This paper is intended partly as a summary of the conclusions that have been arrived at by various authorities on this matter, and also with a view to initiate some discussion on the subject, especially as most of the quartz gold mines in Victoria are operated without having any proper assay plans or any idea of the real values other than those given by the mining managers, who, from experience in the particular cases, are supposed to be able to arrive at a correct idea of the true values of the ore, and, in many cases, oppose any other methods than their own, holding the view that no sampling is equal in correctness to the bulk sample of the battery. The necessity of properly sampling and estimating the ore in a mine in every case, perhaps hardly needs to be urged on the members of the Institute, but is not, apparently, so obvious to others.
It is proposed to confine this paper to the sampling of ore reserves in a mine, and the various points which have to be taken into consideration in arriving at values, from the assayer’s point of view, leaving the methods of sampling metallurgical products out of consideration for the present.
It is advisable to define “ ore reserves ” as ore exposed on three sides at least, and accessible to the sampler on all three.
“Prospective ore” would include all ore not so fully exposed, but which may, from the nature of the case, and the geological conditions prevailing, be reasonably supposed to exist.
In proceeding to sample a mine, endeavour should first be made to get a general idea of the workings from the plans, if any, and from a general inspection of the mine, and of the geological conditions, more particularly the extent and distribution of the supposedly pay ore«as assumed by the mine manager, and of the manner in which the mine is to be worked.
The first thing, then, to be done is to work out a definite scheme on which the sampling is to be conducted. This being determined, the next thing to do is to fix the position and distance apart of the samples. This should be done by the engineer in charge of the sampling party, and the position should be distinctly marked on the drive, rise, shaft, or winze, by a distinct number, preferably with white paint, and the numbers entered in his note-book.
In determining the distance apart the samples are to be taken, the engineer will take into consideration the nature of the ore body. In quartz reefs, carrying gold and silver, three feet apart should be about the proper distance; in the case of those propositions such as low grade sulphide bodies, where the values are more evenly distributed, a much greater distance would be quite sufficient. Wide lodes should be sampled in sections of a width suitable for stoping, say from four to six feet. Patchy lodes should be closely sampled, to determine where the values are.
The object of sampling being to take such a quantity and in such a manner as to represent a true cross-section of the lode, it is best effected by making a groove across the whole width of the lode at right angles to its walls.
The best tools for this are a gad and a hammer. It has been pointed out that a pick tends to break a larger proportion of the softer portions of the lode, and a hammer to break off the projecting hard portions, thus, in one case, getting an undue proportion of the softer portions and, in the other, of the harder, either of which may contain greater or less pro portions of the values. Any ground that is capable of being attacked by hammer and drill is capable of having a groove cut in the manner proposed. The groove may be cut by the working miners if under constant supervision by the sampler. The sample itself should be caught in a box held as close as possible by the sampler to the groove being cut by the miner.
Before taking the sample the face should be thoroughly cleaned so as to remove all fines, which may have been thrown thereon by shots fired in the course of driving or sinking the tunnel or shaft. It is a good rule to take about five pounds weight for every foot in width of the lode. When taking the sample the sampler should note the width of the lode, the stoping width, and any peculiarities that may be observed.
The samples should be broken down and quartered in the usual way by the sampler until a sample of three or four pounds weight is obtained. When this has been done, the sample should be ground down to pass a twenty-mesh sieve and the quartering finished on a piece of glazed American cloth. The manner the writer has adopted is to roll the sample on a piece of the glazed cloth about one yard square, alternately from opposite sides and ends, until it is thoroughly mixed and then to spread it out into an even layer and take a sample with a spatula from over the whole of the surface. Samples taken in this manner have been found to agree very closely when assayed by two independent firms of assayers.
Glazed American cloth should be used, as canvas has been found to retain some of the fines, however much it is swept over. If the sample is perfectly dried before rolling and the rolling is done in such a manner as not to allow the ore to merely slide, it is much more quickly and thoroughly mixed by this method than by scooping it up and pouring it into a cone-shaped pile ; moreover, the fines tend to stick to the sample rather than to the glazed surface of the cloth. If anyone who has not tried this method will make a mixture of material of different colours he will see how quickly and well the sample is mixed.
In making a conical pile and then distributing and flattening it down to a truncated cone with the scoop or spatula, it is very difficult to get the material distributed proportionately all round and to get the different quarters to agree in their assays.
Having thus cut the samples down to about a pound in weight, they are bagged by the sampler and numbered. The better way to do this is by a numbered ticket enclosed in the bag. The engineer when collecting his samples will enter this number (which should not, as a check against any salting, correspond with the number painted on the drive) in his note-book, with the necessary particulars, its number in the drive, its position with reference to his starting point, the width of lode and drive, and any other particulars which he may think necessary.
Calculation of Results
The modern practice is to use the foot-ounce method, which is based on the higher mathematics of the law of averages, and which alone gives due weight to the varying widths of the ore bodies.
This is best illustrated by an example:—
This gives an average, including all samples, of 15 ounces per ton.
Omitting No. 4 it gives 9.22 ounces per ton.
Omitting Nos. 8 and 9 it gives 18.71 ounces per ton.
This is a case where re-sampling of the rich spot should be undertaken, if the engineer has the necessary time at his disposal), and to ascertain whether the high assay is an accidental occurrence or whether the values graduate from that value towards the assay samples on either side of it. Similarly, it would be as well to re-sample the spots whence the low assays came. If he had not the time at his disposal, or the assays were made at a distance after leaving the mine; the only way to arrive at the true value is by the method suggested by Mr. Argall in the Engineering and Mining Journal of New York, of substituting for the high assay the average value of the richer ore; that value in this case would he the value given by omitting samples 1, 4, 8, and 9, and then taking the general average of the whole. Thus, omitting samples 1, 4, 8, and 9, the value of the richer portions is 13 ounces per ton. Substituting this new value for No. 4 and taking the average of the whole as before, we get an average value of 9.76 ounces per ton, which should be very nearly correct. Whenever possible, however, it would be the better plan to re-sample the spots in doubt.
In the application of the results of sampling there are many points to be considered before the engineer can give a definite idea of the milling value of the reserves of ore in the mine, apart from the estimates of available tonnage which can be stoped out.
A calculation should be made as to the proportion of waste which will necessarily have to be included when stoping. A narrow reef does not always hang sufficiently strongly to one wall to enable it to be stripped before breaking down, so that the assay value of the lode would have to be reduced proportionately before the value of the ore going to the mill would be arrived at. If the mine is going to be worked by machine drills a wider width of stope will be necessary than in the case of one worked by hand-drilling. In the case of a lode containing a number of veins of ore with intervening waste matter, a considerable portion of waste would have necessarily to be included, apart altogether from that which would be used underground filling stopes, &c., to obtain the pay ore.
An idea of the amount of this could be arrived at by weighing the ore as it is trucked out and checking by the amount of waste going over the dump, if the ore is one which lends itself to sorting.
The old controversy as to trial crushings versus sampling, plus assay, plus judgment, has, it is considered, been fully answered by leading authorities. In most cases it will be found that the discrepancy arises from imperfect methods of sampling and estimating the values of the ores in the mine. A great deal of’ the sampling, where these discrepancies occur, is done by the underground manager taking a few samples of material which he believes, from inspection and experience, to represent the average of the lode, but which may or may not at all do so. Again, a great many seem never to have heard of the foot-ounce method of obtaining the true average of the lode, which method alone takes note of the varying widths, and base their averages on the arithmetical mean of the samples and set this opposite the arithmetical mean of the widths. Very often no notice is taken of the necessary waste matter that must be included. Again there is, frequently, no check on the mill operations by sampling or assay. A great deal, also, depends on the construction of the mill, whether it is obsolete or otherwise, whether richer or poorer ore has previously passed through; richer ore may help to feed the plates, poorer may gather amalgam from them, according follow they are cleaned up. All these points must be taken into consideration before blaming the sampler and assayer for the discrepancies. Sampling should, properly, be done by someone who has made a special study of the subject and who thoroughly understands it.
In operating a mill the ore should, be regularly sampled as it goes into the bins from which the stamps are supplied. Samples should also be taken and assayed of the tailings and slimes leaving the battery. These operations are best conducted by the many Automatic sampling machines which are readily obtainable or can easily be constructed by the mine carpenter.
This article cannot better be concluded than by quoting from the editorial by the well-known American authority, Mr. T. A. Rickard, in the Engineering and Mining Journal of New York, dated 14th January this year, especially made with reference to Australian methods:
“ That ‘a bulk crushing ’ often does not ‘ live up to the alluring promise of the assays ‘ is true enough, if either be carried out by incompetent persons; but no method is to be gauged by the unskilful use of it, and the experience of earnest professional men in charge of the biggest mines in the world has demonstrated sampling and assay, plus judgment, to be the one practicable manner of arriving at the value and tonnage of ore in a mine.”