The Southern African Institute of Mining and Metallurgy 2014 SOMP Annual Meeting F. Rashidi-Nejad, F. T. Suorineni, and B. Asi Open pit or block caving? A numerical ranking method for selection F. Rashidi-Nejad*, F. T. Suorineni*, and B. Asi† *School of Mining Engineering, UNSW Australia †BBA Consulting Engineers, Canada Mining in the 21st century is facing many challenges, among which are low-grade and high- tonnage orebodies. One great (r)evolution in mining occurred in the early 20th century when Daniel Jackling proposed the concept of economies of scale for large-scale (bulk) mining at Bingham Canyon. At that time, there were many shallow deposits around the world suitable for open pit mining. Hence, this method grew rapidly and the majority of iron, copper, and gold production came from open pit mines. Open pit mining method has many advantages such as increased safety, higher production rates, more flexibility, lower cost, less operational risk, etc., so normally it considered as preferred option; but break-even depth, high stripping ratios, and environmental issues have become significant challenges facing this method in 21st century. In underground mining, block caving is the only method with the costs comparable to surface mining methods, especially open pit mining. A switch from open pit mining to block caving mining could be another great development in this century. However, even though an increasing number of mines are being developed using block caving, this method involves many technological and environmental challenges. The depth of the orebody is one of the most important factors governing selection of the mining method. This paper offers a numerical (quantitative) ranking system for mining method selection when both open pitting and block caving are feasible, which is true for many low-grade and super- large hard rock mining operations. Introduction Both surface and underground mass mining methods have generated increasing interest in recent years, as global demand for raw materials continues to grow. Mining companies are looking for ways to exploit large orebodies faster and more economically. Underground mining of massive, low-grade orebodies has been carried out for decades, but in total there have been fewer than 20 block caving mines (Oancea, 2013). As shown by greenfield projects around the world, production rates of block caving operations can be up to 160 kt/day (Steinberg et al., 2012). According to Chitombo (2010), the caving industry is rapidly moving from weak rock application into strong rock, from shallow operations to depths up to one to two kilometres, and from small block heights to 500 m to 1000 m. As shallow ore deposits accessible through open pit mining become exhausted, block caving mining methods to extract massive low-grade orebodies are gaining increasing attention owing to their merits in terms of safety, tonnages produced, and mining costs that can match those of open pit operations. Underground caving mining methods such as block, panel, and sub-level caving continue to be the premier choice for deeply situated massive orebodies, thanks to the high potential production rates and low operating costs involved. Recent technological developments and improved solutions for designing, planning, and modelling caving operations mean that these techniques can now be applied at greater depths, in more competent rock masses with greater geotechnical challenges than ever before. This does not imply that developments for open pit mining are declining. Bulk mining methods are necessary for the economic exploitation of massive, low-grade porphyry copper, gold, molybdenum, and diamond deposits, and block caving is the method of choice. The process is suitable for the mining of massive deposits, poor cap rock, compact, and highly fissured orebodies (Adler and Thompson, 2011). The rock mass characteristics of the orebody itself and the waste overburden are the governing factors for operating costs of these mining methods. By analysing the advantages and disadvantages of block caving, the risk profile of this type of operation will be understood and the proper approach to succeed in mining these huge deposits will be established. This investigation addresses the question of how a mineral resource is assessed for its suitability for open pit or block cave mining methods. A broad literature review was conducted to gather background information on open pit and block 183 2014 SOMP Annual Meeting cave mining methods, their operations statistics, and challenges. Generic areas of mine evaluation technical risks such as geological resource model risks, geotechnical/hydrogeological model risks, mining method selection, and revenue assumptions, as well as environmental and geopolitical risks, are examined. The two mining methods are compared and evaluated, and finally, a numerical ranking method is proposed. Overview of open pit mining Deposits that are suitable for open pit mining are generally shallow and have a relatively uniform geology. Some deposits can be mined economically by only underground methods. Other deposits are best mined initially as open pits, with production switching to underground as deeper portions of the orebody are extracted. Kable (2013) reported that open pit mines are distributed geographically across the world. Table I lists the ten deepest open pit mines that are currently active and indicates those with underground potential. Table I. Deepest open pit mines Mine Location Product Depth Width Length Underground Production (m)** (km) (km) potential (t/d) Bingham US copper, gold, silver 1200 4.0 - - 150 000 Canyon and molybdenum Chuquicamata Chile copper 850 3.0 4.3 Yes 375 000 (end of 2018) Escondida Chile copper 645 2.7 3.9 - 240 000 Udachny Russia diamond 630 - - Yes Muruntau Uzbekistan gold 600 (final: 650) 3.0 3.5 Yes Fimiston Australia gold 600 1.5 3.8 - 240 000 Grasberg Indonesia gold, copper and 550 - - Yes 240 000 silver Betze-post US gold 500 1.5 2.2 - Nanfen China iron 500 - - Yes Aitik Sweden copper, silver and 430 - - - gold (final:600m) In terms of production, surface mines are almost always larger than underground mines producing the same commodity. This is partly because open pit mines must mine much more waste, whereas many underground methods can mine the same mineral much more selectively (Table I). However, block caving is categorized as a non-selective underground mining method and dilution associated with this method cannot be controlled easily. Dilution is a deficiency of both methods. On the other hand, cut-off grades of open pit and block cave mining methods are not dramatically different, because the mining costs associated with them are not dramatically different. Bingham Canyon produces about 300 000 t of copper, 12.5 t of gold, 125 t of silver, and almost 11 000 t of molybdenum annually (Rio Tinto, 2014). Chuquicamata will be switched to underground production in 2018. The ore reserve under the existing pit is estimated to be 1.7 billion tons grading at 0.7% copper. Escondida produced 1.1 Mt of copper in the financial year 2013, which accounts for about five per cent of global copper production. Escondida's recoverable copper reserve was estimated to be more than 32.6 Mt as of December 2012. At Udachny, the probable contained diamond reserve at the open pit was estimated to be 0.88 t (4.4 million carats) as of July 2013. The Udachny open pit operation is scheduled to close in 2014, and will be replaced by the Udachny underground mine, which is under construction, with more than 108 million carats contained diamond reserves (Kable, 2013). Surface mining has advantages over underground mining regarding recovery, production capacity, mechanization ability, grade control, operational flexibility, low operational and economic risks with possibility of earlier cash flow, as well as safety (Chen et al., 2003). Less additional development is required if resources are increased. Surface mining methods are less complicated and can be assessed and scheduled in simpler manner using off-the-shelf optimization software with easier planning and supervision. Therefore, in suitable deposits, surface mining is more productive, more economical, and safer for workers. However, changes in environmental regulations and societal expectations may lead to fewer large open pit mines, particularly if operators are required to backfill open pits and re-contour waste dumps (Nelson, 2011). Unfavourable weather conditions and depth considerations may result in the development of small, high-grade deposits by very shallow open pits or in the development of high-grade underground mines in place of large open pit mines. Where applicable, large low-grade deposits may be mined by in-situ methods (Hitzman, 2005). As shown in Table I, the size of pits, and consequently the amount of waste removal resulting in environmental concerns in open pit mining may make it less attractive in spite of its advantages of lower development costs, quicker start-up times, and lower accident rates. In open pit mining, the impact on the environment must be considered more 184 Open pit or block caving? A numerical ranking method for selection seriously compared to underground mining. Environmental impact of surface mining includes aesthetics, noise, air quality (dust and pollutants), vibration, water discharge and runoff, subsidence, and process wastes. The sources of these environmental concerns include the mine infrastructure, mineral processing plant, access or haul roads, and remote facilities. If mining will cause a deterioration in the quality of either surface water or groundwater, remedial and treatment measures must be developed to meet discharge standards. These issues are the primary challenges facing surface mining in the 21st century. Overview of block caving mining Block caving is a high-tonnage underground bulk mining method generally applied to large homogeneous ore deposits. Ideally, the ore to be caved should be structurally weak, and the waste overburden should be weak enough to collapse over the ore as the ore is extracted (Figure 1).