Summary:
The mineralised layers of the Waterberg Project:
• The mineralisation is hosted by sulphides that are apparently magmatic in origin.
• The mineralised layers can be relatively thick, often greater than 10 m.
PGM mineralisation within the Bushveld package underlying the Waterberg Project is hosted in two main layers: T Zone and F Zone.
The T Zone occurs within the Main Zone just beneath the contact of the overlaying Upper Zone. Although the T Zone consists of numerous mineralised layers, three potential economical layers were identified: TZ, T1, and T0. They are composed mainly of anorthosite, pegmatoidal gabbros, pyroxenite, troctolite, harzburgite, gabbronorite, and norite.
The F Zone is hosted in a cyclic unit of olivine rich lithologies towards the base of the Main Zone towards the bottom of the Bushveld Complex. This zone consists of alternating units of harzburgite, troctolite, and pyroxenites. The F Zone is divided into the FH and FP layers. The FH layer has significantly higher volumes of olivine in contrast with the lower lying FP layer, which is predominately pyroxenite.
The mineralisation generally comprises sulphide blebs, net-textured to interstitial sulphides and disseminated sulphides within gabbronorite and norite, pyroxenite, and harzburgite.
Within the F Zone, basement topography may have played a role in the formation of higher grade and thicknesses where 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” Zones where the sulphide mineralisation is over 40 m in thickness and within the defined areas average 3 g/t to 4 g/t 4E. Layered magmatic sulphide mineralisation is generally present at the base of the F Zone. As with the T Zone, the sub-outcrop of the F Zone unconformably abuts the base of the Waterberg Group sedimentary rocks and trends northeast from the end of the known Northern Limb and dips moderately to the northwest.
The T Zone includes several lithologically different and separate layers, which were initially recognised in the drilling. With subsequent drilling, it has become clear that the most easily identifiable and consistent are the TZ, T1, and T0 Layers.
T Zone Layering and Mineralisation
The T Zone is a unit that can be correlated and includes five identifiable layers. The three mineralised and economical potential layers are the TZ Layer, the T1 Layer, and the T0 Layer.
Upper Pegmatoidal Anorthosite
The Upper Pegmatoidal Anorthosite (UPA) has a pegmatoidal texture and is mostly anorthositic with some gabbros. This unit is generally not mineralised; however, it was found to have some sulphide mineralisation towards the top of this zone that represents the T0 mineralised unit. The mineralisation is hosted within the mafic crystals of pegmatoidal texture.
The UPA has a thickness range from 2 m to as thick as 100 m and can be correlated in more than 80% of the drill holes. It must be noted that the unit is absent in some drill holes and it also appears more mafic in some instances due to alteration of the anorthositic and gabbroic phases.
T1 Layer Mineralisation
Mineralisation within the T1 Layer is hosted in a 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 unit is mineralised with blebby to net-textured Cu-Ni sulphides (chalcopyrite / pyrite and pentlandite) with very minimal Fe sulphides (pyrrhotite). The thickness of the layer varies from 2 m to 6 m.
Lower Pegmatoidal Anorthosite and Lower Pegmatoidal Pyroxenite
The direct footwall unit of the T1 Layer can be divided into two identifiable units: Lower Pegmatoidal Anorthosite (LPA) and Lower Pegmatoidal Pyroxenite (LPP). These units have an unconformable relationship with one another as both are not always present.
LPA is the first middling unit underlying the T1 Layer. It has the same composition as the UPA but is usually thinner. The LPA thickness ranges from 0-3 m and in some drill holes it is not developed. The LPA is mineralised in some drill holes.
LPP is the second middling unit that underlies the LPA and it is predominantly composed of pegmatoidal pyroxenite. It also ranges from 0-3 m as it is not developed in other drill holes. The LPP is a TZ Layer hanging wall. Mineralisation was not identified in this unit.
TZ Layer Mineralisation
Mineralisation within the TZ Layer is hosted in Main Zone norite and gabbronorite that shows a distinctive elongated texture of milky feldspars. In some instances, the TZ gabbronorite / norite tends to grade into pyroxenite and in places into a pegmatoidal feldspathic pyroxenitic phases, with the same style of mineralisation as in the gabbronorite / norite. The high-grade zones range from 2 m to approximately 10 m in true thickness within these lithologies. Sulphide mineralisation in TZ Layer is net textured to disseminated 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.
F Zone Layering and Mineralisation
A thick package of norite and gabbronorite ranging from 100 m to about 450 m underlies the T Zone and overlies the F Zone.
F Zone mineralisation is hosted in a thick package of troctolite, which usually occurs as thin layers of pyroxenite and/or pegmatoidal pyroxenite and harzburgite. These layers or pulses were identified using their geochemical signatures and various elemental ratios. The initial subdivision was into a harzburgitic layer (FH) which is underlain by a pyroxenitic layer (FP).
F mineralised zone occurs in the ultramafic sequence pyroxenite and harzburgite. In the southern portion, the F zone is typically <10 m thick but in the central portion, the “Super F Zone” thickens to 60 m in true thickness, with grades of 2 to 4 g/t 4E over this interval. The mineralisation generally comprises blebs, net-textured to disseminated pyrrhotite, chalcopyrite and pentlandite with accessory chromite, 70 chalcocite, and pyrite. Chromite crystals are often enclosed in silicates, while chromite itself may host sulphide inclusions and rare chromitite stringers were identified in two drill holes. Magnetite has often replaced sulphides and chromite. PGM are variable with dominant sperrylite and subordinate Pt-Pd bismuthotellurides, Au-Ag alloys, Pd arsenides, and Pt-Rh sulpharsenides.