ore geology reviews · mengler, 2000). structures associated with these deposits such as folds,...

Ore Geology Reviews 69 (2015) 268–284

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Ore Geology Reviews

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Regional dome evolution and its control on ore-grade distribution:Insights from 3D implicit modelling of the Navachab golddeposit, Namibia

Stefan A. Vollgger a,⁎, Alexander R. Cruden a, Laurent Ailleres a, E. Jun Cowan b

a School of Earth, Atmosphere and Environment, Monash University, Australiab Orefind Pty Ltd, Fremantle, Australia

⁎ Corresponding author at: School of Earth, AtmosphUniversity, Clayton, VIC 3800, Australia.

E-mail addresses: [email protected] (S.A. Vollgg(A.R. Cruden), [email protected] (L. Ailleres), j(E.J. Cowan).

http://dx.doi.org/10.1016/j.oregeorev.2015.02.0200169-1368/© 2015 Elsevier B.V. All rights reserved.

a b s t r a c t
a r t i c l e i n f o
Article history:Received 28 November 2014Received in revised form 20 February 2015Accepted 20 February 2015Available online 21 February 2015

Keywords:FoldingOre shootsImplicit modelling3D deposit modellingFold inflection linesOrogenic gold

We introduce a novel approach to analyse and assess the structural framework of ore deposits that fully inte-grates 3D implicit modelling in data-rich environments with field observations. We apply this approach to theearly Palaeozoic Navachab gold deposit which is located in the Damara orogenic belt, Namibia. Compared to tra-ditional modelling methods, 3D implicit modelling reduces user-based modelling bias by generating open orclosed surfaces from geochemical, lithological or structural data without manual digitisation and linkage of sec-tions or level plans. Instead, a mathematically defined spatial interpolation is used to generate 3D models thatshow trends and patterns that are embedded in large drillhole datasets. In our 3D implicitmodel of theNavachabgold deposit, distinctive high-grade mineralisation trends were identified and directly related to structures ob-served in thefield. The 3D implicitmodel andfield data suggest that auriferous semi-massive sulphide ore shootsformed near the inflection line of the steep limb of a regional scale dome, where shear strain reached peak valuesduring fold amplification. This setting generated efficient conduits and traps for hydrothermal fluids and associ-ated mineralisation that led to the formation of the main ore shoots in the deposit. Both bedding-parallel andhighly discordant sets of auriferous quartz-sulphide veins are interpreted to have formed during the later lock-up stage of the regional scale dome. Additionally, pegmatite dykes crosscut and remobilise gold mineralisationat the deposit scale and appear to be related to a younger joint set.We propose that kilometre-scale active foldingis an important deformationmechanism that influences the spatial distribution and orientation ofmineralisationin ore deposits by forming structures (traps and pathways for fluids) at different preferred sites and orientations.We also propose that areas that experience high shear strain, located along the inflection lines of folds can act aspreferred sites for syn-deformational hydrothermal mineralisation and should be targeted for regional scale ex-ploration in fold and thrust belts. Our research also suggests that examination of existing drillhole datasets using3D implicit modelling is a powerful tool for spatial analysis of mineralisation patterns. When combined withfieldwork, this approach has the potential to improve structural understanding of a variety of ore deposits.

© 2015 Elsevier B.V. All rights reserved.

1. Introduction

Ore deposits are economic concentrations of chemical elements thatresult from a number of processes that occur within the Earth's crustand deliver elements from a source region to a deposit location. Theseprocesses can be chemical and/or physical and they control the libera-tion, transport, focusing and trapping of hydrothermal fluids (Cox andEtheridge, 1987; Groves et al., 2000; Ridley, 1993; Robb, 2004;Tomkins, 2010). Most genetic models of gold deposits are ascribed to

ere and Environment, Monash

er), [email protected]@orefind.com

such fluids because of their ability to transport gold and related compo-nents (Phillips and Powell, 2010). Ore deposits form where hydrother-mal fluids are either chemically (e.g., fluid mixing, host rock reactions,pH change) or physically trapped (e.g., rapid change of pressure ortemperature). The importance and efficiency of these two trappingmechanisms is still under debate. However, abundant research hasshown that many gold deposits are hosted in deformed rocks, suggest-ing a strong physical (structural) control on mineralisation. Famousexamples are the lode gold deposits of the Victorian goldfields (Coxet al., 1991, 1995; Leader et al., 2012; Mueller, 2014; Willman, 2006;Willman et al., 2010; Wilson et al., 2013) and the Yilgarn Craton, West-ern Australia (Cox and Ruming, 2004; Groves et al., 1995, 2000;Hodkiewicz et al., 2005; Micklethwaite and Cox, 2006; Ridley andMengler, 2000). Structures associated with these deposits such asfolds, faults, veins and shear zones highlight the importance of a

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269S.A. Vollgger et al. / Ore Geology Reviews 69 (2015) 268–284

changing history of brittle and ductile deformation for economicmineralisation. These changes are important because they influencefluid flow and control the formation of dilational sites that can act astraps for hydrothermal fluids (Leader et al., 2011). It follows that theshape, size, orientation and spatial distribution of ore bodies contain im-portant information about the deformation history of an ore deposit andthe associated interactions between host rock types, fluid flow andstructures (Blenkinsop and Kadzviti, 2006). Work by Blenkinsop(2004) on two lode gold deposits from the Zimbabwe craton, studiesby Bauer et al. (2014) on VMS deposits in the Swedish Skellefte district,3D modelling and analysis of nickel sulphide shoots at the Kambaldadome (Western Australia) by Stone et al. (2005) and investigations onthe vein-hosted gold ore body at Sunrise Dam (Western Australia) byHill et al. (2013) demonstrate that shapes and orientations of ore bodiesor mineralised zones contain important information about the evolu-tion of ore deposits. The analysis of structures that transport and traphydrothermal fluids is therefore of major scientific and economicsignificance. By understanding the genetic relationship between(mineralised) structures and associated physical processes, we canpotentially predict areas with a greater potential for mineralisation insimilar tectonic environments. In this paper, we present a structuralinterpretation workflow that uses 3D implicit modelling to visualiseand analyse geochemical and geological trends from an extensivedrillhole dataset from the Navachab gold deposit, Namibia (providedby AngloGold Ashanti Ltd). Such drillhole data are normally used tocompute block models for the economic assessment of a project. Foreach block of these models, important key values and properties are ei-ther assigned or calculated, such as net present value, metal content,density or rock type. Block models are powerful and well-establishedtools for resource estimation, grade control and scheduling, where themain purpose is to evaluate, maximise and optimise the economics ofa project. However, they are less valuable for understanding the geologyand structural framework of a deposit.

Recent developments in geological modelling software (e.g., 3Dimplicit modelling) allow for the precise and objective generation ofspatial representations of lithological contacts, mineralisation (gradeshells) and structural trends in data-rich environments. Instead of labo-riousmanual cross-section digitisation, spatial interpolations of geolog-ical data are employed to producemodelswithminimised user-inducedmodelling bias. In our case study of theNavachab gold deposit, initial 3Dimplicitmodellingwas followed by structuralfieldwork and data collec-tion in the Main Pit and Eastern Zone 1 of the mine. Field observationsand structural data were compared to the orientation, size and spatialdistribution of implicitly modelled ore bodies and host rock stratigra-phy. Additionally, a 3D implicit structural trend model computed fromregional bedding orientation measurements was used to calculate andvisualise the spatial distribution of shear strain related to the ampli-fication of a regional scale domal structure. This structural trendmodel allowed us to evaluate the regional structural context ofgold mineralisation, suggesting that auriferous ore shoots withinthe Navachab deposit were controlled by the regional-scale, non-cylindrical folding responsible for the formation of the Karibib dome.

Our case study illustrates how 3D implicit modelling in conjunctionwith selectivefieldwork can aid in the extraction of additional structuraland ore-genetic information from existing datasets. At the Navachabgold deposit, it improved and extended understanding of the structuralframework that controlled syn- and post-deformational mineralisation,pointing towards a strong relationship with regional scale folding.

2. Regional geology

The Navachab gold deposit is situated approximately 10 km SW ofthe town of Karibib (Namibia). The current open pit activities representthe only fully operating gold mine in Namibia. However, in recent yearsnew discoveries of gold mineralisation have been reported approxi-mately 20 km and 150 km NE of the present mine site (Helio

Rescources, 2012; Lombard et al., 2013) as part of B2Gold's Otjikoto pro-ject (formerly operated by AuryxGold) and Damara Gold Corp.'s GoldKop target (formerly Helio Resources). These have both been describedas having a similar style of mineralisation to that at the Navachab golddeposit.

TheNavachab gold deposit is located in the S part of the Central Zoneof the Damara belt, also known as the Southern Central Zone. The Cen-tral Zone is one of several tectonostratigraphic domainswithin the Pan-African Damara orogen (Miller, 1983) (Fig. 1). The majority of mineraldeposits are located in the Central Zone and are associated with rocksthat have been subjected to medium- and high-grade metamorphism(Pirajno and Jacob, 1991).

The Damara belt extends to the NE and links with the Lufillianorogen in Zambia–Angola–Congo (Pirajno, 2008). The term Pan-African orogeny is commonly used to describe magmatic, tectonic andmetamorphic activity during the Neoproterozoic (~870 Ma) to earlyPalaeozoic (~550 Ma) orogenic episode (Kröner and Stern, 2004).

The tectonic evolution of the Damara belt started with the initiationof an intracontinental, asymmetric rift between the Congo craton andthe Kalahari craton during the break-up of Rodinia around ~780–750 Ma (Barnes and Sawyer, 1980; Miller and Frimmel, 2009; Milleret al., 2009). The sedimentary fill of the newly opened graben in the S(Nosib Group) exceeds 6 km in thickness and consists of feldspathicsandstones, playa lake and evaporitic sabkha deposits (Miller et al.,2009). Rifting led to siliciclastic and carbonate sedimentation beforeand during the subsequent formation of an oceanic basin (Khomassea). The final width of the Khomas sea is estimated to about 1200–1500 km (Miller et al., 2009). The Matchless Amphibolite is the meta-morphosed remnant of oceanic crust (sea-floor basalt) which formedduring that time, and is host to several VMS and (epigenetic) copper de-posits (Breitkopf and Maiden, 1988).

Reversal of plate motion and formation of a N dipping subductionzone led to the closing of the Khomas sea and the collision of theKalahari craton in the S with the Congo craton in the N around540 Ma to 520 Ma. Convergence culminated in the formation of theCambrian supercontinent Gondwana and caused thrusting and foldingwithin the Damara belt. In the Karibib area, the thrusting and foldingat 560–540 Ma is related to an early phase of crustal thickening owingto continent–continent collision (Longridge, 2012).

Navachab is located in deformed, primarily metasedimentary rocks(calcsilicates, marbles, schists and metapelites) of the several km thickNeoproterozoicDamara Supergroup,which isfloored byMesoproterozoic(~2.0 to ~1.7 Ga) gneisses of the Abbabis Metamorphic Complex (Jacobet al., 1978; Kisters, 2005). Syn- and post-tectonic granitic magmatismis widespread and voluminous in the Damara belt and is reflected aspost-tectonic thermal peaks in thermochronological data (Miller andFrimmel, 2009; Tack and Bowden, 1999). Slab breakoff caused astheno-spheric upwelling and heating of the lower crust and provided heat forthe metamorphism and melting of the crust around 540–535 Ma to pro-duce anatectic red granite (Longridge, 2012). Several diorite andleucogranite intrusions are located within 5 km of the Navachab deposit,but there is no direct contact between the mineralised system and anymajor intrusive body (Steven et al., 2011).

Metamorphic conditions within the Southern Central Zone decreasetowards the NE (along strike) from granulite facies (P ~ 5 kbar, T ~ 700–800 °C, Nex et al., 2001; Masberg, 2000; Masberg et al., 1992) with par-tial melting (Toé et al., 2013) in the SW coastal regions to loweramphibolite-facies (P ~ 3 kbar, T ~ 560–650 °C, Puhan, 1983; Steven,1993) in the Karibib district, as progressively shallower stratigraphiclevels are exposed (Kisters et al., 2004). The temperature-dominatedmetamorphism in the Karibib district has been associated with periodsof voluminous intrusions (Jung and Mezger, 2003; Miller, 1983), whichled to pervasive recrystallisation and obliteration of tectonic fabrics, espe-cially in marbles throughout the Southern Central Zone (Kisters, 2005).

The Southern Central Zone is bound by two roughly parallel, NWtrending, major lineaments which coincide with large-scale facies

Fig. 1.Geological map of Namibia showing tectonostratigraphic domains of the Damara belt after Miller (1983). The Navachab gold deposit is locatedwithin the Southern Central Zone ofthe Panafrican Damara belt, between the Omaruru Shear Zone in the N and the Okahandja Shear Zone in the S.Data source: Geological Survey of Namibia.

270 S.A. Vollgger et al. / Ore Geology Reviews 69 (2015) 268–284

changes (Miller, 1983). The Okahandja lineament (Okahandja shearzone) was the locus of continental break up and represents the Nbounding fault of the S graben during rifting as well as the leadingedge of the overriding Congo craton during convergence (Miller et al.,2009, 2010). The Omaruru lineament (Omaruru shear zone) forms theS boundary of the N graben and has been interpreted to have acted asa growth fault throughout the spreading phase, associated with substan-tial sedimentary-exhalative activity (Miller et al., 2009). The Navachabdeposit lies between these two first-order, major crustal structures(Fig. 1).

Elongated, kilometre-scale NE trending domal structures define theregional structural grain of the Southern Central Zone. One such dome isthe Karibib dome, which hosts the Navachab gold deposit at its steepNW limb, formed by SE-NW shortening (Fig. 2). Kisters et al. (2004) sug-gested that a blind thrust led to the formation of the Karibib Dome as atip-line fold. The moderately steep dipping S limb of the Karibib dome isoverridden from the SE by the Mon Repos thrust zone, where top-to-the-NWmovement pushed crystalline gneisses of the Abbabis Metamor-phic Complex over the younger Damara Sequence (Kisters et al., 2004).SHRIMP U–Pb dating of single zircons from syn-kinematically intruded,foliated diorites and granodiorites (Mon Repos diorite-granodiorite,546 +− 6 Ma and 563 +− 4 Ma) and post-tectonic, undeformedmonzogranites (Rote Kuppe granite, 543 +− 5 Ma) has provided con-straints on the timing of this regional thrusting event (Jacob et al., 2000;Kisters et al., 2004).

3. Main characteristics of the Navachab gold deposit

The Navachab gold deposit has been owned and operated byAngloGoldAshanti Ltd since 1998, butwas acquired byQKRCorporationLimited in 2014 (G. Bell 2014, pers. comm.). At the end of 2012,Navachab had gold mineral resources of 4.4 Moz and reserves of 2.1Moz. The mine produced 46,000 oz of gold in the first nine months of2013. The Navachab gold deposit can be divided into three mainareas: Main Pit, North Pit and Eastern Zone 1 (Fig. 3).

3.1. Lithostratigraphy

The Navachab gold deposit is hosted within NE striking, steeply NWdipping amphibolite facies marbles and schists of the Damara Super-group, which form the NW limb of the kilometre-scale Karibib dome(Fig. 2). The Karibib dome is a shallowly doubly-plunging, NW vergingasymmetric antiform with a length of about 12 km and a width of ap-proximately 4 km. The oldest rocks in the area around the Navachabgold deposit are Palaeoproterozoic gneisses, schists, amphibolites andpegmatitic rocks of the Abbabis Methamorphic Complex (Jacob et al.,1978; Tack and Bowden, 1999), which are exposed in the core of theKaribib dome (Kisters, 2005), but do not crop out at the Navachabmine. The Damaran metasedimentary rocks unconformably overliethe Abbabis Metamorphic Complex starting with arkoses, quartzites,conglomerates and minor metavolcanic rocks of the Etusis formation

Fig. 2. Simplified geological map of the Usakos and Karibib domes after Kisters (2005) and Kisters et al. (2004). Note that the Navachab gold deposit is locatedwithin rocks of the DamaraSupergroup at the NW limb of the Karibib dome. The Karibib dome is overridden by the Mon Repos thrust zone to the S.


(Nosib Group), representing the Damaran rift succession (Miller, 1983).The up to 1500 m thick Etusis formation can be observed as dominantridges to the SE of the Navachab gold deposit (Kitt, 2008), but is onlyexposed within the oxidised zone of the Eastern Zone 1 at the minesite. It consists of highly oxidised and weathered, interbedded layersof sandstone and siltstone. Stratigraphically higher is the Chuos forma-tion, which also crops out in Eastern Zone 1 and consists mainly ofmetaquartzites in the upper part and a diamictite in the lower part.These have been interpreted as glaciomarine in origin and related to thelate Neoproterozoic (746 +− 2 Ma) Sturtian glaciation (Hoffmannet al., 2004). Finely distributed, bedding-parallel sulphides (mainly pyr-rhotite) have been observed in the Chuos formation. The Chuos formationis overlain by the biotite-schist and calcsilicate rock dominated Spes Bonaformation. The contact of the Chuos formation with the overlying SpesBona formation has been mapped in Eastern Zone 1. The Spes Bona for-mation has a true thickness of up to 220 m and is overlain by the up to160 m thick Okawayo formation, which mainly consists of marbles andalso hosts the auriferous semi-massive sulphide ore shoots at its footwallcontact. This basal part is dominated by a cm to dm banded sequence ofcalcsilicate rocks andmarbles. Theuppermost part of theOkawayo forma-tion comprises greyish massive and brecciated (dolomitic) marbles. Thelatter have been interpreted to be sedimentary breccias (Kitt, 2008).

Overlying the Okawayo formation are metasedimentary and minorcalcsilicate rocks of the Oberwasser formation, which is up to 180 mthick in the Main Pit. The Oberwasser formation is compositionallyvery similar to the Spes Bona formation.

The marble-dominated Karibib formation lies on top of theOberwasser formation and is followed bymetapsammitic andmetapeliticrocks of theKuiseb formation. The latter represents theuppermost forma-tion of the Damara Supergroup, but it does not crop out at the Navachabmine.

The Okawayo formation is intruded by meta-lamprophyre sheetsand dykes. U–Pb SHRIMP analyses of titanite grains from a meta-lamprophyre dyke yielded an age of 496+− 12Ma,which probably re-cords a metamorphic overprint (Jacob et al., 2000). At the Navachabgold deposit, the youngest igneous activity occurs as bedding-paralleland mainly highly discordant, subvertical pegmatite dykes of unknownage.

3.2. Gold mineralisation

Mineralisation at the Navachab gold deposit is associated with pyr-rhotite and minor amounts of pyrite, chalcopyrite, arsenopyrite, sphal-erite, maldonite, bismuthinite, native bismuth and bismuth tellurides(Dziggel et al., 2009; Nörtemannet al., 2000;Wulff et al., 2010). Gold oc-curs mainly as native gold, but is also present in minerals such asmaldonite (Au2Bi) and as a solid solution with bismuth (Nörtemannet al., 2000). Stable isotope (O, H, C, S) analyses by Wulff et al. (2010)suggest that gold precipitated from a metamorphic, mid-crustal fluidderived fromDamaranmetapelites that underwent progrademetamor-phism under amphibolite (ca. 550 °C and 2 kbars) to granulite-faciesconditions.

At the Navachab gold deposit, mineralisation occurs in two mainstyles: 1) bedding-parallel semi-massive sulphide bodies, referred toas ore shoots (or massive skarns, Miller, 1983), and 2) bedding-parallel and highly discordant sets of auriferous quartz-sulphide veins.The ore shoots are discrete, elongate zones within a planar contact sur-face that contain higher metal contents than the adjacent parts of thehost rock (Peters, 1993a). The two known ore shoots in the Navachabgold deposit are economically defined by relatively high gold grades of3–7 ppm (Steven and Badenhorst, 2002). One major ore shoot (mainshoot) is located in the Main Pit, the second, smaller and less extensive

Fig. 3.Digital elevationmodel (DEM) of the Navachab goldmine site, which is divided into theMain Pit, North Pit and Eastern Zone 1. In this study, fieldwork focused on high-grade areas(hot colours) within the Main Pit and Eastern Zone 1. See Figs. 6 and 8 for cross section A.


ore shoot (2nd shoot) is located at a structurally higher level and ismined in the North Pit. Both ore shoots are defined by numerous,mineralised semi-massive sulphide lenses that can be up to 5 m highand 1.5 m wide in cross-section (Kisters, 2005). The semi-massive sul-phide lenses are aligned to form a shallowly NNE plunging, high-gradegold trend with a down-plunge extent of at least 1800 m. The semi-massive sulphide lenses are elongated to typically rhomb-shaped bodiesthat are parallel to the compositional layering and locally show evidencefor hydraulic brecciation (Wulff, 2008). The semi-massive sulphide lensesconsist of pyrrhotite (up to 50 vol.%), garnet (10–30 vol.%), clinopyroxene(5–10 vol.%, mainly diopside), minor chalcopyrite, sphalerite, bismuth,bismuthinite, bismuth-tellurides, arsenopyrite, as well as quartz, biotiteand K-feldspar (Dziggel et al., 2009; Nörtemann et al., 2000).

The ore shoots are located in the marble and calcsilicate rock domi-nated, approximately 30 m thick MC unit at the base of the Okawayoformation, near the contact with the underlying (partly silicified)biotite-schist rich Spes Bona formation. Studies of the calcsilicate min-erals (Dziggel et al., 2009; Wulff, 2008) as well as structural evidence

(Kisters, 2005) support a relatively late stage and alteration-related or-igin for the most of the calcsilicate banding within the MC unit.

The second style of gold mineralisation is hosted in flat-lying, verti-cally stacked, quartz-sulphide veins that crosscut the subverticalbedding of the host rocks at high angles. In marble host rocks, quartz-sulphide veins can mostly comprise pyrrhotite with minor quartz,whereas in biotite schist and banded calcsilicate host rocks quartz-sulphide veins mainly consist of quartz with up to 30 vol.% sulphideminerals such as pyrrhotite (Dziggel et al., 2009). Additionally, a rela-tively subordinate quartz-sulphide vein set occurs parallel to bedding,particularly in well-bedded units such as the Oberwasser Formationand Spes Bona Formation (Kisters, 2005).

Several genetic models for mineralisation at the Navachab gold de-posit have been suggested. The close spatial and temporal associationof the deposit to Pan-African granitoids (Nörtemann et al., 2000;Pirajno and Jacob, 1991), the presence of skarn-type alteration assem-blages, as well as an unusual metal association of Au–Bi–As–Cu–Ag(Dziggel et al., 2009) have been used to suggest an origin as an


intrusion-related gold deposit (Thompson et al., 1999). However, recentstructural studies by Kisters (2005), Kitt (2008), and Creus (2011) andgeochemical investigations by Wulff et al. (2010) point to formationas an orogenic gold deposit.

3.3. Structural history

Three main phases of deformation have been described in the Cen-tral Zone of the Damara belt (Barnes and Sawyer, 1980) affecting bothpre-Damaran basement (Abbabis Metamorphic Complex) and themetasedimentary rocks of the Damara Supergroup. However, only twomain deformation phases have been recorded in Damaran rocks withinthe Navachab deposit. The oldest deformation is expressed by abedding-parallel or bedding-subparallel S1 foliation defined by the pre-ferred orientation of mica in metasedimentary rocks, visible within theSpes Bona and Oberwasser formations. Flattened clasts in diamictites ofthe Chuos formation observed in Eastern Zone 1 are also parallel to S1.Transposition of bedding (S0) into the S1 fabric impedes a full character-isation of S1. However, as also observed by Kisters (2005), rare isoclinalintrafolial folds mapped in banded marbles and calcsilicates of theOkawayo formation within the Main Pit indicate that S1 is a compositeS0,1 fabric. S1 is most probably related to an early D1 low angle thrustingevent (Kisters, 2005).

The main deformational phase is D2, which is characterised by up-right, km-scale NE trending folds with doubly-plunging fold axes,such as the Karibib dome. The NE striking axial planar foliation (S2) ofthe Karibib dome dips steeply SE and indicates an asymmetric, NWverging fold geometry that formed during NW-SE shortening. Conse-quently, the NW limb of the Karibib dome dips more steeply than theSE limb.

4. 3D geological modelling

Selecting an appropriatemodelling techniquenot only depends on thegeological complexity and the available data (Guillen et al., 2008), but alsoon the intended function or purpose of the model (Stachowiak, 1973).The two main methodologies used for 3D geometrical modelling of oredeposits, termed explicit and implicit modelling (Cowan et al., 2003;Jessell et al., 2014; Ledez, 2003) are reviewed briefly below.

Before the advent of modern software and hardware, geologicalfindings were hand drawn and stored on paper (2D) as sections andmaps. Even though routinely transferred into digital formats, such sec-tions and maps still form the foundation of traditional 3D explicit geo-logical models (Cowan et al., 2003). Explicit models rely on manualdefinition of geological boundaries by digitisation of sections that arelater linked to generate 3D bodies and surfaces. The data are storeddigitally and can be visualised in 3D, which provides major advantages(Vollgger and Kassl, 2010). It does, however, still underutilise theinherent possibilities of 3Dmodelling. The basic concept of thismethod-ology emerged out of computer aided design (CAD) applications. CADprogrammes were originally developed for industrial design purposes,where the manual (explicit) drawing approach is appropriate becausethe shape anddimension of objects are precisely defined andknownbe-forehand. However, this does not necessarily apply to 3D geologicalmodelling in data-rich and geometrically complex geological environ-ments. Themanual definition of each elementmakes explicit modellingworkflows time consuming and also introduces a strong bias inheritedfrom geological interpretation during digitisation and linkage of cross-sections, resulting in unique and non-reproducible models (Cowanet al., 2002, 2003, 2011; Jessell et al., 2010; Lindsay et al., 2012, 2013a,b). Additionally, manual linking of cross-sections with complex geome-tries regularly forces the modeller to adapt and simplify the model tostay within a practical timeframe (Cowan et al., 2003), hence inhibitingthe graphical representation of the full structural and geological com-plexity of the ore deposit. In summary, themanual approach used in ex-plicit geological modelling is a subjective, time-intensive and a non-

reproducible process in which geological interpretation is inheritedfrom the outset; hence its application for purposes of structural inter-pretation must be viewed with caution.

Alternatively, implicit modelling is a technique where surfaces andvolumes (=closed surfaces) are not explicitly defined, but mathemati-cally described as isovalues of a volumetric scalar field (Calcagno et al.,2008; Carr et al., 1997; Caumon et al., 2013; Frank, 2006; Jessell et al.,2014; Maxelon et al., 2009; Mcinerney et al., 2005) (Fig. 4). Scalar fieldsare computed based on a global interpolation function (e.g., radial basisfunction; RBF) that is fitted to data points. Early applications of the RBFinterpolation scheme were employed by Hardy (1971) to calculate ele-vation contour lines based on irregular topographical data. However,the main development of this scalar field-based technique originatedfrom developments in computer graphics, where the aim was to recon-struct solids and surfaces from scattered point data for applications intomography, LiDAR, animation and visualisation of complex shapes(Carr et al., 1997, 2001; Savchenko et al., 1995; Turk and O'Brien,2002). The first time scalar-field basedmodelling was used in a geolog-ical context was by Lajaunie et al. (1997), who referred to it as implicitform and used it to spatially interpolate structural data. Another geolog-ical application of this interpolation method was by Billings et al.(2002), who used so-called continuous global surfaces to interpolategeophysical datasets. Cowan et al. (2002) first introduced rapid geolog-ical modelling using Leapfrog, a software package that employs RBFs tospatially interpolate large drillhole datasets to build 3D geologicalmodels of ore deposits. This RBF-based interpolation is computationallyexpensive and was initially considered to be impractical (Sibson andStone, 1991). However, mathematical advances resulted in the abilityof RBFs to rapidly interpolate large datasets (Beatson et al., 1999).Cowan et al. (2003) introduced the term implicit modelling to themin-ing industry and highlighted the advantages that arise from an implicit-ly defined 3D model compared to traditional explicit 3D modellingmethods that rely on manual digitisation. Other implicit modellingschemeshave been referred to as implicit surface representation, poten-tial field interpolation or potential field method (not to be confusedwith geophysical modelling of potential field data) in subsequent publi-cations (Calcagno et al., 2006; Chilès et al., 2004; Lane et al., 2007; Ledez,2003; McInerney et al., 2007; Mcinerney et al., 2005).

The proliferation of terminology for 3D geological modellingmethods that share a commonmathematical approach has led to an in-consistent understanding of the term implicit modelling. To clarify, inimplicit modelling, an implicit function (generic format f(x, y, z) = 0)is computed and fitted to a set of spatial data (points, lines, structuraldata) (Fig. 4a). This implicit function describes a volumetric scalarfield, which assigns a scalar value to every point in space (Fig. 4b).Isovalues of this field can be extracted and used to define open or closed3D isosurfaces, which can be envisaged as 3D analogues of 2D contourlines (Fig. 4c). Since the term implicitmodelling refers to themathemat-ical description of a surface, it should only be applied in geologicalmodels that comprise surfaces obtained from a scalar field. To visualisethese surfaces an independent step, that commonly includes themarching cubes algorithm (Lorensen and Cline, 1987), is necessary togenerate triangulated meshes that can be rendered on a screen. Theo-retically, a surface of any resolution could be extracted from an implicitfunction. However, practical and computational limitations will impactthe final resolution of the rendered surface.

In general, implicitmodelling can be used to spatially interpolate nu-merical (e.g., assay) and non-numerical (e.g., lithology) data points todefine scalar fields. In the case of non-numerical data so-called signeddistances are calculated for each data point, which describe the distanceto a surface of interest (e.g., the contact between two geological forma-tions) (Carr et al., 2001; Cowan et al., 2003). This boundary is then rep-resented by an isosurface that smoothly fits all data points with a signeddistance value of 0. In addition to isosurfaces based on lithological data,geochemical data (assay data) can be used to compute isosurfaces (re-ferred to as grade shells) at various threshold grade values. Moreover,

Fig. 4. 3D implicit models are constructed using isosurfaces (c) that are derived from a scalar field (b). This scalar field is defined by the spatial interpolation of points, lines, and structuralmeasurement data (a). The latter influence the gradient of the scalar field and control the shape and orientation of the associated isosurface.


when dealingwithmulti-element datasets, various grade shells for eachelement can be rapidly computed and visualised.

Implicitmodelling also permits the direct integration and processingof planar structural measurements, producing structural isosurfaces byadjusting the gradient of the scalar field, as described in detail byHillier et al. (2013). Computed surfaces are referred to as structuraltrend surfaces and can be used to visualise the structural grain of anarea in 3D. An important characteristic of implicitly defined (iso-) sur-faces is their ability to describe complex geometries, such as overturnedfolds or domes (Cowan et al., 2003; Jessell et al., 2010, 2014; Wellmannet al., 2010).

The direct link between data and the 3Dmodel is a major advantageof the implicit modelling method; updates and changes are straightfor-ward to makewhen additional or new data becomes available, which isan attribute that has partly driven its development (McInerney et al.,2007). This direct link also eliminates the directional bias that is inherentin traditional, manually defined explicit models, which are commonlybased on a series of parallel cross-sections that are often complementedby additional long-sections, surface and level maps. Geological trends orstructures oriented parallel or at low angles to these sections are com-monly overlooked and becomeunder-represented in suchmodels. There-fore, in geologically complex and poly-deformed environments wheremineralisation and structures are expected to have various orientations,an implicit modelling approach should be preferred.

Implicit modelling removes the need for time-consuming rebuildingof wireframes, allowing the continual development of geologicalmodels throughout the life of a project (Mortimer, 2010). In addition,it also supports the testing of multiple hypotheses (Jessell, 2001) in areasonable amount of time by modifying parameters (e.g. isotropicversus anisotropic interpolations), applying structural trends (directlycalculated from structural measurements) or by adapting boundariesor trend. Concatenated files are automatically updated, keeping themodel coherent and giving the geologistmore time to focus on geologicalissues and to evaluate the plausibility of different interpretations. Fur-thermore, implicit modelling is attractive because it makes 3D geologicalmodelling a reproducible task, which is required for the quantification ofgeological uncertainty in 3D models (Ailleres et al., 2010; Chilès et al.,2004; Lindsay et al., 2010, 2012, 2013a,b; Wellmann et al., 2010).

There are, however, some limitations in using an implicit modellingworkflow. Computed surfaces will always try to achieve a smooth fitdue to the nature of the RBF-based spatial interpolation. In data-richenvironments with large amounts of control data this is a less relevantproblem. However, with sparse datasets the resulting model willbecome over-smoothed and will not reflect sudden changes or stepssuch as fault offsets. The lack of control data is generally problematicfor all types of 3D modelling (explicit or implicit). In such under-constrained environments, additional data are commonly manually

introduced based on knowledge and experience to generate a modelthat complies with the geological understanding at the time of creation.To avoid any bias thatmaybe inherited frommodels that have beenmod-ified based on assumptions instead of measured data, the modellingworkflow described in this paper is limited to data-rich environments,where high-data density allows direct computation of boundaries andsurfaces without manual, knowledge driven digitisation. For objectswith a narrowgeometry such as dykes, this criterion is rarelymet and un-aided 3D implicit modelling without manually introduced constraints donot deliver satisfactory results. Another shortcoming is that RBF-basedspatial interpolation tends to generate balloon-like grade shells in areaswhere insufficient control data are available, such as the outer edges ofamodel (e.g., Fig. 8, long section). Furthermore, the type of RBF (Gaussian,multiquadric, or polyharmonic spline) used for the spatial interpolationwill affect the resulting model (Jessell et al., 2014). However, in densedata environments the impact of the chosen RBF appears to be minimal(Hillier et al., 2014). Nevertheless, care must be taken when interpretingsuch models and also when using the calculated grade-shell volumes foran economic assessment of an ore deposit.

Several 3D mining and geological modelling software packages suchas GOCAD (Paradigm, 2014), earthVision (Dynamic Graphics Inc., 2014),GeoModeller (Intrepid Geophysics, 2014), Vulcan (Maptek, 2014), Surpac(Geovia, 2014), Micromine (Micromine, 2014) and Petrel (Schlumberger,2014) have incorporated (semi-) implicit modelling engines.

4.1. 3D implicit modelling of the Navachab gold deposit

In our study we aim to structurally interpret a 3D geological model ofthe Navachab gold deposit, based on available drillhole data and structur-al data extracted from geological maps and collected in the field. It istherefore important to generate objective 3D models that delineate geo-logical boundaries, geochemical anomalies (ore bodies, mineralisedzones) and structural trends. Our 3D implicit modelling workflow is con-fined to areas of high drillhole data density which allows us to directlycompute 3D models without manual interaction (e.g., digitisation). Thisis quite different from a traditional explicit modelling approach, where3D models are based on cross-sections that have assumptions about thegeological and structural framework of the deposit already embedded.The aimof ourworkflow is to identify and analyse trends in large drillholedatasets and relate them to structures observed in the field. We also testthe hypothesis that if mineralisation is structurally controlled or influ-enced, modelled ore-body geometries, orientations and spatial locationswill outline and reveal associatedfirst- or lower-order structures and con-tribute to the understanding of the processes that formed them.

For this study, the implicit modelling software package LeapfrogMining 2.6 (ARANZ Geo, 2014) was chosen because it has the capabilityto process large amounts of data (several thousands of drillholes,


millions of samples) using anRBF-based algorithmon ordinary comput-ing hardware. Data for implicit modelling were extracted from adrillhole database provided by AngloGold Ashanti Ltd in 2012. It includ-ed gold assay data, lithological data (drillhole logs), collar data and sur-vey data for boreholes drilled between May 1986 and January 2012.Maps of the regional geology and digital elevation models (DEM) ofthe region and the mine were also provided and incorporated into the

Fig. 5. 3D implicit modelling workflow applied to drillhole data (a & b lithology, c & d gold assaterpolation (no trend applied)without anymanual digitisation of contacts or cross-sections. In ametric models were combined with field observations to develop a conceptual model, which agold deposit.

3D implicit model. For the 3D implicit structural trendmodel a regionaltopographicmodelwas generated based on SRTM90mdigital elevationdata (source: http://srtm.csi.cgiar.org/).

Data verification and error checking are the first important steps of a3D implicit modelling workflow and are essential for the robustness ofthe resulting 3D geological model. Automated procedures during dataimport and 3D visualisation helped to identify inconsistencies within

ys) and regional structural data (e & f). All datasets were processed using an isotropic in-subsequent step, the three resultingmodelswere used for visual data analysis. These geo-ims to explain the structural evolution and control of mineralisation within the Navachab

http://srtm.csi.cgiar.org/


the extensive drillhole dataset from the Navachab gold deposit(i.e., duplicate samples, missing samples, missing survey information,incorrect positioned drillholes). In summary, 20 out of 2,908 reversecirculation and diamond core drillholes were discarded, providing atotal usable drillhole length of 388.5 km. Three implicit models werecomputed based on lithology codes, gold assay values and beddingmeasurements S0/1.

4.1.1. 3D implicit lithological modelThe six main geological formations that make up the host rocks of

the Navachab gold deposit (Section 3.1) were used to generate the 3Dlithological model (Fig. 5a and b). The known age relationships of theformations were also incorporated into the model. The youngest, over-lying sediments (including alluvium, calcrete and calcretised alluvialconglomerates) were grouped into a unit called cover sediments.Meta-lamprophyre sheets and pegmatite dykes were not included inthe3D implicitmodel due to their narrowgeometry,which requiresman-ual adjustments andmust be modelled separately. However, drillhole in-tercepts were used to investigate and visualise the spatial relationshipbetween pegmatite dykes and gold mineralisation.

The lithological model is exclusively based on drillhole data, withoutany manual digitisation or adjustments that could reflect a subjectiveinterpretation. An isotropic interpolation (i.e., no anisotropic trend ap-plied to the interpolation) was chosen to minimise any bias. A radiusof 60m (adjusted to the drillhole spacing) was applied to generate a vi-sualisation buffer along every drillhole trace (Fig. 5b). Themodel is onlyvisualised inside this buffer, which means that the distance to a sampleor data point is 60m at most to guarantee that subsequent visual inter-pretations were constrained to an area or volume of confidence. The 3Dimplicit lithological model revealed that 1) bedding strikes NE and dips

Fig. 6. Cross-section A of the 3D implicit lithological model and 1.1 ppm gold grade shell of the 3shoot-like, semi-massive sulphide lenses (Main shoot and 2nd shoot) are parallel to bedding anschist-dominated Spes Bona formation. Additionally, bedding-parallelmineralisation can be obsping formations at high angles. Dispersed mineralisation appears to be related to young, NW s

70°–75° to theNW, but steepens to 85°–90° at depth and rotates slightlyto a more E orientation in the NE; and 2) the true thickness of themarble-dominated Okawayo formation varies laterally and verticallyfrom approximately 50 m to 160 m.

4.1.2. 3D implicit assay modelA total of 190,255 gold assay samples from reverse circulation and

diamond core drillholes, sampled from 2 m core segments (99%) andsub-metre segments (1%), were processed into 4 m composites toreduce the number of data points, resulting in 88,580 composites(Fig. 5c). These show a maximum Au grade of 336 ppm, median of0.1 ppm, mean of 0.5 ppm, 90 percentile value of 1.1 ppm and 95 per-centile value of 2.1 ppm (ppm equivalent to g/t). The histogram of4 m gold composites revealed a positively skewed distribution, whichmeans that the frequency of low Au values is much higher than thefrequency of high Au values, and that there are some samples withextreme values. This is very common for gold assay distributions, butis unfavourable when modelling grade values with an interpolant thatuses a weighted sum of the data, which is the case for implicit modelling.This is because itwill place toomuchemphasis onwhat are essentially ex-ceptional values. A nonlinear transformation (log transformation) wasapplied to the gold assay data to reduce this effect.

To minimise any bias in the 3D assay model, an isotropic interpola-tion (i.e., no imposed trend) was used to calculate ore grade shells forcut-off grades of 0.5 ppm, 1.1 ppm and 2.1 ppm, based on the descrip-tive statistical values of the 4 m composites described above (Fig. 5d;for clarity only the 1.1 ppm grade shell is shown).

Subsequent analysis of the 1.1 ppm gold grade shell in combinationwith the 3D implicit lithological model revealed that 1) most goldmineralisation is located in the Okawayo and Spes Bona formations;

D implicit assay model showing distinctivemineralisation trends within theMain Pit. Thed located close to the contact of themarble-dominatedOkawayo formationwith the biotiteerved in the Spes Bona formation. Discordantmineralisation crosscuts the steeply NWdip-triking pegmatite dykes, which have not been modelled due to their narrow geometry.


2) the highest grade and most extensive high-grade gold zones occurclose to lithological contacts; and 3) three geometrically and spatiallydistinct high-grade mineralisation trends can be defined within the1.1 ppm grade shell (Fig. 6). One prominent high-grade mineralisationtrend that can be linked to the semi-massive sulphide lenses is parallelto bedding, located near the contact between the Spes Bona andOkawayo formations and has a width of about 30 m and height of190 m in cross-section. In long-section, it has a 1900 m long shoot-likeappearance that plunges shallowly at higher levels, steepening slightlyat depth (Fig. 8). Additionally, the 1.1 ppm grade shell outlines morebedding-parallel but less extensive gold mineralisation within thestratigraphically lower Spes Bona Formation. This trend resembles theorientation of bedding-parallel quartz-sulphide veins that have beenobserved in the field (for more details, see Section 5).

The second high-grade gold trend dips shallowly ENE, crosscuts thesubvertical Okawayo and Spes Bona formations at a high angle and is lo-cated in both the hanging and footwall of the aforementioned shoot-likemineralisation trend (Fig. 6). Laterally, this shallowly ENE dipping trendcompletely crosscuts the Okawayo formation and continues 50 m intothe Oberwasser formation. The flat-lying trend terminates to the SEwithin the Spes Bona formation and has a maximum horizontal extentof about 100 m. In long section, this mineralisation type appears to beless voluminous and extensive compared to the shoot-like style ofmineralisation. In the field, this type of mineralisation is observed tobe hosted in packages of sheeted, shallowly ENE dipping quartz-sulphide veins that crosscut subvertical bedding at high angles (formore details, see Section 5).

Fig. 7. Field observations from the Main Pit at the Navachab gold deposit. a) Packages of shallowhigh angles. b) Openly folded auriferous qtz-sulphide vein crosscutting subvertical bedding ofdisharmonically folded auriferous qtz-sulphide vein which crosscuts bedding within the marbding-parallel mineralisation trendwithin the Spes Bona formation. e) Subvertical pegmatite dyktion, hence postdating veining. (For interpretation of the references to color in this figure legen

Dispersedhigh-grademineralisationwithin the Spes Bona, Okawayoand Oberwasser formations was also identified, which is impossible torecognise in cross sections (and traditional 3D models that are basedon them) due to its unfavourable orientation. However, a trend withinthe dispersed high-grade mineralisation becomes perceptible during3D visual analysis of the implicitmodel and appears to be spatially relat-ed to subvertical, young pegmatite dykes (Fig. 8). Additionally, gapswithin the 1.1 ppm gold grade shell are spatially associated withdrillhole intercepts of these NW trending, crosscutting pegmatite dykes.

4.1.3. 3D implicit structural trend modelPlanar structural data (bedding S0,1) within an area of about

11 km × 17 km around the Navachab deposit, covering the Usakosdome and Karibib dome have been digitised from a georeferenced geo-logical map compiled by Kisters (2005) and height adjusted using a dig-ital elevation model (DEM), based on SRTM 90 m digital elevation data(Fig. 5e and f). Leapfrog Mining's interpolation of planar structural dataworks best for relatively uniformly sampled data points in geologicalenvironments that are gently, openly or closely folded (i.e., mostdomal structures), but is limited when dealing with tight to isoclinalfolds, unless the data density is very high. In our case study, we comput-ed structural trend surfaces solely based on regional S0,1 measurementsto visualise the 3D structure of the Karibib dome and Usakos dome.Detailed, regional geological maps by Kisters et al. (2004) and Creus(2011) show no evidence for large scale faults cutting through theKaribib or Usakos dome, which could have an important influence onthe topology. However, major thrust faults have been identified at the

ly dipping, auriferous qtz-sulphide veins with sub-metre spacing crosscutting bedding atthe more competent (silicified) biotite schist within the Spes Bona Formation. c) Tightlyles of the Okawayo formation. d) Qtz-sulphide vein (green) related to the modelled bed-es (red) crosscutting flat lying, auriferous qtz-sulphide veins within the Spes Bona forma-d, the reader is referred to the web version of this article.)


SE margin of the Karibib dome, within the Mon Repos thrust zone(Fig. 2), which is located at the SE edge of the 3D structural trendmodel. Movement within this zone is generally sub-parallel to beddingS0,1 along the SE limb of the Karibib dome, hence no significant changesin orientation of structural measurements that might influence themodel are expected. Moreover, the 3D structural trend model aims toonly resolve large-scale (kilometre-scale) structures, meaning thatsmaller-scale features such as minor thrust faults or parasitic folds arenot represented. Nevertheless, the general 3D structure of the chosenmodel area is appropriately reflected by the computed surfaces. An iso-tropic interpolation of the regional beddingmeasurements S0,1was cho-sen as to not force the interpolation to follow amanually-defined trend.The computed 3D trend surfaces were used subsequently to investigatethe spatial location of the Navachab gold deposit within the regionalstructural context. The 3D implicit structural trend model outlines theshape and dimensions of the Usakos dome, Karibib dome and the inter-vening Navachab syncline (Fig. 5f). In NW-SE cross-section, the Karibibdome has an amplitude of ~5 km and a wavelength of ~10 km. On theNW limb of the Karibib dome, structural trend surfaces dip steeply ataround 70° where they intersect topography, but are (sub-)vertical atdeeper levels. The SE limb dips about 50° SE at the surface. The inferredfold axes of the Karibib dome are shallowly double plunging to the NEand SW, and steepen slightly at depth. The mineralised semi-massivesulphide ore shoots of the Navachab gold deposit resemble the orienta-tion of the NE trending fold axis of the Karibib dome.

Fig. 8. Comparison of structural field measurements and the 1.1 ppm grade shell of the 3D impplanar foliation S2, qtz-sulphide veins and S0,1/S2 intersection lineations can be linked to highmineralisation and lead to holes within the grade shell and are related to dispersed mineralisa

5. Structural fieldwork and links to 3D implicit models

Subsequent structural fieldwork was carried out in order to ground-truth the 3D implicit models and to link them to observed geologicalstructures. Spatially interpolated assay data (3D implicit assay model)were overlaid onto themine site digital elevationmodel (DEM) to identi-fy key areas of high-grade mineralisation for targeted fieldwork at theNavachab gold deposit (Fig. 3). Mapping was restricted due to limit-ed exposure and accessibility of high-grade semi-massive sulphidemineralisation as a consequence of ongoing mining activities in theMain Pit andNorth Pit. However,wewere able to link implicitlymodelled1.1 ppm grade shells within the Spes Bona and Okawayo formation to~24° ENE dipping packages of sheeted quartz-sulphide veins with sub-metre spacing located in the Main Pit. Approximately 50% of all quartz-sulphide veins mapped within the Spes Bona Formation have a sub-metre spacing (Fig. 7a), and about 75% of these veins have a thicknessof less than 40mm. Fire assay information from 40 veins sampled withinthe Spes Bona and Okawayo formation (sample size approximately 5 kg)revealed an average gold grade of 13.9 ppm with a maximum value of97 ppm. Statistical analysis of these vein samples indicate that there isno correlation between vein thickness and gold grade.

An hypothetical 1 m × 1 m × 1 m sized block within the Spes Bonaformation (biotite schist), which is crosscut by a single mineralisedvein (40 mm thick, average grade of 13.9 ppm, as detailed above)roughly represents the structural setting of the economic cut-off grade

licit assay model. Orientations from structural data of bedding S0,1, F2 Karibib dome axialgrade mineralisation trends. Crosscutting pegmatite dykes postdate and remobilise goldtion, which has been identified in the 3D implicit assay model.

Fig. 9. Four main stages of buckle-fold development (Cobbold, 1976; Ramsay, 1974).(1) Initial layer-parallel shortening, (2) nucleation of buckling instability, (3) growth oramplification of a buckle-fold, (4) decay or lock up stage.


along the SE face of the Main Pit. Based on previous constraints and anassumed average density of 2900 kg/m3, this 1 m3 block would havean average grade of 0.6 ppm. Consequently, to form high-grademineralisation, as illustrated by the implicitly modelled 1.1 ppm gradeshells, the right combination of vein spacing, gold grade and vein thick-ness is required. However, as noted above, the lack of correlation be-tween vein thickness and Au grade implies that vein spacing must bethe governing quantity. If the frequency of veins exceeds a definedthreshold, high-grade mineralisation becomes visible within the gradeshells of the 3D implicit assaymodel, which in turn outlines areas of sig-nificant economic interest.

Auriferous veins within the Spes Bona and Okawayo formationcrosscut subvertical compositional bedding and intrafolial isoclinalfolds (F1). This suggests that vein development postdates D1 and oc-curred when bedding was already rotated to subvertical attitudes,pointing towards emplacement during the final lock-up stage of region-al scale folding (D2). Additionally, sheeted mineralised quartz-sulphideveins show small-scale upright folds with sub-metre amplitudes andwavelengths and shallowly NNE plunging fold axes. Quartz-sulphideveins within the Spes Bona formation are openly folded (Fig. 7b). Onthe other hand, veins within the Okawayo Formation are affected bytight, disharmonic folding (Fig. 7c). Both vein sets were probablyemplaced contemporaneously, but rheological differences betweentheir country rocks (relatively soft marble versus competent schist)led to variations in the intensity of folding.

Due to the fact that the amplitudes and wavelengths of these foldedveins aremuch smaller than the sample and composite spacing (4m) ofthe drillholes, these small-scale structures are not directly reflected inthe 3D implicit assay model. However, the mean principal vein orienta-tion calculated from field measurements and the attitude of theenveloping surface resemble the orientation of implicitly modelledshallow-dipping, high-grade mineralisation within the Spes Bona for-mation (Fig. 8, cross-sectionA). Because of the disharmonic tight foldingof the veins and the limited exposure on theNW face of theMain Pit, wewere not able to collect representative field measurements of foldedveins within the marble-dominated Okawayo formation.

The horizontal extent of themineralised quartz-sulphide veins is re-stricted andmost probably controlled by changes in host rock lithology.An abrupt termination is not only visible in the 3D implicit assaymodel,but also in the field within the Spes Bona formation in Eastern Zone 1,where flat-lying veins terminate at the contact between biotite schistand interbedded calcsilicate rocks.

We also mapped a set of sub-vertical, bedding-parallel quartz-sulphide veins in the Spes Bona Formation within the Main Pit(Fig. 7d). Because these veins are oriented parallel to the pit face andare widely spaced, their recognition is limited in the field, but theyhave been identified in drill cores by Deltenre (2012). The geometryand orientation of the computed high-grade ore shells within the SpesBona formation also suggests the existence of bedding-parallel goldmineralisation (Figs. 6 and 8).

The S2 axial planar foliation related to the formation of the Karibibdome is best preserved in mica-rich rocks such as biotite schists of theSpes Bona and Oberwasser formations. S2 strikes NE and dips steeplySE, reflecting the asymmetry of the Karibib dome. The lack of an addi-tional (axial planar) foliation trending NW and oriented at high anglesto S2 suggests that the Karibib dome (and Usakos dome) did not formby two different deformation stages, but one non-cylindrical foldingevent. Intersection lineations between S0,1 and S2 are oriented parallelthe fold axis of the Karibib dome and plunge shallowly NNE, and alsoparallel to the orientation of the semi-massive sulphide ore shoots, asindicated by the 1.1 ppm grade shell trend within the MC unit of theOkawayo formation (Fig. 8).

The least competent, marble-dominated Okawayo formation hasbeen intruded by (meta-) lamprophyre sheets, which are generally par-allel to bedding, but also show a number of bedding-oblique splays. Onthe uppermost, non-accessible pit walls in the NE part of the Main Pit,

such splayed meta-lamprophyre sheets have been folded. The foldvergence is converse to F2 parasitic folds, but consistent with asymmet-ric folds that have formed due to the oblique orientation of the meta-lamprophyre sheets in relation to the maximum NW-SE shorteningdirection.

Numerous,mainlyNW-trending subvertical pegmatite dykes (dm toseveral metre thick) intruded into the metasedimentary rocks atNavachab and crosscut auriferous quartz-sulphide veins, semi-massivesulphide ore shoots and the meta-lamprophyres sheets and dykes,therefore postdating gold mineralisation and the mafic intrusions(Fig. 7e). Additionally, it is noticeable that the abundance of pegmatitedykes is greater in the S and N of the Main Pit as well as in the NorthPit. Occasionally, remnants of flat-lying quartz-sulphide veins are pre-served within the crosscutting pegmatite dykes in the field, confirmingthat the dykes postdate vein hosted gold mineralisation (Fig. 7e).

Several young, brittle, shallow SE-dipping faults with minor dis-placement (b1 m) have been mapped on the NW face of the Main Pit.We interpret these to be related to the post-folding relaxation of theDamara orogen or an even younger deformational event. They do notseem to play a significant role in the mineralisation or the main struc-tural framework at Navachab.

6. Discussion and interpretation

One of the most complex structural attributes of many ore depositsis folding, which is a prevalent feature at macro and micro scale andoften described to influence mineralisation (Aerden, 1993; Blenkinsopand Doyle, 2014; Cox et al., 1991; Leader et al., 2010; Morey et al.,2005, 2007; Richard and Tosdal, 2001; Wilson et al., 2013). However,


associated regional scale folding is frequently overlooked as a majorstructural control on mineralisation. Additionally, folding is a ratherunder-appreciated control on ore emplacement compared to faultsand shear zones, even though the formation of these structures isoften a consequence of strain localisation during regional scale folding.

6.1. Relationship between folding and mineralisation

In compressional tectonic settings such as fold and thrust belts, ac-tive folding (buckling) initiates when multilayered rock packages ofdifferent viscosities are shortened parallel to layering (Fossen, 2010).The development of a buckle fold can be divided into four main stages(Fig. 9), namely (1) initial layer-parallel shortening, (2) nucleation orbirth of buckling instability, (3) growth or amplification of the buckle-fold and (4) decay or locking-up (Cobbold, 1976; Ramsay, 1974;Schmalholz, 2006). Mineralisation, on the other hand, is commonlyclassified as pre-, syn- or post-deformational based on structural rela-tionships, petrographical analysis and/or geochronological evidence.This temporal classification of mineralisation can be linked to the fourmain stages of folding, which are thought to influence the spatial distri-bution, trends and relationship of mineralisation. Depending on thefolding stage under which mineralisation occurs, geometrically distincthigh grade ore zones can form at preferred siteswith respect to the fold.Furthermore, due to ongoing tectonic processes these ore zones mightoverprint and overlap each other and can be refolded as well. Theircrosscutting relationships can be used to establish the relative timingof the different generations of mineralisation.

The nucleation of folds is mainly determined by initial heterogene-ities. In numerical models, initial perturbations are introduced to nucle-ate folds (Fernandez and Kaus, 2014; Frehner, 2014; Grasemann andSchmalholz, 2012; Mancktelow, 1999; Schmalholz, 2008; Schmidet al., 2008). Moreover, scaled analogue experiments have highlighted

Fig. 10. Topography based on SRTM90data (grey) intersected by the regional structural trend suwhich is directly related to dip (γ= tan(dip)) of originally horizontal layers, which are deformtheUsakos dome in theWcompared to theKaribib dome in the E. TheNavachab deposit is locateLeapfrog Mining 2.6; shear strain calculated and visualised with Move 2013.1.

the importance of irregularities in localising the site of fold initiation(Abbassi and Mancktelow, 1990; Biot et al., 1961; Cobbold, 1975; Dubeyand Cobbold, 1977). Such heterogeneities are manually introduced priorto experimentation to induce folding during the experiment. These twoexamples emphasise the importance of initial heterogeneities, whichmay also play a role in the development and preservation of certaintypes of mineral deposits. For example, in the case of VMS deposits, thetypical mound shape of such deposits, accompanying faults or the rela-tively soft massive sulphides themselves have the potential to act as het-erogeneities and favour and/or control the nucleation of folds. This mightexplain why VMS deposits like Kidd Creek, Canada (Richardson, 1998) orQue River, Tasmania (Large et al., 1988) are associated with fold closures,although furtherwork is required to evaluate this hypothesis. Alternative-ly, sulphide-rich mineralisation can also be remobilised during deforma-tion and metamorphism. Many sulphide ores are less competent thantheir host rocks (Duuring et al., 2007). When folded, one would expectstretched ore bodies along the limbs or migration and accumulation ofsulphide ore in fold hinges due to mass transport towards low pressuresites.

During the subsequent fold amplification and kinematic growth stageof folding, the highest shear strain is reached along the inflection lines offolds. Maximum values of flexural shear/flexural flow are confined tothese areas, which are also associated with zones of low pressure andmaximum dilation. Such linear zones of enhanced permeability havethe potential to efficiently transport large volumes of fluid through mid-crustal rocks, which is important for the genesis of an economic grademineralisation. Moreover, these linear structures might not only act asfavourable conduits, but also trigger the precipitation of gold because ofchanged physical conditions (e.g., lowered pressure), therefore acting asphysical traps. As described by Peters (1993b), the location and shape ofore shoots are usually controlled by dilatant zones caused by changes inattitude, splays, lithologic contacts and intersections.

rface for S0,1 (structural data fromKisters (2005)). Colours refer to calculated shear strainγed into upright folds (Fossen, 2010). Note the distinctive geometrical differences betweendwhere shear strain reaches peak values. Regional structural trend surface generatedwith


The final lock-up stage of folding occurs when folding can no longeraccommodate layer-parallel shortening. This leads to the formation ofveins and faulting, which is a major feature of orogenic gold deposits.Vein-hosted mineralisation that crosscuts steepened bedding andmineralisation controlled by pre-existing foliations and late thrust faultsform at this stage. Famous examples are the gold deposits of the BendigoZone, where gold is hosted by quartz veins associated with steeply-dipping reverse faults (Leader et al., 2012).

6.2. Structural history and goldmineralisation at theNavachab gold deposit

The 3D implicit structural trend model for bedding S0/1 shows thatthe Navachab gold deposit is located near the inflection line of thesteep NW limb of the regional scale Karibib dome (Fig. 10). DuringNW-SE shortening and associated buckling of the Karibib dome, thiswould have been an area where shear strain reached peak values. Incontrast, shear strain induced by the same process would have reachedminimum values in the hinge regions. To quantify and visualise shearstrain distribution along the Karibib dome, we calculated shear strainbased on the layer dip. For originally horizontal layers that are foldedinto upright folds, shear strain γ is directly related to layer dip (γ =tan(dip)) (Fossen, 2010). This formula was used to determine shearstrain based on local dip values of the triangulated mesh from the 3Dimplicit structural trend model (Fig. 10). The software package Move2013.1 (Midland Valley Exploration Ltd, 2013) was chosen to performadvanced analyses on (triangulated) 3D meshes. It also allows colour-coding of desired properties such as dip, curvature or in our case shearstrain. The results are consistent with the location of the Navachabgold deposit along the NW inflection line of the Karibib dome whereshear strain reached the highest values of 4 to 5 during fold amplifica-tion (Fig. 10). Maximum layer-parallel displacements due to flexuralshear or flexural flow would have been confined to these areas, poten-tially forming low-pressures zones such as shear veins or dilationaljogs, thereby generating suitable conduits and traps for ore bearingfluids. The semi-massive sulphide lenses that delineate the auriferousore shoots were probably controlled by this folding-related strainpartitioning mechanism. This is in agreement with structural fieldworkby Kisters (2005), who relates the emplacement of auriferous semi-massive sulphide lenses to dilational jogs based on their rhomb-like ge-ometry. Additionally, the orientation of calculated S0,1/S2 intersectionlineations (=F2 fold axes) resembles the trend of the central, shallowlyNE plunging 1.1 ppm grade shell that is related to semi=massivesulphide ore shoots, suggesting that they are structurally related tothe formation of the Karibib dome (Fig. 8). No semi-massive sulphidemineralisation is reported within the hinge regions of the Karibibdome, where flexural slip-related shear strain is expected to haveattained minimum values, and where the potential to form associated,dilatant structures is low. Therefore, an epigenetic, syn-deformationalorigin of the semi-massive sulphide ore shoots is more likely.

The potential to form such dilatant structures depends on theamount of shear strain, but also rheological properties of affected lithol-ogies. This could explain the second order control on the localisation ofsemi-massive sulphide ore shoots. As seen in the 3D implicitmodels, themain shoot (Main Pit) and 2nd shoot (North Pit) are both located closeto the contact of the Okawayo Formation with the Spes Bon Formation(Fig. 6). Generally, layer parallel shearing will not be uniformly distrib-uted throughout a sequence (Price and Cosgrove, 1990) and strain willbe preferentially partitioned close to lithological contacts due to relativedifferences in physical properties. In the case of theNavachab gold depos-it, the rheological contrast between relatively softmarbles and competentbiotite schists (party silicified)may have controlled the ‘stratigraphic’ po-sition of the ore shoots.

At the Navachab gold deposit, high grade mineralisation is alsohosted in bedding-parallel and highly discordant quartz sulphideveins. The highly discordant veins dip shallowly to the ENE and are af-fected by sub-metre scale, upright foldingwith fold axis that are parallel

to NNE plunging axis of the Karibib dome. There is no evidence that theaforementioned bedding-parallel flexural shear was active duringfolding of the highly discordant vein set, suggesting that they wereemplaced later during the lock-up stage of the Karibib dome whenhost rocks have already reached steep attitudes and doming relatedflexural slip had mostly ceased (Kisters, 2005).

Ambiguous and mutually-crosscutting relationships between thesubvertical, bedding-parallel and the discordant, flat-lying vein systemsare consistent with contemporaneous emplacement during the latestages of regional fold (dome) lock-up. The highly discordant vein setis oriented sub-parallel to the horizontal principal compressive stressdirection, σ1, expected for the NW-SE regional shortening event andassociated folding. In contrast, bedding-parallel veins are oriented al-most perpendicular to σ1 and were most likely generated under highfluid overpressure conditions, which are required for emplacementalong planes of strong mechanical anisotropy such as bedding (Sibson,1996). Their unfavourable orientation with respect to the main stressfield might also explain their limited abundance and lower thickness.However, we cannot rule out that some bedding-parallel quartz-sulphide veins could have formed as shear veins during amplificationof the Karibib dome, contemporaneously with emplacement of theauriferous semi-massive sulphide ore shoots. Bedding-parallel veinslogged in drillhole N766 located within the Oberwasser Formation areboudinaged. This is consistent with the bulk strain regime that foldedthe discordant vein set and lamprophyre sheets during the late lock-up stage of folding. Boudinage could also explain the lenticular shapeof massive-sulphide bodies that form the ore shoots.

The third type of gold mineralisation is associated with pegmatitedikes that intruded parallel to a NW trending, subvertical joint set,which is interpreted here to have formed in an extensional fracture ori-entation during the lock-up stage of the Karibib dome. These pegmatitedykes are responsible for local remobilisation and redistribution of gold,which has not been described previously at the Navachab gold deposit(Fig. 8). This suggests that earlier deposited gold has been locally re-placed and remobilised by pegmatite intrusions and re-deposited at(mainly) higher levels in the form of dispersed mineralisation.

In summary, results from3D implicit modelling in combinationwithstructural fieldwork suggest that structures formed during the amplifi-cation and lock-up stage of the Karibib dome acted as conduits and trapsfor goldmineralisation and also controlled deposit-scale remobilisation.These deformation related structural features best classify theNavachabgold deposit as an orogenic gold deposit.

7. Conclusions

We have demonstrated that ore body geometries obtained from 3Dimplicit models can be accurately linked to local and regional structuralpatterns. First-order controls are most important for economicmineralisation and are represented in our 3D implicit models for theNavachab gold deposit. Small-scale structures that cannot be resolvedin these models but have been observed in the field were used to com-plement and constrain our structural interpretation. Our workflow fo-cused on minimising the modelling bias by using an isotropic spatialinterpolation in order to utilise the resulting 3D implicit models forstructural interpretation purposes. The results from 3D implicit model-ling in conjunctionwith structural fieldwork suggest that this workflowenhances the identification and (re-)assessment of structural controlsonmineralisation. Our structural interpretation of the Navachab depositbuilds uponwork by Kisters (2005).We demonstrate that the spatial lo-cation of ore shoots at the Navachab gold deposit is systematic and con-trolled by different stages in the development of a regional scale domalstructure (Karibib dome). Additionally, we described the role of cross-cutting pegmatite dykes in locally replacing and remobilising previousvein-hosted and semi-massive sulphide-hosted mineralisation.

The Navachab gold deposit case study highlights the importance offolding as a first order structural control on emplacement of ore bodies.


The 3D structural trendmodel demonstrates a geometrical dependenceof shear strain that potentially controls the localisation of associatedmineralised structures (ore shoots). We propose that inflection lines(where shear strain reaches peak values during fold amplification) ofclose, tight or even overturned folds are preferred sites ofmineralisationand represent pipe-like traps and conduits for ascending fluids of vari-ous origins. Our results have implications for exploration activities insimilar tectonic environments and should be considered in the planningstage of drillhole targets. Additionally, this work indicates that state-of-the-artmodellingworkflows can lead to a better structural insight in al-ready developed and explored mine settings.

Acknowledgements

We thank AngloGold Ashanti Ltd for their generous support as wellas AusIMM for funding S. A. V. (Bicentennial Gold Endowment). Weare grateful to JaneAllen (AngloGoldAshanti Ltd ExplorationManager—Brownfields, Continental Africa Region), Frik Badenhorst and especiallyto Navachab's chief geologist Graham Bell and his team for their abun-dant and helpful support. S.A.V. and A.R.C. also acknowledge MonashUniversity for scholarship and research support. Stereonetswere gener-ated using Rod Holcombe's programmeGEOrient. Academic licenses forthe Leapfrog Mining software package were kindly provided by ARANZGeo Pty. Ltd. In addition, we acknowledge Midland Valley ExplorationLtd. for providing an academic license for the Move 2013.1 geologicalmodelling software package. We are grateful to two anonymous re-viewers for their comments and suggestions, which improved the qual-ity of this paper.

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