Karibib Gold Project – Regional Exploration
Osino’s Karibib Gold Project consists of 12 licences comprising approximately 1,946 km2 in central Namibia near the towns of Usakos, Karibib and Omaruru. This gold bearing district hosts the multi-million ounce Navachab Gold Mine as well as the historical Onguati copper-gold and Goldkuppe small scale gold diggings. Much of the area has never been systemically explored as it is covered by calcrete and/or wind-blown sand.
Early work by Osino, including regional mapping and re-interpretation of the aeromagnetic data identified a large, previously unmapped, deep regional structure – now named the Karibib Fault. This structure can be traced over 70km and has several gold-in-soil anomalies along it and on splays leading off it. Osino’s exploration activities since 2017 have been focussed along this major structure between the company’s Twin Hills project in the northeast and the Navachab Gold Mine. In 2018, additional licences were added to the north, west and south west of Navachab.
The Karibib Project is situated within the Southern Central Zone of the Neo-Proterozoic aged Damara Supergroup, which 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 basin 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 (Navachab and Otjikoto) as well as several other smaller deposits which have been mined or evaluated in the past including Ondundu, Onguati, Sandamap and Epako.
After the discovery of the Navachab gold deposit in 1984, Anglo American carried out a rapid regional exploration programme primarily using stream sampling and targeting similar stratigraphic horizons and basement domes. Known gold occurrences in marbles at Goldkuppe, Albrechtshöhe and Dobbelsberg were followed up with soil sampling as well as limited drilling. Detailed follow-up at Goldkuppe (see separate summary) and Albrechtshöhe was carried out by Helio Resources in 2013. No further regional work was done.
Work Completed by Osino & Leading up to the Twin Hills Discovery
The eastern portion of the Karibib Fault Trend (20km in length) was soil sampled during 2017 and a number of significant soil anomalies including Twin Hills, OJW and Okapawe were discovered along the fault zone, which appears to be mineralized throughout its length. The soil anomalies over outcropping rock are in the range of 100 to 400ppb gold against a background of less than 5ppb and follow the regional northeast trend. The mineralized prospects occur in association with splays, bends and syn-tectonic granite intrusions, typical of fertile orogenic belts.
Acquisition of Ground Magnetic Data Along Karibib Fault
Ground magnetic data was collected on a grid of 50m spaced lines from Twin Hills East to the Navachab Gold Mine property. This data provides lithological and structural detail in an area that is completely covered by calcrete and sand. The detailed magnetic data is of particular value in defining prospective structures under the thick cover to support the geochemical data collected at surface. The ground magnetic data were critical in helping constrain the general structure in the Twin Hills Central and on-strike extensions and the association with the surface geochemistry gold anomalies.
Initial Drilling at Twin Hills East
A short scout drill program (RC and DD) was undertaken during April and May of 2018 with 23 holes at the Twin Hills East prospect and 6 at Okapawe. The drill results at Twin Hills East confirmed the presence of gold within a large-scale hydrothermal alteration system along splays from the Karibib fault. The mineralization is present at surface for 3km in strike length before disappearing beneath calcrete cover to the south west. The gold mineralization is contained within a package of quartz biotite schists and is associated with quartz, sericite, arsenopyrite and pyrite alteration. Gold grades and the intensity of the hydrothermal system appear to increase with depth and towards the south west. This was the impetus for focusing exploration to the southwest – and the discovery of Twin Hills Central.
In the second half of 2018, exploration was focussed on the south west strike of the hydrothermal system where it is covered by calcrete and wind-blown sand. The work program included calcrete sampling, detailed ground magnetics and RAB drilling to test the bedrock under the calcrete cover.
Calcrete Sampling at Twin Hills
The use of surface calcrete as a sample medium was pioneered in Namibia by Osino following the success of the technique for gold exploration in Western Australia. Calcrete sampling and analysis by ultra-low detection assay has proven to be successful and the Twin Hills Gold anomaly was increased in size from 3km to 11km in strike length using this method. A large scale, well-defined core of high grade calcrete samples has been outlined at Twin Hills Central measuring 3km x 300m. The Twin Hills Central gold results coincide with a strong magnetic anomaly in the bedrock due to the presence of pyrrhotite, a magnetic sulphide which is associated with the gold mineralization. Anomalous gold assays and magnetic anomalism are present over the entire 11km strike length.
Surface calcrete sampling to the south west of Twin Hills also identified two additional anomalies as follows:
Quarry – Gold anomaly in marble over 1200m in strike length
Main Road– Gold anomaly in marble over 2000m.
The Main Road anomaly has a highly prospective structural setting as it coincides with a zone of brecciation several hundred metres wide, associated with a set of north-south faults which displace the Karibib Fault and continue for several kilometers northwards into the schists.
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.
Bedrock Drilling to Confirm Calcrete Results at Twin Hills Central and Other Targets
Initially bedrock drilling was carried out on two fence lines with holes 20m apart for a total of 49 holes at Twin Hills Central. The holes were drilled vertically through calcrete to test the top of the weathered bedrock with the objective of confirming that gold anomalies at surface correlate with gold in the bedrock. The average thickness of the calcrete in this area is 19m and the bedrock lithology in all holes is quartz – biotite bearing schists and meta-greywackes, similar to the lithology outcropping at Twin Hills. The maximum gold assay value was 2.3g/t and a further seven bedrock samples had values between 0.3g/t and 1.1g/t gold.
Subsequently, further bedrock drilling was carried out on other targets along strike from Twin Hills Central at Twin Hills West, Clouds and Barking Dog.
Assays of bedrock sampling at Twin Hills West have extended the zone of mineralization to more than 800m strike length including the highest-grade bedrock drilling sample received to date (2.69g/t Au). A prominent arsenic anomaly to the south of the Twin Hills West magnetic zone indicates further size and grade potential. Twin Hills West has now become a sizable gold/arsenic anomaly with significant potential and represents Osino’s second priority on the Twin Hills Cluster, after Twin Hills Central. Twin Hills West is approximately 2km to the west of Twin Hills Central.
Assays received for bedrock sampling at Barking Dog and Clouds prospects also confirm new zones of gold mineralization and represent the 4th and 5th bedrock gold prospects in the Twin Hills Cluster. The Clouds prospect comprises confirmed gold mineralization in two drill lines 800m apart and is located 2km along strike to the east of Twin Hills Central. The Barking Dog prospect comprises two separate zones of confirmed gold mineralization in two drill lines 200m apart. Barking Dog is located approximately 4km along strike to the east of Twin Hills Central.
The gold mineralization intersected to date at Twin Hills is associated with arsenopyrite, which confirms that arsenic (As) can be used as a path finder element.
All bedrock gold prospects discovered to date are made larger and more coherent by using the arsenic (As) assay values as an additional indicator (refer to the figure below).
Arsenic is of particular value as an indicator of gold in this setting for the following reasons:
- Bedrock sampling is equivalent to collecting a blind rock chip sample every 25m, meaning that the chances of collecting a gold bearing veinlet are fairly low. Arsenic often occurs as fine-grained disseminated mineralization in altered zones around the gold bearing veinlets – meaning that the chances of sampling anomalous arsenic are higher than sampling anomalous gold
- Where the gold mineralization is buried and does not subcrop (against the bottom of the calcrete) there may be no gold anomaly present at the top of the bedrock. However, arsenic is more mobile than gold in many situations, and may form anomalous haloes around buried mineralization by leaking up faults and shears.
- Gold mineralization at Twin Hills is associated with three known iron sulphides, namely pyrite, pyrrhotite and arsenopyrite, of which only pyrrhotite is magnetic
- The magnetic anomaly at Twin Hills is related to the presence of pyrrhotite but not all pyrrhotite mineralization is associated with moderate to high grade gold. There are indications that the higher-grade gold at Twin Hills is associated with arsenopyrite. Arsenic may therefore be used to prioritise higher grade zones and define mineralization not related to magnetic anomalies
- It is noteworthy that the bedrock assays around hole OKD004 (65m @ 1.37g/t incl. 31m @ 2.2g/t Au) also show a prominent arsenic anomaly
Shallow Drill Program Along Karibib Fault South West of the Twin Hills Cluster
A program of shallow vertical holes, consisting of fence lines spaced at 800m apart and 100m hole spacing along the fences, was completed towards the end of 2018. This drilling was designed to sample the top of the calcrete layer below wind-blown sand cover up to 3m thick. Several deeper holes were also drilled to determine the calcrete thickness along the Karibib Fault and initial indications are that the thickness increases from Twin Hills towards the southwest.
A total of 259 holes (for 7,152m) of shallow percussion drilling was completed on the Twin Hills West, Barking Dog and Clouds targets during the 2nd and 3rd quarters of 2019. These holes were drilled through a layer of calcrete and sand cover of up to 20m thick to intersect bedrock.
Calcrete assays from the shallow drill program along the Karibib Fault indicate potential for further gold targets with priorities as follows:
Karibib Jog – Gold anomaly in calcrete over schists near jog on Karibib Fault
West End – Anomaly in calcrete over schists just to north of Navachab mining licence. These anomalies look like they continue to the north and further sampling will be carried out in that direction.
Work Planned for Q4 2019 and 2020
During 2019 the following work is planned
- Phase 2 drilling of the Twin Hills Cluster to be completed in 2019 (as mentioned above).
- Phase 3 drilling of the Twin Hills Cluster likely to start in Q1 2020.
- Ongoing follow up of Karibib District targets and continued regional sampling where relevant (e.g. Kranzberg).