University of Texas Bulletin No. 1822: April 15, 1918 The Anticlinal Theory as Applied to Some Quicksilver Deposits BY J. A. UDDEN Bureau of Economic Geology and Technology Division of Economic Geology J. A. Udden, Director of the Bureau and Head of the Division PUBLISHED BY THE UNIVERSITY OF TEXAS AUSTIN Publications of the University of Texas Publications Committee : F. W. GRAFF R. H. GRIFFITH J. M. BRYANT J. L. HENDERSON D. B. CASTEEL I. P. HILDEBRAND FREDERIC DUNCALF E. J. MATHEWS The University publishes bulletins six times a month, so numbered that the first two digits of the number show the year of issue ; the last two the position in the yearly series. (For example, No. 1701 is the first bulletin of the year 1917.) These comprise the official publications of the University, publications on humanistic and scientific subjects, bulletins prepared by the Department of Extension and by the Bureau of Municipal Research and Reference, and other bulletins of general educational interest. With the exception of special numbers, any bulletin will be sent to a citizen of Texas free on request. All communications about Univer- sity publications should be addressed to the Chairman of the Publications Committee, University of Texas, Austin. University of Texas Bulletin No. 1822: April 15, 1918 The Anticlinal Theory as Applied to Some Quicksilver Deposits BY J. A. UDDEN < Bureau of Economic Geology and Technology Division of Economic Geology J. A. Udden, Director of the BureaiTand Head of the Division PUBLISHED BY THE UNIVERSITY SIX TIMES A MONTH. AND ENTERED AS SECOND-CLASS MATTER AT THE POSTOFFICE AT AUSTIN, TEXAS. UNDER THE ACT OF AUGUST 24. 1912 ; The benefits of education and of useful knowledge, generally diffused through a community, are essential to the preservation of a free govern- ment. Sam Houston Cultivated mind is the guardian genius of democracy. . . . It is the only dictator that freemen acknowl- edge and the only security that free- men desire. Mirabeau B. Lamar EXCHANGE CONTENTS Introductory 7 The W. K. Ellis Mine 11 The Marf a and Mariposa Group 14 Study Butte 16 The Christmas Mountain Prospects 18 The Workings on Section 248 20 The Coltrin's Camp Prospect 22 The Laguna Float 22 The Chisos Mine 25 Conclusions . . 30 LIST OF ILLUSTRATIONS Fig. 1. Sketch map showing localities of quicksilver ore described in the text 8 Fig. 2. Map of Mariscal Mountain showing approximate position of the three sections in Fig. 3 10 Fig. 3. Diagrammatic sections from east to west across the Mar- iscal Mountain. Positions of sections shown in Fig. 2 12 Fig. 4. Sketch map of the vicinity of the Marfa and Mariposa and the Old Terlingua Mines showing the position of section given in Fig. 5 14 Fig. 5. Diagrammatic section extending from south-southwest to north-northeast along the line indicated in Fig. 4. Known occur- rences of ore-bearing rock are approximately indicated by solid black 15 Fig. 6. Sketch map of Study Butte showing the position of the section in Fig. 7 17 Fig. 7. Section of Study Butte extending from north to south along the line indicated in Fig. 6. Known occurrences of cinnabar- bearing rock roughly indicated by solid black 18 Fig. 8. Map of Crristmas Mountain showing approximate posi- tion of section in Fig. 9 19 Fig. 9. Diagrammatic section from southwest to northeast across the Christmas Mountain along line indicated in Fig. 8. Known cin- nabar-occurrences indicated by solid black 20 Fig. 10. Map of Section 248, and parts of sections 247, 230, and 229, Block G-4, west of the Chisos Mountains, Brewster County, show- ing location of section a-b, in Fig. 11 21 Fig. 11. Diagrammatic section along the line a-b in Fig. 10. Known occurrences of cinnabar are indicated by solid black .... 22 Fig. 12. Sketch map of the Chisos Mountains showing position of the section in Fig. 13 23 Fig. 13. Diagrammatic section across the Chisos Mountains from west-southwest to east-northeast on the line indicated in Fig. 12 24 Fig. 14. Map of the region around the Chisos Mining Company property showing the Coltrin's camp and the course of the section a-b given in Fig. 15 and of section c-d in Fig. 16 26 Fig. 15. Diagrammatic section running approximately from north to south along the line a-b indicated in Fig. 14. Known localities of ore-bearing rock indicated by solid black 27 Fig. 16. Generalized section along the line c-d in Fig. 24. ... 29 THE ANTICLINAL THEORY AS APPLIED TO SOME QUICKSILVER DEPOSITS* J. A. UDDEN INTRODUCTORY From his general studies of the quicksilver deposits of tho world, Becker came to the conclusion that the source of quicksilver is to be found in deep-lying crystalline rocks, such as granites. On such an hypothesis the distribution of quicksilver in the Terlingua district also would seem to have a natural explanation. Cinnabar is here found in quite diverse kinds of rocks, ranging from the Edwards limestone in the Comanchean to the sandstones of the Rat- tlesnake beds in the Upper Cretaceous. It also occurs in some igneous rocks which intrude these sediments. It has even been reported from Paleozoic rocks in the Solitario uplift, though the present writer has no reliable information on this occurrence. Under all circumstances it would seem that the cinnabar is not limited in its occurrence here to any particular formation. That the source is deep-seated may also be inferred from the fact that the mineral is distributed over a fairly wide area extending from the Mariscal Moun- tains on the east to the Lajitas Mesa on the west, and from beyond the Mexican boundary on the south to the Christ- mas Mountains on the north. The known east and west extension of the district is about thirty miles, and the greatest north and south extension is at least twenty miles. It is also interesting to note that all the quicksilver may be said to occur within the limits of a huge sunken block just west of the main Front Range of the Rock Moun- ains. The sinking of this block must have been accom- panied by a rising of the isogeothermals with reference to the parts of which this block itself consists. The deep- seated heat, which may be looked upon as having been re- *Read before the Geological Society of America at its meeting in Baltimore, December 28, 1918. 8 University of Texas Bulletin sponsible for the upward migration of the quicksilver, is here, as in California, yet in evidence in hot springs. Such waters emerge just east of the district on the banks of the Rio Grande. The fact that the present mines, which repre- sent the best ore so far found, are strung on a line extending from west-northwest to east-southeast, suggests the possible existence of some linear fracture at a depth sufficient to be entirely concealed by the more superficial structures in the "outer crust" of the earth. so, Fig. 1. Sketch map showing localities of quicksilver ore described in text. The date of the deposition of the quicksilver is probably quite late, since the deposits have been found not only in the latest Cretaceous sediments, but also in some of the intrusives which in this region are regarded as belonging to the last half of the Tertiary period. The present writer, who has had opportunity to see most of the quicksilver occurrences in Brewster County, believes that structural conditions clearly affect deposition in this district and have a direct practical bearing on the finding Anticlinal Theory of Quicksilver 9 and development of the quicksilver ores. Briefly stated observations here indicate that the deposition of the quick- silver fumes rising from great depths has resulted from the capture of the fumes and, from their retention, mainly in the form of sulphides, in structures practically resembling those which determine the retention of upwardly migrating liquid and volatile bitumens. The quicksilver deposits in almost every case occur in anticlines and domes or along decided belts in what are known as "structural terraces," or arrested monoclines. Invariably they occur in these struc- tures at levels where the rising solutions have encountered strata that are less pervious than those immediately below. This appears to be true both on a large and on a small scale. The richest cinnabar deposits so far encountered have l>een found in the contact between the Georgetown (which is the upper fifty feet of a thick limestone usually referred to as the Edwards) and the Del Rio clay. The next richest hori- zon has been proved to be the contact between the Buda limestone and the overlying, less penetrable flags and shales of the Eagle Ford. In the Eagle Ford itself, which in this region measures at least some 600 feet in thickness, and which consists of beds rapidly alternating from compart limestone to black shale, the distribution of ore frequently shows a direct relation to these alternations. Even in the small details of these beds it has been found that pockets of cinnabar frequently underlie layers of shale. It may be that there is a coincidence of other circumstances affecting this arrangement in the distribution of the cinnabar. All of these shales are more or less bituminous and contain or- ganic matter. This may have aided in the precipitation of the quicksilver fumes as sulphides. But that the structures themselves have been the most important factor is indicated by the fact that bitumens also have accumulated in the same structures. Oil has been found in the ore in the Eagle Ford in sufficient quantity to materially aid in its reduction in the furnace. I have on a previous occasion briefly described my views 10 University of Texas Bulletin o Scale i a. 3 Miles Fig. 2. Map of Mariscal Mountain showing approximate position of the three sections in Fig. 3. Anticlinal Theory of Quicksilver 11 on the distribution of the quicksilver in this region.* It is my purpose here to present in slightly greater detail than before the evidence which illustrates my views, some of which evidence has come to light through more recent de- velopments of the deposits in this region. I will therefore briefly describe the natural conditions as they exist at the several points where quicksilver ores occur, and also ven- ture to present my interpretation of the significance of the conditions noted. The W. K. Ellis Mine The easternmost mine in this district is the W. K. Ellis mine, located on the north end of the Mariscal Mountain. This mountain is an anticline extending from Mexico across the Rio Grande a distance of about twelve miles north-north- west into Brewster County. It is some five miles wide at the river, and tapers to a fairly sharp point at the north. The crest of the fold at the river consists of the Edwards limestone. This formation at the north dips under the Del Rio horizon which is represented by only a few feet of marly material in turn dipping under the Buda limestone. The northernmost end of the ridge is covered by the Eagle Ford, or the Boquillas flags, which has a thickness in this region of about 600 feet. These in turn go under the Austin chalk and the Taylor marls. The softness of the latter beds has caused them to be eroded away leaving the axial part of the anticline as a prominent ridge. It should be noted that two sills of acidic volcanic rock, one of which is quite glassy, have been intruded between some of the layers of the Eagle Ford beds, and lie folded with these- The "nose/* as petroleum geologists would call it, of the plunging anticline, is itself flexed downward at the north end, and this has resulted in some transverse fissuring of the ridge. In these fissures the best ore was first found, and in one of them the principal ore so far discovered is taken at the present time. Most of the work, both on the * Structural Relations of Quicksilver Deposits. Mining World, Vol. 34, May 13, 1911. 12 University of Texas Bulletin surface and underground, has been done in the middle part of the Eagle Ford. This part of the Eagle Ford consists of calcareous shaly flags that are quite compact and fre- quently highly bituminous. They have been found to con- tain some small pockets of oil, and small "seepages" of solid bituminous material have also been noted. Some ore has Diagrammatic section acrovj The Manscal Anticline near The r\arfh line of section 33 Diagrammatic secTion across The Mariscal AnTiclme near the main shaft of The Ellis Mine in section mmaTic across The'Manscal Anticline abouT ihree mile* south of section 33, "horizontal much -shortened Comar%cheon Limestone Eacjle Ford Shale AubtmChalK Tailor Marl Igneous 5 Fig. 3. Diagrammatic sections from east to west across the Mariscal Mountain. Positions of sections shown in Fig. 2. Anticlinal Theory of Quicksilver 13 been found in the contacts of the shales with the igneous rocks and the igneous rock itself has been found to be slightly impregnated with cinnabar, especially close to its contacts with the shale. Judging by conditions in other mines, richer deposits may be expected where fissures cut the contact of the Buda limestone with the overlying Eagle Ford and also at the depth where they may cut the Del Rio horizon. Here the underlying Georgetown or Edwards fur- nish a more open body of rock than either the overlying Buda or the marly sheet representing the Del Rio, which is quite thin. The workings in this mine have not yet ex- tended down to these deeper horizons. The accompanying illustrations are intended to more clearly represent the geo- logic conditions at this mine. The first mining in the Terlingua district was done on Sections 40, 41, 58, and 59, in Block G-12, in Brewster County. This group of mines lies farthest to the west in the district. It is at the upper fold of a monocline, dipping south, at a point where the Georgetown, immedi- ately below the Del Rio clay, has been laid bare by the re-r moval of this clay and other overlying beds. The Del Ric still underlies California Hill, which is capped by a smaller remnant of the Buda limestone. It goes down under this limestone, which caps the series of hills to the south, called the Tres Cuevas Mountain and the Sierra del Cal. The country north of this line of hills is traversed by several small faults and a larger one which runs from west-north- west to east-southeast. This fault has a downthrow to the north, of from a few to perhaps eighty feet. Following the north side of this fault, a remnant of the Del Rio clay still is seen. The block of rock south of this fault dips to the south and in about a half mile from the fault, the general dip to the south rapidly brings down the suc- cession of Cretaceous beds standing at angles of from twenty to sixty degrees. The strike of this strong dip is shown in the Sierra del Cal. The south dip is continued by a much gentler dip in the same direction for a distance of three miles. To me it is a significant feature that the monocline of the Sierra del Cal forms an arc of a circle, cr a periphery 14 University of Texas Bulletin of a dome, as it were, and that the ground under the Del Rio clay in the subtended segment of this dome has been very generally mineralized. Here is a tract toward which an inverted funnel-like attitude of the impervious Del Rio clay would naturally drive any ascending currents of min- eralizing solutions, gathered from a large collecting area to the south. Fig. 4. Sketch map of the vicinity of the Marfa and Mariposa and the Old Terlingua Mines showing the position of section given in Fig. 5. The ore .in the Marfa and Mariposa mine, and also in the old Terlingua mine, has been deposited mostly in joints, fissures, and cavernous openings that extend down in the upper surface of the Georgetown limestone. The material filling these fissures is locally known as jaboncillo. It is a material of mixed nature, consisting in places largely of clay and in other places of material quite like caliche. It is evident that it has been formed in these fissures partly by precipitation from solutions which have followed the lower surface of the Del Rio clay, and partly also by the Del Rio clay itself which has settled perhaps gradually Anticlinal Theory of Quicksilver 15 into solution caverns, pari passu with their enlargement by solution. The jaboncillo frequently contains fragments of the limestone itself and is in places not unlike a fault breccia, cemented with calcareous material. At the sur- face, this jaboncillo in places changes into caliche, clearly formed at a recent date. Even this caliche contains frag- ments of cinnabar, which apparently have been entombed quite recently in the formation of superficial caliche. The fissures extend to varying depths and no doubt in some place, or places, join "pipes," most probably along fault lines, through which the quicksilver has ascended. The great number of these ore bodies gives out in less than twenty feet below the upper surface of the Georgetown limestone and the mining on most of the hills in the sections men- tioned has been done in shallow pits which stud the land at the present time. In places, ore occurs at greater depths and some fissures have been found with open vugs set with calcite and gypsum crystals. agi, ford Buda Del Rio Edwards Fig. 5. Diagrammatic section extending from south-southwest to north- northeast along the line indicated in Fig. 4. Known occurrences of ore-bearing rock are approximately indicated by solid black It would seem altogether likely that further prospecting may result in the finding of deeper pipes and it would seem quite probable that these should, as already stated, be, found in some of the faulted fissures known to exist. 16 University of Texas Bulletin Study Butte The Study Butte is one of the places where extensive explorations have been made. It is the site of two quick- silver mines which are both in operation at the present time: the Big Bend mine and the Texas Almaden mine, These are adjoining properties, the Texas Almaden lying on the north of the Butte, with the Big Bend on the south. They are at present operated jointly. Study Butte rises from an area of the Taylor marl which is exposed over most of Section 216, east of the center of which the Butte is located. The Butte is clearly the result of differential ero- sion- It rises from a low "flat," where erosion has for some time been in active progress on the soft marls overlying the Austin chalk. The Butte itself consists of a dike-like body of andesite, locally known as rhyolite, which has cut the marls, now mostly eroded, at an angle of about 45 degrees, hading to the north. The dike itself has been penetrated by a mining tunnel from north to south, and along this tunnel the intrusive measures about 300 feet. Both its contacts with the marls show clearly in the drift. The igneous rock has been considerably altered on the north side, some of it having been kaolinized. This oblique dike, if I may call it such, forms the crest of the Butte. Under- lying it on the south side are the upper parts of the Taylor marls, and these have been sapped away, causing the dike to rise in a vertical cliff above it. On the north side the beds exposed belong, as I judge, to the upper part of the Austin chalk horizon, and have been slightly indurated. This has caused a part of them to be left as a cover over the igneous rock on the slope of the dike. At the foot of the hill on the north side, the intruded sediments are about 100 feet deep. Explorations which have been made by the diamond drill show that at this point the intrusive measures 400 feet in thickness and is underlaid by a dark clay. This part of the intrusion is of the nature of a laccolith. At a depth of 500 feet it rests on a black marl or clay, possibly corresponding to the Austin chalk. The remaining over- lying beds on the north side lie nearly flat or show even a Anticlinal Theory of Quicksilver 17 slight dip to the south. On the south side of the intru- sive the Taylor marls seem to lie perfectly horizontal. ! ') v uyy , \ l;p > < V ,>.<" yy / ' '^- -*-'' /' Fig. 6. Sketch map of Study Butte showing the position of the section in Fig. 7. This mine is known for its quantity of low grade ore which has been found on the north side of the dike. Some high grade ore has been found in the contact between the overlying marls and the dike, and also in some fissure veins that cut the andesite. These stand almost vertically, ap parently through the entire depth of the intrusive, on the Texas Almaden property. While the geological condition? in this mine thus differ radically in some respects frofti those in the other mines of the district, one can readily see the similarity of the conditions in both. The quicksilver has evidently followed the fissures in the intrusive up tc its upper contact with the overlying shales- From here it 18 University of Texas Bulletin has partly followed continuations of the same fissures up- ward in the shale, while much of it has also been diverted by the close texture of these shales and has followed th^ slanting contact on the upper surface of the intrusive. Con- siderable ore has been recovered from this contact up to the top of the Butte and along this path the intrusive itself has yielded values in several places. The lower, or south contact of the intrusive, on the other hand, has so far yielded no ore. Toy! or Marl Intrusive Veins Fig. 7. Section of Study Butte extending from north to south along the line indicated in Fig. 6. Known occurrences of cinnabar-bearing rock roughly indicated by solid black. The Christmas Mountain Prospects One of the most interesting occurrences of quicksilver in this region, from a theoretical, not to say didactic, stand point, is to be found in the Christmas Mountain. This mountain forms one of the most perfect dome structures in the region. It is located about twelve miles northwest of the Chisos Mountains. The dome has a length from north- northwest to south-southeast of about five miles and a width of about four miles. The greatest vertical displacement at the center of this dome is at the least 4000 feet. The moun- Anticlinal Theory of Quicksilver 19 tain itself rises above the surrounding country to a height of 2000 feet. It consists of the Edwards limestone which evidently has been lifted up by a laccolitic intrusion. At the foot of the mountain we find the Upper Cretaceous dip- ping outward in all directions away from the uplift at high angles. It is evident that the Upper Cretaceous covered the entire uplift at one time, and has been removed by erosion from its central area. The uplift is somewhat un- symmetrical. The slope to the east and northeast is more gentle than that to the west and southwest. In fact, the west side of the mountain is at some points much sheared and faulted. At the south end the limestones are fissured and the fissures are filled with calcite. 4 Miles Fig. 8. Map of Christmas Mountain showing approxi- mate position of section in Fig. 9. 20 University of Texas Bulletin On the highest point of this mountain there is a small cinnabar prospect. The ore occurs in what appears to b cavernous places in the Edwards limestone. This evidently is part of the mineralization which once existed under tf Del Rio clay, that immediately overlay the limestone before it was removed by erosion- Other small prospects have been reported from fissures on the west side of the moun- tain. At the foot of the mountain where the Buda and the Del Rio clay set in, cinnabar has been found in fissures and cavernous places at several points. Float has been reported also from the east slope of the mountain. It should be recalled that the Christmas Mountain lies in the peri- phery of the quicksilver district, but it seems that the size and perfection of this structure has resulted even here in the accumulation of some of the mineralizing solutions. De\Ro. Fig. 8. Diagrammatic section from southwest to northeast across the Christmas Mountain along line indicated in Fig. 8. Known cin- nabar-occurrences indicated by solid black. The Workings on Section 248 On Section 248, in Block G-4, considerable prospecting has been done, though at present no mining is carried on there. The country which has been prospected on this sec- tion is covered by a relatively quite stony layer of the Austin chalk, from which a less resistant and more argillaceous part of this formation has recently been eroded away. We have therefore here the same conditions as have been noted Anticlinal Theory of Quicksilver 21 I Mile. Fig. 10. Map of Section 248, and parts of sections 247, 230, and 299, Block G-4, west of the Chisos Moun- tains, Brewster County, showing location of section a-b, in Fig. 11. at other places. The mercurial solutions in passing upward have been deflected on the upper contact of the at present exposed, more open layer, with the less pervious and more clayey layer that overlay it at the time the ore was de- posited. The prospecting has been done on the west limb of a low anticline, which runs a course from north to south and whose axis is not far from the east line of the section The east limb of this anticline has a higher dip than the west limb. The ore has been found along some fissures that run vertically to this axis and it has been followed downward for apparently half a hundred feet or more. At several places in one of the worked joints, the country rock has been found to be highly charged with black bitumen 22 University of Texas Bulletin Fig. 11. Diagrammatic section along the line a-b in Fig. 10. Known occurrences of cinnabar are indicated by solid black. The Coltrin's Camp Prospect Coltrin's Camp is a prospect where a little work was done in the early days of mining in this district. It is situated under the south escarpment of the Reed Plateau. The structural conditions are somewhat similar to those on the Marfa and Mariposa property. The plateau is an uplifted part of the Edwards limestone, which rises in a small table land less than one mile wide and not quite four miles in length. On the south side of this plateau the Cretaceous series are abruptly turned down, and the softer formations lying above the Edwards are mostly eroded away. The cinnabar here occurs in some veins vertical to the longer axis of the plateau, which cut the Eagle Ford formation. Here as elsewhere, therefore, it would seem that precipita- tion of ore has taken place at a point where the mineralized solutions, following fissures, encountered relatively imper- vious materials after traversing the more easily penetrable limestones of Comanchean age, which in this region measure more than 1500 feet in thickness. (See Figures 14 and 16.) The Laguna Float Cinnabar has also been reported from the top of the Chisos Mountains. The Eagle Ford underlies the country surrounding the central part of the Chisos Mountains at a depth not far from sea level. In the region west of a point Anticlinal Theory of Quicksilver 23 . X t>. f joS i known as Laguna, on top of the Chisos Mountains, this formation lies at an elevation of 7000 feet above sea level, dipping with a high angle to the northeast. It is not known to the writer where the float reported was found, or in association with what rocks in the upper part of the moun tains it occurred. The Chisos Mountain structure may 24 University of Texas Bulletin I 7 III 5