Table of Contents
- Copper Geology
- Genesis of Porphyry Ore
- Underground Mining-Auxiliary to Steam-Shovel Operations
- Shrinkage Stoping System Adopted
- Orebody Worked on Three Levels
- Main or Motor-Haulage Level
- Order of Working Stopes
- Hand Tramming on Upper Levels
- General Costs
- Hammer and Piston Drills Used
- Water Supply
- Data on Motor Haulage
- Gravity Tram from Mine Portal to Railroad Bins
- Safety of Mining System
The system of stoping as practiced was eminently satisfactory to supplement the steam-shovel operations without injuring the ore reserves of the property through a mixing of capping with ore. It had the advantage that all ore produced was absolutely free from waste, since both stopes and development drifts were discontinued when capping was reached. The assay value of the ore produced could be regulated, and the tonnage materially increased or decreased without affecting to any extent the cost per ton. The mining property of the Utah Copper Co. is situated in the West Mountain mining district, Salt Lake County, Utah, in the Oquirrh Range of mountains.
In a general way the rock formation of the district consists of a series of beds of quartzite and limestone intruded by a body of monzonite porphyry roughly elliptical in shape, with an east-west axis over a mile in length and a north-south axis of about 3,000 ft.
This porphyry intrusion, accompanied by strong mineralizing action and fracturing, resulted in the formation of orebodies in the adjacent sedimentary rocks, and was itself sufficiently mineralized to make it one vast orebody. The Utah Copper Co.’s mining property comprises within its boundaries practically the entire outcrop of the monzonite mass, of a commercial grade.
Genesis of Porphyry Ore
Following an intricate system of fracturing, mineral solutions circulated freely through the porphyry, depositing small quantities of copper and iron, and resulting in a considerable silicification of the monzonite. The quantity of copper originally deposited was undoubtedly too small to have ever given the porphyry a commercial value, had not secondary enrichment, due to the leaching of the copper from the surface of the mass and re-deposition in the sulphide zone, been active over a long period of years.
A large portion of the leached material has probably been removed by erosion, but there still remains a blanket of leached and oxidized porphyry of varying thickness covering the sulphide ore, known as capping. Over certain sections of the orebody, this capping contains commercial quantities of copper carbonates, but most of it contains little or no copper.
To Jan. 1, 1915, a total of 377,690,000 tons of ore had been developed, of which 342,500,000 tons averaging 1.45 per cent, copper still remained to be mined. . The average thickness of the developed ore was 465 ft., while the layer of capping covering the ore averaged 115 ft. Further development will undoubtedly show an increase in the average thickness of the ore, with a corresponding increase in the tonnage of developed ore.
Underground Mining-Auxiliary to Steam-Shovel Operations
As is generally known the Utah Copper mine is primarily a steam-shovel operation, and it will perhaps surprise many that up to April, 1914, a considerable tonnage of ore was obtained by underground mining methods.
During the early years of steam-shovel mining the amount of ore available was naturally limited, since most of the shovels were working in capping, and it was necessary to stope a large tonnage underground in order to keep the mills at Garfield running at capacity.
During the 3-year period from 1911 to 1913 inclusive, a total of 102,719 ft. of drifts, raises, etc., was driven on the property. Most of this development served the double purpose of proving the shape and value of the orebody, and providing the necessary openings for stoping operations. The output of ore from underground operations in these 3 years amounted to 3,071,719 dry tons, of which 247,280 tons came from development and the rest from stopes.
Shrinkage Stoping System Adopted
Realizing that underground mining was to be but an incident in the mining of the orebody as a whole, a system of stoping was adopted which would not affect adversely future steam-shovel operations. In order to fulfill this requirement it was essential that the surface should not be caved, that no large openings be left unfilled, and that the capping should not be mixed with the ore.
The system as finally adopted and successfully operated, consisted in starting stopes on three separate levels or tunnels. The first of these tunnels, at an elevation of 6,733 ft. driven 7 by 7.5 ft. in the clear, was the main or motor-haulage level. The second, at an elevation of 6,883 ft. or 150 ft. vertically above the main level, was equipped for hand tramming only, so all drifts, crosscuts, etc., were driven 5.5 by 6.5 ft. in the clear. The third, at an elevation of 6,983 ft., or 100 ft. vertically above the second, was also a hand-tramming level and driven 5.5 by 6.5 ft. in the clear. These three levels were connected by many manways and raises for dropping the ore from the upper tunnels to the motor-haulage level. An underground shaft centrally located, equipped with a com-
prcssed-air hoist, was used to hoist supplies from the motor-haulage level to the upper levels, but no ore was handled through it. Each level had one or more surface connections, affording good natural ventilation to all parts of the workings (Figs. 1 and 2).
Orebody Worked on Three Levels
By means of these three levels, the orebody was divided into blocks for stoping, the central block, 500 ft. wide (see Fig. 1), was bounded on the main level by two parallel motor drifts, and on the upper levels by two main parallel tramming drifts, directly over those on the haulage level. At intervals of 120 ft. along these motor drifts, raises 5 by 6 ft. were put up on a 55° pitch to the level above and afterward extended to the third level.
In order to make the stopes as safe as possible, to minimize the amount of timber required, and to leave substantial walls for the safety of future steam-shovel operations, it was decided that the standard width of stope should be 16 ft., with 44-ft. pillars between stopes.
The method of starting stopes on the motor-haulage level was somewhat different than on the upper levels although the size of stopes and pillars was the same.
Main or Motor-Haulage Level
Motor drifts were driven on the motor-haulage level, spaced at 50-ft.; centers, and parallel to the main drifts forming the boundaries of the block. At intervals of 60 ft. along these drifts the surveyor marked the center of the stopes as the drifts were driven; if the ground required timbering the tunnel sets were spaced to be suitable for stope-chute sets later on. After the stope-chute sets were completed, a man with a stoping machine drilled both sides directly over the chutes, nearly horizontally and on the center line of the stope, to form a pocket at this point. The next round on each side pointed strongly upward, and from that, point on, the raise was extended on a 60° pitch until the face was 31 ft. vertically above the top of rail. The stoping machine was then taken out, and a No. 9 Leyner machine set up near the top of the raise, and a drift started each way. These drifts were run horizontally following the center line of the stope, and were made 8 by 8 ft. Since their maximum length from any ore chute was only 18 ft., little shoveling was necessary to get the ore to the chute. After this drift was completed for the full length of the stope, the Leyner machine took 4 ft. off each side of the drift, to bring the stope to the standard width of 16 ft. The ore broken in all this work was drawn from the chutes, so that when this stage of operation was completed, an excavation 450 ft. long, 16 ft. wide and 8 ft. high was ready for stoping.
Chute timbers were constructed as follows: Three tunnel sets of 12-by 12-in. timbers were set up, spaced at 5½-ft. centers. Posts 8 ft. long were set in hitches cut in the floor deep enough to make the bottoms of the caps 7 ft. above the top of rail. Caps were cut 10 ft. long, so as to extend 6 in. beyond the side of each post, and blocked tightly against the walls. Planks, 2 by 12 in. by 7 ft. long, nailed under the cap, acted as spreaders for the posts, but no sills were used. The sides were lagged with 2 by 12-in. planks. Enough ground was then broken above the tunnel sets to make room for a short set. The posts for this set were cut 3 ft. 9 in. long of 12 by 12-in. timber. Only two short sets were put in, one on each side of the chute mouth, the top and sides being lagged with split round poles (see Fig. 3).
Manway Raises and Drifts
In alternating drifts, that is, at intervals of 100 ft., manway raises were put up in the pillars, mid way between the stopes. The raises were started in offsets 6 ft. from the center of the track, and driven on a 50° pitch. They were made 4 by 6 ft. and divided
into a chute and a manway by means of stulls and 3 by 12-in. planks. The manways were equipped with ladders, and the chutes equipped with gates, so that the ore broken in the raises could be loaded direct into motor cars. When the manways reached an elevation 31 ft. vertically above the top of rail, manway drifts were started both ways at right angles to the raises, or parallel to the motor drifts. The bottoms of these drifts were 25 ft. above the top of rail, or on a level with the bottom of the stopes. These drifts, at 19 ft. from the manways, broke into the sides of the stopes. At the junction of the drifts with the stopes, 8 by 8-in. sills 8 ft. long were laid 3 ft. in the stope and 5 ft. in the drift and 6 by 6-in. cribbing built upon them, dapped 1 in. so as to leave a space of 4 in. between cribbing. The crib timbers were 4 ft. long, making the manways 3 by 3 ft. in the clear. The cribbing on the drift side was left off for the first 5 ft. to form an entrance into the manway. The manway timbers projected 2 ft. into the stope and 2 ft. into the pillar, thus placing them in solid ground on part of three sides. After the manways were thus completed to within a few feet of the back of the stope, and equipped with air lines, the stope was ready for active stoping.
Usually, in working a stope, a crew consisting of a machine man and a pick man worked from each manway, so that ordinarily five machines were working in each main-level stope. Stoping was done on but one shift a day; so the same crew was responsible for conditions at a given manway. The pickman trimmed down the back of the stope, so as to make it safe for the machine man. In a shift of 8 hr., each stope crew was expected to put in from 14 to 20 holes, 6 ft. deep, depending on the ground, and load and fire them. A round of 15 holes ordinarily broke 150 tons. The motor crew on the following shift pulled about 75 tons, representing the swell of the ore, so that the stope crew
would have sufficient room to work on top of the broken ore. The ideal condition for efficiency and safety in a stope was to keep the top of the broken ore within 9 ft. of the back of the stope. With five machines in a stope, a separate crew of crib men was employed to build the manways, and keep them at all times well above the level of broken ore in the stope. The stopes from the main tunnel were carried to capping, or abandoned 10 ft. below the second level, if the ground above the second level had already been stoped. As soon as a stope was abandoned, the chutes were nailed up or otherwise obstructed, so that no ore could be pulled from them (Fig. 4).
As already mentioned the upper levels consist of main drifts directly above the motor drifts forming the limits of the stope blocks. Crosscuts were driven every 120 ft. at right angles to the main drifts, that is, parallel to the centerline of the stopes. These were driven on a 1 per cent, grade from each main drift, meeting in the center of the block. Each crosscut was connected to the main-haulage level by two raises which came out in the floor of the crosscut 100 ft. from each of the main drifts. On both sides of these crosscuts, stope drifts were driven, at 30-ft. intervals, for a length of 30 ft., or 11 ft. beyond the center line of the stopes. They were then enlarged sufficiently to accommodate the timbers for stope chutes.
Chute Timbering.—The chute timbering consisted of three tunnel sets spaced at 4½-ft. centers, except in every other drift where an additional set was put in at 3½-ft. centers to accommodate the manway. Tunnel sets were of 10 by 10-in. timbers, with posts cut so as to leave 7 ft. between
top of rail and bottom of cap. Collar braces were of 8 by 8-in. timbers, or round poles not less than 7 in. in diameter. Caps for manway sets were 11 ft. long to form the sill for manway cribbing. The chute lip was 3 ft. 9 in. above the top of rail and was made of 3 by 12-in. plank. Chutes were equipped with two gates so as to control the flow of ore easily. Top and sides were lagged with 2 by 12-in. plank, or split round poles (see Fig. 5).
After the stope-chute timbers were in place, the ground above the timbers was drilled with a stope machine; but before blasting, the top was lagged with short split round poles, so that the ore could be loaded direct into a car without shoveling. An 8 by 8-ft. drift was then started on top of the chute timbers with the center line of the stope as one side of the drift, and extended the full length of the stope. Later this drift was widened to 16 ft.
Manways were put up in every second stope drift built of the same dimensions and material as the manways from the main level, and carried to the third level or to capping if struck before the third level was reached. Where the distance to capping was greater than 100 ft. but less than 200 ft., connection was made with the stope on the third level, the old manways abandoned, and new ones started from the third level. Where the ore thickness was greater than 200 ft. above the second level, stopes were started from the third level, run to capping, and abandoned before the stopes from the second level were started.
Order of Working Stopes
In a given block it was the custom to have the stopes on the top level worked out considerably in advance of the stopes started from the middle
level. In this way the upper stopes were abandoned before the stopes from below began to disturb the tramming drifts and stope manways. This was necessary also to avoid cutting off the raises through which the ore was dumped from the upper to the main-haulage level, before the upper stopes were worked out. For the same reasons the stopes started from the middle level were kept well in advance of those started from the main level (Fig- 9).
Hand Tramming on Upper Levels
On the middle and upper levels all ore was hand trammed from the stope chutes to the nearest raise. The raises were so spaced that the maximum length of tram was 150 ft., and the average 90 ft. for all stope ore. Tracks for hand tramming were 18-in. gage, laid with 16-lb. rails and a grade of 1 per cent, in favor of the loaded cars. The stope cross-cuts were double-tracked for short distances near the raises, to permit several cars to run to the same raise without delaying or interfering with each other. Tram cars measured 2 by 2.5 by 4.6 ft. in the clear, and held about 1 ton of porphyry ore. Two trammers were used on a car, and the average tonnage trammed by each crew from a stope was about 65 tons per shift. A tallyman, stationed at each active stope, credited each car dumped to the proper crew.
During the 3-year period, 1911 to 1913 inclusive, 2,824,439 dry toils of ore was drawn from stopes at a cost of 56.6c. per ton loaded into railroad cars at the ore bin. In addition, 102,719 ft. of development work yielded 247,280 dry tons at a cost of $4.95 per foot of development or $2.05 per ton of ore. As the development was carried on entirely in average grade of ore, it about paid for itself from the ore broken. Counting every item of expense, the cost of producing a ton of ore from all sources by underground mining averaged 68.7c. By the system in use, with 16-ft. stopes and 44-ft. pillars, less than 27 per cent, of the ground developed was actually stoped, and as the stopes were full of broken ore when abandoned, the actual production represented less than 50 per cent, of the ore broken, or about 13 per cent, of the ore blocked out by the development work. The cost of production would have been considerably less, if the pillars could have been mined and the broken ore in the stopes pulled. Manway drifts and raises, and stope drifts were charged direct to stoping, all other drifts and raises being charged to development.
All development work was contracted at the following rates for breaking and mucking, not including timbering, the company furnishing the supplies:
- Motor drifts (7 by 7.5 ft. in clear)
$3.50 to $4 per foot (depending on whether dry or wet ground)
- Tramming drifts (5.5 by 6.5 ft. in clear).
$2.25 to $2.50 per foot (depending on length of tram)
- Raises (4 by 5 ft. in clear)
$1.20 to $1.80 per foot (depending on length of raise)
At these rates, the contractors were able to make from $3.75 to $5 per shift, depending on the ground and their own efforts.
These figures include every item of expense incident to the mining of the ore, including the necessary development work. Under the head of miscellaneous is included the proper proportion of all general expenses, such as taxes, insurance, Salt Lake and New York office expenses.
Hammer and Piston Drills Used
For all large drifts, such as motor drifts and stope drifts, and for widening out stopes, No. 7 or No. 9 Leyner water drills were used, the smaller machines being used only in soft ground. For small drifts the one-man 2.5-in. piston machine, of both Ingersoll and Sullivan makes, was used, while hammer drills were used for raising and stoping.
In the principal drifts and crosscuts on each level a 1½-in. pipe line was laid to supply water for the Leyner machines and for fire protection. A reserve supply of water was stored in tanks erected in a drift 50 ft. above the top level, the head being sufficient to carry the water to all parts of the workings. As the two upper tunnels were dry, it was necessary to fill the storage tanks by pumping from the ditch on the main-haulage level, a small compressed-air pump operated an hour or two each day being used for the purpose.
Data on Motor Haulage
The main-level track is laid with 60-lb. rails, 36-in. gage, and a ¼ per cent, grade in favor of the loaded cars. Two General Electric 10-ton locomotives, each pulling a train of seven cars or 60 tons of ore per trip, have handled as high as 4,000 tons in a shift of 8 hr., the average being 2,500 tons. Loading from a chute in which the ore runs freely, a train could make a round trip in 15 min., divided as follows:
Gravity Tram from Mine Portal to Railroad Bins
As the portal of the motor-haulage tunnel is on a hill side, at a vertical distance of 350 ft. above the level of the railroad yard, and at a horizontal distance of 700 ft., a surface gravity tramway is utilized to transport the ore to the yard. Two Stein trams were installed and were so arranged that they can be operated singly or both at the same time.
The surface incline has 60-lb. rails and a 36-in. gage, with an inclination varying from 30° at the top to 18° near the lower end. At the level of the motor-haulage tunnel a 300-ton ore bin gives sufficient storage to assure no delay to the motor trains. The gravity-tram skips are loaded from this bin through air-lift gates.
At the bottom of the incline a circular steel ore bin of 2,000 tons capacity gives sufficient storage to avoid-delay in the operation of the tram even if railroad cars are not furnished for several hours at a time.
The skips on the gravity tram have a capacity of 12 tons, and dump direct into the steel ore bin through doors in the bottom of the skips. Railroad cars are loaded from the steel ore bin by means of air-lift gates, only 4 min. being necessary to load a 70-ton car.
One engineer at the head-house of the gravity tram can easily lower 4,000 tons of ore over this tram in a shift of 9 hr.
The brakes on the drums of these trams are tightened by dead weights, and released when the weights are raised by compressed air. When the supply of compressed air fails, the brakes are automatically applied, eliminating any danger of a runaway.
Safety of Mining System
As many engineers have gained the false impression that a system of mining involving the use of shrinkage stopes is necessarily hazardous, a list of the fatal accidents during the 3 years under discussion may be of interest. The following is a complete list of fatalities for a 3-year period during which time more than 3,000,000 tons were mined at a cost for timbering of less than 5c. per ton:
Most of these accidents were entirely due to the carelessness of the men injured. A noteworthy feature is that not a man was fatally or seriously injured through the use or handling of explosives.