HOFFMANN, D. Statistical size analysis of potholes: an attempt to estimate geological losses ahead of mining at Lonmin’s Marikana mining district. The 4th International Platinum Conference, Platinum in transition ‘Boom or Bust’, The Southern African Institute of Mining and Metallurgy, 2010. Statistical size analysis of potholes: an attempt to estimate geological losses ahead of mining at Lonmin’s Marikana mining district D. HOFFMANN Mineral Resource Department, Lonmin Mapped pothole data from the UG2 and Merensky reefs in Lonmin’s Marikana mining operations were analysed for size variability and abundance. The UG2 Reef potholes have a greater proportion of larger potholes relative to that in the Merensky Reef. However, the proportions of potholes with diameters greater than 70 metres are comparable. Another characteristic identified, is that the maximum pothole diameters were generally found to decrease from west to east for both reef types. A visual inspection of the pothole occurrences revealed that clustering and abundance characteristics can be used to delineate broad domains, which have useful application in quantifying unknown pothole loss in unmined areas. By analysing mapped pothole information and subsequently integrating these features into the grade models, the technical mining systems are now able to benefit from the prediction of unknown geological losses more reliably and serve as a single source of information for mineral resource reporting and mine planning. Introduction which was used to gain some insight into the pothole Potholes within the Merensky and UG2 reefs of the distribution. Thereafter, known potholes were delineated Bushveld Complex are well known geological features that into domains and then the average abundance was applied are problematic in mining. They occur as depressions or to the adjacent unmined areas. The pothole outlines and slumps on the reef horizon normally existing as severe domains were then incorporated into a grade block model to disruptions which prevent economic extraction and are thus obtain a final mineral resource model. The benefit of such characterized as geological losses. Given the adverse an integrated resource model allows the mine design to impact of potholes on ore extraction, considerable effort has exclude areas within known potholes and to quantify the been made by mine geologists to understand the pothole parameters required to mitigate the likely effect of unknown distributions such that their occurrences or abundances may potholes for unmined shaft blocks. be predicted. In mineral resource modelling, potholes may be Geological setting characterized either as known losses, where the data Potholes have generally been described by many geologists acquisition activity has allowed reasonable definition, or as as synmagmatic circular depressions or slumps below the unknown losses, where there is an expectation that potholes PGE-and/or Cr-rich mineralized horizons of the Merensky will be present with some statistical proportion. Typically, and UG2 reefs. Carr et al. (1994) described potholes to be the global average abundance of a shaft block is applied to one of several types of discontinuities that affect the the unmined areas. In the case of known losses, the pothole Bushveld Complex, which range in extreme sizes from dimension and shape may be constrained through partial or several kilometre scale regional gaps to minor ten complete exposure in mine excavations. In contrast, the centimetre discordant structures along the reef. Typically, unknown pothole losses were estimated by applying potholes on a scale that affect mining operations can vary average abundances from areas where potholes were well from five metres to several hundred metres in diameter. In constrained (i.e. from the known pothole loss domains) or some areas, such as the northwest part of the western through the interpretation of high resolution seismic Bushveld Complex, conformable Merensky Reef occurs surveys. Refined mine planning and production forecasting within ‘regional potholes’ despite large-scale elimination of arises from the ability to delineate the potholes into suitable parts of the stratigraphy over areas of several square domains and to estimate their likely abundance within kilometres (Viring, 1998). Within these regional potholes, acceptable error limits. second order potholes occur as exhibited by the local At Lonmin’s Marikana mining operations, the sizes and development of the Merensky chromitite layer below the percentage losses attributable to potholes have been Upper-pseudo Reef chromitite layer (Smith and Basson, analysed along the strike and dip directions of the reefs, 2005). which revealed trends that were useful for geological loss Generally, the reef development within the potholes is prediction. This study examines the pothole size variability irregular in both thickness and dip, and the reef horizon is STATISTICAL SIZE ANALYSIS OF POTHOLES 97 usually unconformable with the immediate footwall Data acquisition and validation stratigraphic units. The Merensky and UG2 reef PGE The information in this study was based on the routine mineralization within potholes is variable and disrupted, underground mapping data collected by mine geological and the high dilution rates associated with the hangingwall personnel over a period of over 20 years. The raw data layers, as well as complex mining configurations to follow consisted of mapped contacts of the potholes both fully and the irregular reef shape results in little or no incentive to partially exposed during the mining process. The pothole extract. The overlying stratigraphic layers that fill the edge or contact is broadly constrained by the interpretation pothole dip towards the centre and are commonly of information from the mine plans, which results in the discordant with the reef layering. The depth and extent of final pothole shape and dimensions to be an approximation the pothole base into the footwall layers, as well as the of its true contact. As a result of this, the true pothole hangingwall unit contact angle to the reef and attenuation of dimensions can be expected to be fractionally smaller or the reef, has been used by geologists at the Impala Mines to larger than the outline shown on the plans. classify UG2 pothole types (Hahn and Ovendale, 1994). The data used for the study consisted of perimeters which Potholes have been exposed throughout the mining defined the mapped pothole outline. The data clean up operations in the Bushveld Complex and can vary proceeded with the removal of incomplete mapped potholes dramatically in their abundance. Geological losses associated usually of small crescent shapes or refining the with the potholes in the western Bushveld Complex range interpretation of potholes where it was thought that the from about 4% to 20% (Figure 1) but may reach 30% in potholes have coalesced or where duplicate data existed. certain areas. The controls on the pothole abundance may be Furthermore, potholes with areas less than 20 m2 (or 5 m examined in terms of distance from the magma’s feeder or diameter) were excluded. The integrity of the final data-set point of entry, or the affect of palaeo-topography, where the relied largely on the diligence applied to the original degree of turbulence within the magma would be greater. In mapping. In total 1 043 Merensky Reef potholes and 3 585 the case of the palaeo-topography effect, several studies have UG2 Reef potholes were defined. correlated the increase in pothole abundance with reef characteristics such as steeper dip, as found at Lonmin’s Western Platinum Mine (Carr, 1994) and Anglo Platinum’s Pothole size and frequency variability Rustenburg Mines (Chitiyo et al., 2008). Mapped potholes are generally near circular features as Along the 26 km strike length of Lonmin’s Marikana shown by the good correlation found between the maximum mining operations, potholes are pervasive on the reef north-south (YINC) dimension and the maximum east-west horizons, but occur with varying abundances (Figure 2). (XINC) dimension (Figure 3). This relationship gradually Average mapped pothole abundances for the Merensky and changes for potholes with diameters greater than 100 metres UG2 reefs were estimated at 9.2% and 7.8% respectively. in both reef types where the larger potholes have a greater However, these may range from between 3% and 12% for XINC length and thus the pothole shape tends towards an individual shaft blocks. The general distribution of these oval elongated in an approximate east-west direction, which potholes appears random; however, local clustering of the is sub-parallel to the reef strike. potholes was evident. These clusters have a northwest- In mining operations, potholes may be characterized into southeast orientation on both reef horizons, which suggests broad categories based on their size. This approach has that the processes responsible for forming these been documented at several mining operations, e.g. the concentrations were active during the formation of both Union Section Mine (Lomberg, 1998) and Rustenburg Merensky and UG2 reefs. Section Mines (Chitiyo et al., 2008) where the size of the Figure 1. Location of the Lonmin mining operations (Western and Eastern Platinum Limited stipple shading) in relation to the surrounding mining district for the south western part of the Bushveld Complex. Also shown are the percentage potholes for the UG2 Reef by mining district. The data from the Anglo Platinum operations were extracted from