BANCROFT LIBRARY THE LIBRARY OF THE UNIVERSITY OF CALIFORNIA With the Author's Compliments Geology of Pachuca and El Oro, Mexico BY HORACE V./WINCHELL, *- Minneapolis, Minn. Paper of the American Institute of Mining and Metallurgical Engineers, No. 1074-M, issued with MINING AND METALLURGY, June, 1921. (New York Meeting, February, 1921) NEW YORK, N. Y. 1921 TRANSACTIONS OF THE AMERICAN INSTITUTE OF MINING AND METALLUR- GICAL ENGINEERS jSUBJECT TO REVISION] No. 1074-M. ISSUED WITH MINING ANP METALLURGY JUNE, 1921 iO I. J I Geology of Pachuca and El Oro, Mexico By HORACE V. WINCHELL, Minneapolis, Minn. (New York Meeting, February, 1921) AN EXAMINATION of the Pachuca and El Oro districts in July, 1920, although cursory and incomplete, disclosed facts of more than passing interest to the student of ore deposits; and inasmuch as the literature on these districts is limited and some of their features are little under- stood, a brief account may prove valuable as reconnaissance observations to be amplified by more thorough and competent observers in the future. Where no complete monograph of important mining camps has been published a partial presentation of the data is often useful, as it some- times leads to fuller and more accurate descriptions, by stimulating discussion. The following notes are presented with the consent of Mr. A. F. Main, managing director of the El Oro Mining and Railway Co., whose fund of information and wide acquaintance with both districts alone made it possible to cover so much ground in a short time. I have also to acknowledge gratefully the aid of Messrs. C. A. Lantz and D. S. Calland of the Santa Gertrudis and Real del Monte companies, respec- tively, in connection with permission to inspect geological features of interest in their mines. PACHUCA, HIDALGO, MEX. This mining camp has been described by Ezequiel Ordonez 1 and by A. F. J. Bordeaux. 2 Reported to have been discovered in 1522, it has produced silver .valued at between $250,000,000 and $375,000,000. Subsequent to the discovery here, by B. de Medina, of the patio process, in 1557, its production was increased, and now, using cyanidation, the output of the camp is nearly one-sixth of the production of the world. Topography and Climate The Pachuca district consists of a series of mountains and valleys, the minor features of which extend in an easterly and westerly direction and reach elevations of 8000 to 10,000 ft. (2438 to 3048 m.) above the 1 Mining District of Pachuca, Mexico. Trans. (1902) 32, 224. 2 Silver Mines of Mexico. Trans. (1908) 39, 360. Copyright, 1921, by the American Institute of Mining and Metallurgical Engineers, Inc. 2 GEOLOGY OP PACHUCA AND EL ORO, MEXICO sea. The range as a whole constitutes a continental divide; its axis runs northerly and southerly and separates the waters that flow westerly into the Pacific Ocean from those flowing easterly into the Gulf of Mexico. On the north and east, the surface falls rapidly down to the tierra caliente and is cut by deep canyons. In this warm country, about 4000 ft. (1219.2 m.) below the summit is a canyon 2500 or 3000 ft. (762 or 914.4m.) deeper that drains the region on the eastern slopes; here are raised coffee and such tropical fruits as bananas. Dense forests, heavy undergrowth, and much soil cover the land. Cloudy weather prevails over a large part of the year; the abundant rains do not run quickly off but are held in the soil and in the vegetation that conceal the underlying rocks. On the western slope we are almost immediately in different atmos- phere and country; the vegetation is relatively sparse and the conditions semi-arid. The rocks are either bare or covered with laterite; they are often weathered and rotted to a considerable depth and the eye has an unobstructed view over large areas. The sky is .clear, the sun beats hotly down, and although there are frequent showers in the afternoon, sharply limited to portions of the area, the waters either run off quickly or are rapidly absorbed by the rotted rocks. According to the best weather data obtainable, there is a marked difference in the amount of rainfall on the two slopes, within the few miles occupied by the operating mines. At Pachuca, the average rainfall for the years 1909 to 1912, inclusive, is given by Mr. Lantz as 11 in. per annum; according to Mr. Calland, the records of the Real del Monte Co. show at Loreto, where the Pachuca mill is situated, a precipitation of 21.22 in. (53.97 cm.); at the village of 1 Real del Monte. 32.76 in. (83.2 cm.) and at Velasco, about 3 mi. (4.8 km.) north of the Guerrero mill of the Real del Monte, 59.33 in. (150.7 cm.). These facts are mentioned because they suggest a difference in the chemical activity of surface waters, which as will be explained later seem to have played an important part in the genesis of these extensive ore concentrations. Geology We have to do here with Tertiary eruptives similar to those that contain so many of the mines of Mexico and the southwestern part of the United States. The underlying basement on which these volcanics were deposited is probably Cretaceous sediments, but none were ob- served in the area covered by these notes. The oldest rock seen is the older andesite. This is a thick flow of inconstant texture and varies from a massive rock with few phenocrysts to a highly porphyritic mass in which are many whitened feldspars and some altered pyroxenes with occasional biotites. Toward the north and west this typical andesite is succeeded by a quartz andesite, or dacite, HORACE V. WINCHELL 3 much more siliceous, lighter in color and with many quartz phenocrysts ranging in size from Jli2 to J6 in. (0.79 to 3.175 mm.) in diameter. Whether the dacite is a phase of the older andesite or a separate flow was not ascertained. No definite line of contact was seen. Both of these rocks contain fragments of darker fine-grained andesite and both at times exhibit marked flow structure. They can hardly have been intrusives, for there is no remnant of any covering rock. Moreover, at times the vesicular or amygdaloidal texture characteristic of rocks cooled near the surface is found. The older andesite appears on the surface over the greater portion of the area thus far developed by mining operations. It can be traced continuously from a point more than 1 mi. (1.6 km.') east of the Guerrero mill of the Real del Monte Co. to a point west of the Bordo shaft, where it passes beneath later eruptives, about 12 mi. Its width on the surface is much less, since its northern margin is south of the northern limit of underground mining operations and may be seen just north of the valley in which are situated the Paraiso and Santo Tomas shafts. Its southern limit of exposure was not traced. Within this older andesite are east-west zones of extensive and profound thermal alteration in which the rocks have been altered over areas probably several miles long and often hundreds of feet wide. The altered rock is softer and lighter in color than the fresh andesite and carries occasional pyrite crystals, with calcite films and veinlets, and even quartz incrustations. It is in these altered zones in the older andesite (and dacite) that the important ore deposits have been found and developed. The succeeding rock is later andesite. This consists of flows, breccias and tuffs, all highly porphyritic and generally less massive than the older andesite. It is, however, sometimes thick-bedded and dense and so similar in appearance as to be distinguished with difficulty except on weathered surfaces. It weathers in various colors from reddish brown to bluish or purplish gray and covers large areas north of the actively mined district. In places, only a thin shell of this rock is left; elsewhere shafts go down through it into the lower andesite. In the Girault tunnel, the contact is well exposed; near the Trompillo shaft there is a steep fault con- tact between the two formations with the Vizcaina vein lying between. This later andesite has not been considered promising ground for explora- tion and has produced but little ore. Whether this prejudice is based on its actual poverty or not is a question that can be answered only by future and deeper development, where this later formation attains great thickness. So far as I could learn, the only workings in it are compara- tively shallow (at least in the central portion of the camp) and do not attain depths where, even in the older andesite, valuable orebodies are first encountered. 4 GEOLOGY OF PACHUCA AND EL ORO, MEXICO Still younger, and overlying the later andesite, is rhyolite. This rock is cream colored and occurs on the surface in thin flows and tuffs, often much weathered and. cut by small arroyos. Rhyolite or quartz porphyry dikes of the same material are also seen on the surface and underground throughout the district from the Santa Gertrudis north to the Capula and from the crest of the main divide above Real del Monte west to and beyond the Bordo. These dikes have various strikes and dips but the larger and more continuous ones strike and dip parallel to the main productive vein systems. On the surface and underground these rhyolite flows and quartz porphyry dikes are cut by the vein fis- sures, which are sometimes particularly well mineralized and contain large stopes where they abut against or rest upon the dikes. At Capula the vein, carrying ore in two or more strands, is in a quartz porphyry dike of great longitudinal extent and from 50 to 100 ft. thick dipping southerly about 50. Cutting later andesite, but of undetermined age relation to the rhyolite is basalt. This rock is seen in east-west dikes from 5 to 40 ft. (1.5 to 12.2 m.) thick and a general northerly dip in the country west of the Pachuca mineralized area. It is not much weathered. It stands up in bold outcrops and is dense and tough and contains abundant olivine phenocrysts. The later andesite near these dikes has been weathered and oxidized, and the red belts have sometimes been mistaken for veins. Vein Systems So far as could be observed, all the veins of the Pachuca camp belong to the same general period of fracturing. Although some veins terminate against or are faulted by others, the evidence tends to show that all of the productive veins thus far developed were formed and primarily mineral- ized atone period, and from the same agencies and structural disturbances. Nevertheless there are two systems of veins; one striking east and west and one striking north and south. The east-west veins are said to have been first discovered and worked, although the north-south veins, which are rich and wonderfully productive, have also been worked for many years. The latter occur in the eastern part of the camp, largely in the property of the Real del Monte Co. Most of the east-west veins have numerous branches extending both northeasterly and northwesterly, forming what has been termed "linked vein" structure, quite typical of rock fracturing on a large scale. Some of t he branches extend through- out the intervening space between the stronger east- west veins; others apparently die out before reaching any great distance. The full extent of the north-south vein system has not yet been disclosed, nor is it known how many such veins remain to be discovered. They seem to terminate on the south against the great Vizcaina fissure, although the Dios te HORACE V. WINCHELL 5 Guie is said to be displaced by the Vizcaina and to extend a short dis- tance south of it, without however containing much ore. The principal veins of the east-west system are the Santa Gertrudis, Fresnillo, Analcos, Vizcaina, Maravillas, Santa Ana, and Polo Norte. La Corteza and El Lobo veins strike northwesterly, while the Veta del Tajo, Cristobal Colon and Florencia strike to the northeast. So far as developed, the north-south veins, in their order from west to east, are the Dios te Guie, San Sabas, Purisima, Santa Ines, Santa Brigida, and Veta de la Reina. The stoping width of these veins varies from 3 to 40 ft., and some of the oreshoots, such as that in the Purisima (N-S) vein are 400 m. long. The large east-west veins contain some of the longest oreshoots ever developed, reaching 1000 m. in the Santa Gertrudis vein and as much in the Vizcaina. The horizontal dimensions of the oreshoots generally exceed the dip length. The bearing of this fact on the question of ore genesis will be suggested later. The dip of the east-west veins is generally to the south, at angles of from 65 to 80. Local north dips are not uncommon. The north-south veins, dip both easterly and westerly; it is said that the former are the more productive. The veins occupy planes of fracture and zones of shearing and are composed of crushed country rock more or less completely replaced by quartz and other vein minerals. The best veins, from the standpoint of productivity, are in and course through the zones of greatest rock alteration, and yet there was occasionally observed a belt or barrier of fresher looking rock just before coming to a vein. Such a belt lies just east of the east-dipping, north-south Purisima vein. There are evidences of two generations of quartz deposition. The first period quartz, which replaced the broken andesite and often pre- sents a banded or curved structure suggestive of its deposition around rock fragments, is white and bony or ivory-like. Upon and around this white, quartz and in its interstices is a darker later quartz carrying silver minerals with pyrite and a little galena and, still rarer, crystals of blende and chalcopyrite. Where not replaced by quartz, the vein filling is sericitic or kaolinic. The quartz is usually broken into fragments, especially where the veins are wide, but is sometimes solid and "frozen" to the hard country rock. Some veins have good walls; some are in " bad ground;" some are accompanied by well-defined planes of movement on one or both walls or in the veins themselves. The best and most abundant ore seems to be in those veins that are quite open to the passage of ground water. There is not a great flow of water, considering the length of underground development. As the rainfall at Pachuca is so much less than that at Real del Monte, it is probable the eastern mines make more water than those in the western camp. It is a singular and significant fact that on the western slope oxidation extends to the lowest productive levels, about 2000 ft. (609.6 m.) from 6 GEOLOGY OF PACHUCA AND EL ORO, MEXICO the surface. This is not intended to imply that no sulfide minerals remain. The sulfide of silver, argentite, is the principal ore mineral, and pyrite is quite abundant; but the quartz is honeycombed, rhodonite and rhodochrosite are more or less altered to psilomelane and pyrolusite, horn silver is apparent and calcite coatings are found on the joints and in cavities. The effect of weathering is naturally greater as one ap- proaches the surface, and indeed, not only the silver but the quartz itself seems to have been dissolved out of the upper portion of the veins and carried downwards. It is only occasionally that quartz and ore persist upwards to the grass roots. Few stopes extend high enough to make their presence known by surface settling, and the average pros- pector acquainted with the camp states without hesitation that good ore can hardly be expected short of 400 or 500 ft. (121.9 or 152.4 m.) in depth. This has a familiar sound, but seems to have more than an element of truth here. Long oreshoots, already fully explored and mined upwards to their terminations sometimes have tongues projecting to the daylight; but the average line of the upper margin of the long shoots is perhaps 150 m. from the surface. Indeed, there seems to be a general parallelism between the surface topography and the configuration of the oreshoots below. This rule may possibly not hold where there is a considerable thickness of recent volcanics, even though such rocks are cut by the vein fissures. It must be admitted that there is no rule with- out exceptions and that no general statement is applicable to every vein in the district. There are stopes that reach the surface; there are ore- shoots that do not persist downwards; there are veins whose production has come from within 500 ft. of the present surface and others that are chiefly barren to that depth and productive below it. Nevertheless there is a remarkably persistent general relation between the surface and the upper limit of the pay ore. This relation becomes still more striking when we consider also the termination of the ore downwards. There seems to be little doubt in the minds of those familiar with Pachuca that the mineralization has a rather abrupt and very definite termination in several fully explored mines, and that the productive area of the veins is confined within a vertical range of about 2000 ft. (609.6 m.). Moreover, in no one oreshoot is the full extent of this zone mineralized. The average height of the pay-ore zone is perhaps 1500 ft. (457.2 m.), in many cases less and in a few cases more My knowledge of the conditions that prevail at the bottom is largely gained from others. It is a matter of common knowledge, however, that the veins scatter, the quartz diminishes, the values fall off rapidly and only occasionally are found any sulfide or base minerals such as blende and galena. This general situation is not in any way exceptional for veins in Tertiary eruptives, but the explanation may not always be the same nor the geological record so easily read. HORACE V. WINCHELL 7 Origin of Ore In considering the possibilities of a mining district an understanding of its geology and something as to the probable genesis of its ores is valuable as a guide in explorations and in appraising its future. The facts given may be summarized as follows: 1. The country rock is a series of Tertiary volcanics. 2. The vein fissures cut the entire series from the oldest to the youngest. 3. Parallel to the veins are quartz porphyry dikes of considerable extent. These are not universally known, but are in sufficiently constant association with the veins to be taken into consideration as possible agents in ore genesis. 4. The country rock is not only widely sheared and fissured but presents evidence of profound alteration by thermal waters over wide zones. 5. The country rock is still further altered by weathering from the surface down to considerable depth. 6. The veins are largely quartz, but this quartz seldom comes to the surface and never in such quantity as in the veins underground. This statement is true of veins and their outcrops whether found on the crests of mountains or in deep valleys. In other words, both quartz and ore lie for the most part some distance below the surface. 7. Oxidation and leaching persist to the lowest levels of silver en- richment. Horn silver, native silver, and argentite (the latter greatly predominating) are the ore minerals. They occur in white quartz, which is itself secondary, or as incrustations or cavity fillings together with pyrite and an occasional speck of chalcopyrite and galena. For the most part, oxides of iron and manganese are present though in diminishing amount as depth is gained. 8. There are seldom any massive sulfide orebodies in the veins be- neath the oxidized ore, nor are there in the upper levels the large masses of oxidized material that often indicate the former presence of heavy sulfide bodies. 9. The major axes of the oreshoots are more nearly horizontal than vertical; so much so that the Pachuca camp is rather unique in this respect. 10. The vein quartz dies out downwards, and even large veins dwindle into a series of scattered and unmineralized stringers. 11. All the mineralized veins belong to one general period and contain the same kind of ore. The products of the camp are silver and gold in the ratio of about 5 gm. of gold to 1 kg. of silver. With reference now to the richly productive portion of the Pachuca camp, the history of ore formation may well have been somewhat as follows : 8 GEOLOGY OF PACHUCA AND EL ORO, MEXICO Having accumulated in large mass, the andesites, both older and latter, slowly cooled, and zones of shearing were produced by shrinkage and subsidence. Through the multitude of cracks and fissures vapors and hot waters penetrated the rock and effected widespread alteration, de- positing at the same time barren pyrite in disseminated crystals over zones of considerable width. When these fissures extended to sufficient depth there was another outburst of magma, this time quartz porphyry and rhyolite, followed by the usual period of hot-spring activity. At this time the first quartz was deposited in and along the fissure already formed and replacing and silicifying the broken andesite along the shear zones. The period of subsidence and fracturing was not yet at an end, for the quartz porphyry itself, after cooling sufficiently, was fractured and fissured and somewhat mineralized. Then, with the dying down of fumarolic and hot-spring activity, came the opportunity for surface waters, which up to this time had been operating solely on the exposed surfaces, to begin working their way downward along the fissures and shear zones, oxidizing and dissolving the scattered sulfides and carrying them to new resting places at lower levels. It is not known how much erosion has taken place since these Tertiary rocks were formed, but that it may well have been thousands of feet is shown by the depth of the valleys in the immediate vicinity. On the western slope of the mountains chemical changes are rapid and with an already altered rock on which to operate the upper parts of the veins were constantly and successively oxidized, leached of quartz, silver minerals, pyrite and gangue minerals while the surface was eroded and carried away. Always the values were held in .the veins and carried downward in advance of the dissipating forces of erosion, and times without number the little films of argentite that had been deposited at a safe depth and were becoming endangered by the slow approach of the surface, were removed still deeper. In this way we have the cumulative result, first, of ages of primary deposition during which perhaps no commercial orebodies were formed, and, second, of a long period of weathering under a hot sun and climatic conditions distinctly favorable to secondary enrichment. The proof of the theory is in the character of the minerals, the leaching and other evidences of the work of descending waters, as well as the shape of the oreshoots, their correspondence with the topography of the surface, the paucity of quartz at grass roots and its diminuendo habit beneath the ore accumulations. In short, all the broad phenomena of the district seem to be in accord with this theory and with no other. It is supported by both the positive proof and the negative facts, by the min- erals found as well as by those that are not present; it is a most excellent example of the formation of large and deep orebodies by secondary sulfide enrichment. But there are other features of interest. HORACE V. WINCHELL 9 Climatic Effects Like forces working on similar materials under similar conditions, for an equal length of time may be expected to produce similar results. Thus, with equal precipitation and evaporation over the entire district, uniform surface gradients on both sides of the divide and similar rocks uniformly sheared and fissured so as to offer equal receptivity to drainage there would probably result a similarity in topography and in the subterra- nean products of weathering agencies. Where the materials are similar but the conditions are known to vary, it is reasonable to attribute heter- ogeneity of products to such variance of conditions; and where the rocks are different, different products may be formed by the operation of similar forces under similar conditions. At Pachuca, the rocks are, in general, similar mineralogically and structurally; the operating forces are similar, although not of equal intensity nor volume; the time factor is practically constant; but the products have a wide variance indifferent portions of the district. Reference has been made to the marked difference in the annual precipitation on the western and eastern slopes of the continental divide, but its effect has not been fully described. The first result to be noticied is in the different sculpturing of the surface. On the eastern slope the topography is rougher, the slope gradients are steeper, the changes in elevation more frequent and abrupt. On the western slope there are many long, smooth," gently sloping hillsides and but comparatively few sharp and deep canyons. As a consequence, the rainfall, if equal over the two areas, would run off faster on the eastern than on the western slope, and a smaller amount would percolate downward into the rocks. But with the much greater rainfall on the eastern slope, it might be expected that the effect of surface waters would be at least as great, and oxidation as deep in Real del Monte as in Pachuca. In fact, considering alone the much greater precipitation, one would expect deeper and more intense oxidation. But the reverse is the case. Sulfide minerals are rarely found within 300 ft. (91 m.) of the surface in the western part of the camp, while they are abundant within 70 ft. of the surface about 1 mi. (1.6 km.) east of the Guerrero mill, on the eastern side of the range. It is probable, as already stated, that there is more water in the mines and in the ground generally, on the eastern than on the western slope. Why then is there such a marked difference in the depth to which oxidation has extended? No doubt many factors enter into the problem. It is evident at a glance that erosion is more rapid, and that it nearly keeps pace with oxidation, on the eastern slope. But there are other reasons why sulfides in that section are so much nearer the surface. It may be ex- plained in part by the fact that the rainwaters are not so active chemically after soaking through the soil. There is perhaps not enough difference 10 GEOLOGY OF PACHUCA AND EL ORO, MEXICO in temperature to make any material difference, although such difference probably exists. It is perhaps more largely due to the fact that the rank vegetation that covers the eastern foot hills deprives the rain of its oxygen and hence the underground waters in that section are compara- tively inert. In this respect the Pachuca camp offers a rare example of the effect of differing climatic conditions on the depth and character of mineralization in veins. In another respect, also, it is interesting. Many observers here and in other districts have noted the fact that there is oxidation below the present water table and have attributed the phenomenon to a change of water level in comparatively recent time. This does not appear to be the only, nor indeed always the more probable, explanation. Where, as is probably the case here, the entire volume of ground water is slowly moving downward, and yet is ever renewed by annual rainfall, there must be oxidizing action until all the oxygen is consumed, and thus, even below the surface of the subterranean water table, extending down- ward perhaps several hundred feet, the sulfide minerals will become oxidized. It is only stagnant water or water that has performed its work and become exhausted, that is inert. Where there is an outlet at some greater depth and the waters are descending, the work of oxidation may and frequently does proceed far beneath its apparently stationary level. EL ORO, 'STATE OF MEXICO For his data on El Oro, the writer is indebted not alone to his personal examination but to a report on the camp by Dr. Waldemar Lindgren, written in 1913. From this report are taken the following more general statements, in order to lay the foundation for points that seem to be of special interest to the economic geologist: "The district of El Oro is situated on the high plateau of Mexico, near its western edge, at an elevation of about 10,000 ft. (3048 m.). On this part of the plateau broad valleys are separated by irregular groups of mountains rising 2000 to 3000 ft. above the depressions. The valleys are filled with volcanic tuff and detritus. The mountains are largely built up of volcanic flows, mainly andesite, but at many places the underlying older rocks are exposed. The latter consist of calcareous shales of Cretaceous or Jurassic age and in places contain an older series of igneous rocks intruded into the shales and exposed by erosion. ' * The geological sequence is then as follows : 1. Calcareous shale with some sandstone and limestone. 2. Older Tertiary igneous rocks intruded into or poured out on these sedimentary shales. 3. Formation of fissure veins intersecting shales and older igneous rocks. HORACE V. WINCHELL 11 4. Epoch of erosion. 5. Late Tertiary and recent igneous rocks, chiefly flows of lava and tuffs, resting on eroded shales, older igneous rocks and veins and showing no mineralization. 6. Recent epoch of erosion. The Formations "Later Andesite. The younger surface lavas are mostly massive, dark gray hornblende-andesites which are oxidized and disintegrated near the surface, but show no mineralization, nor do they contain pyrite. Toward the valley agglomerates and tuffs gradually take the place of the massive rocks. Where the contact with the shales is exposed by mining operations, a few feet of reddish stratified material of fragmental origin often rest directly on the shale. " The thickness of this lava is manifestly affected by the recent erosion. Along the San Rafael lode, south of North shaft, it is less than 200 ft. (60.9 m.), but north of this point it increases to 400 ft. (121.9 m.) and at Tiro Hondo and San Patricio shafts it is about 600 ft. Under the summit of the hill the thickness is 1000 ft. Dikes and intrusive necks of- this lava are found in the adjoining Esperanza mine. " The Older Andesite. The older andesite is a greenish dense rock which has been greatly altered by the vein-forming agencies and now contains much pyrite, calcite and sericite. It occurs as thick intrusive sheets, or as irregular masses in the black shales, also as smaller dikes. On the property of the El Oro Co. none of this rock reaches the present surface or the old surface underneath the younger andesite, but it forms a thick flat body which is about 600 ft. thick; it was first encountered in the northern part of the property along the San Rafael vein about 600 ft. below the capping. Farther south it lies deeper, being near the Interior shaft at about 900 ft. below the capping; and in the Carmen mine its top lies again about 800 ft. below that surface. The lower contact with the shales has been found in the northern part of the mine. Through- out, this andesite sheet, or sill, is faulted by the fissure of the San Rafael, the vertical throw being, in the vicinity of the Somera shaft, about 670 feet. "Down to about the 1300-ft. level, the andesite appears only in the foot wall. Below the 1300-ft. level, it begins to appear in the hanging wall, and continues to form that wall down to the 1600-ft. level, the lowest point reached." (Since Doctor Lindgren's examination the mine workings have developed ground far beneath the lower margin of this sill). "The older series of andesite is represented both by intrusive rocks and lava flows. 11 The Sedimentary Rocks. The predominating rock is a black, bitu- 12 GEOLOGY OF PACHUCA AND EL ORO, MEXICO minous shale, well stratified and often containing much calcite, in fact grading into a calcareous shale and occasionally into a black granular limestone. In places the shale contains embedded masses of a dark gray friable sandstone. This sandstone is more abundant in the deep levels, and is typically present at the station and crosscut of the Somera shaft on the 1300-ft. level. When examined in the field this rock was held to be of tuffaceous origin, but the microscope has shown it to be a pure quartzose sandstone. "The sedimentary rocks lie horizontal, or at slight dips that exhibit no marked irregularity. A total thickness of 1300 ft. of strata is exposed in the workings." The Veins We have at El Oro a series of veins bearing gold and silver minerals in a quartz-calcite gangue. These veins are fissures that were filled by replacement and infiltration. Some of them are evidently fissures of considerable displacement. They cut through the black shales and through the sill of andesite that was intruded horizontally into these shales, and which, as a natural consequence, is both overlain and under- lain by the shale beds. After the veins were formed and mineralized, they were subjected to the action of surface waters for a considerable time, and an unknown extent of their upper portions was removed by erosion. They are oxidized to the depth of nearly 1000 ft., and their silver content at least was secondarily enriched by the action of descending waters. The general strike of the veins is about north 30 west, and their dip at varying angles to the west. They have branches in both foot and hanging walls, but more numerously in the latter. After a long period of weathering and erosion the country was covered by more recent lava flows, which were in turn weathered and eroded until in some localities the underlying shales and one vein, the first discovered Descubridora, are exposed on the surface. These lava flows and volcanic tuffs are not penetrated by the veins which cut the underlying rocks. The veins are, in turn, cut and displaced by north-dipping east-west faults. These faults are unmineralized, except by a little calcite, which may be of recent deposition. Their general effect has been to step the country down to the north. The direction of movement along several of these faults has been diagonally downwards to the east. The andesite sill varies in thickness from 400 to possibly more than 700 ft. ; it has been cut and displaced by the vein fissures with throws of several hundred feet. The most productive orebodies have been found in the veins where they lie within the shale- overlying the andesite sill and adjacent to it where, by reason of faulting movement along the vein fissures, the shale is brought in opposition to the andesite. There is also some ore found HORACE V. WINCHELL 13 where the veins lie wholly in andesite, but not in large quantity anywhere above the lower margin of the lower faulted segment of the sill. Develop- ments indicate that the veins are not enriched for any distance below the andesite. Quartz indeed is found to persist for some distance beneath it, but with diminishing tendency downward, and with smaller oreshoots. 'Later Andes/fe 1 . * ' "O/c/er Andesite Q/der.^ ndesite,, ^>ShJfe and SaZc/sto APPROXIMATE CROSS SECTION AT ELORO MEXICO LOOKING NORTHERLY FIG. l. The quartz scatters in stringers of diminishing strength and dies out. The- lower stretches of the veins contain some of the baser sulfides, such as blende and galena. The commercial orebodies occur in long relatively horizontal ore- shoots better mineralized in the upper levels. Some of these shoots have been definitely bottomed while others are still being pursued through their downward reaching lobes and tails. 14 GEOLOGY OF PACHUCA AND EL ORO, MEXICO Origin of the Ore It is seldom that the facts observable suffice to point strongly to a particular mass of eruptive rock as the source of vein mineralization. In this respect El Oro is of particular interest. The position of the ore- bodies in the ground and their relation to the andesite sill, the dwindling of the quartz and mineralization downward suggest that it was the fountain head of ore deposition. There is nothing to suggest the presence of other sills or masses of intrusive rock and many facts that lead one to doubt their existence. This holds true of all the veins thus far developed. They are large and rich above the sill and poor and small or entirely pinched out beneath it. This could hardly be the case if they had been formed from solutions rising from greater depth. Further careful study should prove or disprove the integrity of this theory. It is at least useful as a working hypothesis for it not only suggests the futility of costly exploration beneath the sill but at once shows the attractiveness of territory overlying the sill still unexplored and now covered by later andesite, which caps the vein-carrying shale formation. Value of Geological Study In these two camps, we thus find geological data that encourage further exploration and indicate where it is most likely to be successful, while in many camps are found facts that, properly interpreted demon- strate the utter folly of large expenditures in the search for orebodies, the existence of which is extremely problematical. Such explorations are now in progress in some of the older western mining camps and hundreds of thousands of dollars are being expended in utter defiance of the easily read geologica-1 history of the districts wherein such work is being prosecuted.