Mining Massive Porphyry Copper Deposits

Mining Massive Porphyry Copper Deposits

The past decade has witnessed the development and successful operation of a half dozen or more immense deposits of low-grade copper ore that occur in the Western States. In every case sufficient capital was available to carry out the development of the mines in the manner decided upon, and it is interesting to note the different methods that were employed.

Utah Copper and Boston Consolidated Mines

The first two “porphyry coppers” were the Utah Copper, and Boston Consolidated, at Bingham, Utah; The Utah Copper started out with a modification of the chute-caving method described in Crane’s Ore Mining Methods, p. 141. The method was not satisfactory and was

stoping system

discarded for stopes and pillars, although very little mining is done by this method because the ore will be handled by steam shovels. The Boston Consolidated mine, now called the Boston mine of the Utah Copper Co., was originally laid out into stopes and pillars. Fig. 9 illustrates the method. Above the main haulageway there is a 30-ft. pillar of ore to protect it. Above this the deposit is divided into a series of vertical stopes 30 ft. wide and 150 ft. high alternating with vertical pillars of the same dimensions. Above these comes another and similar series of stopes and pillars, but the pillars in the upper series come immediately above the stopes in the series below. The plan was to work all the stopes on both levels as shrinkage stopes. When the stopes were all completed drawing would commence from the bottom of the lower stopes. The, upper pillars being over stopes below would settle and crumble so that they would pass down with the ore from the upper stopes when the lower pillars were weakened by blasting. It will be noted that in this method the capping must cave and follow down on top of the broken ore. Just what alterations in the plan of working would have been necessary if the method had remained in use it is difficult to state. The stopes are still being worked but only the excess ore is drawn off; the remaining ore is left to be mined by steam shovels after the capping is removed. The width of stopes has been reduced from 30 to 18 ft., and the pillars increased to 42 ft.

Ray Consolidated Mine

The Ray Consolidated Copper Co. at Ray, Ariz., is controlled by the same interests as the Utah Copper Co. and when their system of mining was laid out they no doubt profited by the earlier experience at Bingham. The orebody is flat and averages a little more than 100 ft. thick. The main haulageways are at the bottom of the ore and above them the ore is divided into vertical stopes 15 ft. wide leaving 10-ft. pillars between as shown in Fig. 10. The stopes are worked as shrinkage stopes to the top of the orebody, then the bottoms of the pillars are blasted and drawing is commenced from under both stopes and pillars. The pillars, being undercut, settle and break up as the ore is drawn. By taking care to draw equally from all the chutes under a block of ground the capping will cave and follow down uniformly on top of the ore and there will be little mixing of the two. The Ray system, from all reports, has been eminently successful. Stopes 15 ft. in width are wide enough so that a large tonnage will break compared to the footage drilled, and all the advantage is had of the shrinkage method of stoping. After the ore in the stopes is broken, the pillars, which contain more than one-third of the total ore, are available for extraction with only a slight expense for drilling and blasting. The cost of mining for the last quarter for which reports have been published was 71c. a ton. This includes a proportion of all general and fixed charges, but does not include an allowance of 12½c. a ton for the retirement of mine-development suspense account.

Miami Mine

The orebody of the Miami Copper Co. is much deeper than that at Ray. In horizontal cross-section it is roughly circular, about 1,000 ft.

section of stopes

across. A tongue of waste rock intrudes into the ore from one side. The irregularities in the orebody under the capping are first worked by square-set stopes and timber is placed to form a mat between ore and waste. Through the main body of ore sublevels are spaced 25 ft. apart vertically as shown in Fig. 11 and on these sublevels drifts and cross-drifts are driven each way 50 ft. apart. The main body of the ore is to be mined with shrinkage stopes and pillars similar to the Ray method except that

cross-section-of-stope

at Miami the stopes were planned to be 60 ft. wide and the pillars 40 ft. In working the stopes the miners do not work on top of the broken ore but approach the stopes through the drifts on the sublevels and break the ore into the stopes by the method of sublevel stoping previously described. The ore breaks readily into small pieces and obviously a 60-ft. shrinkage stope would be very dangerous if worked in the usual way, but by the sublevel method the danger is eliminated. Even under these conditions the last report states that it has been found advisable to make the stopes narrower. After the shrinkage stopes are carried up to the mat, which is next to the capping, the pillars are worked from the top down by top slicing. As this is done the ore from the stopes is drawn and a uniform settlement of the capping results.

The cost of mining at Miami has been about 11.20 a ton. Last year a premature crushing of square sets under the capping started a cave which extended through into the stopes below. The repair of this damage caused an increase in the cost for 1913. The Miami orebody is of higher grade than that at Ray and the Miami method will save a greater percentage of clean ore. The cost per ton is greater and there is a question as to how much the cost can be reduced in the future. To date, a large amount of ore has been taken from the square-set stopes, an expensive operation, but, on the other hand, in the future there will be an increased amount of ore to be mined from the pillars, which is also expensive. It would be interesting to know how much of the difference in cost between the methods at Ray and Miami is due to the difference in cost of labor. At Miami the miners receive $3.50 a day and up. At Ray mostly Mexicans are employed and they are paid, I believe, about $2.50 a day and up.

Inspiration Mine

The Inspiration mine, adjoining the Miami, has not yet begun to produce on a large scale, but it has been developed, in part at least, with the idea of using block caving. The method was described by. Claude T. Rice in Mines and Methods, June, 1909, and is similar to the block caving illustrated earlier in this paper. The individual blocks were to be 75 by 200 by 200 ft. high and were to be isolated by stopes on the sides and breast stopes between ore and capping. The pillars under the blocks instead of being as shown in Fig. 7 were to be very narrow, 75 ft. long and 75 ft. high. Between the pillars narrow shrinkage stopes filled with “ore” would prevent the pillars-from, early crushing. This method, I believe, has never been given a trial and now is to be replaced by the method of block caving which has been so eminently successful at the Ohio Copper mine at Bingham. This method is worthy of a fuller description. The following is abstracted from a description of the method by Clarence G. Bamberger in the Engineering and Mining Journal of Apr. 6, 1912.

Ohio Copper Mine

“ In resume the conditions are: An orebody opened 400 ft. in width, 450 ft. in length, 1,300 ft. in depth, dipping from the horizontal at an angle of 50°. The entire mass being a broken shattered quartzite containing copper, chiefly in the form of chalcopyrite disseminated throughout, inclosed by foot and hanging wall of the same formation with boundaries not clearly defined.

“The preparation of this ground for a caving system of mining has been carried out as shown in Fig. 12. An incline shaft was sunk in the foot wall on the dip of the vein connecting with the different levels for the transportation of men and supplies. Two main ore chutes No. 1 and No. 2 were driven in the foot wall on an angle slightly greater than the dip of the vein. These chutes connect by diagonal raises through the foot wall, with the different levels where extraction is taking place and deliver into bins of large capacity at the Mascotte tunnel or main haulage adit which in turn leads to the concentrator situated at the portal about 3 miles distant. “Lateral main levels were driven across the orebody at the 100-, 300-, 400-, 500-,

cross-section-ohio-copper-mine

750-, and 1-,000-ft. or haulage level. By a network of crosscuts and drifts these different levels have been cut up into blocks 200 ft. square which in mining will again be sub-divided. Between the several levels the ground has been blocked out into sublevels

cross-section-ohio-copper-mine-2

30 ft. apart and again subdivided by crosscuts, drifts and raises into blocks approximately 30 x 50 x 25 ft.

“The actual procedure of extraction can be clearly understood by reference to Fig. 13 which shows in detail the sublevels above mentioned. Thus we have for example the four sublevels, A, B, C, and D. Raises are driven from the crosscuts and drifts on the 300-ft. level through sublevels A and B. From these vertical raises at the various points E, F, G, and H raises are run into sublevel C, radiating like fingers from the palm of the outstretched hand. At the breast of these several raises sublevel D is blasted down, the ore falling by these leads into the cross inclined chutes connecting with the main ore chute, which delivers into bins at the loading station on the haulage-tunnel level.

“The same procedure is carried out on sublevel B which in turn is blasted down; at the same time sublevel C. which has already been cut up by this finger-like network of raises, comes with it as the solid ground below is blasted down. Thus each alternate sublevel is cut up by the numerous raises and the corresponding sublevel above and below is blasted down.

“The actual cost per ton for the different phases of the operation is shown in the accompanying table which is an average covering a period of 31 days in October, 1911, during which period 56,311 tons were mined.

Typical Daily Labor Report. Ohio Copper Co.

description of labor

“From the above data several interesting conclusions can be drawn. Considering the whole working force of 110 men as producing the ore, approximately 17 tons of ore are delivered to bins per day per man. Considering, however, only the men who are actually breaking the ore, approximately 63 tons of ore are delivered to bins per day per man. From the “total operating expense for period” and “total tonnage mined for period,” the cost per ton delivered to bins is shown to be 28.06c., which figure includes development and equipment charges. From “total actual operating expense” and “total tonnage mined for period,” the actual cost per ton for mining delivered to bins is shown to be 21.97c.”

This method of mining is similar to the ideal case of block caving already described, only instead of having to shovel the ore after it is caved a large number of branching raises are brought up from underneath and the ore runs into these and on down to bins at the bottom of the mine without any handling. The raises contain chutes and the ore is drawn evenly from a large area so that the capping will follow down without mixing too much with the ore. While considerable ore is lost the cheapness of the method, is remarkable. I have in my possession the figures for April, 1913, almost two years after the above figures were taken, and the cost of mining including development and equipment is reduced from 28.1c. to 22.2c.

Mine Development

The extensive development necessary for the successful operation of some methods of mining naturally brings to mind the question of what this work costs. In the appendix, I have placed a collection of figures on the cost of development as published in the reports of mining companies. Easily driven drifts and crosscuts will cost $3.50 to $4.50 a foot. Average-sized drifts in hard rock with a small amount of timbering cost $6 to $7 and under unfavorable conditions the cost will be $10 or more a foot. In a 5 by 7 ft. drift costing $6.50 a foot the labor for drilling and blasting will be about $2.15, labor for shoveling and tramming about $1.20, air about 30c. a foot, explosives about $1.25, and miscellaneous 60c. a foot. Raises are as a rule less expensive than drifts and, in very favorable ground, can be driven for $1.50 to $2.50 a foot. The average 4 by 5 ft. raise in hard rock will cost $4 to $5 and raises difficult to drive or requiring close timbering will cost $8 a foot and up. Winzes are not driven often but when they are they cost from $10 a foot up.

Space permits of a consideration of only a few methods from the point of view of development. At Kalgoorlie (J. Cheffirs, Transactions Australian Institute of Mining Engineers, vol. xiii) in a block of ore 500 ft. long and 200 ft. high (the distance between levels), rill stoping required four raises and shrinkage stoping required one; 800 ft. of raises cost about $5 a foot or $4,000. If we assume the vein to average 8 ft. wide a block of ore would contain about 60,000 tons, which means a cost for development of almost 7c. a ton. With shrinkage stoping this cost would be less than 2c. a ton. Of these methods touched upon in the preceding pages the Miami mine has the greatest footage of development work. The scheme of development allows a good many different methods of mining to be used without any changes; it also permits a large tonnage to be produced from a limited area. A block of ore 50 by 50 by 50 ft. contains approximately 50 ft. of raise and 200 ft. of drifts and crosscuts. Estimating raises at $3 a foot and drifts at $5 a foot gives a total of $1,150 for the block. It contains about 10,000 tons of ore, which would make 11½c. a ton as the cost for development; this excludes the cost of shafts, stations, and similar requirements.

At Ray 12½c. a ton is allowed for the retirement of mine-development suspense account but just what this covers is not stated. It evidently covers more than development for stoping.

The method used at the Ohio Copper mine requires considerable development of which a large percentage is raising. Due to the fact that many of these raises were inclined, so that a miner could work at the face without using timber, and that all the ore, after being blasted, runs by gravity to the bins, the cost of the raises was only at the rate of 50 or 60c for each ton of ore mined in driving them.

mining methods

mining methods