Summary:
The Waterberg deposit type falls into the category of magmatic sulfide layered intrusive formed by settling and crystallization of sulphides in a magma chamber during formation. The style of the Northern Limb—the Platreef in particular—is thought to be more complex than that of the narrow stratabound UG2 and the Merensky reefs. The syn- and post-solidification of fluids and the assimilation of the country rock are said to have played a crucial role regarding the mineralization type and style of the Platreef.
Although the Waterberg Project is a separate sub-chamber from the exposed Northern Limb, the mineralization style and type conforms to some of the Platreef attributes. For example:
• The mineralization is hosted by sulphides that are of magmatic origin.
• The mineralized layers can be relatively thick, often greater than 40 m.
Other attributes relating to the Platreef, such as magma contamination by the floor rocks, are not yet apparent on the Waterberg Project. Although the mineralization on the Waterberg Project is deemed to be Platreef-like, other major variations in attributes include the lithostratigraphic positioning of the mineralized zones. This is further compounded by the differences in geochemical profiles, which suggest a type of mineralization never seen before in the Bushveld Igneous Complex (BIC).
Mineralized Zones
PGM mineralization within the Waterberg Project is hosted in two zones which are the lower FZone and the upper T-Zone. The T-Zone is stratigraphically located at the contact between the Main Zone and the Upper Zone. This zone comprises several economically viable mineralized subzones which are the lower TZ, T1, and the upper T0.
The mineralization within the F-Zone is generally hosted in rocks which are stratigraphically located towards the bottom of the mafic-ultramafic succession of the Waterberg Project. These mineralized host rocks are generally lower feldspathic pyroxenite, harzburgite, troctolite, and some occasional gabbronorite. Based on lithological variations, the F-Zone has been subdivided into the lower FP subzone, which is predominantly characterized by feldspathic pyroxenite, and the upper FH subzone, which comprises olivine-bearing rocks such as harzburgite and troctolite.
Within the F-Zone, basement topography is said to have played a crucial role in the formation of higher-grade thicknesses where channelized embayments or large-scale changes in magma flow direction may have facilitated the accumulation of magmatic sulphides. These areas are referred to as the “Super F-Zone” where the mineralization is over 40 m in thickness with an average grade between 3 g/t to 4 g/t 4E.
As with the T-Zone, the sub-outcrop of the F-Zone unconformably abuts the base of the Waterberg Group sedimentary succession, trends northeast from the end of the known Northern Limb, and dips moderately to the northwest.
Description of T-Zone Layering and Mineralization
The T-Zone and its five identifiable layers and subzones can be correlated both on dip and along strike sections. Of the five layers, three mineralized layers are of economic importance.
Upper Pegmatoidal Anorthosite
The upper pegmatoidal anorthosite (UPA) is generally a more felsic, very coarse crystalline igneous rock which is primarily characterized by a pegmatoidal texture. In certain localities, the subzone is more gabbroic with sporadic pegmatoidal gabbronorite. The UPA has a thickness ranging from 2-100 m and can be correlated in more than 80% of the exploration drill holes. The subzone is generally not mineralized; apart from localities where the T0 mineralization migrates from the Upper Zone to the upper portions of the UPA.
T1 Layer Mineralization
Mineralization within the T1 Layer is hosted in troctolite with variations in places where troctolite grades into feldspathic harzburgite. In other localities, olivine-bearing feldspathic pyroxenite grades into feldspathic harzburgite. The 4E grade (g/t) is typically 1-7 g/t with a Pt:Pd ratio of about 1:1.7. The Cu and Ni grades are on average 0.08% and 0.05%, respectively.
The best mineralization occurs in the form of disseminated and blebby to net-textured Cu-Ni sulphides (chalcopyrite / pyrrhotite and pentlandite) with very minimal pyrite. The layer varies in thickness from 2 m-6 m.
Lower Pegmatoidal Anorthosite and Lower Pegmatoidal Pyroxenite
The direct footwall of the T1 Layer can be divided into lower pegmatoidal anorthosite (LPA) and an infrequent lower pegmatoidal pyroxenite (LPP). Where present; the LPA is generally in direct undulating contact with the overlying T1; with thickness ranging from 50 cm to 10 m. This subzone is of similar composition to the UPA but generally thinner. In certain localities, economically viable mineralization occurs towards the bottom of the subzone, closer to the contact between the LPA and the T2 mineralization. Note that the TZ refers to the T2 / LPA mineralization. The mineralization has been seen to transgress both on dip and along strike from the T2 subzone up into the LPA subzone.
The 0 m-3 m thick sporadically occurring LPP is characterized by pegmatoidal pyroxenite. In localities where this subzone has been identified, it occurs as a direct hanging wall to the T2 subzone. To date, mineralization has not been identified in this subzone.
TZ Layer Mineralization
The TZ refers to the transgression of mineralization between the T2 and the LPA layers / subzones. Where this mineralization is in the T2 subzone, it is hosted in gabbronorite and feldspathic pyroxenite, with distinctive elongated milky feldspar laths. In localities where the mineralization has transgressed from the T2 into the LPA, this mineralization is hosted in a very coarse crystalline anorthosite with pegmatoidal texture. The TZ high-grade envelope ranges from 2 m to approximately 30 m in true thickness. Sulfide mineralization within the TZ Layer is nettextured, disseminated, and blebby with higher concentration of sulphides compared to the overlying T1 Layer. The 4E grade (g/t) is typically 1-6 g/t with a Pt:Pd ratio of about 1:1.7. The Cu and Ni grades are typically 0.17% and 0.09%, respectively.
Description of F-Zone Layering and Mineralization
A thick Main Zone ranging from 100 m to about 450 m acts as an interburden between the lower mineralized F-Zone and the upper mineralized T-Zone. The zone is characterized by monotonous gabbronorite which is based on the essential mineral modal percentage ranges from leucocratic to melanocratic gabbronorite. Anorthosite ranging from poikilitic to phenocrystic was also identified within the Main Zone. The F-Zone mineralization, which is stratigraphically located beneath the Main Zone, is hosted in a thick ultramafic sequence comprising feldspathic pyroxenite (FP) and harzburgite (FH). Note that the ultramafic sequence is a mineralized grade envelope within the F-Zone.
To the southern portion of the Waterberg Project, the F-Zone is typically 60 m in thickness more prevalent towards the central portion of the Waterberg Project. These thicker portions, now referred to as the “Super F-Zone,” range in average grades from 3 g/t to 4 g/t 4E over this interval.
The mineralization generally comprises disseminated, blebby, net-textured, and at times semimassive sulphides.
PGE Mineralization
Within the T-Zone, Pt-Pd bismuthotellurides are the dominant PGE minerals accompanied by AuAg alloys, Pd tellurides with rare sperrylite, antimonides, braggite and Pd stannides (McCreesh et al., 2018). The higher Au / PGE, higher native Au content, and the fluid-induced style of mineral remobilization are atypical of the BIC but normal to the T-Zone (McCreesh et al., 2018). As for the F-Zone, sperrylite is found to be the dominant PGE mineral with Au-Ag alloys, Pd-Ni arsenides, Rh-Pt sulpharsenides, minor Pt-Pd bismuthotellurides and rare Pt-Fe alloys (McCreesh et al., 2018).