3 the Skorpion Deposit 3.1 Exploration and Mining History
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3 The Skorpion Deposit 3.1 Exploration and Mining History The Skorpion zinc deposit, owned by Anglo American, is located approximately 40 km north of the Orange River and some 15 km north-northwest of Rosh Pinah Mine in the southern- most Namib Desert, Namibia (Fig. 10). It is hosted by a volcano-sedimentary succession of Neoproterozoic age, the so-called Hilda Sequence (Von Veh 1993, Alchin 1993, Frimmel 2000b, Buxton et al. 2000). The Skorpion deposit is situated rather remotely in the restricted ‘Sperrgebiet No. 1’ (Dia- mond restricted area) of southern Namibia. This protected status of the area persisted until 1975, when Anglo American Prospecting Services (AAPS) and Consolidated Diamond Mines (CDM) entered into an agreement to explore the ‘Sperrgebiet’ for base metals. The Skorpion prospect was discovered during an exploration programme by Erongo Exploration and Mining Co. Ltd. in 1976-77, which included geochemical drainage and gossan sampling. A small outcrop of a crudely banded barite/iron-hydroxide gossan west of Skorpion, the so-called 'discovery outcrop' yielded anomalous metal values of 0.1 – 0.3 % Cu, 0.1 – 2.9 % Pb, 0.1 – 4.1 % Zn, 0.3 – 13.0 % Mn, and 2 – 50 ppm Ag (Corrans et al. 1993). Drainage samples in the area gave elevated values of Pb (250 ppm) and Zn (115 ppm) against background values for Pb, Cu and Zn of 20 - 40 ppm (Corrans et al. 1993). The initial diamond drilling programme proved a measured and indicated resource of 8.3 Mt @ 10.9 % Zn (Corrans et al. 1993). An initial bulk sample pit was excavated in 1979 yielding ore grading approximately 20 % Zn. However, metallurgical testing, at the time, was unable to recover the zinc satisfactorily and the Skorpion prospect was ‘mothballed’. Subsequently in 1996, Anglo American and Reunion Mining formed a joint venture within which Reunion carried out a new and major diamond and RC-drilling campaign during 1997/98. This pro- gramme increased the proven resource to 17.5 Mt @ 10.4 % Zn. Between 1996 and 1998, Reunion Mining commissioned a tailor-made solvent-extraction electro-winning (SX-EW) treatment process from Technicas Reunidas of Spain (the SX component) and Union Miniére of Belgium, now Umicore (the EW component). This technological break-through converted the resource into a minable reserve. Reunion Mining was taken over by Anglo American in 1999 and as a result Anglo American again became the sole owner of the Skorpion deposit. Mining commenced in October 2001 with the stripping of the overburden and exposure of the ore body, which had a resource of 24.6 Mt at 10.6 % Zn by then (Mining Journal 2000). The first zinc metal was produced on site in May 2003. 3.2 Geographical Overview 3.2.1 Present Geomorphology The Skorpion deposit is located within the south-eastern part of the Namib Desert within the 100 km-wide coastal zone west of the Great Escarpment. This desert, which is one of the world’s most arid regions, is underlain by sands of a proto-Namib phase, which started to develop 35 million years ago. The Namib Desert stretches along the entire Atlantic coast and rises to a level of approximately 800 m at the foot of the Great Escarpment in the east. With differences in altitude of more than 1000 m, the Great Escarpment marks the transition to the Central Plateau east of the desert (Christelis & Struckmeier 2001). The Skorpion area consists of undulating sand and gravel plains, isolated rock outcrops, and ranges of rocky hills (Fig. 11). The area is poorly drained, with a few ephemeral drainages 19 radiating away from higher lying features. The area lies about 650 m above sea level. The escarpment lies about 22 km east of the Skorpion area, where the land rises steeply to 800 – 1200 m above sea level. The surface of the current mining area is relatively flat and dips slightly to SE from 665 m in the north to 655 m in the south and forms a wide valley striking NW-SE. The subdued relief of the area with isolated outcrops and inselbergs is the result of the infilling of a rugged palaeo-topography by alluvial and colluvial sediments derived from the retreating escarpment and eroding valley sides as well as more recently, aeolian sand. These alluvial/colluvial sediments rest unconformable on the late Proterozoic host rocks of the Skorpion deposit. Additionally, ancient massive to nodular pedogenic calcrete horizons occur near the surface, although calcite is disseminated through most of the overburden profile as well. Iron-rich silcrete lenses are found locally above the erosional bedrock contact. Fig. 11: Rock outcrop, sand plains and rocky hills in Fig. 12: Sparse vegetation in the Skorpion area. the Skorpion area. Red sands are part of the valley infill. 3.2.2 Climate and Vegetation The area is extremely arid and has mean annual rainfall of less than 100 mm per annum, usually concentrated from March to July. Sometimes, coastal fog from the Atlantic Ocean reaches this part of the Namib Desert, and thus contributes to the very limited precipitation in this area. Mean daily minimum and maximum temperatures range between 4°C and 20°C in July and 15° and 32°C in January, respectively. Aeolian sedimentation processes are active in the southern Namib Desert, where dunes and flat sand plains prevail as the main morphological features. Chemical weathering and soil formation is hampered, mostly due to the lack of moisture. The scarce vegetation consists of grasses, shrubs and succulents growing on the sand plains (Fig. 12). 20 3.3 Geological Framework The current mining activities exposed the rock sequence of the uppermost part of the super- gene Skorpion ore body and thus tectonic structures within the Late Proterozoic sequence. The structures display a high intensity of deformation including folding, faulting, and thrusting in the mining area as a result of a multi-stage deformation process in different periods since the Late Proterozoic. This chapter summarises the main concepts based on studies under- taken by Corrans et al. (1993), Borg et al. (2004), and Dirks (2004). Corrans et al. (1993) described the dominance of Pan-African deformation features like NNW-trending folds, here with a persistent moderate northerly plunge (31°) and related this to thrusting from the west during the late Pan-African event. Additionally, they found major, some of them probably still active, NW-trending faults, which displace the Late Proterozoic Gariep sequence, interpreted as being originally related to the break-up of Gondwana and been in part responsible for the extreme oxidation and deep weathering of the primary sulphide ore zones, locally to more than 800 m depth (Corrans et al. 1993). Two major separate deformational events have also been defined by Borg et al. (2004) and Dirks (2004). The earlier, ductile one, has been related to the main Pan-African deformation event, when the Skorpion precursor ore body was buried and metamorphosed to lower amphibolite facies grade, and has been accompanied by lateral sinistral transpression and pronounced SSE-ward thrusting, which was associated with the closure of the Adamastor ocean around 545 Ma ago (Borg et al. 2004). This multi-stage Pan-African deformation event led to the development of bedding-parallel thrust horizons, and therefore caused a duplica- tion of the stratigraphic pile in places (Fig. 13, Dirks 2004). Additionally, E-verging, tight to isoclinal F1-folds were formed, which have been re-folded by W-verging, open F2-folds (Fig. 14) with a dominant, penetrative E-dipping cleavage. Fold axes of both F1- and F2-folds and prolate tectonic features (rodding) have a shallow to moderate plunge towards NNW (Borg et al. 2004), which is in agreement with Corrans et al. (1993). These early folds have been affected by wide and open D3-crossfolding with NE-SW-striking fold axes with a steep to moderate plunge that caused regional gentle “whale back-type” doubly plunging anticlines exposed prominently in the Skorpion and Rosh Pinah area. Both F2- and F3-folds feature strongly, although in smaller dimensions, in the open pit (Borg et al. 2004). Fig. 13: Duplex arrays and intrafolial, recumbent fold Fig. 14: Pan-African open F2-fold, Skorpion open packages wrapped by low-angle truncation planes mark pit, 630 m level. Pan-African high-strain, thrust horizons. Skorpion open pit. Photo from Dirks (2004). 21 The Pan-African deformed rock sequence has been affected by a later brittle-ductile defor- mation event in a transpressive shallow crustal regime (Borg et al. 2004), which might be correlated to the opening of the South Atlantic Ocean in the Jurassic (Dirks 2004). The Skorpion host rocks have been dissected by steeply dipping brittle-ductile shear zones, which show movement indicators for dip slip, oblique slip, and reverse slip shearing. In plan view of the pit area, this NW- to N-trending brittle ductile shear zone is the most prominent feature forming an anastomosing array (Fig. 16). Commonly, extensional quartz veins mark the centre of some of these shear zones. They are variable folded, boudinaged, and recrystallised (Borg et al. 2004, Dirks 2004). These zones have been interpreted by Borg et al. (2004) as the upper, mar- ginal part of a dextral wrench fault system, which might be part of a positive flower structure (Fig. 15). Almost identical flower structures with a similar sense of move- ment have recently been identified off- shore in seismic sections through the Fig. 15: View of the southern pit wall showing a dextral Orange Basin (Viola et al. 2005) reverse fault set with a sigmoidal shear body and an The youngest tectonic features, which are extensional quartz vein as an example for the reverse found within the Skorpion open pit, are oblique-slip fault zones as part of a positive flower most likely associated with the develop- structure.