Otjikoto East Exploration
The Otjikoto East Exploration Project comprises 5 exploration licenses with a total area of 1,844km2 in north-eastern Namibia. The main part of the Project is made up of three contiguous licenses which cover the eastern strike extension of the Otjikoto Gold Mine geology over a length of approximately 110km. Although the licenses are adjacent to a world class producing gold mine, the area has never been systematically explored and the prospective lithologies are almost entirely covered by calcrete and wind-blown sand.
The Project is situated within the Northern Zone of the Damara Supergroup in central Namibia to the south of Tsumeb. The Damara Supergroup is Neo-Proterozoic in age and is comprised of continental margin carbonates and silts which grade into turbidite sequences representing continental shelf and basin deposits. The Damara underwent a major orogenic event at 550 – 500Ma when the sea was closed along a series of major ENE structures. Hydrothermal fluid movement produced widespread gold mineralization within the carbonates and schists. There are two major producing gold mines within the Damara and several well-known smaller deposits which have been mined or evaluated in the past including Ondundu, Onguati, Sandamap and Epako. The Otjikoto East project is within a zone of convergence of major tectonic boundaries, likely to produce increased hydrothermal fluid flow during and immediately after the Damara orogeny. This setting is ideal for orogenic style gold mineralization.
The Otjikoto deposit is hosted by schists (metamorphosed shales and greywackes) belonging to the Okonguarri Formation, just below the contact with the Karibib marbles (metamorphosed limestones). The Otjikoto East exploration project is focussed on exploring the Okonguarri schists and the overlying contact along the entire 110km strike length. The Osino licenses also cover turbidites belonging to the Kuiseb Formation within a regional syncline.
The Otjikoto Gold Deposit was discovered in 1999 by Anglovaal’s local subsidiary Avdale, when they drilled a prominent magnetic anomaly during a regional base metal exploration programme. After the discovery, the area of similar stratigraphy to the east and south of Otjikoto was tied up under license by Avdale. As the region is largely covered by calcrete and wind-blown sand, the Avdale exploration program was based on geophysical methods – specifically airborne magnetic and electromagnetic surveys. The most prominent magnetic and electromagnetic anomalies were tested with wildcat diamond holes and in some cases with RAB drilling. None of the geophysical targets were positive and Avdale had relinquished the Otjikoto East area by 2010.
Work Completed by Osino to Date
Calcrete and Anthill Sampling
Osino started regional work on the Otjikoto East license (EPL5897) in October of 2017. Initial work involved regolith mapping and reprocessing of the regional magnetic data to assist with lithology mapping under calcrete cover. High priority areas were selected on the basis of stratigraphy and structural complexity and a campaign of calcrete and termite mound sampling was initiated.
Calcrete sampling has been highly successful in Western Australia and has led to the discovery of gold deposits under cover. This technique has also been effective at Osino’s Karibib project in central Namibia, where a gold anomaly has been outlined under a 20m thick cover of calcrete. Anthills are sampled in areas covered by wind-blown sand as they contain fragments of the buried calcrete, brought to surface by the ants.
All the priority regional structural corridors identified in regional interpretations have now been sampled. Approximately 80% of the 110km of the prospective stratigraphy strike length, have been sampled. A total of 11 gold anomalies have been detected by the sampling program to date. Five of these were identified for initial shallow percussion bedrock drilling (see below). Regional sampling will continue in 2020 on the remainder of the prospective stratigraphy not yet sampled.[Calcrete is a hardened, calcium-rich layer in, or on top of, a soil which is formed as a result of climatic fluctuations in arid and semi-arid regions. Calcite is dissolved by groundwater and, under drying conditions, is precipitated as the water evaporates at the surface. Rainwater saturated with carbon dioxide acts as an acid and dissolves calcite and then redeposits it on the surfaces of the soil particles; as the spaces between soil particles are filled, an impermeable crust is formed. This crust can vary from less than 1m to more than 50m thick].
Gold Anomalies Discovered to Date
- Fairview– A multipoint gold anomaly in anthill samples adjacent to an interpreted dome structure. Subsequent pitting and rock chip sampling of limited quartz veins produced two positive assays of 1.16 and 0.90 g/t gold, along with anomalous silver and copper.
- Platform– A linear gold anomaly adjacent to the crustal scale lineament dividing the basin sediments to the south (Northern Zone) and the platform margin carbonates to the north (Northern Marginal Zone).
- Okukumukanti North, South and Linear – Three closely spaced anthill and calcrete sampling anomalies associated with splays off the major structure in the area.
- Omagonde– A low-level gold and silver anomaly within calcrete in the northwest corner of the project.
- Gaidaus– An east–west striking gold anomaly in calcrete, along an interpreted thrust fault between schists and marbles. The historical Okurusu occurrence is adjacent to this anomaly and consists of a small copper – gold gossan in marbles.
- Omahona Fold– A large-scale, low-grade gold anomaly in calcrete along the folded contact between schists and marbles.
- Waldorf South– Several gold anomalies in sand overlying carbonate horizons within Okonguarri schist. Adjacent to a major structure (Otjohorongo Lineament).
- Gesundbrunnen East– Associated with within Okonguarri Formation in the nose of the regional Otjikoto East syncline
- Gesundbrunnen West– Associated with thrust fault within Karibib marbles in the nose of the regional Otjikoto East syncline
Airborne Magnetic and Radiometric Survey
A geological interpretation was carried out using the detailed geophysical information collected during the high-resolution magnetic and radiometric survey flown by Osino in May 2018. Otjikoto East has very limited outcrop and the interpreted geology is essential to prioritize and direct activities on the ground. Exploration is focused on large-scale, fertile structures and prospective stratigraphy.
To date, eight surface geochemistry anomalies have been identified including Fairview, the priority target for follow up and first new gold mineralization discovered in this region since the Otjikoto deposit in the 1990’s.
Shallow percussion drilling – Priority Targets
Priority anomalies and targets at Fairview, Platform, Okumukanti North, Okumukanti South and Okumukanti Linear were drill tested with shallow percussion / rotary air blast (RAB) drilling. A total of 371 holes were completed for 3,603m. Bedrock was intersected at between 1-28m below surface and the average hole depth was 10m (see the figure below). Assay results are expected in Q4 2019.
Work Planned for Q4 2019 and 2020
The work program for 2019 will include:
- Review and follow up of initial shallow percussion bedrock drilling of Fairview, Platform, Okumukanti North, Okumukanti South and Okumukanti Linear targets.
- Completion of the calcrete and anthill sampling program covering the Okonguarri Formation lithologies and the contact with the overlying Karibib marbles.
- Follow-up calcrete sampling programs over gold anomalies.
- Further bedrock sampling of gold anomalies using shallow percussion drilling.