Geology and Quicksilver Deposits 

of the Terlingua District 

Texas 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 312 
Geology and Quicksilver Deposits 

of the Terlingua District 

Texas 

By ROBERT G. YATES and GEORGE A. THOMPSON 

G E 0 L 0 G I CAL SURVEY P R 0 FE S S I·.O N A L PAPER 3 1 2 

A detailed study of the geology of a mining 

district that has produced I50,000 flasks 

of quicksilver 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1959 
UNITED STATES DEPARTMENT OF THE INTERIOR 

FRED A. SEATON, Secretary 

GEOLOGICAL SURVEY 

Thomas B. Nolan, Director 

The U.S. Geological Survey Library has cataloged this publication as follows: 

Yat.es, Robert Giertz, 1910-

Geology and quicksilver deposits of the Terlingua district, 

Texas, by Robert G. Yates and George A. Thompson. Washington, U.S. Govt. Print. Off., 1959. 

v, 114 p. illus., maps, diagrs, tables. 30 em. (U.S. Geological 

Survey. Professional pa.per 312) 

Part of illustrative matter in pocket. 

Bibliography: p. 109-111. 

1. Geology-Texas-Terlingua district. 2. Mercury ores. 3. Mercury 

mines and mining-Texas-Terlingua district. I. Thompson, George 

Albert, 1919- joint author. (Series) 

For sale by the -Superintendent of Documents, U.S. Government Printing Office 

Washington 25, D. C. 
CONTENTS 

Abstract __________________________________________ _ 

Introduction ______________________________________ _ 

Location, culture, and accessibility _______________ _ Physical features ______________________________ _ 

Geographic setting _________________________ _ 

Topography of the district and bordering areas __ Climate and vegetation _________________________ _ Previous work _________________________________ _ 

Present investigation and acknowledgments _______ _ General geology ___________________________________ _ 

Stratified rocks ________________________________ _ 

Devils River limestone _____________________ _ 

Grayson formation _________________ ________ _ 

Buda limestone ____________________________ _ 

Boquillas flags _____________________________ _ 

Terlingua clay _____________________________ _ 

Aguja formation ____________ _______________ _ 

Tornillo clay _____________________ _________ _ 

Chisos volcanics ___________________________ _ 

Intrusive igneous rocks _________________________ _ 

Distribution and form ______________________ _ 

Contact metamorphism _____________________ _ 

Age ___________ --_----------------·--------- Petrography ______________________________ _ 

Classification __________________________ _ 

Analcite-free rocks _____________________ _ 

Rhyolite ______ ____________________ _ 

Quartz soda syenite ________________ _ 

Soda trachyte _____________________ _ 

Soda latite ________________________ _ 

Olivine basalt _____________________ _ 

Analcite-bearing rocks __________________ _ 

Analcite syenite _____________ _____ _ _ 

Analcite-plagioclase syenite _________ _ 

Analcite-orthoclase gabbro (analcite syenogabbro) ____________________ _ 

Analcite basalt ____________________ _ 

Structural geology ________________________ _____ _ 

Regional relations __________________________ _ 

Structural setting ______________________ _ 

Age of the structure ___________________ _ 

General features of the Terlingua district_ ____ _ 

Terlingua uplift ______ _ ~- ______________ _ 

Terlingua monocline ___________________ _ Domes _______________________________ _ 

Wax Factory laccolith and dome __ Contrabando dome _________________ _ 

Black ~esa dome __________________ _ 

Long Draw dome __________________ _ 

Fossil Knobs dome _________________ _ 

Two-Forty-Eight dome _____________ _ 

Origin of the domes ________________ _ 

Faults and fractures ____________________ _ Large grabens _____________________ _ 

Other normal faults and fractures ____ _ 

Relative age and direction of move- ment ___________________________ _ 

Thrust faults ______________________ _ 

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General geology-Continued 

Structural geology-Continued 

General features of the Terlingue district-Con. 

Breccia pipes and other features caused by solution ____________________________ _ 

~aggie Sink pipe __________________ _ 

Two-Forty-Eight pipe ______________ _ 

Aguja pipe _____________ ___________ _ 

Chisos mine pipe __________________ _ 

Other breccia pipes in the Terlingua 

district _________________ - _______ _ 

Origin of solution features __________ _ 

Origin of the breccia pipes __________ _ 

Comparison with breccia pipes and 

diatremes in other regions ________ _ 

Theoretical problems ___________________ _ 

Age of the structures _______________ . ._ 

Analysis of movements _____________ _ 

Origin of the Terlingua uplift and 

Terlingua monocline _____________ _ 

Origin of the grabens _______________ _ 

Quicksilver deposits ________________________________ _ 

History and production _________________________ _ 

Early activity _____________________________ _ 

World War I period ___________________ _____ _ 

Interwar period ____________________________ _ 

World War II period ___________________ -- __ _ 

Summary of production data ________________ _ 

Future of the district _______________________ _ 

General geologic relations _______________________ _ 

Classification and geologic features of deposits ____ _ 

Limestone-clay contact deposits _____________ _ 

Deposits in calcite veins in the Boquillas flags __ Deposits in breccia pipes _____________ _______ _ 

Deposits in igneous rock ____________________ _ 

~inerals and mineralization history __ ____________ _ 

~ineral descriptions ____________________ .:. __ _ Calcite _______________________________ _ 

Clay minerals _________________________ _ 

Silica minerals _______________________ :.. __ 

Iron minerals _______________________ -'- __ _ 

Cinnabar _____________________________ _ 

~ercury minerals other than cinnabar_.:. __ _ 

Hydrocarbon compounds _______________ _ 

Fluorite and barite _____________________ _ 

N atrolite and analcite __________________ _ Sulfates _______________________________ _ 

Summary and conclusions ___________________ _ Origin ________________________________________ _ 

Structural controL _________________________ _ 

Regional controL __________________ - ___ _ 

District controL _______________________ _ 

Local controls _________________________ _ 

Transportation and deposition of the ore elements---------------- - ------------------

Transportation and deposition of the gangue elements------------~----------------------

Descriptions of mines and prospects __________________ _ 

Contrabando dome prospects ____________________ _ 

ill 

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IV CONTENTS 

Descriptions of mines and prospects-Continued Page Descriptions of mines and prospects-Continued Page 

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Fresno mine _____ ______ ___ __________ ___ ____ ____ _ 

Lowe prospect ____ _____________________________ _ 

Prospects in sec. 98 _______ _____________ __ ______ _ 

Sample group ___ ___ _____ __ ___________ ___ __ ____ _ 

Bobs mine _____ ______ __ _____ __ _____ ____ __ _____ _ 

Nat ural Resources, Inc., claims in sec. 58 __ _______ _ 

Monte Cristo and Croesus claims _______ ____ -----

Duncan group in sec. 58 _______ _______ _____ _____ _ 

Margaret D. lode __ ________ ____________ _______ _ 

CanybngroUP---- --- --- ~-- ----------- -- ------

Lafarelle and prospects nearby __ _____ ___ _____ ___ _ 

Rio Grande Quicksilver Co. claims _ _ ___________ _ _ 

Waldron workings in sec. 40 ____________ ___ ______ _ 

Mariposa mine __ __________ ____________ ________ _ 

85 

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Tarrant property in sec. 39 _____________________ _ 

Little Thirty-Eight mine ________________________ _ 

Star mine and neighboring workings ______________ _ 

Rio Grande prospect in sec. 70, Block 341 ________ _ 

Colquitt-Tigner (Waldron) mine _________________ _ 

Le Roi prospect _______________________________ _ 

Chisos mine _________________________________ .:. __ 

90 ' Rainbow mine _____ __ _ ._____ ..: ___ _____________ · ___ _ 

90 Brown prospects in sec. 286--- ---- ----------~----

92 Two-Forty-Eight mine ________ ________________ · __ 

92 Study Butte mine:.. ____ -----------------------~--

92 Prospects east of Study Butte ___________________ _ 

92 ' Selected bibliography ___ · ___________________________ _ 

92 Index ____________ _ ..: ___ ·_ .:.: _ ..:· _______________________ _ 

ILLUSTRATIONS . 

[Plates 1 and lQ-22 in plate volume] 

PLATE L Geologic map and sections of the Terlingua quicksilver district, Texas. 

100 

104 

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109 

109 

113 

2. The Wax Factory laccolith. A, Looking south at northwest termination of laccoiith; B, index sketch of A___ 18 

3. The Study Butte intrusive body. A, Upper contact of body on 150-level; B, flow structures near upper contact__ 18 

4. A, View of the southwestern flank of Solitario do~e; B, vertical aerial photograph of part of the Terlingua 

district----------------------- --------- ---- ------------------------------------------- ------- 34 

5. A, View southeastward along Terlingua monocline at the Fresno mine; B, the Terlingua monocline east of 

Tres Cuevas Mountain __________________ -___________________ ~ __________ _~_________________________ 34 

6. A, View of fault breccia of Boquillas flags on northe~stern fault of the Long Draw graben; B, the westernmost 

fault of the Well Creekgraben near Black Mesa __ ---;-----------~---~-- ----~-------------------- ---- 34 

7. A, View of slumped Gr?.-yson formation and Devils River limestone in a cave-fill zone at the Mariposa mine; 

B, bulbous calcite vein on a fracture, showing strike-slip movement ________ ~~_~_________________________ 34 

8. Quicksilver ore bodies. A, Limestone-clay contact deposit, main stope, Fresno mine; B, calcite vein with 

cinnabar in Boquillas flags on 250-level, Chisos mine-----------------------.,.~----------------- 34 

9. Photomicrographs and sketch from photomicrograph pf thin sections of cinnabar ore. A, Devils River limestone 

with cinnabar replacing calcite; B, cinnabar repl~cing calcareous matrix of Two-Forty-Eight breccia pipe; 

C, cinnabarreplacing clay, Mariposa mine; D, cinnabar replacing clay ______________ .:_____________________ 66 

10. Geologic map and sections, Fresno mine area. · · · 

11. Geologic map and sections, Fresno mine. 1 

12. Geologic map of Duncan prospects in sec. 44, Block G-12. 

13. Geologic map and sections of the Mariposa mine. ' 

14. Map _ofthe principal workings of the Marjposa mine. 

15. Oeologiemap and section~ of part of the Le Roi prospect. 

16. Geologi9 map ofthe_ Qhisos-Rainbow mines. ' 

17. Geologi9 map of the 2~0-level of the Chisos mine. 

18. Geologic map of the 350-level of the Chisos mine. 

19 . . Geologic map of the 450- and 725-levels of the Chiso~ mine. 

20. Geologic sections, Chisos mine. 

21. Geologic map of the workings of the Two-Forty-Eight mine. 

22_. Geologiemap, isometric projection, and sections of the Two-Forty-Eight niirie. 

FIGURE 1. Index map of tP,eBig Bendregion, Texas______________________________________________________________ 2 

. 2. Outline map and E!~ctions of intrusive rocks in the eastern part of the Terlingua district. A, Analcite syenogabbro 

of Leon Mounta!n;_ B, so.d~ ~rachyte body 1 mile n;orth of Maverick Mountain; C, soda trachyte of Maverick 

Mountain; D, A:qa.,lcitesyenogabbro of Cigar Mountain_______________________________________________ 19 

. 3. Map of the Study Bu~te quartz soda syenite intrusiye body and theoretical cross sections showing progressive 

development of the body and accompanying monocline________________________________________________ 21 

4. Sketches maqe froiD: . photomicrographs of thin sections of analcite-bearing rocks. A, . Analcite-orthoclase 

gabpro sill in Two-Forty-Eight mine; B, analcite syenite from top of Sawmill Mountain; C, analcite basalt 

sill southeastof 9igar_Mountain; D, analcite in bituminous impregnated clay from Two-Forty-Eight mine___ 26 

5. Regional structural trends_"' ________ ._""______________ _________________________________________________ 30 

6, Generalized geologic :map_sJwwing structures of Terlingua uplift and the .Solitario____________________________ 32 

7. Generalized structural map of the Terlingua district, 1

sh.owing Terlingua monocli!le a,nd principal graben_______ 33 

i 
CONTENTS v 

FIGURE 8. Vertical sections across Terlingua monocline west of Maggie Sink, at Cuesta Blanca, and south of Maverick 

Mountain__________________________________________________________ ___ ___________________________ 35 

9. Vertical section across Black Mesa dome______ ________ ___ _____ _____ _________ _______ ____________________ 36 

10. Detailed map of normal faults bounding the southwestern side of Long Draw south of the Chisos mine_____ ___ 37 

11. Outline maps of breccia pipes. A, Aguja breccia pipe near the Chisos mine; B, breccia and intrusive body south of 

the Aguja pipe; C, map and vertical section of Maggie Sink pipe___________ ___________ ______________ ____ 40 

12. A, Vertical sections showing theoretical stages in the development of a small graben caused by the flattening 

in dip of a normal fault; B, vertical section of the Fresno graben_____________________________ ____ _____ __ 49 

13. Stratigraphic section showing vertical range of quicksilver deposits of the several types that occur in the Terlingua 

quicksilver district_______ _______ _________________ ___ ___________________ ____ __________ ___ _______ ___ 54 

14. Idealized sections showing development of limestone-clay contact deposit. (1), Initial stage showing enlargement of solution channel; (2), intermediate stage showing filling of solution channel; (3) final, or ore, stage__ 56 

15. Map and sections of cave-fill at Mariposa mine__ ______ _______ __________________________________________ 57 

16. Sketch of calcite vein in Boquillas flags in face of drift, Chisos mine__________ _____________ ____ ____________ 60 

17. Map showing vein pattern in Boquillas flags of Brown prospect, sec. 286, Block G-4_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 61 

18. Map showing calcite vein pattern in Boquillas flags southeast of Susano shaft, Chisos mine_____ ___ ___________ 62 

19. Geologic map of Bobs mine, sec. 44, Block G-12_____ ________________ _____ ______________________________ 88 

20. Geologic map of part of Natural Resources, Inc. claims in sec. 58, Block G-12_________ _____________________ 89 

21. Geologic map of the Hoover and Pilkington prospects, sec. 58, Block G-12_____ _____ ________________________ 91 

22. Sketch map of the workings off the No.1 shaft, Tarrant Mining Co___ ______ ____ __ _____ ___ __ ___ ______ __ ___ 95 

23. Sketch map of the workings off No. 3 shaft, Tarrant Mining Co_____ ______ _______________________________ 96 

24. Sketch map of the Little Thirty-Eight mine__ __ __ __ __ ______________________ _______________ _______ ______ 97 

25. Sketch map of the adit level, Colquitt-Tigner mine_________ __ ______ ____ ________________________________ 99 

TABLES 

TABLE 1. Stratigraphic names in the Terlingua district_____________ ______ ____________ __________ _____________ ____ _ 7 

2. Analysis of sample of Devils River limestone__ ____ ____ __________ ___ _________ _____ ________ ___ ________ ___ _ 8 

3. Fossil collections from the Devils River limestone_ ____________ ______ _____________ ___ ________ ____________ 9 

4. Analysis of sample of clay from the Grayson formation____ ________ __ ____ ____ _____ ____ _______ __ __ __ __ ____ 9 

5. Analysis of Buda limestone____ _______ _________ __ ____ ____ _____ _____ _____ __________ ___ __ ___ ___ _____ __ _ 10 

6. Analyses of :Baggy limestone in the Boquillas flags ____ ____ __ _________ _____ __________ __________ _____ _____ 11 

7. Fossils from measured section of Aguja formation___________ _____ _______________________ ____ ____________ 15 

8. Analysis of Tornillo clay__ _________ ___ _________ _____ _______________ ___ ____ __ ________ ___ _____ _______ __ 16 

9. The classification of igneous rocks in this report________ __ __________ ________ _________________ ___________ 23 

10. Breccia pipes shown on plate L _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 43 

11. Quicksilver production in Texas _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 52 

12. Minerals of the quicksilver deposits___ ____________________ _________ ____________________________ ____ ___ 65 

13. Analyses of altered and unaltered clay from the Grayson formation___________ _________ ___________________ 67 

14. Mercury chloride content of samples from the Terlingua region_____ ____ ______ _____ _______ ______ __________ 70 

15. Analyses of bituminous material from the Terlingua district_______ _______________________________________ 71 

GEOLOGY AND QUICKSILVER DEPOSITS OF THE TERLINGUA DISTRICT, TEXAS 

By RoBERT G. YATES and GEORGE A. THoMPSON 

ABSTRACT 

The Terlingua quicksilver district, which has produced more 

than 150,000 flasks of quicksilver, is in the southern part of the 

Big Bend region of southwestern Texas. It is a narrow, eastwest area about 20 miles long and lies mainly in southwestern 

Brewster County. The district is connected by graded road 

with the nearest railroad, 84 miles north of its center. 

Quicksilver minerals were first discovered in the district in 

the latter part of the 19th century, but there was no substantial 

production until 1900. Although there are about 20 mines and 

many prospects, more than 90 percent of the quicksilver came 

from the Chisos-Rainbow, Mariposa, and Study Butte mines. 

The most productive years were during World War I; since 1946 

the district has been idle. Future production depends upon the 

discovery of new ore bodies-which will be costly-and the 

working of deposits now considered of too low grade to be 

profitable. 

The area is mountainous and arid; its sparse vegetation is 

typical of desert regions. The altitudes range from 2,100 feet 

to 4,200 feet. Only two streams cross the district, and considerable stretches of these are dry during summer months. 

Because of the concentration of the scanty rainfall into torrential 

showers, and because of the wide range in hardness between 

different rocks, erosion is vigorous and has carved a rugged 

terrain. 

The rocks consist of marine sedimentary rocks of Cretaceous 

age, and lavas and associated clastic beds and hypabyssal intrusive rocks, all of T~rtiary age. Cretaceous sedimentary rocks 

beginning with the oldest, are: Devils River limestone (1 500+ 

feet thick), Grayson formation (80 to 200 feet), Buda lim~stone (50 to 100 feet), Boquillas flags (about 1,100 feet), Terlingua clay 

(about 1,000 feet), Aguja formation (about 800 feet of sandstones 

and clays), and Tornillo clay (600 to 1,000 feet). All are fossiliferous. Rocks of Tertiary age, collectively termed Chisos volcanics, include tuffs, clays, conglomerates, and lavas, ranging 

from trachytes through andesites to basalt. Intrusive rocks are 

in the form of dikes, sills, laccoliths, and plugs and range in composition from rhyolite to basalt. Many igneous rocks have a 

high content of soda, represented in their mineral compositions as 

soda orthoclase, sodic pyroxenes, and analcite. 

Domes, grabens, and breccia pipes are unusually abundant 

and well formed. The T~rlingua uplift, an irregular dome with 

a structural relief of several thousand feet, underlies about half 

the district and is believed to have been formed by igneous uplift. 

Smaller domes, found throughout the district, are clearly the result of igneous intrusions. The grabens are bounded by normal 

faults, which together include most of the normal faults of the 

district. The vertical displacement of the grabens below adjacent blocks is as m,uch as 2,000 feet. Masses of breccia wit h a 

pipelike or chimneylike shape have been designated descriptively 

as breccia-filled pipes, or simply breccia pipes. They range 

from 75 feet to 500 feet in diameter, and one has been explored 

vertically for more than 800 feet. The nature of the breccia and 

its walls indicate that the pipes were formed by collapse of bedded 

rocks into solution caverns. 

Quicksilver minerals have been found in veins and tabular 

and pipelike bodies in both sedimentary and igneous rocks. 

Cinnabar is the principal ore mineral but there are minor quantities of native mercury, chlorides of mercury, and other rare 

mercury minerals. Associated minerals are calcite, clay, pyrite, 

hydrocarbon compounds, and, less commonly, several other 

minerals. The deposits are classified as: deposits formed in 

altered rock along the contact between the Devils River limestone and Grayson formation (limestone-clay contact deposits), 

deposits in calcite veins in the Boquillas flags, deposits in breccia 

pipes, and deposits in igneous rocks. Their distribution is 

directly related to fractures and breccia pipes and to the physical 

and chemical character of enclosing sedimentary rocks and is 

believed to be indirectly related to the Terlingua monocline and 

the igneous rocks in the district. 

The quicksilver deposits were formed from hydrothermal 

solutions of igneous origin at temperatures below 300°C and 

pressures locally as high as 30 atmospheres. Deposition of 

mercury minerals is believed to have been from hydrothermal 

solutions as cinnabar and as primary chlorides. Mineralizing 

solutions ascended breccia pipes and fractures and the ore 

minerals were deposited in parts of these channels lying less 

than 2,000 feet below the surface. 

INTRODUCTION 

LOCATION, CULTURE, AND ACCESSIBILITY 

The Terlingua quicksilver district is in Trans-Pecos 

Texas-the extreme western part of the State that lies 

between the Rio Grande and Pecos River-in the Big 

Bend region, which lies within the great southward 

bend of the Rio Grande, so conspicuous on any map of 

the State. The district is in the southern part of the 

Big Bend region and is a narrow belt, 20 miles long, 

that extends eastward from eastern Presidio County 

into southwestern Brewster County (fig. 1). 

Quicksilver minerals have also been found at several 

places in the Big Bend region outside the Terlingua 

district, but these places are not described. 

The population of the district has never been large 

and has been directly related to the highly fluctuating 

mining activity. Mining was greatest during World 

Wars I and II. Shortly after the end of each war 

many mines closed and the mine workers depa.rted. 

At the close of World War II the district contained 

three small company towns, which supported post 

1 
N ,..-·---, 

i ! L _____ _ I ! T E X A ---~ 

10 0 10 20 Miles 

Contour interval 1000 feet 

Datum is mean sea level 

Base from Sectional Aeronautical Chart of the U.S. Coast and Geodetic Survey 

FIGURE I.-Index map of the Big Bend region. 

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INTRODUCTION 3 

offices and a combined population of several thousand. 

By early 194 7 the mines had closed, only one post 

office was open, and the population had shrunk to a 

few hundred inhabitants. The largest town, Terlingua, 

is a short distance east of the center of the district and 

is connected by graded but unsurfaced roads with the 

other two towns, Study Butte, at the eastern end of 

the district, and Bueno Suerte, at the western end of 

the district. 

Terlingua is 84 miles by road from the Southern 

Pacific Railway and U. S. Highway 90 at Alpine. The 

Alpine-Terlingua road is a fair, graded road but is 

impassible for short periods during and immediately 

after heavy rains. The district is also accessible from 

Marathon, a town likewise on the Southern Pacific 

Railway and U. S. Highway 90, by State Highway 

277, which is the principal access road to the Big 

Bend National Park. 

PHYSICAL FEATURES 

GEOGRAPHIC SETTING 

Trans-Pecos Texas is an arid, mountainous region 

and has more geographic affinity to Mexico and New 

Mexico than it has to the rest of Texas. An excellent 

summary statement of the general physical features of 

Trans-Pecos Texas is given by Philip B. King (1937, 

p. 4) who describes them as follows: 

It [Trans-Pecos Texas] is a region of rugged sierras, of high 

plateaus and broad cuestas, and of gently sloping intermontane 

plains. The mountains have no timber except in sheltered 

valleys and on the higher summits. 

In the clear air of the desert the mountain masses loom with 

sharp outlines and clear detail from a distance of many miles, 

and the plains that surround them are deceptively foreshortened . 

More than half of the region is a lowland. These intermontane 

areas are either bolsons (structural depressions filled by mountain 

waste) or destructional plains that slope upward as pediments 

toward the mountain masses from which they have been carved. 

Ephemeral streams, which are dry gravel beds most of the year, 

discharge from the mountains and flow across the plains. Some 

of these drain into bolsons with no outlet to the sea, such as the 

Salt Basin, in the northwestern part of Trans-Pecos Texas. Most 

of the drainage channels, however, lead to the two master streams 

of the area, the Rio Grande and its major tributary, the Pecos 

River. Their waters flow to the Gulf of Mexico. The Rio 

Grande is noteworthy more for its persistence through long 

stretches of desert land than for its breadth or volume. In its 

southeastward course across the area the river traverses a succession of desert basins and passes from one to the next through 

separating mountain barriers in which it has cut narrow and 

imposing canyons. 

The southern part of the Big Bend region corresponds 

only in small part to King's description. In contrast 

to the plains of west Texas, it is dominantly a mountainous region, having some of the most rugged topography 

within Trans-Pecos Texas, but it lacks the broad, 

sloping intermontane plains that characterize the larger 

484889-60--2 

area. Instead, the mountains are surrounded by areas 

of low relief that are as finely dissected as the mountains 

themselves. These intermontane areas are either bare 

rock or are covered by only a thin mantle of debris. 

The thick bolson deposits that are characteristic of 

much of Trans-Pecos Texas are entirely lacking. 

The contrast between the typical Trans-Pecos Texas 

topography described by King and that of the southern 

Big Bend region is particularly evident when one 

approaches the Terlingua quicksilver district from the 

north. Stretching southward from Alpine is a broad 

upland plain (hereafter referred to as Nine Point Mesa 

Plain) having an average altitude of about 3,500 feet. 

The general low relief of this plain is broken here and 

there by isolated mesas and an occasional peak. South 

of Adobe Wails Mountain (fig. 1) the landscape changes 

abruptly; the broad plain is replaced by many intermontane areas of lower altitude bounded by Inountains 

that are no longer mesalike, but rugged and closely 

spaced. This is what is locally called "Terlingua 

Country," which can be roughly defined as the southern 

dra.inage basin of Terlingua Creek. The land-forms 

are no longer smoothly contoured but give the impression of harshness. Close inspection shows that the 

surface is almost everywhere cut by gullies and canyons. 

This, the southern Big Bend region, is an area of active 

erosion where waste from both mountains and intermontane areas is being transported out of the region. 

TOPOGRAPHY OF THE DISTRICT AND BORDERING AREAR 

The Terlingua quicksilver district, whose boundaries 

are geologic and not geographic, is only a small part 

of the Big Bend region. It is therefore desirable t.o 

define its limits before describing its physical features. 

The district contains a belt of quicksilver deposits that 

extends east and west from the town of Terlingua. 

In 1902 the district was loosely defined by the publication of a topographic map, the "Terlingua District 

quadrangle." 1 Since then the belt of quicksilver deposits has been found to extend beyond the west 

boundary of this map. What is here considered as the 

Terlingua quicksilver district (as shown on pl. I) includes all the area covered by the "Terlingua District 

quadrangle" plus the more recently discovered western 

extension of the district. This expanded district lies 

wholly within the boundaries of the Terlingua quadrangle, a topographic map that includes the area covered by the "Terlingua District quadrangle," with 

which it should not be confused. 

The physical features of the quicksilver district cannot be adequately described without including features 

that lie either outside of or extend beyond the defined 

t Prepared and published by the U.S. Geological Survey in cooperation with the 

University of Texas Mineral Survey. 
4 GEOLOGY AND QUICKSILVER DEPOSITS, TERLINGUA DISTRICT, TEXAS 

limits of the district. Fortunately, the local setting 

can be given by a description of that part of the southern Big Bend region that borders the quicksilver district. 

The district and this bordering area are . referred to 

collectively as the "Terlingua area." The Terlingua 

area can be fairly well defined geographically and in 

general corresponds to what is referred to locally as the 

"Terlingua Country." It is bounded on the north 

by the plains of Nine Point Mesa,. on the northeast by 

the Christmas Mountains, on the east by the Chisos 

Mountains, on the south by the Rio Grande, and on 

the west by the Bofecillos Mountains, and the lava 

plateau of eastern Presidio County (see fig. 1). It lies 

in the eastern two-thirds of the Terlingua ,quadrangle, 

the western part of the Chisos Mountains quadrangle, 

aud extends for a short distance north into the areas 

covered by the Tascotal Mesa and Aqua Fria quadrangles. 

Although mountainous, the Terlingua area is surrounded by areas that are in general topographically 

higher. Most of the central part of the area lies between altitudes of 2,500. feet and 3,500 feet, whereas 

the bordering country, averaging more than 4,000 feet 

in altitude, reaches a maximum of 7,835 feet above 

sea level in the Chisos Mountains. The relief within 

the Terlingua area exceeds 3,000 feet, with Fresno 

Peak in the Solitario area having the maximum altitude 

of 5,131 feet, and the Rio Grande, where it leaves the 

area, having the minimum altitude of less than 2,100 

feet. 

The mountains, arranged without apparent system, 

consist of flattopped domes, broad mesas, and monolithic masses of igneous origin that are either knoblike 

or sprawling. There are both large and small examples 

of these three kinds of mountains. The Solitario is 

the best example of a dome mountain and is larger than 

all other mountains of this type, as well as those of the 

other types. Mesas are largest and most prominent in 

the southwestern part of the region and are exemplified 

by Mesa de Anguila (fig. 1) and Lajitas Mesa (pl. 1). 

The mountains of igneous origin are smaller than those 

of the other two types and are most abundant in the 

northeastern part of the region. Good examples of 

igneous mountains are Hen Egg Mountain, Sawmill 

Mountain, and Bee Mountain. 

Because of the lack of systematic arrangement of 

the mountains, the intermontane areas are irregular in 

size and shape. This is particularly pronounced in the 

northeastern part of the district where the mountains 

are intruded igneous masses that are much more resistant to weathering than the soft sedimentary rocks 

that floor the intervening areas. The broadest intermontane area is in the southern part of the Big Bend 

east of the Mesa de Anguila. Even here the surface 

is not a smooth plain, but its general low elevation is 

broken by mesalike hills and entrenched stream 

channels. 

Although the Solitario, the principal topographic 

feature of the Terlingua area, lies completely outside 

the Terlingua quicksilver district, it requires more than 

casual mention because of its influence on the topography, as well as the geology of the western half of 

the district. The southern edge of the Solitario is 

about 3 miles north of the north border of the district 

at its west end. The Solitario is an almost perfectly 

circular topographic feature, 7.5 miles in diameter, 

of two concentric parts. The outer part, or rim, 

consists of an arrangement of hogback hills, from 4,600 

feet to 5,100 feet in altitude, which have steep to gentle 

outward slopes and much shorter clifflike inward 

slopes. The rim encloses a central basin, which is 

a moderately smooth surface with an average altitude 

of abou,t 4,400 feet, that is broken by low hills. The 

interior basin is drained by several stream channels, 

which are enclosed in deep, narrow canyons where they 

cross the rim. The north flank of the Solitario descends 

fairly gently to the plain of Nine Point Mesa, the west 

and southwest flanks descend steeply to Fresno Canyon, 

and the eastern flank descends with moderate steepness 

into the drainage basin of Terlingua Creek. The 

southeast flank is less well defined as it merges with a 

high, intricately dissected, plateaulike surface that 

extends southeastward into the Terlingua quicksilver 

district. 

This plateaulike surface that extends southeastward 

from the Solitario is the principal topographic feature 

of the Terlingua district. It has a general southeasterly slope and is recognizable as far mist as the town 

of Terlingua where its southern and northeastern 

boundaries join. Its southern boundary is a welldefined, southward-facing escarpment, whose general 

east-west alinement is locally sinous and broken by 

reentrants. These are narrow, steep-walled canyons 

such as Croesus CanyoD and broad, northwesterly 

trending, flat-botton1ed valleys such as Long Draw 

(pl. 1). The northeastern boundary is less well 

defined but can be arbitrarily fixed as parelleling the 

Long Draw and lying about 1 mile northeast of it. Its 

surface is dissected by many canyons and spotted with 

small buttes that rise above its general level. Besides 

the valleys that cross it in a northwestern direction, 

it is also crossed by one wide valley of northeastern 

trend; this, Lowes Valley, has a broad, flat floor and 

cliff walls. The most prominent feature that projects 

above the general level of the plateaulike surface is 

Black Mesa, a roughly circular, flat-topped mountain 

that borders the southeast side of the Lowes Valley. 
INTRODUCTION 5 

The country south of this plateau projecting from 

the Solitario is topographically lower and has little 

local relief. In general, it is an irregular arrangement 

of low cuestas with steep northerly slopes and gentle 

southerly slopes. Many small canyons and ravines 

floored with dry stream channels cross the area. 

Southward and eastward the stream channels converge 

into gravel-strewn plains. This cuesta area extends 

along the southern border of the Terlingua quicksilver 

district from Contrabando Mountain eastward (see 

pl. 1). 

Contrabando Mountain and Contrabando dome, 

two contrasting features form the western part of the 

southern border. Contrabando Mountain has a narrow, 

flat top and is a low northwestern extension of Lajitas 

Mesa, which is just south of the district. Contrabaudo dome is a low, circular arrangement of hills that 

have gentle outward slopes and steeper slopes toward 

the stream valleys that cut back into the interior of 

the dome. 

The eastern one-third of the Terlingua distric.t is in 

contrast to the western two-thirds. The plateaulike 

surface extending from the Solitario converges with an 

intermontane surface whose average altitude lies between 2,500 and 2,700 feet. Above this surface 

project numerous isolated mountains that are roughly 

circular in form. Almost all these mountains are 

igneous masses, representing intrusion into soft sedimentary rocks that have since been eroded away. 

Terlingua and Fresno Creeks are the only permanent 

streams in the Terlingua area, and even long stretches 

of them are dry during the more arid months. All the 

runoff reaches the Rio Grande by way of these streams 

except for local areas adjacent to the Rio Grande that 

are drained by small intermittent streams. Terlingua 

Creek, crossing the central part of the region in a southerly direction, drains most of the area, although 

Fresno Creek, also flowing south, drains a much smaller 

area along the western border of the region. 

During and immediately after rain storms Terlingua 

Creek is a boiling torrent, but during most of the year 

long stretches of its channel are completely dry. 

Between the dry stretches are infrequent spring-fed 

ponds from which water trickles downstream to be 

almost immediately lost in the gravels of the stream 

bed. These are the watering holes for the few cattle 

and goats that are grazed in the district. One of these 

permanently flowing sections of Terlingua Creek furnishes the water supply for the Two-Forty-Eight 

m1ne. 

Springs are very rare in the Terlingua area an.d 

none have more than feeble flows. A more common, 

but less trustworthy, water supply is from the tanks, 

or tinajas, as they are called locally. These are both_ 

artificial and natural. The artificial tanks are storage 

basins for rainwater formed by dams built across small 

ravines. The natural tanks are depressions in drainage 

channels formed by solution of limestone and plunge 

basins below small waterfalls. Some tanks are large 

enough to store water sufficient to last between rains. 

CLIMATE AND VEGETATION 

The climate of the Terlingua district is arid or semiarid. No adequate rainfall records have been k~pt 

and there are no accurate data. The annual rainfall 

at Marathon and stations nearby is about 17 inches; 

and as the vegetation in the Terlingua district is representative of a more arid climate, it is probable that 

the rainfall in the Terlingua district is considerably 

less. Much of the rainfall occurs during the late 

summer months and is often torrential. Summer 

rain storms may occur without warning at any time. 

They are highly local; one storm rnay occur within an 

area of only a few square miles, while the sun shines 

brightly in adjoining areas. 

The mean annual temperature is probably about 

70°F. Daytime summer temperatures are high; temperatures of 100°F. are usual and temperatures above 

110° are frequent. Only during short periods are the 

summer nights hot enough to make sleeping difficult. 

The winter climate is particularly invigorating: days 

usually are warm and sunny; the nights, however, 

are cold and temperatures below 20° have been recorded. Snow occasionally falls on the higher peaks 

and more rarely an inch or more covers the entire 

district . 

The vegetation of the Terlingua district is scanty 

and consists mostly of plants especially adapted to 

survive in a climate where water is scarce and evaporation high. Members of the cactus and yucca families 

are most abundant and are found throughout the district. The cacti are mostly small forms that grow 

close to the ground and include representatives of both 

the Opuntia and Cereus groups. The commonest plants 

are many species of yucca, the amaryllid, lechuguilla 

(Agave lecheguilla) and, the lily, sotol (Dasylirion 

wheeleri). Lechuguilla grows particularly well on 

limestone slopes and in many places these sharpspined plants are so closely spaced that walking is 

difficult. Sotol requires more soil than lechuguilla and 

individual plants are widely spaced. Prickly pear 

(Opuntia) and ocotillo (Fouquieria splendens) grow on 

both the hills and on the valley flats. Creosote bushes 

(Covillea) are common on the broad flats and flood 

plains of the larger streams, and mesquite (Prosopsis 

julijlora) and catclaw (Acacia), both thorny shrubs, 

grow along stream courses. The wax plant is locally 

abundant enough to be harvested and treated to ob-
6 GEOLOGY AND QUICKSILVER DEPOSITS, TERLINGUA DISTRICT, TEXAS 

tain the thin coating of wax that covers it. The only 

trees in the district are a few cottonwoods that grow 

along Terlingua and Fresno Creeks. 

PREVIOUS WORK 

The Terlingua quicksilver district apparently escaped 

the attention of geologists until the close of the 19th 

century, when the district was visited by W. P. Blake. 

Blake's interest was concentrated on the recently 

discovered quicksilver deposits, and he gives an excellent summary (Blake, 1896) of the geology of the deposits that were known in 1895. By 1902 increasing 

interest in the deposits led the Texas Mineral Survey 

to send B. F. Hill to make a study of the area. His 

report (Hill, 1902) contributes some valuable historical 

information on what is now the central part of the 

district. 

The work of J. A. Udden, ·(1907a, 1907b, 1911) and 

Udden and others, (1916), beginning a short time after 

Hill's report was published, described the stratigraphy 

of the Big Bend region and was the first serious attempt 

to correlate the deposition of cinnabar with structural 

features. Unfortunately, a paucity of published maps 

detracts from the value of Udden's work; however, of 

all the early work in the district, Udden's is outstanding. Adkins (Sellards, Adkins, and Plummer, 1933) in 

1933 revised and adjusted Udden's stratigraphy into 

closer correlation with that of the rest of Texas. 

Various phases of the geology of the district have 

been discussed by F. L. Ransome, (1917), H. W. 

Turner (1905), and J. T. Lonsdale (1940). The work 

of Ransome is in part based upon data gathered by 

H. D. McCaskey in 1910. The igneous rocks of the 

area have been excellently described by J. T. Lonsdale 

(1940). 

In 1934 Clyde P. Ross, assisted by C. H. Coldwell, 

W. E. Cartwright, H. E. Stocking, and J. A. Connors, 

made a study of the Terlingua quicksilver district and 

the surrounding region. A brief preliminary report 

(Ross, 1941) on this work was published in 1941. A 

more extensive manuscript by Ross is in the files of the 

Geological Survey and was available to the authors. 

Parts of it referred to as "written communication," 

have been incorporated herein. 

PRESENT INVESTIGATION AND ACKNOWLEDGMENTS 

The fieldwork upon which this report is based was 

done between April 1942 and March 1945. Yates, 

accompanied by J. F. McAllister, first entered the area 

in April 1942 to examine several prospects at the request of the War Production Board. Shortly thereafter the War Production Board requested additional 

examinations, covering most mines and prospects in the 

district. In July the party was joined by Thompson. 

In November McAllister was transferred to another 

assignment, but Thompson and Yates remained in the 

field until late December. 

Thomspon returned to the district in February 1943 

and was joined by Randall E. Brown in April and by 

Yates in July. Brown left in August but Thompson 

and Yates remained until February 1944. Jaime 

Fernandez Concha assisted in the fieldwork during 

August and September 1943. During this field season 

the United States Bureau of Mines carried on an exploratory drilling program from June until October, 

testing favorable structures at the Chisos and Fresno 

mines. Geological guidance for this program was given 

by Thompson and Brown. 

Thompson and Yates returned to the district in the 

spring of 1944 to map the areal geology; Yates remained 

only 2 weeks and Thompson 6 weeks. In December 

Yates, accompanied by Edgar H. Bailey, returned to 

the district and completed the mapping of the areal 

geology by March 1945. 

All those who shared in the fieldwork contributed 

greatly to this report, and although their names do 

not appear on the title page, their individual contributions are credited on the maps. Particular mention 

should be made, however, of the contributions of 

James F. McAllister and Edgar H. Bailey. McAllister 

had a large share in the mapping of the Chisos mine and 

several prospects, and Bailey mapped a considerable 

part of the areal geology. Their ideas and suggestions 

on the interpretation of the geology were valuable. 

The manuscript report by Clyde P. Ross, mentioned 

in the preceding section, proved of immeasurable 

value. The sections on stratigraphy, mining history, 

and mine production, as well as miscellaneous mine 

descriptions, are largely revised from Ross's manuscript. 

Ross's observations and conceptions of the geologic 

problems were of great initial help to the authors in 

their study of the area. The opportunity to do detailed 

mapping resulted in the confirmation of many of Ross's 

interpretations, which were, of necessity, based upon a 

less extensive study. However, our studies revealed 

new evidence that conflicts with some of Ross's published ideas (Ross, 1935a, 1935b, 1937, 1941). Unless 

otherwise noted, the present interpretations are based 

on an independent study of the field evidence for which 

the authors assume full responsibility. 

The mining men of the district were, almost without 

exception, extremely cooperative and hospitable to 

the field party. They gave freely of their time and 

information, contributions without which the fieldwork 

would have been difficult. To list all those to whom 

the writers are indebted is not possible, but special 

thanks must be given to the Messrs. W. D. Burcham, 

Harris S. Smith, and Charles Worthington. 
GENERAL GEOLOGY 7 

GENERAL GEOLOGY 

STRATIFIED ROCKS 

The writers' principal interests in the district were 

the structure and quicksilver deposits, and studies were 

concentrated on these phases. This specialized approach would not have been possible without the 

excellent background of stratigraphy supplied by the 

manuscript report of Clyde P. Ross. The descriptions 

that follow are mainly condensations of those of Ross, 

and the measured stratigraphic sections, fossil oollections, and chemical analyses are direct quotations. 

Although the strata in the Big Bend region range in 

age from early Paleozoic to Tertiary, almost all the 

stratified rocks exposed in the Terlingua quicksilver 

district are of Cretaceous age. Rocks of Paleozoic 

age doubtless underlie all the Cretaceous rocks, but as 

these rocks are exposed only in the Solitario, they are 

not discussed in this report. Lavas and associated 

clastic beds of Tertiary age, although very abundant 

east and west of the mapped area, are only sparsely 

represented. 

The Cretaceous rocks in the Terlingua district fall 

naturally into distinct lithclogic units that are easily 

recognized and mapped. Although these units are 

easily distinguished in the field, a comparison of the 

several reports on the district leads to confusion because 

of the duplicity of names. This duplicity has resulted 

mainly from the attempts to apply formational names 

that have been used in the eastern part of the State 

without consideration of differences in lithologic 

character. 

The Cretaceous stratigraphic units in the Terlingua 

district were first named and defined by U elden (1907 a). 

Except for the Buda limestone and the Del Rio clay 

(of former usage), Udden used local names to describe 

the strata. After later studies, Adkins (Sellards, 

Adkins, and Plummer, 1933) revised U elden's stratigraphy by redefinition and by substituting outside 

names for some of Udden's local names. The usage of 

Ross favors the return to some of the local names, a 

return that the writers of this paper have followed 

(table 1). 

Although Adkins' studies established a fairly complete correlation between the local units and better 

known stratigraphic units in the eastern part of the 

State, it seems desirable to deviate somewhat from it. 

This largely results from the fact that the field mapping 

of geologic boundaries requires lithologic, rather than 

paleontologic units. If the paleontologic boundaries 

are not also lithologic boundaries, their mapping is 

difficult if not impossible. With a few exceptions 

Adkins' terminology has been retained, but his formational boundaries have been shifted in several places. 

TABLE I.-Stratigraphic names in the Terlingua district 

Udden (1907a, p . 21-60) Sellards, Adkins, and Names used in this report 

Plummer (1933) 

Surface flows Chisos volcanics 

Rocks of Tertiary age Volcanic rocks 

Burro gravel and tuff 

Crown conglomerate Crown conglomerate 

Navarro group Chisos beds Chisos beds 

Tornillo clays Tornillo formation Tornillo clay 

r/1 r/1 ----------------------------------------- ==' ~ Rattlesnake beds Aguja formation Aguja formation 0 Taylor group ·;::: ~ ~ 

0 r/1 

~ Terlingua beds .Terlingua clay :::::: Taylor formation 

""" ~ ;::; 

0 

~ Austin group 0 Terlingua formation ~ (restricted) Boquillas Upper ~ flags member 0. 

0. Eagle Ford Boquillas flags Boquillas flags p group Lower 

member 

Washita group r/1 Buda limestone Buda limestone Buda limestone ~ 

·c 

~ Del Rio clay Grayson formation Grayson formation r/1 

-- ClJ r/1 ~ Not differentiated in the Ter- Georgetown limestone Devils River limestone ==' ~0 Fredericksburg 0 

~~ ~ lingua district ~ 0 group ~ Edwards limestone 0 ~ s H~ 0 ~ 0 0 
8 GEOLOGY AND QUICKSILVER DEPOSITS, TERLINGUA DISTRICT, TEXAS 

DEVILS RIVER LIMESTONE 

DISTRIBUTION AND CHARACTER 

The Devils River limestone, the oldest rock exposed 

in the Terlingua district, is a thick sequence of limestone beds of uniform character that are so resistant to 

erosion . that they form spectacular cliffs and gorges. 

It is the most abundant rock in the district and, although 

not exposed in the eastern third of the district, its 

exposures total more than half of the area of the 

central and western thirds of the district. 

It is a medium- to thick-bedded, fine-grained limestone, light to dark gray on freshly broken surf~ces, 

and gray on weathered surfaces except where stained 

brown through the oxidation of pyrite, of which it 

contains small amounts. The chemical analysis given 

below (table 2) shows that it is a relatively pure limestone, very low in magnesia content, and contains only 

about 1 percent of silica. The calcium carbonate 

occurs mainly as shells and shell fragments; the 

carbonate is recrystallized, but not extensively-. Although the organic character of the limestone is evident 

in the field where some beds can be seen to consist 

almost entirely of oysterlike fossils, it is strikingly 

evident when the limestone is studied under the 

microscope. Much of the matrix material . around 

megascopic fossils was seen to be tests and fragments of 

tests of Foraminifera, notably species of · Globigerina or 

similar genera. The silica in the rock analyzed is 

probably disseminated in small grains throughout the 

rock; however, some beds contain variable amounts of 

small· and irregular concretions of chert. 

Joints are well developed and the joint pattern has 

been accented by solution along joint planes. In places 

this solution of the limestone ha.s progressed far enough 

to produce caverns and sinkholes. - -. -

The base of the Devils River _limestone is nowhere 

exposed in the Terlingua di~t~i~t; consequently, its 

local thickness could not be -measured. Although 

several sections contain more than 500 feet of beds, 

they doubtless represent only a s~all pa~t of the total 

section, because south of the district, in- cliffs of the ··- . 

TABLE 2-Analysis of sample of Devils River limestone collected 

by Clyde P. Ross from Canyon Group of claims in the northeast 

part of sec. 4-2, Block G-12. 

[Analyst, Charles Milton, November 1936] 

Si02------------------------------------------------

Fe20a---------------------------------------------- MgO _____________________________________________ _ 

Ca0 __________________ ~------~-~ :~ -~--~-- ~------

002---------------- _·_---------_-,-_-:,--:- -::.::: -------------- Il20 ______________________________________________ _ 

Percent 

1. 08 

. 56 

. 06 

54. 62 

42.85 

. 65 

TotaL ____________________________________ --- 99. 82 

Mesa de Anguila, the thickness is more than 1,500 

feet and even here the base is not exposed. The base 

is exposed, however, in the rim of the Solitario. Here 

the Devils River limestone rests conformably upon 

impure limestone containing Glen Rose fossils (Sellards, 

Adkins, Plummer, 1933, p. 305), which in turn rests 

conformably upon a coarse sandstone and conglomerate. 

The conglomerate rests in angular discordance upon 

beds of Paleozoic age. 

The contact of the Devils River limestone with the 

overlying Grayson formation is in all its many exposures sharp and conformable. This division between 

the two formations is not, however, so abrupt a change 

from limestone to clay as the sharp contact suggests, 

because the uppermost beds of Devils River limestone 

are argillaceous and the lowermost beds of the Grayson 

formation are calcareous. These slightly argillaceous 

beds of Devils River limestone appear to be confined 

to the uppermost 50 feet of the formation, which, in 

some places, is also thinner bedded and less resistant 

to solution. Many quicksilver deposits occurred in 

this part of the Devils River limestone. 

This uppermost 50 feet of the formation has locally 

been termed the Georgetown limestone (see table 1), a 

distinction that has not been made on the district 

map (pl. I) because a . consistent mapping of this 

subdivision would be extremely ·difficult. A subdivision 

based on the above described differences in lithologic 

character could have been made in the vicinity of 

California Mountain (pl. 1), but at other places in the 

district the differences are too vague for accurate 

mapping purposes. Under these circumstances it seems 

best to call the entire unit the Devils River limestone, 

a general term proposed by Udden (1907b, p. 56) for a 

similar sequence of limestone beds without mappable 

subdivisions in Val Verde County, Tex. 

AGE 

According to Ross, "Previous investigators (Sellards, 

Adkins and Plummer, 1933, p. 271, 346; Hill, 1902, 

- p. 16) 'consider that none of the beds of the Devils 

River limestone appea.r to be younger than Georgetown 

and that the oldest beds are probably of Fredericksburg age." Fossils are sparse and poorly preserv-ed. 

Of the six collections of fossils (table 3) obtained 

from beds assigned to the Devils River limestone, three 

collections are regarded by J. B. Reeside, Jr., as of 

_ undetermina.te age and the same is true of specimen 

TT 506 examined by L. G. Hen best. Reeside lists one 

collection (USGS 16847) from high in the formation 

as definitely of Edwards age, and another (USGS 

16850) from the lower part of the Devils River as 

probably of Edwards age. 
GENERAL GEOLOGY 9 

TABLE 3.-Fossil collections from the Devils River limestone in the 

- Terlingua region, Texas 

[Numbers, except TT 506, are U. S. Geological Survey Mesozoic locality numbers] 

16846. Top of large sinkhole ncar Colquitt-Tigner mine. 

Kingena wacoensis (Roemer) 

Neithea sp., fragments 

16847. Top of Black Mesa. 

Neithea duplicicosta (Roemer) 

Lucina acutilineolata (Roemer) 

Toucasia cf. T. texana (Roemer) 

Eoradiolites'l sp., fragments 

Boring, undetermined 

16850. From base of scarp on the north side of the Rio Grande 

at the east end of the Grand Canyon of Santa Helena. 

Gryphaea marcoui Hill and Vaughan 

Exogy ra texana Roemer 

Pholadomya near P. sanctisabae (Roemer) 

Lucina'l sp., molds 

Cardium? sp., molds 

Turritella'! sp., molds 

16967. Crosscut of the 200 foot level off the Contratira (Subia 

winze) Mariposa mine. (Collected by H. D. McCaskey.) 

Ostrea (Exogyra'l) sp. 

16980. Main shaft dump, Colquitt-Tigner mine. (Collected by 

H. D. McCaskey.) 

Pecten sp. 

TT 506. Canyon group of claims (studied by L. G. Henbest). 

Globigerina sp. 

Textularian? (not Haplostiche texana) 

Echinoderm plates 

GRAYSON FORMATION 

DISTRIBUTION AND CHARACTER 

Conformably overlying the Devils River limestone is 

an almost structureless clay that had long been known 

as the Del Rio clay (Hill, 1902, p. 18-19). Adkins 

(Sellards, Adkins, and Plummer, 1933, p. 270, 271, 

386-387, 391-393) proposed extending to the Terlingua 

area the name Grayson formation, a name long used for 

beds of similar stratigraphic position farther north. 

The clay of the Grayson formation crops out in the 

same general part of the district as the Devils River 

limestone, but because it is much thinner and has been 

extensively eroded, it covers a much smaller area. It is 

mainly along the flanks of folds and beneath the talus 

slopes of buttes and mesas that the Grayson formation 

has been able to resist erosional stripping. It is not 

only easily eroded but is a constant hazard where it 

forms the walls of mine workings because it loses all 

coherency and completely disintegrates upon wetting. 

Underground the clay appears as a massive rock, 

except near the top and bottom of the formation where 

more calcareous beds give the rock a poorly bedded 

character. In surface exposures the bedding is more 

pronounced, for individual beds weather variously from 

gray to rusty brown. This is due to the oxidized 

pyrite that is present in small quantities throughout 

TABLE 4.-Analysis of sample of clay of the Grayson formation 

collected by C. P. Ross from road to the Mariposa mine in sec. 

39, Block G-12 

Analyst, Charles Milton, November 1936] 

Si02- ____ __ . ____ .. ____ __ _________ _ 

Al203 

__ ___________ _____________ _ 

Fe203 ____ __ ____________________ _ 

Ca0 ____ ______________________ _ 

MgO ____ _______________________ _ 

l'ra20 ____ ___ ___________________ _ 

K 20 _____ _______________ _______ _ 

H 20 _____ ______________________ _ 

Ti02 ____ ____ . _________________ -

C02 _____ ______________________ _ 

SubtotaL ________________ _ 

TotaL ______________ -_--- .. 

Percent 

soluble 

inHCl 

0. 16 

1. 14 

. 29 

12. 77 

1. 56 

11. 94 

27. 86 

Percent 

insoluble 

inHCl 

44. 87 

15.09 

2. 75 

. 23 

. 59 

None 

2. 10 

6. 25 

1. 12 

73. 00 

100. 86 

the rock and is abundant in certain beds. The pyrite 

is best seen in mine workings and in drill cores; it is 

never seen in weathered rock. As indicated by the 

following analysis (table 4) the rock is calcareous. The 

calcium carbonate content of individual beds ranges 

from about 10 percent to more than 90 percent. 

Although the character of the Grayson formation 

(Late Cretaceous) is constant throughout the district, 

its thickness varies considerably. In and near sec. 42, 

Block G-12, it is about 200 feet thick. A diamond-drill 

hole on the Canyon group of claims penetrates 195 feet 

of clay before entering the Devils River limestone. 

Beneath California Mountain the Grayson formation 

is about 185 feet thick. In sec. 70, Block 341, farther 

east, the thickness is a little less than 100 feet. North 

of Clay Mountain the formation appears to be 80 feet 

thick. AGE 

Numerous and well-preserved fossils are common in 

the Grayson formation. Adkins (Sellards, Adkins, and 

Plummer, 1933, p. 393) reported that the basal and 

middle parts of the formation contain an abundance of 

echinoderms (mostly Heteraster), Gryphaea mucronata, 

pyritized micromorphs of ammonites and gastropods, 

and Haplostiche texana. In the upper beds he noted a 

zone of abundant Exogyra cartledgei Bose. 

Hill (1902, p. 19) reports that the most common 

fossils in the formation include Nodosaria [HaplosticheJ 

texana, Exogyra arietina, Exogyra drakei, Gryphaea 

pitcheri (mucronata), and numerous echinoderms. 

According to Ross (written communication): 

Two specimens of Del Rio clay [now called the Grayson formation] were examined by Henbest for possible microfossils. 

One of these from an especially calcareous bed on the Canyon 

group of claims was found to . contain abundant Foraminifera, 

but these cannot be identified closely enough to be of value in 

age determination. The microfossils in a specimen of typical 

Del Rio clay [now called the Grayson form.ation] from an out-
10 GEOLOGY AND QUICKSILVER DEPOSITS, TERLINGUA DISTRICT, TEXAS 

crop on the road east of California Mountain are regarded by 

Henbest as indicative of Comanche age. 

Fossil collections from the Grayson formation in the Terlingua quicksilver district 

[Material submitted by C. P. Ross to J. B. Reeside, Jr., for identification. Numbers 

are U. S. Geological Survey Mesozoic locality numbers] 

16836. From the northwest part of Reed Plateau (pl. 1) close 

to the Walker prospect. 

Exogyra arietina Roemer 

Exogyra cartledgei Bose 

16973. Nodular limestone in Grayson formation and separated 

by fissile shal€ from base of Buda limestone on California Mountain. (Collected by H. D. McCaskey.) 

Exogyra cartledgei Bose 

16975. Near top of Grayson formation on the southwest slope 

of California Mountain. (Collected by H. D. McCaskey.) 

Haplostiche texana (Conrad) 

16977. From the upper few feet of Grayson formation on California Mountain. (Collected by H. D. McCaskey.) 

Haplostiche texana (Conrad) 

BUDA. LIMESTONE 

DISTRIBUTION AND CHARACTER 

In the Terlingua district, it is almost axiomatic that 

wherever there is the Grayson formation there is Buda 

limestone. Preservation of the easily eroded clay of the 

Grayson for any length of time, is possible only where 

the clay is protected by the more resistant Buda limestone. Inturn, the Buda, because of its relation to the 

Grayson, characteristically forms hogbacks and is the 

capping rock in mesas and buttes. 

The Buda limestone is a distinct, easily defined stratigraphic unit. It is a white, well-bedded and well-jointed 

limestone commonly 90 feet thick, but locally as much 

as 100 or as little as 50 feet thick. Its tendency to 

fracture conchoidally, coupled with a spherical weathering of joint blocks, produces a characteristic gravel-like 

detritus. 

The limestone contains more than 96 percent calcium 

carbonate as can be seen from the analysis in table 5 

quoted from Udden (1907a, p. 28). 

TABLE 5.-Analysis of Buda limestone one mile east of Boquillas 

Silica _____________________________________________ _ 

Alumina __ -.- ______________________________________ _ 

Ferric oxide ________ _________________ ______________ _ 

Lime _____________________________________________ _ 

~agnesia __________________ _______________________ _ 

Carbonic acid _____________________________________ _ 

Water (hygroscopic) ____ ____ ____ ___________________ _ 

Water (combined) _________________________________ _ Sulphur ____________________________ ______________ _ 

Percent 

2. 35 

. 21 

. 24 

53. 90 

. 15 

42. 23 

. 18 

. 33 

tr 

Total __________ ___ ___________________ ________ 9Q 59 

AGE 

Identifications by J. B. Reeside, Jr., of five fossil 

collections submitted by C. P. Ross are listed below: 

Fossil collections from the Buda limestone, Terlingua quicksilver 

district 

[Numbers are U.S. Geological Survey Mesozoic locality numbers] 

16838. From one-quarter mile east-northeast · of Coltrin Camp 

on the southwest side of Reed Plateau. (Collected by 

W. E. Cartright.) 

Budaiceras cf. B. mexicanum Bose 

Stoliczkaia sp. 

16839. From the margin of the Buda limestone on Reed Plateau, 

north of the center of sec. 69, Block 341. 

Neithea sp., fragment 

Cardium, apparently unnamed 

Nerinea? sp., fragment 

Budaiceras mexicanum Bose 

16851. . Close to the east end of the exposure of Buda limestone 

on the northeast side of Reed Plateau. 

Sponge? 

Borings of worms? 

Holaster sp. 

Astarte sp., fragment 

Nemodon? sp., possibly new 

Neithea subalpinus Bose 

Turbo? sp., unnamed 

Glauconia? sp., fragment 

Schloenbachia? sp., probably unnamed 

Budaiceras cf. B. hyatti (Shattuck) 

16971. Forty-six feet below top of exposed Buda limestone about 

2 miles southwest of Tres Cuevas ~ountain in the 

northwest quarter of sec. 57, Block G 12. (Collected 

by H. D. McCaskey.) 

Budaiceras sp. 

16978. Location unknown. (Collected by H. D. McCs.skey.) 

Budaiceras sp., fragment 

A specimen of Buda limestone from the Chisos mine 

consists, according to Henbest, largely of the following 

Foraminifera: Globigerina or Globerigerinella, Guembelina?, Textularia?, and dissociated cha.mbers of 

globerigerinids. Similar but even less closely identifiable forms were found by Henbest in two other specimens of Buda limestone. 

BOQUILLAS FLAGS 

DISTRIBUTION AND CHARACTER 

The rocks descdbed on the preceding pages, the 

Devils River limestone (Lower Cretaceous), and 

Grayson formation, and Buda limestone (Upper Cretaceous), are predominantly limestones. The deposition of the lowermost beds of the Boquillas flags, which 

overlie the Buda 'limestone with apparent conformity, 

marks a change in conditions of sedimentation. The 

Upper Cretaceous rocks contain, as one goes up in the 

section, decreasingly less limestone and increasingly 

more clay and sand. This change in rock character 

represents a progressive chan.ge from a marine to a 

continental environment of sedimentation, a change 

that culminated in the deposition of volcanic rocks, 

which continued to be deposited well into Tertiary 

time. 
GENERAL GEOLOGY 11 

As a result of these progressive changes in environment, the upper Cretaceous units grade one into the 

other; consequently, they are not easily delineated and 

have therefore been variously defined and named. The 

writers in mapping the upper Cretaceous rocks divided 

them into formations solely on the basis of lithologic 

character; consequently, formational boundaries are not 

always in agreement with those of earlier workers, \Vho 

established some boundaries on the basis of fauna in the 

desire to have formational divisions that would agree in 

time span with those previously established elsewhere in 

Texas. 

The Boquillas flags, which consist of about 1,000 feet 

of flaggy limestone and shaly beds, are widely distributed throughout the district (pl. 1), and are particularly 

well exposed east of the Chisos mine and in the Contrabando dome south of the Fresno mine. Although the 

basal beds immediately overlying the Buda limestone 

are dominantly limestones, their thin-bedded character 

contrasts so sharply with the thick-bedded Buda limestone that the contact between the two is readily recognized and easily mapped. The formation as mapped 

includes all the beds considered by Ross (1941, p. 121) 

as belonging to the formation, but it is divided into an 

upper and lower member, the plane of division being 

the top of a thin sandy limestone bed that represents a 

faunal zone characterized by an ammonite ( Crioceras), 

found nowhere else in the formation. 

The lower member of the Boquillas is dominantly 

limestone, the beds ranging from 6 inches to 3 feet thick, 

separated by thinner layers of limy shale. The limestones are yellow to creamy white on weathered surfaces 

and gray to black on fresh fracturos. Individual beds 

are faintly laminated and the rock can be readily split 

along the laminations, to produce flagstones, or "flags," 

a property for which it was named. Many limestone 

beds are sandy, containing small amounts of detrital 

quartz grains; others are argillaceous. The analyses 

quoted below show the siliceous and argillaceous character of the flaggy beds and also the presence of "organic" or carbonaceous matter, which is the pigmentation that gives the rock its black color. 

The selected analyses in the following table give some 

idea of the variations in composition of the flags. 

The shaly intercalations are nearly black on fresh 

fracture, and likewise bituminous; they weather to 

light yellow or buff. The upper part of the Boquillas 

contains more shale than the lower part. 

The typical flagstone of the formation is an impure 

limestone, yellow to creamy white on weathered surfaces, and gray to black on fresh fractures. The flags 

range from a few inches to a few feet in thickness, and 

can be cleaved into large flat slabs from 1 to 5 inches 

thick. Some of the flaggy beds are sandy. The 

TABLE 6.-Analyses of jlaggy limestone from the Boquillas flags 

(Udden, 1907a, p. 30, 40) 

0. H. Palm, analyst] 

So·uth of Cuesta Cuesta Blanca Near Colquitt· Blanca Tigner mine 

Silica ____ ___ ______________ 19. 34 7. 80 20. 72 

Alumina ___ _____ __________ 3. 00 1. 30 16 

Ferric oxide _______________ 2. 16 1. 30 . 45 

Lime ___ ____ _______ __ _____ 39. 50 49. 20 43. 15 

Magnesia _________ ____ ____ 1. 00 . 15 . 52 

Carbonic acid _____ ________ 31. 50 38. 50 34.20 

Organic matter ____________ . 90 1. 10 --------- Water (hygroscopic) _______ . 30 . 20 . 10 

Water (combined) _____ ____ 2. 00 . 50 . 32 

----

Totals ______________ 99. 70 100. 05 99.62 

flaggy beds are intercalated with thin layers of shaly 

material, which is also calcareous and generally contains 

some bituminous matter. Most of the shaly material 

is nearly black on fresh fracture but weathers to light 

yellow or buff. The quantity of shaly material increases 

upward in the formation to where the flags disappear, 

and there is a gradation into the overlying Terlingua 

clay. 

The upper limit of the lower Boquillas is placed at 

the top of a 1- to 2-foot bed of sandy limestone about 

500 feet above the top of the Buda limestone. This 

bed contains abundant fossils, determined by Adkins 

(Sellards, Adkins, and Plummer, 1933, p. 451) as 

belemnites, later found by L. W. Stephenson (oral 

communication, 1955) to be Sciponoceras cf. S. gracilis 

(Shumard), Crioceras n. sp., and Scaphites aff. S. 

vermiculus. The most distinctive of these fossils, is the 

large uncoiled amm.onite, Crioceras, which was seen at 

no other horizon in the formation. 

The bed, because of its arenaceous composition, 

forms low hogbacks and is easily recognized as a dark 

line on the air photographs. Its characteristic fossils 

distinguish it from all other similar beds. 

The beds above the Crioceras zone resemble those 

below except there is a tendency for the limestone 

beds to be more shaly, and they are also farther apart. 

This change is progressive and results in a gradation 

into the overlying formation, the Terlingua clay. 

The upper limit of the Boquillas flags is not as definite 

as the lower limit. In mapping this boundary the 

writers followed C. P. Ross, who said (written communication), 

For purposes of mapping, it [the upper contact] is placed at the 

base of the dark, dominantly shaly beds, in which flagstones are 

absent or so thin and sparsely distributed as to be relatively 

inconspicuous. Nearly everywhere the contact as thus defined 

can be recognized with sufficient precision for mapping. Even 

where shaly beds are exceptionally abundant in the upper part 

of the Boquillas flags they can be distinguished from similar 

but generally less calcareous material above by the fact that, on 

the whole, they weather to a lighter, more yellowish color and 
12 GEOLOGY AND QUICKSILVER DEPOSITS, TERLINGUA DISTRICT, TEXAS 

have a conspicuous amount of fl.aggy beds interbedded with 

them. 

The great number of faults that cut exposures of 

Boquillas flags in the Terlingua district make an 

accurate measurement of the thickness of the formation 

extremely difficult. Ross made what he considered a 

fairly accurate measurement of the whole formation in 

the northwest part of sec. 300, Block G-4, where he 

found the thickness to be 1 ,090 feet. 

The subdivision of the Boquillas flags into upper 

and lower members was prompted by a desire to increase 

the degree of accuracy and attendant economic usability 

of the district geologic map and still keep formations 

lithologically distinctive from those above and below. 

This purpose could be best served by the selection ofa 

plane of division somewhere near the middle of the 

formation. The presence of an easily recognized faunal 

zone near this position in the section was indeed fortunate. In a sense it is also fortunate that this zone 

is the one selected by Adkins (see table 1) as the division between his Boquillas flags and Terlingua formation. Its selection, therefore, gives a comm.on boundary, albeit a boundary used in this report between 

members of a formation and a boundary used by Adkins 

between two formations. Adkins' usage is in keeping 

with the commendable desire to correlate a certain 

thickness of strata in the Big Bend region with rocks 

deposited (Eagle Ford) in eastern Texas during the 

same period of time. The writers, however, favor 

making this faunal zone a boundary between formational members rather than between formations because of the similarity of rocks above the zone to those 

below. Accordingly, this division of the Boquillas 

flags into mem hers is not a subdivision of Adkins' 

Boquillas flags, which includes only the lower mem her; 

instead it is an extension of his and Udden's definition 

of the formation to include rocks considered by them 

as more properly belonging to an overlying formation. 

Beds mapped as Boquillas flags therefore include the 

lower part of what Adkins called Terlingua formation 

(restricted). 

AGE 

The Boquillas flags are everywhere fossiliferous, but 

distinctive, weJl-preserved fossils are not common. The 

most conspicuous and most abundant macrofossils are 

shells of Inoceramus, of which there are at least six 

species present in the formation. Some of these shells 

are more than 30 incbes wide and 3 inches thick. Besides the inocerami, there are other pelecypods, a few 

ammonites, and many Foraminifera. 

The · collections of macrofossils made during the investigation of C. P. Ross have been studied by L. W. 

Stephenson and ~he following data are, taken. from ;Ross's 

report. 

Inoceramus labiatus Schlotheim, of Eagle Ford age, 

has been identified from sec. 70, block 341 (USGS coli. 

16837), from Long Draw, from Fossil Knobs (USGS 

coils. 16843, 16844), and from six collections of unknown locations (USGS coils. 16966, 16968, 16969, 

16970, 16983, 16984), made by H. D. McCaskey. All 

the collections containing I. labiatus are from beds 

clearly belonging to the Boquillas flags. In addition 

to I. labiatus, the collections from Fossil Knobs contain 

Inoceramus cf. I. latus Sowerby, Crioceras sp., and problematical markings, such as are usually termed fucoids. 

The fossils in these collections are regarded as of Eagle 

Ford age by Stephenson. 

A collection (USGS coli. 16842) from the lower part 

of a measured (by C. P. Ross) section of Boquillas flags 

in sec. 30, block G-4, contains Inoceramus sp. (many 

fragments) and Ostrea sp. It is regarded by Stephenson 

as of Eagle Ford age. 

Material from this measured section was searched for 

Foraminifera by Lloyd G. Hen best who reports that 

most of the collections from the Boquillas flags do not 

contain diagnostic Foraminifera. Most of the assignments of Austin age to certain of the collections were 

made because of the presence of "species of Hastigerinella of supposed restriction to rocks of Austin age." 

The samples from the lower beds contain no diagnostic Foraminifera. A sample collected about 150 

feet above the base contains Guembelina of the Eagle 

Ford and no forms restricted to rocks of later age. A 

collection from about 400 feet above the base of the 

section measured by Ross is regarded by Henbest as 

of either Eagle Ford or Austin age. The collections 

from the next 200 feet of beds are for the most part not 

diagnostic. One from about 650 feet above the base 

of the section was reported to represent possibly late 

Austin age but more likely represents early Taylor age. 

However, a collection made about 100 feet higher in 

the section is reported by Henbest to contain Hastigerinella alexanderi (smaller than typical specimens) and to 

resemble forms of Austin age. Collections made at still 

higher horizons within the Boquillas flags proved not 

to be diagnostic. 

A specimen collected half a mile southwest of the 

Rainbow shaft in beds regarded as close to the top of 

the Boquillas flags is thought by Henbest to be of 

Taylor age. 

In summary, the fossil evidence indicates that the 

beds designated as the lower member of the Boquillas 

are clearly of Eagle Ford age and that the beds designated as the upper member of the Boquillas are probably predominantly of Austin age, although it is possible 

that the uppermost beds mapped as Boquillas may be 

as young as Taylor. 
GENERAL GEOLOGY 13 

TERLINGUA CLAY 

DISTRIBUTION AND CHARACTER 

Terlingua clay as mapped in the district consists of 

about 1,000 feet of soft structureless clay and a few 

thin beds of impure limestone. Included are only the 

upper three-fourths of the formation Udden (1907a, p. 

33-41) called Terlingua beds; the bottom fourth of the 

formation is included in the writers' upper Boquillas. 

Likewise, it only includes the upper half of what Adkins (Sellards, Adkins, and Plummer, 1933, p. 271) 

called Terlingua formation (restricted), but includes in 

addition all of Adkins' Taylor formation. 

Although the units of Udden and Adkins have a 

greater value in age correlations than the present division, they lack lithologic unity in the Terlingua district. 

As lithologic unity was of primary cartographic importance in this study, the changes in formational 

boundaries appear to be justified. 

The Terlingua clay is only in the eastern part of the 

district and is locally well exposed on Terlingua Creek 

and some of its branches. Because it is soft and almost 

unconsolidated when once exposed, it erodes rapidly to 

produce low rounded hills. In the immediate valley of 

Terlingua Creek this characteristic topography has not 

developed because the hills of Terlingua clay are capped 

and protected by a mantle of caliche-cemented gravels, 

which were once continuous across the stream valley. 

The Terlingua clay as here mapped consists of soft, 

structureless, gray clay that weathers a dirty yellowbrown, a minor quantity of shaly marl, and a few thin 

beds of impure limestone. Exposed clay swells on 

weathering to produce a peculiar popcornlike surface, a 

phenomenon suggestive of a bentonitic composition. 

As stated in the preceding discussion, the lower boundary of the formation is placed "at the base of the dark, 

dominantly shaly beds, in which flagstones are absent 

or so thin and sparsely distributed as to be relatively 

inconspicuous." The shaly beds at the base become 

less calcareous and lose their well-defined bedding 

planes as they grade upward into the structureless clay. 

Although the same clay continues upward into what 

has been mapped as Aguja formation, which is composed of sandstone and clay, the lowermost sandstone 

bed, with which the clay is intercalated, indicates a 

change in conditions of sedimentation sufficient to 

justify a formation boundary at this horizon. 

Except in areas where the clay is baked, measurements of the attitude of bedding must be made on the 

shaly and limestone layers. The baking of the clay 

by underlying igneous intrusions has made apparent 

the otherwise invisible bedding planes in the rock. An 

excellent example of this is just north of Study Butte 

where structureless Terlingua clay can be traced laterally into baked rocks so well bedded that they can easily 

be mistaken for Boquillas flags. 

Because of the paucity of measurable bedding planes 

it is difficult to make more than an estimate of the total 

thickness of the Terlingua clay. A planetable traverse 

made by C. P. Ross over the long ridge northwest of 

Dawson Creek yielded the data for estimating the 

presence of 465 feet of beds. This traverse covered 

only the upper part of the formation and must be 

several hundred feet short of including the total thickness. Another partial section, south of Cuesta Blanca, 

contains about 600 feet of beds. At several other 

places in the district several hundred feet of beds can 

be measured, but nowhere in the district is there a complete, unfaulted section. An estimated total thickness 

of 1,000 feet seems reasonable. 

AGE 

Fossils collected by C. P. Ross and identified by L. W. 

Stephenson, Lloyd G. Henbest, and J. B. Reeside, Jr., 

indicate that the greater part of the Terlingua clay is 

of Taylor age, although the lowest beds may be as old 

as Austin rocks. 

In Long Draw three-quarters of a mile northeast of 

the Colquitt-Tigner mine a collection (USGS coli. 

16835) from a fault block of Terlingua clay contains 

Inoceramus sp., Gryphaea sp., and Foraminifera regarded 

by Henbest as of Taylor age. 

Two small hills on either side of the highway half a 

mile southwest of Study Butte yielded the following 

(USGS coli. 16845): 

Echinoid of the family Spatangidae 

Inoceramus deformis Meek? 

Inoceramus (2 sp.) 

Ostrea congesta Conrad 

Spondylus sp. 

Baculites sp. 

Texanites sp. ( = M ortoniceras Meek) 

Crioceras? sp. 

According to L. W. Stephenson, who made the identifications, this collection is indicative of Austin age. It 

is not possible to tell the exact stratigraphic position 

of the clay in these hills within the Terlingua because 

they are isolated outcrops. It is likely, however, that 

they are low in the section, for the beds from which the 

fossils came are flaggy layers lithologically similar to the 

Boquillas flags. 

Beds of the Terlingua just east of Sierra Aguja 

(about 7 miles south of Terlingua) yielded Inoceramus 

cf. I. vanuxemi Meek and Hayden (USGS coli. 16986), 

regarded by Stephenson as of Taylor(?) age. 

A collection made near the top of the Terlingua clay 

northeast of Leon Mountain, yielded Exogyra ponderosa 

erraticostata Stephenson. This fossil is regarded as of 
14 GEOLOGY AND QUICKSILVER DEPOSITS, TERLINGUA DISTRICT, TEXAS 

late Taylor(?) age by Stephenson. The same species 

was found in similar stratigraphic position in the southwestern part of sec. 252, Block G-4 (USGS coli. 16841). 

The clay e:x:posed in the southeast corner of sec. 297, 

Block G-4, just below the cpntact with the Aguja, 

yielded fossils that are, according to Reeside, fragments 

of a thick-shelled form of Inoceramus (Haploscapha) 

sp., Ostrea congesta Conrad, Spondylus guadalupae 

Roemer?, Anchura, Natica, and other gastropods represented by internal molds, a smooth small Baculiteb, and 

two small corroded ammonite specimens. This assemblage suggests Austin rather than Taylor age, but is 

inconclusive, especially as the identifcation of Spondylus guadalupae is not definite. Foraminifera contained 

in 5 collections from this locality have been examined by 

Henbest, who regards them as of early Taylor age. Of 

the 9 collections made from exposures of the Terlingua 

in the fault zone along Long Draw, 7 are regarded by 

Henbest as of early Taylor, and 2 are regarded by him 

as of Taylor age, without closer assignment. One of 

the collections of early Taylor age, from a point 0.2 

mileS. 55° W. of the main shaft of the Rainbow mine, 

is reported to contain Planoglobulina, highly ornate, 

angular Guembelina, and Kyphopyxa christneri. Two 

collections made near the west end of Study Butte are 

reported as probably of Taylor age. 

A collection of foraminiferal material made from near 

the base of the exposed section of Terlingua clay on the 

ridge near Dawson Creek is reported as probably lower 

Taylor, and another collection from near the middle 

of the section is reported as probably Taylor. The 

collection of supposed early Taylor age contains, according to Henbest, Kyphopyxa christneri which is limited 

to the upper Austin and lower Taylor. Epistomina sp. 

is also present. 

A collection made directly above the top of the 

Boquillas flags sec. 300, Block G-4, is reported by 

Henbest to contain Hastigerinella alexanderi Cushman, 

a species representing a forerunner of Planulina taylorensis, and two or three species of Guembelina. He 

regards the assemblage as indicative of Austin age, 

although the species of Guembflina look like forms from 

the Taylor. Another collection from slightly higher 

beds is assigned an Austin age by Henbest because of 

the presence of Hastigerinella watersi (smaller than 

usual for this species). The Guembelina present agree 

with this assignment, but a species of Globorotalia, 

ordinarily regarded by Henbest as Taylor or younger, 

is also present. 

Henbest notes that the collections of Taylor age 

generally have more nearly complete microfaunas than 

those made in older beds. He determines as lower 

Taylor (lower half) collections with Kyphopyxa christneri (Carsey) plus one or more forms of Taylor age. 

Those collections reported simply as of Taylor age 

contain nothing to indicate the exact part of the Taylor 

to which they are most likely equivalent in age. Several 

collections reported as lower Taylor also contain 

Vaginulina regina Plummer of which Henbest says 

(written communication): 

"This form is limited to the Austin and lower chalky layers 

of the Taylor, according to Plummer. A variety of this species 

has been recorded from the Eagle Ford by Moreman. One or 

two of the collections that contain V. regina also carry Hastigerinella alex.anderi Cushman, currently regarded as an indicator 

of Austin age." 

AGUJA FORMATION 

DISTRIBUTION AND CHARACTER 

The beds of sandstone and clay that overlie the 

Terlingua clay are here termed Aguja formation as 

proposed by Adkins (Sellards, Adkins, and Plummer, 

1933, p. 505-508), who made the proposal because 

Udden's original name, Rattlesnake beds, (Udden, 

1907a, p. 41-54) is preemployed by a formation in 

Oregon of Pliocene and Pleistocene(?) age. 

The Aguja formation is widely, but irregularly, 

distributed in the Terlingua district. It occupies considerable areas in the eastern part of the district but 

has been eroded in the western part. Because it is 

composed of soft clays interlayered with much more 

resistant sandstone, its topographic expression lacks 

the smooth contours of the Terlingua clay. 

As mapped, the base of the Aguja formation is the 

base of the first sandstone bed overlying the Terlingua 

clay. The upper boundary of the Aguja formation, 

which is the base of the overlying Tornillo formation, 

is of only local importance in the area mapped (pl. I), 

because beds mapped as Tornillo were found only in the 

southeastern corner of the district. Here the change 

from dull to brightly colored rocks-a criterion useful 

in areas outside the district-is not sharp; so some 

finer basis of separation had to be sought. A thin 

tuffaceous bed was decided upon as the boundary, and 

all beds below it are included in the Aguja formation. 

These beds apparently contain no tuff. 

In contrast to the underlying formations of clays and 

limestones, the Aguja formation can be described as the 

"sandstone formation" of the Cretaceous rocks. Although it contains argillaceous beds in an abundance 

equal to that of the sandstone beds, the latter are 

conspicuous and characterize the formation; argillaceous 

beds are most abundant in the upper part of the formation. Near the base of the formation .the sandstone 

beds are interbedded with clays that resemble the 

underlying Terlingua clay. Also near the base of the 

formation are two or more thin limestone beds that 

contain abundant fossils. Coaly beds are found near 
GENERAL GEOLOGY 15 

the base but are more abundant in the upper part of 

the section. 

The sandstone of the Aguja formation is of medium 

grain and has considerable range in color. Beds are 

commonly light gray or light brown; yellow and 

reddish beds are fairly rare. Crossbedding may or 

may not be present, and where it is not present, the 

rock is massive or shows fairly well developed normal 

bedding. Beds containing roughly spherical concretions, ranging from a few inches to more than 3 feet 

in diameter, are common. 

The principal component of the sandstone is quartz 

in poorly rounded grains, with lesser quantities of 

plagioclase, chert, biotite, kaolinized material that may 

have been potash-feldspar, and fossil fragments. The 

maximum grain size is a little more than 1 millimeter 

and the average grain size less than 0.5 millimeter. 

Most cementing material is argillaceous, a little is 

calcareous. 

The clay beds in the lower part of the formation 

resemble the Terlingua clay and those in the upper part 

resemble those in the Tornillo formation. On the 

whole, they are more silty and arenaceous than those 

in either of these formations and locally have a shaly 

structure. Coaly material is common throughout the 

clay beds, and although some coal from the Aguja formation has been mined and used locally, there is none 

within the district (pl. 1) that appears suitable for 

shipment. Plant fragments and silicified tree trunks 

are common. 

The only place in the Terlingua district where the 

thickness of the Aguja formation can be measured is in 

the valley of Dawson Creek in the southeast corner of 

the district. Even here exposures are incomplete 

and even an estimate of the total thickness requires 

projections. C. P. Ross, in making a planetable traverse across these beds, found that the lower 260 feet 

consists of sandstone alternating with clay, and also 

three thin limestone beds. Above this lies 30 feet of 

black, highly carbonaceous Ghaly clay and then 400 

feet of crossbedded sandstone. The rest of the Aguja 

is largely concealed. The writers would add an additional 100 feet of clay beds to the section, because it is 

that distance to the tuffaceous bed, that they use as the 

upper limit. 

AGE 

Fossils are abundant in the thin limestone beds of 

the lower part of the Aguja formation and, except for 

saurians, are rare in the upper part of the formation. 

The fossils found in the lower part of the formation 

indicate that these lower beds are of late Taylor age. 

Rocks in the upper part of the formation represent an 

indefinite length of Navarro time. Fossils collected 

by C. P. Ross and identified by L. W. Stephenson are 

given in table 7. 

Another collection (USGS coli. 16840) made high 

in the lower 260 feet unit in the same section, contains 

Ostrea pratti Stephenson. Stephenson regards this as 

of late Taylor(?) age and notes that its known range in 

North Carolina corresponds to the range from upper 

Taylor to lower Navarro. 

A collection of fossils (USGS coli. 16852) from beds 

near the base of the Aguja formation in the southwestern part of sec. 252, Block G-4, contains very abundant 

Ostrea cortex Conrad. Stephenson states that this is a 

characteristic fossil of the Escondido formation of 

Maverick and Uvalde Counties. The Escondido is a 

formation of the Navarro group approximately equivalent to the Kemp clay, the uppermost formation 

(Maestrichtian) of the group. Because fossils of late 

Taylor age have been identified from the lower Aguja, 

this suggests that 0. cortex may have a lower range in 

west Texas than it has in Maverick and Uvalde Counties 

or else it suggests-a less likely possibility-that the 

base of the Aguja is stratigraphically higher here than 

at other fossil localities in the Terlingua district. 

Rather high in the section associated with fossil 

tree trunks are saurian bones. Udden (1907a, p. 51-54) 

TABLE 7.-Fossils from measured section of Aguja formation 

near Dawson Creek 

FIRST LIMES'.rONE BED 

Pelecypoda: 

Anomia sp. 

Cymbophor.a (both large and small species) 

Corbula sp. 

Unidentified pelecypod 

Gastropoda: 

Monodonta? 

Polinices sp. 

Gyrodes sp. 

Pugnellus sp. 

Volutomorpha sp. 

Cephalopoda: 

Eutrephoceras sp. 

Baculites aff. B. taylorensis Adkins 

Vertebrata: 

Shark teeth (2 or 3 species) 

Pelecypoda: 

Exogyra sp. 

Anomia sp. 

Aphrodina sp. 

Cymbophora sp. 

Gastropoda: 

Volutoderma sp. 

Morea sp. 

SECOND LIMESTONE BED 

Cephalopoda 

Eutrephoceras sp. 

Baculites aff. B. taylorensis Adkins 

Placenticeras sp. 
16 GEOLOGY AND QUICKSILVER DEPOSITS, TERLINGUA DISTRICT, TEXAS 

found this characteristic of the formation. According to S. W. Williston, the saurians include Olaosaurus, 

ceratopsid?, Dryptosaurus (several teeth), crocodile 

(new), and several fresh-water turtles. 

TORNILLO CLAY 

DISTRIBUTION AND CHARACTER 

The formation next above the Aguja has been named 

the Tornillo by Udden (1907a, p. 54) and this name 

has been universally adopted. In the area mapped, 

the Tornillo is exposed only in the extreme southeastern 

corner (pl. 1). 

As the name indicates, the Tornillo is composed 

largely of clay, which is locally sandy. In exposure the 

rock is variegated with dull, merging colors, such as 

yellow, brown, red, blue gray, and olive green. It contains a few layers of sandstone that resemble the beds 

in the Aguja. Much of the clay is tuffaceous, and some 

beds are composed almost entirely of tuffaceous material, although the igneous character of the formation 

is not as obvious as that of the overlying Chisos volcanics. U dden estimates that the thickness is at least 

600 feet and possibly as much as 1,000 feet. 

What is probably the characteristic chemical composition of the formation is shown in table 8. 

TABLE 8.-Analyst·s of Tornillo clay from east of Rough Run, 2 

miles east of Dog1·e Mountain. 1 From Udden, (1907a, p. 56) 

[0 . H . Palm, analyst] 

Silica___ __ ___ __ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 64. 14 

Alumina _____ __ __ __ __ _____ _______ ___ __ __ ____ __ __ ___ 18. 81 

Ferric oxide___ ___ ___ __ ____ __ __ __ __________ ____ ____ _ 6. 05 

Lime__ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ . 7 4 

Magnesia_____ ______ _________ __ ___ __ __ ______ ____ ___ . 30 

Potash _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 1. 28 

Soda_____ _____ ___ __ __ _____ _______ _______ __ _____ __ _ . 58 

Water (hygroscopic) _____ _______ __________ ___ · ______ _ 

Water (combined) _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 2. 50 

TotaL___ ___ ___ ____ __ __ ______ _____ __ ___ _____ 100. 40 

1 About 5 miles east of Bee Mountain. 

AGE 

Udden (1907a, p. 60) reports silicified wood and 

saurian bones from the Tornillo clay but other macrofossils have not been recorded. According to C. P. 

Ross (written communication), "The Tornillo clay near 

Dawson Creek contains sparsely distributed Foraminifera which, according to Henbest, include poorly preserved specimens of Globotruncana?, Guembelina, and 

Globigerina. The species of the first two genera are indicative of Cretaceous age with fair assurance, but the 

species of Globigerina are two generalized to be of value.'' 

The formation presumably is to be correlated with some 

part of the Navarro group; Adkins (Sellards, Adkins, 

and Plummer, 1933, p. 271) places it at the top of this 

group. 

CHISOS VOLCANICS 

CHARACTER AND DISTRIBUTION 

The group of lava flows and clastic rocks termed collectively the Chisos volcanics, although abundant both 

to the east and west, are present only locally in the 

Terlingua district. The principal occurrence within 

the district is at the Fresno mine in the extreme western 

part, where they unconformably overlap Boquillas 

flags. Doubtless these flows and related clastic rocks 

once covered the entire district because remnants of 

them are preserved in breccia pipes and related collapse structures. These structures are described in 

considerable detail in a later part of this report. 

In the study of a much larger area than that covered 

by this report C. P. Ross (written communication) 

found that the succession of clastic and volcanic strata 

overlying the Tornillo had . great enough lithologic 

affinity for him to propose extending Udden's definition (1907a, p. 60- 68) of Chisos beds to cover all these 

post-Tornillo beds. This is the sense of Chisos volcanics in this report, for the fragmentary exposures of 

the Chisos volcanics in the Terlingua district permit 

the use of only such loose terminology. 

Ross's studies of the Chisos volcanics in and about 

the Chisos Mountains and west of Fresno Creek (fig. 1) 

led him to the conclusion that, although clastic beds 

predominate in the lower part of the sequence, and 

flows above, there are at several horizons within the 

lavas clastic deposits that are lithologically indistinguishable from those below. The clastic beds 

range from fine nodular clay through sandy beds to 

coarse conglomerate, and their colors are mainly white 

and pale shades of buff, purple, brown, green, and 

gray; a few are brick red. Most of the clay beds are 

found east of the Chisos Mountains and were not 

recognized within the Terlingua district; conglomerate 

is found throughout the region, but is nowhere abundant 

in the district; the sandy tuffaceous beds are the most 

abundant clastic rocks in the district. The sandy beds 

are composed of grains and fragments of both sedimentary and volcanic rocks and an abundance of glass shards 

and pumice fragments. Much of the rock is cemented 

by calcite. The conglomerates are similar in character. 

Lava flows were distinguishable in the Chisos 

volcanics in the Terlingua district only at the Fresno 

mine and possibly at Black Mesa. These are rhyolites. 

The conglomerates and blocks of collapse breccia, 

however, include, besides rhyolites, andesites, basalt, 

and trachyte. 

No conception of the total thickness of the Chisos 

volcanics could be gained from a study of those in the 
GENERAL GEOLOGY 17 

Terlingua district. Studies by C. P. Ross (written communication) in the Chisos Mountains and west of 

Fresno indicate that the aggregate original thickness 

of the flows and intercalated clastic beds must have 

been fully 2,000 feet. 

AGE 

The age of the Chisos volcanics cannot be closely 

estimated from the occurrences of these rocks within 

the Terlingua district. At the Fresno mine the 

volcanics rest in angular unconformity on the Boquillas 

flags; elsewhere in the district breccia blocks of the 

Chisos are in structures that are younger than the 

intrusive rocks. Just south of the southeast corner of 

the mapped area (pl. 1) and south of Dawson Creek, 

the Chisos volcanics · overly the Tornillo rocks in 

apparent conformity. The presence of tuffaceous 

material in the Tornillo suggests that the conformability 

is real. This locally confirms Udden's (1907a, p. 66) 

interpretation of a gradation between beds of the 

Tornillo and Chisos. 

The stratigraphic relations west of Fresno Canyon 

are different. Here the Tornillo is absent and the 

Chisos rest unconformably upon Boquillas or older 

rocks. This unconformity represents too great a 

stratigraphic break to be considered a local condition. 

Consequently, the basal beds of the Chisos here are 

either younger than the basal beds of the Chisos 

Mountains or else there was a pre-Chisos post-BoquiHas 

structural deformation in the western part of the 

district but not in the eastern part. 

Although no fossils indigenous to the Chisos volcanics have been found in the Terlingua district, they 

have been found in similar volcanic sequences farther 

north. About 60 miles north of Terlingua, in Sheep 

Canyon in the Alpine quadrangle, Goldich and Elms 

(1949) found a collection of gastropods in a fresh 

water limestone that have been identified as Eocene in 

age, on the basis of abundant Goniobasis. This 

limestone is in a sequence of volcanic flows and associated clastic rocks that rest unconformably upon 

the Boquillas flags. Higher in the same volcanic 

sequence gastropods were found that were tentatively 

assigned to the Oligocene. Similar fossils from similar 

volcanic sequences have been found still farther north. 

It is very probable that the Chisos volcanics are at 

least in part equivalent to these volcanic rocks to the 

north. From the evidence at hand, it seems probable 

that they range through the uppermost Cretaceous 

well into the Tertiary. 

INTRUSIVE IGNEOUS ROCKS 

The Terlingua quicksilver district is in a province of 

alkalic igneous rocks where the sedimentary strata are 

intruded by many sills, laccoliths, dikes, and plugs 

that range in composition from rhyolite through analcite syenite to olivine basalt. The inkusive rocks 

appear to have solidified at shallow depths; consequently, most a.re fine grained, but some are aphanitic 

and a few are medium grained The rocks can be 

broadly classified into those that contain analcite and 

those that are analcite-free. The analcite-free rocks 

are rhyolite, quartz soda syenite, soda trachyte, soda 

latite, and olivine basalt. The analcite rocks include 

analcite syenite, analcite-plagioclase syenite, analcite 

syenogabbro, and analcite basalt. As almost all the 

rocks are sodium-rich, all probably belong to the same 

general period of igneous activity. 

DISTRffiUTION AND FORM 

Although intrusive igneous rocks are found in many 

places in southwestern Texas, they are more numerous 

and varied in the Terlingua region than elsewhere. 

The eastern part of the Terlingua quicksilver district 

extends into an area where they are particularly abundant. Throughout the rest of the district they occur 

less abundantly. 

The igneous rocks are exceptionally well exposed. 

They are more resistant to erosion than any of the 

sedimentary rocks except the Devils River limestone; 

consequently, in the eastern part of the district, where 

the Devils River limestone is not exposed, they stand 

out in bold relief against the softer, more rapidly eroded 

sediments. In places the sedimentary rocks have been 

so recently and rapidly removed that the intrusive 

bodies appear in almost their original shapes. Contacts between igneous and sedimentary rocks, however, 

are not everywhere exposed; but available exposures 

do show that both concordant and discordant intrusive 

relations exist. 

The concordant intrusions are laccoliths and sills. 

In general, the laccoliths are larger than the sills, and 

some have sills associated with them. The division 

between the two is not sharp, but for purposes of this 

report a laccolith is defined as a concordant, almost 

fiat-lying body whose intrusion caused a pronounced 

bowing of the overlying beds. In most laccoliths the 

bowing is concentrated along their peripheries; above 

their relatively fiat tops bowing is less. 

The discordant intrusions are dikes and plugs.2 The 

dikes range from stringers less than 1 inch thick to 

dikes more than 20 feet thick and more than 1 mile 

long, but average about 5 feet thick and one-fourth 

mile long. Intermediate between dikes and plugs are 

2 The term "plug" is used in this report without genetic implications for any discordant, roughly cylindrical igneous intrusion that may or may not have been 

connected with the surface at the time the rock was in a molten state. Thus, we 

recognize that some features called plugs may be eroded volcanic necks. 
GEOLOGICAL SURVEY PROFESSIONAL PAPER 312 PLATE 3 

A. UPPER CONTACT OF BODY ON 150-LEVEL 

Note steplike character of contact. Ktc, Terlingua clay; Tiqs, intrusive quartz syenite. 

B. FLOW STRUCTURES NEAR UPPER CONTACT OF INTRUSIVE BODY 

Lenticular banding accentuated by oxidation of iron minerals along and near parting planes. 
GENERAL GEOLOGY 19 

B 

A 

c D 

0 1 Mile 

FIGURE 2.-0utline map and sections of intrusive rocks in the eastern part of the Terlingua district. The sections are hypothetical, being based upon surface data and 

analogy with the known subsurface extent of other intrusive rocks in the district. A, Analcite syenogabbro of Leon Mountain; B, soda trachyte body 1 mile north of 

Maverick Mountain; C, soda trachyte of Maverick Mountain; and D, analcite syenogabbro of Cigar Mountain. 
20 GEOLOGY AND QUICKSILVER DEPOSITS, TERLINGUA DISTRICT, TEXAS 

the igneous rock to break along joints and slide down 

the slopes to form the jumble of blocks. 

Sills, or flat-lying concordant intrusives with no 

appreciable thickening in their central part, occur west 

and south of Sawmill Mountain, northwest of Bee 

Mountain, between Willow and Maverick Mountains 

(see fig. 2B), at Contrabando Mountain, in the vicinity 

of the Chisos mine, and at many other places in the 

djstrict. They range in thickness from a few inches to 

more than 200 feet, and in outcrop area from a few 

square feet to more than 1 square mile. Their compositions range from rhyolite to analcite basalt. 

Some intrusive bodies are in part concordant and in 

part discordant. An excellent example is the body of 

fine-grained quartz syenite at Study Butte, which bas 

been described by Ross (1937) as a sphenolith (a wedge·-

shaped intrusion), and which can be studied from drillhole records and mine workings. 

The outcrop of the intrusive body at Study Butte is 

.about 2, 700 feet long and about 400 feet wide and trends 

east. In general, both its north and ·south contacts 

with the enclosing Terlingua clay dip to the north. A 

few hundred feet to the north of the outcrop of the 

body and immediately south of it the clay beds are 

horizontal; whereas those beds adjoining and immediately north of the body dip steeply to the south. A 

monoclinal fold coinciding with these southward-dipping beds extends eastward from the southeastern 

corner of the outcrop and may extend westward from 

the outcrop, but this cannot be determined because the 

rocks west of the intrusion are covered by a thin mantle 

of alluvium. Except at the east end of the intrusion, 

the relations between intrusive and intruded rock are 

discordant on the surface. Data obtained from mine 

workings and drill-core records show that beneath the 

surface their relations are both concordant and discordant. To the north the intrusive is a sill, but its 

south part is dikelike and cuts across the bedding. 

Figure 3 shows the probable way in which the Study 

Butte intrusion was emplaced. A sill-like intrusive advanced southward following bedding planes and locally 

quitting them to follow joint planes (see pl. 3A). As it 

advanced southward it thus came closer and closer 

towards the surface. It made room for itself by lifting 

the overlying strata, except near its southern end 

where both sill and dike mechanisms of lifting and 

dilatation were operative. 

The amount of lifting was less where the sill became 

a dike; consequently a monoclinal fold was formed in 

the overlying beds. Much of this fold has been removed by erosion but that which remains extends eastward to Maverick Mountain, thus indicating that the 

intrusian likewise extends this far. 

It seems probable that the intrusive bodies at 

Maverick Mountain (see fig. 20), Willow Mountain, 

the unnamed mountain between Maverick and Willow 

Mountains, possibly Cigar Mountain, and many smaller 

bodies were emplaced in a manner similar to that at 

Study Butte. They all a.re sill-like bodies that show 

discordant relations with the intruded rocks along at 

least one side. If the body at Cigar Mountain is of 

this type and not a laccolith, the inferred dikelike projection along its western side is not necessarily a deeprooted feeder but may be a connection with a lower 

sill-like mass that lies to the west of Cigar Mountain. 

This inference invites the speculation that many 

isola ted sills are more extensive than their cutcrops 

indicate, and that they were not fed through deeprooted dike conduits, but through relatively short 

connections with other, deeper sills. Thus, apparently 

independent centers of intrusion ma.y be more directly 

connected than surface outcrops indicate. 

Oikes are most numerous in the northeastern part 

of the district (pl. 1), but a few are in the vicinity of 

California Hill and Contrabando dome. Those in the 

vicinity of Contrabando dome are of both silicic and 

mafic varieties of rocks, whereas those in the northeastern part of the district are mainly of mafic varieties. 

For the most part, the rhyolite dikes tend to be short 

and thick and grade into shapes that can be best classified as plugs. The mafic dikes, in contrast, tend to be 

long and thin with very sharp walls. Many of the 

rhyolite dikes contain near their borders numerous 

angular inclusions of baked sedimentary rocks that 

were plucked from their walls. 

Small plugs occur in many parts of the district. 

Many probably have no great downward continuation, 

but are local protuberances from underlying sills and 

laccoliths. Some are along faults, but many are unrelated to faults. Sedimentary rocks in contact with 

them are invariably baked or discolored but do not 

always show evidence of physical deformation through 

brecciation and distortion of bedding planes. 

Small irregular, pluglike intrusives of mafic igneous 

rock occur within the cylindrical bodies of breccia that 

are common in the district. These breccia pipes, 

which are not, at least directly, of igneous origin, are 

described in the chapter on structure. 

The only large plugs are those that form Bee and 

Sawmill Mountains. Each is half a mile in diameter 

and cuts vertically across nearly horizontal Upper 

Cretaceous strata, which are locally turned up sharply 

against the plugs. Relations between intrusive and 

intruded rock are everywhere discordant. The plug 

at Bee Mountain is soda. trachyte and that at Sawmill 

Mountain is an analcite syenite. Bee Mountain is 
GENERAL GEOLOGY 

A 

A A' 

I 

B 

A A' 

500 0 500 

A. Map of the intrusive body. The intruded rocks are Terlingua clay. 

B. An early stage in the emplacement of the partly sill-like and partly dikelike 

body. 

C. Growth of the intrusion by thickening and spreading. The magma made 

way for itself by vertical lifting of the overlying rocks and thus created a 

monocline. Note that the vertical distance through the intrusion is uniform except beneath the fold, whereas the thickness varies. 

N 

A A' 

c 

A A' 

----------

E 

1000 1500 Feet 

D. Continued growth of the intrusion by vertical lifting accompanied by wedging open of the dikelike part at the left. 

E. Present cross section of the intrusive body as determined from all available 

data, including surface, mine workings, and drill boles. 

FIGURE 3.-Tbe Study Butte quartz soda syenite intrusive body and theoretical cross sections showing progressive development of the intrusion and 

accompanying monocline. 

21 
22 GEOLOGY AND QUICKSILVER DEPOSITS, TERLINGUA DISTRICT, TEXAS 

a fairly homogeneous mass that shows well-developed 

columnar structure, particularly at its western side. 

The columns are vertical and extend the full exposed 

height of the plug, flaring somewhat downward. Sawmill Mountain consists of an outer shell of light colored 

syenite and an inner core of darker syenite containing 

more abundant ferromagnesian minerals and analcite. 

There is no indication that either of these bodies is 

a plugged conduit of a volcano now removed by erosion. 

CONTACT METAMORPHISM 

Sedimentary rocks adjoining the intrusive bodies 

were bleached and indurated by the heat of intrusion, 

but with few exceptions they were otherwise little 

changed. In a few places, limestone along contacts 

was silicified, notably that near intrusive rhyolite in 

Lowes Valley and that along the upper contact of the 

Wax Factory laccolith. Analcite was seen in a few 

places in sedimentary rocks adjoining analcite-bearing 

intrusive rocks. The development of typical contact 

metamorphic minerals was observed at only two places: 

C. P. Ross (written communication) noted that calcareous clay next to a smaH basalt intrusion in Long Draw 

had been changed to a calcite aggregate containing 

diopside, and the writers found Arnall garnets in a 

recrystallized calcareous clay at the base of the quartz 

soda syenite intrusion at Study Butte. 

The sedimentary rocks that were most affected by the 

intrusion of igneous rocks are the calcareous shales and 

clays; limestone underwent only minor changes, mainly 

recrystallization of calcite and bleaching. The most 

obvious change in both limestones and clays is a pronounced decrease in the intensity of color, or whitening 

of the rock, which resulted from the carbonaceous 

components being volatized and driven off. 

The physical properties and appearance of the clay 

rocks have been considerably changed by the baking; 

the rocks have been hardened, made more brittle and 

more fissile, and those inconspicuously bedded have 

become well bedded. Some calcareous clays have 

developed a spotted appearance; this is particularly 

evident on weathered surfaces. 

Under the microscope the principal change seen in 

the baked rocks is the recrystallization of calcite. In 

shales that were uniformly calcareous before baking, 

the calcite has been redistributed and recrystallized as 

irregular, somewhat spheroidal aggregates. The material between the calcite aggregates is mainly clay, 

partly in a finely crystalline state, which so contrasts 

with the amorphous clay of the unbaked rock that it 

indicates that there has been considerable recrystallization of the clay. Microfossils are exceptionally well 

preserved, although the calcite that composes and fills 

them is recrystallized. 

The induration is not the result of the addition of 

new substances from the cooling igneous rocks, but of 

the reorganization of original constituents, mainly 

calcite and clay, into a rock of decreased porosity 

whose mineral grains are more tightly interlocked. 

Water, doubtless, was of great importance in this 

conversion. Some of this water was given off by the 

crystallizing magma, but probably most was already 

present in the pores of the rocks intruded. The 

intrusive material heated this water, which circulated 

through the sedimentary rocks, and by processes of 

solution and precipitation, redistributed the calcite 

in the sedimentary rocks. As the calcite was obviously 

redeposited by a replacement process, which requires 

removal of the c1ay minerals to make room for the 

calcite, there was likewise some slight redistribution of 

the components of the clay minerals. 

The thickness of baked rock bordering the intrusive 

bodies varies greatly and depends upon the kind of 

igneous rock and its size or thickness, the kind of 

sedimentary rock, the attitude of the contact between 

igneous and sedimentary rocks, and doubtless upon 

other factors, such as the amount of water in the intruded rocks. Generally, but with notable exceptions, 

the more silicic the igneous rock, the broader the 

aureole of baked rock. Massive limestones are seldom 

bleached more than a few feet away from the igneous 

rock, whereas the most readily baked rocks, the calcareous shales and clays, were baked more than 100 

feet from the contact. The size of the intrusive mass 

was of less importance than the attitude of the contact 

from which the heat was being transferred. Along 

vertical and steeply overhanging contacts of igneous 

rock there is a minimum of baked sedimentary rocks. 

This is particularly true along the peripheries of the 

large plug at Bee Mountain, where Terlingua clay 

only a few feet from the contact is apparently unaffected by intrusion. Conversely, . the rocks that 

overlie even relatively thin sills are highly baked. There 

appears to be great variation in the thickness of baked 

rock that underlies flat-lying intrusive bodies, but in 

general it is less than that above the sill. 

AGE 

The intrusive igneous rocks include the youngest 

rocks in the district exclusive of Quaternary alluvium 

and can be considered of Tertiary age. They intrude 

not only Upper Cretaceous marine sedimentary rocks 

but also lavas and pyroclastic rocks that have been 

correlated with fossiliferous volcanic sediments of 

Tertiary age in neighboring areas. 

Igneous intrusive rocks belong to a late stage in the 

structural history of the area. As they intrude the 

Chisos volcanics that rest on the beveled edges of the 
GENERAL GEOLOGY 23 

major fold in the district, they are later· than the ' feldspar, as an intersertal mineral, and as a filling in 

major folding. The major faults of northwesterly vesicles. 

trend are later than the folding and some of these have 

been intruded by igneous rocks. Examples are at 

Cigar and Sawmill Mountains. Faults of diverse trends 

do cut the intrusive rocks, but all those observed were 

of minor displacements. Numerous dikes of northeasterly trend in the northeast part of the district 

suggest that the regional system of northeasterly 

fractures is earlier than the dikes and that it controls 

their locations. It seems probable that most of the 

intrusive rocks were emplaced after the major faults 

were established, and it seems equally probable that 

some fault movements were contemporaneous with, 

and a result of, intrusion. 

PETROGRAPHY 

The intrusive rocks of the Terlingua region have 

textures typical of small, near-surface intrusions, but 

have mineral assemblages that are uncommon. Finegrained porphyritic textures are by far the most common, but aphanitic and medium-grained textures 

are moderately abundant. The rocks range from 

rhyolitic to basaltic; those of intermediate composition 

are the most abundant and present in the greatest 

variety. The rocks of intermediate composition are 

all sodium-rich, and many contain analcite. Alkalic 

feldspar, either orthoclase, soda orthoclase, or anorthoclase, is common to all the rocks except a few basalts. 

The ferromagnesian minerals in rocks that have alkalic 

feldspar dominant over calcic feldspar are soda varieties, 

as aegerine and riebeckite, whereas those in rocks that 

have dominant calcic feldspar are augite and olivine, 

which almost irvariably occur together. Analcite is 

present in many, but not all, rocks with less than 60 

percent silica,3 and occurs as a replacement of calcic 

a This generalization is made from an interpolation of analyses and modes of rocks 

of the Terlingua district presented by J. T. Lonsdale (1940). 

CLASSIFICATION 

Lonsdale (1940), who has made an intensive study of 

the igneous rocks of the Terlingua region, divides them 

into two main groups: those containing analcite and 

those containing no analcite. This ·division, which is 

followed in this report, although not ideal, is logical, 

because the elements that form the analcite are as 

clearly derived from the magma as are those that 

form the feldspar. This basic division is doubly 

useful, because the analyses of Lonsdale show that 

all the analcite-bearing rocks contain less than 60 percent of silica. It does not follow that all the igneous 

rocks in the district containing less than 60 percent 

silica are analcite-bearing. Lonsdale's subdivision of 

each of the two major groups is not strictly followed 

in this report, mainly because not all the rock types 

he describes are found in the area mapped. The 

refinements of his subdivisions are accordingly unnecessary, and perhaps would be confusing, in this much 

less detailed petrographic discussion, which is of necessity based upon relatively few thin sections and no new 

chemical analyses. The table given below is the 

classification that is followed here. 

Because the intrusive rocks were formed under 

conditions intermediate between the deep-seated, or 

plutonic, environment and the surficial, or volcanic, 

environment, they have textural features characteristic 

of both environments, as well as those characteristic of 

their own. Textures are neither distinctly plutonic nor 

distinctly volcanic, but generally they approximate 

the texture-especially the grain-size-of one of these 

two groups. It must be remembered, however, that 

within the individual igneous masses variation in grain 

size is the rule, and any one igneous mass may have a 

phase most closely akin to its plutonic equivalent and 

another phase most closely akin to its volcanic equivaTABLE 9.-The classification of igneous rocks in this report 

[The approximate percentage of a mineral is given only if it is essential to this classification of rocks; the percentage is not ne~essari~y that of Lonsdale. The r~lative .a~oun 

of a mineral and whether the mineral is essential to the rock name are shown as follows: E, in large amount and essential; e, m small amount and essentml; A, m mter mediate amount and not essential; a, in small amount and not essential] 

Abundance of principal minerals (numbers express percent). 

Names used in this report Correlative names used by Lonsdale (1940) Quartz 

ANALCITE-FREE ROCKS 

Rhyolite _______ - _____ - _____ ___ _ 

Quartz soda syenite ______ .:. ---_--

Potassic rhyolite ___________ - ___ -- Soda rhyolite _______ __ __________ _ Soda trachyte __ - _____ _________ _ Soda la tite ____________________ _ 

Olivine basalt _________________ _ 

Soda trachyte __ _________________ _ 

Trachy-andesite _______________ -- _ 

Olivine basalt ___________________ _ 

E>lO 

e 3-10 

a<3 

ANALCITE-BEARING ROCKS 

Analcite syenite_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ Analcite syenite __________ - ___ - __ _ 

Analcite-plagioclase syenite______ Analcite-plagioclase syenite _______ _ Analcite"'orthoclase gabbro_______ Syenogabbro ____________________ _ Analcite basalt_---_____________ Analcite basalt __________________ _ 

Alkali 

feldspar 

E 

E 

E 

E 30-50 

? 

E<SO 

E<60 

e<25 

--------

Plagioclase Soda ferromag- Augite, olivin 

feldspar nesian minerals or both 

-------- ---------- --------- -------- A --------- -------- A -------- - E 30-50 a A 

E ---------- E 

a<lO a a 

e 1{}-30 a a 

E>30 ---------- E 

E ---------- E 
24 GEOLOGY AND QUICKSILVER DEPOSITS, TERLINGUA DISTRICT, TEXAS 

lent. In such cases, simplification favors giving the 

mass the name of the dominant phase. 

It is necessary because of gradations between rock 

. types to subdivide them on the basis of relative amounts 

(of mineral constituents in a somewhat arbitrary manner 

in order to emphasize interrelations and to avoid 

confusion and ambiguity that would result if more 

rigid rock definitions were strictly adhered to. The 

rhyolite, quartz soda syenite, and soda trachyte are 

all composed dominantly of alkalic feldspars and 

.variable amounts of quartz, with plagioclase feldspar 

in effect absent. These three rocks are separated one 

from another partly on the basis of their quartz content. 

Those with more than 10 percent quartz are termed 

rhyolites; those with from 10 to 3 percent quartz are 

termed quartz soda syenite; and those with less than 3 

percent quartz are termed soda trachyte. In addition, 

rhyolite is separated from quartz soda syenite and soda 

trachyte, which are gradational types and closely 

related, by the presence of orthoclase in contrast to 

soda orthoclase, the dominant feldspar of the two latter 

rocks. Sodium-rich ferromagnesian minerals are present in amounts as much as 20 percent in the quartz 

soda syenite and soda trachyte, but no visible ferromagnesian minerals are in the rhyolite. Only the 

quartz-poor soda rhyolites of Lonsdale are in the 

district, and these are included under quartz soda 

syenites in order to emphasize an affinity that is closer 

to trachyte than rhyolite. The closeness of the relation 

of the rhyolite to the other rocks in the district was not 

learned. 

Felsic rocks containing roughly equal amounts 

(within 40-60 range) of alkali feldspar and plagioclase 

feldspar are termed "soda latite" in substitution for 

the trachy-andesite of Lonsdale. This substitution of 

names is made partly because latite is the commoner 

term, but mostly because trachy-andesite has not only 

been used according to the above definition but 

Rosenbusch has used it also for orthoclase-plagioclase 

rocks carrying a feldspathoid or aegerine or riebeckite. 

The term "trachy-andesite" has therefore been used to 

name two contrasting rock types that fall on both 

sides of the line used to differentiate the rocks of the 

Terlingua district into two main groups. Its retention 

might be confusing. 

Gradational rock types are likewise included within 

the analcite group. As gradations exist between 

analcite syenite, analcite-plagioclase syenite, and 

analcite-orthoclase gabbro (syenogabbro), limits of 

mineral compositions must be defined. All contain 

alkalic feldspar and plagioclase in variable proportions; 

therefore the relative quantities of these minerals is 

a good basis for division~ especially since the same 

ferromagnesian minerals are in all three. Plagioclase 

is less than 10 percent in analcite syenite, between 10 

and 30 percent in analcite-plagioclase syenite, and more 

than 30 percent in analcite-orthoclase gabbro. Analcite 

syenite contains more than 80 percent alkali feldspar, 

and analcite-orthoclase gabbro contains not more 

than 25 percent alkali feldspar. With decreasing 

alkali feldspar, analcite-orthoclase gabbro grades into 

analcite basalt, the aphanitic equivalent of analcite 

gabbro. It is obvious from the foregoing that alkali 

feldspar, usually a soda variety, is an important constituent in practically all the intrusive igneous rocks in 

the Terlingua district. 

ANALCITE-FREE ROCXS 

RHYOLITE 

Intrusive rhyolite occurs at Contrabando Dome, 

Contrabando Mountain, and Lowes Valley, as plugs, 

dikes, and sills. The rhyolite is a white to cream, fine 

to microgranular rock, which has in many places welldeveloped flow structures. It is commonly porphyritic, 

containing rhombs of glassy sanidine, rarely as large as 

5 rom, and much less abundant subhedral to anhedral 

phenocrysts of quartz. No mafic minerals are megascopically visible. 

In thin sections the phenocrysts are seen to be 

enclosed in a groundmass of very fine grained sanidine 

and quartz. No glass was noted. Granophyric quartz 

is common throughout the groundmass, and some 

intergrowths are almost as large as the sanidine phenocrysts. No plagioclase was detected but some sanidine 

phenocrysts show corroded centers, accompanied by 

what appears to be microcrystalline albite. Microscopic examinations revealed no mafic minerals except 

small quantities of iron oxide dusted through the rocks. 

QUARTZ SODA SYENITE 

Quartz soda syenite was found in only the . eastern 

part of the district, and it is the rock that forms Study 

Butte, Willow Mountain, and a dike about half a mile 

northeast of Bee Mountain. It ranges in color from 

light gray to greenish gray and in texture from aphanitic 

glomeroporphyritic to medium-grained porphyritic. 

Although commonly a grained rock, most intrusions are 

partly aphanitic and these phases could be called 

quartz soda trachyte. Flow structures are particularly 

evident in weathered rock where they are outlined by 

films of iron oxide. 

Under the microscope, the phenocrysts are seen to be 

soda sanidine and the groundmass largely laths of the 

same mineral in a trachytic or subtrachytic arrangement. The phenocrysts in the Willow Mountain body 

contain inclusions of aegerine augite, but inclusions are 

absent in phenocrysts from Study Butte. The Willow 

Mountain body has in the groundmass subhedra1 
GENERAL GEOLOGY 25 

prisms of aegerine augite, which compose about 20 

percent of the bulk of the rock. Anhedral quartz 

totaling as much as 5 percent of the bulk of the rock 

is intersertal to the feldspar. Magnetite and pyrite 

are rare accessories. 

The quartz soda syenite of Study Butte differs from 

that of Willow Mountain by having a better developed 

trachytic structure and completely lacking any positively determinable mafic minerals. A few thin sections 

from Study Butte show pseudomorphs of hematite after 

pyrite, which in turn is after a prismatic mineral that 

may have been aegerine. Other thin sections show 

similarly shaped pseudomorphs of calcite and epidote. 

If all these various alteration products were derived 

from the deuteric alteration of the mafic minerals, the 

rock originally contained less than 10 percent of mafic 

minerals. 

Most specimens from Study Butte show a fairly well 

developed orientation of the feldspar laths in the 

groundmass. This structure is particularly well pronounced near the borders of the intrusion. Many 

feldspar laths are slightly bent and cracked. 

The Study Butte rock contains less than 5 percent 

quartz, which occurs both intersertal to the feldspar 

and as crystals of amethystine quartz lining the rare 

vugs. Some thin sections show intersertal glass in one 

part of the section and intersertal quartz in another 

part; an association difficult to interpret. Those 

sections (from near the borders of the rock) showing 

glass also show traces of an intergrowth of orthoclase 

and quartz, which is also intersertal to the groundmass 

feldspar. Although the soda sanidine phenocrysts 

appear to have the same composition as the groundmass 

feldspar, some have ragged borders, suggesting a reaction with the magma. 

The rock at Study Butte is almost a monomineralic 

rock and if given a name better describing its hypabyssal 

character might be called a quartz bostonite. 

SODA TRACHYTE 

Large intrusive bodies of soda trachyte compose 

Maverick and Bee Mountains and the mountain north 

of Maverick Mountain. Several smaller bodies of this 

rock are found in the same general area. These rocks 

are all porphyritic, their groundmass textures ranging from aphanitic to medium grained. Phenocrysts 

are feldspar commonly arranged in clusters. The colors are gray, greenish-gray, and brown on fresh surfaces, and various shades of brown on weathered surfaces. Some of the rare vesicles are filled with calcite. 

The groundmass is mainly lath-shaped soda orthoclase crystals, which are either unoriented or arranged 

in a subtrachytic or trachytic manner. Phenocrysts 

are soda orthoclase and anorthoclase and enclose small 

grains of the mafic minerals. The feldspar content of 

the rock ranges from 70 to 90 percent of the total 

rock volume. The remaining 10 to 30 percent consists of aegerine or aegerine augite, a little quartz, 

and traces of magnetite and apatite. The mafic minerals are euhedral prisms of aegerine augite, fringed 

anhedral crystals of the same mineral, and rare crystals of green augite surrounded by rims of aegerine 

augite. In the finer grained varieties, the mafic minerals of the groundmass are usually fine needles of 

aegerite. Quartz is in anhedral grains, interstitial to 

the groundmass feldspar. 

In general, the soda trachytes differ little from the 

quartz soda trachytes. They contain a smaller amount 

of quartz and more abundant mafic minerals and are 

somewhat darker rocks. 

SODA LATITE 

Rocks classified as soda latite are found southwest 

of the Fresno mine in the Wax Factory laccolith, as 

intrusions near California Mountain, as small irregular 

bodies near the Chisos mine, as small dikes and sills 

south of Sawmill Mountain, and as several small dikes 

and sills elsewhere in the district. · They are somewhat darker than the trachytic rocks, ranging from 

greenish gray to dark greenish gray, and are slightly 

porphyritic, with either feldspar, pyroxene, or amphibole phenocrysts. They range from aphanitic to fine 

grained. 

There are two principal varieties of soda latite: 

hornblende soda latite, exemplified by the body at 

California Mountain, and augite soda latite, exemplified by the Wax Factory laccolith. Only one body 

having the composition of hornblende soda latite was 

noted, but examples of augite soda latite are common 

and of considerable compositional range. Neither 

variety was found in transition to soda trachyte. Soda 

trachytes, with only rare plagioclase phenocrysts, contrast with these rocks, which have 40 percent or more 

plagioclase. 

Hornblende soda latite at California Mountain is 

a greenish-gray, fine-grained to aphanitic rock. Much 

of it is porphyritic, having bladed phenocrysts of green 

hornblende as long as 1 em. Exposures are deeply 

weathered and some of the rock is hydrothermally 

altered to a pinkish-buff to dark-red rock, with white 

kaolinite, pseudomorphic after the hornblende phenocrysts. Phenocrysts of plagioclase are rare. 

Under the microscope the specimens examined were 

seen to be somewhat altered and to contain abundant 

calcite, kaolinite, hematite, and jarosite. All plagioclase phenocrysts are zoned with labroadorite centers 

and more sodic borders (up to An38). Many are rimmed 

by alkalic feldspar. The groundmass shows a pro-
26 GEOLOGY AND QUICKSILVER DEPOSITS, TERLINGUA DISTRICT, TEXA8 

nounced alinement of small andesine laths with interstitial grains of soda orthoclase. Bright-red hematite, 

pseudomorphic after magnetite, is in euhedral to subhedral grains in considerable abundance. Apatite is 

fairly abundant in characteristic prisms. In the more 

highly altered varieties of the rock it is the only unaltered mineral. No hornblende was identified among 

the groundmass minerals but may be represented by 

some of the abundant fine-grained alteration products. 

The augite soda latite southwest of the Fresno mine 

(Wax Factory laccolith) is more mafic than the hornblende soda latite at Californi~ Mountain and on the 

average is coarser grained. In general, it is a mediumgrained dark greenish-gray rock ; but it has a lighter, 

coarser-grained facies, as well as a black aphanitic 

facies. Textural and color differences seen in this 

rock in the field suggested that it may have differentiated to some extent within the laccolith, but unfortunately the intrusion was not well enough sampled 

to prove this hypothesis through laboratory study. 

The rock at the Wax Factory laccolith lacks the 

trachytic texture of that at California Mountain and 

has its feldspar and mafic minerals arranged in an 

allotriomorphic granular texture, which preserves but 

few crystal outlines. Feldspar composes about 80 

percent of the rock bulk and plagioclase and anorthoclase are present in roughly equal amounts. Plagioclase is equidimensional in habit and most grains are 

zoned, with labradorite centers similar to those in the 

California Mountain rock, but with more sodic borders 

(up to An27), and armored with anorthoclase. Anorthoclase is likewise in small grains interstitial 

to the plagioclase and is easily recognized by its fine 

gridiron twinning. The only abundant mafic mineral 

is a gray, almost colorless, augite, which composes 

about 15 percent of the rock bulk. It is enclosed by 

the feldspar grains. Some augite is bordered by 

aegerine augite, and some is altered to hornblende, 

which in turn has locally been altered to biotite. 

Magnetite, mainly in euhedral grains, is about onethird as abundant as augite. 

OLIVINE BASALT 

Olivine basalt-black, aphanitic rocks slightly porphyritic, and having phenocrysts of augite or plagioclase or both-is fairly common as small dikes, sills, 

and irregular pluglike bodies. Some small bodies labeled 

basalt on plate 1 were not ·sampled and studied in thin 

section, but were named after only megoscopic examination, which would not detect alkalic feldspar or analcite 

in the groundmass of the rock. Accordingly, it is 

likely that analcit.e basalts and orthoclase-bearing 

alkalic b~salts are included under the legend for olivine 

basalt. 

The olivine basalt intruded into the breccia pipe 

that is on the road about one-half mile east of California 

Mountain is typical of those rocks of this group that 

most closely approach normal olivine basalt. This 

rock is black, aphanitic, and decidedly prophyritic, 

with phenocrysts of augite and plagioclase. Phenocrysts range from 1 to 10 mm in length. The rock 

contains xenoliths of a medium-grained light~colored 

rock composed of orthoclase, basic oligoclase, and 

quartz, surrounded by reaction zones of an intergrowth 

of diopside and feldspar. 

In thin section the olivine basalt has an intergranular 

texture and is composed of short laths of labradorite 

(An53 to An67) and eq uidimensional grains of augite, 

partly altered to chlorite and calcite. The ground--

mass also includes about 5 percent of· subhedral to 

euhedral magnetite and a smaller amount of apatite 

in long prisms. Both augite and plagioclase are zoned 

with more sodic borders. No orthoclase was seen in 

this rock, but it may be present in the groundmass. 

Equant grains of olivine altered to serpentine minerals 

occur similarly to the augite, but in lesser abundance. 

ANALCITE-BEARING ROCKS 

Analcite rocks, ranging from analcite syenite to 

analcite basalt, are common in the Terlingua district. 

Besides· analcite, these rocks all contain plagioclase 

augite, and olivine, and, with the possible exception of 

the olivine basalt, all contain alkalic feldspar. The. 

analcite is interstitial, both as a replacement of feldspar and as a vesicle filling. Various occurrences of 

analcite in the rocks of the Terlingua district are 

illustrated in figure 4. In quantity analcite ranges 

from a few percent to as much as 30 percent of the rock 

volume. Where it is interstitial and a vesicle filling 

its relative abundance is partly a function of the quantity of - open space present after the nonhydrous 

minerals crystallized. As the volume of open space 

varied greatly from place to place in any one rock mass, 

the analcite content varies accordingly. 

Analcite, a hydrous sodium aluminum silicate containing 8 percent or more water, is usually regarded as 

having affinities with the zeolite group of minerals. 

It is fairly common as a filling in vesicles and cracks 

in mafic igneous rocks; it is less common as a groundmass mineral and as a replacement of plagioclase, 

nepheline, leucite, sodalite, and such in the alkalic 

rocks; it is relatively rare in sedimentary and metamorphic rocks. As late as 1893 (Lindgren, 1893, 

p. 289) it was considered a secondary mineral gen~rally 

deposited by extraneous waters that circulated through 

cooling igneous rocks. According to Scott (1916, 

p. 35), this hypothesis was advanced to explain its 

common occurrence in rock openings, its freshness in 
A 

A. Analcite orthoclase gabbro sill in Two-Forty-Eight mine. Plagioclase 

(white) replaced by aggregate (irregular gray) of analcite and calcite. 

Bladed mineral, biotite, black, ilmenite altered to leucoxene. Plain 

light, X 130. 

c 

C. Analcite basalt sill southeast of Cigar Mountain. Analcite (patterned 

gray) filling with a later zeolite clouded with needles of an unknown 

mineral. Dark gray, augite; white, plagioclase; black, magnetite. 

Plain light, X 130. 

B 

B. Analcite syenite from top of Sawmill Mountain. Analcite (patterned 

gray) replacing orthoclase (white) and cut by calcite veinlets. Note 

limited control of cleavage on replacing analcite. Dark-gray mineral 

with cleavage, aegerine augite; black, ilmenite. Plain light, X 130. 

D 

D. Analcite in bituminous impregnated clay from Two-Forty-Eight mine. 

Gray inclusions in analcite are clouds of minute titanite crystals in 

bitumen stained areas. Plain light, X 30. 

FIGURE 4.-Sketches made from photomicrographs of thin section of analcite-bearing rocks. 

GENERAL GEOLOGY 27 

contrast to highly altered surrounding minerals, and 

its replacement relations with other minerals. At that 

time it was believed by many geologists that magmas 

were in effect dry and did not contain enough water 

to supply the high water-content of analcite. Since 

then it has been demonstrated conclusively that some 

magmas are hydrous enough to supply the water content of analcite. It is therefore possible for analcite to 

form from the alkali-rich and water-rich residuum 

produced during the final stages of crystallization of a 

magma. It is clear that the analcite in the igneous 

rocks in the Terlingua district crystallized from such a 

residuum. 

The final crystallization of this residuum, or residual 

fluid, is in two not clearly separable stages: the first 

stage, the late magmatic stage, is crystallization from 

a water-rich magma; the second stage, the hydrothermal 

stage,4 is crystallization from what is considered a 

water solution. An important difference between the 

two stages is the amount of water present, or in other 

words, the concentrations of the solutions. In the 

late magmatic stage, the solution (the magma) is of 

such a concentration that it is capable upon crystallization of filling with silicate minerals almost all the 

space it occupied as a liquid; the filling of this space is 

a process that does not require circulation of the residual 

fluid. In the hydrothermal stage, the solutions are 

not sufficiently concentrated to be capable of filling 

with silicate minerals more than a small part of the 

space they occupied as liquids; to fill the space with a 

solid requires a circulating supply of solution. 

Because of this difference in the mechanisms of filling, 

one would expect that the form and the texture of the 

minerals developed during the late magmatic stage 

would contrast with the form and texture of the same 

minerals developed during the hydrothermal stage. 

In many igneous rocks, the two stages can be identified 

by interpretations of textures and structures; but in 

the analcite-bearing rocks in the Terlingua district, 

some modes of occurrence of analcite definitely indicate 

a hydrothermal origin; other modes of occurrence can 

be interpreted as either late magmatic or hydrothermal. 

A consideration of the physical enrivonment in which 

the analcite crystallized . suggests that analcite 

crystallizing from a melt need not be any different 

from that crystallizing from hydrothermal solutions. 

At the time the analcite crystallized, the rocks were 

in effect solid; certain parts of the rocks consisted of 

an open mesh of interlocking crystals of feldspar and 

ferromagnesian minerals through which was distributed 

4 The term "hydrothermal stage" is used in this discussion to refer to a stage in 

the crystallization of the small near-surface intrusive bodies exposed in the Terlingua 

district. It is highly local in nature and should not be confused with the hydrothermal processes that produced the quicksilver deposits, which are of later origin 

and only indirectly related to the igneous rocks exposed in the district. 

484889-6().-3 

the residual fluid. The space available for the crystallization of analcite was that between previously formed 

crystals: much of the analcite occurs in such interstitial 

space. If the residual fluid was essentially an analcite 

melt, the interspaces would be filled with analcite as 

the melt crystallized, a process requiring only minor 

movements of fluid. If the residual fluid was of the 

concentration of a hydrothermal solution, the interspaces could also be filled, but only if there was extensive 

movement of fluid through them. Complete filling 

could result, but there would be a tendency for some 

interspaces to be incompletely filled. The fact that 

such incompletely filled interspaces are present, however, does not prove that all the interstitial analcite 

is hydrothermal, because the same features could be 

produced if an analcite melt had been drained off 

before crystallization was complete. One, therefore, 

cannot rule out the possibility that some analcite may 

have formed during the late magmatic stage of 

crystallization. 

Evidence for the deposition of analcite during the 

hydrothermal stage, however, is positive. Its occurrence in vesicles and veinlets in association with zeolites 

and calcite and its occurrence as a replacement of 

plagioclase can only be interpreted as deposition from 

hydrothermal solutions. This replacement of plagioclase, which can be demonstrated because it was only 

partial, invites the speculation that the interstitial 

analcite may be complete replacements of minerals 

other than feldspar-minerals such as leucite or 

nepheline. 

The temperature range within which analcite can 

crystallize, although in the low-temperature field, is so 

great that it is of little help in precisely fixing the 

relative time of formation of the mineral. Bradley 

(1929, p. 1-7) demonstrates that analcite formed 

syngenetically in the Green River shales at temperatures 

below 30° C by the interaction of the products of alteration of volcanic ash and the salts of lake water. The 

experiments of Doelter, Lemberg, and Straub (Morey 

and Ingerson, 1937, p. 654, 640-641, 664-665, 744-745) 

show that analcite can be formed from various mixtures 

in a water system at temperatures ranging from 190° 

to 282° C. The above experimental results put 

neither a maximum nor minimum limit on the temperature of formation of analcite, but they do indicate 

that analcite can readily form at a fairly low temperature and may form most readily during the hydrothermal stage. 

ANALCITE SYENITE 

Intrusive analcite syenite forms a plug at Sawmill 

Mountain and several small sills between Leon and 

Bee Mountains. The plug at Sawmill Mountain con-
28 GEOLOGY AND QUICKSILVER DEPOSITS, TERLINGUA DISTRICT, TEXAS 

sists of two concentrically arranged varieties of analcite 

syenite. The outer part of the plug is a white to gray, 

fine-grained vesicular rock · with a satiny lustre, and 

the inner core of the plug is a grayish-brown, mediumgrained porphyritic rock. 

The inner phase composes the bulk of the plug and is 

a rock consisting of more than 80 percent feldspar, in 

part replaced by analcite, and lesser amounts of olivine, 

aegerine-augite, and magnetite. The phenocrysts are 

zoned plagioclase, anorthoclase, and orthoclase, arranged in clusters. The groundmass is a hypidiomorphic arrangement of orthoclase and anorthoclase 

with small crystals of aegerine-augite that are bordered 

by aegerine. Magnetite occurs in small euhedral to 

anhedral grains. Some feldspar phenocrysts enclose 

small crystals of epidote, which may be ari alteration 

of augite. In places the ferromagnesian minerals are 

altered to an orange-yellow fibrous n1ineral. The 

analcite occurs only in veins and as replacements of 

feldspar, in particular plagioclase. 

A thin section of rock from the border phase shows it 

to consist of a groundmass of orthoclase and rare 

phenocrysts of zoned plagioclase, armored by orthoclase 

arranged in a well-developed trachytic texture. Analcite was not seen as a replacement of feldspar but small 

amounts are in the abundant vesicles. Mafic minerals 

are minute prisms of aegerine-augite bordered-by aegerine, in lesser abundance than in· the center phase. 

Magnetite was proxied by ilmenite, ·which occurs as 

small euhedral grains altered to leucoxene. In neither 

phase of the rock is the plagioclase content greater than 

10 percent. 

ANALCITE-PLAGIOCLASE SYENITE 

Sills of analcite-plagioclase syenite are· numerous 

near Sawmill Mountain, and are exemplified by the large 

sills just west of the mountain (pl. 1). These sills are 

fairly uniform, fine-grained, dark-gray rocks cut by a 

few dikelets of a coarser grained, lighter rock. These 

rocks are similar in mineral composition to the analcite 

syenite, except they contain more plagioclase, commonly 

10 to 30 percent. 

ANALCITE-ORTHOCLASE GABBRO (ANALCITE SYENOGABBRO) 

Prominent examples of analcite-orthoclase gabbro 

are the intrusions forming Cigar and Leon Mountains. 

This is a fine-grained to medium-grained, dark gray 

rock, commonly porphyritic with plagioclase phenocrysts. Amygdules of analcite, zeolites, and calcite 

are abundant in some intrusions. 

Textural varieties include hypidiomorphic granular, 

ophitic, and intergranular. Some thin sections show 

two kinds of texture; for example, a thin section of 

rock from Cigar Mountain contained both ophitic 

and intergranular textures, suggesting two generations 

of augite. 

Feldspars make up about 60 percent of the rock bulk; 

they are in the proportion of 2 or more parts plagioclase to 1 of alkalic feldspar. The plagioclase ranges 

from phenocrysts as calcic as sodic bytownite (An72) to 

groundmass laths as sodic as calcic oligoclase (An25). 

Most of the groundmass plagioclase, however, is either 

calcic andesine or labradorite. Most plagioclase phenocrysts are zoned with more calcic centers and are 

armored with orthoclase, as are many of the groundmass plagioclase laths. There has been a little albitization of some of the groundmass plagioclase. Alkalic 

feldspar is absent as phenocrysts but occurs as subhedral to anhedral grains in the groundmass and as 

armor on plagioclase phenocrysts. 

Mafic minerals compose from 20 to 25 percent of the 

rock and are augite, olivine, biotite, and magnetite or 

ilmenite or both. The augite, faintly purple, is titaniferous and occurs as phenocrysts, small grains, and 

ophitic to the feldspar. The olivine is in small grains 

and is only about half as abundant as the augite. In 

places it is partly or completely altered to serpentine 

minerals. The biotite is a deep reddish brown variety 

and is mostly a reaction product of the augite, but it 

also occurs as rims about ilmenite and analcite. Some 

biotite, however, is subhedral and appears to have 

crystallized directly from the melt. Magnetite and 

ilmenite are in small euhedral to subhedral grains and 

are not commonly together. Ilmenite is generally 

altered to leucoxene. Apatite is always present but is 

commonly less than 1 percent of the rock volume. 

Analcite is more abundant in the analcite-orthoclase 

gabbros than in any of the other rocks. It locally 

makes up as much as 30 percent of the rock volume. 

It is mainly as replacements of feldspar and as vesicle 

fillings and is only rarely interstitial to the plagioclase. 

One thin section of the rock from Cigar Mountain shows 

the space between plagioclase laths rimmed by alkali 

feldspar and the center filled by analcite. Analcite is 

both clear and clouded by inclusions. Some is faintly 

birefringent, but most is isotropic. That in vesicles is 

commonly associated with zeolites and calcite, which 

are always younger than it. 

ANALCITE BASALT 

Small pluglike bodies of analcite basalt are found at 

Clay Mountain and in Long Draw, as well as at several 

other places in the district. They are black, aphanitic, 

slightly porphyriti'C rocks with occasional phenocrysts 

of augite and olivine. Specimens studied under the 

microscope had an analcite content that ranged from 

about 1 to 10 percent. The analcite is almost entirely 

interstitial or in vesicles. 
GENERAL GEOLOGY 29 

The analcite basalt is holocrystalline except near the 

chilled margins of the rock where there is a thin glassy 

selvage. The texture is intergranular and commonly 

there is a seriate range in grain size between the largest 

phenocrysts (20 mm) and the finest ground mass crystals. 

The groundmass consists of plagioclase laths (An53 to 

An62 ), composing from 60 to 70 percent of the rock, and 

irregular grains of augite and olivine and some magnetite and analcite. 

STRUCTURAL GEOLOGY 

REGIONAL RELATIONS 

STRUCTURAL SETTING 

In most of the Big Bend region vigorous erosion by 

tributary streams of the Rio Grande has differentially 

carved the hard and soft rocks into boldly contrasting 

relief and effectively revealed their structure. The 

region (fig. I) may be divided into three areas of contrasting topography and structure; these are, the 

Marathon Basin, the lava-covered plateau south and 

west of Alpine, and the topographically diverse mountains and intermontane areas of the southern part of 

the Big Bend region. The Terlingua district is part of 

the third area. 

The Marathon Basin (King, 1937) is an erosional 

basin formed by the excavation of a broad structural 

dome more than 50 miles in diameter. The basin is 

floored mainly by weak rocks of Paleozoic age and 

rimmed by escarpments of resistant Cretaceous limestones. The northern, eastern, and southern sides of 

the dome are tilted gently outward. The western side, 

according to King, (1937, p. 138-139) "is formed by 

the Del Norte and Santiago Mountains, which structurally constitute a single unit * * *. The rocks of 

the mountains are raised into a sharp monocline or 

anticline, overturned toward the west and broken in 

most places by an eastward dipping thrust fault." 

West of the ~1arathon Basin and south and west of 

the town of Alpine is the lava-covered plateau that is 

the second major topographic and structural division. 

This is a broad lava plain extending over almost the 

entire northern and western part of the Big Bend 

region. Higher mesas and isolated mountains stand 

above this plain; they are erosional remnants of still 

higher lava flows, of igneous instrusions that pierce the 

lava flows, and of fault-block mountains. 

The main features of the third topographic and 

structural division, the southern part of the Big Bend 

region, are the Chisos Mountains, the Solitario, the 

Terlingua uplift, and Mesa de Anguila. The Chisos 

Mountains, which are the largest feature, are composed 

largely of volcanic and intrusive rocks piled up in the 

structurally lowest part of the Big Bend region. The 

most impressive feature is the Solitario. (See geologic 

map, Sellards, Adkins, and Plummer, 1933, p. 119, fig. 9; 

or Lonsdale, 1940, p. 1539, pl. 1.) It is a high, mountainrimmed basin that has been carved from a dome by 

erosional de-roofing to expose the rocks of Paleozoic 

age in its center (pl. 4A). The Solitario is noteworthy 

as a nearly perfect circle. :structurally, it may be 

pictured as a high round dome unsymmetrically atop a 

low irregular dome. 

This low irregular dome is the Terlingua uplift, 5 

which extends south and east from the Solitario for a 

distance of 13 miles and forms the dominating structure 

of the Terlingua district. 

South of the Solitario and the Terlingua uplift is the 

high Mesa de Anguila, the larger part of which is in 

Mexico. The northern boundary of Mesa de Anguila, 

the Santa Helena Canyon fault, is a normal fault with 

a displacement of thousands of feet. 

AGE OF THE STRUCTURE 

The earliest deformation recorded in the Big Bend 

region is in the rocks of Paleozoic age of the Marathon 

Basin and the Solitario, where strong folds and thrust 

faults trend northeastward (fig. 5). This early deformation is pre-Cretaceous and mainly pre-Permian. 

According to King (1937, p. 134), deformation during 

the Paleozoic in the Marathon Basin culminated at the 

end of the Pennsylvanian epoch. 

The Cretaceous rocks, which rest on the eroded edges 

of the deformed rocks of Paleozoic age, were domed 

and faulted during a second epoch of deformation. 

The broad Marathon dome and the sharp fold and 

thrust fault along its western side formed after deposition of the Upper Cretaceous rocks on its flanks and 

probably in part after the volcanic eruptions in Tertiary 

time (King, 1937, p. 139-140). About the same time, 

the Solitario dome and the Terlingua uplift were 

formed. This deformation may have been contemporaneous with the formation of the northwest-trending 

anticlines of the Sierra Madre Oriental, which are well 

developed in Mexico east and west of the Big Bend 

region. The post-Cretaceous deformation was thus 

characterized by trends at nearly right angles to the 

northeast-trending pre-Cretaceous structures. 

GENERAL FEATURES OF THE TERLINGUA DISTRICT 

The structure of the Terlingua district, which is part 

of the southern Big Bend area, is dominated by large 

domes and grabens. On a smaller scale the rocks have 

also been irregularly deformed by intrusion, offset by 

innumerable faults, and pierced by breccia pipes. The 

Terlingua uplift, the largest domical structure, extends 

beyond the district to the north and northwest. On 

5 The Terlingua uplift has previously been called the Solitario uplift by Lonsdale 

and the Terlingua anticline by Ross, but to avoid confusion with the Solitario dome 

and to describe adequately the structure, the new name is desirable. 
30 GEOLOGY AND QUICKSILVER DEPOSITS, TERLINGUA DISTRICT, TEXAS 

(~_\THE 

/-~ ~ 

\-\~LITARIO\ ' \ '\ '-." \ \TERLINGUA __ ) UPLIFT\ 

0 

~ 

29° ~---------------~-----+-------~~--------~--~~~----~~~--~.-------~---­ A 

~ 

EXPLANATION 

Post-Paleozoic rocks 

Paleozoic 

~ rocks 

Contact 

Normal fault 

1-

Thrust fault; T, upper plate 

t 

Anticline 

Map modified after Tectonic Map 

of the United States, AAPG, 1944 

FIGURE 5.-Regional structural trends. Faults and folds trending northeastward are malnly pre-Permian, in age, but some are pre-Cretaceous; those trending 

northwestward ar~ post-Cretaceous in age. The pre-Cretaceous structures, revealed only in the Marathon and Solitario uplifts, differ ln type as well 

as in trend from the post-Cretaceous structures. 

the east it is bounded by the Long Draw graben. Its 

southwestern and southern boundary is an abrupt flexure, the Terlingua monocline, that extends eastward 

beyond the uplift, across the entire district. A number of smaller domes, some of whic4 are superimposed 

on the Terlingua uplift, are prominent structural features of the district. Several grabens, the longest of 

which is Long Draw graben, break the continuity of 

the Terlingua uplift. Flat-bottomed, steep-walled canyons such as the Long Draw and Well Creek Valley are 

topographic expressions of these grabens. The general trend of the grabens is northwest, but the Black 

Mesa graben zone strikes northeast. As major structures, the grabens die out along the Terlingua mono-
STRUCTURAL GEOLOGY 31 

cline, which thus serves as a hinge line for the graben 

faults. Hypabyssal intruded bodies of many different 

shapes have invaded the sedimentary rocks and have an 

important bearing on the structural geology because of 

the deformation that accompanied their emplacement. 

Sills and laccoliths made way for themselves by lifting 

and shouldering aside the enclosing strata; plugs and 

dikes also caused deformation, although in some places 

they may have pushed into preexisting openings. 

Monoclinal flexures over the margins of sills and laccolithic domes are prominent examples of deformation by 

intrusion (fig. 3). Small faults and fractures are numerous in most parts of the district; they are perhaps best 

shown by the drainage pattern on the higher parts of 

the Terlingua uplift, where erosion has emphasized 

them in the exposed limestone (pl. 4B). In some areas 

solution of the limestone has caused collapse, which is 

not limited to the limestone but extends upward into 

the overlying rocks, forming breccia pipes, one of the 

more unusual geological features of the district. 

TERLINGUA UPLIFT 

The Terlingua uplift may conveniently be defined as 

approximately coextensive with the area of Lower Cretaceous rocks shown in figure 6. It covers somewhat 

less than half of the Terlingua district. The regional 

relations of this elongate domical structure have been 

commented upon by Ross (1941; 1935a, p. 558-568), in 

Newhouse (1942, p. 193-195), Lonsdale (1940), and 

King (1937, p. 141). 

Figure 6 illustrates the general outline and size of the 

Terlingua uplift and the So1itario dome, which together 

constitute a single composite unit about 18 miles long 

and 8 miles wide. The rocks along the northeastern 

border dip away at angles from less than 10° to about 

20°. The rocks along the southwestern border are 

tilted much more steeply, forming part of the Terlingua 

monocline. The top of the uplift is relatively flat, 

with a slight eastward plunge, which steepens north 

and east of Long Draw. This steepened plunge and 

the normal faults at Long Draw, which have a net 

vertical displacement of 800 feet between the eastern 

and western sides of the draw, form the eastern end of 

the uplift. 

The difference between the elevation of the rocks in 

the Terlingua uplift and that of the same rocks in the 

surrounding area is measured in thousands of feet. The 

top of the Devils River limestone on Reed Plateau (pl. 

1) is about 2,000 feet above the same horizon southwest 

of Reed Plateau (see section A-A'). Midway between Reed Plateau and the Solitario the comparable 

structural relief is about 3,200 feet. It is estimated 

from an unpublished map by Ross and from the geologic 

map of the Solitario by Sellards, Adkins, and Arick 

(1933), also Sellards, Adkins, and Plummer (1933, p. 

119); and Lonsdale (1940, p. 1539); that the structural 

relief of the Solitario relative to the Terlingua uplift 

is at least 3,500 feet. Thus the combined structural relief of the Solitario dome and Terlingua uplift relative 

to the area southwest of them is roughly 6, 700 feet. 

This may be compared to the uplift of 5,500 feet or more 

in the Marathon dome (King, i937, p. 138) and 9,000 

feet in the Black Hills uplift of South Dakota (Darton 

and O'Harra, 1909, p. 5). 

TERLINGUA MONOCLINE 

The Terlingua monocline forms the southwestern and 

southern flank of the Terlingua uplift (fig. 6), and it 

continues beyond the uplift to the east. It is a zone of 

steep southward-dipping rocks in which the beds rise 

thousands of feet in a horizontal distance averaging less 

than half a mile (fig. 7). Throughout much of its length 

it is marked by a narrow outcrop belt of Buda limestone 

and Grayson formation. Beyond the Terlingua district 

to the northwest it merges into the southwestern flank 

of the Solitario dome. East of Long Draw the monocline is in younger rocks than it is to the west, it is less 

conspicuous in the softer Upper Cretaceous rocks, but 

its magnitude is undiminished. Southeast of Maverick 

Mountain, near the eastern end of the district, the monocline is less regular and well defined, but it may extend 

eastward beyond the district, partly obscured by volcanic rocks and younger sediments. 

At the northwest corner of the Terlingua district, 

the monocline is a folded zone more than a mile wide 

and includes in its outcrop the Devils River limestone, 

the Grayson formation, the Buda limestone, and the 

Boquillas flags (pl. 1). The strike averages N. 35° W. 

and the westward dip of the steeper beds is 40° (pl. 5A). 

A remnant of the Chisos volcanics is left on a high hill 

along this part of the monocline. This remnant of 

Tertiary rocks is composed of flat-lying beds of conglomerate and tuff about 30 feet thick, overlain by 

rhyolitic rocks. It is especially significant for the 

information on age relations that it affords. The basal 

conglomerate lies on the beveled edges of the Boquillas 

flags, which dip between 10° and 35° in this part of the 

monocline. The monoclinal folding and considerable 

erosion must have taken place before deposition of 

these Chisos volcanics. 

Continuing eastward, the monocline curves abruptly 

near the Presidio-Brewster County line and continues 

a more easterly trend for several miles. Here a number 

of faults strike within 20° of east and west, and these 

faults are noteworthy because of an unusual amount 

of strike-slip and reverse movement. The southernmost 

fault in the zone is a reverse fault dipping about 50° N. 

This may have been formed early as a normal fault 
~~}.~{~~~~·~ .... 

v"' '4 ~ 

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"..., ~.,. 'lri»'>JI'\ 

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N 

1 

~ I 

' 

Intrusive igneous rocks 

~ . . Chisos volcanics 

D To~~i~~i~:~~m~~r~i~~u:;,~~~~en~~~ed 

.rJ Devils River, Buda, and Grayson formations undifferentiated 

Unnamed 

~ Rocks 

~ ' ~ \·· . 

' ---------~~~~--- ... 

}_ 

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Sc 

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Uz 

.. .. ~ u 

"' :> 

~ 

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EXPLANATION 

Contact 

Thrust fault; T, upper plate 

~------,-----1·-----.----, 

Fault; hachures on downthrown side; dashed where approxi· 

mately located 

1 0 1 2 3 4Miles 

~ 

s~i~\~ !..\)/, 

h: ~I;5-:~~1 

' 

:;c..;5}c 

""7-' 

. jl -- ¥ - Ten -1.ngua -t distnc 

--' ,,, ~ (0~ "'''"''" ~ ·., .. @,;} 

~ 

Modified after maps by C. P. Ross 

and J. T. Lonsdale 

FIGURE 6.-Generalized geologic map showing structure of the Terlingua uplift and the Solitario dome. Extent of Terlingua district shown by dashed line. 

~ 

~ 

0 

t_:rj 

0 

t"4 

0 

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~ P1 

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1-1 

51 

t_:rj 

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t::::1 

t_:rj 

"d 

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STRUCTURAL GEOLOGY 33 

1 0 1 2 Miles 

FIGURE 7.-Generalized structure map of Terlingua district, showing Terlingua monocline and principal graben. The monocline is delimited as a wne of southerly 

dips exceeding 15°. 

and later tilted to a reverse position, along with the 

strata in the monocline, but more likely it was formed 

as a reverse fault by local squeezing where the strike 

of the monocline changes. 

The faults on the eastward extension of this fault 

zone show only normal movement. The fault zone is 

north of and roughly parallel to the sinuous eastward 

course of the monocline, and along the zone are several 

intruded bodies and breccia pipes and a downfaulted 

synclinal area (on the north side of the monocline west 

of Maggie Sink; pl. I). Remnants of Buda limestone 

and Grayson formation are preserved in the downfaulted syncline (fig. 8). 

From the downfaulted syncline eastward to Tres 

Cuevas Mountain and beyond, the monocline is 

paralleled by an anticline north of it and by a syncline 

north of the anticline. The monocline near Tres 

Cuevas is a half-mile wide zone of southerly dips 

ranging from 20° to 70° (pl. 5B). 

From Tres Cuevas Mountain, the monocline extends 

eastward for n~ miles, with an average strike of S. 70° E. 

and southward dips of as much as 60°. So'uth of the 

Mariposa mine, it makes a major curve to a new 

strike of about N. 65° E., which continues for 17f miles 

and crosses the fault zone of Well Creek Draw (pl. I 

and fig. 4B). This northwest-trending sector of the 

monocline is flatter on the average, although the 

steepest beds dip as much as 60°. The Well Creek 

graben faults can be traced southward to the monocline, where their displacements are negligible compared to their displacements on the Terlingua uplift 

to the northwest. This is a generalization that may be 

extended to most of the normal faults that are so 

conspicuous on the Terlingua uplift, for they weaken 

greatly or die out where they intersect the monocline. 

South of the Waldron and Little Thirty-Eight 

mines, the monocline again makes an abrupt change 

in strike from N. 65° E. to about S. 50° E., and this 

new trend continues southeastward along Reed Plateau 

(pls. 1 and 4B). The steepest beds just east of the 

change in strike dip only 25°, but 1~ miles to the 

southeast, along the southwestern edge of Reed Plateau, 

they have steepened to 75° and locally even more. 

Here the monocline is a zone of steeply dipping beds 

only a little more than half a mile wide. 

At the southern end of Reed Plateau the monocline 

bifurcates, and its main branch extends about S. 60° E. 

The other branch trends N. 55° E.; in it the beds dip 

only about 10°. The southward limit of the main monocline is buried in alluvium, and thus the width, steepness, and amount of faulting of the fold are unknown. 

Three low-angle thrust faults in this area dip southwest 

and approximately parallel the monocline (pl. 1). 

In their exposed parts they displace the rocks vertically 

less than 100 feet. A possible explanation of these 

thrusts is that they are related in origin to the monocline. The rocks north and south of the monocline are 

carried toward each other by the folding, and this 

horizontal component of movement is greatest where 

the fold is steepest. Near the thrust faults, where the 

monocline bifurcates and also becomes more gentle, 

the horizontal component of movement on the fold is 

less, but the thrust faults add to the horizontal movement, making the net movement more nearly constant 

and thereby minimizing the disruption of the rocks 

north or south of the monocline. 

The monocline turns northeastward south of the town 

of Terlingua, along the southern border of the mapped 

area. Across the southeastern end of the Long Draw 

it has a trend of about N. 35° E., and the beds dip as 
34 GEOLOGY AND QUICKSILVER DEPOSITS, TERLINGUA DISTRICT, TEXAS 

much as 35°. The fault zone of the Long Draw graben 

dies out as it crosses the monocline. The part of the 

Terlingua district east of Long Draw is structurally 

lower and is covered by younger rocks. This eastward 

descent is partly owing to the faults of the Long Draw 

graben, which have a net downthrow to the east, and 

partly owing to eastward dip of the strata (pl. 1, section 

A-A'); south of the monocline the eastward descent 

is fully as great and is almost entirely due to eastward 

dip. 

The Terlingua monocline strikes approximately S. 

80° E. from Long Draw to Cuesta Blanca (fig. 8). 

Cuesta Blanca is a notable hill of Boquillas :Bags baked 

gray by a concealed intruded body. The structure of 

the hill, doubtless due to the same body, is that of a 

dome, broken on the northeastern side by the Cigar 

Mountain graben and steepened on the southwestern 

side by coincidence with the monocline (fig. 8). The 

monocline, thus modified, swings around Cuesta Blanca 

as though the latter were a blunt instrument pressed 

southwestward. The result is a monoclinal trend of 

S. 55° E. on the southwestern side of Cuesta Blanca, 

changing to N. 70° E. on the southeastern side. 

The southwest fault of the Cigar Mountain graben 

diminishes in magnitude southeastward and has a 

relatively small displacement where it crosses the 

monocline. It probably dies out within the zone of 

monoclinal folding; the other fault bounding the graben 

dies out north of the monocline. 

The monocline continues its northeastward trend 

from Cuesta Blanca across the southeastern end of the 

Cigar Mountain graben and then swings to a trend of 

almost due east. South dips between 20° and 40° 

characterize a zone more than half a mile wide, but the 

total width is not known because the southern limit is 

outside of the area mapped. East of Cuesta Blanca 

the monocline is topographically less conspicuous, but 

the structural relief is as great as in the western part 

of the district. For example, the structural relief south 

of Maverick Mountain is more than 2,000 feet (fig. 8). 

DOMES 

A dome may be defined as an anticlinal uplift without a distinct trend. Several domes are described here, 

beginning with those at the western end of the district. 

They range in diameter from 1 to 3 miles, and all are 

probably genetically related to the process of intrusion. 

WAX FACTORY LACCOLITH AND DOME 

Only part of the Wax Factory laccolith (Lonsdale, 

1940, p. 1554) extends into the western edge of the 

district (pl. 1). Because this intrusion is well dissected, 

it reveals the doming, crumpling, and offsetting of the 

intruded beds in greater detail than elsewhere in the 

district. The large area of igneous rock at the western 

extremity of the district is the thinning eastern edge of 

the laccolith. Remnants of the top contact are preserved in the small patches of overlying Boquillas flags, 

and a projection of the top and bottom contacts indicates that the eastern edge is but little eroded and is 

nearly at its former limit. 

CONTRABANDO DOME 

Immediately southeast of the Wax Factory laccolith 

is the broad Contrabando dome. It is more than 2 

miles in diameter and has a structural relief of roughly 

500 feet. The dome is crossed by several small faults 

of northeast and northwest trends characteristic of 

faults in the entire district. Rhyolite dikes and plugs 

are intruded discontinuously along two faults; other 

rhyolite plugs are scattered over the dome, and a large 

rhyolite sill is exposed at the eastern extremity of the 

dome. The Boquillas flags are baked throughout the 

area where most of the dikes and plugs are exposed. 

As dikes and plugs elsewhere in the district have not 

baked the surrounding rocks for such great distances 

laterally, the baked rock here must indicate a more 

widespread intruded mass below, probably a laccolith 

or sill. This concealed mass, indicated by the expanse 

of baked rock, is very likely the cause of the dome. 

BLACK MESA DOME 

The Black Mesa dome is 2~ miles northwest of the 

Mariposa mine and adjacent to Lowes Valley. This 

:Bat-topped dome is 1~ miles in diameter and has a 

structural relief of 600 feet (fig. 9). The northwest 

quarter of the dome is not discernable in the chaotic 

structure of Lowes Valley, which was formed at the 

same time or later than the dome. 

The Black Mesa dome was probably formed through 

lifting of the sedimentary rocks by igneous intrusion; 

there are igneous rocks in Lowes Valley but no direct 

evidence of intrusion under the dome was found. The 

theoretical possibility of a nearly circular dome being 

formed by direct horizontal compression is discussed 

and rejected in the section on origin of the domes. 

LONG DRAW DOME 

At the northern end of Long Draw the southern 

third of a dome is indicated by a crescent-shaped belt 

of beds dipping 10° to 30° S. on both sides of, and 

within, the Long Draw graben. Its northeastern flank 

merges with the monoclinal northeastern border of the 

Terlingua uplift. The dome is 3 miles in diameter and 

has a structural relief of 300 or 400 feet. 

FOSSIL KNOBS DOME 

Directly northwest of Terlingua is a low irregular 

domical structure ~mile wide by 1}~ miles long, which 
GEOLOGICAl, SURVEY PROFESSIOKAL PAPER 312 PLATE 4 

A. SOUTHWESTERN FLANK OF THE SOLITARIO DOJ\'IE 

B. VERTICAL AERIAL PHOTOGRAPH OF PART OF THE TERLINGUA DISTRICT 

North is toward the top, scale about 1:50,000. The dark rock covering most of the are3. is Devils River limestone, upon which erosion has strikingly accentuated the pattern of fractures and faults. The sinuous Terlingua monocline can be seen extending across the southern part of the area, and the Long Draw graben is conspicuous near the eastern edge. Photograph courtesy U.S. Army Air Force. 

484889- 60-4 
GEOLOGICAL SURVEY PROFESSIONAL PAPER 312 PLATE 5 

A. VIEW SOUTHEASTWARD ALONG THE TERLINGUA MONOCLINE AT THE FRESNO MINE 

B. THE TERLINGUA MONOCLINE EAST OF TRES CUEVAS MOUNTAIN. THE CHISOS 

MOUNTAINS ARE IN THE BACKGROUND 
GEOLOGICAL SURVEY PROFESSIONAL PAPER 312 PLA'l'E 6 

A. FAULT BRECCIA OF BOQUILLAS FLAGS ON NORTHEASTERN FAULT 

OF THE LONG DRAW GRABEN 

The flat.]ying topmost beds are alluvium cemented by caliche and are not faulted. 

R. THE WESTERNMOST FAULT OF THE WELL CREEK GRABEN NEAR BLACK MESA 

Light-colored Buda limestone on the right is f~ulted against dark Devils River limestone on the left. 
GEOLOGICAL SUR VEY PRO FESSIONAL PAPER 312 PLATE 

A. BULBOUS CALCITE VEIN ON A FRACTURE, SHOWING STRIKE-SLIP 

MOVEMENT 

The vein and fracture end on a bedding plane above the 12-inch scale, the displacement 

being taken up by bedding-plane slippage. 

B. SLUMPED GRAYSON FORMATION AND DEVILS RIVER LIMESTONE IN A CAVE-FILL 

ZONE AT THE MARIPOSA MINE 

The cave-fill zone developed where a vertical fracture intersected the topmost limestone beds. The 

white rock in the lower left of the photograph is Devils River limestone and the darker rock above is 

Grayson formation. The beds were horizontal before solution and slumping occurred. 
GEOLOGICAL SURVEY PROFESSIONAL PAPER 312 PLATE 8 

A. LIMESTONE-CLAY CONTACT DEPOSIT, MAIN STOPE, FRESNO MINE 

Banded rock in center is cinnabar-impregnated cave-fill clay; limestone to left. The darker bands are richer in cinnabar; the black bands contain 50 percent or more cinnabar. 

B. CALCITE VEIN CONTAINING CINNABAR IN BOQUILLAS FLAGS ON 250-LEVEL, CHISOS MINE 

Banded character of the vein is due to thin ribbon§ of gouge and country rock, as well as deposition banding of calcite. 

QUICKSILVER ORE BODIES 

STRUCTURAL GEOLOGY 35 

Buda limestone Devils River limestone 

A 

B 

Boquillas flags 

c 

0 I Mile 

FIGURE 8.-Vertical sections across the Terlingua monocline: A, west of Maggie Sink; B, at Cuesta Blanca; C, south of Maverick Mountain. 
36 GEOLOGY AND QUICKSILVER DEPOSITS, TERLINGUA DISTRICT, TEXAS 

is here called the Fossil Knobs dome. The origin of 

this structure is indicated by its coincidence with an 

area of baked rocks and by one large and three small 

outcrops of intrusive igneous rock within the area. 

A core drill hole near the southeast end of the baked 

area revealed an igneous sill, which is shown in sections 

A-A' and B-B', plate 1. The northeastern border 

of the Terlingua dome is gentle, as though formed by a 

gradual thinning of the sill northeastward; dips do not 

exceed 15~ except locally along a fault, which is probably 

contemporaneous with intrusion. The southwestern 

and northwestern flanks of the dome are abrupt and 

steep, with dips exceeding 35°. This steep border may 

be compared genetically with a better understood fold 

at Study Butte, where mine workings revealed the 

subsurface structure (fig. 3). 

TWO•FORTY-EIGHT DOME 

East of the Cigar Mountain graben and the TwoForty-Eight mine is a broad low dome similar in size 

and shape to Contrabando dome. The upper member 

of the Boquillas flags is exposed over most of the domical 

area, which is bounded on three sides by the overlying 

Terlingua clay and on the fou:rth by the graben. The 

dome has a diameter of 2 to 3 miles, marginal dips as 

much as 15°, and a structural relief of roughly 500 feet. 

Uplift by a sill or broad laccolith is the inferred cause of 

the dome. A sill was penetrated in the Two-FortyEight mine at a depth of 500 feet, but it is less than 

100 feet thick. 

ORIGIN OF THE DOMES 

Although uplift by intrusion is the indicated explanation for the Terlingua domes, some similar domes are 

attributed to horizontal stress. Powers (1921, p. 419) 

considered regional (horizontal) compressive stresses as 

a possible means of accounting for the Solitario dome. 

A consideration of the geometry of circular domes and 

the mechanics of "focusing" regional compressive 

stresses helps to distinguish between the mechanisms 

of vertical uplift and of horizontal compression and 

lends support to the theory of uplift by intrusion. 

The Black Mesa dome may be taken as a specific example; it is chosen because its fairly regular shape makes 

it easy to analyze. Figure 9 shows a cross section of the 

FIGURE 9.-Vertical section across Black Mesa dome. The eroded Grayson formationlis restored to show the shape of the dome. Unfolding the beds increases 

the)ength of the section by 300 feet. 

Black Mesa dome with the Grayson formation restored 

as a datum plane. As shown above the section, the 

Grayson formation if unfolded has a length 300 feet in 

excess of the length of the section; this of course is true 

not only of the Grayson formation but of the whole 

stratigraphic section. The distance of 300 feet represents the minimum shortening of any diameter of the 

dome that must have taken place if the dome were 

formed by horizontal compression. If horizontal compression is assumed, a circle just outside the dome 

would have its diameter and circumference reduced 

during doming by being pushed toward the center of 

the dome. The diameter would be reduced at least 

300 feet and therefore the circumference reduced by 

about 940 feet. How was the circumferential shortening of 940 feet taken up? This shortening is sufficient 

to create three radiating anticlines as high as the dome 

and with comparable dips. To form a circular dome by 

horizontal compression is like pushing the sectors of a 

pie toward its center. It is readily seen that the part 

of each sector outside the presumably doming center of 

the pie would be thrust against neighboring sectors in 

such a way as to form folds or thrust faults radiating 

from the dome. There is good evidence against any 

such radial thrusts or folds at Black Mesa, for the surrounding rocks are well exposed. Nor does it answer 

the question to suggest that the radial thrusting might 

be distributed ()n a number of joints, for the same 

might as well or better be said of the doming itself, 

which is no greater in magnitude than the required 

radial folds or thrust faults. 

If, however, the doming was caused by vertical 

lifting over an intrusive body, there are still some 

questions to be considered. One concerns the apparent 

shortening of 300 feet across the dome. Two interpretations of the apparent shortening are possible: (1) 

The shortening is real and is. balanced by contemporaneous extension on normal faults nearby, or (2) the 

shortening is only apparent and it is actually a lengthening and slight thinning of the strata on the Hanks of 

the dome. The first possibility implies a temporal 

relation of the dome with the adjacent Lowes Valley 

graben and perhaps with its associated intrusive rocks. 

The amount of material raised in the dome is of the 

same order of magnitude as the material depressed in 

the graben. The second possibility requires plastic 

deformation or distributed fracturing, which would be 

concentrated on the Hanks of the dome rather than on 

its fiat top. This stretching on the Hanks of the dome 

would amount to approximately an inch per foot, but 

no direct evidence of it was observed in the field. Although a choice between the two possibilities is impossible to make on the basis of available evidence either 

at Black Mesa or at the other domes, it seems most 
STRUCTURAL GEOLOGY 37 

likely that a combination of the two possibilities entered 

into the formation of the domes. 

The intrusive body that caused the doming of Black 

Mesa may be at a depth of only a few hundred feet or 

it may be very deep, and it may be laccolithic or it may 

by pluglike or bysmalithic. No baked rock or other 

reliable evidence of the proximity of intrusion at shallow 

depth was found on the dome. The closest intrusive 

rocks are rhyolite and trachyte. 

Lonsdale (1940, p. 1551) related the structural relief 

of the domes to the composition of the associated 

igneous rocks, but the present authors found no evidence for this hypothesis. In the larger area .studied by 

Lonsdale, he lists eight thick laccoliths or bysmaliths 

from which the covering rocks have been stripped 

(p. 1555). Although these should correspond to higher 

domes, all but two of them are composed of soda-rich 

or analcite rocks, which are supposed to characterize 

lower domes. The viscosity, rate of crystallization, and 

rate of injection of the magma must have influenced 

the thicknesses of intrusive masses compared to their 

diameters. These factors, however, are not necessarily 

reflected in the final composition of the rock. 

FAULTS AND FRACTURES 

LARGE GRABENS 

Many of the major faults of the Terlingua district 

are steeply dipping normal faults bounding depressed 

tracts, or grabens (fig. 7). The grabens are narrow 

compared to their length; for example, the Long Draw 

graben is more than 12 times as long as it is wide. In 

general the major faults trend northwest, but there is a 

notable exception in the Black Mesa graben zone, whose 

bounding faults trend northeast (pl. 1). The strikes of 

the individual faults vary markedly, making both 

sinuous curves and angular turns. In detail the faults 

are more complex than shown in plate 1; in many places 

they form irregular zones rather than single fault 

planes (fig. 10). The maximum vertical displacement 

is about half a mile, which is comparable to the structural relief across the Terlingua monocline. 

Long Draw graben, with a length of 6 miles, is the 

longest graben in the district; it has an average width 

of half a mile. The vertical displacement is approximately 2,000 feet below the block southwest of the 

graben and 1,200 feet below the block northeast of the 

graben (pl. 6A). The graben dies out southeastward 

at the Terlingua monocline and northward a short 

distance outside the Terlingua district. Areas adjacent 

to the graben are in some places "sagged" toward it, 

and there the displacement on faults is correspondingly 

less. The overall trend of the bounding faults is N. 70° 

W. in the southern part of the graben, N. 30° W. in the 

central part, and N. 20° W. in the northern part. 

100 0 100 200 300 Feet 

FIGURE 10.-Detailed map of normal faults bounding the southwestern side of Long 

Draw south of the Chisos mine. The rocks in Long Draw are displaced downward 

more than 2,000 feet relative to the rocks in the Reed Plateau. The displacement, 

divided between two faults in the northwestern part of the map, Is concentrated In 

one fault at the eastern edge of the map; the transition occurs in the zone of connecting faults and In locally tilted beds. 

There is a noticeable tendency toward parallelism of1 the 

individual, component fault planes with the regional 

fracture pattern. For instance, the most common directions of fractuue immediately west of the northern 

part of the graben ~ are N. 85° E., N. 65° E., N. 20° W., 

and N. 30° W. (pl. 4B); and the grabe,n faults locally 

follow each of these trends except the first. West of 

the Rainbow mine the graben faults on the northern 

side follow a trend of N. 65° E. to N. 80° E. for half 

a mile. There the faults appear to be part of a throughgoing zone of faults extending from the Terlingua 

monocline N. 7 5° E. to the town of Terlingua and perhaps to Cigar Mountain. 

A group of discontinuous grabens, designated as the 

Black Mesa graben zone, extends from west of Black 

Mesa almost to Long Draw, with an overall trend of 

N. 75° E. Although its characteristics are diverse, 

certain general differences from the Long Draw graben 

stand out. The unusual northeast trend has already 

been mentioned. Furthermore, the graben zone is 

neither so long nor so nearly continuous as the Long 

Draw graben; the fault displacements are generally 

less; and there is no marked difference between the 

displacements on the two sides. The northeastern 

part of the zone, near Long Draw, is fairly simple, 

whereas the Lowes Valley part, adjacent to Black 

Mesa, is a chaotic jumble of downfaulted blocks, lacking the relatively straight, linear boundaries of the 

usual grabens. 

• 
38 GEOLOGY AND QUICKSILVER DEPOSITS, TERLINGUA DISTRICT, TEXAS 

The northeastern part of the Black Mesa graben 

zone contains the Grayson formation and Buda limestone, which are faulted into Devils River limestone, 

with a maximum displacement of approximately 400 

feet. The graben block is synclinal with an axial strike 

of N. 55° W. across theN. 65° E. strike of the graben; 

the graben faults die out in both directions away from 

the syncline. Adjacent to this part of the graben on 

the southeast is a square block, which is tilted southward, with faults on all four sides. Two of these faults 

intersect the graben faults without noticeable offset. 

Buda limestone is preserved in the structurally lowest 

part of the tilted block, along the southwestern fault, 

which has a displacement locally in excess of 200 feet. 

Between the northeastern part of the Black Mesa 

graben zone and Lowes Valley, the faults die out; but 

farther southwest, on the flank of the Black 1\tfesa 

dome, the faults reappear. Here displacements along 

the faults are relatively insignificant, but still farther 

southwest, at Lowes Valley, the displacements are 

large. 

The graben in Lowes Valley, is separ~ted into an 

eastern and a western half by a ridge of Devils River 

limestone extending two-thirds of the way across the 

graben. The eastern half contains pyroclastic rocks 

and conglomerates correlated with those of the Chisos 

volcanics, rhyolite that is thought to be intrusive, 

Buda limestone, and a small outcrop of Boquillas flags, 

which is tentatively correlated on the basis of rock 

character with the lower member of Boquillas flags. 

A minimum vertical displacement of at least 800 feet 

is represented by the latter outcrop. The western half 

of Lowes Valley is floored by the Grayson formation, 

Buda limestone, and Boquillas flags. The displacement of the Boquillas flags is at least 1,100 feet downward relative to the surrounding area. Southwest of 

Lowes Valley the rocks in the graben are all Devils River 

limestone and are displaced vertically no more than 

500 feet. This change in displacement, from 1,100 to 

500 feet, is so abrupt that a cross fault, concealed 

beneath the alluvium in the floor of the graben, is 

required to explain it. . 

The Chisos volcanics in the eastern part of Lowes 

Valley are interpreted as having been faulted down 

along with Boquillas flags and Buda limestone, which 

they overlie unconformably farther west. The peculiar 

distribution of the volcanics, which occur in patches in 

the Devils River limestone, might suggest that they 

were deposited in hollows after the graben was completely formed, but this interpretation is untenable, 

because steep dips in bedded pyroclastic rocks indicate 

that the Chisos volcanics were deformed subsequent 

to their deposition. Still another possibility is that the 

igneous rocks lie in a volcanic vent ch9ked with:its 

own debris. This seems unlikely, because the volcanics 

contain bedded stream gravels with limestone pebbles 

of Paleozoic age, and these gravels are characteristic 

of those interbedded with the Chisos volcanics elsewhere. The questionable intrusive rock may have 

been faulted down along with the bedded volcanic 

rocks or intruded later. 

Three grabens, called the Well Creek grabens for the 

intermittent stream that drains the area, extend southeast from Black Mesa to the Terlingua monocline. 

They contain Grayson formation, Buda limestone, and, 

locally, Boquillas flags. The pattern of overlapping, 

convergent, and angular faults bounding these three 

grabens is bes:t grasped by reference to plate 1. The 

displacement on the bounding faults (pl. 6B) is nowhere more than 500 feet and in most places only about 

250 feet. Where the faults die out, monoclinal rolls 

commonly take their places; these rolls become lower 

and lower until they too finally disappear. Some 

graben faults end abruptly, however, with the displacement taken up on a terminal cross fault or fold, as for 

example, the boxlike northwestern end of the graben 

adjacent to the Waldron mine. In summary, the Well 

Creek grabens differ from the Long Draw graben in 

their smaller size and lesser displacement and the fact 

that the blocks east and west of the grabens are not 

differentially displaced as at Long Draw. Otherwise, 

the structural affinities between the Long Draw and 

Well Creek grabens are close. 

The Cigar Mountain graben, striking southeastward 

from Cigar Mountain, is one of the larger grabens of 

the Terlingua district. It has a length of about 3 

miles, a width of 1 mile, and a vertical displacement of 

1,000 to 2,000 feet. Its trend projects into the faulted, 

structurally low area extending northwestward from 

Cigar Mountain. Cigar Mountain itself is a sill or 

laccolith intruded during or after the faulting. The 

southwestern fault of the graben may have served as 

one of the feeding channels for the intrusion, which 

protrudes southeastward as a narrow dike for a short 

distance along the fault. Within the graben, the units 

are the Terlingua clay and Aguja formation. Alluvium 

conceals most of the area. 

OTHER NORMAL FAULTS AND FRACTURES 

The majority of the faults, fractures, 6 and joints in 

the Terlingua district belong to two sets, one trending 

northeast and the other trending northwest. Although 

each of these sets varies considerably in strike, their 

intersections are commonly at nearly right angles. 

The northwest set includes most of the major faults 

• The term "fracture" 1s used for ruptures on which there 1s no measurable displacement but which are more persistent than joints. Some of these fractures can be 

traced for half a mile or more. 
STRUCTURAL GEOLOGY 39 

already discussed, the main exception being the Black 

Mesa graben zone which trends northeast. Among the 

smaller faults and the fractures, those belonging to the 

northeast set are in many places the most persistent 

and conspicuous (pl. 4B). The great majority of all 

the faults and fractures have steep dips (60° to 90°). 

In the southwestern part of the Terlingua district, 

south of the Terlingua monocline and west of the lower 

end of Long Draw, are many small faults splitting and 

crossing in a complex pattern, in contrast to the simpler 

fault pattern north of the monocline. Much of the difference is attributable simply to the different rocks exposed in the two areas. Faults that are single planes 

in the thick-bedded Devils River limestone split into 

multiple faults in the less homogeneous rocks above the 

Devils River limestone. This significant difference 

tends to be magnified in mapping because small faults 

are harder to detect in the relatively massive limestone. 

The northwest-trending faults and perhaps the minor 

folds south and southwest of Tres Cuevas Mountain 

appear to be the dying-out end of the great Santa 

Helena Canyon fault south of the Terlingua district. 

Here as elsewhere in the district, the Terlingua monocline is a relatively unfaulted strip dividing the faults 

north of it from those south of it. 

RELATIVE AGE AND DIRECTION OF MOVEMENT 

Intersections of faults and fractures show offsets of 

either set or both offset; they thus reveal no evidence 

tending to establish separate ages for the northeast and 

northwest sets. Many of the faults and fractures, 

particularly those of the northeast trend, show good 

evidence of oft-repeated small movements. This is 

shown by calcite veins, which occur in most of the 

faults; alternating deposition and movement have produced an interleaving of calcite with thin sheets of 

country rock (pl. 8B). Movements possibly continued 

longer on the northeast set, as suggested by the almost 

total restriction of quicksilver mineralization to this 

set. 

The movements on the large normal faults were 

predominantly dip-slip, as shown by the lack of large 

horizontal displacements at fault intersections and by 

angular changes in strike. However, no way was found 

to detect and measure accurately small horizontal 

displacements. That some horizontal movement took 

place is indicated by horizontal or gently inclined 

grooves, as, for example, those on the Chisos fault in 

the Chisos mine and those on the graben faults near the 

southeastern end of Long Draw. 

THRUST FAULTS 

Thrust or reverse faults are not com.lrlon in the 

Terlingua district, but they occur in at least four 

localities: (1) A reverse fault in the Terlingua monocline 

east of the boundary between Presidio and Brewster 

Counties has been mentioned in the discussion of the 

monocline. This fault strikes parallel to the monocline 

and dips 50° northward. (2) Three thrust faults 

southeast of Reed Plateau have also been mentioned. 

They strike almost parallel with the monocline and dip 

southward. (3) A small thrust fault 1~ miles west and a 

little south of Reed Plateau dips eastward and is offset 

by a normal fault. (4) A mile south of Cigar Mountain 

a thrust fault strikes N. 20° W. and dips 20° to 30° W. 

The faults at the first two localities seem to represent 

minor adjustments related to the formation of the monocline. Alternatively, the faults at the first locality 

might have been caused by intrusive igneous rocks, 

for these crop our near them. The thrust at the fourth 

locality strikes transverse to the monocline but is 

conspicuously parallel to the western side of the Cuesta 

Blanca dome a mile to the east; it may have resulted 

from the pressure of igneous intrusion within the dome. 

In summary, the thrust faults are local in extent, 

small in displacement, and diverse in trend. They are 

probably the result of local adjustments to larger 

structures or to igneous intrusions. 

BRECCIA PIPES AND OTHER FEATURES CAUSED BY SOLUTION 

Solution features are of three main types: limestone 

caverns; filled caverns, or cave-fill zones; and breccia 

pipes. The open caverns, with the exception of a few 

in the Buda limestone, are in the Devils River limestone. The cave-fill zones occur near the top of the 

Devils River limestone, where overlying rocks have 

slumped contemporaneously with solution of the limestone. The breccia pipes have their roots in the Devils 

River limestone but some extend more than 1,000 feet 

above the limestone. 

In general there are two kinds of open caverns: 

vertical caves developed mainly along fractures, and 

horizontal caves developed mainly along bedding planes. 

Examples of vertical caves are in the Little ThirtyEight mine and in the Contra tiro workings of the Mariposa mine. Vertical caves are usually less than 10 feet 

wide but some are locally as much as 50 feet or more in 

width. The length and height reach hundreds of feet. 

Horizontal caves occur in many places, and good examples are in the Waldron mine and in the Contra tiro 

workings of the Mariposa mine. Single rooms in the 

horizontal caves are rarely more than 100 feet in width, 

and though generally low, they may be 50 feet or more 

in height. 

·The antiquity of some of the caverns is established 

by their relation to mineralization. Coatings of cinnabar and coarsely crystalline hydrothermal calcite line 

the walls of many of them, as for example at the 
40 GEOLOGY AND QtJICKSILVEtt DEPOSITS, · TE1lLlNG1JA l>lS'i'RICT, TEXAS 

Waldron, Little Thirty-Eight, Mariposa, and Chisos 

mines. Other caverns are smooth walled and bare; 

ordinary dripstone is rare. One such cavern is a large 

open sinkhole near the Waldron mine. Cave-fill zones 

are commonly associated with open caverns. occurring 

at greater depths along the same fracture, and thus 

the two are probably closely related in age and origin. 

The cave-fill zones (pls. 7 A and 8A) are generally 

formed at intersections of nearly vertical fractures with 

the top of the Devils River limestone; they are filled 

with slumped Grayson formation and residual blocks 

of limestone. They are · usually hundreds of feet long 

and less than 50 feet wide, but in form and size are 

varied and complex, depending partly on the pattern of 

intersecting fractures. Their vertical dimension is 

usually less than 50 feet but in exceptional cases is 

several hundred feet. The fill is generally highly brecciated near the bottom and grades upward into slumped, 

but not brecciated, beds. Fifty feet or more above the 

top of the Devils River limestone, in the Grayson 

formation, the cave-fill zones may be expressed as small 

synclines, and above the Grayson they are not likely 

to be expressed at all except above very large cave-fill 

zones. 

The cave-fill zones are old features; they are much 

older than the present erosion cycle. At the Mariposa 

mine, for example, cave-fill has been invaded by igneous 

rock of probable Tertiary age, which was later fractured 

and the fractures filled with cinnabar. At many other 

places the cave-fill is older than the ore. 

Masses of breccia with a vertical pipelike or chimneylike shape are designated as breccia-filled pipes, or 

simply breccia pipes. They are shown on plate 1 as 

small oval or circular areas. Displacement of the 

breccia is downward, strongly indicating that the pipes 

formed as a result of solution in the Devils River limestone accompanied by collapse-stoping of overlying 

strata. 

Four breccia pipes will be described in detail: Maggie 

Sink pipe, in Devils River limestone 1% miles west of 

Mariposa; Two-Forty-Eight pipe in Boquillas flags at 

the Two-Forty-Eight mine; Aguja pipe in .Boquillas 

flags on the Chisos mine property; and Chisos pipe in 

the Grayson formation and Buda limestone in the 

Chisos mine. 

MAGGIE SINK PIPE 

· Maggie Sink breccia pipe is in Devils River limestone 

about H~ miles west of the Mariposa mine. It is in a 

topographic depression on the crest of an asymmetric 

anticline. Exploration for quicksilver ore to a depth 

of about 600 feet has provided considerable data about 

the pipe. The pipe is roughly elliptical in outcrop, 

having a length of 540 feet and a width of 200 feet 

(fig. 110); the long dimension is oblique to the axial 

/ ....... ...----........................ 

/ ', '\ 

\ 

0 

Breccia I 

A 

/ / 

--/ 

100 Feet 

I 

j 

I ' - I 

----.... 

Breccia // 

/ 

Devils River: Breccia 

limestone: 

I 

I 

I 

I 

I 

I 

I 

I 

I 

SECTION 

c 200 Feet 

FIGURE 11.-0utline maps of breccia pipes. A, Outline map of Aguja breccia 

pipe, near the Chisos mine. The surrounding rocks are Boquillas flags; 

the breccia is broken sandstone and clay of the Aguja formation, displaced 

'downward an estimated minimum of 1,500 feet. B, Outline map of breccia 

and intrusion south of the Aguja pipe. The surrounding rocks are Boquillas 

flags; the breccia is broken Boquillas; and the intrushe rock is basaltic. 

C, Outline map and vertical section of Maggie Sink breccia pipe. The surrounding rock is Devils River limestone; the breccia is broken Grayson 

formation, Buda limestone, Boquillas flags, and possibly some Tertiary 

volcanic rock. The breccia in the outcrop is displaced downward as much as 

500 feet or perhaps more. 

trend of the anticline. Fractures and small faults, 

many filled with calcite, are numerous in the surrounding Devils River limestone, as they are throughout the 

Terlingua district, but significant intersections or concentrations of fractures that might control the localization of the structure are absent. 

The nearly vertical walls of the pipe, which are 

solution-pitted and coated by coarsely crystalline calcite, enclose a collapsed brecciated mass of intermixed 

Grayson formation, Buda limestone and Boquillas flags. 

Clay of the Grayson, which is soft and plastic when 

moist, serves as a matrix for fragments of the harder 

rocks. The brecciated Buda and Boquillas rocks are 

present as randomly oriented fragments ranging in 

diameter from less than an inch to several feet. Some 

blocks of Devils River limestone are present, especially 

near the borders of the pipe, and a few fragments 

similar to Chisos volcanics were found on the surface 

at the southeastern corner of the outcrop. 

Between 1943 and 1946 a tunnel was extended to 

intersect the pipe about 150 feet below the surface, and 

from this tunnel a winze was sunk to explore several 

hundred feet deeper. The winze was not accessible at 

the time of the last visit to the property (1946), but the 

engineer in charge, Mr. Roy Hammond, reported that 

the wan · of the pipe maintained an overall vertical 

attitude to the bottom of the winze, about 400 feet 
STRUCTURAL GEOLOGY 41 

below the tunnel. It is reported also that no ibottom 

to the breccia pipe was found and that the breccia 

contained Buda limestone to the depth explored. At 

the tunnel level, the breccia consisted of the same rocks 

as at the surface, Boquillas flags as well as the older 

strata having collapsed to this depth. 

The results of solution are conspicuous along the 

walls of the pipe. Surfaces that may have been formed 

originally by caving of Devils River limestone during 

formation of the breccia pipe are now pitted, rounded, 

and coated with calcite, some of which contains cinnabar. The breccia near the walls and locally far within 

the pipe is bleached and iron-stained, the Grayson 

formation and Boquillas flags having been affected 

most, whereas the Buda limestone is unaltered. 

Maggie Sink pipe, in summary, is a breccia mass of 

unknown depth, with nearly vertical walls extending at 

least 600 feet below the present surface and at least 1,100 

feet below the surface at the time the pipe was formed. 

It is filled with collapsed breccia of rocks normally 

overlying the Devils River limestone. Some of the 

breccia has moved downward at least 500 feet and 

probably considerably more. 

TWO-FORTY-EIGHT PIPE 

The breccia pipe explored by the Two-Forty-Eight 

mine is on the western side of the broad, low TwoForty-Eight dome. The upper member of the Boquillas 

flags in the vicinity dips gently westward at angles of 

less than 10° and is unbroken except for the breccia 

pipe and fractures of less-than-a-foot displacement 

trending N. 65° E. and N. 20° W. These fractures 

belong to the regional system of northeastward and 

northwestward fractures and are weak here. 

This breccia pipe is more thoroughly explored for a 

greater vertical distance than any other in the district 

(pis. 21 and 22). From a mine shaft more than 600 feet 

deep, exploratory drifts are driven along the contact of 

the breccia. Near the surface, the pipe is nearly circular 

and has a diameter of 115 feet; on the 420 level it is 

elliptical and has a maximum dimension, in a northerly 

direction, of about 200 feet (pl. 21). The breccia consists of Boquillas flags on the upper levels and intermixed Boquillas and Buda rocks below. One block, 

seen on the 556 level, contains the Buda-Boquillas 

contact and has been displaced 25 feet downward. The 

breccia is much like that in a caved mine stope, consisting of blocks and fragments of all sizes, oriented 

randomly. The contact between breccia and enclosing 

country rock is sharp. Blocks are in all stages of detachment and, without exception, show downward movement. The wall rocks are not dragged or bent at the 

contact but are broken off cleanly. This and the intermixture of rocks suggests that the pipe formed by caving 

block by block. Much of the breccia has a matrix of the 

softer shaly parts of the Boquillas flags, but some of it 

has open space with no matrix. In many places, calcite, 

hydrocarbons, and analcite are present. Locally, 

hydrocarbons were abundant enough in the quicksilver 

ore to provide the fuel for furnacing, and in some places 

were so abundant that they caused the furnaces to 

overheat. 

A post-brecciation sill surrounds the pipe on the 500-

foot level, having lifted the wall rocks without lifting 

the breccia. It fingers into the breccia and has baked 

the breccia along the contact. Altered, but probably 

similar, igneous rock is intruded into the breccia explored by the main shaft below the 556-foot level. 

Calcite and cinnabar occur in veinlets in the igneous 

rock, thus revealing a history of fracturing and mineralization after intrusion. 

Some breccia fragments are softened and bleached, 

and here, as at Maggie Sink, the shale is affected much 

more than the limestone. The igneous rock· is .also 

altered and in places is completely changed to clay. 

The alteration of the igneous rock is most intense along 

the border ·of the pipe. Warm water and hydrogen 

sulfide were present in newly opened parts of the mine 

but were exhausted after a few days. 

The breccia in the Two-Forty-Eight pipe is similar 

to the breccia in the Maggie Sink pipe, and the direction 

of displacement is the same. Also, the overall form of 

two pipes is similar. The principal difference between 

the two pipes is the presence of the igneous intrusive 

body at Two-Forty-Eight. This intrusion may account for part of the alteration, and certainly it is 

partly responsible for the structure itself, because 

lifting of the surrounding beds increased the displacement of the brecciated rocks from their normal stratigraphic positions and must have extended the pipe 

upward if it had not already reached the surface. 

In summary, the geologic history of the Two-FortyEight breccia pipe is: (1) formation of a collapse 

breccia due to removal of material at depth; (2) sill-like 

igneous intrusion into the surrounding strata and into 

the breccia itself; and (3) further minor faulting and 

mineralization by calcite and cinnabar. 

AGUJA PIPE 

On the Chisos mine property 1,500 feet east of the 

main, or No. 8, shaft is the Aguja breccia pipe. It is 

in the trough of a , syncline, which has an axial trend 

of S. 23 o W., maximum dips of 30°, and a southwestward plunge. The rocks enclosing the pipe belong to 

the upper member of the Boquillas flags, but the breccia 

within the pipe is sandstone and clay of the Aguja 

formation. Numerous calcite veins filling fractures 

and faults of the regional northwestward and north-
42 GEOLOGY AND QUICKSILVER DEPOSITS, TERLINGUA DISTRICT, TEXAS 

eastward systems traverse this general area, and one 

northeast-trending vein, explored in the shallow Susano 

prospect shaft, cuts the breccia. 

The outcrop of the pipe covers a roughly circular 

area 250 feet in diameter (fig. 11A). The walls are 

almost vertical where exposed and where explored by 

drill holes. The filling of the pipe consists of rocks of 

the Aguja formation, with jumbled blocks of sandstone 

in a clay matrix. Some sandstone blocks are very 

large: as much as tens of feet in diameter. Because 

the Aguja formation normally overlies about 1,200 feet 

of Terlingua clay, which rests upon Boquillas flags, the 

presence of Aguja rocks in a pipe surrounded by the 

upper member of the Boquillas flags indicates a very 

large displacement. The probable displacement is 

estimated to be more than 1,500 feet. This figure 

may be too high, as it depends upon the thickness of 

the Terlingua clay, no section of which is exposed 

nearby; but the displacement must be at least 1,000 

feet. The known geologic history of the Aguja breccia 

pipe includes: formation of the breccia by collapse, 

and repeated movement accompanied by calcite 

mineralization and ending with cinnabar mineralization 

on at least one fracture. 

Another breccia mass crops out 800 feet south of the 

Aguja pipe. It is enclosed in Boquillas flags, consists 

of fragments of the same formation, and contains an 

intrusive body of basalt (fig. 11B). A drill hole 

between the two breccia bodies, but closer to the 

Aguja pipe, passed through a 7 -foot sill of similar 

basalt at the lower Boquillas contact 852 feet below the 

surface. On this evidence, the second breccia mass 

appears to be similar to the Two-Forty-Eight pipe; 

that is, it is a breccia pipe formed by collapse ~;tnd later 

intruded by igneous rock. 

CHISOS MINE PIPE 

The most famous of the Terlingua breccia pipes is 

the Chisos pipe in the Chisos mine (pls. 19 and 20, 

section 0-0'), because from it was mined the largest 

and most valuable single ore body in the district. This 

pipe does not reach the surface but was discovered in 

mine workings at a depth of 550 feet and was mined 

and explored an additional 250 feet downward. It is 

tangent to the Chisos fault in an area where this fault 

begins to fray out into a number of calcite veins along 

fractures with little displacement. There appears to 

have been minor movement along the Chisos fault after 

formation of the breccia. Above the breccia pipe are 

flat-lying Boquillas flags, undisturbed except by the 

Chisos fault. ·Below its· roughly arched roof of Boquillas flags, the pipe is surrounded by near-vertical walls 

of Buda limestone and below that by the Grayson 

formation; information regarding the walls at greater 

depths is lacking. 

The pipe, which is roughly circular and has a diameter 

of about 7 5 feet, is filled by breccia of Boquillas flags, 

Buda limestone, and Grayson formation, all displaced 

downward. The upper part of the pipe, now mined out, 

is reported to have been a breccia of Boquillas flags 

and Buda limestone with interstices filled with calcite 

and cinnabar. The lower part of the pipe, also mineralized in places, is mainly a breccia of Buda limestone in a 

matrix of Grayson formation. Warm water was found 

on lower levels in this part of the mine and was pumped 

to the surface for domestic use. 

The Chisos pipe is thus an irregular vertical cylinder 

filled with breccia that has moved downward to its 

present position. As the overlying rocks are undisturbed, material must have been removed from 

below. This suggests solution of Devils River limestone and contemporaneous collapse-stoping, which 

advanced upward until brecciation increased the volume 

of the rock enough to support the roof. 

OTHER BRECCIA PIPES IN THE TERLINGUA DISTRICT 

A total of 76 breccia pipes is shown on the map 

(pl. 1), and their geology is summarized in table 10. 

They appear in outcrop as a mixed jumble of rock 

fragments from different strata. Some form small 

conical hills or low mounds because the breccia is more 

resistant to erosion than are the surrounding rocks; 

others form depressions because they are less resistant 

than the surrounding rocks. They commonly have a 

roughly circular or roughly elliptical outcrop with a 

preferred northeast elongation, but some are too irregular to be thus described. The oldest rock in which the 

breccia pipes are found is the oldest rock exposed in the 

district, the Devils River limestone. The youngest 

rock in which they have been found is Terlingua clay, 

but the breccia filling includes the younger Aguja formation and Chisos volcanics. The breccia pipes thus 

crop out not only in Devils River limestone where they 

are directly associated with solution features, but also 

in younger rocks where only mechanical collapse--

and no solution-is apparent. As the Devils River 

limestone is the only thick limestone, the conclusion 

seems warranted that the pipes cropping out in younger 

rocks are the upper parts of pipes that extend downward into the · Devils River limestone; this conclusion 

has been proved in some instances by mining or drilling. 

The walls bounding the breccia are generally sharp 

and nearly vertical. They characteristically show no 

drag, but locally show blocks that are partially slumped 

into the breccia. The breccia is composed of randomly 

oriented fragments ranging from a fraction of an inch to 
STRUCTURAL GEOLOGY 43 

TABLE 10.-Breccia pipes shown on plate 1 

[Breccia pipes are listed in order from west to east. Symbols: Kdr, Devils River limestone; Kg,Grayson formation; Kbu, Buda limestcne; Kbol, lower member of 

Boquillas flags; Kbou, upper member of Boquillas flags; Kbo, undifferentiated upper and lower members of the Boquillas flags; Ktc, Terlingua clay; Ka, Aguja 

sandstone; Ti, volcanic rock and conglomerate of Tertiary age] 

Location 

1. West of Fresno _____ _ 

2. West of Fresno, in 

plunging syncline. 

3. Northwest of Fresno_ 

4. Fresno area ________ _ 

5. Northwest of Fresno_ 

6. Fresno area ________ _ 

7. Do ___________ _ 

8. Do ___________ _ 

9. Do ____________ _ 

10. Do ___________ _ 

11. Do ___________ _ 

12. Do ___________ _ 

13. Do ___________ _ 

14. Do ___________ _ 

15. Do ___________ _ 

16. Do ___________ _ 

17. Do ___________ _ 

18. South of Fresno area_ 

19. Fresno area ________ _ 

20. Do ___________ _ 

21. County line _______ _ 22. Do ___________ _ 

23. In structural depression. 24. Do ___________ _ 

25. Do ___________ _ 

26. Do ___________ _ 

27. Monocline at intrusion. 

28. North of monocline __ 

29. Do ___________ _ 

30. Monocline _________ _ 

31. Downfaulted syncline. 

32. Do ___________ _ 

33. South of monocline __ 

34. Downfaulted syncline. 

35. North of Lowes Valley. 

36. Lowes Valley ______ _ 

37. South of Lowes Val38. 

39. 

ley. Do ___________ _ 

Do ___________ _ 

40. Maggie Sink, on an 

anticline. 

41. East of Lowes Valley. 

42. Monocline _________ _ 

43. Mariposa, intruded 

by igneous rock. 

44. East of Lowes Valley._ 

484889-60--5 

Surrounding Observed composition rock at wrface of breccia at surface 

KboL __ _ 

KboL __ _ 

Kg, Kbu 

Kbu ___ _ 

Kdr ____ _ 

Kbu ___ _ 

Kbu ___ _ 

Kdr, Kg_ 

Kg, Kbu 

Kg _____ _ 

Kg _____ _ 

Kg _____ _ 

Kg _____ _ 

Kg_----

Kdr ____ _ 

Kg _____ _ 

Kdr ____ _ 

Kg _____ _ 

Kdr_ __ _ 

Kg ____ _ 

Kdr_ __ _ 

Kdr ___ _ 

Kdr_ __ _ 

Kg_----

Kdr, Kg_ 

Kdr ___ _ 

Kdr_ __ _ 

Kdr_ __ _ 

Kdr ___ _ 

Kdr, Kg_ 

Kg, 

Kbu. 

Kdr, Kg_ 

KboL __ _ 

Kg, 

Kbu. 

Kdr ___ _ 

Kdr ___ _ 

Kdr ___ _ 

Kdr ___ _ 

Kdr_- __ 

Kdr_ __ _ 

Kdr ___ _ 

Kg ____ _ 

Kdr ___ _ 

Kg ____ _ 

Kbu ________ _ 

Kbo ________ _ 

Kg, Kbu, Kbo_ 

Kbo ________ _ 

Kbo ________ _ 

Kbo ________ _ 

Kbu, Kbo ___ _ 

Kbu ________ _ 

Kbu, Kbo, TL 

Kbu, Kbo, TL 

Kbu, Kbo ___ _ 

Kbu, Kbo, TL 

Kbu, Kbo, TL 

Kg, Kbu, Kbo, 

Ti. 

Kg, Kbu, Kbo_ 

Kg, Kbu, 

Kbo. 

Kbo ________ _ 

Kbo ________ _ 

Kg- - - - - - ---- Kg, Kbu ____ _ 

Kg, Kbu, 

Kbo. 

Kg, Kbu ____ _ 

Kbo ________ _ 

Kbo ________ _ 

Kbo_ -------- Kbo ________ _ 

Kg, Kbu ____ _ 

Kg, Kbu, 

Kbo. 

Kg, Kbo ____ _ 

Kbu, Kbo ___ _ Kbo ________ _ 

Kg, Kbu, Kbo, 

Ti (?). 

Ti (?) _______ _ 

Kbu, Kbo ___ _ 

Kbu, Kbo ___ _ 

Estimated 

displacement of fragments of 

youngest 

rock at sur- face (feet) 

50 

50 

300 

100 

300 

100 

150 

50 

400 

400 

200 

450 

300 

450 

300 

300 

200 

300 

50 

200 

350 

200 

100 

250 

100 

200 

300 

300 

300 

300 

500 

300 

100 

Location 

45. Mariposa, intruded 

by igneous rock. 

46. Do ___________ _ 

47. Mariposa area _____ _ 

48. Southeast of Mariposa. 

49. Mariposa area _____ _ 

50. Southeast of Mariposa. 

51. Do ______ ~ ____ _ 

52. Do ___________ _ 

53. Do ___________ _ 

54. Do ___________ _ 

55. Do ___________ _ 

56. Do ___________ _ 

57. Do ___________ _ 

58. Mariposa road, intruded by igneous 

rock. 

59. At monocline ______ _ 

60. Northwest of Waldron mine. 

61. West of Waldron 

mine. 

62. Southeast of W aldron mine. 

63. Southwest of Little 

Thirty-Eight mine. 

64. Southwest of Little 

Thirty-Eight mine. 

65. South of Little 

Thirty-Eight mine. 

66. Do ___________ _ 

67. Do ___________ _ 

68. Southeast of Little 

Thirty-Eight mine. 

69. North of Reed 

Plateau. 

70. Do ___________ _ 

71. Do ___________ _ 

72. Long Draw, along 

graben fault. 

73. Aguja pipe, Chisos 

mine area; in 

plunging syncline. 

7 4. Chisos mine area, 

intruded by igneous rock. 

75. Two-Forty-Eight 

mine, intruded by 

igneous rock. 

76. North of TwoForty-Eight mine. 

Surrounding 

rock at surface 

Kg_----

Kg,Kbu_ 

Kdr, 

Kg. 

Kdr ___ _ 

Kdr, 

Kg. 

Kdr_ __ _ 

Kdr_ __ _ 

Kdr ___ _ 

Kdr_ __ _ 

Kdr_ __ _ 

Kdr_ __ _ 

Kdr_ __ _ 

Kdr ___ _ 

Kdr, Kg_ 

Kbu ___ _ 

Kg,Kbu_ 

Kbu ___ _ 

Kg ____ _ 

Kg_----

Kdr, Kg_ 

Kbu ___ _ 

Kg, 

Kbu. 

Kg ____ _ 

Kdr,Kg_ 

Kdr_ __ _ 

Kg_----

Kdr_ __ _ 

Ktc ____ _ 

Kbou __ _ 

Kbou __ _ 

Kbou __ _ 

Kbou __ _ 

Estimated 

displacement of fragments of 

youngest Observed composition rock at sur- of breccia at surface face (feet) 

Kbu, Kbo ___ _ 

Kbu, Kbo ___ _ 

Kbu, Kbo ___ _ 

Kg - - - - - - - - - -

Kbu, Kbo ___ _ 

Kg, Kbu ____ _ 

Kg, Kbu ___ _ 

Kg, Kbu ____ _ 

Kg _________ _ 

Kg, Kbu ____ _ 

Kg, Kbu ____ _ 

Kg, Kbu ____ _ 

Kg, Kbu ____ _ 

Kbu, Kbo ___ _ 

Kbo ________ _ 

Kbu, Kbo ___ _ 

Kbo ________ _ 

Kbu, Kbo ___ _ 

Kbu, Kbo ___ _ 

Kg, Kbu, 

Kbo. 

Kbo ________ _ 

Kg, Kbu, 

Kbo. 

Kg, Kbu, 

Kbo. 

Kbu, Kbo ___ _ 

Kg, Kbu ____ _ 

Kg, Kbu ____ _ 

Kg, Kbu ____ _ 

Ktc, Ka, Ti_ __ 

Ka _______ --_ -

Kbo ________ _ 

Kbo _________ _ 

100 

100 

300 

50 

300 

200 

200 

200 

50 

200 

200 

200 

200 

300 

50 

100 

50 

250 

100 

300 

50 

100 

200 

250 

200 

150 

200 

1,000 

1, 500 

25 
44 GEOLOGY AND QUICKSILVER DEPOSITS, TERLINGUA DISTRICT, TEXAS 

tens of feet in diameter, and these fragments are displaced below their normal stratigraphic positions in 

every pipe. The amount of downward displacement, 

which can be measured most accurately where the 

lithologic units are thin, reaches a maximum of many 

hundreds of feet. 

Tertiary extrusive rocks are older than the pipes in 

the six places where relative ages were revealed by the 

presence of fragments of Chisos volcanics and conglomerates in the breccia; but Tertiary igneous rocks intrude 

at least five breccia pipes. This evidence of pipes intruded by igneous rocks is conclusive in the Two-FortyEight pipe and also in some of the Mariposa pipes. In 

one of the latter, for example, the intrusive igneous rock 

forms a matrix for a mixed jumble of blocks of the 

Buda and Boquillas formations on one side of the pipe. 

Around the intrusive rock is a halo of baked breccia and 

wall rock. Thus the collapse and intermixture of the 

Buda and Boquillas must have preceded the baking 

resulting from intrusion. 

The distribution of the breccia pipes that have been 

found is not uniform (pl. 1); the majority are in the outcrop area of Devils River limestone. There are probably two reasons for this. First, some pipes like the 

Chisos pipe, may never have reached the surface, and 

the known breccia pipes are thus most numerous in the 

area of Devils River limestone because this is the area of 

deepest erosion. Second, the stratigraphic units immediately overlying Devils River limestone are thin 

and distinctive, and pipes in the Devils River therefore 

contain foreign fragments that make recognition of the 

pipe easy. This is not true of higher stratigraphic 

horizons. 

ORIGIN OF SOLUTION FEATURES 

Because of their general association, the caverns, 

cave-fill zones, and breccia pipes are thought to be contemporaneous features caused by solution of limestone. 

Although they may have formed over a long period of 

time, many are definitely older than the quicksilver ore, 

and several are older than igneous intrusive bodies. On 

the other hand, several breccia pipes contain fragments 

of the overlying stratified Chisos volcanics (tab. 10), 

indicating that these pipes formed after deposition of 

the Chisos, but before deposition of the cinnabar found 

in them. A possible explanation of the association between igneous activity and pipe formation is that many 

of the solution features were formed during the period 

of volcanism and hydrothermal activity. 

Most solution features exposed at the surface or in 

mine workings are now above the ground-water table 

and are dry but caverns and cave-fill zones are found in 

the Chisos mine below the present ground-water table, 

which is about at the altitude of the Rio Grande. The 

Chisos pipe extends at least a hundred feet and the 

Two-Forty-Eight pipe, several hundred feet below the 

water table. Furthermore, the water table must have 

been higher before the streams had cut to their present 

depths. The solution features were therefore formed at 

least partly below the water table. This is in agreement 

with theories of origin of limestone caverns proposed for 

other regions (Davis, 1930, p. 548-550) and is in agreement with the fact that the flow of ground water is not 

restricted to a zone near the water table (Hubbert, 

1940, p. 926). 

The possibility remains that some of the solution 

features are products of thermal waters rather than of 

normal cold ground water. During the time when the 

Chisos volcanics were being deposited and the numerous 

intrusive rocks were being emplaced, normal ground 

water must locally have been heated and augmented by 

magmatic constituents. The ability of hydrothermal 

solutions to form caverns was recognized by Davis 

(1930, p. 487) who says, "In regions that are invaded by 

volcanic magmas it is believed that warm juvenile water 

and carbon dioxide may ascend toward the earth's surface. This rising addition to descending ground water 

of meteoric origin would probably give it a great increase in solvent power while the mixture is still deep 

underground." Walker (1928) states that some of the 

caves at Tintic were formed by ascending hot water. 

In the Terlingua district, an enormous volume of 

coarsely crystalline calcite is present in veins. If this 

calcite was extracted from limestone, the most logical 

source, and deposited in veins closer to the surface, 

caverns must surely have formed in the limestone. 

OFIGJ.N OF THE BRECCIA PIPES 

The breccia pipes are remarkable for their vertical 

cylindrical form, their breccia consisting of mixed 

fragments from various formations, and their great 

penetration into noncalcareous rocks. They differ 

from caverns and cave-fill zones in all of these characteristics and thus present a special problem of 

origin. 

A mechanical process of breaking off and slumping 

was evidently essential in forming the pipes. The 

chemical process of solution was effective only in the 

Devils River limestone, for younger calcareous rocks, 

like the Buda limestone and calcareous layers in the 

Boquillas :flags, are interstratified with shales of low 

permeability and are thus generally isolated from 

solvent waters. Any suggestion that the pipes were 

violently blasted out, a process that may seem attractive because of the large amount of breccia and its 

proximity to igneous activity, is clearly ruled out by 

the lack of any fragments moved upward and by the 

termination of the Chisos pipe, whose top lies 550 
STRUCTURAL GEOLOGY 45 

feet below the present surface. The pipes therefore 

seem to represent the slumping that would be inevitable 

above caverns that grew so broad that their roofs could 

no longer stand. 

The roughly cylindrical shape of the pipes is a natural 

form resulting from collapse, as shown by ordinary 

sinkholes in limestone. Sinkholes in noncalcareous 

rocks several hundred feet above limestone are known 

on the Cumberland Plateau near Chattanooga, Tenn. 

(Stockdale, 1936, p. 515-522; Laurence, 1937, p. 

214-215). The sides of the pipes appear nearly 

vertical but more precisely the sides probably converge 

gradually upward in the form of a very high arch; this 

convergence is evident in the Two-Forty-Eight pipe 

(pl. 22). Similarly in mining practice, the walls of 

block-caved stopes generally stand nearly vertical 

(Gardner, 1930, p. 11). The circular or oval horizontal 

cross section of pipes is explained by the tendency of 

caverns to collapse only at their broadest parts, leaving 

corners and narrower spans intact. Joints and other 

irregularities cause some modification of the ideal 

shape (pl. 21). 

The mixing of fragments from different formations, 

which is a characteristic of the action of most of the 

pipes, can best be explained by contemporaneous 

slumping from two or more formations into a large 

opening near the top of the pipe; the mixing thus gives 

some indication of how the pipes developed. The 

process may be pictured as follows. (1) A cavern in 

the Devils River limestone becomes so broad that large 

blocks begin to peel from its roof. Because the 

broken material occupies more volume, the opening 

below the roof tends to become smaller as the roof 

moves upward, but contemporaneous solution of the 

limestone breccia may keep the opening large. (2) 

When the roof reaches the weak Grayson formation, 

the clay slumps rapidly and may temporarily fill the 

opening. (3) When the Buda limestone is reached, 

an opening may again form below this thin but strong 

rock, and as the arch grows upward, blocks of Buda 

limestone and Boquillas flags peel from the sides and 

top of the arch to form the characteristically mixed 

breccia of these rocks. ( 4) As the top moves farther 

up into the Boquillas flags, the supply of breccia may 

come entirely from this thick formation, as in the upper 

part of the Two-Forty-Eight pipe. The mixture of 

rocks in the breccia may alternatively be explained by 

slow turbulent stirring of the mass of breccia as it 

slides downward, without the existence of a large 

opening at the top, but the degree of turbulence that 

would be required seems unlikely. 

The diameter of a breccia pipe is controlled to a large 

extent by . he strength of the roc.f-s. Pipes are commonly fron1 100 to :300 feet in diameter but some are 

as small as 50 feet and son1e are larger than 300 feet. 

Openings as wide as 50 to 100 feet are maintained 

indefinitely in large caverns and old mine stopes in the 

Devils River limestone. The upper size limit of permanent openings is perhaps 300 to 500 feet, as indica ted by the size of the pipes themselves; openings as 

large as this inevitably collapse to form pipes. Caverns formed at the top of the Devils River and having 

a roof of clay of the Grayson formation are filled by 

c0llapse of the weak roof before the opening ~an grow 

wide; mine stopes as wide as 50 feet in dry clay of the 

Grayson formation are fille ::l by blocks falling from 

their roofs in a period of a few years, and the process is 

much more rapid where the clay is wet. This weakness of the clay of the Grayson formation explains the 

cave-fill zones. Caverns with roofs of the clay can 

never become broad enough to form breccia pipes but 

are filled by slumping clay and residual blocks of limestone as they form; caverns with roofs of limestone rnay 

become very wid e. None of the rocks younger than 

the Devils River limestone can maintain so broad an 

arch as it, and once collapse begins in this limestone, a 

pipe grows upward as long as space i3 available below. 

The amount of rock dissolved to form a breccia pipe 

is large. Assuming the rocks expand 25 percent by 

brecciation, the rocks dissolved must be 25 percent of 

the volume of the pipe. The large pipes must therefore 

have formed above the floors of very large and extensive 

caverns or, perhaps more likely, solution of limestone 

continued actively during the growth of the pipes. As 

long as solution continued, the pipes continued to grow 

and penetrate farther upward. 

COMPARISON WI'l'H BRECCIA PIPES AND DIATREMES IN OTHER 

REGIONS 

Cylindrical bodies of breccia are by no means rare, 

and several explanations for them have been proposed. 

1-Iost ~xplanations fall into one of two general classes: 

(1) subsidence owing to solution of limestone or other 

soluble rocks, or owing to hydrothern1al alteration; (2) 

an explosive origin, commonly followed by subsidence. 

It n1ay be difficult to determine which explains some 

p1pes. 

The filled sinks of southern and central Missouri are 

very numerous, and in many ways they resemble the 

Terlingua breccia pipes (Tarr, 1919, p. 830-841; Grawe, 

1945; Bretz, 1950). They are generally regarded as 

products of solution by ordinary ground water, accompanied or followed by subsidence of overlying rocks. 

Some of the filled sinks contain pyrite, galena, and 

sphalerite; this association of metallic minerals with 

the sinks has not been satisfactorily explained. 

In southeastern ~1issouri there are many breccia 

pipes similar in size and form to the filled sinks just 
46 GEOLOGY AND QUICKSILVER DEPOSITS, TERLINGUA DISTRICT, TEXAS 

described. Igneous rocks have intruded the breccia in 

some of them, and for this reason the pipes of southeastern Missouri have been ascribed to explosive 

volcanism. In describing them, Rust (1937, p. 61) 

says, "It is very common to find limestone fragments 

carrying Devonian fossils in a rock which also contains 

an abundance of granite inclusions. The granite pieces 

have come up several hundred feet and the Devonian 

fossils are at least 3,000 feet below their source beds. 

These two rock types, normally separated by about 

4,000 feet of sediments, are found side by side at the 

intermediate Cambrian horizon of the area." Rust 

explained these conditions as being due to explosions, 

-following which magma rose into the debris-choked 

explosion tubes, incorporating the debris in the tubes. 

In the same area, an ultrabasic intrusive body of pipelike form has been described by Singewald and Milton 

(1930, p. 54). The intrusion has a slightly elliptical 

outcrop with the longer axis measuring 200 feet; drill 

holes indicate that it is about vertical. The surrounding Bonneterre dolomite is brecciated and metamorphosed at the contact, and the borders contain calcite, 

galena, sphalerite, marcasite, chalcopyrite, pyrite, and 

smoky quartz. Singewald and Milton believed it 

analogous to a diatreme in origin. 

Many mineralized breccia pipes have been reviewed 

by Locke (1926), who attributed them to "mineralization stoping" in both calcareous and noncalcareous 

rocks. The process is described as "removal of rock 

along trunk channels by rising solutions during an 

early stage of their activity, collapse and bre.cciation 

of the rock thus left unsupported, and deposition of ore 

and gangue minerals in the brecciated mass." In some 

of Locke's examples, the breccia has a measurable 

downward displacement. Furthermore, he cites two 

examples of breccia pipes in which "the actual downward terminations are visible; there is a rapid convergence of the walls forming a rude keel below which 

the silicified breccia descends only in irregular protuberances. No intrusive plug · appears in either case 

and the hypothesis that such a plug has advanced and 

retreated is inadmissible." (Locke, 1926, p. 446-447.) 

Locke's postulation of hydrothermal solutions has. the 

advantage of explaining the close association of metallic 

deposits with some breccia pipes. 

Some of the pipes cited by Locke as products of 

"mineralization stoping" are listed by W. H. Emmons 

(1938) as examples of diatremes. Emmons has assembled comparative· data on a large number of pipes, and 

he interprets most of them as diatremes or incipient 

diatremes, blown through the rock by gases. . . The 

structures included in his discussion range from vein 

systems believed to represent incipient diatremes to 

volcanic necks. The material within at' least one of the 

pipes is below its normal stratigraphic position; in 

most of them the direction of displacement is not given. 

Limestone occurs in some of the areas containing pipes. 

Volcanic necks and the South African diamond pipes 

attracted the attention of the great French experimenter 

Daubree, who introduced the term diatreme (Daubree, 

1891). In his experiments Daubree used a device 

consisting of a central chamber containing dynamite or 

guncotton and a subsidiary chamber at one end 

containing a cylinder of the rock to be tested, through 

which the explosion gases had to pass in order to 

escape. The explosives developed pressures of 1,100 

to 1,700 atmospheres and temperatures of 2,500° to 

3,200°0. The duration of the explosions ranged from 

0.00002 second for guncotton to 0.003 second for 

dynamite. Each cylindrical rock had been cut in such 

a way that a natural crack occupied a diametral plane 

or else a tiny hole had been drilled along its axis. The 

purpose of using a crack or drilling a hole was to concentrate the explosive action and thus prevent a 

blowout around the outside of .the rock cylinder. The 

holes produced by the explosion had a roughly circular 

or elliptical cross section. In a rock cylinder having a 

diametral crack the exploded hole was zig-zagged like a 

lightning streak within the plane of the crack. In a 

cylinder with a drilled guide-hole the exploded hole was 

fairly straight. Some of the material blown out of the 

holes was fused and some was clastic. It is worth 

emphasizing that in these experiments the holes were 

not punched out but eroded out: the gases began to 

leak along a crack or hole, which was rapidly and 

violently enlarged by tearing and melting away of 

small pieces. Daubree concluded that the effectiveness of high-pressure, high-temperature gases in producing openings was proved, and he felt that the 

experimental results were directly applicable to the 

formation of the South African diamond pipes and to 

volcanic necks. 

There are many excellent examples of breccia pipes 

associated with volcanic activity in northeastern Arizona (Hack, 1942). They occur in a dense cluster 

covering about 800 square miles, but some are far 

removed from the cluster. Their diameter generally 

decreases downward, and the more deeply dissected 

pipes range in diameter from 500 to 3,000 feet. The 

filling is largely of pyroclastic material and it generally 

shows evidence of inward and downward shunping. 

The pipes are thought to be due to explosive volcanism 

and are called diatremes. 

The pit craters of Hawaii (Wentworth and Macdonald, 1953) afford an interesting example of pipes 

associated with volcanic· activity but due entirely to 

subsidence, without explosion. They are circular .to 

ellipsoidal open .pits that· range in size from 100 feet to 
STRUCTURAL GEOLOGY 47 

more than half a mile across and from less than 50 to 

800 feet deep. Their walls are almost vertical. They 

are thought to be formed by collapse resulting from 

the withdrawal downward of underlying magma. 

Many other examples of pipelike structures could be 

given, but these are sufficient to indicate the diversity 

in origin of features that appear superficially similar. 

Subsidence is clearly an adequate process to form pipeshaped openings or masses of breccia; an explosive 

origin is not indicated by the shape alone. The term 

"diatreme" should be modified or applied only to pipes 

where there is clear evidence of formation by explosion. 

Pipes formed originally by subsidence and later intruded 

by igneous rocks may contain blocks pushed up from 

below, or intermixed blocks from above and below, 

without ever having been subjected to explosion. 

THEORETICAL PROBLEMS 

AGE OF THE STRUCTURES 

If our knowledge of the timing of structural events 

were complete and precise, we would derive directly a 

deep insight into their origin. This ideal can only be 

approached, because chronological evidence is fragmentary and incomplete. 

The earliest major structural movement after deposition of Cretaceous rocks was the beginning of the rise 

of the Terlingua uplift. Evidence for this is found on 

the flank of the uplift near the Fresno mine, where 

Chisos volcanics rest with angular unconformity on the 

lower member of the Boquillas flags; the hiatus decreases away from the uplift. Similar evidence can be 

seen on the north flank of the Solitario dome (Erickson, 

1953, p. 1385), where additionally the thick volcanic 

sequence thins and becomes conglomeratic against the 

flank of the dome, indicating continued uplift during 

deposition of the volcanics. The absence of the Aguja 

formation and Tornillo clay at these localities is due 

either to nondeposition or to erosion; the former would 

imply that uplift began in late Cretaceous time, as 

Erickson assumes, but the possibility of erosion is consistent with uplift beginning in Cenozoic time before 

deposition of the volcanic rocks. The volcanic rocks 

have been identified elsewhere by fossils in the tuff 

beds as being Eocene and Oligocene (King, 1937, p. 

117, quoting Baker, Bowman, Berry, and Plummer). 

McAnulty (1955, p. 556) found vertebrate fossils that 

indicate a late Eocene age in the lowermost tuff, sandstone, and conglomerate beds in the Cathedral Mountain quadrangle. Erickson's finding of continued uplift 

of the Solitario dome during deposition of the volcanic 

rocks is consistent with the evidence in the Marathon 

area, where King (1937, p. 140) observed anticlines in 

which the presence of volcanic rocks lead him to conclude that a considerable part of the uplift of the 

Marathon dome took place after the volcanic eruptions 

in Tertiary time. The Terlingua uplift thus began to 

rise sometime between Late Cretaceous and Late Eocene 

times, and it continued to rise for an unknown length 

of Tertiary time. 

The large n0rmal faults appear to have formed 

contemporaneously with the Terlingua uplift. This 

conclusion is based on a genetic interpretation of the 

relations between the Terlingua uplift and the large 

grabens; the grabens die out at the Terlingua monocline (fig. 7), strongly suggesting a time relation as 

well as space relation between the faulting and the 

doming. In the Marathon area King (1937, p. 140) 

judged the normal faults to be later than the folds 

because the folds are cut across and displaced by the 

faults, but this evidence is equally consistent with 

contemporaneity of the faults and folds. A parallel 

may be drawn with the normal faults around salt 

domes: are the normal faults later than a salt dome 

because they cut across it? On the contrary, the salt 

dome and faults are clearly contemporaneous and 

interrelated. Further support for the probable contemporaneity of faults and domical uplifts in the 

Big Bend region has been derived from a study of air 

photographs of the region. These show many smaller 

domes that are bounded partly by monoclines and 

partly by faults, an arrangement that suggests the 

response to lifting was partly by doming and partly 

by simultaneous faulting. 

Intrusive igneous activity, which is intimately related to the structural development, is both older and 

younger than individual normal faults and hence is 

roughly contemporaneous with normal faulting. The 

faults of Long Draw have served as channels for 

intrusive bodies (exposed in the Chisos mine), and 

later movements on these faults have displaced the 

same bodies. The southwestern fault of the Cigar 

Mountain graben cuts a small intrusion and is in turn 

intruded by the Cigar Mountain laccolith (pl. 1). 

Intrusions south of Sawmill Mountain also invade 

faults. The Cuesta Blanca dome is perhaps a good 

example of a laccolithic dome that formed by faulting 

on one side and doming on the other sides, while magma 

rose within it. Because many minor faults and some 

breccia pipes have also been intruded by igneous rock, 

these smaller structures also appear to belong to the 

period of igneous activity. 

In summary, the Terlingua uplift began to develop 

in Late Cretaceous or early Tertiary time. It continued 

to rise during the extrusive volcanism in the Tertiary. 

About the same time, normal faulting and igneous 

intrusion were taking place. The duration of this 

structural period is not known, and it may have continued throughout a large part of the Cenozoic era. 
48 GEOLOGY AND QUICKSILVER DEPOSITS, TERLINGUA DISTRICT, TEXAS 

The subsequent geologic history of the district is 

mainly one of erosion modifying the surface of a structurally quiescent area, which, however, may have 

been subjected to regional tilting and uplifting. 

ANALYSIS OF MOVEMENTS 

Have the rocks in the Terlingua district undergone 

horizontal compression during their structural evolution or alternatively have they been extended? Whence 

came the added volume of material beneath thB domes 

and uplifts? An attempt to answer these questions 

depends upon accurate geologic data from the surrounding region, and such data are available only in 

part. 

The added material beneath the uplifts is believed 

to have come from parts nearby of the Big Bend region. 

On the west side of the Chisos Mountains and extending northwestward to the Terlingua uplift is an area 

where Cretaceous strata are structurally some 3,000 

to 4,000 feet below their average altitude in the Big 

Bend region (King, 1937, pl. 22). Yet in the Solitario 

dome, the same strata are roughly 3,000 feet above, 

and in the Marathon uplift 4,000 feet above, their 

average altitude. This clearly suggests that the 

Terlingua uplift and Solitario dome and in part the 

Marathon uplift are complementary to the structurally 

low area-that material at depth has moved from the 

low area to form the structurally high. Such mobility 

seems possible only by melting, which is reasonable in 

view of the widespread igneous activity in the region. 

Estimates of horizontal compression or extension lead 

to the tentative conclusion that the Terlingua district 

as a whole has been neither compressed nor extended 

but has maintained a nearly constant area. Measurements of horizontal compression or extension have sometimes been rnade on geologic cross sections by unfolding 

and unfaulting the structure and observing the change 

in length. A more suitable method for the Terlingua 

district, where the trends of the folds and faults are 

diverse, is to estimate the changes in area. Thus the 

increase in area owing to normal faulting is the sum of 

the areas obtained by multiplying the length of each 

fault segment by the horizontal component of dip slip 

applicable to that segment. Similarly, the decrease in 

area owing to folding is the difference between the area 

of a deformed stratigraphic reference surface and the 

area of the horizontal projection of the same surface. 

The net increase or decrease in area (areal extension or 

compression) is the difference between the extension on 

normal faults and the compression on folds. Strikeslip movement on faults has no effect on the calculation. 

The analysis was applied to the part of the Terlingua 

uplift that lies in the Terlingua district. The reduction 

in area, which is almost entirely along the Terlingua 

monocline, was estimated by measuring the shortening 

in a series of sections across the monocline, multiplying 

each value by the applicable length along the monocline, 

and then summing the areas so obtained. The reduction 

in area along the monocline from Long Draw to the western end of the district is about 1.4 square miles. Similarly, the extension in area along the faults of the Long 

Draw graben is 0.6 square mile if an average dip of 80° 

is assumed, or 1.2 square miles, assuming an average 

dip of 70°; the intermediate value, for a dip of 75°, is 

0.9 square mile. The other faults on or adjacent to the 

Terlingua uplift, taken together, probably contribute 

about an equal amount to the extension. It thus appears that the extension by faulting is about equal to 

the reduction by folding and the net extension or compression of the Terlingua uplift is near zero. 

The calculations are of course very rough but serve 

to indicate tnat a balance between extension and compression is at least a possibility. Another test may be 

applied in the area where the Terlingua monocline and 

the Long Draw graben are parallel and close to each 

other. The extension on the graben, measured on section A-A' (pl. 1), is 1,100 feet and the shortening on 

the fold is 800 feet. If the faults are a little steeper 

than the 70° shown, the two figures would correspond 

exactly. The extension on faults for the entire length 

of section A-A' is 1,600 feet, and the shortening on 

folds is 1,100 feet. Again, if the faults are a little 

steeper than shown, the two figures would be the same 

and the net extension or compression would be zero. 

ORIGIN OF THE TERLINGUA UPLIFT AND TERLINGUA MONOCLINE 

The Terlingua uplift and the high circular dome of 

the Solitario, which forms the crest of the uplift, exhibit 

the form of a bulge that might be raised by a mobile 

igneous mass. An igneous origin for the Solitario dome 

was inferred by several earlier observers (Lonsdale, 1940, 

p. 1546). Details of the suggested igneous mass are 

completely speculative, however, for no intrusive body 

of requisite size is exposed in the Solitario, where erosion 

has laid bare all of the rocks down to the crumpled and 

thrust-faulted rocks of Paleozoic age. As an example 

of the kind of intrusive mass that might underlie the 

uplift, the Pilansberg complex might be chosen. This 

complex of basic, soda-rich rocks is superimposed on the 

Bush veld complex in the western Transvaal; it has been 

interpreted as a laccolith (Shand, 1929, p. 149). It 

covers about 200 square miles and has a probable thickness of 2,000 to 3,000 feet; the Terlingua uplift covers 

more than 100 square miles and has a structural relief 

of several thousand feet. 

The Terlingua monocline, which forms the abrupt 

southern and southwestern margin of the uplift and 

also extenils eastward beyond the uplift, is parallel in 
STRUCTURAL GEOLOGY 49 

its western part to the anticlines of the Sierra Madre 

Oriental farther west (fig. 5). In its eastern part, however, it corresponds in trend with neither these anticlines nor the older Paleozoic folds exposed in the 

Marathon uplift and the Solitario; no control for its 

east-west strike is obvious. The monocline evidently 

marks the edge, or a line of abrupt thinning, of the 

igneous mass postulated beneath the Terlingua uplift; 

it may be compared with the monoclinal fold over the 

southern edge of the Study Butte intrusive body (fig. 3). 

ORIGIN OF 'lHE GRABENS 

The magnitude and shape of the large grabens 

strongly suggest that, when they formed, material 

muc.b more mobile than ordinary rocks was present 

at a depth no greater than a few miles. The faults 

bounding individual grabens, if extended downward on 

the dips measured at the surface and in mines, converge 

rapidly and join at depths of only about a mile. The 

faults of the Long Draw graben, projected downward 

at 70° (sec. A-A', pl. 1), join at a depth of 1 mile; 

projected at a dip of goo they join at a depth of 2 miles. 

The graben is only half a mile wide, yet its vertical displacement reaches a magnitude of more than a quarter 

of a mile. These relations, particularly the convergence 

of the faults, may well be explained by magma beneath 

the grabens at the time they formed. Dilation of the 

crust by thick dikes below the grabens might be suggested or, alternatively, the large body of igneous rock 

already postulated to explain the Terlingua uplift may 

lie approximately at the depth where the graben faults 

converge. 

Compared to the other large grabens, the Lowes 

Valley structure is distinctive because of its irregular 

outlines, its large displacement, the broken-up character 

of the rocks in it, and its possibly closer association 

with igneous activity. Space for the downfaulted rocks 

could possibly have been provided by solution of 

limestone or withdrawal of magma. The second 

possibility suggests a comparison with calderas or with 

cryptovolcanic structures (Williams, 1941). Grabens 

A 

along rift zones on the Hawaiian shield volcanoes are 

described by Wentworth and Macdonald (1953, p. 11, 

14). They also note that there are all gradations 

between pit craters and grabens. The northeast trend 

of Lowes Valley and the entire Black Mesa graben zone 

is nearly at right angles to the other major grabens but 

parallel to the structures formed during Paleozoic time 

and are now exposed in the Solitario; perhaps the trend 

of the younger gtructure was influenced by the earlier. 

Many structural depressions other than grabens are 

evident on plate 1. These include saucer-shaped 

structural basins, like the one east of the Fresno mine, 

and irregular downfaulted blocks, like the Mariposa 

mine area. Some that are small and composed largely 

of jumbled blocks have been mapped as breccia pipes 

but are actually larger and more irregular than typical 

breccia pipes. Perhaps many of these structures, Uke 

the Lowes Valley structure, were caused by withdrawal 

of support in an underlying magma. 

The large grabens are thus regarded as interrelated 

in time and space with the Terlingua uplift and with 

igneous activity. Their development may be pictured 

as similar to the development of rifts in regions of 

present igneous activity. In Hawaii, tumescence of 

the volcanoes amounting to several feet of uplift is 

observed preceding and during eruptions, and subsidence after eruptions (Wentworth and Macdonald, 

1953, p. 14). At depth the Hawaiian rift zones are 

occupied by hundreds of dikes (p. 11), representing a 

large horizontal extension of the strata. An alternation of tumescence and lesser subsidence, repeated 

many times through a long period of geologic time, is 

clearly capable also of explaining many details such as 

the minor thrust faults in the Terlingua district. 

A small graben in the Fresno mine area, although not 

comparable in size with the principal grabens of the 

district, is significant because of the quantitative 

information on origin that can be obtained from a 

study of it. It was caused by the deflection in dip of a 

single master fault (fig. 12). This fault, which bounds 

the graben on the north, has a dip of about goo in the 

. ~...~ Grayson format1on '\ 

8 

0 100 Feet 

FI'1URE 12.-A, Vertical sections showing theoretical stages in the development of a small graben caused by the flattening in dip of a normal fault. Stages 2 and 3 occur 

simultaneously but are separated for clarity. B, Vertical section of the Fresno graben. 
50 GEOLOGY AND QUICKSILVER DEPOSITS) TERLINGUA DISTRICT) TEXAS 

Buda limestone and about 50° in the underlying 

Grayson forn1ation. During the dip-slip movement of 

20 to 50 feet on thi<s normal fault, the part of the fault 

in the Buda lime<stone tended to open; this allowed a 

segment of the Buda limestone, bounded by a new 

fault on the south, to slide into the potential opening. 

Cross sections of the graben and theoretical sections 

showing its origin are given in figure 12. The development of the large grabens cannot be exactly sin1ilar, 

because, with the exception of the Long Draw graben, 

the displacements on the two sides are equal. 

QUICKSILVER DEPOSITS 

HISTORY AND PRODUCTION 

This section on "History and production" except 

the part "World War II" is condensed from a manuscript report by Clyde P. Ross. Ross gathered his 

information and data from works listed in "Selected 

bibliography" and from local mining men, particularly 

W: D. Burcham, Jack Dawson, Frank Lewis, H. E. 

Perry, and E. A. Waldron. 

EARLY ACTIVITY 

Local stories relate that Indians passing through 

the district going to and from Mexico collected cinnabar and used it as a pigment. Pictographs at 

camping places have suggested such a use and have 

inspired the stories. Ross collected a sample of a red 

pigment from a pictograph at Comanche Springs, an 

old camp site, 3 miles northeast of Lajitas, and had the 

sample tested by Efe Daggett of the Chisos Mining Co. 

The pigment was found to contain no appreciable 

quicksilver. 

The very early history of quicksilver discovery and 

mining is not well recorded. According to Phillips 

(1905, p. 155) reports of the presence of cinnabar in the 

Terlingua region began to trickle out as early as 1850. 

Mexican miners passing through the area in the 

seventies and later are thought to have known about 

the presence of quicksilver. It is reported that a 

merchant from Presidio named Klineman took up 

claims near California Mountain about 1884. There is 

definite knowledge that by 1894 there was some mining 

in the area, for Blake (1896) reports that early in 1894, 

George W. Wanless and Charles Allen, hearing of the 

deposits through reports of Mexicans, visited the area 

and began work in the vicinity of California Mountain. 

Thomas Golby (1924, p. 579-580) reports that he 

found ore in the same locality in 1896. Several other 

Americans including Jack Dawson, J. A. Davies, and 

Louis Lindheim began mining at and close to California. 

Mountain about this time. This last named group of 

men set up a battery of retorts and may have been the 

first producers of quicksilver in the district. 

About 1898 at the mining settlement that had grown 

about California :Nfountain a post office was established 

and named Terlingua. The exact origin and meaning 

of this name is unknown. It is locally supposed to be 

a corruption of some early Mexican name, such as 

Tres Lenguas, that had been applied to some local 

feature. By 1903 there were about 3,000 people in this 

settlement. As more discoveries were made, the 

mining center shifted further to the east and this 

resulted in the transfer of the post office and its name 

to the settlement at the Chisos mine. The retention of 

the name Terlingua by the post office caused the original 

settlement of Terlingua to lose its name and acquire a 

new name-Mariposa. References to "Terlingua" in 

early reports refer to the locality that is now known as 

Mariposa; in this report "Terlingua" refers to the 

settlement later built around the Chisos mine. 

About 1900 Golby and associates erected a 10-ton 

Scott furnace at California Mountain and began production. By 1901 this operation was known as the 

Marfa and Mariposa Mining Co. and was composed of 

two Norman brothers, Montroyd Shayse, and Thomas 

Golby. This company continued mining until 1910 

and although their operations were mainly about 

California Mountain (the Mariposa mine of this report), 

their property extended well beyond this area and ore 

was mined as far away as 2 miles to the northeast (sec. 

38, Block G-12). Although no accurate records for 

individual mines were available, it is estimated with 

fair assurance that between 20,000 and 30,000 flasks of 

quicksilver have been produced from the workings 

about California Mountain. 

Another early operation in this general area was in 

and near the southeast corner of sec. 40, Block G-12. 

From 1901 to 1906 this group of claims was worked by 

the Terlingua Mining Co. A Scott furnace with a 

capacity of 40 to 50 tons of ore a day was built, but 

judging from the dumps of "burned" ore it was not 

long in operation. 

By 1900 cinnabar had been discovered near the 

present town of Terlingua. By April of 1902, the 

McKinney-Parker mine (McKinney-Parker workings 

of the Chisos mine) had yielded a considerable quantity 

of ore from some of which a small quantity of quicksilver had been retorted. This mine was located about 

one-fourth mile southwest of Terlingua. According to 

Hill (1902, p. 38) there were in 1902 numerous openings 

to the northeast of the McKinney-Parker mine show1ng 

the presence of cinnabar. This would be at the site of 

the present Chisos mine, which is supposed to have 

begun operations about this time. The Chisos mine 

soon became the principal producer in the district and 

until 1942 was in almost continuous operation. Before 

1904 the ore was retorted, but that year a 20-ton Scott 
QUICKSILVER DEPOSITS 51 

furnace was put into operation on the property and 

used almost continuously until 1942. 

About the same time quicksilver was discovered 

near Terlingua, it was found farther east at what is 

now called Study Butte, in sec. 216, Block G-4 (Study 

Butte mine) and in sec. 248, Block G-4 (Two-FortyEight mine). Hill (1902, p. 13) reports these discoveries as of January 1902. In sec. 216 the Big Bend 

Mining Co. began operations in 1903 and the TexasAlmaden Co. began soon afterward on adjoining 

ground. Big Bend built a 50-ton Scott furnace, and 

Texas-Almaden built a 20-ton Scott furnace. These 

furnaces shut down after only .a few hundred flasks of 

quicksilver had been produced. In 1905 or 1906 both 

companies ceased operations and the properties remained idle for about 10 years. 

WORLD WAR I PERIOD 

For several years before World War I the Chisos 

mine was the only significant mine in the district. The 

increase in price resulting from the war revived operations in the district, and several of the old properties 

returned to production. By a fortunate coincident, 

the Chisos mine was able to take maximum advantage 

of the price increase, because they had just begun to 

stope the largest ore body in their history. This, the 

Pipe stope, was mined from 1915 through 1918, and 

ore from it was treated in the Scott furnace. A rotary 

furnace was installed and operated on low-grade ore 

from 1918 until1922, when it was judged unsatisfactory 

and its use discontinued until 1942. 

Among other mines that reopened were lVIariposa 

and Colquitt-Tigner. The Rainbow mine was opened 

in 1916, after a shaft was sunk to a depth of 624 feet. 

The Mariposa mine was reopened in 1916 and operated 

until 1919, during which period it produced around 

8,000 flasks of quicksilver. The old Colquitt-Tigner 

mine was reopened and produced a little quicksilver in 

1916. 

In 1915 the (Texas Almaden and Big Bend) properties at Study Butte were reopened and worked as 

a unit until 1920. About $500,000 worth of quicksilver was produced during this time. Lower prices 

and a water problem on the lower levels brought an 

end to mining. 

INTERWAR PERIOD 

The postwar depression of the early twenties with 

its unfavorable market conditions caused the suspension of all mining activity except that at the Chisos 

mine. F. W. Oakes, Jr., became interested in the 

district about 1923 and in time acquired control of many 

of the properties in the western part of the district, 

including the old Marfa and Mariposa property. By 

1927 he was producing quicksilver in retorts and the 

484889-60-6 

following year he built a 30-ton rotary furnace, which, 

upon Oakes' suspension of activities in 1930, was moved 

to the Study Butte mine. 

In 1931 the Tarrant Mining Co. acquired control of 

much of the property that was operated by Oakes. 

They carried out some development at several places, 

notably in sec. 39, Block G-12, in sec. 70, Block 341, 

and on the Monte Cristo claim northwest of the 

Mariposa mine. They produced their first quicksilver 

in August 1933 mainly from ore from the Monte 

Cristo claim. 

In 1928 the Waldron Quicksilver Properties, Inc., 

began development at the Colquitt-Tigner property on 

sec. 38, Block G-12. They operated a 12-ton Scott 

furnace from December 1926 to August 1927, when they 

had to shut down, partly because of law suits. This 

furnace was not operated again until 1936. During 

the intervening period the operations of the company 

were fairly inconsequential. 

The old Rainbow mine near Terlingua was reopened 

by a new company in 1927. In 1934 a 10-ton rotary 

furnace was operating on fairly rich ore. 

In 1928 the Study Butte mine was reopened under 

lease by the Brewster Quicksilver Consolidated, the 

beginning of its period of longest operation. During 

most of the following years the mine was in curtailed 

production, because the low price of quicksilver did 

not permit the pumping of the flooded lower level of 

the mine. However, W. D. Burcham, who was general 

manager from 1928 to 1934, estimates that about 

$400,000 worth of quicksilver was produced during 

these years. In 1936 there was a reorganization and 

a company known as the Southwest Mines took over 

control. 

WORLD WAR II PERIOD 

The increased demand for quicksilver during World 

War II stimulated renewed activity in mining and 

prospecting. The Study Butte mine, which was a 

steady producer, was deepened and extended. ~.1injng 

was carried on from the Big Bend and Dallas shafts and 

shallow shafts north of the butte. The Big Bend shaft 

was sunk to the botton1 of the Study Butte sill at a depth 

of 440 feet. The Chisos mine was active sporadically, 

and most of the ore came from the 250-foot level near 

the No.8 shaft. Some ore was brought from the lower 

levels of the No. 9 shaft during the early war years. 

Prospecting and development work was carried on 

also in the No. 14 and Susano Prospeet shafts on the 

Chisos property. A considerable amount of qu-icksilver was recovered from dismantling and treating the 

old Scott furnace at the Chisos mine. The Two-FortyEight mine was deepened and explored extensively. 

A Herschoff furnace was installed, but production 

remained small. The Mariposa mine was worked at 
52 GEOLOGY AND QUICKSILVER DEPOSITS, TERLINGUA DISTRICT, TEXAS 

intervals and the ore hauled to the Chisos and TwoForty-Eight mines for treatment. Production came 

from shallow open cuts; some material from old dumps 

was furnaced also. During the latter war years the 

Chisos, Two-Forty-Eight, and Mariposa mines were 

operated by one company, the Esperado Mining Co. 

The discovery and development of what is now tbe 

Fresno mine by Harris S. Smith and his partner, 

Homer Wilson, was perhaps the outstanding event of 

this period. The development of the Fresno mine 

extended the belt of productive deposits about 6 miles 

westward. The Fresno mine became a steady producer from an ore body less tban 100 feet below the 

surface. 

The most extensive additional exploration was 

carried on at 11aggie Sink and Contrabando dome. 

At Maggie Sink an adit was extended to intersect the 

breccia pipe at a depth of about 150 feet, and from 

this level 9 winze was sunk several hundred feet 

deeper; no ore was discovered. At Contrabando 

dome, cinnabar showjngs were explored by pits and 

shallow shafts around several small rhyolite intrusions. 

With falling quicksilver prices, all the mines had 

closed by 194 7. 

SUMMARY OF PRODUCTION DATA 

ReHable information on the production of individual 

mines in the Terlingua district is both rare and unsatisfactory. Some production information-whatever is 

available-is given under the descriptions of the indi, 

vidual mines. Judging from the annual statements 

in "Mineral resources of the United States" and estimates made by H. E. Perry, vV. D. Burcham, and 

others for the particular property in which they are 

interested, it appears that the total production of the 

Terlingua district is approximately 150,000 flasks. 

Table 11 was compiled by C. P. Ross from tables 

given in "Mineral resources of the United States" 

and, for recent years, the "Mineral yearbooks." 

The entire production of Texas as given in these 

reports is to be credited to the Terlingua and Mariscal 

districts. At best these figures would be minimum 

figures for the early years when there was probably 

considerable unrecorded production. Since 1922 the 

production of Texas has been combined with certain 

other States, but in general the quantities produced by 

these other States has been small. Production from 

1938 through 1944, which is not given in table 11, 

averaged 1,132 flasks per year. 

FUTURE OF THE DISTRICT 

The future of the Terlingua quicksilver district 

depends largely upon exploration. With the exception 

of the Study Butte mine, known ore bodies are largely 

TABLE !I.-Quicksilver production of Texas 

[Compiled by C. P. Ross] 

Number of Price per 

Year fiasks fiask at San Year 

Fransisco 

1899 ____ 1, 000 $47. 70 1919 ____ 1900 ____ 1, 800 44. 94 1920 ____ 1901_ ___ 2, 932 48. 46 1921_ ___ 1902 ____ 5, 319 43. 20 1922 ____ 

1903 ____ 5,029 45. 29 1923 ____ 1904 ____ 5,336 43. 50 1924 ____ 1905 ____ 4, 723 36. 22 1925 ____ 

1906 ___ - 4, 761 39. 50 1926 ____ 1907 ____ 3, 686 39. 60 1927 ____ 1908 ____ 2, 382 44. 17 1928 ____ 1909 ____ 4, 188 45. 45 1929 ____ 1910 ____ 3, 320 46. 51 1930 ____ 1911_ ___ 2, 326 46. 01 193L ___ 1912 ____ 1, 990 42. 05 1932 ____ 1913 ____ 2, 750 40. 23 1933 ____ 1914 ____ 3, 144 49. 05 1934 ___ - 1915 ____ 4,417 85. 80 1935 ____ 1916 ____ 6, 306 125. 89 1936 ____ 1917 ____ 10, 791 105. 32 1937 ____ 1918 ____ 8, 451 117. 50 

Total flasks produced 146,579 

I Plus Nevada, Oregon, and Idaho. 

2 Plus Nevada, Arizona, Oregon, and Idaho. 

3 Plus Nevada, Arizona, and Idaho. 

'Plus Arizona and Alaska. 

5 Plus Washington and Arizona. 

6 Plus Arizona, Arkansas, and Alaska. 

7 Plus Arkansas, Washington, and Utah. 

s Plus Arizona. 

~Plus Arkansas, Arizona, and Washington. 

1n New York price. 

Number of 

fiasks 

5, 019 

3,436 

3, 283 

2,970 

1 2, 490 2 2, 119 3 1, 560 4 1, 224 4 2, 423 5 4, 316 4 3, 725 4 2, 822 6 3, 711 6 4, 046 7 4, 010 8 3, 059 8 4, 191 7 4, 010 9 3, 514 

Price per fiask at San 

Francisco 

$90. 29 

79. 66 

47.42 

57.78 

65. 68 

68. 69 

82. 67 

91. 60 

117. 61 

125. 34 

124. 00 10 115. 01 10 87. 35 10 57. 925 10 59. 227 10 73. 87 10 71. 99 10 79. 92 

90. 18 

exhausted. Under the stimulus of the high prices of 

World War II the district responded with considerable exploration, but few ore bodies were discovered. 

This exploration did not, however, exhaust the possibilities of finding new ore bodies. Probably all ore 

bodies that crop out have been found, consequently 

undiscovered ones will have to be located by subsurface 

exploration in areas where there are showings of cinnabar and geologic structures that favored deposition. 

Geologic structures that are associated with the known 

ore bodies are described in the general discussion that 

follows. Similar, but unexplored, structures appear on 

the geologic maps that accompany this report and more 

specific recommendations on particularly favorable 

areas are given in the descriptions of individual mines. 

GENERAL GEOLOGIC RELATIONS 

Quicksilver minerals occur at widely separated places 

in the Big Bend region, but the only extensively mineralized area is the long, narrow east-west belt that 

comprises the Terlingua district. With the exception 

of the Mariscal district, southeast of the Chisos Mountains, the occurrences of quicksilver minerals outside 

of this belt have not yet proved to be of economic 

importance. 
QUICKSILVER DEPOSITS 53 

Within the Terlingua district, the deposits are veins 

and tabular and pipelike bodies in both the Cretaceous 

sedimentary rocks and Tertiary igneous intrusive rocks. 

Cinnabar is the principal ore mineral, but there are 

minor quantities of native mercury, calomel, and rarer 

mercury minerals. Associated with these are calcite, 

clay, pyrite, hydrocarbons, and, less commonly, several 

other minerals. 

As the geology of the quicksilver deposits is so closely 

a part of the general geology of the district, it seems 

best to precede the descriptions of the deposits with a 

brief correlation with those features, described in preceding sections, that have a direct bearing upon any 

interpretation of the geology of the quicksilver deposits. 

The ore deposits are directly or indirectly related to 

the Terlingua monocline, to fractures and breccia pipes, 

to the igneous rocks, and to the physical and chemical 

character of the sedimentary rocks. 

Relation between the quicksilver belt and the Terlingua 

monocline.-The east-west trend of the Terlingua monocline is unique among the major structural trends of the 

Terlingua region, which range from northerly to northwesterly. The corresponding east-west trend of the 

quicksilver belt and the closeness of the belt to the 

monocline suggests that there may be a direct relation 

between the two features. However, the hypothesis 

that the monocline was a structure that directly controlled the location of the lodes can be discarded for the 

following reasons: (1) the quicksilver belt is not even 

approximately superimposed upon the monocline, but 

in places is as far as 2 miles to the north, and at its west 

end is actually crossed by the monocline; (2) the irregularities of the monocline are not repeated in the quicksilver belt; and (3) minor fractures that control the 

trends of individual lodes within the quicksilver belt 

are not restricted to the zone of monoclinal folding but 

are regional. Therefore, if there is a genetic relation 

between the monocline and quicksilver belt, the only 

valid hypothesis remaining is that the trends of the 

monocline and quicksilver belt were determined by a 

common factor. In a preceding section, the interpretation of structural data suggests that the Terlingua 

monocline was formed by an igneous intrusion, the trend 

of which is defined by the monocline. Accordingly, an 

extension of this hypothesis interprets the trend of the 

quicksilver belt as either a reflection of the trend of 

the same intrusive body-or of a deeper structure that 

controlled the emplacement of the intrusion, as well as 

the upward course of the rising solutions. 

Relation between mineralized rock and minor structures.-The arrangement, form, and trend of the individual ore bodies within the mineralized belt was determined by minor structures, which existed before 

quicksilver mineralization. The ore bodies are along, 

, 

or adjacent to, fractures and are in breccia pipes, which 

are all structures along which there has been almost 

no movement after mineralization. No relation between minor folds and ore bodies is apparent. 

Almost all the ore bodies are associated with fractures 

that range in trend from N. 20° E. to N. 85° E., consequently the quicksilver belt is a zone of in echelon 

northeasterly trending veins. Fractures of northwesterly trend are as common as those of northeasterly 

trend but almost without exception are barren. Some 

fractures are joints with no visible displacement; others 

are faults with vertical displacements of more than 50 

feet. The ore-bearing fractures range in length from 

less than 100 feet to more than 1 mile, with very little 

relation between lengths and vertical displacement. In 

many places the relation between ore and fractures is 

an indirect one; the ore occurs, not along the fracture, 

but in a cave-fill zone in limestone, which was indirectly 

controlled by the fracture. 

Many breccia pipes in the district are mineralized 

and all may have been potential controlling structures 

for ore deposits. They are generally unrelated to the 

fractures; their distribution is throughout the district. 

On the other hand, all gradations exist between 

fracture-controlled cave-fill zones and breccia pipes, 

which in places are within cave-fill zones. 

Relation between mineralization and igneous intrusion.-Field relations indicate that the igneous rocks 

were emplaced before the quicksilver deposits were 

formed, but they do not show any direct association 

between the deposits and the exposed igneous intrusive 

bodies, either individually or collectively. Cinnabar 

has been found in postintrusion structures in both 

analcite and analcite-free rocks; but appears to be 

unrelated to either. Although there are several sporadic 

occurrences of cinnabar in several places outside the 

Terlingua district, the main mineralized belt is a very 

much restricted area. In contrast to this, the igneous 

rocks are widespread and of fairly general distribution. 

The arrangement of outcrops of igneous rocks, both 

within and without the district, is unrelated to the 

location and trend of the mineralized rocks. The absence of close relations between the outcropping igneous 

rocks and the quicksilver deposits is further demonstrated by: the lack of zonal or other arrangements 

between veins and intrusive masses, the failure of any 

one rock type to occur persistently along the quicksilver belt, and the diversity of rock types that contain 

cinnabar. The association of cinnabar with rhyolite 

plugs at Contrabando dome, as well as with other 

igneous intrusive bodies, is structural. 

As suggested in a preceding paragraph, there probably 

is, however, an indirect genetic relation between the 

exposed igneous rocks and the quicksilver deposits. 
54 GEOLOGY AND QUICKSILVER DEPOSITS, TERLINGUA DISTRICT, TEXAS 

The former may well have been differentiated at depth 

from the same parent magma that was the source of the 

mercury-bearing solutions. 

Relation between mineralization and sedimentary 

rocks.-Ore deposits are most abundant in the sedimentary rocks and have been found in the Devils 

River limestone, Grayson formation, Buda limestone, 

Boquillas flags, and Terlingua clay, with certain of 

these formations and certain parts of them more favorable host rocks than others. The most favorable ore 

horizon is the contact of the Devils River limestone 

with the overlying Grayson formation, where cave-fill 

zones commonly developed. Ore rarely extended more 

than 50 feet below this horizon into the Devils River 

limestone or 10 feet above into the Grayson formation. 

The Buda limestone and Boquillas flags were important host rocks only in the Chisos mine. The only 

ore bodies found in the Terlingua clay were in baked 

beds above the Study Butte intrusion. Ore deposits 

have not been found in rocks stratigraphically higher 

than the Terlingua clay, possibly because the mercury 

was all precipitated from the rising solutions before 

the higher formations were reached. 

The physical characters of the sedimentary rocks 

and possibly their chemical and mineralogical compositions determined their qualities as host rocks. The 

limestones, brittle and hence more permeable when 

Cinnabar found in clay matrix of breccia 1 

~ pipe containing blocks of sandstone Sandst~ne and ~lay of _.;_. _.;_. ~- ~--~ m Aguja formation 

Aguja formation ;:;:::::;:;:::::::::>::::::. 

Terlingua clay 

Boquillas flags 

Buda limestone 

Grayson formation 

Devils River limestone 

1---:_::-_-_--::...-

~-=-=-:::-= E-:::-=-::::--=-

~~~~~ 1 Cinnabar in vein_s in intrusive igneous 

~===~== rock and adjacent baked clay 

Cinnabar in fractures in rhyolite plugs j Cinnabar in calcite veins 

-----,h-

-----,J J 

1V VV V 

Cinnabar in breccia pipes 

Cinnabar and other mercury minerals 

in cave-fill zones 

Cinnabar in intrusive igneous rocks 

Cinnabar in calcite veins 

FIGU RE 13.-Stratigraphic section showing vertical range of quicksilver deposits of 

the several types that occur in the Terlingua district. · 

fractured, formed the best solution channels, whereas 

the relatively imtermeable clays and breccias with 

clay matrices wer~ the best ore hosts wherever they 

could be penetrated by the ore-bearing solutions. 

Time of quicksilver mineralization.-Quicksilver mineralization was late in the geologic history of the area. 

The quicksilver deposits were formed after the folding of the monocline, after the faulting of the grabens, 

and after the extrusion and intrusion of igneous rocks. 

The lack of postore faulting indicates that structural 

movements had died out by the end of mineralization, 

although it is probable that the area has since undergone a general uplift. The present study has not 

yielded data that could be used to correlate quicksilver mineralization with any particular phase of the 

Cenozoic history of the area, but such might be possible if a comprehensive study were made of the physiography and volcanic deposits of the Big Bend region. 

It seems improbable-but it is not impossible-that 

the deposits could be as young as Pleistocene, and it 

seems hardly more probable that they could have 

formed during the early Tertiary. Most likely their 

age is middle or late Tertiary. 

CLASSIFICATION AND GEOLOGIC FEATURES OF 

DEPOSITS 

The Terlingua quicksilver deposits are readily classified. This is fortunate, for descriptions are simplified 

and comparisons and contrasts can be clearly pointed 

out. As all the deposits formed during the same general period of mineralization and by fundamentally 

identical processes, the physical distinctions that separate the several classes are the results of differences 

in the environments of deposition. The environments 

of deposition naturally overlap and intergrade; consequently some deposits have the characteristics of two 

or more classes. It is always possible, however, to 

place a particular deposit in the class to which it shows 

greatest affinity. 

The classification here used is based upon variations 

in environme~t as determined by structural features 

and rock types in which the deposit formed. Because 

a given environment always produced a particular class 

of deposit. it is possible, if the geology of any given 

area is known, to predict the classes of deposits that 

might be found there. The quicksilver deposits are 

divided into the following classes: (I) deposits formed 

along the contact between the Devils River limestone 

and Grayson formation (hereafter called the limestoneclay contact deposits), (2) deposits in calcite veins in 

the Boquillas flags, (3) deposits in breccia pipes, and 

(4) deposits in igneous rocks. Class 4 can be subdivided 

into regular deposits occurring along well-defined fractures and irregular deposits occurring as irregular bodies 
QUICKSILVER DEPOSITS 55 

in brecciated rock. The stratigraphic range of the 

various classes of deposits is shown in figure 13. 

LIMESTONE-CLAY CONTACT DEPOSITS 

The limestone-clay contact deposits are by far the 

most common and the most valuable in the district 

and include all those deposits-elongate in plan and 

relatively thin in cross section-that occur at and near 

the contact between the Devils River limestone and 

the Grayson formation. The limestone immediately 

beneath the clay was dissolved along fractures and 

bedding planes; this allowed the clay to collapse into 

the solution cavity thus produced and formed what is 

hereafter referred to as "cave--fill," an excellent host 

material for cinnabar (see fig. 14). Cinnabar also 

occurs in the undisturbed clay and limestone adjacent 

to the cave-fill zones. The cave-fill commonly has a 

root, which is a narrow, calcite-lined fracture containing small amounts of cinnabar. In some places these 

"fracture-roots," instead of being filled by calcite, are 

differentially widened through solution of their limestone walls, and the opening thus formed is filled by 

detritus from above and locally mineralized by solutions from below. Because these calcite- and detritusfilled fissures extend from the limestone-clay contact 

and are so closely related to the limestone-clay contact 

deposits, they are described here, instead of under a 

separate heading. 

Ore bodies along the contact of the Devils River 

limestone and Grayson formaticn have been worked 

at the Chisos-Rainbow mines, Mariposa mine, and 

Fresno mine. The roots of similar ore bodies, now removed by erosion, have been mined and prospected at 

many places in the Devils River limestone in the westcentral part of the district, ·in particular near Tres 

Cuevas Mountain and north and east of California 

Mountain. 

The quicksilver deposits of the Mariposa · mine are 

typical and well-developed examples of the limestoneclay contact deposits. Although little ore could be 

seen when the field work wa.s done (1945), the e.xtensive surface and underground development provides 

numerous exposures that show the deposits in all stages 

of development. Because the Mariposa mine so well 

serves as the type example for the Terlingua district 

of this class of deposit, the descriptions and interpretations that follow, although confined mostly to this 

mine, can be applied to other places in the district. 

The cave-fill, which contains most of the ore, is in 

long, narrow, relatively shallow troughs, that have a 

keel-like downward projection extending to variable 

depths. In cross section the troughs have either a 

U or V shape with a pronounced upward and out~ard 

flaring of the walls. The . cross-section shape may be 

either symmetric or asymmetric; where symmetric, the 

keel extends downward from the center of the trough; 

where asymmetric, the keel extends downward from 

the side of the trough. The keels of the U -shaped 

forms tend to pinch to narrow calcite-lined fractures 

in shorter distances than those of the V -shaped forms. 

The general shape, size, and character of the cave-fill 

zones is shown in figure 15. 

The dimensions of the cave-fill range through wide 

limits. The upward flaring tops are from a few feet 

to more than 200 feet wide, and in places two or more 

cave-fill zones converge to form even wider zones. 

The average width, however, is probably between 40 

and 50 feet. There is little relation between the width 

of the zone and depth to which it extends. Some of 

the wider zones are only 20 or 30 feet thick, whereas 

some of the narrow zones have keels of cave-fill several 

feet in width at depths of more than 100 feet. The 

great majority of cave-fill deposits do not extend 

more than 50 feet into the limestone. Commonly the 

cave-fill deposits are several hundred feet long, and some 

are longer than 2,000 feet. 

The cave-fill zones occur in areas of horizontal or 

gently inclined limestone, and many appear to occupy 

small local synclines. The synclinal structures, however, are solution features and not tectonic features, 

and as such do not extend below the cave-fill. They 

do, however, extend above the cave-fill zones, and in 

places where the Grayson formation has not been eroded 

a narrow synclinal warp may indicate an underlying 

cave-fill. zone. These synclines were formed by the 

solution of limestone just below the base of the Grayson 

formation and a consequent settling of the overlying 

rocks as solution progressed. Although the beds of 

limestone forming the walls of the cave-fill dip inward, 

only rarely can it be shown that their inward dip was 

produced by solution. Solution features in these beds 

are not apparent, because solution proceeded along 

bedding planes in a uniform manner and gradually 

died out away from the cave-fill zone. The synclinal 

structure in the walls of the cave-fill was, therefore, 

produced by a thinning of the limestone beds, which 

gave each bed a wedge-shape. Below the cave-fill 

this thinning of the beds did not occur and the limestone 

has its normal attitude. The development of these 

synclinal structures is shown diagrammatically in 

figure 14. 

Almost all the cave-fill zones occur within the upper 

50 feet of the Devils River limestone. This part of 

the Devils River limestone, which is lighter in color, 

more argillaceous, and thinner-bedded than the rest 

of the formation, is sometimes locally called the 

Georgetown limestone. It weathers more rapidly 

than the underlying limestone;· probably because it is 
56 GEOLOGY AND QUICKSILVER DEPOSITS, TERLINGUA DISTRICT, TEXAS 

I l I I -I I I I L ld <L_._ l I I I I I I I I I IJ/ T I I I I _] I _l _)n I I I I I I I l 

l I I I I I I I I i .I!::? I l I I I _l I 1 .,I 

..!:>.. ,.,... 

I I I I I I \ I I I I 1 l I I I I l I I I l I I I I I I I I I I l ~ 

I I I I I I I I I I 

l I I l I l l I l I I 

I J I I I I I I I I 

A 

8 

c 

FIGURE 14.-Idealized sections showing development of limestone-clay contact deposit. A, Initial stage showing enlargement of solution channel in limestone and incipient slumping of the Grayson formation; B, intermediate stage showing 

filling of solution channel by increased slumping and addition of cave~ebris. Actually the initial and intermediate 

stages are concomitant and cannot be separated. C, Final, or ore stage, showing cinnabar in cave-fill. 
QUICKSILVER DEPOSITS 

MAP 

100 L_ 

SECTIONS THROUGH CAVE FILL 

0 lOOFeet ~--~----~----L---------------- _J 

EXPLANATION 

Map Sections 

-~ Devils River limestone 

Cave fill 

Contact 

Vertical fracture, dashed 

where approximately located 

Boundaries of uneaposed 

cave fill zones 

N 

FIGURE 15.-Map and sections of cave-fill at Mariposa mine. Map of cave-fill sbowin~t distribution, trend, and controlling fractures in Devlla 

River limestone. Limestone and cave-fill covered along east border of map by alluvium. 

57 
58 GEOLOGY AND QUICKSILVER DEPOSITS, TERLINGUA DISTRICT, TEXAS 

more closely jointed and thinner bedded and perhaps 

more soluble. 

The limestone walls bounding the cave-fill are longitudinally fairly straight and smooth, with curved 

rather than angular irregularities. They are, however, 

locally fluted and embayed by small caverns, which 

are usually lined with calcite crystals. In some 

places the walls are also coated with calcite crystals, 

but more commonly they are altered to a soft porous 

limestone rock. Small calcite veins extend from the 

cave-fill out into the limestone. 

The cave-fill is composed mainly of clay and rounded 

to subangular blocks of limestone. Some clay is 

clearly from the Grayson formation that slumped into 

the limestone caverns, other is from the Grayson 

formation that was disintegrated by water and redeposited in the cavern openings, and a small amount 

is residual from the solution of argillaceous limestone. 

Clays of all three origins are found within single cavefill deposits and in some places probably form intimate 

mixtures. The clays range from the dark gray of the 

unaltered Grayson through shades of red and light 

brown to a light cream. Some is banded and the 

original bedding is preserved by the bands, and some 

has concentric bands of red, pink, and tans that are 

unrelated to bedding. Much of the clay contains 

considerable amounts of calcite, which occurs in layers 

and as very small grains. Clay that contains very 

little calcite and is light cream, mottled with pink and 

red or is dark red, is named "jaboncillo" by the Mexican 

miners, because of its soapy feel when wet. The color 

of the clay is mainly from disseminated hematite, 

which was derived, at least in part, from the pyrite 

that is fairly abundapt near the base of the Grayson 

formation. 

Some cave-fill is a breccia, with fragments of limestone and more indurated Grayson formation in a clay 

matrix. Locally breccias are so well developed that 

Turner (1905) classified the cave-fill deposits as "friction 

breccia lodes" and the keels of the cave-fill deposits as 

"calcite lodes." Friction breccia is a misnomer, as 

the breccia was not produced by fault movements, but 

by solution and collapse. The breccia is commonly 

cut by small calcite veins, and gypsum veinlets are 

common in the slumped Grayson. 

Most of the cinnabar and other quicksilver minerals 

that formed the ore occurred. in the cave-fill but a 

little occurred in the limestone of the walls. The ore 

was found in shoots that occupied only a small part of 

the cave-fill zone; only rarely was the ore deposited 

across the entire zone . . In form the ore shoots were 

more irregular than the ca;ve-fill. They were usually 

in line with the keel of the zone and some extended 

down into the keel, in a few places for distances greater 

than 100 feet. The ore shoots were not continuous 

along the length of the cave-fill zone; a few were several 

hundred feet long, but the average was probably less 

than 50 feet. 

It is difficult to estimate the average grade of ore 

of the deposits of this class, because most of them were 

mined early in the history of the district when few 

records were kept. According to Phillips (1905, p. 160) 

the 60-ton Scott furnace of the Marfa and Mariposa 

mine in 1904 was treating ore that contained less than 

1.5 percent mercury and not infrequently treated ore 

that contained as much as 2.0 to 2.5 percent mercury. 

The cinnabar-bearing cave-fill that was furnaced from 

1942 to 1945 ranged from ore containing less than 0.2 

percent mercury to ore taken from small pockets that 

contained almost 50 percent mercury. 

Cinnabar, the most important ore mineral, occurs 

as cloudlike disseminations and as irregular veins in 

the clay and limestone and as nodules and "beds" in 

the clay of the cave-fill. Some cinnabar occurs as 

fillings in openings in the limestone, but most of it 

occurs as replacements of the clay (see pl. 8A) and to 

a lesser extent as replacements of the limestone. 

Generally the cinnabar is so intimately mixed with the 

clay that it megascopically appears to be amorphous, 

but when examined microscopically it is seen to be 

aggregates of small well-developed crystals. Its distribution is controlled by variations in the texture and 

composition of the cave-fill and by minute fractures. 

The Terlingua quicksilver district is as well known 

to mineralogists for the rare minerals that occur there 

as for cinnabar, which is much more abundant. The 

rare mercury minerals occurring at Terlingua are 

almost entirely restricted to the limestone-clay contact deposits and include native mercury, calomel, 

terlinguaite, eglestonite, mosesite, kleinite, and montroydite, which have been described by Hillebrand and 

Schaller (1909) and by Canfield, Hillebrand, and 

,Schaller (1910). These minerals occur in open spaces 

and only rarely is cinnabar found in close association 

with them. 

The other minerals in the cave-fill deposits are mainly 

clay and calcite and minor amounts of barite, fluorite, 

chalcedonic quartz, and pyrite, as well as the previously 

mentioned hematite, limonite, jarosite, and gypsum. 

These minerals are described and their age relations 

discussed in a later section. 

The ore in the calcite vein which form the keels 

of the limeston(l-clay contact deposits differs from 

that in the cave-fill. Except where clay has fallen 

or washed into the solution-enlarged fractures, the· 

cinnabar is associated with banded calcite, where it 

occurs as crusts and films between the successive bands 

and between the calcite and the limestone walls, as 
QUICKSILVER DEPOSI'l'S 59 

well as within fractures in the calcite. The better ore 

occurs as irregular masses of cinnabar replacing the 

calcite. Although the cinnabar in these calcite veins is 

brilliant red, it is much less pure than that in the 

cave-fill because it contains inclusions of fine-grained 

clay and calcite. Almost all the calcite veins in the 

Devils River limestone contain traces of cinnabar, 

but few even moderately large ore shoots have been 

found in them. Some calcite-filled fractures contain 

pockets of clay that fell or was washed into the solution 

cavitv before or during calcite deposition. All such 

pock~ts of cave-fill were potential hosts for ore bodies 

and ore in such pockets is almost always richer than 

that in the calcite veins. 

In some deposits, such as those of the Little Thirtyeight and Colquitt-Tigner mines, the mechanical redistribution of some of the cinnabar since its original 

deposition has caused the lodes to differ considerably 

from those described above. Cinnabar occurs in these 

deposits as detrital grains and nuggets with a rubblelike 

accumulation of water-rounded fragments of limestone 

in a clay and silt matrix. The deposits occupy either 

vertical or horizontal solution caverns in the Devils 

River limestone. The cave silt and rubble in the 

horizontal caverns is usually stratified, and that in 

the vertical caverns is usually an unsorted jumble 

of material of all sizes. 

A vertical, rubble-filled fissure at the Little Thirtyeight mine has been described by Lonsdale (1929, p. 

630-631) as a placer cinnabar deposit, formed by 

surface waters that washed in detritus from deposits 

nearby at higher altitudes. The observations of the 

writers confirm the interpretation of C. P. Ross, who 

believes this deposit to be of somewhat different origin 

than that advanced by Lonsdale. Ross's interpretation 

(written communication) is as follows: 

In mines such as those on sec. 38, Block G-12, the rubble 

and silt partly filling the caves constitutes much of the ore. 

Some of the clastic material is so intimately associated with 

calcite that it must have come to place before or during the deposition of the calcite. Some, however, shows no direct evidence of 

the date of deposition. In some pockets the fill in the bottom is 

reported to have been relatively high in quicksilver, suggesting 

the influence of gravity. Bones of animals of Pliocene or 

Pleistocene age were found in some of the clastic fill of the 

Little Thirty-eight mine (Turner, 1905, p. 275; Lonsdale, 1929, 

p. 629). The animals presumably got into the fill at a time 

when the cavern was open to the surface and long after primary 

ore deposition had ceased. 

Such factors as those above cited led Lonsdale to suggest that 

the ore in the Little Thirty-eight mine is a mere filling of solution 

openings by debris transported by storm waters. * * * It is 

clear from such observation as could be made by the writer. 

from statements of men familiar with the ore, and from the 

data published by Turner (1905) that much ore, at least, was in 

bands of crystalline calcite and clay similar to those of other 

mines in the district, rather than in loose rubble. 

The most probable explanation of the loose or poorly cemented, 

recent rubble locally present is that it represents the residue 

of insoluble and incompletely attacked material resulting from 

solution by groundwater. * * * This residue has been transported and sorted to a greater or less extent. Where erosion 

has provided suitable connection with the surface, the material 

of this sort may well have been added to by debris transported 

by storm water. When the debris happened to have been 

derived from erosion of an outcrop of quicksilver ore, it may 

have added to the content of quicksilver in the cave filling 

sufficiently so that gravitative concentration could produce 

valuable pockets. 

Cinnabar and mercury chlorides that occurred in the 

horizontally bedded cave detritus at the old ColquittTigner mine had a somewhat different origin. The ore 

minerals here appear to have been formed after a large 

fairly horizontal cavern in the Devils River limestone 

was partly filled by cave rubble. This material that 

partly fills the cave is locally well bedded and some beds 

are sugary-grained limestone. All the cave material is 

highly calcareous. At the time of the writers' visit to 

this mine there was no ore visible, but a little cinnabar 

that was seen was a replacement of the cave material 

and consequently of later origin than the filling of the 

cavern. The discoverers of this cavern reportedly 

found heaps of almost pure cinnabar on the top of the 

cave-fill, which was about 20 feet below the roof of the 

cavern. It seems probable that this cinnabar was 

residual after the removal of the upper part of the fill 

by solution and the concentrating action of streams of 

underground water. There is no reason to believe that 

any of the ore was washed in from surface sources. . 

In summary, the limestone-clay contact deposits 

formed in filled solution caverns that occur near the top 

of the Devils River limestone. The caverns are along 

northeasterly trending fractures that served as channels 

for the rising ore-forming solutions. The best ore 

occurs in the upper parts of the cave-fill, near the base 

of the Grayson formation. Locally cave-fill, in places 

ore-bearing, extends down into the solution-widen.ed 

fracture. Most fractures, however, are filled by calcite 

veins which for the most part contain only very small ' amounts of cinnabar. 

DEPOSITS IN CALCITE VEINS IN THE BOQUILLA.S FLAGS 

The Boquillas flags that crop out between the Rainbow and Two-Forty-Eight mines are cut by numerous 

calcite veins and many of these contain cinnabar, but 

only those at the Chisos mine have yiel~ed co~mercial 

ore. The calcite veins range from thrn strrngers of 

calcite less than 1 inch wide to vein zones more than 10 

feet wide. Almost all the cinnabar-bearing veins have 

northeasterly strikes and steep dips. Some occur along 

minor faults with vertical displacements of a few feet, 

but the walls of most show no visible displacement. 

Cinnabar, the only ore mineral, and pyrite occur in the 
60 GEOLOGY AND QUICKSILVER DEPOSITS, TERLINGUA DISTRICT, TEXAS 

vein calcite and in clay seams in the veins and along the 

walls of the veins. 

The calcite veins range greatly in length and thickness; and, as with the calcite veins in the Devils River 

limestone, there is no relation between length and 

thickness. Many veins are less than 10 feet long and 

these have maximum widths of from a fraction of an 

inch to 18 inches or even more, whereas a vein several 

hundred feet long may have an average width of 2 

inches or less. Although veins more than 1,000 feet 

long are not uncommon, the average length is probably 

between 200 feet and 500 feet. A thickness of 3 feet of 

solid calcite is uncommon, thicknesses of 1 foot to 2 feet 

--

---

._..__ __ _ 

4ft 

----------

-----......... __ ---.......... --.._ 

---..... 

--------- FIGURE 16.-Sketch of calcite vein in Boquillas flags in face of drift, 

Chisos mine. Illustrates tendency of veins to split and feather out. 

of calcite are fairly common, and thicknesses of 2 inches 

to 8 inches are common. 

In some places the veins are multiple; that is, a vein 

zone, from 2 feet to 50 feet wide, has been formed from 

numerous, small, subparallel, closely spaced veins and 

stringers. Vein zones are usually local features not 

continuous for any great distance along the strike. 

Commonly the veins that compose them converge to 

form a single vein. A few veins feather out (see fig. 

16); that is, the vein frays out into small stringers. 

Acute angle splits, or forks, in the veins along their 

strikes are not very common, and where they occur the 

v~in that splits off from the main vein is generally a 

short one. Vein intersections or vein crossings, however, are common. Veins of northwesterly trends, 

which are fairly common but very rarely cinnabarbearing, end against northeasterly trending veins or 

northeasterly veins end against them. There is no 

consistent relation between vein trends and terminations or between vein crossings that could be used to 

interpret the relative ages of the two trends. 

The calcite veins are commonly of uniform strike over 

long distances; irregularities are gentle sinuous curves 

rather than sharp angular bends. Variations in dip, 

however, are more pronounced. Dip angles are 

generally high, probably averaging about 70°, but dips 

of 45° are abundant and dips as low as 30° occur. 

Because of the variation in angle and direction of dip 

there are many vein convergences, with two veins 

joining to form a stronger vein. As changes in dip are 

more common and more abrupt than changes in strike, 

there is commonly a split in the vein at the place where 

the change occurs. Common vein patterns are illustrated in figures 17 and 18. 

Although calcite is commonly present along major 

faults, the calcite veins that contain cinnabar are along 

minor faults or along fractures that have no measurable 

displacement. Minor faults with displacements of 

several feet or less are abundant and the veins along 

these faults are in many places associated with fault 

breccia and gouge. Breccia fragments and gouge 

occur both within the vein and along its walls. Some 

veins, in particular those that are cinnabar-bearing, 

have well-developed sheeted structure, with thin seams 

of gouge cutting through brecciated and rehealed 

calcite. Veins that have not been fractured are 

composed of parallel bands of calcite, but the banding 

is not as pronounced as that in the calcite veins in the 

Devils River limestone. 

The calcite in the veins has the characteristics of 

calcite deposited in open fractures. The veins are -

commonly banded and contain vugs and unoriented 

inclusions of the country rock. The calcite ranges 

from finely-crystalline to coarsely-crystalline. Terminal 

parts of the calcite crystals are found extending into 

the now open parts of the veins and can be seen as 

phantoms in the filled parts of the veins. Modified 

scalenohedrons are most common, but rhombohedral 

forms are fairly abundant. The color is commonly 

light-gray or yellowish brown, but some veins contain 

snow-white or pale apple-green calcite. The gray is 

probably from inclusions of fine clay and the yellow 

brown from inclusions of limonite and hydrocarbon 

compounds. Some calcite contains sufficient hydrocarbon to exude the odor of petroleum froin freshly 

crushed crystals. The gouge in and along the veins 
QUICKSILVER DEPOSITS 61 

FIGURE 17.-Map showing calcite vein pattern in Boquillas flags of Brown prospect, sec. 286, Block G-4. 

also contains hydrocarbon compounds, which locally 

are abundant enough to ignite before the flame of a 

miner's lamp. 

The banding of the calcite, although typical of the 

veins, is not always conspicuous. Some veins have a 

pronounced banding, with striking contrasts between 

the color and grain size of the constituent bands, but 

more commonly the banding is only noticeable through 

close inspection. The inconspicuously banded veins 

have only minor variations in grain size and color of 

the bands. In general, the last formed bands are more 

coarsely crystalline. The banding is not always parallel 

to the walls of the vein. Some bands are lenslike and 

some extend through earlier formed bands at acute 

angles, apparently following fractures that were opened 

after the deposition of earlier calcite. 

Cinnabar is the only mercury mineral that has been 

found associated with calcite veins in the Boquillas 

flags or the Buda limestone. It occurs in very finely 

crystalline aggregates replacing brecciated calcite and 

clay gouge in the veins and to a much lesser extent as 

dissemination in the flaggy limestone of the walls. It 

also fills small fracture openings. The veins that 

contain cinnabar generally have a higher development 

of brecciated calcite and sheeted structure than the 

barren veins. Hydrocarbons, as tar and oil, and pyrite 
62 GEOLOGY AND QUICKSILVER DEPOSITS, TERLINGUA DISTRICT, TEXAS 

, / 

~ , 

N 

I 

I 

I 

/ 

/ 

I 

I 

I 

I 

I 

I 

~ ----

0 100 Feet 

FIGURE 18.-Map showing calcite vein pattern in Boquillas flags southeast of Susano shaft, Chisos mine. 

are associated with the cinnabar, but also occur in parts 

of the veins where cinnabar is absent. 

Pyrite is abundant in the calcite veins, but is only 

rarely seen in outcrop. Even as far below the surface 

as the 200-foot level of the Chisos mine, it is partly 

oxidized to hydrous iron oxides. Much oxidized pyrite 

seen in underground workings may have been oxidized 

after the mine was opened, because melanterite and 

other hydrous sulfates coat the mine walls in many 

places. That the normal depth of oxidized pyrite is 

not so deep as the mine workings suggest, is indicated 

by diamond-drill cores taken from Boquillas flags just 

east of the Chisos mine. These cores contain abundant 

fresh pyrite from a few feet of the surface to the base of 

the Boquillas flags. The pyrite occurs as fillings in the 

calcite veins, as disseminated grains in the calcite, and as 

veinlets and disseminated grains in the wall rocks. Disseminated pyrite is more abundant in the calcareous 

beds than in the argillaceous. 

Some calcite veins extend through the Boquillas flags 

and down into the Buda limestone, but very litt.le ore 

has been found in calcite veins in this lower formation. 

Few fractures in the Buda limestone are completely 

filled with calcite; most are only partly filled and some 

have been enlarged by solution to vertim1l caverns 

several feet wide. In general, the calcite in veius in the 

Buda is whiter and more coarsely crystalline than tha~ 

in veins in the Boquillas flags. 

A related but different kind 'of ore body, is the rich 

550 ore body of the Chisos mine, which is described in 

detail on page 103. This ore body was in .a breccia 

zone at the intersection of the Buda-Boquillas contact 

with the Chisos fault.. Although calcite was only . of 

minor importance, this deposit is included under the 

calcite vein deposits because ·it graded upwards into 

calcite veins in the Boquillas flags. , . 

Almost all the calcite veins end at the base of the Buda. 

limestone or extend only a few feet into the underl}ring 
QUICKSILVER DEPOSITS 63 

Grayson formation. A few calcite veins, however, do 

extend into and through the Grayson formation, but 

they are negligible in abundance and size when compared to those in the Boquillas flags. A vein 2 inches 

thick is uncommon, and even these are not persistent, 

but pinch out in short distances. Calcite veins in the 

Grayson are rarely directly linked to those in the 

Boquillas flags and Devils River limestone. Exceptions 

to this are those along the larger faults. 

In summary, the Devils River limestone and Boquillas 

flags contain numerous calcite veins that are well developed, whereas the Grayson formation contains few 

calcite veins and these are poorly developed. 

DEPOSITS IN BRECCIA PIPES 

Cinnabar has been found in many places in the Terlingua district and has been mined from pipes at the 

Chisos mine, the Two-Forty-Eight mine, and Bobs 

mine. The Chisos pipe contained the most valuable ore 

body ever found in the Terlingua district. Ore found in 

the Two-Forty-Eight pipe and Maggie Sink pipe (Bobs 

mine) has been in small, but fairly rich, pockets. 

The distribution, character, and origin of the breccia 

pipes are given in a preceding section (p. 44-47). Details of individual pipes are given under the mine 

descriptions in the final part of this report. 

Ore that has been found within the breccia pipes either 

fills the pipe locally, as in the Chisos pipe, or is in small 

shoots along or near the bord-ers of the pipe, as in the 

Two-Forty-Eight and Maggie Sink pipes. Intermediate 

types may exist, but none has yet been found. 

The Chisos pipe, beyond containing the largest and 

richest ore body, is unique in the Terlingua district. 

It is the only pipe that does not extend to the surface, 

and it is the only pipe that contained an ore body 

extending from wall to wall. Its uniqueness as a pipe 

that did not extend to the surface is perhaps more 

apparent than real; such pipes could be discovered only 

by subsurface exploration, which has been done in an 

area that is trivial in contrast to the total area of the 

district. The ore body in the Chisos pipe is much 

greater than those found in the other pipes. 

Although the shape, size, and structural relations 

of the Chisos pipe can be determined from the mine 

workings, the character of the breccia and ore can not 

be directly observed. The ore body was mined during 

the first World War period, and was so thoroughly 

removed that in 1942 the writers could find only traces 

of cinnabar in the stope walls. The former Chisos 

Mining Co. had a long established policy of secrecy, 

which has resulted in a great dearth of published 

information on the mine and its ore. In spite of this, a 

fair concept of the character of the ore and breccia can 

be pieced together from information received from those 

who saw the pipe during its exploitation, from observations along the stope walls, and from comparisons with 

other breccia pipes. The following summary description is based upon such information. For a more detailed description the reader is referred to pages 42 and 103. 

The ore body, roughly circular in plan, had a maximum diameter of about 70 feet in its upper part and 

extended vertically for about 200 feet, pinching to a 

diameter of less than 10 feet. It thus had the general 

shape of a stubby carrot. It filled almost the entire 

upper part of the enclosing breccia pipe and in places the 

ore extended out into brecciated wall rock that had not 

collapsed. The lower part of the ore body was entirely 

within the breccia pipe and occupied only about 10 

percent of the volume of this part of the pipe. Although 

the ore body appears to end 200 feet below its top, it 

may have a discontinuous "tap root" extension to 

greater depths, because cinnabar has been found below 

the bottom of the stope. 

The part of the pipe containing the ore body is 

enclosed in Buda limestone and Grayson formation. 

The breccia host of the upper part of the ore body 

consisted of blocks and fragments of Buda limestone 

and Boquillas flags partly mortared with clay from 

the Boquillas flags; that of the lower part consisted of 

fragments and blocks of Buda limestone in a matrix 

of the Grayson formation. The breccia was most open 

near the top of the pipe. The ore consisted of cinnabar 

and minor quantities of calcite, iron oxides, and pyrite. 

The cinnabar was mainly a replacement of the clay 

matrix. 

Ore found in the Two-Forty-Eight and Maggie Sink 

pipes was in parts of those pipes enclosed in rocks 

stratigraphically higher and lower, respectively, than 

the rocks that enclose the Chisos pipe. Both the 

Two-Forty-Eight and Maggie Sink pipes crop out at the 

surface and formerly extended into rocks now removed 

by erosion. It is problematical whether their upward 

growth was arrested before they reached a former 

surface, as that of the Chisos pipe. If their growth 

ended under a rock cover, they may have had ore bodies 

at their apices similar to that of the Chisos pipe. 

The breccia in these pipes, although not entirely 

derived from the same rocks, is similar to that in the 

Chisos pipe: rock blocks and fragments in a clay 

matrix. The breccia in the Two-Forty-Eight pipe is 

in the upper part of the pipe, is open and porous, and 

contains considerable tar. The cinnabar was mainly 

in the clay matrix. 

The greatest differences between the ore bodies in the 

Two-Forty-Eight and Maggie Sink pipes and the ore 

body in the Chisos pipe are in size and structural setting. 

Only pockets of ore have been found in the Maggie 

Sink and Two-Forty-Eight pipes. The Chisos ore 
64 GEOLOGY AND QUICKSILVER DEPOSITS, TERLINGUA DISTRICT, TEXAS 

body occupied a central location in the pipe, whereas 

the ore found in the other pipes occupied marginal 

locations. Ore was found in the Two-Forty-Eight pipe 

either along the border of the breccia or along blocks 

partly detached from the walls of the pipe; that in the 

Maggie Sink pipe has been found either at the contact 

of the breccia with the walls or a short distance within 

the pipe. In both pipes the ore bodies are irregular 

in shape and probably vertically elongate, although 

they have not been explored thoroughly enough to 

prove this. It may be that small irregular ore shoots 

such as the above are also present in the deeper, unexplored parts of the Chisos pipe. If this is so, it could 

be interpreted to mean that the most favorable place 

for ore in a pipe is near its upward termination. However, if a pipe is in rocks of Grayson or younger age 

and contains even a small quantity of cinnabar, it 

seems possible that the pipe may be connected at 

depth with a limestone-clay contact deposit. 

DEPOSITS IN IGNEOUS ROCK 

Cinnabar has been found in igneous rock in several 

places in the Terlingua district but, with the outstanding exception of the Study Butte mine, the occurrences 

have been commercially unimportant. No one kind of 

igneous rock is a more favorable host than any other, 

because cinnabar has been found in quartz soda syenite 

at Study Butte, in analcite syenogabbro at the TwoForty-Eight mine, in soda latite at the Mariposa mine, 

and in rhyolite at Contrabando dome. At these places 

and in these igneous rocks cinnabar fills open spaces and 

replaces brecciated rock. The cinnabar deposits in 

igneous rock ar~ associated with only small amounts 

of introduced calcite and accordingly contrast with the 

deposits in calcite veins in the Bo;quillas flags. Their 

structural settings are in sharp contrast to those of the 

limestone-clay contact deposits. 

In all places where cinnabar has been found in igneous 

rock it has been associated with fractures. The fractures were formed both contemporaneously with and 

after the cooling of the igneous rock. The postcooling 

fractures were far more important to ore deposition. 

Fractures formed in the igneous rocks after the rocks 

cooled are of the same northeasterly trends as the orebearing structures in the sedilnentary rocks, whereas 

those formed during the cooling of the igneous rocks are 

unrelated to any regional trends. Intrusive rocks in 

which cinnabar has been found at both Contrabando 

dome and the Mariposa mine were themselves emplaced 

along preexisting fractures, also of northeasterly trends. 

Renewed movements on these fractures after intrusion 

produced the cinnabar-bearing structures. At the 

Mariposa mine the contact between igneous rock and 

limestone was a controlling structure, but this is also a 

preintrusion structure. 

Postintrusion fractures at Study Butte had greater 

influence on the localization of ore than fractures contemporaneous with the cooling. Here again, as at 

Contrabando dome, these postintrusion fractures may 

have resulted from renewed movements along fractures 

of preintrusion age. The postmtrusion fractures extend for long distances through the igneous rock and 

are unrelated to the contacts, whereas contemporaneous 

fractures are not persistent and are directly related to 

the contacts. The ore-bearing fractures of Study 

Butte are described on p. 107-108. 

As in the limeston'e..:clay contact deposits and calcite 

vein deposits, cinnabar is the principal ore mineral and 

fills open spaces and replaces wall rock. Open-space 

filling was quantitatively more important than replacement. The open spaces that have been filled range 

from openings the thickness of a knife edge to those 

about 1 inch wide. In most places the cinnabar incompletely fills the opening as a crust on both walls, 

with the center of the opening unfilled. Cinnabar 

formed by replacement is rarely found farther than 1 

inch from filled or partly filled fractures. It was seen 

as replacements of both felsic and mafic minerals, but 

far more commonly is a replacement of clay formed 

from the alteration of feldspar. Feldspars are commonly altered to clay minerals along the mineralized 

fractures, but this is not universal and only rarely does 

the alteration extend more than a few inches from the 

fracture. 

Introduced minerals associated with the cinnabar are 

pyrite, e'alcite, and hydrocarbon compounds, all minerals common in the other classes of deposits. With the 

exception of hydrocarbons, the relative abundance of 

these minerals, however, is different. Pyrite is more 

abundant than calcite, a ratio strongly in contrast with 

the relative abundance of these two minerals in the 

other classes of deposits. The ratio of pyrite to cinnabar, however, is possibly no greater; thus the principal 

difference between the mineral composition of thes'e deposits and that of the others is that they are relatively 

deficient in introduced calcite. As the limestone-clay 

contact deposits and calcite vein deposits are in calcareous rocks, the above relations indicate that the 

composition of the wall rocks influences the composition 

of the gangue minerals. 

MINERALS AND MINERALIZATION HISTORY 

The quicksilver deposits were formed late in the geologic history of the Terlingua district. Mineralization-a term here used to include the closely related 

processes that introduced the ore and gangue minerals 
QUICKSILVER DEPOSITS 65 

and altered the wall rocks-took place after the major 

structural movements, after the volcanic activity, and 

after the emplacement of the intrusive igneous rocks. 

Geologic activity after the end of mineralization has 

been principally erosion and oxidation of the rocks by 

superficial agencies; fault movements were negligible 

and can be measured in inches. Although the time 

relations between mineralization, structural movements, and igneous activity have been set forth in preceding sections, they are summarized below, in order 

to establish freshly in the reader's mind the sequence of 

geologic events that determined the environment for 

mineralization. 

The establishing of the environment for quicksilver 

mineralization begins properly with the deposition of 

the Cretaceous sedimentary rocks, which were to 

become the principal hosts to the quicksilver minerals. 

Towards the close of the Cretaceous, the stable conditions that produced the uniform marine limestones 

and clays ended, and the late Cretaceous deposits are 

crossbedded sandstone, coal beds, and red and black 

clays that contain abundant fossil wood and the bones 

of land animals. This change from marine to continental conditions was accompanied by volcanic activity, 

which became intense during the deposition of the Tornillo clay formation and reached its crest with the deposition of the Chisos volcanics. Before the close of volcanism there had been enough structural deformation 

to produce the Terlingua uplift and its attendant south 

flank, the Terlingua monocline. The graben faults and 

the system of northeasterly fractures were at least outlined during this period of deformation. The great 

development of the major faults, however, was after 

volcanism and before the intrusion of the exposed 

igneous rocks. Likewise the host structures of the ore 

deposits, the cave-fill zones and breccia pipes, were 

formed after volcanism and before igneous intrusion. 

After the beginning of igneous intrusion there were but 

few major movements on the graben faults. On the 

other hand the movements that had produced the northeasterly fractures persisted, old fractures reopened and 

new ones formed. These movements continued throughout mineralization but were in effect over by its final 

stages as there is almost no postcinnabar movement. 

MINERAL DESCRIPTIONS 

Mineralization did not introduce a great variety of 

minerals, but it did introduce a few rare minerals, as 

well as several that are common to all the deposits. 

Of the less than 30 epigenetic minerals (tab. 12) that 

have been found in the quicksilver deposits, at least 5 

are supergene and only 6 are abundant and common to 

all deposits. The rest includes rare mercury minerals 

and several minerals that are not necessarily related to 

the quicksilver mineralization. The analcite in the 

Two-Forty-Eight mine, and possibly the natrolite in 

the Fresno mine, although of hydrothermal origin, were 

formed probably independently of the quicksilver minerTABLE 12.-Minerals of the quicksilver deposits 

Class and mineral Composition Distribution Abundance Remarks 

Native elements ___ mercury _________ Hg ___________________ restricted ___ minor ____ _ Fo'-lnd mainly in the limestonecontact deposits associatedwith 

chloride minerals. 

Sulfides __________ _ cinnabar _______ _ pyrite __________ _ 

marcasite _______ _ Haloids __________ _ calomel ________ _ kleinite ____ __ ___ _ mosesite ________ _ fluorite _________ _ 

eglestonite ____ __ _ terling11aite _____ _ Oxides ____ _______ _ quartz and 

chalcedony hematite _______ _ limonite ________ _ 

montroydite ____ _ 

Carbonates ______ _ calcite _________ _ aragonite _______ _ Silicates ___ ·- _____ _ natrolite ________ _ analcite ________ _ kaolinite _______ _ Sulfates _________ _ barite __________ _ 

anhydrite _______ _ gypsum ________ _ 

melanterite _____ _ epsomite _______ _ alunite _________ _ jarosite _________ _ Hydrocarbons ____ _ tars and oils ____ _ 

1 Composition uncertain. 

HgS ____________ ____ _ _ 

FeS2------------ --- --- FeS2------------ -- ---- HgCL _______________ _ 

Hg, NH3 chloride 1 ____ _ 

Hg, NH3, Cl, S03, H01 __ 

CaF 2- _____________ ___ _ 

H g4 CbO ___ - _ - _ - _ - __ - - - Hg2ClO _____ _________ _ 

Si02------------------

Fe20a------ -------- --- Fe20a+ H20- __________ _ HgO ______ _____ ___ ___ _ 

CaCOa _____ __________ _ CaCOa- ______________ _ 

N a2Al2SiaOJO.H20 ___ --- _ NaA1Si2 06.HzO _____ -- __ Alz(Siz05) (OH)4 _____ --- BaS04 _______ ________ _ CaS04 _______________ _ 

CaS04 ·2H20 _________ - _ FeS04 ·7Hz0 _________ - _ 

MgS04 ·7H20 ___ - _- _----

KzAlo(OH)Iz(S04)4------

K2Feo(OH)Ia(S04)4------

widespread _ _____ do ___ _ 

restricted? __ restricted __ _ _ ____ do ___ _ _ ____ do ___ _ _ ____ do ___ _ _ ____ do ___ _ _____ do ___ _ _____ do ___ _ 

widespread _ _ ____ do ___ _ 

restricted __ _ 

widespread _ restricted __ _ _ ____ do ___ _ 

_____ do ___ _ 

widespread _ restricted __ _ _ ____ do ___ _ 

widespread _ restricted __ _ _ ____ do ___ _ _____ do ___ _ 

widespread _ _____ do ___ _ 

abundant __ _ ____ do __ _ 

minor_____ Not easily recognized. 

~~~e~~-~:== = )Restricted to limestone-clay con- minor_____ tact deposits. rare _____ -- _ ____ do __ _ 

minor ____ _ 

abundant __ _____ do___ Oxidation by surficial agencies. 

rare_______ Restricted to limestone-clay con- tact deposits. 

abundant __ minor ____ _ 

rare ______ _ _ ____ do __ _ Only at Fresno mine. 

Only at Two-Forty-Eight mine. 

abundant __ minor ____ _ rare ______ _ 

abundant __ minor ____ _ rare ______ _ _ ____ do __ _ 

minor ____ _ _ ____ do __ _ 
66 GEOLOGY AND QUICKSILVER DEPOSITS, TERLINGUA DISTRICT, TEXAS 

alization. The barite and fluorite of the limestone-clay 

contact deposits may or may not be closely related to 

the quicksilver mineralization. On the other hand, 

the almost constant association of cinnabar, pyrite, 

calcite, kaolinite, and hydrocarbons clearly indicates 

that these minerals share a common origin and are a 

direct result of the processes of the quicksilver mineralization. 

The following mineral descriptions emphasize features 

that illustrate the mode and time of emplacement of 

the various minerals and are not detailed descriptions 

of mineral appearances and properties-or even summaries of their characteristics. For such descriptions 

the interested reader is referred to the standard text 

books on mineralogy and also to papers by Hillebrand 

and Schaller (1909), Canfield (Canfield, Hillebrand and 

Schaller, 1910), and Moses (1903), who have so 

excellently described the rare_ quicksilver minerals. 

CALCITE 

Not only is calcite the most common mineral in the 

Terlingua district, but it is present in the greatest 

variety of forms. It is in all the deposits; it replaces 

all kinds of rock and fills solution and fracture openings 

and is found as amygdules in the volcanic and intrusive 

igneous rocks. It is in almost countless modifications 

of its two basic crystal forms, the rhombohedron and 

scalenohedron. It has many color variations, resulting 

from impurities of iron, manganese, and hydrocarbons. 

The grain size of the calcite ranges from microcrystalline aggregates in the Grayson formation to 

scalenohedrons more than 12 inches long that line 

solution openings in the Devils River limestone. In 

general, the calcite that formed in open spaces is 

more coarsely crystalline than the calcite that replaces 

other minerals. The calcite in limestone along vein 

boundaries and that in hydrothermally altered Grayson 

formation commonly is recrystallized and in larger 

grains. This recrystallized calcite is strikingly evident 

microscopically in thin sections of the Grayson formation, which contain rhombs of calcite consisting of a 

slightly murky calcite core surrounded by an outer 

zone of clear calcite, separated from the core by a 

zone of iron oxide. 

The calcite crystals in the filled veins have very well 

developed terminal faces, representing both rhombohedral and scalenohedral forms and numerous varieties 

of these. Both rhombohedral and the somewhat more 

abundant scalenohedral forms occur in the same veins. 

Variations in the crystal forms suggest repeated changes 

in the physical and chemical conditions of precipitation 

of the calcite. 

Most calcite ranges from white to gray, but a 

considerable quantity is light tan to yellow-brown and 

a much smaller quantity is black, red, pink, and pale 

green. The colored varieties, in particular the tans 

and yellow-browns, are more commonly associated with 

cinnabar than the white varieties. Most yellow and 

brown varieties receive their color from inclusions of 

hydrocarbons, which are so abundant in some d~rkbrown calcite that crushed crystals exude a strong 

petroleum odor. The red varieties are dusted with 

fine hematite. The black varieties are colored by 

manganese oxide, and the pink and pale green varieties 

are probably also colored · by manganese compounds. 

Ross found that all the calcite seen in the quicksilver 

deposits is slightly manganiferous and is in contrast 

to the rare stalactitic calcite of the caverns, which 

contain neither manganese nor hydrocarbon. 

Calcite was deposited throughout mineralization, 

but the great bulk of it was deposited before the ore 

minerals. A little, however, is forming today as 

stalactites in caves, and some formed before the 

beginning of quicksilver mineralization as a postmagmatic mineral in the igneous rocks. All the other 

minerals replace it and many are replaced by it. 

Although calcite crystals have been found that contain 

cinnabar, the common relations are cinnabar replacing 

calcite, cinnabar veinlets cutting calcite, and cinnabar 

crystals crusting calcite. Because calcite was deposited 

throughout mineralization it is not a good index for 

the determination of age relations between minerals. 

It is, however, a mineral that excellently demonstrates 

the recurrence of structural movements along solution 

channels during mineralization. It has been repeat~dly 

sheared and brecciated, healed, resheared and rebrecciated, and rehealed. 

CLAY MINERALS 

Clay minerals are the most abundant gangue material 

in the limestone-clay contact deposits and are in lesser 

amounts in the other classes of deposits. Although 

clay minerals are indigenous to the limestone-clay 

contact deposits, the clays that are intimately associated 

with the ore minerals are believed to be reconstitutions 

of these original sedimentary clays. Determinations 

by C. S. Ross of the clay minerals in a few samples 

indicate that the clays in the unaltered rocks belong 

to the beidellite group and those in the altered rocks 

to the kaolinite group. No precise mineralogic or 

chemical study of the clay minerals was made, consequently conclusions about their occurrence must be 

very general. These kaolinite clays are the principal 

constituent of the jaboncillo, which has been described 

megascopically in a preceding section, and are believed 

to be not introduced, but reconstituted from the original 

sedimentary clays. 

Jaboncillo, when seen in thin section, is a very fine 
GEOLOGICAL SURVEY 

A. DEVILS RIVER LIMESTONE WITH CINNABAR REPLACING 

CALCITE (GRAY) 

Plain light, X 30. 

C. CINNABAR REPLACING CLAY, MARIPOSA MINE 

Clay (gray) contains minute scales of calcite. Plain light, X 130. 

PROFESSIONAL PAPER 312 PLATE 9 

B. CINNABAR REPLACING CALCAREOUS CLAY MATRIX OF 

TWO.FORTY-EIGHT BRECCIA PIPE 

Plain light, X 30. 

D. CINNABAR REPLACING CLAY 

Specimen from dump of Little Thirty -Eight mine. Sketch, X 270. 

PHOTOMICROGRAPHS AND SKETCH FROM PHOTOMICROGRAPH OF THIN SECTIONS OF CINNABAR ORE 

QUICKSILVER DEPOSITS 67 

grained rock composed mainly of tiny flakes of clay 

crystals, numerous small grains of quartz, and minute 

specks of hem.atite, and, if metallized, clusters of small 

cinnabar crystals. In the unaltered Grayson the clay 

is opaque under polarized light; that in the jaboncillo 

is sufficiently crystallized to have a "salt and pepper" 

appearance. The indices of refraction of the clay 

mineral are greater than that of canada balsam (1.54), 

as are those of the kaolinite group. The foregoing 

description is of jaboncillo forrn.ed from the Grayson 

formation from which the carbonate has been completely removed, and accordingly represents the maximum of alteration. Commonly alteration is not so 

nearly complete; partly altered Grayson formation may 

contain 20 percent or more of soluble carbonate. In 

order to evaluate the chemical changes involved in the 

formation of jaboncillo, C. P. Ross selected samples of 

this material and of the rock from which it appeared 

to have been derived and had them analyzed. These 

analyses, along with those samples collected by the 

writers are shown in table 13. 

The analyses bring out differences in composition, 

both between and among the altered and unaltered 

clay of the Grayson formation, that require interpretation. A comparison of the analyses of samples 

1, 3, and 5 demonstrates that there is considerable 

variation from place to place in the composition of 

the unaltered clay. Therefore, it cannot be assumed 

TABLE 13.-Analyses of altered and unaltered clay from the 

Grayson formation 

(1) (2) (3) (4) (5) 

Unaltered Altered Unaltered Altered Unaltered 

Si02----------- 39. 76 63.10 27.38 20.60 45.03 

Ab03---------- 14.29 20. 84 9. 51 7. 02 16. 23 Fe203_________ 1.79 6.30 4.13 4.89 3.04 Feo___________ 1. 73 .14 _______________________________________ _ 

Ti02---------- . 60 .63 1.78 1.60 1.12 

MnQ___ _______ ______________ __ __ ________ . 07 .14 -------------- CaQ __ ________ 17.49 . 02 22.40 27.03 13. 00 

MgQ __ ________ 1.30 .02 4.75 5.94 2. 15 N a20---------- . 42 . 36 __________________________ --------------

K20 ----------- 1. 32 1. 00 -------------- ------------ 2.10 

H20 ---- ------- 5. 46 7. 87 I 4. 54 I 4. 66 6. 25 s______________ ______________ __ __________ 21.66 2. 08 _____________ _ 

C02----------- 15.85 none 22. 77 27.96 11.94 

Hg______ ______ none . 08 none a<. 001 --------------

CL____________ . 05 .17 -------------- ------------ --------------

100.06 100.53 98. 99 L.62 

98.37 

99.89 100. 86 

I Water by Penfield method, uncertain because hydrocarbons known to be present. 2 Total sulfur reported as S. Acid soluble sulfates present in traces only. z Fluorescent screen test for mercury by J. J. Fahey. 

'Less 0=8. 

(1) Unaltered Grayson formation collected by C. P. Ross from the 50-foot level 

of the Mariposa mine. Analyzed by J. J. Fahey. 

(2) Altered Grayson formation (jaboncillo) collected by C. P. Ross from the por- tal of the No. 5 tunnel of the Mariposa mine. Analyzed by J. J. Fahey. 

(3) Unaltered Grayson formation clay collected by R. G. Yates and G .A. Thompson 

from a bed (same bed as sample 4) 3 feet above the base of the formation on the 50-foot 

level of the Mariposa mine at coordinates 100 S. and 500 E. Analyzed by Marie L. Lindberg. 

(4) Altered Grayson formation collected by R. G. Yates and G. A. Thompson from 

the same bed as sample 3 and 20 feet closer to a cave-fill zone. Analyzed by Marie L. Lindberg. 

(5) Unaltered Grayson formation collected by C. P. Ross from the road to the 

Mariposa mine in sec. 39, block G-12. Analyzed by Charles Milton. 

that sample 2 was derived from a clay chemically 

equivalent to sample 1, because the locations of the 

two sample localities are stratigraphically unrelated. 

Yet, it can be assumed with some assurance that 

sample 4 was derived from a clay chemically similar 

to sample 3, because these samples were collected 

from the same bed within a horizontal distance of 

20 feet. All three unaltered clays are calcareous; but 

sample 1 contains 35 percent by weight of carbonates, 

sample 3, 50 percent, and sample 5, 27 percent. The 

remainders are largely clay and quartz, representing 

the alumina and silica of the analyses. 

Despite the great differences between the quantities 

of alumina and silica in the rocks, the ratios of alumina 

to silica are fairly constant throughout both altered 

and unaltered rocks, as can be seen from the following 

tabulation. 

Sample Al20a Si02 Average 

1 14.29:39.76 or 1:2.57 } 3 9.51:27.38 or 1:2.86 1:2.73 (Unaltered rock) 

5 16.23:45.03 or 1:2.76 

2 20.84: 63.10 or 1: 3.02 } 1:2.98 (Altered rock) 4 7.02:20.60 or 1:2.93 

The tabulation also shows that there is an average 

increase of about 9 percent in the quantity of silica 

relative to alumina in the altered rock over that of the 

unaltered rock. As all the alumina and only part of the 

silica are combined as clay minerals, in the ratio of 

1 alumina to 1.18 silica in kaolinite in the altered rock 

and 1 alumina to 1.77 silica in beidellite in the unaltered rock, there is a far greater amount of free 

silica, as quartz, in the altered rock. Although the 

change frorn beidellite to kaolinite produces a decrease 

in the combined silica, making more silica available 

for quartz, it does not change the alumina :silica ratio 

of the rock unless there is an addition or subtraction 

of silica or alumina from the system. 

It is obvious from a comparison of the analyses of 

samples 2 and 4 that these altered rocks are very 

unlike and. therefore may have been produced by 

different processes. Sample 2 is a highly altered rock 

and sample 4 a mildly altered rock. Furthermore, the 

changes of alteration are different; carbonate was 

com.pletely extracted from sample 2 and carbonate 

was addeq to sample 4; 20 percent over that in its 

unaltered equivalent. This may mean that carbonate 

gained by sample 4 is carbonate lost during the production of a rock similar to sample 2. That is, sample 

4 is not a progressive step towards jaboncillo, but is an 

"aureole rock" enriched in carbonate given off from 

centers of 1nore intense alteration. 1 The above interpretation is favored both by the 

analyses of samples 3 and 4 and by what can be seen 
68 GEOLOGY AND QUICKSILVER DEPOSITS, TERLINGUA DISTRICT, TEXAS 

in thin sections of sample 4. A thin section of sample 4 

shows that carbonate replaced the clay. If the replacement process involved either the extraction of the 

total clay molecule or only alumina from the clay 

mineral, there would be a considerable change in the 

alumina: silica ratio of the rock, because about 40 

percent of the total silica in the unaltered rock is in 

the form of quartz. However, if the replacement 

process involved the nonpreferential extraction of the 

total clay molecule and the quartz, which is_intimately 

mixed with the clay as grains for the most part too 

fine to be resolved by the microscope, there would be 

no change in the alumina :silica ratio. This, apparently, 

is what happened; because the difference between the 

alumina: silica ratios of samples 3 and 4 is less than 

2 percent--a difference that could well be a prealteration difference in the compositions of the two samples. 

Because there is no appreciable difference in the 

alumina: silica ratios of samples 3 and 4, it is not possible 

to tell by the analyses whether the clay in sample 4 

is mineralogically different from sample 3. If there has 

been either a complete or partial change from beidellite 

to kaolinite, the silica that was lost from the beidellite 

did not migrate from the rock, but is present as quartz, 

with a consequent increase in the free silica present. 

SILICA MINERALS 

Epigenetic silica, as quartz and chalcedonic quartz, 

is present in small quantities in the limestone-clay 

contact deposits, but it is not present in the calcite vein 

deposits in the Boquillas flags nor in the deposits in 

igneous rocks, with the exception of the occurrences of 

cinnabar in silicified rhyolite in Lowes Valley and Contrabando dome. 

Irregular, elongate masses of jaspery silica occur as 

replacements of Devils River limestone at several 

places between the Mariposa mine and Black Mesa. 

They are small, being rarely more than 5 feet wide and 

30 feet long. They are oriented along fractures in the 

limestone and are not to be confused with the chert 

nodules that are locally abundant in the same limestone. 

They consist of fine-grained quartz and hematite dust. 

No cinnabar has been found in association with these 

jaspery masses. 

Locally the cave-filJ is partly silicified to a moderately 

hard, coherent rock. The silica is in the form of microcrystalline quartz erratically distributed through the 

rock. No quartz veinlets or vugs were observed. 

Silica minerals are more abundant on the west side 

of Lowes Valley than elsewhere in the district. Here 

both rhyolite and Devils River limestone are completely silicified, in some places so thoroughly that it is 

difficult to determine the original rock. The silica 

mineral is quartz, which ranges from cryptocrystalline 

to moderately coarse grained. Crosscutting veinlets of 

quartz of several generations are common. Cinnabar 

occurs along fractures in the quartz. 

IRON MINERALS 

The iron sulfides pyrite and marcasite and the iron 

oxides hematite and limonite are widely distributed in 

the Terlingua district. The oxides are most abundant 

in the limestone-clay contact deposits and the sulfides 

in the other deposits. Although marcasite was recognized at only a few places it may be fairly common, 

but at best it is much less abundant than pyrite. Most 

iron oxide is hydrous, but some has the crystal form and 

streak of hematite. 

Pyrite, of both hydrothermal and authigenic sedimentary origin, is common. Hydrothermal pyrite is 

particularly abundant in the veins in igneous rock at 

Study Butte. Because it oxidizes rapidly, pyrite is 

not particularly evident on the surface or on the walls 

of old mine workings, but unweathered rock of diamonddrill cores taken from both the Boquillas flags and 

Grayson formation contains abundant pyrite. Its former abundance is attested to in many places by pseudomorphs of iron oxide after pyrite. 

It is difficult to tell how much pyrite is authigenic 

sedimentary and how much was introduced hydrothermally during mineralization. The pyrite that is 

almost everywhere near the base of the Grayson formation is unquestionably in part authigenic, especially 

that which is in nodular and concretionary masses. 

Conversely, that which is in veins in igneous rock and 

the Boquillas flags is just as unquestionably hydrothermal. But some pyrite, as for example, disseminated 

cubic crystals in Devils River limestone and Boquillas 

flags, can be interpreted as having formed either hydrothermally or diagenetically with the enclosing sediments. 

In general, pyrite is most abundant in places where 

there is evidence of hydrothermal mineralization. 

Pyrite is older than cinnabar in almost all cases 

where the two minerals were observed in determinable 

paragenetic relations. The one exception noted is an 

incrustation of pyrite on cinnabar that lined a fracture 

in igneous rock at the Study Butte mine. Elsewhere, 

cinnabar was seen as crusts on pyrite and as veinlets 

that cut through pyrite. Cinnabar was not observed, 

however, as a replacement of pyrite. 

Marcasite was recognized, by its crystal form, at 

only the Chisos and Study Butte mines and not in 

sufficient abundance to determine its precise relations 

to the other minerals. Its association with hydrothermal minerals suggests that it is also such a mineral 

and may have been introduced about the same time 

as the pyrite. 
QUICKSILVER DEPOSITS 69 

Hematite and hydrous iron oxides are particularly 

abundant in the jaboncillo of the limestone-clay contact deposits and are common minerals in the other 

deposits. Small concretionary masses may have as 

much as 50 percent iron oxide, but most jaboncillo 

contains less than 10 percent. Most iron oxides, both 

hydrous and anhydrous, that were seen by the writers 

had no crystal form of their own but in many cases 

had assumed the crystal forms of pyrite or jarosite, 

which they pseudomorphed. However, some hematite 

seen by C. P. Ross (written communication) in the cave 

silt of the Little Thirty-Eight and Colquitt-Tigner 

mines is in distinct crystal plates, the form of crystalline 

hematite. In addition, the writers observed in a few 

thin sections of cave-fill from the Mariposa and Fresno 

mines, a few crystals of hydrated hematite in cubelike 

rhombohedrons and hexagonal plates typical of direct 

crystallization of hematite. 

The red of the cave-fill results from fine particles of 

iron oxide that locally cloud the rock. Clouds of these 

particles are in many places arranged in bands that 

follow the bedding. The red of the cave-fill zones contrasts with the yellow-brown of the oxidized pyritic 

zone neat the base of the Grayson in areas where no 

hydrothermal alteration has occurred. This suggests 

that the iron oxide of the quicksilver deposits was 

formed in part by the fluids that formed the ore 

minerals. 

CINNABAR 

Cinnabar is the most abundant ore mineral in all 

classes of deposits, although in a few ore shoots at the 

Mariposa mine it was exceeded by calomel and oxychlorides of mercury. It occurs as massive cinnabar in 

veins, as small grains a.nd groups of grains disseminated 

through the gangue rock, as rare encrustations of 

crystals on rock surfaces, and most commonly as irregular replacements of the gangue rock. Megascopically, much of it appears massive, without crystal form, 

but microscopically, much of the massive cinnabar is 

seen to be aggregates of small, fairly well-formed 

crystals (see pl. 9). 

Cinnabar is both a filling in previously existing openings and a replacement of rock and gangue minerals. 

Commonly only openings less than 0.5 mm. wide are 

completely filled with cinnabar, larger openings are 

only partly filled, and the very large openings rarely 

have encrustations of cinnabar on their walls. 

The introduction of cinnabar by replacement processes was common in the Terlingua district. Most rich 

ore bodies, in particular those that contained large 

1nasses of high-grade ore, were formed in this way. 

In many places it is difficult to separate cinnabar formed 

only by replacement processes from that formed by 

processes of both replacement and filling. However, 

disseminated grains of euhedral cinnabar in limestone 

and clay are as clearly of replacement origin as partly 

filled fractures are of origin by filling. Aggregates of 

grains, with the angular outlines of bounding crystal 

faces, also suggest a replacement origin (see pl. 9A-D), 

as does the branching irregular form of much of the 

cinnabar. Cinnabar formed through replacement is 

commonly controlled by small fractures, from which the 

cinnabar extends outward. Disseminated cinnabar is 

never found far from cinnabar-filled fractures. 

Cinnabar apparently replaced certain minerals more 

readily than others. Pyrite, iron oxides, and jarosite 

were not seen replaced by cinnabar, although it is 

possible that in places they are. Some cinnabar in 

thin section appears to have a cubic form suggesting 

pseudomorphism after pyrite. It seems more probable 

that these apparent cubes are square sections cut 

through nearly cubic rhombhedrons, a form not uncommon to cinnabar. Likewise, the hexagonal outlines 

of some cinnabar crystals found sparsely in the same 

thin sections with jarosite do not necessarily indicate 

that the cinnabar is pseudomorphic after jarosite; instead the section may be cut across a rhombohedron 

to produce a six-sided surface. 11inerals most readily 

replaced by cinnabar are calcite and kaolinite, kaolinite 

being the more readily replaced. Cinnabar was not 

found in contact with fluorite; therefore the relative 

ages of the two could not be directly determined. 

Cinnabar found in the barite gangue at the McQuirk 

workings, south of the Mariposa mine, both veined and 

replaced the barite. 

Cinnabar formed late in the mineralization of the 

deposits. It was preceded by kaolinite, by barite, by 

almost all the calcite and pyrite, and it was followed 

as well as preceded by hydrocarbons. Its relation to 

the other mercury minerals is not clear. The writers 

had the opportunity to examine only one specimen in 

which cinnabar occurred with the chloride or oxycblorides of mercury. In this specimen the apparent resting 

of a crystal of cinnabar on a corroded surface of calomel 

suggests that cinnabar is later than calomel. 

MERCURY MINERALS OTHER THAN CINNABAR 

A greater variety of mercury minerals have been 

found at the Mariposa mine than anywhere else in the 

world. Of the 15 mercury minerals listed by Dana, 

8 are found here and these, exclusive of cinnabar, are: 

native mercury, calomel, eglestonite and terlinguaite, 

montroydite, and kleinite and mosesite. They represent the native element, the chloride (calomel), 2 

oxychlorides (eglestonite and terlinguaite), the oxide 

(montroydite), and 2 complex ammonium chlorides 

(kleinite and mosesite). In addition to those 8 minerals, metacinnabar has been reported by Hill (1902 
70 GEOLOGY AND QUICKSILVER DEPOSITS~ TERLINGUA DISTRICT, TEXAS 

p. 28-29), but this mineral was not seen by the writers 

nor by C. P. Ross (written communication). 

Although larger quantities of most of the foregoing 

minerals have been found in the Terlingua district than 

elsewhere, they were never abundant even here, and, 

with the exception of native mercury, determinable 

specimens have been found only in the limestone-clay 

contact deposits. They were most abundant at the 

Mariposa mine, but calomel and the oxychlorides, as 

well as native mercury, were found in lesser amounts 

in neighboring mines and prospects. During recent 

years it has become increasingly difficult to find even 

specimen samples in the mine workings or on the dumps. 

The writers, however, were fortunate in obtaining some 

good specimens from a pocket in the Perry pit (see pl. 

13) during the winter of 1945. These specimens did not 

contain any cinnabar, although cinnabar was the principal ore mineral taken from this pit. 

These rare mercury minerals are commonly regarded 

as formed by supergene processes, but it is very probable that most of them were formed by hypogene 

processes. Important evidence for this belief is presented by C. P. Ross who says (written communication) of these minerals: 

Most chloride minerals in deposits of metallic ores are of 

supergene origin and it might therefore be assumed that this 

group of seven rare quicksilver minerals [mentioned above], 

which are closely related to each other, have such an origin. 

In this instance, however, chlorine and ammonium compounds 

might equally well have been present in the ground water with 

whic>h the ascending quicksilver solutions are believed to have 

mingled. As such materials are common constituents of hotspring waters it is also entirely possible that the ascending 

solutions themselves contained sufficient CI and NH3 to account 

for the formation of the minerals containing them. The kleinite and 

mosesite, according to the descriptions above cited [see footnote 7], 

furnish direct evidence that they were formed at temperatures 

well above those to be expected during weathering. Optically 

a basal section of kleinite, which is hexagon'l.l, shows double 

refraction, but on heating to about 130°C it becomes singly refracting, being uniaxial, positive. This indicates dimorphism 

and a temperature of formation above 130°C. Similarly mosesite has the form of an isometric mineral but at ordinary temperatures shows double refraction. On heating to 186°C the 

crystals become isotropic in accord with their form. That is, 

mosesite crystallized at an elevated temperature in isometric 

crystals (which are optically isotropic) and changed to the 

dimorphous, doubly refracting condition at temperatures below 

186°C. It seems obvious therefore that weathering would not 

take place at temperatures well above the boiling point of water 

and hence that kleinite and mosesite must be hypogene minerals 

and are not products of weathering. 

7 Descriptions cited refer to work of Oailfield, Hillebrand, and Schaller (1910, p. 206-

208) on mosesite and Hillebrand and Schaller (1910, p. 46). It was found that mosesite 

had an isometric form and was anisotropic but became isotropic on heating to 186°0 

and that kleinite, a hexagonal mineral, Wf)S doubly refracting in basal section but 

became singly refracting on heating to 130°0. The preparation of artificial mosesite 

by Switzer and others (1953) at 25°0 casts some doubt on the validity of the conclusion 

that these inversions represented minimum temperatures of deposition. 

Ross points out that chlorides of mercury are commonly present in small amounts in many cinnabar ores, 

although this fact is not well known. 

J. J. Fahey tested 20 samples of ore and altered rock associated 

with ore from 9 mines in the Terlingua region. Of these, 11 contained no detectable quicksilver. All but one of those containing 

the metal had part of it as mercurous chloride, although only 

one of the samples tested show visible amounts of any of the 

chloride minerals. Samples 2 to 7 inclusive [table 14 of this 

report] have much of their quicksilver as cinnabar. The others 

do not have detectable amounts of this mineral. Specimens of 

cinnabar ore in the U.S. National Museum from Alaska, Arizona, 

Arkansas, California, and Nevada were also found by Fahey to 

contain quicksilver as chloride (Ross, 1935b). * * * In order to 

make these determinations Fahey (1937) devised special methods 

of distillation. The distinction between sulfide and chloride 

would not be made by ordinary assay methods. The methods 

employed are such that all the chlorine present as a compound 

of quicksilver minerals was determined. Such chloride sublimates i~ the form of mercurous chloride and the results are reported in terms of this substance. Any chloride present as 

sodium chloride and similar substances would be nonvolatile at 

the temperatures employed and would not affect the results 

reported. 

Ross's table follows: 

TABLE 14.-Mercury chloride content, in percent, of samples from 

the Terlingua region 

[Determined by J. J. Fahey] 

1. Altered rock from portal of No. 5 tunnel, 

Mariposa mine _____________________ _ 

2. Study Butte mine ____________________ _ 

3. Chisos mine _________________________ _ 

4. Ore, 600-ft level, Rainbow mine ________ _ 

5. High-grade ore from lowest stope in Rainbowmine _________________________ _ 

6. Ore, Mariposa mine near Cruz shaft ___ _ 

7. Upper level, Mariscal mine ____________ _ 

8. Cave-fill in lower stope, Little ThirtyEight mine ________________________ _ 

9. Chloride ore from prospect south of California Mountain ___________________ _ 

Total 

quicksilver 

(Hg) 

0.08 

20. 90 

7. 48 

4. 12 

36. 20 

74.04 

28. 16 

. 60 

1. 13 

Mercurous 

chloride 

(Hg2Cl2) 

0.07 

. 06 

. 05 

. 22 

none 

1. 20 

. 22 

. 05 

. 28 

Ross's conclusion that the rare mercury minerals 

in the Terlingua district were not formed by supergene 

waters is supported by the complete lack of etching, 

pitting, or any other solution features on any cinnabar 

crystals that were examined by the writers. The 

observed cinnabar has crystal faces that are sharp, 

smooth planes that show no effects of resolution. 

If cinnabar bad been the only hypogene ore mineral 

and all the others had been derived from it by supergene processes, it is indeed remarkable that none of 

the crystals examined showed the effects of solution 

by the postulated supergene waters. Also, it is 

difficult to understand how descending waters, which 

would have been high in ferrous sulfate and sulfuric 

acid derived from the decomposition of iron sulfide, 
QUICKSILVER DEPOSITS 71 

could have dissolved the cinnabar, a mineral that is 

considered almost insoluble in such waters. 

The rare mercury minerals were deposted dominantly 

in open spaces and apparently did not replace any 

previously formed material. In this they differ from 

cinnabar. Calomel, kleinite, mosesite, eglestonite, and 

terlinguaite all occur in well-developed crystals. Calomel forms veinlets and crystal encrustations on solution 

and fracture surfaces in limestone and cave-fill material. 

The other minerals were not observed in veinlets, but 

otherwise occur similar to the calomel. Montroydite 

occurs as hairs and growths and as needlelike crystals 

along the cleavage planes of calcite. l\finerals commonly associated with them are kaolinite, calcite, 

gypsum, and iron oxides, which are the same minerals 

associated with cinnabar. Native mercury is commonly associated with montroydite, but the reverse 

is not true. 

Age relations within this group of minerals are not 

too well established. Calomel appears to be the oldest 

and native mercury and montroydite the youngest, 

although at least some montroydite is older than 

terlinguaite. Mosesite, kleinite, terlinguaite, and eglestonite occupy an intermediate position and at least 

locally form in this order. 

Age relations between this group and cinnabar are 

less well known. From evidence cited in a preceding 

paragraph it is believed that cinnabar is younger than 

all except the native mercury and montroydite. Other 

reasons for believing this are discussed under the origin 

of the deposits. 

HYDROCARBON COMPOUNDS 

Bituminous matter (or hydrocarbon compounds) 

is in minor quantities in all the quicksilver deposits 

and is particularly abundant in the Two-Forty-Eight 

mine. It occurs as a yellow to brown pigmenting 

material in vein calcite, as a glossy black, brittle to 

plastic substance in small veinlets, and as a dull black 

material that impregnates rock fragments and fault 

gouge. Its distribution is closely associated with that 

of the quicksilver deposits. 

C. P. Ross collected three kinds of bituminous material from the Two-forty-Eight mine and a samplP of 

bituminous calcite from the Study Butte mine and had 

them examined in the Geological Survey. The results 

of the examinations (table 15) and the interpretation 

of the results given by Ross (written communication) 

follow: 

E. T. Erickson, who did the work, states that the proportion 

and character of the saturated hydrocarbons is similar to that 

given by asphaltic bitumens that have been derived from 

petroleum and that there is sufficient resemblance in the four 

samples to suggest that the petroleums from which they originated were similar in nature. As some paraffin is present in the 

samples, the originating petroleums may have had a tendency 

towards a mixed base (asphaltic paraffin) composition. The 

percentage of saturated hydrocarbons in all the samples is 

critically higher than that of gilsonite and grahamite (Richardson, 

1916, p. 493). The distillate from specimen BC 68, according 

to Erickson, is sufficiently different from the others recorded in 

the table to indicate that it has suffered less modification than 

they have. It contains asphaltic oil and some paraffin and hence 

is apparently not fundamentally different from the other bituminous materials tested. 

TABLE 15.-Analyses of bituminous material from the Terlingua district 

[Analyst, E. T. Erickson, Geological Survey, February 27, 1935] 

Fractions resulting from treatment of asphalt bitumen with an excess of light petroleum ether (boiling point 30°-60° C) 

Weight Asphalt bitumen extracted by Sample of sample heating with 

(grams) chloroform 

(percent of 

sample) 

TT 66 ______ 1. 002 75. 5 

TT 22 ______ 5 21. 7 

TT 23 ______ 42. 06 1. 88 

TT 52 ______ 19. 21 4. 77 

BC 68 ______ 75 . 07 

I Generally black and highly viscous. 2 Viscous amber-colored oil. 

Insoluble fraction 1 

Percent Softens Flows at- at-(° C) (o C) 

28. 7 165 175 

44. 7 160 170 

46. 3 180 190 

30.8 160 170 

-------- -------- --------

Soluble fraction 

Sulfur Saturated Chiefly unsat- (percent of hydro- Qualitative indication of paraffin a urated hydrofraction) carbons carbons 

(percent) 2 (percent) 

1. 25 22. 2 Fairly positive ____________ -_ 49. 1 

1. 50 16. 2 Smaller than TT 66 __________ 39. 1 

1. 61 19. 9 Smaller than TT 66 __________ 33. 8 

1. 32 26. 8 Fairly positive; similar to 42.4 

TT 66 . 

-------- ---------- ---------------------------- ----------

s About one-tenth gram of the saturated hydrocarbon material in a test tube was dissolved with 2 cc of ether, an equal volume of abs?lute alcohol ":as adde.d, and the ~est 

tube was then placed in an ice-salt freezing mixture. In the "fairly positive" test light-colored .flocculent material distinctly appeared (m about 15 mmutes) m the sol~tlon. 

In TT 22 and TT 23 the quantity of the precipitated material was less. The fractionation of the extracted bitumen with light petroleum et~er and the determinatwn of 

saturated hydrocarbons are by methods described by Clifford Richardson in chapter 28 of the "Modern asphalt pavement" 2d ed., 1908, John Wiley & Sons, New York. 

TT 66. Plastic asphalt from the Two-F<?rty-Ei~ht mine. . . TT 22. Dull massive asphalt impregnatmg sedtmentary rock from the Two-Forty-Etght mme. TT 23. Calcite impregnated with bituminous material. From the Two-Forty-Eight mine. 

TT 52. Asphalt in rock from the Study Butte mine. 

BC 68. Carbonaceous shale from near the base of the Boquillas .flags in the northeastern part of sec. 300, Block G-4. 
72 GEOLOGY AND QUICKSILVER DEPOSITS, TERLINGUA DISTRICT, TEXAS 

In order to determine whether the bituminous 

material could be of local origin, Ross collected samples 

of Boquillas flags and Terlingua clay and had these 

samples analyzed for possible hydrocarbon content. 

Only one, BC-68 of the preceding table, a carbonaceous 

shale from the lower part of the measured section of 

Boquillas flags in the northwestern part of sec. 300, 

Block G-4, yielded an appreciable amount of bitumen. 

Of the hydrocarbon in this specimen Ross says (written 

communication): 

A portion of specimen BC-68 weighing 75 grams was treated 

by Erickson with redistilled chloroform and yielded 0.07 percent 

of amber brown, viscous material upon evaporation of the 

chloroform solvent. When this was heated with an excess of 

light petroleum ether, 9.7 percent of an insoluble dark solid 

bitumen was obtained. The filtrate from this treatment after 

prolonged heating and retreating with light petroleum ether 

yielded an additionall4.3 percent of the dark insoluble substance. 

The filtrate from this treatment was found to contain a small 

amount of paraffin. These tests show that the bituminous 

material in the carbonaceous shale is in general similar to that 

in the four specimens listed in [table 15] except that in the shale 

there has been less alteration as evidenced by the smaller quantity of material insoluble in light petroleum ether. 

The only sample _of unmineralized rock that yielded 

measurable amounts of bitumen was taken from rocks 

that are stratigraphically and structurally above many 

of the quicksilver deposits that contain the most 

bitumen. There are, however, limestones and shales 

of Cretaceous and Paleozoic ages that lie beneath the 

lowest known quicksilver deposits and that were not 

sampled and tested for possible bitumen. King (1937, 

p. 143) states that the pre-Carboniferous rocks of the 

Marathon Basin contain abundant bituminous material. 

Regardless of the ultimate source of the bituminous 

material, much of it was deposited definitely later than 

the cinnabar, as tar is found coating the surface of 

cinnabar in vugs. On the other hand, it is very 

possible that some bitumen was deposited earlier than 

the cinnabar. This conclusion is indicated by the 

bitumen that is dispersed through calcite that is itself 

earlier than cinnabar. It seems possible however ' ' that bitumen may have impregnated the calcite along 

submicroscopic cracks and cleavage planes at some 

time after it was formed, and the bitumen is therefore 

not necessarily contemporaneous with the calcite. 

Another possibility that must be considered is that 

bitumen may have been introduced into the deposits 

during an early phase in their formation and was 

redistributed throughout mineralization. 

FLUORITE AND BARITE 

Many of the quicksilver deposits in the west-central 

part of the district contain small amounts of fluorite 

and barite. These two minerals, which are more commonly found apart than together, were seen only in 

deposits in the Devils River limestone and not in 

deposits in any higher stratigraphic units. Their 

relative ages were not determined. Barite is most 

abundant at the McGuirk workings, south of the 

Mariposa mine, and fluorite at the surface workings of 

the Little Thirty-Eight mine. 

The fluorite forms single, euhedral cubic crystals and 

clusters of crystals commonly enclosed in a matrix 

of clay and calcite or studded on fracture and solution 

faces of limestone. It ranges from colorless to pale 

yellow-brown and generally is very transparent. A 

specimen of cave-fill collected from the dumps of the 

Little Thirty-Eight mine contains pseudomorphs of 

calcite after fluorite. A thin section of this specimen 

shows the partial replacement of the fluorite by the 

calcite. This is the only evidence obtained that indicates that at least some calcite was later than the 

fluorite. Age relations between fluorite and other 

minerals was not obtained. 

Barite forms nodular masses (in the cave-fill) and 

euhedral crystals, which range from white to pale 

yellow-tan. The best formed crystals are in vugs in 

the limestone. The white, massively crystalline barite 

appears to be somewhat earlier than the pale yellowtan. In places calcite replaces and veins barite; the 

reverse relation was not noted. The barite in the 

nodular masses is white to gray and cryptocrystalline. 

Contraction cracks in the nodules suggest that this 

barite may have a colloidal origin. 

One specimen, presented to the writers by Frank 

Duncan, excellently demonstrates the relation between 

barite and cinnabar. Megascopically this specimen is 

mainly fine-grained, grayish-white barite through which 

are dissemiDated irregular aggregates of cinnabar. One 

surface of the specimen is covered by a druse of yellowtan barite crystals, which is separated from the grayishwhite barite by a black layer. The black layer is 

barite colored by inclusions of hydrocarbon. The 

skeletal arrangement of the hydrocarbon inclusions 

indicates that they were deposited during the growth of 

the barite crystals. A few tiny veinlets of white to 

yellow-tan barite extend from the drusy barite through 

the grayish-white barite. Cinnabar is not in these 

barite veinlets nor is it in the drusy barite above the 

black layer of hydrocarbon inclusions. 

The foregoing facts, derived from a study of the hand 

specimen, lead one to interpret the paragenesis as a 

continuous deposition of barite, accompanied in its 

early stages by deposition of cinnabar, a deposition 

which ended when hydrocarbons were deposited with 

the later yellow-tan barite. Microscopic examination 

of a thin section of the specimen confirms the above 

facts, but rejects the above interpretation. Megascopic examination did not show that the grayish-
QUICKSILVER DEPOSITS 73 

white barite is a replacement of kao inite and that it 

is locally crowded with kaolinite in lusions, whereas 

the drusy barite and barite veinlets ere deposited in 

open spaces and are accordingly fre of kaolinite inclusions. The microscopic examin tion shows that 

the distribution of cinnabar is con troll d by the distribution of unreplaced kaolinite, which · t replaces. The 

cinnabar not only replaces the kaol nite but also replaces, to a much lesser extent, the arite, as is indicated by replacement veinlets of cin abar that extend 

from cinnabar-replaced kaolinite alo g fractures into 

the barite. These additional facts s pport the correct 

interpretation that the cinnabar was eposited after the 

barite. 

NATROLITE AND ANALCIT 

Two members of the zeolite gr up of minerals, 

natrolite and analcite, were found-b t not togetherin two quicksilver deposits in widel separated areas. 

Natrolite was found in cinnabar or from the main 

stope of the Fresno mine in the extr me western pa.rt 

of the district, and analcite was und in collapse 

breccia that contains cinnabar in the Two-Forty-Eight 

mine in the eastern part of the distric . Both minerals 

are common in the intrusive igneous ocks, with which 

they are genetically related as late m gmatic minerals. 

Their presence in the quicksilver de osits is believed 

to be fortuitous and unrelated t the quicksilver 

mineralization. This belief is corro orated by their 

great abundance in the igneous roc and their great 

rarity in the quicksilver deposits. oth minerals are 

earlier than the cinnabar of the depo its in which they 

occur. 

N atrolite is associated with the qu rtz that has been. 

described in the Fresno mine. The natrolite and associated quartz is in a breccia com osed of angular 

fragments of limestone and calcite ·n a clay matrix. 

The natrolite, seen only in thin sec ion, is in typical 

radiating crystals in what appears to ave beer an open 

space. It is fractured, and a little cinnabar replaces 

the natrolite along the fractures. 

Analcite is very abundant in the u per levels of the 

Two-Forty-Eight mine, where it is i a hydrocarbonimpregnated breccia of limestone and clay. It was not 

observed in the lower levels of them ne, except in the 

analcite syenogabbro that intrudes he breccia. The 

analcite in the collapse breccia is in uhedral crystals, 

as much as 5 millimeters in diameter but that in the 

syenogabbro has no crystal form, bei g present mainly 

as replacement of feldspar. The nalcite crystals 

commonly are fractured. Most are fairly clear, but 

some are stained light brown by hydr carbon and others 

contain inclusions of titanite in hapha ard arrangement. 

Most crystals when examined micro copically show a 

faint birefringence. 

Although cinnabar was not seen in contact with the 

analcite, indirect evidence indicates that the analcite 

formed before the cinnabar and probably has no relation to it. Both the analcite and the cinnabar are 

younger than the breccia pipe. The analcite replaces 

the clay matrix and breccia fragments and the cinnabar 

is controlled by fractures formed during and after the 

filling of the pipe. The igneous rock is later than the 

breccia pipe, which it intrudes, and as the cinnabar is 

in the igneous rock, as well as the breccia, it is therefore 

later than both. Analcite is contemporaneous with 

the analcite syenogabbro, as it is a late magmatic 

mineral in that rock. Accordingly, it seems logical to 

interpret the analcite in the breccia as derived from the 

analcite syenogabbro during its final cooling. This 

proposed correlation between the analcite in the intrusive igneous rock and the analcite in the breccia is 

supported by the titanite inclusions in the analcite of the 

breccia. Pseudomorphs of leucoxene after ilmenite are 

abundant in the igneous rock, and this alteration of the 

ilmenite appears to be contemporaneous with the analcite. This indicates that the titanium of the ilmenite 

was reorganizing into leucoxene (for the most part to be 

identified with titanite) at the same time that analcite 

was forming in the igneous rock. Because similar relations existed between the analcite and titanite in the 

breccia, it is reasonable to assume that the analcite of 

the igneous rock is contemporaneous with the analcite 

of the breccia. The most unlikely possibility that the 

analcite of the breccia pipe was derived from the analcite of the igneous rock by being dissolved by the mineralizing solutions and redeposited by them, however, 

cannot be precluded from the evidence at hand. 

SULFATES 

With the exception of barite, which has been described 

above, the sulfate minerals that were identified in the 

quicksilver deposits are anhydrite, gypsum, melanterite, epsomite, alunite, and jarosite. Gypsum, melanterite, and epsomite are hydrous sulfates formed by 

the weathering of the deposits, and as the latter two 

are unrelated to the mineralization they will not be 

mentioned again. The anhydrite, alunite, and jarosite 

may also have been formed by weathering processes, 

but it is possible that they were formed in part by 

hydrothermal processes. 

Gypsum is a very common mineral and is particularly 

abundant in the limestone-clay contact deposits. It 

forms veins in clay, limestone, and cave-fill, coatings 

on rock surfaces, and impregnations in porous rock. 

In some limestone caverns a snow of fine, powdery 

gypsum covers the walls and floors of the caverns; in 

others, beautiful, curved, stalactitelike "crystals" as 

long as 1 foot hang suspended from the roofs. 
74 GEOLOGY AND QUICKSILVER DEPOSITS, TERLINGUA DISTRICT, TEXAS 

Some gypsum is forming at the present day through 

the reaction of sulfate ground water, produced by the 

oxidation of pyrite, with the limestone. Probably 

most, if not all, the gypsum was formed in a similar way 

and is postcinnabar in age. The possibility remains, 

however, that some gypsum was formed contemporaneously with the cave-fill and breccia pipes, which are 

definitely preintrusion and precinnabar, but this possibility seems remote. 

The presence of anhydrite, found in small veinlets in 

limestone at the Mariposa mine, suggests that some 

gypsum may have formed by the hydration of this 

mineral. 

C. P. Ross collected ''a bright yellow more or less 

pulverulent material from the surface close to the west 

end of the Study Butte mine, which was found by W. T. 

Schaller to be a mixture of jarosite and alunite with a 

little lead." Hillebrand and Schaller (1909, p. 17) 

report jarosite in a brown brecciated mass sent to them 

by W. B. Phillips, who reported its being collected from 

sec. 100 in Block G-12. As there is no sec. 100 in 

Block G-12, the specimen probably came from sec. 100, 

Block G-5, on the northwest side of Lowes Valley, a 

locality from which the writers collected similar specimens. The writers also found jarosite in altered limestone and clays from the Fresno, Mariposa, and Chisos 

mines. When judged by the material examined microscopically, jarosite appears to be a mineral widely distributed in the altered rocks of the Terlingua quicksilver district. 

The jarosite in Lowes Valley is in crystals (as much 

as 0.5 millimeters in diameter) in a brecciated and 

altered rhyolite. The jarosite crystals line and fill vugs 

and form boxwork septi that join the breccia fragments. 

The crystals range from a light yellow-brown to a dark 

resin brown. 

The jarosite in the altered limestones and clays 

throughout the district occurs in minute grains that 

can only be recognized with the aid of the microscope. 

Many grains have been altered to limonite but still 

retain the characteristic hexagonal basal section of 

jarosite. 

Some alunite and jarosite in the Terlingua district 

is an oxidation product of pyrite, but the writers were 

unable to tell whether this oxidation occurred early in 

mineralization or during the present weathering cycle. 

SUMMARY AND CONCLUSIONS 

Although the preceding mineral descriptions are 

meager in data that directly show the age relations 

among the various minerals, they contain enough direct 

evidence to give, when supplemented with indirect 

evidence, a fairly complete history of the quicksilver 

mineralization. However, gaps in this history do appear, gaps that cannot be closed by the combined direct 

and indirect factual data. In such cases, the writers 

have bridged the gaps with inferences, which are clearly 

labeled as such. 

The history of mineralization is divided into two general periods, the first characterized by the deposition of 

nonmetallic minerals and the second characterized by 

the deposition of metallic minerals. How closely related these two periods are can only be inferred, for 

there are no minerals whose deposition was restricted 

to a period transitional between the period of nonmetallic minerals into the period of metallic minerals. 

The deposition of hydrocarbons during both periods, 

however, does suggest-but does not prove-that there 

is a close genetic relation between the two periods, 

even though the acmes of mineral deposition in each 

may be widely separated in time. Minerals that were 

formed during the period of nonmetallic minerals are: 

kaolinite, iron oxides, silica minerals, fluorite, barite, 

and possibly alunite, jarosite, and anhydrite. Minerals 

that were formed during the period of deposition of 

metallic minerals are: calomel, mosesite, kleinite, pyrite, cinnabar, and possibly the oxychlorides and oxide 

of mercury. Minerals that formed during both periods 

are calcite and hydrocarbons. 

The period of nonmetallic minerals begins with the 

formation of kaolinite,. iron oxides, and silica minerals, 

and ends with the deposition of fluorite and barite. 

The kaolinite, iron oxides, and silica minerals were 

probably formed almost contemporaneously. The iron 

oxides could have formed from the authigenic sedimentary pyrite in the Grayson formation. The silica in 

the cave-fill may have been formed at least in part from 

the breakdown of beidellite into kaolinite, but it is not 

possible to tell whether the jaspery silica in the Devils 

River limestone was deposited at this time. Nor is it 

possible to tell whether the deposition of barite and 

fluorite were in part contemporaneous with kaolinite or 

were entirely later. Both minerals replace kaolinite, 

but this does not mean that kaolinization was complete 

before their introduction. Another gap is the age relation between barite and fluorite. From possible temperatures of formation of these minerals it is inferred 

that fluorite was deposited first. This, of course, is 

based upon the assumption that the period of nonmetallic minerals was a period of falling temperatures. 

The position of alunite, jarosite, and anhydrite is 

doubtful. 

The period of metallic minerals includes the deposition of calcite in varying intensity throughout its entire 

duration. The deposition and recrystallization of 

calcite occurred, however, during the . period of nonmetallic minerals and· probably reached its acme during 

the closing stages of this period. It is doubtful that 
QUICKSILVER DEPOSITS 75 

calcite deposition continued uninterrupted into the 

period of metallic minerals. Most calcite was certainly 

deposited before the pyrite and cinnabar. The relations between pyrite and cinnabar are clear; cinnabar 

was not deposited until pyrite deposition had almost 

ceased. The age of the hydrocarbons, unfortunately, 

is not so clear; some hydrocarbon is as old as the barite 

and some is younger than the cinnabar. These relations suggest that the hydrocarbons may have been 

deposited during most of the mineralization, and that 

found in the calcite may be contemporaneous with it 

and not introduced at a later date. 

Age relations within the chloride group of minerals 

are fairly well demonstrated in the specimens examined, 

but the relations between this group and cinnabar are 

not positively determined from the study of the 

specimens. The probable sequence within the chloride 

group is calomel (oldest), mosesite, kleinite, terlinguaite, and eglestonite (youngest). Montroydite 

and native mercury in some specimens are younger 

than terlinguaite. Mercury is enclosed as globules in 

all the calomel, a fact which suggests that deposition 

of this mineral may have been early as well as late, if 

not continuous. Cinnabar appears to be younger 

than calomel in the one specimen that showed these 

minerals in con tact. 

Some minerals found in the deposits are truly supergene. Among these · are gypsum, some of the hydrous 

iron oxides, aragonite, and probably some montroydite 

and some native guicksilver. Some alunite and 

jarosite may also be of supergene origin. 

ORIGIN 

The writers believe that the quicksilver deposits 

were formed by hydrothermal solutions and associated 

gaseous phases. The heat and part of the water are 

believed to have come from a magma of unknown 

depth and unknown composition. The source of the 

quicksilver is problematical; it may have come from a 

magma or it may have been distilled by magmatic heat 

from overlying sedimentary rocks. The postulated 

"ore-bearing" fluids migrated towards the surface 

along interconnected openings of rock fractures that 

they enlarged. Parts of these channels in suitable 

environments became host structures for the deposits. 

Potential host structures were probably no deeper than 

2,000 feet below the surface and were where openings 

were smaller than average. The writers believe that 

the ore minerals were deposited at temperatures below 

300°C and pressures locally at least as high as 30 

atmospheres and that the mercury was probably 

transported at least partly in a gas phase and was 

deposited as both chlorides and sulfide. The common 

gangue mineral, calcite, was probably deposited by 

484889-60-7 

carbonic acid waters that dissolved calcite from limestones, transported it as the bicarbonate, and deposited 

it as calcite under conditions of reduced pressure. 

STRUCTURAL CONTROL 

The term '.'structural control" as ordinarily used 

refers to the structures that directly controlled the 

deposition of the ore elements; as used in this report it 

is broader in scope and refers to structures that gave 

access to and determined the routes of the fluids that 

transported the ore and gangue elements from their 

source to their place of deposition. With this broad 

definition it is possible to discuss structural control 

under three headings: regional control, district control, 

and local control. 

REGIONAL CONTROL 

The Terlingua district, the Mariscal district, and 

occurrences of cinnabar at Christmas Mountains, Mesa 

de Anguila, and across the border in Mexico, constitute 

a metallogenic province. The province requires some 

explanation, or at least invites some speculation. 

Regional controls for metallogenic provinces must 

necessarily be highly theoretical and can only be proposed after the geologic features of a large number of 

similar provinces are compared and interpretations are 

made from those features that are common to all or 

to nearly all. The quicksilver deposits of the United 

States and the rest of the world have, with few exceptions, the common characteristic of being in regions 

where there were earth movements and volcanic 

activity at no great geologic time before quicksilver 

mineralization. 

The Big Bend region, like most other quicksilver 

provinces, is a region where structural movements, 

igneous activity, and quicksilver mineralization are 

interrelated in time and space. As stated in preceding 

sections of this report, structural movements and 

igneous activity are closely related; igneous activity 

has produced not only local but district structures. 

Because the exposed igneous rocks are not the direct 

source of the mercury minerals, it may be postulated 

that both igneous rocks and mercury minerals were 

derived by processes of differentiation at depth from a 

common magma. An alternate hypothesis is that the 

magma was not the source of the quicksilver but was 

only the source of heat that distilled quicksilver from 

deep-seated sedimentary rocks. The magma must have 

underlain much of the Big Bend region because related 

igneous rocks are widespread. The quicksilver deposits, however, are restricted to a small area and. 

therefore the mere presence of the underlying magma 

was not all that was essential for their formation. 

The factors that determined the location of the 

quicksilver province within a particular part of the 
76 GEOLOGY AND QUICKSILVER DEPOSITS, TERLINGUA DISTRICT, TEXAS 

Big Bend region can only be speculated upon. If the 

quicksilver had a magmatic source, there may have 

been a certain local differentiation of the magma that 

permitted a localized migration of the mercury; there 

may have been certain structural conditions that permitted the mercury to be concentrated as mercury 

minerals in restricted channels and not diffused into 

the rocks or lost in the ground water; or there may have 

been a certain critical thickness of rocks overlying the 

magma that permitted the mercury to be concentrated 

and preserved. All these possibilities, as well as others 

that might be suggested, are difficult to weigh. If the 

quicksilver was caused to migrate from a nonmagmatic source by magmatic heat, any of these factors 

but the first might account for the location of the 

province. 

DISTRICT CONTROL 

The district control is more tangible than the regional 

control. The district is closely related in space to the 

west-striking Terlingua monocline but not closely 

enough for the relation to be direct. Reasons for this 

have been pointed out on page 53. The writers 

believe that the quicksilver district and the monocline 

were both controlled by the same underlying structure, 

which was either an intrusive body or a structure that 

controlled an intrusive body. The Terlingua monocline is, nevertheless, the exposed expression of this 

postulated structure and therefore indirectly controls 

the location of the quicksilver belt. 

There is only a general and not a close space relationship between the quicksilver belt and the monocline. 

Although both have general east-west trends and are 

roughly parallel for much of their lengths, the monocline swings northwestward at its west end and is 

crossed by the quicksilver belt, which maintains its 

straighter course. The quicksilver deposits are not 

on the monocline but, with a few exceptions, are updip 

from it. However, the northeasterly fractures that 

control the location of individual ore bodies, although 

not restricted to the belt, are more abundant there and 

their distribution and trends appear to be related to the 

monocline. South of the monocline these northeasterly 

fractures are subordinate in number to northwesterly 

fractures; immediately north of the monocline and 

within the quicksilver belt the situation is reversed, the 

northwesterly fractures, although individually stronger, 

are subordinate in number to the northeasterly fractures. North of the quicksilver belt the number and 

strength of northeasterly and northwesterly fractures 

is about equal. 

These relations between the monocline, fractures, 

and quicksilver belt are best developed in the westcentral part of the district, where the Devils River 

limestone is the exposed rock. Although comparisons 

are being made between areas where the different fracture patterns are in rocks of different physical properties, this should not seriously affect the validity of the 

comparison, because the relative abundance of the 

trends should remain constant regardless of the different 

strengths of the rocks. 

The uneven distribution, or "bunching," of the deposits within the belt cannot be correlated with particular variations in steepness or local trends of the 

monocline nor with the grabens that breach the monocline. However, the belt may have been more evenly 

mineralized than the mine locations indicate, because 

some deposits doubtless have been eroded away and 

others probably have not been discovered. 

From the foregoing it is obvious that the belt does not 

coincide with a major structural high, and it is equally 

obvious that the more intensely mineralized parts of 

the belt (pl. 1) do not correspond with local highs. 

On the contrary, some of the more productive mines, 

notably the Mariposa and Fresno, are in areas that are 

structurally depressed, and others, such as the ChisosRainbow, are on the borders of depressed areas. These 

facts do not support the views of Udden (1918), who 

has applied the anticlinal theory of petroleum accumulation to the Terlingua quicksilver deposits. 

The almost constant association between rock fractures and the quicksilver deposits indicates that fracture 

openings were the best available channels for the ascending ore-forming fluids. The quicksilver deposits occur 

in association with the system of northeasterly fractures, probably because these fractures were, at the time 

of mineralization, the most open connections between 

deeper channels of the fluids and either the surface or 

the circulating ground water. 

LOCAL CONTROLS 

Local controls are the structures with which the ore 

bodies are directly associated. They are the structures 

of those parts of the channels where the mercury 

minerals were deposited. Unfortunately, the term, 

"local control," connotes that wherever similar structures are, ore bodies should also be. This is misleading 

because other factors besides structure controlled the 

deposition of the ore minerals. 

Structures that had a local controlling influence on 

the location, shape, and size of the ore bodies are rock 

fractures, cave-fill zones, and breccia pipes. Obviously 

many potential ore structures were not available to the 

mineralizing fluids and are consequently barren. Likewise, the mineralizing fluids probably circulated through 

many favorable structures that were not mineralized 

because not all the conditions essential for deposition 

were present. 
QUICKSILVER DEPOSITS 77 

The controlling fractures are faults and joints, 

which are either single, multiple or complex. The 

single fractures are joints or faults of small displacements. The multiple fractures are sheeted zones of 

closely spaced shears and faults that have multiple 

planes of movement. The complex fractures are intimately sheared fault zones and breccia zones along 

faults. Single fractures indirectly controlled the location of many of the ore bodies by providing access 

channels to other structures, notably the limestone-clay 

contact deposits, but only at a few places were ore 

bodies formed within a single fracture opening. Multiple fractures are fairly abundant, especially where 

refracturing of calcite veins produced multiple fractures 

for the deposition of the cinnabar. Complex fractures 

are the least abundant of the controlling fractures; a 

few ore bodies occur in fault breccia zones, but they are 

not common. 

Fractures with little or no movement along their 

walls are more favorable ore structures than are faults 

with considerable displacement. No ore bodies, except 

a few small ones at the Chisos mine, have been found 

along any of the major faults. It is true that many 

ore bodies in calcite veins in the Boquillas flags formed 

along faults, but few, if any, of these faults have displacements greater than 20 feet. This apparent 

favoritism for minor instead of major fractures is 

usually explained on the basis of the relative permeabilities of the two structures. Presumably the greater 

the movement along the fracture the greater the tendency for the original irregularities to be planed off 

and the open spaces filled with tight gouge and breccia. 

This generalization might account for the preferences 

seen in the Terlingua district. 

Many fractures in the limestones have been enlarged 

by solution along their walls, but the large, open 

fractures so produced were not favorable for the deposition of mercury minerals. Such enlarged fracture 

openings, and associated large caverns, are best 

developed in the Devils River and Buda limestones. 

Many of these were filled with calcite or clay, but 

a good many are still open and cavernous. The rare 

cinnabar that is found on calcite lining these openings 

indicates that they were open during quicksilver 

mineralization and hence available to the ore fluids. 

None of them, however, were filled with mercury 

minerals. This lack of deposition of cinnabar in 

large openings has been noted by Beyschlag, Vogt, 

and Krusch {1914, p. 459-460) who, in speaking of 

quicksilver deposits in general, say: "With the cinnabar 

deposits on the other hand the filling of the fissure up 

which the quicksilver solutions ascended usually plays 

but a small part, impregnation of the country rock being 

the principal form of the occurrence." In speaking of 

"impregnations of the country rock" these writers 

refer to the filling of pore spaces in sandstone and the 

openings in fractured limestone-all relatively small 

openings. This preference for small openings is 

particularly true in the Terlingua district, because it 

is rare for an opening more than 1 inch wide to contain 

cinnabar. 

The mode of formation of the calcite veins produced 

openings of a size favorable to quicksilver mineralization 

because the volume of open space never became very 

large. Filling of these fractures by calcite was accompanied by intermittent movements along the fracture 

walls and through the calcite previously deposited. 

The calcite vein not only prevented the walls of the 

fracture from closing, but the surfaces of rupture 

developed through it acted as gliding planes for movements parallel to the walls of the fracture. Where 

the planes of rupture were irregular surfaces, movements along them tended locally to dilate the walls of 

the vein and produce open spaces between planes of 

rupture. These open spaces became filled or partly 

filled with calcite, and as the process was repeated the 

walls of the original fracture became farther and 

farther apart. By such a mechanism the walls of the 

original fracture were dilated with no appreciable 

increase in the volume of open space. A sheeted 

structure of multiple fractures resulted, and the open 

space was distributed along numerous shear planes. 

This kind of broken rock appears to be favorable for the 

deposition of cinnabar. 

The relative volume of the channel apparently was 

not nearly so important in determining the place of 

deposition as the ratio between total volume of open 

space and total surface area of channel walls. The 

rarity of cinnabar in single fracture openings wider 

than half an inch and its common presence in zones 

of multiple fractures and breccia-clogged fractures 

attests to this. Lovering (in Newhouse, 1942, p. 6) 

made an even broader generalization by stating that 

replacement deposits in general are commonly associated with tight fissures rather than with open fractures. This applies to the limestone-clay contact 

deposits, even though most but not all the ore is of 

replacement origin. A restatement of the above is 

that any channel with a large surface area and relatively small open space is a more favorable environment for the deposition of cinnabar than a channel 

with a relatively small surface area and a large open 

space. There is, of course, a lower limit to the size of 

openings to which this generalization can be applied. 

These two factors, surface area and contrasts in 

channel cross section, have their greatest influence 

over the stability of circulating fluids where the 

openings are of capillary or near capillary size, which 
78 GEOLOGY AND QUICKSILVER DEPOSITS, TERLINGUA DISTRICT, TEXAS 

is the size of openings that largely controlled the 

replacement ore. The influence of these factors has 

been pointed out by Bain (1936, p. 526), who states 

that the Bernoulli effect influences mineral precipitation under both the Van't Hoff and Le Ohatelier laws. 

Bain states that in capillary openings the adsorption 

phenomena, as studied by Langmuir, control or 

influence metasomatism and the types of metasomatic 

minerals. Subsequently, however, Verhoogen (1948, 

p. 213) showed that surface tension is theoretically of 

much greater importance than the Bernoulli effect. 

The above factors might, at least in part, account 

for the deposition of ore minerals in cave-fill and not 

in the underlying open fractures. The cave-fill zones 

are exceptionally favorable ore structures, but it is not 

certain whether their controlling influence is entirely 

physical or is in part chemical. The replacement of 

the clays by cinna.bar is certainly a chemical phenomenon, but it may be largely dependent upon the 

physical or structural character of the clay. 

It is the clay and not the limestone fragments in the 

cave-fill that directly and indirectly controlled the 

deposition of the ore minerals. This conclusion is 

strongly supported by the common selective replacement of clay by cinnabar and the common association 

between cinnabar and clay in the keels of the limestoneclay contact deposits. The selective replacement by 

cinnabar of certain layers of cave-fill that differ in 

grain size and clay content from neighboring unreplaced 

layers suggests that the controlling influence of the 

clay may have depended largely upon the type of 

channel the clay provided the circulating fluids. 

The cave-fill and Grayson formation immediately 

above cave-fill were not impermeable barriers that 

completely prevented passage of the ascending fluids; 

instead they were channels in marked contrast to those 

in the underlying Devils River limestone. They were 

not impermeable because undisturbed Grayson formation more than 10 feet above cave-fill is cinnabarbearing, and calcite veins, with and without cinnabar, 

are common in the structurally higher Boquillas flags. 

As channels in rocks of low permeability, they contrasted with those in the limestone by being composed 

of innumerable, minute, interconnected openings instead of one relatively large one. These places where 

the fluids could readily pass through the Grayson were 

the more open parts of faults, breccia pipes, and 

fractures in intrusive rocks that crosscut the Grayson. 

The preceding discussion of the probable physical 

environment in the cave-fill is highly simplified and 

does not take into consideration all factors that may 

influence the environment. It does not mention, for 

example, that channels through the cave-fill were 

extremely complex, with innumerable constrictions 

and enlargements where pressures would locally change. 

Nor does it consider the constant changes that would 

be made in channels by solution, filling, and abrasion. 

The discussion is of necessity incomplete; but the 

writers believe that it includes a consideration of the 

structural factors that would most disturb the equilibrium of the ascending mineralizing fluids. 

Much of the preceding discussion of local structural 

controls probably also applies to the ore bodies in the 

breccia pipes, but the application is difficult because of 

the limited opportunity to study these structures and 

their ore bodies. Fractures were important in localizing ore shoots in the Two-Forty-Eight mine, and 

Grayson formation was the host for cinnabar in the 

Ohisos pipe, but aside from this the distribution of the 

discovered ore shoots appears to be decidedly erratic. 

The occurrence of ore bodies in breccia pipes, however, does demonstrate that the collapse breccia was 

a better channel than the flat-lying, well-jointed 

surrounding rocks and that breccia pipes either extend 

to considerable depths or are connected with structures 

that do. It also suggests that there may be a genetic 

relation between cave-fill zones and breccia pipes. 

The breccia pipes are not so tight and impermeable 

as they locally appear, and they were excellent channels 

for the mineralizing fluids. The breccia in the upper 

part of the Two-Forty-Eight pipe is open and very 

porous, and apparently that in the upper part of the 

Ohisos pipe was similar. The breccia in the lower 

part of the Two-Forty-Eight pipe and in parts of 

the Maggie Sink pipe contains abundant clay and 

appears to be a tight structure; but its tightness is 

more apparent than real, for even the tight parts show 

extensive alteration and also permit the inflow of 

ground water in mine workings. Some parts of the 

pipes are obviously more permeable than others. ·In 

general the most permeable parts are along the borders 

of the pipes. 

TRANSPORTATION AND DEPOSITION OF THE ORE 

ELEMENTS 

The state and composition of the transporting 

medium can not be determined by direct observation 

but must be inferred from the products of mineralization and from comparisons of these products with those 

of observable geologic processes and laboratory experiments. The known direct products of quicksilver 

mineralization in the Terlingua district include a 

maximum of 20 minerals, representing combinations of 

11 elements. Some of these 11 elements are indigen~:ms 

to the wall rocks and accordingly, may not have been 

introduced from an outside source, but the mercury, 

barium, chlorine and fluorine were brought in. On 

the other hand, the transporting medium may have 
QUICKSILVER DEPOSITS 79 

carried elements that, although materially affecting 

its character, did not enter into combinations now 

preserved. Many of these mineral products have been 

reproduced in the laboratory, but, for the most part, 

under environments that did not approximate those 

most likely to be found in nature. The laboratory 

experiments, therefore, are most useful in showing the 

chemical and physical possibilities of an inferred 

geologic process. As processes of ore deposition are 

suggested by mineral products that can be observed 

accumulating at the present time, the assumption of a 

magmatic origin of quicksilver deposits directs attention 

towards a comparison with volcanic fumaroles and 

hot springs. 

Fumaroles and hot springs selectively transfer material from a magma to the surface. As they are surface 

phenomena within the zone of possible oxidation, their 

products are not soley those of the magma, but also 

contain components from atmospheric gases, ground 

water, and from the rocks through which they traveled. 

In contrast most ore deposits are believed to have 

formed below the zone of atmospheric oxidation, and 

consequently the primary products were not influenced 

by atmospheric oxidation. Because the Terlingua 

quicksilver deposits are associated with pre-ore caverns, the deposits most likely formed in or above rocks 

that were saturated with ground water; therefore, at 

some stage of mineralization the transporting medium 

was probably diluted with ground water. However, 

as fumaroles ultimately die out and many become hot 

springs, a comparison of the Terlingua quicksilver deposits with fumaroles cannot be neglected. 

The composition and changes in composition of gases 

from volcanoes and fumaroles would be important factors to consider in interpreting the formation of the 

quicksilver deposits if the transporting medium was in 

a gaseous state. Common gases associated with volcanism are steam, carbon dioxide, hydrogen chloride, 

sulfur dioxide, hydrogen sulfide, hydrogen, carbon monoxide, methane, vapors of metallic chlorides, sulfur 

vapor, and gaseous compounds of fluorine and ammonia. Some of these gases, such as methane and 

ammonia, may not be volcanic in origin but freed from 

sedimentary rocks by volcanic heat. All these gases 

are never discharged simultaneously from any one fumarole or volcanic vent, but some of them may appear in 

a certain regular order as the temperature of the discharge decreases. This was pointed out by Deville and 

Leblanc as early as 1858, and has been restated by later 

investigators. Clarke (1924, p. 292), and, more recently, Sapper (1927, p. 82-83) have summarized these 

data. According to Sapper, gases at temperatures from 

200°0 to 1000°0 include S02, HOI, C02, H20, NH,Cl, 

and FeOla, and those below 200°0 include H20, H2S, 

and 002. Changes are primarily from decreasing temperature, which permits interaction between gases and 

between gases and wall rocks. These of course are generalizations based in part on data of doubtful reliability. 

Mineral products of the fumaroles are mainly sublimates of sulfur, ammonium chloride, and metallic 

chlorides. Sulfides and oxides of some metals have been 

found but are not common. Most sublimates are 

readily soluble in water and are therefore unlikely to 

be preserved in an ore deposit. Moreover, the relatively insoluble ore minerals that can be readily transported in gases, in particular the metallic chlorides, are 

commonly considered supergene. However, at least 

two chloride minerals found at Terlingua are not supergene and are minerals capable of gas transport. 

Quicksilver deposits, hot springs, and volcanic rock 

are so commonly associated that a genetic relation has 

long been postulated. The association was pointed out 

as early as 1888 by Becker (1888), who believed that 

quicksilver deposits were formed at shallow depths by 

hot waters similar to those of hot springs now flowing 

in areas mineralized by quicksilver. Becker reported 

that at the time of his visit to Steamboat Springs, Nev., 

and Sulphur Bank, Calif., cinnabar was being deposited 

from hot spring water. Although there is some doubt 

that cinnabar was deposited by the spring waters by 

the process described by Becker, there is little doubt 

that the quicksilver deposits so closely associated with 

the hot springs formed in an environment similar to 

that of the springs. The Terlingua quicksilver deposits 

were certainly not deposited at the surface but they 

may have formed in a system 500 to 1,000 feet below 

discharging hot springs. 

To list, describe, and do cum en t the many places 

where quicksilver deposits are associated with hot 

springs or hot spring sinters is beyond the scope of this 

report (see White, D. E., 1955), but associations arE) 

numerous enough to permit the summarization of what 

se.em to be valid generalizations, which are: (1) hot 

springs are common in many mercury districts mineralized in late Tertiary and Recent age, (2) quicksilver 

minerals are rarely found in superficial hot spring sinters, but commonly were deposited below the surface, 

and (3) the associated hot spring waters have a range 

in composition and are now very low in their content of 

mercury. 

The mineral products of hot springs are more like 

those of quicksilver deposits than are products deposited directly by volcanic gases. Spring deposits 

contain silica and calcium carbonate, two combinations 

common to quicksilver deposits and foreign to most 

fumarolic deposits. Sulfides of the metals, notably 

those of iron, antimony, and arsenic, are commonly 

found in small amounts. Although it is chemically 
80 GEOLOGY AND QUICKSILVER DEPOSITS, TERLINGUA DISTRICT, TEXAS 

possible for almost all the mineral combinations found 

in quicksilver deposits to be transported and deposited 

by gases, only a few of these combinations are typical 

of fumarolic deposits, whereas almost all the minerals 

associated with quicksilver are common to hot spring 

deposits. 

The comparisons between quicksilver deposits and 

the deposits of hot springs or fumaroles indicate that 

the environment in which the quicksilver minerals were 

deposited was· most similar to that of hot springs. The 

comparisons; however, do not preclude the possibility 

that some of the ore elements were transported by a 

gas from the source tO near the place of deposition. 

Nevertheless, to reach the place of deposition any 

postulated gas would most likely have to pass into a 

zone where rock openings were filled with ground water 

The temperature and pressure range through which 

the ore minerals were deposited, although well within 

that of epithermal deposits, cannot be stated definitely, 

but the evidence suggests that the deposits were formed 

at temperatures below 300°0 and at pressures that 

were locally at least as high as 30 atmospheres. Evidence for the temperature of deposition is mainly based 

upon the anomalous optics and conversion temperature 

of kleinite (see p. 70), which appears to have formed 

at temperatures over 130°0. As the paragenetic relations of this mineral indicate that it formed at an intermediate age, the other minerals were formed at both 

higher and lower temperatures. The melting points 

of kleinite (260°0) and calomel (302°0), the oldest ore 

mineral, suggest that the maximum temperature was 

below 300°0. The dissociation of mercuric sulfide into 

its elements at temperatures of 250°C (Krauskopf, 

1951, p. 522) or less is in agreement with this maximum. 

The pressures at which deposition occurred depends 

largely upon the thickness of rock cover and consequently upon the weight of the -column of fluid above 

the point of deposition. Maximum pressures cannot 

be estimated with any accuracy because we do not 

know how far mineralized rock extended above the 

outcrops of ore, and because we do not know whether 

the water table was at or above the upper limit of 

mineralization. Minimum pressures can be postulated 

with more assurance, assuming that the flow was slow 

enough to approximate hydrostatic conditions. Since 

ore deposition, erosion has lowered the surface, perhaps 

1,000 feet, perhaps considerably more. If the water 

table at the Chisos mine was 1,000 feet higher than the 

present surface at the time of mineralization, the hydrostatic pressure at the bottom of the mine (900 feet 

below the present surface) would be on the order of 60 

atmospheres. If the present topographic surface at 

the Chisos mine was the water table that existed during 

mineralization, pressures at the bottom of the mine 

would have been about 30 atmospheres-this is the 

minimum. 

Under these relatively low temperatures and pressures, fluids containing sulfur, iron, chlorine, hydrogen, 

oxygen, nitrogen, and mercury deposited these elements 

in various combinations to form the ore minerals. 

Calcium and carbon were likewise present and combined to form the gangue calcite; as the gangue minerals 

were replaced by cinnabar these elements again entered 

the fluid. Undoubtedly other elements, which did not 

enter into mineral combinations, were present. The 

elements were most likely in the waters as simple ions, 

complexions, un-ionized molecules, and colloid particles. 

Some elements were in molecular combinations, dissolved in the water and possibly free as gas bubbles. 

Conditions favoring separation of the ore elements 

from their transporting medium probably did not exist 

before the entry of the solutions into those parts of the 

channels where the ore minerals occur. Here conditions were such that the solutions were no longer 

stable and deposition resulted. 

Because no samples of these waters are available, the 

state of the ore elements in the solutions is uncertain. 

The problem of determining how the ore elements were 

present is not to be solved by reversing ore deposition 

and redissolving the mineral products-the solutions 

would not be the same. Consequently from here on, 

reliance must be placed upon the compositions of 

known volcanic gases and hot springs, fortified by 

laboratory-verified chemistry, and filled in by assumptions of varying degrees of validity. 

The above was the approach used by George F. 

Becker (1888) in postulating the alkaline sulfide theory 

of quicksilver deposits, a theory that has been widely 

accepted and extended to include deposits of other 

metals. Because cinnabar is the least soluble metallic 

sulfide and can only be dissolved readily in solutions 

of sodium or other alkaline sulfides, Becker proposed 

that such were the solutions from which cinnabar was 

deposited. Becker believed that mercury was present 

in complex ions of sodium sulfide in the waters of 

Steamboat Springs, Nev., which upon cooling precipitated cinnabar. 

The alkaline sulfide theory as stated by Becker is 

not completely acceptable, primarily because it is not 

supported by the composition of hot spring waters 

most closely associated with cinnabar or other sulfide 

minerals. D. E. White (1955, p. 146) in speaking 

of the compositions of hot springs most closely related 

to quicksilver deposits says, 

The thermal waters that appear to be most closely related to 

quicksilver deposits are near-neutral; present springs provide 

no support for the theory that quicksilver is transported in 

alkaline sulfide solutions. In general, the highest dissolved 
QUICKSILVER DEPOSITS 81 

solids and total sulfide content are found in near neutral waters. 

Although thermal waters are known with pH's as high as 10.0, 

the most alkaline waters are relatively dilute, and none js known 

to contain more than a few parts per million of sulfide. 

Other objections of almost equal weight are that the 

alkaline sulfide theory to be effective requires waters 

of a fairly high alkalinity; temperature decreases 

prevent rather than aid the precipitation of cinnabar; 

and cinnabar does not replace pyrite as it should to 

follow Schiirmann's series. This last objection is 

overcome if excess hydrogen sulfide is present; its 

presence would prevent the re-solution of the pyrite. 

J. P. Pollock (1943) has since shown that it is not 

possible to precipitate cinnabar from such a solution 

by either cooling or evaporation of the solution; precipitation can only be accomplished by dilution, 

acidification, or addition of precipitants. Accordingly, 

Pollock (1944) has amended Becker's theory by 

proposing that the mercury found in the spring waters 

analyzed by Becker was colloidal cinnabar that had 

been formed by the rapid dilution or neutralization 

of alkaline sulfide solutions bearing mercury coming 

into contact with deep ground water, or from colloidal 

solution near the source and subsequent flow to the 

point of deposition. 

Recentty Krauskopf (1951) has substantiated the 

theoretical possibility that the transportation of 

mercury in hydrothermal solutions can be in the form 

of the complex sulfide ion. This conclusion was 

reached after he made an analysis of the chemical data 

on mercury and a consequent quantitative delimitation 

of the physicochemical conditions of quicksilver 

deposition. Krauskopf, however, comes to the conclusion that this is not the only way that mercury can 

be transported for he says (p. 521), 

Three possible mechanisms for the transportation of mercury 

in vein solutions stand up under detailed scrutiny: transportation as a complex ion in slightly alkaline sulfide solutions, as 

the volatile chloride, and as mercury vapor. For two other 

mechanisms, supersaturation and colloidal disperson, no definite 

statement can be made about their probable importance or 

lack of it. 

Thompson (1954) in applying Krauskopf's concepts 

to the quicksilver deposits at Terlingua proposed the 

disequilibrium conditions of supersaturation to account 

for the concentration of ore in restricted pockets-a 

feature typical of quicksilver deposits. Thompson's 

proposal has much in common with Pollock's postulated transportation of cinnabar by colloidal solutions 

and has some of the same difficulties, mainly that both 

supersaturated solutions and colloidal solutions are not 

particularly stable under conditions of high temperature and turbulent flow. 

Neither Pollock's nor Thompson's revision of Becker's 

theory is a complete explanation of the origin of the 

Terlingua quicksilver deposits. The presence of calomel as a primary mineral suggests that it is desirable 

to modify the theory still further by proposing that 

the ore-forming system contained a gas phase from 

which calomel was directly deposited. 

Such a proposal would not be necessary if the calomel 

could be considered a supergene mineral that was 

derived through the leaching of cinnabar by ground 

waters of high hydrogen chloride content. Although 

such a mechanism might theoretically produce calomel, 

evidence that it operated at Terlingua is completely 

lacking, as the following discussion indicates. Broderick (1916) has experimentally demonstrated that 

cinnabar can be dissolved by hydrochloric acid solutions but because his results are not quantitative it is ' not possible to evaluate the efficacy of such a solvent 

in the calcite environment that exists at the Mariposa 

mine. In contradiction to the supergene origin of the 

calomel is the complete lack of leaching of any cinnabar 

that was studied. The most valid objection to this 

concept, however, is the distribution of calomel and 

cinnabar which occur in the same general area and at 

the same depth, but almost never in close association. 

Pockets of calomel and other associated mercury 

minerals are found within a few feet of masses of cinnabar, but the pockets themselves are characteristically 

free of cinnabar. These relations do not suggest a 

supergene origin for the calomel. 

.Although calomel and cinnabar are both 

primary minerals, their striking differences in form 

and manner of occurrence suggest that they were 

formed by different processes. Cinnabar commonly 

replaced the gangue minerals; calomel and the other 

mercury minerals apparently did not. Cinnabar rarely 

lines open cavities as crystal druses; the mercury 

chloride minerals almost always occur as crystal linings 

in cavities. The replacement character of the cinnabar 

indicates that it was formed by hydrothermal processes. 

The "open space" character of the calomel and associated minerals would allow deposition from either water 

or vapor. Because the mercury chloride minerals 

show many features typical of sublimates, such as 

chains of perched crystals, long fragile acicular crystals, 

and fine powdery coatings, deposition from a vapor 

phase is the favored process. 

Whether all the kleinite, mosesite, terlinguaite, 

eglestonite, montroydite, and native mercury is of 

primary origin from a gaseous phase is not known. 

Native mercury is so closely associated with calomel, 

being commonly found as tiny droplets on freshly 

broken cleavage planes, that some of it must be primary. 

If this "included" mercury is considered as having been 

held by the calomel in solid solution, the calomel can be 

interpreted as forming from vapors containing both 
82 GEOLOGY AND QUICKSILVER DEPOSITS, TERLINGUA DISTRICT, TEXAS 

mercury and mercury chloride. It is possible that the 

other rare mercury minerals may have formed in part 

by reactions between supergene solutions and calomel. 

The reaction proposed by Switzer and others (1953) 

for the synthesis of mosesite at 25°0, 

4NH,OH + 2Hg2Cl2-72Hg+ Hg2NCl· H20+3H20+ 

3NH4Cl, 

could be typical of calomel attacked by supergene 

waters-providing the waters contain the proper 

solvents. The suggestion that kleinite, terlinguaite, 

eglestonite, and montroydite could also be produced by 

supergene waters, does not preclude the possibility that 

these minerals can be produced at higher temperatures 

by gases. Regardless of whether these minerals can be 

formed by gases, their distribution at Terlingua clearly 

indicates that they were not formed by supergene 

alteration of cinnabar. 

Having concluded that the most feasible mechanism 

for the formation of the mercury minerals in the 

Terlingua district involves a system having both 

gaseous and liquid phases, it is desirable to expand this 

conclusion into a more complete interpretation of their 

origin. The mechanism proposed is essentially a 

system dominated by the vapor phase in the early part 

of its history but gradually becoming dominated by 

liquid-phase relations. 

The basic assumption stated at the beginning of this 

section is that the ore elements were either derived from 

a magma that differentiated to produce the great 

variety of igneous rocks now exposed in the district or 

were distilled from deeply buried sedimentary rocks by 

heat from a magma. Relations between igneous rocks 

and cinnabar indicate that cinnabar is younger than the 

rocks and accordingly could not have formed until 

differentiation of a parent magma was well along or 

completed. If the differentiation of the parent magma 

resulted from or accompanied crystallization, mercury 

did not escape from the magma in any appreciable 

quantities until late in crystallization. Whether gases 

carrying metallic elements can and do escape from a 

magma at such a time is a controversy too theoretical 

and too involved to repeat here; nevertheless it should 

be pointed out that the arguments used against gas 

transfer do not necessarily apply to mercury, which, 

because of its common lack of association with most 

other metals, must have an almost unique cycle. If 

mercury was distilled from sedimentary rocks it 

probably would have migrated under conditions of 

moderately low temperatures in company with gaseous 

organic compounds. 

The composition of the vapor phase can be s~t 

· within only broad limits. Mercury, chlorine, sulfur, 

and doubtless large quantities of hydrogen and oxygen 

as steam were present. If the common fumarole and 

volcanic gases are a clue to the probable composition, 

one can safely infer the presence of sodium, potassium, 

and carbon. Doubtless there were other elements that 

were never fixed in the quicksilver deposits. The 

above elements could occur in many combinations; 

consequently the vapor phase could be exceedingly 

complex, but it would remain a vapor as long as temperatures remained above that at which steam could 

condense into water. 

Metallic mercury in this vapor phase should be 

relatively stable as long as temperatures remain above 

400°0, because, above this temperature, mercury does 

not precipitate with other elements. Cinnabar would 

not form as long as temperatures remained above 

250°0, for Krauskopf (1951, p. 522) has shown that 

cinnabar "cannot even exist in any [probable] system 

that includes moving vapors above a temperature of 

around 250°0." A vapor phase such as this could not 

persist indefinitely; a liquid phase would logically 

develop as ground water entered the system and the 

steam condensed. A liquid phase was probably present 

while temperatures were still too high for cinnabar to be 

deposited. Mercury would be transported in this twophase system in both the gas and liquid phases. In the 

gas phase it could be present as both mercury vapor and 

mercurous chloride; in the liquid phase it could be 

present as the sol, the complex sulfide ion, or perhaps 

some other form. 

With progression up the channel and cooling by 

meteoric water, mercury is condensed to the liquid 

phase. If the quantity of mercury in the gaseous 

phase is greater than that which can be dissolved in 

equilibrium in the water, mercury either precipitates 

or is transported in the liquid by other than equilibrium 

solutions. If the mercury had been deposited from 

equilibrium solutions, it would be expected that deposition would be progressive and that the grade of the ore 

bodies would decrease downward. This, however, is 

not the case; there is no systematic decrease in the 

grade of ore with depth; ore bodies tend to terminate 

within short distances. It therefore appears likely 

that mercury was not deposited immediately once equilibrium was attained, but that it was transported 

further by other than equilibrium solutions. 

Transportation in the liquid phase with greater concentrations than the equilibrium amounts could be accomplished by either supersaturated solutions (Thompson, 1954) or colloidal solutions (Pollock, 1943) or both. 

Although the feasibility of mercury existing in the 

supersaturated or colloid states in a temperature range 

between 100°0 and 250°0 has not been experimentally 

verified, it seems necessary to postulate such transport 
·QUICKSILVER DEPOSITS 83 

to provide a mechanism that will prevent a gradual 

"dribbling out" of cinnabar before structures favorable 

to ore deposition are reached. 

At temperatures below 250°C this sulfur-bearing system, as proposed, would contain mercury in its vapor 

phase as well as in its water phase. The concentration 

of mercury ions need never have been high, because 

whenever mercury ions were lost by precipitation they 

could be replaced from the supply of colloidal cinnabar. 

In the advancement of such a mechanism for colloidal 

transport of sulfide, R. M. Garrels (1944, p. 482) has 

pointed out that "A highly dispersed solid phase is in 

equilibrium with its saturated solution and so has all 

the advantages of a saturated solution, plus a reservoir 

of material in solid form." Garrels suggests that the 

precipitation of galena might proceed in the following 

manner, "If the solution were moving in a trunk fissure, 

its saturated solution would penetrate the wall rock, -

cool and precipitate galena. This would establish a 

precipitation gradient, with the solution in the trunk 

fissure continuously dissolving and migrating outward 

to form the replacement deposit in the wall rocks. The 

final product would give no clue to the colloidal state 

of its origin." A precipitation gradient for the deposition of cinnabar could not be established by a simple 

cooling of the solution as is postulated for galena, because solubility of cinnabar is increased instead of de.;. 

creased with falling temperatures. However, this 

change in the physical characteristics of the channel 

might be adequate in itself to create a precipitation 

gradient. The change in channel is also an enormous 

change in the ratio of surface area to volume; consequently it is accompanied by a corresponding increase 

in the influence of surface tension and adsorption phenomena. Concentrations of solutions within these 

small openings certainly would not be the same as those 

in the trunk channel. Circulations of fluids in the 

small openings would be negligible in contrast to movements in the trunk channel; accordingly ionic diffusion 

would be the main means of moving rna terial necessary 

to form replacement cinnabar. The nature of the 

openings and of the ore indicate that a precipitation 

gradient did exist, regardless of what factors were important in producing this gradient. 

As suggested earlier in this section, the physical properties of the clays may have had a considerable influence on the deposition of the ore minerals. The influence of the clays on the stability of the fluids may 

depend upon the surface tension of the clays; this may 

be able to change the concentration of the gases and the 

solubilities of solutions entering the pores of the clays. 

Verhoogen (1948, p. 210-217), in writing on the geological significance of surface tension, has pointed out 

these factors. In speaking of the gas phase of a magma 

484889-60-8 

he says (p. 214), "the composition of a gas phase permeating the pores of the wall rock will be different from 

that in equilibrium with the magma through a plane 

interface." If we substitute clay for "wall rock," ore 

fluids for "magma" (which is also a solution with a 

gaseous phase), and walls of the open channels for 

"plane interface," we have a statement that is applicable to the deposition of ore minerals. Verhoogen 

says (p. 213) that when a saturated solution of a number of constituents is introduced at constant temperature into small pores, the solution will become oversaturated with respect to some constituents and 

undersaturated with respect to others. Although the 

magnitude of these effects-which depend upon numerous and varied factors-is unknown. it may be that they 

supply the trigger action that would explain the close 

association between clay and cinnabar. 

The preceding discussion has dealt largely with a 

theoretical ore-forming system that would permit the 

deposition of calomel and later deposition of cinnabar, 

but the seemingly anomalous distribution of these two 

minerals has not been fully interpreted. It can be explained, if it is assumed that sulfide was deficient 

during the vapor stage when calomel was deposited in 

the later liquid stage, even a little sulfide present in the 

liquid would react with the calomel to produce cinnabar. 

In the presence of either a neutral or an alkaline 

solution, cinnabar, with a solubility product constant 

of 10-5\ is more stable than calomel, with a solubility 

product constant of 10-18 ; consequently calomel would 

dissolve and cinnabar would deposit. If the system 

were deficient in sulfur, the dissolved mercurous chloride 

might be dispersed in the ground water; if the system 

contained a surplus of sulfur, the mercury of the dissolved calomel could be redeposited as cinnabar. If 

excess sulfur were present as hydrogen sulfide, the 

reaction might be as follows: 

The hydrochloric acid formed would react with calcite 

and thus keep the reaction going in the direction of 

cinnabar. If the conditions of ore genesis are as _proposed, it can be assumed that the calomel that remains 

is calomel that was never accessible to the liquid phase 

of the system when cinnabar was being deposited. 

TRANSPORTATION AND DEPOSITION OF THE GANGUE 

ELEMENTS 

The gangue minerals may have been deposited during 

an early stage of the activity that culminated in the 

deposition of the ore minerals or it may have been an 

early event unrelated to ore deposition. No ~vidence 

to prove either hypothesis was found. -The deposition 

of calcite and hydrocarbons through -the period of ore 
84 GEOLOGY AND QUICKSILVER DEPOSITS, TERLINGUA DISTRICT, TEXAS 

minerals suggests a close association between ore and 

gangue, but as both hydrocarbons and calcite can 

migrate readily, the association means little. The traces 

of cinnabar in many calcite veins suggests an even closer 

affinity between ore and gangue, but the even more 

numerous barren calcite veins may indicate that the ore 

elements merely migrated along channels used at some 

preceding time by the gangue elements. It is extremely 

unlikely that all ore and gangue elements were from the 

same source; but it is very likely that their migrations 

were the result of processes related to the same heat 

source. 

The barium and fluorine of the barite and fluorite 

may be of magmatic origin, but the great bulk .of the 

introduced gangue, which is calcite, was derived from 

the limestones through which the solutions flowed. The 

open channels below the cave-fill deposits attest to the 

solution of limestone, as the calcite veins attest to the 

deposition of calcite. The migration of gangue elements 

from wall rock to deposits is common in the quicksilver 

deposits, as is shown by the close similarity between 

wall rock and gangue compositions in quicksilver deposits in Nevada (Bailey and Phoenix, 1944, p. 15), 

where deposits in siliceous rocks have a silica gangue and 

those in carbonate rocks have a carbonate gangue. 

DESCRIPTIONS OF MINES AND PROSPECTS 

Descriptions of both mines and prospects are included 

in the following pages. As it is not possible to make any 

sharp division between mines and prospects on the basis 

of ore produced-the only logical distinction-these two 

terms are used very loosely. Consequently, some properties are listed as mines, although their workings consist 

of only a few shallow open pits or trenches and their 

productions are not more than one or two flasks of 

·quicksilver. On the other hand, some properties listed 

as prospects, although having small productions, are not 

developed e~ough to be considered as mines. The application of the term "mine" to a property, however, does 

not necessarily imply that the property has been, or 

ever will be, a profitable enterprise. Nor do the lengths 

of the individual descriptions have any bearing on the 

relative merits of the properties. Descriptions of some 

properties are longer than those of others that are just 

as important economically, purely because they are the 

products of relatively detailed examinations made for 

various Federal wartime agencies. Unfortunately, time 

was not available for similar detailed examinations of 

all properties. 

The mining properties are arranged geographically, 

proceeding from west to east. They are located by 

section and block numbers, as well as by geographic 

features and neighboring properties. The public land 

lines of Texas run north and east and in general subdivide the land into mile-square sections, groups of 

which are designated as blocks. However, the sections 

are not always 1 mile square and the blocks have no 

uniform number of sections within them. Nor is there 

uniform numbering of blocks or sections within blocks. 

CONTRABANDO DOME PROSPECTS 

The prospects on Contrabando dome are at the 

western end of the district, 17 miles by road west of 

Terlingua. They are in sees. 104, 105, 108, and 109, 

Block 341. These prospects and the Fresno mine 2 

miles to the north represent cinnabar discoveries made 

chiefly by Harris S. Smith and Homer M. Wilson in 

the 1930's, discoveries which extended the known belt 

of quicksilver mineralization about 8 miles westward. 

Total production of the prospects on Contrabando 

dome is probably not more than 10 flasks. Two flasks 

were produced from one of the prospects in sec. 104. 

In April 1943, B. L. Shoemaker was working two of 

the prospects under lease, but shortly afterward abandoned them. One of these prospects was examined in 

some detail. The workings consisted of a 60-foot shaft 

with two short crosscuts at depths of 35 and 60 feet. 

Exploratory workings at other prospects on Contrabaudo dome are similarly shallow and small in extent. 

A few core drill holes have been put down to greater 

depth, and scattered core may still be seen at some of 

these, but logs of the holes were not available. None 

of the prospects is equipped with furnaces or retorts. 

The country rock of the area is Boquillas flags. The 

lower member of the Boquillas flags is exposed in the 

central part of the dome, and the upper member covers 

the flanks (pl. 1). Small dikes and plugs of rhyolite 

have intruded the flags, particularly along faults 

trending northeasterly. Cinnabar has been found in 

several of the rhyolite bodies and in calcite veins filling 

the faults. 

The shaft of the Shoemaker prospect in · sec. 104 was 

slink on cinnabar· showings along the north contact of 

a plug. A 20-foot drift was driven southwest at a depth 

of 35 feet to the rhyolite-flags contact, where a small 

pocket of ore was mined. Shoemaker reported that 

this pocket assayed 10 to 12 pounds of quicksilver per 

ton. Most of the two flasks of quicksilver produced 

ftom the prospect came from this pocket. A drift 

was driven southward from the 60:...foot level to intersect 

the mineralized zone mined above, but work stopped 

before either the projection of the zone or the contact 

was reached. 

The association of cinnabar with rhyolite is probably 

entirely structural. Both are associated with faults, 

and' many of . the fra.ctures within the rhyolite belong 

to the regional system of northeasterly-trending fractures. The rhyolite is more ·competent than the flags 
DESCRIPTIONS OF MINES AND PROSPECTS 85 

and presumably is continuous vertically through a 

considerable thickness of relatively impermeable shales 

and flags. Autobrecciation, flow banding, and dragging 

of the contacts help to make the rhyolite bodies and 

their borders relatively good hosts. 

Development of mineable ore will probably depend 

upon deeper exploration, although shallow ore bodies 

are a distinct possibility that should be looked for. 

The areal extent of baked rock indicates more widespread intrusion at depth. This might be a sill and 

carry ore similar to that in the Study Butte sill. The 

base of the Grayson formation, if it is not too deep 

because of a section thickened by intrusive rocks, Is 

also a possible locus of ore. 

FRESNO MINE 

The Fresno mine, at the western end of the district, 

is 17 miles by road west of Terlingua. It is near the 

north edge of sec. 106, Block 341. Cinnabar was discovered and exploration begun in the 1930's by Harris 

S. Smith. Mining was begun in 1940 by Smith and 

Homer M. Wilson. The proving of ore at Fresno 

justifies some optimism about chances for future discoveries in the western part of the district, because 

little or no prospecting has previously been done in 

this area. However, practically all prospecting and 

exploration in the Terlingua district stopped soon after 

World War II because of the low price of mercury. 

The mine, which is shallow and dry, is reached by 2 

shafts, each less than 100 feet deep (pl. 10). The 

main stope is nearly flat, 300 feet long, and averages 

about 60 feet wide (pl. 11). Crosscuts total about 500 

feet and are mainly below the stope level. In addition 

to the main workings there are several shallow shafts and 

many pits. Ore was mined from the large pit east of the 

No. 1 shaft and also from veins on the top of the hill 

1,000 feet west of theN o. 2 shaft. Ore was treated in a 

rotary ft...:l·:rnice 30 feet long by 2 feet inside diameter, 

using butane gas for fuel. The rated capacity is 35 

tons per day. Production to April 1943 is estimated 

to be slightly more than 2,000 flasks, from ore averaging 

about 20 pounds of quicksilver to the ton. 

The rocks of the Fresno mine area are, from oldest to 

youngest, Devils River limestone, Grayson formation, 

Buda limestone, Boquillas flags, and Chisos volcanics. 

Erosion has removed little of the Devils River limestone; the mine area is probably underlain by 1,000 to 

2,000 feet of . this formation. The Grayson formation 

is about 120 feet thick but varies from place to place. 

The thickness of the Buda limestone is 130 feet. Only 

the lower part of the Boquillas flags is uneroded and 

it has a thickness as much as 400 feet. The Chisos 

volcanics, consisting of undifferentiated lava flows, 

tuffs, and stream-laid sediments, lies unconformably 

on the upturned Boquillas flags. 

The Fresno mine area is in a structural embayment 

on the monoclinal, south flank of the Terlingua uplift 

(pl. 1). The area is cut by numerous faults most of 

which trend northeasterly to easterly (pl. 10). Oval 

or circular outcrops of breccia pipes occur at several 

places. These breccia pipes are 50 to 200 feet in diameter and consist of fragments of stratigraphically 

higher rocks. Some of the fragments are displaced 

downward at least 300 feet. Calcite is present in some 

of the breccia pipes, both as a breccia filling and as 

small veins, from some of which cinnabar can be panned. 

The contact of the Devils River limestone with the 

overlying Grayson formation is the key to an understanding of the geologic structure in the mine. This 

contact is highly irregular, although the limestone 

beds are nearly flat, dipping no more than 10°. Pockets 

of clay commonly extend 50 feet or more down into the 

limestone, and in places broken limestone and clay 

appear as if stirred together. The No. 2 shaft was 

sunk through 62 feet of clay before reaching limestone, 

although limestone crops out within 10 feet of the 

collar. Pockets of clay mixed with limestone fragments 

were found in the open pits and the main stope. However, the limestone is solid and relatively undisturbed 

in the crosscut only a few feet below the main stope. 

These features originated by solution of limestone and 

collapse of the overlying clay. Thus they are similar 

in origin to the cave-fill zones of the Mariposa mine. 

The cave-fill zones at Fresno are more irregular, however, and also less well exposed. 

Most of the ore mined at Fresno came from an irregular cave-fill zone which was localized in part by 

fractures trending northeasterly to easterly. Much 

of the cinnabar was closely associated with clay, · had 

a laminated structure similar to the unmineralized 

clay, and a finely crystalline to powdery texture. 

It is interpreted to be the result of deposition in and 

replacement of the clay. Associated minerals in order 

of relative abundance are gypsum, calcite, fluorite, 

quartz or chalcedony, and iron oxides. A small amount 

of ore was mined from a calcite vein in Buda limestone 

but the ore was of lower grade. 

LOWE PROSPECT 

The Lowe prospect is on the northwest side of 

Lowes Valley (pl. 1) outside the mapped area, in sec. 

100, Block G-5. It consists of pits, trenches, and short 

adits that were opened early in the history of the 

district by a man named Lowe. The workings are in 

altered and silicified Devils River limestone near small, 

irregular intrusive bodies of rhyolite. For the most 
86 GEOLOGY AND . QUICKSILVER DEPOSITS, TERLINGUA DISTRICT, TEXAS 

part, they follow fractures that range in trend from 

N. 15° E. to N. 30° W. The limestone along and near 

the fractures is silicified, brecciated, and stained brown 

by iron oxides. Associated with the silicified limestone 

is abundant iron oxide, pyrite, calcite, jarosite, and 

manganese oxide, and ·very small quantities of cinnabar. 

The cinnabar was the last mineral to form as it fills 

small fracture openings that cross all the other minerals. · 

This area is the only place in the Terlingua district 

where there is abundant silica associated with the 

cinnabar. 

PROSPECTS IN SEC. 98 

· In sec. ·98, Block G-5 there are a few shallow cuts and 

trenches along narrow cave-fill zones in the Devils 

River limestone. The · zones trend N. 20° W. and 

N. 70° E. No cinnabar wasseen here. The cave-fill 

zones may represent the roots or keels of more extensive 

deposits that have been eroded away. 

SAMPLE GROUP 

Much of sec. 44, Block G-12, is included in the old 

Sample group of 15 claims held by Frank Duncan and 

associates, but in 1944 Duncan's interests were mainly 

in the three adjoining Claims, which lay east and west 

through the center of the section. The topography, 

geology, and mine development of these three claims is 

shown in plate 12. The property is connected by road 

with Terlingua. 

Development work consists of about 100 feet of adit, 

a 25-foot shaft (said to have been 85 feet deep before 

caving), and at least 1,000 feet of relatively shallow 

trenches. A retort capable of burning a charge of 

more than 600 pounds of ore is on the property. No 

reliable information about the production is available, 

but rumors have production as 1,000 flasks of quicksilver during World War I when it was worked by 

Colquitt and Fletcher Tigner. Judging from the extent of the workings, it seems doubtful that a fifth of 

the rumored quantity was produced. 

Most of the rock in the area is massive, fine-grained, 

and somewhat cavernous Devils River limestone, containing irregular nodules of chert concentrated at certain horizons. A little Grayson formation in elongate 

masses about 200 feet long and as much as 50 feet wide 

is enclosed in the limestone east of the retort and at the 

largest workings east of the claims (pl. 12). Some of 

this clay retains stratification, whereas much is structureless ; and in some places contains fragments of 

Devils River limestone. It is the same material that 

is regarded as cave-fill at the Mariposa and other mines 

and got in its present position by collapse and washing 

into open solution-caverns in the limestone. 

The major structure is a broad northwesterly trending· 

anticline; the flat crest and the gently dipping upper 

part of the southwestern flank cross the Claims diagonally. Fracturing was strong enough to brecciate some 

of the limestone and make channels for . mineralizing 

solutions. Some fractures were enlarged by solution of 

the limestone to cavernous passages, which were subsequently lined with calcite or filled to various degrees 

with clay and rubble. Most of the .breccia is of this 

kind. 

Two poorly defined zones of· mineralized rock trend 

northeasterly across the claim; one zone trends from 

southwest of the retort on the Lindbergh claim up the 

ravine to the central part of the north boundary of the 

Spirit of St. Louis claims; and the other trends from 

near the southeast corner of the Spirit of St. Louis 

claim through the northwest corner of the Curtis claim. 

Within these zones few single veins of calcite are more 

than 100 feet long though they are as much as 5 or 6 

feet thick. Most veins, especially those of gentle dip, 

are associated with irregular sheets of jaboncillo. 

The broad zone of calcite and jaboncillo about 200 feet 

north of the retort (pl. 12) seems locally to be nearly 

parallel to the surface and perhaps 4 feet thick, although 

in some places, as revealed by the longest trench, coarse 

calcite extends downward for more than 15 feet. The 

longest continuous vein was traced more than 250 feet 

southwesterly from the road in the eastern part of the 

Lindbergh claim. It has been prospected by a pit 10 

feet deep, which exposes 3 feet of calcite and jaboncillo 

to the bottom. No cinnabar was seen. 

Traces of cinnabar were seen in the western group of 

workings that extend from the shallow pit 250 feet 

southwest of the retort to the trench in the gully bottom 450 feet northeast of the retort, as well as within a 

100-foot strip in the eastern group of workings. Cinnabar localities are shown on the accompanying map 

(Pl. 12). The best showings are in a zone 6 inches 

wide that extends for about 20 feet in the 3-foot calcite 

vein exposed in the adit near the retort;' and in a 

remnant, 6 inches by 3 feet by 2 feet, of the ore body 

removed from the long trench 150 feet farther east. 

Much poorer showings are in the calcite at the eastern 

end of the trench. The calcite and jaboncillo are 4 

feet thick for most of the 100 feet where the gently 

dipping vein goes into the wall. Other showings of 

cinnabar are in calcite and jaboncillo exposed in the 

trenches on .the hillside 250 feet north of the retort, in a 

zone of mineralized rock that dips about 20° south, 

approximately parallel to the surface and rather close 

to the dip of the beds. In the east group of workings, 

an excellent showing of cinnabar in a zone 2 feet wide 

and 6 feet long was seen in the coarse. calcite of the 

trench 350 feet east of the road, and poorer showings 

at the head of the trench 250 feet east of the road. 

The only visible ore is in the retort adit described 
DESCRIPTIONS -· -oF MINES AND PROSPECTS 87 

above and in the two trenches described in the eastern 

group, which if continuous between the trenches, is a 

cinnabar body 80 feet long, dispersed through not more 

than 2 feet of the 6-foot thickness of coarse calcite. 

BOBS MINE 

Bobs mine, in sec. 44, Block G-12, is in the MitchellGillette group of claims and lies between the claims of 

Frank Duncan to the northwest and those of Nat ural 

Resources, Inc., to the southeast. The most recent 

mining activity has been in a breccia pipe in the Maggie 

claim (fig. 19). However, there was considerable 

interest in this general area during and before World 

War I, when numberous trenches, pits, and shallow 

shafts were dug along the ridge of Tres Cuevas 

Mountain, which extends southward towards the 

Natural Resources, Inc., holdings. In 1947 the 

property was held by Robert Pulliam of Alpine, Tex. 

From 1940 through 1942 it was held by the Continental 

Mining Co., which did considerable development work. 

The breccia pipe in which most of the recent development work was done is in the topographic feature known 

as Maggie Sink, which is an elliptical depression, 535 

feet long by 210 feet wide. The south part of the sink 

has been excavated by power shovel to a maximum 

depth of about 30 feet and a shaft has been sunk from 

the bottom of the excavation. An adit has been 

driven from the hillside west of the sink for 540 feet to 

end within the breccia pipe about 135 feet below the 

bottom of the shaft. A winze was sunk from the end of 

this adit. 

The Maggie Sink breccia pipe, enclosed in Devils 

River limestone, consists of blocks and fragments of 

Buda limestone, Boquillas flags, and Chisos volcanics 

set in a matrix of Grayson formation. 

The breccia pipe is on the crest of the northwesterly 

trending asymetric anticline that forms the ridge of 

Tres Cuevas Mountain. The northeast flank of the 

anticline dips from 10° to 15° and the southwest flank 

as much as 70°. Fractures and small faults extend 

both across and parallel to the anticlinal axis. These 

are in two principal sets, one with a northeasterly 

trend and the other with a westerly to northwesterly 

trend. 

The breccia pipe of Maggie Sink is described on 

pages 40-41. A little high-grade ore has been taken 

from it, but until 1946 no commercial ore bodies had 

been discovered. In the surface workings in the west 

part of the pipe, cinnabar was found in j aboncillo and 

calcite that coats the limestone walls of the pipe and 

also in brecciated material within the pipe. In the 

brecciated material the cinnabar is found as disseminations within jaboncillo (altered Grayson), as coatings on fracture planes, and as nodules of nearly pure 

cinnabar. It is reported by Mr. R. B. Davis that one 

nodule taken from the bottom of the shaft weighed 

26 pounds and was estimated to contain 65 percent 

cinnabar. A small pocket containing several tons of 

high-grade ore was discovered in 1944 in the deeper 

workings below the adit level. 

Southeast of Maggie Sink the Devils River limestone 

of Tres Cuevas Ridge is cut by numerous northeasterly 

trending and some northwesterly trending fractures. 

Most northeast-trending and some northwest-trending 

fractures are filled with calcite veins, the most persistent of which are shown on fig. 19. The calcite veins 

range from less than 1 inch wide to several feet wide, 

and from 1 foot to several hundred feet long. They 

characteristically pinch and swell both along their 

strikes and dips. Cinnabar is sparse in these veins and 

its occurrence is similar to that in veins in the Sample 

group of claims. There has been considerable exploration and mining on these veins, and they yielded an 

unknown quantity of ore during early operations in the 

district. For the most part the trenches are shallow 

and short, but a few are more than 100 feet long and 

two shafts have been sunk more than 40 feet. No 

workings, however, are more than 50 feet below the 

surface. 

NATURAL RESOURC~S, INC., CLAIMS IN SEC. 58 

Nat ural Resources, Inc., holds claims in the northwest 

corner of sec. 58, Block G-12, as well as in the southern 

part of the same section. The claims in the northwest 

part are those of the old Tres Cuevas Quicksilver Co., 

which is reported to have mined quicksilver from them 

during World War I. Natural Resources, Inc., explored 

some of the ground in the northwest part of the section 

from 1941 to 1943 and extracted a small amount of ore 

from them. This area is shown in figure 20. Ore re· 

duction facilities include a small retort near the princi· 

pal workings and a Cottrell rotary furnace several miles 

away in sec. 40. 

Development and exploration during World War II 

included extensive trenching of the cinnabar showings, 

sinking several shafts less than 40 feet, tunneling about 

150 feet from the shafts, and drilling shallow holes. An 

estimated 200 tons of ore was mined. The only known 

ore left in place is in the opencut on the west side of 

No. 1 shaft (see fig. 20) ; even this is very limited in 

lateral extent and depth, for at the bottom of the shaft 

there are only a few streaks of powdery cinnabar. According to Mr. A. Gard about 100 tons of ore was treated 

in the rotary furnace in December, 1942, and only 4 

pounds of quicksilver per ton were recovered. He 

believes the low recovery was in part caused by faulty 

furnace operation. 

Thick bedded to massive Devils River limestone is 
88 

100 0 

A 

GEOLOGY AND QUICKSILVER DEPOSITS, TERLINGUA DISTRICT, TEXAS 

.A 

'~ ""~ ~C;. 

1\ 

\\ 

\\ 

\\ 

\\ 

\\ 

\\ 

~ .::::::::-

~ ~ 

\\ 

\\ 

~ 

~ 

\\ 

II 

100 200 300 400 500 Feet 

Contour Interval 10 feet 

~tum is ~pproxiirYie su Je.,.J 

EXPLANATION 

Collapse breccia of Grayson ~ 

~ 

formation, Buda limestone, a:: 

~~i~: ... ":f~~i~~ ~~ ~ 

o js8 0::&..1 

Devils River limestone 0 U 

Fault dashed where 

approximately located 

Strike ·and dip of beds 

I~ I I I---

Calcite vein, dashed where 

approximately located 

~ 

Shaft 

:::::·::::::::::=> 

Portal of adit, dashed where approximately located 

0 Prospect pit 

;;"II IIIII\\ 

,,f///1';\\~\ 

Dump 

Grayson formation and Buda(?) limestone, brecciated 

A' 

Mapped by Robert G. Yates and James F. Me Allister, April 1942 with additions in September 1943 

FIGURE 19.-Geologlc map and section of Bobs mine, sec. •4, Block G-12. 
DESCRIPTIONS OF MINES AND PROSPECTS 89 

EXPLANATION 

All rocks are Devils River limestone 

t60 u 9 

Trench 

D 

Fault or fracture; showing 

dip. U, upthrown side; 

0, downthrown side 

Fault or fracture zone 

· approximately located 

..2 Strike and dip of beds 

1!1 Shaft 

.. -~!( / "Y 

/ 

./ I 

-o 

\ 

100 0 100 200 300 Feet 

Contour interval 20 feet 

Datum is approximate mean sea level 

Geology and topography by 

G. A. Thompson and R. E. Brown 

FIGURE 20.-Geologic and topographic map of part of Natural Resources, Inc., claims in sec. 58, Block G-12. 

the only rock exposed at the workings. It ranges from 

fine to medium grain and some beds are composed almost entirely of fossil shells. The beds are flat to gently 

dipping, except immediately adjacent to faults, but 

south of the workings they are folded abruptly downward to form the Terlingua monocline. In the developed area (fig. 20) the rocks are broken by three 

main fracture systems: (1) vertical fractures with an 

average strike of N. 15° W., (2) steeply dipping fractures with an average strike of N. 40° W., and (3) 

steeply dipping fractures with an average strike of 

N. 60° E. The N. 15° W. fractures are the most 

numerous and persistent laterally but are the least 

mineralized. They commonly end against bedding 

planes and show slickensides that are horizontal or dip 

with the bedding. Vertical displacements on these 

fractures appear to be negligible or measured in inches. 

TheN. 40° W. fractures locally have vertical displacements in excess of 3 feet. The N. 60° E. fractures, the 

best mineralized, have displacements locally of at least 

4 feet but are extremely discontinuous and irregular. 

The fractures along which the best cinnabar showings are localized belong to the N. 60° E . system and 

extend in a poorly defined zone through No. 1 shaft, 

No. 2 shaft, and the adit (see fig. 20). In many places 

the fractures are tight, nearly invisible cracks in one 

limestone bed and coarse breccia and clay zones in a 

bed immediately above or below. This has been caused 

by solution widening of the fractures and subsequent 

filling with debris. Cinnabar occurs in fractures of 
90 GEOLOGY AND QUICKSILVER DEPOSITS, TERLINGUA DISTRICT, TEXAS 

all three systems, but most of it is localized in the 

more brecciated places along the main N. 60° E. fracture zone. Associated with the cinnabar are calcite, 

clay, and iron oxides. Cinnabar is present as fillings 

in breccia, paint along cracks, and disseminations 

1n clay. 

MONTE CRISTO AND CROESUS CLAIMS 

Part of the work done in the early 1930's by the 

Tarrant l\1ining Co. was on a group of four claims in 

the northeast corner of sec. 58, Block G-12, mainly 

in the claim called the Monte Cristo. These claims 

were not examined by the writers and the description 

that follows is that of C. P. Ross. Workings on the 

Monte Cristo claim comprise two small stopes extending from the surface down about 40 feet and a shaft 

begun in March 1934 to prospect ground southwest of 

the stoped area. The principal work done on the 

Croesus claim was the removal of about 500 tons of 

ore from a small stope in the southeastern corner of 

the claim. During the World War II no work was 

done on this claim or the adjoining Croesus claim. 

The older and the higher of the two stopes on the 

Monte Cristo claim is an irregular opening in Devils 

River limestone from which were mined a number of 

pockets of cinnabar ore, in part along fracture planes 

trending N. 40° W., and in part along nearly horizontal bedding planes. Fractures of N. 60° E. trend are 

also present but in this stope appear to have had little 

effect on the ore deposition. Although individual ore 

bodies here were small, they are reported to have been 

of satisfactory tenor. 

In the second stope, which adjoins and is connected 

with that described above, -the ore was along a welldefined zone of fractures that trend N. 55° E. and dip 

70° or more to the southeast. The fracture zone has 

been partly filled by the Grayson formation, colored 

dark green by disseminated pyrite. Cinnabar . was _ 

found within the clay. This stope, much the larger 

of the two, is more than 100 feet long. 

The principal stope in the Croesus claim is on the 

lode trending N. 50° E. shown in the extreme northwest corner of figure 21. This stope, reached by adits 

in the canyon wall, is a broken zone containing much 

of the Grayson formation, dropped or washed down 

from above. 

DUNCAN GROUP IN SEC. 58 

The Frank Duncan group of claims in sec. 58, Block 

G~ 12, is in the northeast part of the section just south 

of the Monte Cristo group. The group consists of 

eight patented claims, but as the better cinnabar showings and the principal development are in the two most 

northeasterly claims, the Pilkington and Hoover,-the 

description is confined to them. Cinnabar showings 

are widespread and numerous, although in 1943 no ore 

in minable quantities was exposed. A few tons of 

ore have been treated in a 6-pipe retort. Development work consists of about 150 feet of tunnels, 900 

feet of trenches, and several shafts less than 20 feet 

deep. The geology, topography, and development is 

shown in figure 21. 

Devils River limestone is the only rock exposed; 

it is thick bedded to massive over most of the area 

but thin bedded where the uppermost beds are preserved. The beds are folded into gentle anticlines and 

synclines, and south of the Hoover claim they are 

flexe~ abruptly downward into the Terlingua monocline; The area is broken by steeply dipping to vertical 

fractures, some of which are filled by calcite. Most 

fractures strike northeasterly, in particular N. 40° E., 

and many of the fractures are arranged in echelon. 

Slickensides and mullions are common on fault surfaces; 

they are usually nearly horizontal and none were seen 

that plunge at an angle greater than 20° from the 

horizontal. Strike-slip movements on the faults were 

accompanied by bedding-plane slips, which resulted 

in lenticular breccia zones and the "capped veins" . of 

the local miners. 

Most northeasterly fractures are mineralized; cinnabar in small quantities accompanied by iron oxides and 

calcite is to be seen in a large proportion of the trenches. 

Commonly the cinnabar is thin films or irregular 

stringers only a few inches long and a fraction of an 

inch thick. Small pockets of ore have apparently 

been mined from the largest workings; but the largest 

pocket probably contained no more than 100 tons. 

Local structures largely controlled the deposition 

of these pockets of ore, most of which are in the overlap 

area of two in echelon vertical fractures or in the zone 

of intersection of vertical fractures with beddingplane slips. Such places are centers of brecciation and 

multiple fracturing, which has been intensified by 

solution and slump. One larger ore shoot was along 

a strong fracture and appears to have formed along a 

more open part of the fracture, as is shown by a filling 

of Del Rio clay that slumped or washed in from above. 

More ore may be found in this area by exploring the 

more strongly mineralized fractures. 

MARGARET D LODE 

The Margaret D lode (sometimes called Maggie D 

lode), located mainly in sees. 41 and 40, Block G-12, 

and extending for a short distance into sec. 58, Block 

G-12, is the longest, most continuous, cinnabar lode 

in the Terlingua district. It extends from the southwestern bounding fault of the Well Creek graben in a 

S. 70° W. direction for almost 7,000 feet. Its north-
DESCRIPTIONS OF MINES AND PROSPECTS 91 

CROESUS CANYON 

LINTON CLAIM 

EXPLANATION 

All rocks are Devils River limestone 

_D u __ _ 

Fault, dashed where approximately 

located ; U, upthrown side; D, 

downthrown side 

Fractures ~--- filled with calcite 

veins, showing dip; dashed 

where approximately located 

Strike and dip of beds 

EB Horizontal beds 

t t Anticline, showing trace 

of axial plane and 

direction of plunge 

t t 

Syncline, showing trace 

of axial plane and 

direction of plunge 

Ad it 

~ 

Shaft 200 

Trench, pit, and opencut 

C[aim lines located 

only approximately 

0 200 400 

00 0'1 LO LO 

u u C1l C1l C/) C/) 

Geology and topography by G. A. Thompson 

and R. E. Brown 

FIGURE 21.-Geologic map of the Hoover and Pilkington prospects, sec. 58, Block G-12 

east end is abrupt against the graben fault, which is 

older, but its southwest end is a gradual pinching out 

in the massive beds of the Devil River limestone. It 

lies mainly in holdings of the Texas Chisos Mining 

Co. and Waldron Quicksilver Properties. 

This lode was worked mainly during the early history 

of the district before 1905. It has been described by 

H. W. Turner (1905, p. 274). Development consists 

of many trenches and pits, several shafts, and a few 

small underground stopes. The trenches range from 

a few feet to several hundred feet long, from 5 to 10 

feet wide, and from 4 to 30 feet deep. The shafts are 

from 30 feet deep to an estimated 100 feet deep. The 

underground stopes are small but comparable in size 

with others in this part of the district. The production 

from the lode is unknown, but it is probably something 

between a few hundred and a few thousand flasks of 

quicksilver. 

The lode is along a continuous fracture that has 

been differentially enlarged by solution on its limestone 

484889-60--9 

walls and subsequently filled by either calcite or cavefill. The only discernible movement along the fracture 

was in a horizontal direction and was probably less than 

10 feet. The central part of the lode appears to have 

been the most productive, because it contains the 

greatest number of workings and the most extensive 

development. Cinnabar and other mercury minerals 

were found sporadically throughout its length in both 

the calcite and cave-fill. The productive parts of the 

lode are separated by unproductive parts, which are 

more extensive. 

Cinnabar was the principal ore mineral, but native 

mercury and the oxychlorides were also found. Abundant calcite, clay, and iron oxides were associated .with 

the cinnabar, and minor quantities of fluorite and 

barite were also found. Much of the calcite was 

bituminous. The ore and its occurrence are typical of 

that described as keels of the limestone-clay contact 

deposits on pages 58 and 59. 
92 GEOLOGY AND QUICKSILVER DEPOSITS, TERLINGUA DISTRICT, TEXAS 

CANYON GROUP 

In the northeast corner of sec. 42, Block G-12, is 

a group of five claims known as the Canyon group. 

Work here began in 1929 and continued intermittently 

through 1934. No work was done during the World 

War II period. The description that follows is largely 

that taken from the manuscript of C. P. Ross. In the 

graben valley which underlies much of the group a 

shaft, 80 feet deep, has been sunk through the Grayson 

formation and into the Devils River limestone a few 

feet. A winding drift more than 400 feet long extends 

N. 60° W. from the shaft. It follows a fault zone 

along which there has been some movement and alteration of the limestone close to the undulating base of 

the Grayson. The zone contains some iron oxide and 

calcite, but the only cinnabar reported to have been 

found was at the bottom of the shaft. Small solution 

openings are present at several places along the mineralized zone. 

LAFARELLE AND PROSPECTS NEARBY 

Near the center of sec. 32, Block G-12, there are 

some old workings mainly made under the direction of 

Lafarelle, one of those active in the early development 

of the district. Nothing appears to have been done 

here for a long time. The description that follows is 

after the manuscript report of C. P. Ross. A prospect 

pit about 30 feet deep and other cuts expose a mineralized zone along a fracture or joint in Devils River 

limestone. The fracture trends N. 80° W., and is 

vertical. Its limestone walls are brecciated and iron 

stained and are coated with calcite and oxidized pyrite. 

No cinnabar was seen. 

Another pit about 1,500 feet to the northwest shows 

a similar zone of brecciation and alteration which trends 

N. 70° E. and is vertical. Near here the walls of the 

fault .on the southwest side of the small graben block 

shown in plate 1 are slightly iron stained and enclose 

veinlets of calcite. 

RIO GRANDE QUICKSILVER CO. CLAIMS 

The Rio Grande Quicksilver Co. operated in the 

Terlingua region only during 1932 and 1933 and produced little quicksilver. Its principal development 

was in the western part of sec. 40, Block G-12, where 

it held some patented claims and a mining lease. No 

work was done on these claims during World War II. 

The description that follows is after that in the manuscript report of C. P. Ross. The Rio Grande Quicksilver Company had a rotary furnace and retorts near 

the base of a cliff. This furnace was operated for a 

short time during 1942 by Natural Resources, Inc., 

which treated ore taken from claims in sec. 58, Block 

G-12. The company also had a mineral prospect 

permit on part of sec. 36, Block G-12, and mineral 

leases on parts of sec. 44, Block G-12, and sec. 70, 

Block 341. 

A shaft, northwest of the furnace and on the plateau 

at the top of the cliff, is reported to be 100 feet deep, 

but in March 1934 the lower 50 feet were inaccessible. 

At the 50-foot level 150 feet of workings cut 3 calcitelined fractures in the Devils River limestone. Short 

drifts on the 25-foot level are connected with the 

surface through stopes along a 4-foot zone of brecciated 

limestone containing calcite, clay, and a little cinnabar. 

The zone trends N. 65° E. and is nearly vertical. It 

has been followed from the shaft for about 60 feet to 

the northeast and 150 feet to the southwest. In the 

cliff face below and northeast of these workings and 

close to the furnace, a short tunnel cuts several minor 

calcite and breccia seams of northwest trend. 

WALDRON WORKINGS IN SEC. 40 

Waldron Quicksilver Properties holds claims covering 

much of sec. 40, Block G-12. The principal development consists of a set of very irregular drifts and stopes 

totalling a few hundred feet of work that is located 

directly across the draw from the Rio Grande furnace. 

The workings extend to a depth of about 80 feet. 

When visited by C. P. Ross in August, 1934, no work 

had been done for about 20 years. Good ore is reported 

to have been mined, and the mine faces showed a little 

ore. Work was resumed here later in 1934, but continued for only a short time and the property was idle 

during World War II. 

According to C. P. Ross (written communication) 

the principal workings follow a steep zone of brecciated 

and altered Devils River limestone which trends 

N. 60°-65° E. Calcite in small crystals coats some of 

the breccia and coarser calcite bands line and in part 

fill vugs in the limestone. In the upper workings 

exploratory drifts disclose calcite, lining and partly 

filling openings on joints of N. 60° E. and N. 40° W. 

trends. 

MARIPOSA MINE 

The Mariposa mine ranks second in production of 

quicksilver in the Terlingua district. It is in sec. 59, 

Block G-12, and is about 7 miles by road west of the 

Terlingua post office (see pl. 1). The property is 

controlled by the Esperado Mining Co. of Houston, 

Tex. 

Since 1895 the Mariposa mine has produced between 

20,000 and 30,000 flasks of quicksilver. Most was 

produced before 1911, but some during World War 

I and a little between 1933 and 1945. In the spring of 

1945 the Esperado Mining Co. was reworking some of 

the old dumps and mining some ore from the Perry pit. 
DESCRIPTIONS OF MINES AND PROSPECTS 93 

The productive ground, a rectangular area roughly 

3,500 feet long and 1,000 feet wide, centers about 

California Hill (pl. 13). The principal subsurface 

workings (pl. 14), about 3 miles of drifts, stopes, and 

crosscuts, are under California Hill, but much of the 

ore was removed from pits, trenches, and shafts northeast and west of the hill. Of the 121 shafts deeper than 

20 feet, 6 are more than 100 feet deep and 2 of these, 

the No. 5 (Cruz) shaft and the White shaft, are 300 

feet or more deep. The Contratiro winze (pl. 14) 

extends 200 feet below the 100 level of the mine. 

Several thousand feet of drifts and crosscuts extending 

from this winze from the deeper shafts explore the mineralized ground through a vertical range of about 300 

feet, but almost all the ore was removed from the 

uppermost workings. 

The Buda limestone, Grayson formation, and Devils 

River limestone are the sedimentary rocks in the Mariposa area. An erosional remnant of Buda limestone 

caps California Hill and protects the underlying 

Grayson formation from rapid erosion. The Grayson 

forms the flanks of California Hill and covers the Devils 

River limestone in the southwest part of the area (pl. 

13). Devils River limestone, stripped of the Grayson 

formation, crops out in the eastern and western ends of 

the area (pl. 13). The sedimentary rocks have a 

general northwest strike and an average dip of about 

5°SW. 

Soda latite dikes and sills intrude the Devils River 

limestone and Grayson formation in the southwe,st part 

of the area. In general the dikes occur in the limestone 

and the sills in the clay. The large sill that forms 

the southern part of California Hill connects with a dike 

that is explored by workings in the vicinity of No. 11 

shaft. Another soda latite dike crops out 170 feet 

northeast of Goldby shaft. 

Faults along the northwest, southwest, and southeast 

borders of the mapped area (pl. 13) bound a depressed 

block of Devils River limestone. Vertical displacements on the faults at no place exceed 100 feet. The 

planes of fault movement are largely obliterated by 

later solution of the limestone walls. 

Most fractures in the depressed block trend northeast, but a few trend northwest. The northeasterly 

fractures range in trend from N. 40° E. to N. 85° E., 

but most trends are either N. 60° E. or N. 75° E. Only 

a few fractures have vertical displacements; however, 

horizontal grooves indicate that most movement was 

probably in a horizontal direction. East and west of 

California Hill, the fracture pattern is expressed in 

elongate outcrops of Devils River limestone that are 

separated by narrower zones of cave-fill. The cave-fill 

occupies solution caverns that developed in the upper 

beds of Devils River limestone adjacent to the northeasterly fractures. The cave-fill as mapped (pls; 13 

and 14) includes many blocks and small pillars of limestone and some small areas of highly altered limestone 

or extremely cavernous limestone partly filled with 

calcite. The cave-fill zones Bxtend under California 

Hill where they are explored by the mine workings. 

The cave-fill zones at the Mariposa mine are type 

examples of the limestone-clay contact deposits in the 

Terlingua district and are described on pages 55-59. 

The ore bodies were distributed within and along the 

borders of the cave-fill zones as veinlike bodies and as 

irregular tabular bodies and pockets. Their general 

form in some places roughly corresponded to that of the 

cave-fill which enclosed them; however, in only a few 

places did they extend from wall to wall of the cave-fill. 

As all known ore bodies have been mined out, little can 

be said of their detailed character and exact dimensions. 

According to old reports, some ore was mined from ca,lcite veins and a little from the limestone, but doubtless 

most ore came from altered clay in the cave-fill zones. 

No record of the average grade of ore from the Mariposa mine is available. In the early years of the mine 

only the better grade ore was treated and the lower 

grade ore was put on the dumps, most of which have 

since been reworked. According to Hill (1902), in 1901 

ore that was being treated in retorts averaged from 8 to 

25 percent quicksilver, and ore that was being treated in 

the furnaces about 3 percent. During this early period 

many rich pockets were mined; one contained more than 

20 tons of ore that averaged over 50 percent quicksilver. 

Figures given by Phillips (1905) for ore that was being 

treated in the Marfa and Mariposa Scott furnace in 1904 

differ somewhat from . those of Hill. According to 

Phillips, the furnace was treating ore that averaged less 

than 1.5 percent quicksilver, but not infrequently 

treated ore that contained as much as 2.5 percent quicksilver. Material that was mined from 1942 to 1945 

ranged from ore that contained less than 0.2 percent 

quicksilver to ore taken from small pockets that contained almost 50 percent quicksilver. 

The principal ore mineral is cinnabar, but important 

quantities of native mercury and other rare mercury 

minerals were mined. The cinnabar occurs as disseminations and as solid masses in the clay matrix of the 

cave-fill and to a much lesser extent as veinlets and 

disseminations in the limestone. 

In March 1945, the Mariposa mine had no appreciable 

ore reserves. At that time a small body of ore was 

being mined from the Perry pit and a very small amount 

of dump material was being screened by hand and 

trucked to the Chisos mine for furnacing. The total 

production of the mine, a maximum of 30,000 flasks of 

quicksilver since 1895, came principally from the cavefill zones. The geologic map (pl. 13) shows that about 
94 GEOLOGY AND QUICKSILVER DEPOSITS, TERLINGUA DISTRICT, TEXAS 

75 percent of the cave-fill area has been mined or 

intensively explored. Less than 50 percent of the 

remaining cave-fill area is unexplored or inadequately 

prospected by widely spaced trenches and pits. A 

large number of the prospect · pits and trenches shown 

on plate 13, represent exploration done since 1935. 

Even though the ground with the best indications of 

cinnabar was explored during this period, few ore 

bodies were found. The unexplored ground that 

remains shows less mineralization than did the explored places; therefore, it is not likely to be so productive. 

Small ore pockets probably remain in the area 

shown by the geologic map (pl. 13), but because of their 

probable small size and scattered occurrence, the cost 

of exploration may not be justified. These pockets can 

most economically be searched for by sinking pits on 

the better showings of cinnabar, as determined by 

panning the surface clay of the cave-fill. The ground 

that is covered by alluvium east of the Perry pit and 

that in the west end of the mapped a-rea (pl. 13) deserve special attention in any exploratory program. 

Small quantities of cinnabar can be panned from 

much of the cave-fill clay, but this material is of too low 

grade to be worked, even at the high prices of the 

World War II period by existing mining practices. 

Future price conditions and new technology may make 

possible the mining of much ofthis material. 

It does not seem advisable to explore the Devils 

River limestone at depths more than 50 feet below the 

base of the Grayson formation. In the mapped area 

(pl. 13) more than 25 percent (about 1 mile) of the total 

underground workings are below this depth and, judged 

from· the number and size of the stopes on these lower 

levels, the ore mined probably did not compensate for 

the cost of its discovery and extraction. 

TARRANT PROPERTY IN SEC. 39 

The main camp, a 10-ton rotary furnace, and retorts 

of the Tarrant Mining Co., of Forth Worth, Tex., were 

in sec. 39, Block G-12. Mine workings are scattered 

over the southwestern part of the section, mainly in the 

grabenlike depression occupied by upper Well Creek. 

The Tarrant Mining Co. mined and treated ore in the 

district during 1933 and 1934 but did not operate 

during World War II. - In addition to the property 

in sec. 39, this company had holdings in sees. 58, 38, 

and 60 of Block G-12, and sees. 69 and 70 of Block 341. 

The more extensive underground workings (according 

to C. P. Ross's manuscript} in sec. 39 include those of 

the No. 1 shaft located at the base of the cliff near the 

southwest corner of the · section (fig. 22). The shaft 

is 130 feet deep. More than 2,400 feet of exploratory 

drifts have been opened on the 84-foot level, largely in 

unaltered · Devils River limestone just below the 

undulating base of the Grayson formation. A drift 

southwest of the shaft follows a fracture of northwest 

trend. This appears to correspond to a narrow vein of 

bituminous calcite on the level below. More than 100 

feet north of the shaft, drifts and small stopes extend 

for more than 300 feet along a zone of brecciated and 

altered limestone of which the Grayson formation 

locally forms the southeast wall. The average trend is 

N. 55° E. At one place there is a 2-foot band of 

kaolinized material, possibly a much altered dike, 

which makes an acute angle with the fracture zone. 

The parts of the drifts near the shaft explore a zone 

of altered Devils River limestone and partly altered 

the overlying Grayson formation. The trend of the 

zone, as developed, averages about N. 50° E., but on 

the 84-foot lev.el, especially west of the shaft, there is 

little fractured rock along the zone. Northeast of the 

shaft the Grayson formation within the altered zone is 

flexed downward toward the southeast. Between this 

point and the 130 level there are small stopes. Evidently the ore mined here lay in part along the lower 

con tact of the shale and in part formed irregular masses 

in the altered limestone. On the 130 level below the 

stopes there are fractures in the limestone, of which 

the principal one trends about N, 45 ° E. and dips 

steeply northwest. A little work has been done below 

the 130 level, presumably on a continuation of this 

fracture, but this part of the mine was inaccessible in 

1934 as well as in 1940. Most of the 25 feet of drifts 

on the 130 level are in fresh Devils River limestone. 

About half a mile north of the workings about the 

No. 1 shaft, the No. 3 shaft, which starts on top of a 

hillock, goes down nearly 250 feet and has more than 

1,100 feet of drifts at the bottom, as shown in figure 23. 

The shaft and drifts are in unaltered Devils River 

limestone. The drifts explore a system of joints, 

which locally show thin calcite veins and solution vugs. 

At one. place a little iron oxide is visible. 

Numerous shallow workings are scattered over the 

southwestern part of sec. 39. Trenches on the high 

ground west of the No. 1 shaft mark a continuation of 

the mineralized fracture system so conspicuous near 

California Mountain. Others along Well Creek and 

its branches between Clay Mountain and the No. 2 

shaft (pl. 12) and along Well Creek east of this shaft, 

explore seams of both · the N. 40° W. and N. 60° E. 

systems, and locally mineralized bedding planes in the 

nearly flat Devils River limestone. · At the workings 

at the No. 2 shaft, work was in progress in March 1934. 

Stoping here extended nearly 20 feet below the surface 

on a zone of calcite seams with cinnabar in Devils 

River limestone. The zone trends N. 10°-30° W. and 

dips 60° SW., flattening below. Soine calcite seams 
DESCRIPTIONS OF MINES AND PROSPECTS 95 

50 

Geology by C. P. Ross and C. H. Coldwell, 

March 1934 

50 100 Feet 

EXPLANATION 

[:· .. ·.t<g >:j 

Grayson format1on 

G Devils River limestone All levels below 84·foot level are in Devils River limestone 

u --,---o Fault, show1ng dip; U, upthrown side; D, downthrown side 

Fault fissure

~ 

, showmg dip; 

direction of displacement, if any, not known 

.....!' 

Strike and dip of beds 

~ 

Shaft 

C8:) 

foot of raise or winze 

0 Head of raise or winze 

-

Stope 

841evel 

_____ :"\ __ _ 

130 level 

(-·-·-

Intermediate level 

-=t ::.::.::. ::.: Inaccessible workmgs, from company maps 

FIGURE 22.-Sketch map of the workings off the No. 1 shaft, Tarrant Mining Co. property. 

are vertical and others follow the nearly flat bedding 

of the limestone. The maximum width of the zone in 

the stope was about 3 feet. 

LITTLE THIRTY-EIGHT MINEs 

The Tarrant Mining Co. controls the Little ThirtyEight Mine in the southern part of sec. 38, Block G-12, 

but did very little work there. This lode has been 

known since early in the history of the district and 

yielded considerable ore during the World War I 

period. The workings are shown in figure 24 and 

include a vertical shaft 250 feet deep, with levels off it 

every 50 feet. Stopes and natural openings connect 

the levels, especially in the northeast part of the mine. 

Below the shaft, irregular skidways, with some drifting 

and stoping off them, follow the lode down to an 

additional depth of more than 160 feet. The lode 

s After written communication by Clyde P. Ross. 

trends N. 65° E. and dips 80° SE. Crosscuts on the 

200 and 250 levels explore the hanging wall. The 

drifts and crosscuts total about 2,000 feet. In addition, open spaces, in part artificial, extend more than 

100 feet to the northeast beyond those drifts that lie 

between the surface and the 200 level as a minimum 

depth. 

The lode consists of an opening alqng one of the 

series of northeasterly joint fissures in the Devils River 

limestone, enlarged by solution and lined and partly 

filled by calcite. Locally the distance between limestone walls is more than 15 feet, but the average is only 

a few feet and in places it narrows to a few inches or 

even closes entirely. The greatest widths between 

walls are in the eastern part of the mine, especially on 

the upper levels. In some places the larger openings are 

scores of feet long. Elsewhere there are roughly circular 

solution pipes, 6 feet and less in diameter. Some of _these 
96 GEOLOGY AND QUICKSILVER DEPOSITS, TERLINGUA DISTRICT, TEXAS 

extend out into t4e wall rocks. The open places left 

after calcite deposition, probably somewhat enlarged by 

renewed solution, have been in part filled by rubble. 

Within the workings most of the clastic fill and some 

of the calcite have been removed in mining. In some 

of the calcite-lined openings in the northwest part of 

the mine which may never have been filled, the calcite 

is corroded and small stalactites formed. · In most places 

the calcite bands and combs are roughly parallel to the 

walls of the main fissure, but locally bands lie horizon tal 

with a tendency to curve upward at their edges. Exceptionally, as in the northeast part of the 50 level, bands 

of calcite crystals blanket masses of poorly cemented 

rubble, having crystallized after filling of the opening by 

rubble had begun. 

Most of the material filling the openings along the 

fissure is rubble, cemented to varying firmness by a 

material resembling caliche. The rubble consists of 

angular to slightly rounded fragments of limestone, 

calcite, and shale, in part held together rather loosely, 

EXPLANATION 

__e_ 

A~~r:rr~~Dfsi~~o~; t~~1fss~~e is nearly vertical with variable direction of dip; partly filled with calcite 

90 

Vertical fissure; ~artly filled with ca1<:1te 

~ 

Shaft 

181 

Foot of~~ 

50 0 

in part so firmly cemented as to require blasting to 

mine. Many fragments are altered and many are 

jaboncillo. In addition it is reported that the material 

mined locally contained bands of jaboncillo roughly 

parallel to the walls. Much of the cinnabar was in such 

bands. The fragments commonly range from a fraction 

of an inch to a few inches in maximum dim·ension, with 

fine-grained material in the interstices. According to 

report, much of this rubble-fill contained enough cinnabar to be high-grade ore and most of it yields some 

cinnabar on panning. Apparently most of the cinnabar 

is in grains in the finest part of the interstitial material 

rather than in the larger fragments or in the calcite. 

In the northeast part of the 50 level, calcite bands and 

clastic material are so intermingled as to show that the 

two modes of filling alternated. 

At the surface the northeast part of the lode is masked · 

by caliche-cemented conglomerate fully 10 feet thick. 

Low, irregular rooms have been excavated along the 

contact between the conglomerate and the limestone 

50 lOOFeet 

._____._ _ _.__ __ ___._---=-Geo-:1-ogy--:by:--~C. P. Ross end C. H. Coldwell, March 1934 

FIGURE 23.-Sketch map of the workings oft No.3 shaft Tarrant Mining Co. · 
DESCRIPTIONS OF MINES AND PROSPECTS 97 

50 

----------- Approximate projection of open 

space from surface to 200 level. Open below the 200 level, but not accessible 

EXPLANATION 

Inclined workings, chevrons point down 

f8J Foot of raise or winze 

0 Head of raise or winze 

Underground workings 

0 50 100 Feet 

Mapped by C. P. Ross and C. R. Coldwell, April 1934 

FIGURE 24.-Sketch map of the Little Thirty-Eight Mine. 

on which it rests. About 250 feet southeast of the 

shaft (fig. 24) a series of trenches follows a lode in the 

limestone for about 600 feet. A little cinnabar and 

fairly abundant fluorite and barite were seen along this 

lode. This may be the same mineralized joint fissure 

that is explored in a drift off the long crosscut on the 

200 level, although at that level it shows nothing but a 

little calcite. 

STAR MINE AND NEIGHBORING WORKINGS 

The northern part of sec. 70, Block 341, contains 

many trenches. The principal recent development is on 

the Star claim in the northeast part of the section. 

Here the Tarrant Mining Co. penetrated a block of 

Grayson formation with a shaft 80 feet deep and explored the Devils River limestone below with about 250 

feet of drifts. According to C. P. Ross (written com-
98 GEOLOGY AND QUICKSILVER DEPOSITS, TERLINGUA DISTRICT, TEXAS 

munication) one of the drifts follows for 60 feet to the 

northeast, a zone of rusty, brecciated limestone with 

calcite seams. The same company also did a little 

work close to the road northeast of the shaft. A few 

hundred feet farther west and just north of the main 

road trenches expose a mineralized fracture zone in 

Devils River limestone almost continuously for about 

900 feet. The zone trends N. 70° E. and stands vertical 

or dips chiefly south, having well-marked nearly horizontal striae on the outermost fractures, which are 

several feet apart. At its southwest end this fracture 

zone ends against a zone 20 feet wide of closely spaced 

fracture planes lined with calcite that is in part bituminous. The fracture planes strike N. 45° W. and stand 

vertical or dip about 75° SW. There is little suggestion 

of displacement along either the northeasterly or northwesterly fractures. 

RIO GRANDE PROSPECT IN SEC. 70, BLOCK 341 

The Rio Grande Quicksilver Co. did some development near the center of sec. 70, Block 341. According 

to C. P. Ross (written communication), at this place a 

fault in the Devils River limestone strikes N. 75° E. 

and dips 85 ° N. Grooves on the fault plane are 

inclined 20° E. There are some brecciated limestone, 

calcite, iron staining, and evidence of solution along 

the fault. Some of the solution cavities have been 

enlarged artificially at a point where there is a calcitelined cross seam that strikes N. 30° W. and dips 80° to 

90° E. In the depression on the downthrown side of 

the fault there is a shaft that appears to be fully 7 5 feet 

deep. In 1942 this property was held by A. E. Owens, 

but no work had been done on it for many years. 

COLQUITT-TIGNER (WALDRON) MINE 

The main camp, Scott furnace, and retorts of Waldron 

Quicksilver Properties, Inc., are at the old ColquittTigner mine in the northwestern part of sec. 38, Block 

G~ 12. This company is one of the largest property 

owners in the district. It has claims in sees. 38, 40, 60, 

and 70, Block 12, and sees. 248 and 298, Block G-4, and 

holds the whole of sees. 35 and 37, Block G-12, and part 

of sec. 70, Block 341. The company did some mining 

between 1925 to 1927 and again for a short time beginning in 1934. The latter work was mainly in sec. 248, 

Block G-4, and sec. 40, Block G-12 (see Waldron 

workings in sec. 40, p. 92). The ore was trucked to 

the plant at the Colquitt-Tigner mine. The workings 

in sec. 248 were actively developed during World War 

II and became known as the Two-Forty-Eight mine 

(pp. 106_:_107). The Waldron camp is unique among 

the mining camps in the central part of the district 

in that it has its own domestic water supply, a covered 

concrete tank, in a stream nearby. The description that 

follows is from C. P. Ross's written communication: 

The Colquitt-Tigner mine contains about 900 feet of drifts on 

the main level, which is entered by an adit in the hillside close 

to the furnace and is connected with the surface on the crest of 

the hill by shafts and cuts (fig. 25). Several winzes, inaccessible 

at the time of visit, connect with lower workings. Stopings have 

been done at intervals along the adit and also in the caves in the 

northwestern part of the main level. The workings explore a 

set of complementary joint planes in the Devils River limestone. 

Most joints of northeasterly trend are accompanied by a little 

iron staining and other alteration in the limestone and by calcite. 

It is evident that there was marked local enlargement of these 

joint openings by solution, in part, at least before the deposition 

of the calcite. The interconnected caves in the northwestern 

part of the workings are some of the larger solution cavities in 

the district. They have a combined length of about 340 feet, 

a maximum width of about 60 feet, and are known to extend to 

a depth of about 200 feet. Most of this cavity was originally 

filled with a fine-grained, fairly distinctly banded sediment 

containing sulfide and chloride of quicksilver in such quantity 

that it could be mined as ore. The upper 20 feet or so was 

unfilled. 

LE ROI PROSPECT 

The Le Roi prospect, which consists of 8 claims in 

sec. 70, Block G-12, is abcut 1}~ miles from Terlingua 

by road and less than 1 mile southwest of the Chisos 

and Rainbow mines. Superficial prospecting has been 

done by about a dozen shallow trenches, a shaft about 

50 feet deep, and an adit 35 feet long. No ore has been 

found and only traces of cinnabar can be panned from 

the prospects. 

Most of the claims are along or near the base of the 

cliff of Devils River limestone that forms the south 

wall of the Long Draw graben. North of the cliff and 

graben fault are badly faulted and crumpled Grayson 

formation, Buda limestone, Boquillas flags, and Terlingua clay (see pl. 15). Vertical displacement on the 

Long Draw gr.aben fault is roughly 1,500 feet. Much 

of this displacement is on one fault, but at the eastern 

end of the Speed Wagon claim it is distributed over' a 

complex of faults. This complex of faults converge 1n 

the northeast cor;ner of the Speed Wagon claim to forrn 

a single fault trending southeastward. At the poipt 

of convergence a northwesterly fault splits from the 

graben fault to bound a block of Boquillas flags between 

the Kraben fault ~nd the Terlingua clay that floo'rs 

Long Draw. The beds of Boquillas flags composing 

this fault block are warped into northward-plunging 

folds. 

Some of the major faults and most of the minor 

fracture~ are associated with calcite veins. Calcite is 

the only abundant mineral in the veins; iron oxides in 

the vein zone, however, suggest that pyrite or other 

iron sulfides were introduced locally. Gypsum is common locally, especially in the Grayson formation. The 
DESCRIPTIONS OF MINES AND PROSPECTS 99 

50 0 

EXPLANATION 

~ 

Shaft at surface 

~ 

Foot of raise or winze 

C2J 

Head of raise or winze 

Underground workings 

Mapped by C. P. Ross and C. H. Coldwell, August 1934 

50 100 Feet 

FIGURE 25.- Sketch map of the adit level, Colquitt-Tigner mine. 

calcite is coarsely crystal1iDe and some is notably bituminous. No cinnabar was seen in the veins, but traces 

were panned from tht- longest trench in the eastern 

part of the Speed Wagon claim and, according to Mr. 

Frank Duncan, it can be panned from most of the other 

prospects, a fact which indicates that although the 

mineralization was sparse it was widespread. The zone 

most heavily mineralized with calcite veins is at the 

eastern end of the Speed Wagon claim, where irregular 

areas of calcite in Devils River limestone are well 

exposed across the gully bottom for a distance of 

about 150 feet. An offshoot of this calcite zone is in 

the slot-shaped part of the gully for at least 50 feet 

further upstream. The calcite lode that crosses the 

gully extends more than 200 feet up the southwestern 

bank and continues beyond the Le Roi property. In 
100 GEOLOGY AND QUICKSILVER DEPOSITS, TERLINGUA DISTRICT, TEXAS 

some places the vein zones that compose these lodes 

are as much as 10 feet wide, within a lode-width of 

about 50 feet. Calcite is more abundant on the north-

*est bank of the gully and is covered by rubble on the 

shoulder above it; the erratically mineralized ]ode 

widens at the shoulder to about 150 feet. 

The next major zone of calcite veins is at the mouth 

of the gulch southwest of the shaft, and it is entirely 

in Devils River limestone. Although erratic lodes and 

more widely spaced networks of veins crop out over 

most of the gulch for 200 feet across the mouth and 

back 150 feet, they tend to be alined in two directions, 

approximately north and approximately east. Single 

calcite veins are as much as 3 feet thick for short 

distances. Jaboncillo is commonly associated with 

them. 

: The veins in the Boquillas flags at the western end 

of the mapped area (pl. 15) are more persistent than 

those in the eastern zones; nevertheless they are commonly less than 3 inches thick, although for short 

qistances they are as much as 15 inches thick. The 

eastern group of two groups of veins extends intermittently from the trench at the fault between Boquillas 

flags and Terlingua clay near the north boundary of 

the Oakland claim, under terrace alluvium to the bed 

of the Long Draw, where they are exposed in the 

southeast cliff, and then beyond intermittently for a 

distance of 350 feet, or a total distance of 850 feet. 

.4-lined with this zone are two veins, 1 foot and 2 feet 

thick, respectively, that go diagonally S. 25° E. up the 

steep slope. The larger one is prominent for 150 feet, 

but much less distinct on the other side of the ridge. 

At the steep bank of the creek in Long Draw the veins 

follow fractures of 1- or 2-foot displacement that tend 

to converge downward and towards the northwest. 

The western group of veins does not cross the creek. 

Although these prospects are near the Chisos and 

Rainbow mines, they are on structures and veins that 

are entirely independent of those of these mines. 

Both the Le Roi property and the Ohisos-Rainbow 

properties are associated with bounding faults of the 

Long Draw graben, but there the resemblance ends. 

The Ohisos-Rainbow productive area lies on the upthrown side of the northeast bounding-fault, whereas 

that area of the Le Roi property that is mineralized by 

calcite veins and sparse cinnabar lies mainly on the 

downthrown side of the southwest bounding-fault. 

There was no ore found on the downthrown side of the 

Chisos-Rainbow fault. Further structural differences 

are the absence of faults in the rocks on the upthro\vn 

side of the southwest bounding-fault; faults were conspicuously absent at the Ohisos mine. This difference 

may mean, however, that the best places for finding 

commercial ore bodies on the Le Roi property are below 

the calcite mineralized area on the downthrown side of 

the fault. In this area the most favorable potential 

ore horizon would be the Devils River-Grayson contact. 

CHISOS MINE 

The Chisos mine, which is a-t the town of Terlingua, 

is the outstanding all-time producer in the Terlingua 

district, having yielded two-thirds of the quicksilver 

produced. Most of the mine workings are in sec. 295, 

Block G-4, but they also extend for a short distance 

into sec. 70, Block G-12, where they connect with the 

workings of the adjoining Rainbow mine. The mine 

was in almost continuous operation from the time of its 

discovery, about 1902, to 1943, and during that time 

produced about 100,000 flasks of quicksilver. 

The mine, consisting of about 23 miles of workings, 

is accessible by three main shafts (3, 8, and 9;plate 16), 

but No. 8 shaft is the only one that has been used in 

recent years. Besides these shafts, there are several 

others that connect only with the upper levels and 

the Susano, No. 14, and No. 16 shafts that have no 

connection with the workings of the Chisos mine proper. 

The lateral extent of the workings is 3,000 feet east and 

west and 2,500 feet north and south; the vertical extent 

is 840 feet in the eastern part of the mine and 800 feet 

in the western part of the mine. There are 17 levels, 

roughly at 50 foot intervals, and many sublevels. 

The workings below the 800 level in the eastern part of 

the mine and below the 725 level in the western part of 

the mine are small in extent and were flooded in 1942 

when the writers mapped the property. Practically 

all the remaining workings were accessible at that time. 

Maps of a few typical levels are shown as plates 17, 18, 

and 19. 

Furnace facilities at the Ohisos mine were a 20-ton 

Scott furnace, built in 1903 and razed in 1943, and a 

rotary furnace, which had a rated capacity of 100 tons 

of ore per day. The rotary furnace, installed in 1918, 

was partly destroyed by fire on July 24, 1942, but was 

repaired the following year. Although it is rated as a 

100-ton furnace, it has never been operated at even 

one-half this rated capacity. 

The earliest discovery of cinnabar in the area ·of the 

Ohisos mine was about 1897 near the present. No. 9 

shaft. Workings that developed from this discovery 

were known as the McKinney-Parker mine, which has 

since become a part of the Ohisos property. Discoveries were made east of here about 1902 near the present 

Lafarelle shaft, and these discoveries led to the 

development of the Chisos mine proper. 

Early mining at the Ohisos was done from surface 

workings and later from the South Lafarelle shaft, 

which connects with the 87 level in what is now the 

northeastern part of the mine. Later the No. I, No.3, 
DESCRIPTIONS 'oF MINES AND PROSPECTS 101 

No.8; and No.9 shafts were sunk. By 1912 the No.9 

shaft was down to the 500 level, the No. 3 to the 350 

level, the No. 8 to the 250 level, and the No. 1 to the 

150 level. In 1911 and 1912 production came mainly 

from the No. 3 shaft and in 1913 and 1914 from the 

No. 3 and No. 1. Between 1915 and 1919 rich ore 

from the 550 level and Pipe stope made the best production record in the history of the mine; the peak was 

reached in 1917 when 7,200 flasks of quicksilver were 

produced. In 1917 the No. 9 shaft was sunk to the 

750 level, and production from the early 1920's came 

partly f:rom lower levels off this shaft. As the Chisos 

600 level was gradually extended northward from the 

No.9 shaft, ore was followed into the Rainbow property. 

In 1937 the Chisos production declined sharply, and 

since that time production came mainly from dumps. 

When the mine closed in August, 1942, the Chisos 

furnaces were being supplied partly from low-grade ore 

trucked from the Mariposa mine, partly from a very 

small ore body on the 250 level of the Chisos mine, and 

partly from old dumps. 

The Chisos mine was operated until 1942 by the 

Chisos Mining Co., which was owned by Howard E. 

Perry of Portland, Me. The estate of Mr. Perry 

(legally separate from the Chisos Mining Co.) owns 

sec. 296, Block G-4, sec. 69, Block G-12, and part of 

sec. 70, Block G-12, all of which are adjacent to sec. 

295. In November 1942 the Chisos Mining Co. was in 

litigation and was finally purchased by the Esperado 

Mining Co. of Houston, Tex. 

The rocks, structures, and quicksilver deposits are 

more varied in the Chisos mine than in any other mine 

in the district. Both sedimentary and igneous rocks 

are present. The sedimentary strata include Devils 

River limestone, Grayson formation, Buda limestone, 

Boquillas flags, Terlingua clay, and sandstone and clay 

of the Aguja formation, as well as cemented and 

uncemented Recent alluvium. Only the Boquillas 

flags, Aguja formation, Terlingua clay, and alluvium 

are exposed at the surface. The igneous rock is intrusive and is probably ·a soda latite, although its altered 

character prevents an exact determination. The sedimentary rocks are gently folded and in places strongly 

faulted. Cinnabar was mined from limestone-clay 

contact deposits, calcite veins in the Boquillas flags and 

Buda limestone, and from a breccia pipe. 

The Devils River limestone, Grayson formation, and 

Buda limestone can be seen only in the mine workings. 

The Devils River limestone is below the lowest workings 

in the eastern half of the mine, but is penetrated by 

workings in the western half of the mine as well as in 

the adjacent Rainbow mine. The Grayson formation 

is best exposed in the western part of the mine, but it 

has also been penetrated by workings near the No. 8 

shaft. It ranges from 145 to 185 feet thick; probably 

most of this variation · is the result of flowage adjustment to structural stresses rather than to depositional 

differences. Because this clay disintegrates upon contact with water it is a major mine hazard. The Buda 

limestone is penetrated by mine workings and drill 

holes in both the eastern and western parts of the mine. 

It is about 90 feet thick. The Boquillas flags is exposed 

both on the surface and in mine workings. On the 

Chisos property it is more than 900 feet thick, but an 

exact section could not be measured because of numerous faults. The Boquillas flags are extensively weathered as far down as the 200 level; below this, weathering 

is confined to the immediate wall rock of the veins, 

where ground water could most easily move. The 

Terlingua clay is well exposed in Long Draw south of 

the Chisos mine. In the mine it can be seen only on the 

west side of the McKinney-Parker fault. Its occurrence down to and below the 350 level indicates that 

in this area it has a minimum thickness of at least 400 

feet. 

The only igneous rock that crops out is south of the 

Susano shaft (pl. 16); however, in the underground 

workings sills and discordant dikelike masses of igneous 

rock can be observed in many places. They are most 

common in the western part of the mine, but are also 

found northeast of the No. 8 shaft and in drill holes in 

the Susano area. A drill hole 7 50 feet northwest of 

No. 16 shaft goes through igneous rock from 370 feet 

to 459 feet below the surface. Specimens of igneous 

rock that were collected for microscopic examination, 

although appearing fresh in the hand specimen, were 

found to be greatly changed from their original composition by deuteric alteration. They were questionably 

determined to be a soda latite. The fresh-appearing 

rock is greenish black, aphanitic, and locally porphyritic, having phenocrysts of pyroxene. Underground 

it is in most places altered near its borders to a 

soft, white rock that has lost most of its igneous 

characteristics. 

The Chisos mine is on the northeast side of the Long 

Draw graben and its ore bodies are structurally related 

to this feature (see pl. 1). The fault that determines 

the northeast limit of the graben also determines the 

southwest limit of the mineralized ground. This fault 

is not a simple structure but is composed of several 

parts. The rocks northeast of the graben fault are 

folded into gentle synclines and anticlines that are 

broken by numerous minor faults, some of which are 

splits from the graben fault. All the faults in the area 

are pre-ore structures. 

The graben fault extends across the southern edge 

of the mapped area (pl. 16) in a general northwesterly 

direction. Its course is characterized by abrupt angular 
102 GEOLOGY AND QUICKSILVER DEPOSITS, TERLINGUA DISTRICT, TEXAS 

bends where occur splits from the main fault. The 

graben fault also determines the southwestern limit of 

the ore bodies of the Rainbow mine, and that part of it 

which goes through the Rainbow property is called the 

Rainbow fault. Just northwest of the No. 9 shaft 

the Rainbow fault splits to form the Chisos fault, which 

extends southeasterly past the No. 9 shaft towards the 

No. 3 shaft, and the McKinney-Parker fault, which 

extends southerly towards the McKinney-Parker workings. At the McKinney-Parker workings, the McKinney-Parker segment of the Chisos fault again splits 

and the branch that extends to the southeast, the Cliff 

fault, finally passes off the south border of the area 

shown in plate 16. 

Because of mine dump and alluvial cover the Rainbow fault is poorly exposed; wherever it can be seen it 

is marked by a well-developed breccia zone and forms 

the boundary between the Boquillas flags and Terlingua 

clay. Displacement is between 800 and 1,000 feet. 

Section D-D' of plate 20 shows that it has an average 

dip of about 65°, with the downthrown block to the 

south. A calcite vein and fault that extend N. 75° E. 

through the northwest Rainbow surface workings are 

probably an adjustment expression of the angular bend 

in the Rainbow fault west of the Rainbow shaft. 

Likewise the wide breccia and calcite zone at the east 

end of the Rainbow fault is probably an adjustment 

expression of the junction of the Rainbow, McKinneyParker, and Chisos faults. 

The McKinney-Parker segment of the graben fault 

dips 65° to the west and the west side of the fault has 

been displaced downward at least 600 feet. (See 

section C-C', pl. 20.) 

The Cliff fault segment has less displacement than 

either the Rainbow or McKinney-Parker faults; at its 

west end the displacement is between 200 and 400 feet 

and where it leaves the mapped area (pl. 16) its displacement is only a few feet. 

The Chisos fault cannot be seen on the surface, but 

on the 50 level it passes just east of the No. 9 shaft and 

splits into a number of ·smaller faults before it reaches 

the No. 3 shaft. However, on the 650 level it extends 

as far east as the No. 8 shaft. On the upper levels of 

the mine between the No. 3 and No. 8 shafts the 

Chisos fault is expressed by a number of small nonpersistent faults and calcite veins. The vertical 

component of displacement along the Chisos fault is 

variable; east of No. 3 shaft it is only a few feet, 

immediately west of No. 3 shaft and to within 150 feet 

of No. 9 shaft it is 85 feet, and at the west end of the 

fault, 100 feet north of No. 9 shaft, it is about 250 feet. 

The abrupt change in displacement at the No. 3 shaft 

is the result of cross faults that divide the south block 

into two parts; the change near the west end of the 

fault is the result of drag produced by movement on 

the McKinney-Parker fault. Horizontal and lowangle flutes and grooves indicate some horizontal 

movement along the fault. 

A series of faults with an average trend of N. 30° E. 

are north of the No. 8 shaft, and they may be the northeast expression of the Chisos fault. Calcite veins 

have formed along almost all these faults, which form 

the northeast vein system of the Chisos mine. Displacement on these faults is in general small, probably 

not more than a few feet on most, although one, the 

250 Vein fault has a displacement greater than 30 feet. 

Dips of the fault planes range from 40° to vertical. 

The faults die out to the northeast and tend to converge 

with the Chisos fault to the southwest. They tend 

also to converge at depth. . 

There are numerous other minor faults in the area 

southeast and east of the exploited part of the Chisos 

property. The most persistent of these have strikes 

ranging from N. 60° E. to N. 75° E. and dips ranging 

from 25° to 80°. Probably none have displacements 

greater than 40 feet. 

In the area about the Chisos and Rainbow mines 

fold~ are definitely subordinate to faults. All the folds 

are open structures with dips of the limbs rarely 

exceeding 30°. The two most prominent folds in the 

area covered by plate 16 are synclines. The axis of 

one roughly corresponds to the course of the valley at 

the head of Grace Canyon a,nd plunges in a southeasterly 

direction. The axis of the other trends N. 20° E. and 

passes through the Aguja breccia pipe. In general, 

folds in the western part of the area trend northwesterly 

and folds in the eastern part of the area trend northeasterly. Of particular interest is a zone of sharp 

monoclinal flexure that passes through the northcentral 

part of the area, about 300 feet south of the No. 16 

shaft. This zone trends N. 60° W. and is locally 

more than 100 feet wide. It may mark the blunt 

termination of a small underlying sill. 

None of these folds appear to have any direct 

relation to the ore bodies that have been discovered. 

The minor faults, however, had a marked influence O!l 

the localization of the ore bodies; they served as 

channels for ore-forming fluids that formed two of the 

following three classes of deposits that were found in 

the mine. The three structural classes are: (1) deposits 

in calcite veins in the Boquillas flags and the Buda 

limestone; (2) deposits along and near the contact of 

the Devils River limestone with the Grayson formation; 

and (3) breccia pipe deposits. The dividing lines 

between classes of deposits are not everywhere clear-cut 

and hence some overlapping exists: for example, breccia 

in varying amounts is associated with all three, types 

of deposits. 
DESCRIPTIONS OF MINES AND PROSPECTS 103 

The Chisos mine is the only place in the Terlingua 

district where economic deposit of cinnabar have been 

mined from calcite veins in the Boquillas flags. For 

that reason it is the type example for this class of 

deposit and the general description in a preceding 

section is essentially that of the veins in the Chisos 

mine. Most cinnabar-bearing calcite veins at the 

Chisos mine are arranged in a system of northeasterly 

trending veins that lie north of the No. 8 shaft and 

tend to converge southwestward with the eastern end 

of the Chisos fault, as previously described. A few 

other cinnabar-bearing calcite veins have been worked 

in the Chisos mine, notably those along and near the 

Chisos fault. The size, shape, and relations of the 

calcite veins can be seen in the mine maps and sections, 

and detailed descriptions of them are unnecessary. 

General descriptions are given on pp. 59-63. 

The 550 ore body, although classified with the calcite 

veins, differs enough from them to warrant separate 

description. This ore body was mined mainly from 

the 550 level, from which it receives its name. On 

this level it is divided by a zone of barren rock into two 

parts. The western part of the ore body, in the 

vicinity of No. 3 shaft, was 550 feet long, and the 

eastern part of the ore body, in the vicinity of No. 8 

shaft, was 500 feet long. The ore body averaged about 

30 feet wide and 40 feet high. The separate parts of 

the ore body plunge towards each other beneath the 

barren zone to converge on the 600 level and form a 

single ore body with a length of 200 feet and an average 

width of 30 feet. Below the 600 level the walls of the 

ore body steepen to form an ore body described below 

as the Pipe ore body. 

The 550 ore body was in a breccia zone of the Chisos 

fault. On the 550 level the western part of the ore body 

was in a breccia zone between Boquillas flags on the 

south side of the fault and Buda limestone on the north 

side of the fault. The eastern part of the ore body, 

where the fault displacement is less than 10 feet, was 

in a breccia zone entirely within the Boquillas flags 

just above the contact with the Buda limestone. N arrow calcite vein deposits extended upward into the 

Boquillas flags at the east end of the 550 ore body 

and in the vicinity of No. 3 shaft. 

Cinnabar was in the breccia as a filling in interstices, 

as a replacement of the clay matrix of the breccia, and 

as stockworks of veinlets extending into fractured 

wall rocks. Veinlets of cinnabar extending into fractured wall rock that bordered the breccia locally increased the minable width of the ore body to more than 

40 feet. Associated with the cinnabar were minor 

quantities of calcite and iron oxide. 

The Pipe ore body was the downward extension of the 

550 ore body, but structures of these two ore bodies ar e 

very different. The Pipe ore body had a vertical, 

cylindrical form, whereas both parts of the 550 ore 

bodies, although somewhat cylindrical in form, were 

inclined at low angles. The origins of the breccias 

in the two ore bodies are likewise different; that in the 

550 ore body was dominantly a fault-friction breccia, 

perhaps modified by solution and slump, and that in the 

Pipe ore body was entirely a collapse breccia, produced 

by the solution of underlying limestone. 

The Pipe ore body was a body of remarkably highgrade ore that extended vertically downward from the 

600 level to the 800 level. The breccia pipe that 

contains the ore body extends deeper than the 800 

level, but little ore has yet been found below this level. 

However, cinnabar has been found as deep as the 850 

level according to Charles Worthington (oral communication) who says that the Esperado Mining Co. 

in 1946 found a 6-inch streak of high-grade ore in core 

drilled from the bottom of an old winze on the 850 level. 

The Pipe ore body had an irregular, roughly circular 

shape, with a diameter of 70 feet on the 650 level, 

which decreased to about 30 feet on the 800 level. 

However, on the 800 level the ore body was only a small 

part of the breccia pipe. The Grayson formation 

surrounds the breccia pipe on most levels, but near its 

top Buda limestone is the enclosing rock. Although 

the Chisos fault is at the north edge of the pipe, displacement on the fault at this place is less than 10 feet. 

The breccia in the pipe includes Buda limestone blocks 

and fr:agments throughout, some of which have moved 

downward more than 150 feet. In the lower part of 

the pipe the blocks and fragments of limestone are in 

a matrix of clay from the Grayson formation; but in 

the upper part of the pipe blocks and fragments of 

both Buda limestone and Boquillas flags form either 

an open breccia or a breccia with a matrix of clay 

derived from the clayey layers of the Boquillas. The 

cinnabar was largely a replacement of the clay, but in 

the upper part of the pipe it was an interstitial filling 

in breccia openings, an occurrence similar to much of 

the ore in the 550 ore body. 

Ore bodies mined from the lower levels in the western 

part of the mine are in marked contrast to those described above. These ore bodies belong to the limestone-clay contact class of deposit. They were flatlying shoots along the contact between Devils River 

limestone and Grayson formation. In general, they 

are in a zone less than 50 feet thick on both sides of the 

contact. The zones of mineralization, ore shoots, and 

the ore itself are similar to those described at the 

Mariposa mine. Although the ore bodies were pockety, 

they were, in the aggregate, large and rich. 

There were several limestone-clay contact deposits 

in the Chisos mine; the largest and most profitable was 
104 GEOLOGY AND QUICKSILVER DEPOSITS, TERLINGUA DISTRICT, TEXAS 

the North ore body, which extended northeasterly on 

the 600 level from the point on the Chisos fault where 

it changes trend from west to northwest. This ore 

body was mined for a total distance of 1,600 feet on 

levels that ranged from the 580 to the 700. Part of 

this change in elevation of the ore body is due to the 

dip of the favorable ore horizon, the limestone-clay 

contact, and part .of it is due to two northwesterly 

pre-ore faults that drop the contact progressively towards the northeast. These two faults divide the ore 

body into three parts. The southwest part extended 

northeasterly from the Chisos fault, as stated above, 

for 400 feet, gradually working upward to the 580 level 

where it continued northeasterly for another 400 feet. 

At this point it is joined by an east-west ore body that 

extends eastward from the Rainbow mine. Also at this 

point the cont:act is dropped down to the 630 level by 

one of the northwesterly faults. The ore followed the 

contact, and on the 630 level the ore body continues in 

a ·northeasterly direction for another 400 feet, where 

another east-west Rain bow ore body joins it. Here 

the contact, followed by the ore, gradually works down 

to the 650 level, where it continues for 300 feet and is 

again stepped down, on the other northwesterly fault, 

to the 700 level. According to maps of the Chisos 

Mining Co., the ore was explored and mined on the 

700 level for 100 feet. This level was inaccessible in 

1942. 

Another limestone-clay contact deposit lies between 

the 650 and 750 levels a little east of the No. 9 shaft. 

It extends northeasterly from the Chisos fault. It is 

small in comparison with the North ore body. 

When the writers examined the Chisos mine in the 

summer and fall of 1942, there was very little ore that 

could be seen. It is reported by Mr. F. H. Fovargue, 

a former mine superintendent, that some ore remains 

in places that were inaccessible at the time of · the 

examination. Notable among these places is the ground 

around the ore body described in the preceding paragraph; however, it is doubtful that any large tonnage 

of ore remains in this part of the mine, as little favorable 

unexplored ground remains. 

Within the mine proper no large, unexplored areas 

where geologic conditions appear favorable for the discovery of new ore bodies remair: unexp]ored. However, 

away from the mine proper, there are several places 

where geologic conditions and surface mineral~zation 

appear favorable for the presence of underlying ore 

bodies. The most favorable place is the area between 

the Susano and No. 14 shafts, east of the main workings. 

Although this area was partly and unsuccessfully 

explored by a joint drilling program of the U. S. Geological Survey and U. S; Bureau of Mines during the 

summer of 1943, its possibilities were by no means 

exhausted. 11ineralization is evidenced by calcite veins 

and by showings of cinnabar in_ the following places: 

on dumps from the No. 14 shaft, in the adit 350 feet 

northeast of No. 14 shaft and on dumps therefrom, i~ 

the shaft of the Susano prospect and on dumps from 

it, and from the small adit 200 -fpet southwest of this 

last shaft. These showings of cinnabar are in a. zone 

of calcite-mineralized faults, trending N. 60° E., which 

extend from the Aguja breccia pipe southwestward to 

and beyond the No. 14 shaft. 

A particularly favorable structure in this area is the 

Aguja breccia pipe, which consists of blocks of sandstone of the Aquja formation in a clay matrix. The 

Susano prospect shaft is at the southeast edge of this 

breccia pipe. Cinnabar was seen in a calcite vein . in 

the drift from this shaft and, ·according to Mr. ~ovargue (oral communication), a few tons of ore -were 

mined from the vein. Two drill holes that intersect 

the eastern border of the pipe at depths of 115 and 185 

feet found traces of cinnabar (but no ore) both In the 

Boquillas flags and breccia of the pipe. 

In 1943 a single hole was drilled to the Devil& RiverGrayson contact in this area, and the core contained 

neither ore nor altered rocks at this horizon. ·' This 

single drill hole does not disprove the existenc~, ~f ·ore 

bodies at this horizon, as any comparison of the size o~ 

the favorable structures with the average size of . ore 

bodies at Terlingua will show. Core from thi$:. drill 

hole showed that the Devils River-Grayson contact is 

about 1,140 feet below the floor of Grace Canyon; this 

would be about the maximum depth of the horizon in 

this general area because the collar of the hole wa~ near 

the axis of the synclinal structure of the area. Because 

the most favorable ore horizon is .at such a depth, exploration and possible mining would be expensive. 

Probably the most practical method of finding an . ore 

body at deeper horizons in this area. would be to ·sink on 

the traces of cinnabar-in the Aguja breccia pipe. 

RAINBOW MINEo 

The Rainbow mine is one of the deepest and most 

productive mines in the Terlingua district. Cinnabar 

was discovered on the Rainbow ground about 1900, and 

it is reported that $10,000 worth of quicksilver was obtained from shallow cuts and tunnels at that time. 

During World War I a shaft (Rainbow) was sunk more 

than 600 feet but without finding ore. During the early 

thirties drifting from near the bottom of the shaft re--

sulted in the discovery of several rich ore bodies, and 

the ore from these was treated in a 1 0-ton furnace. 

The mine was closed for lack of ore in 1942. 

The early near-surface workings follow calcite veins 

of different trends in the Boquillas flags. These are 

9 After Clyde P. Ross's written communication. · 
DESCRIPTIONS OF' ·MINES AND PROSPECTS 105 

similar to those of the adjacent Chisos mine and their 

distribution can be seen on plate 16. 

The Rainbow shaH has a reported depth of 670 feet. 

Drifts extend off it at intervals of 50 feet and less, 

throughout a depth of about 635 feet, but most of the 

development and all stopes are on and below the socalled 600 level and above the 635 level. On the 550 

level there is a long crosscut through barren ground, 

but all drifts on the levels above are short. None except those on the 50 level, which totals more than 300 

feet, total as much as 100 feet. 

The shaft for much of its depth follows a fault zone, 

which trends about N. 60° E., with local departures 

from this trend. On the 635 level the fault zone is a 

little 1nore than 100 feet south of the shaft. The vertical displacement on the fault zone is probably more 

than 100 feet. The faults in the Rainbow shaft are 

probably subsidiary faults to the Rainbow fault, which 

lies a little to the south, where it has a vertical displacement of 800 to 1,000 feet. 

On the 600 and 635 levels the contact between the 

Grayson formation and the Devils River limestone has 

been explored and at least three ore zones of the lirnestone-clay contact variety of deposit have been found. 

The most northerly of these trends on the average 

about N. 80° E. and has a length of more than 700 feet. 

It follows a rolling lower contact of the Grayson formation along or in the vicinity of steeply inclined fractures. Many of these fra.ctures have little or no displacement along them; one in the northwestern part of 

the mine, however, has a probable displacement of 

nearly 50 feet. 

The second ore zone is about 250 feet south of the 

first and roughly parallel to it. This zone lies along a 

sharp monoclinal flexure, accompanied by fractures 

along which the beds bordering the Grayson-Devils 

River contact are bent downward towards the south at 

angles of 25° to 50° and locally more. In the eastern 

part of this ore zone the contact flattens and then 

slopes gently downward, both to the northeast and 

southwest of the main drift. Northeast of the drift 

some good ore was stoped along a group of poorly 

defined fractures trending about N. 65° E. 

The third ore zone is in the extreme southeast part 

of the mine. The ore here had been found and stoping 

in it had begun from the east by the Chisos Mining Co. 

before it was also found by a drift from the Rainbow 

shaft and before the boundary between the properties 

of the two companies had been fixed. This ore zone 

is in an area of extensive fracturing at and somewhat 

below the base of the Grayson. The principal fractures trend about N. 35° E. and most of them nre 

nearly vertical. Some steep fractures trending N. 60°-

700 E. are also present. The ore mined was altered 

limestone and clay, with cinnabar along fractures and 

in irregular replacements. Much of it was of exceptionally high grade. 

BROWN PROSPECTS IN SEC. 286 

A group of eight full claims and six fractional claims 

in the southwest part of sec. 286, Block G-4 are held by 

Guy E. Brown of Houston, Tex. This property is about 

1 mile northeast of the Chisos mine. A shaft, about 100 

feet deep, and several short trenches and shallow pits 

represent the only prospecting on the claims. No ore 

has been mined from the property and only traces of 

cinnabar have been found. 

The only exposed rocks belong to the upper part of 

the Boquillas flags. In general the rocks are gently 

flexed with dips rarely exceeding 10°; but in some places, 

notably in disturbed zones along minor faults, the dips 

are as much as 50°. Joints and minor faults are numerous and most of them are filled by calcite (see fig. 17). 

The common thickness of the calcite-filled fractures, or 

veins, ranges from 1 inch to 12 inches, but in places the 

veins are as much as 3 feet thick. The calcite veins are 

very similar to those at the Chisos mine, but they are 

not extensions of the ore-bearing structures of this mine. 

In the southwestern part of the property the veins are 

arranged in two systems; one trends northeasterly and 

the other northwesterly. Vein intersections are locally 

offset, but more commonly, intersections are places 

where the persistency of the vein breaks down and the 

distribution of calcite is very irregular. Practically no 

prospecting has been done in this part of the property. 

In the northeastern part of the property the most 

prominent veins trend northeasterly. On the most 

prominent of these, a shaft has been sunk to an approximate depth of 100 feet, but it was not accessible when 

the writers were in the district (1942-45). A sample 

taken from the vein at the co1lar of the shaft contained 

only traces of cinnabar. To the northeast the veins of 

this area are covered by alluvium. 

In the northwestern part of the property three welldeveloped veins have trends of N. 70° E., N. 35° E., 

and N. 85 ° E. The N. 70 ° E. vein has been prospected 

by a pit about 12 feet deep, but neither of the other 

veins has been prospected. Near the western border of 

the property theN. 35° E. and N. 85° E. veins converge. 

Near the point of convergence theN. 85° E. vein splits, 

but the lack of continuous exposures prevents a clear 

understanding of this splitting. 

The showings of cinnabar in the calcite veins exposed 

at the surface may indicate that limestone-clay contact 

deposits exist at depth, but the search for such bodies 

would be expensive. The Devils River-Grayson contact is between 800 and 1,000 feet below the surface. 

Another unfavorable aspect of the property is the lack of 
106 GEOLOGY AND QUICKSILVER DEPOSITS, TERLINGUA DISTRICT, TEXAS 

outstanding geologic structure where exploration could 

be concentrated. 

TWO-FORTY -EIGHT MINE 

The Two-Forty-Eight mine is in sec. 248, Block G-4, 

2 miles east of the town of Terlingua. It is owned by 

E. A. Waldron of Alpine, Tex., and was last worked 

(1946) by the Esperado Mining Co. of Houston, Tex., 

which had the property under lease. Cinnabar was discovered here before 1902, but by 1934 only 700 feet of 

subsurface workings had been driven. Since 1940 the 

Esperado Mining Co. has extended the mine workings 

to more than 7,000 feet and has erected a Herreschoff 

furnace capable of treating 40 tons of ore per day. 

The mine workings consist of 13 levels, which extend 

from the main shaft at irregular intervals. The deepest 

mine level is at 804 feet below the collar of the main 

shaft. A second shaft is 50 feet deep. Maps of levels 

are shown in plate 21. 

The workings are in and close to a vertical breccia 

pipe enclosed in fiat-lying, slightly fractured Boquillas 

flags and underlying rocks. Near the surface the 

pipe is 100 feet in diameter and roughly circular in 

plan, but as it extends downward it gradually enlarges 

and becomes elongate in plan. 

The breccia pipe is enclosed in Boquillas flags from 

the surface to a depth of 465 feet where the flags are 

intruded, 8 feet above their base, by a sill of finegrained analcite syenogabbro. This sill, 60 feet thick, 

is younger than the pipe, with which it has intrusive 

relations. T.he pipe on the 556 level of the mine is 

enclosed in Buda limestone, 90 feet thick, which is 

conformably overlain by Boquillas flags and conformably underlain by Grayson formation. The lowest 

level of the mine, the 804 level, is in ~he Grayson 

formation but the Devils River limestone is probably 

only a short distance below this level. Below the 556 

level tongues of altered analcite syenogabbro irregularly 

intrude the pipe. In the northwest and southeast 

crosscuts on the 556 level soft clay separates the 

breccia pipe from the enclosing Buda limestone. This 

clay appears to be an altered dikelike body of analcite 

syenogabbro. Similar altered igneous rock was found 

in the shaft below the 556 level. 

Above the 420 level the breccia filling consists of 

blocks of Boquillas flags, below the 500 level it consists 

of blocks of both Boquillas flags and Buda limestone 

and continues so to a depth of about 650 feet where 

blocks of these rocks are enclosed in a matrix of Grayson 

formation. In the upper levels of the mine much of 

the breccia has a soft clay matrix of comminuted and 

altered shale of the Boquillas flags, but in some places 

the breccia blocks have no matrix. Tar, some of 

which is brittle but much of which is fluid, and calcite 

fill or line many of the open spaces between the blocks. 

Some of the calcite contains tar. Analcite is common 

in the breccia in the upper part of the pipe. 

It is evident from an inspection of the geologic map 

of the surface of the property (pl. 22) that the location 

of the breccia pipe is not controlled by intersecting 

faults. All the more persistent fractures in_ the immediate vicinity of the mine are shown on this map, 

and none of these fracture.s have vertical displacements 

of more than 1 foot. Minor fractures, however, 

locally controlled the shape of the breccia pipe, as can 

be seen on the maps of the various mine levels (pl. 21). 

Locally such fractures, or joints, have· outlined large 

blocks of wall rock that have slumped into the pipe. 

Such blocks can be seen along the periphery of the 

pipe in all stages of detachment and slump; in particular along the west side of the pipe as shown in 

section A-A', plate 22. The detachment and slumping of these blocks produced excellent structures for 

the deposition of cinnabar. 

The quicksilver deposits of the Two-Forty-Eight 

mine are readily divided into two groups. The more 

important group includes deposits in marginal fissures 

of the breccia pipe and within the breccia. The other 

group comprises fillings along fractures outside of and 

away from the breccia pipe. In both groups the only 

important quicksilver mineral is cinnabar. 

The deposits in fractures outside the pipe are small 

and low in grade; they consist of fillings by cinnabar and 

calcite, with small quantities of pyrite and iron oxides. 

Most mineralized fractures trend N. 65° E, but some 

trend about N. 15° W. In places the fracture walls 

are coated by thin films of cinnabar, which locally 

widen to veinlets a quarter of an inch or more wide. 

The widest veinlets are where the fractures cross 

limestone beds in the Boquillas flags, which are rarely 

more than 2 feet thick. Cinnabar is found in some 

places in shaly partings along bedding planes on both 

sides of fractures. The igneous sill contains cinnabar 

in small fractures · and irregular disseminations. 

Deposits in marginal fissures and within the breccia 

are best developed between the 270-foot and 360-foot 

levels. An ore body on the 320-foot level is typical of 

this class of deposit. On this level the smooth curve 

of the western border of the pipe is broken by a large 

block of wall rock that juts into the pipe. This block 

is detached and slightly slumped towards the pipe. 

The fissure thus developed at the back of the block is 

filled with small breccia fragments and is mineralized 

with cinnabar. The ore is limited to the length of the 

block. Pockets of ore fill marginal fissures along other 

large blocks. Where ore is found within the pipe, 

the associated breccia commonly contains many large 

blocks, as on the northern side of the pipe on the 360 
DESCRIPTIONS OF MINES AND PROSPECTS 107 

level. The interior of the pipe has been explored by 

both mine workings and diamond-drill holes, but no 

ore has yet been found far from the pipe walls. 

At the writer's last visit to the mine, in February 1945, 

the most favorable ore horizon in the district, the Devils 

River limestone-Grayson formation contact, had not 

been reached. As the breccia pipe was doubtless 

caused by solution in the Devils River limestone and 

subsequent collapse of the overlying rocks, it is probable 

that solution also produced at the top of the Devils 

River limestone deposits of cave-fill, which were very 

excellent hosts for cinnabar at the Mariposa, Rainbow, 

and Chisos mines. The possible presence of such 

cave-fill zones depends in large part on the existence of 

northeasterly fractures cutting the limestone where it 

borders the breccia pipe. 

Although the Two-Forty-Eight mine is in one of the 

most geologically favorable ore structures in the district, exploration and mining is not without its hazards. 

Some of these hazards, none of which are insurmountable, are the unfortunate location of the main shaft 

within the breccia pipe, heavy ground in much of the 

breccia, the pockety character of the ore that has been 

found, and the presence of hydrogen sulfide gas, considerable water, and high temperatures. The location 

of the main shaft within the breccia make difficult the 

mining of any ore bodies that might be found near it, 

as for example, that on the 320-foot level. Heavy 

ground around the shaft and in the drifts and crosscuts 

requires considerable timbering, an expensive operation 

in the Terlingua district. The pipe has been sufficiently 

explored by drill holes and mine workings to indicate 

that it contains, at least above the 500-foot level, no 

large ore body similar to that in the Chisos pipe. 

The ore that has been found in the pipe, and probably 

the ore that may be found in the pipe, is in small 

disconnected ore bodies. The vertical extent of these 

has not been proven and it is possible that they may be 

sinuous chimneys of ore along the borders of the pipe. 

In places there was hydrogen sulfide gas which was 

at times a hazard to mining. The gas, however, appears to be entrapped in open pockets in the breccia 

and is dissipitated a short time after these pockets are 

tapped by mine workings. High temperatures in the 

lower workings make mining difficult, but forced ventilation will lower these temperatures. The mine water 

will decrease with continuous pumping, but a greater 

water problem can be expected in the Devils River 

limestone. 

STUDY BUTTE MINE 

The Study Butte mine, in sec. 216, Block G-4, is the 

third largest producer of quicksilver in the Terlingua 

district. It is 5 miles east of Terlingua and marks 

essentially the eastern end of the district. Cinnabar 

was discovered early in 1902 and was being actively 

mined as early as 1905. Substantial production did 

not begin, however, until World War I, when from 1915 

to 1920 quicksilver valued at $500,000 was produced. 

The Study Butte mine consists of two adjacent proper.:. 

ties, the Big Bend and the 'rexas Almaden, which 

have been worked both individually and as a unit. 

During World War II both properties were worked 

under lease by the Texas Mercury Co. of San Antonio, 

Tex. 

Before the mine closed in 1944 ore was being treated 

in 2 medium-sized furnaces . . The mine workings include 4 principal shafts-only 3 of which had been used 

in recent years-many minor shafts, and more than 

3 miles of horizontal workings on the 4 main and 

numerous sublevels. The deepest shaft is the Big 

Bend, which early in 1944 was slightly more than 440 

feet. The other 3 main shafts are the Dallas, Fortuna, 

and No. 10, which reach to the 300-foot level, 250-foot 

level, and 200-foot level, respectively. All are vertical, 

except the Fortuna, which is inclined at an angle of 80°. 

The horizontal workings are mainly on the 150-foot, 

200-foot, 250-foot, and 300-foot levels, which are 

named from their approximate depths below the collar 

of the Big Bend shaft. 

The workings of the Study Butte mine are almost 

entirely in the Study Butte intrusion of quartz soda 

syenite, which forms the hill that gives the mine its name. 

This body, described on pages 20 and 25, is dikelike in its southern outcropping part and a sill in its 

northern part; the dikelike part dips steeply to the 

north to converge at depth with the sill part, which is 

horizontal. The intrusion is in Terlingua clay that is 

almost horizontal, except where sharply flexed to 

form a monoclinal fold above the intrusion. Terlingua 

clay adjoining the intrusive is highly baked except 

along the steep southern contact. The intrusion is 

cut by numerous joints and fairly persistent fractures, 

some of which were filled with cinnabar, pyrite, and 

calcite, and small quantities of hydrocarbons. In 

general, the mine workings follow these ore-bearing 

fractures, which are in both the concordant and discordant parts of the intrustion. 

Fractures are of both northeasterly and northwesterly 

trends, but the northwesterly fractures are not only 

less persistent but for the most part are barren of cinnabar. Fractures of east-west trend, which parallel the 

elongation of the discordant part of the intrusive, are 

common locally, in particular along the upper, inclined 

contact of the intrusive, where shallow ore bodies have 

formed along them. Vertical displacements along these 

fractures are in no place more than a few feet. In 

places the contact between the intrusion and Terlingua 
108 GEOLOGY AND QUICKSILVER DEPOSITS, TERLINGUA DISTRICT, TEXAS 

clay is offset slightly along fractures, in other places 

fractures end at the contact or cross it without offset. 

The main ore-bearing structures are groups of closely 

spaced parallel fractures that are continuous both 

vertically and horizontally for several hundred feet. 

They range in dip from 60 ° to vertical, with dips of 

80° the most common. · Each group of mineralized 

fractures is locally referred to as a "vein," terminology 

that is retained in this description. Individual fractures that constitute each vein are overlapping and 

discontinuous. The width of individual fractures before 

mineralization ranged from a knife-edge opening to 

about 1 inch. The width of a mineralized zone, or 

vein, ranges from a fraction of an inch where the vein 

has narrowed to a single fracture, to several feet where 

the vein includes several widely spaced fractures. 

Compared with other vein deposits in the Terlingua 

district these veins are remarkable for continuity of 

quicksilver mineralization, although by no means are 

all the fractures mineralized or do any of them contain 

ore throughout their entire horizontal and vertical 

extents. Shoots of high-grade ore are most common 

riear contacts between igneous and sedimentary rock 

or where fractures are numerous and closely spaced. 

Mineralized fractures are not restricted to the igneous 

rock; they are also in the baked Terlingua clay that 

overlies the intrusive body. Mineralized fractures in 

the Terlingua clay are generally continuations of those 

in underlying igneous rock. In early mining operations, 

this fractured baked clay just above the contact with 

the igneous body yielded bodies of high-grade ore, 

both at the surface and on the 150 level. All the 

raises on the 150 level were to explore this contact 

and many of them connect with stopes above the contact. The ore was in no sense in tabular blanket 

deposits below or along the contact, but instead was 

localized in ore shoots that commonly were higher 

than wide and entirely within the shale above the 

contact. Most of them were found by following thin 

veins up through the igneous rock and into the shale, 

where the veins commonly widened into high-grade ore 

bodies in zones of multiple fractures in the baked clay. 

These ore bodies extended from 10 to 40 feet above the 

contact of the clay and the igneous rock. 

The mine workings are in three almost unconnected 

blocks of mineralized ground, which are located as 

follows: (1) south of the Big Bend shaft, (2) north of 

the Big Bend shaft, between the Dallas and No. 10 

shafts, and (3) west of the Big Bend shaft. 

The block south of the No. 10 shaft includes a smaller 

area than the other two blocks, but extends through a 

greater vertical range. Its greatest development by 

mine workings is on the 300 level, where the workings 

have a maximum extent of about 300 feet north and 

south and 300 feet east and west. It has been worked 

on the 100, 150, 200, 250, and 300 levels and several 

sublevels. The ground between the levels and between 

the 100 level and the surface is connected by raises 

and by stopes. Connections with the surface above 

the 100 level are inaccessible. 

Ore shoots in this block are along a series of closely 

spaced veins, which in general strike from N. 5° E. 

to N. 20° E. Above the 200 level, individual veins 

are not well defined, and some cinnabar-bearing fractures trend northwestward. Below the 200 level the 

veins are well defined and have been identified by the 

following names: Shafter, Brown, Big Fracture, Caballo, and Nuevo, given in order of increasing distances 

south from the Big Bend shaft. Between the 300 and 

200 levels these veins abut to the southwest against 

Terlingua clay at the southern limit of the intrusive 

body. Through this distance ore has been stoped up 

to the clay contact. Above the 200 level this contact 

has not been explored and the exploited ore shoots are 

to the north, entirely within igneous rock. The zone 

of exploited ore shoots extends from the 200 level 

vertically upwards to the upper contact of the sill, 

which in this area is a local fiat-lying "terrace." The 

ore shoots probably continued into the overlying 

Terlingua clay, but because of the inaccessibility of the 

uppermost workings this could not be definitely 

determined. 

The exploited ground north of the Big Bend shaft is 

an elongate block, about 1,500 feet long, that extends 

in a general N. 60° E. direction between the No. 10 and 

Dallas shafts. It also exter:.ds about 300 feet northeast 

of the Dallas shaft and about 300 feet southwest of the 

No. 10 shaft. This ground has been worked at the 

surface, and on the 150, 200, 250, and 300 levels, but a 

large part of it is unexplored. Workings are most 

extensive on the 150 and 200 levels and least extensive 

on the 250 and 300 levels. All levels are connected 

with the Dallas shaft and all except the 150 are connected with the Big Bend shaft. Ground that is 

explored and exploited by the 150 level extends from 

the Dallas shaft to, and 200 feet beyond, the No. 10 

shaft. This level is mainly in igneous rock in the sill 

part of the instrusion, a few feet below the upper 

contact of the sill. A small part of it, southwest of 

the Tank and Dallas shafts that was inaccessible in 

1944 appears to be in Terlingua clay. The common 

trend of the veins on this level, as on the other levels 

is about N. 60° E. Many small stopes are above the 

150 level and some of them extend to the surface and 

into the overlying Terlingua clay. The 200 level is 

entirely in igneous rock and the workings are mainly 

between the Dallas and Fortuna shafts. Work on the 

250 level is mainly along two veins; one near the Dallas 
DESCRIPTIONS OF MINES AND PROSPECTS 109 

shaft and the other near the Fortuna shaft. That on 

the 300 level is almost entirely on the vein near the 

Dallas shaft. 

The center of the third block of mineralized ground 

is about 400 feet southwest of No. 10 shaft. The 

workings here, almost entirely surface pits and cuts, 

are in a narrow east-west zone about 500 feet long. 

The workings follow a system of east-west toN. 75° E. 

fractures and are entirely within the intrusion, although 

the now eroded contact of the intrusive with the overlying Terlingua clay was formerly only a few feet 

above the surface in this area. A few inaccessible 

winzes of unknown depth lead from these workings, 

but it is doubtful that any of them are deeper than 50 

feet. Workings from other parts of the mine do not 

connect with those in this area, nor do they extend 

below them to explore ground that might possibly 

contain downward continuations of the exploited ore 

bodies. 

Although no specific estimates of ore reserves can 

be given for the Study Butte mine, it is reasonably 

certain that the possibility of developing reserves of 

low-grade ore is very favorable. This is indicated by 

ore on numerous faces in different parts of the mine, 

by company drill-hole records of ore below the present 

workings, and by the presence of much unexplored 

ground that is geologically favorable for ore. 

The most favorable unexplored ground lies below 

the 300 level. Practically no exploration by mine 

workings has been done below this level. In the 

southern part of the mine in the vicinity of the Big 

Bend shaft it is about 130 feet to the locally flat, 

terracelike, contact of the intrusive rock with the 

underlying Terlingua clay. Two hundred feet north 

of the Big Bend shaft, on the 300 level, this contact is 

about 300 feet below the 300 level. This contact 

on the "terraces," was geologically favorable for the 

formation of ore bodies. Places particularly favorable 

are these where the contact is locally irregular. 

The possibilities of finding ore at horizons that were 

productive in other parts of the district appear remote. 

The most favorable horizon, the contact of the Devils 

River limestone with the Grayson formation, is at least 

1,800 feet below the collar of the Big Bend shaft; if 

there are more sills below the Study Butte intrusion, 

this contact could be much deeper. 

Operators of the Study Butte mine have always 

had a water problem. Constant pumping is necessary 

to hold the water out of the deeper levels of the mine. 

It is reported that when the water is not pumped it 

rises until it seeps into the bed of the creek nearby. 

MiniPg at deeper levels will probably have more water 

to contend with. The intrusion acts as a local aquifer, 

the joint and fracture openings allowing free circulation. 

Although the unfractured parts of the intrusion are 

dry, these parts are unmineralized; consequently, 

mining must be in its open and wet parts. 

PROSPECTS EAST OF STUDY BUTTE 

East of Study Butte along the north foot of Maverick 

Mountain are a number of abandoned prospects. For 

the most part they consist of adits driven into the 

intrusive soda syenite of Maverick Mountain near 

its northern contact. No cinnabar was seen on the 

dumps from these adits. It is reported, however, that 

some ore was mined from this area during the early 

history of the district. 

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Udden, J. A., Baker, C. L., and Bose, E., 1916, Review of the 

geology of Texas: Texas Univ. Bull. 44, 164 p. 

Verhoogen, Jean, 1948, Geological significance of surface tension: 

Jour. Geology, v. 56, p. 210-217. 

Walker, R. T., 1928, Deposition of ore in pre-existing limestone 

caves: Am. Inst. Mining Metall. Engineers Tech. Pub. 

154, 43 p. 

Waters, A. C., and Krauskopf, K. B., 1941, Protoclastic border 

of the Colville batholith: Geol. Soc. America Bull., v. 52, 

p. 1355-1418. 

Wentworth, C. K., and Macdonald, G. A., 1953, Structures and 

forms of basaltic rocks in Hawaii: U. S. Geol. Survey Bull. 

994, p. 17-21. 

·white, D. E., 1955, Thermal springs and epithermal ore deposits in Pt. 1 of Bateman, A. M., ed., Economic Geology, 

p. 99-154. 

Williams, Howel, 1941, Calderas and their origin: Calif. Univ. 

Dept. Geol. Sciences Bull., v. 25, no. 6, p. 239-346. 

Willis, Bailey, 1946, Normal fault structures and others: Am. 

Assoc. Petroleum Geologists Bull., v. 30, p. 1875-1887. 

Zies, E. C., 1929, The Valley of Ten Thousand Smokes-1, The 

fumarolic incrustations and their bearing on ore deposition: 

Nat. Geog. Soc. Contr. Tech. Papers, Katmai series, v. 1, 

no. 4. 
INDEX 

Page Page 

Acknowledgments__ ____________________ ___ _______ __ ____________ ___________ ___ 6 Domes, age ____________________________________________ . ________ -------------- 47 

Age of mineralization____________________________________ __ _____________ ______ 54 described. ____ --------- ___________________________ ------- ___ -------· ------ 34-36 Aguja formation, described. __________________________________________________ 14-16 theories of origin _________ _____ _____ __ _____ . __ __________ ----_-------------_ 36-37 

Analcite basalt, petrography------- ___________________________________________ 28-29 

bearing rocks ____________ ------ ___________________ ___ ____ ____________ ____ _ 26-29 F orthoclase gabbro, petrography __ . __ ._____________________________________ 28 Faults, age of normaL ______________________________ -------------------.------ 47 

plagioclase syenite sills. __ ----- ______ .____________________________________ 28 large graben.-------------- _________ __ -· _____________________ ------------- 37-38 

syenite, petrography----------- ___ . _______________________________________ 27-28 thrust. ____ ------· _. ___ ___ _ __ __ _ _ _ _ __ __ _ _ _ __ _ ____ ___ _ __ ____ _ __ ____ _ __ _ ____ 39 

Analysis, bituminous materiaL .. _.______ _______ _______ _____ _________________ 71 Faults and fractures, as ore controls. __ _______ ______ -------------------------- 76-77 Boquillas flags ___________________ . __ ______ __________ _____ ___ _____ . _ ___ _ __ _ 11 See also Fracture. Buda limestone ________ -----_--___________________________________________ 10 general discussion. ___ ______ ______ ___ ____ _________ ______ ___ ________ ------- 37 

clays from Grayson formation. _~ ------- _________ . ____ ___ __________ _______ 67 relation to mineralization ______ --------------------- ______ ---------------- 53 

Devils River limestone______________ ____ ___ ________ __ _____ ____ ___________ 8 sets and age of normaL __________ ________ _____ __ ----- ___ ------------------ 38-39 

Grayson formation __ ---------- ___ .--------------- _________ _____ _________ _ Fluorite, occurrences. _______________ ____ ____ ____ ______ --- __ -------------- 85, 91, 97 

Tornillo clay __ ----------------___________________________________________ 16 Folding, age ___________ _ ---------------- ___ . ___________ ----------------------- 47 

B Fossils from Aguja formation, listed_____ __ _______ ____________________________ 15 

from Buda limestone listed. ____________ __________ - ____ - __ - __ ------------_ 10 

Big Bend region, topography_ _______ __ _____________ _____ _______ ________ _____ _ 29 from Devils River limestone, listed. ______ . ____________________ ----------_ 9 

Black Mesa dome. __ __ -------------_ -_--- --- ___________ ___ __ --- ______________ 34, 36 from Grayson formation, listed. _______ ._-- ____ --------------------------- 10 

graben zone. See Grabens. from Terlingua clay, listed _______________________ ------ ___ _ --------------- 13, 14 Block, explained _______ .______________ ____________ ___ _____________ ____________ 84 Fossil Knobs dome_---------------------- _________ --------------------------- 34 Boquillas flags, described _________________________________________ ------------ 10-12 Fracture, defined for this report ____________ ---------------------------------- 38 

Breccia pipes, Aguja (Chisos mine) ________ ---------------------------------- 41-42 Future of district _______ __ _____ ------- _______ ____ ___ ----_-_--_ -- __ ------------ 52. 

as structural control of ore __ ________ -----------------------------· ________ 78 

Chisos, ore in ______________________ ___ ______________ _ -------------------- 63,103 G 

Chisos mine ____________ __ -----------· ____ _______________ ~--- ------------ 42 

compared with diatremes __________ --------------------------------------- 45-47 Gangue elements, transportation and deposition ____________ ________ _____ _____ 83-84 

list of known_____________________________________________________________ 43 Geography and topography of area __________ --------------------------------- 3-5 

Maggie Sink, ore in ___________________ ____ -------------------------------- 63-64 Georgetown limestone __ _______ ------- ________ ____ _ - __ - ____ -----_-_----------- 55 

Maggie Sink, structure _____ ------------- ______ --------------------------- 4G-41 Also .~ee Devils River limestone. 

Two-Forty-Eight, ore in _____ - ______ . ________ . ____________________________ 63-64 Grabens, Black Mesa ___ ______________ ____ __ ___ __ ----------------------------- 37 Two-Forty-Eight, structure ________ . __________ ._______ ___ ________________ 41 Cigar Mountain __________________ ____ ----------------- ------------------- 38 

origin ____ ------------------------------._-- ___ __ ______ ______ _____ _________ 44-45 in Lowes Valley---------------------------------------------------------- 38 other ___ -----_------------ _____________ ---- ____ ------------ _______________ 42, 44 Long Draw _______ ____ ____________ ___ ____________________ ----------------- 37 

Buda limestone, described_________________________ ___________________________ 10 map ______________ ___ ______ ___ __ _______ _____ _ .. ____________________________ 33 

c 

Cave-filL __________________________ _ ----------------------------- 39, 40, 44, 55, 57-59 

Chisos Mountains _________ ----------- __ --- ______ ___ _____ ----_-_-___________ 29 

~~~n~~:.~:=~~~~~~---~ ~:: ~ ~ ~~~~~ ~~ ~~~~ ~~~~ ~~ ~~~~ ~~ ~~ ~~~~ ~~~~ ~ ~~~~~~~~~ ~ ~~~ I Cinnabar, in calcite veins ___ _ -------- __ -- ------ ________________ _ ------------- 59 

occurrence in cave-filL __ _ . ___ ---_- __ . ____ _____ ____ .----_________________ __ 58 

See also under Minerals. 

Climate and plants ___________ __ --_-- _______ ________ ___ ----_.________ ______ __ _ 5-6 

Concordant intrusions _____ . ________________ __ _________ ._---_________________ _ 1.7 

Contact metamorphism____________ __ ___ _____________________________________ 22 

Contra ban do dome ______________________ . _· __________ _ . __ _ _ __ __ __ _ _ _ _ _ _ __ _ _ _ _ _ 34 

Calcite lodes ________ ___ _____ -------------_ ______________________________ __ ____ 58 

veins. See Cave-fill deposits and Deposits, calcite veins in Boquillas flags. 

theories of origin ____ ______ _____ ___ _______ ____ __ ---_- ___ ------------------- 49--50 

Well Creek ____ _____ ________ _____ __________ __ ----------------------------- 38 

Grayson formation, described _____________________ --------_---.--------------- 9-10 

in cave-fill deposits. ___ _____ ___ __ .---------------------------------------- 58 

H 

Henbest, Lloyd G., cited _______________ ________________ ______ ___ ___ _______ 12, 13,14 

' Host rocks, relative importance ______ ----------- ___ --·-------------.---------- 54 

· Hydrocarbons, abundance in quicksilver ore________ _____ ______ _______________ 41 

in calcite veins ______________________ -------.--------- ------- -------------- 60-61 

in igneous-rock deposits _______________ - __ -- ___ ---------------------------- 64 

See also under Minerals. 

Hydrothermal stage, defined for this report___________________________________ 27 

Hydrothermal solutions, transportation of mercury in _________________________ 80-81 

Capillary-sized openings, as structural controL _________ ___ __ ___________ _____ _ i7-78 

Cave-fill, as structural control of ore ________________________ ---------------- 78 I-K 

Cave-fill, clays in.------------------------------------------------------------ · 58 ore in _______________________________________ __ ___ _____ -- ______ _______ ___ . 53-54 

See Limestone-clay contact deposits. 

· Igneous intrusion, relation to mineralization __________ ___ ____ _________________ 53-54 

Igneous (intrusive) rocks, classification (table)________________________________ 23 

described ____ _____ __ . ______________ . ______________ ---------------.-------- 17-29 

D • Jaboncillo ______ __ ______ . _______________ - __ ----------------------------------- 58 

Davis, W. M., quoted .. -------c------cc·------· ------------------------------ 44 See Minerals, clays. 

Daubr6P, A., cited ____ __ -----------------------__ __ __________________________ _ 46 Keels, of cave-fill deposits _______ __ -------------------------------------------- 55 

Deposits, classification and geologic features ______ __ ____ ---------------------- 54-64 King, Philip B., quoted .. ---------------------------------------------------- 3, 29 

district structural controL_____ ___ __________ ______ __ ______________ ______ __ 76 Krauskopf, K. B., quoted .. ________________ ----- __ ----------------------- 81 

in breccia pipes _________________________________ __ ____ ____ ___ __________ ___ 63-64 

in calcite veins in Boquillas flags ____ _____ -------------------------------- 59-63 L 

in igneous rock ______ ----------------------·_____ ______ ____ __ _____ _____ ___ 64 Laccoliths __ ------------------- ____ ___ . ____ __ ---- _----- -------- -------------- 17-18 local structural controls ____________________ --· ______ - ___ ______ _____ _______ 76-78 Laccolith, Wax Factory __ ------ ______ .----------- ---------------------------- 34 

origin of quicksilver_- ------------------ __ __ ___ ______________ _______ ______ 75-84 Limestone-clay contact deposits ____ __ _ --- ------------------------------------ 55 

regional structural controL-----~----- _______ _______ __ - __ --·---- __ -------- 75-76 Location of area ____ --------- __ ---- __ _ -- --------- ---------- ------------------ 1-3 

Devils River limestone, as host of cave-filL _______ __ ----------------- ______ 55,5 7-58 Locke, A., quoted __________________ __ .--------------------------------------- 46 described. _____ ___ ______________________ ____ ___ _____ --__ ______ ____ ______ __ 8-9 Long Draw dome _______ ------------------------------------------------------ 34 

Dikes _____ . ________ . __________ ._.------..... ____ . _. __ _____ . ______ . __ ----·· 17-18, 20 graben. See Grabens. 

Discordant intrusions._------------------------------· ____ --------------· ____ 17-18 Lowes Valley, graben ___ ______ ___ ______ ---------- ----------------------------- 38 

113 
114 INDEX 

M Page 

Magmatic crystallization_____________________________________________________ 27 

Marathon Basin, structural setting _______ -------------------·________________ 29 

Minerals, alunite_---------------_____________________________________________ 73 

analcite ___________________________________________________________________ 27, 73 

anhydrite ________________________________________________________________ 73 

barite . -----.-------- _____________________________________________________ 72-73 

calcite._________________________ ___ _______________________________________ 66 

calomeL ____ -- _________ ______________ __________________________________ 58, 69, 81 

cinnabar____________________________ __ ____________________________________ 69 

clays minerals _____ -------------- ____ --------------_------------- _________ 66-68 eglestonite_" ______________________________________________________________ 58, 69 

epsomite ____________________________________________________ . _ __ _____ ___ _ 73 

fluorite. ___ _______________________________________________________________ 72 

gypsum __ -------------------------_______________________________________ 73 

hematite_________________________________________________________________ 58 

hydrocarbon compounds _________________________________________________ 71-72 

iron minerals_--------------------- ____ _______________________ 58, 59, 61, 62, 68-69 

jarosite.--------------.:_-- ___ ------------------------- ____________________ 58, 73 

kleinite _________ ------------ ______________________________________________ 58, 69 

limonite. ____ --------_____________________________________________________ 58 

manganese oxide ..... ----------- ________________ ----------------------____ 86 

melentrite _________ .- _____ ---------- ______________________________________ 62, 73 

mercury, native ______ ---------------------------------------------- 58, 69, 91,93 oxychlorides __________ . ____ _______ _ ____ ____ ___ ___ _ _______ ___ _ __ _______ 91 

montroydite _____ ------- _. ______________ . _________________________________ 58, 69 

mosesite ___ ---------.-------------.--- ____________________________________ 58, 69 

natrolite ... ---------------------.. ____________ . ____ __ _ _ _ __ _ __ _ _ __ _ __ _ ___ _ _ 73 

of quicksilver deposits (table)_______________________________________ _____ _ 65 

silica. ___ ------ ___ ----------- _______ . ______ . ____ .. _____ __ ____ _________ ._____ 68 

terlinguaite .. _________ --------- ____________ ._ . c _____________ . ______________ 58, 69 

titanite __________________________________ .________________________________ 73 

titanium .. ____ .------------------------------_____________________________ 28 

Mineralization ____ ---------------------------------~-- _______________ 54, 64-{i5, 74-75 

Mines and prospects Big Bend property. See Study Butte mine. 

Bobs mine ___ -------------------------"--------------------- ______________ 87, 88 

Brown properties in sec. 286 .. _ --------------------------- ______________ 105-106 

Canyon group of claims___________________________________________________ 92 

Chisos mine ______________ -----------------~------ ______________________ 10G-104 

Colquitt-Tigner (Waldron) mine _________________________________________ 98,99 

Colquitt Tigner (old), mine, origin of ore ___ ------------------------------ 59 Contrabando Dome prospects ____________________________________________ 84-85 

Croesus claim_------------------ _____ --------- __ .________________________ 90 

Curtis claim. See Sample group. 

D and G 1 and 2 claims___________________________________________________ 88 

Frank Duncan group of claims------------------------------------------- 90 Fresno mine ____ ._______________________________________________________ __ 85 

Hoover claim. (See also Frank Duncan group of claims)_________________ 91 

Lafarelle and other prospects nearbY-------------------------------------- 92 

Le Roi prospect_ __________________________ ----· ___ -------------------- ___ 98-99 

Lindbergh claim. See Sample group. 

Little Thirty-Eight mine _____________________________ -------------------- 95-97 

origin of ore ______ ----------------------------------------------------- 59 

Lowe prospect ___________ ----------- ______________ ------------------------ 85-86 

Maggie claim. See Bobs mine. 

Manhattie claim _____________________________ ----------------------------- 88 

Margaret D lode _______ --------------------------------------------------- 9G-91 

Mariposa mine. _______ --------------------------------------------------- 92-94 

as type example of cave-fill deposits·---------------------------------- 55-59 

Mitchell-Gillette group of claims. See Bobs mine. 

Monte Cristo claim _______ ------------------------------------------------ 90 

Natural Resources, Inc., claims .... ------------------------------------ 87, 89-90 

Pilkington claim (see also Frank Duncan group of claims)________________ 91 

Pipe ore body. See Chisos mine. 

Prospects in sec . . 98.--------------- _ -------------------------------- ______ 86 Rainbow mine ______ ----------__________________________________________ 104-105 

Rio Grande Quicksilver Co. claims and prospects _________________________ 92,98 

Sample group _____________ -_- _____________________________________________ 86-87 

Speed Wagon claim. See Le Roi prospect. 

Spirit of St. Louis claim. See Sample group. 

Star mine ______________ ---------- __ ----------------- ______ ---------------- 97-98 

Study Butte mine _____________________ ------- ____ ----------- ____________ 107-109 

Tarrant Mining Co. property (see also Little-Thirty-Eight mine, Star 

mine)------------------ ______ ------ ________ --------- ______________ 94-95 

Texas Almaden property. See Study Butte mine. 

Two-Forty-Eight mine. _____________________________________ ------_____ 106-107 

Waldron Quicksilver Propertie~ claims (see also Colquitt-Tigner mine)____ 92 

N-0 Page 

Nine Point Mesa Plain, defined______________________________________________ 3 

Olivine basalt, petrography--------------------------------------------------- 26 

Ore horizon, most favorable__________________ ________________________________ 54 

Origin of gangue elements, transportation and deposition _____________________ 83-84 

mercury minerals, gas-liquid mechanism proposed ________________________ 82-83 

quicksilver deposits, similarity to fumarole and l:ot spring deposits _______ 79-81 

temperatures and pressures of deposition______________________________ 80 

transportation and deposition of ore elements. ________________________ 78-83 

p 

Petrography of intrusive igneous rocks _______________________________________ 23-29 

Plugs _______________________________ _______________________________________ 17-18, 20 

Previous geologic work __ ----------- ___________________________________ c______ 6 

Production of quicksilver----.-________________________________________________ 5G-52 

Q 

Quicksilver belt. ____________________ _________________________________________ 52, 53 

deposits. See Deposits. 

minerals. See under Minerals. 

Quartz bostonite. See Quartz soda syenite. 

Quartz soda syenite, petrography-------------------------------- ___________ 24-25 

R 

Reeside, J. B., Jr., cited .. -------------c-------------------------------------- 8,13 

Rhyolite, petrography------------------------------- ____________ ------------ 24 

Ross, Clyde P ., manuscript report on district.________________________________ 6 

quoted and cited ________________________ 9, 11, 5G-52, 59, 70, 71, 72, 90, 92, 94, 98,104 

Rust, G. W., quoted_________________________________________________________ 46 

s 

Section, explained .. ___ -------- _____ -------_. ___ -----c----------------- ___ ____ 84 

Sedimentary rocks, relation to mineralization_________________________________ 54 

Sills. _ .... _______ . __ . ___ . _ ... ___ ... ___ . _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 18, 20 

Soda latite, petrography --c----- -------- ------------------------- _____________ 25-26 

Soda trachyte, petrography--------------------------------------_____________ 25 

Solitario, The, description and structure _________________________________ 4, 29, 30,32 

igneous origin ___ --------------------_____________________________________ 48 

Solution features, breccia pipes. .See Breccia pipes. 

cave-fill. See Cave-fill. 

limestone caverns .... ---------------------------------------- __________ 39-40, 44 

Stephenson, L. W., cited ... ----------------------------------------------- 12, 13,15 

Stratified rocks, general description ... __ --------------------------------- cc _ __ 7 

See also under formatiowtl names. 

Stratigraphic names of the district.____________ _______________________________ 7 

Structure, age _______ ---------------------------------------------------______ 29 

Structural control, channel cross section: surface area _________________________ 77-78 

Also see under Deposits. 

Structural features, age _______________________________ ------------------------ 47 

analysis of movements _________________________ _________________ --------__ 48 

relation of mineralization to minor------------------------------------- .__ 53 Structural geology ________________________ . _______________ . ___________________ 29-50 

Structural trends, regionaL ______________________ --------------------------___ 30 

Study Butte intrusive body __ ---------------------------------------"-------- 20,21 

T 

Terlingua clay, described _________ -------------------------------------------- 13-14 

Terlingua· Country, defined. _______ ------------------------------------------ 3, 4 

. Terlingua district, structural setting__________________________________________ 29 

: Terlingua uplift._------------------------------------------------- _____ 29, 30, 31, 32 

age and movement._---------------------------------------------- _______ 47-49 

Terlingua monocline, origin __ ------------------------------------------ ______ 48-49 

relation to quicksilver belt .. ---------------------------------------------- 53 

structure. ____ ----------------------_.-------.-.-------------------- ___ 31, 33-35 

Tornillo clay, described ______ ------------ __ -----------------------------______ 16 

Trans-Pecos Texas, described ... ---------------------------------------------- 1, 3 

Two-Forty-Eight dome _______ . ___ - __ .-. ____ . ________ ----_-----_. ___ -- ___ ._.__ 36 

V-W 

Vei~s, calcite. See under Cave-fill and Deposits. 

Wax Factory laccolith, general character •..• ---------------------------------- 18 

White, D. E·., quoted ... ----------------------··------------------------------ SG-81