on a draft version of themanuscript are acknowledged, as are constructive assessments by D.Wyman and one anonymous reviewer. F. Pirajno's tireless editorial ef-forts and efficient handling of this paper were also greatly appreciated.

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