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The Southern African Institute of and 2014 SOMP Annual Meeting F. Rashidi-Nejad, F. T. Suorineni, and B. Asi

Open pit or block ? A numerical ranking method for selection

F. Rashidi-Nejad*, F. T. Suorineni*, and B. Asi† *School of Mining , UNSW Australia †BBA Consulting ,

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 , , 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 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 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 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 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 resource is assessed for its suitability for open pit or block mining methods. A broad literature review was conducted to gather background information on open pit and block

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cave mining methods, their operations , 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 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 . 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 (, 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, 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).

Figure 1. Block cave mining (Hamrin, 1998)

International surveys have shown that sub-level caving and block caving are the preferred methods in underground hard rock mining, often supported by preconditioning of the virgin rock mass with hydrofracturing and dynamic weakening using explosives (Ghose, 2009). Block caving involves excavation of natural support from beneath the ore, causing the structure of the orebody to fail and collapse into the excavated void under the force of gravity and local geomechanical stresses. The broken ore is then removed from under the caved section through a drawpoint arrangement, subsequently removing support from ore and overburden at increasing height above the initial excavation, and eventually extending the cave upward to the surface. The attractive aspect of block caving is that only a relatively small portion of the ore must be drilled and blasted prior to extraction. Once the cave initiates, production continues without further primary until the ore column above is exhausted.

Cave mining statistics Woo et al. (2009) compiled a database of more than 90 cave mining operations throughout the world, including both historical and producing mines. Table II presents active block cave mining operations as well as potential block cave mining operations. Figure 2 shows a breakdown of the database by continent, mining method, and commodity. The biggest mining companies are transitioning from open pit to underground mining, and Woo et al. estimate that within 10 years, about 40 per cent of copper production will come from underground mining operations. By 2018, about 50 per cent of the world’s ore production will be from underground mines (Oancea, 2013).

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Table II. Block cave mining operations

Mine Owner Product Country Production rate (t/d) Active block cave mining operations New Afton New Gold Copper-silver-gold BC, Canada <11 900 Henderson M Colorado, 32 000 US Deep Ore Zone Copper Indonesia El Teniente Codelco Copper-molybdenum Chile 95 000 Northparkes Rio Tinto Copper-gold Australia 28 000 Palabora Rio Tinto Copper 30 000 Philex Mining Padcal Copper-gold Philippines Tongkuangyu Copper China Kimberley and Petra South Africa 17 400 Cullinan Diamonds Finsch Petra Diamonds South Africa 17 000 Diamonds Potential block cave mining operations Bingham Canyon Rio Tinto Copper Utah Oyu Tolgoi Rio Tinto Copper-gold Mongolia 90 000 Argyle Rio Tinto Diamond Australia Resolution Rio Tinto and Copper Arizona BHP Chuquicamata Codelco Copper Chile 120 000 Grasberg Freeport Copper Indonesia 160 000-240 000 Olympic Dam BHP Copper-gold Australia 25 000 Mount Keith BHP Nickel Australia Telfer Newcrest Copper-gold Australia Mining Mount Lyell Vedanta Copper Tasmania Resources Debswana Debswana Diamond Botswana

Figure 2. Cave mining by (a) continent, (b) mining method (BC = block cave, PC = panel cave, SC = sublevel cave), (c) resource mined, and (d) activity (Woo et al., 2009)

186 Open pit or block caving? A numerical ranking method for selection

Two of the world’s largest copper mines will soon move underground. The Grasberg mine in Indonesia (Figure 3), operated by Freeport-McMoRan Copper and Gold, will reach its open pit limit by 2016 (Chadwick, 2010), while Codelco’s Chuquicamata mine in Chile is expected to do the same in 2018 (Kable, 2013).

Figure 3. Perspective view of the Grasberg District orebodies (Brannon et al., 2012)

As indicated by Brannon et al. (2012), the three currently active mining operations in the Grasberg Mining District are the Grasberg open pit (160 kt/d), the Deep Ore Zone (DOZ) block cave mine (80 kt/d), and the newly commissioned Big Gossan open stoping operation (target 7 kt/d). The Grasberg concentrator has a capacity of about 240 kt/d. After open pit mining is concluded in 2016, production from the district will remain at 240 kt/d but will be entirely from underground. The Grasberg block cave mine will be the primary source of mill feed at that time, with a target 1 of 60 kt/d (Figure 4)

Figure 4.Grasberg development production plan (Casten and Johnson, 2013)

Challenges in open pit mining and block caving Some of the biggest open pit mines worldwide will reach their final pit limits in the next 10 to 15 years (Fuentes, 2004). Furthermore, there are many other mines which are planning to change from open pit to underground operation due to increasing environmental requirements (Chen et al., 2003). As an example, the predicted copper output from Rio Tinto’s open pits and underground mines can be seen in Figure 5. For low-grade mines, block or panel caving and stoping methods are usually used after the transition to achieve a high production rate at low cost (Arancibia and Flores, 2004).

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Figure 5. Predicted copper output from open pits and underground – Rio Tinto production share (Cross, 2005)

Block caving is a capital-intensive mining method, requiring significant initial investment early in the mine life for infrastructure and primary development. Once in place, the method’s high upfront costs are offset by high production rates and low operating costs (relative to other underground methods) over a considerable length of time, resulting in a low overall cost per ton. Block cave mining is among the least costly of all underground mining methods per ton of ore extracted. While the startup costs are comparable to those of an open pit mining operation, block caving operations have a significantly longer lifespan, usually in the 15 to 20 year range. For example, Freeport’s Deep Mill Level Zone (DMLZ) underground mine is expected to be fully developed over 12 years (Casten and Johnson, 2013). Also, four subterranean extending 1 500 km will be constructed under the surface of the new Chuquicamata underground mine that will begin operations in 2018. The underground mine will be developed at an estimated cost of $2 billion and will produce an estimated 120 000 t/d (Hatch, 2012). Another factor that differentiates block caving from open pit mining operations is that for a block caving operation, on average, 70 per cent of capital expenditure is incurred before any revenue is generated (Oancea, 2013). Long lead times and high capital expenditures mean that block caving carries certain risks for investors, including geopolitical risk, long-term commodity prices, and cost inflation. Block caving has several advantages. It has high productivity, which is comparable to open pit methods. Unit costs are low and production rate is high. It is safer in comparison with other underground mining methods, and is without the waste dump requirements of a large open pit. The production cost of block caving method per unit ore is comparable to that of open pit mining. However, block caving entails certain disadvantages. It is more capital intensive and a long lead time is required for development, construction, and decommissioning of the mine. Intermittent secondary blasting is necessary which results in more personnel exposure to open drawpoints, high risk of production interruption, more repair required due to blast damage, and negative impact on draw control. If overburden fragmentation is greater than expected it leads to ore loss and increased dilution. The subsidence resulting from block caving has a negative impact on surface facilities. There are issues with cave management and control with numerous steep structures that transect the deposit. Cave management and problems with geological structures can lead to the loss of developed production areas. As mentioned previously, one of the most important consequences of block cave mining is the potential for surface subsidence or settling. Surface subsidence is caused by the material above the orebody gradually moving downward to replace the ore that has been mined. Caving-induced subsidence may endanger mine infrastructure and is a major concern for operational safety. Using industry standard engineering practices, it is possible to predict both the cave and subsidence zones based on orebody knowledge gained during exploratory geological investigations. However, the best understanding of caving and subsidence will come only once mining begins.

The initial capital costs for a block or panel cave is always higher than for other underground mining methods, which generally phase in capital costs over time. Large, modern-day caving projects typically report total capital costs of anywhere from US$500 million to in excess of US$10 billion for ‘mega-’. However, operating costs for block or panel caves are much lower than for other underground methods, and are comparable to open pit mining costs. Typical operating costs are in the order of US$10-20 per ton (Lovejoy, 2012). Figure 6 illustrates estimated capital costs as well as operating costs for both caving and open pit mines with different stripping ratios.

188 Open pit or block caving? A numerical ranking method for selection

Figure 6. Capital (a) and operating (b) costs for open pit and caving mines

Underground mining is more acceptable from an environmental and social point of view (for instance, an underground operation will often have a smaller footprint than an open pit of comparable capacity) (Chadwick, 2008). For near-surface deposits with a vertical extent, open pit mining is usually used first, but with increasing mining depth there is a point where a decision has to be taken whether to continue deepening the surface operation or change to underground mining (Flores, 2004). This transition point from open pit to underground operation (break-even depth) is currently the focus of much research.

Technical risks in project evaluation There are two categories of risk in project evaluation – risks of completing the evaluation process on time and within budget and risks of evaluation achieving the correct outcomes. The major generic areas of technical risks in mining evaluation are the geological resource model, the geotechnical/hydrogeological model, mining method selection, and environmental and geopolitical risks. These factors are discussed in the following sections.

Geological resource models Failure of resource modelling to predict the correct ore tonnage can result in financial impact up to mine closure. Incorrect structural interpretation can lead to incorrect mining methods or development location. Berry (2009) indicated that geological inputs are interpretations, not facts. The fact that that resources are a non-homogeneous rock mass (structurally and mineralogically) causes another technical risk in mining evaluation. Furthermore, in providing geological resource models, the ‘unknown unknowns’ (logging consistency) as well as the ‘known unknowns’ (drilling density) need to be considered. Further risks recognized by industry include: • Standard categories of resources to identify levels of confidence • Development of sophisticated geostatistical models to improve estimation confidence • Evolving range of quality assurance measures for and resource preparation activities. Resource evaluation usually continues well into the life of an operation. It is unlikely that a resource will ever be completely understood before mining commences.

Geotechnical and hydrogeological models Geotechnical and hydrogeological studies are among the most important components of mining evaluation. Failure of models can result in anything from pillar failure to mine flooding that may lead to multiple fatalities. Structural and hydrogeological models that are based on drilling, interpretation, and data-sets are also limited. The economic consequences of inadequate models can range from financial loss to mine closure. Developed models need to be adequate for global decision-making during evaluation of underground block caving projects as well as open pit mining with final pit wall development from the start. It is clear that a lower confidence level during evaluation results in higher probability of changes being required during operation. There is another risk in terms of geotechnical and hydrogeological models, which is called collapse risk. For instance, if large blocks collapse from the roof of a cave, production drifts/galleries can also collapse and mining activity can be halted. Such a massive collapse happened in 1999 at Northparkes in Australia, causing an airblast that shot through the mine killing four workers (Hebblewhite, 2009). In early 2010, Henderson Mine experienced a significant convergence

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event in a production drift that eventually resulted in the total collapse of 36 m of drift and another 52 m of severely squeezed that completely halted production in the middle of the cave panel (Carlson et al., 2012). Deep underground mines are also plagued by other negative events, including rockbursts. These are caused by the sudden release of the high energy stored in compressed rocks as well as fault-plane movements caused by mining operations. Examples of slope failure include the Cadia Hill open pit south wall bench failure that occurred in 2006 (Sainsbury et. al., 2007), Chuquicamata open pit west wall bench failure in 1967–1969 (Lorig et. al., 2009), and one of the world’s largest landslides, which tumbled 150 Mt of rock and dirt down the northeastern pit wall of Kennecott Utah’s Bingham Canyon (Kosich, 2013).

Mining method selection Incorrect mining methods can result in significant disruption due to method changes being required, as well as loss of reserves. This is more serious if an inflexible mining method was initially selected and development started. The economic consequences of incorrect mining method (open pit instead of underground or vice versa) range from financial impost to mine closure. Furthermore, determination of the transition zone, calculation of stability charts, as well as designing the conversion from one method to another are categorized as the important risks in mining method selection. In some cases at operational level, the method might be correct but parameters incorrect, such as equipment size, bench height, and lift/panel/level intervals. Even highly conservative design parameters entail the risk of proper mining method selection. Eventually, inflexible design and labour shortages pose additional risks. One of the most important issues is that, unlike other mining methods, block caving is inflexible; once started, it cannot be changed. In other words, if the initial design of a block caving operation is faulty, at least some of the capital invested in the mine will likely be at risk. In addition, a block caving operation cannot be put on care and maintenance as the company is then at risk of losing all of its capital investment. Another issue is that there are not many qualified block caving experts and miners, so personnel must be trained from scratch, sometimes over a short period of time. For example, as the open pit mining operation at Chuquicamata in Chile reaches its end, some 3000 to 3500 miners will be retrained over three years for the upcoming underground block caving operation (Oancea, 2013). In the pre-production stage at the New Afton block caving operation in Canada, 80 per cent of the workforce was inexperienced and had to be trained on the job (Bergen et al., 2009). Therefore, it is important to consider potential delays and additional labour costs due to training in evaluating the transition from open pit to underground mining. There is another risk in terms of influence of stress path during the transition from open pit to block cave mine. At the Palabora copper mine in South Africa (Severin and Ebrehardt, 2012), three years after the transition to block caving in 2004, a 100 Mt pit slope failure occurred, encompassing the full 800 m height of the northwest wall. This serves as an important lesson in the complex interactions that may develop between underground mass mining operations and overlying steep rock slopes (Figure 7).

Figure 7. Palabora northwest wall failure (left), and 3-D spatial relationship between the underground operations, open pit, and fault (right) (Severin and Ebrehardt, 2012)

Environmental risks Geography and play important roles in establishing the risk profile of a block caving operation. Block caving operations located in the vicinity of inhabited areas are not desirable as they create significant risks for water resources, infrastructure, buildings, and lives due to ground subsidence. Block caving mines located in wet climates have a higher risk of flooding than those located in temperate or desert climates. Managing water inflows and dewatering the mine and surrounding area will definitely have adverse impacts

190 Open pit or block caving? A numerical ranking method for selection

on groundwater in the long term, and possibly on the local flora and fauna. These types of environmental impacts are not overlooked by regulators, NGOs, and local communities and can result in longer approval times. On a related note, an often-overlooked aspect of deciding whether block caving is a suitable mining method is the region’s seismicity. Some regions, like Bingham Canyon, are quite stable and present little seismic risk, while other sites are characterized by high seismicity – for example, El Teniente in Chile (Potvin et al. 2010) or Olympic Dam (BHP Billiton, 2011). An earthquake that might not adversely affect an open pit mining operation is capable of substantially damaging a block caving operation through a massive cave-roof collapse. One of the most common locations for a block caving operation is below an existing open pit. In this case, the block caving operation could pose serious danger to the open pit mine, especially if the underground operation starts before open pit mining is completed.

Geopolitical risk The large capital cost and long lead times associated with block cave mines highlight the importance of a correct geopolitical risk assessment of a prospective block cave project. Friendly mining jurisdictions where open pit operations have gone undisturbed for decades and where mines are in transition to underground bulk mining are good places to invest.

Mining method selection process Boshkov and Wright (1973) developed a classification system for underground mining method selection. Their system assumes that the possibility of surface mining has already been eliminated. The classification provides up to four methods that may be applicable. Hartman (1987) developed a flow chart selection process for defining the mining method, based on the geometry of the deposit and the ground conditions of the ore zone. This system is similar to that proposed by Boshkov and Wright, but is aimed at more specific mining methods. The classification includes surface and underground methods, coal, and hard rock. Another classification system proposed by Morrison (1976) divides underground mining into three basic groups of rigid pillar support, controlled subsidence, and caving. In this system, the ground conditions have already been evaluated to determine the type of support required. A process for the selection of an appropriate mass underground mining method is presented by Laubscher (1981). More recently, Laubscher (1990) modified the classification to relate rock mass rating to the hydraulic radius. In addition, the classification proposed by Nicholas (1981) determines feasible mining methods by numerical ranking and thus is truly quantitative. The first step is to classify the ore geometry and grade distribution. The rock characteristics of the ore zone, hangingwall, and footwall are similarly classified. Lineberry and Adler (1987) developed a classification system for mining methods by following geological and geotechnical studies. The available techniques for mining method selection are limited, because selection is based solely on the known physical parameters and rock strength characteristics. Sometimes several mining methods may appear to be equally feasible. Some mines use more than one method to adapt to the orebody shapes and conditions. Typically, in selection of an appropriate mining method, more than one mining method or variation will need to be evaluated. Nelson (2011) and Carter (2011) emphasize this important issue. Some of the factors influencing the selection of a surface or underground mining method include: • Style and geological characteristics (size, shape and depth) of the deposit • Geologic structure and geomechanical conditions • Productivities and machinery capacities • Availability of experienced workforce • Capital requirements and operating costs • Ore recoveries • Safety • Prevailing regulatory environment • Environmental impacts, during and after mining • Reclamation and restoration requirements and costs • Societal and cultural expectations. The optimum mining method will always be the one that maximizes the economic returns while keeping the environmental impact within acceptable levels, maintaining acceptable working conditions (especially in regard to levels of safety risk) for employees, and satisfying statutory obligations (including resource recovery stipulations). Collectively, these goals will also satisfy the objective of efficient use of the mineral resource. In order to further determine which method(s) is the most suitable, the input variables of mining costs, mining rate, labour availability, and environmental regulations should be considered in more detail.

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A numerical ranking system for open pit and block caving mining evaluation Based on the discussion comparing open pitting and block caving, and in order to further determine which method is the most suitable, the authors have tried to identify the most important factors/criteria by considering physical parameters as well as mining costs, mining rate, labour availability, and environmental regulations to develop a numerical (quantitative) ranking system for the two mining methods. The principle of this classification is based on the modified classification proposed by Nicholas (1981).

Criteria to consider for evaluation The criteria were selected in order to evaluate the two mining methods in terms of the main drivers of the mine and problems identified. These are discussed under the headings of resource geometry, mining rate, safety, cost, and other factors. The following criteria can be assumed identical for evaluation of the open pit and block caving mining methods:

• Resource geometry • Environmental and social • Required time • Geotechnical • Safety • Technical • Hydrology and • Workforce skills • Financial

Principles for selecting a mining method The two mining methods are compared in terms of the above criteria. A score is allocated to the factors using the factor rating system in Table III. The value of the score represents the suitability of a given characteristic for a particular mining method. A value of 3 or 4 indicates that the characteristic is preferred for the mining method. A value of 1 or 2 indicates that a characteristic is probably suited to that mining method, while a value of 0 indicates that a characteristic will probably not favour the use of that mining method, although it does not rule it out entirely. A value of –153 would indicate a characteristic that completely eliminates consideration of that method.

Table III. Score and weight scale

Criteria Weight scale Criteria Weight scale Criteria Weight scale Resource Geometry 0.15 Geotechnical 0.15 Safety 0.08 Environmental/Social 0.10 Worker skills 0.05 Hydrology and hydrogeology 0.05 Technical 0.12 Financial 0.20 Required time 0.10

Different criteria carry different weights; these weights were determined by considering the impact of each criterion on the performance and main problems experienced on the mines. The proposed weightings for each category are based upon the findings of recent NI 43-101-compliant technical reports. Table IV shows the respective weights of the criteria. The score was multiplied by its weight and these values were added to give an overall performance rating. The mining method with the higher score will be the better performing one and the more suitable.

192 Open pit or block caving? A numerical ranking method for selection

Table IV. Weights of criteria

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Weights of criteria A selection of technical reports prepared in accordance with the requirements of Canada’s National Instrument 43-101 (NI 43-101), Standards of Disclosure for Mineral Projects, of the Canadian Securities Administrators (CSA) for mining projects in different stages such as preliminary economic assessment, pre-feasibility study, and feasibility study were reviewed in detail to adjust the ranking system developed in this research. (Table V). In Table VI, the score sheet of one of the case studies has been evaluated. As can be seen, in this case, open pitting is more profitable than the block caving method.

194 Open pit or block caving? A numerical ranking method for selection

Table V. Trade-off result based on reviewed technical reports

Mine Commodity Country Mining method Trade-off result Fox OP OP Misery OP OP Ekati Diamond Canada Pigeon OP OP Koala N. BC BC Cadia Valley Cadia Hill OP OP Ridgeway (Lift 1 and 2) Gold, copper,sSilver, and BC BC Australia Cadia East (Lift 0, 1 and Mo PC N/A 2) Telfer - Copper and gold Australia OP/SLC N/A Gosowong Gosowong OP OP Gold and silver Indonesia Kencana C&F N/A Wafi-Golpu Wafi Gold and silver - - Papua New Golpu (Lift 1 and 2) Copper and gold BC BC Guinea Nambonga Copper and gold - - Oyu Tolgoi Southern Oyu OP OP Hugo Dummett BC BC Copper and gold Mongolia Heruga BC BC Oyu Tolgoi OP OP Kemess - Copper, gold and silver Canada BC BC New Afton B1, B2 and B3 Copper, gold and silver Canada BC BC Blackwater - Gold and silver Canada OP OP El Morro Gold and copper Chile OP/BC OP/BC Rainy River Gold and silver Canada OP/CAF OP

Table VI. Score sheet of the case study

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Conclusion With the rate of discovery of near-surface mineral deposits declining, exploring for deep-seated deposits and finding innovative ways to mine them is the solution to the ever-growing need for . Exploring and finding rewarding underground deposits is a job that can be successfully completed by juniors, but designing and bringing such challenging projects into production is a task that requires capital and technical expertise. There are many questions that an might need to be answered before deciding to undertake a proper evaluation. Mining in the 21st century is steadily gravitating towards large-scale low-grade deposits utilizing either open pit or cave mining methods. It is therefore considered useful to develop a classification method by numerical ranking of engineering properties of the orebody and host rock mass, together with technical, financial, environmental, and social issues to assist in deciding between open pit and block cave mining methods. This paper presents an attempt to develop a numerical ranking system for open pit and block caving that could be helpful in making the choice between the two methods. The authors admit that the proposed method is quantitative, but should be used as a first-pass approach similar to the other qualitative and quantitative ranking systems. The ranking is devised by using a ‘smart’ approach to identify the most important factors, including resource geometry, geotechnical, technical, hydrology and hydrogeology, environmental/social issues, and safety, financial, and timing factors. Case studies have been conducted for more than 20 mining projects. In most of the projects the model was properly matched with the result of technical reports, and for some of them, the definition and assigned scores of some criteria should be scrutinized. Since this research is ongoing, the procedure will be fine-tuned as more data become available and experience is gained.

Acknowledgements The authors would like to thank Ms. Isabelle Leblanc, Mr. Jean-François St-Onge, and Mr. Patrice Live from BBA for their invaluable contributions to this work.

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