I The Promontorio Silver-Mine, Durango, Mexico. KRANCIS CHURCH L.INCOLN, NEW YORK, N. Y. A Paper Read before the American Institute op Mining Engineers, at the Toronto Meeting, July, 1907. AUTHOR'S EDITION. 19 0 8. subjeW to eevision. [TRANSACTIONS OF THE AMERICAN INSTITUTE OF MINING ENGINEERS.] < The Promontorio Silver-Mine, Durango, Mexico. BY FRANCIS CHURCH LINCOLN,* NEW YORK, N. Y. (Toronto Meeting, July, 1907.) I. Situation and Surroundings. The Promontorio mine is situated at the northern end of the Sierra San Francisco de Coneto, in the town of Promon- torio, Partido of El Oro, State of Durango, Mexico. As shown in the sketch-map, Fig. 1, the nearest railroad station is Chin- acates on the Mexican International Railroad, 82 miles north of the city of Durango. The mine is 16 miles north of the sta- tion by air-line. It is reached by means of a good wagon-road which first crosses the Guatimape Plain, passing the ranches of San Antonio and San Julian, to Estacion — a distance of about 15.5 miles — and then entering the mountains crosses the Sierra to Promontorio — a further distance of 11.5 miles- making in all a distance of 27 miles by road from railroad to mine. Promontorio is just beyond the summit of the range, at an elevation of about 8,000 ft. above sea-level, or 1,350 ft. above the Chinacates station. The Castillo de San Francisco, the highest peak in the Sierra, has an altitude of 10,000 ft., and the Promontorio road crosses the summit of the ridge at an ele- vation of 9,000 ft. by a pass just below this peak known as the Puerto del Almagre. The Promontorio mill is situated at Santa Ines, 2 miles by tram or 3 miles by wagon-road to the north of Promontorio and about 600 ft. lower. The Sierra San Francisco de Coneto decreases in height towards the north and west till it comes to an end at the Mel- chor Arroyo on the ranches of Melchor and Pamos. Beyond this arroyo another range, the Sierra de la Candela, begins. In the vicinity of the Promontorio mine the hills are very rugged and the surface is furrowed by ravines which contain running streams during the rainy season — July, August and * Fellow in Geology, Columbia University, New York, N. Y. [1] 84 THE PROMONTORIO SILVER-MINE, DURANGO, MEXICO. September — but are dry throughout the rest of the year. Fig. 2 is a view of the Promontorio mine, looking east, showing the 4% Longitude West 105 %\ ( ^ ^ \c ( Estacion \ \ San Julian Santiago \^^^ Antonio^ Papasquiaro%, >.-(v"^ so' from XGreenwicli Ramos/ v^o-^ ( 'te' if (/santa Ines PROMONTORIO \ Pi^^ ' m ® Potrillos Chinacat N \ 1 \ s Scale 1 =1,200,000 Canatlan ^ >t 50l Ot >I o| DURANGO ®'^24°M. Fig. 1.— Map of Part or Burango, Mexico. Eefugio shaft-house in the ravine, the San Joaquin dump on the hill, the administration buildings on the left, and the power- [2] THE PROMONTORIO SILVER-MINE, DURANGO, MEXICO. 85 house on the right. Fig. 3 is another view, looking northward. The nearest permanent flowing water is the Melchor Arroyo, a good-sized stream which might be utilized for power. A con- siderable number of large pine-trees are still standing on the slopes of the hills immediately adjacent to the mine, while a short distance away, oaks and other small trees are fairly [3] 86 THE PROMONTORIO SILVER-MINE, DURANGO, MEXICO. abundant as well, promising a readily available supply of mitie- timbers and tire-wood for a long time to come. II. The Country-Eock. The country-rock of Promontorio is a rhyolite-porphyry. The ground-mass is glassy, showing flow-lines, and the pheno- crysts are large quartzes and orthoclases, together with [4] THE PROMONTORIO SILVER-MINE, DURANGO, MEXICO. 87 smaller and somewhat altered hornblendes. Small grains of magnetite are rather plentiful, and a much lesser number of little crystals of pyrite are to be seen. The rock is, therefore, a typical rhyolite-porphyry, and undoubtedly belongs to the Tertiary rhyolites so common throughout the Sierra Madre of Mexico. ; Of special interest are the inclusions. The principal ones are small, dark, angular fragments, less than an inch in diameter, which are distributed rather plentifully in some parts of the porphyry. Microscopic examination shows these inclusions to be fragments of andesite and dacite, which were evidently broken from older rocks and brought up from a depth by the rhyolite-porphyry. Occasional rounded pieces of binary granite, from several inches to a foot in diameter, are also to be ob- served in the porphyry. These may have been broken off from underlying-rock like the andesite and dacite ; or they may be simply segregations, since they are composed of quartz and orthoclase, the commonest phenocrysts of the porphyry. According to Ordonez,^ the succession of Tertiary eruptives in Mexico is similar in all districts, and may be summarized as follows : Group No. 6, Basalts — Andesitic basalts. ; Group No. 5, Dacites — Andesites. Group No. 4, Ehyolites. Group No. 3, Andesites — Dacites. Group No. 2, Diorites— Diabases. Group No. 1, Granites — Granulites. If this order is correct, the Promontorio rhyolite-porphyry be- longs to group ^o. 4. The dark inclusions in this porphyry show that group l^o. 3 is represented in depth by both andesite and dacite ; the binary granite, if not a segregation, is repre- sentative of group 1^0. 1 ; but inclusions indicating the ex- istence of group ITo. 2 have not been discovered. The Promontorio country-rock has a well-developed joint structure. It is split into sheets which range from less than an inch to more than a foot in thickness and are of con- siderable length and breadth. The strike of these sheets ik 1 Las Rhyolitas de Mexico, Boletin del Instituto Geologico de Mexico, No. 14, p. 66 (1900). [5] 88 THE PROMONTORIO SILVER-MINE, DURANGO, MEXICO. about ^N". 20° W. (magnetic), and their dip nearly vertical but inclining slightly towards the northeast. The only other rock in the vicinity of Promontorio is a rhyo- lite which caps the range. At Coneto, 28 miles SE., there is an outcrop of andesite of similar character to that of the in- clusions in the porphyr3\ III. The Vein. The Promontorio vein strikes through the rhyolite-porphyry N. 55° W. (magnetic). Its dip is vertical at the surface, inchn- ing towards the SW. in depth. Thus it cuts the joint-planes of the porphyry at an acute angle in both strike and dip. The Promontorio vein proper has been followed beneath ground for a horizontal distance of 2,660 ft., but it cannot be traced so far upon the surface because of the covering of soil. Below ground the vein still continues towards the SE., while to the NW. it forks, and the West vein, proceeding from the hang- ing-wall of the Promontorio vein proper, has been followed an additional distance of 968 ft. The "West vein can be traced on the surface much farther, and is probably identical with the LaLuz vein tunneled in a prospect far to the 'NW. of Pro- montorio. To the SE. no vein has been discovered which can be identified with the Promontorio vein. Either the vein has pinched out rapidly in that direction, or, as seems much more likely, the vein is older than the rhyolite which makes its ap- pearance to the SE. and has been covered by it. There are no parallel veins near the Promontorio, but there are numerous cross-courses, all of which fault the E"W. part of the Promontorio vein toward the SW., the horizontal displace- ments varying from 1 to 13 ft. These cross-courses are in some instances simple faults, while in others they have become mineralized and constitute veins. The most noteworthy ex- ample is the Yeta Dolores, which strikes K 42° E. (magnetic) and can be traced as a well-defined quartz-vein all the way from Santa Ines to its junction with the Promontorio vein — 2 miles — and for some distance beyond. Another important cross-course is known as the Yeta Atravesada. It is much less marked than the Yeta Dolores, strikes K 28° E. (magnetic) and is possibly identical with the Los ^aufragos vein which has been explored in a prospect to the SW. of Promontorio. [6] THE PROMONTORIO SILVER-MINE, DURANGO, MEXICO. 89 All the cross-courses dip steeply to the SE., which makes their junctions with the Promontorio vein pitch steeply in this direc- tion also. IV. The Ore. The Promontorio vein is frequently divisible into three dis- tinct parts: 1, a hanging-wall portion of vein-matter; 2, an intermediate portion of more or less altered country-rock, and 3, a foot-wall portion of vein-matter. The principal value of the ore is in silver, and sometimes one, sometimes another, por- tion of the vein is richest. In general, however, either the foot- or the hanging-wall portion contains the most silver, and the intermediate portion is most likely to he ore when both foot- and hanging-wall portions are rich. Thus the Promontorio ore consists of vein-matter and min- eralized country-rock. The characteristics of the fresh coun- try-rock have already been described. It has been mineralized in two ways: 1, by silicification and impregnation with small scattered grains of the same sulphides as are found in the vein-matter ; and 2, by the precipitation of secondary minerals in joint-cracks and decomposed spots. The common primary vein-minerals are quartz, galena, and sphalerite, less pyrite, a very little chalcopyrite, and minute quantities of bornite, chalcocite, and covellite. The rare pri- mary vein-minerals are tetrahedrite, chalcocite, argentite, and native gold. The oxidized vein-filling consists of quartz, kao- lin, hematite, wad, and limonite, with occasional films of mala- chite and linarite and remains of the sulphides. The min- erals which have contributed to the secondary enrichment are native silver, chalcocite, and a little chalcopyrite. The native silver does not contain even a trace of gold. Secon- dary enrichments occur both in oxidized portions of the vein and in the country-rock of the walls and horses. Considering the frequency with which free silver is still en- countered in the Promontorio mine, the rather even grade of the ore is somewhat surprising. In a systematic sampling of the mine, the highest result obtained was 263.6 oz. of silver per ton, and only 12 out of 1,059 assays exceeded 150 oz. per ton. The ratio by weight of gold to silver in the shipping-ore varies from 2 : 1,000 to 3 : 1,000 parts. The presence of small [7] 90 THE PROMONTORIO SILYER-MINE, DURANGO, MEXICO. amounts of copper-minerals always indicates a higli silver-con- tent, but in all cases where neither copper-minerals nor native silver can be identified in the vein-filling, assays are necessary to distinguish between ore and waste. When the assays are plotted upon the mine-map, it is seen that the ore is arranged in shoots which, like the junctions be- tween the Promontorio vein and cross-courses, all pitch steeply toward the SE. These shoots sometimes parallel the faults, while in other cases they are cut by the faults or occur in unfaulted parts of the vein. They are usually long and nar- row, extending from 15 to 100 ft. along a level and cutting many levels on their pitch. After continuing downwards for a number of levels, shoots sometimes pinch out. 'New shoots may come in along the line of the old ones, or make their ap- pearance in intermediate positions. The shoots are either primary or secondary, both pitching in the same general direction. The primary shoots are distinguish- able by their comparatively high content of sulphides, by their lack of secondary minerals, and by their habit of being cut off by faults unless occurring in unfaulted parts of the vein. On the other hand, the secondary shoots are recognizable b}^ their low content of sulphides, by the presence in their richer portions of the secondary minerals, native silver, chalco- cite, and chalcopyrite, and by their tendency to follow closely well-defined faults. The primary ore-shoots are dominant in the lower levels, the secondary in the upper. The secondary ore-shoots reach their maximum development in the neigh- borhood of the fourth level, where the Yeta Dolores shoot ex- tends horizontally for 460 ft. and the Yeta Atravesada shoot for 456 ft. i^one of the cross-veins have developed ore at a distance from the Promontorio vein, although small amounts of ore have been taken from some of them near their junctions with the Promon- torio vein. Considerable prospecting has been carried on, but no other mine has been discovered in the district. [8] THE PROMOXTORIO SILYER-MIXE, DURAXGO, MEXICO. 91 Y. The Eelatioxs or the Primary Veix-Mixerals. Polished surfaces of the sulphide ore were prepared aud ex- amined microscopically under the direction and after the methods- of Dr. William Campbell, for whose assistance I de- sire to express my thanks. The specimens consisted, for the most part, of three minerals, galena, sphalerite, and quartz, Fig. 4. — Low-Geade Syxphide Op.e. Olagnified 80 times; . Eelatioxs of Sphaeeeite, Qeaetz, axd Gaee^'a. whose relations are illustrated in Fig. 4. This shows that the sphalerite was formed first. It was fissured, and subsequently quartz was deposited in the fissures, and also in crystals on the exterior. Finally came galena, filling the vugs in the quartz-vein and molding itself about the previously deposited sphalerite and quartz. - Economic Geology, toI. i., pp. 751 to "56 (1906). [9] UNIVERSITY- OF ILUNOIS LIBRARY AT URBANA CHAMPAIGN 92 THE PROMONTORIO SILVER-MINE, DURANGO, MEXICO. Pyrite is an important component of the sulphide ore, though by no means so plentiful as sphalerite, galena, or quartz. The relation of pyrite to these minerals is clearly indicated in Fig. 5. Pyrite is there shown to be incrusted with sphalerite, which is, in turn, incrusted with quartz-crystals, while galena occupies all the remaining space, just as it does in Fig. 4. Pyrite, there- fore, clearly antedates sphalerite, and was the first vein-mineral to be formed. Fig. 5. — Low-Ghade Sulphide Ore. (Magnified 80 times). Eelations of Pyrite, Sphalerite, Quartz, and Galena. Chalcopyrite is a much rarer constituent of the primary ore. Its usual mode of occurrence, illustrated in Fig. 6, is in grains attached to the sphalerite and surrounded by galena. In this illustration one grain is shown surrounded by bornite and an- other is intimately associated with covellite. In other speci- mens chalcocite has been found in similar relations with chal- copyrite. These associations indicate that the minute amounts of bornite, chalcocite, and covellite found in the ore are pro- bably secondary and derived from chalcopyrite. Chalcopyrite has not been found contiguous to quartz, but since it is entirely [10] THE PROMONTORIO SILVER-MINE, DURANGO, MEXICO. 93 absent from quartz-veins in sphalerite and present in galena- veins in this same mineral, it is reasonable to infer that it is Fig. 6.— Medium-Grade Sulphide Ore. (Magnified 80 times). Relations OF Sphalerite, Chalcopyrite, Covellite, Bornite, and Galena. Fig. 7.— Medium-Grade Sulphide Ore. (Magnified 80 times). Relations OF Pyrite, Quartz, Bornite, Chalcocite, and Galena. later than the quartz. The secondary copper-minerals asso- ciated with grains of pyrite are illustrated in Fig. 7, which [11] 94 THE PROMONTORIO SILVER-MINE, DURANGO, MEXICO. shows that the bornite is younger than the quartz' and older than the chalcocite. The order of succession of the minerals in the primary ore of Promontorio is therefore: 1, pyrite ; 2, sphalerite; (period of crushing) ; 3, quartz ; 4, chalcopyrite ; 5, galena. Rich primary minerals are exceedingly rare at the Promon- torio mine. They play no recognized part in the mine's pro- duction, although they are probably present in minute amounts in the ordinary primary ore. Fig. 8.— High-Grade Sulphide Ore. f Magnified 200 times). Keeations of Quartz Gold, Tetrahedrite, Chalcocite, and Argentite. A specimen of rich ore obtained from the fourth level is ap- parently of primary origin. It consists of numerous metallic gray stringers and veinlets in milky white quartz. Polished surfaces, examined under the microscope, show the cavity- fillings to consist of tetrahedrite, chalcocite, argentite and native gold, arranged as illustrated by Figs. 8 and 9. (These minerals were identified by methods which I developed and shall soon publish.) Quartz was deposited, then shattered, and native gold de- posited upon it in small isolated crystals. Tetrahedrite came [12] THE PEOMONTORIO SILVER-MINE, DURANGO, MEXICO. 95 next, forming detached ci^stals also, which occasionally sur- rounded gold. A crust of chalcocite then formed over all, and what gold had not already been involved in tetrahedrite became surrounded by chalcocite. This is indicated by Fig. 9. The section of veinlet there represented was completely filled with chalcocite, but in most instances the central parts of the crevices were left open and were filled later with argentite, as shown in Fig. 8. Fig. 9. — High-Grade Sulphide Oe,e. (Magnified 200 times). Eelations OF Quartz, Gold, and Chalcocite. The order of succession in this rich piece of ore was there- fore: 1, quartz; (period of crushing); 2, native gold; 3, tetra- hedrite ; 4, chalcocite ; 5, argentite. It will be noticed that the period of crushing came just after the deposition of quartz instead of just before, as in the ordi- nary primary ore. The difierence in conditions thus clearly, indicated probably caused the deposition of particularly rich ore at the point whence this specimen was taken. [13] 96 THE PROMONTORIO SILVER-MINE, DURANGO, MEXICO. YI. Geological History of the Promontorio District. The Promontorio rhjolite-porphyry was extruded during Tertiary time through underlying andesite, dacite and, perhaps, binary granite. Pressure during cooling developed sheet- jointing. After the consolidation of the porphyry a disturbance took place which resulted in the formation of an extensive N W.-SE. fault, through which heated ore-bearing solutions rose. This disturbance was probably caused by an intrusion of molten igneous rock that faulted the overlying layers and gave forth plutonic emanations, which, ascending through the fault, de- posited the primary ore of the Promontorio vein. The conditions were such that first pyrite, then sphalerite was deposited. A slight rearrangement along the fault led to the shattering of some of the pyrite and sphalerite, and at the same time brought about a change of conditions, so that quartz and, soon after, chalcopyrite were formed. Finally, galena was deposited in all remaining cavities, and the deposition of pri- mary minerals came to an end. After the mineralization of the Promontorio fault another upheaval took place, which was probably contemporaneous with the extrusion of the rhyolite that caps the Sierra San Francisco. This caused the formation of a series of NE.-S W. faults, sev- eral of which intersected the Promontorio vein. These faults were slightly mineralized, perhaps by lateral secretion from the porphyry, but certainly in a different manner from the Promon- torio fault. The district was covered by rhyolite for a while, but, when this was eroded, the only partly tilled cross-faults formed con- venient channels by which surface-waters were tapped oft* and, entering the Promontorio vein in the neigborhood of these cross-faults, rearranged its contents. Rich deposits of second- ary minerals were thus formed in the vicinity of the cross- courses. VII. Mining. The Promontorio mine was discovered by Joaquin Contreras in 1880, and purchased by its present owner, the i^Tegociacion Minera de Promontorio, S. A., in 1887. It is at present the only real mine in the whole Sierra San Francisco de Coneto district. It should be noted, however, that the famous Po- [14] THE PROMONTORIO SILVER-MINE, DURANGO, MEXICO. 97 trillos tin-deposits are situated in these mountains, and that a system of silver-gold veins at Coneto has been worked inter- mittently for a long period of time. When mining began at Promontorio, it was necessary to freight the ore 260 miles to the railroad at Fresnillo. At that time, shipping-ore had to contain at least 240 oz. of silver per ton. In 1892 the railroad reached the city of Durango, making it profitable to ship 90-oz. ore; and when, in 1900, the branch line was opened to Chinacates, 60-oz. silver-ore could be shipped at a profit. The mine was first opened by the San Joaquin shaft, on a hill-side (see Fig. 2), and later by the Cinco Senores workings, still further ap the hill. The present main working- shaft, the Refugio, is in the ravine beside the hill upon which the older openings are situated. The shaft-house is visible in Figs. 2 and 3. This shaft has been sunk to a depth of 675 ft., of which the first 280 ft. are in the vein, and 14 levels run from it. Levels 1 to 9 are 40 ft. apart; levels 9 to 14 are 71 ft. apart. Level 1 is a tunnel which starts at the mouth of the shaft and passes into the hill to the SE. where it connects with the early workings mentioned above. This level has the furthest extent towards the SE. of any. The other levels are all drifts run in both di- rections on the vein for longer or shorter distances. Level 9 extends furthest towards the NW., passing the fork in the Pro- montorio vein into the West vein and finally connecting with another shaft, the Santa Maria, at a depth of 250 ft. From the SE. face of the first level to the l^W. face of the ninth level is a horizontal distance of 3,628 ft. A cross-cut from a point in the Santa Maria ravine a short distance below the Santa Maria shaft intersects the fourth level and converts it into a tunnel. When I visited the mine in the fall of 1906, the method of operation was as follows : All the material mined was trammed out to a patio or sorting-yard by means of the cross-cut from the fourth level. Rock mined above the fourth level was dropped to it, and rock mined below was raised to it by the hoist at the Refugio shaft. On the patio the produce of the mine was sorted to " shipping " ore, running 60 oz. of silver per ton, and better. The rejected material was thrown [15] 98 THE PROMONTORTO SILVER-MINE, DURAXGO, MEXICO. on the milling-clump when it ran better than 20 oz., and on the waste-dump when worse. Since I left Promontorio, a mill has been started, and it is probable that ore of the grade formerly shipped is now being milled together with the lower- grade material. The power-house, situated in a small arroyo, across from the main shaft (see Fig. 3), contains six tubular boilers, which burn wood and sapplj steam to the hoist and compressor, and to en- gines operating generators. The electricity generated supplies power at the patio, illuminates electric lamps, and operates the pumps. A 120-kw. alternating-current generator supplies the power to four 3-in. Worthington electric pumps. In the dry season the pumps are worked for 8-hr. a day, and raise about 96,000 gal. of water to the fourth level daily. Most of this water comes from the fourteenth level, a distance of 555 ft. In the rainy season, it is necessary to run the pumps 14-hr. a day, thus raising 168,000 gal. per day. Fire-wood delivered at Promontorio costs $2.50 per cord and lumber $17.50 per 1,000 ft. The wages paid per day are : . §0.75 Laborers, ..... 0.375 to 10.75 . 1.25 Shift Bosses, . 1.00 to 1.25 Timbermen and Carpenters, 0.875 Blacksmiths, ..... . 1.25 . 1.25 . 1.00 Firemen, ..... . 0.625 Labor is, on the whole, plentiful. Some difficulty is experi- enced in keeping sufficient men at the mine during the periods of sowing and harvesting of crops, for the Durango laborer prefers farming to mining. Another peculiar condition has to be met in cold weather, when all hands want to work on night- shift because their cabins are too cold to sleep in at. night, but are warmed to a more comfortable temperature by the sun during the day. With the exercise of a little tact there should be no difficulty in obtaining all the labor desired. [16] THE PROMONTORIO SILVER-MINE, DURANGO, MEXICO. 99 YIIL Milling. Milling operations did not prove a great success at Promon- torio in the early days of the mine. One brand-new mill, which had never turned a wheel, was completely destroyed by the col- lapse of a dam. Another mill was erected later, but failed to give good extractions, as the tailings-dumps bear witness. Mr. Gordon Wilson carried on a long series of milling-tests upon Promontorio ore, and came to the conclusion that the best results could be obtained by concentration, followed by sliming and cyanidation of the tails. His experiments indicated that 45 per cent, of the values could be extracted by concentration in 10 per cent, of the weight, and that, of the remaining 55 per cent., 89 per cent, of the silver, and practically all the gold, could be extracted by sliming and cyaniding for a period of 10 days. This would mean a total extraction of 94 per cent. Mr. Wilson, therefore, constructed a 50-metric-ton concentrating and cyaniding mill at Santa Ines, which has now been in oper- ation for several months, and, I am informed, is performing work. The dam which supplies water for this mill is situated mid- way between Promontorio and Santa Ines. It is 100 ft. high, and was constructed at a cost of $50,000. IX. Production. The smelter-returns on shipments from Promontorio are re- ported to have been about $5,000,000. Unfortunately, no rec- ords of the quantities of gold and silver produced were kept during the early bonanza days of the mine. From Dec. 5, 1896, to Aug. 18, 1906, there were produced and sold 5,689,618 oz. of silver and 15,857.4 oz. of gold. During this period of re- corded production the Promontorio mine lost its position as " one of the largest producers of silver in Mexico,"^ and became one of the many minor producers. At present there is but little shipping-ore in sight in the mine, but there are large reserves of good milling-ore, and, with new and effective milling methods in full operation, we may soon expect to see the Promontorio mine make its way to the front once more. 3 In galls, Trans., xxv., 149 (1895). [17]