Summary:
The Navachab gold deposit is located in the Pan-African Damara Orogen and hosted by greenschist-amphibolite facies calc-silicates, marbles and volcaniclastic rocks. The rocks have been intruded by granite, pegmatite and aplitic dykes and have also been deformed into a series of alternating dome and basin-like structures.
Navachab Gold Mine is a classical carbonate-hosted replacement skarn deposit with strata bound style of mineralised rock. Gold mineralisation at Navachab Gold Mine occurs in:
- A steeply inclined to sub-vertical, shallow north north-east (NNE)-plunging, lens-shaped ore body confined to the Massif Central (MC) unit (interlayered calc-silicates and marbles) in the basal parts of marbles of the Okawayo Formation; and
- Sets of shallow north-west (NW) to north-east (NE) dipping mineralised sheeted quartzsulphide-gold veins that are developed both in the hanging wall (marbles of the Okawayo Formation (MDM) and biotite schist of the Oberwasser Formation (US), as well as in the footwall (biotite schist of the Spes Bona Formation (LS) of the MC unit.
The veins have recently comprised much of the ore mined at Navachab Gold Mine. According to Katharina Wulff, Nick M. Steven, Kim A.A. Hein, Judith A. Kinnaird, 2017, within “the veins, quartz, pyrrhotite and chalcopyrite are the most common minerals. Microscopically, minor amounts of clinopyroxene (often replaced by secondary actinolite and calcite) as well as sphalerite, arsenopyrite, native bismuth, native gold, bismuthinite and an unidentified Bi-Te-SeS-mineral were observed (Nörtemann et al., 2000; Wulff, 2009; Dziggel et al., 2009a, 2009b).
The gold commonly occurs as small grains (few µm to 0.1 mm) of free gold surrounded by quartz, or in contact with native bismuth and other bismuth minerals.”
The mineralisation at Navachab forms a sheet-like body which plunges at an angle of approximately 20° to the northwest. The mineralisation is predominantly hosted in a sheeted quartz vein set (approximately 60% of tonnage) and a replacement skarn (approximately 40% of tonnage). The mineralisation in the main pit is hosted by a northeast to southwest striking metamorphosed sequence of calc-silicates, marbles and volcaniclastic rocks that dip at 70° to the west. The gold is very fine-grained and associated with pyrrhotite and minor amounts of pyrite, chalcopyrite, arsenopyrite, sphalerite, maldonite and bismuthinite. An estimated 90% of the gold occurs as free gold and the remainder is present in minerals such as maldonite (Au2Bi). Silver is also present with a gold to silver ratio of approximately 15 to 1.