THE MINERALOGY OF THE MAPIMl'
MINING DISTRICT, DURANGO, MEXICO

byVictor Joseph Hoffmann

A Dissertation Submitted to the Faculty of the
DEPARTMENT OF GEOLOGYIn Partial Fulfillment of the Requirements
For the Degree of
DOCTOR OF PHILOSOPHY
In the Graduate College
THE UNIVERSITY OF ARIZONA

19

6 8

THE UNIVERSITY OF ARIZONAGRADUATE COLLEGE

I hereby recommend that this dissertation prepared under mydirection by _________ Victor Joseph Hoffmann____________________
entitled

The Mineralogy of the Mapimi Mining District,______

Durango, Mexico______________________________
be accepted as fulfilling the dissertation requirement of thedegree ofDoctor of Philosophy_______________________________

7/2____________ ‘

Dissertation Director^/

Date

z

/

__________________~

After inspection of the final copy of the dissertation, thefollowing members of the Final Examination Committee concur in
its approval and recommend its acceptance:*f, A> Q ~/wn n

rT 2.7,
7 /f/7u•fs----- ----- -' ------- 7

Z Z / 9<$7

This approval and acceptance is contingent on the candidate's
adequate performance and defense of this dissertation at the
final oral examination. The inclusion of this sheet bound into
the library copy of the dissertation is evidence of satisfactory
performance at the final examination.

STATEMENT BY AUTHORThis dissertation has been submitted in partial
fulfillment of requirements for an advanced degree at
The University of Arizona and is deposited in the
University Library to be made available to borrowers
under rules of the Library.
Brief quotations from this dissertation are
allowable without special permission, provided that
accurate acknowledgment of source is made. Requests for
permission for extended quotation from or reproductionof this manuscript in whole or in part may be granted by
the head of the major department or the Dean of the
Graduate College when in his judgment the proposed useof the material is in the interests of scholarship. In
all other instances, however, permission must be obtained
from the author.

SIGNED:ACKNOWLEDGMENTS
The author thanks Professors John W. Anthony,under whose direction this study was made,Evans B. Mayo, Spencer R. Titley and John M. Guilbert,
for their many helpful suggestions, criticism, anddetailed examination of the manuscript.

The author

also wishes to thank Drs. F. F. Apian, H. Schweigart
and Mr. R. L. Labosky for their assistance with manyof the mineralogical and chemical aspects of the study.
The author is indebted to the Mexican Government
for permission to collect at the mines and to take thesamples from their country for study.

Many of theresidents of Mapimjf and miners were most helpful inobtaining samples from the mines and offered informationon the mine locations and their access.

Mr. Salvador Davilaof Mapimi assisted the author with the collection of samples
and acted as an interpreter and guide.
The author is also indebted to Union Carbide
Corporation for the use of their equipment and supplies
and for many chemical analyses.

iii

Many mineral specimens were obtained from theSmithsonian Institution from both the collections ofOjuela mine specimens obtained by Foshag in 1934 andfrom the more recent acquisitions.

Mr. John S. Whiteof the Smithsonian staff was most helpful in locatingthese cataloged samples for use in the study.

TABLE OF CONTENTS
Page

LIST OF ILLUSTRATIONSlx

LIST OF TABLESXIUABSTRACTXVIINTRODUCTION

1

Methods of Investigation

2

Previous Investigation

3

Location and Accessibility

4

Climate and Vegetation

6

Physiography.7

History

9

Production and Present Mining

12

STRUCTURE AND PETROLOGY21

PRIMARY MINERALIZATION

31

ZONING43

DESCRIPTION OF INDIVIDUAL MINERALS

46
46
46
47
48
48
49
50
51
51
54

bismuthcoppergold
silversulfurcerargyrite
fluoritenantockite
bindheimite
braunitechalcophanite

55

V

vi
TABLE OF CONTENTS— Continued

Page

DESCRIPTION OF INDIVIDUAL MINERALS
cryptomelane
cupritedelafossite
goethitegroutite
hausmannite
hematitehydrohausmannitehydrohetaerolite
lepidocrosite
litharge
magnetiteraanganite
miniummolybdic ochremurdockite
plattnerite
psilomelane
pyrolusite
ramsdellite
stibiconite
tenoritevalentinite
cassiterite
arsenopyrite
berzelianite
clausthalite
bismuthinite
boulangerite
bornitechalcocite
chalcopyrite
covellite
emplectite
enargitegalenajamesonite
marcasitemolybdenite
pyargyrite
pyritepyrrhotite(Contd.)
55
55
56
56
57
57
58
59
59
60
60
60
61
61
62
62
62
63
64
64
64
66
66
67
67
69
71
71
71
72
72
73
73
75
75
77
78
80
80
82
82
83

vii
TABLE OF CONTENTS— Continued

Page
DESCRIPTION OF INDIVIDUAL MINERALS(Contd.)sphalerite
stannitestibnitetetrahedrite
tennantite
umangiteankeritearagonite
aurichalciteazuritecalcitecerussite
dolomitehydrozincite
malachiterhodochrosite
rosasitesideritesmithsonite
anglesite
baritebrochantite
celestitechalcanthite
copiapite
gypsumhydrous sulfatesjarositemelanterite
plumbojarosite
adamiteapatitearseniosiderite
austinite
carminite
chenevixite
conichalcite-cuprian austinite
crandellite
descloizite
duftite -j3
dussertite
ferrimolybdite

83
84
85
86
86
87
87
87
88
90
90
91
93
93
94
95
95
96
96
98
98
99
100
100
102
103
104
112
114
114
116
121
123
125
126
128
129
134
134
136
138
140

viii
TABLE OF CONTENTS— Continued

Page
DESCRIPTION OF INDIVIDUAL MINERALS(Contd.)hedyphanekoettigite
legrandite
mimetite-pyromorphite
paradamite
scoroditevanadinite
whitlockite
wulfenite
chrysocollahemimorphite
kaolinite
quartzsauconite

140
142
142
146
148
151
153
154
155
157
158
160
161
162

PARAGENESIS

163

MINE WATERS

173

STABILITY RELATIONSOF MINERALS IN THE
OXIDIZED ZONE184

SOURCE OF ELEMENTS IN THE OXIDATION ENVIRONMENT217

SELECTED BIBLIOGRAPHY

223'

LIST OF ILLUSTRATIONS
FigurePage

1.

Location of study area

5

2.

The Bufa de Mapimi as seen from the roadleading to the America Dos Mine showing
the massive bedding of the Aurora Limestone

83.

The city of Mapimi as seen from the prospect
pits near the El Padre Mine. (La Esperanza)

84.

View of the suspension bridge that connects
the mines of Ojuela with those to the south

115.

The abandoned town of Ojuela, looking north15

6.

The headframe and loading bins at theAmerica Dos Mine

15

7.

A view of Ojuela and the headframe of the
Socavon shaft

18

8.

A view of Ojuela during the period of
active mining (1908)

19

9•

A similar view of Ojuela as above taken in
1965

19

10.

The Sarnoso stock, looking south from thenorthern contact with the limestone

25

11.

The manganese ore deposits near San Pedro

25

12.

Cross

36

13.

Cross section - the San Jorge-San Juanore bodies

36

14.

Cross

section - the Cumbres ore body

38

15.

Cross

section - the Cumbres ore body

38

16.

Photomicrograph showing the alterationof boulangerite to jamesonite to bindheimite
and goethite

53

section - the 0juela-Paloma ore

ix

body

X
LIST OF ILLUSTRATIONS— Continued
Figure17.

Page

Photomicrograph of arsenopyrite from lower
sulfide zone at America Dos mine. Uncrossed
nicols70

18a. Photomicrograph showing the alteration ofchalcopyrite and galena to covellite. Nicols
uncrossed-oil. immersion-green filter
18b. Same field as above - crossed nicols
19.

Photomicrograph showing the replacement ofclausthalite by emplectite • and bismuth, Nicols
uncrossed-oil immersion-blue filter

20a. Photomicrograph showing the alteration ofboulangerite to jamesonite along fractures.
Nicols uncrossed-oil immersion-blue filter
20b. Same field as above.
blue filter

Crossed nicols-oil immersion-

21.

Photomicrograph showing the pseudomorphic
replacement of enargite by marcasite

81

22.

Photomicrograph showing umangite and chalcopyritein sphalerite

81

23.

Aurichalcite on a matrix of goethite

89

24.

Radiating clusters of aurichalcite growing onhydrozincite

89

25.

Photograph of cerussite twinned on {110 } and{130} on a matrix of rosasite and nantpckite

92

26.

Photograph of a cluster of adamite on
limonitic matrix

118

27.

Photograph of cuprian adamite on calcite fromthe America Dos mine

118

28.

Photograph showing fibrous-appearing
arseniosiderite with hemimorphite

124

29.

Photograph of a small pocket of carminite
crystals from a sample taken from a dump near
the Socavon shaft

124

xi

LIST OF ILLUSTRATIONS— Continued
FigurePage

30.

Photograph of a crust of conichalcite

131

31.

Photograph of botryoidal crusts of brownishgreen duftite-p with calcite

131

32.

Photograph of small crystalline mass ofkoettigite on a limonitic matrix

143

33.

Photograph of a radiating cluster oflegrandite crystals in a small vug ingoethite

143

34.

Photograph of mimetite with cuprianaustinite

147

35.

Photograph of paradamite

147

36.

Photograph of a cluster of lath shaped
hemimorphite crystals

159

37.

Paragenesis of Primary Minerals at Ojuela

166

38.

Paragenesis of Secondary Minerals at Ojuela

167

39.

Photomicrograph showing the alteration of
cryptomelane to pyrolusite

168

40.

Photomicrograph showing euhedralhydrohetaerolite crystals with small bandof hydrohausmannite (?)

168

41.

Photograph of veinlet of groutite inpyrolusite

169

42.

Paragenesis of Minerals from San Pedro

170

43.

Photomicrograph showing pyrrhotite, stannite,
galena, enargite, bismuth, and bismuthinite

169

44.

Paragenesis of Minerals from the Monterrey
mine

172

45.

Plot of Eh vs. pH of mine waters

174

Xll
LIST OF ILLUSTRATIONS— Continued

FigurePage

47.

Eh-pH characteristics of natural mine waters
distinguishing between waters from theprimary ores and those draining oxidized ores

181

48.

Plot of SO^ vs. pH

182

49.

Plot of CO2 vs. pH

183

50.

Eh-pH diagram of the system.

51.

Eh-pH diagram of the system, Hg-S-Og

189

52.

Eh-pH diagram of the system, Cu-S—C—H2O

193

53.

Eh-pH diagram of the system, Pb-S-C-H20

197

54.

Eh-pH diagram of the system, Zn-S-C—HgO

201

55.

Composition triangle for the Zn0-Si02“H20system showing compatible phases (temp.
<210°C.)

203

56.

Eh-pH diagram of the system. Ag-Cl-H^O

204

57.

Eh-pH diagram of the system, Cu -C1-H20

205

58.

Eh-pH diagram of the system, Cu H20-02

206

59.

Eh-pH diagram of the system, Mn-HgO

209

60.

Eh-pH diagram of the system, Mn-C-H20

210

61.

Eh-pH diagram of the system, Zn-Ca—As—C—HpO

214

h 2o -cGeologic Map of a Portion of the Torreon
Quadrangle, Durango, Mexico showing the Locationof the Mines of the Mapinn District

188"In Pocket"

LIST OF TABLES

Table

Page

I

Modal Analyses of Sodic Granite from
Sarnoso Stock26

II

Spectrographic and Chemical Analysesof Bindheimite

54

III

X-ray Diffraction
and Valentinite

IV

Chemical Analysis of Aurichalcite

88

V

Chemical Analysis of Hydrozincite

94

VI

Chemical Analysis of Rosasite

95

VII

Chemical Analysis of Chalcanthite

101

VIII

Chemical Analysis of Copiapite

103

IX

Chemical Analysis of Goslarite

106

X

Chemical Analysis of Bianchite .

107

XI

Chemical Analysis of Hexahydrite

108

XII

Chemical Analysis of Epsomite

109

XIII

Chemical Analysis of Halotrichite

110

XIV

Chemical Analysis of Pickeringite

111

XV

Chemical Analysis of Jarosite

113

XVI

Chemical Analysis of Melanterite

115

XVII

Partial Chemical Analysis of
Plumboj arosite

116

XVIII

Chemical Analyses of Adamite

120

XIX

Chemical Analyses of Apatite fromthe San Ilario Mine

122

Data for Stibiconite

xiii

68

xiv
LIST OF TABLES— Continued

Table

Page

XX

Chemical Analysis of Arseniosiderite
from Mapimi

125

XXI

Chemical Analysis of Carminite from
Mapimi

127

XXII

X-ray Diffraction Data for
Chenevixite and Mimetite

130

XXIII

Partial Chemical Analyses ofConichalcite and Cuprian Austinite

133

XXIV

X-ray Diffraction Data of Apatite
and Crandellite

135

XXV

Chemical Analysis of Duftite-0 from
Mapimi Compared with Conichalcite andMottramite

137

XXVI

Comparison of X-ray Diffraction Dataon Duftite- |3 , Mottramite, andConichalcite

139

XXVII

X-ray Diffraction Data of Mimetite
and Hedyphane

141

XXVIII

Chemical Analyses of Legrandite

144

XXIX

Chemical Analyses of MimetitePyromorphite

146

XXX

Chemical Analysis of Paradamite(Switzer, 1955)

149

XXXI

X-ray Powder Diffraction Data for
Paradamite

150

XXXII

Chemical Analysis of Scorodite
from Mapimi

152

XXXIII

X-ray Diffraction Data on Scorodite
from Mapimi (from Axelrod in Allenand Fahey, 1948)

153

XXXIV

Chemical Analysis of Hemimorphite

160

XV

LIST OF TABLES— Continued
Table

Page

XXXV

Eh-pH Measurements of Mine Waters in
the Upper Oxidized Zones of the
Mapimi' District

177

XXXVI

Eh-pH and Partial Chemical Analysesof Mine Waters in the Mapimi District178

XXXVII

Range of Ionic Activities Obtained
from Mine Waters

180

XXXVIII

Distribution of Primary Minerals
According to the Elements they Contain

219

XXXIX

Distribution of Secondary Minerals
According to the Elements they Contain220

ABSTRACTThe Mapimi mining district in Durango, Mexico has
been mined almost continuously since the discovery in 1598of rich silver ores.

The district is noted for the great

variety and number of mineral species in the zone ofoxidation and the accessibility of mine dumps and under­
ground workings presented an excellent opportunity to
study the deposits.

Of the 110 mineral species identified

in the study, approximately 20 are new for the district.
The district is located in the eastern portion of
the Central Plateau province near the edge of the Bolson de
Mapimi.

The deposits are related to a number of quartzmonzonite and quartz diorite intrusives and the ore occurs
as replacements of the Cretaceous Aurora Limestone.

Ore

deposition has been controlled by folding, faulting, joint­
ing, and the chemical favorability of the limestone.
A zonal distribution of the ores is well developedin the deposits.

Proceeding from the source of mineraliza­

tion, copper, lead-zinc, lead-silver, and the carbonate
zones of mineralization are encountered.

The oxidationof any of these zones is characterized by a distinct
secondary mineral assemblage.

xvi

xvii

Thermodynamic calculations based on Eh and pH
measurements and chemical analyses of mine waters wereused to determine the chemical stability fields ofvarious minerals.

The mineral assemblages noted in thedeposits are generally predicted by the Eh-pH diagrams
established.
The arsenical nature of the primary ores isreflected in the number of copper, lead, zinc, and ironarsenates in the oxidized ores.

Oxygen and chlorine,

which are the only extrinsic elements in the zone ofoxidation, can be attributed to meteoric waters.

INTRODUCTION
The occurrence of a large number of mineral species
at the mines near MapimI presents an interesting problem
regarding conditions of their formation.

The present study

is an attempt to explain many of the assemblages with regard
to primary mineralization and the oxidation and alterationof the primary minerals.
For the study it was necessary to collect samples
from all accessible areas in the district and to identify
mineral species and assemblages.
intermittently during 1963-65.

Field work was conducted
Additional specimens wereobtained from the Smithsonian Institution and from numerous
mineral dealers and collectors.
Mineral species were determined by the use of
optical and X-ray diffraction techniques.

Chemical analyses

supplement many mineralogical descriptions if the minerals
are members of isomorphic series or if significant chemicalvariation was indicated by either emission spectrographic
or X-ray spectrographic studies.
The investigation of the present environment was
conducted in the hope of satisfactorily explaining themineral assemblages observed and elemental distributions
through various zones of mineralization.

1

2

Methods of Investigation
Methods of mineral identification included X-ray
diffraction, X-ray spectrographic, emission spectrographic,
electron microprobe, and optical techniques.

Approximately

150 thin sections, polished sections, and thin polishedsections were prepared for the study.

Chemical analyses

supplement many of the mineral descriptions and X-ray
diffraction data are included for many of the less common
minerals or for those minerals whose lattice parameters
vary with ionic substitution.

Specific gravity determina­

tions were made with the use of the Berman density
balance and with the Cohn Electrobalance modified forspecific gravity measurements.
Eh and pH measurements of mine waters were madewith the Beckman Eh-pH meter at the sample sites.

The

measurement of Eh and pH at the sample site eliminatedthe possibility of contamination from a container orchanges due to variations in atmospheric conditions
from the sample site to the location at which analysis
might be made.

Previous Investigations

Literature related to the Mapimi district is notextensive.

Many of the papers cited briefly discuss: only

certain features of district or the mineralogy.
Naumann (1898), Villarello (1906, 1909, 1910),
Angermann (1907), Paredes (1908), Bennett (1908) , and Rice
(1908) briefly describe the geology of the district,mining and milling methods, costs, and production figures.
Spurr (1923), Prescott (1926), Fletcher (1929), andHayward and Triplet (1931) discussed the principles of ore
emplacement at Ojuela.

Santillan (1936) includes a

discussion of the district and a bibliography in his
publication on the geology of the State of Durango.
Singewald (1936) described the geology northeast of Mapimi
and deCserna (1956) described the regional tectonics inrelation to the Sierra Madre Oriental to the east.
Not until the work of Clemans and McLeroy (1962)
was any comprehensive discussion of the geology of theregion found.

In their work, they incorporated thescant geologic literature with a fairly detailed regional
investigation.

Their study is part of a program aimed

at obtaining complete geologic coverage of Mexico.

Ageologic map of the Torreon quadrangle by McLeroy with

4

accompanying geologic resume was published in 1963.

A

similar map by Pantoja-Alor on the adjacent San Pedro delGallo quadrangle to the west was also published in 1963.
Bernstein (1964), in his book on the Mexican
mining industry of the period 1-890 to 1950, discusses
the production and mining methods of the district as wellas giving an insight into the political and economic
factors controlling or affecting the mining.
Location and Accessibility
The town of Mapimi is located in the northeasternpart of the State of Durango approximately 65 kilometers
NW of Torreon, Coahuila and 820 kilometers south of
El Paso, Texas (Figure 1).

It can be reached from theqast by a paved road (Highway 30) from Bermejillo, adistance of 25 kilometers, or from the west from LaZarca,a small village on the Parral-Durango Highway (Highway 45).
During more active periods of mining in the
district, a railroad connected Mapimi with Bermejillo,a station on the Mexican Central Railroad.

A small spurlead from Mapimi up the canyon to the Ojuela mine, adistance of 3.5 kilometers.

A similar line connected themines of the Descubridora district with the Mexican Central

5

GULF OFMEXICOFIGURE 1

Location of study area,

republic of Mexico.

6

Railroad at the station of Colon.

The Descubridora

district is approximately 35 kilometers to the northwest of
Mapimi and about 2 kilometers WNW of the village of
Buruguilla.
Numerous unimproved roads, usually passable only
by truck or four-wheel drive vehicle, and trails which
can be traversed on foot or horseback provide access tomost of the mines in the region.

An unimproved road con­nects Mapimi with the village of Dinamita about 25 kilo­
meters to the southeast and the several mines located nearby.
A regional geologic map is included inside the back cover.
Climate and Vegetation
The average annual precipitation varies from
20 to 35 centimeters, most of which falls during therainy season from July until October.
are common during this period.

Thunderstorms

Temperatures in the area

range from about 35°c. in the summer to about 5°C. inwinter.

The hot summer days are usually followed bydelightfully cool evenings.
Clemons and McLeroy (1962) describe the sparsevegetation found in the valleys and mountain slopes of
the region.

The vegetation is designated by botanists as

Chihuahuan Desert Shrub.

The most characteristic speciesof the flora is Larrea tridentata (creosote bush).

Associated with it are Acacia (huizachal), several speciesof Agave (lechugilla and maguey), Dasylirion wheeleri(sotol) , Euphorbia anti-syphilitica (candelilla),
Fouquieria splendens, at least four species of Opuntia(cholla), Prosopis (mesquite), Yucca, and others.
Quercus intricate (scrub oak) is found in the higher
portions of the district, especially on the western sideof the Bufa.

Corn and beans are the principal crops ofthe area.
Physiography
Mapimi is situated at the base of the northernend of the Bufa de Mapimi, an elongated mountainapproximately 3 kilometers wide,the axis of which trends
about 40°NW.

Seen from the western side of the city of

Mapimi, its profile as a whole resembles a woman's face.
The eastern side of the mountain is steep and almost
impassable, while the western slope is considerably
more gentle.

The crest of the Bufa rises to an elevationof about 2700 meters, some 700 meters above the valley
floor.

The Ojuela mine is located on the east side of

the mountain about 400 meters below the crest of themountain at the foot of an almost sheer-walled limestone
cliff (Figure 2).

8/

FIGURE 2

FIGURE 3

The Bufa de Mapimi as seen from the roadleading to the America Dos Mine showing
the massive bedding of the Aurora
Limestone.

z
The city of Mapimi as seen from theprospect pits near the El Padre Mine
(La Esperanza). The large slag dump
is approximately 600 meters in length.

Ojuela is separated from the mines of the CampoSur by a large ravine which trends northeast.

Thesuspension bridge described in more detail in the chapter
dealing with the production of the district crosses thisravine.

No perennial streams are located within the

district.

The streams in the area carry water only duringor shortly after, rainstorms.
The Bolson de Mapimi extends to the east toward
Saltillo in the state of Coahuila and northward toward the
Rio Grande River.

The photograph of Figure 3 was takenfrom the north end of La Bufa at the El Padre mine
(La Esperanza) looking toward Mapimi and the Bolson de
Mapimi.

Mapimi is located at the southwestern entranceof the Bolson de Mapimi.
History
The mining district of Mapimi, the oldest districtin the State of Durango and certainly one of the oldest
in the entire Western Hemisphere, has been mined almost
continuously since the discovery of rich ore in the year
1598.

The district is also one of the most famous of themany Mexican mineral areas(Villarello, 1906).

10

The Spaniards discovered the district in 1598, andthe town of Mapimi prospered partly on account of ore
discovery and partly for an abundance of fresh springwater, which made the city one of the main watering stops
along the long road from Mexico City

to Chihuahua.

The town of Ojuela is located near the apex of twolarge ravines near the Ojuela mine, a distance of somefour kilometers southeast of Mapimi.

Ojuela was partiallyoccupied until relatively recently.

The present miners

live;mainly in Mapimi.

At its peak, Ojuela had apopulation of almost 3,000.

Mapimi, during one of the more

active periods of mining, boasted a population in excess
of 8,000.

At the present, however, the population

approaches only 1,000.
Ore was discovered on the northern side of aneast-trending arroyo near what is now the abandoned cityof Ojuela.

The "boca

de mina," the original discovery

area, can be seen in Figure 4.

The original hand-hewn

steps in the limestone are still visible.
The Spaniards profitably mined the many veins of
the district until 1821, at which time the mines weretaken over by the Mexicans.

Mining continued at adecreasing rate until 1867.

Civil war had a devastating

FIGURE 4

View of the suspension bridgethat connects the mines of
Ojuela with those to the south.
The "Boca de Mina" can be seen
to the right of the shadow of
the bridge.

effect on the mines.

Not only were many of the minesabandoned, but machinery, which was at a premium andrequired long periods of time for replacement, wasdestroyed.
The largest mine of the district is the Ojuela.
According to Villarello (1906), the name Ojuela wasprobably given because of the leaf-like texture of thegalena found at the mine. Today, through a spelling
corruption, the name is written Ojuela instead ofHojuela.

This mine is one of the deepest in Mexico

and also one of the most productive, having accounted
for the wealth of Mapimi during its peak near the turnof thepresent century.

Production and Present Mining
Few production figures exist for the almost
continuous exploitation over a period of about 370 years.
No known records were kept by the early Spaniards who
extracted the silver and gold from the rich mines of
Mapimi.

This early production of rare and precious

metals was taken only from the upper zones of oxidationand mineralization.

Hayward and Triplett (1931)

attribute a production of 3,817,562 tons for the period
from 1893 to 1931 assaying 3.7 g. gold, 462 g. silver

13

and 14.9 per cent lead.

Probably not more than 5 percent of this tonnage was sulfide ore.
The Mexican War of Independence from 1810 to1821 put an end to Spanish exploitation of the Mexican
mineral wealth.

Production during the War and immediately

following was seriously curtailed and many mines filledwith water, timbers rotted, shafts collapsed, machinery
rusted, and roads fell into disrepair.

The locations ofisolated mines were even forgotten when prospectors
abandoned their work.
After a few years, mining in Mexico began torecover slowly.

In 1823 the legal prohibitions against

foreigners owning mines in Mexico were eased.

An

excellent discussion of mining laws, production and mining
ventures in Mexico can be found in the book by Bernstein(1965) on the Mexican Mining Industry.

The ownershipof the mines prior to their acquisition by A. B. Sawyer
in the late nineteenth century-is unclear.

According toBernstein, Sawyer
sold them to the Durango-Mapimi Mining
Company of Council Bluffs, Iowa, while
he remained in charge of operations.
Durango-Mapimi united the principal
mines and smelted some 20 tons of ore
a day. Despite the fact that the ore
ran about 20 per cent lead and 30 ouncesof silver with some gold values, highinitial costs precluded any profits'
and the American and Mexican capitalists
who had financed the venture gave up.

Charles Reidt reorganized and modernized
the equipment, developed some promisingareas with capital provided byJose Maria Bermejillo, a Spanish capital­
ist of Mexico, and found a bonanza. Jacob
Langeloth, who was buying ores in Mexico
at the time, promoted the new company in
the United States. Through Langeloth, itbecame closely connected with Minerals y
Metals, S.A. and the American Metal
Company, a concern controlled by theMetallgesellschaft of Frankfurt-am-Main.
The Cia. Minera de Penoles was organizedon March 1, 1887, and by 1903 it wasreferred to as the largest independentbase-metal enterprise in Mexico.
In 1893-94 Penoles introduced electricity andbuilt a railroad to connect the mines to the smelter.
An arsenic plant was constructed that produced 1500
tons of refined white arsenic a year.

A suspension

bridge was built across the arroyo that separates the
Ojuela shafts from the mines to the south (Figures 4
and 5).

The bridge was built by the John A. Roebling

Company in 1899 and was at that time the second long­est suspension bridge in the world (Rice, 1908).

The

bridge is 325.83 meters long, 1.83 meters wide and isabout 80 meters above the bottom of the arroyo.

Thereis a decrease in elevation from the south to the northend of 14.60 meters (Villarillo, 1906).
of the bridge is presently ina

The flooringstate of disrepair

and crossings must be made carefully but the over-all
structure remains strong.

FIGURE 5

The abandoned town of Ojuela looking north.
The large dark area toward the bottom centerof the picture is the "Boca de Mina," theoriginal glory hole dating to the earliest
mining in the district.

FIGURE 6

The headframe and loading bins at theAmerica Dos Mine.

15

FIGURE 6

16✓

The mines and smelter at Mapimi were one of themost highly mechanized operations in Mexico.

Penoles

was one of the earliest companies in Mexico to provide
decent housing and sanitation facilities for thelaborers.

Some medical attention was also available.

This was in contrast to the miserable conditions thatexisted in most mining camps in Mexico at the time.
Most of the production came from the Ojuela mine, which was
one of the deepest, as noted, and most productive mines in
Mexico.

The peak of production occurred near the turnof the last century.

In 1899, the district had a

production of over $4,000,000 from as many as 216 mines.
As late as 1913, almost six per cent of the mineralZ

production of Mexico came from the Mapimi district.
In 1921, Penoles began to dismantle the Mapimi
smelter and Mapimi was abandoned as a smelter site
because the decline in its siliceous ore production had
made it more profitable to concentrate operations at
Torreon or Monterrey (Bernstein, 1965).

By 1923-24, theproduction of the mines began to decrease rapidly and in
the 301s , production had essentially ceased.

The increase in prices and demand brought on byWorld War II saw a brief rebirth of mining but thecessation of hostilities was accompanied by a decreased
demand and again production slackened.
almost abandoned.

Ojuela was

Present mining is conducted by acooperative which selectively mines small areas in themine.

The production now consists of only a few tonsof copper, lead, zinc, manganese, and silver ore that ishauled to Torreon by truck.

This small production iseffected by fluctuations of metal prices.

At the timeof the last visit to the district (October, 1965),
approximately 50 men were employed in the mines.
Figure 6 shows the headframe and loading facilities
at the America Dos mine which is the largest producer
in the district at present.
Figure 7 is a reproduction of an illustrationfrom the Engineering and Mining Journal (1908) showing
the town of Ojuela during the period of active mining
shortly after the turn of the century.

Figure 8 also

shows the town of Ojuela as it appeared at that time
and the photograph in Figure 9 shows Ojuela as it istoday.

Most of the roofs and many of the walls havecollapsed and wind and rain are taking their toll on
the crumbling adobe.

FIGURE 7

A view of 0juela and the headframe of the
Socavon Shaft (lower right hand corner).
(Reproduced from The Engineering andMining Journal, No. 7, August 15, 1908,
p. 310 - by permission.)FIGURE

H

00

19

FIGURE 8

A view of Ojuela during the period of
active mining.
(Reproduced from The
Engineering and Mining Journal, No. 7,
August 15, 1908, p. 312 - by permission.)

FIGURE 9

A similar view of Ojuela as
above taken in 1965.

More complete production records from various
periods of mining in the district can be found in the
numerous references cited by Bernstein (1965) and
Dahlgren (1883) .

STRUCTURE AND PETROLOGY
Mapimi lies in the western portion of the
Sierra Madre Oriental or the eastern part of the Central
Plateau province.

Farther to the west is the Sierra

Madre Occidental.

The Sierra Madre Oriental is character­ized by considerable structural complexity with manytightly folded anticlinoria and synclinoria.

The entire

district is situated within the structural feature called
the Mapimi anticlinorium which has an approximate N30°W
trend.

To the west lies the Rosario synclinorium and to

the east, the Mapimi synclinorium.

To the north, theanticlinorium plunges into the merging Rosario and
Mapimi synclinoria.
A large granitic stock,the Sarnoso, has been
emplacedin the center of the anticlinorium, causing

considerable widening of this structure.

The Sarnoso

stock is completely surrounded by Aurora Limestone ofLower Cretaceous age.

The limestone has undergone

varying degrees of contact metasomatism.

The contact

usually weathers more deeply than either the unaltered
limestone or the granitic intrusion.

21

Rocks in this region range in age from Triassic(Clemons

and McLeroy, 1962) to Recent.

Clemons andMcLeroy discuss the lithologic units in considerable
detail; the reader interested in the stratigraphy isreferred to this paper which includes a complete
correlation chart for the entire northern Mexico region
and part of southwestern United States.
The sedimentary unit of significance in the
Mapimi district is the Aurora Limestone.

This limestone

ranges from 300 to 600 meters in thickness.' Clemons andMcLeroy (1962) state that the individual beds range inthickness from 30 centimeters to 9 meters and average
about 0.5 meter.

The Aurora Limestone in the vicinityof Ojuela has massive bedding which is more common in
the lower part of the formation (Figure 2).
The Indidura Formation of Upper Cretaceous age
is found at Ojuela as a series of intensely folded,
black-to-gray, laminated limestones and shales.

A sharpcontact exists between the Indidura Formation and theunderlying Aurora Limestone.

Nowhere was economic

mineralization observed within or related to theIndidura Formation.

23

The Caracol Formation (?) is exposed along theroad about 5 kilometers east of Mapimi as an extensive
series of resistant ridges.

The formation has been

metamorphosed to a grayish hornfel at this location.
Emission spectrographs of several samples of the Caracol
Formation (?) from the exposures east of Mapimi and fromthe type section at Arroyo Caracol show little variationin trace element content, especially the base metals.
Sarnoso Stock
An outcropping of a highly sodic granite occurs
to the northwest of Dinamita, a small village approxi­
mately 24 kilometers southeast of Ojuela.

The intrusive

has been variously described as a laccolith (Kellum,1935) and a stock (Clemons and McLeroy, 1962).

The

nature of the contact, which is fairly well exposed aroundthe circumference of the intrusive, is strongly suggestiveof a stock.

The contact with the surrounding sedimentary

units is usually steeply dipping, generally greater than
75° away from the intrusive.

A few places were noted

where the contact dipped below the intrusive at a low
angle.

At these localities a laccolithic shape is sug­gested but considering the entire unit, the intrusive
is more stocklike.

The area of outcrop of the stock isapproximately 75 square kilometers.

2k

The intrusive is strongly weathered.
been facilitatedby

Weathering has

two steeply-dipping joint sets , one

N-S and the other almost E-W; a third set of joints isalmost horizontal.

Spheroidal weathering of the intrusive isa result of the pronounced

jointing (Figure 10).

The

weathered surface of the intrusive is brown or tan, but
fresh surfaces are generally gray in color.

The rockweathers to a coarse sand composed of quartz andargillitized feldspars.
The intrusive is predominantly a sodic granite.
However, small masses or local segregations of otherrock types are common throughout the stock, especiallynear the contact with the surrounding sediments.

These

rock types include quartz diorite, monzonite, hornblendediorite, and diorite.

No orientation of the inclusions

was evident.
The principal rock type of the intrusive iscomposed of quartz, microcline, albite, and orthoclasewith minor amounts of biotite, hornblende, chlorite, andpyroxene.

Accessory minerals include apatite, magnetite,ilmenite, zircon, sphene, and monazite.

The rock is

usually equigranular, but porphyritic zones are notuncommon.

Where porphyritic, the phenocrysts are sub-hedral orthoclase or zoned plagioclase crystals.

The

minerals of the groundmass are anhedral with the exception

FIGURE 10

The Sarnoso stock looking south from thenorthern contact with the limestone. The
shearing and jointing of the intrusive
are clearly visible.

FIGURE 11

The manganese ore deposits near San Pedro.
The darker zones in the photograph areareas of manganese mineralization.

25

FIGURE 10

FIGURE 11

26of the accessory minerals.

The microcline exhibits the

typical quadrille structure.

Some perthitic intergrowths

were noted as well as graphic intergrowths of quartz andorthoclase.

The feldspars have a sanded or cloudy

appearance due to argillic and sericitic alteration.

The

hornblende and biotite are occasionally altered tochlorite and iron oxide.

The composition of the intrusive

becomes more dioritic toward the southern part of thestock.Modal analyses of samples from the center of the
intrusive to the outer contact are shown in Table I.
TABLE I
Modal Analyses of Sodic Granite from Sarnoso Stock
4-18-64-17

4-18-64-23

4-18-64-21

4-18-64-16

microcline

20

26

23

22

quartz

28

26

21

18

albite(perthitic)

25

22

26

30

orthoclase

22

19

20

16

5

6

9

12

1

1

2

biotite>hornblendeJ

diopside27

As seen above, there is a progression from left
to right which corresponds to traverse samples from thecenter to the outer contact.

Field examination of thestock indicates that as the outer contact is approachedthe mafic mineral content of the rock increases.
At the contact a number of hybrid rocks areencountered.

They include such minerals as diopside,

epidote, grossularite, actinolite, oligoclase,
vesuvianite, wollastonite, and cordierite (?).

The

contact between the limestone and the intrusive is usually
sharp and well defined, but small zones of tactite or calcsilicate rock are occasionally developed.

This tactite orcalc-silicate rock is composed of varying amounts ofgrossularite, diopside, actinolite, vesuvianite, pectolite,
phlogopite, apatite, clinozoisite, quartz, and calcite.
The rock is pale green in color.
The limestone near the contact has beenrecrystallized and is white in color.
been quarried.

At places it has

The limestone, for considerable distancesfrom the intrusive contact contains small acicular
crystals of tremolite or wollastonite.
Just east of the village of Dinamita, several
mines are situated on contact deposits of iron oxide.
£t the present time, however, these deposits are not being
exploited because of their small size and generally low

grade.

Most of the high grade zones have been removed.

The mines at this location include the La Lucha and Minatyinera.
East of San Pedro, on the western side of the
intrusive, a rather large occurrence of manganese ore is
being mined (Figure 11).

The ore is brought to thesurface, hand-cobbed, and separated into various types
and grades of ore.

In 1965 the daily production wasapproximately 35-40 metric tons mined at a depth ofabout 50 meters.

The ore minerals include pyrolusite,

cryptomelane, psilomelane, and manganite with minor
groutite.

These minerals will be discussed in moredetail in the chapter on mineralogy.

Gangue minerals

are calcite, quartz, and rhodochrosite.

El Vergel Stock;
A similar intrusive, the El Vergil stock, issituated to the west of the Sarnoso stock.

Composition^

ally, the two bodies are similar ^nd a common origin isassumed.

The outcrop area of this stock is approximately

one-half square kilometer.
The rock weathers in a manner similar to the
Sarnoso intrusive and has roughly the same appearance inhand specimen.

The rock is phaneritic, and individual

crystals^are anhedral to subhedral, having an average
size of about 3 millimeters.

A few feldspar crystals up

to 1 centimeter have been noted.

A range of values isgiven in the modal analysis below.
Per Cent
Range
Albite20-40

Quartz20-30

Microcline

15-25

Orthoclase

5-15

Oligoclase-andesine

2-10

Biotite-muscovite

1- 3Hornblende

1- 3

30

Accessory minerals include magnetite, ilmenite,
zircon, apatite, and monazite.

The feldspars are partiallyaltered to sericite and clay.

The mafic minerals alter tochlorite and iron oxides.

PRIMARY MINERALIZATION
The principles of the origin of limestone
replacement deposits in the Mexican Province have beenpresented by Prescott (1926)

after years of detailed

studies in the many mines in northern Mexico.

During his

studies he noted many similarities in structures and min-eral assemblages from district to district.

Eventually it

became clear to him that certain factors were common for
all of the districts.

These common factors of ore

deposition are briefly summarized as follows:1.

The ore bodies are continuous from thepoint of entrance into the favorable lime­stones at depth, to the surface, to thepoint of egress from the favorable beds,
or until they gradually become extremely
attenuated.

2.

The area of the cross section of the ore
body at any point will not be greater thana small multiple of the area of the normal
cross section near by, nor smaller than alarge fraction of that cross section.

31

32

3.

The progress of the mineralizer is nearly
always upward from its source.

4.

The analysis of each ore body is distinctive
and, broadly considered, invariable.

5.

The ore bodies are essentially chimneys or
pipes, whether standing vertically and obvi­
ously fulfilling the accepted idea of a chim­
ney or pipe, whether inclined at any angle,
or even lying horizontally; and, as in pipes,
the ore deposited in the portions farthestfrom the source has passed through and inside
the walls of the ore body nearer the source.

6.

Between the point at which the deposits startin depth and the finishing point at thesurface, or where the mineralizer enters
unfavorable overlying formations, the courseof a single ore body will be controlled byany one, any combination, or all of thefollowing factors:(a)selective action of certain
limestones,(b)

variation in composition of thesedimentaries,(c)structure of the limestone,(d)posture of the beds,(e)pre-mineral fissures

33

7.

The first favorable limestone encountered by
the ascending mineralizer is often by far themost productive, though, under certain condi­
tions , the last favorable limestone also may
be unusually productive.

The term "favorable bed" or "horizon" has been
applied in descriptions of almost all limestone replace­ment deposits in both the United States and in Mexico.
What constitutes a favorable bed is variable from district
to district, but Fletcher (1929) recognizes, for example,
at Sierra Mojada, some 250 Km to the south, that the beds
in which the mantos commonly occur have an almost constantcomposition.

One such horizon contained an average of

14 per cent MgO, varying locally from that magnesium
content by only 1 per cent at most.

Generally, analyses

vary between 13 and 20 per cent MgO, which is a typicalrange for the magnesian content of the favorable beds formost of the similar limestone replacement deposits.
Fletcher further found that the beds both immediately
above and below one of these magnesian strata at Sierra
Mojada had an MgO content of only 1 to 2 per cent.

Thus,

at Sierra Mojada the favorable bed would be considered
to be the dolomitic limestone.

34

The favorable bed concept has often been considered
to be the controlling factor for mineralization; themineralizing solutions came up along fractures until thefavorable horizon was encountered and then mineralization
proceeded along this unit.

The detailed studies of

Prescott, previously mentioned, show that other factors
must be considered before any clear understanding of theore distribution can be obtained.

Olson (1966) described

the favorable bed at the Glove Mine in southern Arizona
as a fairly pure limestone with little if any MgO.

But

the pipelike ore bodies are also localized along zones
of permeability formed by faults and fault intersections
within the favorable limestone.

Here it is clear that

more than just the presence of a favorable bed is involvedin the ore localization.
At Mapimi, six distinct types of ore controls wererecognized by Prescott (1926 ) .

The great variation inore controls at Mapimi may be laid to the complexity ofthe structure which is contorted by strong pre- and post­mineral faulting and pre-mineral folding.

The various

types of ore controls described by Prescott are presented
as follows:35Type I

Simple chimney crossing Strata.

Ojuela-Paloma ore body.
The Ojuela-Paloma ore body is an example
of a simple mineralized chimney cross­cutting nearly horizontal beds.
Approximately 3000 feet of verticaldevelopment has been made with little
diminishment in cross sectional area withdepth.

Figure 12 illustrates a meanderingchimney without definite control by either
fissures or beds.
Type II

Chimney, manto and chimney.

San Jorge-

San Juan ore body.
Figure 13 is a projection of the San JorgeSan Juan ore body.

The ore body begins asa chimney following a pre-mineral fault.
The ore body forms a manto about 200 meters
above the water table and follows the bed­
ding in the limestone for about half akilometer.

The ore body then crosses thefissure and again chimneys to the surface.
The manto phase is controlled by the
texture and/or composition of the mineral­ized horizon.

36?

100 2QO

300 4QO

500

FIGURE 12

Cross section - the Ojuela-Paloma ore
body - Ojuela mine - Ojuela (fromPrescott, 1926).

FIGURE 13

Cross section - the San Jorge-San Juan ore
bodies - Ojuela mine (from Prescott, 1926).

Type III

Inclined chimney and flat manto guided bystructure.

Cumbres ore body.

The Cumbres ore body shows the effect ofstructure on ore deposition.

The orebody lies along or near the axis of an
anticline as shown in Figures 14 and 15.
The description from Prescott follows:
From the water level up to thefourth level it consists essentiallyof a group of inclined mantos, eachof which is in reality an independentore body, with its own standard
analysis and cross section. On the
fourth, the mantos, following the
anticline, encounter a pre-mineral
fault fissure striking at a^ slightangle to the axis of the anticline,
and follow this upward for about
one hundred meters, where the body
mantos to the south and is followedwith diminishing cross section for
at least half a mile.
Type IV

Ore body on beds on limb of an
anticline.

San Carlos ore body.

From the bottom the San Carlos ore bodyfollows a pre-mineral fault.

At about

the twelfth level the ore body begins tofollow the limestone bedding, which atthis point is dipping at about 40°, andcontinues upward along the limb of the
anticline.

The San Carlos ore body shows

38

100
_4

FIGURE 14

FIGURE 15

kV#f«

200

meters

Cross section - Cumbres ore body. Projection
N 32°W., looking NE. (from Prescott, 1926).

Cross section - Cumbres ore body. Section
through A— A' (from Prescott, 1926).

3QQ

39

the effect of the selective action ofcertain beds, usually dolomitic, onore deposition.
Type VOre body follows fissures as series ofvertical pipes.

Santa Rita ore body.

From approximately the seventh level tothe surface a number of pipes
follow the Santa Rita fault in almost
vein-like form.

Prescott (1926)

describes the occurrence as follows:
The fissure continues strong andwell marked to the deeper levels;
but the ore bodies below theseventh level make off to thewest, and mantos on the twelfthlevel horizon. Below that level
they continue downward as chimneys
in the limestone.
We have here,
then, an ascending mineralization
which reverses the condition of
the San Jorge in that this finishes
above on a strong pre-mineral fault
fissure, whereas San Jorge starts
below on such a one. Both show
the basic indifference of the ore
channels with regard to pre-mineralfissures.
Type VI

Controlled by all types.
ore body.

San Diego

The description given by

Prescott again best describes the
San Diego ore body.

l+o
San Diego starts as a strong manto
below water level, crosses the post­mineral San Diego fault, mantos forone hundred meters more, goes up asa chimney for fifty meters, thenfollows the beds, crosses the San
Diego fault, and continues on thebeds, following faithfully the struc­ture developed, a huge Z, and finallyabove the first level strikes the pre­mineral San Vicente fault which it
follows to the surface.
At depth, the ore bodies approach
simple chimneys in form.

These chimneys

are situated along a line that bears
little or no relation to known struc­tural features of the limestone.

The

San Diego fault roughly parallels thearrangement of chimneys but this fault
is post-mineral and cuts all structure
and pre-mineral faults at an angle, which,according to Prescott, represent re­adjustment along some geologic feature of
the underlying rock.

This geologic

feature in the underlying rock along withthe fractures and structures in themineralized limestones caused the local­
ization of the chimneys.

The variationin ore body controls indicates thatmineralization follows the course ofleast resistance from the source to thesurface.
The mineralization at Mapimi is unique in thatthe range of mineralization from the intrusive throughthe ore zone and into the pegmatitic low temperature vein
system is exposed.

The zoning, thus, can be fairly

accurately determined.
Drill cores have penetrated an alaskite porphyry
intrusion at a depth of 1000 meters near the base of
La Bufa.

As this intrusive was approached, the amount ofcalc-silicate minerals increased greatly. ' These calcsilicates include wollastonite, tremolite, diopside andgarnet.

The alaskite, which consists of quartz, ortho-

clase and plagioclase with only a few per cent of biotite
and hornblende, is not exposed at the surface within the
district, but it does outcrop about 12 kilometers to thenorth where it intrudes a shale series.

The absence ofore mineralization at this location may be laid to theintrusion into the unfavorable host rock, the shale.
Spurr (1923) considers the intrusion of thealaskite and also possibly the diorites as being thecause of the doming and folding of this and other similar
districts in Mexico.

The ore zones are directly related

to the folding; the mineralization is often localizedalong the axis of anticlines and synclines.

The

mineralization, therefore, was certainly later than thefolding.
The diorites referred to by Spurr vary incomposition from quartz monzonites to hornblende diorites.
They are usually present as dikes and irregular shaped
intrusives that have been considered laccolithic.
Mineralized veinlets are truncated by the diorite dikes,
showing that the diorites are younger than the stage ofore mineralization.

Like the ore mineralization, thedistribution of the diorite dikes is controlled to an
extent by the folding in the sediments.
Dikes of hornblende diorite are encountered in
the mines at Mapiml.

Similar dikes are also common at

Penoles, some 50 kilometers west of Mapimi.

They also

intrude the same lithologic unit as is encountered at
Mapimi.

Spurr envisions the diorites and the alaskite

to have been derived from a single magma, whose upward
and lateral pressure was the cause of the domal uplift.
The vertical extent of mineralization from the
top of the alaskite to the summit of La Bufa, where thecarbonate veins are still exposed, is almost 2,000
meters.

The top of the silver-lead zone corresponds

roughly with the present topography at the base of
La Bufa, some 1,000 meters above the top of the intrusive

ZONING
Four distinct zones of mineralization can beobserved at Mapimi.
(1)

Copper-bearing contact zone(2)

Zinc-lead zone

(3)

Silver-lead zone

(4)

Barren carbonate zone

Each of the zones is characterized by distinct mineralogical assemblages.

Certainly

telescoping of zones occur
readily distinguishable.

some overlapping andbut generally the zones are

The contact mineralized zone

consists of various calc-silicates along with arsenopyrite,
pyrite and chaclcopyrite and lesser amounts of galena,
sphalerite, enargite, and molybdenite.
the principal sulfide.

Arsenopyrite is

The pyrite is cupriferous.

The zinc-lead zone is characterized by a definite
increase in sphalerite and galena with a corresponding
decrease in calc-silicates and arsenopyrite.

Pyrite,chalcopyrite, enargite, marcasite, and molybdenite arealso present.

Exsolution blebs of chalcopyrite insphalerite are common.

The enargite is being

pseudomorphically replaced by marcasite.

43

kk
The silver-lead zone consists of argentiferous
galena with lesser amounts of sphalerite, pyrite,
chalcopyrite, enargite, molybdenite, stibnite andarsenopyrite.

In this zone, the arsenopyrite content has

decreased to less than a per cent.
The barren carbonate zone contains numerous
veinlets of calcite, siderite, quartz and stibnite. Small
amounts of pyrargyrite are occasionally encountered inthese veinlets.

The carbonate vein zone is the latest stageof mineralization.

The minerals are typical of low tempera­ture deposition; they represent the incorporation into
crystalline phases of the volatile components of the orebearing solutions.

The veinlets extend considerable

distances from the source.

Stibnite and stibiconite havebeen mined on a very small scale at the San Ilario mineon the west side of La Bufa.
Oxidation has obliterated many of the primaryminerals of the silver-lead zone and the zinc-lead zone.
The oxidation products of the silver-lead zone include
cerussite, wulfenite, mimetite, anglesite, goethite, andjarosite.

Cerargyrite is the only silver mineral

recognized in this oxidized zone.

The oxidation of the zinc-lead zone has produced
such minerals as adamite, paradamite, hemimorphite,
mimetite, aurichalcite, rosasite, hydrozincite,arseniosiderite, and goethite.

The noted occurrence oflegrandite is found in this zone.

With increasing depth

in the oxidation zone, the amount of secondary copper
minerals increases.
Oxidation of the carbonate zone results in thealteration of stibnite to stibiconite.

Siderite grains

frequently have coatings of iron oxide resulting from theoxidation of the iron carbonate.

DESCRIPTION OF INDIVIDUAL MINERALS
Bismuth

Bi
Irregular shaped masses of bismuth were noted inalmost all of the polished sections of ore from theMonterrey mine.

It is associated with galena, sphalerite,

pyrrhotite, magnetite, stannite, chalcopyrite, enargite,
umangite,

emplectite, and marcasite andcommonly

it isseen to be replacing galena and pyrrhotite.
In polished sections the bismuth is characterized
by its low hardness, complex twinning observed under crossed
nicols, and the strong anisotropism.
CopperCu
Subhedral crystals and fine wires of native

copper have been observed on a few samples of ore in theoxidized zones at both Mapimi and at the Descubridora
mines. The copper was no doubt formed in small local areas
where strong reducing conditions prevailed.
The native copper is associated with tenorite,
cuprite, goethite, and delafossite.

Much of the copper

has been pseudomorphically replaced by covellite.

1*6

Small wires and crystals of copper are found in
gypsum which occurs as late fracture fillings in oxidized
ores. Where exposed, the copper has altered to cuprite
and malachite.
Gold

AuGold has been found as minute irregularly shaped

grains associated with hematite, goethite and various
manganese oxides in the upper oxidized zones at Mapimi.
A few masses of iron and manganese oxides were reportedly
selectively processed for their high gold content.

In

the zone of oxidation, gold has been observed only in thepresence of iron and manganese oxides.
Analyses of primary ores by spectrophotometrictechniques show a variation of gold values from less than
1 ppm to 8 ppm.

The higher values are associated with

pyrite in the more arsenical ores.

No native gold or gold

minerals have been detected during the examination ofnumerous polished sections of primary ores from the
district.
Careful micropanning of tailings, which were usedas fill in worked out stopes and crosscuts in the Ojuelamine, failed to concentrate any gold values even though a
few parts per million were occasionally noted in assaysof the fill material.

Analysis of pyrite and arsenopyrite

148
concentrates obtained from this fill material by the
micropanning technique showed an appreciable increase ofgold in the concentrate.

This indicates that either the

pyrite or the arsenopyrite is auriferous.
SilverAg
Small wires and flattened plates of silver up to

two millimeters in diameter have been noted by the writerin the oxidized lead ores from the San Juan mine near
Ojuela.

Similar occurrences, however, have been reported

from most of the oxide lead ores of the district.

Thesilver is associated with cerussite, anglesite, andmimetite and usually has a black oxidized coating.
Examination of polished sections of the oxidized ores
showed the silver to be partially replaced by a dullgray mineral which is probably cerargyrite.
SulfurS
Sulfur occurs as bright yellow masses in thecelestite ores from El Vergel.
gypsum and celestite.

It is associated with

Spectrographic studies of asulfur concentrate revealed only trace amounts of Ca, Sr,and As.

49

Cerargyrite

AgCl

Like gold, the majority of the silver extracted
from the district has been recovered from the upper zonesof oxidation.
different.

The associations, however, are distinctly

Cerargyrite occurs as small greenish-grayanhedral grains and masses associated with mimetite,
cerussite, anglesite, and wulfenite.

It is usually diffi­

cult to recognize because of its small size and its
similarity in color to many of the other oxide minerals
with which it is associated.

Chemical analyses of threehand-picked oxidized lead ore samples that contained novisible mimetite gave the following Ag and halide values
compared with theoretical cerargyrite.

The halides include

Cl, B r , and I.
Sample No.

% Ag% Halides

Ag/Halides

10-10-65-17

0.026

0.008

3.2

10-10-65-23

0.110

0.039

■'

2.8

■
r.'

5—7—65—6

0.198

0.071

2.8

AgCl

76.26

24.74

3.1

The principal halide noted was Cl, but an X-ray
spectrographic scan of the silver halide precipitate
showed a considerable amount of Br to be present.

It

can be seen that the Ag/halide ratio is very close to thetheoretical ratio of Ag to halides found in cerargyrite.
Thus, the cerargyrite can be considered to be the silver
bearing phase.
Cerargyrite is found predominantly in theoxidized lead ores but copper and zinc may be present.
The silver content of the oxide copper and zinc ores alone
in the absence of lead is low.
Fluorite

CaF2
Fluorite is found at many locations in the

district.

The color varies from colorless to deep purple

and many hues of blue, green, pink, and brown.

Typicallyit has a cubic habit but the octahedron {ill 3 , anddodecahedron {.110} are not uncommon.

Usually octahedral

and dodecahedral development is limited to the paler
colored varieties, especially light pink or green orcolorless.

Much of the darker material is coated withfine druses of descloizite and plattnerite.
The associations of fluorite are numerous.

It

is commonly found with wulfenite, barite, calcite,
aurichalcite, plattnerite, murdochite, and hemimorphite.
At one location on the first levelof the Ojuela mine astriking group of specimens was found for which theparagenesis could be easily determined.

Cubic crystalsof a pale blue fluorite partially enclosed wulfenite,

51

barite and hemimorphite.

Small rosettes of aurichalcite

were partially enclosed in a colorless encrusting calcite.
Minute crystals of plattnerite and murdochite were
scattered over the entire assemblage.
Nantockite

CuCl

Nantockite occurs in intergrowths with rosasite,
aurichalcite, plattnerite, malachite, and cerussite on thesurface of cerussite pseudomorphs after hemimorphite (?)
on samples labled "Mina Ojuela" in the U.S. National
Museum (U.S.N.M. R-16854, John S. White, PersonalCommunication).
Other occurrences of nantockite have probably been
overlooked many times because the identification would re­quire a close microscopic examination or X-ray diffraction
studies of the rosasite and aurichalcite coatings.
Bindheimite

Pb2Sb206 (0,0H)

Fibrous to massive, yellow-brown bindheimite
has been noted in several areas in the district.

The

most noteworthy occurrence is in the San Juan Poniente
stope, 14th level of Mina Ojuela where it is found withcolorless to white adamite, mimetite zand hedyphane.

52

The composition of the bindheimite varies
considerably.

A sample collected by Foshag in 1924 was

obtained from the U.S. National Museum (U.S.N.M 92284).
A chemical and spectrographic analysis of this sample
showed considerable quantities of Zn, As, Fe, and SO3 to
be present (Table II).

An electron microscope examinationof the sample indicates that these impurities are probably
present as the minerals adamite (or paradamite), goethite,
and anglesite, which undoubtedly formed contemporaneously withor shortly after the bindheimite.

After deduction of theZn and As as adamite, Fe as goethite, and an amount of Pb
sufficient to account for the SO3 and CC>2 as anglesite andcerussite, the composition is found to be very close to thetheoretical composition of bindheimite.
Other samples of bindheimite contain a quantity of
Sb in excess of that required for this mineral.

Natta andBaccaredda (1933) report that stibiconite and bindheimite
are isostructural.

Thus, the excess Sb could be accounted

for on this basis.
Bindheimite was probably formed as an alteration
product of tetrahedrite, boulangerite, enargite, galena,and stibnite.

A suggestion of the original morphologies

of boulangerite, jamesonite, or stibnite crystals has .
been observed (Figure 16).

A spectrographic analysis

of a bright orange-yellow material showed about five per

■

-<<

FIGURE 16,

Photomicrographs showing thealteration of boulangerite(white) to jamesonite (lightgray) to bindheimite (mediumgray) and goethite (dark gray)Nicols uncrossed-oil immersion
green filter.

54
TABLE II
Spectrographic and Chemical Analysesof Bindheimite

Pb2Sb207JJ20

MapimiBindheimite*

PbO

56.65

47.9

AgO

-

1.3

Sb2°5

41.06

46.8

H2°

2.29

4.0

Total

100.00100.0

Recalculated to 100% after deductionof adamite, goethite, anglesite, andcerussite.
cent silver to be present.

The X-ray diffraction patterngave only the bindheimite pattern, but the d-spacings ofstetefeldite, the silver antimonate, are similar to thoseof bindheimite and the stronger bindheimite pattern would
have obscured the pattern of stetefeldite.
minerals are probably isostructural

The twoand a complete sub­stitution between the two end members no doubt could exist.
Braunite

Mn+^Mn+|si0^2
Braunite is one of the earliest manganese mineralsof the manganese ores near San Pedro.

Most of the braunite

has been replaced by cryptomelane and only a few irregular55

shaped grains, seldom more than a few micra in greatest
dimension, remain.
Chalcophanite

M n ^ M n ^ O g •2H20

Chalcophanite occurs as thin velvety grayishblack crusts on goethite.

Chalcophanite is associatedwith adamite, smithsonite, and calcite at many of themines in the district.

The best specimens of chalcopha­nite are reported as being from the Esperanza stope which
can be reached from either the San Juan or the America
Dos shafts.
Cryptomelane

KMn+2Mn+gC>2g •2 ^ 0

Cryptomelane is the major constituent of themanganese deposits on the western side of the Sarnoso
stock.

The cryptomelane is associated with pyrolusite,manganite, and psilomelane.

Crytpomelane is partiallyreplaced by psilomelane along fractures and an occasional
thin rim of groutite is also observed on weatheredsurfaces.
Cuprite

CU2O
Masses of cuprite are seen as alteration productsof chalcopyrite and with botryoidal and pulverulent goethite
Small, euhedral, blood-red crystals of cuprite also occur

56
in small vugs on botryoidal goethite along with copper
and tenorite.

These crystals are highly modified cubes.

The cuprite alters to tenorite, native copper,
malachite, and chrysocolla, and is relatively abundantin the oxidized copper ores.
Delafossite

CuFeOgSmall brown botryoidal crusts and rosettes andelongate crystal clusters of delafossite are common onmasses of goethite, cuprite, and tenorite.

The dominant

form of the elongate crystals is probably the scalenohedronor rhombohedron.
The formation of delafossite clearly began toward
the end of the goethite crystallization and continued after
cessation of goethite deposition.
GoethiteFeO(OH)
Goethite is the most common and widely distributed

iron mineral in the oxidized zone.

It occurs as the matrix

for almost every secondary mineral assemblage noted at
Mapimi.

The goethite forms as an alteration product ofpyrite, arsenopyrite, marcasite, and pyrrhotite.

Traces ofarsenic were noted in almost all spectrographs of goethite,
but the arsenic content was greatest in the deeper portions

57

of the mines.

This increase in the arsenic content of thegoethite reflects the increase in arsenopyrite in the
primary ores with depth.
The color of the goethite varies from a paleorange-brown ocherous material to almost black botryoidal
crusts and masses.

The more compact brown-black botryoidal

material usually contains only very minor amounts of traceelements while the pulverulent goethite may contain almost
traces of almost all of the elements found in the ores.
Pseudomorphs of goethite after pyrite are common
and a few pseudomorphs after arsenopyrite were also noted.
Siderite and ankerite are often coated with goethite andhollow goethite casts ofrhombohedral siderite wereobserved on many samples.
GroutiteMnO(OH)
Groutite occurs as an alteration product of

cryptomelane in the manganese ores of the district.

It

can be recognized in polished sections and distinguishedfrom manganite by its unique bireflection.
Hausmannite

Mn^O^

Hausmannite has been found in a few polishedsections of manganese ores from the mines at San Pedro
and the American Dos mine.

Hausmannite is associated58
with cryptomelane and pyrolusite both of which are partiallyaltered to hausmannite.
Hematite

Fe203
Hematite is the principal constituent of the ironores mined at the LaLucha mine near Dinamita.

The hematite

occurs as large red masses near the contact of the Sarnoso
stock with limestone.

The distribution of hematite appears

to be localized along the contact where the dip of thecontact deviates from the almost vertical attitude observed
at most exposures.
The hematite is associated with various calc- andiron-silicates that were formed in the contact zone.
Spectrographic analyses of the hematite revealed only tracesof Mn, T i , Si, Cu, and Al.

Arsenic and antimony, which arealmost always present in the iron and manganese minerals
at Ojuela, are conspicuously lacking.
A few minor occurrences of hematite were noted in
the mines near Ojuela.

Here the hematite occurs withvarious copper and iron oxides, including cuprite, tenorite,
delafossite, and native copper and, occasionally, covellite.
A similar association was noted at Descubridora.

59

Hydrohausmannite(Mn+2/Mn+^)„ (0,0H)„
3-x*

Hydrohausmannite occurs as thin bands ofirregular shaped grains in hydrohetaerolite.

Grain bound-:aries are serrated, and it is difficult to determine if thehydrohausmannite is being replaced by hydrohetaerolite.
Wadsley (1955) discusses the characteristics and relation­
ships of hydrohausmannite to hydrohetaerolite.
Hydrohetaerolite--------------(Zn,Mn+3)„ (0,0H)y,3-x%

Hydrohetaerolite occurs as elongated euhedral
crystals along cavities and fractures that were filled bymanganite and as masses of enhedral grains that replace
cryptomelane.

Hydrohetaerolite is associated withhydrohausmannite, cryptomelane, and manganite in the
manganese ores at San Pedro and is relatively common insome zones of manganese mineralization of the America
Dos mine and a small occurrence of manganese near thesouth end of the suspension bridge at Ojuela.
Hydrohetaerolite has strong pleochroism andanisotropism when viewed in polished section.

The identifi­

cation of the mineral was verified by X-ray diffraction
studies.

6o

Lepidocrosite

FeO(OH)

Small platelets and rosettes of lepidocrosite
up to one millimeter across have been found on samples
from the San Rafael mine and as small irregular-shaped
masses in oxidized ores from many localities in the
district.

The lepidocrosite

is associated with goethite,quartz and calcite.
The crystals of lepidocrosite

are transparent

and are dark reddish-brown to brown in color.
sections the lepidocrosite

In polishedcan be distinguished fromgoethite, with which it is associated, by its more
reddish internal reflections and slightly higher reflectance,
Litharge

PbO
The occurrence of litharge is restricted to minuteinclusions in wulfenite.

The presence of abundant litharge

inclusions gives a turbid appearance to the wulfenite.

The

more colored varieties of wulfenite appear to have thegreatest number of litharge crystals, but some of thesecrystals show very few inclusions.
Magnetite

Fe^O^

Small irregular masses of magnetite are found nearcontact zones.

The magnetite is associated with pyrrhotite

and numerous other sulfides and silicates including diopside6lvesuvianite, grossularite, and sphene are also present.
Much of the magnetite is surrounded and partially replacedby pyrrhotite, leaving only small magnetite cores.

Towardzones of oxidation the magnetite shows some alteration tohematite and goethite.

Larger masses of magnetite are also

found near the intrusive at Dinamita.

Here the magnetite

has been exploited on a small scale at the LaLucha minealong with other iron oxides.
Manganite

MnO(OH)

Manganite is one of the last manganese minerals toform in the manganese ores at San Pedro and Mapimi.

Itoccurs as fracture fillings which postdate the depositionof all manganese minerals except groutite.

Euhedralcrystals of manganite are common in vugs.
Manganite can be distinguished from groutite by
the unique bireflection of the latter mineral perpendicular
to the crystal elongation.
MiniumPb^O^
Fine pulverulent orange coatings of minium wereobserved on bright orange wulfenite crystals and on small
cavities in oxidized lead ores associated with adamite,
wulfenite, mimetite.

The matrix is usually composed ofgoethite and bindheimite.

62

The X-ray powder diffraction patterns of the minium
have very broad and diffuse lines which indicates a small
particle size but the patterns were sufficiently clear topermit a positive identification.
Molybdic Ochre(Mo-bearing iron oxides)

Small yellow aggregates of molybdic ochre seldom
greater than 1 mm. in diameter were noted in larger massesof goethite and/or bindheimite.

X-ray diffraction patternsof the material were not diagnostic, but semiquantitative
spectrographic analyses showed Mo to be the major
constituent.
Murdockite^UgPbOg

Small, brilliarit octahedra of murdockite, similar
to the type material described by Fahey (1955), are foundwith plattnerite and hemimorphite On a matrix, of limonite.
The crystals of murdockite are seldom more than one milli­meter in longest dimension.

The only form noted on the

crystals is o{lll) .
Plattnerite

PbOg

Plattnerite is the tetragonal dimorph of leaddioxide and is designated the B-pb02 phase.

Zaslavskii,

63

Kondrashov, and Tolkachev (1950) describe the or-PbOg phase,
which is the orthorhombic dimorph, but it is not known tooccur in nature.
Excellent crystals of plattnerite have been notedin the oxidized lead ores at Mapimi.

White (1966) studied

the species and discussed the composition, morphology andphysical properties from the deposits.

Plattnerite occurs

as black acicular crystals on a matrix of goethite.

Itis associated with hemimorphite, smithsonite, calcite,rosasite, hydrozincite, aurichalcite, and murdockite.
The crystals of plattnerite from Mapimi are simple:
only five forms were recognized by White.

These include{010} ,

Twinning on {Oil }{110} ,

{131}, . {Oil} ,and {001} .

was commonly, noted.
Trace amounts of Fe, Ca, Mo, Al, Na, Si, Cu, Tl, Ti,
Mg, Mn, Ba, B, Cr, and Ag were detected in spectrographic
analyses reported by White on hand-picked material from
Mapimi.
1,0-

Psilomelane

44

* “

(Ba,Mn"*"^)Mn ^Og (OH) 2

Psilomelane occurs as botryoidal crusts and masses
on pyrolusite and cryptomelane.

Psilomelane proceeds theformation of cryptomelane and pyrolusite and partiallyreplaces both minerals.

In polished section, the botryoidal

crusts are seen to be composed of numerous acicular crystals
arranged in radiating clusters.

64

Pyrolusite

MnOg

Pyrolusite occurs as an alteration product of
cryptomelane in the manganese ore deposits at San Pedro
and in the upper zone of oxidation at Mapimi.
In polished section, the pyrolusite is seen to havea fibrous structure and moderate anisotropism.

It isassociated with hausmannite, psilomelane, manganite, andgoethite.
Ramsdellite

MnOg

Small, irregularly shaped grains of ramsdellite inpyrolusite were observed in polished sections of manganeseores.

In polished section it can be distinguished frompyrolusite by its slightly different color parallel tothe elongation and faint red internal reflections under
oil immersion (Ramdohr and Frenzel, 1956).
Stibiconite

Sb+3Sb+2(0,0H,H20)?

Pseudomorphs of stibiconite after elongate
prismatic crystals of stibnite are common at numerous
localities on the western slope of the Bufa de Mapimi.
Several small mines are found in this area, the largest ofwhich is the San Ilario.

The stibnite was a replacement ofrelatively unaltered limestone and in late calcite-sideritequartz veinlets.

Stibiconite is also found associated with

65
bindheimite, as previously mentioned, and as a replacementof stibnite in the sulfide ores at the America Dos mine.
It is probably present in many other localities, but theearthy appearance of the stibiconite makes it difficult torecognize.
Vitaliano and Mason (1952), in their paper on
stibiconite and cervantite, present data on a sample ofstibiconite from the Ojuela mine.

They give values of

1.762 for the index of refraction and 5.00 for the density.
Examination of several stibiconites from the San Ilario
mine showed the refractive index to vary from 1.772 to
1.820.

The examination of the stibiconite revealed thepresence of a second phase which is probably valentinite.
The presence of valentinite was tentatively confirmed by
Vitaliano and Mason,apartial chemical analysis of

Ojuela stibiconite from their paper follows:
Sb

=

66.4

H20- =

0.34

H20+ =

3.00

Vitaliano and Mason also state that both therefractive index and density "show a direct relationshipto the antimony content and an inverse relationship to thewater content.

These relations are not linear, however,evidently because of the complex variability of the chemi^
cal composition coupled with a greater or less degree of
defectness in the structure."Vitaliano and Mason discredit cervantite claiming
it to be stibiconite and other antimony oxides but
Gruender, Paetzold and Strunz (1962) showed that material
from Romania and Bolivia gave X-ray powder patterns
identical with that of synthetic Sb20^, thus re-instating
cervantite.

Diffraction patterns of the antimony oxides

from Mapimi are generally diffuse,and positive identified
tion of cervantite could not be made.
Tenorite

CuO
The occurrence of tenorite is restricted to theupper zones of oxidation.

The tenorite is dark blackish-brown to black in color and has a resinous to pitchy
luster.

The brownish cast of the tenorite is attributed

to admixed iron oxides.

The presence of iron has beenverified by X-ray spectrographic studies.

Tenorite iscommonly found as alteration coatings on cuprite.
Valentinite

SbgOg

X-ray diffraction data obtained by Vitaliano andMason (1952) suggest the presence of valentinite admixed
with stibiconite from Ojuela.

A sample of stibiconite

from the Asterillio mine was crushed and subjected to aheavy liquid separation to remove quartz and calcite.
The data obtained from the X-ray diffraction pattern of

67

the sink fraction are shown in Table III compared with
the individual patterns of valentinite and stibiconite.
It can be clearly seen from the diffraction patterns thatthe samples does contain both valentinite and stibiconite.
Cassiterite

SnOg

Small acicular crystals of blackish-brown
cassiterite have been found at the Monterrey mine incontact metasomatized limestone.

The only other ore

minerals found with the cassiterite are magnetite andpyrrhotite.
Small amounts of tin were frequently noted in
the spectrographic studies of almost all of the oxide
ores of the district.

It is probable that the tin ispresent as cassiterite resulting from the oxidation ofstannite.
Arsenopyrite

FeAsS

Large, euhedral crystals of arsenopyrite, up to
one centimeter long, have been found associated with pyrite,
galena, and sphalerite.
altered to goethite.

Commonly the pyrite is partially

These large arsenopyrite crystals

were not found in place in the mines, but were fairly
abundant on many of the dumps from the deeper portions of
the mines.

68
TABLE III
X-ray Diffraction Data for
Stiblconite and Valentlnit©

Stiblconite
and Valentinite(Mapimi)Id(X)Valentinite
ASTM - 11-691
I

d(A)

9

5.9

1

4.6

17

4.56

2

3.5

25

3.494

2

3.2

20

3.173

5

3.15

100

3.142

9

3.10B

80

3.117

10

2.95

5

1

10

2.737

14

2.648

2.58

2.37

10

2.456

4

2.371

2

2.272

4

2.203

4

2.107

10

2.058

8

2.045

4

1.98

6

1.970

1

1.92

12

1.929

2

1.888

6

1.835

8
20

1.618
1.804

( 1.826 ^ 1.60
2

1.74
10

1.675

Stiblconite
Vitaliano andMason (1952)
I

d(&)

90

5.93

70

3.09

100

2.96

40

2.57

10

2.36

30

1.98

60

1.81

30

1.74

69

The crystals are of simple habit, consisting onlyof prisms and the basal planes.

Small crystals of pyrite,with the predominant pyijitohedron form, are occasionallyfound on the crystal faces of the arsenopyrite.
A spectrographic 'analysis of selected crystalsof arsenopyrite showed the following traces:
Wt. %

Wt. %
Ag
Al
Ca
Cu
Mg
Mn

.04-.4.04-.4008-.08.08-.8.01-.1.04-.4

Pb
Sb
Si
Sn
Zn

.2-2
.04-.4.08-.8. 01-.1.03-.3

Arsenopyrite was also observed in many of thepolished sections of the primary ores, especially thosefrom the deeper portions of the mineralized
close to the intrusiv.es..

zones andThe arsenopyrite is associatedwith pyrite, chalcopyrite, galena, and sphalerite(Figure 17).

On weathering, the arsenopyrite alters

to a yellow-brown ocherous material and numerous lead,copper, and zinc arsenates, which are common throughout
the entire district.
Berzelianite

CUgSe

Irregularly shaped grains of berzelianite havebeen found in the primary ores of the Monterrey mine.
Berzelianite is associated with chalcopyrite, umangite,

70■ /Q ° * L

i

FIGURE 17

Photomicrograph of arsenopyrite
from lower sulfide zone atAmerica Dos mine. Uncrossed
nicols.

71

bismuth, pyrrhotite, sphalerite and clausthalite and canbe recognized in polished section by its weak bireflectance,
pale bluish color and the association with other selenides.
Clausthalite

PbSe

Clausthalite occurs as small anhedral grains which
are commonly partially replaced by emplectite.

Clausthalite

is associated with umangite, sphalerite, pyrrhotite, bismuth,and enargite.

It is similar in appearance to galena but

microchemical tests revealed the presence of selenium.
Bismuthinite------------

Bi_S_
2 3

Small euhedral crystals and irregular-shaped
masses of bismuthinite replacing bismuth have been
noted in the polished sections of ores from the Monterrey
mine.

The crystals vary in size from about 2 p, to 30p

The bismuthinite is associated with bismuth,umangite,
emplectite, pyrrhotite, stannite, chalcopyrite, marcasite,
pyrite, galena, and sphalerite.

A reaction rim ofbismuthinite commonly separates enargite and galena fromlater bismuth.
Boulangerite

Pb^Sb^S^^

Boulangerite occurs as compact fibrous masses in
the primary lead ores at Mapimi.

Boulangerite is

associated with pyrite, stibnite, galena, and calcite and72

is partially replaced by jamesonite along fractures.

Italters to bindheimite, anglesite;and sulfur (?).
Spectrographic studies of a specimen of boulangerite
from the U.S. National Museum (U.S.N.M. R-1059) showed thematerial to contain about one per cent silver.

Thesilver

may be present either in substitution for lead in theboulangerite or as a separate silver or lead-silver sulfosalt such as miargyrite (AgSbSg) or owyheeite (PbgAggSbgS^g).
BorniteCu^FeS^
Like chalcocite, bornite is a relatively raremineral in the mines at Mapimi.

Bornite occurs as thinrims on chalcopyrite and, in one specimen, on enargite.
This rimming of enargite was observed on only one polishedsection of the many studied.
anMuch of the chalcopyrite has

iridescent coating which may be bornite.

Chalcocite

CU2S

At Mapimi, chalcocite is a relatively rare mineral.
It occurs as a partial replacement of chalcopyrite or othercopper-iron sulfides.

The rims of chalcocite are commonly

only a few micra thick but are characterized by theirstrong reaction to HNO3 observed in microchemical tests.
Usually, the chalcocite is later than either bornite orcovellite but the reverse case has been observed.

73

Chalcocite has only been noted in the partially oxidized
zones of the America Dos mine in the Mapimi district.
Chalcopyrite

CuFeSg

Chalcopyrite occurs as irregularly shaped masses
that partially replace earlier sulfides such as arsenopyrite and pyrite.

The amount of chalcopyrite apparently

increases away from the contact zone but has greatly
decreased before the main zone at lead mineralization.
Chalcopyrite is associated with pyrite, galena, sphalerite,
arsenopyrite, enargite, tetrahedrite, and marcasite and small
blebs of chalcopyrite in sphalerite are common.

Chalcopyrite

is generally the most important primary copper mineral at
Mapimi.
Covellite

CuS

Coveilite has been found on many samples from thevarious mines of the district.
assemblages are noteworthy.

Several distinct

In one assemblage thecovellite is seen along fractures and grain boundaries ingalena which is associated with other copper minerals(Figure 18).

A second association is as partial replace­ments of chalcopyrite and other copper sulfides.

i

J

FIGURE 18a

Photomicrograph showing thealteration of chalcopyrite (cpy)and galena (gn) to covellite (cv).
Nicols uncrossed-oil immersiongreen filter.

FIGURE 18b

Same field as above.
nicols.

Crossed75

A most striking occurrence of covellite is a
replacement of arborescent crystal masses of native copperon tenorite, cuprite, delafossite, and goethite.

Thephase diagram of Figure 52 clearly predicts the assemblages.
Emplectite

CuBiS2

The relatively rare mineral, emplectite, has beenfound in many of the polished sections of ore from theMonterrey mine where it is associated with pyrrhotite,
galena, sphalerite, bismuth, enargite, bismuthinite, andumangite.

Emplectite may be partially replaced along

grain boundaries by bismuth and bismuthinite.
In polished sections the emplectite can be recognizedby its low hardness, weak anisotropism, weak bireflection,andviolet cream color (Figure 19).

Klaprothite, with which it

may be confused, has higher bireflection and stronger
anisotropism.
Enargite

Cu^AsS^
Enargite is one of earlier sulfide minerals in the

primary mineral assemblages at Mapimf.

It is associated witharsenopyrite, pyrrhotite, marcasite, galena, and chalcopyrite.
Enargite is commonly replaced by marcasite (Figure 21).

Muchof the copper ore from deeper portions of the mines consisted
primarily of enargite.

,■FIGURE 19

Photomicrograph showing the
replacement of clausthalite (cl)by emplectite (em) and bismuth(bi). Nicols uncrossed-oil
immersion-blue filter.

77

GalenaPbS
The principal lead mineral of the district isgalena, which is generally quiteicoarse in texture.

Duringthe examination of polished sections of the ores, curved
grain boundaries and fractures were observed which may
indicate some post-deposition strain.

As previously

mentioned, the Ojuela district derived its name from theleaf-like texture of the galena ore, ojuela being a colloqui­alism of the word hojuela,meaning.leaflet (Villarillo, 1909).
In the higher grade lead ore zones only minor
amounts of Cu and Zn are associated with the gajLena.

Alter­

ation of anglesite and cerussite along grain boundaries andfractures is common even to depths of over 700 meters.
Generally, the galena occurs in small pods up to
several centimeters in diameter surrounded by a rind ofanglesite and cerussite in a dull ochreous mass which may
include goethite, plumbojarosite, finely disseminatedcerussite, mimetite, wulfenite, massicot, and, occasionally,bindheimite.

In the unoxidized zones and zones near thesource of mineralization, the galena is finer in grain
size and is associated with sphalerite, pyrite, marcasite,
chalcopyrite, arsenopyrite, enargite, and tetrahedrite.
In small prospect pits on zones peripheral to themain mineralization the association of galena with fluorite,
barite, aurichalite, and calcite has been noted. Here theonly form observed on the galena was the cube. It is

78

probable that this stage of lead mineralization considerably
postdates the main period of ore deposition.
Chemical analyses for Ag have been made of fourgalena samples that were taken at distances progressively
farther from the probable source.

The results are shown

below.
Sample No.

% Ag

10-10-65-102

0.37

10-10-65-105

0.31

10-10-65-106

0.11

10-10-65-108

0.06

It is clearly shown that there is a definite
decrease in silver content of the galena with increasing
distance from what is thought to be the source.
Jamesonite

Pb^FeSb^S^

Jamesonite occurs as a replacement of boulangerite.
Replacement is initiated along fractures and grain bounda­
ries in the boulangerite.
to bindheimite.

The jamesonite itself is altered

Figure 20 shows the partial replacementof boulangerite by jamesonite and of jamesonite bybindheimite.

79

,

■

FIGURE 20a

Photomicrograph showing thealteration of boulangerite(bl) to jamesonite (jm) along
fractures. Nicols uncrossedoil immersion-blue filter.

FIGURE 20b

Same field as above. Crossed
nicols-oil immersion-blue filter.

80

Marcasite

FeS2

Marcasite is a common constituent in many of the
primary ores of the district.

It is usually one of theearlier sulfides to form and it frequently replaces
pyrrhotite and enargite (Figure 21).

Intimate inter­growths of pyrite and marcasite replacing pyrrhotite arecommon in ores from the Monterrey mine.
Molybdenite

MoS2

Although not abundant in the district, molybdenite
is a widely distributed mineral in the unaltered sulfide
zones.

It occurs as distorted tabular crystals androsettes with sphalerite, pyrite, and galena.

Molybdenite

was noted in the tactite zone developed near the intrusive
contact in association with sphalerite and diopside.

Nolarge masses of molybdenite were noted.
An unusual occurrence of molybdenite in cryptomelane
and pyrolusite was noted in a manganese ore pile at thesouth end of the suspension bridge at Ojuela.

Here, themolybdenite rosettes were enclosed by the cryptomelane
and pyrolusite.
Spectrographic analyses of molybdenite showed notrace amounts of rhenium to be present.

Sufficient material

for a chemical analysis for rhenium was not available.

81

,

,

FIGURE 21

Photomicrograph showing the pseudomorphic
replacement of enargite (en) by marcasite.
Nicols uncrossed-oil immersion-blue filter.

FIGURE 22

Photomicrograph showing umangite(urn) and chalcopyrite (cpy) insphalerite (si). Nicols uncrossedoil immersion-blue filter.

82
The amount of wulfenite in the oxidized zones
suggests that molybdenite might have been more abundant in
the upper sulfide zones.

This assumption, however, is notwholly warranted when one considers the numerous otheroccurrences of wulfenite derived from the oxidation ofprimary lead ores that contained little or no molybdenite.
Pyargyrite

Ag^SbS^

Pyargyrite was found in the upper lead silver andthe barren carbonate zones in the Mapimf district.

At

no place is the mineral present in quantities to permit
exploitation for it alone but it is relatively widespread.
Pyargyrite is associated with galena, pyrite, chalcopyriteand sphalerite in the lead-silver zone and with stibnite,
calcite, siderite,

ankerite, barite and fluorite in thebarren carbonate veins.
PyriteFeS2
Pyrite is a widely distributed mineral in theunoxidized ores at Mapiml.

It is associated with othersulfide minerals that form the massive sulfide ore
deposits.

There is an apparent increase in the pyrite/

arsenopyrite ratio away from the probable source ofmineralization.

83

The pyritohedral form is predominant on thespecimens examined although the cube, octahedron anddiploid have been observed.

Pyrite frequently replacesarsenopyrite and is often replaced by sphalerite, galena
and chalcopyrite.

Pseudormorphs of goethite after pyrite

are common.
Pyrrhotite

FeS

Pyrrhotite occurs as one of the earliest minerals
in the massive sulfide ores in many mines of the district.
At the Monterrey mine, the pyrrhotite is associated withchalcopyrite, galena, and sphalerite, and large euhedral,
dark-green, lathlike crystals of diopside up to three
centimeters in length.
The pyrrhotite may be partially replaced bylater sulfides and replaces magnetite.

Generally, the more

pyrrhotite in the ore the lower the arsenic content of theore.

Apparently, the pyrrhotite has formed in place ofarsenopyrite in the arsenic deficient environment.
Sphalerite

ZnS

Sphalerite is a widely distributed mineral in the
primary ores at Mapiml.

It occurs pale golden-brown todark-brown masses associated with chalcopyrite, galena,
pyrite and arsenopyrite and other sulfides.

The altera­tion of sphalerite has resulted in the formation ofsmithsonite, hemimorphite,and the numerous zinc arsenates%
for which the mines near Mapimi are so well known.
As indicated by the variation in the color of thesphalerite, there is a considerable range of compositionsin the material from Mapimi.

The cell edges of sphalerites

from various portions of the mines were determined and the
FeS contents were calculated using the method described by
Skinner, Barton, and Kullerud (1959).

The compositionsof the sphalerite obtained in this manner are given below.
Mole Per Cent FeS

Sample Location

1.5

Upper silver-lead zone

3.7

Silver-lead zone

7.1

Zinc-lead oresr-San Juan mine

7.7

Zinc-lead ores-San Juan mine

12.8

Copper-zinc ore-San Juan mine

16.4

Monterrey mine

Stannite

CugFeSnS^Stannite has been recognized in the primary ores

from the Monterrey mine.

It occurs as small,irregular-

shaped grains associated with chalcopyrite, galena,
sphalerite, bismuth, bismuthinite, and other sulfides.
The stannite replaces sphalerite and is partially replacedby galena and bismuth.

85

In polished section, stannite is greenish-yellow
under oil immersion and has moderate to strong anisotropismand distinct bireflection.

It can be distinguished fromchalcopyrite by its more greenish color, stronger
anisotropism, and lower reflectance than chalcopyrite.
Stibnite

SbgSg
Stibnite is a widely distributed mineral in theupper zones of mineralization, especially in the numerous
veinlets that cut La Bufa.

The stibnite is associatedwith calcite, barite, siderite, ankerite, and quartz in the
low temperature veinlets and frequently shows varying
degrees of alteration to stibiconite.
of stibiconite after

stibniteComplete pseudomorphs

are not uncommon.

Euhedral crystals of stibnite are extremely complex.
Stibnite and stibiconite have been exploited at theAsterillo mine.
Stibnite is also found in the massive lead sulfideores.

It occurs as fine acicular crystals with galena,boulangerite, and jamesonite.

In the alteration of thesesulfides, stibnite is the first to react.

Thus, in polishedsections of stibnite-bearing sulfide ores the stibnite
commonly exhibits an alteration rim along surfaces.

Themassive bindheimite ores of the San Juan Poniente stope haveno doubt formed as a result of complete oxidation of themassive lead-antimony sulfide ores.

86
Emission spectrographs of a hand-picked sample
showed minor amounts of Pb and Fe and traces of Ag and
Cu to be present.
Tetrahedrite (Cu,Fe,Ag) 2.2S1:>4S13
Tetrahedrite is a minor but widely distributedmineral in the sulfide ores at Mapimi.

It is associatedwith galena, sphalerite, chalcopyrite, enargite, andmarcasite.

The tetrahedrite occurs as small irregularshaped masses in galena near the grain boundaries ofgalena with other sulfides and sulfo salts.
Spectrographic analyses of small grains of themineral showed several per cent silver to be present.
In addition to silver, minor amounts of Fe, Pb, Zn, andAs were also detected.
Tennantite(Cu,Fe,Ag)i2A S 4S13

Tennantite occurs as a minor constituent of the mas­sive sulfide ores where it is associated with galena, sphal­
erite, pyritetand other sulfides.

In polished section it isdifficult to distinguish from tetrahedrite but micro­
chemical tests proved the presence of abundant As.
minor Sb is present in the mineral.

Only

87

Umangite

Cu^Seg
The rare copper selenide, umangite, has been

observed in the polished sections of ores from the Monterrey
mine.

Umangite is associated with galena, sphalerite,

bismuth, emplectite/and other sulfides.

Paragenetic studies

indicate that it was probably the last primary mineral tohave formed, being proceeded only by secondary goethite.
In polished section under oil immersion the umangite
has a brownish cream color, strong bireflectance,and extreme
anisotropism (Figure 22).
Ankerite

CaFe(COg)2
Ankerite occurs in the low temperature stibnite-

bearing veins which are common in upper mineralized zones.
It is associated with calcite, barite, fluorite, quartz,
and stibnite.

Surfaces of ankerite are frequently stained

tan or brown because of the oxidation of its ferrous iron.
Aragonite

CaCOg

White stalagtites of aragonite are found near thenumerous water courses within the mine workings.

Thestalagtites are presently forming,usually on recrystallized
calcite and dolomite.
Spectrographic analyses revealed the presence ofvariable amounts of Zn, Fe, Mn, and Sr.

88
Aurichalcite(Zn,Cu)^(OH)^(CO^)^The Ojuela mine has produced spectacular specimensof aurichalcite.

The color varies from pale to bright

blue, and the crystals are acicular in habit (Figures 23-24).
Aurichalcite is associated with numerous secondary carbonatesof lead, zinc, and copper.

Paragenetically, the aurichalcite

proceeds hydrozincite and precedes the formation of rosasite.
Spectrographic analyses of aurichalcite showed
traces of Al, Si, Co, As, Pb, and Fe to be present.

Achemical analysis is presented in Table IV.
TABLE IV
Chemical Analysis of Aurichalcite(Zn,Cu)5 (0H)g (C0„)o

Aurichalcite (Mapimi)

ZnO

54.08

47.10

CuO

19.92

25.11

co 2

16.11

16.16

h2°

9.89

10.03

100.0098.40

The optical properties of the aurichalcite are
given below:
X =1.660*0.002

Y =1.747*0.002

Z =1.752*0.002

biaxial (-)
2V-small

89

FIGURE 23

Aurichalcite on a matrix of goethite(U.S.N.M. R-15008 - photographcourtesy of U.S. National Museum).
Specimen is approximately 2-1/2" x 3".

FIGURE 24

Radiating clusters of aurichalcite
growing on hydrozincite. Individual
crystals are approximately 2 mm long.

90
Azurite Cu^(OH)2 (C03 ) 2

Lustrous,dark blue crystals and dull blue crusts ofazurite are found in the upper zones of oxidation of the orebodies.

Azurite is also found in shear and fault zones

associated with gypsum.

Occasionally, the azurite is coatedwith a pale blue,water soluble mineral, probably chalcanthite.
More frequently the azurite alters to malachite and smalleuhedral pseudomorphs of malachite after azurite have been
observed.
CalciteCaCOg
Calcite is a widely distributed and common mineral

at Mapimi.

It varies in color from colorless to white topink and gray.

Individual crystals may exhibit numerous

forms or may consist only ofp{1011} .

Mineralogical associations are too numerous todiscuss.

Paragenetically, calcite is usually one of thelatest minerals to have formed.

The only minerals thatappear to proceed calcite formation are desloizite andoccasionally plattnerite and hemimorphite.
Notable occurrences of calcite are seen at the
La China and La Reina mines.

Here spectacular specimens

are found with white highly modified crystals and crystal
clusters of calcite.

91

Numerous observations on scores of specimens from
Mapimf indicate that there may be some correlation between
morphology and mineral assemblages.

It is suggested thatthe simpler crystals consisting of only one or two forms
are associated with zinc mineral assemblages with few or nocopper or lead species.

Calcite crystals associated withcopper and lead minerals tend to be highly modified.

Few

exceptions to this generalization have been noted but theauthor is not familiar with any theories which might predict
this type of occurrence and offers none himself.
Cerussite

PbCO^

Cerussite is usually found as an alteration productof anglesite.
observed.

Three types of cerussite occurrences were

■The principal type is a banded alteration rim

around galena separated from the unaltered galena bybanded anglesite of variable thickness.

The second typeof cerussite occurrence is as clusters of twinned crystals
having a reticulated appearance.
{110}

and

{ 130 } .

Twinning is on both the

The third type of cerussite occurrence

is as very fine euhedral twinned crystals associated withocherous masses of goethite, plumbojarosite, mimetite,
wulfenite,and massicot.

Twinning on {130 } (Figure 25)

is the common type observed (Goldschmidt, 1902).

92

FIGURE 25

Photograph of cerussite twinned on {110)and {130} on a matrix of rosasite andnantockite. Twin measures approximately
4 mm across.

93

Pseudomorphs of cerussite after anglesite andhemimorphite (?) have been noted.

The pseudomorphs after

hemimorphite (?) are associated with rosasite, plattnerite,
nantockite, malachite, and aurichalcite.
Dolomite

CaMg(COg)2
The principal ore deposits around Mapimi occur as

replacements of dolomitic limestones.

Solid masses ofsugary white dolomite form small water courses in dense ,gray dolomitic limestone.

Euhedral crystals of dolomite

line fractures in the limestone that have later been filledby quartz.
Hydrozincite

Zng(OH)g(CO3)2

Thick,white pulverulent coatings of hydrozincitewith occasional colorless acicular hydrozincite crystals
are found on a matrix of goethite in the oxidized areas of
the high grade zinc ores.

The hydrozincite is associatedwith aurichalcite, hemimorphite, and plattnerite.

Hydro­

zincite proceeds the aurichalcite and plattnerite and isusually contemporaneous with hemimorphite.
Spectrographic analyses of both the pulverulent
coatings and the colorless crystals showed trace amountsof Cu, Fe, Si, Al, and As to be present.

A chemical

analysis of hydrozincite is given in Table V.

9k

TABLE V
Chemical Analysis of Hydrozincite
Hydrozincite (Mapimi)

ZnO

74.12

70.61

o
o
to

6 (C°3)2

16.03

16.31

h 2o

9.85

10.30

CuO

1.35
98.57

100.00Total

The hydrozincite from several localities in themines showed very little variation in optical properties.
The refractive indices are given below.
X =± 0.002Y •- 0.004

Z ;

-

0.002

The hydrozincite exhibits a pale blue fluorescence in theultraviolet light.
Malachite

CUgfOHjgfCOg)

Malachite is a widely distributed mineral in theoxidized ores of the Mapimi district.

It occurs as

acicular crystals, radiating clusters, and masses.

Samplesfrom an unknown location in the Ojuela mine consist of
banded malachite and calcite, giving a striped appearance
to the rock.

Malachite is associated with many of theoxidized minerals, especially in the deeper portions of
the mines.

95

Rhodochrosite

MnCOg

Pale pink grains of rhodochrosite are associatedwith the manganese mineralization near San Pedro, where
it was probably formed by the replacement of limestoneduring manganese deposition and contact metasomatism.
Original structure of the limestone is occasionally pre­served.

The alteration of rhodochrosite results in theformation of pyrolusite and psilomelane.
Rosasite(Cu,Zn)2 (OH)2 (C03)
Rosasite occurs as small botryoidal crusts goethiteor other oxidized copper or zinc minerals.

Many of the botry­

oidal masses have cores of malachite that grade from green
in the center to blue toward the outer surface.

This grada­tion probably represents a solid solution series between
malachite [CU2(OH)2 (CO3)] and rosasite [(Cu,Zn)2 (OH)2 (CO3)].
A chemical analysis of massive rosasite from an
ore pile at the San Juan mine is shown in Table VI.
TABLE VI
Chemical Analysis of Rosasite(Cu,Zn)2 (OH)2 (CO3)Wt. %Mapimi
Wt. %

CuO

41.15

45.84

ZnO

30.99

26.91

C02

19.77

19.28

h 2°

8.09

8.5

100.00100.53Total

96
Siderite

FeCOg

Siderite is found in small veinlets above the zoneof commercial mineralization as tan euhedral crystals
associated with calcite, quartz, stibnitefor antimony oxides.
Frequently, the siderite is surficially altered to goethiteand complete pseudomorphs of goethite after rhombic crystalsof siderite have been observed.
Massive siderite is also present in the carbonate
zinc ore zones where it is associated with smithsonite,goethite and hemimorphite.
Smithsonite

ZnCOg

Smithsonite of three types is present in numerous
mines of the district.

A first generation occurs as apartial replacement of the recrystallized limestones.
This material is of a pale grayish-tan color, a consequenceof inclusion of very fine grains of iron oxides along
grain boundaries and cleavages.

It is often difficult

to distinguish from iron stained limestone, but its higher
specific gravity (4.4 vs. 2.7) usually serves for field
identifications.

This method of determination is employedby the miners of the district in the upgrading of the zincores.

(One miner, when handed 8 pieces of limestone

partially replaced by smithsonite and 4 of limestone,
rapidly separated the two types without even looking at

the specimens.

A later examination of the pieces in thelaboratory gave an average bulk specific gravity of 2.8
for the limestone and 3.1 for the partially replaced
limestone.

X-ray spectrographic scans of the samples

revealed the presence of zinc in all of the samples
selected by the miner as containing zinc.

It was present

only in traces in those he rejected.)
The replacement of the recrystallized limestone
progressed along grain boundaries.

This is shown by thevariation of refractive indices from the core to the edges
of individual grains.

Replacement of the calcite has

seldom gone to completion.

A contemporaneous formationof siderite with the smithsonite has also been noted.
The second type of smithsonite occurs as pale
gray-to-white botryoidal incrustations on goethite and as
highly modified crystals.

This smithsonite is almost

always associated with blades of hemimorphite.
Paragenetically, smithsonite is usually later than thehemimorphite, and is commonly perched upon it.
A third type of smithsonite is found as pale blue
crystals of a simple rhombic habit on a matrix of earthy
goethite.

It is associated with colorless calcite andacicular crystals of plattnerite.
are later than the smithsonite.

Both of these minerals
Spectrographic analysisof the smithsonite showed the presence of minor amountsof copper and iron.

98

Anglesite

PbSO^

Anglesite occurs as an alteration product on thegalena.

Commonly it in turn is partially altered tocerussite.

Most of the galena shows at least someanglesite alteration on crystal surfaces or fractures.
Euhedral anglesite is not common but a few vugs in galena
masses have been found that contain highly modified
adamantine crystals of anglesite up to 50 mm. in length.
On these crystals the predominant forms noted were the{ L01},{ 001} , {011} , and {ill} .

Numerous smaller faces arealso present.
BariteBaS04
Barite is a widely distributed mineral in the upperzones of mineralization in the primary sulfide ores, andas a secondary mineral in the oxidized zones at Mapimi.
Blades of colorless to white barite can be found inrecrystallized limestones near small carbonate-bearing
veinlets considerable distances from the principal minesof the district.

Pale blue euhedral crystals, up to three

centimeters across, have been found on the first and second
levels at Ojuela.

The barite is associated with aurichal-cite, plattnerite, murdockite, calcite, fluorite, andhydrozincite.

99
The blue barite probably represents a late stage ofbarite mineralization.

An early generation of barite isfound with sulfide ores.

Here, the barite is one of thelater minerals to form but precedes some pyrite deposition.
Colorless to white barite is also associated withthe manganese mineralization at San Pedro and in the
numerous manganese prospects near Mapimi.
Brochantite

Cu^(SO4)(OH)g

Dark-green acicular crystals of brochantite havebeen noted in the upper oxidized zones at Ojuela.

The

brochantite is associated with cerussite, malachite, andgoethite.

Many of the specimens of "brochantite" from

Mapimi, observed by the author in various collections, have
proven to be malachite.

The brochantite is partiallyaltered to a pale-green malachite.
Optical properties for brochantite from the dumps
below the San Juan mine are as follows:
Optically (-)
2V ^ 80°X= 1.724 -0.004

Y= 1.768 -0.004

Z= 1.796 -0.004

100
Celestite

SrSO^

The occurrence of celestite at Mapiml isrestricted to the uppermost zones of mineralization
where it is associated with fluorite, barite, calcite,
and various manganese oxides.

Small colorless crystals

on partially recrystallized limestone were observed at
the El Padre (La Esperanza) mine which is approximately
midway between Ojuela and Mapiml.
Huge pale blue euhedral crystals of celestite
up to 15 centimeters in length have been obtained atmines near the El Vergel intrusive, about 20 kilometers
southwest of Ojuela where celestite has been periodicallymined.

The crystals are morphologically simple consist­ing of predominant d {10.1], o (Oil }, m {210} , and 1 {102 )forms.

Occasionally, the c {00l3and b {010}forms arepresent.

The celestite is associated here with calcite,

gypsum, and sulfur.

A similar occurrence of celestite

is presently being exploited (1965) near Laguna Matamoras,
Coahuila.

Excellent specimens from both localities areon display at the Smithsonian Institution.
Chalcanthite

CuSO^'SH^O

Chalcanthite has been noted in several places in
the mine workings.

The mineral is principally found along

fault and fracture zones in the gougue material.

Commonly

101
it tends to be intermixed with melanterite and the color
varies from clear sky-blue to a pale brownish-green.

The

brown coloration is due to finely dispersed goethite orother iron oxides formed by the partial breakdown of theiron-bearing sulfate.

The chemical analysis ofchalcanthite is shown in Table VII.
TABLE VII
Chemical Analysis of Chalcanthite

CuO

1

2

3

31.87

32.61

33.94

1.40

1.83

FeO
so3

32.06

32.11

31.77

h 2o

36.07

34.03

31.41

100.00100.15

98.95

Rem.
Total
1.

CuS04 *5H20

2,3.

Mapimi, Durango, Mexico

Chalcanthite having a fibrous structure and of apale blue color was also noted on a sample of partially
oxidized chalcopyrite near the working face of the San
Carlos stope.

The temperature at this face was very highand the humidity was extreme.

The high temperature in

the deeper portions of the mine is probably due in part tothe oxidation of sulfides.

102
CopiapiteF e ^ F e * 3 (S04 ) g (OH) 2 •20H20

Copiapite is present on pyrite on several of theold mine dumps within the district.

With color varying

from pale yellowish white to pale yellow, it forms as
botryoidal coatings in partially sheltered areas, espe­
cially on the undersides of pyrite-bearing boulders.
Spectrographic analyses of several samples showed consid­
erable amounts of arsenic to be present.

This is probably

due to the concurrent alteration of arsenopyrite.

Achemical analysis of copiapite from Mapiml is shown in
Table VIII compared with analyses taken from Palache,
Berman,and Frondel (1951).
X-ray diffraction patterns of the material arepoor but of sufficient quality to permit positive
identification.

The fine particle size of the copiapite

made determination of the indices of refraction impossible.
Average minimum and maximum values are 1.518 and 1.580,
respectively, which fall within the range of values given by
Winchell and Winchell (1959):
X = 1.506-1.540
Y = 1.528-1.550
Z = 1.575-1.606

103

TABLE VIII
Chemical Analysis of Copiapite
1

2

3

5.75

6.2

4.01

CuO

0.4

1.00

ZnO

tr.

FeO

Fe2°3

25.55

23.9

23.21
3.62

A12°3
S03
H2°

38.43

37.4

38.44

30.27

28.7

27.76

Rem.
Total
Fe''Fe40.68
100.00

96.6*

98.72

(SO4)e (0H)2 -20H20

Mapimi, Durango, Mexico
3.

Cuprian copiapite, Island Mountain, California*

No analysis of arsenic was made but aconsiderable amount of arsenic was detectedin spectrographic analysis.

GypsumCaS0^'2H20
Gypsum occurs as a very late stage mineral in

the upper oxidized zones.

It is generally colorless orwhite and is found as cavity fillings in goethite andcalcite-lined vugs.

Euhedral crystals of gypsum are rare

but beautiful, colorless terminated crystals have beenfound.

Crystals as large as one centimeter have been

10U
collected but more typically they are only a few
millimeters in length.

The most common forms are the{010}, { 120} and {111}.

The crystals are tabular on{010}.
Inclusionsof native copper and clay are common

toward the center of the crystals.

Gypsum was probably

one of the last minerals to form having been proceeded
only by the hydrous iron, copper, and zinc sulfates.
Small fractures in the limestone and volcanic rocks
throughout the district are commonly filled with whitetranslucent gypsum or mixtures of gypsum and calcite.
A zone of caliche in the soils of the region is also
noted which consists of calcite and gypsum.

Gypsum-coatedrocks in the dry stream beds that drain the mineralized
areas are common.
Hydrous Sulfates
Pulverulent, white to pale bluish-to-white
efflorescences were noted in several areas in the district,
especially near water seepages in the San Juan mine.

Anumber of samples were examined by X-ray diffraction
techniques.

Six hydrous sulfates were positively identi­

fied in these areas and several others might be present.

The identified species are:hexahydrite

MgS04•6H20

bianchite

ZnS04 *6H20

epsomite

MgS04-7H20

goslarite

ZnS04 *7H20

pickeringite

MgAl2 (S04)-22H20

halotrichite

FeAl2 (S04)•22H20

Chemical analyses of these minerals were made andthe results compared with published data.
are shown in Tables

These results

IX through XIV compared withanalyses from Palache, Berman,and Frondel (1951).

Sev­

eral other analyses were made, but the results are notincluded in the present study because it has been
determined that they probably represent mixtures of two
or more minerals.
Associations of the above mentioned minerals
included the following in their relative order ofabundance:hexahydrite - epsomite - goslarite
hexahydrite - epsomite - bianchitehexahydrite - goslarite
goslarite - epsomite
hexahydrite - halotrichite (?) - epsomite
bianchite - goslarite

io6
TABLE IX ;Chemical Analysis of Goslarite
1

2

3

4

5

6

MgO

0.22

1.93

2.10

0.81

1.58

CaO

trace

CuO

trace

trace

trace

23.90

22.18

23.24

23.12

22.91

1.83

2.63

1.31

1.12

0.95

0.06

0.22

ZnO

28.30

FeO
NiO
MnO

trace

trace

0.30

1.24

0.54

S°3

27.84

26.70

26.90

26.84

27.29

27.76

h 2o

43.86

42.18

42.37

42.90

42.42

41.84

3.69

4.50

99.71

100.30

Rem.
Total
1.
2,3,4.
5,6.
7.

100.00

94.83

96.01

96.69

ZnS04*7H20
Goslarite - Mapimi, Durango, Mexico
Ushaw Moor, Durham, England
Ferroan Goslarite

Freiberg, Saxony

TABLE X
Chemical Analysis of Bianchite

ZnO

1

2

3

30.19

28.47

20.01

MgO

1.32

MnO

trace

CuO

trace

FeO

1.46

8.84

S°3

29.70

28.77

30.13

h 20

40.11

39.33

39.92
•

Rem.
Total

100.00

99.35

1.02
99.92

1.

ZnSO'4'6H20

2.

Mapimi, Durango, Mexico

3.

Raibl, Venezia Giulia

TABLE XI
Chemical Analysis of Hexahydrite

MgO

1

2*

3

17.64

16.80

17.15

ZnO

1.42

FeO

0.19

CuO

trace

MnO

trace

S03

35.04

34.22

34.52

h 20

47.32

46.16

46.42

Rem.

1.78

Total

100.00

98.79

99.87

1.

MgS04 *6H20

2.

Mapimi, Durango, Mexico

3.

Bonaparte River, British Columbia*

After correcting weight percentages
for included calcite

109

TABLE XII
Chemical Analysis of Epsomite

MgO

1

2

3

4

5

16.36

15.02

16.26

13.99

12.76

ZnO

0.74

1.03

2.82

FeO

trace

trace

0.015

CoO

0.068

NiO

trace
1.12

MnO
S03
h 2o

2.67

4.28

32.48

31.69

32.41

31.84

31.54

51.16

50.11

51.32

50.92

48.41

0.113

Rem.
Total

100.0098.68

100.103

1.

MgS04 *7H20

2.

MapimI, Durango, Mexico

3.

Ashcroft, :British (Columbia

4.

Mapiml, Durango, Mexico

5.

Schemnite, Hungary

trace
100.4599.893

TABLE X III
C h em ical A n a ly s is o f H a l o t r i c h i t e
1

2

3

1.44

0.85

7.39

6.86

0.96

0.47

11.11

10.54

0.92

1.12

35.97

35.63

36.42

44.51

42.90

43.63

MgO
FeO

8.07

MnO
a 12°3

11.45

Fe2°3
S°3
h 2o
Rem.

0.37

Total

100.00100.39100.26

1.

Fe ''al2<S04>4 *22H20

2.

Mapimi, Durango, Mexico

3.

Roccalumera, SicilyTABLE XIV
C h em ical A n a ly s is o f P i c k e r i n g i t e

MgO

1

2

3

4

4.69

4.03

4.35

4.16

FeO

trace

MnO

trace

a 12°3

11.87

11.71

1.02
12.30

0.24

0.15

Fe2°3

11.64

S°3

37.29

36.92

37.84

36.86

h 2o

46.15

45.31

44.66

46.10

insol.

0.59

0.41

98.12

99.74

100.43Rem.
Total

100.001.

MgAl2 (S04)4 *22H20

2.

Mapiml, Durango, Mexico

3.

Quetana, Chile

4.

Portland, Conn.

112

The multitude of areas in the workings having
efflorescent coatings would lend themselves to an ex­
haustive study of the conditions of formation of theminerals such as Eh, pH, ionic ^activities,and temperature.
The chapter dealing with mine waters clearly shows that awide variation in mineral assemblages could exist.
The sulfates found in the deposit commonly occur
on a partially dolomitized limestone, frequently with
gypsum.
As the samples of the various sulfate minerals
were collected, they were placed in bottles with tightly
fitted tops in order to prevent dehydration upon contact
with the less humid air at the surface.
Jarosite

KFe3 (S04)2 (OH)g
Jarosite is a widely distributed mineral commonly

found in the upper oxidized portions of the mines.

It

usually occurs as large, yellow-buff pulverulent masses
associated with bindheimite, mimetite, and goethite.
Euhedral brown crystals on a matrix of botryoidal goethite
have also been noted.

The optical properties of themassive jarosite vary greatly, indicating a considerable
range in composition.

The indices vary within the limits

listed below:
E = 1.722 to 1.800
0 = 1.824 to 1.880

Sectoral development was frequently noted, especially inthat material having the lower indices.

Semiquantitative

spectrographic analyses indicate that the increase inrefractive indices is accompanied by a corresponding
increase in the lead content of the jarosite.

A chemical

analysis of a sample of massive material (U.S.N.M. 92279)
is given in Table XV.
TABLE XV
Chemical Analysis of Jarosite
KFe3 (SO4)«) (OH) &(Mapimi)

K2°

9.41

8.62

Fe203

47.83

47.05

PbO

1.36

S°4

31.97

29.7

H2°

10.79

10.4

Total

100.0097.13

Minor amounts of Ag, Al, As, Ba, Bi, Ca, Cu,
Mg, M n , Mo, Sb, Si, Sn, and Zn were detectedin spectrographic analyses.
The indices for this particular analyzed
material are E = 1.800, 0 = 1.880.

The euhedral crystals of jarosite on goethite are
golden brown in color and are transparent.

The indices

are E = 1.722, 0 = 1.824.
The jarosite has probably been formed by theoxidation of pyrite accompanied by the alteration of galena
or bournonite and other sulfides or sulfosalts.
Melanterite

FeSO^•7H20

Like chalcanthite, melanterite is found along
fault

and fracture zones in the gouge material.

in color from pale-green to bluish-green.

It varies

Small brownish

blebs of goethite are frequently dispersed throughout themelanterite masses.

A chemical analysis of a pale-green

mass of melanterite is presented in Table XVI.
Plumbojarosite

PbFe6 (SO4)4 (OH)

Many of the oxidized lead ores from district containmoderate amounts of pulverulent yellowish-brown masses ofplumbojarosite with bindheimite, mimetite, and wulfenite.
The plumbojarosite has formed as an alteration product oflead and iron sulfides.
A small amount of fairly pure material wascollected for chemical analysis.

Sufficient sample for acomplete analysis could not'.be obtained but a partial
analysis is given in Table

XVII.

TABLE XVI
C h em ical A n a ly s is o f M e la n t e r it e
1

2

3

trace

MgO
25.84

21.23

25.23

CuO

2.73

2.21

ZnO

.22

MnO

trace

FeO

S°3

28.80

29.17

31.14

h 2o

45.36

43.34

41.42

100.0096.69

100.00

Rem.
Total
1.

FeSO 4 *7H2°

2.

Mapimi, Durango, Mexico

3.

Cuprian melanterite. Zmeinogorsky
mine, Altai, U.S.S.R.

116

TABLE XV.II
Partial Chemical Analysis of Plumbojarosite
Plumbojarosite (Mapimi)
Nd^O

0.18

k 2o

0.23

PbFeg (SO/,)^ (OH) i?

a 12°3

<0.05

Fe2°6

35.26

42.37

PbO

23.61 .

19.74

AgO

0.03

CuO

0.12

Total

59.48

62.11

Spectrographic analysis of the plumbojarosite showed thepresence of minor amounts of Zn, Ca, and Ba and tracesof Bi, Ni, and Ge.
AdamiteZn2(AsO^)(OH)The Ojuela mine is noted for its fine specimens ofadamite.

The adamite occurs as single individuals or

as radiating crusts or rosettes usually on a matrix oflimonite.

Mayers and Wise in Mrose (1948) recount thediscovery of an occurrence of adamite in the Las Palomas
ore body.

117

In June 1946, the writers had thegood fortune to discover a remarkablepocket of adamite in a small manwayof the Las Palomas ore body, just above
the 11th level. Enroute to a stope
containing fine specimens of wulfenite
and green mimetite, our lamps fell upona pocket in the limestone. We saw aminiature grotto, some four feet indiameter and as many deep, its interior
carved into fantastic shapes and theentire surface covered with smoothly
undulating waves of sparkling yellow
crystals; it was a glorious sight, as
though we were gazing upon a mineral
specimen of unimaginable splendor.
It must have been exciting, indeed, to have foundsuch an occurrence.

The author is familiar with theexhiliration felt when encountering the numerous muchsmaller vugs covered with fine crystals and masses ofsuch minerals as hemimorphite, aurichalcite, rosasite,and even calcite (Figures 26 and 27) .
Commonly the crystals are elongated parallel to[010] .

The dominant form is d {101}.

Additional forms

include t (120 }, m {110 }, h{210), and b (010}.

Crystalsthat have been shortened parallel to the b-axis have apseudo-octahedral habit.
The optical properties determined by Mrose (1948)
follow and are compared with data obtained during the present
study on various types of adamite encountered at several
localities in the mines.

FIGURE 26

Photograph of a cluster of adamite on
limonitic matrix (U.S.N.M. C-5813 photograph courtesy of U.S. National
Museum). Specimen is approximately
2-1/2" x 3-3/4".

FIGURE 27

Photograph of cuprian adamite on
calcite from the America Dos mine.
Crystal in center is approximately
4 mm across.

119

Mrose (1948)

10—10—65—8

10—10—65—11

10—10—65—65

color

greenish-yellow

colorlesspale yellow

pale bluesign(+); partly (-)(+)

Indices
X = 1.722-0.003

1.713-0.003

Y = 1.742-0.003

1.735-0.003

Z = 1.763-0.003

1.754-0.003

1.719-0.003

1.716-0.003
1.734-0.003

1.757-0.003

1.755*0.003

Chemical analyses of several types of adamite
compared with data presented by Mrose (1948) are given in
Table XVIII.
Adamite is associated with calcite, smithsonite,
conichalcite, hemimorphite,and legrandite.

Williams (1963a)

determined the stability relations of adamite, legrandite,and austinite.

Based on the activities of Zn = 10“^,Ca = 10“6, As = 10-3, and CO2 = 10”^, the stability fieldof adamite is very large (Figure 61) and one could predicta great number of adamite-containing assemblages.

Theeffect of Cl, Cu, Fe, and various sulfur species, however,would probably tend to restrict the stability field of
adamite.

120

TABLE XV III
Chemical Analyses of Adamite

ZnO

Theoretical
Zn?(AsOa)(OH)

Mrose(1948)

10—10—65—8

10-10-65-11

56.77

56.78

56.43

57.06

0.03

0.03

Al2°3
CuO
FeO
MnO

trace

AS2O5

40.09

38.96

39.98

39.45

h 2°

3.14

3.53

3.43

3.17

Rem.
Total

0.26
100.00

G

99.53

—

—

99.87

99.71

10-10-65-67

10—10—65—68

4.435
10-10-65-65 10—10—65—66

ZnO

54.63

55.43

54.77

56.01

AI2O3

0.06

0.13

0.01

0.01

CuO

3.03

0.01

0.16

FeO

0.01

trace

MnO
A s 2°5
h 2o

Rem.
Total
G

0.02
38.31

40.01

39.32

39.94

4.01

3.62

3.26

3.42

—

—

—

—

100.05

99.22

97.52

99.38

4.42

121

ApatiteCa5 (P04)3 (Cl/0H/F)
Few phosphates have been recognized in the Mapimi

district as being related to the hydrothermal ore
mineralization or to the oxidation or secondary enrichmentof the deposit.

Apatite, however, is found on the westernside of the Bufa de Mapimi and is presently being mined ona small scale at the San Ilario mine.

A few tons per dayof hand-picked and cobbed ore are separated into two grades,
lump and fines.
The occurrence of the apatite is unusual; it isfound in fractures and cavities in weakly recrystallized
limestone where it occurs both as massive fillings orbotryoidal crusts.

Banding is common.

The origin of

the material is difficult to determine because accesscould not be obtained to most of the mine.

The impressionof the author is that it is of organic origin.
The composition of the apatite is variable.

The

samples were crushed to -200 mesh and treated with 1:20 HC1
for approximately 20 minutes in order to dissolve any
admixed calcite.

The analyses of four samples arepresented in Table XIX.
Pseudomorphs and casts of apatite after an unknown
bladed mineral are common.

The original mineral may havebeen gypsum but no gypsum has been observed in any of thesamples studied.

122

TABLE XIX
Chemical Analyses of Apatite from the San Ilario Mine
10 -10-65-55a

10-10-65-55b

CaO

55.11

54.45

55.91

53.47

P 2O 5

39.64

38.43

36.77

38.11

C02

2.78

4.71

4.18

3.89

H-O 6
^125°C.

0.50

h 2o

2.45

2.01

2.73

3.03

FeO

0.03

0.01

0.08

0.11

MgO

0.20

0.12

0.09

0.22

MnO

0.76

0.38

0.63

0.72

F

0.48

0.61

0.37

0.53

Cl

trace

0.21

0.05

0.08

Total

101.95100.93100.81

100.16e1000°C.

-

10-10-65- 56 10-10-65-:--

10-10-65-55a - white dense "vein" ,apatite10-10-65-55b - white porous "vein" apatite
10-10-65-56- white and gray banded apatite

10-10-65-57- tan botryoidal apatite

123

Spectrographic analyses showed trace amounts of
Al, B, Ba, Cr, Cu, Na, Ni, Pb, Sb, Si, Sr, Ti, and Zn in
all of the samples collected from this property.

The

only minerals recognized at the San Ilario mine are trace
amounts of crandellite which is admixed with earthy
apatite.
Reportedly, there are many similar occurrences of
apatite in the district from which a minor production has
been realized.

Only a few were visited and the mineralogy

appeared to be the same as that at the San Ilariomine.

Samples were not obtained because of the general unsafe
appearance of the mines:

surface samples were too con­taminated to be used for chemical analyses.
Arseniosiderite

Ca3Fe4 (As04)4(OH)g

Arseniosiderite was noted on numerous specimens,
especially those of hemimorphite from the America Dos
mine.

The arseniosiderite varies in color from lightbrown to a blackish brown.

Commonly, it partiallyreplaces the hemimorphite or is contemporaneous with it.
Pseudomorphs of arseniosiderite after scorodite are re­ported by Foshag (1937).

Occasionally the arseniosiderite

is associated with small sheaf-like clusters of carminite.
Goethite is a common alteration product (Figure 28).

12k

FIGURE 28

Photograph showing fibrous-appearing
arseniosiderite with hemimorphite.
Length of fiber is about 5 mm.

FIGURE 29

Photograph of a small pocket ofcarminite crystals from a sample
taken from a dump near the Socavon
shaft. Individual crystals areapproximately 1 mm in length.

125

Sufficiently pure material for a chemical analysis
could not be obtained because of admixed goethite.

Thechemical analysis of Table XX is from Foshag (1937).
TABLE XX
Chemical Analysis of Arseniosiderite from Mapimi
Foshag (1937)

TheoreticalA s 2°5

42.67

43.56

PbO

0.28

Fe2°3

32.71

FeO

0.12

CaO

14.44

MgO

0.61

h 2o

9.34

insol.

0.40
100.57

Austinite

30.26

15.94

10.24

100.00CaZn(AsO^)(OH)

Colorless to white crystals of austinite areassociated with various manganese minerals at theAmerica Dos and the San Juan mines.

The austinite

occurs as radiating clusters of acicular crystals.

Themorphology of these crystals is difficult to determine,
but the rhombic prism 0.10} and the positive{111} ornegative disphenoid 0.11} have been recognized.
crystals are seldom more than 0.7 mm. long.

Individual

126

Optically, the colorless and white austinites arebiaxial positive and have a negative elongation.

The

refractive indices are:X = 1.762.005

Y = 1.765+ .003

Z = 1.787

jh

.005

Z-Y = 0.025
OO

2V (calc.)=

Pure material sufficient for a chemical analysis
could not be obtained; however, spectrographic analyses ofsmall quantities of austinite revealed traces of Mn, Cu,
P, and Fe.

Comparison with standards indicated that thesetrace elements were present in the order of less than 0.5%.
Some of the Mn might be due to the inclusion of manganeseoxides.
Carminite

PbFe2(ASO4)2 (OH)2/

Excellent specimens of carminite from Mapimi werecollected on a small dump near the North Shaft of the
Ojuela mine in the summer of 1927 by Drs. Harry Berman andW. F. Foshag.

The material consisted of blocks of massive

to crystalline scorodite containing seams and pockets ofarseniosiderite and small areas of dussertite and carminite(Foshag, 1937).

Samples of this material were obtained

from the U.S. National Museum for the present study.

127

The carminite from the museum specimens occurs
as small acicular or bladed crystals or sheaf-likeaggregates up to 2 mm. in length.

The crystals are a

deep carmine red in color and have a high luster (Figure 29).
Scorodite and dussertite form the matrix.

The crystals areelongated or flattened parallel to (010 }.

The only forms

recognized are {010}, {110}, and {011}.
The chemical analysis given in Table XXI was takenfrom Foshag (1937).
TABLE XXI
Chemical Analysis of Carminite from Mapimi
Mapimi

Theoretical

PbO

37.30

35.38

CaO

0.44

MgO

0.06

FeO

0.21

Fe2°3

23.43

Al2°3

0.96

A s 2°5

33.98

36.44

2.9

2.85

H2°
h 2o -

0.10

insol.

0.58
99.96

25.33

100.00128

The pleochroism is pronounced, X = pale yellowish red,
Y and Z = dark carmine red.

The mineral is biaxial positive

with a moderately large 2V and has parallel extinction andnegative elongation.

Slight variations in the refractive

indices were noted.
present study

FoshagX =2.065*0.005

2.070

Y =

2.070*0.005

2.070

Z =

2.080*0.005

2.080(1937)

Birefringence is low.
Carminite was found by the author associated withlarge masses of cerussite, anglesite, mimetite, wulfenite,and plumbojarosite from the dump at the San Juan mine andwith jarosite near the locality described by Foshag (1937)
at the North Shaft of the Ojuela mine.

The average

crystal size of carminite was only 0.2 mm. in length andcould be identified in thin section by its strong
pleochroism.
Chenevixite

Cxi^Fe^ (AsO^) (OH) ^ •HgO

Small, pale olive-green botryoidal crusts ofchenevixite were found on pale yellow mimetite masses fromthe San Juan mine.

The crusts seldom exceeded 0.1 mm. indiameter and were made up of numerous crystals ofchenevixite.

The material is intimately intergrown with

129

mimetite and many X-ray patterns show lines characteristicof both minerals.

X-ray data are presented in Table XXII

compared with the standard ASTM pattern for chenevixite andone from Mapimi mimetite.
Conichalcite-Cuprian Austinite

CaCU(AsO^)(OH)

Conichalcite and cuprian austinite, previously
described as "barthite," a name applied by Henglein and
Meigen (1914) for material from Guchab-Ostberg near Otavi,
S.W. Africa, is common in the oxidized zones at Mapimi
and Descubridura.

The mineralogical associations areusually simple, consisting of malachite, calcite, and,occasionally, mimetite on a matrix of dense goethite.

Themalachite and calcite generally proceed the formation of
the conichalcite and cuprian austinite.
The mineral occurs as small botryoidal crusts on
the goethite (Figure 30).

The color varies from a paleolive green through a bright green to a dark olive green.
Individual crystals were too small for complete optical
data, but partial data follow

for four samples.

Pleochroism is strong from pale green to green to blue green.
Many samples showed overgrowths of pleochroic conichalcite
on colorless austinite.

These materials were carefully

avoided when determining the optical properties and thesamples submitted for chemical analyses were checked to be
sure that only one mineral phase was present.

130

TABLE XXII
X-ray Diffraction Data for
Chenevixite and Mimetite 'chenevixite
Mapiml
I
1

dU)
10.0

Chenevixite
ASTM 14-68

Mimetite

I

d(l)

I

A(i)

10

9.98

tr,

6.39

(Mapiml)

1

5.4

5

5.37

1

5.0

tr.

4.94

io

3.00

2

4.4

10

4.44

40

4.46

tr.

4.20

40

4.21

tr.

3.83

20

3.8i

30

3.68

50

3,42

70

3.33

10

6

35.4

3.31

100

3.53

40

3.30

tr.

3.17

3

3.66

10

3.06

i oo

3.06

3

2.97

10

2,97

100

3.01

70

2.93

tr.

2.85

tr.

2.77

10

2.7 4

20

2,35

50

2.31

30

2.23

20

2.15

100

2.09

60

2.04

8

2.37

70

2.56

0

3,47

70

2.46

1

2.39

5

2.38

2

2.31
5

2.23

1

2.22

5

2.22

1

2.13

5

2,14

3

2.10

1

2.07

5

2.06

FIGURE 30

Photograph of a crust of conichalcite(U.S.N.M. - 114670 - photograph courtesyof U.S. National Museum). Individual
spherules are approximately 2 mm indiameter.

FIGURE 31

Photograph of botryoidal crusts ofbrownish-green duftite - p with calcite.
Width of field is approximately 20 mm.

132

The individual crusts of cuprian austinite andconichalcite were hand picked and further separated fromgoethite by the use of the Frantz Isodynamic Separator
and an ultrasonic vibrator.

Sufficiently pure samples

suitable for chemical analyses could be obtained by thismethod.

These chemical analyses are presented inTable XXIII.
Staples (1935) described austinite from Gold Hill,
Tooele County, Utah.

Fischer (1944) examined similarmaterial of the type described by Henglein and Meigen andfound it to consist of a colorless phase with overgrowths
of a bright green mineral.

The central portions of thecrystal were determined to be a zinc arsenate (austinite)
and the outer green mineral a copper arsenate, which
Fischer called "higginsite" (Palache and Shannon, 1920).
The X-ray powder pattern and unit cell dimensions of"higginsite" were shown to be almost identical withconcichalcite (Struntz, 1939) and Richmond (1940).
Since conichalcite has priority over "higginsite,"
Berry (1951) suggests that the latter name be dropped.
Thus, the material described by Henglein and Meigen wasa mixture of austinite and conichalcite.
Although the name barthite had priority overaustinite, Fleischer(1945) recommended that the namebarthite be dropped because of the poor description of theoriginal material.

TABLE XXIII
Partial Chemical Analyses of Conichalcite and Cuprian Austinite

CaZntAsO.)(OH)- — —-- 4 ----

cuprianaustinite
10-10-65-4

cuprianaustinite
10—10—65—4a

conichalcite
10—10-65—4b

CuO/ZnO

0

.78

1.75

2.06

693.

CaO

21.46

20.18

20.45

21.30

19.50

21.61

14.02

20.19

20.03

34.63

30.65

17.91

11.54

9.70

.05

CuO
ZnO

31.13

PbO

conichalcite
10—10—65—5

CaCu(AsOj)(OH)
or

0.15

AS2°5
h 2o

43.96

44.2

43.7

44.27

3.45

3.8*

4.2*

3.47

1 0 0 .0 0

1 0 0 .1 1

99.08

1 0 0 .0 0

1.77*0.01

X

1.76*0.01

1.78*0.01

1.79+0.01
1.80*0.01Y
1.80±(k0tZ*

1.79±(K01r-

determined, by thermogravinetric analyses-

1.80±».Ol

1.82*0.01

w
u>

134

Fischer mentions that a series between austinite
and conichalcite probably exists.
from Table XXIII suggest
Crandellite

The chemical dataa similar conclusion.

CaAlg (PO^) 2 (OH) 5

Crandellite associated with earthy apatite wasfound at the San Ilario mine.

The presence of crandellite

was suspected when spectrographic scans showed consider­able amounts of Al in the earthy apatite.

X-ray

diffraction patterns of this material confirmed that
crandellite was present.

The d-spacings of the earthy

apatite are compared with those of apatite and crandellite
in Table XXIV.
The optical properties could not be determinedbecause of the small crystal size.

The material is alsotransluscent and contains unidentified inclusions whichfurther make optical identification difficult.
Descloizite(Zn,Cu)Pb(VO^)(OH)

Descloizite occurs as minute dark brown tabular
crystals and incrustations on fluorite, wulfenite andbarite.

The descloizite appears to be much more common

in the upper oxidization zone, especially on the firstlevel of the Ojuela mine where it is associated withbarite, fluorite, wulfenite, aurichalcite, hydrozincite,
plattnerite, murdockite. and rosasite.

135
TABLE XXIV
X-ray Diffraction Data of
Apatite and drandellite“earthy” apatite(Mapimi)____Id( hapatiteASTM 12-263

crandellite
ASTM 5-0615

I

d(A)

I

dU)

10

8.32

1

6.7

5

6.73

1

6.0

5

6.05

5

5.7

40

5.70

4

4.9

4

4.87

30

4.86

5

3.5

6

3.55

40

3.50

2

3.4

14

3.39

10

3.15

1

3.1

5

3.09

10

2.95

100

2.94

4

2.88

60

2.861

5

2.87

10

2.80

100

2.779

2

2.70

10

2.69

6

2.630

1

2.50

5

2,52

1

2.45

10

2.43

20

2.20

20

2.23

50

2.16

5

2.11

5

1.99

5

1.936

40

1.932

5

1.84

5

1.804

40

1.75

4D

2.15

1

2.10

4

2.19

4

2.145

4
1

2.00

2

1.96

5D

18

1.963

1.93

1

1.84

1

1.81

10

1.909

20

1.836

4
3

2.044

1.76

1.774

136

Decloizite is the most abundant vanadium-bearing
mineral found in the Mapimi district.

It is occasionallyfound on fluorite from the La China and La Reina mines which
are located on the mountain range directly to the west of
La Bufa.

These mines are worked specifically for fluorite.

Duftite-p(Pb,Ca)(Cu,Zn)[(As/y)04](OH)

Duftite- 3 (Guillemin, 1956) occurs as dark olivegreen incrustations and botryoidal masses on pale yellow
wulfenite (Figure 31).

The matrix material is usuallygoethite and manganese oxides.

Other mineral associates

include calcite and mimetite.
The optical properties of the mineral weredifficult to determine because of the small size of theindividual radiating crystals.

Maximum and minimum

indices are shown below compared to data on duftite from
Tsumeb (Barth and Berman in Palache, Berman, and Frondel,X

present study

Tsumeb

1.99*0.01

2.03*0.01

YZ

2.06*0.01
2.09*0.01

2.08*0.01

Chemical analyses of two samples of duftite-gfrom Mapimi have been made.
in Table XXV.

The results are presented

TABLE XXV
Chemical Analysis of Duftite - P from Mapimf
Compared with Conichalcite and Mottramite
TsumebGuillemin (1956)
Wt. %Mapimi 1
Wt. %Mapimi 2
Wt. %

PbO

43.4

43.7

42.8

CaO

3.4

4.0

4.3

1.6

1.8

ZnO

Conichalcite
CaCu(AsOa)(OH)Wt. %Mottramite(Cu,Zn)Pb(V04) (OH)
Zn:Cu ;= 1:1
Wt. %
55.30

21.61
10.08

CuO

19.9

18.7

18.1

30.65

A s 2°5

30.1

20.8

20.7

44.27

7.3

8.3

3.2

3.4

3.4

3.47

2.23

100.0

99.5

99.4

100.0

100.00

V2°5
H„0
2
Total

9.86

22.53

137

Minor amounts of Ag, Sb, P, Mn and Mg were noted inspectrographic studies of the material.
Guillemin (1956), in his study of the mineralogyof the copper arsenates, phosphates, and vanadates, dis­cusses structure and compositon of duftite - cy and - 0 .
He states that there is an isomorphic series between
duftite and conichalcite and a partial series withmottramite.

The X-ray diffraction data for duftite - 0 ,mottramite and conichalcite are compared in Table XXVI.
The original duftite described by Pufahl (1920)
from Tsumeb is duftite - or. Fleischer (1957) suggeststhat the terms duftite -or and duftite - 0 be dropped andthe name duftite restricted to the material that has beencalled duftite - or, and that duftite -0
Dussertite

be renamed.

BaFe^(AsO^)2(OH)^HgO

Dussertite, the hydrated basic arsenate of bariumand iron, is found on specimens obtained from the U.S.
National Museum.

It is associated with scorodite,arseniosiderite, and carminite.
pale to dark green.

The color varies from

Foshag (1937) determined the opticalproperties of the Mapimi material which he described as
being uniaxial to abnormally biaxial with 2V = 15o-20°,
negative, E = 1.845, O = 1.870.
Sufficient material for a chemical analysis was
not available.

139
TABLE XXVI
Comparison of X-ray Diffraction Data on
Duftlte-B, Mottramlte, and Conichalcite

duftitc- 3
HapimfI d(A) hkl
6
2
2

7
2
3
k

3
5
10

6
6

5.01
4.79
4.69
4.19
4.00
3.79
3.52
3.33
3.22
3.19

2.97
2.93

5 2.88Oil
101

4o 5.03

Oil

020
111

30 4.60
50 4.2130 3.95

020

120
200
210

30 3.56

111

120
210

121
201

220

• 130

100 3.26

4o 2.99

201

Oil
101

80 5.07!
•; conichalcite
j Berry (1951)jBisbee| I
dUU hkli
110
20 5.77
Oil j 10 4.921
Oil

4.l6

111

4o 4.66
60 4.24
40 3.99

020
111
120

30 4.101
10 3.905

3.76
3.49
3.3S

200
121

40 3.54
30 3.31210
121

20 3.690
20 3.424
10 3.246

120
200
210
121

3.1?
5 3.07201
030

100 3.24

201

90 3.118

201

5 2.96002

40 3.03
5 3.022I1002
10 2.887

220

70 2.91130

80 2.87

130 100 2.842

130

90B 2.62

131,2218o 2.68
8o 2.66

112
221

022

80 2.59131

30 2.51
30 2.4230 5.00.10 4.75
6o
•
30
5
5
100

\

210

220

102

80 2.85 130,102

221

80 2.65

9
3
3
5

2.66

131
022

60 2.57 131,022

2.52

2

2.44

310

10 2.48

2.63
2 .6 lmottramlte
Kingsbury &Hartley (1956)
TsumebIdUU hklduftite” $duftite* 0
Guilleman (1956 ) Claringbull (1951)
TsumebBrandy Gill
I
dUU hkl
1<HU hkl

221

310

122? 10 2.38 301.202
2.35
212
50 2.28
SB 2.27
212? 50 2.26
2 2.23 231,320 10 2.23
231

310

5 2.473io
5 2.41230
1:2
5 2.396o 2.30212
30 2.24 320,231
30 2.22
140

1

212

10 2.19
1

2.12

222

U 2.08 327,132, 30 2.09
141 40 2.05
1 1.98240 10 1.96
1 1.94330 10 1.93
400
1/2 1.91
312 50 I.87
5 1.87

2 1.77
k 1.75

7 1.64
1 1 .6 l
3 1.5930 2.1240 2.08
10
10
10
10

2.00

1.97
1.93
1.91

50B 1.868

10 1.85
10 1;82
10 1.81! 10 1.78! 10 1.77

i 5 1.75
j 30 1.71

50 1.750

j 50 1.64

60

1.637

1 50 1.62

i 30 1.58
j40 1.586

222

111

50 2.594 112,221
40 2.551
131
10 2.466
022
20 2.375 310.230
5 2.336
10 2.282

122
202 •

20 6.047 222.132

321
1327
240
330
400
31S

50 2.10321
50 2.08 141,132
3o 1.967

5 1.959240

20 1.912

30 1.871
5 1.837

50B 1.762
5 1.721
1 20 1.657
332?! 80 1.648: 40 1.615

10 1.84$ 312,113
20 1.828
10 1.806

331
042

30 1.7lb

420

203

402
!

35 1.605 332.133
402
10 1.558

Ferrimolybdite

hydrated Fe-molybdate

Small yellow grains of ferrimolybdite wereobserved on only one specimen of partially oxidized
sulfide minerals. Here a small rosette of molybdenite waspartially altered to ferrimolybdite.

It is suspectedthat much of the ferrimolybdite is overlooked in the large
ochreous masses which include goethite, jarosite, andbindheimite because spectrographic analyses of thesemasses frequently show
Hedyphanesmall amounts of molybdenum.

(PbCa)g(AsO^)^Cl

Small translucent white crystals of hedyphane
are found on mimetite and bindheimite samples from the14th level of the San Juan Poniente stope obtained fromthe U.S. National Museum (U.S.N.M. 92284).

The material

was too small to permit accurate microscopical determina­tion.

An average index of refraction is about 1.98 or 1.99
X-ray diffraction patterns of this material arecompared with hedyphane from Franklin, New Jersey and withmimetite from Mapimi.
Table XXVII.

This comparison is shown in

An emission spectrogram

was made of a small

quantity of the sample in which a considerable amount of
Ca was found.

The presence of abundant Ca and thesimilarities of the X-ray diffraction pattern to that ofknown hedyphane lend certainty to the identification ofthis mineral.

l4i
TABLE XXVII
X-ray Diffraction Data of Mimetite and Hedyphane
Mimetite(Mapimi)Hedyphane(New Jersey)(ASTM-14-213)

Hedyphane(Mapimi)Id( a )Id( A.)

I

d(A)

10

5.00

20

5.08

30

5.02

40

4.46

10

4.40

20

4.39

40

4.21

40

4.18

50

4.14

20

3.81

30

3.68

40

3.66

50

3.63

50

3.42

70

3.33

30

3.34

20

3.31

100

3.06

100

3.03

100

3.01

100

3.01

60

2.96

70

2.95

70

2.93

50

2.92

70

2.92

10

2.76

20

2.55
30

2.46

20

2.45

50

2.31

40

2.31

30

2.30

30

2.23

40

2.25

40

2.21

20

2.15

100

2.09

40

2.08

50

2.08

60

2.04

30

2.02

20

2.02

30

2.02

80

1.98

50

1.97

60

1.96

70

1.95

30

1.947

40

1.94

Koettigite

Zn3 (AsC>4)2 *8H20

Koettigite is a rare mineral in deposits at/

Mapimi and at present, only a few specimens fromthe locality are known.

Koettigite occurs as palelavender crystals elongated parallel to [001] (Figure 32).
The mineral is associated with adamite and calcite.
Dr. Peter B. Leavens of U.S. National Museum,
was kind enough to determine the optical propertiesof koettigite from a specimen in that collection (U.S.
N.M. R-15277).
X = b

The data are presented below:

X = 1.628 (calc, from Y, Z using 2V = 74°)
Y = 1.646-0.003
Z = 1.681*0.003

optically +ZAc ~ 38°
Legrandite

ZngfAsO^)(OH)"HgO

Legrandite occurs as elongated bright yellow
crystals in the zone of oxidation along with several otherarsenates including carminite, arseniosiderite, dussertite,adamite, paradamite, and scorodite.
The composition of legrandite from the Ojuela minehas been determined by Finney (1963) and Desautels (1963).
The chemical analyses are given in Table XXVIII.

FIGURE 32

Photograph of small crystalline mass ofkoettigite on a limonitic matrix (U.S.
N.M.-R 15277 - photograph courtesy of
U.S. National Museum). Length of cluster
is approximately 25 mm.

FIGURE 33

Photograph of a radiating cluster oflegrandite crystals in a small vug ingoethite (U.S.N.M.-115515 - photographcourtesy of U.S. National Museum).
Cluster measures 25 mm across.

FIGURE 33

TABLE XXVIII
Chemical Analyses of Legrandite

ZnO

Zn_ (AsOji) (OH) 'H^O
2
*
^
53.42

Mapimx(Finney, 1963)
51.02

Mapimi(Desautels and
Clarke, 1963)
50.1

■)
/

Nuevo Leon, Mexico(Drugman and Hey, 1932)
46.70

53.2

FeO

1,4 f
MnO
A s 20 5

37.71

0.21

1.7

37.73

37.7

8187

9.18

8.8

42.02

0.20

Ca

0.05

Cd

0.20

Mg

0.50

Si

0.20

9.36
2.14

0.75

^2^3
Al

0.2

insol.
Total

0.05

0.1

h 2o h 2o +j100.00

99.94

100.0

Spectrographic analyses showed traces of F e , Mn, Al,
Ca, Cd, Mg, and Si.

100.27

145
The chemical data on the type material by Drugman
and Hey (1932) were shown to be in error by Desautels and
Clarke (1963) .
Spectacular crystals of legrandite have been observedin lots of specimens found in 1962 (Figure 33).

Some ofthese crystals measured up to 60 millimeters in length andare elongated parallel to [001].

The morphology, crystallo­graphic elements and optical properties have been describedby Desautels and

Clarke(1963).

The most dominant formis m{110}and is frequently accompanied by p(ill}.

The

forms, c {00l}and a{l00}are occasionally observed.

The

smaller sized crystals are commonly doubly terminated.
The optical properties of Ojuela mine legrandite
from Desautels and Clarke (1963) are given below.
X = 1.702*.002
Y = 1.709^.002
Z = 1.740-.002
biaxial (+)
orientation:x = b; ZAc = 40°

pleochroism:
Xcolorless to yellow

Ycolorless to yellow

Zyellow

2V= 50°dispersion r<v distinct

Mimetite-Pyromorphite

Pbg(AsO^)gCl-

Pbg(PO4)3CI

Mimetite is a common mineral of the oxidized leadores that are presently being exploited at the San Juanmine.

The mineral occurs as pale yellow to greenish

yellow crystals and masses associated with cerussite,
wulfenite, plumbojarosite, goethite, bindheimite, andanglesite.

Small colorless transparent crystals of

mimetite have been observed.

Spectrographic analyses

indicated that these colorless crystals contain lessthan 0.1 per cent PgOg

(Figure 34).

Several samples were chemically analyzed.

The

results are shown in Table XXIX.
TABLE XXIX
Chemical Analyses of Mimetite-Pyromorphite
10—8—65—3
Wt. %10—8—65—4
Wt. %10—8—65—5
Wt. %Pbg(As 04 ,P04)ClP:As = 1:1
Wt. %

PbO

75.8

73.6

74.1

79.81

CaO

0.76

1.44

1.04

P2°5

1.44

0.85

8.26

7.61

A s 2°5

18.5

22.7

12.8

10.61

Cl

3.19

2.13

2.41

2.54

Total

99.69

100.72

98.61

100.570 = Cl0.57

Minor amounts of V, Cd, Ag, Mo, Cu, and Fe are alsopresent.

14?

FIGURE 34

Photograph of mimetite (pale greenishwhite) with cuprian austinite (green
spherulites) (photograph courtesy of
U.S. National Museum).

FIGURE 35

Photograph of paradamite (type materialU.S.N.M.-112518 - photograph courtesyof U.S. National Museum). Specimen isapproximately 1" x 1".

148
Paradamite

ZiigfAsO^lfOH)
The type locality of paradamite is the Ojuela

mine.

Switzer (1955) described paradamite from a suiteof secondary minerals submitted for identification.

The

mineral is associated with mimetite and adamite on amatrix of limonite (Figure 35).
Paradamite is triclinic dimorph ofwhile adamite is orthorhombic.

(AsO^)(OH)

The physical propertiesof paradamite taken from Switzer (1955) on the typematerial (U.S.N.M. 107443) are given below
Optically (-)
2v = 50°
X = 1.726
Y = 1.771
Z = 1.780
Perfect (010) Cleavage
Specific Gravity = 4.55
and the chemical analysis and X-ray diffraction data arepresented in Tables XXX and XXXI, respectively.

Finney(1966) has determined the unit cell of the type material.
The rarity of the species in contrast to therelative abundance of adamite suggests that the physicalchemical environment of paradamite is very restricted.

TABLE XXX
Chemical Analysis of Paradamite (Switzer, 1955)
Zno (As01)(OH)
ZnO
AS2°5
H2°
FeO

56.77

56.22

40.09

40.17

3.14

3.44
0.45
0.12

Fe2°3
Total

Paradamite

100.00100.40150

Table XXXI gives the X-ray diffraction data for paradamite.
TABLE XXXI
X-ray Powder Diffraction Data for Paradamite
d(A)

I

dU)

I

6.33

10

2.18

2

5.44

6

2.13

1

4.77

2

2.11

7

3.99

3

2.08

5

3.71

10

1.99

1

3.34

5

1.95

1

3.16

6

1.91

2

3.08

7

1.88

1

2.99

9

1.84

1

2.84

9

1.82

2

2.76

1

1.79

4

2.73

1

1.76

2

2.57

6

1.72

4

2.53

1

1.67

3

2.49

8

1.58

4

2.45

3

1.55

2

2.39

3

1.54

4

2.32

1

1.51

3

1.48

4

151

Scorodite

Fe (AsOJ •2H90

No scorodite was recognized by the author on any ofthe material collected at the numerous mines in the district.
Samples of massive and crystalline scorodite have been ob­
tained from the U.S. National Museum (U.S.N.M. 95771) for thepresent study. . The scorodite is associated with carminite
and dussertite in cavities.

The.scorodite is of a pale, graygreen color and has a resinous to subadamahtine luster.
According to Foshag (1937), "the crystals of
scorodite are simple in habit, consisting of the pyramid,p {Lll}only slightly modified by a {100) and small triangularfaces of d {120}, so that the crystals have an almost simple'octahedral' habit."
In transmitted light, the scorodite is almost
colorless or pale, greenish gray.

The 2V is large, about

70-80°f/and the mineral is biaxial positive.

Refractive

indices are as follows:
Foshag (1937)
X

=

1.784

Y

=

1.795

Z

=

1.810

Z-X

=

0.026

The Al^Og substitutes for FegOg to form;a solid solution
series with mansfieldite A1(AsO^)*2H2O.

The chemical

analysis from Allen and Fahey (1948) of scorodite from
Mapimi is presented in Table XXXII.

TABLE XXXII
Chemical Analysis of Scorodite from Mapimi
Foshag (1937)
wt. %Insol.
A s 2°5
Fe2°3
R12°3

Allen and Fahey (1948)
wt. %Fe(A s 0 4) *2H
wt. %

48.96

49.52

49.79

34.20

34.79

34.60

0.12

1.60
0.06

Sb2°5
CaO
T102
MgO

0.54
0.02
0.22

Si02
H2°
Total

0.30
14.53

15.44

15.61

100.17100.13100.00

H

vn
ro

153

The X-ray data for the Mapimi material given in
Table XXXIII are from Axelrod in Allen and Fahey (1948).
TABLE XXXIII
X-ray Diffraction Data on Scorodite from Mapimi(from Axelrod in Allen and Fahey, 1948)

Vanadinite

d (1)

I

2.50

ms

6.16

m

2.31

mw

5.56

vvs

2.18

mw

4.95

ms

2.13

mw

4.44

vvs

2.11

mw

4.06

m

2.04

mw

3.78

m

2.00

m

3.50

m

1.836

mw

3.36

m

1.797

mw

3.16

vvs

1.753

mw

3.05

s

1.664

ms

2.98

ms

1.645

mw

2.84

mw

1.583

mw

2.75

mw

1.513

mw

2.67

m

1.473

m

2.58

vs

1.401

mw

Pb5 (VC>4)gCl

Small,bright brownish orange crystals of vanadinite,
approximately 0.1 mm. in length, were found on a specimen
consisting of pale yellow transparent crystals of wulfeniteand colorless rhombs of calcite on alimonitic matrix.

A spectrographic analysis of a small quantityof thevanadinite showed minor amounts of As and Zn to be present
in quantities of approximately two and one per cent,respectively.
Normally,the vanadium-bearing phase at Mapimi
is mimetite where the vanadium substitutes for arsenic.
A very high As:V ratio in the mines would account for thegreater abundance of mimetite over vanadinite sinceconsiderable V can substitute for As in mimetite.

Thus

vanadinite would only be found where the arsenic content
was either very low or lacking. A study of the matrix
material of the vanadinite-bearing specimen showed only
a trace of arsenic to be present.
Several specimens of vanadinite supposedly from
Mapimi have been obtained from various mineral dealers.
The authenticity of the locality of the material isseriously questioned.

Many of the miners who have lived

and worked most of their lives at Mapimi do not recall
ever having seen any such specimens at any of the minesof the district.
Whitlockite

Cag(PO^)2

Whitlockite is found at the San Ilario mine aslate fracture fillings as stalagtitic masses.

The colorvaries with porosity from pale yellowish-tan to yellowishbrown ; the darker the color, the greater the density.
Individual grains are generally less than 0.01 mm.
and have abundant dark inclusions.

The optical properties

are given below:
O = 1.611-.002
E = 1.613-.002
uniaxial negative (-)
Wulfenite

PbMoO^

Wulfenite is a widely distributed mineral in themassive oxidized lead ores.

It is associated withmimetite, duftite, bindheimite, cerussite, hemimorphite,
aurichalcite, adamite, plattnerite, and calcite.

Thecolor ranges from colorless to yellow, orange, or brown.
The habit varies from thin tabular crystals with thepredominant (00l) form to elongate crystals exhibiting only
the Coio} and {00l} .

The elongate crystals are usually darkorange-brown in color, while the tabular crystals exhibit
considerable color variations.
Botryoidal crusts of dark-green duftite coat paleyellow tabular crystals of wulfenite.

Highly modified

colorless calcite crystals are perched on both thewulfenite and duftite and are obviously later than eitherof the lead minerals.

Small opaque or translucent

inclusions have been

noted in many of the wulfenite crystals examined micro­
scopically.
litharge.

These have tentatively been identified as
A similar occurrence of litharge has been

described by Williams (1962) in bright orange wulfenite
crystals from the Chicago mine, Pima County, Arizona.
The variation in color of the wulfenite crystals
has been discussed by Williams (1962 and 1963, andpersonal communication).

This color variation is

attributed to structural defects based on the color's
inverse absorption surface which is flattened on the{001} axis. Williams (1966) also studied the relation­
ship of habit and morphology of wulfenites to theirconditions of formation and minor element content.

157

Chrysocolla

CuSi03 •2H20

While the mineral chrysocolla is not an abundant
mineral in the mines of the district, it is widely distrib­
uted, notably in the numerous prospect pits that cover thearea.

Chrysocolla occurs as small cavity and fracture

fillings and as pseudomorphs after malachite, rosasite, andaurichalcite.

Some of the pseudomorphs still have cores of

either malachite or rosasite.

The pseudomorphs afterrosasite are ball-shaped and those after malachite oraurichalcite are usually acicular.

The color is usually

pale blue to almost white and generally the mineral ispithy.

The occurrences of chrysocolla seem to be

restricted to near the surface.
X-ray diffraction studies of the material suggestthat it is almost amorphous in structure.

X-ray spectro-

graphic studies of several samples show considerable
variation in Cu, Zn, A1 and Si content.
There are numerous reports of large masses ofchrysocolla having been found in the past.

This

chrysocolla was supposedly of a fine blue color, resembling
turquoise, and quite hard.

Fine cut stones are said tohave been made from it.
The occurrence of turquoise has been reported fromthe district, but the writer feels that the material
described was actually chrysocolla.

158

Hemimorphite

Zn^fOHlgSigO^'HgO

Hemimorphite is a widely distributed mineral in
the deposits at Mapimi.

It is found from near the surface

to depths at which unoxidized ore is encountered.

The

habit varies greatly from small individual lathlike
crystals (Figure 36 ) to large rosettes up to four centi­
meters across composed of highly modified elongate
crystals.
The color varies from colorless to white, and thehemimorphite may be strongly stained with iron oxides.
Hemimorphite commonly occurs with aurichalcite, rosasite,
calcite, and adamite.

The paragenesis of these minerals

is often difficult to discern because of complex interngrowths of any or all of these minerals.

Many samples,

however, were noted on which the hemimorphite was clearly
the last mineral to have formed.
Roy and Mumpton (1956) have shown that hemimorphite
is the stable phase in the ZnO-SiOg-HgO system below
250°C. at 20,000 psi with sufficient HgO.

The tempera­

tures of formation at Mapimi were probably much lower than
this maximum temperature.

No willemite has been detected.

A sample of colorless hemimorphite was chemicallyanalyzed.

The results are presented in Table XXXIV.

FIGURE 36

Photograph of a cluster of lath shaped
hemimorphite crystals (photographcourtesy of U.S. National Museum).
Cluster is approximately 20 mm long.

TABLE XXXIV
Chemical Analysis of Hemimorphite
Zn^Si^07 (OH)o »H20

Mapimx

ZnO

67.5

67.05

Si02

25.0

25.04

H,0

7.5

7.71

100.0

99.80

Total

The refractive indices are:X = 1.612-0.002
Y = 1.616-0.002
Z = 1.637*0.002
Variations from these indices are rare.

The average

specific gravity is 3.14.
Figure 55 has been modified from Roy and Mumpton
to show the stability relations of sauconite, hemimorphite
and willemite.
Kaolinite

Al2Si205 (OH)4

Kaolinite occurs in small veinlets and fault gougezones in the upper oxidized zones of mineralization.

Thekaolinite is associated with manganese and iron oxides
and pulverized rock material.

The crystallinity varies

greatly as indicated by the range of peak intensities andline broadening on X-ray diffraction graphs.

l6 l
Quartz

SiC>2
Silicification has been a minor phase ofmineralization at Mapimi.

Occasionally, silicified zones

in the limestones and dolomites have been noted by thedevelopment of a jasperoid or chert, which gives a dense
appearance to the rock.

The original textures are usuallypreserved.
A few clusters of colorless to white quartz crystals
have been found.

The crystals are frequently coated byiron oxides and have a slightly corroded appearance.

Quartzcrystals are more common near the upper portions of themines rather than at any appreciable depth.

Many of theprospect pits and shallow mines in the district have drusy
quartz overgrowths.
The amethystine variety of quartz has been found inseveral of the more shallow mines north of the principal
mines at Ojuela (The El Padre or La Esperanza).

Individual

crystals seldom exceed one centimeter in length.
Genetically, the quartz crystals appear to be verylate, although silicification of the limestones anddolomites was probably early.

That two generations ofsilicification are present does not seem unreasonable;
the first
latestvery early and the second

corresponding to thestage of mineralization, since quartz druses areseen to proceed almost all of the secondary copper andzinc minerals.
Sauconite(Zn,Al)3 (OH)2 (Si,Al)401Q

Light gray, pulverulent coatings of sauconite were
found on specimens near the Socavon shaft.

The sauconite

was associated with hemimorphite and various manganeseoxides.

The X-ray diffraction pattern of the material had

several diffuse lines which corresponded satisfactorily
with published data for sauconite (ASTM-8-444).

X-ray

spectrographic studies revealed the presence of minor
amounts of Al, Mn, Fe, and Mg.

PARAGENESIS
Despite the number of mineral species observed in
the Mapimi district, the paragenesis for local areas is
not too difficult to discern.

The mines of principal

interest in the district are mines at Ojuela and theparagenetic relationships described in the following pages
will deal primarily with these mines because most of theminerals found in the district were detected here.

Theparagenesis of the Monterrey mine near Ojuela is uniqueand deserves a brief description as do the manganese ores
near San Pedro.
The paragenetic relationship of minerals wasdetermined by microscopic examination of specimens andstudy of thin sections, polished sections, and thinpolished sections.

The examinations of thin and polishedsections were most rewarding in understanding theparagenesis because replacement and alteration features
could be more clearly determined than by the examinationof minerals in hand specimen.

The works of Short (1940),

Cameron (1961), and Oelsner (1966) were used as referencesin determining many of the individual minerals and theirparagenetic relationships in polished sections.

163

164

The paragenesis at the Ojuela mine can be discussed
for two groups of minerals - the primary sulfide ores andthe secondary copper, zinc, and lead minerals.
From the examination of various suites of minerals
paragenetic charts could be prepared.

These charts only

serve in a general manner to show the approximate orderof mineral formation for the majority of the samples
studied.

Exceptions to the paragenetic sequences shown

are common and might be explained as being due to local
conditions which were not prevalent throughout the entire
deposit.
Many correlations from different mineral suites
were made on the basis of a few minerals that were common
to both assemblages although this may present an erroneousparagenetic classification because certain mineral species
have a very broad field of stability as can be seen in
the following chapter dealing with stability relationships.
A few species that consistently showed definite
paragenetic relationships will be discussed briefly.

At

Mapimi, hydrozincite was always seen to be the first of thebasic zinc and copper carbonates to form.

Hydrozincite

is followed by aurichalcite and malachite and, finally, byrosasite.

Malachite commonly forms the cores of botryoidal

crusts or spherulites of rosasite.

165

One group of specimens showed a clear paragenesisof copper minerals from tenorite to cuprite, to native
copper to covellite.

Such a paragenetic sequence can bepredicted from the Eh-pH diagrams of the following chapter
with decreasing pH and oxidation potential.
Galena alters to anglesite which in turn isreplaced by cerussite.

Mimetite frequently accompanies

cerussite formation, and wulfenite usually precedes or iscontemporaneous with mimetite.

Plattnerite is one of thelatest minerals to form and is accompanied by murdockite.
The paragenetic classifications of the primary and secondary
minerals from the mines at Ojuela are shown in Figures 37
and 38, respectively.
The paragenesis of manganese minerals is fairly
well understood.

The first manganese mineral to form wasprobably braunite followed by cryptomelane, pyrolusite,and psilomelane (Figure 39).

Ramsdellite is contemporaneouswith pyrolusite, and hausmannite, hydrohausmannite, andhydrohetaerolite precede the formation of psilomelane.
Manganite fills fractures and vugs lined with euhedralhydrohetaerolite (Figure 40).

Groutite is an alteration

product of manganite and a late fracture filling mineral(Figure 41).

A probable paragenetic sequence of themanganese minerals is presented in Figure 42.

166decreasing age
arsenopyrite
pyrrhotite
pyriteenarglte
sphalerite
marcasite
galenadhalcopyrite
molybdenite
tetrahedrite
tennantite
stibnitecovdllite
borniteboulangerite
jamesonite
chalcocite
pyatgyrite
sideriteankerite
qdartzcalciteFIGURE37

Paragenesis of Primary Minerals at Ojuela

16?

decreasing.age
baritegoethitebindheimite ---fluorite---

cerargyrite
adamitecupriteanglesite
wulfenitemalachite
mimetitecerussite
hemimorphite
tenoriteaurichalcitehydrozinciterosasitebrochantite
coppermassicotlitharge
plattnerite
murdockite
legrandite
descloizite
smithsonite
calcitearseniosideritedelafossite
jarosite
FIGURE 38
Paragenesis of Secondary Minerals at Ojuela

FIGURE 39

Photomicrograph showing the alterationof cryptomelane (cryp) to pyrolusite(pi). Nicols uncrossed-oil immersionblue filter.

FIGURE 40

Photomicrograph showing euhedralhydrohetaerolite crystals with small band ofhydrohausmannite (?). White is manganite.
Nicols uncrossed-oil immersion-blue filter.

168

L

so ^

j

FIGURE 40

FIGURE 41

Veinlet of groutite in pyrolusite. Nicols
uncrossed but rotated 90° to give maximum
bireflection. Oil immersion-green filter.

FIGURE 43

Photomicrograph showing pyrrhotite (po),stannite (st), galena (gn), enargite (en),bismuth (bi), and bismuthinite (bm).
Nicols uncrossed-oil immersion-blue filter.

169

,

/00^( .

FIGURE 41

170

decreasing age
rhodochrosite--------

braunite----------------

hematite■ ■copper— .. -.

goethite
cryptomelane---- -----■

pyrolusite... -----------hydrohausmannite------hydrohetaerolite---------

psilomelane----

hausmannite-----

manganite
groutitemolybdenite— ----- --ramsdellite---------FIGURE 42

Paragenesis of Minerals from San Pedro171

The mineralogy of the Monterrey minefor the district.

is unique

The mineral assemblages are characterized

by such relatively rare minerals as umangite, emplectite,
bismuth, clausthalite,and stannite occurring with magnetite,pyrrhotite, marcastie, pyrite sphalerite, galena, bismuthinite, chalcopyrite,and.enargite. (Figure 43).
The ores from this mine are distinguished from mostof the other mines of the district by their high bismuth andselenium low arsenic and antimony contents.

The onlyarsenic mineral detected in the number of polished sections
examined was enargite which is only a minor constituent of
the ores.
The early magnetite is partially replaced bypyrrhotite, which in turn is altered to intergrowths ofpyrite and marcasite.

Sphalerite and chalcopyrite arelater than the magnetite and pyrrhotite and partiallyreplacethem.

chalcopyrite.

Galena and clausthalite both replace
Enargite formation is about contemporaneouswith that of galena and clausthalite.
The initiation of deposition of bismuth minerals
began toward the end of galena formation and frequently
a thin reaction rim of bismuthinite separates bismuth
from galena.

Emplectite replaces galena but a few islandsof galena remain in the emplectite grains.

Umangite andberzelianite were clearly the latest minerals to form
(Figure 44).

172

decreasing age
magnetite— —— —

pyrrhotite
pyritecassiterite
marcasite
chalcopyrite
sphalerite
stannitegalenaclausthalite
enargitebismuthbismuthinite
emplectite
berzelianite----------- -umangite
FIGURE 44

Paragenesls of Minerals from the Monterrey Mine

MINE WATERS
Samples of mine waters from 34 locations in the
numerous workings of the district were collected for Eh andpH determinations and trace element analyses.

The resultsof these analyses are shown in Tables XXXV and XXXVI and aplot of Eh versus pH is presented in Figure 45.

The rangesof ionic activities obtained from mine waters are given
in Table XXXVII.
The Eh and pH of the mine waters were determined at
the sample site,a

50 ml. sample was also taken in stopperedbrown glass bottles and sealed with sealing wax in order toprevent leakage or exposure to changes in atmosphere.
The usefulness of the Eh and pH values of somesamples is open to criticism because of the contamination
by both human and animal urine.

Several locations thatare actively being worked were not sampled because ofobvious contamination.
The analyses of the water samples from the various
localities in the mine permit one to construct Eh-pH di­agrams for various systems.

Certain maximum and minimum

activities can be used for the construction of these
diagrams based on the analyses of present mine waters.
Assemblages actually noted in the mines may appear to bemetastable when considered in the fields presently173

+ 500

+ 400

+ 300

+ 200

□

FIGURE 45

Plot of Eh vs. pH of nine waters

[]

175

determined, but certainly the environment has been changed
by mining, and the opening of the mines for over 350 years
has permitted rapid oxidation.
The points enclosed by squares in Figure 45 represent
samples that were taken in the lower levels of the mine.
There is a general progression of values to a lower pH andincreasingly negative Eh with depth.

These samples weretaken from three consecutive levels.

The circled points

were taken in upper mine levels.

Unfortunately, only Eh-pH

readings were taken of the upper mine waters and no samples
were obtained for chemical analysis.

The plots of thesepoints coincide well with a part of the region of the Eh-pH
diagram considered by Baas Becking, Kaplan and Moore (1960)
to be characteristic of oxidized mine waters (Figure 47).
Both sets of Eh-pH values from Mapimi have been
overlain on the diagram of Baas Becking, Kaplan and Moore
which shows the oxidized and unoxidized stability fields.
(Figure 47).

Little mention of the host rock primary mineral

assemblages is given by these authors who only mention thatthe mine waters were obtained from pyritic ore bodies in
New South Wales and Western Australia.

Values from Sato

(1960) on work in Utah and Arizona were also included in
the study.
Plots of SO4 and CC>2 content of the mine waters
versus the pH were made (Figures 48 and 49).

An obvious

trend can be seen in the plot of SC>4 versus pH.

The S04176
content decreases with increasing pH from a value of0.033% S04 at pH 5.0 to 0.0005% at a pH of 9.
The plot of the CC^ content versus pH did not show
any pronounced trend, but in general the COg content is higher
at the lower pH values.

Unfortunately, the sensitivity ofthe method used for the determination of chlorine content was
not sufficient to determine if any variation does exist.
The effect of a high-carbonate environment such as
is encountered at Mapimi would act to limit the lower pH
range of the waters.

The rate of flow of waters through themineralized. zone will also greatly affect the Eh-pH of themine waters.

The more rapidly moving the waters, the more

restricted the Eh-pH values would become.
Mining of the deposits, especially for such a long
period of time as those of Mapimi have been mined, would
permit an easier circulation of waters than would probably
have existed before mining began.

With the exception of

the hydrous sulfates, most of the minerals encountered at
Mapimi were probably formed before mining commenced.
The present Eh-pH conditions, therefore, would notnecessarily reflect earlier conditions that were responsible
for the deposition of the mineral species observed.
The restricted occurrences of many minerals can be
ascribed to local" environments which may have resulted,
from stagnation or. locally higher ionic concentrations.

TABLE XXXV
Eh-pH Measurements of Mine Waters in the Upper
Oxidized Zones of the Mapimi District
SampleNumber

Eh

pH

24

+400

4.1

25

+380

4.7

26

+470

9.9

27

+160

4.3

28

+310

5.9

29

+500

6.1

30

+330

8.6

31

+390

2.7

32

+440

8.0

TABLE XXXVI
Eh-pH and Partial Chemical Analysesof Mine Waters in the Mapial District
1

2

3

4

5

6

7

8

9

10

11

12

Eh

-lUO

-8o

-130

-n o

-220

+10

-l4o

-110

-90

-120

0

-120

PH

7.7

8.9

8.2

8.2

7.1

9.4

8.0

8.1

8.6

8.2

8.8

8.2

Sample Number

i Ca

.010
.006
.056
.0140
.0025
.0015
.00102.00613.0015%
<.001
<.001
<.001
.0001
.0001
.0001
.00004.00006.00007
.0014
.0016
.0018
.0006
.0007
.0007
.00048.00031.00037
.005
.005
.005
.0009
.0009
.0009
.00039.000555 .00061
<.001
<.001
.002
.0001
.0001
.0003
.00006.00007.00013
.001
<.001
<.001
.0001
.0001
.0002
.00006.00009.00007
.0005
.0177
.0207
.0001
.0022
.0018
.00136.00079.00007.002' .04?
.009
.0015
.0003
.0077
.00626.00136.00027
.02
<.02
<.02
.0028
.0056
.0028
.00254.00248.00456

molality
activity$ Mn
molality
activity$ Mg
molality
activity$ Fe
molality
activity% Zn
molality
activity% Cu
molality
activity£ so.
molality
activity% HC03molality
activity* Cl
molality
activity- logy (when- logy (whenionic strength1? = 4)
= l)

.004
.022
.001
.002
.042
.031
.0002
.0010
.0005
.0105
.0077
.0055
.00400.00474.00058.00321.00032.00013
<.001
<.001
<.001
C.001
.001
<.001
.0001
.0001
.0001
.0001
.0002
.0001
.00006.00006.00006.00006.00009.00005
.0008
.0008
.0026
.0037
.0009
0017
.0004
.0011
.0015
.0003
.0007
.0003
.ooo64.00014.00097.00016.00040.00025
.008
.006
.004
.006
.006
.007
.0014
.0011
.0011
.0011
.0013
.0007
.00064.00081.00032.00082.00071.00057
.014
.002
<.001
<.001
.019
.007
.0021
.0001
.0011
.0001
.0029
.0003
.00006.00014.00167.00071.00133.00005
.004
.001
.002
.001
.001
.003
.0006
.0002
.0002
.0002
.0005
.0003
.00038.00010.00013.00029.00017.00009
.0082
.0080
.0092
.0110
.0129
.0007
.0008
.0001
.0010
.0011
.0009
.0013
.00006.00052.00050.00065.00051.00067
.002
<.001
.090
.002
.013
.033
.0054
.0001
.0021
.0147
.0003
.0003
.00026.00026.00470.00178.01205.00009
<.02
<.02
<.02
.04
<.02
.03
.0028
.0028
.0028
.0028
.0113
.0085
.00244.00250.00926.00721.00251.00245.007.0017.00089<.001.0001.00005.0026.0011.00057.002.0004.00021.030.0046.00240.001.0002.00010.0080.0008.00042.071.0116.00986.02.0056.00476

.002
.059
.0147
.0005
.00648.00030
<.001
.008
.0001
.0015
.00066.00006
.0022
.0031
.0013
.0009
.00057.00055
.004
.005
.0009
.0007
.00043.00040
.001
<.001
.0002
.0001
.00004.00012•<.001
.007
.0001
.0011
.00004.00067
.0060
.0027
.0006
.0003
.00018.00026
.042
.010
.0016
.0069
.00610.00131
.02
•03
.0056
.0085
.00456.00751.48332.12083.38367.09592.82559.20640.79600.19900.65605.16401.53963.13491.43635.10909.45957.11489.55281.13820.64992.16248.49477.12369.81976.20494

.0146

.0092

.0426

.0396

.0269

.0182

.0119

.0132

.0191

.0264

.0153

.0420

TABUS XXXVI (Contd.)
Eh-pH and Partial Chemical Analysesof Mine Waters in the Mapimi District
13

14

15

16

17

18

19

20

21

22

23

Eh

-20

+10

+50

-100

-230

-200

-180

+10

-80

+10

-80

9.1

9.2

9.2

7.9

6.6

5.0

6.8

9.1

8.7

9.0

8.4

PH

.010
.0025
.00136.001.0002.00011.0029.0012.00065.003.0005.00027.004.0006.00033<.001.0001 .
.00054.0337.0035.00190.041.0067.00575.02.0056.00481.062.0155.00641.001.0002.00008.0016.0007.00029.005.0009.00037.006.0009.00037.002.0003.00012.0210.0022.00091.056.0092.00738.02.0056.00449.017.0042.00217C.001.0001.00005.0019.0008.00041.005.0009.00047.002.0003.00016.001.0002.00010.0006.0001.00005.066.0108.00916.06.0169.01433.084.0210.00868<.001.0001.00004.0011.0005.00021.006.0011.00045.001.0002.00008.002.0003.00012.0020.0002.00008.007.0011.00088<.02.0028.00225

Sample Number$Camolality% Mn
molality
f*4vi tvo p V X V X UJf
C*s»

56 Mg

molality
S C U l V X ujr$

Fe
molality$Znmolalityo
4 tt 4 4 -y
S v ul. V X V jartivitv

C * V U X V X vjf% Cu
molalityn
4 ir4 +-\rac
V l V X VJ
% S0U

molality
r*4-4v4 i"v

aV V A . V X V J
S$ hco3molality»Cvxvxuy< Cl

molality0
4 ^r4 4*vf
acvxvxuy- logY (when= 4)- logy (when Zd = l)ionic strength

.068
.0170
.00692.010.0018.00073.0020.0008.00033.005.0009.00037<.001.0001.00004<.001.0001.00004.0022.0002.00008.072.0118.00943.02.0056.00447

.022
.029
.0072
.0055
.00410.00312
<.001
.001
.0001
.0002
.00006.00011<.0002
.0002
.0001
.0001
.00006.00006
.002
.007
.0004
.0013
.00074.00023.001.007.0011 ‘ .0002.00062.00011
.001
.001
.0002
.0002
.00011.00011
.0010
.0007
.0001
.0001
.00006.00006
.004
.004
.0007
.0007
.00061.00061
.02
.02
.0056
.0056
.00486.00486

.008
.031
.0020
.0077
.00104.00406
<.001
<.001
.0001
.0001
.00005.00005.OOlti.0010.0004.00021.00037
.001
<.001
.0002
.0001
.00010.00005
.002
.007
.0011
.0003
.00016.00058
<.001
.001
.0001
.0002
.00011.00005
.0226
.0119
.0024
.0012
.00125.00063
.078
.033
.0128
.0054
.01088.00460
.06
<.02
.0169
.0028
.01436.00239

.0007

.007
.003
.0017
.0007
.00048.00107
<.001
<.001
.0001
.0001
.00006.00007
.0010
.0021
.0005
.0009
.00032.00062
.007
.005
.0013
.0009
.00082.00062
.002
<.001
.0001
.0003
.00019.00007.003 ■ .009
.0014
.0005
.00088.00034
.0050
.0005
.0001
.0005
.00032.00007
.002
.009
.0003
.0015
.00027.00137
<.02
<.02
.0028
.0028
.00250.00255.89889.22472.56851.14213.56285.14071.64125.16031.65115.16279.61188.15297.88363.22091.65848.16462.88363.22091.37523.09381.45957.11489

.0505

.0202

.0198

.0257

.0265

.0234

.0488

.0271

.0488

.0088

.0132

180

TABLE XXXVII
Range of Ionic Activities Obtained from Mine Waters
Maximum

MinimumCa

8.7 x 10 -3

1.3 x IQ-4

Mn

7.3 x 10 -4

4.0 x 10"5Mg

9.7 x 10-4

6

x IQ'SFe

1.0 x 10-4

5

x 10-5Zn

2.4 x 10~3

4

x 10-5Cu

8.8 x 10 -4

4

x 10“5so4

1.9 x 10 -3

6

x 10-5HCO-

1.2 x 10 -2

9x 10“5

1.4 x 10 -2

2.2 x 10"3l8l

FIGURE 47

Eh-pH characteristics-of natural mine waters'x,
distinguishing between waters from primary ores'
and those draining oxidized ores (from Baas Becking,
Kaplan, and Moore, i960).
Values in circles are taken from Table XXXV. Values insquares are from Table XXXVI.

182

::zz

Xi ! Uuo .06.

FIGURE 48

Plot of SO

vs. pH

183

FIGURE 49

Plot of C 0 2 vs. pH

STABILITY RELATIONS OF MINERALS IN THE OXIDIZED ZONE
The calculation of molalities and activities ofions in natural mine waters along with Eh and pH determina­
tions of these waters permits the construction of Eh-pH
diagrams for various mineral species.
The descriptions of the diagrams and theirconstruction given by Carrels and Christ (1965) serve asa basis for the construction of diagrams for the present
study.

Free energies and equilibrium constants of ionicspecies and minerals were taken from Carrels and Christ(1965), Latimer (1952), Pourbaix (1966), Hem (1963), andBricker (1965).

Comparison has also been made to themineral equilibrium diagrams edited by Schmitt (1962).
The free energy data given in these references
are for 25°C. and one atmosphere total pressure.

Obviously,

deviations from these conditions must have existed during
the oxidation of the primary ore bodies at Mapimi.

Con­sidering the depth of oxidation alone, the hydrostaticpressure was many times the atmospheric pressure.

Carrelsand Christ (1965, pp. 306-350) discuss the effect oftemperature and pressure on mineral equilibrium and showthat for small increases in either temperature or pressure,the free energies change only slightly.

184

In general, the185effect of temperature or pressure increase is an increasein the free energies.

In the present study, standard con­ditions of 25°C. and one atmosphere total pressure wereused for all calculations.

The Eh-pH plots consider only

simple systems of three or four ionic species.

The catalyt­

ic effect of other ions, which would be undoubtedly present
in almost any natural environment, cannot be evaluated.
The equations used to delineate stability fields
have been included and the numbers encircled on the
diagrams correspond to the equation for the boundary line
considered.

Transparencies of each of the Eh-pH diagramsin the text are included in the envelop attached to therear cover.

These transparencies can be overlaid on anyof the figures in the text to show fields of mutual
stability.
The equations used for the construction of the
Eh-pH diagrams include the following:1.

I = 1/2 Eir^Z?
where I = ionic strength,m^ = molality of the ion= ionic charge

2.

-log Yi = AZ? /j
1(Debye-Huckel equationfor very small ionicstrengthwhere y = activity coefficient
A = 0.5085 @ 25°C.

= constant characteristic of thesolvent (here considered to be water,
Garrels and Christ, 1965)

186

I = ionic strength= molality of the ion
3.

a = ymwhere a = activity
y = activity coefficientm = molality

4.

A F°

where A= - RT Ink= change of free energy of a reaction

R = Gas constant = 1.987 cal./deg. moleT = °K = 273.15°
5.

K = aprod.
areac.
K = thermodynamic equilibrium constantaprod. = activity of products
areac. = activity of reactants

6.

E° =

n

? Ink

E° = voltage of reaction when all
substances involved are at unit
activityn = number of electrons involved
y = constant (faraday) = 23.06 kcal.
per volt-gram equivalent
7.

Eh = E° +EE. 7 Inkn

Eh = potential of a half-cell, referred
to a standard hydrogen half-cell(EMF of the standard half-cell istaken as zero at all temperatures).

18?

The diagrams showing the limits of the domains ofrelative predominance of dissolved carbonate and sulfur
species have been included (Figures 50 and 51) , along withpertinent equations taken from Pourbaix, 1966).

The

overlaying of these diagrams on any of the Eh-pH plots
for the various minerals will show which of the carbonate
or sulfur species are in equilibrium with the minerals orions shown.
The equations used to calculate the limits ofdomains of relative predominance of dissolved carbonate
species are:1.

h 2c o 3= h c o 3"+ H+

log]fp§jf = " 6.38 + pH
2.

HC03~ = C03 = + H+
logis§%l] = - 10-34 + pH'

and for sulfur species:
1.

= S£ + 2H* + 2e
Eh = .141 - .0291og [H2S] - .059pH

2.

S® + 4H20 = S04+ 8H+ + 6e

Eh = .357 + .OlOlog [S04"_] - .078pH
3.

Sc + 4H2° = HS04~ + 7H+ + 6e
Eh = .338 + .OlOlog [HS04“] - .069pH

185+ 0.1”

+ 0.6

x.
H2CO3

+ 0.4

H CO,

Ox.

x
X
X

x

X

- 0.4

x
—

0.6

L i m i t s otoftherelativeof t h edomains

predominanceXdissolved substances-0.1-

FIGURE 50

Eh-pH diagram of the system, H20-C

x

189

Total

dissolved

Sulfur species

FIGURE 51

=

1 0'3

Eh-pH diagram of the

190
4.

HS~ + 4H20 = S04"" + 9H+ + 8e
Eh = .252 - .067pH

5.

H^S + 4H20 = S04~~ + 10H+ + 8e
Eh = .303 - .074pH

6.

HS04" = S04”

+ H+

pH = 1.91
7.

H S = HS” + H+
2
pH = 7.00

The pertinent equations for the construction of
the Cu -S-C-H20 diagram follow:
1.

H0S_
A aq = S2
u + 2H+ + 2e
Eh = .141 - .0291og[H2Sag] - .059pH

2.

S” + 4H20 = S04"" + 8H+ + 6e
Eh = .357 + .0101og[S04""] - .078pH

3.

S° + 4H O = HSO,“ + 7H+ + 6e

c

2

4

Eh = .338 + .0101og[HSO4“] - .069pH
4.

5.

Cu2S + H2S = 2CuS

+ 2H+ + 2e

Eh = .08 - .059pH- .0291og[H2Saq]

Cu2S + HS+ H+ + 2e= 2CuS

Eh = - .13 - .029pH
6.

7.

- .0291og[HS~]2CuS + 4H20 = Cu2S + SO4+ 8H+ + 6e

Eh = .378 + .OlOlog[S04]- .079pH

2CuS + 4H00 = Cu S + HSO" + 7H+ + 6e

Eh = .359 + .OlOlog[HS04“] - .069pH
8.

Cu2S + 4H20 = 2Cu° + S04_" + 8H+ + 6e
Eh = .506 + .OlOlog[SO4]- .079pH

191

9.

Cu_S + 4H_0 = 2Cu++ + SO
+ 8H+ + lOe224
Eh = .438 + .00591og[SO^ ] + .01181og[Cu+*]- .047pH

10.

CUgS + 4H20 = HS04" + 2Cu++ + 7H+ + 10e
Eh = .427 + .00591og[HS04“] + .OllSlog[Cu++]- .0 4IpH

11.

Cu° = Cu
c

++

+ 2e

Eh = .337 + .0291og[Cu++]
12.

2Cu° + HgO = cu20 = 2H+ + 2e
Eh = .47 - .059pH

13.

Cu20 + 2H+ = 2Cu++ + H20 + 2e
Eh = .205 + .059log[Cu++] + .059pH

14.

Cu20 + H20 = 2CuO + 2H+ + 2e
Eh = .67 - .059pH

15.

Cu 2 (0H)2C03 = 2CuO + HCO3” + H+
11.54 = - log [HC03~] + pH

16.

Cu20 + HCO3" + H20 = Cu2 (OH)2C03 + H+ + 2e
EH = .338 - .029pH - .0291og[HCOg-]

17.

Cu20 + H^COg + H^O = Cu2 (0H)2C03 + 2H+ + 2e
Eh = .526 - .0291og[H2C03] - .059pH

18.

Cu4 (OH)6S04 + 2H2C03 = 2Cu 2 (0H)2C02 + S04+ 2H+ + 2H20
7.13 = - log[S0419.

] + 21og[H2C03] + 2pH

Cu20 + S04"" + 4H20 = Cu4 (OH)6S04"" + 2H+ + 4e
Eh = .428 - .0151og[S04] - .029pH

192

20.

4Cu++ + S04"" + 6H20 = Cu4 (0H)gS04 + 6H+
15.3 = 6pH + 4log[Cu++] + log[S04~~]

21.

From

CuO + H20 = Cu02

+ 2H+

32.21 = - log[CuC>2] + 2pHan examination of the diagram for the

Cu-S-C-H^O system (Figure 52), it can be seen that brochantite
occupies a very small stability field to the left of thelarge field of malachite stability.

This small stability

range could account for the minor amounts of brochantite
found at Mapimi.

An increase in cupric ion or sulfate ion

will permit a larger stability field of brochantite.

Suchconditions could be met locally in areas of oxidation ofcopper sulfides.
If the carbonate ion concentration is increased, themalachite field enlarges at the expense of tenorite.

Chal-canthite would only form under conditions of strong oxidationand low pH.

Such an environment would be encountered near

rapidly oxidizing sulfide

masses.

The copper sample briefly discussed in the previous
chapter clearly shows the effect of reducing environment.
The earliest mineral to form was tenorite followed bycuprite, native copper, and covellite.

This sequence

would correspond to a decrease in pH,and a shift in con­ditions from an oxidizing to a reducing environment isenvisioned.

Most commonly the environment becomes moreoxidizing resulting in the reverse paragenetic sequence.

CuO,"

CuO
tenorlte

Gug S

c h a I c oc l t e

1 atm
Total dissolved
Cu++ = 10-3
C

= 10

CuO,--FIGURE 52

Eh-pR diagram of the system

Cu-S-C-H.,0194
The equations for the construction of the
Pb-S-C-HgO system are:1.

PbS = Pb++ + S° + 2e
Eh = .354 + .0291og[Pb++]

2.

PbS + 2H+ = Pb++ + H2S- 7.2 = log[Pb++][H2S] + 2pH

3.

PbS + 4H20 = Pb++ + HS04“ + 7H + 8e
Eh = .342 + .0071og[Pb++][HS04“] - .052pH

4.

Pb++ + HSO ” = PbSO + H+
4
4
pH = 6.0 + log[Pb++] [HS04~~]

5.

PbS + 4H20 = PbS04 + 8H+ + 2e
Eh = .298 - .059pH

6.

PbCO- + S O , " + 2H+ = PbSO. + H_CO_
3
4
4
2 3
2pH = 11.62 - log [H2C03] + log[S04" ]7.

PbS + H9C0 o + 4H-0 = PbCO_ + SO,
Eh = .384 + .0071og[S04+ 10H+ + 8e

] - .0071og[H2C03]- .074pH

8.

PbS + HC03“ + 4H20 = PbC03 + S04+ 9H+ + 8e

Eh = .337 + .0071og[S04” ] - .0071og [HC03~]- .066pH
9.

3PbC03 + 2H20 = Pb3 (OH)2 (C03)2 + HCO^" + H+
pH = 11.86+ log[HC03-]

10. 3PbS + 2HC03“ + 14H20 = Pb3 (OH)2 (C03)2 +3S04"
Eh = .367 + .0071og[S04" ]+ 28H+ + 24e- .0051og [HC03~]- .069pH

195

11.

3PbS + 2C03

+ 14H20 = Pb3 (OH)2 (C03)2 +3S04“" + 26H+ + 24e

Eh = .316 + .0071og[S0412.

Pb3 (OH)2 (C03)2 + H20 = 3PbO + 2C03
55.0 = 4pH - 21og[CO313.

] - .OOSlog[CO-""]- .064pH+ 4H+]

PbS + 5H20 = PbO + 10H+ + SO^+ 8e

Eh = .450 + .0071og[S04""] - .074pH
14.

Pb° + H20 = PbO + 2H+ + 2e
Eh = .252 - .059pH

15.

Pb° + 2H20 = HPb02~ + 3H+ + 2e
Eh = .702 + .0291og[HPb02"] - .089pH

16.

PbS + 4H20 = Pb° + S04““ + 8H+ + 6e
Eh = .518 + .OlOlog[S0417.

] - .077pH

PbO + H20 = HPb02~ + H+
pH = 15.2 + log[HPb02~]

18.

3HPb02" + H+ = Pb304 + 2H20 + 2e
Eh = - .390 - .0871og[HPbO2“] + .029pH

19.

3PbO + H20 = Pb304 + 2H+ + 2e
Eh = .960 - .059pH

20.

pb304 + 2H20 = 3Pb02 + 4H+ + 2e
Eh = 1.127 - .059pH

21.

Pb3 (0H)2 (C03)2 + H20 = Pb304 + 2C03""+ 6H+ + 2e
Eh = 2.469 + .0591og[C03"~] - .177pH

196

22.

Pb3 (OH)2 (CO3)2 + 2H20 = Pb304 + HCO^
+ 5H+ + 4e
Eh = 2.52 + .0151og[HCO3“] - .075pH

23.

Pb3 (0H)2 (C03)2 + 4H20 = 3Pb02 + 2HC03”+ 8H+ + 6e
Eh = 1.411 + .0201og[HC03*"] - .078pH

24.

PbC03 + 2H20 = Pb02 + HC03" + 3H+ + 2e
Eh = 1.527 + .0291og[HCO3"*] - .088pH

25.

H2Saq = S® + 2H+ + 2e
Eh = .141 - .0291og[H2Saq] - .059pH

26.

S® + 4H20 = S04"” + 8H+ + 6e
Eh = .357 + .0101og[SO4““] - .078pH

27.

S°
c + 4H.0z = HSO 4 “ + 7H+ + 6e
Eh = .338 + .0101og[HSO4“] - .069pH

The Eh-pH plot for the Pb-S-C-H20 system. Figure 53,
shows that the field of massicot stability lies in the highpH range of the diagram.

The minor amount of massicot

observed in the district suggests that high pH values were
not frequently attained.
The presence of considerable amounts of plattnerite
indicates that strong oxidizing environment existed.

Thefrequent association of aurichalcite and hydrozincite withplattnerite suggests that the field of stability ofaurichalcite and hydrozincite must partially overlap theplattnerite stability field.

The stability field of smith-sonite does partially overlap the plattnerite field.

197(23) *

PbS04 ©
anglesite

©\

PbCOj

ceru ssite

massicotgalena

25 C

1atm

Total

dissolved
Pb

=

10*6

S

=

10 " 3C=

10-3

HPbO," = 10'2

FIGURE 53

Eh-pH diagram ofthe system, Pb-S-C-H20

198

Hydrocerussite was not detected in the present
study but it can be seen from Equation 9 that an increasein carbonate species would shift the boundary of thehydrocerussite with cerussite to the right (the higher
pH range) and the result of increasing the carbonate
activity in Equation 12 is a shift of the massicot andhydrocerussite boundary to the left, further restricting
the stability field of hydrocerussite.
Williams (1962) mentions that an increase in theactivity species in solution would restrict the hydro­
cerussite stability field to a pH of about 11 and the
direct alteration of cerussite to lead oxides reflects an
infinitesimal rate of reaction from cerussite to hydro­
cerussite as compared with the rate of change of pH, Eh,or
partial pressure of carbonate species.
Temple(1959)

in his study of the Leadhills-

Wanlockhead lead and zinc deposits, noted that brochantite,
CU4(OH)6 (SO4), and caledonite, Pb5Cu2 (S04)3 (CO3 )(OH)g, do
not occur together in the deposits.

He states that linarite

and caledonite are formed from solutions in which lead
predominates over copper, but of the two minerals, only
linarite is formed from solutions in which copper exceeds
lead.

Brochantite is formed under the latter conditions,and is the only mineral formed from solutions deficient in
lead.

The absence of lead-copper sulfates or assemblages

that contain both lead and copper sulfate minerals suggests

199

that the solutions from which the secondary minerals wereformed had either very high or low lead to copper ratios.
The exact lead to copper ratio to give the requisite
conditions is not known.
Another factor that probably limited the number ofsulfate species was rapid increase in the carbonate content
of the solutions.

A comparison of the Eh-pH diagrams for

the Cu-S-C-HgO and Pb-S-C-H^O (Figures 52 and 53) shows thatthe stability fields of brochantite and anglesite coincide.
Thus, a paragenetic sequence such as was found in the
Leadhills-Wanlockhead or Mammouth, Arizona deposits from
galena to anglesite-caledonite-linarite-brochantite would
represent a gradual increase of copper with respect tolead with essentially no change in either the Eh or pH.
The sequence noted at Map ini£, from galena toanglesite and cerussite and eventually to plattnerite
indicates that deposition was controlled by an increaseof pH and, possibly, a decrease of potential.
The following are the equations used to construct
the Eh-pH diagram for the Zn-S-C-H^O system.
1.

S°+ 4H,0= HSO 4 “ + 7H+ + 6e
CZ
Eh = .338 + .OlOlog[HS04“] - .069pH

2.

S” + 4H20 = S04~" + 8H+ + 6e
Eh = .357 + .OlOlog[S04“-] - .078pH

3.

HgS = S£ + 2H+ + 2e
Eh = .142 - .0291og[H2S] - .059pH

200

4.

ZnS + 2H+ = HgS + Zn++ZpH =-4.16 - log[Zn++] - logtH^S]

5.

ZnS = Zn++ + S°
c + 2eEh = .265 + .0291og[Zn++]6.

ZnS + 4H20 = Zn++ + S04~*" + 8H+ + 8e
Eh = .334 + .0071og[Zn++] + .0071og[SO4]- .059pH
7.

Zn++ + HC03" = ZnC03 + H+
.51 = log[Zn++] + log[HCOg ] + ph+

8.

ZnS + HC03"* + 4H20 = ZnCOy + SO^+ 9H+ + Be

Eh = .338 + .0071og[S04~"*] - .0071og [HCO3"]- .066pH
9.

ZnS + CO+ 4H-0 = ZnCO, + SO
+ 8H+ + 8eJZ-34
Eh = .262 + .0071ogtS04"_] - .0071og[C03""]- .059pH
—

10.

11.

—

ZnCO

+ 2H_0 = ZnO+ C0o
+ 4H
3
2
2
3
4pH = 50.50 + log[Zn02 "*] + log [CO3ZnS + 6Ho0
2 = Zn0o2]+ SO 4 + 12H+ + 8e

Eh = .636 + .0071og[Zn02] + .0071og[S04]- .088pH
The Eh-pH diagram for the Zn-S-C-HgO system(Figure 54) is simple, consisting primarily of the ZnS,
ZnCOg, and Zn

fields.

An increase in either zinc ion

activity or carbonate activity would increase the stability
field of the smithsonite at the expense of Zn02

and Zn++.

201

ZnC03
smlthsonite

ZnS^
sphalerite

25°C

1 atm

Total dissolved
Sulfur species=10

Carbonate species= 10

Zinc species= 10

FIGURE 54

Eh-pH diagram of the system, Zn-S-C-H20

202

The hydrous zinc sulfates are formed in a low ph - high Ehenvironment.

As previously mentioned, the association ofplattnerite with hydrozincite and aurichalcite indicatesthat the fields of stability of these three minerals un­
doubtedly overlap.
An increase in either the Zn++ or carbonate ion
activity would shift the Zn++-ZnC0g boundary Equation 7 tothe left and the ZnCO^-ZnOg

boundary Equation 10 to theright greatly increasing the stability field of thesmithsonite.
The composition diagram (Figure 55) taken from Roy
and Mumpton (1956) shows that in the triangle with apicesof sauconite, hemimorphite and water, sauconite andhemimorphite would be compatible phases.

The presence of

these two minerals near the Socavon shaft indicates thatthe chemical composition of the solutions that formed
the minerals must have fallen within the composition
range described.
The plots of the Ag-Cl-H90, Cu-Cl-H 0, and
CU-H2O-O2 systems are presented in Figures 56, 57, and 58,
respectively.

The pertinent equations for their construc­tion follow.

For the Ag-Cl-HgO system:

1.

Ag° + HC1 = AgCl + H+ + e
Eh = .222 - .0591og[HC1] - .059pH

2.

Ag£ = Ag+ + e
Eh = .799 + .0591og[Ag+]SI02

ZihclleFIGURE 55

Composition triangle for the
ZnO-SiO^-HgO system showing
compatible phases (Temp.<210°C.)♦
Dashed lines indicate probableassemblages in the presence ofa deficiency of HLO.
(From Royand Mumpton, 19567)

20b

25 C

1 atm

Total di s s ol ve d
Cl s p e c i e s - to"

A i+ = 10

PH
FIGURE 56

Eh-pH diagram of the system, Ag-Cl-HgO

CuCI
na n t o c k i t e

25 C

1 atm

Total dissolved
Cl s p e c i e s= 10- 4Cu+ + = 1 0 '5

PH
FIGURE 57

Eh-pH diagram of the system, Cu-Cl-HgP

CuO

205

206

CuO
t e n or I te

Cupritenative c o p p e r

Total

dissolved
C u = 10 '4of the system, Cu-I^O-O

3.

AgCl + H+ = Ag+ + HCl
pH = - 9.75 - log[HCl][Ag+]for the Cu-Cl-HgO system:
1.

Cu° + HCl = CuCl + H+ + e
Eh = .128 - .0591og[HCl] - .059pH

2.

Cu° = Cu++ + 2ecEh = .673 + .0291og[Cu++]

3.

CuCl + H+ = Cu++ + HCl + e
Eh = .546 + .0591og[Cu++][HCl] + .059pH

4.

2CuCl + 3H20 = Cu2 (OH)Cl + HCl + 2H+ + 2e
Eh = .928 + .0291og[HCl] - .059pH

5.

CuCl + 2H20 = Cu02

+ HCl + 3H+ + e

Eh = 2.905 + .0591og[CuO2"~][HCl] - .177pH
6.

Cu2 (OH) 3C1 + H20 = 2Cu 02”
4pH = 2log[CuO^

7.

+ HCl + 4H+] + log[HCl] + 6 6 . 7

Cu2 (OH) 3C1 + 4H+ = 2Cu++ + HCl + 3 ^ 0
4pH = - 21og[Cu++] - log[HCl] + 12.95

and for the Cu-H20-02 system:
1.

2Cu° + H20 = Cu20 + 2H+ + 2e
Eh = .47 - .059pH

2.

Cu° = Cu++ + 2e
Eh = .337 + .0291og[Cu++]

3.

2Cu++ + H20 = Cu20 + 2H+ - 2e
Eh = .202 - .0591og[H+] + .0591og[Cu++]

208

4.

Cu^O + H20 = 2CuO + 2H+ + 2e
Eh = .67 - .059pH

5.

Cu++ + H20 = CuO + 2H+
7.92 = 2pH + log[Cu++]

6.

CuO + H20 = Cu02
32.21 = - log[Cu02+ 2H+] + 2pH

From an examination of Figures 56, 57, and 58, one
would predict the association of cerargyrite with nantockite
or cuprite or tenorite.

These associations, however, were

not noted by the author.

The cerargyrite is commonly

associated with either conichalcite or malachite indicating
a greater tendency to form an arsenate or carbonate rather
than the chloride or oxide.
The large stability field of cerargyrite would permit
the association of cerargyrite with almost any of the oxidemineral assemblages described.

The silver and chlorine inmost of the samples of oxide ores can be attributed to thepresence of cerargyrite.
Figures 59 and 60 for the systems Mn-HgO andMn-C-H20 have been constructed using the following
equations.
For the Mn-HgO system:
1.

Mn++ + 2H20 = Mn(OH)2 + 2H+
20.88 = - 1.361og[Mn(OH)2][H+] + 1.361og[Mn++]209

Mn( OH) .

To t a l d i s s o l v
M n - 10PH
FIGURE 59

Eh-pH diagram of the system, Hn-H20

210

Mn C 0 3
r hod o c h r o s l teMn(OH),
2 5° C

1 atm

Total

dissolved

Carbonatespecies= 1 0"3

M n+ + = 1O'4

FIGURE 60

Ehdiagram211

2.

3Mn++ + 4H20 = Mn^O^ + 8H+ + 2e
Eh = 1.82 - .232pH - .0291og[Mn++]

3.

3Mn(OH)2 = Mn304 + 2H+ + 2H20 + 2e
Eh = .466 - .059pH

4.

2Mn++ + 3H20 = Mn203 + 6H+ + 2e
Eh = 1.497 - 1.74pH - .0591og[Mn++]

5.

2Mn304 + H20 = 3Mn203 + 2H+ + 2e
Eh = .842 - .059pH

6.

Mn203 + H20 = 2Mn02 + 2H+ + 2e
Eh = .953 - .059pH

7.

Mn++ + 2H20 = Mn02 + 4H+ + 2e
Eh = 1.225 - .0291og[Mn++] - .116pH

and for the Mn-C-I^O system:
1.

Mn++ + HC03“ = MnC03 + H+
.52 = pH + log[HC03“] + log[Mn++]

2.

MnC03 + 2H20 = Mn(OH)2 + C03
26.24 = 2pH - log[C03

3.

+ 2H+

]

3Mn(OH)2 = Mn304 + 2H+ + 2H20 + 2e
Eh = .466 - .059pH

4.

3MnC03 + 4H20 = Mn304 + 3C03“" + 8H^ + 2e
Eh = 2.78 + .0871og[C03“"] - .232pH

5.

3MnC03 + 4H20 = Mn304 + 3HCC>3” + 5H+ + 2e
Eh = 1.87 + .0871og[HCO3“] - .145pH

6.

2Mn304 + H20 = 3Mn203 + 2H+ + 2e
Eh = .842 - .059pH

212

7.

2MnC03 + 3H20 = Mn203 + 2HC03+ 4H

Eh = 1.53 + .0591og[HCO3~] - .118pH
8.

Mn203 + H20 = 2MnOg + 2H+ + 2e
Eh = .953 - .059pH“-f*+ 2H90 = MnO, + HC03 + 3H +3z^
Eh = 1.24 + .0291og[HC03~] - .088pH

9.

MnCO

10.

Mn++ + 2H20 = Mn02 + 4H+ + 2e
Eh = 1.225 - .0291og[Mn++] - .116pH

Bricker (1965) studied the stability fields ofmanganese minerals in relation to known manganese deposits.
Several of the suites described by Bricker have been
examined by the writer in order to become more familiar
with the optical properties and paragenetic relations.
The Eh-pH diagrams for the manganese species do
not include many of the less common manganese oxide mineralssuch as groutite and ramsdellite because their free energies
have not been determined.

Manganosite, MnO, and bixbyite,

MngOg, are unstable in the supergene environment.

Bricker(p. 1345) shows the stability field of the Y-MnOOH lying
between the Mn02 and MngO^ fields.

Thus, it would occupy

the stability field of bixbyite, which has been described
as being metastable.
Pyrochroite, Mn(OH)2 /Oxidizes rapidly.

Thus, nopyrochroite was found in the strongly oxidizing conditions
encountered in the upper oxidized zones at Mapimi.

213

Hausmannite, MiigO^, is also unstable under strongoxidizing conditions. Bricker has shown that hydrohausmannite is a mixture of 0-MnOOH and MnOg and proposes
the name feitknechtite for the naturally occurring B-MnOOH.
The stability fields of feitknechtite and hydrohetaerolite
probably overlap as predicted by the intimate associationof the two minerals.
Pyrolusite, 0-Mn02, and ramsdellite, MnC>2 , represent
the strong oxidizing environment.

Faulring, Zwicker, andForgeng (1960) show that both birnessite,

G-MnOg, andnsutite, y-MnOg, undergo thermal transformation into e-MnOg
in the presence of high potassium concentrations. The+4+2zabundance of cryptomelane, K(Mn , Mn )8^16 both at Mapimi
and San Pedro a.ttests the high concentrations of potassium.
For discussions of other thermal transformations of various
manganese oxides, the reader is referred to the works of
Fleischer, Richmond, and Evans (1962), Ramsdell (1932),
Klingsberg (1958) , and Grattow and Blemser (1961).
Figure 61 has been taken from Williams(1963b) who

calculated the free energies of austinite, legraridite, andadamite by experimentally determining the equilibrium
constants for the dissociation of the three minerals andthe pH in equilibrium with the dissociated arsenate.

The

free energies for the smithsonite, zincite, calcite, andthe various arsenate ion species are available in work of
Garrels (1960) or Latimer (1952).

C a Z n ( A s 0 4)(OH)
austinite

21k

25 C l a t m
Total

dissolved

Arsenic species
C a r b o n a t e s p e c i e s = 10z i nc i t e

Ca = t oc a l c i t e X-

PH
FIGURE 61

Eh pH diagram of the system, Zn-Ca-As-C - h 2o

215

The large stability field of adamite explains thewidespread distribution of this mineral and its varied
associations.

The stability field of austinite is limited

to the high pH environment but the catalytic effect ofother ions in solution could change the lower pH limit.
No legrandite field is shown in the diagram but Williams
mentions that it is possible to have a narrow field of
stability lying between the adamite and austinite fields
by manipulating the activities of the zinc and arsenate
species.

The rarity of legrandite would seem to emphasize

the fact that it requires a delicate balance of zinc andarsenate activities for stable existence (Williams, 1963b).
Krauskopf (1958) discusses the solubility of gold
and the conditions necessary for its solution and trans­
portation.

The Eh and pH of the present mine waters would

permit a solubility of gold of 10”^ mole/liter (when Cl" =10 ^ mole/liter) as determined from solubility tables
listed in Krauskopf.

He considers the reaction:

2Au + 12H+ + 3Mn02 + 8C1~= 3Mn++ + 2AuCl4 + GHgO
E° = + 0.28, F° = - 38.OKcal, and K to1028.
The positive electromotive force, negative free energychange, and the large equilibrium constant all indicatethat gold is readily soluble in a mixture of MnOg and
1M HC1.

The pH of waters in the past and more specifically,

during the period of oxidation of the massive sulfide
bodies near the surface, was undoubtedly much lower than216

at present, or than was attained in deeper portions ofthat mine, which would have a greater solubility of thegold.

The presence of enriched gold values in the upper

oxidized zones suggests that a fairly low pH was attainedin this environment.
Krauskopf (1958), Emmons (1910), and Brokaw (1913)
discuss the necessity of chlorides for the solubilizationof gold.

Evidence for the presence of chlorine can beseen in the abundance of such minerals as mimetite andpyromorphite.

SOURCE OF ELEMENTS IN THE OXIDATION ENVIRONMENT
The distribution and relative abundance of elements
have long been the subjects of speculation.

The conceptof metallogenic provinces has been discussed by Petrascheck(1965) and Turneaure (1955) .
Any discussion of the origin of mineral deposits
would have to include the factors governing deposition,such as favorability of environment, which would include
structure, lithology, and physical-chemical conditions,and the nature of the ore-bearing solutions.
The distribution and behavior of elements have been
discussed by Clarke (1924), Rankama and Bahama (1950),
Mason (1952) and numerous papers have been written on the
problems of ore transport, solubility, and ionic mobility
at low temperature (Barton, 1957),(Krauskoof, 1951),(Sato, 1959).

(Garrels, 1954),
Thus, the discussion ofprimary mineralization will not be included in the present
study.
Numerous elements are present in the zone ofprimary mineralization and the oxidation and alterationof the primary minerals might liberate elements which could
be transported and deposited in a supergene environment.
The elements found in the primary minerals in quantities
sufficient to account for their abundance in the zone of
217

218

secondary mineralization include the following:

Fe, Zn,Cu, Pb, Sn, Ag, Au,'Bi, Si, Al, K, Na, Ca, Mn, Mo, As, S,
C , Se, and Sb.

Thefee elements are present in the zone ofprimary mineralization, either from the hypogene minerals
or from the wall rock.
The addition of chlorine and oxygen is attributed
to waters of meteoric origin which circulated through theprimary ores and caused the oxidation of these minerals
and the mobilization of their components.
The minerals in the primary zone have been listed
under the elements of which they are composed inTable XXXVIII and the secondary minerals resulting fromthe oxidation of the primary ores are shown in Table XXXIX.
The abundance of arsenopyrite in zone of
hypogene mineralization is reflected in the numerous
arsenate species of copper, lead, iron, and zinc found in
the zones of oxidation.

Many parent sulfides could account,for the sulfate minerals and alteration of boulangeriteand other antimony sulfides would result in the formationof the bindheimite.

The action of meteoric waters on the

carbonate wall rocks probably supplied carbonate ion
species which produced quantities of secondary carbonate
minerals.

The association of secondary silver minerals

with cerussite and anglesite in the zone of oxidation is

219

TABLE XXXVIII
Distribution of Primary Minerals According
to the Elements they Contain
Fe

Cu

Pb

Zn

magnetiteberzelianiteboulangerite

SPHALERITEARSENOPYRITE

borniteclausthalite(tennantite)borniteCHALCOPYRITEGALENA(tetrahedrite)

CHALCOPYRITE

covellite

jamesonite

jamesonite

emplectite

marcasite

enargiteAsboulangerite

PYRITE

stannite

ARSENOPYRITE

jamesonite

pyrrhotite

tennantite

ENARGITE

pyrargyrite

stannitetetrahedrite

tennantite

stibnitetennantite

umangiteSb

tetrahedrite
Caps = major occurrence
underlined = moderate occurrence
not underlined = minor occurrence

tetrahedrite

220
TABLE XXXIX
Distribution of Secondary Minerals According
to the Elements they Contain !
Fe

Cu

Zinc

Pb

delafossite

copper

BINDHEIMITE

chalcophanite

GOETHITE

nantockite

lithargeaurichalcite

hematitecupriteminiumhydrozincite

lepidochrosite

delafossite

murdockiterosasitemolybdic ochre

tenoriteplattnerite

smithsonite

sideriteaurichalcite

CERUSSITE

adamitecopiapite

azuriteanglesite

austinitejarositemalachiteplumbojarosite

descloizite

melanteriterosasite

descloizite

koettigite

plumbojarosite

brochantite

duftitelegrandite

arseniosiderite

chalcanthite

MIMETITE

paradamite

carminite

chenevixite

vanadinite

HEMIMORPHITE

chenevixite

conichalcite

wulfenitesanconite

dussertite

cuprian austinite

ferrimolybdite

descloizite

scoroditeduftitechrysocollaAsadamiteSbarseniosiderite

BINDHEIMITE

carminitestibiconite

chenevixite

valentinite

conichalcite
duftitedussertite
koettigite
legrandite
MIMETITE
paradamite
scoroditeCaps « major occurrence
underlined = moderate occurrence
not underlined = minor occurrence

221

undoubtedly the result:of the alteration of theargentiferous galena.
The incorporation into crystalline phases of the
carbon dioxide liberated by the reaction of the intrusives
with the limestones may have resulted in the deposition ofcarbonate minerals in the upper zone of primary low
temperature veinlets.
It is difficult to speculate as to the amountof molybdenite that was present in the primary ores.
Wulfenite is common and the amount of the molybdenite
present would probably have been sufficient for theformation of wulfenite.

The source of molybdenum has been

considered by Williams (1962) for environments in whichmolybdenite is only a minor phase or is totally absent.
He points out that since small amounts of molybdenum arepresent in groundwater and normally is not fixed by rock­forming cations, it would tend to remain in solution until
it could be fixed by lead under conditions of oxidation.
Vanadates are not common at Mapimi; only minor
amountof vanadinite and descloizite have been observed

and there appears to have been little substitution of
vanadium for arsenic in the arsenate minerals.

Spectro-graphic analyses of biotite and hornblende from theintrusives in the region showed that these minerals
contain up to approximately 700 ppm of vanadium.

The

shales contain about 50 ppm and traces of vanadium were222

also detected in the limestones (limit of detection forvanadium ~5 ppm).

Even though the vanadium is present

in abundance in the intrusive, its mobility in groundwater
is not certain.
In general, the diversity of mineral species inthe zone of secondary mineralization at Mapimi should beconsidered with regard to the number of elements present
that are available to form mineral species.

Many minerals

which were expected to be found were not present.

Theabsence of these minerals merely indicates that conditions
for their deposition and stability were never met or
maintained.

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Villarello, J. D., 1909, The Mapimf District:
Vol. 31, pp. 62-63.

Mining World:

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Nauk SSSR, Vol. 75, pp. 559-561.

104c00'
2 6° 0 O'

EXPLANATION133
S E DIME N T A RY

ROCKS

r
Qo I

c
o>

Alluvium<

Valley fill, stream deposits, talus

CO
O

Ot

V

r

Terrace Alluvium

Unconsolidated and caliche -cemented gravel

To
Ahuichila Formation

Conglomerate, limestone, s i It stone, tuff,
bentonite, gypsumr

UNCONFORM ITY

3

o

Kcl

o
re

Caracol(?) Formatione> <oHornfels, shale

Ki

V

r

j
X,

I noioura Formation

Shale, limestone

U N CONFORM ITY

Comanche S eries

Kco, Undivided; Kcc, Cuesta del Cura limestone -,Ka, Aurora limestone; Kip, La Pena formationOimestone)

L,

V

^

C oahuila S e r i e s

Kcs, Undivided; Kco, Cupido limestoner
L a Casita F ormation

Sandstone, shale, dolomite- <v

Zuloaga Group

Includes La Gloria formation (sandstone)
and Zuloaga limestone

IG N EO U S

ROOKS

Tv
VOLCANICS

Rhyolite and andesite flows, tuff, breccia

T,____ LINTRUSIVES

Granite, quartz monzonite, diorite,
diabase, rhyolite

SYMBOLSCONTACT--- 1____ *H i g h- a n g l e

fau ltDashed where approximate ; dotted where concealed;
dum bell on downthrown sideS J R I K E AND DIP OF

BEDS

VerticalB eds

str ikeS t ri ke andof

Dip of

Mine or

Overturned Beds

Q uarry-N-

10 K i l o m e t e r s
M odifiedfrom

C lem o n sandM c L e r o y ,

1962

SCALE

G E O L O G I C M A P O F A P O R T I O N OF T H E T O R R E O N Q U A D R A N G L E , D U R A N G O , M E X I C O S H O W I N G T H E L O C A T I O N O F T H E M I N E S O F T H E M A P I M I D I S T R I C Tim^•tfcslerllt

2I*C 1
T ilS l l l W H i #C i r l m l e *#####*

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