Copper and silver mineralisation are hosted within the Ghanzi-Chobe Fold and Thrust Belt that forms the southern portion of a much larger Pan African Mobile Belt, stretching from Namibia through Botswana, Zambia and into the Democratic Republic of Congo. The Ghanzi-Chobe Belt is also known as the Kalahari Copper Belt which consists of deformed metavolcanic and meta-sedimentary rocks that were deposited in the late Mesoproterozoic to early Neoproterozic eras. The belt is host to several well-known strata-bound sediment-hosted copper deposits and represents an emerging, world-class copper-silver district of untapped potential in southern Africa.
Rocks of the Ghanzi-Chobe Belt are exposed along the north-east trending Ghanzi Ridge which make up the basal metavolcanic sequence known as the Kgwebe Formation. The Kgwebe Formation is overlain by the Ghanzi Group metasediments. This Group, from oldest to youngest, consists of the Kuke, Ngwako Pan, D’Kar and Mamuno Formations (as seen in the “Khoemacau Regional Stratigraphy” figure below).
The Khoemacau deposits are characterised as a structurally controlled strata-bound sediment-hosted copper deposit. Copper-silver mineralisation typically occurs at the stratigraphic and/or structural boundary between the contact of the oxidised Ngwako Pan sandstone Formation and reduced D’Kar siltstone Formation. The boundary is both a chemically reduced and structurally controlled trap environment. Host rocks are unconformably overlain by unconsolidated Kalahari Sand and calcrete up to 60m thick.
The lower ductile siltstones and carbonaceous units of the D’Kar Formation are the main host for most of the copper and silver mineralisation. The D’Kar rocks are composed of shallow marine sediments deposited >981 +/- 3 Ma (millions of years ago) and consist of finely laminated and chemically reduced mudstones and siltstones intercalated with carbon rich limestone and thin lagoonal black shale.
The overall sedimentary basin architecture and structural association of the basin remains intact. Basement normal faulting during early subsidence and extension formed a series of horst and grabens across the project area where organic rich, shallow waters exhibiting an abundance of sulphur were restricted to basement margins. Prior to mineralisation, copper bearing basinal brines were confined to the lower oxidised sandstone and upper bimodal volcanic basalts. Metals were leached from the red-bed and basalt. During deformation and basin inversion during the Damarion Orogeny (~550Ma (millions of years ago)), metal enriched brines migrated along basement structures, major faults and lithological contacts depositing copper at the redox and structural trap sites.
Structure and Mineralisation
The entire region has been subject to compression, folding and thrusting along north-east trends resulting in structurally repeated stratigraphically controlled mineralisation over hundreds of kilometres. The structural orientation and related permeability are key aspects in the mineral trap site development. Deposits generally occur at the margins of basement structures where the stratigraphic redox boundary is controlled by sediment deposition and structural geometry. Flexural slip along bedding on the limbs of parasitic folding were important primary fluid pathways. Brittle fractures, and tectonic breccia at local and deposit scale are the dominant secondary structural mechanisms.
Although mineralisation differs slightly at each deposit, economic grades are dominantly related to shearing, folding and tensional failure along and close to the Ngwako Pan and D’Kar redox contact. Disseminated and hydrothermal vein-hosted sulphide mineralisation styles combine to produce continuity of high-grade copper and silver mineralisation over tens of kilometres. These higher-grade copper sulphide zones typically contain disseminated cleavage parallel lenticles and massive quartz-carbonate and breccia veins hosting chalcopyrite, bornite and chalcocite mineralisation.
Sulphide assemblages are commonly zoned. The sequence is developed vertically upward from the base of the D’Kar Formation and can be seen to develop horizontally along strike at some deposits. The typical zonation sequence consists of low sulphur, low iron, copper sulphides (chalcocite and bornite) and passes upward with increasing iron content (chalcopyrite and pyrite). This sulphide zonation coincides with copper solubility precipitating of low soluble sulphides at the first reductant while chalcopyrite and pyrite remain in solution.
Starter and Expansion Projects Geology
Exploration to date has focused on delineating four high-grade deposits with economic copper-silver mineralization:
- The Zone 5 deposit, discovered in 2012, was successfully drilled to 1,200m below surface, delivering 92Mt of resources in the period 2012 to 2015. In 2019, grade control, or stope definition, drilling was undertaken from surface to facilitate detailed mine design planning as part of the development of the underground operations (refer to the Starter Project for further detail on Zone 5).
- Additional resources development and exploration drilling has recently focused on the Zone 5 North, Zeta NE and Mango NE deposits all located in the north-east (shown in the diagram below) of the Project license area (refer to the Expansion Project for further detail on these expansion resources).
Regional Exploration & Resource Drilling
Regional exploration has included extensive soil geochemistry testing, geophysical surveying, drilling, geological interpretation, geological mapping, structural studies, litho-geochemical sampling, three dimensional interpretation, reverse circulation (RC) and diamond core drilling (DDH), petrographic studies and mineral resource estimation. Recent exploration activities included surface geology and reconnaissance field mapping, geophysical aero electromagnetic surveying and drilling at various targets across the project area. Field mapping, logged drill core and geophysical interpretation were used to assemble a regional and deposit scale three-dimensional model delineating local structures, lithology and mineralisation to aid in further drilling targets and provide detail on mineral and grade continuity.
A summary of all drilling completed at the Project area, extending from the far north, north-east area to the Banana Zone in the South, up to the end of December 2020 is summarised in the following table. The hole type is defined as either Reverse Circulation (RC) or Diamond Drilled Hole (DDH) (DDH may also refer to diamond core drilling holes that may have been pre-collared with RC).
|Company||Year||Hole Type||No. of Holes||Metres (m)|
|US Steel||Pre 2007||DDH||62||12,434|
|US Steel Total||62||12,434|
|DML||2006 - 2013||DDH||985||90,266|
|2007 - 2013||RC||1,286||55,797|
|Hana||2007 - 2012||DDH||737||116,161|
|2008 - 2012||RC||597||69,096|
|Khoemacau||2013 - 2020||DDH||789||293,354|
|2013 - 2020||RC||169||27,320|
|Project Drilling Total||4,742||679,946|
2020 Regional Exploration
The most recent greenfields exploration has focused on identifying targets with favourable ore controls through the use of Airborne Electromagnetic (AEM) geophysics, multi-element geochemistry, basin architecture, structural modelling and diamond drilling.
Airborne Electromagnetic (AEM) Survey
An AEM high resolution survey was flown in October 2019 and processed during the fourth quarter of 2020 to aid in target location of buried dome features and depth of structures, across the Company’s license area. The processed data used the AarhusGeo method and was able to identify clear conductors, calculate accurate depths and clear several artefacts from the profiles.
The data was able to successfully identify the lower D’Kar units as strong conductors and clearly highlight the reduced contact between the D’Kar and Ngwako Pan formations.
The prominent conductors identified were hosted within the lower D’Kar and located at Chalcocite Zone, North East Fold, Banana Peel, Zone 4, Zone 6 and Zone 6 East. These conductors that coincide with surface structures are interesting targets for further exploration work and diamond drilling programmes.
The Banana Zone is a double plunging antiform located approximately 60km to the south-west of Zone 5 (refer to the following figure “Khoemacau License area displaying location of Banana Zone”). The entire reduced contact between the D’Kar Formation and Ngwako Pan Formation is continuously mineralized for 64km. The north-west limb dips at 55 to 60 degrees while the south-east limb dips steeper at 75 to 80 degrees. The fold hinges both plunge shallowly at 35 to 42 degrees.
The Banana Zone was extensively explored between 2010 and 2012 with additional exploration drilling, geophysical surveys and geotechnical studies completed from 2013 to present. Drilling successfully identified four deposits of economic copper-silver mineralization: Chalcocite, North East Fold, South Limb Definition and New Discovery. Additional potential exists along the Southern and Northern Limbs. Mineral Resources at Banana Zone were reported in August 2014.
Since 2015, the Company focussed drilling and development of Zone 5. However, as outlined earlier exploration continued at the Banana Zone through the use of Airborne Electromagnetic (AEM) surveying and the subsequent analysis and interpretation of the data.
These deposits share similar broad characteristics of alteration, host lithology and ore mineralogy. However, each deposit has specific characteristics as outlined below:
Chalcocite Zone is located on the south-west fold closure of the Banana Zone (refer to the following figure “Surface plan map of Banana Zone’s five sub-zone’s”). The fold nose plunge dips at 35 degrees to the south-west displaying open folds and shallow dipping stratigraphy. Mineralization extends along the southern limb of the fold for approximately 2km, and along the northern limb for almost 1km, having a total strike length of 7km. The deposit contains a significant portion of the total contained copper with grades averaging 0.6%.
Mineralization is hosted within the alternating sequence of sandstones and siltstones at the contact between the D’Kar and Ngwako Pan Formations and is almost exclusively disseminated chalcocite over a substantial thickness. The zone lacks veining and associated massive bornite typically seen at neighbouring deposits in the north. One high grade copper domain was defined using a cutoff of 0.5% Cu. The mineralized zone averages 4m in thickness on the north limb and 15m on the south limb with dips ranging from 35 to 60 degrees, respectively.
North East Fold
The North East Fold deposit sits on the north-eastern fold hinge of the Banana Zone and includes 1km of strike extending south-west along the northern limb (refer to the following figure “Surface plan map of Banana Zone’s five sub-zone’s”). The fold axis plunges at 42 degrees toward the north-east and the northern limb dips at 55 degrees to the north-west.
Mineralization along the north limb has been divided into two continuous high grade copper domains surrounded within a wider halo of lower grade mineralization. The two high grade domains are based on a 0.5% Cu cutoff with true thicknesses that range from 2 to 8m. The upper hanging wall grade shell is above the Ngwako Pan - D’Kar contact by 15m. It sits predominately within the alternating siltstone and sandstone unit and does not continue down the dip of the northern limb below 300m from surface. The lower of the two domains sits at the redox contact within the lower marl unit. This grade shell has been intersected in drill core down dip to 400m below surface along the north limb, and down the plunge of the fold axis.
The fold closure is structurally complex displaying several fractures formed due to folding. It contains the same two mineralized domains as the north limb but includes two additional grade domains within the Ngwako Pan oxidized sandstone. These two mineralized zones sit approximately 10 and 30m below the contact. All four high grade domains are separated by lower grade copper and silver. The entire network of mineralization ranges in width from 40 to 60m and makes up a stockwork-style of stacked breccia dominated lenses. Each of the domains true thickness ranges from 2 to 16m.
Close spaced drilling defined a narrow carbonate quartz vein using a 4% Cu cutoff and averages 8% Cu. The vein sits within the hanging wall host rocks of the lower marl and alternating siltstone and sandstone units. High grade copper, silver and molybdenum mineralization are confined within a quartz-calcite vein that ranges in thickness from 0.3 m to 6.5 m over a strike length of 1km down the plunge of the fold nose. The vein runs parallel to sub parallel to bedding, pinches and swells and cross cuts into other mineralized grade shells.
South Limb Definition
South Limb Definition has a strike length of 3.5km and has been drilled to a maximum vertical depth of 400m from surface (refer to the following figure “Surface plan map of Banana Zone’s five sub-zone’s”). The deposit displays continuous mineralization and dips steeply at 75 degrees to the south-east and strikes at 45 degrees.
The true thickness averages 5m using a 0.5% Cu grade domain. Copper grade averages 1.3% and is defined by one high grade shell. The mineralization is found within fractures and veins as chalcopyrite and bornite hosted within the alternating sandstone and siltstone unit.
The New Discovery deposit is on the northern limb of the Banana Zone with an economic strike length of 1.6km (refer to the following figure “Surface plan map of Banana Zone’s five sub-zone’s”). The deposit has been drilled over 3.5km strike and to a maximum vertical depth of 650m below surface. The mineralization strikes at 236 degrees and dips at 55 degrees to the north-west.
High grade mineralization is defined in two copper grade domains using a 0.5% Cu cutoff. The hanging wall domain sits approximately 25m above the Ngwako Pan contact and hosted within the alternating siltstone and sandstone unit. Mineralization is predominantly vein hosted displaying micro-folds and extensive quartz-carbonate veining over an average true width of 5m. The footwall domain is hosted in the lower marl unit and sits at the contact with the Ngwako Pan Formation. Both domains average 30m width, contain predominately bornite and chalcocite mineralization with minor chalcopyrite along shear planes and fractures. Copper averages 1%.
2020 Drilling Programme
Four targets; Zone 9, South Selene, South Dome and Kgwebe Central, were identified and drilled with high potential for economic mineralisation based on regional ore controls and conceptual models:
- sediment starved, organic rich, shallow water environments or near evaporite (sulphur source);
- underlying oxidized and altered sandstones, bimodal volcanic and paleo basement (copper source);
- magnetic and gravity highs indicative of basement faulting, major structures and metal enrichment (high permeability); and
- regional litho-stratigraphic lineaments as copper- bearing fluid traps (structural or chemical trap).
A total of 22 diamond core holes for 4,248m of drilling was successfully completed at these four above mentioned prospects (refer to the following diagram, Khoemacau regional drilling locations (4,248m completed in 2020)).
Zone 9 was drilled over 1.5km strike to 170m below surface. The target was identified as a fold closure adjacent to a basin high. A total of six holes defined the high-grade copper sulphides along the basin shelf. The overall architecture showed intact facies distribution with the basin deepening toward the north-east. Abundant north, north-east trending structures and parasitic folds are seen on the aeromagnetics. Further drilling and follow up is warranted at this prospect to define continuity along strike and depth.
South Selene was drilled over 1.5km strike to a total depth of 260m below surface. The target was identified as a tight syncline hosting strong sulphides within stromatolites and veins indicating a carbonate platform and basin high. The southern limb has been interested by 9 diamond drill holes over varying widths and grade. The prospect intersects economic grades over an average of 6.5m width. Further core drilling will be completed to define continuity in higher grade intervals along strike and depth.
The South Dome was also drilled over a 1.5km strike length to a depth of 250m below surface. The target was identified as a fold closure adjacent to a basin high. A total of 7 core holes defined high-grade copper sulphides along the basin shelf. Holes intersected massive quartz calcite veins hosting multiple stacked horizons of strong sulphide mineralisation. South Dome is located on the strike extent of the contact that host the Zone 5 and Mango NE resources.
The Kgwebe prospect area was drilled over a strike length of 2km to a depth of 250m below surface. The area is structurally complex and was targeted as a tight syncline hosted between two basement highs showing only stratigraphic surfaces, sedimentary wedges and truncations. A total of 10 holes were drilled. Mineralisation was not continuous but did intersect high-grade copper sulphides over thick intervals averaging 7m. Further modelling is required to define the controls and total potential of this prospect.