UNITED STATES DEPARTMENT OF THE INTERIOR 

Harold L. Ickes, Secretary

GEOLOGICAL SURVEY 

W. C. Mendenhall, Director

Bulletin 935-C

TIN DEPOSITS OF THE REPUBLIC OF MEXICO

BY

WILLIAM F. FOSHAG AND CARL FRIES, JR.

Geologic Investigations in the American Republics, 1941-42 

(Pages 99-1W

UNITED STATES

GOVERNMENT PRINTING OFFICE 

WASHINGTON : 1942

For sale by the Superintendent of Documents, Washington, D. C. Price 40 cents

CONTENTS

. Page

Abstract.................................................. 99

Introduction.............................................. 100

Distribution of the tin deposits...................... 100

Climate and topography................................ 101

Accessibility......................................... 101

History and production................................ 103

Field and laboratory work............................. 105

Acknowledgments....................................... 106

Tin deposits.............................................. 107

Deposits associated with granite...................... 107

Location and history.............................. 107

General features.................................. 108

Rock formations................................... 109

Limestone..................................... 109

Granite....................................... 110

Fan deposits.................................. Ill

Secondary deposits of gravel.................. 112

Placers........................................... 113

Heavy minerals in the gravels................. 113

Grade of the deposits......................... 116

Possible production........................... 117

Deposits associated with lead-zinc-silver ores........ 117

Deposits associated with extrusive rocks.............. 118

Geology........................................... 119

Vein deposits..................................... 122

Occurrence.........'........................... 122

Mineralogy and paragenesis.................... 123

Alteration.................................... 125

Origin........................................ 129

Mining and concentration...................... 133

Possible production........................... 137

Placers........................................... 137

Occurrence.................................... 137

Mining and concentration...................... 139

Possible production........................... 140

Summary of possible tin production.................... 141

Mines, deposits, and districts examined................... 142

Michoacan............................................. 142

Los Cabires mine.................................. 142

Hidalgo............................................... 144

Deposits near Zlmap&n............................. 144

 Guanajuato............................................ 145

District west and northwest of Dolores Hidalgo.... 145

 Vein deposits................................. 145

Placers....................................... 146

Deposits near Tlachiquera......................... 151

Deposits near San Felipe.......................... 152

Queensland mine................................... 153

El Santin mine.................................... 155

Deposits north of San Luis de la Paz.............. 158

Aguascalientes........................................ 158

Deposits near Calvillo............................ 158

Jalisco............................................... 160

Deposits near Paso de Sotos and Teocaltiche....... 160

Los Amigos mine................................... 161

III
IV CONTENTS

Page

Mines, deposits, and districts examined Continued.

Zacatecas............................................. 162

Deposits west of the city of Zacatecas............ 162

Deposits between Fresnillo and Sain Alto.......... 162

Durango............................................... 164

Deposits south of the city of Durango............. 164

Deposits on the Cerro de los Remedies............. 165

Deposits in the Sierra de Cacaria................. 167

Vein deposits................................. 168

Placer deposits............................... 170

Deposits in the Sierra de San Francisco........... 170

Vein deposits................................. 171

Placer deposits............................... 174

ILLUSTRATIONS

Page

Plate 25. Map of Mexico showing distribution of reported

tin districts................................ 102

26. Geologic sketch map of the Guadalcazar district, San Luis Potosi....................... 110

27. A, Small abandoned tin mine near San Felipe,

Guanajuato; B_, Small tin placer near Tlachiquera, Guanajuato............................ 138

28. Plan and section of Los Cabires mine, Tepuxtepec, Michoacan............................. 142

29. Geologic map of placer area west of Dolores

Hidalgo, Guana juato.......................... 146

30. Geologic sketch map of Cerro de los Remedies,

Durango...................................... 166

31. Sketch map of tin district in Sierra de San

Francisco, Durango............................ 170

Figure 10. Geologic map of placer area along Arroyo de

Galvez, west of Dolores Hidalgo, Guanajuato.. 147

11. Geologic map of placer area along Arroyo del

Estanlto, southwest of La Quemada, Guanajuato 150

TABLES

Page

Table 3. Production of tin in Mexico................... 105

4. Heavy minerals in the gravel deposits in the

Guadalcazar and Realejo "basins,............. 114

5. Mineral composition of the rocks and tin veins

in some of the tin districts of Mexico..,... 122,

6. Chemical analyses of vein material, cassiterite, and bars of smelted tin................ 126
TIN DEPOSITS OF THE REPUBLIC OF MEXICO

By William F. Foshag and Carl Fries, Jr.

ABSTRACT

The tin deposits which are reported to occur in 19 States 

of the Republic of Mexico are all on the high lava-covered plateau extending from the border with the United States to the 

State of Oaxaca. They are of three types: (1) Deposits associated with granite. (2) deposits associated with lead-zincsilver ores, and (3; deposits closely associated with extrusive 

rocks. The total production of metallic tin between 1800 and 

1940 has probably been somewhat more than 6,000 metric tons; 

the annual production between 1936 and 1940 has averaged 330 

tons. About half of the tin came from deposits of type 2 and 

the rest from deposits of type 3; none has yet been mined from 

deposits of type 1.

The only district known in which tin deposits are associated 

with granite is near Guadalcazar, San Luis Potos£. Here granite 

is intruded into Cretaceous limestone and crops out in a roughly 

circular area a mile and a quarter (2 km.) in diameter. Cassiterite is apparently a late accessory mineral in the granite 

and may occur in some of the lead-silver veins mined in the 

peripheral zone of the intrusive body, but no tin deposits of 

economic value are known. Large deposits of gravel derived from 

the granite contain cassiterite, cinnabar, gold, and silver; 

parts of these deposits may contain valuable metals in sufficient quantity for profitable placer mining.

The only known Mexican tin deposits associated with leadzinc-silver ores are at the San Antonio mine, near Santa Eulalia, Chihuahua. Tertiary clastic and volcanic rocks unconformably overlie the lead-zinc-silver deposits, which replace Cretaceous limestone, and dikes of rhyolite and andesite intrude all 

the rocks. The exact mode of occurrence and origin of the tin 

are unknown; the tin may be related in origin either to the rhyolite dikes or'to the lead-zinc ores. Although the San Antonio 

mine has yielded about half the tin produced in Mexico, it has 

not produced any tin since 1939, and its tin ore is said to be 

exhausted.

Probably more than a thousand tin deposits, most of them 

small veins but some of them placers, are known in volcanic 

rocks on the plateau of Mexico, largely in the States of Durango, Zacatecas, and Guanajuato. All these deposits and some 

in the United States--in the Black Range of southwestern New 

Mexico and in northern Lander County, Nevada are very similar 

in occurrence and mineral composition and differ, so far as 

known, from all other tin deposits yet described. Most- of the 

deposits are in late Tertiary rhyolite flows, but a few are in 

latite and andesite that underlie the rhyolite. The veins consist largely of specularite intimately associated with cassit99
100 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS, 1941-42

erite. The cassiterite forms pockety incrustations on walls of 

irregular, branching, steeply dipping fissures, and is nowhere 

disseminated through large bodies of rock. The deposits, in 

general, are believed to have formed within 2,000 feet (600 m.) 

of the surface and shortly after extrusion of the lavas. Most 

of the cassiterite is believed to have come from small bodies 

of crystallizing rhyolitic magma that had been intruded near the 

surface, though some of it may have come from crystallizing 

flows. The deposits appear to have been localized by minor 

faults near the centers of eruption of the rhyolitic lavas.

Production of tin from the veins in the lavas has been sporadic; most of the veins have yielded less than 5 tons of tin 

and only a few have yielded more than 20 tons. Pour attempts 

to mine deposits on a large scale have proved unprofitable, 

apparently owing to the small size of the individual ore bodies. 

No valid estimate of reserves can be made. No deposit has been 

proved to contain more than 120,000 tons of rock containing 0.5 

percent tin, and most of the deposits probably contain a much 

smaller tonnage of rock of that grade. There may be as much as 

10,000 tons of tin in rock containing 0.1 percent or more of 

tin, but no method has yet been developed by which material of 

such low grade can be profitably mined.

Most of the tin produced between 1936 and 1941 came from 

placers derived from the veins. Most of these placers are 

extremely small and are mined entirely b'y hand methods. One of 

the deposits, indeed, may be more than 1,000,000 cubic yards in 

volume, but the results of extensive sampling indicate that it 

contains less than half a pound of tin to a cubic yard. There 

may be more than 10,000 tons of tin in placers containing half 

a pound or more of tin to a cubic yard, but most of it is in 

small deposits that cannot be worked profitably with machinery 

and that can be mined only during the rainy season from July 

to December.

Under the mining conditions and at the prices prevailing in 

1941 the probable production of tin from veins and placers in 

the lavas is not likely to exceed 400 tons a year, and even at 

double the 1941 price of tin the probable production is not 

likely to exceed 800 tons a year.

\ 

INTRODUCTION

Distribution of the tin deposits

Tin deposits have been reported to occur in 19 States of the 

Republic of Mexico, from the United States boundary on the north 

to the State of Oaxaca on the south. The deposits are most 

thickly distributed in the central part, in the States of Durango, Zacatecas, San Luis Potosi, Aguascalientes, Jalisco, Guanajuato,.and Quere'taro (see pi. 25); a few deposits each have 

been reported in Sonora, Chihuahua, Coahuila, Nuevo Leon, Nayarit, Michoacan, Hidalgo, Mexico, Morelos, Puebla, OaXBCa, and 

Guerrero. There are probably more than a thousand individual 

deposits scattered throughout this belt, but they occur in more 

or less distinct groups. Only the groups are located on the map,
TIN DEPOSITS OP THE REPUBLIC OP MEXICO 101 

Climate and topography

The tin deposits are almost wholly on the lava-covered semiarid central plateau, at elevations between 4,000 and 10,000 

feet (1,200 and 3,000 m.). Although rainfall is extremely light 

during the greater part of the year, a rainy season, beginning 

in May or June in the southern part of the country and in June 

or July in the northern part, lasts until mid-October. During 

the rainy season water runs in many of the arroyos, but during 

the remainder of the year most of them are dry. Although there 

is an occasional light snowfall over the northern part of the 

plateau during the winter, frosts rarely occur south of San 

Luis Potosi except at high altitudes. Temperatures during the 

summer are not excessive, owing in part to the high altitude of 

the plateau and in part to the frequent rainfall.

North of the city of San Luis Potos£ the plateau is characterized by interior drainage; more or less isolated mountain 

ranges and mesas rising from 1,000 to about 5,000 feet (300 to 

about 1,520 m.) above the general level separate the various 

basins, some of which contain saline or playa lakes. South of 

San Luis Potosf, except for a few basins of interior drainage, 

of which the valley of Mexico is best known, the drainage flows 

eastward into the Gulf of Mexico or westward into the Pacific 

Ocean. Westward drainage leaves the plateau through precipitous 

canyons (barrancas), some of which are nearly as deep as the 

Grand Canyon of the Colorado River; eastward drainage flows 

through somewhat shallower canyons.

Accessibility

An excellent all-weather highway connects Laredo, Tex., with 

Mexico City via Monterrey, Victoria, and Zimapan. (See pi. 25.) 

From Mexico City surfaced highways extend south to Acapulco, on 

the Pacific Ocean, west to Guadalajara, and east to Veracruz, 

on the Gulf of Mexico. A road extends from Guadalajara north-
102 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS, 1941-42

westward along the-coast through Cullacan, Guaymas, and Nogales 

to Tucson, Ariz., but it is now not passable to through traffic 

except from Guaymas to Tucson. A surfaced highway runs from El 

Paso, Tex., to Chihuahua, Mexico. Prom Chihuahua to Mexico City 

there is a road passing through Durango, Zacatecas, Aguascalientes, and Quere'taro, which is unsurfaced except for short 

stretches near those cities. This road can be travelled, with 

some difficulty in the dry season, but it is impassable for 

automobiles in the rainy season.

A road, of which it is planned to make a highway, from 

Monterrey westward to Mazatlan, on the coast, is surfaced as 

far as Torreon; it is passable from there to Durango during the 

dry season but is impassable for automobiles between Durango 

and Mazatlan. Prom Guadalajara to the border of the United 

States, a distance of about 900 miles, there is not a single 

road that crosses the barranca country between the plateau and 

the west coast. The highway from Victoria, or from Antiguo 

Morelos, on the Laredo-Mexlco City highway, to San Luis Potosi 

is surfaced and passable throughout the year and an extension 

of it from there to Guadalajara is projected. A surfaced highway extends eastward from the Mexico City highway to Tampico, 

on the Gulf of Mexico..

It is possible to travel in trucks or cars with high clearance on wagon roads over much of the plateau country during the 

dry season; during the rainy season most of the plateau country 

is virtually inaccessible by automobile. Many of the tin deposits, located mainly along the western edge of the plateau, can 

be reached by automobile during the dry season, but some can be 

reached only by a day's ride on horseback from the nearest road.

A main railroad line extends southward from Tucson, Ariz.,

through Guaymas, Cullacan, Mazatlan, and GfuadalajflFfl tO MeXlCO 

City; another extends from El Paso, Tex., to Mexico City through 

Chihuahua, Torreon, Zacatecas, Aguascalientes, Leon, and Quere-
TIN DEPOSITS OP THE REPUBLIC OP MEXICO . 103

taro. From Mexico City a line.extends eastward to Veracruz. 

There are railroads from Laredo and Brownsville, Tex., to Monterrey, from which a main line continues southward through San 

Luis Potosf and Quere'taro to Mexico City; a branch line extends 

from Monterrey to Tampico and another to Torredn and Durango. 

Branches run from San Luis Potosl to Tampico and to Aguascalientes, and several others extend from the main lines for short 

distances. Pour seaports are on the railroads: Tampico and 

Veracruz on the Gulf of Mexico, Guaymas on the Gulf of California, and Mazatlan on the Pacific Ocean.

History and production

I/ 

Ingalls records the fact that the Indians knew of tin in

Mexico before the Spaniards arrived in the New World. Accord27 ing to Hanks,-'the Spaniards began to mine tin in the Ame'ricaPotrillos district in Durango as early as 1790. A summary of

the history of tin mining during the nineteenth century is given

3/ 4/ 

by Ingalls. When Garcfa reviewed the history of tin mining

up to 1926, he found that there had been little change in either 

methods or scale of mining from those described by Ingalls. 

Even in 1941, except for a few relatively extensive deposits, 

described farther on in this report, mining methods were the 

same as during the past two centuries. Most of the tin ore has 

been recovered on a small scale, with hand tools and without 

explosives, by washing the arrpyo gravels and the soil on hillsides near small vein deposits, and by crushing and washing 

material mined from small veins.

I/ Ingalls, W. R., The tin deposits of Durango, Mexico: Am. Inst. Min. 

Eng. Trans. , vol. 25, p. ll|6, 1896.

2/ Hanks, H. G., On the occurrence of durangite in the tin-tearing 

region of Durango, Mexico: Am. Jour. Sci., 3d ser., vol. 12, p. 276, 1876.

2/ Ingalls, W. R., op. cit., pp. 1U6-163.

Jf/ Garc£a, J. A., Monograffa del estano: Bol. minero, vol. 22* pp. 6-29, 

1926.
104 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS, 1941-42

Records of the production of tin in Mexico prior to 1916 are

meager, only a few figures appearing in published reports.

5/ 

Ingalls quotes Humboldt as reporting that 4.602 metric tons of

metallic tin was produced in 1802 in the administrative district

6/ Mexico in 1003. Manzano -1 states that 44 tons of ore was produced in 1882 from a deposit near Santa Maria del Rio, S. L. P.,

and that it yielded possibly 10 tons of metallic tin. According

7/ 

to Ingalls, a deposit that was being operated in 1883 near

Teocaltiche, Jalisco, had a maximum monthly production of about 

3 tons of metallic tin during the short period it was operated. 

Mines in the America-Potrillos district in Durango were most

during that period was a few thousand tons of ore; he does not 

state the tin content of the ore, but it was probably much less 

than 10 percent. Prom these few figures it seems reasonable to 

infer that the average annual production from 1800 to 1916 was 

about 10 metric tons of metallic tin, or about 1,100 metric tons 

in all. The table on page 105 shows the production of tin in 

Mexico compiled from statistics given in Boletfn Minero and 

Boletin Petroleo y Minero, publications of the Mexican government. The figures are incomplete, and some of them are undoubtedly too low, but they are probably of the right order of magnitude .

5/ Innalls, Y;. H., pp. cit., p. 147.

&_/ Manzano, J. P., Informs sobre la exploraci6n.de la zona minera de 

Santa Mnrfa del Hfo, S. L. P.: Bol. agriculture, minerfa 6 industries, 

vol. 3, No. 2, p. 101, 1893.

7/ Ingalls, ;. H., op. cit., p. 160.

8/ Rangel, Manuel, Apuntes sobre la distribuci6n de minerales en el 

Lstadb de Durango: Soc. geol. mexicana, Bol. 7, p. Ill, 1910.
TIN DEPOSITS OP THE REPUBLIC OP MEXICO 105

Table 3. Production of tin in Mexico

Year

1800-1915 (estimate).

1916.... .............

n QI 7

1918.................

1919

1920 (first half)....

1921 (first half)....

T Qpp

1923. ................

1QP4

1925.................

1926.................

1927................. 

1 QPA

Metric 

tons

1,100.000

?Q?

Q PI 4

13.537

1.588

4Q?

No record

No record

fl R4Q

i o1^

2.224

No record

Year

1 Q?Q

i Q^n

1931......... ........

1932.................

1933 (Jan. to March) .

1934.................

1 Q^R

1 Q^R

1937.................

1938.................

1939-:*..... ...........

1940*................

Total. . . .(about)

Metric 

tons

269.560

773.171

751.430

120.378

15.829

630.749

373.476

379.480

255.458

.293.910

350.729

5,400.000

I/ More than half the tin produced between 1930 and 1939 ceme from the 

San Antonio mine, near Santa Eulalia, Chihuahua. 

* From Mexican Department of National Economy..

Field and laboratory work

The authors were in Mexico for 3 months, from the first of 

January to' the end of March 1941, and visited as many of the 

tin-bearing districts as time permitted, devoting special attention to those in which tin was being mined or which appeared to 

be promising. Prom 1 to 10 small mines were examined in each 

of 19 more or less separate districts, or 44 mines in all. Pannings were made of placer gravels in the arroyos of 7 districts, 

and pan concentrates of gravels, as well as specimens of ore and 

country rock from each district, were collected for laboratory 

study. Owing largely to scarcity of water during the dry season, only half a dozen of the mines visited were being worked, 

and placer deposits were being mined in only three of the districts visited. No deposits were mapped, partly because the 

time allotted for the field work did not permit it, and partly 

because the size of°the deposits did not seem to warrant it.

Brief laboratory investigations have been made in order to 

identify the rocks and minerals collected and to determine 

roughly the tin content of some channel samples collected from
106 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS, 1941-42

the most promising deposits. Miss Jewell J. Glass of the Geological Survey collaborated in this work. Some of the results 

are presented in subsequent sections.

Acknowledgments

The field work was done by the Geological Survey, United 

States Department of the Interior, and the Smithsonian Institution, in cooperation with the United States Department of 

State, and with the permission and aid of the Government of Mexico. The authors wish to thank the staff of the Institute geologico de Mexico for expediting the field work and for the many 

valuable suggestions they have offered. Further aid in the 

field was given by more persons than can be enumerated here, but 

the authors must acknowledge particularly the assistance and 

courtesies extended by P. W. Avery, Antonio Cano, Carlos Preymann, Manuel Heyser, S. G. Jenkin, Jesus de Lara, D. B. McAllister, E. Mouret, Francisco Salazar, Rafael Solfs, and Martin 

Sutti; they are indebted also to many persons for valued information, especially to W. E. Brock, Colonel Gordon McCoy, E. M. 

Flynn, Francisco Galarza, Major H. K. Kicklighter, J. F. Morton, 

the Cia. Minera de Penoles, S. A., and members of the Consular 

staff of the United States in Mexico. The many kindnesses 

extended the authors by the Mexican miners and ranchers are 

also gratefully acknowledged. For aid in the office the authors 

are indebted to many members of the Geological Survey, particularly to J. J. Glass for assistance in petrographic and mineralogic studies; to R. C. Wells, J. J. Fahey, and George Steiger 

for chemical and spectrographic work; and to H. G. Ferguson and 

F. C. Calkins for constructive criticism of the manuscript.
TIN DEPOSITS OP THE REPUBLIC OP MEXICO 107 

TIN DEPOSITS

The known tin deposits in Mexico are of three types: (1) 

Deposits associated with granite, (2) deposits associated with 

lead-zinc-silver ores, and (3) deposits closely associated with 

extrusive rocks (lava flows, breccia, and tuff). Deposits of 

the first and second types are thus far known to occur in only 

one district each, but more than a thousand small deposits of 

the third type are known in the more or less separate districts 

scattered over the plateau. The deposits of the first two types 

are younger than Cretaceous, though it is not known how much 

younger; the deposits in the lavas are largely late Tertiary. 

Erosion of the tin-bearing veins has given rise to many small 

placers, for the most part of recent origin.

Deposits associated with granite

Location and history 

i

The only tin deposits in Mexico known to be directly associated with granite occur near the town of Guadalcazar, San 

Luis Potosf, in a famous old mining region 60 miles (96 km.) 

by road northeast of the city of San Luis Potosf and 12 miles 

(19 km.) northeast of the Villar station on the Tampico branch 

of the Mexican Central Railroad. The district is easily reached 

by car during the dry season over 11 miles (18 km.) of narrow 

dirt road from a point on the San Luis Potosi-Victoria highway 

48 miles (78 km.) from the city of San Luis Potosf. The ancient 

mines «near Guadalcazar, in which stone hammers have been found, 

were probably worked for mercury by the Indians before the colonization which took place between 1614 and 1620. This district 

contained important silver mines as early as 1622, and it later 

became the largest producer of mercury in Mexico. Gold has been 

washed from placer deposits in the district intermittently for 

many years, but cassiterite was not recognized there until about
108 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS, 1941-42

1937. Since then parts of the gravel deposits have been tested 

for tin, mercury, gold, and silver, but no tin had been mined up 

to December 1941.

General features

The district is .dominated by the Sierra de Guadalcazar, a 

westward-trending mountain range flanked on the east by the 

Abrego basin, on the south by the Guadalc&zar basin, and on the 

northwest by the Realejo basin. The highest point in the range 

is Cerro San Cristobal, which is 7,382 feet (2,250 m.) above sea 

level and about 1,500 feet (460 m.) above the Guadalcazar basin. 

(See pi. 26.) This peak forms the center of a small roughly 

circular body of granite intruded into Cretaceous limestone, 

which surrounds the granite on all sides except the northwest, 

where fan deposits lap directly against the granite. The granite is a little higher than the surrounding limestone and is 

outlined by arroyos that, have developed along the greater part 

of the contact. Debris eroded from the contact zone was deposited in large alluvial fans along the sides of the contiguous 

basins. Subsequent erosion lowered the arroyo bottoms, dissected the fans, and caused the formation of secondary deposits 

of gravel. The basins are now drained largely through sinks 

(resumideros) in the underlying limestone.

The western part of the Sierra de Guadalcazar is a gently 

rolling upland, which gradually decreases in altitude westward 

from Cerro San Cristobal and which appears to be about in the 

same plane as the tops of the granite hills. This upland is 

underlain by limestone and is characterized by sink holes, collapsed slopes, and deep caverns open to the surface. It 

undoubtedly represents an old karst topography that was formed

prior to erosion of the steep-walled canyons. A broad, shallow 

valley at a high elevation, into which the Arroyo Las Papas has 

been cut, south of Cerro San Cristobal, and a narrow bench on the 

limestone along the front of the range east of Arroyo Las Papas, 
TIN DEPOSITS OF THE REPUBLIC OF MEXICO 109

facing the Guadalcazar basin, probably are remnants of this old 

topography.

Broad, gently 'sloping alluvial fans extend out from the 

Sierra de Guadalcazar into the Abrego, Guadalcazar, and Realejo 

basins. The distribution of these fans is such that they could 

not have been formed by the present drainage but must have been 

built by ancestral streams flowing at higher levels. The easternmost fan in the Guadalcazar basin was built at the mouth of 

a high valley that corresponds in position to Arroyo Minas 

Viejas. This fan has a gentle, slope toward the soiitheast and 

forms the low divide that separates the Guadalc&zar and Abrego 

basins. A second large fan was built by drainage carried 

through a high valley having the approximate position of Arroyo 

Las Papas, and a third fan was constructed at.the mouth of 

another high valley a little farther west. These fans have 

coalesced to form a .continuous alluvial apron along the mountain front. The small Realejo basin is filled with a large 

fan that slopes gently toward the northwest, away from the /granite. These old fans are.more closely related to the old upland 

surface than to the present more youthful topography.

All the arroyos are steep-walled and narrow, and the old 

topography and the fan deposits are being actively dissected, 

parts of the fans having typical badlands topography. During 

this recent period of erosion the soft, highly altered contact 

zone of the granite has also been extensively eroded, and at the 

edge of the Realejo basin this altered granite has a badlands 

topography similar to that of the eroded fans.

Rock formations

Limestone. The oldest formation in the district is a

i/

cherty, light-gray limestone of middle Cretaceous age. Its

2/ Wittich, Ernesto, and Ragotey, Federlco, La geologla de la regi6n 

minera de Guadalcazar, San Luis Potosi: Bol. ninero, vol. 12, p. 665, 

1921.
110 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS, 1941-42

thickness exceeds 1,500 feet (450 m.). In places along its contact with the granite, the limestone is partly marmorized and 

contains lime-bearing silicate minerals, such as diopside, 

andradite, epidote, and vesuvianite. Some of the silicate bodies contain sulfides of iron, lead, silver, copper, zinc, and 

antimony. The large mercury deposits at San Antonio and Trinidad, as well as some of the other sulfide deposits, are wholly 

within this limestone. The numerous caverns that have formed 

in the limestone in the upland areas and in the broad basins are 

due to the relatively high solubility of this rock.

Granite. The granite is a light-gray rock, whose texture is 

variable, but most of which is porphyritic., with a medium- to 

fine-grained groundmassc The principal minerals are quartz 

and orthoclase, which occur both as phenocrysts and as smaller 

crystals in the groundmass. There are smaller quantities of 

biotite, sericite, muscovite, oligoclase, hornblende, and chlorite. A specimen from near the top of Cerro San Crist6bal (see 

pi. 26), apparently representative of the granite in that area, 

proved to contain a small quantity of microscopic grains of 

fluorite, zircon, hematite, apatite, epidote, tourmaline, garnet, topaz, brookite/ cassiterite, scheelite, and molybdenite. 

A sample from the north side of Cerro Piedras contained, besides 

the common accessory minerals, a few grains of scheelite but 

apparently no cassiterite. Samples of granite taken at three 

places on the north side of Arroyo Las Papas between Casa de los 

Arcos and Socav6n Dolores each contained cassiterite, and two 

also contained scheelite.

Tourmaline occurs in patches scattered through much of the 

granite and also with quartz in narrow veinlets cutting the 

granite. Ill-defined pegmatitic veinlets containing beryl, 

topaz, and dumortierite are present in some places, and Wittictl 

reports that danburite and axinite have been found.

IO/ Wittich, Ernesto, and Rogotzy, Federico, op. cit., p. 669.
UNITED STATES DEPARTMENT OF THE INTERIOR 

GEOLOGICAL SURVEY BULLETIN 935 PLATE 26

GEOLOGIC SKETCH MAP OF THE GUADALCAZAR -DISTRICT, SAN LUIS POTOSI 

CROQUIS GEOLOGICO DEL DISTRITO DE GUADALCAZAR, SAN LUIS POTOSI

300 0

Adapted from unpublished sketch and claim location maps of 

Cia de Winerales de Potosi, S.A.,and Cia Metalur^ica Mexicans,

Adaptado de un croquis y mapas meditos de /as pertenencias 

la Cia de Minera/es ye Potos^S.A^y /<? Cta Metd/urg/cg Mexi

EXPLANATION 

EXPLICAGJON

A/uviones y abanicos aluviales

(Contienen'depdsitos de placer)

(U+i i mi

Gray limestone 

Call'za gris

Contact accurately jocated 

Contacto de/ineado corfexacti tud

Contact approximately located 

Contacto delineado aproximadamente

Landmark 

Punto sobresaliente

B

Shaft 

Tiro r""

/Adit/ 

Socavdn

X

Mine entrance 

BOCB mina

X

Prospect 

Cata

^ ^ t I 

Direction of^ drainage

Oirecio'n del 

c/errame del agua

cg Mexicans

1500 Feet

100 0 500 Metros V. S. GOVERNMENT PRINTING OFFICE : 1942 O - 480115 (faces p. 110)
TIN DEPOSITS OP THE REPUBLIC OP MEXICO 111

A small sample taken from one of these veinlets near Casa de los 

Arcos contained a few grains of cassiterite but no scheelite. 

Along the southern border of the granite, there are a few dikes 

of fine-grained aplite with patches of tourmaline, and a sample 

from one of these at the junction of Arroyo La Luz with Arrpyo 

Las Papas contained cassiterite and scheelite. It thus appears 

that small quantities of cassiterite and scheelite are widely 

distributed as late accessory minerals of the granite.

The border zone of the granite, except along the northeast 

side, is a soft rock in which the feldspars are more or less 

completely altered to clays. Samples of this rock taken at 

four places contained small quantities of cinnabar. Most of the 

ore deposits mined are in this peripheral altered zone, as shown 

in plate 26, and contain gold and sulfides of iron, zinc, lead, 

silver, antimony, and copper. Pluorite and quartz form a large

"> x_-

part of the gangue in these ores at some places. The limesilicate minerals are abundant immediately adjacent to the limestone contact.

Fan deposits. The deposits of gravel along the east, south, 

and northwest base of the Sierra de Guadalcazar were derived 

mainly.from the limestone-granite contact zone. The exposed 

thickness of gravel in the walls of the arroyos that have dissected the deposits is about 70 feet (21 m.), but the maximum 

thickness is somewhat greater, for most of the arroyos have not 

cut down to the underlying limestone. The bedding in the deposits is indistinct in most places, and the observed dips are 

generally less than 10°.

The deposits are composed in large part of poorly cemented 

coarse sands with a reddish clay matrix. Owing to this reddish 

color, the dissected parts of the fans are known locally as 

tierras coloradas, or red ground. Texturally the material 

changes within short horizontal distances from sand to silt or 

from sand to fine-grained conglomerate. A section through 53

480115 O - 42 - 2
112 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS, 1941-42

feet (16 m.), which is exposed along one of the arroyos and 

which is characteristic of the deposits, shows: Caliche-cemented 

coarse angular sand, 3 feet (0.9 m.); coarse angular sand, 2 feet 

(0.6 m.); brown bentonitic clay, 3 feet (0.9 m.); gray sand, 2 

feet (0.6 m.); iron-cemented sand, 2 feet (0.6 m.)j silty sand, 

4 feet (1.2 m.); and coarse sand with small pebbles, interbedded 

with 6-lnch (0.15-m.) lenses of brown soapy clay, 25 feet (7.6 

m.). Beds of fine-grained gravel are most abundant near the 

heads of the fans and along their axes, where clay occurs only 

as part of the matrix rather than as separate beds. There are 

boulder beds in only a few places. Beds of bentonitic clay are 

most abundant and thickest along the lateral margins of the 

fans.

The deposits are almost, wholly derived from the granite. 

They contain only a small proportion of limestone pebbles, even 

though the material was transported across the limestone; Erosion may have been slow enough to permit the limestone to be 

largely removed by solution, leaving only fine clays; or it is 

possible that some of the limestone pebbles, small in amount to 

begin with, may have been dissolved from the gravels after their 

deposition. Heavy minerals, to be described later, form between 

0.5 and 1.0 percent of the material.

Secondary deposits of gravel. Erosion of the fans has 

resulted in extensive dissection of parts of the deposits. A 

cover of from 1 to 5 feet (0.3 to 1.5 m.) of secondary gravel 

was left on top of the primary gravel at many places while the 

arroyo bottoms were being lowered, and the present beds of the 

arroyos generally contain 1 to 3 feet (0.3 to 0.9 m.) of the 

secondary gravel. This material differs from the primary gravel 

mainly in that the clay has been washed out, resulting in a 

greater concentration of the heavier and more resistant minerals. Limestone pebbles are more abundant in the gravel in
TIN DEPOSITS OF THE REPUBLIC OF MEXICO 113

the bottoms of the arroyos that now drain the limestone-granite 

contact zone than they are in.the primary fan gravels.

Placers

 Gold has been recovered on a small scale intermittently for 

many years from the secondary deposits of gravel in the bottoms 

of the arroyos in the fan gravels in the Guadalcazar basin. 

This work has been done entirely by individual miners, who pan 

the gravels by hand when water flows in the.arroyos during the 

rainy season. So far as known, the grade of the primary fan 

gravels is too low for profitable mining. Some attempts have

been made to diatill mercury from the pan concentrates of the

ll/ 

secondary gravels on a small scale. An examination of the

heavy minerals proves that all the gravel deposits sampled contain mercury, tin, and tungsten; assays show small quantities of 

gold and silver.

Heavy minerals in the gravels. Vertical channel samples of 

the primary gravels were taken by the authors from the walls of 

10 arroyos that cut the fan deposits in the Guadalcazar and 

Realejo basins; none were taken in the Abrego basin. These 

samples ranged from 20 to 60 feet (6.6 to 20 m.) in length and 

averaged about 35 pounds (16 kg.) each in weight. After screening out the gravel larger than a quarter of an inch (0.6 cm.), 

the undersize was concentrated by hand-panning to a ratio of 1 

part to 25. Rough pannings were also made of the secondary 

gravel from the beds of some of the arroyos. All these concentrates were separated in methylene'iodide (sp. gr. 3.3), and the 

heavy minerals were identified by their optical or crystallographic properties. Qualitative results of the mineral studies, which are presented in table 4, show a wide distribution 

of the mercury, tin, and tungsten minerals.

ll/ Wittich, Erneeto, Observed ones e.cerca de placeres de cine'brio y 

oro, encontrados en el Distrito de Guadalcazar, San Luis Potosf: Bol. 

minero, vol. 10, p. 256, 1920.
Table 4. Heavy minerals -> in the graVel deposits in the Gua

Ay%/^T>Qy^-? "I" O

Bindheimite ........

Brookite. ..........

Gassiterite. .......

Epidote ............

Cio ~\ OTI Q »

Hematite 

( specularite ) . . . .

Limonite (some 

goettiite) ........

Tvr* T Q r*Vi "1 "t~ ^

P r» w A 1 1 "1 "t~ P

Pyrite. ............

Rutile. ........... o

ClrtVi^fll T 1-O

Siderite. ..........

Sphalerite .........

Svabite. ...........

Titanite. ..........

T**~IT\Q *7

Wolframite .........

Zircon. .............

til

X

R

ft

X

y

X

R

X

R

X

ay

R

R

X

X

X

y

X

U3

R

R

R

X

X

X

X

X

X

x

G4

R

R

R

X

y

X

X

X

R

R

Ub

R

X

R

R

R

X

X

R

X

X

X

X

X

G6

R

X

R

R

R

X

X

X

y

R

X

X

(jy

R

R

R

R

y

X

X

R

y

R

(18

X

R

R

R

X

X

X

R

R

X

G13

R

R

R

R

R

R

X

X

R

X

MG7

X

X

R

X

X

X

X

R

X

X

G9

R

y

R

R

R

R

X

y

X

R

R

X

Gil

R

X

R

R

R

R

X

X

X

X

X

X

X

R

R

R

X

G15

R

R

R

R

R

X

R

X

MG6

R

R

X

X

X

X

X

R

R

R

M
Gl Arroyo Cieneguilla; 60-foot (18.3-m.) vertical channel sample 

G2 Arroyo Soldado; 40-foot (12.2-m.) vertical channel sample of p

G3 Arroyo Difunta; 22-foot (3.7-m.) vertical channel sample of pr

G4 Arroyo Papas; 20-foot (6.1-m.) vertical channel sample of prim

35 Arroyo Final; 45-foot (13.7-m.) vertical channel sample of pri

G6 Arroyo Roble; 20-foot (6.1-m.) vertical channel sample of prim

G7 Arroyo Escondido; 45-foot (13.7-m.) vertical channel sample of 

G8 Arroyo Profundidad; 45-foot (13.7-m.) vertical channel sample 

G13 Arroyo Santa Elena; 20-foot (6.1-m.) vertical channel sample o

MG7 Arroyo Santa Elena; 15-foot (4.6-m.) vertical channel sample o

G9 Secondary gravel on low bench at north edge of town of Guadalc

Gil Secondary gravel from bed of Arroyo Santa Elena. 

G15 Do. 

MG6 Secondary gravel from bed of Arroyo Mihas Viejas at junction w

MG8 Secondary gravel from bed of Arroyo Dolores.

MG10 Secondary gravel from Guadalcazar basin just south of village 

MG11 Secondary gravel from Guadalcazar basin along ridge just south 

161 Secondary gravel from bed of Arroyo Cieneguilla. 

162 Secondary gravel from bed of Arroyo Sangria. 

163 Secondary gravel from bed of Arroyo Difunta. 

164 Secondary gravel from bed of Arroyo Las Papas. 

165 Do. 

166 Secondary gravel from beds of Arroyos Pinal and Roble. 

I/ Heavy minerals are those nith a specific gravity freater than 3.3. 

2/ The letter R in the table si-nifies that the mineral is rare in the «re.ve

small to moderate quantity of the mineral in the gravel. 
116 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS, 1941-42

The mineral cassiterlte, which occurs mainly in small crystals and crystal fragments but also in sparse fragments of 

banded wood tin, appears to be the only tin-bearing mineral 

present. The color of the cassiterite ranges from red to yellow 

to dark brown or nearly black. The mercury is in the mineral 

cinnabar, which occurs as rounded bright- to dull-red crystal 

fragments. The tungsten-bearing minerals are wolframite and 

scheelite, the former occurring as shiny black rectangular and 

platy crystal fragments and the latter as white to cream-colored grains with greasy luster. The wolframite in the concentrates is similar in appearance to the specularite. The gold 

forms small warty grains. No silver minerals have been recognized, but some of the silver shown by assays may be in the 

galena, which occurs as subrounded grains partly incrusted with 

white anglesite.

Grade of the deposits. Rather detailed studies of the quantities of tin, mercury, gold, and silver in parts of the gravel 

 deposits were made by the Cia. Minera de Penoles, S. A., in 

1938, and by the Cia. Mlnera de Guadalcdzar, S. A., in 1940 and 

1941, and tests were being conducted by the San Luis Mining Co. 

in December 1941. The studies by these companies and by the 

authors show that the primary fan gravels north of the town of 

Guadalc6zar contain from 0.005 to 0.03 percent tin and between 

0.0008 and 0.04 percent mercury; they also.contain from 0.0006 

to 0.003 ounce (0.017 to 0.085 gm..) of gold and about 0.2 ounce 

(5.7 gm.) of silver per metric ton. About 40 percent of the 

gold and silver, 35 percent of the mercury, and 30 percent of 

the tin appear to be in the fines, which form about 20 percent 

of these primary gravels. No quantitative tests for tungsten 

have been made.

Rough channel samples of the primary fan gravels taken by 

the authors indicate that the gravels in the Realejo basin contain more tin and mercury than those in the Guadalcazar basin.
TIN DEPOSITS OF THE REPUBLIC OF MEXICO . 117

Preliminary tests indicate that the secondary gravels in the 

beds of the arroyos contain appreciably larger quantities of 

the metals than the primary fan gravels.

Possible production. The total amount of alluvial material 

in the Guadalcazar and Realejo basins may be more than 75,000,000 

tons, of which all but a few million tons are in the Guadalcazar basin. It is doubtful whether, at 1941 prices, the valuable metals tin, mercury, gold> and silver could be extracted 

at a profit from the primary gravels in the Guadalcfizar basin, 

not only because of the low grade of this placer material but 

also because too great a percentage of the metals are in the 

fines. However, the primary gravels of the Realejo basin seem 

to be of a somewhat better grade.

The secondary gravels, which are confined mainly to the 

Guadalcazar basin, appear to contain -enough of the metals for 

profitable mining, and their volume is moderately large. 

Although these deposits have not been sampled in*detail, it is 

believed that they are well worth testing.

It is probable that enough water for placer mining could 

easily be obtained by pumping, since wells in the Guadalcazar 

basin reach the water table 15 to 30 feet (4 to 10 m.) below 

the surface.

Deposits associated with lead-zinc-silver ores

Deposits associated with lead-zinc-silver ores are known to 

occur in only one district in Mexico, and only in a part of a 

single mine the San Antonio mine at Santa Eulalia, 12 miles 

(20 km.) east of the city of Chihuahua. Tin was first discovered in milling and smelting some of the ore in 1926. A 

process was devised for recovering the tin, and by 1939 about 

3,000 tons of tin had been produced, which is about half of 

Mexico's total production. No tin has been recovered from the 

mine since 1939, the tin ore having then apparently been
118 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS, 1941-42

exhausted, although the other metals are still being produced.

The authors did not examine the deposit 

12/ The dominant rocks in the region, according to Prescott, '

are Cretaceous limestones, which are overlain by beds of Tertiary clastic and volcanic rocks. Sills of diabase and a larger 

intrusive mass of diorite have been discovered in the mine workings in recent years. All the formations are cut by dikes of 

light-colored fine-grained aplitic rock, called "rhyolite", and 

of a dark-colored dioritic rock, called "andesite". One of the 

largest of the "rhyolite" dikes is exposed in the San Antonio 

mine. The lead-zinc-silver ores are described by Prescott as 

replacement deposits in the limestone. They consist largely of 

pyrite, pyrrhotite, galena, sphalerite, and iron-lime-silicate 

minerals. No description of the exact mode of occurrence of the 

tin has been published so far as known.

In 1927 Poshag collected from the ore pile specimens of rock 

similar in appearance to the "rhyolite" dikes; these specimens 

consisted of porous quartzose rock containing small crystals of 

brown casslterite and black specularite. The cassiterite may, 

however, be more closely related to the lead-zinc ores than to

the dikes. In the Sullivan mine, at Kimberly, British Columbia,

IV according to Schwartz,-=^ cfassiterite occurs in a lead-zincsilver deposit very similar to the Santa Eulalia deposit and is 

closely associated with the sulfide ores.

Deposits associated with extrusive rocks

Most of the tin deposits of Mexico are in extrusive rocks. 

About half the total production of tin in Mexico has come from 

these deposits and from placers associated with them. As the 

general features described in the introductory section of this 

report refer mainly to deposits in extrusive rocks, it will not

\2j Prescott, Basil, The main mineral zone of the Santa Eulalia district, Chihuahua: Am."lost. Win. Eng. Trans., vol. 51, pp. 57-99, 1916.

\jf/ Sch^artz, G. M., Microscopic features of Sullivan ore: Eng. and 

Min. Jour., vol. 122, pp. 375-377, 1926.
TIN DEPOSITS OF THE REPUBLIC OP MEXICO 119

be necessary to describe them here. These deposits appear to 

form a unique type. Two districts in the United States in which 

deposits of this type are known have recently been described by

Fries. One of these is in southwestern New Mexico, the other 

in northern Lander County, Nevada. ' Apparently no other such 

deposits have been described, though some specimens received by 

LIndgren from Bolivia suggested to him that similar deposits may 

occur in acidic lavas there. is/

Geology

The volcanic rocks that contain the tin deposits of the 18 

districts that have been investigated in the States of Michoacan, Guana juato, San Luis Potosi, Aguascalientes, Zacatecas, 

and Durango are predominantly rhyolitic. In the Jaula district, 

In Guana juato, the veins are in latite, and at El Santin mine 

in the same State they are in andesite, but in both localities 

these rocks are overlain by rhyolite. Throughout the plateau, 

particularly along its west side and in its northern part, the 

rhyolitic lava flows are interbedded with volcanic breccia and

tuff. The pyroclastics are not abundant, however, In the tin ' " i

districts. Most of the tin-bearing veins are in the lava flows; 

a few are in breccia; none of those examined are in rocks

clearly identified as tuff. The age of the rhyolitic rocks ',

17/ according to Ordonez, 'probably ranges from Miocene to Pliocene, thus being about the same as that of the tin-bearing rhyolite in New Mexico and Nevada.

14 / Fries, Carl, Jr., Tin deposits of the Black Range, Catron and 

Sierra Counties, N. M., a preliminary report: U. S. Cool. Survey Bull. 

922-M, pp. 355-370, 1940.

15/ Fries, Carl, Jr., Tin deposits of northern Londer County, Nov.: 

U. S. Geol. Survey Bull. 931-L, pp. 279-294, 1942.

16_/ Lindgren, Waldemax, Mineral deposits, 4th ad., p. 659, New York," 

McGraw-Hill Book Co., 1933.

IT/ OrdoTiez, Ezequiel, Las rhyolitas de Mexico: M6xino Inst. geol. 

Bol., No. U, pp. 66-67, 1900.
120 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS, 1941-42

The rhyolite flows are in general distinctly layered. The 

layers are nearly horizontal in most of the places where the 

lavas are interbedded with pyroclastic rocks, but in the vicinity of the tin-bearing veins this attitude is the exception 

rather than the rule. There the layers generally have irregular 

strikes and steep dips, which change markedly within short distances. No attempts were made to interpret in detail the meaning of this confused structure, but in most places it was 

clearly a result of original flow rather than of later tectonic 

movements. This peculiar structure and the relative scarcity of 

pyroclastic rocks suggest that the tin-bearing areas are near 

the centers of eruption of the lavas.

The rhyolite is reddish brown to light gray or white; the 

latite and andesite are dark reddish brown. Lithophysae, vugs, 

and cavities of all shapes are abundant in some parts of all the 

flows. The rocks are generally lighter in color around the 

cavities than in the rest of the groundmass, apparently owing 

to coarser crystallization; some of the rocks consequently have 

a spotted or mottled appearance. In all the tin districts the 

tin-bearing rocks are porphyritic, and their groundmasses are 

completely devitrified, finely crystalline, and more or les.s 

spherulitic. The phenocryst-s, which make up from about 5 to 

nearly 30 percent of the rocks examined, are largely sanidine 

and quartz, but most of the flows also contain a few percent 

of plagioclase phenocrysts, ranging in composition from .calcic 

albite to sodic andesine in different flows. In general the 

flows contain less than 1 percent biotite or hornblende, or 

both, now mostly altered to hematite, quartz, and feldspar. 

Most of the alteration apparently occurred just before and 

during the crystallization of the groundmass. The phenocrysts

in the andesite are chiefly calcic andesine or sodic labradorite, and less than 2 percent of them are biotite and pyrox-
TIN DEPOSITS OP THE REPUBLIC OP MEXICO 121

ene. A brief description of the rocks in each of 15 of the 

districts visited is given in table 5.

The fine-grained groundmass of most of the rhyolite flows is 

composed largely of sanidine and quartz, but in some of the 

flows the silica forms tridymite and cristobalite, which may or 

may not be accompanied by quartz. A fine dust of hematite and 

tiny crystals of specularite form a small part of the groundmass. Magnetite is scarce in most of the rocks. In most of the 

rhyolites the only nonopaque microscopic accessory mineral is 

zircon, but the andesite and latite contain both zircon and 

apatite. The most abundant mineral in the lithophysae and cavities of the rhyolitlc lavas is quartz, but lithophysal tridymite is abundant in some of the flows and opal and chalcedony 

in others. Small quantities of lithophysal sanidine, specularite, topazj pseudobrookite, cristobalite, zeolites, and fluorite 

also occur in parts of some flows.

Overlying the tin-bearing formations are lava flows and 

pyroclastic rocks ranging in composition from rhyolite to basalt. The moat recent of these are largely basalt. Still more 

recent deposits consist of fan gravels, which lap against and 

surround the ridges and ranges of volcanic rocks that rise above 

the general level of the. plateau. Many of these deposits of 

detritus have been deeply dissected by arroyos, in which secondary deposits of gravel have been formed. Some of the gravel 

deposits contain placer tin near the veins.

The lavas and pyroclastic beds of the Mexican plateau 

country are for the most part nearly horizontal. None of these 

late volcanic rocks are known to be folded, although they appear 

to be gently tilted in places. Many of the lava-capped ridges 

and ranges undoubtedly owe their elevation to normal faulting, 

but no Important faults were observed close to any of the tin 

deposits.
122 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS, 1941-42

Vein deposits

Occurrence. The tin-bearing veins are largely incrustations 

on the walls of narrow open fissures, and they appear to occur 

mainly in the more open parts of the fissures. These fissures 

are in most places extremely irregular in strike and dip and 

pass within short distances into tight joints. In some places 

they branch at various angles and connect with other parallel 

or diverging fissures. Most of the veins are less than 50 feet 

(15 m.) in length, a few of those examined are as much as 200 

or 300 feet (60 or 90 m.) long, and one vein appears to be continuous for about 1,000 feet (300 m.) on the surface. The majority of the deposits have been mined to depths of from 20 to 

50 feet (6 to 15 m.), where the vein material seems to pinch 

out. The deepest exploration known extends about 300 feet (90 

m.) from the surface. The thickness of the vein material varies 

along the fissures and is generally less than half an inch (1.3 

cm.), but some bodies as thick as 10 inches (25 cm.) have been 

mined. These thicker bodies, which have the form of small 

lenses or pockets, are locally enlarged parts of the veins, but, 

as the fissures leading to sone of them are very obscure, they 

have been mistakenly regarded as local segregations from the 

lavas. In places the incrustations follow branching and reticulating fissures and form lodes from 1 to 5 feet (0.3 to 1.5 m.) 

wide. The length of these lodes appears to be generally less 

than 20 feet (6 m.) and the depth about the same.

The vein minerals were deposited for the most part in open 

fissures, but they occur locally in the wall rocks of all the 

deposits, filling cavities and tiny openings or replacing the 

wall rock to a slight extent. No large bodies of rock have 

been found through which the vein minerals are uniformly disseminated, and all dissemination of these minerals appears to be 

limited to the immediate vicinity of the small tin-bearing fis-

<?

sures, ordinarily extending only an inch or two (2.5 to 5 cm.)
Table 5. HIneral composition of the rocks and tin veins in some of the tin districts of Mexico I/

Locality (Numbers 

refer to numbers in 

text and on pi. 25. )

1. Los Cablrea mine,

Tepuxtepec, Michoacfin.

2. Tin deposit 15

miles (24 km. )

west of Dolores

Hidalgo, on road

to Guana Juato,

Gto.

3. Tin deposits 2

miles (3 km. )

east of Tlachiquera, between

San Pellpe and

Ledn, Gto.

4. Tin deposits in

Cerro del Toro,

northwest of

Tlachlquera, between San Pellpe

and Le6n, Gto.

5. Tin deposits 10

miles (16 km. )

north of San

Pelipe, Gto.

6. 'Queensland mine 

near Santa Barbara, northwest 

of San Pellpe, 

Gto.

7. El San tfn mine 

near Santa CatarJiia, southeast 

of San Luia de

la Paz, Gto.

8. Tin deposlta on 

Hacienda Sauceda, about 25 

miles (40 km.)

north of San

Luis de la Faz,

Gto.

9. Tin deposits on 

road from San Luis Potosi to San

Luis de la Paz,

between Santa Domingo, 3. L. P.,

and Jaula, Gto.

10. Tin deposits 10 

miles (16 km. ) 

southwest of Calvillo, Aguascallentes.

11. Tin deposits at 

La Quemada Indian 

ruins, about 40

miles (64 km. )

southwest of the

city of Zacate-

. cas.

12. Tin deposlta 25 

miles (40 km. ) 

northwest of

Presnlllo, Zacatecaa.

13. Cerro de loa 

Remedloa at the 

city of Durango .

14. Cacaria district northwest 

of city of Durango; rocks 

from El Dlablo 

and Las Auras 

mines.

15. Potrillos 

district in 

Sierra de San 

Francisco, Durango .

Rock name

Porphyritic

rhyollte.

....do.....

1

....do.....

....do.....

....do.....

....do.....

Porphyrltlc 

andesite.

Porphyrltlc 

rhyollte .

Porphyritic 

latlte.

Porphyritic 

rhyollte.

....do.....

....do.....

....do.....

....do.....

do

Rock description

Reddiah brown to light gray; in

places spotted, vuggy, llthophysal; largely well layered;

coarsely porphyritic; spherulltic; lava flows.

Reddiah brown to light gray; In

places vuggy and llthophysal;

In part well flow-layered;

porphyritic; highly spherulitic in part; lava flows.

Reddish brown to light gray; In

places spotted, vuggy, llthophysal; moderately well layered; coarsely porphyrltlo;

lava flows.

Light reddiah brown to light

gray; in places vuggy and

llthophysal; porphyrltlc,

with some spherulites In

the crystalline groundmasa;

flow-layered; lava flows.

Reddish brown to light gray; in

part vuggy and llthophysal;

moderately coarse porphyry;

groundmaaa crystalline, with

spherulites; in part well

flow-layered; lava flows.

Reddish brown to light gray; In 

part vuggy and lithophysal;' 

moderately coarse porphyry, 

with some spherulites in finely crystalline groundmaaa; in

part well layered; lava f Iowa .

Dark reddiah brown to dark gray; 

In part vuggy; porphyrltlc, 

with aome spherulites In finely crystalline groundmass; In

places spotted; in part well

layered; lava flows.

Reddish brown to light gray; In 

part vuggy and llthophysal; 

moderately coarse phenocryats; aome spherulltea In

finely crystalline groundmass; well flow-layered, lava

flows .

Light brown; in part vuggy; 

finely porphyritic; well layered; probably lava flow.

Reddish brown to light gray; In 

part vuggy and lithophysal; 

porphyrltlc, with aome spherulltes in groundmass; in part 

well flow- layered; lava

flows .

Reddish brown to light gray; In 

places vuggy and lithophysal; 

fine-grained porphyry, with

some spherulites in crystalline groundmass; well flowlayered; lava flows.

Reddish brown to light gray; in 

part vuggy and lithophysal; 

porphyritic, with aome spherulites in groundmass; In part

well flow- layered; lava flowa.

Reddish brown to light gray; in 

places vuggy and llthophyaal; 

fine-grained porphyry, with

spherulltea; in part well

flow-layered and in part flow

breccia; mainly lava flows.

Reddish brown to light gray; in 

places vuggy and lithophysal; 

moderately coarse porphyry; 

groundmaaa apherulitic In 

placea; generally well layered; lava flows.

Reddish brown to light gray; In 

places highly vuggy and lithophysal, in other places' dense 

and firm; generally a very 

fine-grained porphyry; in

places well layered; appears

to be mainly lava flows.

Phenocrysta

Percent

15

10

oc GO

10

20

20

15

15

10

15

10

10

10

15

5

Minerals 

( In order of

Sanldine

Quartz

Ollgoclase

Biotlte

(Hydroblotlte)

Quartz

Sanidlne

Hornblende?

Sanldine

Quartz

Ollgoclase

Biotlte?

Quartz

Sanidlne

Alb it e

Biotite

Sanidine

Quartz

Biotite? or

hornblende?

Sanidlne 

Quartz 

Biotlte? or 

hornblende?

Calcic andesine 

Biotite 

Pyroxene

Quartz 

Sanidlne 

Biotite

Andeaine 

Sanidlne

Biotlte

Sanldine 

Quartz 

Albite

Hydroblotlte

Sanldine 

Quartz 

Biotite?

Sanldine 

Quartz 

Biotlte

Sanldine 

Quartz 

Oligoclase

Biotite

Sanidine 

Quartz 

Hydrobiotite

Sanidlne 

Quartz 

Biotlte 

Hornblende

Minerals in 

groundnaas =*

Sanidlne

Quartz

Tridymite

Crlstobalite

Hematite

Sanidlne

Quartz

Hematite

Sanidine

Quartz

Hematite

Tridymlte?

Quartz

Sanidine

Hematite

Tridymlte?

Sanidlne

Quartz

Tridymite

Hematite

Sanldine 

Quartz 

Tridymlte 

Hematite

Plagioclase 

Sanidlne 

Hematite

Sanldine 

Tridymite 

Quartz 

Hematite

Sanidlne 

Plagioclase

Hematite

Sanldine 

Quartz 

Hematite

Sanidlne 

Quartz

Sanidlne 

Quartz 

Hematite

Sanldine 

Quartz 

Hematite

Sanldine 

Quartz

Hematite

Sanidlne 

Quartz 

Hematite

Microscopic 

accessory 

minerals

Zircon

....do.....

....do.....

....do.....

....do.....

....do.....

Apatite 

Zircon

Zircon

Apatite 

Zircon

Zircon

....do.....

....do.....

....do.....

....do.....

....do.....

Minerala In 

vugs and ]/ 

lithophysae -'

Specularite

Tridymite

Topaz

Quartz

Chalcedony

Opal

Specularite

Tridymite

Quartz

Chalcedony

Opal

Specularite

Topaz

Tridymlte

Quartz

Chalcedony

Opal

Specularite

Tridymlte

Quartz

Chalcedony

Opal

Specularite

Pseudobrooklte

Topaz

Tridymite

Quartz

Sanldine

Specularite 

Pseudobrooklte 

Sanidlne 

Tridymlte 

Quartz

Specularite 

Tridymite 

Quartz 

Others not

determined.

Specularite 

Topaz 

Tridymite 

Quartz

Chalcedony

Opal

Specularite 

Quartz

Chalcedony

Opal

Not determined.

Specularite 

Tridymlte 

Quartz

Specularite 

Topaz. 

Quartz

Specularite 

Pseudobrooklte 

Topaz

Sanldine

Tridymite

Quartz

Chalcedony

Opal

Pluorite

Not determined.

Specularite 

Pseudobrooklte 

Topaz 

Sanidlne 

Tridymlte

Quartz

Chalcedony

Opal

Zeolites

Plucrrlte

Minerals in 

the veins I/

Magnetite

Specularite

Cassiterite

Cristobalite

Tridymlte

Quartz

Opal

Montmorillonite

Magnetite

Specularite

Caaslterlte

Criatobalite

Tridymite

Quartz

Mimetite

Opal

Zeolites

Montmorillonite

Magnetite

Specularite

Caasiterite

Crlstoballte

Tridymite

Chalcedony

Quartz

Opal

Clays

Magnetite

Specularite

Cassiterite

Clays

Magnetite

Specularite

Cassiterite

Topaz

Cristobalite

Tridymite

Quartz

Huebnerlte?

Chalcedony

Opal

Clays

Magnetite 

Specularite 

Cassiterite 

Tridymlte 

Quartz

Chalcedony

Opal

Clays

Magnetite 

Specularite 

Cassiterite 

Tridymlte

Quartz

Zeolites

Opal

Montmorillonite

Magnetite 

Specularite 

Cassiterite 

Tridymite

Quartz

Opal

Chalcedony

Magnetite 

Specularite

Casalterite

Chalcedony

Quartz

Opal

Montmorillonlte

Magnetite 

Specularite 

Cassiterite

Chalcedony 

Quartz

Opal

Magnetite 

Specularite 

Cassiterite

Tridymlte

Quartz

Chalcedony

Opal

Magnetite 

Specularite 

Casalterite

Quartz

Chalcedony

Opal

Magnetite 

Specularite 

Cassiterite

Sanidlne

Cristobalite

Tridymlte

Quartz

Chalcedony

Opal

Pluorite

Montmorillonlte

Magnetite 

Specularite 

Caaaiterite 

Cristoballte 

Tridymite 

Chalcedony 

Opal

Mimetite

Montmorillonite

Magnetite 

Specularite 

Cassiterite 

Topaz 

Sanidlne

Crlstobalite

Duranglte

Tridymite

Chalcedony

Opal

Zeolites

Pluorite

Clays

Alteration 2/

In placea highly porous;

contains zeolites and minor

quantities of clay minerala;

phenocrysta unaltered.

In places highly porous;

contains some zeolites and

minor quantities of clay minerals; phenocrysts unaltered.

In places somewhat porous;

rock generally fresh.

In places plagioclase altered

to a mixture of quartz and clays;

sanldine fresh; rock somewhat

porous near aome veins.

Wall rock porous in places;

groundmasa containa aome clays;

along one vein there was brecciatlon and recementatlon with

opal and chalcedony; feldapars freah.

Rock generally denae and firm 

near veins, but in a few places 

porous; some clays in groundmass; phenocryats not altered.

In places plagioclase phenocrysts 

altered to claya; in aome zones 

feldspars in grbundmass strongly 

altered; alteration not along all

veins; zeolites present in places.

Rock somewhat porous near some 

veins; apparently some clays In 

groundmass; phenocrysts unaltered.

Some phenocrysts largely altered 

to kaolinlte; some blotlte altered in part to chlorlte; montmorlllonite and other clays,

possibly some zeolites, in groundmass.

Rock along some veins highly 

porous; along other veins there 

has been slight brecclatlon and

recementatlon with chalcedony and 

opal; apparently some clays in

groundmass of porous rock.

None observed; phenocrysts 

entirely fresh.

Rock along some veins porous; 

possibly aome clays In groundmass; 

phenocrysts unaltered.

Rock porous In places; along one 

vein some phenocrysts partly altered 

to clay; clays In groundmass in some

placea; some tridymlte inverted to

quartz; some sanldine in the veins

altered to clays; fluorlte In groundmass; in most placea all phenocryats

unaltered.

Rock generally denae and firm, 

but porous In a few places along 

veins; in placea groundmaaa contains 

claya; phenocryata In wall rock along 

veins generally unaltered.

Rock generally dense and firm, but 

highly porous In some places ; where 

porous, rock containa zeolites, fluorite, 

and clay minerala; phenocryata generally 

unaltered.

I/ The minerala listed are only' fhbse actually found taring a brief examination of the deposits, and it should not be assumed that all the lists are complete.

"Z/ Owing to the extremely small size and the intergrowth of the minerals in the groundmasa, it is not possible to identify all the minerals by ordinary methods. There may be a much wider 

distribution of oristobalite and tridymlte than is indicated. A small quantity of magnetite occurs in moat of the rocka.

3/ There la alao a amall quantity of llmonlte in many placea, most of it apparently aupergene. U. 3. GOVERNMENT PRINTING OFFICE: 1942 O - 4801 IS (faces p. 122)
TIN DEPOSITS OP THE REPUBLIC OF MEXICO 123

on either side of the fissure. In a few places where two fissures containing vein material were separated by only a few 

Inches of wall rock, all this rock was found to be impregnated 

with small quantities of the vein minerals.

There has been minor faulting along at least one of the tinbearing fissures in each district visited, but none of the veins 

examined appear to be on major faults. Possibly, however, all 

the tin-bearing fissures may be related to extremely small 

faults. In a few places the rocks are sheeted, but in general 

adjacent blocks are not relatively displaced by more than a 

small fraction of an inch (less than a centimeter). In one 

place a slickensided wall incrusted with vein minerals was 

found, indicating that movement preceded mineralization; in 

other places crushed vein material gives evidence of slight 

movement after mineralization.

Mineralogy and paragenesis. The veins consist of incrustations that are composed for the most part of specularite and 

cass_it_erj.t.e-.. Specularite is generally more abundant than cassiterite. These are the only minerals In parts of many veins; 

other parts of the same veins contain cristobalite, tridymite, 

quartz, chalcedony, and opal, usually in small quantity, though 

the last two are abundant locally. Other vein minerals include' 

magnetite, sanidine, topaz, fluorite, zeolites, montmorillonite 

and other clay minerals, calcite, and llmonite. Mimetite <has 

been found in three localities, and the rare mineral duranglte 

has been found in one. Wolframite has been reported to occur in 

some of the veins in Durango, but the report has not been verified. A few tiny crystals collected by the authors from one 

locality in Guanajuato have the optical properties of huebnerite, but not enough material was found for chemical tests. The 

identity of two other minerals collected by the authors is 

unknown, owing to the. extremely small quantities found, but one
124 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS, 1941-42

appears to be closely related to mlmetite, the lead chloro-arsenate. Pyrite and other sulfide minerals are completely lacking.

The cassiterite occurs in small, well-formed crystals and in 

botryoidal, banded masses known as wood tin and called rinones 

(kidneys) or guijilos (little pebbles) by the local miners. 

Both varieties are harder than a knife blade and have a specific 

gravity close to 6.5. The color of the crystals is generally 

bright red, but some are" orange, greenish yellow, smoky, or 

brown. The banded wood tin varies in color from place to place 

in the same vein and also from layer to layer in a single piece; 

the colors range from nearly white through all shades of yellow, 

greenish yellow, and reddish brown to nearly black. Both vari- v 

eties of cassiterite occur in all. the veins, but the wood tin 

is much the more common. Both varieties incrust and are intimately intergrown with the specularite and have in places 

replaced it pseudomorphously. This association of the two minerals is so close that they cannot be separated completely by 

crushing as fine as 200 mesh probably not even at 500 mesh.

Published chemical analyses, most of which are presented in 

table 6, show that small quantities of copper, lead, zinc, antimony, bismuth, or arsenic occur in the deposits; very little 

sulfur is reported in four analyses. Spectrograms of concentrates of samples collected by the authors from five tin districts,, were made by George Steiger in the laboratories of the 

Geological Survey. The elements thus looked for were: Lead, 

zinc, antimony, bismuth, silver, beryllium, arsenic, boron, 

tungsten, and germanium. All the samples contained one or more 

of all these elements except tungsten and germanium, neither 

of which was found. Qualitative chemical tests showed small 

quantities, also, of manganese and titanium in all the samples. 

The only separate mineral recognized in which one of the minor 

metallic elements found in the deposits forms an essential part 

was mimetite. It is possible that the metals occur in submicro-
TIN DEPOSITS OF THE REPUBLIC OF MEXICO 125

scopic mineral grains in the vein material, or some of them 

may replace the iron in specularite. None of them form sulfldes, for the quantity of sulfur in the veins is negligible. 

Some may form rare arsenic minerals, enclosed in the wood tin, 

as suggested by Genth =2/in his description of cassiterite from 

Durango.

The specularite, magnetite, cassiterite, topaz, sanidine, 

cristobalite, tridymite, and some of the quartz in the incrusta- 

"tlons appear to have been deposited at nearly .the same time. 

The specularite and magnetite were in general deposited first, 

and the cassiterite next, but at one place both specularite and 

cassiterite deposited on cristobalite were found. Some of the 

specularite seems to have been deposited after some of the cassiterite. Some crystals of cassiterite are coated with a film 

of wood tin, but some of the fibrous wood tin is encrusted with 

small crystals of cassiterite. The'Other minerals enumerated 

above are intergrown with or deposited on the specularite and 

cassiterite. The mimetite, durangite, opal, chalcedony, zeolites, fluorite, clay minerals, calcite, limonite, and $ome of 

the quartz are all clearly later than the other vein minerals. 

In a few pla-ces the vein material has been brecciated and recemented with opal or chalcedony, some of which, together with 

some of the clay minerals and of the calcite and limonite, may 

be supergene (deposited by descending surface water). None of 

the cassiterite is supergene, so far as could be determined.

Alteration.--The country rock close to many of the tin veins 

is more or less altered, the most strongly altered rock observed 

being the andesite adjacent to some of the veins at the El Santin mine, in Guanajuato; but in a few places it is no different 

from any of the surrounding lava. Although much of the rhyolite 

is highly vesicular, indicating that it contained large quantities of gases when extruded, its porosity is in general espe13/ Genth, P. A.. Contributions to mineralogy: Am. Philos. Soc. Proc., 

vol. 2U, pp. 30-31, 18S7.
126 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS, 1941-42

r

r\j

r

t<

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w

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 d

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H

........ O'CDCn»«CM«'<<t<««»tO«CO««»

........ m «HCO «CM «CM m CM 

^_ ^^^ -^^ ^_

........ in«Qjrl««rj«cO« O uj

. ....... cQ«a)C^««co»<^*« m 10 

/ > ^ s Irt rt /1^ 7^

******* rn*r^r^** [ * Hr . (y L 

. ....... 05«dC««CO«CM«. . d 'CM 

***** * *"«; Q> CP CV * J^7 *^*ir^"*

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a. Vein material containing cassiterite, specularite, miraeti

Mina del Diablo, Cacaria district, Durango.

b. Bright-red fibrous crystalline*cassiterite from Durango. 

c. Red wood tin from Coneto district, Durango.

d. Dark reddish-brown fibrous cassiterite from Coneto distric

e. Average of two analyses of bright-red finely crystalline 

district, Durango.

f. Dark reddish-brown wood tin from Guanajuato, Guanajuato.

g. Average of three samples of yellowis.h fibrous wood tin fro

h. Fibrous, concentrically laminated, yellowish wood tin from 

3. Concentrates from the Queensland mine, Santa Barbara, Guanajua

Nuevo Le6n. Bromly, A. H., Tin mining and smelting at Santa Barbara, 

Eng. Trans., vol. 36, p. 233, 1905. (Recalculated as oxides.) 

4. Wittich, Ernesto, El estano en la Sierra de' Guanajuato: Soc. 

5. Stream tin. Analysts, John H. Banks & Son. New York. Sampson

placer deposits near Dolores Hidalgo, Guanajuato, for natural Product

6. Stream tin. Analysts, Ledoux & Co., Inc., New York. Sampson, 

7. Partial analysis of shipment of 15 tons of concentrates from E

Williams, Harvey & Co., Liverpool, England. (Recalculated as oxides
128 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS, 1941-42

dally great near the tin veins, where the groundmass appears to 

be weakly leached. In these places the rock is so friable that 

it can be easily crushed. In some places the groundmass of the 

altered rock contains small qxiantities of clay minerals, which 

appear to be mainly of the montmorillonite group but in smaller 

part kaolin; in other places the groundmass contains zeolites or 

fluorite or both. Generally the feldspar phenocrysts are unaltered, but in a few places, particularly where there has been 

faulting, they are almost completely altered to clay minerals. 

Silica was deposited almost entirely'within the fissures, but

it extends an inch or two (2.5 to 5 cm.) into the walls along

/ 

some parts of the veins. Some of the quartz phenocrysts and a

few sanidine phenocrysts are secondarily enlarged.

Although alteration of the vein minerals is not common, many 

crystals of specularite are weakly corroded. Part of this corrosion may have occurred at the time the cassiterite was deposited; but in some places the corrosion was clearly later, for 

cavities have been left where the crystals of specularite have 

been dissolved out of the mixtures of the two minerals. In one 

place the sanidine in the incrustation is partly altered to 

clay, whereas that in the wall rock is unaltered. In a few" 

places the tridymite has inverted to quartz but has retained its 

characteristic crystal form. The cassiterite apparently was 

highly resistant to attack by later solutions.

It is clear that alteration and leaching of the wall rock 

occurred after the deposition of the specularite and cassiterite. In some places the incrustations cover small cavities 

that would have been filled up if they had been present before 

the deposition of the specularite and cassiterite. Had the wall 

rock been porous prior to the deposition of the cassiterite, 

this mineral should be widely disseminated throughout the rock, 

but very little disseminated cassiterite has been found. The
TIN DEPOSITS OP THE REPUBLIC OP MEXICO 129

minerals deposited in the altered rock are the same minerals 

that were last deposited In the veins.

Origin. As no one deposit has been thoroughly explored, 

the origin of this type of tin deposit must be Inferred from 

scattered bits of evidence gathered not only in Mexico but in 

southwestern New Mexico and in northern Nevada. It is believed 

that the source of the tin was largely rhyolitic magma that was 

Intruded near the surface of.the earth rather than deep down; 

some deposits were possibly derived in minor part from the lava 

flows themselves. It is-believed, further, that the veins were 

deposited in fissures formed by minor structural adjustments 

after extrusion of the lavas and during the waning stages of 

volcanism. The mineralization is thought to have occurred during one continuous cycle, in whose initial stages residual 

gases, depositing cassiterite from the crystallizing rhyolitic 

magma, escaped at temperatures probably above 600° C., and in 

whose final stages hot waters filled the fissures, forming 

zeolites and clay minerals. As sulfide minerals are absent 

from the deposits, it would seem that, when temperatures had 

dropped low enough to permit the presence of aqueous solutions 

at the existing pressures, the magmatic fluids must have contained little but water.

The veins apparently were formed soon after the extrusion 

of the Thyolite, for In a few places they are overlain by»unmineralized basalt and in other places by other unmineralized lavas 

probably extruded after mineralization. The cassiterite was 

clearly deposited at relatively shallow depths. The lavas have 

not been either folded or metamorphosed, and field relations 

indicate that no great thicknesses of rock have been eroded 

from the deposits. The fissures in which the incrustations 

were deposited are not in general filled with the vein minerals.- 

They are extremely irregular and discontinuous, many of them 

branching or feathering out in a manner characteristic of fis-
130 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS, 1941-42

sure veins formed at shallow depth. Although some of the veins 

are at least 1,000 feet (300 m.) below the present upland surface and may have been formed at a depth of 2,000 to 3,000 feet 

(600 to 1,000 m.), others may have been formed within a few 

hundred feet (less than 100 m.) of the surface. Evidence from 

outcrops and from explored veins indicates that some groups of 

veins have a vertical range of at least 400 feet (120 m.), but 

exploration has been insufficient to prove how much greater the 

range may be.

The fact that the veins occur only in certain parts of the 

lavas, there being vast quantities of rhyolitic lava that .do not 

contain tin veins, indicates that certain localizing factors 

were operative. One such factor appears to have been proximity 

to the vents from which the lavas were extruded; some of the 

deposits clearly occur near centers of extrusion, whereas few 

deposits occur in the thin flows, with interbedded pyroclastic 

rocks, which apparently are distant from these centers. Faulting is known to have occurred along some fissures before the 

incrustations were formed, and faults occur in some part of each 

deposit examined. Although none of these appear to be of large 

displacement, it is believed that they provided channelways 

extending from some depth below the present outcrops of the 

veins, and that these faults resulted from structural adjustment 

consequent upon the extrusion of the lavas rather than from 

later earth movements.

The source of the tin is believed to have been the rhyolitic magma. .All the tin deposits are closely associated with 

rhyolitic extrusive rocks, and most of them are actually in such 

rocks. Spectrograms of the freshest rhyolite from two districts 

show about 0.0005 percent tin, indicating that tin is a minor 

element of the rhyolite in those districts. All the minerals 

of the incrustations, except the arsenic minerals and possibly 

cassiterite, occur also in the lithophysae of the rhyolite
TIN DEPOSITS OF THE REPUBLIC OF MEXICO 131

flows, as normal late products of crystallization deposited 

after the lavas solidified. Some of these minerals are abundant throughout all the rhyolite flows, while others are of 

rare occurrence. The relative scarcity of tin in the rhyolite 

may account for the apparent absence of cassiterite in the 

lithophysae.

The tin-bearing fluids that formed the minable deposits 

must have been concentrated in small areas or directed along 

a few conouits. The relatively small extent of the mineralization indicates that the fluids were of small volume and therefore that the bodies of magma from which they came were either 

small or poor in fluids. The localization of the deposits indicates that the mineralizing fluids did not come entirely, though 

they may have come in part, from the flows themselves; it seems 

probable, rather, that they came chiefly from crystallizing rhyolitic magma that never reached the surface. Some of the deposits may have been formed,.while the lavas were cooling, by 

intermingled fluids from both sources. The solutions that 

formed the tin veins in the latite and andesite of eastern 

Guanajuato are believed to have had their source in the reservoir of rhyolitic magma from which the rhyolitic flows overlying 

these more basic lavas came.

The early formed vein minerals are believed to have been 

deposited from gases at moderately high temperatures, possibly 

at first above 600° Co and later at somewhat lower temperatures. 

The minerals in the incrustations that are generally believed to 

form at high temperature are magnetite, specularite, cassiterite, sanidine (the high-temperature form of potash feldspar), 

cristobalite and tridymite (generally formed at fairly high 

temperature), topaz, garnet (in New Mexico and Nevada), and 

bixbyite (only in New Mexico). The fact that the early metallic 

vein minerals are oxides and mixtures of oxides, not accompanied 

by sulfides, indicates that the environment was an oxidizing one
132 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS, 1941-42

and that the temperatures were high enough to permit the sulfur,

which must certainly have been present in the ore-forming flu19/ 

ids, to be carried away in the gases. Zies points out that

in the fumarolic incrustations of the Valley of Ten Thousand 

Smokes in Alaska the sulfides and sulfates were formed abundantly at the vents where temperatures had dropped below the 

boiling point of water, and only rarely at the high-temperature 

gas vents, where the compounds formed most abundantly were 

oxides. Although these fumarolic incrustations were formed at 

the surface, whereas the tin-bearing incrustations under consideration were formed below the surface, the physical conditions 

under which the two sorts of deposits were formed were probably 

not very different, except that the gas pressures in the fissures 

during the formation of the tin deposits were greater.

Most of the alteration and leaching of the wall rocks along 

the tin veins occurred after the deposition of the cassiterite 

(see p. 128). Some of this alteration may have been effected in 

part by hot gases, but in the final stages it probably was the 

work of liquids, as indicated by the occurrence in the veins and 

altered rock of clay minerals, chalcedony, opal, and zeolites. 

This hydrothermal stage appears in most of the deposits to have 

been of short duration, for in general its effects are slight. 

The liquids must have been nearly neutral and have carried very 

little material in solution, since most of the specularite is 

unattacked, ealcite is rare in the deposits, and little silicification has taken place. In discussing the lavas of the San

20/ 

Juan region in Colorado, Larsen points out that where the

lavas are strongly altered by hydrothermal agencies tridymlte 

and cristobalite invert to the more stable form, quartz. Such 

inversion is rare in 'the tin deposits.

jj/ Ziee, E. 0., The Valley of Ten Thousand Smokes; I. The fumarolic 

incrustations and their bearing on ore deposition: Nat. Geog. Soc,, Contributed Technical Paners, Xatmai series, vol. 1., No. U, pp. 5-61, 1929-

?Q/ Larsen, E. S., Irving, John, Gonyer, F. A., and Larsen, E. S., jd, 

Petrologic results of a study of the minerals from the Tertiary volcanic 

rocks of the San Juan region, Colo. 2. The silica minerals (by E. S. Larsen): Am. Mineralogist, vol. 21, pp. 692-69U, 1936.
TIN DEPOSITS OP THE REPUBLIC OP MEXICO 133

Mining and concentration. Many of the small tin deposits 

have been mined with profit to the operators, mostly without the 

aid of machinery or even of explosives. The mining has consisted 

in the gouging out of rich vein material from between relatively 

barren walls. The openings in most of the mines are, in the 

main, only wide enough for one man to squeeze through, though 

they widen somewhat where pockets of ore have been found. Some 

of these mines' are reported to have yielded as much as 50 tons 

of metallic tin, but most have yielded probably less than 5 tons. 

Renewed work in some of the abandoned mines has revealed additional small pockets of ore. "Dead work" has not usually 

proved profitable. A typical small mine is illustrated in plate 

27, A, a photograph of a mine 10 miles (16 km.) north of San 

Pelipe, Guanajuato.

The ore obtained from the veins is crushed by hand and is 

washed and concentrated on gently sloping rock or earth platforms (planillas) or in shallow wooden pans (bateas). Some of 

the miners may lose more than 50 percent of the cassiterite during these operations, but in general the recovery is somewhat 

higher. The concentrates consist almost entirely of cassiterite 

and specularite and contain from 15 to 70 percent tin, averaging 

around 45 percent. Some of the kidneys of wood tin contain as 

much as 75 percent tin, but none are completely free from iron.

The amount of cassiterite disseminated in the wall rock of 

the veins in rhyolite is extremely small, and the cassiterite. 

is generally confined to a very narrow zone immediately adjacent 

to the vein. Since the veins are almost without exception less 

than 10 inches thick and are usually mere incrustations on the 

walls of narrow fissxires, punctuated erratically by small pockets or kidneys of ore, very little of the material that must be 

mined to extract the ore contains recoverable tin. Workings 

must therefore be kept to minimum dimensions and regular methods 

of development by machine methods obviously will failo
134 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS, 1941-42

There are few figures showing grade of ore and production 

for deposits that have been mined on a large scale, but those 

available suffice to show that such mining has nowhere been 

profitable. Bromly reports that around 1900 an attempt was made 

at Santa Barbara, Guanajuato, to mine with the aid of machinery 

a deposit that proved to average only 0.4 percent tin to a depth 

of 130 feet (40 m.). He remarks, "On the whole, it would be 

correct to describe the deposit as one that carries a small percentage of iron contaminated with tin." '

The Pittsburgh & Mexico Tin Mining Co., according to Rangel, 

spent more than $200,000 (United States currency) on the deposits in the America-Potrillos district in Durango around 1907 but 

recovered only a few thousand tons of ore. The operation

failed, it is said, because of poor management and lack of knowl22/ 

edge. The amount of tin recovered is not recorded, but the

grade of the ore was presumably low, as no further large-scale 

mining has been done.

The Durango Tin Mining Co. of St. Louis, as Ingalls was 

informed by Mr. H. Winninghoff, operated mines in the Cacaria 

district in Durango from 1881 to 1882; the most extensive work 

was in the Diablo mine, which was opened to a depth of 273 feet 

(83.2 m.). It is said that the vein material contained about 

3 percent tin; how much barren rock had to be mined to obtain 

the vein material is not known. The enterprise was abandoned as 

unprofitable. Ingalls makes the comment that "The delusion as 

to the extraordinary richness of the Durango tin fields is 

largely due to the assay of specimens of the remarkable nodules 

of cassiterite found there." ' No further large-scale mining 

has been attempted in the Cacaria district, although several 

small veins have been mined or prospected.

21/ Bromly, A. K., Tin mining and smelting at Santa Barbara, Guanejuato, 

Mexico; An. Inst. Min. Eng. Trans., vol. 3^i p. 229, 1905.

22/ Rengel, Manuel, Apuntes sot>re la distril>uci6n de minerales en el 

Bstado de Durango: Soc. geol. roexicana, Bol. 7i PP. 110-111, 1910.

2J/ Ingelle, W. R., The tin deposits of Durango, Mexico: An. Inst. Min. 

Eng. Trsns., vol. 25, p. 159, 1896.
TIN DEPOSITS OP THE REPUBLIC OP MEXICO 135

It is doubtful whether any of the deposits can be profitably 

mined and milled by bulk methods. Judging from the few published descriptions of deposits that have been mined and from 

studies of samples collected by the authors from the most promising deposits in several districts, it seems very doubtful 

whether any deposit contains as much as 120,000 tons of ore averaging more than 0.5 percent tin; indeed, it is doubtful whether 

many deposits averaging even 0.1 percent tin contain that much 

ore. The small size of the individual deposits thus precludes 

large-scale raining by bulk methods, for few of the deposits contain enough tin to pay for the equipment needed for efficient 

large-scale operation.

The recovery of tin from these low-grade ores is likely to 

be low, partly because the abundance of specularite and its 

intimate association with the cassiterite would prevent a clean 

separation, and partly because much cassiterite would be lost in 

the slimes when the ore was milled. Furthermore, the smelting 

of concentrates high in iron, such as these would be, results in 

loss of some of the tin or in increased costs. At the Queensland mine, near Santa Barbara, Guanajuato, where rock assaying 

0.4 percent tin was crushed and concentrated by means of tables, 

the concentrates, according to Bromly,^/contained only 29 percent of the tin. When a sample of these concentrates, containing about 15 percent tin, was panned down to half its weight and 

smelted with charcoal in a small furnace constructed at the 

mine, only 40 percent of the tin in the concentrates, or 12 percent of the tin in the ore, was recovered as metal. The recovery from ore of that grade would undoubtedly be much higher with 

more efficient, modern methods of milling, gravity concentration, and smelting, but the losses would probably still be high.

Tests conducted by the Bureau of Mines on rock from deposits 

of this type in southwestern New Mexico, averaging 0.03 percent

Bromly, A. H., op. cit., pp. 230-232.
136 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS, 1941-42

tin, gave concentrates ranging from 0.05 to 0.8 percent tin, 

with a recovery of from 10 to 47 percent. Although no attempt 

was made to concentrate this material further nor to smelt the 

concentrates, it was considered doubtful whether concentrates

could be made that would be suitable for smelting and yet give

25/ a satisfactory recovery of tin. '

Although ,every commercial method of recovering cassiterite 

from its ores is based largely on separation by gravity, flotation is also being applied to some ores. The standard procedure 

for treating the cassiterite-sulfide ores of Bolivia is to 

depress the cassiterite and to float off the sulfides in concentrates made by gravity methods. Processes also have been developed for floating off cassiterite in ores containing quartz, 

feldspar, sericite, and tourmaline. Gaudin 'outlines a process that gives concentrates containing 50 percent tin from an 

ore of this type assaying 0.8 percent tin; the recovery is 70 

percent of the tin in the ore. Experiments were being conducted 

at Mexico City in 1941 on the flotation of cassiterite in ores 

from the tin deposits in rhyolite. It is said that 90 percent 

of the tin can thus be recovered, but the process has not yet

been used commercially.

\. 

A process for recovering tin as a byproduct from the ieadzinc ores of the Sullivan mine in British Columbia, Canada, was

developed and put into operation in March 1941, by the Consol27/ idated Mining & Smelting Co. at Trail. ' The material treated,

which consists of the flotation tailings remaining after the 

recovery of zinc and lead, contains only 0.05 percent tin; but 

by concentrating to a ratio of more than 2,000 to 1, concentrates containing 67 percent tin are obtained. These, when 

smelted in an electric furnace, yield plus-99 percent "standard"

25/ Strategic minernle investigations; progress report on exploration of 

tin deposits: U. S. Bur. Mines Inf. Circ. ?15U, p. 5, March 19^1.

26/ Gaudin, A. M., Flotation, p. 379, New York, McGrat-Hill Book Co., 

1932.

27/ Banks, H. H., Tin at the Sullivan concentrator: Canadian Inst. Min. 

Met. Eng. Irene., vol. Wt, pp. 611-622,
TIN DEPOSITS OF JHE REPUBLIC OP MEXICO 137

tin.. The recovery, of tin is' about 45. percent of that shown by 

assay in the' "tailings ore." It should be emphasized that the 

lead and zinc pay for all costs of mining and milling and that 

the tin need pay only for the. additional cost of its recovery

from the tailings, whereas the tin deposits in rhyolite, now

» 

under consideration, contain no appreciable quantity of any

other metals of commercial value.

Possible production. At the present stage of tin metallurgy 

it is doubtful whether any of the tin deposits in rhyolite can 

be mined profitably on a large scale. The future value of the 

deposits depends on the development of efficient, low-cost methods of recovering tin from low-grade ores by means of processing 

plants that, owing to the small size of the individual deposits, 

would have to be small and portable. Until such methods are 

developed, the production of tin from these deposits will continue to come from many small mines and will not be appreciably 

greater .than during the past few decades probably no more than 

a few hundred tons a year. It has been suggested by persons who 

have bought cassiterite concentrates from the miners that the 

lending of tools and equipment, to be paid for in concentrates 

subsequently mined, would stimulate production somewhat.

Placers

Occurrence. Most of the tin-bearing veins exposed at the 

surface have given rise to small placers, many of which are 

eluvial; that is, they consist merely of the soil mantle that 

overlies and extends downhill from the outcrops. These soil 

deposits are thin, because of the steep slopes and arid climate, 

and none of them contains much tin, as the supply of cassiterite 

at the source is small. Most of the cassiterite is carried down 

the hillsides into small arroyos, where it accumulates in small 

alluvial placers. Where denudation is the dominant process, the 

thickness of the gravel in the arroyos is at most only a few
138 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS, 1941-42

feet (about one meter). This gravel is likely to be rich near 

the outcrops of the veins, but the rich gravel is generally of 

small extent; little of the gravel that lies more than a mile 

(1.6 km.) from an outcrop is worth mining. Most of the tin placers in Mexico are of this type. One of them is shown in plate

o

27, B, a photograph of a small arroyo that is being mined a few 

hundred feet (about a hundred meters) downhill from the outcrop 

of some small veins.

In a few districts where the tin veins are widely scattered 

rather than more or less isolated, cassiterite is carried down 

the small tributary valleys to the main arroyos. The Coneto 

district, in the Sierra de San Francisco, Durango, is the best 

example of such a district. Here the bottoms of the main 

arroyos are from 100 to 300 feet (30 to 90 m.) wide and contain 

gravel as much as 15 feet (4.6 m.) in thickness. The cassiterite is concentrated mainly in the lower part of the gravel, 

within 1 or 2 feet (about 0.5 m.) of bedrock. At a distance of 

about 2 miles (3.2 km.) beyond the nearest outcropping vein, 

dilution with gravel from barren areas has reduced the percentage of cassiterite below the point of profitable recovery.

Thick deposits of gravel have accumulated at the bases of 

some of the rhyolite ridges containing tin deposits, notably in 

the valley of the Laja River, which extends along the east side 

of the Sierra de Guanajuato, near Dolores Hidalgo, Guanajuato. 

These deposits consist of coalesced alluvial fans, more than 500 

feet (150 m.) in aggregate thickness, composed of debris eroded 

from the higher country. Tests made by the authors have shown 

that these fans contain cassiterite in places, but nowhere have 

they yet been found to be rich enough for profitable mining.

aggrading conditions under which they were formed are not 

favorable to the concentration of heavy minerals.

In some places where the fan gravels have been deeply eroded, cassiterite has been reconcentrated on the bottoms of the
GEOLOGICAL SURVEY BULLETIN1 935 PLATE 27

A. SMALL ABANDONED TIN MINE NEAU SAN FELII'E, GLAN AJLA'I U.

IS. .SMALL UN I'LACEK NEAK TLAC1UO.LEK A, GLANAJLATO.
TIN DEPOSITS OP THE REPUBLIC OP MEXICO 139

arroyos in secondary gravels, which have been further enriched 

with cassiterite freed by further erosion of the tin veins. 

Probably because of this two-fold enrichment, the gravels yield 

a profit in the little small-scale mining that has been done. 

But even these reworked gravels, though richer than any of the 

other gravel deposits and in part adapted to dredging or to the 

use of a dragline, have not been proved by the tests made thus 

far to be worth mining on a large scale.

In most of the placers worked hitherto the gravels are less 

than 4 feet (1.2 m.) thick, but in some they are as much as 15 

feet (4.6 m.) thick. Boulders as much as 3 feet (0.9 m.) in 

diameter are common where the gravels are thin, but in the 

thicker deposits most of the material is less than 1 foot (0»3 

m.) in diameter, though only a minor percentage consists of 

clay. The heavy minerals are chiefly specularite and magnetite; 

cassiterite is generally much scarcer. In many places topaz is 

abundant, but, being much lighter than caositerite, it can be 

washed out of the concentrates with relative ease. There are

x

small quantities of zircon and other heavy silicate minerals, 

which also can easily be eliminated from the concentrates. As 

in the vein material, cassiterite and specularite are so intimately intergrown that they cannot be completely separated. 

Mining and concentration. The placers have been mined 

entirely by hand, not even the simplest machinery being 

employed. The gravel, like the vein material, is concentrated 

by washing it first on inclined rock or dirt platforms (planillas) and then in shallow wooden pans (bateas). As concentrates 

containing much iron bring low prices, the miners wash the concentrates vigorously to eliminate as much of the specularite as 

possible. In so doing they lose a part of the cassiterite but 

obtain concentrates containing from 45 to 65 percent tin. The 

loss is naturally much greater if the concentrates are fine than 

if the cassiterite occurs as large kidneys, and therefore most
140 .GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS, 1941-42

of the mining is done near the veins, where the material is 

coarse. ..

' As "the. cgratle; of .the gravel must be fairly high to permit 

this, waatefui; method of mining, large tonnages undoubtedly 

remain1 .o^;.'grayei.'that contains more than half a pound of tin to 

a cubic yard (-0.1 percent) (the lower limit for minable gravel 

in the Malay States). Such gravel might "be worked by mechanical 

methods, but, as almost all the deposits are small, any mechanical equipment used in mining and milling them should be small 

and portable. Small jigs and tables could probably be used to 

obtain rough concentrates, which would then, like those from the. 

vein material, have to be treated further to separate the cassiterite from the iron minerals. In this respect' the Mexican 

deposits differ from those in the Malay States ^ ;wher'e concentrates of nearly pure cassiterite can re£tdfiy;be vriiiade, and 

therefore the lower limit for minable gravel would: probably be 

somewhat higher in Mexico than in Malaya.

Water supply for placer operations presents a serious problem over much of the plateau country during the dry season; 

hence most of the placer mining is done between July and January, when small streams of water flow in the larger arroyos. 

As the miners eliminate the coarse material before washing the 

gravel and wash.only small quantities of fairly rich material, 

using the same water repeatedly, not much water is required, 

A few tanks for storing water have been made by building earthen 

and stone dams, and additional tanks would undoubtedly aid somewhat in maintaining a supply during at least a part of the dry 

season. In most of the tin districts, enough water for smallscale mechanical concentrating plants could be obtained during 

the dry season from shallow wells in the larger arroyos.

Possible production. It is impossible to estimate reserves 

of tin in the placers, because the number of small placer deposits, though large, is unknown and none have been sampled and
TIN DEPOSITS OP THE REPUBLIC OP MEXICO 141

blocked out in an engineering sense. It is clear that several 

thousand tons of tin can be obtained from placers that contain 

half a pound or more of tin a cubic yard (more than 0.1 percent 

tin). Even with a moderate amount of stimulation, however, it 

is doubtful whether the' annual production could exceed a few 

hundred tons with present methods of mining. The use of small 

portable power jigs or tables would increase the production, but 

it is doubtful whether any placer containing enough cassiterite 

for profitable mining is large enough for dredging or the use of 

a dragline or scraper.

Summary of possible tin production

Production of tin in Mexico is not likely to exceed 400 tons 

of metallic tin a^year if the price of tin remains at 50 cents 

(United States currency) a pound (5.50 pesos a kilogram) and 

mining conditions and methods continue as in the past. The 

production might be doubled by making loans to miners and by 

paying higher prices for concentrates. The large pla-cer deposits at Guadalcazar, San Luis Potosi, are of extremely low grade, 

and it is not yet certain whether any tin can be produced from 

them. The tin ore in the lead-zinc-silver deposits at Santa 

Eulalia, Chihuahua, is said to be exhausted, and unless more of 

such ore has already been found no tin is likely to be produced 

there in the near future. The vein deposits in rhyolite and the 

placers related to them, though numerous, are all small and of 

low grade. Production from them cannot be greatly increased 

unless machinery can be employed in mining and milling, and no 

satisfactory methods for doing this have yet been devised. If 

such methods could be devised, all vein deposits containing 0.1 

percent or more of tin could perhaps be mined. The total amount 

of recoverable tin in the known vein deposits might then exceed 

10,000 tons. Large-scale working of the gravel deposits con-
142 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS, .1941-42

taining half a pound a cubic yard (28 grams a metric ton) or 

more might yield another 10,000 tons.

MINES, DEPOSITS, AND DISTRICTS EXAMINED

The number of individual tin deposits in Mexico is so great, 

and exact information concerning most of them is so scanty, that 

only those deposits or districts actually visited by the authors 

are described in this report. The most recent compilation of 

the location and occurrence of the deposits known in 1926 was 

made by Garcia £§/and presented in his monograph on tin. All 

other important references have been cited on previous pages of 

this report. Brief notes on the rocks and on the mineralogy of 

the veins in the individual districts described in the following 

sections are given in table 5. The numbers in parentheses that 

follow the headings refer to those on plate 25 and table 5.

Michoaca'n 

Los Cabires mine (1)

Los Cabires mine is near the crest of Cerro Frfo, about 10 

miles (16 km.) northwest of the village of Tepuxtepec in the 

district of Contepec, and is only a few miles south of the 

Guanajuato-Mlchoaoan State line. The mine is at the head of a 

deep arroyo known as the Canada de los Lobos. A small mill, a 

repair shop, bunk houses, and stables have been erected in the 

valley at an altitude 600 feet (180 m.) lower than the mine, 

which is reached from the mill by a good pack trail. A dirt 

road extends from the mill to Tepuxtepec, which is at the end 

of a branch railroad line connecting with Mexico City. A dam 

impounding about 31 billion cubic feet (900,000,000 cubic 

meters) of water when the reservoir is full has been built at 

the edge of Tepuxtepec, where there is a small plant for generating light and power. Several hundred tons of ore have been

28/ Garcia, J. Aurelio, Monografia del estafio: Bol. minero, vol. 22, 

pp. £29. 1926.
UNITED STATES DEPARTMENT OF THE INTERIOR 

GEOLOGICAL SURVEY

First, level 

Nivel primer

Nivel intermedia 

(-5 to -//meters)

:<. / Lower

crosscut 

*". /Crucero inferior 

.- *' ( 33 meters)

 --- >^-__,(~22 me^ rs)

Zero level 

Nivel cero

CO.O meters)

Q

\

.1

Second level 

Nivel seo-uncfo

""" ... (-14-meters')

1

yCrucero in fiarior

Y

Plan 

Piano

Intermediate level 

Nivel intermedia

Q

Second level 

Nivol Sec/undo

Zero level f| 

Nivel cero '

Solid lines are in plane of section;

broken lines are projected. 

Las lineas continuas r-epresentan el

corte en. el piano de la seccion. Las,

lineas espacicLdas representan. proyecCiornes.

Section A-A'

BULLETIN 935 PLATE 28

EXPLANATION 

EXPLICACION

First level 

/VtVeZ prim ero

Intermediate level 

Nivel intermedia

Second level 

Nivel segundo

Zero level 

Nivel cero

Lower crosscut 

Crucero inferior

Major fractures, some containing 

cassiterite; shown by same patterns 

as the levels on which they occur

fracturas mayores; algunas contienen 

casiterita; indicadas por el misrno 

tipo de linea que se usa para el nivel 

en tjfue ocurren

B

Shaft 

Tiro

Inclined shaft 

Tiro inclinacto

Country rock is porphyritic rhyolite

La. roca de la region, es 

riolito. porfidico.

25

PLAN AND SECTION OF LOS CABIRES MINE, TEPUXTEPEC, MICHOACAN 

PLANO Y PERFIL DE LA MINA LOS CABIRES, TEPUXTEPEC, MICHOACAN

too Feet

25 Metros

Adapted from unpublished maps by W.Gerzabek for 

American Smelting 8, Refining Co., 1939

Adaptado de mapas ineditos por W. Gerzabek para. 

American Smelting <J Refining Co., 1939

U. 3. GOVERNMENT PRINTING OFFICE: 1942 O - 480115 (faces p. 142)
TIN DEPOSITS OP THE REPUBLIC OP MEXICO 143

mined, but the mine is still in a stage of development and no 

tin has yet been produced.

The crest of the Cerro Frfo is about 9,800 feet (3,000 m.) 

above sea level and about 1,000 feet (300 m.) above the valley. 

The upper 400 feet (120 m.) of the ridge in the vicinity of the 

mine is composed of rhyolite flows, which are underlain by 200 

feet (60 m.) of bedded tuff. Other volcanic rocks underlie the 

tuff. ' The rhyolite at the mine is well layered, and its layers 

dip steeply into the hill. The flows exposed in the Caftada de 

los Lobos, east of the mine, appear to be nearly horizontal.

Small tin veins that crop out on the hillside have been 

'explored by means of three adits and several drifts and shafts. 

(See pi. 28.) The vertical range of exploration is 150 feet 

(46 m.), and the total length of the workings is about 1,200 

feet (365 m.). The rock is more or less strongly fractured, and 

many of the fractures contain sporadic incrustations of cassiterite. The strike of most of the fissures varies from east to 

northeast, but a few strike northwest; the dip averages about 

60° S. The fissures apparently show no regularity of pattern, 

many of them seeming to branch and feather out. There is no 

positive evidence of displacement along most of the fractures, 

and sllckensldes are rare.

The incrustations in the tin-bearing fissures are composed 

largely of botryoidal wood tin with a little specularite, and in 

some places there are drusy crusts of small red cassiterite 

crystals and black specularite. Tridymite incrusts some of the 

cassiterite and also occurs in fissures without cassiterite. 

The wall rock appears to be essentially barren, although in a few 

places cassiterite is disseminated in the walls for a few inches. 

The incrustations, to judge from the few available exposures, 

pinch out somewhat in the lower part of the mine, where the wall 

rock is less altered than above. There is no apparent change in 

the mineralogical character of the veins with depth.

480115 O - 42 - 4
144 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS, 1941-42

The rhyolite throughout most of the mine is fairly firm and 

dense, but in places it is altered to a porous, friable, lightgray rock. Although some of the fissures are partly filled with 

clay minerals, the wall rocks do not seem to contain much clay. 

The devitrified groundmass of the rhyolite along the vein has 

undergone much leaching, and a small quantity of zeolites has 

been introduced into the altered rock, but there apparently is 

no silicification. It is notable that the sanidine, a mineral 

that in general is readily altered, is here completely fresh.

As far as can be determined by the limits of exploration, 

the mineralized body forms an elliptical stockwork, which has 

a maximum diameter of about 150 feet (46 m.) and dips about 60* 

SW. The vertical extent of mineralization is not less than 150 

feet (46 m.). A block of-rock containing about 120,000 tons has 

been explored, but as this material has not been adequately 

sampled its tin content is uncertain. Judging from assays made 

for the owners and from a study of samples taken by the authors, 

this block probably averages less than 0.5 percent tin and may 

average less than 0.1 percent. Thorough sampling should be done 

before attempting to mine the material blocked out. Several 

tons of tin could undoubtedly be recovered by mining the veins. 

Owing to the steepness of the arroyo leading from the vein outcrops at Los Cabires, no placer appears to have been formed near 

the mine.

Other vein deposits in the Cerro Fr£o have been reported, 

but none of them have yet been mined.

Hidalgo 

Deposits near Zimapan (18)

Gasaiterite-bearing veins have recently been discovered in 

rhyolite about 7 miles (11 km.) west of Zimapan, an important 

lead-silver mining center, on the Laredo-Mexico City Highway.
TIN DEPOSITS OF THE REPUBLIC OF MEXICO 145

The topography in the region is extremely rugged, and at present 

the tin locality can be reached only over a rough mountain trail, 

The deposit has not yet been mined.

The rhyollte caps Cretaceous limestone on the high mountains 

.that form one side of the canyon of the Moctezuma River. It is 

a dark reddish-brown fine-grained rock, differing markedly from 

the lighter-colored, porphyritic rhyolite in most of the other 

tin districts. The cassiterite forms small broken kidneys in a 

brecciated zone partly cemented with chalcedony and opal. Clays 

and fine-grained yellow mimetite (?) are abundant in some of the 

fractures. Owing to the small amount of prospecting, the size 

of the deposits and the extent of the mineralized area are 

unknown.

Guanajuato 

District west and northwest of Dolores Hidalgo (2)

Small tin deposits occur on the west side of the northwardtrending Sierra de Guanajuato west and northwest of Dolores 

Hidalgo. The mountain range consists of folded and metamorphosed pre-Tretiary igneous and sedimentary rocks overlain by 

Tertiary volcanic rocks, including rhyolite. The tin deposits, 

are in the rhyolite. Thick deposits of gravel lap against the 

rhyolite on the east side of the mountains and extend eastward 

to the Laja River, which flows southward through Dolores 

Hidalgo. The gravels are deeply dissected by steep-sided, flatbottomed arroyos floored with secondary deposits of gravel, some 

of which contain placer cassiterite. Tin-bearing veins and placers have been mined in this district for many years, but at 

most only a few tons of tin has been produced annually.

Vein deposits. Small vein deposits have been mined along 

the road from Dolores Hidalgo to Guanajuato at a point about 15 

miles (24 km.) west of Dolores Hidalgo. All the old workings 

are merely shallow cuts in the bedrock, none apparently extend-
146 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS, 1941-42

Ing more than 10 feet (3m.) below the surface. All the vein 

material in sight has been removed, and the deposits offer no 

promise of containing any large amount of tin.

A small mine known as the San Marcos has recently been 

opened in the rhyolite on the northwest side of the Guadalupe 

dam and reservoir, about 2 miles (3 km.) northeast of the deposits along the road. The vein has been stoped for a length of 15 

feet (4.6 m.) and to a maximum depth of 12 feet (3.7 m.). Not 

all the vein material has been removed, but that exposed is 

scarcely more than an inch (2.5 cm.) wide. As the wall rock is 

essentially barren, it is doubtful whether much tin can be , 

recovered from the deposit. A few other small veins crop out 

nearby.

Placers.--Some of the gravel deposits that have yielded 

placer casslterite are on the Trancas estate, near the Guadalupe 

dam in the Canada de Gonzales; others are in the Arroyo de 

Galvez, on the north side of the Dolores Hidalgo-Guanajuato road 

and 14 miles (23 km.) west of Dolores Hidalgo. (See pi. 29 and 

fig. 10.) These deposits were extensively sampled from 1926 to 

1928 by E. H. S. Sampson for the Natural Products Producers 

Corporation of New York. The placers have not been mined on a 

large scale, but local miners now and then recover a little cassiterite. T-he district is easily reached by automobile.

The gravels consist of debris deposited in coalesced alluvial fans extending eastward from the front of the mountains. 

The contact of the gravels with the older volcanic rocks is near 

the Guadalupe dam. The gravels are partly consolidated and well 

exposed in both the Canada de Gonzales and the Arroyo de Galvez 

and are at least 500 feet (150 m.) thick. They are composed of 

fragments ranging in size from fines to boulders 3 feet (0.9 m.) 

in diameter. The Canada de Gonzales has a nearly flat though 

terraced bottom. The stream bed is mainly in secondarily deposited gravel averaging about 10 feet (3 m.) thick, but some of it
UNITED STATES DEPARTMENT OF THE INTERIOR 

GEOLOGICAL SURVEY

EXPLANATION 

EXPLICACION

Alluvium 

(Contains some placer deposits)

. Aluvion 

(Contiene depositos de placer)

Conglomerate 

Conqlomerado

Porphyritic rhyollte with interbedded tuffand breccia 

(Rhyolite contains tin veins) 

Riolitaporfidlca intercalada entfe 

lechos de toba y brecha (La riollta 

contiene vetas de estano)

X

Prospect shaft 

Tirode cateo

Contact approximately located 

Contacto delineado apro&imadamerrte

BULLETIN 935 PLATE 29

Prospect adit 

Socavon de cateo

Contact inferred 

Contacto inferido

Elevations approximate 

Elcvaciones aproximadas

 . ' i. X^'-v- 

. -vv V:--:-*^5i! m. 6 220Om.

]!&\ - x .' '?' ' "V.; «

Adapted from unpublished^ map by Ernest J. h.Sampson, 1926 

Adaptado deunmapa insditopor Ernest J. H. Sampson, 1926

3000 Feet

1000 Metros

Piano de referenda es el nival del mar

GEOLOGIC MAP OF PLACER AREA WEST OF DOLORES HIDALGO, GUANAJUATO 

MAPA GEOLOGICO DE LOS PLACERES DE ESTANO AL OESTE DE DOLORES HIDALGO. GUANAJUATO

U. S. GOVERNMENT PRINTING OFFICE: 1942 O - 480115 (faces p. 146)
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EXPLANATION

EXPLICACION

!;. ]

1 .' .' *J

Alluvium

(Contains some placer deposits]

A/e/iS/on

(Contiene depositor de placer)

^% ' F7^ 

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Figure 10. Geologic map of placer area along Arroyo de Galvez west 
148 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS, 1941-42

An area covering about 80 acres (32 hectares) in the Canada 

de Gonzales and just below the Guadalupe dam (see pi. 29) was 

prospected by Sampson ^/by means of 73 pits, 3 short adits, and 

3 shafts. Channel samples a foot square (0.09 square meter) and 

a foot (0.3 m.) deep were taken at 3-foot (0.9-m.) intervals 

from one or more of the walls of each pit. After screening on 

a 1-inch (2.5-cm.) screen and washing the undersize in Klondike 

rockers, the concentrates were collected and weighed. Precautions were taken to recover as much of the heavy material in the 

gravel as possible, and the tailings were occasionally rewashed 

in order to check the recovery. Some of the heavy minerals 

undoubtedly were lost in washing but the quantity lost was probably no greater than that recovered.

Some of the concentrates were assayed for tin and gold. The 

concentrates averaged 0.03 percent of the total gravel. Assuming that half the heavy minerals were lost, their original average weight would be 0.06 percent. Assays of the concentrates 

ranged from 0.45 to 13.43 percent tin, probably averaging less 

than 10 percent. Assuming that the concentrates co.ntained 10 

percent tin, the average tin content of the gravel would be 

about OoOOG percent, or nearly 0.2 pound a cubic yard (60 gm. a 

metric ton). As some of the pits were partly in the primary 

gravel, which in general does not contain as much tin as the 

secondary gravel, the grade of the secondary gravel is probably 

somewhat higher than the figure given. The gravels appear to 

contain also about 0.003 ounce of gold a cubic yard (0.57 gm. a 

metric ton).

The authors briefly examined these deposits and estimate 

that the volume of secondary gravel may be about one million 

cubic yards (760,000 cubic meters). From the results of the

§2/ Sampson, 3. H. S., The tin end gold-besring grevels of the Stn Joaquin de Tre.neas, Sen Jose de la Quamcda, snd La Palma. estetes, Guenajuato, 

Mexico: Unpublished private report for Nsturel Products Producers Corporation, Mew York, l^2Z.
TIN DEPOSITS OP THE REPUBLIC OP MEXICO 149

sampling by Sampson, the authors infer that this gravel probably 

contains between 0.1 and 0.5 pound of tin a cubic yard (28 to 

140 gm. of tin a metric ton), of which possibly half might be 

recovered. Possibly 0.0003 ounce of gold a cubic yard (0.06 gm. 

of gold a metric ton) about one-tenth of the gold shown by 

assay might be recovered.

A small permanent stream flows through the Canada de Gonzales, and an abundant supply of water could be obtained from 

shallow wells.

An area covering about 5 acres (2 hectares) near the old 

mill in Arroyo de Galvez, 2 miles (3 km.) west of Paredones, was 

also tested by Sampson.£°-/ (See pi. 29 and fig. 11.) The secondary gravel here averages about 6 feet (1.8 m.) in thickness. 

The total volume of gravel is about 45,000 cubic yards (34,000 

cubic meters), and the grade is somewhat lower than in Canada 

,de Gonzales probably less than 0.1 pound of tin a cubic yard 

(28 gm. of tin a metric ton). A supply of water can be obtained 

from wells in the gravel.

The secondary gravels farther downstream in both the Canada 

de Gonzales and the Arroyo de Galvez contain cassiterite, but 

their tin content appears to be much lower than that of the gravel farther upstream.

Placer cassiterite is mined intermittently southeast of the 

Guadalupe dam in the Arroyo de Salta Pena Colorado, which is on 

the south side of the ridge traversed by the road to Guanajuato. 

(See pi. 29.) The arroyo carries drainage from areas containing 

tin veins, and its bottom, though fairly narrow, contains thin 

secondary deposits of gravel at several places. Pannings made 

by the authors show that some of this gravel contains as much 

as 1 percent tin, but its volume is small. One bench or terrace 

150 feet (46 m.) wide is covered with 2 feet (0.6 m.) of gravel 

that may contain a few pounds of tin a cubic yard (more than a

30/ Sanroson, E. H. S., op. cit.
150 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS, 1941-42

kilogram a metric ton). The total volume of material is about 

12,000 cubic yards (9,000 cubic meters). Another terrace half 

a mile (0.8 km.) downstream is a little larger but appears to

 £ I

I-5§

E,n

&

contain a lower percentage of tin. A small supply of water can 

"be obtained from the gravel in the bottom of the arroyo.

Local miners have washed cassiterite from placer gravel in 

the Arroyo del Estanito, which is known by the name Arroyo 

Blanco where it joins the Laja River at La Quemada, a village
TIN DEPOSITS OP THE REPUBLIC OP MEXICO 151

4 miles (6 km.) northeast of the placer area and 18 miles (29 

km.) northwest of Dolores Hidalgo. The Guadalupana mercury 

mine is a quarter of a mile (0.4 km.) upstream from the placer 

area. The geology here is much the same as in the Caflada de

Gonzales. (See fig. 11.) The placers in both places were sam31/ 

pled by Sampson, who tested a total area of 189 acres (76.5

hectares) by means of 104 prospect pits, 2 trenches, and 2 short 

adits. The average depth of the pits was about 12 feet (3.7 

m.). The authors estimate that the average depth of the secondary gravels in the area is about 7 feet (2.1 m.) and that, 

therefore, about half the material sampled by Sampson was primary gravel. The secondary gravel sampled has a volume of probably 2,000,000 cubic yards (1,500,000 cubic meters). Sampson's 

figures are interpreted as indicating that the gravel may average about 0.1 pound of tin a cubic yard (28 gm. a metric ton) 

and jthe primary gravel somewhat less. There is a little cinnabar in some of the gravel. The arroyo does not contain water 

during the dry season, and although some water might be obtained 

from wells it does not appear to be plentiful.

Deposits near Tlachiquera (3 and 4)

Tlachiquera lies nearly halfway between San Pelipe and Le<5n 

and is west-northwest of Dolores Hidalgo. A ton of cassiterite 

concentrates a week is said to be produced in the region during 

the rainy season, most of it from placer deposits. There is a 

small tin-smelting furnace in the village, but it was not being 

used in February 1941.

Small tin deposits occur in the rhyolite flows on Cerr'o de 

los Charcos about 2 miles (3 km.) east of the village. Beds of 

tuff crop out on the south side of the hill about 200 feet (60 

m.) below the tin veins; they may be interbedded with the flows. 

Although half a dozen small vein deposits are scattered over the

3I/ Sampson, E. H. S., op. cit.
152 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS, 1941-42

upland, only four were examined. The length of the veins mined 

ranges from 30 to 100 feet (9 to 30 m.), and their maximum depth 

is about 40 feet (12 m.). Apparently the veins were small, for 

the average width of mining was about 3 feet v (l m.) and most of 

the material mined is barren rhyolite, now piled on the dumps. 

The veins appear to be mined out. A few small placers, such as 

that shown in plate 27, B, have been mined in arroyos draining 

this area.

Other small veins occur in rhyolite flows on Cerro del Toro, 

northwest of Tlachiquera. These veins appear to be even smaller 

than those east of the village, for one was mined to a depth of 

only 3 feet (1m.) and another to a depth of 6 feet (2 m.). 

Small mercury mines have been operated in the vicinity. It is 

reported that, a few weeks before the authors' visit, two miners 

recovered a few hundred pounds (about 150 kg.) of cassiterite 

from the arroyo leading from one of the tin deposits examined. 

Many of the small arroyos in this region contain a little cassiterite.

Deposits near San Pelipe (5.)

Half a dozen tin veins have been prospected and mined in the 

rhyolite flows on a mesa known as Boquillas, 10 miles (16 km.) 

north of San Pelipe. No .pyroclastic rocks were observed in the 

vicinity of the tin veins. The broad, nearly flat upland is cut 

by deep, steep-sided arroyos, which drain into the broad valley 

wherein San Pelipe lies. Some of the tin veins, such as the one 

shown in plate 27, A, are on top of the mesa; others are on the 

steep sides of the arroyos. The mine shown in the photograph 

has a maximum depth of 20 feet (6 m.) and is about 100 feet 

(30 m.) long. The veins have an average strike of N. 60° W. and 

dip about 65° SW., tut in detail they are irregular and branching. The wall rock is little altered and quite firm. No vein 

material is visible in the mine, but some is in crusts on blocks
TIN DEPOSITS OP THE REPUBLIC OF MEXICO 153

In the dump. Barren rock would have to be mined in order to 

find more ore.

The Esperanza mine, which was prospected in 1940, is about 

10 feet (3 m.) across and 15 feet (4.6 m.) deep. The flow layers of the rhyolite strike N. 20° E. and dip steeply toward the 

west. The deposit consists of a mass traversed by many irregular fractures in which vein material has been deposited. Some 

of the small blocks of rock that are taken out are almost completely incrusted with a thin layer of specularite and cassiterite, but the central parts of these blocks are essentially 

barren, the vein minerals being virtually confined to the fractures. The ore had not all been mined out at the time of the 

authors' visit.

Another small deposit along a steeply dipping, slickensided 

fracture was mined to a depth of 20 feet (6m.) for about 30 

feet (9m.) along the strike. The vein material on the dump 

appears to be largely specularite, and no ore was visible in 

the mine. A small deposit prospected near the bottom of one of 

the arroyos consists of a zone of silicified breccia about a

foot (0.3 m.) wide, in which there are small fragments of nod- \

ules of wood tin and crystals of a specular hematite. Several 

other deposits that were visited had apparently been mined out.

Queensland mine (6)

. The Queensland mine is on a low, gently sloping rhyolite 

ridge extending northwest from the base of the cliffs of rhyolite that form the southeastern part of the Sierra del Pajaro. 

It is about 2 miles (3 km.) south of Santa Barbara^ a small village 9 miles (15 km.) west of San Felipe, and 18 miles (29 km.) 

by road from San Felipe. It was opened and operated around 1900 

by an English-controlled company but was soon closed down 

because of the low grade of the ore. The amount of ore mined is
154 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS, 1941-42

not recorded, but, to judge from the extent of the workings, at 

least a few tons of tin must have been produced,, The mine workings were inaccessible in 1941.

The deposits are in massive flows of rhyolite, and no pyro32/ clastic rocks were observed in the vicinity. Bromly 'reports

that the veins were mined to a depth of 130 feet (40 m.) and for 

'a maximum length of 330 feet (100 m.) along the strike. There 

are three nearly parallel groups of northward-trending veins, 

the main veins dipping 70° to 80° E., and one group was extensively mined. It is said that the veins appeared to die out, 

branching in many places and following joints and crevices that 

ramified in every direction. Some of the tin fissures mined 

were partly filled with a barren waxy clay. The specularite 

and cassiterite were frozen to the walls in some places; elsewhere they appeared to be loosely held in the clayey matrix, 

as if broken from the walls. The wall rocks proved to be virtually barren. Bromly believes that a large part of the rhyolite near the deposits contains traces of tin but that the grade 

of tin is much too low for profitable mining. The ore in the 

upper part of the veins averaged from 2 to 3 percent tin, but 

the average for all the ore mined to a depth of 130 feet (40 m.) 

was only about 0.4 percent.

The cassiterite was intimately associated with speculari'te, 

and it was found impossible to separate the two without losing 

a large part of the cassiterite. A Krupp ball mill, a Wilfley 

table, and a Wilfley slime-table were used in milling the ore. 

The concentrates from 0.4-percent ore contained only 15 percent 

tin indicating a recovery of only 29 percent from the ore. The 

concentrates were smelted in a small furnace constructed at the 

mine.

It seems doubtful whether there is any more ore readily 

available in the mine. An examination of the dumps indicates

32/ Bromly, A. H., Tin raining and smelting st St.nta Barbara, Gu.anejufi.to, 

Mexico: An. Inst. Min. Eng. Trans., vol. 36, pp. 22J-233, 1905.
TIN DEPOSITS OP THE REPUBLIC OP MEXICO 155

that most of the rhyolite near the veins is dense and firm, and 

it is improbable that cassiterite is disseminated through any 

significant volume of this rock. Bromly says "Doubtless, better 

knowledge would have given better results, but r strongly doubt

whether the Queensland ores, even if sufficiently abundant

33/ 

instead of the reverse, would have commercial value." Small

placers have been mined in shallow arroyos near the Queensland, 

but most of these appear to be worked out.

El Santin mine (7)

t 

El Santin mine is on the south side of the Mesa de la Fajas

In the southeVn part of the Sierra Gorda; it is about 4 3/4 

miles (7.8 km.) by trail south-southeast of Santa Catarina, a 

small village in the valley of the Tierra Blanca River. Santa 

Catarina lies 42 miles (68 km.) by road southeast of San Luis de 

la Paz, a city at the end of a branch line of the railroad. A 

fair road extends from San Luis de la Paz to Victoria, but travel by truck from there to Santa Catarina, a distance of 16 

miles (26 km.), is possible only at times when the trail has

just been repaired. There is no road from Santa Catarina to the

34/

mine. The tin deposits, according to Antunez, are at an altitude of 6,400 feet (1,940 m.), about 900 feet (270 m.) higher

than the village. About 20 tons of tin has probably been proi

duced from the mine.

Cretaceous limestone is said to crop out in the valley at 

a few places near Santa Catarina. The limestone is overlain by 

at least 2,000 feet (600 m.) of volcanic rocks, which include 

flows of andesite in the lower part and flows of rhyolite near 

the to«p. The tin deposits consist of several northwest-trending 

groups of veins in the andesite about 1,000 feet (300 m.) below 

the top of the Mesa de la Pajas. Faulting has occurred along 

some of the tin-Tbearing fissures, a few of the walls being

33/ Bromly, X. H., op. cit., pp. 232-233.

2*/ Xntuflez E., Francisco, Mina II Santin y anezae: Unpublished report, 

1939-
156 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS, 1941-42

slickensided, but there is little or no brecciation along moat 

of the veins. The authors could not determine whether the 

faulting preceded or followed mineralization.

In detail the tin veins are similar to most of the others 

in the lava flows. They are highly irregular and ramify along 

'Joints and fractures in the country rock. Some of them are 

merely tight seams, many of which open into pockets more than 

a foot wide; some of these pockets have yielded as much as 220 

pounds (100 kg.) of cassiterite. The vein material consists 

in part of arborescent and stalactitic aggregates of small, 

crystals of cassiterite, with only minor quantities of specularite. These bodies are apparently unique; at least none so 

well developed have been found elsewhere. The upper parts of 

the veins are said to have contained mainly the ordinary banded

 v

wood tin. Minor quantities of clay partly fill some of the 

fissures. In some places along the veins the wall rock is 

strongly altered and contains a large percentage of clay, but 

in other places there appears to be virtually no alteration. 

Tests of specimens of the wall rock collected by the authors 

prove that cassiterite is not disseminated through large bodies 

of the andesite, although there is a little cassiterite in the 

andesite immediately adjacent to some of the tin-bearing fissures. A spectrogram of one specimen collected near a vein 

showed no tin.

The workings at El Santln consist mainly of an irregular 

inclined shaft with a maximum depth of 130 feet (40 m.), several 

shallow shafts, and an adit about 260 feet t,80 m.) long. Short 

drifts, winzes, and raises of all shapes extend out from the 

main shaft at different levels; mining was done only where vein 

material was found. The largest stope is about 12 feet (3.7 m.) 

in diameter and 10 feet (3 m,) high, and the smallest passages 

are just large enough for a man to squeeze through. The stopes 

represent places where groups of closely spaced fissures con-
TIN DEPOSITS OF THE REPUBLIC OP MEXICO 157

taining vein material were found. The adit, intended to intersect the ore zone in depth, started from a point near the bottom

of a small arroyo, a few hundred feet (about 100 m.) southeast

* 

of the head of the shaft, but it had not been completed in March

1941. No tin veins were cut in the adit. Several small shafts 

on the surface show that the fissures are mineralized at intervals along the strike for a distance of at least 300 feet (90 

m.). The width of the zone of mineralized fractures may be 30 

feet (9 m.).

It is impossible to make any close estimate of the amount of 

tin in the deposit at El Santfn, for no channel samples have 

been taken and no assays have been made. The maximum limits of 

the workings, however, outline roughly a body of rock 300 feet 

(90 m,). long, 30 feet (9 m,) wide, and 130 feet (40 m.) deep. 

This block contains about 43,000 cubic yards (32,400 cubic 

meters), or possibly 75,000 metric tons. It certainly averages 

no more than 0.5 percent tin, and it may.average less than 0.1 

percent. At the higher figure the block would contain 375 tons 

of tin and at the lower figure 75 tons. It is doubtful whether 

more than half the tin could be recovered, even if it were possible to mine such low grade ore. Thus the limitations of the 

deposit are evident.

A small arroyo leading from El Santln mine contains placer 

cassiterite, which is mined during the rainy season. These gravels are only a few feet (1 to 2m.) thick and quite bouldery. 

There may be as much as 10,000 cubic yards (15,000 metric tons) 

of gravel containing possibly 1 pound of tin a cubic yard (0.035 

percent tin). A supply of water could probably be obtained from 

wells in the main arroyo a few hundred feet (about a hundred 

meters) below the mine.

Many other vein deposits have been worked in the district 

around Santa Catarina, but none were visited. The Peril ta mine, 

on the Cerro del Rincon del Carrizal, is said to have yielded
158 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS, 1941-42

over 20 tons of tin during the past 20 years. The natives 

intermittently mine the small placer deposits in the district,

but the annual production of tin is only a few tons.

/

Deposits north of San Luis de la Paz (8 and 9)

Small deposits of tin have been mined on the Hacienda Sauceda, 25 miles (40 km.) by road northwest of San Luis de la Paz 

and 6 miles (10 km.) by trail east of the main group of buildings on the hacienda. The area is one of low relief near the " 

head of one of the main arroyos in the district. The veins are 

in flows of rhyolite and apparently contain very little cassiterite. Two prospect pits, about 6 feet (2 m.) deep, excavated 

along the veins showed only thin seams of vein material. Most 

of the cassiterite produced in the area is mined from the 

arroyos below the vein outcrops. The concentrates from the 

gravel contain many well-formed crystals of topaz, some of which 

are as much as 2 inches (5 cm.) long. Virtually all the mining 

is done during the rainy season.

Other small tin veins between Santo Domingo, S. L. P., and 

Jaula, Gto., along the road from San Luis Potosi to San Luis de 

la Paz have been explored by two shallow pits. These veins are 

in latite. Apparently the miners did not find enough cassiterite for profitable mining.

Aguascalientes 

Deposits near Calvillo (10)

Several tin veins have been explored on the north slopes of 

the Sierra del Laurel south of Calvillo. A road extends from 

Calvillo to the Rancho Jaltiche de Arriba, a distance of 9 miles 

(15 km.), and the tin deposits are about 4 miles (6 km.) by 

trail from the ranch and perhaps 800 feet (240 in.) higher. The 

main workings, known as La Virgen mine, are on a hillside near
TIN DEPOSITS OP THE REPUBLIC OF MEXICO 159

the bottom of a steep-sided arroyo, and several veins cropping 

out on the same hillside have been explored. A few hundred 

tons of rock had been mined from La Virge.n when the mine was 

visited. No attempt had been made to recover the cassiterite 

from this low-grade ore, but several hundred pounds (possibly 

200 kg.) of vein material had been picked out by hand.

The veins are in flows of white and highly porous rhyolite 

whose flow layers have variable strikes and steep dips. Heavy 

concentrates panned from, the crushed rock suggested that cassiterite might be disseminated through this porous zone, but assays 

of three rough channel samples collected from different parts of 

the workings prove that the cassiterite is not evenly disseminated, for one sample contained about 0.2 percent tin and 

another only about 0.002 percent. There is no direct relation 

between the degree of alteration and the quantity of tin; the 

sample of the most altered-appearing and porous rock contained 

the smallest quantity of tin. No veinlets were observed where 

the samples were taken, but the cassiterite is probably in thin 

seams or tight fractures cutting through the rock. The miners 

report that they found small veins and pockets of ore while 

making excavations.

The maximum depth of the La Virgen workings is aboiit 15 feet 

(4.6 m.), and the short drifts outline an area about 50 feet 

(15 m.) in diameter. The samples taken from this area contain 

on the average less than 0.5 percent tin. The general average 

is probably somewhat higher than this, because of the small 

pockets and veins of cassiterite scattered through the rock, 

but it seems unlikely to be much^higher. Moreover, unless further work reveals a much larger body of tin-bearing rock than 

has thus far been blocked out, the deposit is too small to be 

mined by bulk methods even if the grade were fairly high.

All the other veins prospected near La Virgen mine are in 

moderately dense, firm rhyolite. One of the veins has been

480115 O - 42 - 5
160 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS, 1941-42

slightly brecciated and receinented with chalcedony and opal, "but 

the others are merely thin incrustations on fissure walls and 

contain a high percentage of specularite. Owing to the steepness of the arroyo below the tin veins, no gravel deposits have 

been formed near the mine, and no significant amount of workable placer has been found elsewhere in the area.

Jallaco 

Deposits near Paso de Sotos and Teocaltlche

Tin deposits have been mined for many years in northern 

Jalisco near the villages of Paso de Sotos and Teocaltiche. 

None of these deposits were being mined when the authors visited 

this region, and it was not feasible to enter any of the mines. 

Some concentrates, however, that had been sold in Paso de Sotos 

were examined and found to be fairly typical of those from the 

rhyolite tin deposits, though rather high in iron. These concentrates were reported to have come from small placer and vein 

deposits in the rhyolite on the south side of the Sierra del 

Laurel. Alluvial material was panned at several places in one 

of the arroyos heading in the Sierra del Laurel, but no cassiterite was visible in the heavy concentrates.

35/ Ingalls 'reported that Los Vallecitos mine, near Teocaltiche, which in 1883 had been opened to a depth of 100 feet 

(30 m.) and for a length of 650 feet (200 m.), produced a maximum of about 3 tons of metallic tin a month. Cassiterite is 

still being mined from the rhyolite flows in the hills around 

Teocaltiche during the rainy season, and the concentrates are 

sold to buyers in the village.

35/ Ingalls, W. R., The tin deposits of Durango, Mexico: Am. Inst. 

Min. Eng. Tracs., vol. 25, p. 160, 1896.
TIN DEPOSITS OP THE REPUBLIC OF MEXICO 161 

Los Amigos mine

Los Amigos mine is in rhyolite flows on the Cerro de la 

Pefia, 19 miles (30 km.) by road northeast of Encarnaci6n. It 

was operated by the Cia. Minera Los Amigos, S. A., until May 

1932, when work was discontinued. As the workings are said to 

be inaccessible, the mine was not visited by the authors, and 

most of the following statements are from a report by Martfnez.^2/ The mine is said to have produced 80 tons-of concentrates in 1924 and metallic tin valued at 28,890 pesos between 

May 1931 and February 1932.

The mine, according to Martinez, consists of an inclined 

shaft 390 feet (120 m.) long, four levels, several winzes, and 

a few crosscuts. The total length of all the workings is 2,800 

feet (853.m.), and the maximum depth, of the mine is 253 feet 

(77 m.). The ore was crushed, jigged, tabled, and smelted in 

plants built near the mine. Ores from the Minitas region, on 

the Cerro de la Pachona, which is separated' from the Cerro de 

la Pefla by the Arroyo de la Pena, were also treated in the mill. 

The mill did not operate properly, so that the concentrates were 

not clean and the cassiterite was not all extracted from the 

middlings and tailings. The percentage of tin recovered was 

not known, however, since assays of heads and tailings were 

lacking. Martlnez concluded that the project could not be considered profitable.

Raines and Partridge, who were associated with the Cia.

Minera Los Amigos, estimated that in 1933 the mine had reserves

37/ of 9,000 tons of ore containing 1 percent tin, 'but as this

estimate was not based on systematic sampling it may have been 

too high. As far as known, no ore has been mined from the 

deposit since 1933.

36/ Martfnez, Juan, La mina LOB Amigos, de la Cia. Minera LOB Amigot, 

S. A.: Hev. industrial, vol. 1, pp. 390-391, 1933. 

££/ Idem, p. 390.
162 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS, 1941-42

Zacatecas 

Deposits west of the city of Zacatecas (11)

A small tin deposit occurs near La Quemada, which is about 

40 miles (64 km.) southwest of Zacatecas. The deposit is on the 

south end of a rhyolite hill on which there are ruins of prehistoric stone dwellings. The largest excavation, which is 10 

feet (3 m.) deep and 30 feet (9 m.) long, reveals a single massive face of rhyolite partly incrusted with specularite and cassiterlte. The incrustations also extend out along some less 

well defined cross fractures. The surrounding rock is light 

gray, quite dense, and apparently unaltered. The main fissure 

is nearly vertical and cuts sharply across the steeply dipping 

flow layers of the rhyolite. Apparently the vein was too narrow to be mined; at any rate it evidently has not been worked 

for several years.

Many other deposits have been reported In the region southwest of Zacatecas but 'none were examined.

Deposits between Presnillo and Sain Alto (12)

Many small tin deposits are scattered over the Sierra de 

Chapultepec, the central part of which is 37 miles (59 km.) by 

road northwest of Fresnillo and about 15 miles (24 km.) south 

of Sain Alto. The Sierra de Chapultepec is underlain by preTertiary rocks, which crop out at places in the lower parts 

of the deeper valleys. The tin deposits are in the rhyolite 

flows which form the upper part of the mountain range. The tinbearing region is drained largely by the San Francisco River, 

on the south side of the mountains. Raise 'reports that about

62^- tons of cassiterite concentrates a year, or roughly 30 tons

r

of metallic tin, were being smelted at Las Cuevas, a small village near Sain Alto, around 1894. It is said that about 8 tons

38/ Halse, Edward, The occurrence of tin ore at Sain Alto, Zacatecas, 

with reference to similar deposits in San Luis Potosi and Durango, Mexico: 

M. Inst. Min. Eng. Trans., vol. 29, p. 506, 1899.
TIN DEPOSITS OP THE REPUBLIC OP MEXICO 163

of cassiterite concentrates a month is now being produced from 

this region during the rainy season; the annual production may 

be about 20 tons of metallic tin, or about the same that it was 

45 years ago.

The largest tin deposit visited is at the Leona mine, which 

is on the Cerro de Leona, near the crest of the Sierra de Chapultepec. About 20 men were working the mine in February 1941. 

The authors were not permitted to enter the mine, but the men 

were then said to be working at a depth of 60 feet (18 m.), and 

the maximum length of the workings was apparently about 30 feet 

(9 m.). The shaft is on a well-defined vertical fissure, which 

is at right angles to the steeply dipping flow layers of the 

rhyolite. The rhyolite that was being mined was nearly white, 

and moderately porous though fairly firm. The miners said that 

an ore pocket had been found which they had been mining for a 

week. The largest pieces of vein material, about an inch and a 

half (4 cm.) thick, were composed largely of specularite. The 

quantity of ore mined from the pocket was not revealed but was 

probably small.

Some concentrates from the Mufieca mine, said.to be about 6 

miles (10 km.) southeast of the Leona mine, were found to consist mainly of cassiterite. This mine is in the rhyolite flows 

north of the village of El Aguila. Several small veins are said

to have been prospected in its vicinity.

39/ Halse 'describes three small tin deposits in the Sain Alto

district, which were being mined in 1895 but have apparently 

long since been forgotten, for the inhabitants knew nothing of 

them in 1941. The La Desparramada deposit, about 16 miles (25 

km.) southeast of Sombrerete, was mined for a length of 50 feet 

(15 m.) and to a depth of 40 feet (12 m.). It is a vein in 

rhyolite, striking N. 38° W. and dipping 75° NE., and containing 

wood tin and specularite. The workings of El Refugio mine,

Halse, Edward, op. cit. , pp. 502-50S.
164 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS, 1941-42

which is about 10 miles (16 km.) southwest of Fresnillo, 

extended to a depth of 30 feet (9m.) along intersecting veins 

in rhyolite. The El Calabrote deposit, 20 miles (32 km.) west 

of Fresnillo, was mined for a length of 63 feet (19 m.) and to 

a depth of 50 feet (15 m.). It is in rhyolite cut by irregular, 

steeply dipping fractures; the cassiterite in the fractures is 

mainly wood tin.

Many small placer deposits in the region are mined during 

the rainy season. One of these, in an arroyo near the Leona 

mine, was being worked in March 1941. The concentrates contained well-formed crystals of topaz, besides cassiterite and 

a little specularite. The arroyo may contain 1,000 cubic yards 

(1,500 metric tons) of gravel averaging a few pounds of tin a 

yard (possibly 0.15 percent tin). Another placer deposit, in 

the valley of the San Francisco River, is said to contain cassiterite, but althoxigh this deposit has not been tested it probably contains no large body of gravel that is not of extremely 

low grade.

Durango 

Deposits south of the city of Durango

None of the deposits between the town of Chalchihultes, in 

Zacatecas, and the city of Durango were visited by the authors, 

but concentrates from several were examined. Only a few tons 

of tin a year is produced from this region, and little work

appears to have been done on its deposits since 1897, when they

40/ were described by Ingalls. ' Some ore consisting mainly of

specularite is said to have come from the rhyolite near the 

crest of the mountains at a place that is half a day's horse-. 

back ride possibly 12 miles (20 km.) west of Chalchihuites. 

It'was said that the deposits at that place had not been extensively mined, owing to the narrowness of the veins and their

IjO/'Ingalls, W. R. , Notes on the tin deposits of Mexico: An. last. Min. 

Eng. Irons., vol. 27, pp. U2S-U29, 1898.
TIN DEPOSITS OF THE REPUBLIC OP MEXICO 165

high iron content. Some concentrates, consisting of small 

pieces of wood tin moderately low in iron, that were being sold 

in Vicente Guerrero were said to have been washed from small 

placer deposits in the rhyolite on the Cerro Blanco, -about 28 

miles (45 km.) southwest of that village. Vein deposits have 

been mined intermittently on the Cerro Blanco and in the Sierra 

de Michis northwest of Mezqultal, a town in the valley of the 

Mezquital River, west of the Cerro Blanco. It was also said 

that tin deposits had been mined in the lavas near the crest of 

the Sierra de Sacrificio, northeast of Vicente Guerrero.

Deposits on the Cerro de los Remedies (13)

Tin deposits have been mined intermittently for at least 

50 years on the Cerro de los Remedies at the southwest edge of 

the city of Durango. This hill is composed of gently dipping 

well-layered flows of rhyolite (see pi. 30). These deposits 

have been mined intermittently and have yielded more than 10 

tons of tin.

Most of the tin veins are on the northeastern side and on 

the summit of the hill. The main workings are on La Vanguardia 

claim (see pi. 30) and consist of a narrow trench 500 feet (150 

m.) long and 4 feet (1.2 m.) in average width, excavated to a 

depth of 3 to 95 feet (1 to 29 m.). The trench is along a 

poorly defined, nearly vertical fault, the footwall of.which is 

less fractured than the hanging wall. The fault is exposed in 

two small pits 500 feet (150 m.) farther along its strike, making its total known length 1,000 feet (300 m.). Cassiterite 

and specularite locally incrust the walls of the fault and occur 

in some of the fissures branching from the fault. As the maximum width of the trench is about 6 feet (2 m.), the vein material apparently was not deposited far from the fault. Some of 

the faulting occurred after the cassiterite was deposited, as

evidenced by slickensides on incrustations firmly frozen to the ^~VV 

walls. The rhyolite along a-part of the fault contains small
166 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS, 1941-42

quantities of clay minerals, but only a few of the sanidine 

phenocrysts are altered, and these only slightly,,

Most of the cassiterite was mined from the vicinity of the 

"shaft", the deepest part of La Vanguardia open cut (see pi.

30), where fairly large pockets of ore are said to have been

«/

found. Jenkin reports that the largest pocket contained as

much as 8 tons of vein material assaying possibly 50 percent 

tin. The last operator had constructed some concrete planillas, 

on which the crushed ore was concentrated. It is said that for 

a short time 14 men were producing daily about 700 pounds (320 

kg.) of concentrates assaying about 50 percent tin. According 

to Jenkin, a pocket which might contain several tons of highgrade ore had. just been \mcovered in the bottom of the "shaft" 

when work was halted because of the death of the owner of the 

mine. This discovery, however, could not be verified by the 

authors, the bottom of the "shaft" being flooded at the time of 

their visit.

The north "shaft" had been excavated along a tin-bearing 

fissure to a depth of 62 feet (19 m.) when city authorities 

halted the work for fear that the blasting might damage the 

city's water reservoir nearby. The rhyolite in the shaft is

light-colored, and some of it is moderately porous, but in gen42/ 

eral it is little altered. Jenkin - states that a pocket of

vein material weighing 450 pounds (204 kg.) and containing 60 

percent.of tin was excavated at a depth of 26 feet (8 m.). 

Another pocket of vein material of the same grade weighing 855 

pounds (388 kg.) was excavated at a depth of 40 feet (12 m.). 

The fissures in the bottom of the shaft are said to contain thin 

incrustations that have not been mined out.

The south "shafts", one of which is about 30 feet (10 m.) 

deep and the other 100 feet (30 m.) deep, were excavated along

Ul./ Jenkin, S. G., Unpublished report on the tin deposits of Cerro de 

loa Bemedios, Durango, 

H2/ Idem.
UNITED STATES DEPARTMENT OF THE INTERIOR 

GEOLOGICAL SURVEY BULLETIN 935 PLATE SO

Worth shaft^- ^, 

Tfro.de/nqrte: -\

Summit of hi 

' N -SCumbre de la loma XL

EXPLANATION 

EXPLICACION

(Contains some placer deposits)

(Contiene depositos de placer)

Porphyritic rhyolite, with 

some interbedded breccia

Riolita porfidica interca/ada entre 

/ecnos de toba y brecha

Prospect trench 

Cat a

B

Shaft 

Tiro

900 Feet

300 Metros

Adapted from unpublished map by S.G.Jenkins, 1940 

Adapt ado de un mapa ine'dito por5.GJenkinsi !940

GEOLOGIC SKETCH MAP OF CERRO DE LOS REMEDIOS, DURANGO 

CROQUIS GEOLOGICO DEL CERRO DE LOS REMEDIOS, DURANGO

V. 3. GOVERNMENT PRINTING OFFICE: 1942 O - 480115 (faces p. 188)
TIN DEPOSITS OP THE REPUBLIC OP MEXICO 167

poorly defined fissures containing thin incrustations of specularite and cassiterite at the surface. A zone of flow breccia, 

possibly representing the division between two superposed flows, 

was cut by the shafts, and the rhyollte In general is highly 

vesicular and lithophysal. Although the porosity of the rock 

is high, the sanidine is entirely fresh and clays apparently 

are quite subordinate or absent in the groundmass. Incrustations composed of a mixture of chalcedony, quartz, and fluorite 

occur on the walls of one of the fissures. According to Jenkln 

small bodies of vein material were found in excavating the deeper shaft, and yielded a maximum daily production of about 65 

pounds (30 kg.) of ore containing 40 percent tin. The authors 

collected a channel sample across the width, of the shaft at 95 

feet (29 m.). A separation of the crushed rock in the laboratory 

of the Geological Survey gave only 0.1 percent heavy minerals, 

and of this only about 1.0 percent was cassiterite, which 

probably came from a tight fissure containing a thin film of 

vein material. Thus it is evident that the quantity of tin in 

the wall rock is negligible.

Incrustations have been found in other fissures on the hill, 

but none of them except those already mentioned have been 

explored to a depth of more than 10 feet (3 m.). Small quantities of placer cassiterite have been mined intermittently 

around the base of Cerro de los Remedies. During the summer 

and fall of 1941 the alluvium north of La Vanguardia claim was 

extensively mined, but the qxiantity of cassiterite recovered is 

not known.

Deposits in the Sierra de Cacaria (14)

Tin deposits have been mined in the Sierra de Cacaria, about 

25 miles (40 km.) northwest of the city of Durango. The district can be reached by automobile by way of Otinapa and R£O 

Verde, a total distance of 65 miles (104 km.). Vein and placer
168 GEOLOGIC INVESTIGATIONS'IN THE AMERICAN REPUBLICS, 1941-42

deposits are scattered over a large area in the rhyolite flows 

that cover the upper, mesa-like part of the mountains; some can 

be reached by automobile but others only on horseback or on 

foot. The authors examined three groups of mines and three 

placers considered to be representative of the district. It is 

reported that about a thousand men mine the placers during the 

rainy season, and that during this period the production'of 

concentrates reaches a maximum of about 3 tons a week. The 

annual production in the district, though not definitely known, 

may exceed 30 tons of metallic tin, so that the district is one 

of the most important in Mexico.

Vein deposits. The largest mine in the district is El 

Diablo, which was operated by the Durango Tin Mining Co. in 

1881 and 1882. It is on a vein that occupies a fairly well

defined northeast-trending fault zone in the rhyolite flows.

43/ Genth ' quotes Kleinschmidt as saying that the fault zone can

be traced on the surface for a nile (1.6 km.), but the authors 

were unable to trace it so far, and its tin-bearing part is 

apparently no more than 1,000 feet (300 m.) in length. The mine 

consists of a vertical shaft 273 feet (83 m.) deep, to which a 

connecting adit has been driven from the bottom of an arroyo 

below the collar of the shaft, together with some narrow stopes 

along the vein. Except for the adit, which was driven in 1935, 

these workings were inaccessible in 1941.

Shallow excavations near the shaft expose a zone of breccia 

about 5 feet (1.5 m.) wide between walls of more massive rock. 

Specularite.and cassiterite incrust some of the pieces of breccia and occupy some of the irregular fissures in the precciated 

zone, but apparently they were not deposited in the less-fractured walls. Most of the rock excavated is fairly dense and 

firm. The more open of the fissures that were mined apparently 

contained a good deal of clay, clearly formed after the cassit-

*)3/ Oenth, F. A., Contributions to mineralogy: An. Philos. Soc. Proc., 

vol. 24. p. 2k t 1887.
TIN DEPOSITS OP THE REPUBLIC OP MEXICO . 169

44/ erite was deposited. Ingalls 'was informed by Winninghoff

that the vein material (presumably the rock in the brecciated 

zone) did not average more than 15 percent ore, of which possibly 25 percent was cassiterite. The quantity of tin produced 

from the mine is not recorded, but it was not large enough to 

cover the cost of mining. The vein has not been mined out completely, but no estimate can be made of the quantity and grade 

of the ore remaining.

Two small excavations on the Las Auras claims expose an illdefined vein trending northwest. The workings consist of a 

stope about 8 feet (2.4 m.) long and 20 feet (6m.) deep and a 

small cut in the bedrock near the bottom of a small arroyo. The 

rhyolite in the excavations is moderately fractured and fairly 

porous, and in places the groundmass contains a little clay. 

The cassiterite seems to be concentrated in the fissures along 

which the pit was excavated, and it apparently is associated 

with a high percentage of specularite. A horizontal channel 

sample taken across 4 feet (1.2 m.) of rhyolite in one of the 

excavations was found, in the laboratories of the Geological 

Survey, to contain 0.5 percent tin, but the quantity of rock 

containing as much as this is probably small.

At Los Alamitos mine an old vertical shaft had been sunk to 

a depth of about 60 feet (18 m.), and two intersecting veins had 

been irregularly stoped for from 200 to 300 feet (60 to 90 m.) 

southeast and southwest from the shaft. No vein material was 

observed in the accessible part of the workings, but apparently 

the veins were narrow and pockety, for large bodies of the rock 

along their strike were left unstoped. A new, smaller excavation about 8 feet (2.4 m.) deep and 10 feet (3 m.) in diameter, 

near the old shaft exposes moderately fractured rhyolite in 

which a few of the fissures contain specularite and cassiterite.

, W. R., The tin deposits of Durango, Mexico: Am. Inst. Min. 

Eng. Trans., vol.' 25, p. 259, 1896.
170 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS, 1941-42

This body of tin-bearing rock "is probably small, and its grade 

is obviously low. The production from these deposits is not 

known.

Placer deposits. Small placer deposits in the arroyo below 

El Diablo mine and others below Las Auras and Los Alamitos 

mines, have been worked. The largest one examined is in the 

Canada de las Auras, a shallow, flat-bottomed valley on the 

rhyolite upland which had been extensively prospected for a 

length of half a mile (0.9 km.) and is probably tin-bearing for 

a greater distance. At one place a prospect pit in the gravel 

exposes a vein of cassiterite in the rhyolite bedrock, indicating that at least some of the cassiterite in the gravel has not 

been transported far. Material on the dumps of some of the pits 

was panned by the authors and the concentrates were found to 

contain enough cassiterite to suggest that the gravel might be 

mined profitably. Most of the gravel is smaller in size than 2 

inches (5 cm.). The width of the valley bottom may average 

about 100 feet (30 m.) for a distance of half a mile (0.9 km.), 

and the thickness of the gravel may average about 3 feet (1 m.). 

Thus the area examined contains at least 20,000 cubic yards of 

gravel (15,000 cubic meters). Although the grade is not definitely known, the authors believe it may be higher than half 

a pound of tin a cubic yard (0.02 percent tin). The other placers in the district were not examined, but some of them, to 

judge from reports, may be at least as large as that in the 

Canada de las Auras.

Deposits in the Sierra de San Francisco (15)

The Sierra de San Francisco lies along the northeast side 

of the plains of Guatimap6 and northeast of Patos, a village 

that lies about 100 miles (160 km.) north-northwest of the city 

of Durango and is connected with it by a branch railroad line. 

Tir occurs in what is commonly known as the Potrillos district,
UNITED STATES DEPARTMENT OF THE INTERIOR 

GEOLOGICAL SURVEY BULLETIN 935 PLATE 31

EXPLANATION 

EXPLICACION

X

Tin mine 

Mina de estano

Trail 

Camino de herradura

N

\

VAdapted from unpublished map by Gustave 

Favela for Cia. Miners de Coneto, S. A.

Adaptado de un maps inedito por Gustavo 

Favela ara Cia. Minera de Coneto, 5. A.

/y Ojo de

X/ A £UJ 

/

'/^

/ / 

O/

$> 

^

X

> : ^-^ /

: ,tf C i X'' ' ': \

V* ./TX

 / ^ 4) \

^/ v-- //V x-

/ 6; X y ' K

/ y

/ ^ -*1 / __--^ o 1

' "' A A / O i / \

/ ^v^--

«i ^^...-~-/ i 

« / x

A ^

i\

SKETCH MAP OF TIN DISTRICT IN SIERRA DE SAN FRANCISCO, DURANGO 

DEL DISTRITOESTANIFERO EN LA SIERRA DE SAN FRANCISCO, DURANGO

3,000 Metros

U. S. GOVERNMENT PRINTING OFFICE: 1942 O - 480115 (faces p. 170)
TIN DEPOSITS OP THE REPUBLIC OP MEXICO 171

on the northeast slope of the range. The district is not accessible by automobile. The shortest trail by which it can be 

reached is about 6 miles (10 km.) long and extends from the 

Rancho Tobozo, located at the southwest foot of the mountains, 

to the small tin-mining settlement of America in the Arroyo de 

America. From Tobozo the trail rises about 2,000 feet (600 m.) 

over a pass near the crest of the range and drops down about 

1,000 feet (300 m.) to America. A wagon road from Tobozo to 

America was constructed about 40 years ago, but it has been 

washed out and would have to be completely rebuilt and possibly 

relocated to be passable for motor trucks.

The lower part of the Sierra de San Francisco consists of 

pre-Tertiary rocks and early Tertiary andesite, which contain 

the important silver deposits that are being mined near Promontorio, about 12 miles-(19 km.) northwest of the tin district. 

All the known tin deposits, however, are in the late Tertiary 

rhyolitic rocks which form the upper part of the mountain range. 

All the veins examined are in what appear to be flows, and none 

were seen in rocks definitely identified as tuff, although thick 

beds of tuff and breccia occur southeast of the tin-bearing 

zone. The maximum monthly production in the district is 

reported to be about 50 tons of cassiterite concentrates containing about 50 percent tin, and the annual production of metallic tin may be as great as 100 tons. The district is now the

most productive in Mexico.

Vein deposits. The largest mines in the Potrillos district

are the Candelaria and the Grant. (See pi. 31.) These mines

45/ 

were inaccessible in 1941, but Ingalls, who described the

workings as far as they were developed in 1894, showed that the 

ore was mined from narrow, pockety veins similar to the many 

that have been described earlier in this report. The deposits 

were later worked by the Pittsburgh & Mexico Tin Mining Co.,

Ingalls, W. R., op. cit. (vol. 25), pp.
172 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS, 1941-42

but after mining several thousand tons of ore at a loss the

46/ 

company discontinued operations. The maximum depth of the

workings is said to be about 330 feet (100 m.).

The authors took a horizontal channel sample across 6 feet 

(1.8 m.) of the rhyolite in a small adit near the main shaft of 

the Candelaria mine, and this sample was found to contain about 

0.005 percent tin. Little if any of the rhyolite on the dxunps 

has the appearance of being any richer than this sample, and, 

even though material on dumps is presumably waste, it seems fair 

to infer that cassiterite is not disseminated in commercially 

recoverable quantities through any large body of the rhyolite 

near these deposits. The amount of tin recovered from the large 

mines is not known.

The Varocitos mine, in the hill above the Candelaria mine, 

consists of an open cut along two parallel fissures containing 

vein material. The veins have been stoped at irregular intervals for about 600 feet (180 m.) along the strike; the stopes 

have a maximum width of 6 feet (1.8 m.) and depths varying from 

5 to about 30 feet (1.5 to 9 m.). So far as could be seen, the 

cassiterite is virtually confined to the fissures. The rhyolite 

is moderately firm and dense along the greater part of the 

stoped veins, but in a few places it is porous and contains 

fluori.te and zeolites. The amount of tin recovered is not 

known.

The Golondrina mine is an old working on the west side of 

the Arroyo de America, north of the village of America. The 

deposit consists of pockety veins along irregular, branching 

fissures in moderately firm rhyolite containing small quantities 

of clay. The veins form a northwest-trending lode, which has 

been stoped at irregular intervals for about 300 feet (90 m.) 

along the strike; the maximum width of the stope is -about 6 feet 

(1.8 m.). A shaft said to be 260 feet (80 m.) deep had been

U6/ Rangel, Manuel, Apuntes sobre la distribuci&n de minerales en el 

Estado de Durango; Soc. geol. mexicana, Bol. 7. p. HI, 1910.
TIN DEPOSITS OP THE REPUBLIC OP MEXICO 173

sunk at one end "but was inaccessible in 1941, The vein material 

apparently contains a large percentage of specularite, for miners who were reworking the dumps found little but specularite, 

and some veins exposed in the open cuts along the lode were also 

largely specularite. The quantity of tin recovered is not 

known.

A short distance north of the Golondrina mine a vein had 

been discovered, only about a week before the authors' visit, 

under the gravel in the Arroyo de America in the course of 

placer mining. A shaft had been sunk into the rhyolite bedrock 

to a depth of 10 feet (3 m.), and the miners were concentrating 

orange-colored casslterite from the soft, porous rock along the 

vein. Much of the cassiterite was being lost in panning-, and 

some of it was so fine that it colored the water orange.

A small inclined shaft 15 feet (4.6 m.) deep, located on the 

east side of Arroyo de America at its junction with Arroyo de 

Potrillos and at an altitude about 100 feet (30 m.) above the 

bottom of the valley, exposes an irregular vein 2 inches (5 cm.) 

in maximum width. The vein contains specularite, thin pencillike crystals of topaz, and a little cassiterite. The mine was 

abandoned because of the leanness of the vein.

The Orosco mine, a mile and a half (2 km.) north of the 

Candelaria mine, consists of a group of half a dozen shafts 

and some small cuts excavated at irregular intervals for about 

1,000 feet (300 m.) along the strike of a moderately well 

defined fracture zone in the rhyolite. The deepest shaft was 

60 feet (18 m.) in depth. The miners who were working in this 

shaft had discovered a pocket of ore from which they said they 

had been recovering about 220 pounds (100 kg.) of concentrates 

daily for'three weeks. The concentrates probably contained 

about 40 percent tin. Other miners who were working in shafts 

on either side of this one were getting nothing at that time,
174 GEOLOGIC ...INVESTIGATIONS IN THE AMERICAN REPUBLICS, 1941-42

although they had previously found small pockets of ore. This 

deposit seems very similar to the Grant and Candelaria deposits.

The'Veintinueve de Junio mine, about three-quarters of a 

mile (1.2 km.) east-northeast of the Orosco mine, consists of a 

low harrow adit 460 feet (140 m.) long, driven along an illdefined fissure in moderately hard rhyolite. Pockets of botryoidal cassiterite weighing as much as 55 pounds (25 kg.) are 

said to have occurred along the fissure. The width of the 

largest pocket mined is said to be 8 inches (20 cm.). Two small 

mines nearby, obviously not on the same fissure, were being 

worked, but the quantity of cassiterite being recovered was 

small.

Several other small mines were visited, but as they had long 

been abandoned no information was obtained as to their production. Some mines were being operated northeast of the small 

village of. Sapiorfs, and it was reported that a large pocket of 

high-grade ore had been discovered in one of these, but the 

authors did not visit the mine nor verify the report. Many 

other small deposits in this district have been mined and forgotten.

Placer deposits. Most of the tin produced in the Sierra de 

San Francisco in recent years has been recovered from placer 

deposits. It is said that about 2,000 miners work these placers during the rainy season. Assuming that 100 tons of tin a 

year is produced in the district, the average annual production 

of each miner is worth less than $50 (United States currency). 

The largest deposits are in the Arroyo de America, the Arroyo de 

Sapior£s, and the Arroyo de Liendres, and many of the tributaries to these arroyos contain small placers. Other deposits 

are merely soil mantle on the hillsides below the vein deposits. 

It is probable that most of the placer deposits in the district 

are already known but that many of them are left unworked 

because it has been possible to mine deposits of higher grade.
TIN DEPOSITS OP THE REPUBLIC OP MEXICO 175

As the recovery of cassiterite from the gravel is poor and the 

small dumps cover patches of unworked ground, seemingly workedout deposits may at some future time be reworked.

The only part of the Arroyo de America that may contain a 

volume of gravel large enough to warrant mining with machinery 

extends 3,300 feet (1 km.) upstream and 5,900 feet (1.8 km.) 

downstream from the settlement of America. The average thickness of the gravel in this 8,200-foot stretch may be 5 feet 

(1.5 m.), and its total volume about 100,000 cubic yards (76,000 

cubic meters). The percentage of tin in this gravel cannot be 

closely estimated for want of accurate sampling, but it is certainly low perhaps too low for profitable mining by any method. 

The abundance of large boulders, moreover, is detrimental to 

mining. There is another body of gravel in this arroyo in the 

vicinity of the Golondrina mine, but it is only about one-third 

as large. There are also gravel deposits near the junction of 

Arroyo de America with Arroyo de Potrillos, but they appear to 

be of extremely low grade. Water flows in the arroyo during 

most of the year. '

In the Arroyo de Sapioris, which is merely the upper part 

of the Arroyo de America, gravel deposits extend from a point 

about 4,900 feet (1.5 km.) upstream from the settlement of 

Saploris to a point 8,200 feet (2.5 km.) downstream from Sapioris. The arroyo in this section may contain over 200,000 

cubic yards (152,000 cubic meters) of gravel, having a maximum 

thickness of about 25 feet (7.6 m.) and consisting principally 

of material less than 4 Inches (10 cm.) in diameter. The grade 

of this gravel also is unknown, but it may contain more than 

half a pound of tin per cubic yard .(0.02 percent). Water flows 

in the arroyo only during the rainy season.

The Arroyo de Llendres, In which water flows during most of 

the year, contains gravel deposits for a distance of 6,500 feet 

(2 km.) upstream from its junction with the Arroyo de Tejabanes.

.-480115 O - 42 - 6
176 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS, 1941-42

(See pi. 31.) The maximum thickness of the deposit is about 15 

feet (4.6 m.) and averages about 8 feet (2.4 m.). Most of the 

material is less than 4 inches-(10 cm.) in diameter. The volume

**.

of gravel in this deposit may approach 200,000 cubic yards 

(152,000 cubic meters). The tin content of the deposit is 

unknown, but it may exceed half a pound of tin a cubic yard 

(0.02 percent).

O