The Otjikoto Mine is held and operated by B2Gold Namibia (Pty) Ltd., which is 90% owned indirectly by B2Gold Corp. and 10% owned by EVI Mining (Proprietary) Ltd., a Namibian empowerment company.
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Summary:
The Otjikoto deposit is located within the Damara Mobile Belt, within the northern portion of the northeasterly-striking “Intracratonic Branch” of the belt and is an example of an orogenic-style gold deposit.
No definitive deposit model has been accepted for the Otjikoto and Wolfshag deposits, and the deposit genesis is still debated. Models that have been suggested include:
- Orogenic lode gold deposits: based on the fact that the systems are open to depth and show both lateral and vertical continuity down plunge on shoots, which is typical of Archean and Neoproterozoic orogenic systems;
- Structurally-controlled end member of an iron oxide–copper–gold (IOCG) system: based on the presence of high concentrations of magnetite at Otjikoto coupled with the low concentrations of quartz, both of which are uncommon in orogenic deposits.
The Otjikoto area is predominantly underlain by lithologies belonging to the Neoproterozoic Swakop Group. The Okonguarri Formation hosts the gold mineralization and is overlain and underlain by glacial diamictite horizons of the Ghaub and Chuos Formations, respectively. The Okonguarri Formation consists primarily of thick units of dark grey carbonaceous marble, biotite-schist, graphitic schist, and calc- silicate horizons. The schist units are derived from semi-pelitic, pelitic, marl and psammitic units in a turbiditic sedimentary package. The rocks in the Otjikoto area have experienced at least three phases of moderate to tight folding and some thrust faulting. They have also been affected by extensive metasomatism, followed by prograde regional metamorphism that has reached upper greenschist to lower amphibolite facies.
Otjikoto deposit
The deposit has a strike extent of 2.6 km and has been drill tested to 475 m depth below surface.
Most of the gold is hosted by a north–northeast striking sheeted vein system that is more than 40 m wide. Individual veins are thin (<10 cm) and consist of sulphide (+ magnetite)–quartz + carbonate veins developed in schist (variably albitised) and granofels of the Upper and Middle Okonguarri Formation. High-grade mineralized shoots occur on the short limbs and within the hinges of intrafolial folds between planar beds.
The major lithologies are:
- OTD biotite schist: hosts minor bedding-parallel veins with irregularly distributed gold values; Upper Okonguarri Formation;
- OTC albitite–hornfels unit: hosts most of the mineralized vein system; basal unit of the Upper Okonguarri Formation;
- OTB calcitic marble: unmineralized, 6–10 m thick; Middle Okonguarri Formation;
- OTA albite biotite schist–albitite: hosts minor bedding-parallel veins with irregularly distributed gold values; 20–30 m thick; Middle Okonguarri Formation;
- Footwall calcitic marble: unmineralised, 15–25 m thick; Middle Okonguarri Formation;
- Footwall biotite schist: unmineralised; Middle Okonguarri Formation.
The sheeted vein swarm lies at an angle of 20° to 30° to a set of north–northeasttrending linear structures, and the majority of the veins lie parallel to an S0/S1 transposition foliation which approximates bedding. The folded veins and related highgrade gold mineralization occur in a series of distinct en-echelon zones oriented at approximately 010° to 020° north–northeast and plunging at 10–15° (average 12°) to the south–southwest.
Vein concentrations range from one to 30 veins per metre, with a higher vein concentration within the Central and West shoots. Vein contacts are commonly irregular. Brecciation within and adjacent to the veins is common.
In the shallower and northern portions of the deposit, the veins are pyrite-rich, while in the southern and deeper levels of the deposit the veins are pyrrhotite-dominant. Veins proximal to the OTB marble, the bottom or footwall veins tend to be carbonate-rich while those in the rest of the deposit contain a mix of quartz–calcite and iron carbonate. Pyrite within the pyrite-dominant veins tends to have a vuggy texture.
Chalcopyrite is rare, and small (<10 µm) grains of maldonite (Au2Bi) were observed during metallurgical testwork on the pyrite-dominant veins. Marcasite is also present in the shallower, pyrite-dominant vein systems.
Magnetite represents an important constituent of the veins, ranging from trace amounts to 100% of individual veins. Granular to massive magnetite is more common than bladed “feathery” magnetite. The granular magnetite veins are commonly hosted by the intensively albitized metasediments.
Within the oxide zone the sulphides commonly weather to hematite and/or limonite.
Quartz, calcite, dolomite, siderite, biotite and ankerite are the main gangue minerals. Other important gangue minerals in the veins include garnet, amphibole and to a lesser extent chlorite and muscovite/sericite.
Silicate + sulphide vein assemblages show very variable proportions of the constituent minerals. Garnets within and on the haloes of veins tend to be larger than the host rock garnets.
Gold occurs within the vein system as coarse native gold with a size variation from 5 µm to 400 µm, with the median at about 100 µm. No specific location for gold has been noted. It has been observed adjacent to and within sulphides, along fractures, adjacent to and within garnets, within magnetite, on the edges of amphiboles and chlorite, and as free gold in quartz and carbonate.
Wolfshag deposit
The deposit has a strike extent of 2.1 km and has been drill tested to 700 m depth below surface.
The Wolfshag deposit is hosted within the fold duplicated OTA horizon enveloped between the Footwall (west) and OTB (east) marbles, and stratigraphically below the Otjikoto OTC zone. Albitite is the major lithology within the zone, with minor albite–biotite schist and marble bands in places.
The deposit consists of a series of fold-duplicated mineralized zones alphabetically subdivided from WA to WE into either west–northwest or east–southeast-verging fold closure zones. The zones steepen and narrow to the south. High-grade shoots within the mineralised zones are associated with parasitic folds occurring within the larger fold structure. The shoots plunge at 15° to 20° to the south–southwest, subparallel to the Otjikoto deposit shoots. The dip of the shoots is parallel to the axial planes of the folds (5 to 20°) to the east–southeast.
Gold mineralization can be vein-hosted, or represent replacement or disseminated styles.
Vein-hosted mineralization consists of pyrite–calcite + magnetite veins. These shear which tend to be brittle fracture-fill-related, with ragged edges, and most commonly parallel to subparallel to the compositional banding. Locally the veins are folded which is related to higher gold grades. Shear veining usually contains a mix of both replacement and vein style mineralization with both brittle (brecciation) and ductile deformation textures.
The replacement style mineralization ranges from moderate, disseminated, to massive pyrite and/or magnetite replacement of calcareous bands and/or marbles. Replacement mineralization tends to be lower grade and is commonly gradational with the shear vein style.
Disseminated mineralization tends to be lower grade with fine- to medium-grained magnetite more dominant than fine-grained pyrite.
Antelope deposit
Mineralization has been intersected by drilling over a combined plunge of approximately 2,000 m. The Antelope deposit mineralization is characterized by sheeted quartz–pyrrhotite veins, which have been overprinted by deformation focused along two main marble beds. The shoot-like geometry of the Antelope deposit mineralization derives, in part, from the thickening of quartz–pyrrhotite–gold mineralization in the hinge zones of centimetre- to metre-scale folds. Mineralized shoots plunge shallowly north–northeast, suggesting a subtle inflection in the stratigraphy that hosts the Otjikoto deposit, where ore zones plunge shallowly south–southwest.