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 Stillw ........

The two principal mining methods are:
• mechanised ramp and fill (both overhand and underhand) (80% – 90%)
• sub-level extraction by long hole, open stoping with hydraulic backfilling (10% – 20%).

Both Stillwater and East Boulder Mines employ the following underground mining methods which are suited for the variable steep dips and narrow widths of the J-M Reef:
- Captive Cut and Fill stoping, utilising either conventional, Alimak TM or raise boring to create accesses, and the resulting stopes are also known as captive stopes - this method is being phased out;
- Ramp and Fill using overhand or underhand approaches; and
- Sub-level extraction by long hole open stoping with subsequent backfill. Longitudinal and transverse stoping methods are variations of the long hole stoping method in use at the mines.

The mining method mix is adjustable and largely driven by mineralisation grade, ground conditions encountered and the requirement to minimise dilution. Ramp and Fill stoping (which includes on-reef sub-level sill development) is the predominant mining method at both mine-sites. The Ramp and Fill method allows for maximum selectivity for separating ore and waste. Sub-level extraction long hole stoping is utilised typically in narrow continuous ore zones. Captive Cut and Fill stoping is only used on rare occasions when the other two methods are not practical ceased in 2021. Except for open stoping, the mining methods employ high-qual ........

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