CHAPTER XVII PROSPECTING AND ESTIMATING ORE 0 NE of the fundamental reasons for the extreme rapidity with which the Porphyries developed after the trail had been blazed at Bingham was the amenability of the deposits to prospecting and proving of the ore either by churn-drilling or by diamond-drilling. At Ray churn-drill data formed the principal basis for early calculations of ore reserves; and in 1918, Inspiration floated its first bond issue for $6,000,000 on the strength of the estimate of 45,000,000 tons of sulphide ore assaying just over $2 per cent copper. The money was obtained in spite of the fact that no one ever had actually laid eyes on more than a minute fraction of that ore. The customary method of developing a copper deposit before the Porphyry era had been to sink shafts or drive adits and then to follow and expose the veins or lodes with horizontal and vertical underground workings-drifts, crosscuts, raises, and winzes. The excavated material sometimes was sampled in bulk; but always the walls of the openings would be channeled at intervals to obtain small samples representative of the ore left in place. This process was known as "blocking out" ore. It was costly and it required a great deal of time to prove the existence of immense tonnages. The early development at Utah was done in this way; likewise, at Miami. This fact helps to explain why only 8,000,000 tons of ore was reported as being developed when J. Parke Channing wrote the report on which the original Miami financing was based in March, 1908. Fortunately, his 2,000,000 tons averaged 3 per cent copper. At Ray, Seeley W. Mudd estimated 3,000,000 tons of ore in June, 1908, developed principally by underground working; but by the end of 1909, Henry Krumb was able to report 50,000,000 tons as the fruit of a little more than a year of churn-drilling. 355
Transcript
CHAPTER XVII
PROSPECTING AND ESTIMATING ORE
0 NE of the fundamental reasons for the extreme rapidity with which the Porphyries developed after the trail had been blazed at Bingham was the amenability of
the deposits to prospecting and proving of the ore either by churn-drilling or by diamond-drilling. At Ray churn-drill data formed the principal basis for early calculations of ore reserves; and in 1918, Inspiration floated its first bond issue for $6,000,000 on the strength of the estimate of 45,000,000 tons of sulphide ore assaying just over $2 per cent copper. The money was obtained in spite of the fact that no one ever had actually laid eyes on more than a minute fraction of that ore.
The customary method of developing a copper deposit before the Porphyry era had been to sink shafts or drive adits and then to follow and expose the veins or lodes with horizontal and vertical underground workings-drifts, crosscuts, raises, and winzes. The excavated material sometimes was sampled in bulk; but always the walls of the openings would be channeled a t intervals to obtain small samples representative of the ore left in place. This process was known as "blocking out" ore. I t was costly and it required a great deal of time to prove the existence of immense tonnages. The early development at Utah was done in this way; likewise, a t Miami. This fact helps to explain why only 8,000,000 tons of ore was reported as being developed when J. Parke Channing wrote the report on which the original Miami financing was based in March, 1908. Fortunately, his 2,000,000 tons averaged 3 per cent copper. At Ray, Seeley W. Mudd estimated 3,000,000 tons of ore in June, 1908, developed principally by underground working; but by the end of 1909, Henry Krumb was able to report 50,000,000 tons as the fruit of a little more than a year of churn-drilling.
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3.56 THE PORPHYRY COPPERS
With respect to a vein or lode deposit, development work serves a dual purpose; i t discloses the existence of the ore and, at the same time, i t provides the workings through which the ore may be attacked, and, when broken, transported either to the bottom of a shaft or to a main exit adit. Development, literally, means the uncovering or unfolding of something that has been uncertain; the working out in detail. On this basis, i t might be argued that a systematic drilling campaign would develop the details of extent and richness of an orebody, the existence of which had been demonstrated by previous explora- tory or prospect drilling. Some authorities draw the following distinction: prospecting they define as the search for a mineral deposit; exploration as the work of exploring an orebody, when found, with a view to gaining a knowledge of its size, shape, position, and value. Others reserve exploration for the exami- nation of large regions in search of a likely place to prospect.
With respect to the Porphyries the custom is to use the terms prospecting and exploration interchangeably to designate drill- ing that is designed to prove the orebody. The word develop- ment usually is reserved for the process of driving the haulage drifts and crosscuts and the raises and other workings above the haulage level, preparatory to the actual mining of the ore.
The churn drill was developed about the close of the last century as a more economical substitute for the diamond drill on the Mesabi iron ranges in Minnesota. Progenitors of the churn drill were the drills called Keystone and Cyclone, which were used in testing the sand and gravel of placer deposits.
At the time of the examination of the Nevada Consolidated on behalf of the Guggenheims, a number of engineers, including A. Chester Beatty, Seeley W. Mudd, Mark Requa, and Henry Krumb. were discussing the possibility of finding some quick, cheap method by which it could be determined whether ore underlay some of the undeveloped outcrops in the Ely district which, to all appearances, were similar to those that capped the developed orebodies. Had geophysical methods of the present day been known they would have met the need. One of the engineers recalled that in the Joplin district in Missouri sludge from churn-drill holes was panned to see if there was su l~hide
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mineral present. Could a.similar procedure be adopted a t Ely? There was no suggestion a t that time that the grade of ore could be determined; the idea merely was to ascertain quickly and .at relatively small cost whether sulphide minerals occurred beneath the outcrops.
After the property was taken over by the Guggenheims two churn drills were purchased and drillers were brought from Missouri to run them. As the chalcocite in the Ely district is much more finely disseminated than the "jack" in Joplin, little information was derived from inspecting or panning the sludge. Consequently, tomato cans full of the sludge were sent to the assay office to be dried and assayed. The result afforded a rough guide as to the character of the material penetrated.
FIRST SYSTEMATIC CHURN-DRILLING AT RAY
In 1908, following the panic of 1907, Jackling and Mudd found themselves short of funds at Ray, yet they were particu- larly anxious to know whether large bodies of disseminated ore really existed in the district. W. Y. Westervelt, consulting engineer for the minority owners, had proposed the use of churn drills to Sherwood Aldrich, president of the Ray company, and Aldrich passed the suggestion along to Mudd. Recalling the experience a t the Nevada Consolidated, Mudd accepted the suggestion, recognizing that by careful handling and sampling of the sludge a close approximation as to the grade of the ore encountered could be made. This idea was followed and soon after the drilling campaign was started Henry Krumb, as an outside, independent engineer, was retained to organize and supervise the work. He placed Ralph Nowland in charge of a staff of young engineers whose duty it was to sample the sludge and keep accurate and adequate records. As related in Chapter IX, it was here that the technique of conducting this important work was developed.
Ray was the scene of the first carefully organized churn-drill prospecting with respect to copper mines; but Inspiration was the first property where churn-drilling was depended upon almost exclusively. And even here tbe engineers had the advan-
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tage of knowing that their orebody really was an extension of Miami's. New Cornelia was prospected by the use of diamond drills; and the new Clay orebody a t Morenci has been diamond- drilled. However, probably 90 per cent of the Porphyry ores have been proved by churn drills, and current prospecting designed to delineate the orebodies more fully is for the most part done with that machine rather than with the diamond drill.
The churn drill in principle is an exceedingly simple device for boring a vertical hole into comparatively soft rock. Formerly, it was known as a "rope and drop" drill. Essentially, it comprises a "string of tools " consisting of a tempered steel cutting bit, attached to a stem which gives it weight, a set of "jars" and a "sinker bar" all attached to a drill rope either of hemp or woven wire cable. This assembly is raised by a derrick and suitable walking beam and dropped to deliver 30 to 50 strokes per minute. The source of power may be either a steam or gasoline engine or an electric motor. Each blow shatters some rock in the bottom of the hole. Holes range in diameter from 3 to 26 in. and can be sunk as deep as 1500 ft. or even more, tapering as they go down. The drill operates "wet" and the cuttings form a sludge which is bailed or pumped out a t intervals. In drilling copper deposits, the sludge, usually from successive 5-ft. vertical sections of hole, is carefully collected and preserved because it is the sole source of the information upon which is based the determination of tonnage, grade, and mineralogical character of the ore, as well as of the geology of the ground penetrated.
The diamond drill consists of a "line" of hollow rods with a bit a t the working end, which actually bores into the rock by virtue of rotation a t a speed of from 300 to 800 r.p.m. The mechanism may be actuated by a steam engine or gasoline engine or by an electric motor. Black, or industrial, diamoilds are "set" on both the outside rim and the face of the annular bit of soft steel, the result being the constant cutting of a cylindrical core of the rock passed through. At the same time, fine cuttings necessarily are produced, and these are washed to the surface by a stream of water pumped through the hollow drill rod, and returning to the surface through the annular space between the rod and the walls of the hole. Drill diameters range
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from 135 to 2% in., yielding cores of about 1 to 2 in.; and holes have been dri1Ied as deep as 5000 ft., although such depths are not usual and have not been needed a t the Porphyries.
CHURN DRILL On the brow of a bench of the Sacramento Hill pit a t Bisbee
I n order to recover the core it is necessary to withdraw all of the drill rods from the hole at intervals of 3 to'8 ft. of advance. Naturally, this retards the rate of progress and increases the cost per foot. as greater depth is attained.
A feature of the diamond drill is that i t can bore upward, downward, or a t any angle, whereas the churn drill must make a
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"down" hole. Incidentally, the diamond drill may wander from its projected course by as much as 60 deg. or more, so that a survey of the hole must be made. One advantage of the diamond drill is that it provides a better specimen of the rock passed through for the study of the geologists. However, the recovery of core is by no means complete. At Morenci 50 per cent is lost and this makes it necessary to save the sludge, just as in churn- drilling, and to combine the assays of the core and sludge by careful calculation in order to arrive a t an assay to represent the grade of a particular interval of hole. The desired result of any drilling is, of course, to obtain an assay that would be obtained if a complete and perfect cylinder of rock from each 5-ft. section of hole were recovered and sampled for assay.
PORPHYRIES SUITED TO DRILL EXPLORATION
I t is quite evident how well suited the Porphyry deposits are to prospecting or exploration by either churn or diamond drills. The deposits, typically, are large in horizontal extent and lie close to the surface, so that they can be penetrated without requiring holes of great depth, as would many orebodies of other types. Being essentially horizontal "blankets," drillholes are much more likely to tell a true story than if the orebodies were bent and folded and pitched a t various angles. The rock may be classed as soft rather than hard; and yet it does not often tend to crumble or slough badly. When it does, it is necessary to line the hole with steel casing. Indeed, this is done frequently as a measure of insurance, where no great difficulty would be experi- enced if the precaution were not taken.
Perhaps most important are the low average grade of the ore, the tendency to continuity and uniformity, and the absence of extraordinarily rich minerals. For an ore containing free gold, churn-drilling, particularly, is not entirely satisfactory, because the ore contains minute particles of great richness. The erroneous exclusion of a few such particles or the inclusion of too many in the sludge tends seriously to vitiate results. Gold placers usually are prospected by churn-drilling, in spite of inherent sources of inaccuracy, because no other method is as practicable.
PROSPECTING AND ESTIMATING ORE
In spite of all these favorable conditions, however, i t should not be concluded that the work of drilling a big copper orebody, collecting the data, and drawing correct conclusions is not attended by many sources of possible serious error. The problem is a t least twofold:
1. To obtain thoroughly accurate and complete logs of each drillhole, usually bored a t the corners of squares made by the intersecting of coordinates spaced a t 200-ft. intervals.
2. To interpret, correlate, and combine these data correctly. Of the two problems, the second no doubt is the more difficult.
The first requires careful manipulation and accurate record- ing; the second entails, in addition, the exercise of sound engineering judgment. Naturally, the greatest source of error lies in the possible contamination of the sludge with material that may break off from the walls above the section being drilled. This may make the sample either too low or too high. If, because of the crumbling character of the ground, the danger is too great, steel casing may be used in churn-drill holes or grouting with portland cement may be resorted to in diamond-drill operations. Another possibility is that richer portions of the rock may be more friable or soft or for some other reason may break more easily than the leaner portions, thereby yielding a false sample.
It is necessary, of course, that the hole be cleaned thoroughly between sample intervals so as to minimize the intermixing of samples. An elaborate technique of handling the sludge, sampling, and drying it has been developed. One important detail is the necessity for avoiding, in the process of drying, excessive temper- atures that might tend to oxidize sulphide minerals. If sulphur were "roasted " off the resulting copper assay naturally would be erroneously high; and the same would be true if water of crystallization were expelled from any of the constituent minerals. The supervision of drilling operations is generally intrusted to the engineering or to the geological staff, and a young engineer keeps close track of the work, directs the sampling, and keeps the detailed records. The immediate result of a drilling campaign is a set of permanent drillhole logs usually
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comprehensive and elaborate. They contain part or all of the following and frequently various other items:
1. Graphic chart showing adopted assay for each 5-ft. interval.
2. Notes on physical character of rock, including type, hardness, etc., and the color of sludge (as an index to oxidation).
3. Notes on mineral content of rock or ore and general geology.
4. Size of bit used; size and footage of casing placed. 5. Footage of core recovered (if diamond-drilled) . 6. Weight of sludge per foot drilled (as a key to caving of
hole). Generally speaking, the cost for diamond-drilling is some-
what greater than for churn-drilling, although A. C. Stoddard of Inspiration is authority for the statement that "in accuracy, speed, and cost, diamond-drilling compares most favorably with churn-drilling." In recent years diamond drills have supple- mented the others a t Inspiration to a considerable extent. At New Cornelia, the decision to use diamond drills, according to General Greenway and Ira B. Joralemon, was based on the availability of a contractor who would supply the equipment and do 1500 ft. of preliminary drilling in a hurry. First results warranted the continuation of prospecting and a contract was let for diamond-drilling at $6 per foot against an estimated cost of $3 for churn-drilling.
At Miami, where the new low-grade orebody was churn- drilled in 1924, the cost was reported as $2.57 per foot. At Chino, a churn-drill hole 965 ft. deep cost $2.74 per foot in 1928. In another porphyry mine in Arizona, 16,000 ft. of hole diamond- drilled in 1925 cost $4.16 per foot. As is always the case in comparisons of this kind, allowance should be made for different conditions and different accounting methods. Nevertheless, it can be said that under conditions usually obtaining in the Porphyry Coppers, churn drills are more satisfactory.
Between the portable steam rigs used to drill holes with a starting diameter of 6 in. and the machines now in use a t Utah Copper lies a big difference. Here two standard oil rigs, operated by electricity, are in use. When a depth in excess of 1500 ft.
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is anticipated, the hole is started with a R6-in. bit and the first 80 to 100 f t . is lined with R6-in. stovepipe casing. By the time the bottom is reached, a casing as small as 4 in. may be used, but usually the entire hole is cased. The object is to assure the greatest possible accuracy in sampling. A. Soderberg gives the
TABLE 30
COST OF CHURN-DRILLING AT UTAH
A DEEP KOLE DRILLED WITH h STANDARD ELECTRIC OIL RIG
Spudded-in with 23-in. casing. Average progress per day, 3.23 it. Elapsed time, 455 days. Average drilling progress per day, 3.92 ft.
Total depth drilled, 1468.8 ft. Coat per foot
Moving and setting up drill . . . . . . . . . . . . . . . . . . . . . Labor, power and water supply. $1.14
Actual drilling . . . . . . . Amount paid contractor plus labor and supplies. 13.33
data in Table 30, showing the cost of a 1468-ft. hole drilled a t the Utah Copper mine to be $20.49 per foot.
The foregoing data reflect quite a different undertaking from the drilling involved in the Arizona mine previously cited where the per-foot cost was just one-eighth of $90.49. I t can scarcely be regarded as representative of the practice a t the Porphyry mines, but it reflects the large scale on which operations are conducted by t,he Utah Copper Co.
CALCULATION OF ORE RESERVES
However, irrespective of the detail of drilling methods, the final result is a set of vertical holes penetrating the orebody a t points usually arranged a t the corners of the squares on a ROO-ft. checkerboard. The distance may vary, and the location of the holes may be irregular. What shall be done wit,h the data?
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James E. Harding, who participated in the drilling of the Andes deposit, points out (Engineering and Mining Journal, vol. 117) that a complete set of cross-sectional drawings should be made from the drill logs. "Mapped imagination" he calls it, because the section is essentially the result of deductive reasoning rather than of actual observation. I n addition to sections on 200-ft. coordinate planes, models of glass or wood help in visualizing the deposit. Aside from the calculation of ore reserves, the location of shafts, haulageways, ore passes, in fact, the entire program of mine development, may be best laid out after study of the deposit and its geology through carefully made cross-sections and models.
The problem of estimating the tonnage and average grade of the ore in the deposit theoretically should not be difficult; but practically it confronts the engineer with an intricate problern. If the entire mass were known to be workable ore, it would be necessary merely to divide the deposit into prisms- triangular or quadrilateral-with the drillholes forming the vertical edges, and to ascertain the volume by application of the prismoidal formula. Tonnage would then be determined by using a factor, usually between 12 and 13, determined as the number of cubic feet occupied by a ton of ore in place. The grade of the ore in each prism would be computed by averaging the assays from each hole and combining the averages according to stand- ard formulas.
However, the irregularities in grade-which occur even in Porphyry deposits of which uniformity is a distinctive character- istic-and the geologic structure, which shows bands or zones of lean rock or of richer ore, complicate the problem of combining assays and lengths so as to give calculated results that accord with the facts. Particularly, if the ore is to be mined by power shovel, so that selectivity can be readily exercised, the problem arises of fixing the boundaries of masses of ore to include in the calculation. Whether open-cut or underground methods are to be used, the question of determining the limits of profitable ore is important. Here the study and correlation of the vertical cross-sections becomes of special value.
To complicate the matter still further comes the question of
PROSPECTING AND ESTIMATING ORE 365
what price of copper is to be used in arriving at the "cutoff" between ore and waste-the cutoff being the copper content below which material cannot meet the direct cost of mining and treatment. But even if a substantial body of copper-bearing rock is below the cutoff grade, it may be profitable to mine it because of its position in relation to workable portions of the deposit.
The point might be further elaborated, but enough has been said to indicate the difficulties that arise. It will be informing to investigate what actual mining has disclosed as to the accuracy of estimates based on drilling.
In general, experience has been that the average grade of the ore extracted from a given block has been lower than the expectancy; whereas the tonnage frequently has been higher, as a consequence, for the most part, of unavoidable dilution with low-grade material.
An illustration of this point is found in the data in Table 31 showing the expected and actual recoveries a t Copper Queen. They relate to the ore averaging 1 per cent or more in the benches of the pit above the 5210-ft. elevation. Incidentally, in making the original estimates, each steam-shovel bench was treated as a separate mine, so to speak, with the object of
Copper content, pounds
Tons ore Copper content, per cent
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minimizing probable error. In anticipation of being able to mine selectively with shovels, low-grade material was omitted from the estimates. At the time the accompanying figures were prepared, the benches concerned had been entirely mined. In the table " 'Expanded' estimate" refers to a revised set of figures made in 1924, after two years of operation. At that time, based on the results so far obtained, it appeared that tonnage mined would be 23.5 per cent greater than originally expected; but that the average grade of the ore would fall 12.0 per cent below. According to H. M. Ziesemer, chief engineer of the Sacramento Hill division, three possible sources of error were: (1) faulty sampling of churn-drill holes; (2) faulty work in combining assays and in calculating estimates from drillhole logs; a.nd (3) greater dilution than anticipated in the course of mining. It seems likely that the first is the least potent factor. The second offers room for error in method used; and also in the matter of judgment exercised.
Perhaps in the effort to be conservative and to avoid possible failure of the mine to yield as much and as rich ore as forecast, the estimates were deliberately made low by the application of a factor of safety. However, the fact that the total tonnage of ore as mined exceeded the estimates by 26.9 per cent shows the importance of dilution as a factor in the miscalculation. It is probable that the discrepancy that arose was an element in the decision of the Phelps Dodge officials to use diamond drills in prospecting the Clay orebody at Morenci.
One other open-cut project that has been virtually completed is the mining of the oxidized or carbonate ores a t New Cornelia. The figures in Table 32 make interesting comparison, but should be interpreted in the light of the following facts:
The original estimate used a cutoff of 1 per cent copper. Later this was lowered substantially, with the result that a large tonnage that had not been considered amenable to profitable treatment was mined. In 1929, the average grade of the ore leached was only slightly higher than the cutoff assumed at the start, so that much low-grade material must have been included.
Comparisons of recoveries obtained by underground caving methods with the expectancy calculated from drilling are shown
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T ~ L E 33
CHURK-DRILLING RESULTS AT AJO COMPARISON OF EXPECTED AND REALIZED RECOVERIES OF OXIDIZED ORE8
in Table 33. These data really have more significance so far as the probable accuracy of the sampling and estimating the ore is concerned than do the data for New Cornelia, for the reason that they apply to definite blocks of ore. Though a considerable range of variation exists, all of the data coincide to this extent: the realized tonnage in each instance is considerably in excess of that anticipated, whereas the average grade is substantially lower. The least satisfactory results are shown for Inspiration; and the wide discrepancy in grade may account in part for the present apparent leaning of the Inspiration management toward diamond-drilling as a method of prospecting. Error as to average copper content in the original estimate, based on churn-drilling,
T ~ L E 33
CHURN-DRILLING BEFORE UNDERGROUND MINING COMPARISON OF MPECTED AND REALIZED RECOVERIES AT SUNDRY PORPHYRY MINE8
could be responsible. However, the true explanation of the discrepancy is that, in the process of mining, certain portions of the richer ore were "lost" and waste and low-grade material were mined unavoidably, thus bringing the tonnage figure up to 102.4 per cent of the expectancy. Inspiration commenced pro- duction in 1915 when both the demand and price of copper were soaring. The mine was "strained" at a time when it should have been accelerating production gradually. Circumstances justified what really was poor mining from an engineering standpoint as explained in Chapter XI.
The most serious dilution occurs in the last item, which refers to recent operations in Miami's "new" orebody. F. W. Maclen- nan explains that the reported net recovery of 102.4 per cent of the copper is accounted for by the drawing of some "partition" ore and copper-bearing capping or fill from old stopes. He does not attribute the discrepancy to errors in sampling churn-drill holes or to faulty interpretation of the data.
As a matter of fact, most engineers who have had experience in drilling Porphyry Copper deposits, either by means of churn drills or diamond drills, are of the opinion that reliable results can be obtained. Losses, when they develop, are accounted for by shortcomings in the process of mining. Diamond-drilling, where the ground is uniform and relatively hard, probably gives more accurate information; churn-drilling is more economical and somewhat more rapid at depths to which it is applicable, and is more accurate when the character of the ground is such that only a small proportion of a diamond-drill core would be recovered. Even though the diamond-drill sludge is recovered the quantity is much smaller than where a churn drill is used.
The results of many hundreds of feet of churn-drilling at both Ray and Andes were carefully tested by following the holes with raises. Careful sampling of the raises in 5-ft. intervals gave assays that checked the churn-drill assays for the corre- sponding vertical sections with remarkable accuracy. And, as was pointed out at the start, this method of proving ore reserves has been a vital factor in the speed and economy of Porphyry exploitation.
