Summary:
The recognized stratigraphy underlying the Marikana Operations comprises the Main and Critical Zones of the RLS. The Main Zone predominantly comprises gabbro-norite and norite rock types, whereas, in the Upper Critical Zone, pyroxenite, norite, anorthosite, and chromitite lithologies are found.
The Upper Critical Zone stratigraphy of the RLS, which contains the units of economic interest, the Merensky and UG2 Reefs, comprises well-developed cyclic units divided into six sub-units as follows:
- Bastard Pyroxenite;
- Merensky Reef;
- Merensky Footwall;
- UG2 Hangingwall;
- UG2 Chromitite Layer/Reef;
- UGI Chromitite Layer.
Merensky Reef
The Merensky Reef varies in thickness and PGM mineralization down-dip and along strike across the Marikana Operations. The pyroxenite thickens from ±0.3m in the west to greater than 15m in the east of the operations. The bottom contact of the Merensky Pyroxenite is defined by a laterally consistent and well-developed 5-10mm thick chromitite layer (Lower Chromitite), which is almost always underlain by a 1 to 3cm thick anorthosite layer. The lower contact of the Merensky Pyroxenite with the underlying anorthosite is sharp and dimpled (Farquhar, 1981) and cross-cutting the layering in the footwall where present. The top contact of the Merensky Pyroxenite may be sharp but is most often gradational over a 10 to 20cm into the overlying spotted anorthosite.
A 1 to 2mm thick chromitite layer (Upper Chromitite) is often developed 50 to 100cm below the top contact and often has a few centimetres of pegmatoidal development immediately above and below it. In some areas, up to three chromitite layers can be present in this pegmatoidal zone.
A coarse-grained feldspathic pegmatoidal pyroxenite (Merensky Pegmatite) underlies the Merensky Pyroxenite towards the west of the operations. The bottom contact of this unit is also defined by a 1 to 10mm thick chromitite layer (Basal Chromitite).
Different facies of the Merensky Reef at the Marikana Operations are locally distinguished, based on the lithology and morphology of the reef as well as the number and position of the chromitite layers associated with the pyroxenite.
UG2 Reef
The UG2 Reef includes a main chromitite seam, which varies in thickness from 0.7m to 1.3m across the Marikana Operations. The top contact is sharp, planar and laterally very consistent, while the bottom contact is cuspating. The basic mining parameter of the UG2 chromitite seam is to select the composite between the top and bottom contact of the main chromitite seam, with 10cm of footwall material.
At the Marikana eastern shafts, Saffy and E3, thin pyroxenite lenses are often present in the upper part of the UG2 chromitite seam. The lenses can be laterally consistent for tens of metres. Occasional anorthosite or mottled anorthosite partings are less common, are normally thicker than the pyroxenite lenses and often associated with potholes.
On the western shafts at Marikana, a continuous layer of pyroxenite separates the UG2 into two layers. This is referred to as "Split Reef". The internal pyroxenite is 30cm to 70cm thick on the Western side of K3 Shaft, but thickens to the west and north and will have a significant influence on mining in the K4 Shaft area. The grade of the UG2 Split Reef is negatively affected due to dilution caused by the internal pyroxenite.
The immediate hanging wall to the UG2, hangingwall 1B(HW1B), is a pyroxenite package varying in thickness from Om in the west to 18m at EPPL. The grain size of the HW1B pyroxenite is generally finer than that of the overlying HWIA pyroxenite. Large oikocrysts of pyroxene are typical and characteristic of the HW1B unit. The pyroxenite unit contains several chromitite layers locally known as the UG2A Chromitite Markers. Geologically these chromitite layers are considered to be analogous to the "Triplets" described in other areas. The UG2A unit consists mostly of two prominent chromitite layers (a few centimetres thick) which, together with the pyroxenite in-between have a thickness ranging from 10cm to 30cm. HWTB may also contain several thin chromite layers or disseminated chromite.
The stratigraphic sequence of the Upper Critical Zone at Marikana Operations consists of alternating pyroxenite, norite, anorthosite and chromitite layers. The UG2 Reef underlies the Merensky Reef by 130m to 230m with the middling increasing from west to east. Both Reefs outcrop for a distance of 27km along strike within the Marikana Operations area. The regional dip varies between 10 and 13 degrees with a general dip direction of north-northeast.
Localised geological discontinuities associated with the Merensky and UG2 Reefs include potholes, faults, joints, shears zones, dykes and IRUP. The Merensky Reef is also disrupted by the occurrence of a very fine-grained pyroxenite, locally referred to as "Brown Sugar Norite".
Mineralogy
Merensky Reef
The Merensky Reef mineralogy comprises major silicate minerals: pyroxene, plagioclase, and biotite.
These minerals form secondary minerals such as talc and chlorite in structurally disturbed and weathered areas. PGM mineralization is closely related to thin chromite layers (1mm to 5cm thick). PGM and sulphide mineralization can also occur in the immediate footwall rocks.
The dominant platinum group minerals are -30% Pt-Pd sulphides (braggite-cooperite), -11% PGM tellurides and arsenides, ~6% sperrylite and minor PGM alloys. Platinum group mineral grain sizes have two size ranges in the Merensky Reef: 10 to 30µm and 50 to 350µm.
The platinum-group minerals of the Merensky Reef occur in three textural associations:
- Enclosed in or attached to base metal sulphides (38 97 %). This is a common Occurrence on the western limb.
- Enclosed in silicate (3 - 62%) and further north along the western limb past the regional Swartklip facies (62%).
- Enclosed in/or attached to chromite or Fe-oxide.
UG2 Reef
The UG2 Reef is composed of 60 to 90% (by volume) chromite, 5 to 25% orthopyroxene, 5 to 15% plagioclase and accessory amounts of other minerals, including clinopyroxene, base metal and other sulphides, platinum-group minerals, ilmenite and magnetite. The UG2 Reef often has a mottled appearance due to the presence of large poikilitic bronzite crystals. The UG2 Reef contains much less sulphide minerals compared to the Merensky Reef.
The base metal sulphides are predominantly pentlandite, pyrrhotite, pyrite and chalcopyrite. PGM minerals identified in the UG2 are Cooperite, Laurite, Braggite, Sperrylite and Pt alloys (Pt-Fe & PT-As).
Platinum group mineral grains in UG2 Reef can be classified into one of the following categories according to their textural setting:
- Locked in base-metal sulphide;
- Locked in chromite;
- Locked in silicate;
- At grain boundaries of base metal-sulphides, silicates and chromite.