Some ore deposits found in jasper-slates and in the horizon between these and their associated clay-slates; in the latter they are true contact-deposits. They are found in this jasper- and clay-slate formation under varied conditions, yet, in all, certain characteristic laws are preserved. The angle of dip varies from 15° to 20°, and up to 85° and even 90°, with the horizon. The ore is rather soft, bluish-black in color, and is a hematite. It varies in thickness from a few inches to one hundred feet, or more. It is mainly “Bessemer” ore, containing 57 to 63 per cent, of metallic iron, and .03 to .07 per cent, of phosphorus.
In the mining nomenclature of this range two conditions of the ore-lens are described by the terms “dip” and “pitch.” The former is in common use in defining the relation of the ore-plane to the horizon ; the latter indicates the posture of the ore-body as it inclines east or west from its position at the opening of the mine. In the Menominee Range the “pitch” is usually westward, and varies from 27° to 45°.
It will thus be seen that the “pitch” of the ore-body is next in importance to the dip in enabling the mining engineer to locate the works intelligently.
Some of the ore-deposits outcrop to the surface boldly. Other deposits are found with the upper edges beneath the surface. Other conditions being equal, the latter can be relied upon for the greater depth, as the original volumes have not been reduced by the eroding or denuding agencies which cut down the former to varying depths, and consequently reduced volumes. But the conditions indicating large or deep deposits vary so much that they are not yet well understood.
Along the ore-bearing group of jasper and clay-slates the ore-deposits are found in three or more horizons, the Norway siliceous limestone formation affording the miner a base from which he can estimate the probable place of the ore in these measures.
Naturally, the deposits outcropping at the surface invited the first mining operations.
It might reasonably be inferred that, with the accumulated mining knowledge of the past, an advanced system would have been introduced at the very opening of operations on the Menominee Range; but such was not the case. The first mining methods were very crude. In the experience of ten years they have been rapidly advanced, until now they are fully abreast of the best modern mining practice. In all this upward movement in mining methods it is evident that necessity in a great degree compelled progress.
As human progress has been illustrated by the stone age, the bronze age and the iron age, so the growth of mining in this range has had its distinct periods of open pit-work, irregular timbering, Nevada system of framed timber, and the recent system of rock- filling.
The open-pit mining embraced ore-deposits outcropping at or near the surface. These were mined with skips or derricks in a very crude way, so as to afford the utmost output, without reference to the economy of subsequent working or the stability of the mine.
This unplanned mining was also the result, in part, of the want of experience in the depth and posture of the ore-bodies.
There were justifiable instances in this open-pit mining, such as in the west end of the large Norway Mine, where an outcrop of ore over 100 feet thick and 150 feet deep was mined to great advantage. The Cyclops pits have been mined in the open way, near the surface, with horses and carts, and in the deeper portions, with the usual derrick and tubs, banking over 100,000 tons of excellent ore. But these are exceptional cases. The open mining work at other places proved wasteful and dangerous in every way.
As the mining deepened, regular skip-ways were established and protected by timbering. At intervals down these skip-ways of 60 to 100 feet, levels were driven on each side and at right angles to the skip-ways. These are known as the levels of the mine, and are numbered 1, 2, 3, 4, etc., in descending order. This second period evolved some attempts at securing the hanging-walls by round timbers, placed across the exhausted space of the ore-body and at right angles to its bedding-plane. This early effort at timbering was also supplemented by leaving ore-pillars at certain places to steady the hanging-walls. Cribbing was also used in places where a crush or creep made its appearance.
The third period, induced by increasing depth and its accompanying pressure, compelled, in most mines, the application of some system of timbering. The time at which the improved plan of timbering was introduced at the several mines along the range varied from many different causes.
In the large Norway mine the hanging-wall is of jasper-slates, tough and firm, and can be easily sustained with little support. In fact, this mine is made up mostly of a series of great caverns of ore, each affording 10,000 to 20,000 tons, with heights of stopes from 20 to 60 feet. Posts and crib-timbering were used in the early years of this mine. Mines in the ore-bodies, with jasper-slate foot-walls and clay-slate hanging-walls, were the earliest to require careful timbering and the first to compel systematic working.
Captain John Curnow introduced into the East Vulcan Mines of the Penn Iron Mining Company the plan of timbering known as the “Nevada system,” which consists in filling the space exhausted in the ore-body with a series of frame cubes. These cubes are constructed with white pine timber of excellent quality, 12 to 15 inches square, framed into squares of 7 feet from center to center. The drawings (Fig. 2) will show the value of this system of timbering as related to strength, facility in erecting, and its adaptability to all thicknesses of ore-deposits or variations in hanging and foot-walls.
When the walls of the ore-deposit are tolerably firm and not easily softened by exposure to the moist atmosphere of the mine, this system possesses great strength ; but where the hanging-wall is of soft clay- slates, as in West Vulcan, softening rapidly on exposure to the air in the mine, this system with its large timbers affords only temporary support. When the crush begins, the upright posts lose their vertical position, and many of the timbers are crushed into splinters. Collars and cross-braces are sometimes used to arrest a squeeze; but a brief respite only is secured in this way.
The flexure of a soft hanging-wall indicates the early crushing of timbers in the portion of the level exhausted of the iron-ore. It becomes then a struggle on the part of the miner to remove as much as possible of the ore out of the creeping level before the final crush comes to close out all mining operations.
It has thus been found that for certain qualities of mine-walls this elaborate system of timbering fulfils its office satisfactorily, but that under a softening hanging- or footwall it is not reliable for a sufficient time to afford opportunity for exhaustive mining in each level, or to protect ways to the adjoining level.
The depth to which this system of timbering can be carried safely depends on the firmness of the hanging- and footwalls more than on the increased pressure from depth, although the latter is also an important factor.
It may be noted that with this timbering it became practicable to make exhaustive mining in each level, securing only the skip-ways by pillars of iron ore. Even this ore-support can be avoided when stopes or shafts are used.
The latest and modern method of “ rock-filling” was compelled by the conditions already described, as in the West Vulcan mine of the Penn Iron Mining Company.
The drawings of this mine (Figs. 3 and 4) exhibit the general posture and westward pitch of the ore-deposit. At the eighth level the ore is 600 feet long and the average thickness about 25 feet.
The longitudinal section exhibits the “levels,” with the method of working the stopes and removing the pillars. This mine is operated by a shaft and a skipway or slope, as shown on the cross-sections. The large new shaft is 665 feet deep to the ninth level. The western pitch of the ore is clearly shown on this section.
The cross-sections looking west show the dip of the ore-body down to the ninth level, now the working-level. Cross-section B shows the main shaft in the hanging-wall.
The timbering, mainly after the Nevada system, has been carried to the eighth level. Rock-filling is now being used in the ninth level, from which 130,000 tons of ore, it is estimated, will be mined this year.
The drawings, Figs. 5, 6 and 7, show the method of this filling. From the main shaft “ A,” a drift cuts the ore-body and 25 feet into the foot-wall of firm jasper-slates. From this point a rock:tunnel is driven in the foot-wall east, as shown on plan, to shaft “B.” Along this rock-tunnel ports are made at intervals of about 100 feet into the ore-body. From these ports the mining of the ore begins on the bottom of the ninth level.
The first cut of this ore is mined about 8 to 10 feet high and the spaces from which the ore has been removed are filled with rock. This rock-filling follows the mining and affords absolute safety and inflexible support to the walls of the mine. The broken rock for filling is conveyed down the winzes B, and the ore through the shutes D, which are built up as fast as the filling is made upwards.
It will thus be seen that the tunnel in the foot-wall secures absolute safety to the main-ways of each level; it is out of the crush range in any event. The winzes or rock-mills are also ventilators and greatly improve the sanitary condition of the mine.
The ore-body is, under this system, attacked by the miners on double face at each portal C, from the main rock-level or tunnel in the foot-wall. The mining by sections upwards is simply a repetition of the first 8 or 10 feet; the rock-filling following the mining work and filling the spaces from which the ore has been removed as rapidly as room for the mining operations will permit.
Some timbering will be required occasionally in this rock-filling, especially in sustaining the filling in upper levels, as it is approached from below in removing the last cut of ore.
It may be noted here, that this rock-filling system has not been evolved from any special progress in the mining art, or in the line of its economy over the Nevada system of timbering, but has been compelled by the absolute necessity of the case in West Vulcan and all other mines in this range where it has been introduced.
It may be said that sufficient work has not been done to submit an accurate statement of the relative expenses of these two systems.
However gratifying this comparison would be, if it indicated economy on the side of the rock-filling, yet this is not the vital factor governing the change of method from timber to rock, but rather the latter is a compelled requirement to secure the stability of the hanging-wall of the mine. And further, it may be pointed out that such a statement at any time would not afford a permanent basis of comparison, from the fact that the magnificent pine forests of this section of Michigan are fast disappearing under the great drain for these mines and eastern markets, so that the timber-supply is becoming year by year enhanced in value by scarcity and lengthened haulage. It is, therefore, a constantly shifting factor of expense in mining operations.
The rock, on the other hand, is inexhaustible; and, in spite of some little variableness in the cost of breaking it up and sliding it down into the mine, is a much less fluctuating factor of expense than the wood for timbering. The cost of timbers and timbering, per ton of ore mined at West Vulcan during 1887, was 38 cents.
The cost of rock-filling, with its necessary use of some timbering, per ton of ore mined, is estimated at 20 cents per ton—14 cents for rock-filling, and 6 cents per ton for the timbering required with rock-filling. These estimates require verification by a few years of actual work.
It may be pointed out here, that the rock-filling affords a permanent support, which is not liable to decay as is the timbering system, and will not require renewals during the progress of the mine- workings.
The great caverns of ore in the Norway mine compelled rock-filling about two years ago, so that Captain John Oliver, of Norway, was one of the pioneers in the application of this system.
The Chapin was also forced to use this rock-filling system to maintain the walls of its great ore-deposit, and was the first on this range to apply it.
Many of the mines, with firm walls and at moderate depths, continue to use the timbers in steadying their works, but the rock system must commend itself rapidly to all these mines from its great safety, stability, and facility for exhaustive mining.
There are other important factors in connection with the general application of this system of rock-filling. The location of the shafts or slopes to the mines is one. The first planning of these slopes or skip-ways placed them in the ore-body, requiring large pillars of ore for their maintenance. Where the ore is of moderate thickness (10 to 15 feet), this method is not so open to criticism; but when the ore-body is 20 to 50 or 100 feet thick and rather soft, it has serious drawbacks in the great pillars of ore which must be set apart for this purpose, and in their tendency to crumble, especially in the event of a creep or crush, which follows in a greater or less degree in exhaustive mining.
The use of skip-ways in the foot-walls, or the sinking of shafts in the hanging-walls, are matters of the greatest importance in assuring safe and economical mining. It has been shown that slopes or skip-ways in the ore-body are open to serious objections in large deposits of ore.
It is quite possible to sink the slopes or skip-ways in the foot-wall, especially when this is composed of firm jasper-slates. They could be driven in the foot-wall, and a sufficient distance under the plane of the ore, say 10 to 20 feet, so as to assure a permanent rock tunnel under all conditions of creep or crush. But this system has some exceptions, as in the case of the East Vulcan mine, with its ore-body flexed and pitching, as shown in the longitudinal section, Fig. 8. It will be evident that a slope or skip-way in the footwall slates of an ore-body so placed would have been impracticable. But there are many deposits where a slope or skip-way could be driven in the foot-wall of an ore-deposit, and be readily made to conform to the flexures met, in ordinary cases, in these foot-walls.
This method would be costly at first, but it would ultimately be found to assure great safety, and permit exhaustive mining, since no pillars would be required for its protection.
This East Vulcan mine, after passing through the early stages of mining, adopted the vertical shaft for winding and pumping. It is now 500 feet deep. At 400 feet deep, the ore-body pitched rapidly westward, requiring a long rock-tunnel from the shaft to the ore-body. It will readily appear that with the continued westward pitch, the lengthening rock-tunnels at each level could not be maintained, as their increasing cost would debar their use. Hence, in this case, after testing the pitch of the ore-body, it was concluded that a new shaft, 800 feet west of the old one, should be put down. Work on the new shaft is now in rapid progress. It is designed to produce the tunnels in the levels of the old workings, until they reach the new shaft, to assure good ventilation, and also to explore the intervening mining ground.
This new shaft will pass through the ore-bodies, and reach a point at which its levels or rock-tunnels will become too expensive if the present westward plunge of the ore continues. But it is also probable that the ore-body will change its present rather flat pitch. In any case, it is evident that sinking this new shaft in soft slates will be much less expensive than continuing the old shaft with its increasing lengths of rock-tunnels in jasper-slates.
This case has been selected as affording an example that has furnished an abundant series of perplexing mining problems to its managers.
West Vulcan mine exhibits (Fig. 4) an example of approaches by slope or skip-ways, partly in foot-wall slates, and a vertical shaft in the soft slates of the hanging-wall. The cross sections of this mine will show the dip of the ore-body, and the relations of the shafts to it.
The location of the main shaft A in the hanging-wall was the result of several conditions bearing on the ultimate economy of mining its ore. It is near the siding of the railroad, affording a ready way for delivering the ore into railroad cars, and also for receiving coal by the same way for the boilers, and timbers for the mine. This location also reduced largely the height of the pumping column, and the shaft was sunk in soft slates rapidly and cheaply. Aside from these special conditions, the locating of a shaft in the hanging-wall cannot be commended, as there is generally some shifting in the hanging-wall ground, endangering shaft and machinery.
It thus appears that a slope or skip-way in the foot-walls, with levels also in the rocks, is, in most instances, the safest and most economical plan.
It is evident, also, that the limited amount of exploring performed at most ore-deposits, fails to afford an adequate knowledge of the posture and pitch of the ore, and hence no comprehensive plan of mining operations can be laid down; the result is a shifting, undecided, and, in many cases, badly located system of ways, that must result in abnormal expenditures in operating them.
It has also been a matter of discussion whether the usual method of mining by beginning operations at or near the surface and working downwards, is the best plan. It is true that it affords a ready output of ore, and a quick return of money ; but, unless permanent rock-ways are established, it involves increasing expenditure in the downward workings. It is submitted, that in deposits of ore of moderate thickness, a slope could be cut in the ore to the bottom of the deposit, and workings commenced there, entirely exhausting the ore in the progress of the working upwards. The exhausted spaces below would afford a ready place for slates or other mining refuse, and could, if necessary, be supplemented with additional rock-filling. Even should the hanging-wall swell or buckle, no serious injury could result, as such a crush would be arrested at each level.
The methods of shaft- and slope-timbering on this range are fully up to the best practice. In tunneling, compressed air and power- drills are used, giving rapid progress in extending the mine-ways. The winding and pumping plants at Norway and West Vulcan mines are models of comprehensive designs for this special work, in magnitude, efficiency, stability, and harmony of their parts. The shafts at these two large mines are 10 by 16 feet in size, and 400 and 700 feet deep, respectively. The double-acting plunger-pumps are 16 in. in diameter and 10 ft. stroke into single water column, throwing, when necessary, a continual discharge of water. The pumping-engines are 38 in. by 28 in. each, and the ponderous gear-wheels for driving the pumps are 30 ft. 8 in. in diameter. The winding-machinery, boilers, etc., are on a proportional scale.
Norway mine will probably produce 100,000 tons of ore this year, 1888, and West Vulcan will probably yield 130,000 tons of ore.
Improved systems of mining and modern plants of winding- and pumping-machinery are becoming rapidly introduced in this range, especially as greater depths in mining are reached.
The western energy and push are admirably illustrated in the progress of improvement of mining methods in the Menominee Range. It is also hopeful to observe that the comfort of the miners is not neglected, and that at Norway and West Vulcan mines comfortable dry-houses for the use of the miners have been established, where not only the wet mining-suits are dried, but bathrooms are provided also.
Reading-rooms have also been established at these mines. At Norway a hospital is open for the reception of miners for treatment under a skilled physician and surgeon.
The occasional fears of exhaustion of the ore-deposits of this range that marked its early experience, are now measurably abated under the compensatory elements of occasional discoveries of new deposits which sustain its annual output with assuring regularity.