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 figure below).
The Project area is 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.
Exploration to date has focused on delineating four high-grade deposits with economic copper-silver mineralisation. The Zone 5 deposit was recently drilled for grade control mine planning and has advanced into development with underground operations and +20-year LOM underway (refer to the Starter Project for further detail on Zone 5). Additional resources and exploration drilling then focused on Zone 5 North, Zeta NE and Mango NE deposits all located in the north-east (shown in the diagram below) of the Project licence area and have several similarities to Zone 5 (refer to the Expansion Project for further detail on these expansion resources).
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 drilling completed at the Project area 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 - May 2020||DDH||789||293,354|
|2013 - May 2020||RC||169||27,320|
|Project Drilling Total||4,742||679,946|
Recent exploration work focused on identifying targets with favourable ore controls through the use of AEM geophysics, multi-element geochemistry, basin architecture and structural modelling. Four targets were identified 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 approximately 4,300m of drilling was successfully completed at four prospects (figure 7). The drilling defined economic grade intersections with 1km strike length.
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.