A prospector may be, or become, a "mining expert" and be called upon to make examinations of mining properties, whether prospects or developed mines, so a few suggestions may prove useful. Mining properties of the precious metals are generally of two kinds, those containing ore deposits in place, such as fissure veins and blanket deposits and placers, the latter being gold-bearing. In both cases, and especially in the former, the character, position and other relations of properties are infinitely varied, so that no hard and fast rule can be given to suit all cases ; certain rules, however, will generally apply. A mining engineer receives a letter from a company telling him to go to such and such a country or region and examine and report on a certain property that has been offered them. Such a mandate is usually accompanied by a letter or report from the owner or parties offering the property, giving the owner's description of the same, or else the report of some expert on it. As a general rule such reports give the most favorable view, and in some cases must betaken "cum grano salts." To the mining engineer they give some sort of an idea as to what the property may be like. As to its value, etc., that he proposes to find out for himself. The company sometimes asks him to examine with a view to verifying or modifying or contradicting such reports. The region, the country, the character of the deposits, the local conditions, may in all probability be comparatively new or strange to him. Prior to starting he may make inquiries in mining circles if anything is known about the region or district. If there are" any published mining or geological reports or maps, he will consult these. Finally he starts out with as little baggage as possible, usually a small hand bag, containing a few necessaries; a tapeline, geological pick, clinometer and compass and note or sketch book. His dress is generally a suit of corduroys, leather gaiters and strong boots.
As he enters the region by rail or on horseback he notices the main geological features, whether the rocks are granitic, sedimentary, or eruptive. Finally he reaches the camp, calls on the owner or superintendent and rides up with him to visit the mine. If he should "lay over" for the afternoon in the village, he may as indirectly as possible try to pick up any gossip there may be afloat relating to the property. He is at once impressed with the accessibility or inaccessibility of the property, and estimates the probable cost of bringing down the ore to the mill or to the railway track, and observes the proximity or absence of timber and water power. At last he reaches the mine, dines at the boarding house, and is then taken over the premises by the superintendent. His first attention is directed to the surface character of the property, its topography, whether rolling, smooth or precipitous, whether it is high above the valley or near down to it, whether the mine is high or low as regards the water level or drainage system of the neighborhood, whether the property is conveniently situated for working the mine and transporting the ore, etc.
Accessibility is an important matter. In some regions, such as in the San Juan district (Colorado) for example, mines and prospect holes are sometimes on the top or sides of mountains or precipices, thousands of feet above the valley below, located at spots one would think only an eagle could reach; prospect tunnels, too, are driven where there appears scarcely a foothold for a squirrel. No spot, however, seems too inaccessible for the prospector. At a glance the engineer sees that in a property situated in such a region, accessibility is one of the first and often most formidable problems to be considered. To some of these mines are long zigzag trails cut in the side of the mountain. The engineer calculates how much the owners of the donkey or "burro" train will charge to bring that ore down to the valley or mill. He argues that a mine at that almost inaccessible height, ought to carry a good deal of pretty high grade ore to pay even for transportation by the " burros," let alone the cost of freight afterwards to distant smelting works.
On the other hand a mine whose workings open out within easy access of the valley or railway track, could afford to carry less valuable ore. Then there is timber and water power to be considered, the former for timbering the workings of the mine, the latter for running a stamp mill, or for supplying steam power to the engines of the mine. If there be no water power, and the vein carries free gold, the ore must be carried down to the nearest stamp mill. In a young or "virgin" property or prospect, the engineer will look out for a convenient site for such a mill, under a developed property; if there be a mill on the premises, he will examine and report on its capacity and suitability for treating the ores. A mill site must, of course, be selected close to some water power. In some districts there is a superabundance of water, in others a serious lack of it, or the supply is meager at certain seasons, or is frozen up in winter. Some mines are quite dry, but generally they will supply enough mine-water from their workings to afford steam power.
He observes the character and dip and direction of the veins if exposed on the surface, examines any prospect holes on them, and takes a few samples for assay. He will consider the nature of the ore deposits, whether they are in fissure veins in crystalline rocks, or blanket deposits in sedimentary rocks, or contacts at junction with eruptive porphyries. A great variety of local and minor details have to be noticed which can hardly be specified. Having looked over the surface, he enters the tunnel or workings with the superintendent. As he passes along, the latter is likely to call his attention to this or that spot, as especially good, and naturally he rather overlooks the poorer parts of the mine. He may suggest the advisability of taking samples from such favorable spots. The engineer, however, takes little heed, as, if he were to confine his attention to, and sample only these choice portions, he would obtain an incorrect estimate of the average run of the mine.
Moreover in some cases it is well to be on the look out, lest these points to which special attention is called by the miner, be previously "salted" or "put up" for the expert, and charged in various ways by rich ore. Having traversed the workings and obtained a general idea of the position of the vein and ore bodies, and taking an inventory of the amount of development, length of drifts, shafts, etc. (the latter he can obtain from a map of the mine in the superintendent's office, a copy of which he will send to his company) he asks the superintendent to leave him and his assistant and vacate the mine, as he does not wish any one except his assistant to be with him whilst he is taking samples for assay.
SAMPLING:
Now begins his hard and most telling work, the time and labor depending
very much upon the size and amount of development in the mine, or the degree
of accuracy necessary. Taking a large strip of muslin, he cuts part of it up
into small pieces about the size of a pocket handkerchief, these are to
contain his samples for assay, when quartered. Then he takes the remainder
of the muslin, or better still an ordinary candlebox, this to catch the mass
of small fragments he detaches from the vein with his pick. Now with a light
pick or with a chisel and hammer he begins, either from the entrance or end
of the tunnel, to detach small portions of the rock, cutting a rough groove
across the vein. Sometimes the tunnel occupies the whole width of the vein
in which case he will have to make a circular groove clear around the
tunnel, across floor, roof and walls; the fragments from his work drop into
the candlebox or onto the muslin. According to the length of the workings or
the need for great accuracy, he repeats this operation at intervals which
may be every 5 feet, 10 feet or 20 feet; at intervals of say 20 feet, he
masses and mixes together all the samples, breaks them up as fine as he can
on a shovel and divides the result into four parts, throws away three parts
and retains one. This he reduces to a fine powder and wraps up in the small
muslin pieces, ties it up and seals with sealing wax, marking on it the
number and other notes, such as 10 feet from entrance, etc., with an
indelible pencil. This work he continues untill he has reached the end of
the tunnel, which if it be 100 feet long will give him from 20 feet
intervals, five little sacks of powdered ore for assaying. As a check upon
this work and for reference in case of any accident to or any tampering with
his samples in transit, he will occasionally take a "grab" sample from his
broken rock before quartering it. Here and there, too, he may take a chunk
of some peculiar rock such as a porphyry or some peculiar streak in the
gangue, these he will put in his coat pocket and keep on his person.
It is sometimes important in a vein to find out what rock or portion of rock carries the most value. For instance, on one occasion we examined a vein said to carry gold clear across its entire width of some 50 feet. Now this so called vein proved to be a decomposed dyke of porphyry impregnated with pyrite and free gold, and through the dyke ran a net-work of little narrow quartz veins or veinlets. On sampling, whilst we took samples clear across the whole width of the vein, we kept those fragments which came from the quartz veinlets apart from those which came from the porphyry gangue. The result was we found the porphyry, constituting of course the main element, to be barren and the gold to be concentrated in the quartz veinlets constituting a minimum of the width. So in a vein there will generally be parts richer than others, "pay streaks" as they are called, which it is important to distinguish, also certain metallic minerals in the vein carrying greater values than others. Thus the pyrites, if undecomposed, may prove too poor to treat for gold, or in a silver mine, streaks of gray copper may be very rich, whilst bodies of coarse galena may be very poor. In a gold mine it is important for the engineer, if he can, to find out to what depth surface decomposition or oxidation has penetrated, because in this brown rusty matter will likely be most of the "free gold;" whilst when the unoxidized pyrites makes its appearance the ore is no longer free ore, but must be treated by some process other than that of a stamp mill, and with the incoming of pyrite the palmy days of the gold mine may be at an end. Sometimes, however, though the oxidized brown goethite gossan may play out and succeed to white quartz, the latter, if it be not too hard, white and "hungry," may still continue to carry free gold in it. Again in the veins, with their descent into depths, greater or less richness may occur or different varieties of ore set in, or absolutely barren quartz, so if there be shafts or tunnels driven on the vein a distinction should be noted with descent as to values found at different levels, also as to character and richness of the ore above and below water line; the latter corresponds to the average drainage level of the country.
This completes his underground examination. Whilst in the mine he may make a rough sketch or two of the vein showing the general disposition of the ore bodies or any peculiarities. On emerging and carefully securing his samples beyond reach of their being tampered with, he selects a convenient point, possibly on a neighboring hill facing the property, and takes a general sketch of the property in pencil also makes a pencil sketch and ideal section of the hill, showing the position of the vein and its workings, the amount of ore stoped out and the amount presumably in place intact; to estimate the latter is often a difficult and uncertain problem. He may make some sort of estimate as to the reasonableness or not of the price asked and give his estimate ; he can form, however, no true estimate of the value of the ore bodies till he has had time to assay his samples, for these are the crucial test of the value of the property. In writing up his report at his leisure, which will most likely be read at a general meeting of the company, he cannot be too clear, simple and explanatory in his account and its details, as it is to be remembered that the company is likely largely to be composed of men unacquainted with mining and mining terms ; he must therefore not take it for granted that they know what "stopes," "adits" and "gouge," etc., are, but explain as he goes along, accompanying his remarks with rough sketches to make his meaning clear and put the members of the company as much on the ground as possible. We ourselves have found that it is not necessary, generally to make elaborate notes in the field or to write pages of reading matter then, provided we make many sketches and on them put down items such as length of workings, etc., etc. The sketch is generally the notes, andwhen the engineer returns home, his sketches will recall vividly all he has seen and from these he will write his report. Upon certain matters, however, such as involve numbers, he should be very accurate in writing notes and not trust to treacherous memory for them.
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Sampling Mines For Investment And Avoiding Salting
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Gold And Silver Basic Prospecting Methods
