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Navachab

Namibia

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The Navachab gold deposit is located in the southern Central zone of the Pan-African Damara orogen, 10 km SW of Karibab, and ~120 km NW of Windhoek in Namibia (#Location: 21° 58' 52"S, 15° 45' 53"E).

The Damara belt in central Namibia is part of the Pan-African Damara orogen, which separates the Kongo and Kalahari cratons. The belt has been subdivided into several distinct tectonostratigraphic zones on the basis of lithology, structure and metamorphism, principally the Northern, Central, Southern and Southern marginal zones. The Navachab gold mine is situated in the southern part of the Central zone, characterised by a low-pressure, medium- to high-temperature metamorphic terrane made up of supracrustal rocks of the Damara sequence, various generations of I- and S-type granitoids (Jung and Mezger, 2003; Jung et al., 2003), and Mesoproterozoic gneisses of the Abbabis Metamorphic Complex. The southern Central zone is characterised by elongate, kilometre-scale, NW-vergent, noncylindrical anticlines (Kisters et al., 2004), the cores of which are commonly composed of older gneisses and/or Pan-African granitoids. These cores are surrounded by infolded, high-grade metamorphosed rocks of the Damara sequence (Poli and Oliver, 2001). Metamorphic grade ranges from amphibolite facies in the east, to granulite facies near the Atlantic coast to the west (Masberg et al., 1992; Ward et al., 2008; Kisters et al., 2009). Peak granulite-facies metamorphism was at ~525 and 504 Ma, followed by several high-temperature metamorphic events that lasted until ~470 Ma, related to emplacement of late- to post-tectonic granitoids (Nex et al., 2001; Jung and Mezger, 2003; Jung et al., 2003).

The Navachab gold deposit is hosted by a steeply dipping, near-vertical succession of meta-sedimentary rocks, which include biotite-schist, marble and calc-silicate rock of the Okawayo, Spes Bona and Oberwasser Formations (Kisters et al., 2004; Nörtemann et al., 2000), on the NW limb of the Karibib dome, a NE-vergent, non-cylindrical anticline. The assemblages representing peak metamorphism of the biotite schist and marble comprise biotite + K -feldspar + plagioclase + quartz ± actinolite ± muscovite and calcite ± dolomite ± graphite. The calc-silicate rocks are composed of alternating layers of clinopyroxene- and calcite-rich rocks, containing variable amounts of calcite, anorthite, K feldspar, clinopyroxene and quartz (Wulff et al., 2010).

Abundant dykes and sills of lamprophyre, aplite pegmatite intrude these rocks (Kisters et al., 2004; Kisters, 2005; Dziggel et al., 2009). The characteristic high K2O and SiO2 of the aplites suggest a sedimentary origin (Wulff, 2009).

Gold mineralisation is hosted by: i). quartz-sulphide veins that crosscut the metasedimentary sequence, and can be subdivided into a conjugate set of sheeted quartz veins, which dip at shallow angles to the NW and NE, and at least three minor vein sets of various orientations; ii). cigar-shaped, bedding-parallel, semi-massive sulphide lenses that can be traced down shallow plunge directions for at least 1800 m, within layered calc-silicate rocks that are termed the marble-calc-silicate unit (Kisters, 2005; Kolb, 2008; Dziggel et al., 2009). Both the intersection lineation of the sheeted quartz veins, and the semi-massive sulphide lenses plunge at ~20° to the NNE, parallel to the fold axis of the Karibib dome in the mine (Kisters, 2005; Dziggel et al., 2009; Wulff, 2009). This geometry suggests that mineralisation is closely related to the folding of the Karibib dome (Wulff et al., 2010).

The age of mineralisation, has not been unequivocally determined. Structural data and U-Pb zircon dates of the Rote Kuppe granite and Mon Repos granodiorite (Jacob et al., 2000), suggest gold mineralisation was emplaced at ~550 to 540 Ma (Kisters, 2005), which is consistent with an upper intercept zircon age of 544±39 Ma for an aplite dyke that crosscuts the mineralisation. However, quartz-sulphide veining and meta-lamprophyre dyke have been dated at 494±8 and 494±12 Ma, respectively (Jacob et al., 2000), although these may reflect metamorphic crystallisation during a later thermal event (Wulff et al., 2010).

Gold generally occurs in the free form, and the mineralized zones are characteristically polymetallic assemblages of pyrrhotite-chalcopyrite-sphalerite-arsenopyrite-native bismuth-gold-bismuthinite-bismuth telluride, representing the addition of significant Au, Bi, As, Ag, Cu, Fe and Mn (Nörtemann et al., 2000; Dziggel et al., 2009a; Wulff, 2009).

Veins vary in thickness and mineralogy reflecting the composition and rheological behavior of the wall rocks. In marble they are only a few mm thick, strongly folded and dominated by sulphides. In contrast, in biotite schist and calc-silicate rock, the same veins may be up to several tens of cm thick and are mainly composed of quartz (Dziggel et al., 2009).

The associated alteration halos in biotite schist are characterised by actinolite-quartz assemblages; garnet-clinopyroxene-K feldspar-quartz in marble and calcsilicate rocks; and a garnet-biotite phase that is also developed in biotite schist and calcsilicate rock, and in rare cases also in marble (Wulff et al., 2004; Dziggel et al., 2009). The semi-massive sulphide lenses in the banded calc-silicate rocks of the marble calc-silicate unit are accompanied by garnet-clinopyroxene-K feldspar-quartz and garnet-biotite alteration. Alteration halos sandwiching subhorizontal veins are asymmetric, being much more pronounced facing upward, where they follow lithologic contacts and foliation planes away from the mineralisation. This suggests the host sequence was near vertical during the main phase of mineralisation (Dziggel et al., 2009).

Isotopic studies of the ore and alteration, led Wulff et al. (2010) to the conclusion that gold was precipitated in equilibrium with metamorphic fluids during peak metamorphism at ~550°C and 2 kbars, consistent with isotopic fractionations between coexisting calcite, garnet and clinopyroxene in the alteration halos. They concluded that the most likely source of the mineralisation was a mid-crustal fluid in equilibrium with Damaran metapelites that underwent prograde metamorphism at amphibolite- to granulite-facies grades. Although they found no isotopic evidence for the contribution of magmatic fluids, they suggest such fluids may have made an important contribution to the overall hydraulic regime and high apparent geothermal gradients (~80°C/km) in the mine area.

The deposit was brought into production in December, 1989. Production to the end of 2011 is estimated at 47 t of Au.

Reserve and resource figures include:
Mineral resources as at 31 December, 2011 (Anglogold Ashanti, 2012)
    Measured resource - 18.35 Mt @ 0.71 g/t Au,
    Indicated resource - 99.78 Mt @ 1.22 g/t Au,
    Inferred resource - 16.41 Mt @ 1.15 g/t Au,
    Total resource - 134.54 Mt @ 1.14 g/t Au, for 154.01 t of Au

Ore reserves as at 31 December, 2011 (Anglogold Ashanti, 2012)
    Proved reserve - 6.31 Mt @ 1.09 g/t Au,
    Probable reserve - 44.18 Mt @ 1.29 g/t Au,
    Total reserve - 50.49 Mt @ 1.26 g/t Au, for 63.76 t of Au

The most recent source geological information used to prepare this summary was dated: 2010.     Record last updated: 4/9/2012
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.


Navachab

  References & Additional Information
 References to this deposit in the PGC Literature Collection:
Dziggel A, Wulff K, Kolb J and Meyer F M,  2009 - Processes of high-T fluid–rock interaction during gold mineralization in carbonate-bearing metasediments: the Navachab gold deposit, Namibia: in    Mineralium Deposita   v.44 pp. 665-687
Wulff K, Dziggel A, Kolb J, Vennemann T, Bottcher ME and Meyer FM,  2010 - Origin of Mineralizing Fluids of the Sediment-Hosted Navachab Gold Mine, Namibia: Constraints from Stable (O, H, C, S) Isotopes : in    Econ. Geol.   v105 pp 285-302


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