Drilling is employed extensively, either as the principal exploratory method or to supplement exploration by underground and surface workings. Three principal types of drills are employed for this purpose—churn drills, core drills, and hammer drills. In ground too soft to core and in which a churn drill would stick, a rotary drill with a fish-tail bit is useful for obtaining sludge samples.
In churn drilling, a string of tools with a cutting bit at the lower end is suspended from a rope or cable and is alternately raised and dropped by hand or power-driven mechanism, chopping a hole in the rock. Water is used in the hole during drilling, and the cuttings are removed with a bailer, usually at regular intervals of about 5 or 10 feet of hole. These cuttings constitute samples of the material passed through and may be examined mineralogicallv and assayed.
Core drills are rotary drills that cut an annular groove about a central core of rock. This core is broken off and removed in sections as the hole is deepened, each section of core comprising a sample that can be analyzed and likewise reveals the composition, texture, and structure of the rocks penetrated. The cores may be split in half longitudinally by means of a core splitter, one half being used for assay purposes and the other retained as a more or less permanent visual record of the formations for inspection and correlation with the cores from other holes.
There are three general types of core drills: (1) Those employing a bit of soft steel set with a number of cutting elements, usually carbons (“black diamonds”) or bort, (2) those of the shot-drill type, in which the annular groove is cut by grinding the rock with chilled shot or other hard material under a rotating shoe or bit, and (3) those employing a toothed cutter (sometimes used in soft rocks).
The hammer drill is employed principally in underground work. It is the common heavy rock drill using ordinary hollow drill steel and standard bits, except that for drilling long holes the machine usually is equipped with strong independent rotation and sectional steels are employed. The sludge is caught as it issues from the hole during drilling and is used as a sample or series of samples for visual examination and assay.
Each type of drill has its limitations and its particular fields of usefulness. Although short holes occasionally are drilled at steep angles with churn drills, they are not suited to drilling other than vertical holes. One great advantage of the diamond drill is that holes may be drilled at any angle from the vertical, up or down. Deep diamond-drill holes usually deviate from a straight line, and methods have been devised to straighten curved holes. It is also possible to turn a hole at any desired depth by means of deflecting wedges, and there are interesting examples of drilling several branching holes from a predetermined point in an original hole.
The churn drill has found special favor in drilling a number of the large so-called disseminated “porphyry” copper deposits in south-western United States. In the Tri-State lead and zinc district the churn drill has been used almost exclusively. The diamond drill is not well-adapted to the work in this district because, on account of the cherty, fractured, often bouldery and vuggy nature of the ores and surrounding wall rocks, the percentage of core recovered is low and consumption of carbon or bort by excessive wear, breakage, and loss is high. In the Lake Superior iron-ore districts a combination diamond- and churn-drill machine, known as the Mesabi rig, has been used extensively. When drilling is done in soft formations that will break and thus not return a core, vertical holes may be drilled with a chopping bit screwed on the end of the regular string of rods. When passing through hard formations, the chopping bit is removed and replaced by a diamond bit.
In the Southeastern Missouri lead belt the diamond drill has been employed almost exclusively for drilling the disseminated ores, which are homogeneous, give a good core recovery, and drill easily with this type of drill. In the Mascot (Tenn.) area both churn and diamond drills have been used extensively.
Until quite recently, diamond-drill bits were set almost exclusively with carbons (“black diamonds”), but today bort is used more than carbons in the United States and Canada. About 4 to 10 or more times as many stones are required when bort is used as when carbon is used, and the cost of setting the bits is therefore greater. The cost per carat for bort, however, is about one-tenth that of carbon, or less. In many places it has been found that a bit set with bort will drill more footage than one set with carbon, that it will give better core recovery in some kinds of ground, and that the cost per foot of hole is much less. The loss of a few stones by breakage or of an entire bit is not nearly as costly as when carbon is employed.
Standard diamond-drilling outfits are built with steam, gasoline, electric, or compressed-air drive. Ordinarily, they are built in sizes ranging from those taking a 5/8-inch core and limited to 150 feet of drilling depth to machines that will take a 2 1/8-inch core and drill to a depth of 5,000 feet. Other core sizes and capacities also are built for special work. The smaller outfits are sectionalized (knock-down), so that the largest and heaviest individual part can be transported under the most difficult conditions. The smaller rigs can readily be taken into and operated in stopes and drifts.
In 1930 the Diamond Core Drill Manufacturers Association (United States and Canada) established new standard sizes of bits and casing for four sizes of bits based upon a series of four nesting casings, so that the bit for one size hole will pass through the casing that will go in the next larger size hole. These sizes are as follows:
The hammer drill has its best application in exploratory drilling underground, where relatively short holes are to be drilled, core recovery would be poor with a core drill, or core samples have no particular advantages over sludge samples. The hammer drill with special strong, independent rotation, using sectional steel, has been used to drill holes up to 272 feet in depth. Hammer drilling has been discussed by Knaebel and by Jackson and Knaebel in earlier Bureau of Mines publications.
The advantages of drill exploration have sometimes been underrated in certain districts, whereas in others the reliability of ore-reserve estimates based on samples obtained by drilling has perhaps been overrated.
Drilling can often be employed to determine, at a fraction of the cost of underground exploration and in much less time, the position, outline, and approximate volume of ore in a lode or massive deposit; and, under favorable conditions, a close approximation to the average grade of ore may be made from drill samples. Drilling is particularly useful in determining various structural features, the position of faults, contacts between different types of rocks and shear zones, and obtaining other information of great use in planning underground exploratory workings.
Light, compact diamond-drill rigs and hammer drills are useful for drilling from underground stations in tracing the extension of veins or important structural conditions below the lowest level of the mine, and in probing for probable or possible blind lodes out in the walls of known deposits. By judicious use of drilling and interpretation of drilling results, considerable money often can be saved that otherwise would be wasted in drifting, crosscutting, and sinking.
In flat-lying bedded deposits or veins and ore bodies of considerable horizontal area, churn drills often can be employed to outline the deposits roughly, and, if the particles of valuable minerals are fine and uniformly distributed in the rock, churn-drill samples often will give very accurate information concerning the grade of the ore. Churn-drill samples may become contaminated with waste or salted if the hole caves, and allowance must be made for this in calculating the grade of ore from them or from sludge samples obtained in diamond drilling or hammer drilling. Moreover, in broken ground drilling water may carry away part of the cuttings into fissures and seams in the rock.
If the mineralization is erratic, less reliance can be placed on drill samples as a basis for estimating grade of ore than where mineralization is uniform. Even where ore occurrence is very erratic, however, drilling may be useful in determining quickly and cheaply the presence or absence of a vein or lode in a given zone, or of structural conditions favorable for the occurrence of ore.
Where a high percentage of core can be recovered, the core drill gives the most complete and most reliable information of any type of drilling. In precious-metal veins, where the valuable mineral is distributed very erratically, too much reliance should not be placed on sludge or even core-sample assays. The hole may pass through a particularly rich spot in a generally lean area or close to a rich zone in barren material. Results of drilling, therefore, must be checked by means of excavated workings, particularly where mineralization is erratic, before a high degree of accuracy is possible in estimating the average grade of the ore in this type of deposit.
The subject of drill exploration has been discussed by Jackson and Knaebel, who give examples of practice and present data comparing the estimates of ore grades based on drill samples, with grades of ore actually mined later.
Table 6 is a compilation of data on cost of churn drilling, table 7 on costs of diamond drilling, and table 8 on costs of hammer drilling.
Drilling costs vary with conditions, such as character of the rock formations drilled, accessibility of drill stations, depth of the individual holes, wage rates and cost of equipment and supplies laid down at the operation, unit cost of power, kind of power available, and water supply. Under like conditions, the cost per foot of hole will be less if a large total footage is to be drilled (thus distributing installation and organization costs over more feet of hole) than for a small total footage, and, provided moving from one set-up to the next is not too expensive, will be less for holes of moderate depth than for deep holes.
From certain of the data in table 6, the total costs of large churn drilling-exploration campaigns may be deduced. Thus, 60,000 feet of drilling at Bisbee, Ariz., at $1.97 per foot cost $118,200; at Miami, Ariz., 44,500 feet at $3.26 per foot totals $145,070; 57 holes totaling 31,100 feet at Burro Mountain, N. Mex., at $2,177 per foot cost $67,705; 62 holes totaling 14,260 feet at Silver Bell, Ariz., cost $35,935.
At Globe, Ariz., the Old Dominion Copper Mining & Smelting Co. developed and at one time employed an electrically driven underground churn drill. Four holes 12 inches in diameter were sunk for ventilation, and seven holes, starting with 8¼-inch bits and finishing with 6¼-inch or smaller bits, were prospect holes. The four 12-inch holes totaled 637.5 feet and cost $4,775; the seven prospecting holes totaled 1,124 feet and cost $5,052, including all items of cost such as moving, preparing drilling stations, and initial cost of equipment. Out of a total of $9,827, $2,382 was initial cost of equipment; $876, the cost of alterations to equipment; and $492, the cost of additions to equipment, a total of $3,750.
The following table shows the costs of churn-drilling eight different tracts in the Tri-State lead and zinc district:
The figures in the foregoing tables indicate that although, on the average, diamond- and churn-drilling costs are higher than hammer-drilling costs, the costs incident to the different methods of drilling are, broadly, of the same general order, especially when each is employed under conditions favorable to it. It has been pointed out that each type of drill has its peculiar limitations and each its particular field and that in certain kinds of ground or with certain depths or angles of holes one or the other may not suit at all.
Moreover, for sampling purposes, the reliability of results obtained rather than the cost per foot of hole should be the first consideration. The cost tables have been presented partly to show the range in drilling costs but are particularly significant in showing that where there is a choice of methods, the spread between costs by the different methods need not necessarily be great, and that the use of a cheaper drilling method to reduce the cost per foot of drilling is not justified if reliability of results would be sacrificed thereby. Misleading samples will be far more costly in the long run than any additional cost required to obtain dependable samples.
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