Abstract

An area consisting of UG2 Spit Reef provided a unique challenge to extract the reserves by means of a safe and feasible mining strategy. The proposed strategy was required to ameliorate the exposure of personnel to geotechnical risk whilst providing a feasible scenario from a cost perspective.

The initial full channel mining strategy would extract the reserves at stoping widths in excess of 2.0 m at perceived low levels of risk and low grade. A paradigm shift was required to deviate from this historic method.

The proposed mining strategy entailed undercutting the internal waste layer resulting in the extraction of the UG2 reserves at stoping widths of less than 1.2 m at quantified low risk levels and high grade. No successful attempts had previously been recorded where the UG2 Split Reef had been undercut within a highly discontinuous rock mass area such as the Spruitfontein Fault zone. This resistance to change was devolatilized by the introduction of a rigid areal support medium concomitant with several other support unit alterations. This was nevertheless received with negative criticism by a mining team intent on resisting change. The benefit though of extracting the bottom reef portion exclusively, even with inflated support costs resulted in a beneficiated grade (channel grade) to in excess of 6.0 grams per tonne processed versus the 2.6 grams per tonne from the full channel mining extraction strategy.

The proposed mining method and support strategy was presented in March of 2015 and implemented in March of 2017. This paper intends to share the findings pertaining to geotechnical design, support performance and design back-analysis from monitoring results.

1.
Introduction

Lonmin Platinum Marikana operations are situated along the Western limb of the Bushveld Igneous Complex in the North West Province of South Africa. Karee 3 Shaft (K3) is the largest of the 12 shafts on the Lonmin Marikana lease area. The shaft is able to hoist on average 12 400 t of UG2 and Merensky ore per day (285 200 t per month). The shallower reserves of both Merensky and UG2 Reefs have nearly been depleted and what remains (Fig. 1) are the reserves along both the eastern and western boundaries of the shaft as well as the deeper reserves (825 m below surface) along the sub-decline. Both reef bodies dip from South to North on average 11° with the Merensky Reef overlaying the UG2 Reef (145 m separation). The Spruitfontein Fault can be seen along the western boundary of the shaft (Fig. 1). This fault consists of 2 major faults in conjunction with several sympathetic faults and associated jointing concentrated in an area colloquially referred to as the Spruitfontein Fault zone. The western UG2 Reef horizon is also affected by the presence of Iron Rich Ultramafic Pegmatiod (Fig. 1 - IRUP indicated in maroon which is mostly unmined) and Split Reef (indicated in blue). Pothole features (indicated in yellow) that displace the reefs are also prevalent over the entire K3 shaft property along both UG2 and Merensky horizons.

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