General Geology and 

Phosphate Deposits 

1 of Conception del 

Oro District, Zacatecas 

Mexico

GEOLOGICAL SURVEY BULLETIN 1037-A

Prepared in cooperation with the Institute Nacional 

para la Investigacion de Recur sos Miner ales, under 

the auspices of the Technical Cooperation 

Administration of the Department of State and 

the Foreign Operations Administration

General Geology and 

Phosphate Deposits 

of Conception del 

Oro District, Zacatecas 

Mexico

By CLEAVES L. ROGERS, ZOLTAN DE CSERNA, EUGENIC TAVERA, 

and SALVADOR ULLOA

GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS

GEOLOGICAL SURVEY BULLETIN 1037-A

Prepared in cooperation with the Institute Nacional 

para la Investigacion de Recur sos Miner ales, under 

the auspices of the Technical Cooperation 

Administration of the Department of State and 

the Foreign Operations Administration

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1956
A

UNITED STATES DEPARTMENT OF THE INTERIOR 

Douglas McKay, Secretary

GEOLOGICAL SURVEY 

Thomas B. Nolan, Director

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

Washington 25, D. C.
CONTENTS

Page

Abstract _________________________________________________________ 1

Introduction. _____________________________________________________ 2

Purpose and scope of the investigation___________________________ 2

Geographic features of the area,_________________________________ 2

History of the district__________________________________________ 5

Earlier work in the area,_ _ _____________________________________ 6

Fieldwork and acknowledgments_____________________________ 7

Sedimentary rocks _________________________________________________ 8

Basement rocks _______________________________________________ 11

Jurassic system,_______________________________________________ 11

Zuloaga limestone_____________________________ .___________ 13

La Caja formation.__________________ ______________________ 15

Unit A______ ______________________________ 17

Unit B____ _____________________________________ 18

Unit C____________________________________________ 20

Unit D____ __________________________________ 21

Paleoecology_ _________________________________________ 23

Cretaceous system,______________________________________ 24

Taraises formation,_________________________________________ 24

Cupido limestone.___________________________________________ 25

La Pena formation._________________________________________ 25

Cuesta del Cura limestone-_____________ ___________________ 26

Indidura formation,_______________________________________ 28

Caracol formation ____________________________________________ 30

Parras shale_ _______________________________________________ 32

Tertiary system _________________________________________________ 33

Mazapil conglomerate______________________________________ 33

Quaternary system,_____________________________________________ 34

Lake beds and conglomerate-_______________________________ 34

Alluvium.________________________________________________ 35

Igneous and metamorphic rocks _____________________________________ 35

Tertiary intrusive rocks ________________________________________ 35

Granodiorite and diorite_____________________________________ 35

Concepciou del Oro and Providencia stocks.______________ 35

Nochebuena stock_______ ______________________________ 37

San Feliciano phacolith(?) ______________________________ 38

Hornblende-pyroxene andesite and associated aplite____________ 39

Dacite porphyry_________________________________________ 41

Andesite _________________________________________________ 43

Rhyolite__ ______________________________________________ 43

Contact metamorphism and mineralization _______________________ 44

^ Concepcion del Oro and Providencia stocks___________________ 44

iVochebuena stock _________________________________________ 46

San .Feliciano phacolith(?) __________________________________ 47

in
IV CONTENTS

Igneous and metamorphic rocks Continued Page

Tertiary extrusive rocks_ _______________________________________ 47

Agglomerate (?) ____________________________________________ 48

Rhyolite flows_______________________________________ ______ 50

Basalt flows_____--_____________-___---_-_---__-__________ 50

Structural geology __________________________________ 51

Folds___..--__-_-_-----_______ ________ ______________ 51

Faulting______________________________________________________ 52

Major structural units __________________________________________ 53

Canutillo anticline._____________________________ 53

La Caja anticline,_________________________________________ 55

Santa Rosa anticline,________________________________ 59

Santa Rita anticline. __ ___-_________________________________ 61

Geologic history.___________________________________________________ 62

Phosphate deposits_________________________________________________ 67

The phosphate mineral (fluorapatite)_________-__--_________"______ 67

Lithology, petrography, and chemical composition.-_______________ 70

Calcareous phosphorite.____________________________________ 70

Cherty calcareous phosphorite___________-__-________________ 74

Phosphatic cherty limestone________________________________ 76

Phosphatic calcareous chert_____ _____________ 79

Phosphatic limestone.________---____-____-_-_-_--__________ 80

Phosphatic calcareous siltstone______________________________ 82

Phosphatic rocks adjacent to phosphatic member______________ 82

Origin__ _____________________________ 83

Reserves ______________________________________________________ 87

Possible utilization of phosphate rock_ ___________________________ 89

Phosphate industry in Mexico____ ___---_-_____---__-________________ 92

Bibliography __ ____________________________________________________ 94

Metric equivalents,________________________________ 98

Index____________________________________________________________ 99

ILLUSTRATIONS

PLATE 1. Geologic map and structure sections of the Concepcion del

Oro district, Zacatecas_________-_-_______-_-__-_-___ In pocket

2. Map showing sample localities, columnar sections, analytical

data, and reserves of phosphate rock ________________ In pocket

Page 

FICURB 1. Index map of Mexico showing location of area studied ______ 3

2. Aerial view, looking westward, of northern part of area

mapped._________ ___________________________________ 4

3. Correlation of Burckhardt's stratigraphic units and those of

the present writers____________________________________ 9

4. Correlation of Jurassic and Cretaceous formations of northcentral and eastern Mexico ____________________________ 10

5. Age determinations of index fossils of the Upper Jurassic Ci " -* ' 

Mexico, according to Burckhardt and Iinlay_____________ 12

6. Flat-lying Zuloaga limestone in the western part of the Sierra

de La Caja_________________________________ _________ 13
CONTENTS V

' Page 

FIGURE 7. View of the north flank of the Sierra de Santa Rita, showing

La Caja formation____________________________________ 16

8. View of a typical contact between^the^La Caja and Taraises

formations_______ _______________ ___________ 17

9. View showing typical lithology of unit D of the La Caja

formation.__________________________ 22

11). Cuesta del Cura limestone, showing wavy bedding and chert__ 27 

1L. View showing lower beds of the Iiididura formation, north

flank of the Sierra de Santa Rosa_______________________ 29

12. View of the Caracol formation, south flank of the Sierra de

Canutillo________________________________ 30

13. View of the Parras shale near village of Bonanza.__________ 32

14. View of the dacite intrusive body of Las Parroquias, Sierra de

Santa Rosa__________________________________________ 41

15. View showing crumpled, metamorphosed Cuesta del Cura

limestone,___________________________________________ 46

16. Aerial view, looking westward, of the Sierra de Santa Rosa_ _ 49

17. View of the scarp formed along Canutillo fault north of

Novillo,s_ _ ___________________________________________ 54

18. View of Cerro de La Campana, with Concepci6n del Oro in

foreground, showing Jurassic and Lower Cretaceous strata__ 56

19. Longitudinal sections showing differences in thickness of the

La Caja formation in the Sierra de Santa Rosa___________ 64

20. Exposure of phosphatic unit of La Caja formation in Canon

de La Canela, Sierra de Santa Rosa_ ___________________ 70

21. Photomicrograph of calcareous phosphorite.____ __________ 72

22. Photomicrograph of calcareous phosphorite ________________ 72

23. Photomicrograph of calcareous phosphorite, under crossed

nicols ______________-__________---_-____--_- 73

24. Photomicrograph of cherty calcareous phosphorite__________ 75

25. Photomicrograph of phosphatic cherty limestone. __________ 76

26. Photomicrograph of phosphatic calcareous chert____________ 79

27. Photomicrograph of phosphatic limestone. _________________ 81

TABLES

TABLE 1. Chemical analyses of rock samples from the La Caja formation,

Puerto Blanco, Sierra de Santa Rosa____________________-_ 19

2. Modal composition of granodiorite_________________________ 36

3. Modal composition of representative specimens of diorite______ 38

4. Partial chemical analyses of two samples of wollastonite from

the Sierra de La Caja___________________________________ 47

5. Results of spectrographic analyses of 30 samples of phosphate

rock from the Concepci6n del Oro district _________________ 69

6. Partial chemical analyses of five samples of calcareous phosphorite from the Sierra de Santa Rosa____________________ 74

v. . _ 7. Partial chemical analyses of phosphatic rock from the Sierra de

Santa Rosa___________________________________________ 78

8. Results of greenhouse experiments testing the effects of Coiicepcion del Oro phosphate on yields of wheat-_____________ 91

GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS

GENERAL GEOLOGY AND PHOSPHATE DEPOSITS OF 

CONCEPCION DEL ORO DISTRICT, ZACATECAS, MEXICO

By CLEAVES L. ROGERS, ZOLTAN DE CSERNA/ EUGENIC TAVERA/ 

and SALVADOR ULLOA :

ABSTRACT

The Concepcion del Oro district comprises an area of about 1,300 square kilometers in northern Zacatecas and southern Coahuila, Mexico. The district lies 

near the southern margin of a broad belt of strongly asymmetrical mountains, 

which trend generally westward and are characterized by a moderately rugged 

terrain. Four small anticlinal ranges have been mapped. They are generally 

overturned toward the north and in some places are characterized by fan folding. 

They contain several important thrust faults and many normal faults, some of which 

have large displacement.

The stratigraphic sequence, about 3,600 meters in thickness, includes the 

Zuloaga limestone and La Caja formations of Late Jurassic age, and the Taraises 

formation, Cupido limestone, La Pefta formation, Cuesta del Cura limestone, 

Indidma formation ,Caracol formation, and Parras shale, of Cretaceous age. 

The rocks are predominantly calcareous up to the Cuesta del Cura limestone, 

above which they become increasingly clastic. Above these marine formations 

is the Mazapil conglomerate of early Tertiary age which consists of mixed sedimentary and pyroclastic material. Flows and pyroclastic rocks ranging in composition from rhyolite to basalt are considered to be mainly of middle to late Tertiary 

age. The Tertiary rocks are overlain by lacustrine deposits, conglomerate, and 

alluvium of Quaternary age.

Intrusive rocks are of Tertiary age and range from stocks of granocliorite and 

diorite to small stocks, sills, and dikes of andesite, dacite, rhyolite, and aplite. 

The deep-seated intrusive masses of granodiorite and diorite strongly metamorphosed the adjacent sediments and in places highly deformed them. Commercial deposits of copper, lead, zinc, silver, gold, and iron are associated with the 

stocks and contact metamorphic zones.

Marine phosphorites occur principally in one unit or member of the La Caja 

formation. Five lithologic types are present in this member, calcareous phosphorite being the most important commercially and containing the highest percentage of PsOs. The phosphate mineral is fluorapatite, occurring as primary 

pellets of generally structureless, cryptocrystalline collophane and as secondary 

material replacing pellets, fossils, and the rock matrix. The fluorapatite may

1 Institute isacional para la Investigacion de Eecursos Minerales.
2 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS

contain carbonate, and spectrographic analysis indicates the presence of 28 

elements in addition to those reported in the chemical analyses. The principal 

impurities in the phosphatic rock are chert and calcite, the chert predominating 

in the lower part of the phosphatic member, and the calcite predominating in its 

upper part. The deposits probably accumulated very slowly in shallow clear 

agitated waters, and the richest material may have formed on a low submarine 

ridge.

The phosphatic member ranges from less than 1 meter to about 20 meters in 

thickness, and the grade of the phosphatic rock decreases in inverse proportion to 

its thickness. The phosphatic member is considered to have commercial possibilities only in the Sierra de Santa Rosa, where it averages 2.15 meters in thickness. 

In that area, reserves are calculated to amount to some 5,000,000 metric tons 

averaging 21.1 percent P20s, or about 13,500,000 metric tons averaging 16.2 

percent P2Os.

INTRODUCTION

PURPOSE AND SCOPE OF THE INVESTIGATION

The primary purpose of the present study was to evaluate the 

phosphate deposits of the Concepci6n del Oro district, in the hope 

that a major phosphate field might be present in that area.

Although Mexico has a rapidly growing fertilizer industry, it can 

meet only a part of the country's needs. Large-scale production 

of superphosphates is contingent upon the discovery of an adequate 

source of raw material within the country. It is expensive to import 

phosphate rock or commercial fertilizers from the United States, and 

the deposits now being exploited in the State of Nuevo Leon are small 

and scattered, although they are high in grade.

The investigation was not limited to the phosphate deposits, but was 

extended to include areal geologic mapping of the district. The authors 

hope that the results of this work will be useful to the local mining 

industry, which produces a diversity of precious and base metals, 

including gold, silver, copper, lead, zinc, and iron. The geology in 

many of the principal mining areas is complex and difficult and, in 

large part, has never been adequately mapped. Also, it is hoped that 

this study may lead to a better understanding of some aspects of the 

regional geology.

GEOGRAPHIC FEATURES OF THE AREA

The area studied has an extent of about 1,300 square kilometers. 

Although it is situated mainly in the northern part of the State of 

Zacatecas, it extends a short distance into Coahuila. (See fig. 1.) 

The principal town in the district is Concepci6n del Oro, an important 

metal-mining and smelting center of 5,400 inhabitants. It is connected with the city of Saltillo, Coahuila by the narrow gage Coahuila 

and Zacatecas Kailroad. Saltillo lies 125 kilometers to the northeast 

of Conception del Oro. Branch lines connect Bonanza and Melchpj: 

Ocampo with the Coahuila and Zacatecas main line at Avalos, and the 

Compania Minera de Petioles operates a short narrow gage line
GEOLOGY, PHOSPHATE DEPOSITS, CONCEPCION DEL ORO DIST., MEX. 3

connecting Avalos with their plants at Terminal. Although the Coahuila and Zacatecas Kailroad is mainly a freight line servicing the local 

mining industry, it maintains passenger service operating daily, except 

Sunday, between Saltillo and Concepcion del Oro.

The closest paved highway is in the vicinity of Saltillo, and roads 

within the mapped area are dirt or gravel and generally poor. Only 

the roads being used for trucking ore receive any maintenance, and

FIGURE 1. Index map of Mexico showing location of the area studied.

some roads may be temporarily impassable during the rainy season. 

However, a modern highway from Saltillo to Concepcion del Oro is 

under construction and will be completed in 1956. Eventually this 

road will be surfaced with asphalt and will become a part of the 

federal highway network. The area contains numerous foot and burro 

trails, which form important connecting links for the local inhabitants. 

The area is situated in the northern half of the Mesa Central, a high 

semiarid plateau containing many scattered mountain ranges of relatively short extent. It lies close to the southern margin of a broad 

belt of closely spaced, westward-trending mountains and valleys
GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS
GEOLOGY, PHOSPHATE DEPOSITS, CONCEPCION DEL ORO DIST. ; MEX. 5

which form the dominant topographic features of the region (fig. 2). 

The mountains are characterized by a terrain only moderately rugged.

The mapped area is crossed by four parallel ranges, which from 

north to south are known as the Sierras de Canutillo, La Caja, Santa 

Rosa, and Santa Rita. The eastern part of the Sierra de La Caja is 

known as the Sierra de Concepci6n del Oro, while the eastern part of 

the Sierra de Santa Rosa is called the Sierra de San Jose. The Sierras 

de San Jose and Conception del Oro are in part joined by a large 

granodiorite stock to form a continuous U-shaped range open toward 

the west. The area is bounded on the east and west by the Bolsones 

de San Carlos and Cedros, which are large, northwestward-trending 

intermontane basins.

The maximum relief in the area is about 1,400 meters, and the 

highest mountain summit, Cerro de La Sorpresa, has an altitude of 

3,175 meters.

This region of Mexico has a semiarid climate, most of the precipitation occurring in the summer rainy season. Annual rainfall totals 

about 500 millimeters. The mapped area contains scattered shallow 

wells, and a few springs emanate from the igneous rocks and some of 

the larger fault zones, but the farmers are in many places precariously 

dependent upon the collection of surface water in small earth reservoirs known as tanques.

Diurnal changes in temperature are relatively great. In winter 

the days are pleasantly warm, while the nights are cool, and temperatures occasionally dip below freezing. The summers are generally 

hot. The mean annual temperature ranges between 11° and 13°C.

The plains and lower mountain slopes support a sparse vegetation, 

classed as xerophytic, containing a great variety of desert shrubs and 

cactus. At higher elevations there may be open pine or cedar forest.

The principal source of income in the area is the mining industry, 

which, directly or indirectly, supports a majority of the inhabitants 

and has resulted in a greater population density than characterizes the 

surrounding region. However, many settlements in the area are predominantly agricultural communities, which are engaged mainly in 

the cultivation of corn, beans, and wheat and in the raising of goats. 

Other activities include the gathering of fiber for rope, wood for charcoal, and candelilla for wax. Guayule has been another important 

resource, although the rubber factory in Cedros has been idle since 

the end of World War II. In general, the farming is on a bare subsistence level.

HISTORY OF THE DISTRICT

Although the Indians may have exploited the ore deposits of the 

Concepci6n del Oro district on a small scale, the mining history of the 

area really began in the early days of the Spanish conquest. The
6 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS

oldest known mines are the San Eligio and Albarradon in the Sierra 

de La Caja, which were discovered about 1530 by Francisco de 

Urdinola, an officer in the army of Cortes. For his discoveries he was 

rewarded with a title and was known as the Marques de Aguayo. 

About 1548 the Keal de Minas de Mazapil was established.

The earliest ore bodies worked were mainly silver, but by the end 

of the 16th century gold had been discovered near the present site of 

Concepci6n del Oro. The rich copper ores of Araiizazu were left 

undisturbed, as the Spaniards were interested only in the precious 

metals.

During the 18th century some of the rich bonanzas were exhausted, 

and this, together with the War of Independence in the beginning of 

the 19th century, greatly reduced activity in the district. Kecovery 

began about 1889, when the Mazapil Copper Co. was organized by 

a group of English investors. Old mines were reopened, new mines 

were discovered, and construction of the Coahuila and Zacatecas 

Kailroad made the region easily accessible for the first time. By 

1906, monthly production from Aranzazu amounted to 7,000 metric 

tons of ore averaging 5 percent copper. A number of smaller companies were also active in this period, and in 1915 the Compania 

Minera de Penoles, S. A., a subsidiary of the American Metals Co., 

undertook exploitation of the deposits of silver, lead, and zinc in the 

Providencia area.

During the Mexican Kevolution, activity in the district was again 

greably reduced and real prosperity returned only with the advent of 

World War II. This prosperity has continued, and although the rich 

oxide ores were exhausted long ago, many sulfide bodies remain, and 

new discoveries are still being made.

EARLIER WORK IN THE AREA

The first reference to the district to be found in the literature is a 

description by Joseph Burkart (1836) of a visit to Mazapil, which is 

believed to have been made about 1830. He describes briefly an 

iron deposit and some of the metalliferous deposits then being exploited to the northeast of Mazapil.

The first detailed geologic work in the area was done by Carlos 

Burckhardt, who mapped parts of the Sierras de La Caja, Concepcion 

del Oro, and Santa Kosa. The results of his excellent studies were 

published in 1906 as excursion guides for the 10th session of the 

International Geological Congress (Burckhardt, 1906a, 1906b, 1906c). 

Although earlier workers refer to the presence of fetid and oolitic 

rocks in the area, Burckhardt (1907) was apparently the first to 

recognize the phosphorites, and he described them briefly. Additional material 011 the district is included in his volume on the Mexican 

Mesozoic (1930).
GEOLOGY, PHOSPHATE DEPOSITS, CONCEPCION DEL ORO DIST.; MEX. 7

In the late 1930's, a phosphate sampling program was undertaken 

in the area by Ings. Jesus J. Falomir and German Garcia Lozano on 

behalf of the Mexican Government. Although their investigation 

was rather limited, it produced additional information on the extent 

and grade of the deposits (Falomir and Garcia, 1939).

In the course of the past 70 years many papers have appeared 

describing individual mines or camps, but these have contributed 

little to a knowledge of the regional geology.

FIELDWORK AND ACKNOWLEDGMENTS

This study is one of the projects carried on in Mexico under the 

joint auspices of the United States Geological Survey and the Mexican 

Institute Xacional para la Iiivestigacion de Recursos Minerales. The 

project was originally suggested by Jenaro Gonzalez Reyna, chief 

geologist of the Institute. It was carried on under the overall supervision of Carl Fries, Jr., chief of the Geological Survey party in 

Mexico, in close cooperation with Gonzalez Reyna. It was part of 

the geologic program in the American Republics sponsored at its 

inception, 011 the part of the United States, by the Interdepartmental 

Committee on Scientific and Cultural Cooperation of the Department 

of State, but in November 1951 it passed to the Technical Cooperation 

Administration and in 1953 to the Foreign Operations Administration.

The fieldwork was carried on intermittently between December 

1949 and July 1952, and only the senior author was engaged continuously on the project. Field mapping was done on aerial photographs taken by the Comision Cartografica Militar of the Secretaria 

de la Defensa Nacional; these photographs have an average scale of 

1:20,000. The Sierra de Canutillo, which lies along the northern 

border of the area, was mapped by Ulloa. The Sierra de La Caja 

was mapped by Rogers and Tavera, and its southeastern extension, 

known as the Sierra de Concepcion del Oro, was mapped by Rogers. 

The Sierra de Santa Rosa was mapped by Rogers, Tavera, and 

Ulloa, and its southeastern extension, the Sierra de San Jose, was 

mapped by Rogers. The Sierra de Santa Rita and the basalt flows 

to the northeast of Terminal were mapped by de Cserna. A considerable amount of geologic reconnaissance in areas contiguous to 

the Concepcion del Oro district was carried on during the same period.

The fieldwork was greatly aided by several residents of the area. 

Among these were Ing. Leonides Gonzalez of the Mazapil Copper Co., 

Walter H. Triplett and Charles Y. Manderfield of the Compania 

Minera de Penoles, and John G. Barry, a consulting geologist with 

long experience in the district. The owners of the Santa Rosa mine, 

the Nochebuena mine, and the guayule rubber factory in Cedros 

hospitably provided living quarters during some parts of the work. 

Antonio Pardo and Daniel Rodriguez were loyal and capable helpers
8 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS

in the field, and Adolfo Iiiterial M. was helpful and cooperative in 

many ways. Many other residents of the district were most generous 

in their assistance.

Much assistance has been given by colleagues of the United States 

Geological Survey. The triangulation necessary for the compilation 

of a planimetric map was begun by Kenneth Segerstrom of the Geological Survey and was later finished by Eugenio Tavera, Salvador 

Ulloa, and Pedro Keyes Soto of the Institute Nacional para la 

Investigacion de Recursos Minerales. R. W. Imlay visited the area 

briefly in March 1952 and studied the greater part of our invertebrate 

fossil collections. In June 1952 the party was visited by Carl Fries, 

Jr., and V. E. McKelvey, who offered many valuable suggestions. 

C. S. Ross, James Owens, Charles Milton, Norman Herz, and Richard 

Kellagher examined many of our thin sections and made helpful 

comments. Ruth Todd examined shale samples for microfossils. 

Specimens of fossil wood were identified by R. W. Brown.

Valuable assistance was also given by these persons: D. H. Dunkle 

of the United States National Museum examined our vertebrate 

material, S. A. Northrop of the University of New Mexico identified 

one specimen of fossil wood, a number of invertebrate fossils that 

were not studied by Imlay were identified by the late F. K. G. Mullerried of the Institute de Geologia and by de Cserna, and Eduardo 

Schmitter of the Institute de Geologia kindly assisted us in the solution of some petrographic problems.

SEDIMENTARY ROCKS

The sedimentary rocks exposed in the region are mainly offshore 

marine f acies of Late Jurassic and Cretaceous age which were deposited 

in the Mexican geosyncline. The only younger sediments found in 

significant quantities are a few small masses of Tertiary continental 

rocks, consisting of conglomerate, arkose, siltstone, and tuff, and a 

thin alluvial mantle covering the valleys and lower mountain slopes. 

(See pi. 1.)

All but the uppermost part of the stratigraphic column was carefully worked out by Burckhardt, mainly based on ammonites, and 

is clearly described in his early reports on the area. Although the 

authors have adopted the nomenclature developed by Imlay and other 

geologists of the University of Michigan, who have carried on stratigraphic studies and reconnaissance mapping in a vast area lying to 

the north and northwest of the Concepcion del Oro district, the work 

of Burckhardt was useful in many ways. A correlation between the 

present writers' stratigraphic units and those of Burckhardt was 

easily made and is shown in figure 3, which also gives the approximate thickness of the formations.
GEOLOGY, PHOSPHATE DEPOSITS, CONCEPCION DEL ORO DIST., MEX. 9

AGE UNDETERMINED 

Green sandstone with brownish and blackish shale. 

No fossils

UPPER CRETACEOUS 

Platy limestone and shale, alternating with bluish 

limestone. Some sandy beds. Many Inoceramus, 

Twrilites, and Ammonites

MIDDLE CRETACEOUS 

Grayish laminated limestone with bedded and lenticular 

black chert. Many uncoiled ammonites

LOWER CRETACEOUS 

Alternation of yellow marl and marly limestone with 

grayish, bluish, or slightly brownish limestone. 

Parahoplites abundant

Grayish limestone with interbedded irregular chert. 

Few identifiable fossils

Yellowish marl alternating with grayish or brownish 

limestone. Holcostephanus abundant

UPPER JURASSIC 

Whitish marly limestone with bedded black chert

Grayish phosphatic limestone 

Reddish phosphatic limestone

Claystone with Waagenia 

Black compact limestone with Haploceras fialar 

Brownish laminated limestone with Aucella

Claystone and shale with Idoceras

 Massive grayish limestone with Nerinea

QUATERNARY

TERTIARY

CRETACEOUS UPPER 

CRETACEOUS LOWER 

JURASSIC UPPER 

Alluvium 0-30 m

Mazapil conglomerate 200 + m

Parras shale 1300 -f- m

Caracol formation 800 + m

Indidura formation 180 ± m

Cuesta del Cura limestone 280 ± m

La Pefia formation 140 ± m

Cupido limestone 300 ± m

Taraises formation 140 ±m

Unit D

UnitC

Unit B

Unit A

Zuloaga limestone 400+m

FintiEE 3. Correlation of Burckhardt's stratigraphic units and those of the present writers.

Mexican and European geologists working in Mexico have generally 

separated the Cretaceous into Lower, Middle, and Upper divisions, 

whereas geologists from the United States separate it into two parts. 

Although bhis twofold division is still not universally accepted in 

Mexico, it has been adopted in this report as the more convenient usage.

According to Imlay,2 the reasons for rejecting the usage of Middle 

Cretaceous in Mexico are that it does not reflect a natural lithologic 

division, it does not have any particular value in mapping or in dis2 Written, communication.
10 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS

cussions of geology, it does not correspond exactly in time with the 

Middle Cretaceous in Europe, and it has not been generally used even 

in Europe. The beds in Mexico that Burckhardt (1930, tables 15 

and 16, p. 210) called Middle Cretaceous are correlated on the basis 

of ammonites with the middle and upper Albian and the lower Cenomanian of Europe. The Middle Cretaceous in Europe has been 

employed generally for the Albian and Cenomaiiian stages, although 

some European geologists have included the Aptian, some have excluded the Cenomanian, and some have included even the Turonian. 

The controversy was settled as far as most geologists were concerned

FIOURE 4. Correlation of Jurassic and Cretaceous formations of north-central and eastern Mexico.
GEOLOGY, PHOSPHATE DEPOSITS, CONCEPCION DEL ORO DIST., MEX. 11

when the International Congress at Zurich in 1885 divided the Cretaceous into Lower and Upper subsystems and placed the Gault (middle 

and upper Albian) in the Lower Cretaceous. It was chance that both 

Carlos Burckhardt and Emil Bose (1923a, 1923b), who contributed 

greatly to the knowledge of the Cretaceous of Mexico, were educated 

under paleontologists who employed a threefold subdivision of the 

Cretaceous.

Figure 4 shows the correlation between the stratigraphic section in 

the Concepci6n del Oro district and sections in eastern and northern 

Mexico. Additional correlation charts are available in the writings 

of Imlay. Although broad stratigraphic studies have been carried 

on in many parts of the country, the matter of correlation is a difficult 

and continuing problem. As stratigraphic knowledge increases, adjustments will undoubtedly be made in correlations, and unnecessary 

and duplicating formational names will be discarded.

BASEMENT ROCKS

Although the Zuloaga limestone is the oldest formation exposed 

within the mapped area, the underlying rocks must come close to the 

surface at some points. In two deeply eroded mountain ranges 

a few kilometers to the west, known as the Sierras de Candelaria and 

El Borrego, the Zuloaga limestone rests unconformably upon the 

Huizachal formation, a series of predominantly reddish siltstone, sandstone, and conglomerate beds that lack identifiable fossils but may 

be late Callovian or early Oxfordian in age, or possibly both. The 

redbeds have remarkable differences in thickness, probably owing 

mainly to erosion, and farther to the northwest, in the vicinity of 

Caopas, they appear to be lacking over a fairly extensive area in 

which the Zuloaga limestone rests directly upon older phyllitic and 

schistose rocks of undetermined age.

Kedbeds have been reported by Imlay (1938b, p. 1659) as underlying the Zuloaga limestone in an area 40 kilometers to the east 

of Concepci6n del Oro. A section approximately 6 meters thick is 

exposed along an anticlinal axis in the mountains lying immediately 

to the west of La Ventura, Coahuila.

JURASSIC SYSTEM

Most of the index fossils that were used by Burckhardt in dating 

the Upper Jurassic sedimentary rocks in Mexico were previously 

known in Europe or in South America. His ideas concerning these 

fossils were applied to his Mexican studies without a thought for 

possible discrepancies in age in such widely separated regions. As 

a result, the work by later paleontologists has slightly modified the 

findings of Burckhardt, as shown in figure 5, which, with some 

simplifications, is adapted from Mullerried (1951).

368566 56 2
12 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS

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GEOLOGY, PHOSPHATE DEPOSITS, CONCEPCION DEL ORO DIST., MEX. 13

ZULOAGA LIMESTONE

The Zuloaga limestone was defined by Imlay (1938b, p. 1657), who 

designated the Sierra de Sombreretillo as the type locality. About 

550 meters of this limestone formation is exposed in that area, and 

about 400 meters is exposed in the Concepcion del Oro district. The 

unit apparently thins to the west, for the complete section above the 

redbeds in the Sierra de Candelaria is only 275 meters, and it may 

thin to the east as well, for in the vicinity of La Ventura the complete

FIGURE 6. Flat-lying Zuloaga limestone close to an anticlinal axis in 

the western part of the Sierra de La Caja. The small cave is formed 

in a calcareous siltstone layer.

section above the redbeds is only about 365 meters. Within the 

mapped area the Zuloaga limestone forms the cores of the anticlinal 

ranges and composes most of the high central ridges and mountain 

summits (fig. 6).

The upper part of this formation is predominantly a massive darkgray limestone, whose beds range generally from 0.3 to 3 meters in 

thickness; whereas the middle and lower parts contain many light- to 

medium-gray beds, which may be reddish to yellowish weathering and 

in many places show incipient brecciation. In general, the middle and
14 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS

lower parts are thinner bedded than the upper limestone and contain 

several argillaceous beds. The latter are only rarely exposed, but 

in many places their contacts with the adjacent limestone are characterized by a relatively dense growth of vegetation and the beds are 

thus easily traced. These argillaceous beds appear as sharply defined 

lines on the aerial photographs and may be utilized in many places 

as structural guides in the field. They are as much as 4 meters thick 

and consist generally of a brownish poorly indurated calcareous siltstone, which may be fossiliferous. Gray to brownish shales are 

found in some places, and in several localities the lower part of the 

formation was found to contain platy dark-gray limestone with 

intercalated lenticular black chert. Scattered relatively small nodules 

of gray to black chert are found in many beds throughout the formation, but they are quantitatively unimportant. Some limestone beds 

contain a host of coral fragments, which are poorly preserved and can 

rarely be identified. They are never found in place but have clearly 

been abraded and transported. Although the coral-bearing layers 

are not confined to a particular horizon, they have not been observed 

within a hundred meters of the La Caja contact. The massive 

limestone composing the upper part of the formation is characterized 

by many stylolitic partings of relatively large amplitude.

In a few localities the Zuloaga limestone contains small areas of 

dark-reddish-gray to brownish finely crystalline dolomite. The 

dolomitization may follow individual beds but it is commonly irregular 

and clearly seems to be a result of secondary alteration, probably by 

hydrothermal solutions.

It is difficult to obtain fossils from the limestone, but they are 

easily obtained from a few argillaceous beds in the middle and lower 

parts of the formation. With the exception of Thamnasteria and 

Nerinea, those listed below were collected from the calcareous siltstone.

Thamnasteria imlayi Wells

Astarte aff. A. smackoverensis Imlay

Astarte sp.

Astarte (Coelastarte) cf. A. ratheri P. de Loriol

Gryphaea sp. juv.

Trigoniat sp.

Nerinea sp.

Hatpagodes sp.

Arcid pelecypod

Terebratulid brachiopod

The Zuloaga limestone is apparently overlain conformably by 

units A, B, C, and D of Oxfordian, Kimmeridgian, and Portlandian 

ages of the La Caja formation, and on the basis of its stratigraphic 

position it has been assigned to the upper Oxfordian. Although it 

is lacking in characteristic fossils thai mighl allow it to be dated 

more accurately, it has a faunal assemblage closely resembling those
GEOLOGY, PHOSPHATE DEPOSITS, CONCEPCION DEL ORO DIST., MEX. 15

of the upper Oxfordian units of other parts of the Gulf region (Imlay, 

1943b, p. 1446-1450).

The mollusks and coral fragments hi the Zuloaga limestone indicate 

deposition in an epineritic environment, and the corals are indicative 

of warm waters. According to Imlay (1943b, p. 1484), "the uniform 

appearance of this unit throughout most, if not all, of the Mexican 

geosyncline indicates a uniform climate, few rivers emptying into 

the sea, and few highland areas."

LA CAJA FORMATION

The La Caja formation is composed mainly of calcareous siltstone, 

but it contains other rock types, including the phosphorite beds that 

were studied in detail in this examination.

The type locality of the formation has been designated by Imlay 

(1938b, p. 1659) as the Vereda del Quemado on the southern flank of 

the Sierra de La Caja. That section was described by Burckhardt 

(1906b, and 1930, fig. 14) and was visited and restudied by Imlay.

The formation occurs in many widely separated areas of Mexico 

and will probably be found throughout much of the central part of 

the Mexican geosyncline. It has been found in the Sierrita del Chivo 

of southeastern Durango; near Simon and in the Sierra de 

Ramirez, which lie to the west of Concepeion del Oro in the DurangoZacatecas border area; in the Sierra Madre Oriental west of Linares, 

Nuevo Leon; in the vicinity of Ciudad Victoria, Tamauiipas; and in 

the Sierra de Catorce of northern San Luis Potosi. It is the offshore 

equivalent of the La Casita formation, into which it is gradational in 

the area to the north of Concepcion del Oro. Much of the available 

information concerning these formations has been summarized by 

Ruiz Elizondo (1950), but many other data, obtained largely by 

petroleum geologists, have not been published. Little or nothing is 

known of the sedimentary rocks of Kimmeridgian and Portlandian 

ages in the several parts of the geosyncline that have not received 

even reconnaissance studies.

Within the area mapped the La Caja formation comprises a series 

of predominantly thin- to medium-bedded rocks ranging in composition from argillaceous limestone to shale but consisting mainly of 

siltstone; many chert layers occur in its middle and upper parts. The 

formation is sharply set off from the underlying Zuloaga limestone 

and the limestone beds of the overlying Taraises formation.

The La Caja formation is exposed generally hi two narrow parallel 

belts flanking the Zuloaga limestone of the anticlinal cores, and, being 

easily eroded, it is characterized by the formation of saddles and 

strike valleys. It weathers rapidly, and exposures are generally poor. 

In most places the formation supports a scantier vegetation than the
16 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS

adjacent limestone and appears as relatively barren strips, which 

may generally be identified from great distances (figs. 7 and 8.)-

The La Caja formation has been measured in many places, although 

such measurements are difficult to make, owing partly to a lack of 

exposures and partly to the incompetent nature of the rocks, which 

in many places are highly squeezed and contorted. It has a thickness 

ranging from 40 to 150 meters. In general the thickest sections are 

in the Sierra de Santa Rosa, from which the formation thins gradually

FIGURE 7. View of the north flank of the Sierra de Santa Rita southeast of San Francisco. The barren 

strip is underlain by the La Caja formation (Juc) and the ridge above is composed of Zuloaga limestone 

(Juz). White areas are caliche.

in all directions. In a northerly direction the formation averages 

about 80 meters in thickness in the Sierra de La Caja, 50 meters in 

the Sierra de Caiiutillo, and between 40 and 90 meters in the Sierras 

do Zuloaga and Sombreretillo. In the Sierra de Santa Rita to the 

south it has an average thickness of about 65 meters.

In his studies of the Sierras de Santa Rosa and La Caja, Burckhardt (1906a, 1906b, 1906c) divided the formation into seven units, 

which are based primarily on faunal divisions, whereas the authors 

have established four lithologic divisions based upon mappability. 

Although these divisions are in this report referred to as units A, B, 

C, and D, it is believed that they may eventually be designated as 

members. A correlation between these units and those described by 

Burckhardt is given in figure 3.
GEOLOGY, PHOSPHATE DEPOSITS, CONCEPCION DEL ORO DIST., MEX. 17

UNIT A

Unit A has a thickness ranging from 12 to 44 meters, and, like the 

formation as a whole, it reaches its maximum thickness in the Sierra 

de Santa Rosa.

The lowermost beds, in at least part of the district, are Oxfordian 

in age. They consist of a thin section of gray to brownish poorly 

indurated siltstones overlain by more resistant thin- to mediumFIGURE 8. View of a typical contact between the La Caja (left) 

andTaraises (right) formations. The La Caja formation is characterized by poor exposures.

bedded siltstones and cherts. As the base of the La Caja formation 

is generally concealed by talus or alluvium, these rocks are rarely exposed. 

In the sections measured, however, they ranged from less than 1 meter 

to about 8.5 meters in thickness. Near the top of the cherty rocks 

is a 10-centimeter bed of lustrous black crystalline limestone, which 

locally is highly bituminous. This bed is apparently persistent over 

a wide area, and Imlay has suggested that it may represent extremely 

slow deposition, perhaps being equivalent stratigraphically to the 

gypsiferous beds of the Olvido formation. These beds are moderately
18 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS

fossiliferous in places, the lower siltstone beds yielding several species 

of small pelecypods and the upper section containing Trigonia aff. 

T. hudlestoni Lycett.

The remainder of the unit is of early Kimmeridgian age. It 

consists mainly of thin-bedded brownish to grayish-brown poorly 

lithified calcareous siltstone containing numerous large spherical to 

dis-'oidal limestone concretions. The concretions are as much as 

1 meter in longest dimension and are composed of dark-gray to black, 

hard to brittle limestone. They may be fossiliferous, and some have 

furnished remarkably well preserved specimens of Idocems. One concretion has yielded a fragmentary fossil that has been tentatively 

identified by S. A. Northrop of the University of New Mexico as 

fossil wood. It is a segment with an indicated diameter of about 10 

centimeters, and its surface is covered with gypsum crystals. Although similar concretions may be found in other parts of the formation, they are abundant only in this unit. The siltstone generally 

contains a few beds of grayish to brownish often slightly reddish 

weathering laminated argillaceous limestone, which emits a fetid 

odor when freshly broken. The siltstone is also characterized by 

thin partings of a friable rust-brown claystone. Wherever exposed, 

this rock is broken into innumerable small fragments with smooth 

curving surfaces and a slightly waxy luster. A mineralogical examination shows it composed mainly of mica and montmorillonite, and 

some quartz and calcite and small reddish-brown nodular masses 

containing manganese and iron. It is similar in appearance to some 

bentonitic shales, but C. S. Ross of the Geological Survey reports 

that it is not of volcanic origin.

The siltstone is thin- to medium-bedded and of varied shades of 

medium gray and brown. It ranges from soft and crumbly to medium 

hard and is strongly calcareous, as shown by chemical analysis (table 

1, sample 6). It is similar in composition to the siltstone of the 

middle and upper sections of the formation.

UNIT B

Unit B has a thickness ranging from 20 to 65 meters and averaging 

about 35 meters. Like the underlying unit A, it reaches its maximum thickness in the Sierra de Santa Rosa. It is middle and late 

Kimmeridgian in age, and over most of the area mapped it contains 

a diversity of types of rock.

The lowermost part of the unit is composed of a thin series of 

brownish to grayish-brown argillaceous limestone beds of a thickness 

of 1.5 to 4.0 meters. The limestone is medium to thick bedded and 

medium hard, generally offering better exposures than the adjacent 

siltstones. Some beds are only moderately fossiliferous, whereas 

others are composed in large part of ammonite remains. The fossil
GEOLOGY, PHOSPHATE DEPOSITS, CONCEPCION DEL ORO DIST., MEX. 19

material is largely a single ammonite species, Glochiceras fialar (Oppel), 

although Aucella, a pelecypod, was found by Burckhardt in the 

lower part of the section. The limestone emits a strongly fetid odor 

when freshly broken and is described by Burckhardt as marking the 

base of the phosphorite. Sampling, however, has shown the rock to 

be only feebly phosphatic, having approximately the same tenor 

as the siltstone lying above and below it. The limestone may be 

interbedded with calcareous siltstone and gray to brown friable claystone, and in many places it contains lenticular layers of a granular 

crumbly rock consisting mainly of coarsely crystalline calcite in an 

argillaceous groundmass. Also present in places are lenses of coarsely 

fibrous fetid calcite, whose crystals lie normal to the bedding. Both 

types of calcite are found in other parts of the formation, but they 

occur most commonly at this horizon.

TABLE 1. Chemical analyses of rock samples from the La Caja formation, Puerto 

Blanco, Sierra de Santa Rosa

[Analyses made in laboratories of the Institute de Geologia by Albcrto Obregon, reported Apr. 22, 19531

S102- - - --_-_

A12O3 * -

FejOs - -_ 

FeO__ ______ 

MnO___ ___ ________________________

MgO. _ _ __ _

CaO-_____, __________ _______ _

Na.0 ______________________ ________

K.O. _ _. _______

TiO2 -

P205 . 

C02 _____________________ __________

H2O-__ _________________________________

H2 0+__ _ __ .

Sample no.

1

10581

8.40 

.70 

1.15 

.11 

.02 

.25 

49.21 

.21 

.11 

.20 

.58 

37.95 

.14 

.89

2

10582

39.44 

1.01 

.86 

.09 

.01 

.08 

31.48 

.29 

.20 

.15 

.65 

23.73 

.17 

1.74

3

10583

85.40 

2.06 

1.43 

.14 

.01 

.12 

4.56 

.25 

.35 

.15 

1.62 

2.06 

.10 

1.80

4

10584

63.37 

2.36 

1.13 

.13 

.03 

.54 

17.22 

.29 

.10 

.11 

.18 

13.35

1.45

5

10586

67.86 

2.12 

1.43 

.13 

.02 

.41 

14.04 

.39 

.18 

.24 

.29 

10.47 

.22 

2.11

6

10589

65.23 

3.72 

2.02 

.19 

.01 

1.12 

13.20 

1.03 

.37 

.36 

.48 

9.30 

.16 

2.44

1. Silty limestone from upper part of unit D, from bed 2.9 meters below contact with the Taraises formation.

2. Highly calcareous siltstone from basal part of unit D, from bed 3.5 meters above phosphatic member.

3. Calcareous chert from upper part of unit B, from bed 2.1 meters below phosphatic member.

4. Hard calcareous siltstone from central part of unit B, from bed 14 meters below phosphatic member.

5. Soft calcareous siltstone from lower part of unit B, from bed 26.2 meters below phosphatic Hicmber.

6. Calcareous siltstone from lower part of unit A, from bed 24 meters below unit B.

The argillaceous limestone is overlain by a section of thin-bedded 

gray to brown calcareous siltstone (table 1, sample 5) 5 to 20 meters 

thick. This rock is rather similar lithologically to the underlying 

member, although it contains relatively few concretions and a smaller 

number of shaly partings. Since much of the siltstone is soft to 

crumbly, exposures in most places are poor. This material is succeeded rather abruptly by a hard medium- to thick-bedded mediumbrown calcareous siltstone, which breaks with a blocky subconchoidal 

fracture. It is similar in composition to the rest of the formation 

(table 1, sample 4) but has undergone a higher degree of lithification.
20 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS

The hard brown siltstone grades upward into a heterogeneous series 

of rocks containing many cherty layers. The chert and calcareous 

chert (table 1, sample 3) are thin-bedded to massive dark brownishblack to black hard to brittle rocks, which crop out in many localities 

where the rest of the formation is concealed. Although it is easy to 

obtain an exaggerated impression of their abundance, they actually 

compose only 25 to 30 percent of the member. The cherty rocks are 

interbedded with cherty limestone, calcareous siltstone, phosphorite, 

and gray to brown friable claystone, and they generally contain many 

small limestone lenses and concretions. Near the top of the series 

these lenses are composed of a hard dark-gray to reddish-gray finely 

crystalline phosphatic limestone, which is characterized by a smooth 

light- to medium-gray weathered surface. At a lower horizon the 

cherty rocks generally contain several yellow- or brownish-weathering 

lenses of hard to brittle finely crystalline dark-gray to brownish limestone, which is largely nonphosphatic. Unlike the overlying lenses, 

these are generally unsymmetrical and may be characterized by a 

pronounced irregularity of form. Large concretions similar to those 

of the underlying unit are also rather common at this horizon. Although the cherty rocks contain relatively little phosphate, the interbedded siltstone and limestone maj be moderately to strongly phosphatic, and a few cherty limestone layers contain 10 to 15 percent of 

phosphorus pentoxide. The phosphorite beds are thin, and many 

are highly conglomeratic, being composed of fragments of black phosphatic limestone in a matrix consisting mainly of coarsely crystalline 

calcite and massive apatite.

Unit B, as described above, is thickest in the Sierra de Santa Rosa. 

Outward from that area the cherty fades tends to disappear gradually, 

and the member grades into a relatively homogeneous series of calcareous siltstone beds similar to those of the underlying unit A. 

The fossiliferous argillaceous limestone persists at the base of the 

member, however, and the limestone lenses characteristic of the upper 

part of the member are also found over a wide area. The cherty facies 

disappears within a relatively short distance in an easterly direction 

and is almost completely lacking in the Sierra de Conception del Oro. 

To the north and south it diminishes more gradually, and to the west 

it has been traced as far as Pico de Teyra, where it is fairly thick. 

Pico de Teyra lies approximately 40 kilometers to the west of the area

mapped.

UNIT c

Unit C contains the highest concentration of phosphatic material, 

the grade of the rock generally decreasing in inverse proportion to 

thickness of the member. Where it is thick the phosphate is dispersed 

and beds of commercial grade are lacking. It ranges from early to 

late Portlandian age.
GEOLOGY, PHOSPHATE DEPOSITS, CONCEPCION DEL ORO DIST., MEX. 21

In the Sierra de Santa Rosa the unit ranges from 1.35 to 3.40 meters 

in thickness and averages about 2.20 meters. It contains three 

general lithologic types, which are only briefly described here but are 

discussed more fully in a later section of the report. The lower beds 

are composed mainly of hard to brittle medium- to dark-gray phosphatic calcareous chert and cherty phosphorite, while the overlying 

beds are medium-hard reddish-gray calcareous phosphorite. Both 

sections contain numerous lenses of gray to reddish-gray finely crystalline phosphatic limestone. In some localities these lenses change into 

fairly persistent beds, which may be characterized by'pinching and 

swelling. All the rocks described above give off a strongly fetid odor 

when freshly broken. One of the phosphorite beds has yielded fossils 

which have been examined by D. H. Dimkle of the United States 

National Museum and identified as reptile bones. They closely 

resemble various reptilian ribs of Jurassic age but cannot be classified 

even as to order. The structure of the bones, however, suggests a 

terrestrial rather than a marine reptile.

Outward from the Sierra de Santa Rosa the phosphatic unit thickens 

within a short distance, reaching a maximum of about 20 meters, and 

it grades into a series of thin- to medium-bedded phosphatic siltstone 

and phosphatic calcareous chert beds, having only a few scattered 

layers that can be classed as phosphorite. The lenticular phosphatic 

limestone is abundant everywhere.

The phosphatic facies diminishes in an easterly direction, and in 

the Sierra de Conception del Oro it can hardly be distinguished from 

the underlying beds of unit B. It persists, however, and has been 

recognized in the vincinity of Iturbide, Nuevo Leon, a village in the 

Sierra Madre Oriental which is situated 150 kilometers to the east of 

the area mapped. It also diminishes to the north and south but is 

more persistent in a westerly direction.

UNIT D

Unit D is the uppermost member of the La Caja formation and has 

a thickness ranging from 2 to 50 meters. Like the other members, it 

reaches its maximum thickness in the Sierra de Santa Rosa. It is late 

Portlandian and Tithonian in age.

Most of the unit is thin- to medium-bedded medium hard brownish 

to brownish-gray calcareous siltstone (fig. 9), which contains many 

thin beds and lenses of black chert. The siltstone becomes increasingly calcareous toward the top, and the uppermost beds are mediumto thick-bedded silty limestone (table 1, samples 1 and 2). In some 

localities the member contains a few concretionary bodies and beds of 

reddish-gray laminated fetid argillaceous limestone. The siltstone 

has an ashy-white weathered surface, which is its most distinctive 

feature.
22 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS

In the Sierra de Santa Rosa the rocks immediately overlying the 

phosphorites are soft thin-bedded brownish to grayish-brown phosphatic calcareous siltstone beds with a total thickness of from 2 to 8 

meters. They contain some beds of calcareous phosphorite and fetid 

argillaceous limestone, but such layers are relatively rare. These beds 

grade upward almost imperceptibly into the harder, more calcareous

FIGURE 9. View showing typical lithology of unit D of the La 

Caja formation. The rock is calcareous siltstone with numerous 

layers of black chert. The picture was taken in the northwest 

corner of Valle de Santa Rosa.

siltstone described above. Beyond the Sierra de Santa Rosa this 

facies is only poorly developed, if present at all.

Unit D has little lithologic variation in the Concepcion del Oro 

district and has apparently been found over a wide area. The 

rocks that weather whitish have been noted in the Sierrita del Chivo, 

the Sierra de Ramirez, in the vicinity of La Ventura, and even farther 

to the east in the Sierra Madre Oriental. The chert is less persistent, 

however, and may not extend far beyond the boundaries of the area 

mapped.
GEOLOGY, PHOSPHATE DEPOSITS, CONCEPCION DEL ORO DIST., MEX. 23

PALEOECOLOGY

A list of representative fossils collected from the La Caja formation 

within the area mapped is given below.

Unit A

TarameUiceras sp.

Idoceras cf. 7. santarosanum Burckhardt 

Belemnopsis? sp.

Fossil wood(?). A segment with an indicated diameter of about 10 

centimeters; surface covered with gypsum crystals

Unit B

Hybonoticeras (Waagenia) sp. 

Torquatisphinctes sp. 

Idoceras cajense Burckhardt

? sp. juv.

Subdichotomoceras? sp. juv. 

Glochiceras fialar (Oppel) 

Isocyprina sp. 

Aucella sp. 

Vermetus (Burtinella) cornejoi Castillo and Aguilera

Unit G

Aspidoceras? sp. 

Mazapilites zitteli (Burckhardt) 

Epivirgatitesf cf. E. nikitini Michalski 

Virgatosphinctes cf. V. aguileri (Burckhardt) 

Aptychus sp. 

Lamellaptychus sp. 

Meleagrinella sp.

Cucullaea phosphoritica Burckhardt 

Lucina sp. 

Astarte sp. 

Aucella sp.

Isocyprina? cf. /. coteroi (Castillo and Aguilera) 

Grammatodon sp. 

Corbis sp. 

Anomia? sp.

Vermetus (Burtinella') cornejoi Castillo and Aguilera 

Cap-shaped gastropods 

Cerithioid gastropods 

Fish vertebrae

Reptile bone fragments suggesting terrestrial forms 

Coniferous wood fragment

Unit D

Pronicerasf sp. 

Substeuroceras sp.

Berriasella aff. B. calisto (D'Orbigny) 

Kossmatia sp.

Cylindroteuthis cf. C. clavicula Anderson 

Belemnite not determined 

Aucella cf. A. elderensis Anderson
24 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS

The predominance of ammonites in most of the La Caja formation 

suggests deposition in an infraneritic environment. Imlay (1943b, 

p. 1495) believes that the abundance of ammonites and relative 

scarcity of benthonic mollusks may be due to an excessively muddy 

environment, however, rather than to any great depth of water. He 

suggests that the large amount of shale and organic matter in the 

sedimentary rocks of Kimmeridgian-Portlandian age of the Mexican 

geosyncline was the result of climatic changes in the bordering land 

area. The climate may have been less arid than during Oxfordian 

time.

The authors have found several lines of evidence in their studies of 

the La Caja formation which strongly indicate deposition in shallow 

waters, which during some periods were strongly agitated. These 

include the occurrence of conglomeratic layers in the phosphatic 

member and the upper part of unit B, and the occurrence of capshaped gastropods in the phosphatic member. These small gastropods are abundant in some beds. The marked differences in thickness 

that characterize the various members of the formation may be due 

to diastems, which would also reflect temporary shallow-water conditions, or may be the result of deposition on an undulating surface 

which lay close to sea level and may actually have been emergent in 

places.

CRETACEOUS SYSTEM 

TARAISES FORMATION

The Taraises formation, which contains limestone and some subordinate siltstone, was defined by Imlay (1936, p. 1111) and is named 

for Canon de Taraises in the western part of the Sierra de Parras. It 

is Valanginian and Hauterivian in age. Within the area mapped it 

has an average thickness of 140 meters.

It appears to rest conformably upon the silty limestone with intercalated black chert that forms the upper part of the La Caja formation. The contact with the underlying rocks may represent a diastem, 

however, as no Berriasian rocks have been found. Valanginian 

fossils are present in the lowermost Taraises beds.

The formation consists of alternate thick-bedded to massive limestone and poorly lithified gray to yellowish or brownish siltstone. 

The limestone is generally light to medium gray and weathers to a 

characteristic shade of blue or bluish gray. In many places weathered 

surfaces also show yellowish discoloration. The limestone beds 

contain many rosettes of oxidized pyrite and a small proportion of 

nodular gray and black chert, which is sparsely distributed through 

the formation. The siltstone is highly calcareous and locally contains 

a small proportion of iioncalcareous friable brown claystone. Some 

beds are fossiliferous.
GEOLOGY, PHOSPHATE DEPOSITS, CONCEPCION DEL ORO DIST., MEX. 25

In general, the limestone beds have a thickness averaging about 30 

centimeters. In some localities, however, the formation contains 

a unit of thicker bedded limestone at or near the base. In this unit 

the beds may be as much as a meter in thickness, and the siltstone 

layers are largely or wholly lacking.

The fossils collected from the Taraises formation are 

Distoloceras aff. D. michaelis (Uhlig)

? sp.

Crioceras sp.

Olcostephanus aff. 0. raricostatus (Bose) 

aff. 0. atherstoni (Sharpe)

All are ammonites which, along with the lithology, may indicate 

deposition in an infraneritic environment.

CUPIDOILIMESTONE

The Cupido limestone was first described by Imlay (1937, p. 606) 

from exposures in the middle part of the Sierra de Parras. Paleontologic evidence is scant, but the stratigraphic position indicates a 

late Hauterivian and Barremian age.

The formation comprises about 300 meters of massive limestone and 

is one of the most important ridge formers in the region. The beds 

range generally from 0.25 to 1 meter in thickness, and the rock is a 

medium-gray limestone with a slight pinkish to reddish coloration on 

weathered surfaces. The formation contains many pyritic concretions 

and a large proportion of nodular to lenticular gray chert, which 

becomes increasingly abundant from the base upward. A few 

nodules contain various amounts of coarsely crystalline limestone. 

The limestone beds are characterized by well-developed stylolites of 

relatively large amplitude. Locally, a few thin layers of calcareous 

siltstone occur.

The Cupido limestone is almost completely lacking in fossils; the 

few ammonites found are too poorly preserved for generic identification. The lithologic character indicates normal limestone deposition 

in an infraneritic environment.

LA PENA FORMATION

The La Pena formation, which consists mainly of alternate limestone and siltstone beds, was defined by Imlay (1936, p. 1119), who 

designated the north flank of the Sierra de Taraises in the vicinity of 

Hacienda de La Pena as the type locality. The Sierra de Taraises is 

in the western part of the Sierra de Parras.

The formation is more easily eroded than the underlying Cupido 

limestone and the overlying Cuesta del Cura limestone, and it is 

commonly marked by a strong development of arroyos and saddles. 

It contains several types of rock, and the resulting differences in 

vegetation may be apparent on the aerial photographs as fairly well
26 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS

defined lines or bands. In the field the formation is generally easily 

identified at a distance by a brownish or reddish coloration, which 

offers a sharp contrast to the gray of the adjacent limestones. Within 

the area mapped it has an average thickness of about 140 meters.

The lower and middle parts of the formation consist of alternate 

massive limestone and poorly lithified siltstone beds. The limestone 

is light- to medium- and dark-gray, weathers reddish to yellowish, 

brownish, or olive green, and contains a considerable proportion of 

gray to black chert in irregular nodular masses and lenses. Some 

nodules contain carbonate, and where this has been leached the 

remaining chert has a spongy appearance. Pyrite concretions are 

common in the limestone. In general, the limestone is thicker bedded 

at the base than in the rest of the formation and contains fewer 

argillaceous beds. The siltstone is highly calcareous, is soft and 

crumbly to medium hard, and is yellowish to buff and gray. It contains a few partings of a fissile yellow green to rusty-brown shale. 

The siltstone is subordinate to the limestone and in most places is not 

exposed. The upper part of the formation contains limestone and 

siltstone similar to the underlying material, but it also includes a large 

proportion of thin-bedded to platy light-gray argillaceous Iimeston3, 

and occasional layers of light- to medium-gray medium- to thickbedded laminated limestone. The platy limestone and the thicker 

bedded laminated limestone contain thin lenses of black chert, giving 

the formation an appearance similar to the upper part of the La 

Caja formation. The sequence contains a few thin layers of fissile 

greenish to brownish shale.

The La Pena formation contains a few fragmentary external molds 

of ammonites, one of which was identified as Parahoplites sp. The 

stratigraphic position and fossil evidence indicate that the formation 

represents probably the entire Aptian. The lithologic character and 

ammonite fauna suggest deposition in moderately deep water. According to Imlay (1938b, p. 1690), the black chert represents a type of 

sedimentation which in northern Zacatecas continued almost without 

interruption until the end of the Albian.

CUESTA DEL CURA LIMESTONE

The Cuesta del Cura limestone was first described by Imlay (1936, p. 

1125) from exposures in the western part of the Sierra de Parras, where 

it lies between the Aurora limestone and the Indidura formation. 

It is a resistant formation and is inferior only to the Zuloaga and 

Cupido limestones as a ridge former. At the same time, it is highly 

incompetent and almost everywhere is characterized by extensive 

drag folding. It has an average thickness of 270 meters.

Within the area mapped the formation contains two well-defined 

units. The lower one is composed predominantly of a laminated wavy-
GEOLOGY, PHOSPHATE DEPOSITS, CONCEPCIOJXT DEL ORO DIST., MEX. 27

bedded medium- to dark-gray limestone with fairly common thin 

lenses of black chert, but it also contains many layers of a massive 

light- to medium-gray limestone (fig. 10). This unit has a thickness of 

50 to 60 meters and may be equivalent to the Aurora limestone of the 

Sierra de Parras, which is described by Imlay (1936, p. 1124) as a 

reef facies.

The upper unit of the formation is composed almost wholly of 

laminated wavy-bedded limestone and lenticular black chert, but it

FIGUEE 10. Cuesta del Cura limestone, showing wavy bedding 

and lenticular black chert.

also contains many partings of a platy argillaceous limestone, which 

may show reddish to violet tints when weathered. The beds range 

generally from 15 to 30 centimeters in thickness and are the only rocks 

in the stratigraphic section which have wavy bedding.

Fossils were observed in the upper part of the formation in many 

localities, but few were collected, as they are poorly preserved. They 

consist mainly of small uncoiled ammonites, with some high-spired 

gastropods.,

368566 56 3
28 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS

The ammonites were identified as follows

Hysteroceras? sp.

Crioceras? sp.

Ancyloceras cf. A. zacatecanum Burckhardt

On the basis of its stratigraphic position, in addition to the ammonites that were found in the formation, the Cuesta del Cura limestone probably ranges in age from early Alhian into the Cenomanian. 

The lithologic character and faunal evidence suggest deposition in 

waters of moderate depth in the infraiieritic zone.

INDIDURA FORMATION

Within the Concepci6n del Oro district the Indidura formation 

consists mainly of siltstone, shale, and platy or laminated limestone. 

The unit was first defined by Kelly (1936, p. 1028) from exposures in 

the Las Delicias region of Coahuila, where the name was applied to 

about 30 meters of shale, rubbly limestone, and platy limestone 

directly overlying the Aurora limestone and containing fossils of late 

Albian, Cenomanian, and Turonian age. It has been described by 

Imlay (1936, p. 1126) in the western part of the Sierra de Parras, 

where it lies between the Cuesta del Cura limestone and the Parras 

shale and has a total thickness of about 650 meters. Imlay (1938b, 

p. 1665) has also described it in the Melchor Ocampo area, where it 

overlies the Cuesta del Cura limestone and underlies the Caracol 

formation, as in the Conception del Oro district. It is clear that the 

sedimentary rocks that have been placed in the Indidura formation 

include not only several different facies but are not everywhere 

stratigraphically equivalent, and probably the nomenclature should 

be revised, as suggested by Imlay (1938b, p. 1692).

In the Conception del Oro district the Indidura formation is generally set off from the Cuesta del Cura limestone by an abrupt change 

from wavy-bedded limestone to thin-bedded highly argillaceous rocks. 

In some areas, however, the boundary may be represented by a transitional zone 20 to 30 meters thick. The formation appears manily 

along the lower mountain slopes or at the base of the ranges. Where 

it occurs at higher elevations, the exposures are generally better and 

are characterized by scanty vegetation, wilich may render the formation easily visible in the field and also on the aerial photographs.

The formation has an average thickness of about 180 meters, and, 

in the area mapped, it contains two distinct lithologic units. The 

lower one is mainly thin-bedded or platy argillaceous limestone and 

calcareous siltstone, having many layers of medium- to thick-bedded 

laminated dark-gray to brownish limestone (fig. 11). The platy 

argillaceous limestone is light gray and may weather to pinkish, red, 

or violet. It is commonly interbedded with a relatively soft, gray to 

yellow-gray or brownish siltstone, which may contain partings of a
GEOLOGY, PHOSPHATE DEPOSITS, CONCEPCION DEL ORO DIST., MEX. 29

fissile rusty-brown shale. The upper unit has an average thickness 

of 50 meters and is largely a homogeneous highly fissile dark-gray 

calcareous shale that weathers yellow. It is easily eroded and in 

many places is marked by the formation of shallow saddles.

FIGURE 11. View showing lower beds of tthe Indidura formation 

cropping out on the north flank of the Sierra de Santa Rosa. The 

Sierra de La Caja is in the background.

Fossils are limited mainly to the lower unit; the best specimens 

are external molds of bivalves from the beds of laminated dark-gray 

limestone. The following were identified 

Inoceramus cf. 7. labiatus Schlotheim 

Inuceramus sp.

Several species of foraminifera were found in the shale of the upper 

unit and were identified as

GumbeHna? sp. 

Gldbigerina sp. 

Globorotalia? sp. 

Globotruncana sp.

On the basis of fauna! evidence and stratigraphic position, the 

Indidura formation is probably Cenomanian and Turonian in age.
30 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS

The fauna and lithology indicate a shallowing of the sea after deposition 

of the Cuesta del Cur a limestone.

CARACOL FORMATION

The Caracol formation was named by Imlay (1937, p. 616) for 

exposures in the middle part of the Sierra de Parras. He described 

it as a series of devitrified tuff, shale, and limestone directly overlying 

the Indidura formation. It occupies the same stratigraphic position 

in the Concepcion del Oro district, and its boundary with the Indidura 

' formation is drawn at the lowest bed of arkosic sandstone.

FIGURE 12. View of the Caracol formation cropping out on the 

south flank of the Sierra de Canutillo, showing differential erosion 

between shale and resistant sandstone beds.

The formation is found only in the synclinal valleys separating the 

mountain ranges and has a minimum thickness of about 800 meters. 

It is mainly an alternation of shale and resistant sandy beds, the 

shale predominating. (See fig. 12.) The shale is dark gray to black, 

weathers yellowish or yellowish gray, and breaks characteristically 

into sharp splintery fragments. It is slightly to moderately calcareous 

and locally may contain some admixture of sandy material.
GEOLOGY, PHOSPHATE DEPOSITS, CONCEPCIOJXT DEL ORO DIST., MEX. 31

Although the hard beds have been described as tuff in the type 

locality (Imlay, 1937, p. 616), the specimens collected in the Concepcion del Oro district and examined petrographically by the authors 

can be more accurately described as calcareous arkosic sandstone, 

perhaps approaching graywacke in composition. This view is confirmed by C. S. Ross,3 of the Geological Survey, who reviewed several 

of the slides. The thin sections contain no material that is demonstrably volcanic in origin but are composed mainly of angular to 

moderately rounded quartz and feldspar grains embedded in a carbonate matrix. The feldspar exhibits considerable variation in the 

degree of alteration, and some grains appear to have been completely 

kaolinized. The sections also contain scattered rock fragments of 

quartzite, slate, or fine-grained schist, and small quantities of biotite, 

chlorite, sericite, magnetite, and limonite.

In the field the sandstones are hard thin-bedded to massive rocks, 

which on fresh surfaces range from gray to brown and greenish brown 

or green, but generally weather to a distinctive shade of brown.

The sandstone beds are more abundant in the lower part of the 

formation than toward the top and seem to be more numerous between the Sierras de Zuloaga and Santa Rosa than in the areas to the 

north and south. On the north flank of the Sierra de La Caja they 

compose about 50 percent of the lower part of the formation, and 

individual layers may be as much as 3 or 4 meters in thickness. In one 

area on the northeast flank of the Sierra de Concepcion del Oro the 

middle part of the Caracol formation contains a zone of lenticular 

dark-gray to blackish limestone, which weathers light gray. Some of 

the limestone is thin to medium bedded, with slightly wavy laminations, and some is massive and characterized by a rough, peculiarly 

pitted, weathered surface.

The first megafossils to be found in the Caracol formation were 

collected by Ulloa in the valley to the north of the Sierra de Canutillo, 

but they have proved to be of little stratigraphic value. They include 

fragmentary oysters and echinoids, and a few ammonites which probably belong to the family Peroniceratidae and suggest a Coniacian or 

Santonian age for the formation.

Shale samples yielded only a few specimens of Foraminifera. These 

were identified as

Globorotaliaf sp. 

Haplophrogmoidesf sp. 

Globigerina? sp. 

Gumbelina? sp.

Although the fossil evidence is inconclusive, the Caracol formation, 

according to Imlay (1944a, p. 1160), is probably of Coniacian and 

Santonian age and is correlative in its lowest part .with the upper

s Written communication.
32 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS

part of the Indidura formation near Parras, Coahuila. The lithology 

seems to indicate rapid deposition in shallow water, and the apparent 

scarcity of marine life may reffect unfavorable bottom conditions.

The Parras shale was described by Imlay in the western part of 

the Sierra de Parras, where it lies*between the Indidura and Difunta 

formations. In the Concepcion del Oro district it is the uppermost 

Cretaceous unit and is present only in the Bonanza Valley. The 

formation is difficult to measure, as the bedding is indeterminable in

FIGURE 13. View of the Parras shale near village of Bonanza. The shale is overlain by lake-bed deposits 

and conglomerates of probable Pleistocene age.

many places, but it appears to have a thickness of about 1,300 meters.

The boundary with the underlying Caracol formation is transitional and is drawn where the sandstone beds become relatively 

scarce. The formation is made up mainly of a splintery black calcareous shale, which may weather slightly yellowish (fig. 13). The 

sands tone layers occur in zones or groups sparsely distributed through 

the formation.

No megafossils were found in the formation, but shale samples 

yielded a few specimens of Foraminif era. The following w ere identified.

Bulimina? sp. 

Virgulina? sp. 

Globigerinaf sp.
GEOLOGY, PHOSPHATE DEPOSITS, CONCEPCION DEL ORO DIST.,, MEX. 33

The Foraminifera are all minute and apparently do not represent 

the type of planktonic population that is characteristic of the Indidura 

and Caracol formations. The apparent lack of abundant planktonic 

species would suggest that, although the shale was deposited under 

marine conditions, it was not in an area to which oceanic currents had 

access.

The Parras shale of the Concepcion del Oro area probably corresponds to only the upper part of this formation in the Parras Basin. 

It represents the same type of deposits as the Caracol, except that it 

contains less sandy material. It is probably largely Santonian in age.

TERTIARY SYSTEM

MAZAPIL CONGLOMERATE

Six small to moderately large areas within the Concepci6n de 

Oro district are underlain by a sequence of continental deposits that 

contain tuffaceous material and are of Tertiary age. Originally the 

beds may have formed a continuous mantle over the region, but they 

are now present only where they have been downfaultecl into the older 

rocks and have thus been preserved from erosion.

The largest area is that underlying the northern part of the Mazapil 

valley to the north of Mazapil and extending east to within about 1.5 

kilometers of Salaverna. Outcrops are relatively scarce except in 

the large reentrant on the south flank of the Sierra de La Caja, where 

several of the deeper arroyos expose most of the sequence. The 

authors refer to the unit as the Mazapil conglomerate, and this area 

is designated as the type locality. The rocks have a minimum thickness of 200 meters.

Along the northern side of the area, where the rocks are in fault 

contact with the older Mesozoic sedimentary rocks, the lowest exposed 

beds consist mainly of 3Tellowish to brownish loosely consolidated 

calcareous siltstone or impure sandstone, but include some layers of 

lenticular and concretionary limestone, which is generally medium 

dark to dark brownish-gray and is probably of fresh-water origin. 

One of the beds observed was highly conglomeratic.

Overlying the yellow sedimentary rocks and separated from them by 

an angular unconformity is a poorly sorted reddish conglomerate, 

which in the exposures along the Mazapil-Novillos trail has a thickness 

of about 40 meters. The rock fragments in the conglomerate range 

from pebble size to boulders but are generally coarse, the boulders 

having a maximum diameter of about 1 meter. The rock fragments 

are angular to moderately rounded and are mainly limestone, although 

in places the conglomerate contains platy fragments from the La Caja 

formation and fragments of sandstone and shale, which have apparently been derived from the Caracol formation or Parras shale. 

The matrix is generally a reddish material consisting mainly of calcite
34 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS

stained by hematite, although it may contain some sandy and argillaceous material, and close to the fault boundary it may contain 

secondary silica. On the south side of the area, and in the grabens, 

this conglomerate forms the lowest beds exposed, and in the area 

northwest of Mazapil it has been seen resting directly upon the Caracol 

formation.

The red conglomerate is overlain by a considerable thickness of 

loosely consolidated green arkose or arkosic sandstone, which consists 

mainly of rounded quartz and feldspar grains, andesitic rock fragments, and an interstitial green earthy mineral that has been identified 

as celadonite by C. S. Ross.4 The materials are essentially those of 

a reworked volcanic ash. Some of the rock is calcareous and in some 

places it contains a few thin conglomeratic layers in which rounded 

pebbles and cobbles of limestone are common. Crossbedding has 

been observed in several localities.

The green arkose is overlain by a varicolored heterogeneous sequence of sedimentary rocks which range from green or bluish green to 

yellow, reddish brown, and red, and include conglomerate, arkose, 

siltstone, mudstone, and tuff, all fairly calcareous. The arkosic rocks 

contain angular to rounded grains of quartz, feldspar, calcite, epidote, 

and magnetite in a predominantly carbonate matrix. The siltstone is 

generally reddish and probably much of it contains reworked highly 

altered volcanic material. The mudstone is gray to greenish and only 

a few centimeters in thickness. The tuff is predominantly reddish 

highly acid rock and is probably rhyolitic in composition.

The sedimentary rocks are gently folded, extensively faulted, and 

in one graben area are overlain by a rhyolite flow. All these criteria 

indicate a Tertiary age, and the rocks may well correlate with the 

red conglomerate at Guanajuato and similar deposits in various parts 

of central and southern Mexico, as well as with red conglomerates 

near the city of Zacatecas (Edwards, 1954) and in the Santa Maria 

del Oro district of northern Durango. The red conglomerate at 

Guanajuato has yielded a few vertebrate fossils which indicate a 

late Eocene or early Oligocene age (Fries, Hibbard, and Dunkle,

1955, p. 8-9).

QUATERNARY SYSTEM

LAKE BEDS AND CONGLOMERATE

In the Bonanza valley, near Bonanza, a few remnants of lake beds 

are overlain by conglomerate (fig. 13). The lake beds consist of 

poorly consolidated calcareous mud or silt about 4.5 meters thick. 

The conglomerate, which has a maximum thickness of about 14 

meters, is composed of strongly cemented pebbles and cobbles of 

limestone and igneous rocks and contains several layers of lake-bed

* Written communication-
GEOLOGY, PHOSPHATE DEPOSITS, CONCEPCION DEL ORO DIST., MEX. 35

material. These deposits dip slightly to the north but are otherwise 

undisturbed. They may be Pleistocene in age, and possibly they 

represent the same humid cycle of erosion as the Mayran formation, 

which was described by Imlay (1936, p. 1135) in the western part of 

the Sierra de Parras. Remnants of a strongly cemented conglomerate 

have been found in other parts of the area mapped, but it is impossible 

to prove whether they are all of one age.

ALLUVIUM

A thin mantle of alluvium covers the floors of the synclinal valleys 

within the area mapped. It consists of silt, sand, and gravel washed 

down from the adjacent mountain ranges. Most of the alluvium is 

moderately well bonded by an argillaceous lime cement.

IGNEOUS AND METAMORPHIC ROCKS

TERTIARY INTRUSIVE ROCKS

Many intrusive bodies, in the form of stocks, sills, dikes, and possibly 

one phacolitb, are found in the three southerly ranges of the Concepci6n del Oro district. In composition they range from rhyolite to 

andesite, and in size from sills to bodies having an area of 10 square 

kilometers or more. Many small bodies that were found in the field 

could not be mapped on the scale employed, and others may have 

been overlooked. The various lithologic types are described below 

in probable chronological order of intrusion.

GRANODIORITE ANT> DIOKITE

Granodiorite and diorite are the oldest igneous rocks to be found 

in the district and are quantitatively the most important. Occurrences are limited mainly to the Sierras de La Caja and Santa Rita, 

where these rocks form three major stocks, a possible phacolitb, and 

a large number of smaller bodies, which may be sills or dikes but are 

generally irregular in form.

CONCEPCION DEI ORO AND PROVIDENCIA STOCKS

The Concepci6n del Oro and Providencia stocks are found in the 

Sierra de Concepcidn del Oro, and, although they have invaded mainly 

the anticlinal core, they extend locally into the adjacent synclinal 

structures. At the surface, they are separated only by a thin septum 

of Zuloaga limestone, and they probably coalesce at a relatively 

shallow depth to form a single large intrusive body.

The fresh granodiorite is a gray to greenish-gray or slightly pinkish 

predominantly medium-grained rock, with fairly common small plagioclase phenocrysts whose maximum length is about 1 centimeter. 

These phenocrysts are subhedral and, along with some large- to 

medium-sized grains of quartz and mafic minerals, are in many places 

enclosed in a finer grained groundmass composed mainly of quartz,
36 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS

plagioclase, and ortlioclase. In some places the rock is nearly equigranular. On weathered surfaces it is generally reddish to brownish. 

The modal composition is shown in table 2.

TABLE 2. Modal composition of granodiorite 

[Very small amount indicated by letter x]

Composition of representative specimens

R-953

61.6

6.1

22.8

3.1

3.5

.2

R-984

61.6

6.4

8.1

9.4

3.6

.2

R-986

70.6

10.9

14.8

1.6

.3

.2

R-826

61.5

15.3

14.4

.4

.6

X

R-759

61.2

10.9

12.6

14.2

1.0

X

X

R-777

73.5

8.6

6.2

6.2

.5

2.3

2.6

Average composition of six thin sections

Plagioclase

65.0

Orthoclase

9.7

Quartz

13.0

Mafic 

minerals

10.1

Magnetiteilmenite

2.0

Other 

accessories

0.2

R-953 and R-984 are from the central part, and R-986 is from the border area of the Coiicepcion del Oro stock. 

R-826 and R-759 are from the central part, and R-777 is from the border area of the Providencia stock.

In general, these stocks are granodioritic in composition, although 

in some areas close to the borders they may grade to tonalite or 

diorite. Some of the rock may be more silicic, however, as suggested 

by Triplett (1952, p. 585), who reports that a specimen taken from 

the Providencia mine workings was identified as quartz monzonite 

by Frank F. Grout. The predominant mineral is andesine plagioclase, which has an average composition of An40 . Some of the phenocrysts are strongly zoned. Most of the simple twins observed are 

the Carlsbad type, but polysynthetic twinning is more common. 

Much of the plagioclase has been rather highly altered under hydrothermal conditions, and this makes accurate determination of the 

composition difficult. The potash feldspar is orthoclase, which is 

also extensively sericitized and kaoliiiized. In one thin section it 

contained a meager microperthitic intergrowth. The quartz content 

is much more variable than the feldspar content, and in the specimens 

studied it ranges from 6.2 to 22.9 percent and averages about 13.0 

percent. The commonest mafic minerals are biotite and pyroxene, 

and hornblende is relatively scarce. The pyroxene, which is mainly 

diopside, ranges from neutral to pale green and may be slightly 

pleochroic. The hornblende is relatively light and is generally 

pleochroic from yellow green to a medium dark green, although it 

may be brownish or bluish locally. It may be intergrown with
GEOLOGY, PHOSPHATE DEPOSITS, CONCEPCION DEL ORO DIST., MEX. 37

biotite, which, in part at least, seems to be an alteration product. 

Biotite is also closely associated with the pyroxene in many places. 

In the thin sections containing both pyroxene and hornblende, much 

of the hornblende seems to be a magmatic mineral formed later from 

the diopside. There is some chloritization and other alteration of 

the mafic minerals, but such changes are not extensive, and the 

minerals remain relatively fresh. The commonest accessory minerals 

are magnetite, ilmenite, sphene, apatite, and zircon. Epidote occurs 

in a few sections.

The granodiorite stocks contain a large amount of alaskite in dikes 

and small to moderately large bodies of different shapes. The dikes 

are found in various parts of the intrusive bodies, and, although they 

are fairly common, they are relatively small, usually measuring only 

a few centimeters in thickness. The larger bodies are generally found 

along or close to the borders of the stocks, either in the stocks or in 

the adjacent country rock. In most places the alaskite is light gray 

to white, but it may be yellow or buff. It is fine to medium grained, 

but predominantly fine grained, and in many places it has a granulitic 

texture. It is composed mainly of quartz and orthoclase and contains subordinate plagioclase, mostly as phenocrysts. The feldspars 

may be highly altered. The rock is conspicuously deficient in mafic 

minerals. Small amounts of biotite and muscovite, and a little 

magnetite, sphene, and zircon were found in the thin sections that 

were studied. The alaskite is clearly younger than the granodiorite 

and probably represents a late-stage acid differentiate.

No pegmatites or lamprophyres have been observed in association 

with the Concepci6n del Oro and Providencia stocks, or with any of 

the other granodiorite bodies in the area mapped.

The granodiorite contains many dark-gray dioritic inclusions, 

which are rounded to somewhat angular and are generally small, 

measuring less than 30 centimeters long. One thin section of this 

rock was found to consist mainly of andesine, with rather abundant 

biotite and some clinopyroxene and magnetite. Only a very little 

quartz and potash feldspar were present. According to Bergeat 

(1910, p. 20), these inclusions are not xenoliths but are segregations 

of the early-formed minerals and have a composition closely corresponding to that of the enclosing rock, except they are more basic. 

He reports that they contain relatively abundant magnetite, sphene, 

and apatite.

NOCHEBUENA STOCK

The Nochebuena stock is in the western part of the Sierra de La 

Caja and is elongated, measuring about 4.5 kilometers in length and 

about 1.5 kilometers in maximum width. Unlike other stocks, it 

has not invaded the anticlinal core but is found on the northern limb 

of a fold.
38 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS

The rock is predominantly medium grained and is medium gray to 

greenish gray or slightly pinkish. Although it contains feldspar 

phenocrysts, they are relatively small and scarce, and the porphyritic 

texture is not pronounced. The rock is diorite, as shown in table 3.

TABLE 3. Modal composition of representative specimens of diorite 

[Very small amount indicated by letter x]

R-652

78.5

3.4

5.2

6.9

3.1

.7

2.1

R-634

57.6

6.5

2.2

13.0

14.9

1.7

4.0

R-652 is from the interior, and R-634 is from the border area of the Nochebuena stock.

The plagioclase is andesine, having a composition of about An40 . 

Potash feldspar and quartz are present only in small quantities. In 

one thin section the orthoclase was found to be microperthitic. The 

hornblende seems to be mainly uralite formed by the alteration of a 

pale-green clinopyroxene.

In some places close to its margins, the diorite contains many small 

areas of recrystallized and silicated limestone, although nearly all are 

too small to be shown in plate 1.

Several smaller granodiorite and diorite bodies are found in the 

Sierra de La Caja, some in the vicinity of the large intrusive masses 

and some in the area between the Providencia and Nochebuena stocks. 

A few in the second area have invaded the breccia zone along the La 

Caja thrust. These rocks have not been studied in thin section, but 

megascopically they are similar to the large igneous masses. The 

number and wide distribution of the granodiorite intrusive bodies 

suggests that the greater part of the La Caja anticline may be underlain by a batholith.

SAN FELICIANO PHACOIITH(?)

The San Feliciano phacolith(?) is in the axial region of the Santa 

Rita anticline, and field evidence suggests that it is largely concordant 

with the overlying Zuloaga limestone. The authors suggest that it 

may have been intruded between the Zuloaga limestone and the 

underlying redbeds. It may be considerably larger, however, than 

is indicated by the exposures. Limestones to the southeast are partly 

altered for a distance of about 2 kilometers from the intrusive mass, 

possibly reflecting the presence of an underlying igneous mass. In 

this area the rocks have a strongly reddish coloration, owing to an 

abundance of finely disseminated hematite. It is also possible that
GEOLOGY, PHOSPHATE DEPOSITS, CONCEPCION DEL ORO DIST., MEX. 39

San Feliciano valley, a large reentrant, is underlain by granodiorite, 

and granodiorite has been reported as underlying the synclinal valley 

in the area immediately to the east. It was found by diamond 

drilling close to the Gemelos gold prospect,5 and a granodiorite exposure is present on the northeast side of the valley near the base of 

the Sierra de San Jose. If a single igneous body actually underlies 

this extensive area, it is not a phacolith but a stock only locally 

concordant with the overlying sediments.

The intrusive mass ranges in composition from granodiorite to 

diorite and is a medium-gray fine- to medium-grained rock with a 

hypidiomorphic texture. The plagioclase, which has a composition 

ranging from An65 to An40 , forms phenocrysts or relatively large 

subhedral grains in a finer groundmass consisting mainly of quartz, 

orthoclase, plagioclase, and mafic minerals. The phenocrysts may be 

strongly zoned, having a labradorite core rimmed by andesine, 

although a few grains appear to contain a labradorite zone lying 

between a core and an outer shell of andesine. Clinopyroxene, biotite, 

and hornblende are found in subhedral prismatic grains, the first two 

somewhat more abundant than the last. The pyroxene is a pale-green 

diopside, which may be slightly pleochroic, and the hornblende is 

generally pleochroic from yellow green to a medium dark green or 

bluish green. The accessory minerals include magnetite, hematite, 

apatite, sphene, and zircon. The hematite occurs as irregular grains 

and in small veinlets cutting the other minerals. The feldspar? are 

extensively altered, and the biotite is generally much chloritized. 

Epidote was noted in several thin sections, and in one section the 

chlorite contains small aggregates of dark nearly opaque needlelike 

crystals which may be rutile.

HORNBUENT>E-PYROXENE ANDE8ITE AND ASSOCIATED APLITE

An area on the north flank of the Sierra de La Caja and extending 

into Bonanza valley contains many sills and a few dikes of andesite 

and some aplite. The area is indicated on plate 1 by a special symbol 

because the sills are too numerous and small to map individually on 

the scale employed. The area has a length of 8.5 kilometers and a 

maximum width of 2.5 kilometers and lies mainly within the Caracol 

formation and Parras shale. Although the igneous rock is largely 

confined to the region described, a few sills occur in other parts of the 

valley and have been found as far east as Concepci6n del Oro.

The sills range from a few centimeters to 12 meters or more in 

thickness, and in many places they cut across the adjacent sediments 

on a small scale. Minor brecciation of the country rock occurs 

locally along the borders of the sills, which may contain many angular 

fragments of shale. Most country rock immediately adjacent to the

5 Oral communication from W. H. Triplett, Cia. Minera de Penoles.
40 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS

andesite is altered to a hornfels. The most extensively metamorphosed'area is in the Cerros Colorados, rugged isolated hills west of 

Bonanza. There are many sills in that area, and the consequent 

metamorphism has produced a rock highly resistant to erosion.

The andesite sills are medium dark gray to dark greenish gray and 

highly porphyritic. Specimens of only four were examined petrographically and of this number only one appears to be a fairly fresh 

andesite, whereas the others are highly altered. The first is composed 

of plagioclase phenocrysts and altered mafic grains in a fine-grained 

groundmass containing mainly plagioclase with a little accessory 

magnetite and little or no quartz. The phenocrysts are relatively 

fresh and consist of sodic andesine or calcic oligoclase, but the mafic 

minerals are wholly altered to aggregates of carbonate and chlorite.

The remaining sills were found to contain phenocrysts of plagioclase, 

hornblende, and pyroxene in a fine-grained groundmass consisting 

mainly of plagioclase, and minor amounts of quartz and carbonate 

and some accessory magnetite, limonite, sphene, apatite, and zircon. 

Some of the feldspar may be andesine or oligoclase, but a large part 

of it, including both the phenocrysts and the material in the groundmass, has apparently been albitized by magmatic solutions introduced 

later. It is rather strongly sericitized. The hornblende is pleochroic 

from yellow green to olive green and may form well-developed prismatic crystals, which in one sill have a preferred orientation, reflecting 

the direction of flowage. The pyroxene is a pale-green augite and 

may be slightly pleochroic. Both of the mafic minerals are extensively 

altered, largely to an aggregate of chlorite, carbonate, and some 

quartz. Much of the quartz in these sections is obviously secondary, 

and possibly none is an original rock constituent. The limonite is 

probably derived from the weathering of pyrite, which is sparsely 

disseminated through the rock. These highly altered sills differ 

texturally from the first sill described above. Individual grains in 

the groundmass are poorly defined, and the original texture seems to 

have been largely or wholly obliterated by the alteration processes.

The phenocrysts make up a large proportion of the rock, perhaps 

forming more than 50 percent by volume of some sills, and the magma 

must have been a highly viscous mixture of crystals and liquid injected 

under high pressure. This may account for the brecciation that is 

frequently seen along the borders of the rock, and may serve to 

explain several small sills in which foreign material, including shale 

and igneous rock fragments, is actually predominant.

The eastern part of the area contains several sills of a light-gray or 

buff to creamy-white color. One was examined petrographically and 

was found to contain a few plagioclase phenocrysts, largely kaolinized, 

in a fine-grained equigranular groundmass consisting mainly of sodic 

plagioclase with a little quartz and many isometric grains of pyrite.
GEOLOGY, PHOSPHATE DEPOSITS, CONCEPCION DEL OEO DIST., MEX. 41

Perhaps these sills represent a slightly acid differentiate of the andesitic 

magma and can be classed as aplites.

The Cerros Colorados and surrounding area contain many gold 

prospects, probably related genetically to the igneous rocks.

The sills here described may be related to sills described by Imlay 

(1938b, p. 1671) as intruding the Caracol and Indidura formations to 

the north and northeast of Melchor Ocampo, which lies in the synclinal 

valley separating the Sierras de Zuloaga and Sombreretillo. An area 

of about 13 square kilometers contains vertical or nearly vertical thin 

sills identified as andesite porphyry and a light-colored felsite porphyry.

FIGURE 14. View of the dacite intrusive body of Las Parroquias, Sierra de Santa Rosa. The 

picture was taken from Puerto Blanco.

Imlay suggests that these sills may mark the position of a fairly large 

igneous body lying at depth.

DACITE PORPHYRY

Dacite is found mainly in the Sierra de Santa Rosa, where it forms 

14 sills or sill-like intrusive bodies sufficiently large to be mapped. 

It occurs in both limbs of the anticline and has invaded mainly the 

comparatively thin-bedded rocks of the La Caja and Taraises formations, although one large sill has intruded the Cuesta del Cura limestone and another has intruded the Zuloaga limestone. The rock 

differs somewhat in composition from place to place and may actually 

range from a dacite to a latite.

The intrusive body of Las Parroquias is by far the largest and measures approximately 5 kilometers in length by 900 meters in maximum
42 GEOLOGIC INVESTIGATIONS IN THE AMEEICAN REPUBLICS

width (fig. 14). Although grossly sill-like in form, it has strongly 

crosscut the adjacent formations. Unlike the granodiorite, it has not 

invaded the sedimentary rocks by magmatic stoping, but has forced 

its way in by pushing the rocks aside, as can be clearly observed on 

the geologic map (pi. 1). The body has crosscut the La Caja and 

Taraises formations and the Cupido limestone, which have been offset 

a maximum distance of about 750 meters. If the offset segments 

could be restored to their original position, however, little, if any, of 

the section would be found to be missing. The magma emplacement 

was responsible for two moderately large faults, one at each end of 

the intrusive mass.

The fresh dacite is a light- to medium-gray rock, easily weathered 

to a soft, rotten material with a brownish or reddish coloration. It 

contains many large lath-shaped phenocrysts of plagioclase, which are 

enclosed in an aphanitic groundmass. The phenocrysts have a 

maximum length of 4 or 5 centimeters and commonly are zoned from 

labradorite in the core to an outer shell of andesine or calcic oligoclase. 

They seem to have a largely random orientation, and no primary flow 

structures could be clearly determined. Petrographic study shows 

that the plagioclase phenocrysts, along with a few large grains of 

quartz and moderately large subhedral grains of hornblende and 

biotite, are enclosed in a finely crystalline groundmass of potash 

feldspar, plagioclase, and quartz. The biotite is partly altered to 

chlorite and epidote. The potash feldspar is probably sanidine. A 

little magnetite, sphene, and zircon are present, but accessory minerals 

are relatively scarce. Calcite is found in most thin sections. Rosenbusch (Burckhardt, 1906a) reports that the groundmass may contain 

secondary feldspar and quartz, which have formed through devitrification of an original glass.

The limestone adjacent to the dacite is bleached or finely recrystallized, but the metamorphic zone is generally narrow. There is little 

or no evidence of ore mineralization in the vicinity of the Parroquias 

intrusive body or other dacite bodies.

The only other mass of dacite in the Sierra de Santa Rosa studied 

petrographically by the authors was the westernmost sill on the north 

flank of the anticline, which has a length of 1.75 kilometers and a 

maximum thickness of about 300 meters. It is a medium-gray to 

reddish-gray or violet rock containing small phenocrysts of plagioclase, 

quartz, and biotite in an aphanitic groundmass consisting of a finely 

crystalline mixture of potash feldspar, plagioclase, and quartz. The 

feldspar is approximately 60 percent plagioclase and 40 percent 

potash feldspar.

A thin section from the compound sill that is exposed in Can6n de 

La Canela was studied by Rosenbusch (Burckhardt, 1906a, p. 26), 

who described it as a highly altered rock composed of irregular
GEOLOGY, PHOSPHATE DEPOSITS, CONCEPCION DEL ORO DIST., MEX. 43

angular fragments of mica dacite having a glassy or cryptocrystalline 

groundmass, angular quartz grains, and fragments of a hornfelslike 

mixture of biotite, quartz, and feldspar. It also rather commonly 

contains calcite. Rosenbusch could not determine whether the rock 

was a dacite with many inclusions or a breccia. However, field 

study has shown this body to be clearly intrusive.

A few small dacite bodies, including sills and dikes, have intruded 

the loosely consolidated rocks of the Mazapil conglomerate in the 

Santa Rosa graben and in the downfaulted area lying along the 

north side of the Mazapil valley. In the second area only two 

bodies were sufficiently large to map, and a specimen from only one 

was studied in thin section. A few wholly altered plagioclase phenocrysts and a few biotite grains are enclosed in a fine-grained, highly 

altered groundmass containing plagioclase, potash feldspar, and quartz.

ANDESITE

The only body of later formed andesite to be found within the area 

mapped is that exposed on the crest of the Sierra de La Caja and 

lying immediately to the west of the high summit known as Cerro 

de Los Carneros. This body is roughly circular and has a diameter 

of about 525 meters. The rock ranges from grayish violet to yellow 

and yellowish white and seems to have a slightly porous texture, 

which is accentuated on weathered surfaces. The single thin section 

obtained by the authors is composed of very small, highly altered, 

plagioclase phenocrysts and some muscovite laths in a glassy or 

cryptocrystalline groundmass with a strongly developed flow banding. 

The plagioclase could not be determined. Rosenbusch also studied 

the rock petrographically (Burckhardt, 1906a, p. 27) and described 

it as a highly altered vitrophyric amphibole andesite. The flow 

structure strikes approximately parallel to the borders of the intrusive 

body and dips inward at moderate to steep angles. The rock is 

closely jointed parallel to the flow structure, imparting a platy appearance. The mass has intruded the Zuloaga limestone, which is almost 

completely unmetamorphosed. It is believed to have filled a volcanic 

vent, although there is no trace of the andesitic volcanic rocks which 

must have been ejected. Apparently they have been completely 

eroded.

RHYOLITE

A small rhyolite stock crops out in Canon del Potrero on the north 

side of the Sierra de Santa Rita. It may be considerably larger 

than is indicated by the exposure, for apparently it underlies the 

Zuloaga limestone immediately to the west. A second small rhyolitic 

mass is present on the northeast flank of the Sierra de San Jose 

about 0.8 kilometer southwest of the La Laja mine. A few additional 

minor rhyolitic intrusive masses are present in the Sierra de La Caja

368566 56 1
44 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS

but have not been indicated on the geologic map (pi. 1). They are 

closely associated with rhyolitic flow remnants which have been 

mapped in the Salaverna-Providencia area.

The rock in Can6n del Potrero is light reddish brown and has a 

porphyritic texture. The phenocrysts are potash feldspar and quartz. 

They range in size from 0.5 to 1.0 millimeter and are contained in a 

finely crystalline groundmass. The feldspar is highly altered but 

was probably sanidine. Biotite and hornblende can be recognized, 

as well as accessory hematite. Petrographic study shows that the 

rock underwent some brecciation before crystallization was complete, 

and this was followed by the introduction of silica, probably as 

chalcedony.

CONTACT METAMOBPHISM AND MINERALIZATION

The deep-seated intrusions of granodiorite and diorite strongly 

metamorphosed the adjacent sediments. Since the authors were able 

to make only a cursory investigation of the contact metamorphism 

and ore mineralization, these subjects are discussed only briefly 

in the present report. However, they would like to recommend 

that a detailed study be made of the district, for they believe that 

it would greatly benefit the local mining industry.

To date the only careful investigation of the contact metamorphism 

is that made by Bergeat (1910). His study was limited to a small 

area in the vicinity of Concepcion del Oro, however, and although 

his paper is based on a great deal of careful petrographic and analytical 

work, he devoted relatively little time to field observations.

CONCEPCION DEL ORO AND PROVIDENCIA STOCKS

Contact metamorphism formed a thin garnetized border in the 

limestone adjacent to the Concepcion del Oro and Providencia stocks. 

This belt occurs only intermittently around the edges of the stocks 

and ranges in thickness from a few centimeters to 15 meters or more. 

The predominant mineral is andradite, which is generally green to 

yellow green, but locally may be red or brownish. The garnet may 

be accompanied by calcite, quartz, wollastonite, diopside, tremolite, 

vesuvianite, epidote, scapolite, and more rarely, zoisite and hornblende. Although the boundary between the garnetite and granodiorite is generally sharp, in some places it is poorly defined. The 

garnet rock in places has been invaded by small apophyses or small 

isolated bodies of the intrusive rock.

In the garnetite zone, or close to it, are found some of the most 

important ore deposits. These contain mainly chalcopyrite (with 

gold and silver), pyrite, chalcocite, magnetite, hematite, and sphalerite, together with a little tetrahedrite and other minerals in small 

amounts. Some deposits are irregular in shape and apparently inde-
GEOLOGY, PHOSPHATE DEPOSITS, CONCEPCION DEL ORO DIST., MEX. 45

pendent of structural controls, and others form pipes or chimneys 

lying along fractures, fracture intersections, or the intersections of 

fractures with beds favorable to replacement.

Beyond the garnetite border is a metamorphic aureole in which the 

limestone is recrystallized and is replaced only locally by silicate 

minerals. The aureole has a width ranging from several hundred 

meters to more than 1 kilometer, and toward the outer border the 

metamorphic effects die out gradually and almost imperceptibly. 

Most limestone is transformed into medium- to coarse-grained white 

marble, although in some areas away from the contact, black or 

alternately black and white recrystallized limestones are relatively 

common. Certain limestone beds, such as those of the Taraises 

formation or those in the lower part of the Zuloaga limestone, seem 

especially to produce black marble. This color is probably related 

in part to the carbonaceous content of the rock, which is recrystallized to graphite. In the white marble the carbonaceous material 

may have been burned off rather than recrystallized and preserved. 

As Bergeat (1910, p. 46) has pointed out, however, the intense black 

coloration does not always correspond to a relatively high carbon 

content. For example, the uppermost unit of the La Caja formation, 

which in the areas close to the granodiorite contact is generally a 

recrystallized black limestone rich in wollastoiiite, contains very 

little carbonaceous material. The lower units of the La Caja formation, which differ little in composition from unit D, except they are 

less calcareous, are transformed into hard buff-colored hornfels rich 

in wollastonite and quartz, which contrasts sharply with the overhang black rocks.

The silicates within the metamorphic aureole may be sparsely 

disseminated through the limestone, but in many places theyr form thin 

discontinuous layers along the bedding planes, and possibly along 

some stylolitic partings. In cherty formations, such as the Cuesta 

del Cura limestone, many chert lenses are completely garnetized. 

In some areas close to the igneous contact the thin garnet layers formed 

along bedding planes appear to have been folded and crumpled and 

are broken into many discontinuous segments (fig. 15). Bergeat (1910, 

p. 49) believes this may have taken place during reorystallization of the 

limestone and may be the result of pressure exerted by the ascending 

magma.

Within or close to the aureole there are many mesothermal ore 

deposits consisting mainly of galena, sphalerite, and pyiite, which 

carry silver and gold. Although contemporaneous with the contact 

metamorphic deposits, they were formed at lower temperatures. 

Many of these mesothermal deposits are associated with fractures 

and various types of flexures and may occur where such structural
46 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS

elements are found in conjunction with beds favorable to replacement 

or sedimentary contacts.

According to Bergeat (1910, p. 35), the margin of the granodiorite 

in the Concepcion del Oro stock is in many places transformed into a 

thin garnetiferous selvage, whose thickness is only a few centimeters, 

in which the garnet, diopside, wollastonite, quartz, and minor calcite 

are pseudomorphic after plagioclase, hornblende, and pyroxene.

FIGURE 15. View showing crumpled, metamorphosed Cuesta del Cura 

limestone close to contact with grarodiorite in Sierra de Conception 

del Oro. g, garnet; c. bleached chert.

NOCHEBTJENA STOCK

Along the limestone contacts of the Nochebuena stock there is 

considerable replacement by garnet and other contact-metamorphic 

minerals, but there is no strongly developed garnetite zone comparable 

to that bounding the Concepcion del Oro and Providencia stocks. 

A wide metamorphic aureole is present, however, and within or close 

to this zone are several metalliferous deposits that are being mined 

for silver, lead, and gold. At the east end of the intrusive body the 

Caracol and Parras sedimentary rocks are metamorphosed to yellow 

or buff and, locally, greenish masses of hornfels and argillite.
GEOLOGY, PHOSPHATE DEPOSITS, CONCEPCION DEL ORO DIST., MEX. 47

The east end of the stock, along with the small intrusive body lying 

adjacent to it and invading the La Caja thrust, has produced a wide 

belt of fairly massive wollastonite in the Cuesta del Cura limestone. 

The wollastonite ranges from fine grained to coarsely crystalline, and 

the grains are as much as 2 centimeters in length. The fibers may 

form radial aggregates or lie roughly parallel, but more often they 

seem to have a random orientation. They may contain disseminated 

calcite, garnet, diopside, and possibly other minerals, but for the most 

part these silicates are concentrated along the chert lenses, which may 

be almost completely garnetized. In general, the wollastonite close 

to the contact is white, coarse grained, and contains relatively few 

impurities, and that occurring away from the contact is gray to bluish 

gray, fine grained, and contains a large quantity of foreign material, 

particularly calcite. Partial chemical analyses of grab samples of 

these two general lithologic types'are*given in table 4.

TABLE 4. Partial chemical analyses of two samples of wollastonite from the Sierra

de La Caja

[The analyses were made in the laboratories of the Institute de Geologia by Alberto Obregon P., reported

June 20,1953]

Laboratory no.

10700 10701

SiOa _ 47.36 26.76

AhOs - 4.69 5.29

FesOs (total iron)_________________________________________________ .63 .79

CaO__________________ ____________________ __ __ ______ 41.79 55.83

MgO. ____________________________________________________ .12 .05

No. 10700 represents a thickness of about 10 meters and was taken immediately adjacent to the diorite contact. 

No. 10701 represents a thickness of about 20 meters and was taken immediately above the first sample.

The authors believe that the rock represented by no. 10700 may 

have commercial possibilities. Wollastonite and garnet are easily 

separated by high-intensity magnetic- or gravity-separation methods, 

and the garnet might be salvaged as a byproduct for the abrasives 

industry.

SAN FELICIANO PHACOLITH(?)

The granodiorite in the Valle de San Feliciano has recrystallized 

the adjacent limestone to a distance of a few meters from the contact, 

but it has not strongly silicated it. The only metallization in the 

vicinity of the intrusive body produced hematite, found mainly along 

fracture zones and along the high-angle thrust fault lying immediately 

to the northwest. It is being mined on a very small scale for use in 

the paint industry in Monterrey. The granodiorite contains many 

inclusions of Zuloaga limestone, which in general have been marmarized 

only close to the margins.

TERTIARY EXTRUSIVE ROCKS

Extrusive rocks occupy several small areas in the Sierras de La 

Caja and Santa Rosa, and in the adjacent valleys. They include 

flows, agglomerate (?), and some tuff, and they range in composition 

from rhyolite to basalt.
48 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS

AGGLOMERATE(?)

In the center of the Santa Rosa anticline (fig. 16) an area slightly 

oval in shape and about 2.75 kilometers long is underlain by a highly 

altered rock chat has been described as an altered porphyry stock 

(Burckhardt, 1906a, p. 23-24; Barrera, 1927, p. 53 54). Field study 

and petrographic examination of this material, however, strongly 

suggest a volcanic origin.

In one area on the western margin of the Valle de Santa Rosa the 

rock is characterized by well-developed layering, which dips away from 

the fault contact at an angle of 25° to 30°. However, most of the 

material seems to be relatively massive, with layering only faintly defined or wholly lacking, and in many places it is obviously composed 

of angular rock fragments ranging from less than a centimeter to more 

than a meter in longest dimension. The fragmental structure lends 

credence to the theory that the rock is an agglomerate or volcanic 

breccia. The fresh rock is generally white to pinkish, but weathered 

surfaces are strongly reddish. A few xenolithic blocks of marmarized 

limestone have been seen in the material, and one very large limestone 

block was penetrated by a diamond drill in the western part of the 

area.

The rock is so strongly altered that little idea of the original composition and texture can be gained from a petrographic examination. 

The thin sections that were studied consisc mainly of quartz and chalcedony grains, with considerable muscovite and sericite, in a groundmass composed of fine-grained secondary silica. A. few outlines of 

original feldspar grains can be observed, but these have been completely altered to sericite or kaolin, and the quartz, which occurs as 

large, irregular grains, may be the only original rock constituent. 

Several of the specimens studied by Rosenbusch (Burckhardt, 1906a, 

p. 26-27) appear to have been similar, although another one he describes is mainly an allotriomorphic quartz and feldspar aggregate 

formed by alteration of a spherulitic matrix. This material has a 

few large crystal outlines filled by chlorite; these probably represent 

original grains of feldspar and biotite.

Kaolin and secondary silica may be deposited along fractures in 

the agglomerate (?) or in larger, more irregular areas. The kaolin is 

apparently of good quality and might possibly merit a brief investigation to determine commercial possibilities. On the south side of 

Cerro Colorado the agglomerate (?) contains deposits of turquoise, 

which occur along fractures close to the fault contact and must have 

been formed by hydro thermal solutions. The phosphoric acid required for the deposition of turquoise may have been derived from the 

phosphatic beds of the La Caja formation.

The authors believe that the agglomerate^?) may occupy the site 

of a volcanic orifice and may have been dropped down into the sedi-
Poettorxtrf

^ff: 

l ' ' \ iOafyin cfte fa* *. Boeds "'

H 

O t~< O

OKj

*aW

o

CD

^w IS

FIGURE 16. Aerial view, looking westward, of the Sierra de Santa Rosa. Photograph furnished by United States Air Force.

CD
50 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS

mentary rocks by cauldron subsidence. It has a known thickness of 

about 400 meters but continues to an unknown depth in the central 

part of the valley. Since all the intrusive rocks in the vicinity are 

dacite, perhaps this rock was also dacitic in composition, and possibly 

this was the only point at which the dacite magma reached the surface. 

Along the eastern margin of the agglomerate (?), the Zuloaga limestone is partly marmarized and in some areas is mineralized, containing 

gold, silver, and lead ores. However, along the remainder of the contact there is evidence of only minor alteration or mineralization. 

This wTould suggest that the ore-bearing solutions moved upward 

locally along the fault contact but may have had no genetic relation 

to the volcanic rock. There are little or no mineralization effects 

associated with the dacite, but, on the other hand, some features of 

the ore in the agglomerate (?) and adjacent limestone at Santa Rosa, 

such as the presence of fluorite and manganese, suggest an origin 

during the metallogenetic epoch that later accompanied widespread 

extrusion of rhyolite.

RHYOLITE FLOWS

A thin rhyolite flow lies unconformably above the Mazapil conglomerate in Valle de La Caja, and small remnants of other flows cap 

several hill summits on the south limb of the La Caja anticline in 

the vicinity of Salaverna. The rocks are generally reddish or pinkish 

but locally may be gray or yellowish gray. Phenocrysts are relatively rare and consist mainly of quartz grains enclosed in a finely 

crystalline groundinass containing sanidine, plagioclase, and quartz. 

Mafic minerals are very rare. Most of the rock is slightly vesicular, 

but it has no strongly developed flow structure. One thin section 

appears to be a leucotrachyte, containing mainly sanidine and a 

considerable amount of hematite and some quartz. No mafic minerals 

are present.

BASALT FLOWS

Basic volcanic rocks appear in two rather widely separated areas. 

They underlie the flat-topped hills, which are known as Las Mesas, 

at the western end of the Sierra de Santa Rosa, and they form a range 

of low but rather conspicuous hills in the Bonanza valley northeast 

of Terminal. In both localities a series of basalt flows overlie loosely 

consolidated arkose, which may be a reworked volcanic ash.

In the Las Mesas area the volcanic rocks have a maximum exposed 

thickness of more than 200 meters and dip to the west at an angle of 

about 5°. There is no evidence of faulting in the area, and the dip 

probably represents an original slope. The arkosic sediments are 

covered by talus from the overlying flows and are rarely exposed. 

Those which were seen are light-gray to reddish or yellowish crumbly 

poorly cemented rocks, and only two specimens were examined
GEOLOGY, PHOSPHATE DEPOSITS, CONCEPCION DEL ORO DIST., MEX. 51

petrographically. One was found to be composed of rounded grains 

of plagioclase, quartz, and chalcedony, and rounded rock fragments 

in a chalcedonic cement. The rock fragments consist of basalt or 

basalt and chalcedony. The other rock is a well-sorted loosely 

consolidated sand containing angular to moderately rounded grains 

of highly altered feldspar, chalcedony, and quartz, and some carbonate and magnetite. One flow was examined in thin section and 

proved to be a basalt containing phenocrysts of twinned basic plagioclase and a few grains of olivine that are largely replaced by iddingsite.

In the western part of the Mazapil valley several low hills are 

composed of arkose resting upon beds of Caracol age. The flows are 

missing, but the hills are littered with bombs of scoriaceous lava, 

which in places have been cemented by caliche.

In the Bonanza valley the volcanic rocks dip to the northeast at an 

angle of 15° to 20°. They are bounded on the north by a large normal 

fault known as the Canutillo fault (p. 54), have a total thickness of 

about 300 meters, and apparently rest upon beds of the Caracol 

formation, although the actual contact is covered by alluvium. Five 

basalt flows cap the [hills and have a total thickness of about 150 

meters. They range in color from dark reddish to gray or almost 

black. One examined petrographically was found to contain abundant phenocrysts of plagioclase, olivine, and hypersthene in a finely 

crystalline groundmass. The plagioclase forms euhedral grains, 

which are generally twinned according to the albite law and may be 

strongly and intricately zoned. The olivine is pale greenish and 

relatively fresh. Magnetite or ilmenite is very common. The 

groundmass consists mainly of plagioclase in microlites and small 

irregular grains. Locally, the microlites appear to define flow lines, 

although the flow structure is poorly developed.

STRUCTURAL GEOLOGY

FOLDS

The Concepcion del Oro district is near the southern margin of a 

broad belt of closely spaced asymmetrical folds which extend in a 

westerly direction across the southern part of Coahuila and the 

northern part of Zacatecas. The area mapped contains four anticlinal ranges separated by narrow synclinal valleys. The three 

southerly ranges, which are arcuate and convex to the northeast, 

occur on a major curve in the regional trend from northwest to west, 

or slightly south of west.

The three southerly ranges, Sierra de La Caja, Sierra de SantaRosa, and Sierra de Santa Rita, are moderately to strongly overturned toward the north and northeast and in part are fan shaped. 

The Sierra de Canutillo, however, is slightly overturned toward the
52 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS

south with a tendency toward fan folding in the anticlinal core. The 

Sierra de Zuloaga, which lies to the north of the Sierra de Canutillo 

and which has been mapped by Imlay (1938b, p. 1675, pi. 7), is rather 

strongly overturned toward the south. All these anticlinal folds plunge 

to the west beneath the Bolson de Cedros, a narrow alluvial basin 

bounded on the west by a series of anticlinal ranges with a northwestward trend. These folds lie at a sharp angle to those found 

within the area mapped and must reflect another abrupt change in 

the regional strike. The Santa Rita anticline plunges to the southeast beneath the Bolson de San Carlos, a second northwestwardtrending alluvial basin. The other ranges, however, terminate 

abruptly on the east, and it is possible that the Santa Rosa and La 

Caja anticlines may continue under the basin in a southeasterly 

direction, to reappear beyond the basin in the form of other anticlinal 

ranges. The authors believe that the Bolson de San Carlos may be 

a structural basin formed by block faulting, although little attention 

has been given to this problem.

The folds in the Concepcion del Oro district and adjacent areas are 

small and rather tightly packed and may extend only to a relatively 

shallow depth. The redbeds of the Huizachal formation have been 

involved in the folding, but clearly to a lesser extent than the overlying sediments, and for that reason they are only rarely exposed in 

the anticlinal cores in this region. During the Laramide orogeny 

there must have been widespread slippage along the surface separating 

the Huizachal rocks from the overlying Zuloaga limestone, which 

permitted decollement in these anticlines.

FAULTING

Faulting is more common in the Concepcion del Oro district than 

in the surrounding region and has produced a complex structural pattern in some places. Three important thrust faults have been found 

within the area. They occur along the north flanks of the Sierras de 

La Caja and Canutillo and in the center of the Santa Rita anticline; 

they dip southward. A similar fault has been mapped by Imlay 

(1938b, pi. 7) on the north limb of the Zuloaga anticline, but apparently thrusting is rare in the broad folded belt to the north and northwest in the Sierra de Parras. Only three overthrusts have been 

mapped in the Sierra de Parras (Imlay, 1937, pi. 10), and they are all 

in the western half of the region.

Most of the faults in the district are normal faults which appear to 

fall into three general categories; faults associated with major intrusions, transverse faults, and block faults with associated graben 

structures.

Many faults in the vicinity of the major igneous bodies apparently 

were formed by the pressure of upwelling magmas during the period
GEOLOGY, PHOSPHATE DEPOSITS, CONCEPCION DEL ORO DIST., MEX. 53

of emplacement. Most faults of this type have small displacements, 

and many are found on the immediate periphery of the igneous 

bodies. They may have random orientation but in most places fall 

into fairly well defined sets. Such fracture sets may be parallel to 

the borders of the intrusive bodies, may lie perpendicular to the borders, or may have some orientation between these two extremes. In 

the mineralized areas these faults have played an important part in 

the localization of ore. A few large faults close to the intrusive masses 

may have displacements of as much as several thousand meters.

Many vertical or steeply dipping transverse and oblique faults are 

present in the district. In general, they are small, although they may 

have effected lateral offsets of formational boundaries amounting to 

100 meters or more. Most are simple gravity faults, probably formed 

during the teiisioiial phase that followed folding. A few can be 

classed as tear faults, which are related to the dominant overturning of the anticlinal flanks.

Several long, relatively narrow grabens, nearly parallel to the 

regional trend, are bounded by longitudinal faults dipping at moderate 

to high angles. The three smallest grabens are comparatively short 

and broad and are completely bounded by faults. The largest structure, which occupies a part of the Mazapil valley, is bounded on the 

north by a fault but may not be on the south. The authors believe 

that the small fault troughs or grabens, which are among the most 

noticeable structural features of the Concepcion del Oro district, as 

well as the younger Canutillo fault, belong to a major system of block 

faulting in the northern part of the Mesa Central. Although this 

type of faulting has not been widely recognized, much of the previous 

geological work in the region has been of a reconnaissance nature, and 

it may simply have been overlooked.

MAJOR STRUCTURAL UNITS 

CANUTILLO ANTICLINE

The Sierra de Canutillo is the smallest of the anticlinal structures 

within the area mapped. It has a trend of approximately N. 80° W. 

and lies along the boundary between the States of Zacatecas and 

Coahuila. It has a length of 17.5 kilometers and a maximum width 

of about 4 kilometers. It ends abruptly on the east, but on the west 

it plunges steeply under an alluvial plain. The fold is slightly asymmetrical, the axial plane dipping steeply to the north. Dips on the 

north limb are generally 65° to 70° N., but on the south limb they are 

as much as 80° S. The anticlinal core, which is composed of massive 

Zuloaga limestone, shows a tendency toward fan folding. This folding is present in the Cerro del Guayabo, immediately to the west of 

Puerto de Canutillo, and can be seen from the road that goes through 

the pass.
54 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS

In the eastern part of the range a thrust fault has been traced for 

approximately 3.5 kilometers. It terminates on the east against the 

younger Canutillo fault and on the west against a second normal 

fault. Exposures in the area are poor, but the fault appears to dip to 

the south at a low angle, possibly not more than 10° or 15°. Maximum displacement occurs at the base of Cerro del Guayabo, where 

Zuloaga limestone is apparently resting upon beds of the Caracol 

formation.

The Canutillo fault is a large normal fault which can be traced for 

14 kilometers along the south flank of the anticline and is responsible 

for the fault-line scarp that forms such a conspicuous topographic 

feature in that area (fig. 17). Two kilometers west of Puerto de

FIOURE 17. View of the scarp formed along Canutillo fault north of No v m^s. oiiestu del Cura limestone 

(Klcc) is in contact with the Zuloaga limestone (Juz). The saddle is formed on La Caja formation (Juc).

Canutillo the fault has dropped beds of Caracol age against the 

Zuloaga limestone, a stratigraphic throw of more than 1,100 meters. 

The fault dips 80° to 85° S. Although the fault disappears under alluvium at the east end of the Sierra de Canutillo, it must curve to the 

southeast and persist for 10 kilometers or more, for it is this structure 

that has tilted the volcanic rocks lying to the northeast of Terminal. 

The volcanic rocks form a range of low but rather conspicuous hills, 

and the basaltic flows capping these hills dip to the north at an angle 

of 15° to 20°.

The Canutillo fault belongs to the youngest group of faults to be 

found within the area mapped. They must be Pliocene in age, as 

they are younger than the basic volcanic rocks, which were probably 

extruded in late Tertiary time. The character of the Canutillo 

fault-line scarp indicates an early stage of evolution.
GEOLOGY, PHOSPHATE DEPOSITS, CONCEPCION DEL ORO DIST., MEX. 55

In addition to the faults described above, the Canutillo anticline 

contains several transverse faults, and in two localities the incompetent bends of the La Caja formation have been squeezed and faulted 

out for appreciable distances by reverse faults.

LA CAJA ANTICLINE

The La Caja anticline is represented topographically by the Sierras 

de Concepcion del Oro and La Caja, which are actually a continuous 

range with different names for the eastern and western segments. 

This structural feature has a length of about 45 kilometers and is 

arcuate in shape, curving gradually in a westerly direction from N. 45° 

W. to approximately west. The anticline ends abruptly on the southeast, at the edge of the Bolson de San Carlos, but on the west it 

plunges at a low angle beneath the Bolson de Cedros.

The Sierra de Concepcion del Oro, the part of the fold southeast of 

Providencia, is the more complex segment of the anticline, owing 

largely to the intrusion of two large granodiorite stocks. This segment 

is strongly overturned toward the northeast, and in the vicinity of 

Concepcion del Oro it is nearly recumbent, the north limb of the fold 

dipping 5° to 20° SW. (fig. 18). In this area the south limb has an 

average dip of about 45° SW., and on both limbs the dips steepen 

gradually outward from the anticlinal core until they reach a maximum of 75° to 80° SW. Both limbs of the fold steepen gradually to 

the west, and in the vicinity of Terminal a,nd Providencia the north 

limb is dipping generally 70° to 80° S.

In the part of the Sierra de La Caja lying between the Providencia 

stock and the Nochebuena granodiorite stock, the fold is not strongly 

overturned in either direction, although locally, in and close to the 

anticlinal core, there may be fan folding, and locally there is overturning of the limbs of the fold. Much of the north limb is concealed 

beneath the La Caja overthrust, but in exposed areas the rocks dip 

generally 50° to 80° N. In part, the south limb has approximately 

the same range of dips to the south. As elsewhere, dips in the Cuesta 

del Cura limestone and the Caracol formation differ in many places, 

owing to the extensive drag folding. The same is true, although to a 

lesser extent, of the Indidura formation and the incompetent sections 

of the remaining stratigraphic units.

The west end of the anticline, extending from the Nochebuena 

stock to Cedros, is known locally as the Sierra de Cedros; it is 

a normal symmetrical anticline, with the dips on both limbs generally 

ranging from 45° to 70°. In the axial region of the fold the rocks are 

nearly horiztontal, although the axis plunges from 5° to 10° W.

The La Caja thrust extends about 15 kilometers along the north 

flank of the Sierra de La Caja, although the actual fault outcrop is 

sinuous and considerably longer. For most of its length the edge of
56 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS

is W
GEOLOGY, PHOSPHATE DEPOSITS, CONCEPCION DEL ORO DIST., MEX. 57

the overtlirust plate forms a prominent fault-line scarp, one of the 

most noticeable topographic features of the range. On the west the 

fault apparently ends at the Nochebuena stock, although, owing to 

poor exposures, it cannot be traced all the way to the edge of the stock. 

On the east it dies out rather abruptly only 0.5 kilometer short of the 

Providencia stock. However, several smaller related faults in this 

area may continue to the granodiorite stock. Four of these faults are 

intersected by the El Tesoro adit, which runs in a southwesterly direction from the stock through the Taraises and La Caja formations and 

continues for about 170 meters into the Zuloaga limestone. The 

faults have southwesterly dips ranging between 55° and 70°, and the 

two largest have breccia zones measuring several meters in thickness. 

At the surface, several of the fault zones contain dikes or small 

lenticular bodies of alaskite.

In general, the La Caja fault appears to dip 45° to 50° S., although 

locally the dip may steepen to 75° or 80°, and in one relatively small 

area lying to the south of Novillos the thrust plainly dips to the north 

at a very low angle. The fault is everywhere characterized by a large 

breccia zone, which ranges generally from 30 to 100 meters in thickness. 

A few small to moderately large granodiorite bodies occur along the 

fault zone; only the largest of these have been mapped. They are 

clearly formed later than the thrusting, although one has been brecciated, owing apparently to localized later movement along the fault. 

Maximum displacement occurs in the vicinity of Canon del Gato, 

where Zuloaga limestone rests upon the Caracol formation, thus indicating a stratigraphic throw of more than 1,100 meters.

A peculiar reverse fault is found along the crest and southern flank 

of the La Caja range to the southwest of Providencia. It appears to 

be a high-angle thrust dipping to the north and may owe its origin to 

emplacement of the Providencia stock.

The principal area of large-scale normal faulting is found in the 

vicinity of the Concepcidn del Oro granodiorite stock, which apparently 

exerted an active upward thrust that strongly disrupted the overlying 

sediments. The Concepcion del Oro fault is a peripheral fault, somewhat arcuate in form, which lies to the northeast of the stock and 

runs nearly parallel to the igneous contact. The rocks on the northeast side of the fault have low southwestwarcl dips and represent the 

strongly overturned limb of the anticline. On the other side of the 

fault the rocks are generally steeply inclined, and this segment represents a part of the underlying recumbent syncline, which was pushed 

upward by the invading magma. The fault plane is nearly vertical, 

and the fault may have a maximum throw of 900 meters or more.

The Concepcion del Oro stock invaded the core of the La Caja 

anticline mainly, but a part of the igneous mass extended into the 

adjacent syncline, and in this area also, the invading magma disturbed
58 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS

the overlying sediments in a highly complex manner. The rocks lying 

along the trough of the syncline were thrust upward and disrupted, 

and the fold axis was offset about 1,200 meters to the southwest. 

These dislocations have resulted in a peculiar topographic feature. 

The La Caja and Santa Rosa anticlines are now connected by a ridge 

which crosses the intervening synclinal valley and divides it into two 

segments. The major segment is that lying to the west, which is 

known as the Mazapil valley. The ridge has a width of 2 kilometers, 

and in this upthrust area the synclinal trough is occupied by Cuesta 

del Cura limestone, the younger beds of the Indidura and Caracol 

formations having been eroded. The ridge has been dissected by 

headward erosion from the small valley to the southeast, and the divide separating this system from the Mazapil valley is little more than 

100 meters in width. The synclinal axis has been offset along a fault 

zone that extends for about 5 kilometers S. 65° W. from Can6n de 

Los Angeles and then turns sharply to the northwest to follow the La 

Caja formation. The offset was not effected along a single fault, but 

along a complex zone containing several large faults. Several large 

sedimentary blocks were rotated and moved along the zone for considerable distances. The La Caja formation has been offset for more 

than 2 kilometers along the zone.

Two small grabens are present along the crest of the La Caja anticline. A high relatively broad, shallow valley, known as Valle de La 

Caja, has been developed upon the larger graben, and to the west 

this valley drains into a narrow steep-walled canyon cut along the 

anticlinal axis. The fault block has a length of about 2.25 kilometers 

and a maximum width of 1.5 kilometers; the fault has dropped Mazapil 

conglomerate into the Zuloaga limestone. The Tertiary rocks are 

folded into a gentle westward-trending syncline, with maximum dips 

of about 35°. The folding may have been effected earlier than the 

formation of the graben structure, but it seems more likely that the 

gentle syncline is a product of drag along the faults bounding it. In 

the northern half of the valley the conglomerate, siltstone, and tuff 

are covered by a thin rhyolite flow, which has been breached by the 

larger arroyos crossing it. The flow antedates the fault and is tilted 

to the south along with the underlying rocks. Can6n del Gato, a 

deep, relatively narrow valley, has been cut along the second graben, 

which has a length of only 1 kilometer. Tertiary conglomerate, siltstone, and tuff have been dropped deep into the Zuloaga limestone of 

the anticlinal core and have been tilted to the south or southeast. 

The lowest rock exposed is a coarse boulder conglomerate with a gray 

to slightly reddish matrix. Exposures are poor, as most of the Tertiary rocks are covered by alluvium.

The largest of the downfaulted blocks containing Mazapil conglomerate is found along the south flank of the range. It underlies the
GEOLOGY, PHOSPHATE DEPOSITS, CONCEPCION DEL ORO DIST., MEX. 59

northern side of the Mazapil valley in the area directly north of 

Mazapil and may be about 9 kilometers long by 2 to 2.5 kilometers 

wide. The area is bounded on the north by a fault or fault system, 

which in part is easily traced, but the southern border of the area is 

covered largely by alluvium. In one area to the northwest of Mazapil, 

however, the red conglomerate rests unconformably upon beds of the 

Caracol formation, and the yellowish siltstone, sandstone, and limestone which underlie the conglomerate along the north side of the 

structure are wholly lacking. In this locality, at least, there is no 

fault bounding the southern margin of the formation. The Tertiary 

rocks have a thickness of about 200 meters and are fairly well exposed 

in the large reentrant lying to the north of Mazapil. They have been 

folded into a shallow westward-trending syncline. Close to the fault 

on the north, dips may be 60° or more to the south or southwest, but 

away from the fault they are 15° or 20°. Along the southern edge of 

the area the rocks dip 5° to 30° N. Although it is probable that the 

folding is due in large part to drag along the faults at the boundary, 

there may have been some folding of the Tertiary rocks before the 

graben was formed. In one locality on the north side of the valley 

the beds are overturned, and such overturning is difficult to explain 

by faulting alone. Around the borders of the reentrant the conglomerate and associated rocks are bounded by a relatively complex system 

of faults, of which the largest are shown on the regional map (pi. 1). 

The rocks have been intruded by a few small dikes and one sill of 

highly altered gray to violet dacite. Some dikes have invaded fault 

zones.

SANTA ROSA ANTICLINE

The Santa Rosa anticline is represented by a small mountain range 

45 kilometers long. The western part of the range is known as the 

Sierra de Santa Rosa (fig. 16), and the eastern section is called the 

Sierra de San Jose. The fold is arcuate in shape, curving gradually in 

a westerly direction from a strike of N. 40° W. to a strike averaging 

a little south of west. The anticline ends abruptly on the southeast, 

at the edge of the Bols6n de San Carlos, but on the west it appears to 

plunge beneath the Bols6n de Cedros at a low angle, although in 

that area it is partly concealed beneath basic volcanic rocks.

The anticline is moderately overturned toward the north along its 

entire length. The strata on the northern limb of the fold dip from 

50° to 75° to the south and southwest and average about 65°. In 

most of the Sierra de San Jose the anticlinal core, which is composed 

of Zuloaga limestone, is fan folded, the southern half of the core 

dipping generally 40° to 45° NE. This overturning extends outward 

to the La Pena formation, but the younger rocks in most places dip 

normally to the southwest. On the west the fan folding largely dies

368566 56 5
60 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS

out in the vicinity of Santa Rosa, and to the southeast it dies out a 

few kilometers short of the end of the range. In the Sierra de Santa 

Rosa there is a tendency toward fan folding in some areas, but it is 

less strongly developed. In general, the rocks on the south limb of 

the fold are steeply inclined toward the south, although they flatten 

gradually away from the anticlinal core. They may have an average 

dip of about 65° S.

The Zuloaga limestone is believed to be compressed into a double 

anticline along the entire length of the fold. The formation contain? 

no characteristic key beds that would furnish definite proof of this, 

but the distribution of the siltstone layers and coral-bearing bedp 

would seem to bear it out. Furthermore, the Zuloaga limestone, which 

is tightly and even isoclinally folded, forms a belt with an average 

width of 1,800 meters. As the total thickness of the formation may 

be little more than 400 meters, the section would appear to be repeated 

four times, rather than twice as in the normal single fold.

In the southern half of the Mazapil valley, about 1.5 kilometers 

to the north of the Sierra de Santa Rosa, is an isolated hill known as 

Cerro Gordo. The hill is 2.5 kilometers long and has an average 

width of about 1.25 kilometers. Structurally, the hill is a small 

anticline (section A-A', pi. 1) slightly overturned toward the south. 

On the south side of the hill the rocks are partly altered and slightly 

mineralized near the contact of the Cuesta del Cura limestone and 

the Indidura formation. Possibly the fold was formed by the upward 

pressure of an unexposed intrusive body.

A long, narrow graben is present in the core of the Santa Rosa 

anticline. It can be traced for about 6.5 kilometers, mainly along 

Can6n de Las Lajas, and on the west it disappears under the alluvium 

that floors the valley. It is very narrow, having a maximum width 

of about 375 meters. The rocks within the trough are poorly exposed, 

but seem to consist mainly of red Mazapil conglomerate, with only a 

small proportion of siltstone or tuffaceous material. Bedding is 

obscure, although in one locality adjacent to the southern boundary 

fault the conglomerate is clearly dipping 45° N. In geneial, the 

graben structure appears to lie along a synclinal axis in the Zuloaga 

limestone.

In the eastern part of the Sierra de Santa Rosa an oval area 2.75 

kilometers long occupying the center of the anticline is underlain by 

altered volcanic agglomerate (?). The broad depression known as 

Valle de Santa Rosa has been eroded in this material, and the agglomerate (?) forms a conspicuous reddish hill known as Cerro Colorado 

(fig. 16) that rises about 400 meters above the level of the valley. 

The contact of the volcanic material and the surrounding sedimentary 

rocks is^largely concealed by alluvium or other debris, but where 

exposed it is a fault zone. In the Santa Rosa mine the fault plane
GEOLOGY, PHOSPHATE DEPOSITS, CONCEPCION DEL ORO DIST., MEX. 61

dips 30° to 50° W. and appears to steepen gradually with depth. 

The agglomerate (?) may once have blanketed a large region, but it is 

now wholly eroded except in this single area, where it has been preserved by faulting. The reason for the faulting is unknown, although 

the authors suggest that a volcanic vent may have been present in 

the area, and that the peculiar structure was caused by collapse of an 

underlying magma reservoir. No ring dikes have been observed in 

the area.

SANTA RITA ANTICLINE

The Santa Rita anticline is represented by a smill arcuate mountain 

range known as the Sierra de Santa Rita, which has a length of 46 

kilometers. It lies parallel to the Santa Rosa anticline and has a 

strike curving in a westerly direction from N. 20° W. to approximately 

west.

The fold is an asymmetrical doubly plunging anticline, in part 

moderately overturned toward the north. The south limb of the fold, 

however, has a normal inclination and an average dip of about 50° to 

the south and southwest. In the vicinity of San Feliciano valley the 

fold is overturned to the southeast, although in a southeasterly direction this limb steepens gradually to the vertical and then assumes a 

normal inclination to the east. This section of the fold plunges steeply 

beneath an alluvial plain at an angle of 30° to 35°. To the west of 

San Feliciano valley the amount of overturning toward the north decreases gradually until the fold is nearly symmetrical. The western 

end of the fold plunges beneath the Bolson de Cedros at an angle of 

approximately 5°.

To the west of the San Feliciano phacolith(?) the Zuloaga limestone, 

which occupies the anticlinal core, seems to develop a double fold 

similar to that in the Sierra de Santa Rosa. The more northerly anticline is overturned toward the north and is generally isoclinal except 

in the area to the west of the rhyolite stock in the Canon del Portrero, 

where it is asymmetrical and may have been modified by intrusion of 

the rhyolite, which apparently continues into the anticlinal structure 

to the west of the outcrop. The other anticline is also overturned 

toward the north and is generally isoclinal, except in the vicinity of 

the rhyolite stock, where it develops a fan structure.

The hills in the vicinity of La Cigiiena represent a part of the west 

limb of a small northward-trending anticline. The fold is asymmetrical, the axial plane dipping steeply to the east.

The granodiorite in the Valle de San Feliciano is believed to be much 

larger than is indicated by the outcrop area. During the process of 

emplacement the overlying sediments were probably arched strongly 

upward, thus producing the small to moderately large normal faults 

present in the vicinity of the body.
62 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS

A major fault can be traced from the San Feliciano intrusive body 

on the east to the lower end of Valle de Santa Rita on the west, a distance of about 19 kilometers. It is believed to be a high-angle thrust 

dipping to the south, although, except on the west, stratigraphic 

displacement has been relatively small. A thick breccia zone is seen 

in the few places where the fault is exposed.

The upper part of Valle de Santa Rita, a troughlike valley with a 

length of 7.5 kilometers and a maximum width of about 400 meters, is 

an anomalous topographic feature within the Zuloaga limestone, the 

principal ridge former in the region. It is apparently a graben structure, similar to a parallel structure in the Sierra de Santa Rosa, which 

formed along the zone of weakness created by the thrust fault. A 

few outcrops of Mazapil conglomerate occur along the southern margin 

of the valley, but exposures of arkose, siltstone, or tuffaceous beds are 

wholly lacking. The valley may have been thoroughly scoured, and 

all the rocks overlying the basal conglomerate, as well as most of the 

conglomerate itself, were removed by erosion.

GEOLOGIC HISTORY

The redbeds of the Huizachal formation, which do not crop out in 

the Concepcion del Oro district, are widely exposed in the area lying 

immediately to the west. They seem to have been deposited in arid 

basins in many sections of Mexico and elsewhere in both North and 

South America (Imlay, Cepeda, Alvarez, and Diaz, 1948, p. 1753- 

1761). The source of the redbeds in central and eastern Mexico was 

probably in moist uplands in the western and north-central parts of 

the country. Imlay suggests that deposition was not necessarily 

continental but may have been in large basins of highly saline waters. 

Such an origin is suggested by the generally even bedding and by the 

association of redbeds with salt deposits in southern Mexico.

During the Late Jurassic the formation cf the Mexican geosyncline 

began as a mobile belt or trough along the outer margin of the Coahuila Peninsula, a southerly extension of the North American continent 

in Mesozoic time and perhaps a continuation of the Ouachita erogenic 

belt. Although the Coahuila Peninsula must have had its origin in 

late Paleozoic time, it was probably rejuvenated in the Mesozoic, and 

it was this foreland area which formed the source of the Jurassic and 

Lower Cretaceous sedimentary rocks represented within the area 

mapped. Upper Cretaceous sedimentary rocks were derived from the 

west, however, as the foreland was covered by Late Cretaceous seas.

Marine transgression began in late Oxfordian time, when the 

Zuloaga limestone was deposited on an erosion surface truncating the 

redbeds. The Zuloaga sediments were laid down in a shallow epicontinental sea and consisted mainly of limestone, with a relatively 

small proportion of clastic material. In Kimmeridgian and Port-
GEOLOGY, PHOSPHATE DEPOSITS, CONCEPCION DEL ORO DIST., MEX. 63

landian time the character of the deposits changed and became predominantly clastic, possibly reflecting climatic changes in the adjacent 

land areas. During this period intermittent erogenic movements may 

have affected the geosyncline as well, as Burckhardt (1930, p. 72-73) 

has suggested in discussing the area west of San Pedro del Gallo, 

Durango, where the vertical or steeply inclined beds of Kimmeridgian 

age are overlain by Portlandian and Cretaceous sedimentary rocks 

dipping at a low angle toward the west. This orogeny may have been 

related to the Nevadan disturbance, which probably had a widespread 

influence in the region of the geosyncline. According to Imlay,6 

however, the apparent unconformity near San Pedro del Gallo should 

be thoroughly verified in the field, as there has been tight folding and 

some thrusting in the area, and this feature may actually be a thrust 

plane.

Although no erosional unconformity separating the beds of Kimmeridgian and Portlandian ages has been found within the Concepcion 

del Oro district, exposures are generally poor, and local unconformities 

might well exist. The marked differences in thickness of the La Caja 

formation, and of the units within the formation, suggest that the 

sediments were deposited on an uneven surface (fig. 19), and that the 

sea bottom was characterized by a series of gentle valleys and ridges, 

which may have been the product of minor tectonic movements. The 

ridges must have had low relief, and a few may have emerged and 

formed low, relatively small islands from which the reptile remains in 

the La Caja formation may have been swept. This type of submarine 

topography may have impeded the circulation of sea water in some 

areas. Imlay reports that in the Sierras de Parras, Atajo, and Jimulco 

which lie to the north and northwest of the area mapped, the Upper 

Jurassic sedimentary rocks contain abundant gypsiferous and carbonaceous beds, which imply lagoonal conditions and, therefore, some 

sort of barrier to the south. Within a limited area such as the Concepci6n del Oro district, it is difficult to determine whether the hollows 

and ridges were elongated in a particular direction or were irregular in 

form, although there is some suggestion of a northwestward trend.

Apparently there was no emergence at the end of the Jurassic, 

although the boundary with the Cretaceous seems to be marked by a 

diastem, for sediments representing the Berriasian are absent. This 

diastem, like the earlier oscillations, may be related to the Nevadan 

orogeny. During the Early Cretaceous, normal limestone was deposited in a moderately deep sea, with some layers of clastic materials 

being deposited from time to time, probably reflecting temporary 

changes in climate or elevation of the adjacent land areas.. At the 

beginning of the Late Cretaceous there was an abrupt change to 

predominantly clastic sedimentation, and from that time onward

6 Written communication.
64 GEOLOGIC INVESTIGATIONS LN THE AMERICAN REPUBLICS

E oom <

a <=
GEOLOGY, PHOSPHATE DEPOSITS, CONCEPCION DEL ORO DIST., MEX. 65

the ratio of clastic material to chemically or biochemically precipitated 

lime continued to increase. During this period the land masses to the 

west must have been rising slowly, providing increasingly greater 

quantities of mud and silt for the shallow seas. From Coniacian to 

Campanian time, but mainly in the Coniacian, large quantities of 

arkosic sand were added to the calcareous muds then being deposited.

During the Laramide orogeny, probably started in latest Cretaceous 

time and continuing into the middle Eocene (Muir, 1936, p. 140), the 

Mexican geosyncline was compressed by forces acting from the south 

and west, almost at right angles to the margin of the Coahuila 

Peninsula, which served as a stable buttress. The sedimentary rocks 

were thrown into a series of narrow, elongated folds, most of which were 

parallel to the.borders of the old land masses and were overturned 

toward the north and east, although the deformation also produced 

many fan structures and some overturning toward the south and west. 

The strong overturning culminated with some thrusting. Toward the 

end of the orogeny some anticlinal structures were invaded by large 

masses of granodiorite or diorite, which metamorphosed the surrounding sediments and in some areas strongly deformed them. Following 

emplacement and cooling of the magmas many localities were mineralized. As the compressional phase subsided, mountain building 

ended and was succeeded by a phase in which tensional stresses predominated. These stresses produced first many relatively small 

transverse faults, but during later stages were responsible for fairly 

extensive graben and block faulting.

Block faulting began in late Eocene time and formed basins in which 

the red conglomerates and associated sediments accumulated. Fault 

movements continued into the early part of the Oligocene but probably 

halted soon after deposition ended. The unconformity separating 

the red conglomerate from the underlying rocks in the area north of 

Mazapil is evidence that fault movement occurred during deposition 

of the Mazapil conglomerate.

The last major chapter in the diastrophic history of the region was 

a second period of major block faulting that occurred in late Miocene 

or early Pliocene time and that started deposition of middle Pliocene 

to Pleistocene valley fills. The Canutillo fault, formed after the 

extrusion of basalt, is probably Pliocene in age and related to these 

relatively recent fault movements. Other contemporaneous faults 

may have formed the basins that bound the area mapped on the east 

and west, as well as several other basins in the region.

Although graben structures apparently originate in several different 

ways (Taber, 1927, p. 581-590), those associated with anticlines, 

domes, and monoclinal flexures are probably the product of tensional 

stresses due to differential uplift. Generally, such structures are 

relatively small and extend only to a shallow depth.
66 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS

Since the end of the Laramide orogeny the geosynclinal belt has 

remained above sea level, and the anticlinal ranges within the geosynclinal belt have been subjected to continuing strong erosion. Although 

this time period has generally been characterized by increasing aridity, 

it may have been punctuated by occasional brief humid cycles. Such a 

humid cycle may be represented by the red limestone-boulder Mazapil 

conglomerate, which has been preserved in several of the graben areas. 

This conglomerate may be correlative with the red conglomerate at 

Guanajuato, which apparently is late Eocene or early Oligocene in age.

At some time in the Eocene and following emplacement of the deepseated granodiorite stocks, many sills of andesite, along with some 

associated aplite, intruded the rocks of the Caracol formation and 

Parras shale on the south side of the Bonanza valley. Similar rocks 

in the Melchor Ocampo district have been described by Imlay (1938b, 

p. 1671), who presents evidence indicating that the intrusion occurred 

toward the end of folding, but before final overturning. Deposition 

of the Mazapil conglomerate was accompanied by volcanic activity, 

which added andesitic and rhyolitic ash to the sediments.

Following deposition of the Mazapil conglomerate the region was 

intruded by a magma ranging in composition from dacite to latite. 

This rock is found mainly intruding limestones in the Sierra de Santa 

Rosa, although a few sills and dikes of dacite have been found intruding 

the loosely consolidated rocks of the Mazapil conglomerate in the 

Santa Rosa graben and in the downfaulted area lying on the north side 

of the Mazapil valley. The dacitic magma may have reached the 

surface through a volcanic vent at the present site of Valle de Santa 

Rosa. If the continental deposits of the Mazapil conglomerate are late 

Eocene or early Oligocene, as a tentative correlation with the 

red conglomerate at Guanajuato seems to indicate, the dacites are 

probably Oligocene in age.

The remnants of the other igneous rocks present in the Concepci6n 

del Oro district suggest three major periods of volcanic activity. The 

first is represented only by the round andesitic body that occurs on the 

crest of the Sierra de La Caja, in the area lying immediately to the east 

of the high summit known as Cerro de Los Carneros. This rock may 

fill a volcanic orifice, suggesting that andesitic rocks reached the 

surface. None of the ejected material has been found, however, and it 

may have been completely eroded.

During the second period of eruption small rhyolitic bodies intruded 

the Santa Rita, Santa Rosa, and La Caja anticlines, and thin flows 

covered at least part of the Sierra de La Caja. One flow unconformably overlies rocks of the Mazapil conglomerate in the Valle de 

La Caja graben and has been downfaulted with those rocks. The 

other flows occur only as small remnants capping several mountain 

summits in the eastern part of the range.
GEOLOGY, PHOSPHATE DEPOSITS, CONCEPCION DEL ORO DIST., MEX. 67

During the last stage of volcanic activity a series of basalt flows was 

extruded and may have covered much of the district, although the 

flows now remain only in two relatively small areas. The age of these 

rocks is unknown, but they are certainly more recent than the acid 

volcanic rocks and may have been extruded in late Miocene or early 

Pliocene time. They seem to have originated after the topography 

had largely assumed its present form, and although they have been 

downfaulted in the Bonanza valley, there is no evidence of faulting in 

the Las Mesas area. Since the volcanic rocks in the Las Mesas area have 

not been preserved by graben faulting, it seems doubtful if they could 

have survived erosion for a long period of time. They are underlain 

by loosely consolidated arkose, which is easily eroded, and the flows are 

continually being undermined and reduced in size.

The possible sequence of igneous rocks and tentative age assignments 

are summed up as follows

Pliocene.

?__-___________ Basalt flows

Miocene_ ______ Rhyolite intrusive and flow rocks

?______________ Andesite

Oligocene_____ Dacite and agglomerate(?)

?______________ Pyroclastic rocks (Mazapil conglomerate) .

Eocene_________ Hornblende-pyroxene andesite and

associated aplite.

Eocene_ ______ Granodiorite, with dioritic and alaskitic facies.

PHOSPHATE DEPOSITS 

THE PHOSPHATE MINERAL (FLUORAPATITE)

The phosphate mineral in the Concepci6n del Oro rocks is dense 

cryptocrystalline material with a concretionary habit. This material, which contains various impurities, is generally known as collophane. Part of it is primary, and the rest is secondary. Primary 

phosphate forms small generally structureless pellets ranging in size 

from 0.05 millimeter to more than 1 millimeter; secondary phosphate 

replaces primary pellets and fossils and is disseminated through the 

rock matrix. The phosphatic material ranges in color from light 

gray or almost colorless to pinkish, brown, and intensely black. The 

color, and also a characteristic fetid odor, may be dependent upon 

the quantity of organic matter present. The material has a hardness 

of from 3 to 4 and a specific gravity ranging from 2.5 to 2.9. In 

thin section the phosphatic material is commonly light to medium 

dark brown and, owing perhaps to its color and fine grain, appears 

to be isotropic. Only occasional grains are anisotropic.
68 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS

The phosphate mineral belongs to the apatite series, which has the 

general formula Cas (PO4) 3 (F, Cl, OH) (Palache, Berman, and Frondel, 1951, p. 882), and can be classed as the member fluorapatite, 

Ca5 (PO4) 3F. Fluorine is present in a proportion equal to 10.1 percent of the P2 O5 content. Altschuler and Cisney (1952) have found 

that marine phosphorites are generally composed of carbonatian 

fluorapatite, and a study of several chemical analyses of the Concepcion del Oro deposits indicates that they also may be slightly to 

moderately carbonatian. Possibly the anisotropic material seen in 

thin sections contains more carbonate than the bulk of the phosphatic material.

Owing to the presence of a simple cation, usually calcium, and an 

anionic radical, PO4 , as well as a simple anion, usually fluorine, 

apatite is particularly susceptible to substitutions. Minor quantities 

of magnesium, manganese, strontium, lead, sodium, uranium, and 

the rare earths may substitute for calcium, and these and other substitutions largely explain the abundance of the minor metals found in 

many phosphorites, including those of the Concepcion del Oro district. 

The presence of the carbonate may result by substitution of CO3 OH 

for PO4 (Palache, Berman, and Frondel, 1951,rp. 882).

Many phosphatic samples collected in the Concepcion del Oro 

district were analyzed semiquantitatively by spectrograph by the 

United States Bureau of Mines and the Geological Survey. Twentyeight elements were identified in addition to those found in chemical 

analysis. All are present in minute quantities, and apparently all 

are too dilute to have commercial value. Table 5 summarizes spectrographic analyses of 30 samples, including 10 samples each of the 

calcareous phosphorite, phosphatic calcareous chert, and phosphatic 

limestone. The rarer elements show little variation in percentage 

in these different lithologic types. The table brings out several 

points, among which are the almost complete lack of potassium in 

the phosphate rocks and the relative abundance of lanthanum, strontium, and yttrium. Other spectrographic analyses, not included in 

the table, show that some phosphorite also contains the elements 

beryllium, cobalt, bismuth, columbium [niobium], niobium, and 

ytterbium.

Twelve samples of phosphate rock that'were analyzed by radiation 

and chemical methods specifically for uranium ranged from less than 

0.001 to 0.006 percent in equivalent uranium (eU). Uranium and the 

rare earths are present in the phosphate mineral and are therefore 

most abundant in the highest grade phosphorites. Zinc, nickel, and 

molybdenum probably also occur in part in the phosphate mineral, 

although they seem to be almost as abundant in the weakly phosphatic rocks as in the phosphorites.
GEOLOGY, PHOSPHATE DEPOSITS, CONCEPCION DEL ORO DIST., MEX. 69

TABLE 5. Results of spectrographic analyses of 30 samples of phosphate rock from 

the Concepcidn del Oro district

[Analyses by P. R. Barnett and R. G. Havens, Geological Survey]

Element

Si... ... ... 

Al 

Fe... ... ... ...

Ti__ 

Mn ... ._ ... _

P ... ... 

Ca ... 

Mg ... 

Na... ... ... 

K ........... .............

Ag

B . -_ 

Ba

Cr_. ........................

Cu ... ... 

Ga ... ... ...

La ... ... 

Mo... .

Ni....... ...............

Pb_ ... ... . .... .. 

Be.. ... ... ... ... 

Sn - . .

Sr__ 

V . 

Y

Zn....__.___._ . . _. 

Zr__ __ . ..........

Number of analyses with indicated range in percent

>10

11

28

1-10

19 

3 

6

30 

2

0.1-1.0

27 

24

23 

3 

1

1

8

0.01-0.1

26 

21

7 

25

1 

16 

3

10

22 

22 

19

18

0.001-0.01

4 

9

2

3 

11

14 

27 

26

11 

2

28 

5 

6

8 

11

29

0.0001-0.001

20 

4

4 

2 

1 

8 

2 

3 

13 

5

1

NOTE. Looked for but not found: As, Au, Be, Bi, Cd, Ce, Co, Ge, In, Ir, Hf, Hg, Li, Kb, Os, Pd, Pt, 

Re, Ru, Sb, Sm, Ta, Te, Th, Tl, U, W.

Although sampling has shown that nearly all the La Caja formation 

is at least feebly phosphatic (table 1), by far the greatest concentration 

of phosphatic material is in unit C. In general, the grade of the rock 

decreases in inverse proportion to the thickness of the member. 

Where it is thick the phosphate is so dispersed that beds of commercial 

grade are lacking. Although the unit may range from less than 1 

meter to 20 meters in thickness, the total phosphate present generally 

ranges only from 20 to 40 meters-percent (thickness in meters times 

percent of P205). The overall range within the area mapped, however, 

is from 10 to 70 meters-percent.

In the Sierra de Santa Rosa, which contains the only deposits with 

commercial possibilities, unit C ranges from 1.35 to 3.30 meters in 

thickness, and has an average thickness of 2.15 meters.

Outward from the Sierra de Santa Rosa, the phosphatic member 

thickens within a short distance, having a maximum thickness of about 

20 meters in several places, and the phosphorites grade into a series of 

thin- to medium-bedded siltstones containing many lenses of phosphatic limestone. The siltstones are highly calcareous, and many are 

cherty as well. They are soft to moderately hard brown to grayishbrown rocks containing differing proportions of phosphate and are 

commonly interbedded with scattered thin layers of calcareous phosphorite. Such rocks rarely average more than 5 or 6 percent P2O5.
70 GEOLOGIC INVESTIGATIONS IN THE AMERICAN EEPUBLICS

LITHOLOGY, PETROGRAPHY, AND CHEMICAL COMPOSITION

The phosphate deposits are predominantly chemical or biochemical 

sediments, containing very little detrital material. The lower part of 

the phosphatic member consists mainly of phosphatic calcareous 

chert, phosphatic cherty limestone, and cherty phosphorite; the upper 

part is calcareous phosphorite (fig. 20). Both parts contain many 

lenses of phosphatic limestone and locally may contain phosphatic 

siltstone.

The phosphate occurs mainly in small round to oval or irregular 

pellets, which are embedded in a matrix consisting of calcite, silica,

FIGURE 20. Exposure of phosphatic unit of La Caja formation in Canon de La Canela, Sierra de 

Santa Rosa, cp, calcareous phosphorite; pi, phosphatic limestone; ccp, cherty calcareous phosphorite; pec, phosphatic calcareous chert.

and apatite. The silica has probably migrated in part from adjacent 

cherty sediments.

The principal lithologic types present in the phosphatic member 

and adjacent rocks are described in order of importance.

CALCAREOUS PHOSPHORITE

Calcareous phosphorite, which is the most abundant rock type in 

the phosphatic member, as well as the richest in P2O5 , composes the 

upper section of unit. C in the Sierra de Santa Rosa, where it reaches 

its greatest size. It has a maximum thickness of 1.25 meters and an 

average grade, within the reserve areas, of 21.1 percent P2O5 . It disappears within a short distance to the east of Valle de Santa Rosa 

and diminishes to the north and south as well. In places on the south 

flank of the Sierra de La Caja it may appear in two or three beds
GEOLOGY, PHOSPHATE DEPOSITS, CONCEPCION DEL ORO DIST., MEX. 71

rather widely separated by phosphatic siltstones and other rocks. 

Such layers have a maximum thickness of 85 centimeters, but in most 

places they range only from 10 to 25 centimeters. The calcareous 

phosphorite has not been seen farther to the north, and on the south 

it occurs only on the north flank of the Sierra de Santa Rita, where it 

reaches a maximum thickness of about 55 centimeters.

Most of the calcareous phosphorite is thick bedded to massive and 

moderately hard. It is medium dark reddish gray and has a strongly 

speckled appearance, owing to an abundance of light-pinkish or 

pinkish-white phosphate pellets, which are enclosed in a groundmass 

consisting mainly of very dark crj^stalline calcite. Secondary calcite 

commonly occurs in small veinlets, which may carry free fluorite. 

The- rock has a strongly fetid odor when freshly broken.

In most places the rock is somewhat conglomeratic and contains 

small scattered fragments of phosphate rock darker than the host 

rock. These fragments may be sharply angular but are generally 

subangular to rounded or somewhat tabular in form, and, although 

they are easily visible on weathered surfaces, they are never conspicuous.

Petrographic study reveals that the rock consists mainly of small 

apatite pellets or replacement oolites embedded in a coarsely crystalline 

calcite matrix (figs. 21, 22, and 23). The pellets are well sorted and 

range from 0.1 to 1.0 millimeter in diameter, although they are generally not much larger than 0.5 millimeter. Many are wholly or partly 

replaced by calcite or chert, or both. This replacement was apparently followed by redeposition of apatite as matrix material and, 

in many places, as concentric laminations on the pellets, giving them 

an oolitic appearance. Many pellets have a rounded or oval form, 

but some are irregular, and they are lacking in nuclear impurities. 

The apatite is brown massive cryptocrystalline material, which is 

almost uniformly isotropic. The brown color may be due to the inclusion of organic matter. The chert seems to be present almost wholly 

as a replacement of apatite and may have migrated from the adjacent 

sediments, where it is relatively abundant. The rock is extensively 

fractured, and in thin sections many minute fractures were filled with 

secondary calcite.

The rock fragments were found to have the same constituents as 

the host rock, although generally in widely different proportions. 

They are mainly a fairly intimate mixture of apatite, calcite, and 

chert, although apatite is predominant. They contain fewer pellets 

than the host rock. They may be well denned, but in many places 

they have indefinite outlines and appear to merge gradually into the 

matrix, owing apparently to the movement and redeposition of apatite 

and other minerals. Some are partly rimmed by calcite or by a 

sheath of secondary apatite. Probably many of these fragments are
72 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS

FIGURE 21. Photomicrograph of calcareous phosphorite. A part of a large rock fragment or 

nodule can be seen on the left. The fragment contains a few apatite pellets but is composed mainly of massive apatite, with minor calcite and chert. The remainder of the 

picture shows round to ova] apatite pellets in a coarsely crystalline calcite matrix. 

Some of the pellets exhibit replacement by calcite, ca, or secondary apatite, ap.

FIGURE 22. Photomicrograph of calcareous phosphorite. Apatite (ap) pellets in a matrix 

which is predominantly calcite but contains a considerable amount of massive apatite. 

The pellets show replacement by calcite, ca, chert, c, and secondary apatite, ap.
GEOLOGY, PHOSPHATE DEPOSITS, CONCEPCION DEL ORO DIST., MEX. 73

FinuRE 23. Photomicrograph of calcareous phosphorite, 

nicols. The apatite is dark under crossed nicols.

Like figure 22, but with crossed

nodules that, in varying degrees, have been transported and subjected 

to abrasion.

The calcareous phosphorite contains fairly abundant ostracodes and 

some Foraminifera (Globigerinids?) , 7 which are generally phosphatized. None could be identified specifically.

The chemical composition of the calcareous phosphorite is shown 

in table 6. These and other analyses indicate that the principal 

mineral constituents of the calcareous phosphorite may have the 

average percents given below.. No detrital minerals, except very 

minor quartz, were recognized in the thin sections.

Apatite_____________________________________________ 54. 8

Calcite__________._______________________ 33. 1

Chert_________________.________________ 10. 5

Miscellaneous material.________________________________ 1. 6

Total_______________________________ 100. 0

In the area immediately east of the dacite intrusive body of Las 

Parroquias the phosphatic member contains a calcareous phosphorite 

not found elsewhere. It occurs in two layers totaling 77 centimeters 

hi thickness, separated by a lens of phosphatic limestone. The layers 

are composed of a soft to moderately hard, thin- to medium-bedded,

7 'The calcareous phosphorite and other phosphatic rocks were examined for microfossils by C. TellezGiron de A. and A. Ayala of Petroleos Mexicanos, and by. Ruth Todd of the U. S. Geological Survey.
74 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS

light- to medium-gray rock having a strongly speckled appearance 

and containing an abundance of gray to black apatite pellets in a lightcolored highly calcareous groundmass. Petrographic study reveals 

that the rock consists of light-gray oval to ellipsoidal apatite pellets 

in a calcite and chert matrix. The pellets are well sorted in some beds 

and poorly sorted in others. They have a size range of 0.05 to 0.8 

millimeter and an average diameter of about 0.3 millimeter. They 

differ from the pellets in the usual calcareous phosphorite, which 

overlies this rock, in their pale-gray color, which perhaps indicates a 

low organic content, and in their lack of concentric laminations. The 

pellets are discrete, and there is relatively little evidence of alteration 

in the thin sections, although a few of the pellets have been partly or 

wholly replaced by calcite or chert. The matrix is a mixture of carbonate and chert, which occur in varying proportions in different beds. 

Some fossil material is present and is largely replaced by apatite. 

One thin section contains rotaliform Foraminifera, in addition to the 

usual megafossils. The rock has an average grade of 24.7 percent 

P2O5 and an average acid-insoluble content of 29.0 percent. The grade 

falls off very rapidly toward the east, and the rock is transitional to a 

phosphatic calcareous siltstone. Only 800 meters to the east it has a 

total thickness of 1.52 meters and an average grade of 14.9 percent 

P206 .

TABLE 6. Partial chemical analyses of five samples of calcareous phosphorite from

the Sierra de Santa Rosa

[U. S. Geological Survey analyses by Outtag, Caemmerer, Barlow, Rowe, and Tucker; reported

June 13,1951]

Sample no.

PR5-1-.- 

PR9-2 _

PR14-1-.. __

PR30-4- -_

PR33-3--.---_

Laboratory no.

36970

36999

37042

51280

51292

Strati-' 

graphic 

thickness 

(meters)

1.10

1.05

.90

on

P2O5

10 7

99 7

20.1

20 9

CaO

47.50

47.50

48.75

C02

14.92

14.30

17 84

16.86

14.20

Acid 

insoluble

15.5

1ft 9

6.6

9.8

11.6

F

9 nfi

2.01

2.28

A12O3

1.10

1.10

.67

.67

.86

Fe2O3

0.35

.40

.23

.46

.58

V2Os

0.01

.01

.02

.02

.03

CHBRTY CALCAREOUS PHOSPHORITE

The cherty calcareous phosphorite is limited mainly to the south 

flank of the Sierra de Santa Rosa. On the east it disappears beyond 

Valle de Santa Rosa, and on the west it lenses out in the upper part of 

Canon de Los Mimbres. It has a thickness ranging from 16 to 40 

centimeters, averaging 25 centimeters, and has an average grade of 

20.1 percent P2O5.

The rock is medium dark reddish gray to dark reddish black and 

obviously siliceous. No bedding is discernible, but the rock parts 

easily along irregular closely spaced planes which are roughly parallel 

with bedding in the adjacent sediments. It is hard and only moder-
GEOLOGY, PHOSPHATE DEPOSITS, CONCEPCION DEL ORO DIST., MEX. 75

ately fetid. It contains an abundance of black pellets and fossil 

remains in a lighter colored groundmass.

Petrographic study shows that the rock is composed of brown 

apatite pellets, fragments of phosphate rock or nodules, and phosphatized fossil remains in a matrix consisting mainly of cryptocrystalline isotropic chert with subordinate calcite (fig. 24). The pellets are 

oval to ellipsoidal and poorly sorted. In sections examined they range 

from 0.05 to 0.9 millimeter in diameter and average about 0.25 milliFIOURE 24. Photomicrograph of cherty calcareous phosphorite. A part of a rock frag 

ment or nodule can be seen in the lower part of the photograph. The fragment is composed of a few apatite pellets, one chert, e, pellet and a fossil fragment, /, in a matrix of 

massive apatite. The groundmass is composed of apatite, ap, pellets and several phosphatized microfossils, mf, in a predominantly chert matrix. Small hematite, ft, grains 

are visible in both the rock fragment and groundmass.

meter. Like the calcareous phosphorite, this rock is slightly conglomeratic. The foreign fragments are small and scattered, however, and 

are very inconspicuous except under magnification. They are subrounded to rather angular, and some are composed essentially of 

apatite and chert pellets cemented by brown finely crystalline apatite. 

Others contain little or no pelletal material, but may be composed of 

rather intimate mixtures of apatite, calcite, and chert. These constituents occur in varying proportions, but apatite is generally greatly 

predominant. Reddish iron-oxide fragments are in many places 

associated with the cementing material in the rock fragments, but they 

are also fairly abundant in the rock matrix.

368566 56 6
76 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS

The rock contains many fossils largely replaced by apatite. They 

are mainly small pelecypods, but the rock includes a number of 

cerithioid gastropods. Microfossil study reveals an abundance of 

Radiolaria, which are altered and cannot be identified specifically.

A partial chemical analysis of one sample of cherty calcareous 

phosphorite is given in table 7. This and many additional analyses 

for phosphorous pentoxide and acid insoluble indicate that the principal mineral constituents of this rock may form the average percents 

given below. No detrital material was recognized in thin sections. 

Apatite_____________________________________ 52

Chert________ _________________________________ 27

Calcite_____________________________ 18

Miscellaneous material. __________________________________ 3

Total. _______________________________ 100

PHOSPHATIC CHERTY LIMESTONE

The phosphatic cherty limestone can be traced along the southern 

flank of the Sierra de Santa Rosa for about 9 kilometers. It does 

not occur to the east of Valle de Santa Rosa, and on the west it lenses 

out in the lower part of Canon de La Canela. It has a thickness

FIOTJRE 25. Photomicrograph of phosphatic cherty limestone. A partially phosphatized fossil, /, and a part of a rock fragment or nodule appear in a groundmass containing scattered apatite, ap, pellets in a rather intimate mixture of calcite, ca, and 

apatite, with the calcite predominating. The rock fragment or nodule is mainly 

massive apatite, with some calcite and scattered small detrital quartz, g, grains.
GEOLOGY, PHOSPHATE DEPOSITS, CONCEPCION DEL ORO DIST., MEX. 77

ranging from 10 to 25 centimeters and averaging about 17 centimeters, and has an average grade of 14.0 percent P2OS .

The rock is medium gray with reddish or brownish tints. It is soft 

to moderately hard and is generally easily broken with a hammer. 

It contains an abundance of black pellets and fossil material in a 

lighter colored groundmass and is moderately fetid.

Petrographic study shows the rock to consist of dark-brown apatite, 

small oval apatite pellets, fossil remains, and irregular rock fragments 

or nodules, all cemented by a mixture of crystalline calcite, chert, and 

light-brown apatite (fig. 25). The rock fragments are composed of 

phosphate pellets and detrital quartz grains in an apatite matrix. 

The fossils are generally phosphatized, but some are partly replaced 

by calcite, and some are partly replaced by secondary silica, commonly as feathery fringes.

No microfossils were found in the specimens studied, although 

megafossils are abundant, consisting mainly of small cerithioid gastropods and small pelecypods. Fish and reptile remains also occur 

in this rock.

Analyses indicate that the principal mineral constituents of the 

phosphatic cherty limestone may form the average percents given 

below. The quartz occurs in part as detrital material, but mainly 

as secondary chert, which may have migrated from the adjacent 

sediments.

Calcite___------______- _____________________________ 38

Apatite___________________________________________ 36

Quartz______________________________________ 24

Miscellaneous material- _______________________________ 2

Total_____________________________________ 100
78 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS

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GEOLOGY, PHOSPHATE DEPOSITS, CONCEPCION DEL ORO DIST., MEX. 79 

PHOSPHATIC CALCAREOUS CHERT

Phosphatic calcareous chert occurs mainly in the Sierra de Santa 

Rosa. It disappears to the east of Valle de Santa Rosa, but on thewest it can be traced to the end of the range. A similar rock is found 

on the southern flank of the Sierra de La Caja and on the northern 

flank of the Sierra de Santa Rita, but it forms relatively thin layers 

and is clearly transitional to other rock types. It ranges in thickness

FIOTTRE 26. Photomicrograph of phosphatic calcareous chert. Scattered, irregular apatite, ap, pellets in a matrix of calcite and isotropic chert. Several apatite pellets have 

been replacad by calcite, ca.

from a few centimeters to 1.68 meters and has an average grade of 

about 12.5 percent P2O5 .

The rock is medium bedded to massive and is medium gray, although 

much of it exhibits slight reddish or brownish tints. It is hard to 

brittle and moderately fetid when struck with a hammer. It contains 

an abundance of black pellets and many black phosphatized fossils, 

which are enclosed in a hard, gray matrix. In general, the rock 

becomes increasingly phosphatic and calcareous, and less siliceous, 

from the base upward. It contains many short lenses of phosphatic 

limestone, particularly in the lower part of the zone.

Petrographic study reveals that the rock is composed of small 

pellets, fossil remains, and phosphatic rock fragments or nodules in 

a matrix of chert and calcite (fig. 26). The pellets are oval to irregular and are poorly sorted. They have a size range from 0.05 to 0.7
80 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS

millimeter and average about 0.2 millimeter. They consist mainly 

of dark-brown apatite, but some have been extensively replaced by 

chert. The fossils are replaced by brown apatite, and in a few places 

by chert. The rock fragments or nodules are small, scattered, and 

rounded to subangular. They are inconspicuous except in thin 

sections and resemble the fragments found in the cherty calcareous 

phosphorite. Some consist of phosphate pellets cemented by finely 

crystalline apatite, but others contain little pelletal material and 

may have variable proportions of chert and calcite intimately mixed 

with the apatite. The matrix of the phosphatic calcareous chert is 

predominantly a light-brown to colorless cryptocrystalline chert, 

which is generally isotropic but may be anisotropic. Crystalline 

calcite is common but rather variable in proportion. A small quantity 

of detrital quartz was seen in the matrix in one thin section.

The silica in the cherty calcareous phosphorite and phosphatic 

calcareous chert may be primary in origin, but to some extent it has 

migrated within these layers and has been redeposited, for it is found 

replacing the apatite pellets, and less commonly the fossils. Perhaps 

it has migrated also to adjacent layers of calcareous phosphorite and 

phosphatic limestone.

The faunal assemblage in the phosphatic calcareous chert appears 

to be similar to that in the cherty calcareous phosphorite, including 

fairly numerous small pelecypods and small cerithioid gastropods. 

Radiolaria are also present but cannot be identified specifically.

A partial chemical analysis of one sample of phosphatic calcareous 

chert is given in table 7. This analysis and many additional analyses 

for P2O5 and acid insoluble indicate that the principal mineral constituents of the rock may form the following average percents.

Quartz-________________________________ 45. 0

Apatite_________________________________ 32. 5

Calcite_________ __________________ ____ 19. 5

Miscellaneous material_________________________________ 3. 0

Total--__________________________________ 100. 0

PHOSPHATIC LIMESTONE

Phosphatic limestone occurs abundantly throughout the area 

mapped and in the surrounding region as well. It may be found in 

any part of the phosphatic member and is commonly present in the 

cherty beds forming the upper section of unit B. It is a medium-dark 

to dark-gray or reddish-gray phosphatic limestone, which develops 

a characteristically smooth light-gray or bluish-gray weathered 

surface. In many places it forms short, relatively thick lenses, 

and in some localities these develop into fairly persistent beds, which 

are commonly characterized by pinching and swelling. The beds 

range from a few centimeters to more than a meter in thickness.
GEOLOGY, PHOSPHATE DEPOSITS, CONCEPCION DEL ORO DIST., MEX. 81

The phosphatic limestone has the lowest phosphate content of all 

the rock types within the phosphatic member, and, unlike the other 

rocks, the lenses and beds of phosphatic limestone may have great 

differences in the total phosphate present. The phosphate content 

not only differs from one bed to the next, but also laterally within a

FIGURE 27. Photomicrograph of phosphatic limestone. Small round apatite pellets 

in a finely crystalline calcite matrix. The small rounded rock fragment or nodule near 

the center is largely massive apatite, which is cut by a calcite, ca, veinlet.

given bed. The phosphatic limestone is the most strongly fetid 

rock in the phosphatic member.

Petrographically, the rock is a finely crystalline limestone containing 

scattered small apatite pellets, a little chert, and a minor proportion 

of detrital quartz fragments (fig. 27). The apatite ranges from pale 

brown to almost black. Many pellets have been replaced by calcite.

The rock contains many fossils replaced by apatite. None of the 

microfossils could be identified specifically, but they are largely 

Radiolaria. Small cerithioid gastropods are present in some beds.

Partial chemical analyses of two samples of the phosphatic limestone are given in table 7. The acid insoluble is mainly silica and is 

present partly as detrital quartz and partly as secondary chert which 

may have migrated from adjacent cherty rocks.
82 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS 

PHOSPHATIC CALCAREOUS SILTSTONE

Although the phosphatic member in the Sierra de Santa Rosa is 

almost wholly composed of the five lithologic types described above, 

phosphatic calcareous silts tone may be interbedded with them locally. 

This rock is not markedly different in appearance from other siltstones 

in the formation, except that it is relatively rich in apatite pellets and 

may contain a few thin beds of calcareous phosphorite.

Partial chemical analyses of two samples of phosphatic calcareous 

siltstone are given in table 7. Sample PR25-7 represents a layer 40 

centimeters thick of a moderately hard, thin- to medium-bedded, 

grayish-brown phosphatic calcareous siltstone containing scattered 

thin beds of calcareous phosphorite. Sample PR4-6 represents a 

layer 20 centimeters thick which lies between a lens of phosphatic 

limestone and a layer of cherty calcareous phosphorite. It is a soft, 

brownish, predominantly thin-bedded, phosphatic calcareous siltstone. Both samples are noticeably higher in vanadium content than 

the phosphorites and phosphatic cherts. They are also higher in 

content of alumina and iron oxide.

PHOSPHATIC ROCKS ADJACENT TO PHOSPHATIC MEMBER

In the Sierra de Santa Rosa the calcareous siltstones immedi 

ately overlying the phosphatic member have been sampled in a few 

localities and were found to be weakly to moderately phosphatic. 

Nowhere are they sufficiently rich to have commercial possibilities, 

however. They grade upward into normal siltstones containing little 

more than a trace of phosphatic material.

The upper section of unit B, which underlies the phosphatic member, also contains many phosphatic layers, of which only a few were 

sampled, as they are too thin and too low in grade to be exploited. 

They differ in lithologic character and include phosphatic cherts, 

phosphatic calcareous cherts, phosphatic limestones, phosphatic siltstones, and a few thin calcareous phosphorites. Some of the phosphorites are highly conglomeratic. One conglomeratic bed was 

examined petrographically and was found to be composed of large 

brown phosphatic rock fragments in a matrix of apatite and calcite. 

The rock fragments are phosphatic limestone composed of poorly 

sorted apatite pellets, fragmental pellets, and fossil remains in a cement 

of coarsely crystalline calcite. A few of the pellets are replaced by 

chert. The rock matrix is composed of massive brown secondary 

apatite and coarsely crystalline calcite, with abundant apatite pellets 

and fossil remains disseminated throughout. Small quantities of 

chert and brown iron oxide are also present in the matrix. The rock 

was examined for microfossils and may contain rotaliform and agglutinated Foraminifera and globigerinids, although none could be 

identified genericaUy.
GEOLOGY, PHOSPHATE DEPOSITS, CONCEPCION DEL ORO DIST., MEX. 83

ORIGIN

The outstanding work of recent years on the formation of marine 

phosphorites has been that of Kazakov (1937, 1950), who has discarded the traditional theory of biolith-diagenetic origin and substituted the hypothesis that phosphate is chemically precipitated from 

sea water. The deep waters of the ocean basins contain an enormous 

store of phosphate, and he believes that where these waters well up 

onto the continental slopes, the phosphate may be precipitated by an 

increase in the pH of the ascending waters as the temperature increases 

and the partial pressure of carbon dioxide decreases. He states that 

calcium carbonate reaches the saturation point first and is precipitated, and this is followed by simultaneous precipitation of calcium 

carbonate and calcium phosphate. He believes that phosphate is 

formed between depths of 50 and 200 meters. It is not precipitated 

in shallower waters, where most of the available phosphorus is utilized 

by the planktonic population, and conditions of supersaturation are 

not attained at depths greater than 200 meters.

Kazakov's theory has been accepted in large part by McKelvey 

(McKelvey, Swanson, and Sheldon, 1953) and other geologists of the 

U. S. Geological Survey working on the Phosphoria formation of the 

Northwestern United States, and, with some important modifications, 

it has been used by them to explain the origin of those deposits. 

McKelvey and his associates made the following three modifications 

of his theory. First, the solubility curves of apatite and carbonate 

overlap but do not coincide precisely, so that either apatite or carbonate may be precipitated independently. Also, apatite is generally 

precipitated at a lower pH and temperature than is calcium carbonate, 

rather than the reverse as indicated by Kazakov. Second, phosphorites are not completely a product of chemical precipitation, but a 

significant amount of phosphate is probably derived through the 

decay of organic matter. When sea water is essentially saturated 

with phosphate, the phosphate derived from decaying ogranisms is not 

resorbed by the water and probably accumulates with the bottom 

sediment. And third, the depths at which phosphate is being deposited on the present sea bottom (Dietz, Emery, and Shepard, 1942) 

suggest that the zone of phosphorite formation may lie between 200 

and 1,000 meters, rather than in the shallower area described by 

Kazakov.

Marine phosphorites include two major facies: the platform deposits, which are associated mainly with limestone and glauconitic 

sandstone, and the geosynclinal deposits, which are associated mainly 

with chert, black carbonaceous shale, and smaller quantities of 

limestone.

McKelvey and his associates have pointed out that in the geosynclines, areas of bedded chert correspond fairly closely to the areas in
84 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS

which phosphorites are found, a point that was overlooked by 

Kazakov. The significance of the chert and phosphorite relationship 

is still unknown, although some important information may be furnished by studies now under way. Facies relationships in the Phosphoria formation suggest that the chert may have formed by diagenetic 

recrystallization of silt-sized quartz, which is most abundant in a 

zone intermediate between nearshore sandy detritus and deepwater 

argillaceous detritus. Yet, the silica may have been precipitated 

directly from sea water, owing to the physicochemical environment, 

or precipitation from sea water may have been brought about by 

organisms.

Calcium phosphate probably accumulates very slowly on the sea 

floor, and the formation of phosphorites is dependent upon a slow 

rate of deposition of clastic sediments and the inhibiting of other 

chemical precipitates, such as calcium carbonate, by the physicochemical environment. Clastic material may be scarce owing to 

distance from the shoreline, or to the low elevation or arid climate of 

the adjacent land masses, or because some areas are not reached by 

silt-bearing currents.

Kazakov's theory may apply equally well to most other major areas 

of marine phosphorites, although it is not yet widely known or accepted. The present writers believe that this theory, with some of the 

modifications suggested by McKelvey and others, may also apply 

to the Mexican deposits. Since their knowledge of the Mexican 

phosphorites is limited to a small area of several thousand square 

kilometers, it is impossible to test adequately many of Kazakov's 

ideas. A reconnaissance study is recommended of the phosphatic 

facies of the La Caja formation in a broad area extending from the 

longitude of Torre'on, Coahuila, on the west, to Monterrey and 

Linares, Nuevo Le6n, on the east. This study should reveal to what 

extent those ideas are applicable to the Mexican phosphates.

The Mexican phosphorites are a geosynclinal facies, and in the 

Concepcion del Oro district they are associated mainly with highly 

calcareous siltstone and chert, containing some limestone. This 

suggests that higher grade deposits might be found in areas of lower 

lime content, where the terrigenous material is finer and accumulated 

more slowly, and this idea should be thoroughly tested. It is interesting that the Concepci6n del Oro deposits, like many similar deposits, 

are accompanied by a cherty facies, which is in large part coextensive 

with the richest phosphorite beds.

At the present time the writers would disagree with Kasakov and 

McKelvey in only one important respect. There is abundant evidence that the Mexican deposits were formed in shallow waters rather 

than in the zone lying between 50 and 200 meters as suggested by 

Kazakov, or in the still-deeper zone proposed by the Geological
GEOLOGY, PHOSPHATE DEPOSITS, CONCEPCION DEL ORO DIST., MEX. 85

Survey geologists. McKelvey 8 agrees, however, that the depth of 

water is still an open question, and he points out that many of the 

phosphorites in other parts of the world show signs of shallow-water 

deposition.

In the Concepci6n del Oro area the La Caja formation was apparently deposited on an uneven surface, as previously suggested (p. 

63 and fig. 19), and the sea bottom may have been characterized by a 

series of gentle valleys and ridges, which were perhaps the product of 

minor tectonic movements related to the Nevadan orogeny. The 

ridges must have had low relief, and a few may have emerged to form 

relatively small low islands. No direct evidence for the existence of 

islands can be presented, as no erosional unconformities have been 

found within the area mapped. Good exposures of the beds of 

Kimmeridgian and Portlandian ages are generally rare, however, and 

local unconformities might well exist. Two lines of evidence suggest 

the presence of islands during deposition of the beds. One of the 

large limestone concretions in unit A has yielded a fragment of fossil 

wood, and one of the phosphorite beds in unit C has yielded bone 

fragments that were probably derived from a terrestrial rather than a 

marine reptile. As the Concepcion del Oro area must have lain 100 

kilometers to the south of the mainland during Kimmeridgian and 

Portlandian time, it is difficult to see how the presence of plant and 

land-reptile remains could be explained except by islands.

Imlay reports that in the Sierras de Parras, Atajo, and Jimulco, 

which lie to the north and northwest of the area mapped, the Upper 

Jurassic sediments contain abundant gypsiferous and carbonaceous 

beds, which imply lagoonal conditions and, therefore, some sort of 

barrier to the south. Perhaps such a barrier was furnished by the 

type of submarine topography here described.

The writers' fossil collections from the phosphatic member have 

been studied by Imlay, and his opinions on the ecology provide 

valuable information regarding the conditions under which the phosphorites must have been deposited.

The phosphatic material contains representatives of a great variety 

of free-swimming and benthonic organisms (see p. 23). Free-swimming forms include ammonites, belemnites, and fish, the fish represented by vertebrae and perhaps also by fragments of teeth and bone. 

The benthonic forms include various pelecypods, brachiopods, gastropods, worms, and many irregular worm tubes. The shells of all the 

free-swimming organisms are broken or smashed; whereas the shells 

of most of the benthonic organisms are entire and undeformed.

The pelecypods and the cap-shaped gastropods indicate a firm 

substratum and shallow agitated marine waters. The absence of 

such burrowing pelecypods as Pleuromya and Trigonia is also evidence,

8 Written communication.
86 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS

even though indirect, of a firm sea bottom. Grammatodon in the 

western interior of the United States and in Alaska is generally 

associated with thin beds of limestone or sandstone in a softer claystone or siltstone sequence. Aucella is most abundant associated 

with conglomerate, sandstone, or thin limestone. Its occurrence in 

limestone is as a coquina in a mass of siltstone, and some of the 

Aucellas in conglomerate and sandstone appear to have been reworked 

by wave action in much the same manner as pebbles. The cap-shaped 

gastropods, by comparison with similar living genera, indicate shallow 

much-agitated waters. The presence of many broken or smashed 

remnants of swimming organisms, including fish, also suggests agitated 

water and slow deposition.

Some of the phosphatic beds contain a host of rather minute 

gastropods and pelecypods, which the authors originally believed to 

be strongly suggestive of a dwarf fauna. However, this assemblage 

has been studied by Imlay, who describes it as containing an abundance of tiny immature forms. The mature pelecypods are normal 

in size, and the mature gastropods, consisting of cap-shaped varieties 

and various high-spired gastropods, including specimens of the family 

Cerithidae as well as smooth species, although very small, are also 

normal in size. The abundance of immature forms suggests that 

there was little, if any, winnowing action by currents. The presence 

of many cerithioid gastropods is indicative of clear waters and, 

therefore, a scarcity of clayey material.

Although the phosphatic sediments must have been formed in 

shallow waters, the complete absence of Gryphaea and Exogyra from 

these beds and their abundance elsewhere in the nearshore deposits 

of the Mexican sea suggests either that the phosphatic sediments 

were formed below the shallowest part of the neritic zone or that 

there was some factor which inhibited the growth of these oysters in 

areas where phosphate was being formed.

Deposition of the phosphatic beds in central Mexico appears to 

have been extremely slow, if their relatively slight thickness is compared with equivalent beds in the gulf region of the United States 

or in Cuba or California. The presence of many fossils in such thin 

strata is not good evidence that there was an abundance of life at any 

one time that might have completely utilized the phosphatic material 

in the sea water. Such an abundance of fossils merely proves that 

conditions were favorable for the preservation of fossils.

An interesting faunal relationship that may bear on the origin of 

the phosphate is the presence of the pelecypod Aucella and the absence 

of the gastropod Nerinea in the phosphorite beds. It is well known 

that Nerinea was characteristic of reefs and reefhke deposits of the 

Mediterranean region during Late Jurassic and Early Cretaceous 

time and that Aucella was characteristic of littoral and shallow-water
GEOLOGY, PHOSPHATE DEPOSITS, CONCEPCION DEL ORO DIST., MEX. 87

deposits of the Boreal region during Late Jurassic and early Neocomian time. During those times Aucella rarely ranged south of 

central Europe and California and Nerinea rarely ranged north of 

about the same latitudes. Therefore, the occurrence of Aucella 

at two levels in the beds of Kimmeridgian to Portlandian ages in 

Mexico and the scarcity of Nerinea (except in the Kimmeridgian of 

the Malone Mountains of Texas) in beds above the Oxfordian in 

Mexico, might be construed as evidence that the waters of the Mexican 

sea were cooler during Kimmeridgian to Portlandian times than they 

had been earlier in the Jurassic.

As described in the foregoing pages, many of the phosphatic beds, 

particularly the calcareous phosphorite forming the upper part of 

unit C, are somewhat conglomeratic and contain small rounded to 

angular rock fragments or reworked nodules which may contain 

essentially the same constituents as the host rock but are clearly 

foreign in origin. This feature would appear to support the paleontological evidence and to indicate deposition of the phosphorites in 

shallow agitated waters. The sediments accumulating on the sea 

bottom must have been subjected to strong wave action and perhaps 

at times were actually exposed to the air.

It seems evident then, that the phosphate beds were deposited 

extremely slowly in shallow clear waters that may have been slightly 

deeper than the shallowest part of the neritic zone. There was 

probably considerable wave action but little current action. The 

life on the sea bottom where the phosphorite was forming includes 

genera that prefer or tolerate a firm substratum and probably were 

not abundant at any one time.

The concentration of phosphate in the Conception del Oro district 

seems to be related more to nondeposition of clastic material than to 

any other factor. Perhaps the site of the Sierra de Santa Rosa, where 

the concentration is greatest, remained above wave base longer than 

the surrounding areas, and for that reason received less silty material.

RESERVES

The phosphatic member of the La Caja formation was sampled in 

73 localities within the area mapped, and also in several adjacent 

areas which will be reported in future studies. In addition, other 

parts of the formation, particularly the sections immediately underlying and overlying the phosphatic member, were sampled in several 

localities, and in one area in the Sierra de Santa Rosa the full thickness 

of the formation was tested. The sample localities are not evenly 

spaced, as only natural exposures of the phosphatic rocks were 

utilized, and the material taken consisted of chip samples, as the 

extreme hardness of the cherty rocks and the moderate hardness of 

some of the other rock types make channel sampling difficult. The
88 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS

sample localities and sampling results are shown in plate 2. The 

analyses were made by the United States Bureau of Mines in its 

laboratories at Albany, Oreg., and by the United States Geological 

Survey in its laboratories at Washington, D. C.

Deposits of possible commercial value are limited to five areas in 

the Sierra de Santa Rosa (pi. 2), and the discussion of reserves that 

follows is therefore limited to those areas.

Computing reserves is a simple matter, and the results obtained 

are believed to be reliable, for, with the exception of the phosphatic 

limestone, individual beds within the phosphatic member are consistent in thickness and grade for fairly long distances. In calculating 

the tonnages, each sample locality was considered the center of a 

block. The volumes were obtained by multiplying the length of the 

block by the average width by a depth of 150 meters downdip, and 

the resulting figures were converted to metric tons by multiplying 

by the average specific gravity of the rock composing the block. 

The authors do not know whether it would be feasible to mine the 

phosphatic zone commercially to an average depth of 150 meters 

down the dip, and hence that figure must be considered as arbitrary 

until a study is made of mining conditions and costs. Specific 

gravities of the principal lithologic types occurring in the phosphatic 

member were determined in the laboratories of the Institute Nacional 

para la Investigaci6n de Recursos Minerales. The average specific 

gravity of the phosphatic member was found to be 2.64.

Until beneficiation or metallurgical tests have been made of the 

phosphate rock of the Concepcion del Oro district, and a detailed 

analysis has been made of the costs involved in mining, processing, 

and marketing the rock, it is impossible to determine what the cutoff 

grade would be. For this reason, the reserves have been estimated in 

three ways. First, the full thickness of the phosphatic member was 

used in the calculations; this gave a total of 13,505,000 metric tons 

averaging 16.2 percent P2O5 or 35.3 percent tricalcium phosphate. 

Next, it was assumed that the full thickness of the phosphatic member 

would be mined, but that the lenses and beds of phosphatic limestone 

would be discarded as waste. This gave a total of 11,043,000 metric 

tons of rock averaging 18.3 percent P2O5 or 39.9 percent tricalcium 

phosphate. Finally, it was assumed that only the calcareous phosphorite would be mined and that 90 centimeters would be considered a 

minimum mining width. This gave a total of 5,036,000 metric tons 

of rock averaging 21.1 percent P2O5 or 46.0 percent tricalcium phosphate. Although the calcareous phosphorite has an average thickness 

of 90 centimeters or more in the reserve areas, in some localities the 

thickness may fall a few centimeters under the minimum, and this 

might require the mine operator to take a small amount of wall rock. 

He would have to determine whether to take the hard phosphatic
GEOLOGY, PHOSPHATE DEPOSITS, CONCEPCION DEL ORO DIST., MEX. 89

limestone underlying the phosphorite or the comparatively soft 

phosphatic silts tone overlying it. Possibly these materials could be 

included with the calcareous phosphorite without unduly diluting it.

The reserves data and the areas mentioned are given on the map hi 

plate 2 (see also fig. 16). Area 1 is located in Canon del Aire on the 

north flank of the range and is easily accessible by a broad trail from 

Mazapil which can be negotiated by trucks. The block ends on the 

west against a small graben structure containing Mazapil conglomerate, and on the east it ends at a dacite intrusive body.

Area 2 lies to the south of the large dacite intrusive body of Las 

Parroquias. It can be reached by a trail from Mazapil or by a trail 

from Valle de Santa Rosa, but most of the area is not easily accessible. 

The block ends against a segment of the dacite intrusive body on the 

west, and it is also cut out by the dacite on the east. Area 3 is a small 

block in the Puerto Blanco area, which is crossed by the trail leading 

from Santa Rosa to Mazapil. On the west it ends against the dacite of 

Las Parroquias, and it is limited on the east by low proportion of phosphate in the rocks. Area 4 occupies the upper part of Can6n de Los 

Mimbres and the lower part of Canon de La Canela; it is easily accessible by the trails leading from Santa Gertrudis to Mazapil. It is 

limited on the west by low proportion of phosphate in the rocks and 

ends 011 the east against a dacite sill. Area 5 is the largest block, 

having a length of about 6.35 kilometers. It terminates on the west 

against a dacite sill and on the east against the fault bounding Valle 

de Santa Rosa, which is underlain by altered agglomerate (?). On the 

west it can be reached by the trail that follows Canon de La Canela, 

and on the east it is easily reached from Valle de Santa Rosa or by the 

Chorreadero trail. The central part of the area, however, lies at a 

high altitude and is rather inaccessible.

POSSIBLE UTILIZATION OF PHOSPHATE BOCK

In the phosphate industry, phosphate rock is usually graded according to B. P. L. (bone phosphate of lime), the content of tricalcium 

phosphate (Ca3 (FO4)2). In the areas in the Sierra de Santa Rosa for 

which reserves have been estimated, the B. P. L. content of the phosphatic member averages about 35 percent, and the content of the 

calcareous phosphorite averages about 46 percent; whereas the grades 

of marketable phosphate rock in the United States range from 66 to 

77 percent B. P. L. Some deposits, such as those of the soft unindurated Pliocene Bone Valley formation in Florida, are beneficiated 

to this grade by washing and flotation. As beneficiation or metallurgical tests have not been made of the hard phosphate rock of the 

Concepci6n del Oro district, it is not known whether the rock can be 

concentrated for the manufacture of superphosphate, but it seems 

unlikely that beneficiation will be feasible at low cost.
90 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS

There would be no difficulty with a high iron and alumina content 

of the rock. Most phosphate rock is sold with a maximum of 3 or 4 

percent of iron plus alumina, arid analyses indicate that the Concepcion del Oro rock generally averages well below 3 percent. Five 

samples of the calcareous phosphorite averaged only 1.28 percent, but 

a few samples of the cherty rocks composing the lower part of the 

phosphatic member averaged about 2.40 percent.

As fluorine is present in the Concepci6n del Oro rock in a proportion 

slightly greater than 10 percent of the P2O5 content, it might eventually 

be recoverable as a byproduct, particularly if it could be made into 

CaF2 . However, no commercial processes for the recovery of fluorine 

from this type of phosphorite have been put into practice as yet.

The Concepcion del Oro deposits contain (p. 68) several minor 

metals. These metals form important elements of the phosphate 

fertilizers, for some are essential to plant growth.

Superphosphates are produced by treating the finely ground rock 

with sulfuric acid of 50 to 60 percent strength, which breaks down the 

calcium fluoride and carbonate and leaves a mixture of soluble phosphate and calcium sulfate. The process depends upon the following 

basic reaction

Ca3 (PO4) 2 +2H2SO4+6H2O^CaH4 (PO4) 2+2H2O+2(CaSO4 2H2O)

As phosphate rock and sulfuric acid are bulky comparatively low 

value products that are expensive to ship, it is advantageous to have 

both available in the same area, or to have adequate means of cheap 

transportation for them. The Concepci6n del Oro deposits are in a 

fortunate location, for, with the installation of an acid plant, the 

smelter of the Mazapil Copper Co. in Concepcion del Oro could 

produce 200 to 300 metric tons of sulfuric acid daily. This company 

is engaged in smelting sulfide ores, and large quantities of sulfur, 

which are present in the stack gases, are not being utilized.

If the Concepci6n del Oro rock cannot be beneficiated for the 

manufacture of superphosphate by the sulfuric-acid process, it might 

be beneficiated for the manufacture of elemental phosphorus by 

electric-furnace methods. Rock used in this process in the United 

States contains about 24 percent P2O,r} or 52 percent B. P. L. Although 

crude phosphoric acid and triple superphosphate for fertilizer can be 

produced more economically by the sulfuric-acid process, electricfurnace methods have some advantages in the manufacture of pure 

phosphoric acid and chemical-grade phosphate salts (Waggaman and 

Bell, 1950). The success of an electric-furnace operation is, of course, 

dependent upon the development of cheap electric power. Although 

there is obviously no possibility of developing hydroelectric power in 

the Concepcion del Oro district, it might be feasible to install a 

thermoelectric plant. Such a plant might be run with coal from the
GEOLOGY, PHOSPHATE DEPOSITS, CONCEPCION DEL ORO DIST., MEX. 91

coalfields of Coalmila, which would require a rail haul of 400 to 450 

kilometers, or possibly it could be operated by natural gas, which is 

available in the city of Saltillo, Coahuila, 125 kilometers to the northeast of Concepcion del Oro.

If the Concepcion del Oro rock can be used in no other way, it 

might be pulverized and applied as a fertilizer in the raw state. An 

increasing quantity of phosphate rock is being used in this manner, 

and approximately 10 percent of the present world production is 

being used for direct application to the soil. The phosphorus in rock 

phosphate is less easily available to plant life than that contained in 

superphosphates, but if used in a suitable crop system and in a longrange soil-building program, it might benefit some types of soils, 

particularly acid soils low in content of iron, and, of course, phosphate. 

It is mainly the presence of CaF2 that limits the availability of phosphorus in rock phosphate. If the material could be defluorinated, 

its value would be considerably greater.

Several greenhouse experiments have been made with pulverized 

calcareous phosphorite from the Concepcion del Oro district, and the 

results are presented in table 8. The rock was crushed to the size of

TABLE 8. Results of greenhouse experiments testing the effects of Concepcion del Oro 

phosphate on yields of wheat

Treatment

Weight of 

first harvest 

(grams)

Weight of 

second harvest (grams)

Total weight 

of first and 

second harvests (grams)

Average 

number of 

plants harvested

Soil number 1, pH 7.4

Superphosphate added at rate of 500 kg per

Florida phosphate rock added at rate of 1 ton

Florida phosphate rock added at rate of 2 tons

Florida phosphate rock added at rate of 4 tons

Concepcion del Oro phosphate rock added at

Concepcion del Oro phosphate rock added at

Concepcion del Oro phosphate rock added at

0.24

.26

.18

.13

.12

.18

.19

,18

7.4

8.9

7.8

8.0

9.4

8.1

7.6

7.6

7.6

9.1

7.9

8.1

9.5

8.2

7. 7

7.7

15

14

14

15

15

14

14

14

Soil number 2, pH 5.5. Old red soil formed on volcanic rocks

Superphosphate added at rate of 500 kg per

Florida phosphate rock added at rate of 1 ton

Florida phosphate rock added at rate of 2 tons

Florida phosphate rock added at rate of 4 tons

Concepcion del Oro phosphate rock added at

Concepcion del Oro phosphate rock added at

Concepcion del Oro phosphate rock added at

0.15

.27

.30

.29

.23

.22

.22

4.0

9.1

8.4

10.1

10.3

c ft

7.5

6.4

4.1

9.3

8.6

10.4

10.5

5.8

7.7

6.6

14

15

14

15

14

15

16

15

3,685,66 56-
92 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS

 100 mesh and applied directly to the soils, without beiieficiation or 

treatment of any kind. To show the relative value of the Concepcion 

del Oro rock, simultaneous experiments were conducted with a standard 

superphosphate, with Florida pebble phosphate, and with soils to 

which no fertilizing material had been added. The Concepcion del 

Oro rock produced a substantial increase in wheat yields, although it 

was less effective than the other materials used. The Florida rock 

that was used contained about 50 percent more phosphate.

PHOSPHATE INDUSTRY IN MEXICO

Although Mexico has a rapidly growing fertilizer industry, the 

production is still insufficient to meet the nation's needs. Largescale production of superphosphates is contingent upon the discovery 

of an adequate source of raw material within the country. The 

present industry depends in part upon the Nuevo Leon deposits, which 

are small, scattered, and rather variable in grade, and in part upon 

phosphate rock imported from the United States. The fertilizers 

being produced are costly and are, in general, beyond the reach of the 

small independent farmer.

The fertilizer needs of Mexico have been demonstrated by a number 

of experimental programs, although further experimental work is 

desirable. For example, the Oficina de Estudios Especiales of the 

Secretaria de Agricultura y Ganaderia conducted a series of field tests 

during 1945 in the States of Puebla, Tlaxcala, Mexico, Morelos, Queretaro, Guanajuato, and Jalisco, which showed that 22 percent of the soils 

being tested were seriously deficient in phosphorus. These soils gave 

an average increase in corn yields of 1,026 kilograms per hectare when 

superphosphate was added at the rate of 250 kilograms (or 40 kilograms of P2O5) per hectare. Some of these soils were deficient in 

nitrogen as well as phosphorus, but it was found that the addition of 

nitrogen in the form of ammonium sulfate, without the simultaneous 

application of phosphate, was ineffective.

In 1953, the mining of phosphate rock in Mexico was limited to 

three small fields in the State of Nuevo Leon. The northernmost field 

is in the Sierra de Sabinas, a few kilometers to the northwest of Sabinas Hidalgo, and contains two mines operated by Fertilizantesde Mexico, S. A. 

This company also owns the two small mines that make up the Ayancual district, about 60 kilometers east of Monterrey. The production 

from these fields,'amounting to 4,000 or 5,000 metric tons annually, 

was shipped to the company's fertilizer plant in El Vergel, Durango, 

which is a few kilometers to the northwest of Torreon. Superphosphate is produced, using acid obtained from the Mexican Zinc Co. 

plant in Nueva Rosita, Coahuila.

The rock in the Sabinas and Ayancual fields occurs in relatively 

small scattered deposits, which are irregular in form and consist of
GEOLOGY, PHOSPHATE DEPOSITS, CONCEPCION DEL ORO DIST., MEX. 93

two distinct lithologic types. The predominant material is a reddish 

or brownish-red earthy laminated rock, which occurs as cave deposits 

and was apparently derived from accumulations of bat guano (Perez 

Martinez and Wiggin, 1953). Below this material, fractures in the 

limestone bedrock are filled with a hard to brittle white to yellowish 

or buff rock, which is commonly banded and is clearly secondary in 

origin. This rock is much richer than the earthy material, and the 

two are mixed in varying proportions by the company, so as to maintain a relatively uniform end product. A mineralogical study of the 

deposits has revealed that they are composed mainly of beta tricalcium 

phosphate and should probably be classed as martinite (Cady, Hill, 

Miller, and Magness, 1952). Only a very small proportion of .apatite 

is present.

The third phosphate field, which was developed in 1952, lies near 

the village of Dulces Nombres in the southeastern part of the State 

of Nuevo Leon, only a few kilometers west of the border with Tamaulipas. The field is on the eastern slope of the Sierra Madre Oriental 

and lies at an altitude of about 2,300 meters. Several small mining 

companies and individuals were exploiting these deposits, which are 

small scattered pockets occupying cavities and fractures in the limestone bedrock. The pockets are generally tabular, but they may be 

irregular in shape. The deposits consist mainly of hard, banded, buff 

to light-brown material having a maximum grade of 80 percent 

tricalcium phosphate and less than 0.75 percent fluorine. The 

authors visited the field and sent a sample of the material to W. L. 

Hill of the Agricultural Research Service, United States Department 

of Agriculture, who referred it to J. G. Cady for mineralogical examination by X-ray spectrometer and petrographic microscope. Hill 9 

reported that the rock is largely apatite rather than beta tricalcium 

phosphate, as the authors had thought. The material may have had 

an origin similar to that of the Sabinas and Ayancual deposits, although 

the caves in which the deposits formed have been completely eroded. 

Some layers contain accumulations of bones, which presumably are 

bat remains.

In 1953, production in the field was at the rate of 200 to 300 metric 

tons a week. The rock was being shipped to two principal markets: 

a part was exported to the United States for the manufacture of animal 

feeds, and most of the rest was shipped to Beick Felix y Compafiia in 

Mexico City for the manufacture of superphosphate. This company 

was buying phosphate rock at the rate of about 3,300 metric tons 

annually and was treating this material with acid obtained from its 

own plant. The rock has a grade ranging from 45 to 75 percent 

tricalcium phosphate.

9 Written communication.
94 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS

Guanos y Fertilizantes de Mexico, S. A., which receives support 

from the Nacional Financiera of the Mexican Government, is the leading fertilizer manufacturer in the country. The following data regarding its production of superphosphates were obtained with the 

kind assistance of Dr. Alfonso Romero, in charge of company research. 

The company operates a modern plant in the city of San Luis Potosi, 

which between July 1, 1953 and June 30, 1954 produced 36,519 metric 

tons of superphosphate. The raw material, imported from Florida, 

is pebble phosphate with a minimum grade of 70 percent tricalcium 

phosphate and less than 4 percent iron and alumina. It is treated 

with sulfuric acid obtained from the American Smelting and Refining 

Co. plant situated in the same city. In October 1953, the company 

began operating a new plant at Cuautitlan on the outskirts of Mexico 

City. By June 30, 1954, this plant had produced a total of 28,204 

metric tons of superphosphate. The raw material is Florida pebble 

phosphate, and the sulfuric acid is produced in the same plant. The 

company also operates a small plant in Guadalajara, Jalisco; production amounts to several thousand tons annually of fertilizers derived 

from bird guano, obtained from small islands close to the west coast 

of Baja California. Total production of superphosphates in metric 

tons by Guanos y Fertilizantes de Mexico, S. A. during the last five 

fiscal years is summarized as follows

1950________________________________________________ 13,335

1951 _______________ __________________________ ___ 12, 604

1952_.______________________________________________ 28, 589

1953 ________________________________________________ 53, 963

1954________________________________________________ 64, 723

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GEOLOGY, PHOSPHATE DEPOSITS, CONCEPCION DEL ORO DIST., MEX. 95

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 1907, Sobre las rocas fosforfticas de las Sierras de Mazapil y Concepcion del 

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1930, Etude synthetique sur le Me"sozoi'que Mexicain: Soc. paleont. Suisse

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Burkart, Joseph, 1836, Aufenthalt und Reisen in Mexico in den Jahren 1825 bis

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Castanedo, Jose, 1927, Los distritos cupriferos de Mazapil y Concepcion del Oro:

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Chase, Thorington, 1909, Concepcion del Oro district, State of Zacatecas, Mexico:

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Colwell, W. E., 1947, Fertilizante comercial conteiiiendo nitrogeno y fosforo para

aumentar los rendimientos del maiz: Oficina de Estudios Especiales, Secretaria de Agricultura y Ganaderia, Folleto de Divulgacion no. 1. 

Dietz., R. S., Emery, K. O., and Shepard, F. P., 1942, Phosphorite deposits on

the sea floor off southern California: Geol. Soc. America Bull., v. 53, p.

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Dorado Santibanez, Arnaldo, 1952 Fertilizantes fosfaticos, Tesis Profesional para

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Dovalina, Jose, 1928. Existencias de minerales fosfatados en Mexico, in Les

re&erves mundiales en phosphates: Internat. Geol. Cong., Madrid, 14th sess.,

v. 2, p. 787-806.

Edwards, J. D., 1954, Studies of some early Tertiary red conglomerates in central Mexico: U. S. Geol. Survey Prof. Paper 264-H. 

Falomir, J. J., and Garcia Lozano, German, 1939, Los depositos de fosfatos en

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Flores, Teodoro, 1916, Los criaderos de fosfato de calcio en los alrededores de

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Fries, Carl, Jr., Hibbard, C. W., and Dunkle, D. H., 1955, Early Cenozoic vertebrates in the red conglomerate at Guanajuato, Mexico: Smithsonian Misc.

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Garcia Lozano, German, 1939, Fosforitas en el Estado de Nuevo Leon: Revista

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Gonzalez R., Jenaro, 1946, La industria minera en el Estado de Zacatecas: Coniite

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Ibarra, Jesus, 1930, Informe general de la region minera comprendida en las

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 1937, Geology of the middle part of the Sierra de Parras, Coahuila, 

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 1938a. Ammonites of the Taraises formation of northern Mexico: Geol. 

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 1938b, Studies of the Mexican geosyncline: Geol. Soc. America Bull., v. 

49, p. 1651-1694.
96 GEOLOGIC INVESTIGATIONS IN THE AMERICAN REPUBLICS

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 1940a, Neocomian faunas of northern Mexico: Geol. Soc. America Bull.,

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GEOLOGY, PHOSPHATE DEPOSITS, CONCEPCION DEL ORO DIST., MEX. 97

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Potential markets: Ind. and Eng. Chemistry, v. 42, p. 269-292.
METRIC EQUIVALENTS

TEMPERATURE LINEAR MEASURE 

C° F° Centi- | nches Meters Feet Kilo- Mi | es 

meters meters g

70 -n

-

60

50^

I

40^

-

30 

20 

10-:

I

o :

~

10 _

on

100 

 150

- 140 9°~

r 130 so-

- 120

 110 70 

 100

-90 6°~

- 80 50- 

- 70

 60 40 

f-r\

30 

 40

-30 20 

 20

 10 10 ~

-0 n

" 40 1000-^

-

900^

800 

- 30 :

700 

:

600 

-

- 20 500 -=

400^

300 

- 10 =

200^

100-^

n n

r 100 -q

'_ -

 3000 90 _:

- 80 

 2500 =

70 

~

 2000 60 _i

~ :

: so 

 1500 -

- 40-^

 1000 30 ~^_

20 

 500 i

: 10^

n n

h 

 60

 50 

-

 40

: 

_

^30 

I 

~ 2° 

_

- 

 10

~ 

 

n 

1 cm. = 0.3937 inch 1 meter = 3.2808 ft. 1 km. = 0.6214 mile 

1 inch = 2.5400 cm. 1 ft. = 0.3048 meter 1 mile = 1.6093 km.

1 sq. meter = 1.20 sq. yd.

1 hectare = 2.47 acres

1 cu. meter = 1.31 cu. yd.

1 k

1 I
INDEX

Page

A

Acknowledgments. _ ------------------------

Agglomerate (?), distribution_______________

lithologic character, ._____ ___ -

mineralization..-- -_-_--__--_-----______

thickness..-. ______________ ____ _

Alluvium.__________________________________

Ancyloeeras zacatecanum-. ____________________

Andesite intrusive body______________________

Anomia sp_____._________________________

Anticlines. See Canutillo, La Caja, Santa

Rita, and Santa Rosa anticlines. 

Apfychussp-- --------------------------------

Aspidoceras sp..------------------------------

Astarte (Coelastarte) ratheri __________________

smackoverensis.-- _________________

23

23

14

14

14,23

23

B

Basalt flows, distribution___________________ 50, 51

lithologic character. _-_-__-__.-.__ 50-51 

thickness.________________________________ 50,51

Basement rocks____-_-_---_---__--_-------_-_ 11

Belemnopsis sp__. __ _ ____ .__.._-._ 23

Berriasella calisto. ____________________________ 23

Bibliography. _.._. _. _ _ _ _.. _ 94-97 

Bulimina sp.. . _ --_ . 32

Canutillo anticline, age of Canutillo fault.___ 54 

displacement along Canutillo fault_ _... 54 

La Caja formation faulted out ------------ 55

location. ____________..._._.___________. 53

thrust fault_-------------_.______ 54

Caracol formation, age ._-___-__.________ 31-32

fossils..._.._.--____________________ 31

lithologic character..,._____________. 30-31

thickness..___--__ __ _____________ 30

Contact metamorphism, garnetization_._ 44-45,46 

general..._..-----_--____-_____._..___ 44

locations___......._______________.. 44-47

mesothermal alteration,__________________ 45-46

recrystallized lirpestone_ _--.--._________ 45,47

relation to ore deposits...--------_.. 44-45,47

Corbis sp.____________________________________ 23

Cretaceous system. See Taraises formation, 

Cupido limestone, La Pefia formation, Cuesta del Cura limestone, Indidura formation, Caracol formation, and Parras shale. 

Crioceras sp___.___ _-____.______, 25,28

Cucullaea phosphoritica, ______________________ 23

Cuesta del Cura limestone, age_---_----_-__._ 28

fossils___-________________________ 27-28

lithologic character._________________ 25-27

thickness__________________________ 26

Cupido limestone, age_.

lithologic character.

thickness.._.___ 

Cylindrotei this davicula.

Page

25

25

25

23

Dacite porphyry sills, distribution... 41-42, 43, pi. 1

lithologic character,..-__--_-_--- _____ 42-43

F

Faults, as boundaries of grabens________._.._- 53

associated with intrusions....------------ 52-53

categories________________ ,--_---__ 52

thrusts..___-.-------.---.____-____. 52

transverse- __ -- __ --- 53

Fieldwork and mapping.. _---_--__-_----- 7, 8

Fluorapatite. See Phosphate mineral.

Folds, general - _-__ -------------- 51-52

G

Geographic features, climate .___... -_ 5 

population and transportation..__________ 2-3

topography...___ __.__ -------------- 3-5

vegetation and agriculture__ . 5

Geologic history- - - .. 62-67

Globigerinasp ------------------------- 29,31,32

Globorotalia sp____---_-.----_-,------------- 29,31

Globotruncana sp.------ --------- ----- 29

Glochicerasfialar- ------------------------ 19, 23

Grammatodon sp_____ _-------_ ----------- 23, 86

Granodiorite and diorlte Intrusive bodies, 

Concepcion del Oro and Providencia stocks, distribution ______ 35

Concepcion del Oro and Providencia

stocks, lithologic character.... 35-37

metamorphism.___--_------------_--- 44-46

modal composition..__----_-__------- 36

Nochebuena stock, distribution ______ pi. 1

magnitude.._ -- - 37

metamorphism__ -- - 46-47

modal composition.-.--.---.---------- 38

types...---------._----------- ---------- 35

San Feliciano phacolith(?), lithologic

character _ _-_ - 39

metamorphism _ _ - 47

size and shape__ - -_ ---_ - 38-39 

Qryphaea sp.____ _ - - _ -.- 14, 86

Gumbelina sp__. __ 29, 31

H

Haplophrogmoides sp..----------------------- 31

Harpagodes sp..-. -------------------------- 14

99
100 INDEX

Page 

History of the district-..-... . ----- 5-6

Hornblende-pyroxene andesite sills, distribution,...__.__._....__ 39, pi. 1

lithologic character........__.___._ 40-41

thickness______________________... 39

Hybanoticeras (Waagenia) sp___............. 23

Hysteroceras sp--....__..._-_-.-_-_-.-.-..-... 28

Idoceras cajense.. _ 

santarosanum _ 

sp.___________

___._-_._-___---.-_.-. 23 

. _- 23 

_._____-_____-_-___._. 18

Igneous rocks, sequence of deposition......... 66-67

See also particular rock type.

Indidura formation, age_-------__.__......... 28,29

fossils. .________________________________ 29-30

lithologic character.______________...... 28-29

thickness.......__________________________ 28, 29

Inoceramus te&ia£ug_____-___________._________ 29

sp_.__________ ___._________.___-_._- 29

Isocyprina coteroi.

Jurassic system, age determinations,--.----.- 11-12 

fossils.-..,_.___.__.__._...________ 12

See also Zuloaga limestone and La Caja 

formation.

Kossmatia sp_ 23

La Caja anticline, Concepcion del Oro fault.. 57 

displacement along La Oaja overthrust__. 57 

graben structure_--__-_._._-___.________ 58-59

intrusion of granodiorite stocks__ 55, 56-57, 58 

La Caja overthrust..____.--___._._.__ 55, 57

location....___--_-___.__.._.__._...... 55, 56

La Caja formation, condition of deposition_ 24, 63 

faulted out in Canutillo anticline......... 55

fossils__---__.___...._.__-_._.._ 23-24

lithologic divisions.....___.......__ 9,16

mode of deposition......_.............. 24

occurrence_........._................. 15

possible source of phosphoric acid. ..... 48

thickness___....._-.-___-__.__.______ 16, 64

topographic expression___............._ 15

unit A, age_----_-_._______-____.._..... 17,18

chemical analysis_.---.._........... 19

fossil wood.__..___.______.. 18, 23, 85

fossils...._..__.__._____....__. 18, 23

litholoRic character___.._._____.__-_ 17,18

thickness......._._____________....-.. 17

unit B, age.__.__......_..________.__... 18

chemical analyses..... .-------...... 19

fossils..._______....____ 18-19,23

lithologic character....._____. 18,19-20

other phosphatic rocks__........... 82

phosphatic limestone................. 80-81

thickness..------_--..__________.___ 18

unit C, age________._.____._._.___ 20

fossils____._._.._..._-..___________. 23

grade...,_________--........_._______. 69

Page

La Caja formation, unit C Continued

lithologic character_---_-__----------- 21

occurrence..........__------------ 21

relation of thickness to phosphate content--.--- - - .. 20 

reptile remains...._.__..._..__.-. 21, 23, 85

thickness..--_.---__-__--._-.--------- 21

See also Phosphate member, 

unit D, age..--_.__.___-_.--_--------- 21

chemical analyses..__.____._.-_._. 19

fossils.. ___ -.__. .. _- - 23 

lithologic character_.---.._--.-.--.--. 21-22 

occurrence.---------.--------.-------- 22

thickness..---.._-____--_-----.------- 21

See also Phosphate member. 

Lake beds and conglomerate, character and

distribution..._____________ 34-35

thickness..... -__-__.-__. ...----.--- 34

Lamellaptychussp..------- ____.____---__- 23

La Pena formation, age_____._-_--_-. 26

lithologic character.-.__-._---__-._- ... 26 

thickness.-.----------.------.--------- 26

topographic expression. ._----.-__-_--- 25-26

Lucina sp......... ---------------------- 23

M

Mazapil conglomerate, age_______-_..-----. 33,34

distribution....-________. -.. 33

lithologic character.....______... 33-34

thickness. - - - -- - - 33 

Mazapilites zitteli..---.----.-..--............. 23

Meleagrinelta sp.___-.._._.__._---. ._-.-- 23 

Metamorphic rocks. See Contact metamorphism. 

Metric equivalents. _.... ----------- 98

Mexican geosyncline, compression and folding. 65 

erosion.__------_---_------..------------- 66

origin____--.---___.-_.---.-_--._-.--- 62 

Mining history........_---__---_---_--.------ 5-6

N

14,86,87

O

Olcostephanus atherstoni.-................ ..... 25

raricostatus-.......... _.._________-_-..- 25

Origin of phosphate deposits, chert-phosphorite relationship...._. ______ 83-84

depth of water......____-_______. 83,84-85,86,87

effect of clastic material_ _ _ ____-_...-.. 84, 87

fossil evidence of conditions__-________-. 85-87

geosynclinal fades._.-__________...-. 83-84,85

Kazakov's theory.. ____._.._..._ _ --- 83,84

lagoonal conditions........._______.-... 85

modifications of Kazakov's theory---..-.. 83,84 

not biolith-diagenetic_._____._.... 83

precipitation from sea water...._--_-._ 83,86,87

presence of islands..__ _____-_____-__---_-- 85

rate of deposition..___..._____.____ 84,86,87

wave action..._.___________-___-.--_._ 85,86,87

Parahoplites sp.
INDEX 101

Page 

Parras shale, age...______ ____________________ 33

fossils__________________________________ 32-33

lithologic character--_---_-_--_______---_- 3:

thickness.----__--_--_---_--__-_-_-_-.____ 32

Phosphate industry, grade of phosphate used.. 93, 94 

location of mining areas_________________ 92-93

production of phosphate. ___------_____ 92,93, 94

soil deficiencies in Mexico-----__---__-___ 92

source of raw material------__-_--._-_-_ 92

Phosphate member, adjacent phosphatic rocks. 82

calcareous phosphorite, chemical analyses. 73, 74

distribution.____________________ 70-71,73

grade.---_-_----------------________ 70, 74

lithologic character___________.____ 71, 73-74

petrographic description. __________ 71-73, 74

thicknesS------_--_--___._______ 70-71, 73, 74

cherty calcareous phosphorite, ehemical

analyses..._...________.. 76, 78

distribution. _______________________ 74

grade____.____-_-__--_--__________ 7.4, 78

lithologic description,________________ 74-75

petrographic description._____________ 75

thickness_--__-_._____________________ 74, 78

outcrop_--_-_..____________________. 70

phosphatic calcareous chert, chemical

analyses.--..____.________ 78, 80

distribution. _______________________ 79

grade --_----_------- _------. 78, 79

petrographic description...._------- 79-80

thickness.__________________________ 78, 79

phosphatic calcareous siltstone, chemical

analyses.-_---_---.--_..____ 78

grade.--_-__-_--_--_--_---_________ 78

thickness_________..___.____.__.__ 78,82

phosphatic cherty limestone, chemical

analysis..-. _____________ 77

distribution.................._..... 76

grade..._____......__..._____ 77

petrographic description.............. 76, 77

thickness.-_-------_-_--_______-______ 77

phosphatic limestone, chemical analyses.. 78 

grade______________________ 78

lithologic character...._____________ 80-81

petrographic description___.__________ 81

thickness.._________________ 78,80

utilization. See Utilization of phosphate

rock. 

Phosphate mineral, classification...________ 68

description...______.._______________ 67

grade of phosphate member..___________. 69

presence of minor metals.-_______________ 68

spectrographic analyses_.._.._________ 68, 69

thickness of phosphate member.._______. 69

uranium content..____..___________ 68

Pleuromya sp_.------.___________.._________ 85

Proniceras sp_.--.____________________________ 23

Purpose of investigation...................... 2

Quaternary system. See Lake beds and conglomerate and Alluvium.

Page

R

Reserves, computation.____________________ 88

grade._____________________ 88, pi. 2

location and accessibility of five areas.. 89, pi. 2 

method of sampling...................... 87

sample localities________________________ pi. 2

tonnage.___....__________________ 88, pi. 2

using different grades__________________ 88

Rhyolite flows. ______________________ 50

Rhyolite stock, distributioii-_-_________-.____ 43

lithologic character.............._...... 44

Santa Rita anticline, double folding in Zuloaga

limestone_______________.. 61

graben structure_____.__________---__ 62

intrusion of granodiorite______......... 61, 62

location. _____________________ 61

Santa Rosa anticline, double folding in Zuloaga

limestone___________.___ 60

fanfolding...____________________ 59-60

fault zone..____________________ 60-61

fold formed by intrusive body_____.--____ 60

graben structure...___......_.--.... 60

location.______________.___________ 49,59

Scope of investigation.-______.._.______ 2

Sedimentary rocks, conditions of deposition.. 62-66 

general-,...__.........__.._____... 8-11

stratigraphic correlation___..._....._. 9,10

See also particular formations.

Structural features. See Folds, Faults, and 

particular anticlines.

Subdichotomoceras sp.---.- -----_------ 23

Substeuroceras sp____---.---_________----___ 23

T

Taraises formation, age______.__-----__-----_- 24

fossils.--. . 25

lithologic character. .________------------_ 24,25

thickness ___ _ _ __ _ . .. __ __ ___ - 24,25

Taramelliceras sp_. _ _______._ _ _. 23

Tertiary extrusive rocks. See particular rock

types and 47. 

Tertiary intrusive rocks. See particular rock

types.

Tertiary system. See Mazapil conglomerate. 

Thamnasteria imlayi. ... ... _ __

Torquatisphinctes sp-----___--------_------

Trigonia hudlestoni. ------------------ --- --

14

23

18

14,85

U

Utilization of phosphate rock, beneflciation 89,90 

B. P. L. content..- 89,90

comparison with other phosphates _.__. 91-92 

iron and alumina content .. -- 90 

possible sulfuric-acid production.. _ _ _ -. 90 

production of superphosphate ------- 89,90,91
102 INDEX

Page 

Utilization of phosphate rock Continued

recovery of fluorine__-____--_---. 90 

results of greenhouse experiments _____-.__ 91 

without treatment..---______________ 91-92

Virgatosphinctes aguileri......

Vermetus (Burtinetta) cornejoi. 

Virgulinasp-.---. _.--____

Page

Zuloaga limestone, age and condition of deposition. -_-__-______--..- 14-15

double folding-_____- ........... 60,61

fossils.--.------.------------------------- 14-15

lithologic character.._-_---------------- 13-14

marmarization--------------- --------- 50

thickness.-__-___ -_. -------- 13

o