Applied Mining Geology

Applied Mining Geology

The geologic notes are taken as soon as possible after the ground is broken so that any mistake in the mining may be corrected at once, or any particularly advantageous procedure may be suggested before any useless work is done. Taking the notes underground is a comparatively simple matter, but a few necessary precautions may be pointed out. The essential to success is that the notes shall show exactly what geological facts are disclosed. The observer must discriminate carefully between important and unimportant exposures in making the record, especially as regards the relation between veins and stringers, between faults and minor slips, or between the characteristics of veins of different ages. A simple color scheme has been adopted for making this record. Red pencils are used to indicate vein filling, which may be ore, barren pyrite, in fact any metallic minerals or quartz, and the record of the minerals present is found in the written notes. Blue coloring indicates evidence of faulting, either as the definite planes of movement or the crushed granite resulting from such movements. The sketches are always supplemented with copious notes as to the dip and strike, character of mineralization, and condition of the surrounding granite. A loose-leaf system has been adopted for the notes, which allows keeping them in the form of a card index. The scale used for the notes is 40 ft. to the inch, although 20 ft. to the inch is found to be a better scale in places where the detail is especially complicated.

In the office the principal working maps are drawn to a scale of 40 ft. to the inch. They are prepared on tracing cloth, one level to a sheet, and made to register perfectly with each other. The notes are platted on these sheets as soon as the surveys are posted, and the same color scheme is used as that for the original notes. After each platting, these maps are studied in conjunction with all the recorded data of that particular vicinity, and ideas for future development or for alterations of the present plan suggest themselves. The details studied on these maps, which often lead to useful suggestions underground, are such matters as the position of a block of drag ore at the intersection of a fault and a vein ; the sudden change in the character of a vein; the relation of dip and strike of a vein to those of a fault cutting it, and the correlation of faults, veins, and ore shoots from level to level.

A set of maps drawn to a scale of 100 ft. to the inch is also maintained in the office for each mine. This scale makes it possible to show a whole mine level on one sheet without having it inconveniently large, and many useful correlations of faults and veins in widely separated parts of the same mine are made with this set. All the details consistent with the smaller scale are platted on these maps, although the written notes are usually not transferred from the larger working maps. Each mine foreman and superintendent is provided with a copy of the 100-scale geologic maps of his mine and has them posted from the office set at regular intervals.

Next in importance are the vertical cross-sections, of which Fig. 1 is typical. They are made on a scale of 100 ft. to the inch and are taken at various intervals, along lines depending on , the strike of the veins to be studied. Since the veins and faults in this district present such a variety of strikes, it is obvious that no cross section can be prepared which will cut all of them at right angles. Therefore, the true dip for a vein whose line of strike meets the plane of the


section at an acute angle cannot be used directly in making that section. In order that the geological relations on the section may be kept geometrically correct there is a convenient instrument in use which automatically reduces the true dip and angle of intersection to the angle of dip which must be platted. Vertical sections have frequently been instrumental in clearing up complicated geological structure, and many savings and discoveries have been made by studying them.

There is also maintained in the office a set of 200-scale tracings covering the entire district. Only the principal geologic features appear on them, but the proper relative importance of the veins and faults is maintained. This set of maps is particularly useful in correlating and identifying veins in widely separated mines.

Before presenting examples of a few of the typical discoveries made through the aid of geologic maps and sections, some of the numerous minor uses of geologic information should be pointed out.

In laying out a certain drain tunnel on one of the lower levels where only limited development work had been done, it was found that the Bell fault would be encountered somewhere between the points to be connected. It was further noticed that the drain tunnel would meet the fault planes at an acute angle, and, since the fault zone is 75 ft. wide, the tunnel would have to lie within it for a distance of 320 ft. It was therefore decided to run the tunnel to a point as near as possible to the Bell fault without cutting it, then to cross-cut the fault at right angles and continue the work in-the foot- wall of the fault. The problem for the Geological Department, then, was to locate the fault on this partly developed level as nearly as possible, and to turn the tunnel in accordance with the determined position. Several cross-sections and projections were made from the known positions of the fault, and these checked fairly well for the desired position. When the course of the tunnel was finally turned, the Bell fault was cut at right angles only 15 ft. from the turn. This work will result in a minimum cost of timbering and maintenance of this drain tunnel.

The pump stations in Butte are exceptionally large underground excavations, and it is desirable to locate them in ground which is as free as possible from faults and fault veins. The Geological Department has been called upon to make examinations for this purpose and in one case it was found advantageous to change the location of one of these pump stations. The position originally selected for it would have been crossed by certain faults and much difficulty would have been met both in the excavation and in future maintenance.

Where a, raise was designed to connect two levels, it has frequently been possible to select a starting point such that the raise would be continuous in Ore, whereas in any other position it would have to cross a fault before reaching the level above. In the West Gray Rock mine, it was estimated by projections that the faulted segment of a certain vein between two faults would only be a few feet in length on the 800-ft. level. The development work at the time was limited on this level, and the calculations had to be made from positions known on the 1,000-ft. level, where the faulted segment was considerably longer. In order to make the raise in ore continuously from the 1,000- to the 800-ft. level, it had to be placed in accordance with these calculations. The result was entirely successful, and the raise through this wedge-shaped block of ore reached the 800-ft. level at its very apex.

In another case, the geologic cross-sections and plans showed that the La Plata fault was nearly parallel in strike to an east-and-west vein. The dip of the vein, however, was steeper than that of the fault, so that in the stopes the fault generally encroached upon the work and presented all the appearance of a hanging-wall of the vein. The stope became narrower and the operators considered it a pinched place in the vein. The stope would probably never have reached the level above if the geologic conditions had not been understood. When the ore was finally cut off raises were continued from the top of the stope, but designed to cross through the fault and to make the fault a foot-wall rather than a hanging-wall. By this means, the ore above the fault was encountered at its lowest point and the stope continued to the level above. This procedure underground was suggested by studying that portion of the section, Fig. 1, which shows the La Plata fault and the Gray Rock east-and-west vein between the 1,200- and the 1,300-ft. levels. Fig. 1 is a cross-section through the West Gray Rock mine, and is typical of the 100-scale sets of sections carried for all the mines.

An important discovery made through a somewhat different agency is represented in Fig. 2. The main drawing is a portion of the 40-ft. scale map of the 1,800-ft. level of the Mountain Con mine, and the small sketch is a reproduction of the 20-ft. scale notes upon which information the discovery was made. It will be seen that without any geologic notes the drift No. 1824 would appear to be a drift on a continuous vein, and in fact it was stoped as such. The notes, however, displayed a significant difference in character between the westerly end, from which the work progressed, and the easterly end. The notation on the map indicates this difference. The westerly portion exhibited the character of a quartz-pyrite vein, which it was known to be, but as the drifting continued the exposures were characteristic of the northwest fault vein system, notwithstanding the apparent continuity of the vein. This led to an investigation of the stope and the small sketch here reproduced was made. It showed these differences clearly to the geologist, and also the departure from the stope of the clay portions of the fault.

Accordingly, the cross-cut No. 1869 was run, and it encountered the faulted portion of the quartz-pyrite vein now being mined in drift No. 1828. As this drift


was advanced westerly it intersected the fault vein as shown, and proved the correctness of the theory. After this discovery the development of several other levels became merely a matter of projection.

Fig. 3 represents a portion of the 40-ft. scale maps of the 1,100- and 1,200-ft. levels of the West Gray Rock mine. It will be seen that the south-dipping east-and-west vein on the 1,100-ft. level lies north of the north-dipping Corra fault while on the 1,200-ft,. level


these positions are reversed. The cross-sections clearly showed a displacement of the vein by the fault. The ore above the 1,100-ft. level was stoped to the 1,000 continuously in good ore and a stope was being worked above the 1,200, but, as shown on the maps, this stope could not reach the 1,100 on account of the fault. The ore on the 1,100 did not come down to the 1,200, for the same reason. The small section A-A shows these, relations. It was therefore required to determine a point on the 1,200 from which a vertical raise could be run to intersect the vein as nearly as possible at its line of intersection with the fault. In other words, a vertical raise was to be so located as to reach the very bottom of the ore below the 1,100. A large-scale cross-section was drawn, making due allowance for the drag ore near the fault, and a course was given to the surveyor for the lateral drift No. 1218 in accordance with this section. The raise was to have one offset from the sill and to be vertical. The result of the work is shown in the small section in Fig. 3. Practically the bottom of the ore was encountered in this raise.

In Fig. 4 there is reproduced a portion of the 100-ft. scale maps of three levels of the High Ore mine, also a portion of the 40-ft. scale notes which led to the proper development of these levels. The map marked 300 Modoc shows a wide vein of good ore known as the Modoc vein. It was supposed that the entire drift was on this vein until the geologic notes showed that such was not the case. The portion near the shaft was barren; and the dip shown there was opposite to that, at the east end. Bands of fault clay passed out of the drift as shown in the notes. It was decided that there were two veins exposed in this drift. At this time the drift on the level below ; (700 High Ore, see Fig: 4) had been abandoned at the point C. By carefully projecting the Modoc vein with its south dip from the 300 Modoc and also the other vein which is called the Edith May with its north dip, it was found that the line of intersection of these, two veins would cross the level of the 700 High Ore at a point beyond the face of the old drift. Drifting was therefore continued in order to reach this determined point, and a certain amount, of the north wall of the Edith May vein was broken so as to see the ground north of it. Within a few feet of the predicted intersection the Modoc vein was found and was stoped continuously to the level above. The next move naturally was to get the faulted portion south of the Edith May vein, which was accomplished as shown on the map. This information could now be extended to the next level below. The development work on the 900 High Ore had only reached the point D, Fig. 4, but the line of intersection of the Modoc vein and the Edith


May vein was now well known on two levels and in the stopes, so it could be carried to the 900 with considerable accuracy. The underground work was therefore continued on the 900 with this point in view, and the map shows the accuracy with which the intersection was located. Nearly all of the ore thus developed on these three levels has been stoped.


In Fig. 5 is shown a portion of the 100-ft. scale map of the 1,400 level of the Mountain View mine. In order to understand the geologic work done in this case, the reader’s attention must be called to drifts Nos. 1429, and 1447. The remaining numbered drifts had not been run until the ore in them was discovered through the application of geologic knowledge of the conditions. The veins disclosed in these first drifts were, considered to be faulted segments of the same vein. Later, however, in correlating with other parts of the mine, the Middle Fault (see Fig. 5) was identified, but the direction of the throw on this fault was known to be to the left instead of to the right, as the former interpretation would require. Therefore, the drift No. 1447 must contain the faulted portion of a vein lying north of No. 1429, and further, the faulted segment of No. 1429 must lie south of No. 1447. The map shows that this reasoning was correct, as the missing portions of the vein were found in drifts Nos. 1406 and 1410, respectively. The next step was to reach the south vein above the Rams Fault (see Fig. 5), which was done in drift No. 1457. Thus everything was accounted for, and three valuable ore bodies were won.

The examples of geologic work given above are typical of the work carried on in the Butte mines, although many more might be cited. Of course, all of the suggestions made by the Geological Department do not result in the discovery of ore bodies, but a failure to find the vein predicted is unusual. When the underground work reaches a vein as predicted, but cuts it in a barren place, calculations on the pitch of the ore shoot in the plane of the vein are made, and the downward extensions of such shoots have been located.

Graphic determinations of the direction and amount of movement in the planes of the principal faults have been made, and the knowledge of these displacements, both vertically and horizontally, is very helpful in locating faulted veins, or identifying faulted segments of veins. It has been recognized that the amount of horizontal displacement of several veins by any particular fault may vary considerably, and this variation has been studied graphically. The relation between the amount of horizontal displacement, on one hand, to the dips of the fault and vein and the angle between their strikes, on the other hand, has been studied, and has resulted in useful deductions. In order to show the variety of horizontal displacements possible on one fault, owing to differences in angles of dip and angles of intersection, Fig. 6 has been prepared. The four diagrams represent the four possible cases, namely :

A. Dip of fault steeper than dip of vein, but in opposite direction.
B. Dip of fault flatter than dip of vein, but in opposite direction.
C. Dip of fault flatter than dip of vein, but in same direction.
D. Dip of fault steeper than dip of vein, but in same direction.

In all cases a normal downward displacement of 100 ft. is assumed


and the veins meet the fault at angles of from 10° to 90°. The various directions and amounts of horizontal displacement are shown. In one case it will be seen that the horizontal throw increases positively in direction to a maximum, and then becomes negative. The angle of intersection of the vein with the fault that gives the turning point, is called the critical angle, and a means of determining it graphically for any set of dips has been devised. It must be understood, however, that these graphical determinations have their limitations and must be applied accordingly.

Although this paper has dealt largely with problems related to structural matters in the Butte district, much attention is given to the study of the complicated mineralogical and chemical problems involved, but they fall beyond the scope of this paper.

B. H. Dunshee, Butte, Mont.:—I merely want to testify to the value of the Geological Department so far as the Anaconda Copper Mining Co. is concerned. The information that the mining men get from the Geological Department is exceedingly valuable. Many times in mining underground we come to a question that is obscure; we don’t know exactly where we are or what to do. Instead of going at it blindly and perhaps wasting time and money searching for the continuation of the vein, we merely go to the Geological Department, and the geologists come to our assistance, and almost every time their conclusions are correct; the work they do is intelligent, and again I want to testify to the value of the accurate maps of the veins and fault systems as worked out by our Geological Department.


The object of this paper is to present a brief outline of the methods of geologic mapping employed in the Geological Department of the Anaconda Copper Mining Co., at Butte, and to show by means of a few typical examples the practical nature of the results obtained. The extremely complicated geological conditions early encountered in Butte mining led mine operators to realize the necessity of accurate detailed geologic mapping as an aid to successful development of the ore bodies. The organization of the Anaconda Geological Department was undertaken in 1900 by H. V. Winchell, who, in conjunction with D. W. Brunton, worked out the essential methods of procedure which are, in the main, followed at the present time. Since the year 1906 the geologic work has been under the direction of Reno H. Sales, and in some respects the scope of the work has been slightly enlarged. An excellent article describing the general equipment of the geological department was presented by D. W. Brunton at the British Columbia meeting of the Institute in 1905. His idea, that data collected underground and recorded should be put. to practical use and not remain a mere “ inventory of the company’s underground possessions,” is the keynote to the success of applied geology in Butte.