Husab Mine - Rossing South
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The Husab uranium mine, which exploits the Rössing South deposit, is located 7 km south of the Rössing mine, separated from it by the Khan River Canyon, and is ~52 km ENE of Swakopmund in Namibia (#Location: 22° 33' 40"S, 15° 3' 26"E).
The deposit was discovered in 2008. The operating company, Swakop Uranium, started development of the mine in February 2013, and production commenced at the end of 2016. Swakop Uranium was initially held by public companies, but since 2011 has been a private entity, mostly held by Chinese interests. A a consequence, publically available information since 2011 is sparse.
Rössing South shares many geologic characteristics with the Rössing deposit, although it is located on the opposite limb of the regional Kuiseb syncline. Mineralisation is hosted within alaskites* as at Rössing, within the same host sequence. For detail of the geology of the Rössing deposit, see the separate Rössing record.
The Husab deposit lies within a NE trending Mesoproterozoic to earliest Palaeozoic intracontinental rift zone, the Damara Orogenic Belt, that extends from the Namibian coast northeast into Botswana and Zambia, separating the stable Kaapvaal and Congo Cratons. Extension was initiated between ~1000 and 900 Ma, with the rift basin being filled by a thick rift and then sag phase succession of sedimentary and volcanic rocks. Final inversion took place in the late Neoproterozoic to early Palaeozoic during a period of major thrust faulting and granitoid intrusion resulting in closure of the basin. Many of the regional extensional structures involved in the initial rifting were inverted and have continued to influence the tectonic development of the Namibian region up until the Jurassic, when the African and South American continents rifted apart to produce the South Atlantic Ocean. Mineralisation at Husab is associated with structural and intrusion-associated settings formed during and following the major thrust deformation/basin inversion that closed the orogen.
The Husab Project is located in the Central, or Swakop Zone of the Damara Orogen. This zone is bounded by two NE trending structures, the Omaruru and the Okahandja lineaments to the NW and SE respectively. It represents a palaeo-arch or NE trending anticlinal hinge zone, where Damara and pre-Damara rocks are exposed as a series of elongated domes following inversion of the rift between the Congo and Kalahari Cratons. These domes may be due to interference folding or more likely reflect metamorphic core complexes generated during post-orogenic rebound. Extensive inliers of Palaeoproterozoic basement of the Abbabis Metamorphic Complex are exposed in these elongated dome-like windows, surrounded by folded younger Damara Sequence lithologies. These older metamorphic complex lithologies were subjected to medium to high grade amphibolite to granulite facies metamorphism, to produce a succession that is dominantly composed of metasedimentary rocks, paragneiss, orthogneiss and ortho-amphibolite, all of which are extensively intruded by pegmatites.
The Damara sequence has been divided into three broad successions, the:
• Nosib Group - composed of metamorphosed sandstone, quartzite and minor conglomerate, representing a rift phase succession, deposited as a consequence of the uplift and erosion of adjoining basement during the initial rifting event. The Nosib is sub-divided into the ~3000 m thick Etusis Formation quartzites, arenites and arkoses, and the ~1100 m thick Khan Formation, which is largely calc-silicate rocks with lesser clastic components.
• Lower Swakop Group, which discordantly overlies the Nosib Group, and comprises an alternating succession of dolomite, marble, schist and schistose diamictite. The succession represent the post-rift thermal sag phase of deposition reflecting cooling of the mantle emplaced below the thinned crust resulting from the rift extensional phase. It includes, from the base, upward, the ~200 m thick Rössing Formation - mainly carbonates, wackes, quartzites and mica-schists, discordantly overlain by the Khomas Subgroup units that comprise the ~700 m thick Chuos Formation mica schists, calc-silicates and carbonates, and the overlying Arandis and 1000 m thick Karibib Formations mica-schists, calc-silicate rocks and marbles.
• Upper Swakop Group and upper Khomas Subgroup which comprises the >3000 m thick Kuiseb Formation. This formation is predominantly developed within the southern portion of the Damara Orogen and is mainly composed of a thick succession of flysch-type meta-sedimentary rocks, generally comprising biotite schist with subordinate calc-silicate rocks and carbonaceous schist. It also includes a laterally extensive unit of metamorphosed mafic volcanic rocks, the Matchless Amphibolite Member, which has compositions similar to those of mid-ocean ridge basalts.
All the above sequences are cut by a wide variety of syn-and post-tectonic intrusive rocks, including the Goas Diorite Suite, a series of small disconnected plutons to the south of Karibib. Other late to post-tectonic intrusives include the 590 to 580 Ma Salem type granites, commonly represented by grey biotite granites to granodiorites locally intruding Damaran rocks. Another group are the 'red' granites, a heterogeneous group of foliated and massive intrusions, often occurring as small bodies within the Swakop Group marbles and schists. Other, mainly post-tectonic intrusive types include the leucogranites and alaskites, some of which are uraniferous. These characteristically occur as vein to dyke-like or anastomosing intrusions below and around the lowest Damaran marbles, although massive or plug-like occurrences are also mapped. Some of these grey granites carry low grade uranium mineralisation and have been intruded by uranium bearing alaskite, as at one location at the Rössing Mine.
The Damara Orogen was terminated by a major south-vergent thrust deformation, producing a pervasive NNW to NW dipping foliation. This thrust contraction is interpreted to have transported sections of the sequence up to 200 km to the south over basement rocks.
Damaran rocks are covered by the Kalahari Sands in northeastern Namibia, but are continuous through Botswana into Zambia, where equivalent rocks are represented by the Muva and Katanga Groups of the contiguous Lufilian Orogen.
The area surrounding the Husab deposit is dominated by a series of NNE to NE trending regional scale antiforms and synforms which are consistent with the structural architecture that defines the entire Central/Swakop Zone of the Damara. These folded metasedimentary rocks form dome-like structures, cored by gneissic and metasedimentary rocks of the Abbabis Formation. Basement rocks are masked to the NE and south by stranded cover sequences of flat-lying calcrete and alluvial deposits that are part of a broad NE trending valley marginal to the Khan River.
The series of semi-ovoid domal cores of basement gneisses within the Central Zone of the Damara Orogen are flanked by Damara Sequence sedimentary rocks that have been complexly folded and faulted. The whole sequence is extensively invaded by syn- and post-tectonic granitoids and pegmatite swarms. Cross-cutting Mesozoic dolerite dykes are also evident locally.
A NW-SE section across the Rössing South deposit area shows a basement antiformal core of the Abbabis Metamorphic Complex to the NW, overlain progressively to the SE by the Etusis and Khan formations, then by the Rössing Formation hosting Alaskite intrusions. These are, in turn, overlain by the Chuos and Karibab formations, and then the Kuiseb Formation in the core of the Kuiseb Syncline. The SE limb of this synformal structure repeats the Karibab and Chuos formations, which are juxtaposed across a steep SE dipping fault with the tight Rössing South Anticline. This anticline has two limbs of Rössing Formation hosting alaskite, cored by Khan Formation rocks. A faulted synformal core to the immediate SE is followed by a Rössing Formation limb intruded by grey granite. Airborne magnetic data trace the folded stratigraphy from the Rössing mine area to the Rössing South/Husab vicinity to confirm the interpreted section described above. In the latter area the host stratigraphy is largely covered by sands of the Namib Desert which have hindered past exploration.
Basement domes to the east and west of the Husab project area, the latter along the Khan River, are predominantly composed of metasedimentary rocks. However, basement associated with the cores of the Husab Mine Dome/antiform and the Ida Dome to the SSW and are gneissic.
The Rössing South deposit is hosted on the limbs of the Rössing South antiform. Regional experience has shown that the unconformable contact between the Khan and Rössing Formation contact has been preferentially intruded by the leucogranites (alaskites) containing primary uranium
mineralisation at the Rössing Mine to the north and at Ida Dome to the south.
Two NNE-SSW trending mineralised zones are initially being mined as the 2.5 x 1.3 km Zone 1 pit, and the smaller 1.5 x 1.0 km Zone 2 pit, <1 km to the SSW. Additional zones 3 to 5 are indicated over the 15 km strike interval on the limbs of the Rössing South Antiform that also includes zones 1 and 2. Mineralisation lies below shallow, ~50 m thick alluvial and aeolian sand cover.
The stratigraphic sequence at Zone 1 is a south-plunging antiform with gneisses of the Khan Formation overlain by a mixture of metasedimentary and altered calc-silicate lithologies of the Rössing Formation, which is, in turn, overlain by biotite schists and gneisses of the Chuos Formation. Alaskite bodies have generally intruded sub-parallel to foliation.
Primary Uranium Mineralisation
Mineralisation at Rössing South occurs within intrusive uraniferous alaskites of the Central or Swakop Zone of the Damara Orogen. This alaskite mineralisation is interpreted to be associated with 'a zone of abnormally high heat flow that has produced leucogranite melts from pre-Damara basement, and preferentially enriched uranium over thorium with respect to source rocks' (Spivey and Penkethman, 2009). These magmas are interpreted to have been predominantly passively emplaced as sheeted bodies, and less frequently, as stocks., and occur in zones of high grade metamorphism characterised by high temperature- low pressure metamorphism' (Spivey and Penkethman, 2009).
Mineralised alaskites are interpreted have been preferential emplaced in dilatational zones created during regional deformation, localised at or near the first occurrence of carbonate rocks within the sequence. The change from a clastic to carbonate sedimentary regime may have acted to change local redox conditions, and/or to trap and focus mineralised fluids beneath marble caps (Spivey and Penkethman (2009).
Mineralised alaskite in the Husab deposit area occurs at or near the contact between the Khan Formation and calc-silicates of the Rössing Formation, with the bulk located within the latter. The Rössing Formation varies from 50 to >200 m in thickness and encloses multiple mineralised alaskites with widths of from a few to >100 m, averaging ~20 m. For example, on an individual section in Zone 1, the Rössing Formation is 280 m thick and is intruded by 8 alaskite bodies, generally parallel to the margins of the formation. Individual mineralised intervals ranged from 3 to 145 m, with an average thickness of 23 m. A total of 42 mineralised zones (grouped into 39 domains) were created for Zone 1, with individual strike lengths ranging from 125 to 1100 m, and for several hundred metres down dip.
The mineralisation from Zones 1 to 4 is a near-continuous mineralised area extending along ~8 km of strike length and up to 2 km across strike.
Metallurgical test work indicates that uraninite is the dominant primary uranium mineral, occurring as discrete grains with minor occurrences on margins, rims, veins and inclusions. Varying amounts of coffinite have been identified with trace to minor brannerite and trace thorite. Elevated uranium grades of as much as 0.96% U3O8 have been identified in some samples associated with beta-Uranophane, an oxidation product of uraninite.
Little refractory uranium mineralisation (e.g., betafite) has been identified. Initial leach testing of 38 µm feed and residue of alaskite composite samples concluded that uraninite, coffinite and thorite had leached and the brannerite showed signs of chemical attack but had not leached at the conditions of the test. Follow up work determined that the brannerite, which accounts for ~2% of the total uranium in the sample could be leached at higher acidity and temperature. The majority of uranium remaining in leach residues is in the form of brannerite with inclusions of uraninite in gangue. The alaskite ore type composites are composed of the rock forming silicate minerals microcline, albite, quartz, mica (muscovite and/or biotite/phlogopite) with some chlorite (as clinochlore) with
the distribution varying for different ore type. Minor phases observed were a Ca-Mg-Al-Fe-silicate (ferroan diopside), pyrite, titanite, apatite and ilmenite. Zircon is also evident in the calc-silicate and biotite schist composites.
Secondary Uranium Mineralisation
In addition to primary alaskite hosted uranium mineralisation, the Husab district contains occurrences of secondary uranium vanadate (carnotite), similar to that at the Langer Heinrich deposit. Exploration drilling has intersected calcrete hosted uranium mineralisation in a flat lying paleochannel.
Ore Reserves and Mineral Resources
Published Mineral Resources at a 100 ppm U3O8 cut-off at 10 August, 2010 (Inwood et al., 2011) were:
Indicated Resource - 122 Mt @ 0.0450% U3O8 for 55 000 t U3O8;
Inferred Resource - 41.3 Mt @ 0.0420% U3O8 for 17 400 t U3O8;
Indicated Resource - 118.8 Mt @ 0.0520% U3O8 for 61 800 t U3O8;
Inferred Resource - 26.8 Mt @ 0.0520% U3O8 for 13 900 t U3O8;
Inferred Resource - 43 Mt @ 0.0250% U3O8 for 10 700 t U3O8;
Inferred Resource - 14.4 Mt @ 0.0570% U3O8 for 8 200 t U3O8.
Indicated Resource - 241 Mt @ 0.0480% U3O8 for 117 000 t U3O8;
Inferred Resource - 125.5 Mt @ 0.0400% U3O8 for 50 000 t U3O8;
Ore Reserves published by Swakop Uranium in 2011 (in October 2011 edition of Swakop Vission, a company newsletter) were:
Proved Reserve - 25.3 Mt @ 0.0482% U3O8 for 12 200 t U3O8;
Probable Reserve - 123.4 Mt @ 0.0460% U3O8 for 56 745 t U3O8;
Proved Reserve - 37.4 Mt @ 0.0628% U3O8 for 23 495 t U3O8;
Probable Reserve - 93.9 Mt @ 0.0561% U3O8 for 52 660 t U3O8;
Proved + Probable Reserve - 280 Mt @ 0.0518% U3O8 for 145 100 t U3O8.
* Alaskite: A light coloured alkali-feldspar-granite, containing few, if any, mafic minerals, in which quartz constitutes 20% to 60% of the felsic minerals and in which the ratio of alkali feldspar to total feldspar is greater than 90%.
The information in this summary is largely drawn from: Inwood, N., Le Brun, S., Craig, S., Cheyne, R., Valenta, M., Browner, H. and Amos, S., 2011 - Husab Project, Namibia - May 2011 Project Update; An NI 43-101 Technical report compiled by Coffey Mining Pty Ltd for Extract Resources Limited, 261p.
The most recent source geological information used to prepare this summary was dated: 2010.
This description is a summary from published sources, the chief of which are listed below.
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