Contents

ABSTRACT 3

INTRODUCTION 5

STOPING PRACTICE 6

MINING LAYOUTS 9

ADITING 9

SHAFT ON SEAM (PILOT WINZE) 9

INCLINED FOOTWALL SHAFTS 11

PREVIOUS MECHANISATION ATTEMPTS 12

FUTURE STOPING 13

FUTURE LAYOUTS 15

VERTICAL LAYOUTS 15

CASE STUDY 3-4 MILE SECTION AT ARTHUR’S LUCK 16

OPTION 1 – INCLINE SHAFT ACROSS SEAMS 17

OPTION 2 – VERTICAL SHAFT ACROSS SEAMS 18

OPTION 3 – DECLINE RAMPS 19

3rd International Symposium

May 6 - 8

2008Zimbabwe holds at least 12% of world chromite resources, the majority being hosted on the Great Dyke and mostly situated below present mining depths. Zimasco’s chromite resources as at February 2007 were 107.5 Mt of which 98% lies on the Great Dyke.

Zimbabwe’s contribution to the World market in 2007 was a chromite output of 0.9Mt, of which 0.5Mt was produced by Zimasco. Chromite production in Zimbabwe has not grown over the past decade as a result of the costly and difficult nature of mining on the Great Dyke and because of a controlled economy, which has stifled new investment.

This paper describes the current mining practice, previous mechanization attempts on the Great Dyke and possible future mining methods.

Chromite Seam mining practice on the Great Dyke in Zimbabwe

3rd International Symposium

On Narrow Vein & Reef Mining

May 6 – 8, 2008

CHROMITE SEAM MINING PRACTICE ON THE GREAT DYKE IN ZIMBABWEWalter Nemasasi,

Zimasco (Pvt) Ltd, 6th Floor Pegasus House, Samora Machel Avenue, Harare, Zimbabwe.

ABSTRACT

The chromite resources of Zimbabwe are estimated at 900 million tonnes, the majority being hosted on the Great Dyke. Most lie below present mining depths. Zimasco’s ore resources as at February 2007 were reported as 107.5Mt at 40.91% Cr203 and 2.12 Cr: Fe ratio. 98% of this resource (105 Mt) lies on the Great Dyke and the remaining 2% is in podiform deposits off the Dyke in and around Shurugwi.The Great Dyke is a linear NNE-trending body of mafic and ultramafic rocks, 550 km in length and between 4 and 11km wide. It was formed 2460 million years ago by a series of separate magma intrusions. In the Mutorashanga area, 8 separate seams (seam numbers 4 to 11), averaging 12 cm in thickness are known to exist. In the Ngezi and Lalapanzi areas the two main separate seams (seam numbers 1 and 2) average 26 cm and 22 cm respectively. Dips in Mutorashanga vary from 26° to 38° and 11° to 18° in Ngezi and Lalapanzi. (Fig. 1)

Chromite production in Zimbabwe has not grown over the past decade as a result of the costly and difficult nature of mining on the Great Dyke and also because of the controlled Zimbabwean economy, which has stifled new investment.

This paper describes the current mining practice on the Great Dyke particularly the resue variations in the stoping method and the different mining layouts. A brief synopsis of previous attempts at mechanisation using “Coal” cutting, Continuous mining, and Trackless mining techniques is also presented, with some brief post mortems of why these trials did not survive the test of time.

In conclusion the paper offers possibilities for future mining methods that take cognisance of local infrastructure.

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GREAT DYKE OF ZIMBABWE Fig. 1

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INTRODUCTIONZimbabwe hosts 12% of world chromite resources and produced 4% (0.9Mt) of world production in 2007. (Figure2)

Fig. 2

In 2007 Zimasco (Pvt.) Ltd, produced 0.52Mt of the 0.9Mt of chromite produced by Zimbabwe. The remainder came from Zimalloys, Maranatha, Oliken and a couple of other smaller producers. From a reserve of 50Mt, figure 3 shows the distribution of extraction when compared with the reserve base.

Fig. 3

Strip mining on the dyke is carried out to 22 metres of vertical high wall in the Ngezi and Lalapanzi areas which are amenable to this mining method because of the relatively flat terrain, with provisions for a portal left every 500 metres on strike. These portals will in future be mined to 500 metres on dip according to current designs. There is no strip mining in Mutorashanga because of the hilly terrain and steep seam dips.

Initial exploitation of the seams in Mutorashanga is by aditing the resource in the hills down to the lower ground level after which sub declines are sunk to exploit the deeper resources. Dyke Underground mining then proceeds to 500 metres on dip. 80% of dyke underground mining is done in Mutorashanga.

Surface mining is used to exploit podiform deposits at Valley and in Shurugwi to a depth of about 80 metres.

Underground podiform mining, which has been going on for more than a century in Shurugwi, has been conducted using sub level open stoping.

in Mt in % RANK in Mt in % RANKS. Africa 5500 72.4 1 8.7 39.9 1Zimbabwe 930 12.2 2 0.9 4.1 6Kazakhstan 320 4.2 3 3.8 17.4 2Finland 120 1.6 4 0.6 2.8 7India 67 0.9 5 3.3 15.1 3Turkey 20 0.3 6 1.3 6.0 5Brazil 17 0.2 7 0.6 2.8 8Others 626 8.2 8 2.6 11.9 4Total 7600 100.0 21.8 100.0

RESOURCE OUTPUTCOUNTRY

PRODUCT TYPE Reserve (Mt) in %2007 Output

(Mt) in %Dyke Surface Strip 10.5 21 0.18 34Dyke Underground Mining 37.0 74 0.08 16Podiform Surface Mining 0.5 1 0.10 20Podiform Underground Mining 1.0 2 0.16 30Eluvials 1.0 2 0.00 0TOTAL 50.0 100 0.52 100

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Chromite deposits on the Great Dyke have 2 distinct occurrences: -

A soft host rock (ucs 10 Mpa & hardness 4) containing friable chromite ore. This soft rock, which can be drilled using auger machines, is predominant in Mutorashanga.

A silicified hard serpentinite (ucs 42Mpa & hardness 5) containing hard lumpy chromite ore. This rock requires jackhammer drilling and is predominant on the rest of the dyke.

STOPING PRACTICEThe essential considerations in the extraction of chrome seams are the removal of the seam with minimum fragmentation of the material and with minimum contamination by waste.

The method of exploitation almost universally employed is that of Resue Stoping on breast faces (for dips ≤30º) and up-dip faces (for dips >30º).

The generally adopted technique is shown in Figure 4 below.

Fig. 4

Drilling and blasting the hanging wall waste down the full length of the stope face. The waste derived from the blast is packed between timber props to fill the stope from footwall to hanging wall (Figure 5). Excessive waste (30%) due to swell is lashed into the seam drive for tramming to a waste pass and subsequent hoisting to surface for dumping. The stope floor is swept clean before breaking the chrome.

The chrome seam exposed on the footwall is broken to induce separation and lifted with as much care as possible to avoid fragmentation. (Figure 6)

Support consists of 2 rows of props close to the face, and back filling behind.

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Resue stoping on breast faces is in most cases practised on advance stoping with Retreat mining only employed in areas where the ground is considered blocky and unstable. Up-dip stoping is seldom applied where dips are less than 30º. In such cases, local faulting/jointing will be the determining factor.

WASTE PACKING Fig. 5

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CHROME LIFTING Fig. 6

The shift cycle is explained in the table below.

Chrome lifting - Team Leader and 4 men 6 hours

Drill Charging and Blasting - Team Leader and 2 men 4 hours

Stope Waste Lashing - Team leader and 4 men 7 hours

Seam Drive Waste lashing - 2 men 5 hours

On an optimal panel length of 20 metres and an average of 19.8 metres advance per stope per month, this translates to 26 centares per man per month.

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MINING LAYOUTSADITING

Artisinal miners drive adits on seam at 20m intervals on dip on the mountainside using auger drills (Figure 7). The ore and waste swell is trammed to the mountainside for lowering using rudimentary aerial ropeways and dumping respectively.

ADITS ON A MOUNTAINSIDE Fig.7

SHAFT ON SEAM (PILOT WINZE)

Incline shafts are sited on the outcrop at intervals of 500m and sunk on dip carrying the seam ±1m from the footwall.

At 20m intervals seam drives are developed and a tramming loop mined in the hanging wall provides short passes for storage of ore and waste.

These shafts (Figure 8) generally produce (±630t) from six stopes and carry two stopes as spare. They are equipped with 70 hp hoists and mining progresses on dip to a maximum depth of 500m before the shaft is re-sited. There are 16 such shafts operating in Mutorashanga, three in Ngezi and one in Lalapanzi. Shaft output in each area is based on the parameters in the table below.

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Seam Thickness

Stopes % Rec Chrome

Seam Cont. SG Monthly Tonnage

N/Dyke 0.12m 6 0.8 77% 3.6 632

M/Dyke 0.26m 6 0.8 90% 3.4 1512

S/Dyke 0.22m 6 0.8 90% 3.4 1280

PILOT WINZE Fig.8

Tramming in the drives is by hand, using 1.5t cocopans

The simple layout of an on-seam shaft is most common on the dyke because: -

Class 2 artisans can maintain the shaft.

The development off-reef is minimised, thus reducing negative exposure to blanks.

There is minimal mine planning and survey as day-to-day face advance direction is determined by seam behaviour.

INCLINED FOOTWALL SHAFTS

In cases where seam continuity has been good both on strike and on dip and plans concluded to increase shaft output to +2000 tonnes, a footwall shaft has been mined below the pilot winze. (Refer to Fig. 9 below)

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FOOTWALL SHAFT - 11 # LALAPANZI Fig. 9

The Pilot Winze now serves as the main return airway.

On every 5th level, tramming crosscuts are developed into the footwall of the seam at 500m intervals along strike. From there ore and waste passes are developed into the reef horizon.

The Footwall Shaft is mined on grade at initially ±15m below the pilot winze. The vertical distance between the two shafts increases with depth as the Footwall Shaft is mined on grade and the Pilot Winze follows the reef horizon whose dip flattens with depth.

The advantages of this layout are that: -

Activities on the reef horizon are separated from those in the shaft system

Hoisting for four sublevels is done from one main hoist station.

The disadvantage being: -

A high development rate per tonne ore

This exploitation method has been used extensively at 11 Shaft in Lalapanzi.

PREVIOUS MECHANISATION ATTEMPTSSeveral attempts at mechanisation using mainly coal-based technology have been made.

1960’s Vanad mine Coal Cutter Trials

1987 Joy Coal Cutter Studies

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1990 – 1994 Roadheader ET110 plus Joy 14CM5 Continuous Miner

1994 – 1996 Vacuum Cleaning of stopes

1994 – 1999 Trackless mining - Skidsteer trials at Darwendale & Ngezi

2006 Stope productivity improvement.

Trials were made at Vanad Mine (AAC – Zimbabwe) in the 60s to win chromite using a coal cutter. In 1987, JOY concluded a study on using a 10RU universal coal cutter that “would eliminate ‘stopes’ and therefore drastically reduce the moving of large quantities of rock..” Tests indicated that cutter wear in chromite was going to be 18 times higher than in coal. The project was not implemented.

Between 1990 and 1994 a project promoted by the Government of Zimbabwe introduced an ET 110 Roadheader for mining declines and a JOY 14CM5 continuous miner for stoping. The project was called off in April 1994 after 1342 meters of heading had been developed. Not much stoping was done. High maintenance costs, low machine availability, ore dilution on the stope cutting and inability of the machines to manage the steep dips negated against this project.

Trackless mining was introduced between 1994 and 1999 in Darwendale and Ngezi on declines mined at an apparent dip of 10 degrees). Uniloaders/Skidsteers were introduced for underground mining. They failed because of the steep dips, an abrasive ore which resulted in excessive tyre wear and a poorly prepared environment for trackless mining. .

A study in conjunction with AEL Zimbabwe was conducted in 2006 to increase average stope advance per month within current mining practice from 15 metres to 20 metres.

Three major constraints were identified in this study:

Stope lashing was taking +10 hours to complete and therefore a daily blast was not possible. Because holes were marked at 80º to the face inclined towards the bottom drive, waste tended to heave to the bottom of the stope requiring a lot of effort to use it to build the stope pack on the top part of the stope. Furthermore, the muck pile had little throw resulting in the bulk of the broken rock accumulating at the face to be cleaned rather than the back area.

+150mm rock in the muck pile was less than 50% of the broken rock. Miners were thus forced to use smaller rock sizes for building the waste pack, which increased numbers of rock handled, and therefore the time to complete the task.

Swell waste from the stopes was being lashed onto the footwall of the drive below before being re-lashed into a cocopan.

These constraints were addressed by:

Increasing the stope lashing crew from 3 to 4, changing the direction of blast holes by 320º so that the direction of muck throw was upwards in the stope, increasing the burden and spacing of the holes from 0.4 metres and 0.75 metres to 0.5 metres and 0.8 metres respectively and carrying the footwall of the reef drive 1.2 metres below the seam horizon to facilitate direct loading into cocopans from the stope.

This initiative reduced stope-lashing time to 7 hours, allowed a daily blast and resulted in stope advance increasing from 15 metres/month to 19.8 metres/month.

FUTURE STOPINGWhy does one need to change the current stoping method?

60 % of the stopes available in Mutorashanga are manned. Fewer people are prepared to do this backbreaking work. This is an important consideration in future stoping methods.

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The approach to stoping has been based on the assumption that waste packed in the back area plays an important role in roof support. Geotechnical core logging has shown that ground competence improves with depth. The upper portions (0-30 metres depth) have a rock classification ranging from 4A-3B. Fracture frequency is higher than 10 and the rock is weathered. Below 30 metres, the rock classification is 2B or better with fracture frequency per metre falling below 3 and the rock is not weathered. Headings mined for the continuous miner, 3.2 metres width are still standing after 15 years. All this evidence supports the current thinking that the packed waste does not provide active support and that if all waste is hoisted out, timber and mechanical prop support will hold the roof.

Trials are now in progress to blast the seam in the centre of the panel in classical narrow reef breast stoping at a stoping height of one metre. With no separation of reef and waste in the stopes, all material is scraped into a box hole and hoisted to surface for segregation in a DMS plant. A history of stope movement and costing will be built before roll out.

RESUE vs SCRAPER MINING Fig.10

FOOTWALL DRIVE FOR SCRAPER MINING Fig. 11

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Figure 11 and figure 14 depict the primary development layout alternatives under cosideration. The layout as in figure 11 has been used at Hartley in Zimbabwe and is widely applied in the South African platinum industry. Figure 14 is the traditional layout for track mining on the Dyke.

FUTURE LAYOUTSLayouts under design are being considered to take cognisance of future mining as follows: -

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1. 2.5 times higher capacity shafts will be required for the same ore tonnage when all the waste is hoisted to surface.

2. Where possible, shafts will be designed to cater for more than one seam. 3. Vertical shafts will be required to exploit the deeper sections of the dyke where the bulk of the

resource is. The syncline of these deposits varies from 100 metres in Lalapanzi to 1400 metres in Mutorashanga. Mining depths in the same areas based on exploiting 500 metres on dip

A case study is presented which considers these options on a specific area in Mutorashanga.

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CASE STUDY 3-4 MILE SECTION AT ARTHUR’S LUCK

Description of Area and Resource Estimate: -

The 3 - 4 Mile area is situated about 5km south of Mutorashanga. Seam numbers 5 to 9 are present within Zimasco’s claims. Over a strike length of 3km and to a maximum depth of about 1.1 km the mineable resource is estimated to be at least 2.3 million tonnes after leaving an 80m surface pillar, which also takes into account any previous Artisinal mining.

Resource estimates are based on the following assumptions: -

Seam width 0.1m; Geological Factor 0.9; Recovery Factor 0.85 and SG of 3.5

The resource estimated for the individual seams are as follows:

Seam No. Strike Down Dip Tonnage

No.5 3000 175 140000

No.6 3000 450 360000

No.7 3000 550 440000

No.8 3000 750 600000

No.9 3000 975 780000

TOTAL PRODUCT 2320000

Three mining options are presented for the case study, these being: -

Option 1 – Incline Shaft

Option 2 – Vertical Shaft

Option 3 – Decline Ramps

At a mining rate of 1500t per seam, mining 4 seams, the life of mine is estimated to be 32 years.

The long sections for each of these options are shown in Figures 14 to 16 below.

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OPTION 1

INCLINE SHAFT ACROSS SEAMS Fig. 12

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OPTION 2

VERTICAL SHAFT ACROSS SEAMS Fig. 13

3D VIEW OF INTER-LEVEL CONNECTION Fig. 14

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OPTION 3

DECLINE RAMPS Fig. 15

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CONCLUSIONZimasco’s podiform deposits will be mined out by 2015 and by 2020 the bulk of production will be coming from dyke underground mining. The number of stopes will increase from the current 59 to 132 stopes (697 000ca).

Stoping in Mutorashanga is going to be dominated by scraper mining in the short to medium term, the limiting factor on introducing low profile mechanised mining being the dip of the seams and the depth of the syncline. Infrastructure will change to multiple seam serving decline or vertical shafts.

Trackless mining will be easier to introduce in the Ngezi and Lalapanzi areas with declines sunk at a suitable apparent dip. Having two seams approximately 60 metres apart, this is a good area to put a decline in between the seams.

The rate at which infrastructural upgrade and execution of these mining designs is going to be implemented will depend on a good ferrochrome price outlook and a favourable country environment.

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ACKNOWLEDGEMENTS

The author would like to thank Zimasco (Private) Limited for permission to present this paper. The author is grateful to George Blaver for providing 3D drawings of Lalapanzi and to various Zimasco personnel, who helped in providing data, critiqued the ideas presented and helped assemble the paper.

REFERENCES

1. Butcher, D. W Dyke Mechanisation ProgrammeUnpublished, 1991

2. Ministry of Mines Chromium MiningPublication No. 3Government of Zimbabwe

3. Moore, D and Relvas, L Investigation Into The Possible Use Of A Joy Universal Coal Cutter In A Zimbabwe Chromite DepositOctober 1997

4. Szwedzicki, T and Bull, G A Report on Geotechnical Core Logging and Rock Strength Testing

5. Takundwa, G and Mtemeri, M Underground Seam Productivity ImprovementUnpublished, 2006

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