The use of hammer drills for obtaining samples in underground work has increased in recent years. This method has advantages where holes are short, ground conditions preclude the use of diamond drills, satisfactory core recovery is impossible, or sludge samples are accurate enough for the purpose in view. Holes may be drilled at all angles from horizontal to vertical, up or down, but best results usually are obtained with holes drilled above the horizontal. The samples are small sludge samples, and previous statements concerning churn drilling are applicable here. Due to the smaller size of the holes in test drilling, the results of caving may be more pronounced than with churn drilling, especially as test holes seldom are cased.
Test-hole drilling with ordinary hammer drills is employed to check cut, channel, and grab samples or as a substitute therefor and is especially useful for obtaining samples of ground 5 to 20 or 25 feet beyond existing faces, particularly where ore boundaries are gradational or small seams of ore diverge from the main vein for short distances. Longer holes, up to over 250 feet, are drilled with heavy machines, as previously mentioned, as a substitute for diamond drilling where conditions are such that core recovery would be unsatisfactory.
The usual practice is to take a separate sample for each change in the formation or in the character of ore. These samples may be accurate unless lax methods are employed for collecting the sludge as it issues from the drill hole. If only structural information is desired, refinements in collecting the samples are not important; but if samples are desired for assay, it pays to take pains to assure collection of all the sludge and to prevent loss of slimes in the overflow from the collecting receptacle. The reliability of the samples will depend largely on the care taken in collecting the cuttings; and, as stated previously, it is better to take no samples at all than to be misled by faulty ones.
Drilling is done either wet or dry. When drilling is done dry, the samples may be caught in canvas bags fastened around the collar of the hole with the drill steel passing through an opening in the side of the bag. Open receptacles are sometimes employed, but loss of fine particles in the air may seriously affect the reliability of the samples.
When drilling is done wet, the samples are caught in boxes or cans placed on the ground below the hole, in small pans held below the collar of the hole by a helper, or in sludge boxes equipped with baffle plates for retention of the sludge. It is not always easy to place these receptacles so as to catch all the sludge, and with either small cans or boxes the overflow water may carry away slimes because of incomplete settling. By drilling a short hole with a large bit, sealing a pipe in the hole, and then drilling through the pipe with smaller bits, all the sludge usually can be caught and diverted to the settling device. A more elaborate method, which may be warranted when drilling deep holes, is shown in figure 6. As in churn drilling, it is important to provide ample settling time to collect the slimes with the sample, especially if there is an appreciable difference between the assay value of the fine and coarse cuttings, as often there is.
Accuracy Of Hammer Drill Sampling
In this connection, some results reported by operators doing hammer-drill sampling are of interest.
At the Teck-Hughes mine, Kirkland Lake, Ontario, 5-foot holes are drilled in the sides of the drifts at 10-foot intervals to test the walls and assist in keeping the faces in the best ore. The veins or lenses occur in shear zones in acid syenite, porphyry, and basic syenite, cut by parallel fault planes. The samples are accurate enough to be used in conjunction with cut samples in preparing assay plans upon which ore-reserve estimates are based.
At the Pilares mine the ore mineral is principally chalcopyrite, which occurs as open-space filling in brecciated latite, andesite, and monzonite porphyry. The sides of drifts and raises are sampled by test-drilling with dry stoper drills, drilling holes 5 feet deep. The average grade of ore blocks is estimated by averaging the assays of the dry-stoper samples, and for several years the grade of ore delivered to the mill very closely approximated the estimated grade.
Maclennan states that at Miami, where the copper minerals occur finely disseminated in schist, “stoper-drill samples [are] the most accurate samples for drifts.”
At Fierro, N. Mex., deep-hole prospect drilling was used for a time, but the cost was higher than for diamond drilling and the information obtained was not as reliable. Holes up to 20 feet long continue to be used in stopes, raises, and drifts for exploration and sampling purposes, and the grade of the ore cut is estimated from the samples. The iron ore occurs as a replacement of limestone beds.
The long-hole drill has been used for underground prospecting in the Tri-State district for several years. The practice in this district has been described by Netzeband.
The zinc and lead minerals occur in zones of shearing, shattering, and brecciation in limestone beds, particularly where there has been considerable silicification of the original beds. Drilling is done wet, and the cuttings are diverted into a powder box by a spout. The experience in this district, according to Netzeband, has been that no great reliance can be placed on assays of the cuttings, though this may be due in part to the lack of close supervision of the sampling by competent observers.
At Gilman, Colo., drill cuttings from each 6 feet of hole are caught in a powder box, from which a sample of the sludge is cut from top to bottom. The ore is of a grade that does not necessitate close sampling and is so friable that the sample is usually contaminated. A composite assay of the sludge from an entire hole is regarded as accurate enough, however, to be used in ore-reserve calculations.
At Edwards, N. Y., prospect drilling with hammer machines is a regular feature of routine exploration work. Here, zinc ore occurs as a sulfide replacement of silicified, folded dolomite beds. The sludge from the holes is assayed, but, since the ore minerals are very brittle, the samples are salted and generally assay about 25 percent too high.
In general, it may be said that where the ore minerals are not friable and have about the same toughness and hardness as the gangue or are disseminated in small particles uniformly through the matrix, hammer-drill samples may be accurate if precautions are taken to catch and settle all the sludge. If the ore minerals are brittle, occur in layers or bands, and are of very different hardness than that of the gangue, assay results will be high, due to salting. In such cases, an inch of rich ore cutting across the hole may salt several feet of sample or even the whole sample beyond the rich section. Even in such instances, however, samples may be valuable as indicative of mineralized formation if conditions are understood and too much reliance is not placed on the assay returns.