The Rössing Uranium Deposit: a product of late-kinematic localization of uraniferous granites in the Central Zone of the Damara Orogen, Namibia

Journal of African Earth Sciences 38 (2004) 413–435 www.elsevier.com/locate/jafrearsci

€ssing Uranium Deposit: a product of late-kinematic The Ro localization of uraniferous granites in the Central Zone of the Damara Orogen, Namibia I.J. Basson a

a,*

, G. Greenway

b,1

Department of Geological Sciences, University of Cape Town, P.B. Rondebosch 7701, South Africa b R€ossing Uranium Mine, Private Bag 5005, Swakopmund 9000, Namibia Received 18 September 2003; accepted 22 April 2004

Abstract The R€ ossing granite-hosted uranium deposit in the Central Zone of the Pan-African Damara Orogen, Namibia, is situated in the ‘‘SJ area’’ to the south of the R€ ossing Dome. The coincidence of a number of features in this area suggests that mineralization is closely linked to late-kinematic evolution of the R€ ossing Dome. These features include: (1) the rotation of the dome’s long axis (trend of 017
), relative to the regional F3 trend of 042
; (2) southward dome impingement, concomitant with dome rotation, producing a wedge-shaped zone of alkali-leucogranites, within which uranium mineralization is transgressive with respect to granites and their host lithologies; uranium mineralization and a high fluid flux are also confined to this arcuate zone to the south and southeast of the dome core and (3) fault modeling that indicates that the SJ area underwent late-D3 to D4 brittle–ductile deformation, producing a dense fault network that was exploited by leucogranites. Dome rotation and southward impingement occurred after a protracted period of transtensional tectonism in the Central Zone, from ca. 542 to 526 Ma, during which I- and S-type granites were initiated in a metamorphic core complex. Late-kinematic deformation involved a rejuvenation of the stresses that acted from ca. 600 to 550 Ma. This deformation overlapped with uranium-enriched granite intrusion in the Central Zone at 510 ± 3 Ma. Such late-kinematic, north–south transpression, which persisted into the post-kinematic cooling phase until at least 478 ± 4 Ma, was synchronous with left-lateral displacement along NNE-trending (‘‘Welwitschia Trend’’) shears in the vicinity of R€ ossing. Latekinematic deformation, causing block rotation, overlying dome rotation and interaction of the more competent units of the Khan Formation with the R€ ossing Formation in the dome rim was pivotal in the localization of uranium-enriched granites within a highly fractured, high-strain zone that was also the site of prolonged/high fluid flux.  2004 Elsevier Ltd. All rights reserved. Keywords: Leucogranite; Uranium; Mineralization; Late-kinematic; Damara Orogen; Namibia

1. Introduction Whereas diapiric upwelling is confined to the lower ductile crust, upper crustal granitic melt movement is governed by the prevailing tectonic setting, such as continental collisional orogens and active continental margins (Ramberg, 1981; Brown and Rushmer, 1997; Petford et al., 2000). Upper crustal granites and their * Corresponding author. Tel.: +27-21-650-2921; fax: +27-21-6503783. E-mail addresses: [email protected] (I.J. Basson), [email protected] (G. Greenway). 1 Present address: Snowden Consulting, P.O. Box 2613, Parklands, 2121, South Africa. Fax: +27-11-782-2396.

0899-5362/$ - see front matter  2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jafrearsci.2004.04.004

host structures essentially reflect the influence of an applied differential stress on magmas, a process that may locally be more important than the effects of gravity (Petford et al., 2000). The scale at which granite plutons and their feeders are examined is crucial, for instance, despite regional compression or transpression, granites commonly intrude into local transtensional zones (Vigneresse, 1995a,b; Brown and Rushmer, 1997). There is limited non-experimental data on the physical and mechanical properties of magmas, consequently, direct observation of upper crustal or near-surface features should provide the basis for most granite intrusion models (Delaney et al., 1986; Bouchez et al., 1990; Hutton et al., 1990; Vigneresse, 1995a,b; Fernandez and Castro, 1999). Information on the third dimension of

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granites (their form below the visible surface) is directly relevant to developing upper crustal intrusion models. In addition, studies that document the four-dimensional evolution of granites and their feeder zones are particularly useful in understanding the short- to long-term evolution of granitic magmas in the upper crust (e.g. McCaffery and Petford, 1997; Petford et al., 2000). The R€ ossing Uranium Mine, situated approximately 60 km NE of Swakopmund, Namibia, is the only economic granite-hosted uranium deposit being exploited at present. The Paleozoic R€ ossing deposit is, by virtue of its age, host rocks, size, grade and U–Th ratio that yields extremely low radioactivity (Berning et al., 1976; Barnes and Hambleton-Jones, 1978; Berning, 1986), exceptional in both African and global contexts. The R€ ossing deposit typifies ‘‘intrusivealaskite’’ types; indeed, the term ‘‘R€ ossing-type’’ has entered the literature (Berning, 1986; Guilbert and Park, 1986). In keeping with recent developments, the term ‘‘granite-hosted’’ is used here, rather than ‘‘intrusive-alaskite’’ or ‘‘alaskite-hosted’’, allowing a more generic overview of the granites of the Central Zone of the Damara Orogen. Besides its economic importance, exploitation of the R€ ossing deposit provides exceptional three-dimensional access to granites and their controlling structures. Previous studies on genetically similar granite-hosted uranium mineralization in Namibia concentrated either on proposed spatial correlations between uranium mineralization and inferred or interpreted regional aeromagnetic lineaments (Corner, 1982, 1983, 2000; Jacob et al., 1986), the mineralogical and chemical processes operative during mineralization and granite intrusion (Barnes and Hambleton-Jones, 1978; Cuney, 1979; Marlow, 1981; Miller, 1983a; Kerr, 1990; Mestres-Ridge, 1992; Bowden et al., 1995; Nex, 1997; Herd, 1997; Nex et al., 2002) and the metamorphic setting of sheeted granite emplacement (Jacob et al., 1986, 2000; Nex, 1997; Nex et al., 2001a). This study seeks to determine the processes behind the structural ‘‘uniqueness’’ of R€ ossing Uranium Mine, in particular the means whereby voluminous leucogranites were localized in the vicinity of R€ ossing and structural reasons for their relatively high volume and exceptional uranium enrichment compared to other granites in the vicinity. This study firstly establishes a tectonometamorphic context during granite intrusion in the southern Central Zone of the Damara orogen, including granites in the Goanikontes area approximately 30 km to the west of R€ ossing, followed by a definition of local and regional structural (predominantly ‘‘D3 ’’) trends and features. Much of this contribution is necessarily a synthesis of previous work, however new structural data from the southern portions of the R€ ossing Dome is presented which highlights the means by which late-kinematic ‘‘D4 ’’ deformation was

pivotal in the formation of the R€ ossing deposit. This data was obtained during structural mapping of the area over a four-month period by the first author (IJB) and a four-month geotechnical survey of faulting in the open cast R€ ossing mine (IJB and C. Myburgh). The study incorporates observations from seven years of on-site experience and lithological and ore body modeling by the second author (GG).

2. The Damara Orogen The Neoproterozoic to early Paleozoic Damara Orogen consists of two main sectors, the N-trending coastal branch, and the NE-trending Intracontinental Branch (Fig. 1a, insets). The 400 km-wide Intracontinental branch between the Congo and Kalahari Cratons is divided into four major NE-trending tectonostratigraphic domains (Fig. 1, insets; Martin and Porada, 1977; Miller, 1983a,b). These structural domains are (from NW to SE): the Northern Platform, consisting of a thick sequence of Otavi Group carbonates; the Northern Zone, consisting of folded and northward-thrusted rift volcanic and sedimentary rocks and related intrusive bodies (e.g. Henry et al., 1990); the Central Zone; the Okahandja Lineament Zone, which separates the dominant dome-and-basin pattern of the Central Zone from the linear structures of the Southern and Southern Marginal Zones; and the Southern Zone itself, comprising SE-thrusted accretionary prism sequences (Kukla et al., 1991). The Central Zone contains voluminous granites and gneisses, including basement Abbabis augen gneiss, synmetamorphic Red granite (Jacob, 1974; Fig. 1b), the Salem granite suites and late- to post-kinematic intrusions such as the Donkerhuk granite (Sawyer, 1978; Nex and Kinnaird, 1995; Bowden and Tack, 1995). Domal structures with cores of Abbabis gneiss or Etusis Formation rocks (see Tables 3 and 4) are relatively widespread in the Central Zone compared to the Northern or Southern Zones (Barnes and Downing, 1979; Coward, 1980; Corner, 1983; Miller, 1983a,b; Kr€ oner, 1984; Jacob et al., 1986; Oliver, 1995). The Omaruru Lineament separates the northern Central Zone from the southern Central Zone. The latter contains the R€ ossing, Goanikontes and Ida Dome areas, which host notable uranium-enriched, sheeted leucogranites. The southern Central Zone consists of high-temperature/low-pressure metasedimentary rocks (upper-amphibolite to granulite facies), widespread granite intrusions, inliers of basement gneiss and NE- to NNE-trending domes. Significant occurrences of subeconomic to economic uranium mineralization, hosted by late- to post-kinematic granites (Berning et al., 1976; Berning, 1986; Bowden and Tack, 1995; Bowden et al., 1995; Nex, 1997) are confined to the southern Central Zone, seemingly related to NE- and NNE-trending

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415

Fig. 1. (a) Total Magnetic Intensity (TMI) image, with apparent sunshading from the NW, of the central portion of the Damara Orogenic Belt, with insets showing its general position in Africa, and zonal sub-divisions, including its two main branches (intracontinental and coastal). WL ¼ (proposed) Welwitschia Lineament. (b) Geology and superimposed major lineaments interpreted from regional aeromagnetics. NP/CC ¼ Northern Province/Congo Craton, NZ ¼ Northern Zone, nCZ ¼ northern Central Zone, sCZ ¼ southern Central Zone, SZ ¼ Southern Zone, SMZ ¼ Southern Marginal Zone, SF/KC ¼ Southern Foreland/Kalahari Craton, OLZ ¼ Okahandja Lineament Zone. W ¼ Windhoek; S ¼ Swakopmund; WB ¼ Walvis Bay. (a) and (b) are adapted from Smith (1965), Corner (1982, 1983, 2000), Miller (1983b), Oliver (1995) and Eberle et al. (1995).

mantled domes between the Omaruru and Okahandja lineaments.

3. Stratigraphy of the R€ ossing area Nash (1971), Henry (1992), Lehtonen et al. (1996) and Anderson and Nash (1997) have described the basal Swakop and Nosib Groups, that rest unconformably on the Abbabis Metamorphic Complex/Basement, to the SE of the Omaruru lineament in the vicinity of R€ ossing (Table 1). The highly metamorphosed pre-Damara Abbabis Complex, consisting of augen, migmatitic, biotite, sillimanite and granitic gneiss, biotite schist and

amphibolite, is exposed in domes in the Swakop and Khan River areas (e.g. Marlow, 1981; Lehtonen et al., 1996). The basement complex is polycyclic, with a minimum age of 1038 ± 58 Ma for the Kibaran granitoid-gneiss (U–Pb SHRIMP zircon date: Kr€ oner et al., 1991). The Etusis Formation is restricted in extent due to its original sedimentation on an irregular paleotopography. Oliver and Kinnaird (1996) suggest that the Basement Complex is not present in the R€ ossing Dome, which instead probably consists of migmatized metapsammites and metapelites intruded by late- to post-kinematic granites (Downing, 1983; Jacob et al., 1986; Bowden and Tack, 1995; Oliver and Kinnaird, 1996).

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Table 1 Lithostratigraphy of the Etusis, Khan and R€ ossing Formations in the vicinity of R€ ossing Group and nature

Formation

Member/unit

Thickness (m)

Description

Swakop (pelitic and calcareous)

R€ ossing

Metaquartzite

>100

Upper metapelitic gneiss

40–50

Upper marble

50–70

Lower metapelitic gneiss

30–40

Lower marble

20–50

Medium-grained quartzite, coarsening towards base Diopside-biotite (-scapolite) gneiss; thin lenses of coarse-grained metaquartzite and marble; grading upwards into cordierite gneiss Serpentinitic and diopside-quartz-bearing marble; lenses of biotite-diopside granofels and cordierite-biotite gneiss. Basal conglomerate Cordierite-biotite-sillimanite gneiss; grading upwards into biotite-hornblende schist Dominant serpentinitic and minor graphitic marble; lenses of biotite-diopside granofels and pelitic schist

Amphibolite

10–20

Upper gneiss Pyroxene-garnet orthoamphibolite gneiss

70–100 <120

Lower gneiss

70–150

Upper metaquartzite

>300

Nosib (fluviatile)

Khan

Etusis

Amphibole–biotite schist with relict pebble bands or conglomerate at its base Amphibole- and pyroxene-gneiss Pyroxene-garnet gneiss and pods of hornblende-oligoclase ortho-amphibolite, often migmatized Strongly banded clinopyroxene-amphibole gneiss; migmatized in high-strain zones Arkosic and micaceous gneisses, feldspathic quartzites and biotite schist; extensively intruded by late- to post-kinematic granites and migmatized in high-strain zones

Upper biotite schist Lower metaquartzite Lower biotite schist Abbabis metamorphic complex/basement

–

–

Augen-, migmatitic-, biotite-, silimanite- granite-gneiss; biotite schist and amphibolite

The quoted thicknesses are approximate. Many of the lithologies are partially or pervasively migmatized. Summarized after Nash (1971), SACS (1980), Coward (1983), Downing (1983), Martin (1983), Lehtonen et al. (1996) and Nex (1997).

The Etusis Formation locally consists of a Lower Biotite Schist Member, a Lower Metaquartzite Member, an Upper Biotite Schist Member and an Upper Metaquartzite Member (Nash, 1971; Lehtonen et al., 1996; Oliver and Kinnaird, 1996; Table 1). The Upper Biotite Schist and Upper Metaquartzite Members are truncated and overlain by the intraformational R€ ossing SJ Shear Zone (Oliver, 1994, 1995). The gneisses of the Khan Formation (Nosib Group), comprising locally banded migmatitic lithotypes that reflect a protracted tectonothermal history, gradationally overlie (Nash, 1971; Berning et al., 1976) or interfinger (Downing, 1983) with the Etusis Formation. The Khan Formation consists mainly of amphibole–clinopyroxene gneisses, interpreted to reflect the change to a more calcareous and less clastic sedimentary protolith (Downing, 1983; Martin, 1983). The uppermost portion of the Khan Formation shows increased amphibole in place of pyroxene, culminating in a thin unit of amphibole–biotite schist with minor, discontinuous pebble horizons (Nash, 1971; Table 1).

The R€ ossing Formation paraconformably and disconformably overlies the Khan Formation. In the vicinity of the R€ ossing Mine it is divided into a lower serpentinitic marble, a metapelitic gneiss and an upper siliceous and serpentinitic metacarbonate unit interbedded with granofelsic/schistose layers, which is succeeded by metapelitic gneiss subunits (Nash, 1971).

4. Regional geophysical lineaments Lineaments in the Damara Orogen are defined by the alignment of prominent negative magnetic anomalies related to layers within shallow suboutcropping or outcropping Nosib Group metasedimentary units (Corner, 1982, 1983, 2000; Eberle et al., 1995), changes in the orientation of kinematic fabrics and by structural corridors (c.f. O’Driscoll, 1981, 1986). Geophysical modeling by Corner (1982, 1983, 2000) found that many magnetic lineaments are late-kinematic geanticlinal ridges dividing the NE-trending intracontinental branch of

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417

western portion of the Central Zone. To the east of the Welwitschia Lineament, F3 domes and folds have predominantly NE trends, while to the west, F3 domes and lithologies trend predominantly NNE. Recent offshore aeromagnetic data show that the Welwitschia Lineament continues to the SW, essentially demarcating the boundary between the southern Central Zone and the Southern Zone (Fig. 1; Corner, 2000). Although there are no published ages for the initiation of this late-kinematic lineament, its crosscutting relationship with respect to the products of earlier deformation events (specifically those of ‘‘D3 ’’––see later), pre-existing lineaments and lithological trends is evident from regional and local aeromagnetics (Corner, 1982, 1983, 2000; Eberle et al., 1995; Anderson and Nash, 1997). Late-kinematic, left-lateral, strike-slip movement along the Welwitschia Lineament is also suggested by its truncation of the N-trending Okahandja Lineament (Fig. 1; Corner, 1982, 1983, 2000). Multiple, parallel to sub-parallel, NNE-trending (‘‘Welwitschia Trend’’), left-lateral shears occur in the vicinity of the R€ ossing Dome (see Fig. 2), a feature documented by Anderson and Nash (1997) and Lord et al. (1996). Corner (2000) observed that mafic dykes, possibly of Karoo-age, have utilized this trend, as is shown in the Landsat interpretation of Fig. 2c. In this study the term ‘‘Welwitschia Trend’’ will be used to refer to the similar orientation of interpreted or observed late-kinematic, NNE-trending, left-lateral shear zones in the southern Central Zone and in the vicinity of R€ ossing.

5. Regional structural evolution

Fig. 2. (a) Total Magnetic Intensity (TMI) image over the R€ ossing Dome (RD) and R€ ossing Mountain (central bilobate form). The inset is a First Vertical Derivative (FVD) of the TMI. (b) LandSat Image of a portion of (a). (c) Two main trends are evident from the interpretation of (a) and (b): NE trending lithological banding/foliation and NNE-trending lineaments (parallel to the NNE ‘‘Welwitschia Trend’’), which are delineated as shears due to their offset of banding/foliation (q.v. Anderson and Nash, 1997). The inset shows a Rose diagram of the trends of NNE to NE-trending (probably late-Cretaceous; Lord et al., 1996) dykes (N ¼ 236) in the area between R€ ossing and Goanikontes.

the Damara Orogen into regional ridges and grabens. Corner (1982, 1983), Jacob et al. (1986) and Eberle et al. (1995) proposed a spatial correlation between uraniferous granite plutons and regional magnetic lineaments, particularly the NNE-trending Welwitschia Lineament (Fig. 1). This lineament, interpreted by Corner (1982, 1983, 2000), marks a change in structural style in the

Smith (1965), Sawyer (1978), Martin (1983), Coward (1980), Buhn and Stannistreet (1992), Anderson and Nash (1997) and Nex et al. (2001a,b) provide overviews of regional deformation events in the southern Central Zone, which includes the R€ ossing, Khan River, Goanikontes and Swakop River areas (Table 2). Original sedimentary layering (S0 ) is preserved in the form of bedding and cross-bedding within the Khan and Etusis formations in the Goanikontes area. Planar bedding and pyritic quartzite bands are preserved in the marbles of the R€ ossing Formation. The dominant fold orientations and associated foliations produced during the first (D1 ) and second (D2 ) deformation events are not readily discernable, although at least one model proposes that F2 trended NW, prior to F3 interference folding along NE-trending axes (Table 2 and references therein) to produce the domes of the Central Zone. Alternative theories have been proposed (e.g. Coward, 1980; Jacob et al., 1983; Oliver, 1995). S1 and S2 migmatitic banding, particularly evident in the areas to the west of the R€ ossing Dome, are preserved within the Khan and Etusis formations, while S1 and S2 occur as laminar flow

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Table 2 Deformation events of the R€ ossing area, Central Zone, Damara Orogen, summarized from Smith (1965), Sawyer (1978), Martin (1983), Coward (1983), Buhn and Stannistreet (1992), Anderson and Nash (1997) and Nash (1971) Deformation event

Major fold trend

R€ ossing Formation

Khan Formation

Etusis Formation

Original layering

–

D1

F1 : unknown

Planar bedding in marbles and pyritic bands S1 and S2 laminar foliation in pelites

Not evident in R€ ossing area S1 and S2 migmatitic banding to the west of the R€ ossing Dome

Relict heavy mineral layering in orthoquartzites S1 and S2 migmatitic banding

D2 D3

F2 : NW (?) F3 : NE

S3 schistocity superimposed on migmatitic banding

D4

(‘‘F4 ’’): NNE R€ ossing Dome Axis Orientation and ‘‘Welwitschia Trend’’

S1 and S2 transposed into subparallelism with S3 ; flow folding in marbles Small, isolated folds with axial planar foliation (S4 ) Inferred NNE-trending leftlateral shears cross-cutting and re-orienting NE-trending F3 folds (shears later intruded by Karoo dolerites)

Non-systematic brittle– ductile deformation of migmatitic banding Not discernible

foliations in R€ ossing Formation metapelites (Table 2). D3 deformation produced highly ductile flow folding in the marbles of the R€ ossing Formation. An S3 gneissosity to schistocity (dependant upon the proportion of phyllosilicates), as observed in the R€ ossing Dome and the R€ ossing deposit, almost pervasively replaced migmatitic S1 and S2 banding. The Etusis Formation shows non-systematic brittle–ductile deformation of S1 /S2 migmatitic banding (Table 2 and references therein). Based on crosscutting relationships between dated granites and ductile, mid-crustal shear zones, the peak of Damaran transpressional deformation (D1 and D2 ) and metamorphism occurred from ca. 600 to 550 Ma (Bowden et al., 1999). This was followed by constrictional tectonics, possibly including doming by the model of Oliver (1995) and Poli and Oliver (2001) at ca. 542–526 Ma, in turn succeeded at approximately 510 Ma (U–Pb date on monazite) by the peak of granite plutonism (Briqueu et al., 1980; Allsop et al., 1983; de Kock and Walraven, 1994; Nex, 1997). This was followed by a ca. 505–478 Ma period of cooling and successive closure of minerals at their respective blocking temperatures (Bowden et al., 1995, 1999; Jacob et al., 2000), possibly extending to ca. 429 ± 17 Ma (Clifford, 1967). Nex et al. (2001b) cite evidence for a post-534 ± 7 Ma/pre-508 ± 2 Ma regional Central Zone heating event (termed M2), which annealed the great majority of fabrics and minerals in sheeted granites, metasediments and even mylonites within high-strain zones. M2 was superimposed on the first regional tectonometamorphic event (M1), which has been assigned a 571 ± 64 to 534 ± 7 Ma date range (Nex et al., 2001b). Tack et al. (1995) assign an 39 Ar–40 Ar biotite and hornblende stepheating date of 465 ± 2 Ma to the Khan River Detachment, thereby implying rapid cooling within high-strain

Inferred NNE-trending left-lateral shears crosscutting and re-orienting NE-trending F3 folds

zones. The M2 event of Nex et al. (2001b) was observed in samples from R€ ossing (this study), which displayed signs of extensive annealing. As such, commonly used kinematic indicators that may have formed during D3 or D4 are not readily discernable and are therefore not useful in defining late-kinematic processes. Fig. 3 highlights structural features in the vicinity of R€ ossing. Pervasive F3 folding or constrictional deformation produced an average fold orientation of 042–27
in the area to the SW of R€ ossing (Jacob and Kerber, 1997; great circle fit to contoured S3 foliation poles, N ¼ 1057). Jointing formed during late D3 has a mean trend of NW–SE (Fig. 3; Jacob et al., N ¼ 717). It is apparent from Fig. 3 that there is a ‘‘discordance’’ of approximately 25
between the regional trends of F3 folds (042
) and the long axis of the R€ ossing dome (017
). That is, the dome’s long axis is 25
anticlockwise from the mean fold axis orientation defined by Jacob and Kerber (1997). This, and the concentration of widespread granites to the S, SW and SE of the dome suggest that the post-D3 evolution of the Dome (i.e. D4 ) was somehow pivotal in localizing deformation, granite bodies (e.g. Vigneresse, 1995a,b) and potentially uranium-enriched fluids. Previous studies on D4 suggest that it is commonly expressed as small, isolated E–W trending folds with a concomitant axial planar foliation, particularly evident to the south of the R€ ossing Dome (e.g. Martin, 1983; Coward, 1983). The effects of D4 deformation in the Khan and Etusis formations have not been well defined, although late-kinematic shears and/or regional lineaments that cross-cut and re-orient NE trending F3 folds (Fig. 2), appear to have been operative during the closing stages of D3 and during D4 deformation (Corner, 1982, 1983, 2000; Anderson and Nash, 1997). Anderson and Nash (1997), in their integrated

I.J. Basson, G. Greenway / Journal of African Earth Sciences 38 (2004) 413–435

419

Fig. 3. Compilation of mapping by various researchers and this study. The source of data is shown in the inset at bottom right: (a) Smith (1965); (b) Jacob (1974) and Anderson and Nash (1997); (c) Greyscale Landsat––this study and Anderson and Nash (1997); (d) aerial photo mosaic––this study and Anderson and Nash (1997); (e) Oliver and Kinnaird (1996); (f) this study/the subject of this contribution. Also shown at bottom right are contoured, equal area, lower hemisphere stereonets from Jacob and Kerber (1997), both contoured at 1% surface area and 1% intervals: (1) S3 data showing a best-fit F3 fold axes of 042–27
; and (2) J3 joint data, which essentially defines a NW–SE trending joint set. The approximate source of this data is indicated as ‘‘D3 data’’ on the map. Note the angular difference between the trend of the R€ ossing dome and the regional F3 trend.

lithostructural study of the R€ ossing area, concluded that polyphase folding, culminating in F3 , was followed by the development of a post-D3 to D4 deformation system of Welwitschia Trend sinistral strike-slip zones (Fig. 2). The effect of Welwitschia Trend shears have not been taken into account in the oft-quoted structural summary in Buhn and Stannistreet (1992). It is not within the scope of this study to re-interpret the origin of D3 /F3 domes and folds or NNE-trending shears, rather new mapping and data focuses on the nature of late/post-D3 dome evolution and post-doming D4 deformation to the S, SW and SE of the R€ ossing dome, in the SJ and R€ ossing Mine areas (Fig. 3, inset). As a qualitative link between tectonism, specifically D3 and D4 , and the nature of granite intrusion has been proposed by Nex and Kinnaird (1995) for the nearby Goanikontes area, it is worth briefly reviewing this study.

6. Granite intrusion at Goanikontes Nex and Kinnaird (1995) and Nex et al. (2001a) investigated a high-strain zone in the Goanikontes area, approximately 30 km to the west of the R€ ossing area. Six granite types (Types A–F; Table 3) were recognized at Goanikontes and were classified according to field appearance, crosscutting nature, mineralogy and petrology. An overall compositional trend, with an increase in alkali feldspar content over time, from early monzogranite to alkali feldspar leucogranite, concomitant with a broad increase in the concentration of primary uranium mineralization, was recognized by Nex and Kinnaird (1995) and Nex et al. (2001a). Uranium mineralization peaked in Type D, with average (100 counts per second) and maximum (400 counts per second) scintillometer readings that are an order of

420 Table 3 Salient features of granite/leucogranite types recognized in the Swakop River area (Goanikontes), summarized from Nex and Kinnaird (1995) and Nex et al. (2001a,b), and the R€ ossing Area (this study) Type

Colour

Texture

Mineralogy/other features

Goanikontes: location and morphology (Nex and Kinnaird, 1995; Nex et al., 2001a,b)

R€ ossing: key location

NW dome limb of Khan Formation <0.1 m wide, fine-/mediumgrained saccharoidal texture, largely conformable to S3 foliation; ptygmatic fold axes are parallel to the F3 axial-planar foliation (Fig. 6) NW and SE dome limbs of Khan Formation 0.1 to 2 m wide, often strongly boudinaged with weak imposed S3 foliation at margins (Fig. 6) NW and SE dome limbs of Khan Formation Etusis Formation in the dome core 0.1–0.5 m wide, frequently at high angle to foliation and preintruded granites, may show pure shear and boudinaging with the partial development of weak S3 foliation (Fig. 6) Dilated S4 in NW dome limb of Khan Formation, fault network in R€ ossing Mine, highly attenuated (high strain) Khan Formation ‘‘streaming’’ to the NE from the eastern edge of the dome core 0.2 to 3 m wide, partially replaced by Type E and F granites in R€ ossing (Figs. 7–9) Dilated S4 in NW dome limb of Khan Formation, fault network in R€ ossing Mine, highly attenuated (high strain) Khan Formation ‘‘streaming’’ to the eastern edge of the dome core Approx. 30 m wide massive tabular bodies in Mine (Figs. 7–9)

Pale pink

Fine- to mediumgrained, homogeneous saccharoidal

Dominantly white feldspar

Narrow (<0.75 m) irregular sheets and veins, weakly foliated, boudinaged and folded by D3 , infrequent occurrence

B

Dominantly white

Fine-grained to pegmatitic (variable)

Garnetiferous, with tourmaline and biotite as infrequent accessory minerals

Sheets boudinaged and folded by D3 , common outside high strain zone, weakly foliated

C

White to pale pink

Medium-grained pegmatitic with clear interstitial quartz

Two feldspar populations, accessory magnetite, ilmenite and tourmaline

Abundant, dominant in relatively undeformed cover sequence, occasionally boudinaged and occurs in F3 flexures

D

White

Medium- to coarsegrained, granular texture more variable than other leucogranite types

Host rock to primary uranium mineralization, white feldspar, characteristic smoky quartz, betauranophane occasional betafite and apatite

Extremely irregular and anastomosing, concentrated in high strain zones along the Khan/ R€ ossing contact (i.e. sheared amphibole–biotite schist)

E

Red to pink, very variable colour with ‘‘oxidation haloes’’

Extremely variable–– fine-grained to very coarse-grained pegmatitic

Similar to Type D or consists almost entirely of smoky/black quartz and pink feldspar

Irregular, tabular to bifurcating bodies in high strain zones, generally emplaced parallel to foliation in basement rocks

Typical morphology (this study) Pre-D4

Pre-D4

Pre-D4

D4 (509 ± 1 Ma; U–Pb monazite age, Briqueu et al., 1980)?

D4

I.J. Basson, G. Greenway / Journal of African Earth Sciences 38 (2004) 413–435

A

Deformation event

The types and sequence of granites are very similar in both areas, although the proposed rotation and southward movement of the competent core strongly controls granite forms and structures at R€ ossing.

F

Red (distinctive)

Coarse-grained pegmatitic

Pink, coarse perthitic feldspar, milky quartz, accessory biotite, magnetite and ilmenite

Tabular bodies with parallel sides, cross-cuts all other structural features

Fault network in R€ ossing Mine, Dome core - Etusis Formation; massive extensive outcrops to SE of mine approx. 30–40 m wide massive tabular bodies (Figs. 7–9)

D4

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421

magnitude higher than those of the other granite types (Nex et al., 2001a). The exact structural context of granites is commonly obscured in high-strain areas, which may coincide with zones of enhanced migmatization (Nex et al., 2001a). Granites at Goanikontes and R€ ossing may be divided into those deformed by D3 and D4 (i.e. Types A, B, and C), and those which are post-D3 (i.e. D4 Types D, E and F; Table 3). Granite Types A and B at Goanikontes are pale pink to white, irregular to boudinaged, notably folded by D3 , display a weak S3 foliation, particularly at their margins and may occur in high-strain zones. Type C granites are white to pale pink and comprise the dominant granite within the cover sequence, being occasionally boudinaged and emplaced in F3 ‘‘flexures’’. White Type D uranium-enriched granites, distinguished by their characteristic grey or smoky quartz, take irregular to anastomozing forms and are typically confined to high strain zones and the Khan–R€ ossing contact. Type E granites dominate in high-strain, post-D3 zones, taking the form of bifurcating bodies with highly variable grain size and colour. Lastly, distinctively red Type F granites are post-kinematic, having parallel sides and cross-cutting all pre-existing structures (Table 3; Nex and Kinnaird, 1995; Nex et al., 2001a). Nex et al. (2001a,b) suggest that granite Types B to F were intruded after ‘‘M2’’ regional contact metamorphism (534 ± 7 Ma to 508 ± 2 Ma). This sequence of granite intrusion, in particular the division of granites between pre- and post-D4 deformation was also recognized in the R€ ossing area (Table 3).

7. Granites of the central zone Granitic rocks of the Central and southern Central Zones are described in Haack and Hoffer (1976), Miller (1983b), Jacob et al. (1986), Kerr (1990), Bowden et al. (1995), Nex and Kinnaird (1995), Nex (1997), Jacob et al. (2000) and Nex et al. (2001a). Granite complexes are categorized and described according to their age, morphology and dominant mineralogy, with the polycyclic basement granite–gneisses forming the oldest group with a minimum age of 1038 ± 58 Ma (Kibaran granitoid–gneiss; Kr€ oner et al., 1991; Table 4). The Mon Reposdiorite, Rotekuppe monzogranite, Ida Dome granitoids, foliated Red granite and unfoliated white– grey granite at Goanikontes, the Okongava diorite, the Salem granite, gneisses at Goanikontes, the Donkerhuk granite (and the Khan Formation gneisses at Goanikontes) yield intrusion, syn-metamorphic or anatexis dates from 563 ± 4 Ma to 505 ± 4 Ma (Table 4). Uranium-enriched granites in the Goanikontes area yield 508 ± 2 Ma (U–Pb uraninite date: Briqueu et al., 1980), 509 ± 1 Ma (U–Pb monazite date: Briqueu et al., 1980), approximately 510 Ma (errorchron Rb–Sr whole

422

Table 4 Summary of dates of intrusion-related anatexis, metamorphic and tectonic/kinematic events of the Central Zone (Rb–Sr whole-rock isochron/errorchron dates have largely been excluded) Dating method/mineral

Event description from reference

Age (Ma)

Reference

Interpretation (Bowden et al., 1995, 1999; Jacob et al., 2000)

Interpretation (Nex et al., 2001a,b, based on Goanikontes area)

Kibaran granitoidgneiss, Khan River (Abbabis)

U–Pb SHRIMP: zircon cores

Crystallization of basement granite-gneiss

1038 ± 58

Kr€ oner et al. (1991)

Minimum age of polycyclic pre-Damara basement

G1 Orogenesis, continental collision and crustal thickening Intrusion of Type A granites Peak Damaran Metamorphism

U–Pb SHRIMP: zircon rims U–Pb SHRIMP: zircon

Damaran metamorphic overprint Granite intrusion

571 ± 64

Kr€ oner et al. (1991)

563 ± 4 to 546 ± 6

Jacob et al. (2000)

U–Pb SHRIMP: zircon

Granite intrusion

543 ± 5 to 539 ± 6

Jacob et al. (2000)

Granitoids, Ida Dome

U–Pb SHRIMP: zircon

Early post-collisional

ca. 542–526

Bowden et al. (1999)

Foliated Red Granite, Goanikontes

U–Pb single ziron

Syn-metamorphic anatexis

534 ± 7

Briqueu et al. (1980)

P1 ca. 600–550 Ma–– peak transpressional tectonism and metamorphism––oblique N–S collision of Kalahari and Congo Cratons (doming?) P2 ca. 542–526 Ma––transtensional tectonism and metamorphism (doming?)

Unfoliated white-grey granite, Goanikontes

U–Pb: monazite

Syn-metamorphic anatexis

517 ± 7

Briqueu et al. (1980)

Okongava diorite

U–Pb: zircon evaporation U–Pb: zircon

Early-tectonic

516 ± 6

Syn-metamorphic intrusion Syn-metamorphic growth

512 ± 40

de Kock and Walraven (1994) Allsop et al. (1983)

510 ± 3

Briqueu et al. (1980)

Mon Repos diorite, Navachab Rotekuppe monzogranite, Navachab

Salem granite, Goas Khan Formation gneisses Goanikontes

U–Pb: monazite

Alkali leucogranite (‘‘alaskite’’), Goanikontes Alkali leucogranite (‘‘alaskite’’), Goanikontes Donkerhuk granite, Otjimbingwe

U–Pb: uraninite

Intrusion/post-tectonic doming

508 ± 2

Briqueu et al. (1980)

U–Pb: monazite

Intrusion/post-tectonic doming

509 ± 1

Briqueu et al. (1980)

U–Pb: zircon

Intrusion/late-tectonic

505 ± 4

Kukla et al. (1991)

Meta-lamprophyre sill, Navachab

U–Pb: SHRIMP, titanite met. overgrowths

Mineralization/cooling below closure

496 ± 12

Jacob et al. (2000)

Syn-metamorphic red granite intrusion

P3 ca. 510 Ma––peak of granite plutonism

G2 Decompression, D3 deformation and F3 dome formation, continued red and grey granite intrusion Intrusion of Type B & C granites Constrictional deformation in high strain zones Post-decompression isobaric annealing and crystallization

This study: reactivation of N–S compression, causing left-lateral movement along NNE-trending shears with dome rotation and southward impingement, particularly at R€ossing

G3 Intrusion of Type D granite and main uranium mineralization

Intrusion of Donkerhoek granites (q.v. de Kock and Walraven, 1994)

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Lithology/locality

Sheeted leucogranites of the Goanikontes area (Nex et al., 2001a,b) and the interpretation of tectonic events by Bowden et al. (1995, 1999) and Jacob et al. (2000). Different ages for U-granite intrusion have been proposed by Briqueu et al. (1980) and Von Backstr€ om and Jacob (1978). The date of 1038 ± 58 Ma (Kr€ oner et al., 1991) for the polycyclic pre-Damara Basement is probably a minimum.

429 ± 17 Biotite schist, Khan-Swakop River

K–Ar: biotite

Cooling below 300
C

Hawkesworth et al. (1983) Clifford (1967) Ar/39 Ar: hornblende 40

Mineralized quartz veins, Navachab Diorite, Otjozondjou

U–Pb: SHRIMP, titanite

Mineralization/cooling below closure Cooling below 500
C

478 ± 4

Jacob et al. (2000) 494 ± 8 to 500 ± 10

P4 ca. 505–478 Ma (extended to ca. 429 Ma?)–– late/post-tectonic intrusion, cooling and successive closure of minerals at their respective blocking temperatures

Cooling and localized resetting of Rb–Sr ages

G4 Intrusion of Type E & F granites

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423

rockdate; Kr€ oner, 1982) and 468 ± 8 Ma (Rb–Sr biotite date: Von Backstr€ om and Jacob, 1978; Kr€ oner et al., 1978). The latter intrusions are synchronous with the final stages of post-kinematic cooling or 468 ± 8––the age of regional cooling through the biotite blocking temperature. As many Rb–Sr dates are probably unreliable, Table 4 does not place any significance on Rb–Sr whole-rock isochron or errorchron dates. Bowden et al. (1995) suggested that granite plutonism that produced the granites at R€ ossing peaked at approximately 510 Ma, on the basis of a syn-metamorphic U–Pb monazite growth date of 510 ± 3 Ma for Khan Formation gneisses (Briqueu et al., 1980). This was followed by minor, repeated intrusions of granite (some of which are uranium-enriched) and hydrothermal activity until approximately 478 ± 4 Ma (Ar–Ar hornblende cooling age: Hawkesworth et al., 1983; Table 4), whereupon Rb–Sr mineral blocking temperatures were reached. Clifford (1967) extended the postkinematic cooling period to 429 ± 17 Ma (K–Ar biotite cooling below 300
C). Late tectonism and retrograde metamorphism of the southern Central Zone was evidently a remarkably protracted event (potentially up to 80 my in duration, from 510 Ma to approximately 429 Ma, Table 4), a concept supported by Haack (1976) and Jacob et al. (1986), who attribute this to regional radioactivity during that time.

8. Local structural interpretation The total Magnetic Field (TMI) image and its First Vertical Derivative, a high resolution, greyscale Landsat image encompassing the R€ ossing Dome and R€ ossing Mountain, and a mosaic of seven aerial photographs were structurally interpreted to provide a basis for ground-truthing and a context for mine-derived data. The interpretation was combined with that from Anderson and Nash (1997). In general, foliations are defined by alternating bands within the R€ ossing and Khan Formations, while faults and shear zones are defined by an observable offset or deformation of lithological layering or banding (Anderson and Nash, 1997; Corner, 2000). While the dominant lithological trend is NE (S3 /F3 : Table 2; Fig. 3), the trend of post-S3 /F3 structures, which crosscut and offset D3 structures are predominantly NNE-trending (Fig. 4; Welwitschia Trend). Several of these inferred shears host 035
–050
trending mafic/dolerite dykes (Lord et al., 1996; Corner, 2000), which are clearly evident next to the road between R€ ossing and Swakopmund to the west. Dolerite dykes in the southern Central Zone also trend NNE to NE (Fig. 2c, inset), sub-parallel to the long axis of the R€ ossing Dome (017
). A pervasive or dominant fabric, designated S3 in accordance with regional data and mapping (Fig. 3)

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the dome rim and near a shear zone to the east of the dome, is a second fabric, termed S4 . The shear to the east of the dome is ‘‘bent’’, as are the adjacent F3 fold axes (Anderson and Nash, 1997). According to this study, the left-lateral shear to the east of the dome translates locally into a thrust in the area just S/SE of the dome (Fig. 4 and see Fig. 9). F3 fold axes are furthermore ‘‘concatenated’’, i.e. they are closer together to the S/SE of the dome. Large granite bodies occur to the east, and in a wedge-shaped zone to the south of the dome. In order to test these preliminary interpretations and to identify these structures, key areas around the dome were mapped. These will be described, with the presentation of photographs and structural data, in order to compile a final model of late-kinematic (D4 ) deformation and thereby to propose its control on mineralization within tectonometamorphism of the southern Central Zone of the Damara Orogen. 8.1. Area 1: Core of R€ossing Dome (Etusis Formation) The R€ ossing Dome core, as mapped by Nash (1971) and Oliver and Kinnaird (1996), consists of highly migmatized and metasomatized Etusis Formation. Relict heavy mineral layering is evident in unmetasomatized areas (Fig. 5). S01 foliation in the metaquartzites and biotite schists is arranged around the center of the dome, yielding a NE-trending fold axis (mapping by Oliver and Kinnaird, 1996). The azimuth of an interpreted ‘‘L1 ’’ (probably L3 , given the sequence of deformation) stretching lineation in the Etusis Formation metaquartzites trends NNE (q.v. mapping by Oliver and Kinnaird, 1996). At the SE margin of the dome core, a brittle–ductile, spaced cleavage in the Etusis Upper metaquartzite trends at approximately 046
. Overall, the effects of D4 are not obvious within the Etusis Formation. 8.2. Area 2: NW rim of R€ossing Dome (Khan Formation)

Fig. 4. (a) Interpretation of the tectonic fabric and major shear zones, from a 1:35 000 scale black and white aerial photograph mosaic. Features of interest include NNE-trending shears that crosscut the preexisting (S3 /F3 ) lithological banding/foliation. Note that two foliations are evident in the Khan Formation in the NW limb; one which is tangential or conformable to the curve of the dome core (nominally S3 ) and a second one which trends NE, that occurs at an angle of up to 15
to the dominant foliation (nominally S4 ). (b) The interpretation of the same area by Anderson and Nash (1997) shows the ‘‘bending’’ of F3 fold axes around the S/SE tip of the dome core, ‘‘concatenation’’ of F3 fold axes in the same area and the local translation of a NNE-trending shear zone (termed Fault 14 or F14 in open pit terminology) into a thrust.

essentially defines the curve of the dome. Crosscutting this at an acute angle, particularly in the NW portions of

In accordance with more regional mapping (Table 2; Fig. 2) and Landsat and aerial photograph interpretation, the dominant foliation in the Khan Formation is termed S3 , defined by a strong schistose or gneissose foliation that all but obscures the S1 and S2 migmatitic banding. Two foliations occur in the NW margin of the R€ ossing Dome, one following the curve of the dome margin (S3 ), the other at approximately 15
to this in a clockwise sense (S4 ), although this varies somewhat due to the curvature of the dome rim (Figs. 4 and 6a). The average orientation of S3 in this segment of the dome rim (strike and dip, wherein the direction of dip is always 90
clockwise from the strike, which is measured as an absolute value clockwise from 0
or true north) is 208/67
(N ¼ 97), while the average orientation of S4 is 221/74
(N ¼ 34). Jointly, the foliations in outcrop show

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425

Fig. 5. Features of Area 1 (Dome core, consisting mainly of Etusis metaquartzite). The core of the dome contains metasomatized granites as small, irregular lenses and bodies within the Etusis metaquartzite (EMQ). No systematic orientation of granite bodies is discernable. The lens cap is 6 cm in diameter (a, b), while the hammer in (c) is 45 cm long. Contoured stereonets at right are from Nex and Kinnaird (1995).

a typical, acute, sinistral S–C format (Figs. 4,6a–c). In the central part of the NW limb of the dome, dilated S3 and S4 contain thin quartzofeldspathic lenses (Fig. 6b and c). It is significant that large, irregular granite bodies locally follow the trend of both foliations (see Fig. 12a). Small, ptygmatic folds in thin, S3 -parallel Type A granite stringers have an overall NNE trend (Fig. 6d) parallel to the L3 derived by Nex and Kinnaird (1995). Larger bodies of pre-F3 granites display boudinaging and extension parallel to S3 (e.g. Type B granite, Fig. 6e), the boudin axes being distributed along an S3 plane by ongoing D3 pure shear. The average orientation of boudin axes, expressed as direction of plunge and plunge is 307–62
(N ¼ 59). A great circle fit to their distribution shows an expected NNE trend. Type C granites developed folding and ‘‘cusps’’ where their margins locally adopted an incipient S3 cleavage/foliation (Fig. 6f, ‘‘IC’’), indicating that although their relative timing was broadly synchronous with D3 or F3 folding, they did not develop intra-foliation folding, ptygmatic folding or boudins due to D3 pure shear in F3 fold limbs, to the same extent as granite Types A or B (see Table 3). 8.3. Area 3: Southern tip of the R€ ossing Dome (Khan and R€ossing Formations) S3 and S4 may not be differentiated to the south of the dome, that is, they are conformable. Orientations of ‘‘S3=4 ’’ foliation, banding and lithological contacts indicate that the main F3 fold axis curves around the southern tip of the R€ ossing Dome, with the distance between limbs narrowing dramatically to the NE (q.v.

Fig. 4) of the dome. ‘‘Chocolate tablet’’ structures, indicators of pure shear acting at a high angle to the S3=4 foliation, are prominently developed to the S/SE of the dome (Fig. 7b). These resulted from thin, relatively competent pyritic quartzite bands within the R€ ossing marbles that underwent flattening and oblate strain. S3=4 is steeper on the northern limb than on the southern limb (average of 72
in the NE wall of the pit, average of 50
in the SE wall of the pit, N ¼ 390). As the inferred left-lateral shear to the east of the dome is approached, banding and S3=4 foliation of both the N and S limbs of the major F3 fold become more tightly constrained towards the NE portion of the pit (Fig. 7; N ¼ 100). Deformation of the large F3 fold to the south of the R€ ossing Dome is clearly shown in a 3-D model of the lithologies that contain the R€ ossing open pit; the northern limb of this F3 fold displays deformation (‘‘impingement’’) and attenuation of lithologies in its NE extent (Fig. 7f). Rare mesoscale F4 fold axes trend E–W. The almost pervasive granite intrusion to the south of the dome is wedge-shaped in plan view (Figs. 3 and 4), with the tip consisting of near-massive granite intrusions, extending to the NE (Fig. 4). Granites in the western extent of the wedge-shaped intrusion area occupy the curved S3 foliation within the Khan Formation. The local translation of the inferred NNE-trending shear in the east into a reverse fault/high angle thrust is evident on the south face of the open pit (Fig. 7, stereonet). This feature is colloquially termed ‘‘Fault 14’’(Figs. 7 and 8b and e) and displays both horizontal and steeply dipping slickenlines, the latter with ‘‘step features’’ which suggest low-angle reverse faulting, with the hangingwall of the fault moving upwards to the south (Ferre, 1998a,b, 2001).

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Fig. 6. Features of Area 2 (NW dome rim, consisting of Khan Formation). Features include the S3 /S4 cleavage relationship: (a––discernable in aerial photograph interpretations) and the dilation and infilling of this cleavage (DS3 and DS4 ––b and c). Granite types (a, b and c) display different intrusion modes, all of which are pre-D4 , (d) thin, ptygmatically folded Type A granite; (e) boudinaged Type B granite which has adopted an incipient cleavage at its margins; and (f) late pre-D4 Type C granite which shows incipient cusps and asperities on its margins. The hand lens is 2 cm in diameter (a, c, e); the compass-clinometer is 12 cm long, (b, d) while the lens cap is 6 cm in diameter (f).

Brittle–ductile to brittle faulting is remarkably abundant to the south of the dome. A four-month faultmodeling project, aimed at delineating and characterizing these faults, was undertaken in the early part of 2001 by the first author (IJB). The data for this project included more than 4000 fault intersections from boreholes, approximately 750 of which indicate extreme brecciation and fault movement. Of the total recorded fault intersections, a minority were oriented in 3-D space by a chalk or clay imprint method during drilling, although these commonly display errors (±5
) due to minor borehole deviation. A further 449 well-constrained faults, recorded during line and pit mapping, augmented this data set. Three main fault types, based on their orientation and sense of offset, were discerned by this approach, from MineSight modeling and from

data in geotechnical reports (e.g. Ferre, 1998a,b, 2001; Basson, 2001a,b; Fig. 8). The crosscutting configuration of this fault network has segmented the Khan and R€ ossing Formations into numerous triangular to irregular blocks. Granite emplacement along all three of the fault types is ubiquitous in the mine, creating a mesh or network of planar to sub-planar granite bodies (Figs. 7 and 8d). (a) Concave-north, curved, brittle–ductile shears run asymptotically into the larger NNE-trending shear zone to the SE of the open pit (Figs. 8a, b, e, and 9a), with the distance between these splays decreasing towards the main shear (i.e. these are also ‘‘concatenated’’). These splays, which also show a sinistral sense of offset on aerial photographs,

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427

Fig. 7. Features of Area 3 (south of dome core, consisting of Khan and R€ ossing Formations): (a) folding in pyritic quartzite layers in R€ ossing marbles in the south side of the open pit––height of face ¼ 120 m; (b) chocolate tablet structure, in pyritic quartzite layers in the R€ ossing marbles, indicating oblate strain––the lens cap is 6 cm in diameter; (c) large, 80 m high granite intrusion in the NW lobe of the open pit; (d) planar to wedgeshaped form of granites in the S wall of the open pit, indicating their structural control by splays off the large NNE-trending shear and concave-north faults––height of front bench is 15 m; (e) metasomatism of Type E granitic fluids by Type D granite, with the development of a characteristic ‘‘oxidation halo’’; (f) 3-dimensional model of the large F3 fold in which the open pit is situated, constructed from drilling and mining data as discussed in the text. The northern limb of this F3 fold, in the vicinity of the southern tip of the R€ ossing Dome, displays D4 deformation and southward-directed ‘‘impingement’’. The position of the R€ ossing open pit, with respect to the amphibole–biotite schist, is shown in Fig. 8.

essentially comprise synthetic shears to the main inferred NNE-trending sinistral shear (Figs. 4 and 9a). (b) Concave-south curved faults are symmetrically distributed on either side of the southern tip of the dome. Those to the east of the southernmost extremity of the Etusis metaquartzite display a sinistral sense of offset, while those to the west display a dextral sense of offset (Fig. 8a, c, e). (c) NNW-trending faults show minor offsets, dilation and alteration. These faults commonly act as water channels in the R€ ossing Mine and are usually filled with friable, porous secondary calcite

(Fig. 8a, d, e). Such faults include reactivated D3 joints (J3 , Fig. 3), which have been rotated anticlockwise by the same amount as the discordancy between the regional F3 trend and the long axis of the R€ ossing dome (i.e. approximately 15
; Fig. 3). Large-scale metasomatism of Type E granitic fluids into red Type D granites, producing a characteristic ‘‘oxidation halo’’ is common in the open pit (Fig. 7e). Similar features are not readily evident elsewhere around the dome.

428

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the shear trends NNE in its northeastern exposure. Biotite- and amphibole-quartz S3 gneissosity/schistocity (depending on composition) and the planar sheared and entrained Type D and E granites in the Khan Formation in the SE dome limb display markedly similar orientations (028=66
 N ¼ 24 and 030=67
 N ¼ 27, respectively) to the east of the dome. The mode of granite intrusion to the east of the dome, that is, highly planar contacts with a small range in orientation, should be contrasted with granites of the NW limb, which are massive, commonly non-planar, display ptygmatic to closed folding, have highly irregular to boudinaged margins and may occupy dilated S3 and S4 foliations. 8.5. Summary of granite intrusion modes

Fig. 8. (a) Geotechnically important faults within and around the R€ ossing open pit/Area 3. The lithology indicated is the amphibole– biotite schist, which occurs at the contact of the Nosib and Swakop Groups; (b) splays off the major shear zone to the east of the mine; (c) North-dipping, concave-north ‘‘indentation’’ faults, with complementary sense of shear, on either side of the southern tip of the dome core; (d) NNW, N–S and NNE-trending brittle, dilational faults, including originally NW-trending D3 joints (J3 ) that have been rotated anticlockwise with the long axis of the R€ ossing dome; and (e) available orientation data for line-mapped faults (equal angle stereonet, N ¼ 449), distinguished by their orientation, sense of shear, morphology and infill (Ferre, 1998a,b, 2001; Basson, 2001a,b; R€ ossing geotechnical data).

8.4. Area 4: SE Rim of R€ ossing Dome (Khan Formation) The shear zone to the east of the mine (Fig. 4 and 9a) shows left-lateral displacement in the field and is ‘‘bent’’ around the southern tip of the dome (N ¼ 89), such that

A relative intrusion sequence (summarized in Tables 3 and 4) may be determined by the crosscutting relationships and the morphology of sheeted granites. Granites that intruded prior to D4 (Types A, B and C) display notable boudinaging, folding within parasitic F3 folds and invariably adopted a weak S3 foliation towards their margins (e.g. Fig. 6e and f). The distribution of boudin axes on the margins of D3 granites (Fig. 6) indicates D3 /F3 pure shear, flattening and boudin axis rotation (towards a common average azimuth of 307  62
 N ¼ 59). Granite Types D, E and F have shallower dips and predominantly NE trends (223/58
) in the NW rim of the dome, indicating their exploitation of dilating S3 and S4 in local tensional zones. The latter granites show little or no boudinaging and have highly irregular to planar margins. Post-D3 to D4 granites to the east of the dome trend NNE to NE, parallel to the nearby shear zone and the S3=4 foliation in this area (Fig. 9b). Plots of the margins of all granites encountered in the study display an unusual pattern on stereonets (Fig.

Fig. 9. Features of Area 4 (SE of R€ ossing Dome core); (a) the major NNE-to NE-trending, locally brittle–ductile shear zone to the S/SE of the dome tip, exhibiting splays that indicate left-lateral movement––lens cap is 6 cm in diameter; (b) closely-spaced, multiple, highly planar, conformable Types D–F granites to the S/SE of the dome tip––width of outcrop approximately 70 m.

I.J. Basson, G. Greenway / Journal of African Earth Sciences 38 (2004) 413–435

Fig. 10. Equal angle and equal area (contoured) lower hemisphere stereonet projections of poles to major sheeted granites in the R€ ossing Dome area. Pre-D4 (Types A–C) granites have steeper overall dips and largely trend NNE, while D4 (Types D–F) granites have shallower overall dips and trend NE to NNE. The combined contoured plot clearly indicates that the NW and SE-dipping granites fall along two great circles that diverge from the edge of the stereonet, suggesting intrusion during rotation.

10; N ¼ 268); data diverge from the stereonet margin along independent great circles.

9. Uranium mineralization at R€ ossing Uranium-enriched, late- to post-kinematic (post-D3 to D4 ) granites, specifically Types D and E (Tables 3 and 4), are remarkably extensive in the R€ ossing deposit. They contain dispersed xenoliths and skialiths of host rock to the south of the R€ ossing Dome and in the shear zone to the east of the dome (Berning et al., 1976; Berning, 1986). Granites in the R€ ossing Mine intruded preferentially into the biotite–amphibole gneiss and amphibole–biotite schist of the Khan and R€ ossing Formations (Table 1). The main, primary uranium mineral is magmatic uraninite, which is included in quartz and feldspar, occurs interstitially between these minerals or shows a preferential association with biotite and zircon (Berning et al., 1976; Cuney, 1979; Berning, 1986). Lesser high-Nb + Ti betafite occurs as inclusions in quartz and feldspar (Berning, 1986). Secondary uranium mineralization, due to hydrothermal or surficial weathering, takes the form of uranophane, beta-uranophane, gummite, torbernite/metatorbenite, carnotite, metahawaiweeite and thorogummite (Berning et al., 1976; Berning, 1986; Cuney, 1979; Marlow, 1981). Mineralization and granite crystallization processes have been detailed by Cuney (1979), Marlow (1981), Kerr (1990), Bowden et al. (1995) and Nex et al. (2002). Fluid inclusions from the R€ ossing SJ and SH areas (see Fig. 12) show that the former contain higher H2 O contents, higher H2 O/CO2 ratios and higher total fluids compared to the SH area (Nex et al., 2002). This is interpreted to represent a higher, localized fluid flux in the SJ area, which caused U redistribution from urani-

429

nite into secondary beta-uranophane (op. cit). Nex et al. (2002) suggest that the sub-vertical (fault-controlled, this study) orientation of the tabular granite sheets facilitated greater fluid movement, compared to the relatively large or massive SH area to the west. This study supports this conclusion, but points to the lateossing dome as a kinematic D4 movement of the R€ means of preferentially maintaining high strains, and thereby greater solution of lithologies in the R€ ossing SJ and mine areas. The localized fault network to the S/SE of the dome undoubtedly contributed to the large-scale permeability of the area.

10. Ore body modeling The present In-situ Resource Model utilizes three drilling/sampling information databases: (a) Underground Diamond Drilling: NW-trending arrays of drill holes, on 15 m centers along three crosscuts, drilled during the exploration bulk sampling project. (b) Reverse Circulation Drilling: Vertical drill holes on a 20 m · 20 m grid within the open pit to further improve the definition of the ore body and resource model confidence. (c) Surface Diamond Drilling: More recent inclined drill holes with hole spacing ranging from 60 m · 60 m to 120 m · 120 m or greater to define the limits of the main ore body. Both underground and surface diamond drill holes were sampled over an average length of 1.5 m, while reverse circulation drilling was sampled over 3 m lengths. Sample assay data was then composited to 15 m lengths, which equates to the R€ ossing bench mining height of 15 m. Analysis for U3 O8 is by fused pellet XRF; primary and secondary mineralization have not been distinguished. The new resource block model was created using MineSight software on a block model matrix with block dimensions of 40 m · 20 m · 15 m in the easterly, northerly and vertical directions, respectively, by selective merging of the estimates from the above data sets, using the respective confidence levels of each estimation method. A number of factors complicated model construction. Firstly, the strike of lithologies and granite bodies vary substantially (Fig. 11), particularly in the NW of the deposit (NW ‘‘prong’’ or ‘‘lobe’’ in mining terminology). Secondly, due to the irregular fault- or lithology-controlled nature and multiple phases of granite intrusion, the granite dykes and sills cannot be modeled as simple lithological solids, necessitating the interpolation of uranium grades by kriging. Due to the change in strike of the host lithologies, the deposit was sub-divided into smaller areas of more

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Fig. 11. Highly simplified U grade block model of the R€ ossing Open pit along three sections (a––800W; b––400E; c––1200E). The present pit outline (upper solid thin line) is superimposed on the block model. A plan of Bench 12 is shown in (d). U grade is contoured as a percentage of maximum grade (<25% to 100%). The amphibole–biotite schist of the Khan Formation is included to illustrate the transgressive nature of the mineralized zones with respect to both strike and dip of host lithologies, which are concordant with the amphibole schist.

consistent strike direction (Fig. 11, inset), firstly by dividing it into two curved segments on either side of the amphibole–biotite schist member (Table 1). The Khan Formation occurs to the north of the amphibole–biotite schist, while the R€ ossing Formation occurs to the south (also see Fig. 7f). The amphibole–biotite schist was included in the R€ ossing Formation for the purposes of the orebody model. Each of the arcs was further subdivided into three sectors of more consistent strike (Fig. 11, inset), producing six geographical areas. Borehole data were coded into the block model for each of the six volumes, which allowed separate statistics and variography. Semi-variogram iso-contours of the directions of maximum continuity allowed the strike, trend and dip directions of uranium mineralization to be determined for each geographical area. The variography was necessarily confined to U3 O8 graded areas that were lithology independent. These directions, indicating the major, minor and vertical axes of maximum mineralization continuity were then used in the development of semi-variograms for each of the main directions/axes in each geographical area. Nugget and sill values of the semi-variograms in different directions are similar, but their associated ranges are different. These semi-variograms were then used in the kriging interpolation of uranium grades into a 3D-block model, producing the best representation of the geometric trend of uranium mineralization without constraining interpolation to lithological boundaries, granite intrusions or a particular layer of metasediment. Mineralization therefore occurs in an arc around the S/SE margin of the dome, cross-cutting both the dip and strike of granites and host lithologies (Fig. 11).

11. Discussion The movement of high-viscosity granitic magma in a hydrostatic environment is not feasible on geological time-scales, rather, granite segregation and movement is better facilitated by deformation (McKenzie, 1985; Brown and Rushmer, 1997; Petford et al., 2000). A number of modeling experiments point to the primacy of the role of deformation in granite movement. Beach (1973) and Fyfe et al. (1978) recognized that fluid and magma is focused into localized deformation zones or faults, while Brodie and Rutter (1985) proposed that reaction and/or faulted zones commonly undergo reaction-controlled softening, compounded by high pore fluid pressures (Etheridge et al., 1984). A post-extensional structural catalyst is therefore needed for magma mobilization and intrusion at R€ ossing and Goanikontes. We suggest that this catalyst is D4 dome movement and interaction with surrounding, initially less competent, dome ‘‘rim’’ rocks. Domal structures of the Central Zone, showing long:short axis dimensions of 20 km:10 km to 40 km:10 km, trend predominantly NE to the east of the Welwitschia Lineament and predominantly NNE to the west of this lineament (Fig. 1; Corner, 1982, 1983, 2000; Kr€ oner, 1984; Oliver, 1994, 1995). Theories on the formation of domes in the Central Zone include simple interference folding (Smith, 1965; Barnes and Downing, 1979), diapiric rise of granite basement into overlying metasedimentary rocks that were folded during F3 (Ramberg, 1972; Kr€ oner, 1984; Barnes and HambletonJones, 1978), shear-induced sheath folding (Coward, 1980, 1983; Cobbold and Quinquis, 1980) and doming

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over metamorphic core complexes (Crittenden et al., 1980). Oliver (1994, 1995) and Poli and Oliver (2001) proposed that the Central Zone of the Damara Orogen is a deep metamorphic core complex, similar to the type originally proposed by Crittenden et al. (1980) and Lister and Davies (1980), wherein basal detachment and dome formation without interference folding took place in the middle crust, prior to widespread leucogranite intrusion. In the model of Oliver (1994, 1995) and Poli and Oliver (2001) (D3 ?) horizontal constriction from all directions formed cusps in the overlying relatively competent metasedimentary sequence, which protruded downwards into the more ductile basement (Smith, 1977). The intervening basement material thereby adopted a domed form in an overall constrictive or transpressional regime. Such a model is supported by scattered ‘‘basement rodded’’ L–S tectonites (Oliver, 1995; Poli and Oliver, 2001). Oliver (1995) proposes that SW-directed motion of the cover sequences occurred above the Khan River detachment after D3 dome formation, steepening dips on the SW side of the majority of Central Zone domes. This study suggests that steepening of dips (e.g. the northern limb of the large F3 fold hosting the mine) was caused by rotation and southward movement of the R€ ossing Dome. Fig. 12 highlights the co-incidence of features documented in this study. Fig. 12a, which contains details from Anderson and Nash (1997), granite outlines traced from high-resolution enlargements of the relevant aerial photographs, mapping performed in this study, and detailed granite outlines in the open pit prior to mining, highlights the granites in the central and southern part of the dome rim. Granites on the NW limb of the dome intruded into dilated S3 and S4 foliations, suggesting the creation of a locally tensional zone during D4 . The large wedge-shaped granite intrusion, co-incident with the open pit, mimics the shape of the large host F3 fold, that is, the intrusive ‘‘wedge’’ narrows, as does the F3 fold, towards the shear zone to the SE of the dome (Fig. 7f). Granites and the large F3 fold are entrained into this left-lateral shear zone, which is coincident with the amphibole–biotite schist. F3 fold axes are ‘‘bent’’ around the southern tip of the dome. Thrusts and

b Fig. 12. Co-incident features in the southernmost portions of the R€ ossing dome: (a) granite distribution and deformation, bending/ concatenation of F3 fold axes, includes granite distributions from Anderson and Nash (1997) and this study. Lithologies and symbols as per Fig. 3; 1––R€ ossing and R€ ossing SJ area, local compressional or transpressional zone (this study) with high fluid flux, high absolute H2 O, higher H2 O/CO2 ratio and total fluids (Nex et al., 2002); 2–– R€ ossing SH area, lower absolute H2 O contents, lower H2 O/CO2 ratio and low total fluids (Nex et al., 2002); 3––Local transtensional area and intrusion of granites into dilated S3 and S4 ; (b) highly faulted zone, with the open pit shaded (summary of Fig. 8); and (c) ‘‘arcs’’ of uranium mineralization, from ore body modeling, around the S/SE tip of the R€ ossing Dome.

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reverse faults developed to the SE of the dome, one of which was particularly problematic in mining of the southern wall of the open pit (‘‘Fault 14’’). This structure, which displays a variable dip (Figs. 7 and 8) is the local translation of the NNE-trending shear into a reverse fault (q.v. Anderson and Nash, 1997). Late-D3 to D4 brittle–ductile and brittle faulting is remarkably abundant, relative to the other parts of the dome, to the south of the dome (Figs. 8e and 12b). Fault types include splays off the large shear zone to the SE of the dome, concave-north curved faults that indicate dome core ‘‘indentation’’ (q.v. Le Pichon et al., 1992; Hauck et al., 1998) and WNW to N–S trending brittle dilational faults, many of which are the D3 joints of Jacob and Kerber (1997), that have been rotated anticlockwise (Fig. 8). Enhanced U-mineralization is not confined to lithologies, lithological contacts or planar, fault-controlled granites within the open pit (Fig. 11). Rather, economic mineralization is transgressive to both lithologies and granites, occurring in two merging arcs around the southern tip of the dome and extending towards the shear zone to the east (Fig. 12c). This wedge-shaped zone is visible as a separate magnetic high to the south of the dome in Fig. 2. F3 dome formation necessarily occurred towards the latter part of the ca. 600–550 Ma period, which encompassed peak transpressional tectonism and metamorphism caused by the oblique collision of the Kalahari and Congo Cratons (Table 4; Oliver, 1994, 1995; Bowden et al., 1999). The formation of shears surrounding the dome, dome rotation, and final southward impingement relate to a N–S compressional regime similar to that of the ca. 600–550 Ma peak transpressional event, that was caused by N–S collision of the Kalahari and Congo Cratons (Bowden et al., 1999; Table 4). This late-kinematic, transpressional stress regime occurred at the end of the ca. 542–526 Ma transtensional tectonomorphic event, prior to the ‘‘earlykinematic’’ 516 ± 6 Ma granite (de Kock and Walraven, 1994), the ‘‘late-kinematic’’ 505 ± 4 Ma granite of Bowden et al. (1999), the peak of granite plutonism at ca. 510 Ma and the intrusion of uranium-enriched leucogranites at 508 ± 2 Ma (Briqueu et al., 1980; Table 4).

Regional transtensional tectonism from ca. 542–526 Ma, which produced the voluminous granites in the Central Zone, must have preceded localized D4 deformation. Syn-rotation/impingement emplacement of these granites, including Type D granites defined by Nex and Kinnaird (1995) and Nex et al. (2002), is indicated by their intrusion into dilated S3 and S4 foliation in transient tensional areas within the Khan Formation along the central NW rim of the dome (Figs. 6 and 12a). Type D and E granites also intruded into a dense fault network, the nature and situation of which is genetically linked to dome rotation and southward dome impingement. D4 shearing also occurred along multiple NNEtrending (Welwitschia Trend) shears in the vicinity of R€ ossing, which were later occupied by dolerite dykes. The maintenance of a high strain zone, causing a prolonged high fluid flux regime, with an abundant fault network to the S and SE of the dome core, is proposed to have been instrumental in firstly localizing these voluminous granites and, secondly, localizing the fluid flow which concentrated uranium in the form of betauranophane.

Acknowledgements IJB gratefully acknowledges the staff of the Geology and Surveying Sections of R€ ossing Uranium Mine for their unfailing assistance in the gathering of archival and new data and for access to mine property in the vicinity of the R€ ossing Dome. The authors thank the management of R€ ossing Mine for permission to publish this study. Christie Myburgh, Braam Saayman, Dave Matthews, Keith Seely and Dan Katjivikua of the Technical Services Division are thanked for their assistance with Excel, MineSight and AutoCAD data processing and examination. Branko Corner very kindly processed and supplied the base maps on which Figs. 1 and 2 are based, in addition to providing valuable discussion on the nature of lineaments in Namibia. John Moore, Judith Kinnaird, Guy Charlesworth, Mark Barton, Graham Oliver, A.B. Kampunzu and an anonymous reviewer provided helpful reviews of earlier versions. Discussions with Paul Nex, Roger Jacob and Russell Sweeney greatly improved the final product.

12. Conclusion D4 rotation and southward impingement of the R€ ossing Dome, within a N–S compressional regime between the Kalahari and Congo Cratons, occurred from approximately 516 ± 6 Ma to at least 505 ± 4 Ma in the southern Central Zone of the Damara Orogen. D4 overlaps with the intrusion of late-kinematic, uraniumenriched granites into a dense fault network in a high strain zone at the southern tip of the R€ ossing Dome.

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