Summary:
The Stillwater Complex is a large layered igneous complex resulting from magma intrusion through regional transverse faults into highly deformed Archaean sedimentary rocks. However, the flat-lying part of the Stillwater Complex occurs at significant depth below surface, which makes the exploitation of the J-M Reef in this part of the complex uneconomic.
The magma intrusion and emplacement relating to the Stillwater Complex were accompanied by fractionation and accumulation of magmatic crystals that gave rise to the conspicuous magmatic layering observed in the complex. The magmatic layering is reflected by the changes in mineralogy, mode, grain size and texture across the stratigraphic profile of the complex. However, the overall texture of the lithological units in the Stillwater Complex is typified by subhedral to euhedral cumulate grains in a framework of post- cumulus interstitial material including oikocrysts. The mineralogical, modal, grain size and textural variations formed the basis for subdividing the Stillwater Complex into five major series (from bottom upwards) as follows: the Basal Series, Ultramafic Series, Lower Banded Series, Middle Banded Series and Upper Banded Series. The Ultramafic Series (UMS) is further subdivided into the Bronzitite Zone and Peridotite Zone. The Lower Banded Series hosts the J-M Reef targeted at Stillwater and East Boulder Mines.
The steep dipping nature of the lithological layers in the exposed part of the Stillwater Complex is due to uplift and tilting associated with the Laramide Orogeny. Faults and dykes are the most common geological structures. Most of the regional faults affecting the Stillwater Complex have been ascribed to the Laramide Orogeny and are grouped according to trends as follows:
- Northwest to southeast striking thrust faults;
- East to west striking south dipping steep reverse faults; and
- East to west trending vertical faults.
These are also the most common fault and dyke trends observed at Stillwater and East Boulder Mines. Numerous diabase and felsic dykes that cut and offset the J-M Reef at the mines are known from surface mapping, underground drilling and mining. These dykes dilate the J-M Reef, but do not destroy the PGM mineralisation and have limited (up to 30ft) contact alteration zones along which poor ground conditions are common. However, these ground conditions do not present significant obstacles to mining and are dealt with using established support procedures.
The local stratigraphy at Stillwater and East Boulder Mines resembles the regional stratigraphic sequence of the Stillwater Complex. Much of the area covered by the Sibanye-Stillwater held or leased Mining Claims is underlain by the Lower Banded Series that hosts the J-M Reef and, to a lesser extent, the Ultramafic and Middle Banded Series.
Mineralisation Style
The J-M Reef exploited at the Stilwafer and East Boulder Mines is a world class primary magmatic stratiform PGM deposit occuring mainly within a troctolife (as well as dunite/harzburgite and anorthosite) in the OB-| zone of the Lower Banded Series. The reef package, which hosts the J-M Reef, is defined by rock fabric as well as lithology while the JM Reef is identified as the high tenor, reef-type subzone comprising disseminated to massive sulphide mineralisation. The high tenor mineralisation may also occur in footwall lenses (footwall mineralisation) in gabbronorite and norite below the OB-I zone while some areas of the reef package are poorly mineralised (Jenkins et al., 2022).
The J-M Reef has retained most of its primary magmatic characteristics, particularly its broad lateral continuity, very coarse textures (e.g. pegmatoidal and poikilitic textures) and consistent ore and silicate mineral abundances. The combination of visible disseminated copper-nickel sulphide minerals (0.25% to 3% modal abundances) within a complex cumulate of silicate minerals, consistent stratigraphic location (OB-I zone) and lithological sequences (footwall, reef and hangingwall) as well as reliable lithological markers facilitate the visual identification and delineation of the J-M Reef for sampling purposes. Sampling and laboratory analysis provide the definitive data required to confirm the presence of the J-M Reef and to determine its PGM tenor. Historically, reef intersections that did not have visible sulphide minerals were not sampled but were assigned a zero grade. However, current protocols require the sampling of all reef intersections irrespective of the sulphide mineral abundance.
Hangingwall lithologies are typically fine-grained to medium-grained, poikilitic anorthosite and leucotroctolite as well as rare occurrences of fine-grained norite or gabbronorite (Jenkins et al., 2022). The contact between the J-M Reef and hangingwall is suo-planar or sharp and is identified based on the textural change from very coarse (pegmatoidal) in the reef to fine-grained to medium-grained in the hangingwall. Progression from the reef package to the hangingwall package does not necessarily coincide with a change in rock type.
The ore mineralogy of the J-M Reef is dominated by disseminated chalcopyrite, pyrrhotite and pentlandite, with minor pyrite, moncheite, cooperite, braggite, kotulskite, Pt-Fe alloy and various arsenides within a complex cumulate of olivine, plagioclase, bronzite and augite. Pd is the dominant PGM in the J-M Reef and occurs primarily (80%) as a solid-solution in pentlandite as well as in sulphides (15%) and moncheite (5%). Pt occurs primarily (67%) in sulphides, as a metal alloy (isoferroplatinum, 25%) and in moncheite (telluride mineral, 8%).
Length and Width
Sibanye-Stillwater holds title over the entire 28-mile strike length of the J-M Reef. For evaluation purposes, the J-M Reef is defined as the Pa-Pt rich stratigraphic interval mainly occurring within the OB-| zone and characterised by disseminated to massive sulphide mineralisation, a variable thickness ranging from 3ft to 9ft (average éft) and average combined Pd and Pt (2E) grades of 0.602 per ton (opt) to 0.80pt. Locally, it forms keel-shaped footwall zones, which transgress the footwall mafic rocks, commonly reaching thicknesses of 18ft and greater. Of the two PGMs, Pd is the most significant resulting in average in sifu Pd:Pt ratio of 3.4:1 and 3.6:1 for Stillwater and East Boulder Mines, respectively. Ongoing metallurgical accounting has determined Pd:Pt ratio of 3.5:1 and 3.6:1 for Stillwater and East Boulder Mines, respectively, which are used for all evaluations. Other associated PGMs (e.g., Rh, Ir, Ru and Os), Au and base metals (Cu and Ni) occur in low abundances and are generally not evaluated.