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Regional Dome Evolution and Its Control on Ore-Grade Distribution: Insights from 3D Implicit Modelling of the Navachab Gold Deposit, Namibia

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Ore Geology Reviews 69 (2015) 268–284 Contents lists available at ScienceDirect Ore Geology Reviews journal homepage: www.elsevier.com/locate/oregeorev Regional dome evolution and its control on ore-grade distribution: Insights from 3D implicit modelling of the Navachab gold deposit, 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, Australia b Orefind Pty Ltd, Fremantle, Australia article info abstract Article history: We introduce a novel approach to analyse and assess the structural framework of ore deposits that fully inte- Received 28 November 2014 grates 3D implicit modelling in data-rich environments with field observations. We apply this approach to the Received in revised form 20 February 2015 early Palaeozoic Navachab gold deposit which is located in the Damara orogenic belt, Namibia. Compared to tra- Accepted 20 February 2015 ditional modelling methods, 3D implicit modelling reduces user-based modelling bias by generating open or Available online 21 February 2015 closed surfaces from geochemical, lithological or structural data without manual digitisation and linkage of sec- fi Keywords: tions or level plans. Instead, a mathematically de ned spatial interpolation is used to generate 3D models that Folding show trends and patterns that are embedded in large drillhole datasets. In our 3D implicit model of the Navachab Ore shoots gold deposit, distinctive high-grade mineralisation trends were identified and directly related to structures ob- Implicit modelling served in the field. The 3D implicit model and field data suggest that auriferous semi-massive sulphide ore shoots 3D deposit modelling formed near the inflection line of the steep limb of a regional scale dome, where shear strain reached peak values Fold inflection lines during fold amplification. This setting generated efficient conduits and traps for hydrothermal fluids and associ- Orogenic gold ated mineralisation that led to the formation of the main ore shoots in the deposit. Both bedding-parallel and highly 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 mineralisation at the deposit scale and appear to be related to a younger joint set. We propose that kilometre-scale active folding is an important deformation mechanism that influences the spatial distribution and orientation of mineralisation in 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 as preferred 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 using 3D implicit modelling is a powerful tool for spatial analysis of mineralisation patterns. When combined with fieldwork, this approach has the potential to improve structural understanding of a variety of ore deposits. © 2015 Elsevier B.V. All rights reserved. 1. Introduction such fluids because of their ability to transport gold and related compo- nents (Phillips and Powell, 2010). Ore deposits form where hydrother- Ore deposits are economic concentrations of chemical elements that mal fluids are either chemically (e.g., fluid mixing, host rock reactions, result from a number of processes that occur within the Earth's crust pH change) or physically trapped (e.g., rapid change of pressure or and deliver elements from a source region to a deposit location. These temperature). The importance and efficiency of these two trapping processes can be chemical and/or physical and they control the libera- mechanisms is still under debate. However, abundant research has tion, transport, focusing and trapping of hydrothermal fluids (Cox and shown that many gold deposits are hosted in deformed rocks, suggest- Etheridge, 1987; Groves et al., 2000; Ridley, 1993; Robb, 2004; ing a strong physical (structural) control on mineralisation. Famous Tomkins, 2010). Most genetic models of gold deposits are ascribed to examples are the lode gold deposits of the Victorian goldfields (Cox et al., 1991, 1995; Leader et al., 2012; Mueller, 2014; Willman, 2006; Willman et al., 2010; Wilson et al., 2013) and the Yilgarn Craton, West- ⁎ Corresponding author at: School of Earth, Atmosphere and Environment, Monash ern Australia (Cox and Ruming, 2004; Groves et al., 1995, 2000; University, Clayton, VIC 3800, Australia. Hodkiewicz et al., 2005; Micklethwaite and Cox, 2006; Ridley and E-mail addresses: [email protected] (S.A. Vollgger), [email protected] (A.R. Cruden), [email protected] (L. Ailleres), jun.cowan@orefind.com Mengler, 2000). Structures associated with these deposits such as (E.J. Cowan). folds, faults, veins and shear zones highlight the importance of a http://dx.doi.org/10.1016/j.oregeorev.2015.02.020 0169-1368/© 2015 Elsevier B.V. All rights reserved. S.A. Vollgger et al. / Ore Geology Reviews 69 (2015) 268–284 269 changing history of brittle and ductile deformation for economic Rescources, 2012; Lombard et al., 2013) as part of B2Gold's Otjikoto pro- mineralisation. These changes are important because they influence ject (formerly operated by AuryxGold) and Damara Gold Corp.'s Gold fluid flow and control the formation of dilational sites that can act as Kop target (formerly Helio Resources). These have both been described traps for hydrothermal fluids (Leader et al., 2011). It follows that the as having a similar style of mineralisation to that at the Navachab gold shape, size, orientation and spatial distribution of ore bodies contain im- deposit. portant information about the deformation history of an ore deposit and The Navachab gold deposit is located in the S part of the Central Zone the associated interactions between host rock types, fluid flow and of the Damara belt, also known as the Southern Central Zone. The Cen- structures (Blenkinsop and Kadzviti, 2006). Work by Blenkinsop tral Zone is one of several tectonostratigraphic domains within the Pan- (2004) on two lode gold deposits from the Zimbabwe craton, studies African Damara orogen (Miller, 1983)(Fig. 1). The majority of mineral by Bauer et al. (2014) on VMS deposits in the Swedish Skellefte district, deposits are located in the Central Zone and are associated with rocks 3D modelling and analysis of nickel sulphide shoots at the Kambalda that have been subjected to medium- and high-grade metamorphism dome (Western Australia) by Stone et al. (2005) and investigations on (Pirajno and Jacob, 1991). the vein-hosted gold ore body at Sunrise Dam (Western Australia) by The Damara belt extends to the NE and links with the Lufillian Hill et al. (2013) demonstrate that shapes and orientations of ore bodies orogen in Zambia–Angola–Congo (Pirajno, 2008). The term Pan- or mineralised zones contain important information about the evolu- African orogeny is commonly used to describe magmatic, tectonic and tion of ore deposits. The analysis of structures that transport and trap metamorphic activity during the Neoproterozoic (~870 Ma) to early hydrothermal fluids is therefore of major scientific and economic Palaeozoic (~550 Ma) orogenic episode (Kröner and Stern, 2004). significance. By understanding the genetic relationship between The tectonic evolution of the Damara belt started with the initiation (mineralised) structures and associated physical processes, we can of an intracontinental, asymmetric rift between the Congo craton and potentially predict areas with a greater potential for mineralisation in the Kalahari craton during the break-up of Rodinia around ~780– similar tectonic environments. In this paper, we present a structural 750 Ma (Barnes and Sawyer, 1980; Miller and Frimmel, 2009; Miller interpretation workflow that uses 3D implicit modelling to visualise et al., 2009). The sedimentary fill of the newly opened graben in the S and analyse geochemical and geological trends from an extensive (Nosib Group) exceeds 6 km in thickness and consists of feldspathic drillhole dataset from the Navachab gold deposit, Namibia (provided sandstones, playa lake and evaporitic sabkha deposits (Miller et al., by AngloGold Ashanti Ltd). Such drillhole data are normally used to 2009). Rifting led to siliciclastic and carbonate sedimentation before compute block models for the economic assessment of a project. For and during the subsequent formation of an oceanic basin (Khomas each block of these models, important key values and properties are ei- sea). The final width of the Khomas sea is estimated to about 1200– ther assigned or calculated, such as net present value, metal content, 1500 km (Miller et al., 2009). The Matchless Amphibolite is the meta- density or rock type. Block models are powerful and well-established morphosed remnant of oceanic crust (sea-floor basalt) which formed tools for resource estimation, grade control and scheduling, where the during that time, and is host to several VMS and (epigenetic) copper de- main purpose is to evaluate, maximise and optimise the economics of posits (Breitkopf and Maiden, 1988). a project. However, they are less valuable for understanding the geology Reversal of plate motion and formation of a N dipping subduction and structural framework of a deposit. zone led to the closing of the Khomas sea
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