Mining Methods

Mining Methods

Presented by:
David Wortman,
P.Eng.,
Retired Mining Engineer

Notes by:
J.V.Tully,
Senior Mining Engineer,
Fluor Daniel Wright Engineers Ltd.

INTRODUCTION

The decision to place a mineral property into production is taken after the property has passed through several phases of exploration and development. It is during these phases of development that geologists determine the three dimensional shape of the deposit and calculate its contained tonnes and grade. This data, together with other pertinent data that is required to determine how the deposit will be mined, is collected and utilized when a detailed feasibility study is undertaken.

During the feasibility stage, many factors are considered when determining whether or not to place a property into production. Some of the more important considerations include:

• Recoverable tonnes and grade
• Mining method
• Metallurgical characteristics
• Capital and operating costs
• Present and future metal prices
• Environmental concerns
• Financial risk, sensitivity to costs,grade etc.

The purpose of this paper is to discuss the mining portion of the feasibility process by describing the various ways in which underground mines can be developed, and some of the most common mining methods that are used in Canadian mines today. The conditions under which such methods may apply and the relative cost and productivity differences will be illustrated.

Much of the material in this presentation has been excerpted from the SME Mining Engineering Handbook (1973), and the SME Underground Mining Methods Handbook (1982).

Fluor Daniel Wright kindly covered the cost of my time to prepare the text, and typed the manuscript. The manuscript benefited from review by John Nilsson, Ken Midan of Fluor Daniel Wright, and Nancy Reardon of the B.C and Yukon Chamber of Mines.

Mining Methods

I-1. UNDERGROUND MINE ACCESS AND DEVELOPMENT

In determining how to best access a mine, the following factors have to be taken into account:

- Surface topography
- Depth to the top and bottom of the ore zone
- Plunge and dip of the deposit
- Ground conditions surrounding the ore zone
- Planned present and future production requirements
- Planned method of mining and stope development
- Planned equipment fleet and ventilation requirements

The primary access to an underground mine is by either vertical or inclined shaft, adit, or ramp, as illustrated in FIGURES 1 through 3. The size of these openings is variable, and is dependent on such factors as daily production requirements, equipment sizing, and ventilation considerations.

In areas with very little relief or limited rock outcrop a shaft is sometimes the only cost effective method of accessing a mine. (FIGURE 1). Shafts are appropriate for deposits that are steeply dipping and that extend to considerable depth. The advantages of a shaft in this situation is that the amount of lateral development from the shaft to reach the ore on any one level is relatively small. This will result in decreased level development costs and shorter ore haulage costs to tram the ore to the ore pass and loading pocket. Another advantage is that it is probably the fastest way to access the ore zone and get the mine into production.

The disadvantages of shafts, apart from their high costs for sinking and capital outlays, are that they are bottlenecks as far as transferring equipment between levels is concerned. This could be important if the mining fleet consists of large equipment and the mining method requires that this equipment be frequently moved between levels. Another disadvantage is that shafts are usually designed for a certain production capacity, which cannot readily be increased substantially without serious capital additions and loss of production.

The cost of sinking and equipping a production shaft, including the capital for the headframe and hoist etc., depends on a number of factors which include: the size of the shaft, the size of the hoist,the amount of ground support required, whether the shaft is concrete lined or open, the amount of water encountered, and the ultimate depth. Total costs can range anywhere from $8,000 to $19,000 per meter.

In mountainous terrain, an adit is often a very cost effective means of accessing an orebody, as illustrated in FIGURE 2. In this case, it is important to be able to access the proposed portal sites for the adits with relatively short, inexpensive external roadways for the transportation of personnel, equipment and materials. Another factor, is is to insure that the cost of constructing the portals is not excessive. It is also important that the dip of the orebody can be accommodated by successivly lower adits in order to minimize the development cost.

The advantages of an adit access are that adits are relatively cheap to drive ($1,300 to $1,600 per meter for a 4.0 x 4.5 meter sectional heading), and that when connected by short raises, they can be effectively ventilated. Adits face the same disadvantages as levels off shafts in that equipment cannot be readily transferred from one level to the next without first exiting the mine.

The third method of mine access development is by a ramp system, as illustrated in FIGURE 3. The development of high production, rubber tired, multiple boom drill jumbos, versatile rock loading equipment and low profile heavy duty trucks has greatly advanced the productivity of ramp development. Ramps can be driven at grades ranging up to 20%, but are usually restricted to no more than 16% if they are to be used for truck transport of stope muck. The average cost of ramp development is in the order of $1,600 to $2,500 per meter, depending on ground conditions. If the ground conditions in the area where the ramp is being driven are poor, and require excessive bolting or other ground support systems, the overall cost of the ramp could increase considerably. The economic depth limit to which a single ramp access mine can extend is governed by haulage costs, equipment selection, productivities, and ventilation considerations.

The best access for a mine is thus very property specific and dependent on a number of factors. In reality, a large number of mines today are actually a combination of two or more of the methods described above. For example, it is not uncommon to install ramp systems between levels of a shaft accessed mine. Such an arrangement is usually employed in deep mines utilizing large mobile equipment fleets that move between several levels on a regular basis.

The relative costs of accessing a mine to a depth of 180 meters which is illustrated in Figures 1 through 3, is shown on TABLE 1. These costs show only the access costs, and do not reflect the most appropriate method. Factors such as mining methods, haulage costs, ventilation requirements,and equipment requirements are not considered. The cost of any one of these factors could result in the cheapest access method not to be considered, as it would lead to much higher costs elsewhere, or not be compatible with the mining method.

TABLE 1

Comparative costs of accessing a mine to a depth of 180 metres with main levels at 45 metre centers.

ACCESS	METHOD	$/Meter	Length	Total Cost
SHAFT	Sinking Plus Headframe	$12,000	250 m	$3,000,000
	Plus Hoist Shaft Stations Total of 4	$15,000 each		$60,000
		TOTAL Shaft Access:		$3,060,000
ADIT	Driving Adits (4)	$1,400	600 m	$840,000
	Portal Construction 4	$75,000 each		$300,000
	Access Road	3 Km at $25,000/km		$75,000
		TOTAL Adit Access:		$1,215,000
RAMP ACCESS	Driving 15% Ramp	$1,800	1,100 m	$1,980,000
	Portal 1 Only at	$75,000		$75,000
	Access Road	1 Km at $25,000		$25,000
		Total Ramp Access:		$2,080,000
NOTE: These costs are for comparative purposes only. Actual unit costs will vary greatly depending on ground conditions and a number of other factors.

II-1. UNDERGROUND MINING METHODS

The criteria that must be evaluated in the selection of an underground mining method are summarized below:

• Size and shape of the deposit
• Grade of the deposit and metal distribution within it
• Ground conditions in and around the deposit
• Nature of the contact between the ore and surrounding waste
• Depth of deposit below ground
• Production requirements

All of the above factors will have an economic impact on the deposit. The mining method ultimately chosen must be the one that affords the best trade off between cost options and insures that the deposit will be extracted in the safest, most efficient manner possible. The selected method must result in a production rate that is sustainable throughout the life of the mine, and yet be flexible enough to accommodate any changes such as the discovery of additional ore, or increased production rates.

We will begin our look at underground mining methods by starting with relatively small sized deposits which are selectively mined and progress to the large deposits where bulk mining methods are applicable.

II-1. SELECTIVE MINING METHODS

Selective mining methods are used to extract ore with a minimum amount dilution by waste rock. They typically apply to narrow vein, precious metal deposits characteristic of the Canadian Shield, and high- grade, Bonanza type deposits of the Cordillera and western USA. The mining, or stoping methods that we will review under this heading are Shrinkage, Cut and Fill, Room and Pillar, Resuing and Square Set methods.

II-1-A. Shrinkage Stoping

Shrinkage stoping is applicable to ore zones that are dipping at least 55 degrees and that range in width from roughly 1.2 to 4.5 meters. The enclosing waste rock must be competent and not subject to failure so that when the ore is drawn from the stope, dilution is kept to a minimum. Another major requirement is that the contact between wall rock and the ore zone be relatively sharp without any abrupt changes in either strike or dip along the stope interval. A typical shrinkage stope is illustrated in FIGURE 4.

The stope is commonly accessed by crosscuts driven into the ore body at regular intervals from a drift driven in the footwall, or from headings driven along the length of the ore. The stope is mined by drilling short holes (2 to 3 meters) along the length of the vein and blasting the ore down, or by a series of short horizontal holes, commonly called breasting. Access to the next lift is gained by standing on the broken ore, and repeating the process until the upper level is reached. During the mining phase, only enough muck is drawn out of the bottom of the stope to permit the miner to access the stope, and to drill off the next lift. Typically, during the mining stage of the stope, approximately 40% of the total broken muck is drawn off.

From FIGURE 4 it can be seen that a considerable amount of development work is required to prepare a shrinkage stope for production. In addition, the productivity of the method is not high during the mining cycle, since the bulk of the muck must remain in the stope until the stope is finished.

Productivities within shrinkage stopes are largely dependent on the width of the ore zone, and can vary from 15 to 30 tonnes per manshift. Mines using this method as a sole source of ore typically produce between 200 to 800 tonne per day, with mining costs varying from $28 to $35/tonne.

II-1-B. Cut and Fill Stoping

This type of mining method is used in ore zones where the strength of the wall rocks is such that the ground will not stand unsupported over long dip intervals. (FIGURE 5). The method is also used in ore zones where the hanging wall and footwall ore contacts are quite irregular, or have erratically mineralized lenses in the walls that only make ore grade on an intermittent basis. The method is also applicable for use in wide ore zones that would not stand unsupported if opened up over their full width. Wide ore zones can be recovered by mining across the width of wide zones. A pillar is left beside the just mined out stope, and a second stope is mined across the zone parallel to the first. When the cemented backfill in both stopes has cured, it is possible to extract the remaining pillar ore between the two stopes. This assures that 100% of the ore is recovered. With such a method, it is essential that the fill used in the initial stopes contains sufficient cement to ensure that they will not fail when the pillar between them is mined.

Fill material can consist of waste rock, sand and gravel, or, more commonly, mill tailings. The coarse fraction of the mill tailings are usually delivered to the stope from the mill in a mixture containing up to 25 -30% water which, after placed in the stope, drains off, leaving the tailings in place. It is common practice to add cement to the top portion of the fill in a stope in order to provide a solid roadbed for equipment in the stope, and to keep the fill from diluting the ore from the next cut. In transverse cut and fill stopes it is necessary to add cement to the total volume of fill, eliminating the need to leave pillars between the stopes.

Relatively narrow and short cut and fill stopes are mined in a similar manner to shrinkage stopes by using hand held machines and blasting the muck down in 2.3 to 3.0 meter lifts, filling the void and repeating the process. If the stope is relatively long, both mining and filling operations can occur simultaneously. FIGURE 5 represents a typical cut and fill stope that illustrates the method and shows the associated services required for access and ventilation. Broken ore is normally removed from the stope through a steel lined mill hole that is carried up through the fill each lift.

If the ore zone is in the order of 4.0 to 9.0 meters wide, productivity improvements can be gained by utilizing rubber-tired equipment rather than electric or pneumatic scrapers or slushers to transfer the broken ore to the mill holes. In addition, drilling operations can be conducted utilizing mobile drilling rigs rather than hand held machines. It is possible to modify the method from that shown in Figure 5 so that access can be gained to each stope lift through a drift driven off a ramp located in the footwall of the ore. The advantage of such an arrangement is that it eliminates the need to carry a mill hole through the fill and permits mobile equipment, which would be captive in the stope during its life, to be utilized elsewhere in the mine.

One of the advantages of cut and fill mining is that it permits the selective mining of irregular shaped ore lenses with a minimum of dilution. It is often possible to separate a waste section occurring within the ore and leave it behind in the stope for fill. Cut and fill mining has an advantage over shrinkage methods in that there is very little time lag in getting the broken ore to the mill once it is mined. For ore zones that are characterized by heavy or slaby ground, cut and fill mining is often the only choice available to the mine planner.

The disadvantage of this method is that stope productivity is often cyclical due to the need to delay mining operations while the stope is being filled. To overcome this, it is necessary to have a number of extra stopes developed to rotate the mining crews into while waiting for fill. In addition, fill material must be available upon demand. Any breakdown in the mill affecting the production of fill material or any major breakdown in the fill distribution system, will result in reduced production.

Productivities for cut and fill stopes can range from 15 to 30 tonnes per manshift for conventional stopes, and from 35 to 50 tonnes per manshift if the stope is mechanized. Operating mines utilizing cut and fill methods as their only mining method would typically produce at a rate between 500 to 1000 tonne per day. Mining costs range from $30 to $40/tonne and are quite variable from mine to mine, depending on conditions.

II-1-C. Room and Pillar Stoping

This type of mining is applicable to ore zones that are relatively flat lying, and which extend over a considerable area. Due to the large areal extent that such deposits cover, it is necessary to leave portions of the ore (pillars) in place during the mining sequence to prevent the mined out area from collapsing. Room and Pillar mining is illustrated in FIGURE 6.

In deposits that cover a very large area, and in which the thickness and grade of the deposit is relatively uniform, the pillars are laid out on a regular interval and have a uniform size, cross-sectional area and spacing. The distance between the pillars is dependent on the quality of the roof rock,the amount of stress within the enclosing rock, and the number of fault structures in the area. Under ideal conditions, spans between pillars of up to 30 metres or more are possible.

Deposits that are more irregular in shape and/or thickness are usually mined by a method known as "random room and pillar" mining. In such deposits the spacing and size of pillars is not determined in advance,but depends on such things as local ground conditions, ore thickness, or the grade of the deposit locally.

Room and pillar stopes are usually highly mechanized and thus very productive. Self propelled multiple boom drilling jumbos and large capacity trucks and loading equipment can be used with this type of stoping method as the access is good and there is ample room for movement. As a result, productivities can range up to 100 plus tonnes per manshift, but are commonly in the 35 to 70 tonnes per manshift range. Mines utilizing room and pillar mining methods as their sole means of mining typically range from 1,500 to 10,000 tonnes per day. Mining costs depend on ore thickness and the degree of mechanization and can vary from less than $12 to up to $20/tonne.

The advantage of room and pillar mining is that it is highly mechanized and thus very productive. It may be possible to leave lower grade zones within the ore body as pillars, and all the ore that is broken is immediately available for milling. The disadvantage of the method is that it has an overall recovery rate of the ore zones that rarely exceeds 80%, and is frequently in the 60% range.

II-1-D. Resuing

Resuing is a highly selective means of mining extremely narrow, high grade veins. The application of resuing is restricted to use in veins that can be as narrow as 15 centimetres or less. This type of mining is not commonly used in today's mines, as it has a very low productivity and thus is very expensive on a unit cost basis. The recent upsurge in the gold price, however, is causing operators to review this type of mining as a means of extracting certain very high grade deposits. The method is illustrated in FIGURE 7, which represents a cross-sectional and plan view of how resuing is carried out.

The method works best when the vein being mined has a clay slip or well developed shear plane on one or both contacts that permits the vein to be readily separated from the enclosing waste. In the example shown, the method involves mining the hangingwall portion to the minimum width possible to expose the vein, and then breaking down the vein. The waste material is left in the stope, as in shrinkage stoping, to provide access for the next lift. The vein material is picked up (commonly by hand) and dumped down mill holes that are usually located on 5 to 15 meter centers, and which are built up through the waste. If the contact between ore and waste is very weak, it is usually possible to bar the ore down in slabs without having to blast, thus assuring that little of the vein material is lost in the broken waste muck. If the ore has to be blasted, it is then necessary to hand sort the blasted vein material in the stope, which results in lower recoveries and higher costs. In some instances it may be feasible to lay down conveyor belting on top of the waste prior to blasting and to use a slusher to move the ore to the mill hole.

The advantage of resuing is that it is a dilution-free way of mining. There is very little waste included within the ore,thus the grade is usually very high. Since so little waste is included, processing costs are lower. The main disadvantages of the method are the low productivities and associated high costs, which can exceed $150/tonne. Productivities commonly range from less than 0.5 to 1.0 tonnes per manshift, and mines using this method as a sole mining source usually produce in the less than 50 to 100 tonnes per day range.

II-1-E. Square Set Stoping

As a principal mining method, square set stoping is generally used only in ground that is so weak that it must be totally supported, and where the shape of the deposit is such that any other mining method would result in unacceptable dilution of the ore. The method is also applicable for recovering high-grade pillars between cut and fill stopes and for pillars above and below access drifts.

In square set stoping, small blocks of ore, approximately 2 by 2 by 2 meters in size (approximately 20 tonnes) are blasted and extracted, and the opening thus created is timbered before the next set or section is taken. This method is illustrated in FIGURE 8. A set of timber consists of a vertical post and two horizontal members mutually at right angles that are cut to fit interlocking ends of adjoining timbers. (Figure 8). Successive sets are installed to form a complete cellular timber support structure. Adjoining timber sets are framed to support the ground and form a continuous structure of horizontal floors composed of rectangular frames supported at their corners by posts. The ore is generally removed on the upper floor and dropped to the lower floor where it is scraped into a chute. Stope floors are usually filled with either waste rock or hydraulic tailings once mining activity on the floor is complete.

The advantage of the method is that it permits the removal of high grade ore with minimal dilution in areas of bad ground where all other mining methods would result in excessive dilution. The method is not widely used today, as it is labour intensive, very expensive, and requires specially trained crews. Cut and fill mining methods can frequently be used in todays mines instead of square set mining.

Productivities in square set stopes depend on a number of factors, but generally range in the 10 to 20 tonnes per manshift. Mines using square set mining methods as their only mining method commonly produce in the 500 to 1000 tonne per day range. Mining costs are variable, but are commonly in the $75 to $85/tonne range.

II-2. BULK MINING METHODS

Bulk mining methods are mining schemes that are designed to extract large tonnages of ore at minimal costs. They apply to large ore zones that are either quite low grade or cannot be extracted at a profit using the mining methods described above. The increased mining dilution that results from bulk mining methods is offset by the reduction in operating costs and increased productivity. The mining methods that will be reviewed under this section are: Sub-level Open Stoping, Vertical Crater Retreat Stoping, Sub level Caving and Block Caving methods.

II-2-A. Sub-Level Open Stoping

This type of mining method applies to large ore zones that are characterized by relatively regular ore- waste contacts and good ground conditions. The method is often referred to as Blast Hole Stoping or Longhole Stoping. Sub-level stoping methods utilizing a ring pattern of drilling is shown on FIGURE 9. The ore block is readied for mining by driving sub levels in the ore at intervals ranging from 12 to 18 meters. From these subs, nominal 5 cm diameter blast holes are drilled in a ring pattern to the ore limits. Mining usually commences at one end of the stope from a slot raise and continues along the stope and from one sub to the next until the stope is completed. Broken muck is drawn from the stope through draw points located in the footwall of the stope. Access to the sub levels can be either from a manway raise located at the far end of the stope, or from a ramp system located in the footwall of the ore zone.

A variation on the method, in which holes are drilled between subs in parallel rows rather than in fans, is shown in FIGURE 10. By using such a drilling pattern, it is possible to increase the distance between sub levels to as much as 40 meters,and to increase the diameter of the drill holes (up to 25 cm). Such an arrangement results in reduced development costs and usually reduced explosive costs. Recent improvements in long hole drilling equipment makes it possible to locate sub levels on 35 to 40 meter centers. With this new equipment it is possible to drill long, straight holes on a routine basis without hole deviation that can cause serious problems in the blasting of the ore.

Since sub-level stoping creates large openings that remain unfilled without support during the life of the stope, it is essential that the ground conditions are good so that mining dilution is minimal. The advantage of the method is that it is highly productive and has a relatively low cost. It results in a mining recovery rate that is usually better than 90%, and produces acceptable dilution rates. The disadvantage of the method is that it requires a considerable amount of development work and time to prepare a stope for production. Fortunately most of the development that is required is within the ore zone. Once mining is completed it is usually necessary to backfill the stope to maintain the overall stability of the area.

Productivities in sub-level stopes vary with the stope width and sub-level interval and range between 50 to 100 tonnes per manshift. Mines producing from sub-level stopes as a sole source of production generally operate at from 1500 to plus 5000 tonnes per day. Mining costs are very dependent on ore width and a number of other factors and commonly range from $15 to $25/tonne.

II-2-B. Vertical Crater Retreat Stoping

This mining method is similar in principle to sub- level stoping in that the development of the stope and the method in which the ore is drawn from the stope are similar. The main difference is that in vertical crater retreat methods (commonly referred to as VCR mining), the blast holes are detonated from the bottom,in several stages rather than in one shot as in sub-level methods.

Under ideal circumstances, the sub levels from which the blast holes are drilled can be spaced up to 60 plus meters apart as compared to 20 to 40 meters for sub-level methods. In addition , the size of the blast holes can range up to 16 cm in diameter. The development of the stope, which is illustrated in FIGURE 11 is essentially the same as in sub-level stoping. The basic difference is that only the bottom portions of the holes are loaded and fired at any one time. The stope is brought up blast by blast, with the back of the stope kept level all the way. Explosives are placed in the bottom portions of the holes each lift by loading them from the top sub level.

The main advantage of VCR over sub-level stoping is one of costs. VCR results in an appreciable saving in development work as fewer sub levels are required, and there is an overall reduction in the consumption of powder for secondary blasting of oversized material in the draw points as the method produces better rock fragmentation. The main disadvantage of the method is that as the stope nears the top of the sub level, ground conditions often deteriorate due to the successive blasting from each lift. This frequently results in premature closure of the stope without full extraction of the ore.

Productivities from VCR stopes vary with the width and size of each blast, and range from 50 to 100 tonnes per manshift in most instances. Production from mines using VCR mining methods as a sole source of production ranges from 1500 to 3000 tonnes per day. Mining costs are a function of width, hole size, and a number of factors that are site specific. They commonly range from less than $10 to $20/tonne.

II-2-C. Sub Level Caving

Sub level caving methods are best suited to steeply inclined, medium width ore bodies enclosed by relatively weak ground. The tendency for the ground to cave when only a small portion of it is opened up is essential for this method to be effective. The capping rock must cave and follow the ore down during the mucking process so that serious weight problems on the enclosing strata can be avoided and dangerous collapse air blasts do not occur. The method is commonly used in near surface ore deposits when mining progresses underground from the bottom of an open pit as illustrated in FIGURE 12.

The method consists of driving a series of sub-levels commencing at the top of the orebody. A starting vertical slot is cut at one end of the zone to be mined and a series of holes drilled in a ring pattern are blasted into this opening. The swell muck from the blast is drawn off after each blast. The pattern of drilling, blasting and mucking is repeated as the stope retreats. The muck left in the stope on the upper sub-levels is recovered on the lower levels as mining progresses to depth. Some dilution of the ore is inevitable with this method from both wall rock dilution and from pulling the caved muck. Careful draw control is extremely important with this type of mining so that the mixing of caved waste with the ore and loss of ore encapsulated in waste is minimized.

The advantage of the method is that it is a high-production, low cost means of mining relatively weak ore zones in poor ground, with no loss of ore in pillars. In addition, the method can be highly mechanized. The major disadvantage of the method is that it results in some ore loss (10 to 15%) and can result in serious problems if the caving extends well into the wall rocks and jeopardizes access ramps etc.

Productivities from sub-level caving stopes vary considerably but are in the range of 100 to 150 tonnes per manshift. Mines using this type of mining as a sole source of production typically produce in the 5,000 to 15,000 tonne per day range. Mining costs are variable and usually range between $18 to $25/tonne.

II-2-D. Block Caving

Block caving methods result in the lowest cost per tonne, highest production rate operations of all underground mining methods. As with sub-level caving operations the ground must be relatively weak so as to permit caving and the orebody must have sufficient horizontal area to cave freely without excessive dilution by the enclosing waste rock. The method is not selective in that it is not economically feasible to separate ore from waste in the draw points and thus the requirement for uniformity of grade. The method is illustrated in FIGURE 13 and FIGURE 14.

Block caving operations are initiated by first undercutting the area to be caved and establishing a series of drawpoints through which the caved ore is drawn off. The method of creating the initial cave area depends on a number of factors, but generally involves opening up sufficient area at the bottom of the ore so that the overlying mass becomes unstable and caves. The initial openings are usually accomplished by "belling" out the ore, as shown on FIGURE 13, from a series of short raises driven up from closely spaced parallel drifts driven below the ore zone. Once the caving begins, broken muck is continuously drawn off as the caving progress upwards. If rock is drawn off faster than the ore is caving, a void will be produced that could result in a very dangerous situation. Under such circumstances large blocks of ore may fall out of the back, resulting in destructive air blasts. The control of the "caving line" is thus very critical to the successful operation of a block caving operation. This is a difficult task, as the actual level of the broken muck in a stope can never be actually seen.

The main advantages of block caving are its very low cost per tonne and very high production rates. The main disadvantages are the high development cost required to prepare the cave block for production, and production problems associated with improper control of the cave line. Another problem with the method is maintaining the draw points through which the broken muck is drawn. In some caving operations it is necessary to continually reinforce or replace these openings, which can greatly affect costs and productivity.

Productivity in block caving operations ranges from 150 to 350 tonnes per manshift and daily production from block caving operations can range from 30,000 to over 60,000 tonnes per day. Mining costs are in the $4.50 to $12/tonne range.

PRODUCTIVITY AND COST SUMMARY

In most producing mines in Canada today, ore is produced by a combination of several methods, rather than one method alone. The resulting overall cost of production at any one mine is thus dependent on not only the individual direct costs of the mining method, but to a significant degree, on how well the methods "mesh"in terms of sharing equipment, supervisors and common development headings.

The following Tables are intended to show the "average" productivity and costs associated with the various mining methods discussed above. There are many mines in Canada that operate either well below or above these cost ranges and whose productivities are either better or worse than those shown.

I strongly recommend that you do not attempt to determine the potential viability of any mining property on the basis of the numbers presented here. There are many additional costs that go into the total operating costs that are not included. These costs are very site specific and would be meaningless if generalized or averaged. Such cost items include ventilation, water pumping, development headings required to ready a tonne of ore for mining, and supervision.

The costs presented also assume average ground conditions, good access and infrastructure and no adverse climatic or geological conditions.

TABLE A is a summary of the average productivity and mining cost associated with various mining methods and illustrates the average daily production from mines using such methods as their major source of stope muck.

TABLE A is a summary of the average productivity and mining cost associated with various mining methods and illustrates the average daily production from mines using such methods as their major source of stope muck.

TABLE A: SUMMARY - UNDERGROUND MINING METHODS

Method	Tonnes / Manshift	Avg. Tonnes/Day Milled	Avg. Mining Cost/Tonne
Resuing	0.20 - 0.50	50 - 100 +	$150
Square Set	10 - 25	500 - 800	$75 - $80
Shrinkage	20 - 28	200 - 800	$28 - $40
Cut and fill	12 - 48	500 - 1200	$30 - $45
Room and Pillar	15 - 150	1500 - 8000	$12 - $20
Open stoping	20 - 115	1500 - 5000	$15 - $35
VCR Stoping	50 - 175	1500 - 3000	$10 - $25
Sub-Level Caving	65 - 180	1500 - 4000	$15 - $35
Block Caving	300 - 500	10000 - 60000	$8 - $12

OPEN PIT MINING METHODS

Open pit mining methods are ideal for extraction of near surface ore bodies having substantial horizontal dimensions and little or no overburden cover. The method is flexible, allowing for large fluctuations in production schedules at relatively short notice, and can be highly mechanized. As such it is the most productive of all the mining methods. Given favourable stripping ratios and climatic conditions open pit mining produces ore at a fraction of the cost of underground methods. The method requires relatively few men who can be readily trained and supervised, and has a lower accident frequency rate than underground operations.

With open pit mining methods, grade control can be easily accomplished by separating lean or waste sections from the ore and mining them separately. This is accomplished by assaying the cuttings from the blast holes drilled on each bench and determining which portions of the bench make ore.

Open pit mines are developed by excavating rock along a series of regularly spaced horizontal lifts or benches. Access roads and ramps connect the benches which allow haulage trucks to remove materials from the pit as it is deepened. Mining of the pit requires careful planning so that sufficient overburden and waste are stripped from the area to permit mining to proceed at an even pace. A typical open pit is illustrated on FIGURE 15.

I-1. OPEN PIT MINING - PRODUCTION AND COST SUMMARY

Just as in underground mining, operating costs for open pits are variable from one mine to the next and are dependent largely on site specific factors. Mining costs for open pit mines are commonly broken down into the unit activities of drilling, blasting, loading, hauling and general. The last category includes pumping, maintenance and labour supervision. Each of these costs can be shown as a percentage of the total mining cost, but again this is a generalization and individual mines can show considerable variations to the percentages presented here. Factors affecting these ratios include rock hardness, length of haulage for ore and waste, equipment sizing and compatibility,and type of blasting agents required. In most cases, haulage cost is the largest factor in the total mining cost.

The average operating costs for some typical mines at various daily operating rates is shown on TABLE B. The relative average percentage of the total operating cost that each component represents is illustrated under each heading. The size of operations range from a relatively small operation producing 1,500 tonnes of ore per day to a very large operation that produces in the 90,000 tonne per day range. The stripping ratio (S.R.) has been kept constant at 2:1 (waste to ore) in the cases shown to illustrate the comparative costs. Stripping ratios can vary substantially from less than 1:1 to 10 or 20:1 in some instances. The amount of waste stripping which can economically be carried out depends on ore grade and can form the basis for a decision regarding mining the deposit by open pit or underground methods.

TABLE B: TYPICAL OPEN PIT MINING COSTS 

T.P.D.	S.R.	Drilling	Blasting	Loading	Hauling	General	TOTAL
ORE	W/O	8%	10%	17%	43%	22%
1,500	2:1	0.67	0.83	1.41	3.56	1.81	8.28
2,500	2:1	0.56	0.70	1.19	2.99	1.54	6.98
5,000	2:1	0.44	0.56	0.94	2.39	1.22	5.55
25,000	2:1	0.27	0.35	0.58	1.47	0.75	3.42
50,000	2:1	0.21	0.28	0.47	1.19	0.60	2.75
90,000	2:1	0.19	0.24	0.40	1.03	0.54	2.40

MINING TERMS - DEFINITIONS

Adit - A horizontal or nearly horizontal opening into a mine for access, haulage or ventilation. Has only one portal as opposed to a tunnel which has a portal at each end.

Anfo - Ammonium nitrate and fuel oil blasting agent. Usually free pouring and in prill form. Has replaced dynamite in many applications, as it is safer and cheaper.

Back - The underground mining term for the roof or overhead portion of any underground opening.

Capital Cost - Initial - Total investment prior to production.
Preproduction - May mean same as Initial or may only include mine development costs .
Replacement - The cost of periodic replacement of mine equipment.
Deferred - The cost of future additional equipment or mine development, e,g, lengthening a conveyor system or deepening a shaft to accommodate advance in a mine.

Crosscut - Any horizontal opening made at right angles or nearly right angles to the strike of the orebody or to the main direction of advance, as opposed to drifts, which are driven along strike.

Cutoff Grade - Lowest grade of ore shipped to the concentrator, lower grades to waste dumps or leaching pads. In calculating ore reserves, several cutoff grades may be used to evaluate the effect on reserve tonnage, mine life and economics.

Decline (Incline) - Any sloped opening interconnecting levels and/or the surface used to transport men, materials, ore/waste. The term(s) are usually applied to headings advanced by utilizing mobile equipment and are sloped at between approximately 5 to 20% grades. Declines are advanced downwards and inclines are advanced upward.

Drift - In underground mining, a horizontal passageway driven along the strike or long axis of the orebody.

Feasibility Study - Studies conducted to estimate the profitability of proposed mining operations. Usually includes ore reserves estimate, mine design, equipment selection, cost estimates; and may include processing, infrastructure, and financial calculations. May range from completely factored estimates to +/- 10% accuracy.

Hangingwall - In an inclined orebody, the upper wall rock, especially in dipping deposits.

Jumbo - Articulated, mobile drilling machine used to advance headings. Can contain from one to three or more drills that are controlled by one operator. Jumbos can be electric or diesel powered, and are usually mounted on rubber tire carriers, which make them very mobile.

Level -- Underground mine workings at given elevation above sea level or distance below the shaft collar. Levels are often spaced at constant intervals, compatible with the mining scheme. The term may also include a bench in a pit.

Muck - The product that results when solid rock is blasted (broken rock). Either ore or waste.

Ore Pass - A vertical or inclined opening, through which ore is transported to a lower elevation or haulage level.

Productivity - Output of an item of equipment or suite of equipment for a given time period, expressed in weight or volume per hour, day etc. Also applies to output per man, calculated by dividing the total tons output by total men in personnel group being considered.

Raise - Vertical or inclined opening advanced upward from a level, subsequently used as a manway, ore pass, or for ventilation.

Shaft - Vertical or inclined opening that starts at surface or underground. It is used to convey men and/or materials into the mine and/or hoist ore (waste) and /or men from the mine and/or conduct ventilation air into or from the mine.

Stope - Production of ore from stopes (chambers) underground, in contrast to development or service functions.

Waste - Non-ore material mined.