Porgera underground mine and its mining methods G.G.Yowa (August, 2017) Senior Instructor, Mining Eng. Dept., PNG University of Technology. PNG

([email protected]) 1. Underground (UG)mines in PNG The underground mines that were in operation in PNG in the last 10 years are Porgera, Tolekuma, Irumafimpa, and Mt Crater Mines. The Tolekuma and Mt Crater Mines are currently on care and maintenance. The Irumafimpa mine was back into operation under a new owner last year (2016) after care and maintenance. The Wafi Project proposed to utilise underground method and also Ok Tedi looks at mining the bottom of the orebody with underground method.

Fig 1: Mines and projects in PNG (MRA, 2007 modified) Legend where; C=current Mines, P= Potential Projects, A= Advanced Exploration and C=Closed Mines

2. Porgera underground mine 2.1 Background Porgera Mine is located in the Enga Province in the highlands of Papua New Guinea (PNG). The mine consist both open pit and underground operations. The commodity mined is gold and the by-products are silver and copper.

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2.2 Mine geology The three main lithologies are the black and brown sediments, altered sediments and diorite intrusion complex. The black sediments are Chim Formation composed of grey to black calcareous mudstones and clayey siltstones. The light-coloured mudstones are called the brown sediments. The low grade contact metamorphism in the vicinity of the intrusive dykes and stocks has formed the altered sediments (slates). There are also extensive sections of highly blocky and brecciated rockmass resulted from the intrusion mechanisms. The intrusive complex (Porgera Intrusive Complex or PIC) consist the hornblende diorite and the augite hornblende diorite. The mineralisation occurs within the PIC and the contact boundaries with the mudstones and dissipates within the mudstones in the form of a ‘horsetail’. The mineralisation occurs on the footwall diorite of the Roamane Fault. The hydrothermal mineralisation is formed from the magmatic phases of the PIC. The hydrothermal veins are rich quartzroscoelite deposits. The orebodies are associated with splay faults and strike at NE-SW directions up to 300m. The width of the orebody ranges from 5-30m and dip 55-85 degrees. 2.3 Mine geotechnology The assessment and characterisation of the rock mass is done using Rock Quality Designation (RQD), Rock Quality (Q-System) developed from NGI (Norwegian Geotechnical Institute), Rock Mass Rating (RMR) and Geological Strength Index (GSI). Table 1: Summary of rockmass classification (W Claeys, 2007)

Stability assessment of the trike of the stope is determined using the Modified Matthews Stability Graph Method. The stability number N and modified stability number N’ are utilised to quantitatively determine the stability of the excavation. The figure 3 below shows a modified stability graph that determines the stable strike of the stope hanging wall.

Fig 2: Matthew Potvin Stability Graph (W Claeys, 2007)

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3. Underground mining methods There are several variations of sublevel stoping methods that are applied in Porgera. Below are the variations; i) ii) iii) iv) v)

Blind uphole retreat method Primary-secondary transverse method Pure avoca method Modified avoca method Sublevel stoping and post-fill

3.1 Blind Uphole Retreat Method. Blind Uphole Retreat Method is commonly applied on the crown or the ends of the orebody where the top drilling level is uneconomical to develop. Only the bottom ore drive is developed to access the stop for drilling, blasting and bogging. The slot is located at the end of the ore block. The first 2 or 3 rings are dumps as slash rings to help in pulling out the slot as well as allowing the charge crew to be position safely away from the open brow. Several rings (2-4) rings are fired per blast and the bogger completely mucks out the tonnes. The next set of rings is fired and the cycle continues. In some cases where the recommended strike length is reached, a rib pillar is usually left behind and a new slot is drilled and fired for the next panel. The pillar left behind is for support of the walls. Some pillars can be recovered like the ones at the cross cuts or access where access is possible. Other rib pillars inside the open stope are normally sterilised (removed from reserve). The figures 3, 4 and 5 below show the plan, the cross section, and the longsection views of the Uphole Retreat Method.

Fig 3: Blind Uphole Retreat plan view

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Fig 4: Blind Uphole Retreat x-section view

Fig 5: Blind Uphole Retreat longsection view 3.2 Primary-secondary transverse method The transverse method is utilised on wide orebodies with widths >10m. The swell section along the strike of the orebody results in wider orebodies while the pinch sections result in narrow orebodies. Additional cross section drives are developed from the FWD that gets connected to the orebody and each stope has its own cross cut. The primary stopes are mined out and backfilled with either paste or cemented aggregate fill (CAF). After the fill is cured, the secondary stopes are mined out and filled with rock fill (RF). The primary stopes are filled with paste or CAF to maintain a competent wall contact while mining the adjacent stopes, the secondary stopes. RF is used to fill the secondaries as there is no requirement for any of the more competent fill (paste and CAF) provided that there are no other stopes adjacent to be mined. The figures 6, 7 and 8 below show the plan, the cross section, and the longsection views of the Primary-Secondary Transverse Method.

Fig 6: Sublevel transverse method plan view 4 | Page

Fig 7: Sublevel transverse x-section view

Fig 8: Sublevel transverse longsection view In the figure 8 above, the P stands for primary stope and S stands for secondary stope. Moreover, some stopes can be call tertiary stopes when they are adjacent and not directly along the direction of the strike of the main mineralisation. These tertiary stopes (T) are mined out after the secondaries. In the case were the tertiary stopes are on the Hangingwall of the main stoping strike, tertiary developments like paste development are done to access the stopes. That is illustrated in figure 9 below.

Fig 9: Sublevel transverse tertiary stopes plan view 3.3 Pure avoca method Avoca can be either Pure Avoca (or sometimes termed True Avoca) or Modified Avoca depending on the direction of mining and filling. In a Pure Avoca Method the mining retreats towards the access while the backfilling advances from the opposite end and chasing after the stoping brow. There has to be two accesses available and are developed

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from the FWD for a Pure Avoca Method. One access is used production blasting while the other opposite access is used filling waste materials. The filling is stopped when the brow reached. The brow must not be choked in order to have void to fire next rings. The cycle repeats until the stope is completely mined and filled.

for for is the out

The figure 10 below shows a typical plan of level development for a Pure Avoca Method. There are two accesses into the stoping blocks (ore blocks in green colour) and they are developed from footwall drives (FWD) east and west of north or south. Some additional infrastructure development can be added like ore pass drive, and paste reticulation drive depending on the mine site operation plans.

Fig 10: Pure avoca development layout-plan view. The figure 11 below shows a typical cross section view of a Pure Avoca Method.

Fig 11: Pure avoca cross section view (x-section). The figure 12 below shows the longitudinal section (longsection) view of a Pure Avoca Method.

Fig 12: Pure Avoca longsection view.

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3.4 Modified avoca method A Modified Avoca Method has a single access serving blasting and backfilling. There is an absence of FWD.

production

The rings are fired to the recommended strike length and the ore is completely bogged out. The void is then filled completed with waste rock fill. The brow in this case is completely choked off. In order to fire the next rings of the next stope, the waste materials have to be bogged out to create the void and this process is called ‘reslotting’. Once the brow is cracked, the next set of rings are fired and bogged until the require strike length is reached. The process repeats until the entire level is mined out.

Fig 13: Modified Avoca development layout-plan view.

Fig 14: Modified Avoca longsection view. 3.5 Modified avoca chock blast method In some cases, chock firing is applied where the next set of rings are fired against the rockfill without reslotting the rockfill. Since rockfill have some void within (15-30%), the rings are fired against the RF aiming to compact or chock the blast. This method is usually not preferred as it introduces more dilution from the RF waste as well as in many cases back break and damages are induces on the holes on the adjacent ring. 3.6 Sublevel stoping and post-fill method This method can also refer to Avoca Methods and Sublevel Stoping with CAF and paste filling. When rockfill is used as a backfilling material, then the method can be one of the two Avocas. However, when either paste, CAF or CRF is used, then the method is varies slightly as a new slot raise has to be established for the next stope panel and

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in that case, the method is appropriately called a Sublevel Stoping and post fill method. 4. Development Jumbo drills are used to establish all the developments. The development sizes can be 4.5m width by 5.0m height, or 5.0m width by 5.0m height or 6.0m width by 5.0m height. In some cases, the width of the drive can be as wide as 10-15m. Usually, the wide drives are a result of stripping for ore drives or stripping for infrastructure set up.

Fig 15: Double boom jumbo. 5. Longhole drilling The stope slot and rings are drilled with Simba and Solo rings. Simba is an Atlas Copco Product while Solo 1006 is a Tamrock Product.

Fig 16: a) Simba Rig

b)

Tamrock 1006 Rig

The blastholes ranges from 79mm, 89mm, and 102mm. The common hole diameter is 102mm (4inch). The reamer holes on the slot are 204mm or 250mm. (Occasionally 305mm may be used for reaming service holes). 6. Blasting Blasting is the process of applying explosives to fragment the ore into desirable sizes to be hauled to the surface. Blasting is done on lateral development vertical developments and stope rings and slots. 6.1 Stope blasting (production blasting) In a production stope, the slot is fired and opened and the rings are fired after. The success of the stope blast depends on the success of the slot. When the slot fails, then a new slot is re-drilled and blasted. In the case of uphole slot blast, some slots do not pull to the designed depth and a new slot is drilled. Uphole stopes are a real challenge to pull and the recovery is less than 100% compared to downhole stopes. Some of the common explosive products used in stope are; ammonium nitrate (ANFO), emulsion, package explosives (busters), detonators (electric and non-electric dets), green cord, boosters, Ikon dets, zbar, devil dets. Most explosives are Orica and Dyno Products. Other accessories used in the blast include spiders, liners, collar pipes, stem aggregates, chock block, firing lines and so forth.

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The figure 17 below shows explosive column of a ring. The charge collar and stemming length is spaced so as to maintain the charged toe spacing and thus the recommended powder factor.

Fig 17: Cross section of ring charge column. The figure 18 below is a slot pattern.

Fig 18: Slot drill plan view. The figure 19 below shows the blasting sequence of a slot. The hole in the centre (between the reamers) is the first hole initiated.

Fig 19: Slot blast sequence plan view. A downhole slot is fired in lifts. The sequence of the blast may remain similar with the subsequent firings until the slot is pulled through. That is the beauty of downhole slots as there is continuous access to charge the several lifts from the top drive (sill) until the final cap is fired through. In an uphole (inverse) slot, the slot is fired in a single shot. The slots cannot be fired in lifts as the collars of the holes will have been fired and it becomes totally and completely unsafe to go under a fired ground. Thus, an uphole slot is fired once and t becomes a real

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challenge pulling to the depth. Many uphole slots have to be redrilled or a new slot is drilled when the initial slot fails. 6.2 Development drilling & blasting "Burn cutʺ pattern is used for the drilling out the face. There are four (4) reamer holes of 74mm diameter and 50-70 blastholes of 45mm diameter. The numbers of blastholes depend on the size of the dive. A smaller size hole such as 4.5mW x 5.0mH requires fewer holes than a 6.0mW x 5.0mH. There are different names attributed to the holes on a face as shown in figure 20 below.

Fig 20: Face round, burn cut pattern section view The explosive products used in blasting a round includes; LP Dets (long period detonators), ANFO, Isanol (50/50), Green cord (10g/m), primers (package explosives), starter detonator, and z-bar. Other miscellaneous charge cylinder.

items

include

bell

wire,

air

compressor,

and

Below are the stages involved in firing a round: i) The det is inserted into the primer and stem into the hole. The primer is normally 10-30cm away from the toe of the blasthole. ii) The ANFO is blown into the hole via hose that is connected to the high pressure charge cylinder and the cylinder is usually mounted on an Integrated Tool carrier (IT) or a Scissors Truck (ST). iii) When all the holes are loaded, the green cord is run within the face of the heading and all the end tail "J Hooksʺ on the dets is affixed to the green cord. 10 | P a g e

iv) v) vi)

The two ends of the green cord are rolled to about 10m from the face and is attached and taped with the electric detonator with insulating tape. The end wire ends of the electric det (electric detonator), is connected to the main firing line. The blast is set off at the office on the surface at the end of the shift when everyone is out of the mine.

Fig 21: a)

IT (internet)

b) An ST (internet)

The holes within the burn cut are given longer delay to make sure the material thrown out and a continuous void is present for the subsequent holes are blasted. The blasting sequence is show in figure 23 below.

Fig 22: Face round, blasting sequence 7. Material movement Truck and loaders are utilised for all the bogging and hauling of the blasted materials. Elphinstone trucks (CAT Products) of 45t and 50t payload capacity are employed to move dirt. The R1700, R1800 and R1900 loaders (CAT Product) are used for mucking. The materials stockpiled.

are

hauled

through

the

decline

to

the

surface

and

There is a weight bridge at the surface that the trucks travel on to record the tonnes hauled. The weight bridge is connected to a live computer that records all the data. The data is processed daily, weekly and monthly to determine the weighted average payload of the load over the time period determined.

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Fig 23: a) R1900 Loader

b) AD55 Truck

8. Backfilling There are four (4) types of backfill material used and they are the paste, rock fill (RF), cemented aggregate fill (CAF), cemented rock fill (CRF). Currently, the common fill used are paste and rockfill. Occasionally CAF is used when the paste plant is down. The CRF is not commonly used recently due to the introduction of CAF in 2004. With paste filling, paste reticulation lines are established from the paste plant to the underground workings where the paste travels and fills up a stope. With RF, trucks are used to haul the dirt to the open stope to be backfilled. With CAF, trucks are used to load the CAF from the plant located underground and are hauled to the stope location and backfilled. With the CRF, loaders are used to mix the rock with the cement that is poured from an agitator truck (AG). The loaders haul move the CRF to the open stope.

Fig 24: An AG truck Prior to backfilling a stope, a backfill clearance note is issued. There are several items that needs to be installed including and depends on the type of fill required. If a stope is to be paste filled, then the following are required including paste walls, lightings, reticulation lines and any back support. If a stope is to be rock filled, then the following are required including bund wall, lightning, secondary ventilation. 9. Summary Porgera Mine is a medium to large operation. The underground ore production rate is around 3000t/day. The development metres are targeted at 600m/month. There are several orebodies that yields reserve that see the life of mine to extend to 2020 and beyond. The main mineralisation is hydrothermal veins and they are a result of the PIC that intrudes the sedimentary rock units. The veins and the veinlets pinch and swell and once encountering the sediments, the mineralisation dissipates like a horse tail. The mining method applied is sublevel stoping method with several variations including, blind uphole retreat method, primary-secondary transverse method, pure avoca method, modified avoca method and sublevel stoping and fill.

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Reference: 1. Claeys.W.D, (2007), Underground Ground Control Management Plan, Revision 6 (Porgera internal document, pp 8-14. 2. Mackenzie.W, & Cusworth.N, (2007), The Use and Abuse of Feasibility Studies. Project Evaluation Conference, Mel, Vict, p3(cited www.enthalpyconsulting.com/admin/html/archivos) 3. Mineral resource authority (MRA-PNG), PNG Mines and Potential Projects,(cited https://ramumine.files.wordpress.com/2016/11/pngmining-projects-map1.jpg 4. Miller.S, (2008), Drill and Blast Review-Barrick Porgera Joint Venture, p 11. 5. Porgera UG, (2013), Mid-Year Reserve COG Calculations,(cited from the report from the mine technical team) 6. Trevor. Historical Overview of Mining In PNG, www.infomine.com/library/publications/docs/Neale.pdf

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