In looking to develop a mineral prospect, a number of factors need to be determined and weighed in balance, including geology and mineralogy at the time, and ore valuation forecasts over the various horizons for possible life-time periods of the potential mine. Into the planning investigations also goes potential mining methods and sizing for mine production, the openings, productivity and costs.
With respect to geology and mineralogy, key information to obtain includes: the scale of the prime and any adjacent prospects; the length, dip and maximum depth of each mineralised zone or seam, and distances to them; any known discontinuities, swelling or narrowing of the zone or seam; any sharp changes in mineral grades, and between the ore and waste, within the zone; if the ore oxidises during extraction; distribution of valuable minerals as well as those that might be harmful in mining or processing; any alteration zones.
Other key areas for information gathering during this site investigation work, includes: the geology and structural features of the general rock mass covering and surrounding the prime mineral prospect, including water, gases or even oil; then, more locally, the type and quality of the rock at the prospect, including strengths, weaknesses, inherent stresses, temperature, acidity, all together helping to inform the rock quality designation (RQD).
As to mining methods, the deposits can be mined by a variety of methods. Among the major mining methods are: room-and-pillar; vein; shrinkage stoping; sublevel open stoping; bighole stoping; vertical crater retreat; cut and fill stoping; longwall mining; sublevel caving; and block caving. Various tunnelling and shaft excavation methods can also be employed to get underground initially and then to advance on the deposits; to branch to new parts in the field with further prospects and then, from those adits, to excavate shafts, drifts, declines, roadways, sumps and winzes among the many other mine features.
But, from these collective points, there are key further aspects of mine planning that must be considered, including: effective understanding of induced changes through excavation and their relative size, shape and position; ground support methods and ground monitoring. Overlaying all of the factors are needs of health, safety and appropriate environmental conditions and controls.
Stillwater PMG Complex
The hot spot for platinum mining in North America is in the Beartooth Mountain Range, in Montana, at the neighbouring Stillwater and East Boulder mines, where palladium is also extracted from ever growing stretches of the 45km long ore body – the "J-M" reef. The mines are owned by Stillwater Mining Co.
The world class reef runs parallel to a river valley that is almost 50km north of Yellowstone National Park. The richness of resources along the east-west running reef gives it strategic importance in global supplies. It is one of the only significant sources of the platinum metals group outside of Russia and South Africa; the mine produces four per cent of the world’s platinum supply and nine per cent of the palladium.
As development of the mine has progressed to new parts in the large reserve blocks with main access adits (footwall laterals) and so extending the life of the site, then miners have had to reach farther out into the reef for platinum, palladium and the minor amounts of rhodium available. There are also significant amounts of iron, copper and nickel, plus traces of gold and silver. The reef has been found to be highly homogeneous, and longer development bores are needed to link to, and open up, the blocks.
Stillwater Mining also operates a smelter and refinery in Montana. In recent years, production of the platinum group metals has been around half a million ounces annually, and based on historical drilling data the company has estimated there could be another 115 million ounces of undeveloped mineral resources within the area controlled by the mine.
Based on expansion plans issued in 2010-11, Stillwater Mining expects to add approximately 200,000 5,700kg ounces of platinum group metals by 2014, which would be an increase of about 40 per cent, and still mean the untapped, estimated resources would equate to 150-160 years of annual production at those higher levels of output.
Geology in the mine area comprises a layered succession of ultramafic to mafic rocks with bands of norite, gabbro and anorthosite. Crystal settling allowed the heavier mineral ore to settle to the bottom with the lighter siliceous minerals remaining near the tip. Originally horizontal, the typically long narrow reef was tilted and fractured by crustal displacements, and the platinum group minerals are found in layers averaging 2.4m thick though can be down to only 1m, approximately.
Opening the mine
The J-M reef was discovered in the 1970s and production began in the mid- 1980s. Progressively, the mine opened up with drill and blast drives but very soon the first TBM was in the ground, in 1988. Since then, and in addition to use of TBMs for the footwall laterals, the mine has expanded with typical drill and blast, or small boring equipment, for excavation and advancement methods such as ramp and fill, sub-level stoping and captive cut and fill.
Typically, the TBMs are used to construct the main adits and also to bore some crosscut access tunnels. In total, the three TBMs that were used – the fourth is just about to start – have bored more than 32km of tunnel, sometimes at a relatively quick pace (the Jarva shield driving 7.7km during 1991) to very slow, such as the first Robbins machine excavating 2.2km over eight years, according to the mine owner’s schedule.
However, more recently, the Robbins TBM, which was the third shield on the reef, has been boring the new footwall lateral for the Graham Creek section of the reef, adjacent to East Boulder Mine. With completion of the drive expected around within six to eight months, the mine development work at Graham Creek will then switch to adding new ventilation raises and other infrastructure to allow sustainable ore production to commence. The mining company has commented that most of the development cost is on raise boring or drilling next to the TBM. However, the shield was already on site as an owned asset, or sunk cost.
For the latest Robbins TBM, the primary objective is to bore the Blitz tunnel from Stillwater Mine alongside the reef and help define what resource there is and how to approach it. The Blitz project also has another tunnel being excavated by conventional mining, and will eventually intersect a new entrance to the mine from the suface.
The TBM will bore at least 7km to help detect the edges of the reef along the eastern side of the mine. The tunnel will also be used as an access and rail haulage tunnel for the long-term. Expected to be bored over about three years, there is the possibility the Blitz tunnel may be extended, and the TBM will drive farther, but it all depends on how the quality of the reef plays out beside the initial Blitz tunnel.
Aquired and refurbished for the Blitz job, the TBM has two drills mounted for miners to consecutively probe and sample the complex mafic norite geology by diamond core drilling. The rock mass will be pierced ahead, above and to the side of the TBM at intervals of 150m, and so help to establish the edge of the reef as it is steered to transit the boundaries. The length of the drill holes is to be 182m. "We will drill and log the core right there while we are drilling it, then interpret the results, so it will be concurrent with boring," says Justus Deen, the mine’s head of technical services.
With the TBM’s path being adjusted less by absolute pre-planned survey control and more with adjustments to optimise its position against the perceived distance to the reef, the control is like civils in that local actions are taken to deal with geological discoveries. Unlike mining though, such as in this case, in civils the adjustments are often to put distance between the shield and zone in question, often where there is potential for problems or at least sufficiently less favourable ground, or anticipated greater risk. In mining, the shield is brought closer – but not too close – to the prize.
Platinum and palladium are in base metal sulphides, and that ore is located in a distinct layer of igneous rock – "so if we penetrate the right rock types then we know we are in the right place," says the mine’s chief geologist, Mike Koski. The metals, he adds, are typically found in only half the core samples, "so the rock types will guide us." To that end, Koski continues, "we don’t want to get too far away from or too close to the ore body."
Over the initial planned length of the TBM-bored Blitz tunnel, the mining company expected to undertake almost 37km, in total, of core drilling to help locate the boundaries of the J-M reef along that section. Approximately 1.1km of core drilling, in total, is to be executed at every 150m interval.
In opening the footwall lateral parallel to the axis of the reef, the TBM drive is also, in effect, an exploratory tunnel. Yet, the planning for future exploitation of the reef goes beyond establishing its boundaries around the TBM through diamond core drilling. Conventional geotechnical probing for the TBM bore – to look for weak ground in the expected mainly competent rock with clay infill at faults and shear zones, and also dykes and water – will also help to fine-tune the geological map to nail the good spots for later breakout with the branching, side access tunnels to pierce the reef, and those to avoid.
Koski says the plan is to undertake the geotechnical probing ahead at slightly greater intervals (just over 165m) compared to the core drilling. He adds, "we will know we are placing the footwall laterals in the best possible ground."
Ore mining along footwall laterals will get underway, from the TBM tunnel, while the shield continues to advance ahead into the block. To prepare for the rapidly following active mining stage, the final rail grade with ballast, wood ties and track in the tunnel will be set behind the backup train each time core drilling is underway. Prior to the final placement, the TBM backup will have run on temporary steel sleepers from the bridge area.
Northparkes Copper Mine, Australia
For the last few years, among those business initiatives high on the agenda for mining giant Rio Tinto has been its "Mine of the Future" initiative that it believes could cut mine development time by up to 40 per cent. Launched to boost safety, effectiveness and capabilty in finding, developing, mining and recovering copper and other ores, the initiative has also seen much R&D cooperation with some excavation equipment manufacturers.
For the development strand, tunnel boring system trials are getting underway at Northparkes, a copper mine in a volcanic belt in New South Wales, Australia. The mine is held by a Rio Tinto and Sumitomo Group JV, and the aim is to develop future block cave mines in safer, faster and better ways.
Block cave mining involves undercutting the ore zone to allow collapses under gravity into ecavated, cone-shaped funnels for collection and transport to the surface. It is a mass mining method, and, says Rio Tinto, can be intensely automated and is the lowest cost underground mining method.
The first important mineral discovery at Northparkes was in the 1970s and the ore deposits are typical copper-gold porphyry system. Mine development took off in the early 1990s with open pits and, later, block cave mining – Australia’s first use of the method.
The initial mine in the E26 mineralisation zone, the first to be exploited, was E26 Lift 1, which was developed in 1995. It quickly ramped up to become the world’s most productive underground hard rock mine, reaching 42,600t of ore per underground employee (including contractors), according to Rio Tinto. Annual productivity peaked in 2000 at more than 50,000t per employee.
Open cut mining resumed for a few years and also some small underground works (pilar-wrecking, slots below Lift 1 extraction level) were undertaken to bridge output during the establishment of E26 Lift 2 block cave mine, starting in 2001. The Lift 2 mine was commissioned in 2004 and extended four years later. Meanwhile, construction of the third major block cave mine, E48 Lift 1 got under way and is now in full production. The E48 zone is expected to last another 12 years.
Work is now advancing on the proposed ‘Step Change’ expansion of the existing block cave layout to create a greater, interlinked underground network to boost output and productivity. A 2010 study indicated that the expansion could see ore extraction from Northparkes reach 30 million tonne per year from multiple block caves. Future block cave zones to be tied by the tunnels include E26 Lift 3, E48 Lift 2, E22 and GRP314.
A USD 114M pre-feasibility study into the expansion plan included underground diamond core drilling to establish a fuller picture of the layout and boundaries of the mineralisation zones. In total, approximately 155km of core drilling was performed. Next, and just getting underway at Northparkes, are the in-field verification trials of the new Mobile Tunnel Miner (MTM) – a cross between a roadheader and TBM – developed by Aker Wirth in reference to Rio Tinto’s "Mine of the Future" initiative, launched in 2008.
Rio Tinto said "mine design is our most important value creation," – and "protection lever", it added. The trials of the Aker Wirth MTM tunnel boring system have been integrated into the expansion studies at the mine.
Northparkes is one of seven main copper interests held wholly or through equity share by Rio Tinto. With many prospects to exploit through block cave mining, the company has set up the Rio Tinto Centre for Rapid Underground Construction, based at the Centre for Excellence in Mining Innovation in Ontario, Canada, for training and to build technical capability. Other underground mine expansions that will tap the resource include Kennecott in Utah and Oyu Tolgoi, in Mongolia. At Kennecott, the company is pushing underground with near term development of the North Rim Scarn zone which has both copper and gold. Pre-feasibility studies up to USD165M have been approved up to 2014 and it is anticpated that, following development tunnelling, first production could be in about six years.
Starting 2013, the Kennecott mine will also host the trials-in- the-field of the Atlas Copco Mobile Miner. Rio Tinto says the mobile miner is expected to allow more than 10m/day of tunnel construction, nearly twice the rate of conventional methods. Oyu Tolgoi – a "world class asset" – will see a switch from open pit to complete underground development in the transition from Phase 1 to Phase 2 of the mine.
Underground extensions are also seen for the massive Grasberg mining complex. However, it will be a variant on block caving – a ‘panel cave mine’ where the active mining area progressively migrates along a ‘panel’.
Rio Tinto’s Resolution mine is planned to be one of the major underground mines in the future even though the top of the mineral resource is about 1.5km below the surface with the production level at approximately 2km deep. The mine has copper ore with significant molybdenum.
John McGagh, head of innovation with Rio Tinto, says: "More mining is moving underground as deeper ore bodies are identified and open pits come to the end of their lives."
Petra Diamonds, South Africa
Block caving is also a key underground mining method for diamonds, such as at the Cullinan and Kimberley mines owned by Petra Diamonds in South Africa. The technique is also used at the company’s Finsch mine where it is supplemented by sublevel caving, which uses smaller, intermediate-level excavations that last for shorter periods.
With their shorter livespans, the sub-level caving operations do not need the same degree of permanance in infrastructure support as required by block caving.
However, these jabs at the ore have slighly higher operating costs as they call for relatively more drill and blast excavation, the company notes. The advantage for Petra, therefore, at Finsch mine is the greater flexibility afforded to production, such as being a bridge for output while new areas are opened up. For example, ensuring production levels are maintained while the mine development effort is moving across to a new major ore block.
At Petra’s Koffiefontein mine the front caving method is used, which saw an initiating excavation to the centre of the ore body and then the slot is opened up, or has an "overdraw" system used as the miners retreat out in stages.
Lower levels are also excavated with to function as the semi-permanent drawpoint, and these are developed ahead. Depending on the mine, the initiating slot may not be at the centre of the ore body but, instead, can be against the ore body boundary.
Elsewhere in the country – at the Helam, Sedibeng and Star mines – the company employs the complex ‘fissure’ or ‘dyke’ mining method where erosion has worn down kimberlite ore bodies to leave only the extremely narrow (less than 1m wide), magmatic vertical root zones.
The mining recovery is complicated by scale and geometry in the slender dykes but also due to variation in the types of kimberlite caused by magmatic intrusions at different spots over various periods of history.
Cullinan is one of the world’s largest diamond mines and also a significant source of prized blue diamonds. Petra Diamonds bought into the mine, in Guateng province, in 2008 with a purchase from De Beers and quickly increased its stake by aquiring Al Rajhi’s holding, and it now holds the controlling share. Mine began at the site 110 years ago but it wasn’t until 1946 that mining switched from open pits to underground.
Mining at a depth of 747m currently, the mine plan is for 18 years and the potential life of the operations could last another 50 years, it estimates. The current expansion plan, to take annual production to 2.4Mcts (including 17 per cent from tailings processing, which is being stepped up) by FY2019 will see a new block cave mined once reached by mine development infrastructure in the western side of the orebody. Development and mining at this block could be boosted by new work on a major decline. Also on site, Petra has had shaft deepening work undertaken by contractor Murray & Roberts Cementation.
Comprising three neighbouring kimberlite pipe mines, the Kimberely complex has its origins in the late 19th Century and were amalgamated into De Beers in 1890. Petra Diamonds Ltd acquired its controlling (74 per cent) stake in 2010. The mine plan is for 10 years with potential life of a further 12 years from this mature asset, which is currently being mined at depths of 845m to almost 1km.
But in 2005 the mine had been closed by De Beers. However, working under the existing licence during a care and maintenance period from September 2007, Petra rehabilitated the deep underground mines and completion the purchase just over two years ago. The company is implementing a development plan that would double production from current levels to 135,000 carats annually by FY 2016.
