GEOLOGICAL AND GEOCHEMICAL EXPLORATION CHARACTERISTICS OF MEXICAN TIN DEPOSITS IN RHYOLITIC ROCKS

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Authors Lee Moreno, José Luis

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LEE MORENO, Jose Luis, 1939- GEOLOGICAL AND GEOCHEMICAL EXPLORATION CHARACTERISTICS OF MEXICAN TIN DEPOSITS IN RHYOLITIC ROCKS.

The University of Arizona, Ph.D., 1972 Geology

| University Microfilms, A XEROX Company, Ann Arbor, Michigan i

TfflS DISSERTATION HAS BEEN MICROFILMED EXACTLY AS HECEIVED. GEOLOGICAL AND GEOCHEMICAL EXPLORATION CHARACTERISTICS

OF MEXICAN TIN DEPOSITS IN RHYOLITIC ROCKS

by

Jose Luis Lee Moreno

A Dissertation Submitted to the Faculty of the

DEPARTMENT OF MINING AND GEOLOGICAL ENGINEERING

In Partial Fulfillment of the Requirements For the Degree of

DOCTOR OF PHILOSOPHY WITH A MAJOR IN GEOLOGICAL ENGINEERING

In the Graduate College

THE UNIVERSITY OF ARIZONA

19 7 2 THE UNIVERSITY OF ARIZONA

GRADUATE COLLEGE

I hereby recommend that this dissertation prepared under my direction by Jose Luis Lee Moreno entitled Geological and Geochemical Exploration Characteristics

of Mexican Tin Deposits in Rhyolitic Rocks be accepted as fulfilling the dissertation requirement of the degree of Doctor of Philosophy

f A/w- ^ /^// Dissertation Director Date

After inspection of the final copy of the dissertation, the following members of the Final Examination Committee concur in its approval and recommend its acceptance:'"

Ccy.

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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. PLEASE NOTE:

Some pages have indistinct print. Filmed as received.

University Microfilms, A Xerox Education Company STATEMENT BY AUTHOR

This 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 bor­ rowers 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 re­ production of 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 use of the material is in the in­ terests of scholarship. In all other instances, however, permission must be obtained from the author. ACKNOWLEDGMENTS

The author wishes to thank Drs. W. C. Lacy and W. C. Peters of the Department of Mining and Geological Engineering, The University of Arizona, for their valuable advice and critical review of this paper.

Reading of the manuscript by Drs. J. F. Abel, J. M. Guilbert, and

J. S. Sumner also proved to be of great value.

The field and laboratory work were done as a part of a project sponsored by the Mexican Government through the Consejo de Recursos

Naturales No Renovables. The author especially wishes to thank Ing.

Guillermo P. Salas, Director General of this organization, for his sug­ gestions and interest in the development of this study. Thanks are also accorded to Ing. Ruben Pesquera V., Exploration Manager of the Consejo de Recursos Naturales No Renovables.

Most of the trace element analyses were done by the author at the Department of Geochemistry of the Consejo de Recursos Naturales

No Renovables in the city of San Luis Potosi, and some were done by the

Bureau de Recherches Geologiques et Minieres of France. The Physical

Chemical Laboratories of the Consejo de Recursos Naturales No

Renovables did all the chemical and some petrographic analyses of the

Mexican rocks studied. Mr. John L.Lufkin of the Geology Department,

Stanford University, kindly made available the results of the chemical analyses and petrographic descriptions of some rock specimens from the

Black Range tin district of New Mexico.

iii The encouragement of my wife Alicia and the happy presence of my daughter Lori contributed greatly to the completion of this work. TABLE OF CONTENTS

Page

LIST OF ILLUSTRATIONS vii

LIST OF TABLES viii

ABSTRACT ix

INTRODUCTION 1

Objectives 1 History of the Area 3 Location . 3 History 4

GEOLOGIC STUDY 8

Regional Geology 8 Local Geology 9 Tuffs and Ignimbrites 10 Breccias 10 Flow-banded Rhyolites 11 Multiple Linear Regression Analyses 12 Juan Aldama District 12 Sierra de Pinos 23 Tin Deposits and Districts Examined 24 America-Sapioris District 24 Avino Area 29 La Barrosa Mine 29 Los Remedios Mine 30 El Naranjo Mine 30 Juan Aldama District 32 Sierra de Pinos 39 Mineralogy and Origin 45 Mineral Species 45 Solution Chemistry 50 The Emplacement of Rhyolitic Rocks 52 Apparent Geologic Controls 54

PRODUCTION TECHNOLOGY 57

GEOCHEMICAL STUDY 60

Orientation Survey 60 Tin in Natural Surface Waters 60

v vi

TABLE OF CONTENTS —Continued

Page

Tin in Vegetation 62 Tin in Stream Sediments 63 Tin in Soils 66 Trace Elements in Mineralized Rocks 71 Trend Surface Analysis at Juan Aldama 74 Geochemical Experiment at Sierra de Pinos 80 Stream Sediment Survey 84 Soil Survey "B" Horizon 84 Soil Survey "C" Horizon 85 Trend Surface Analysis 86 Diamond Drilling at the Mina Nueva Anomaly 95

CONCLUSIONS 98

SELECTED BIBLIOGRAPHY 104 LIST OF ILLUSTRATIONS

Figure Page

1. Index Map of States of Zacatecas and Durango Showing Location of Tin Bearing Areas in pocket

2. Geologic Map and Stream Sediment Geochemical Survey of Sierra de Pinos, Zac., Mexico in pocket

3. Location Map of Mesa del Venado in the Juan Aldama Tin District 34

4. Geologic Map and Location of Tin Mining Works of Mesa del Venado in Juan Aldama, Zac 75

5. Hand-contoured Map of Observed Tin Values in Soils of Mesa del Venado 79

6. First Order Contour Maps of Mesa del Venado 81

7. Second Order Contour Maps of Mesa del Venado 82

8. Third Order Contour Maps of Mesa del Venado 83

9. Geochemical Soil Survey "B" Horizon Area Picacho-Adjuntas in pocket

10. Geochemical Orientation Survey on the Area of Mina San Jose 87

11. Geochemical Orientation Survey in the Area of Mina Nueva 88

12. Geologic Map and Location of Tin Mining Works of Sierra de Pinos 90

13. Hand-contoured Map of Observed Tin Values in Soils of Sierra de Pinos 91

14. First Order Contour Maps of Sierra de Pinos 92

15. Second Order Contour Maps of Sierra de Pinos 93

16. Third Order Contour Maps of Sierra de Pinos 94

vii LIST OF TABLES

Table Page

1. Tin Production in Mexico, 1800-1969 5

2. Chemical Analyses of Rhyolitic Rocks from Various Mexican Tin Districts 13

3 . Chemical Analyses of Rhyolitic Rocks from the Black Range Tin Area in New Mexico 14

4. Chemical Analyses of Tin-barren Mexican Rhyolitic Rocks 15

5. Description of Rock Samples Included in Table 2 .... 16

6. Descriptions of Rock Samples Included in Table A .... 20

7. Average Chemical Composition of Some Tin- bearing and Barren Mexican Rhyolitic Rocks 25

8. Trace Element Analyses of Various Cassiterite Samples from the Sapioris District 47

9. Distribution of Tin in Different Size Fractions of Soils, Stream Sediments, and Banks from Durango and Zacatecas 64

10. Distribution of Tin Values Downstream from La Barrosa Mine 66

11. Mineralogical Composition of Alluvial Soil Samples from Various Tin Districts 70

12 . Trace Element Content of Mineralized Rocks of Mesa del Venado, Juan Aldama, Zacatecas 72

viii ABSTRACT

The study program was undertaken in central Mexico with the following three main objectives:

1. To investigate the occurrence of tin in rhyolitic rocks in

Mexico to make some contribution toward an understanding of

the origin and economic significance of this type of deposit,

2. To define certain diagnostic criteria to guide future exploration

work in this environment, and

3. To determine and compare the relation between the presence of

tin and of other elements and between the presence of tin and

the chemical composition of the host rock.

The work included geologic observations of many tin prospects of the States of Durango and Zacatecas and geochemical investigation of the response obtained from various materials derived from them.

Tin occurs in the rhyolitic environment in three main types of deposits:

1. In vein deposits in tuffs and rhyolites which are commonly nar­

row but which occasionally contain small ore shoots yielding a

few hundred kilograms of tin ore with grades from 20 to 30 per­

cent. These deposits are usually mined in a primitive fashion

by local miners, called "gambusinos ." Exploitation of this

type of ore body would not be economic under any other mining

system.

ix X

2. Epigenetic disseminations in tuffaceous lenses. These deposits

usually have large tonnages and low grades. Here the cas-

siterite is found filling lithophysae and any other empty space

available. Due to lack of either tonnage or grade in the known

deposits of this type, there is not a single one being mined in

Mexico. However, it is thought that some deposits might exist

with the required tonnage and minimum grade to permit exploita­

tion by open pit methods.

3. Void fillings in brecciated tuffs and rhyolites. These deposits

are similar to those in tuffs, except that their morphological

constitution is different.

All deposits examined during this study are of small extent, suggesting that mineralizing solutions were not abundant in these local- itie s.

A multiple regression analysis of the chemical components of barren and productive rhyolites shows that only Si02 and AI2O3 seem to be slightly (but not significantly) more abundant in barren than in tin- bearing rhyolites. Si02 anc* CaO content seems to be related directly to tin content in mineralized rhyolites. Zinc and gallium show a good direct correlation with tin in a group of 12 mineralized rock samples . Lead and copper do not seem to have any conclusive correlation. Fluorine content is very high in all of the examined samples, but no correlation with tin content seems to exist. Ytterbium and yttrium are also found to have some correlation with tin, but the meaning of this is not yet clear.

Stream sediment geochemical reconnaissance detected most significant targets but failed to disclose properly the mine of El Huiza-.he. This is thought to be due to the highly silicified matrix of the ore at this mine, which did not allow cassiterite to be liberated into the stream, as well as to the weakness of the local erosional activity.

In practice, positive stream sediment anomalies should be investigated, but lack of anomalous values does not necessarily mean lack of tin deposits.

Residual tin values in the soil over any of the deposits found in this environment are generally erratic, although, in all instances, they reflected the presence of a bedrock source.

Metallurgical methods for the recovery of very fine grained cassiterite have not been successful in Mexico. Development of any new large-tonnage, low-grade tin deposits in Mexico will be largely dependent on the discovery of a suitable metallurgical method to extract the tin values.

Well-planned geochemical prospecting may assist greatly in the exploration of tin deposits associated with rhyolitic rocks. However, this approach may be misleading, if applied loosely and without ade­ quate scientific control. INTRODUCTION

The classical goal of geochemistry, in the words of V. M. Goldschmidt (1954), is "to discover the laws which control the distribution of the individual elements ." For many of the rare metallic elements the most striking fact of distribu­ tion is their local concentration into the rocks that we call ores. The laws controlling this extreme concentration are most elusive, as economic geologists discovered long ago. In the pursuit of these laws, where geochemistry ana economic geol­ ogy have a common purpose, the contributions of geochemists have been illuminating but hardly definitive.

Konrad B. Krauskopf (1970) The Source of Ore Metals

Oblectives

Tin deposits in Mexico occur along a well-defined belt that extends northwest-southeast from Sonora to Oaxaca and includes ap­ proximately one thousand individual prospects scattered throughout 19 different states of the Republic of Mexico. Most of them are located in three central states, Durango, Zacatecas, and Guanajuato.

Typically, these deposits occur in smail veins in Tertiary acidic lavas and in their resulting placers. The deposit of Quadalcazar,

San Luis Potosi, is the only one known in Mexico to contain tin asso­ ciated with granitic rocks.

Tin consumption in Mexico has largely depended on imports from the United States and Bolivia., Total imports of this metal have in­ creased steadily from 198 tons in 1962 and 1527 tons in 1966 to 2899 tons in 1969. On the other hand, production of domestic tin has de­ creased from 1164 tons in 1964 to 308 tons in 1966 to only 498 tons in

1969.

1 2 Although there are many tin deposits being worked in Mexico, they are small. The largest has 150,000 proven tons of ore with grades no higher than 0.4% tin. Most tin deposits in Mexico are worked by gambusinos (miners who use rudimentary mining and metallurgical methods), mostly by hand-sorting methods, which yield poor recoveries.

It is interesting to note, however, that all of the tin mining activities are concentrated within areas perfectly delimitable that have never been the subject of well-planned scientific exploration.

Two basic premises underlie the present study: (1) Mexican tin imports are on the increase, while production has decreased; and (2) there exists a well-defined tin province in Mexico which has produced considerable amounts of this metal but which has never been explored by modern scientific methods. This study was planned as an orientation survey for future explorations for tin in Mexico. The main objectives of the work are as follows:

1. To investigate the occurrence of tin in rhyolitic rocks in

Mexico, in order to try to make some contribution toward an

understanding of the origin of this type of deposit and its

economic significance.

2. To define diagnostic criteria for future exploration work in this

environment.

3. To determine the relation (if any) between the presence of tin

to other elements and to the chemical composition of the host

rock.

For convenience, the work was divided into two main stages.

One included the sampling and reconnaissance of various known tin 3 districts in Durango and Zacatecas; the other, specific studies of the tin-producing areas of Juan Aldama and Sierra de Pinos in the State of

Zacatecas.

History of the Area

Location

The area selected for the regional reconnaissance includes a

southern portion of the State of Durango and a western part of Zacatecas.

It contains tin deposits scattered over an area of approximately 40,000 square kilometers (Fig. 1, in pocket) . The town of Juan Aldama is lo­ cated in the northwestern part of the State of Zacatecas, approximately

156 km south of the city of Torreon on paved highway 49. The mining district of Juan Aldama begins 3 km southeast of the town and extends through most of the Sierra de Flores along a belt of 15 by 8 km, broadly bordered by parallels 24°05' and 24°20' N. and meridians 103°10' and

103°25' W. The area chosen for a trend surface analysis is located on the northwestern part of the Sierra (which shows more evidence of min­ eralization) on a grid 1000 by 1000 meters. This area is accessible year round via highway 49, which connects San Luis Potosi, S.L.P. and Ciudad Jimenez, Chih. A branch of the National Railways System between Durango and Felipe Pescador passes by the town of Rio Grande,

Zac., 40 km south of Juan Aldama. In this town there is a dirt landing strip for light aircraft, which is generally well maintained.

Most of the prospects included in our reconnaissance are a few hours drive from the city of Durango, mostly by dirt roads, which are 4 open to all traffic during the dry season and limited to four-wheel-drive vehicles and pickup trucks during the rainy season.

Another trend surface analysis was done on soils of the south­ east portion of Sierra de Pinos, which is located northwest of the city of San Luis Potosi, S.L.P. It has an approximate extension of 40 by 15 km. Its western part has produced quantities of precious metals minerals since Spanish times. Its eastern portion has produced small amounts of tin during the present century.

The tin area of Sierra de Pinos can be reached from the city of

San Luis Potosi, S.L.P., on paved highway 49 to Zacatecas, south at kilometer 70, a wide dirt road 30 km long leads to the town of Pinos,

Zac. From Pinos, there are several narrow dirt roads east to the tin areas. These are passable by four-wheel-drive vehicles during the rainy season and by any field car during the dry season.

History

Tin has been produced in Mexico since the times of the con- quistadores (16th century). Hanks (1876) reports tin mining by the

Spaniards in the America-Potrillos district of Durango as early as 1790.

Ingalls (1896) briefly reviews tin mining during the nineteenth century but indicates that production did not exceed 1500 tons of metallic tin.

Foshag and Fries (1946) quote "Boletin Minero" ana "Boletin de Petroleo y Minero" on Mexican tin production from 1916 to 1942. Table 1 shows the production of tin in Mexico from 1800 to 1969, compiled from various sources but mainly from publications by Consejo de Recursos Naturales

No Renovables (C.R.N .N .R.). 5

Table 1. Tin Production in Mexico, 1800-1969

Year Tons (Metric) Year Tons (Metric)

1800-1915 1,000.000 (Estimate) 1937 379.480

1916 0.292 1938 255.458

1917 9.214 1939 293.910

1918 13.537 1940 350.729

1919 1.588 1941 216.327

1920 No record 1942 370.248

1921 0.492 1943 433.532

1922 No record 1944 322.024

1923 No record 1945 177.736

1924 8.849 1946 267.817

1925 1.033 1947 174.422

1926 2.224 1948 185.304

1927 No record 1949 364.570

1928 No record 1950 368.031

1929 No record 1951 289.058

1930 269.560 1952 419.413

1931 773.171 1953 483.482

1932 751.430 1954 354.630

1933 120.378 1955 614.719

1934 15.829 1956 507.887

1935 630.749 1957 481.263

1936 373.476 1958 439.000 6

Table 1. Tin Production—Continued

Year Tons (Metric) Year Tons (Metric)

1959 369.000 1965 466.000

1960 371.000 1966 808.000

1961 487.000 1967 597.000

1962 528.000 1968 528.000

1963 1,072.000 1969 498.000

1964 1,164.000

Total 18,181.390

1800-1940 Compilation by Foshag and Fries (1946).

1941-1949 Gonzalez Reyna (1956).

1950-1959 Consejo de Recursos Naturales No Renovables (1963) .

1960-1966 Consejo de Recursos Naturales No Renovables (1967) .

1967-1969 Secretaria del Patrimonio Nacional, Public files. 7

Little has been published on tin in Mexico. The most compre­ hensive work is by Foshag and Fries (1946) . These authors compiled existing data and complemented it with some of their own field observa­ tions. The Consejo de Recursos Naturales Mo Renovables has published some bulletins with information on various tin fields. Fries and Schmitter

(1948) made a detailed study of the placer deposits of Guadalcazar,

S .L .P ., and of the general geology of that area.

Smith, Guiza, and Segerstrom (1957) published a short paper on selected tin deposits of Durango and analyzed the economic potential of each. Finally, Bracho (I960, 1961) presented data on some tin areas of the States of Durango ana Zacatecas. His work includes brief geo­ logic descriptions and assay data on the mines of Sierras de Chapultepec and Juan Aldama in Zacatecas, La Ochoa in Durango, ana Cosio in

Aguascalientes. Other papers have been published that make reference to tin deposits in Mexico but do not specifically deal with them.

It appears that, although some attention has been given to tin deposits in Mexico, the efforts to increase production of this metal have apparently been unsuccessful. It is possible—unless new metal­ lurgical methods for the recovery of tin from low-grade ores are developed

—that Mexico will continue to depend on imports to cover its tin needs. GEOLOGIC STUDY

Regional Geology

All the tin deposits visited for this study are in rhyolitic rocks.

The rhyolites are usually layered and are usually interbedded with tuffs and breccias of the same composition. In some places they rest directly over green andesites, possibly belonging to the Miocene Palmas forma­ tion. In others, they overlie a sedimentary formation constituted by thin layers of sandstones with small lenses of shales at the bottorr •; a red conglomerate formed by fragments of metamorphosed Paleozoic -ediments and of volcanic rocks at the top. This formation probably corresponds to the "Conglomerado Mazapil" of the Concepcion del Oro district.

In the Juan Aldama and Pinos areas the rhyolitic rocks lie dis­ cordantly over a thick sequence of limestone and clay sediments of pos­ sible Late Cretaceous age, which may correlate with the Caracol and

Indidura Formations of the Zacatecas mining district. The first consists of devitrified tuffs, shales, and limestones, and the second of laminated argillaceous limestone with thick interbedded horizons of shale.

The age of the rhyolitic rocks of Mexico is not precisely known.

From stratigraphic correlation, Ordonez (1900) assumed them to be of

Miocene to Pliocene age, since the green andesites and the red con­ glomerates that often underlie the rhyolites are considered to have been emplaced during late Miocene times. It is also thought that the rhyolitic rocks range from Miocene to Pliocene because they lie undisturbed

8 9 directly over rocks, such as the andesites and conglomerates) that were affected by late Mesozoic and early Cenozoic tectonic movements.

The age of the Mexican rhyolites is, thus, about the same as that of the tin-bearing rhyolites of the Black Range area in New Mexico, reported by Kottlowski, Weber, and Willard (1969) to range from 26 to 37 million years, making them Oligocene in age.

Local Geology

In general, the sequence of rhyolitic rocks of the Mexican tin districts consists of a layer of porphyritic rhyolite at the base directly overlying the various older rocks mentioned above. These rhyolites are usually massive and show little banding. They are overlain by a horizon of pyroclastic rocks commonly layered, with various degrees of consoli­ dation, some parts remaining almost unconsolidated and others slightly or thoroughly welded. Tuffs and ignimbrites are the two most common rock types of the pyroclastic horizon.

In the upper part of the sequence there is a flow -banded por­ phyritic rhyolite much harder than the underlying pyroclastics. The transition from soft to hard rock commonly results in the formation of small cliffs or benches along these contacts. Columnar structures are often seen on the faces of these cliffs.

All through the sequence, but mostly in the pyroclastic horizon, there are bodies of rhyolitic breccia. It is also common to find dikes of vitrophyre scattered irregularly through the tin areas.

Figure 2 (in pocket) is the geologic map of a portion of the

Sierra de Pinos which has the typical sequence of the rhyolitic rocks in

Mexico. 10

Tuffs and Ignimbrites

The tuffs and ignimbrites are typically of a light-pink to gray color, but on occasions they show reddish or purplish shades. They contain quartz, feldspars (commonly sanidine and oligoclase) , devitrified glass, specularite, and small rhyolite fragments. Very rarely they carry minute amounts of zircon, opal, tridymite , cristobalite, and topaz.

These last minerals are thought to have been introduced in the rock after its consolidation.

The pyroclasties are highly porous and contain a great number of lithophysae; these cavities range in size from 1 mm up to 10 cm (the largest ones were observed in the Potrillos district in Durango) and very often they have in their interior many fine concentric bands or sheets of white to pink color with a pattern like the petals of a rose. Under the microscope, these fine sheets seem to be formed by a central mass of tridymite and possibly orthoclase with many gas voids. A layer of very small and almost perfect acicular and bipyramidal quartz crystals covers the central tridymite mass. Other materials which were also observed filling the lithophysae were cassiterite, specularite, calcedony, opal, and cristobalite. Rarely, the lithophysae were void.

Breccias

Rhyolitic breccias are important in Mexican tin districts be­ cause they often contain tin mineralization. They consist of fragments of ignimbrites, tuffs, or porphyritic rhyolite in sizes that range from a few centimeters to three meters. In the mineralized areas, the breccias show intense kaolinization, jasperization, hematitization, and some silicification. When present, cassiterite appears surrounding the brec­ cia fragments or in minute fractures within the rock fragments.

The breccias observed seem to be of three different kinds:

1. Talus breccias formed by the cementation of fragments of rhyo-

litic rocks broken up by erosion processes. These bodies very

rarely have any economic importance, and the small amount of

tin that they may bear is only that derived from small veins in

the parent rock. A good example of this formation exists in the

Mesa del Venado in Juan Aldama, Zacatecas.

2. Breccias formed at the edge of rhyolitic flows as a result of the

accumulation of material scraped from the lava paths are easily

recognizable because their fragments are of heterogeneous

materials and commonly include blocks of the underlying rocks.

3. Tectonic breccias formed either along single structures or at

the intersections of two or more faults. Grabens and horsts

also result in the formation of these bodies. The breccias of El

Huizache, La Boveda, and El Granate mines in Sierra de Pinos

are of this type.

Flow-banded Rhyoiites

These rocks occupy the upper levels of the volcanic sequence.

They are mostly flow banded and porphyrinic, but there are also areas where fine-grained rhyoiites exist, which are reddish to pink when fresh and change to lighter colors, including white, where altered. Their phenocrysts are mainly sanidine and quartz, which make up to 25 per­ cent of the rock. The groundmass is mostly composed of plagioclase and quartz with small percentages of magnetite and a green mineral 12

(possibly chlorite), which seems to have formed from alteration of horn­ blende. In some areas, particularly near mineralization, the silica appears as tridymite and cristobalite; specularite is also commonly present and usually seems to be genetically related to the presence of cassiterite. Small amounts of fluorite were observed in Juan Aldama and in some areas of the America-Sapioris district. Lithophysae are also present in many rhyolites, but they are not as abundant as in the tuffs.

Even though fracturing and faulting do not follow a distinctive trend, there seems to be a dominant northeast orientation for the struc­ ture of these rocks. A secondary northwest trend also seems to appear.

Multiple Linear Regression Analyses

In general, the rhyolitic rocks of the Mexican Plateau contain more than 70 percent of SiC^, although a few samples analyzed had less than that amount. Table 2 shows the results of chemical analyses of several samples of Mexican tin-bearing rhyolites. Table 3 shows the results obtained from rhyolitic rocks from the Black Range tin district of

New Mexico. As a comparison, Table 4 also includes the results of chemical analyses of barren rhyolitic rocks of Mexico. Tables 5 ana 6 summarize the rock descriptions and mineral compositions of the samples included in Tables 2 ana 4, respectively.

Juan Aldama District

Using the information obtained from the analyses of the Mexican rocks, multiple regression analyses were made, using a computer pro­ gram originated from group class work at The University of Arizona and Table 2. Chemical Analyses of Rhyolitic Rocks from Various Mexican Tin Districts

(Analyses by Consejo de Recursos Naturales No Renovables)

Chemical Composition (%)

Sample No. and Location Sn S1C>2 AI9O3 Fe2C>3 FeO MnO MGO CaO K2O NazO TIO2 P2O5 BaO s co2 H2O

No. 1 Sierra de Pinos 0.0072 70.3 11,.7 4.3 2.3 0.18 tr 0.8 2.85 0.32 — 0.19 0 .0 0.0 1.1 0.57

No. 2 Sierra de Pinos .0121 66.2 11,.4 4.3 1.8 0.24 1.41 2.5 1.58 3.50 — .17 .0 .0 4.3 1.68

No. 3 Sierra de Pinos .0069 73.3 12.7 4.7 2.7 0.18 tr 0.8 2.56 4.82 — .80 .0 .0 0.6 0.71

No. 4 Sierra de Pinos .0060 74.0 10.8 6.1 2.4 0.18 tr 0.7 1.99 3.57 — .36 .0 .0 .6 .60

Mo. 5 Sierra de Pinos .0071 72.0 10,.5 6.2 2.6 0.24 tr 1.0 1.89 1.43 — .11 .0 .0 1.0 .50 lio. 6 Sifcrra de Pinos .0059 73.6 9.6 5.2 1.3 0.30 tr 0.7 1.55 1.97 — .12 .0 .0 1.0 .50

No. 7 Sierra de Pinos .0060 71.7 11,.3 0.3 2.6 0.36 tr 1.0 1.99 1.20 — .34 .0 .0 0.7 .40 No. 8 £1 Naranjo, ZdC. .0087 70.7 15,.0 3.7 1.0 tr 0.22 0.9 4.52 2.22 0.17 tr .0 .0 .9 tr No. 9 £1 Naranjo, ZaC . .0148 71.4 13,.6 1.2 1.2 tr .30 2.0 2.29 1.55 .05 0.0 .0 .0 .5 0.70

No. 10 El Inliernito Sapiorls .0066 74.8 14,.1 2.3 0.4 tr .20 0.5 2.51 2.SI .0 .0 .0 .0 .0 .20

No. 11 C. de Elena, ]. Aldama .0060 74.2 13,.7 1.5 1.2 tr tr tr 3.50 3.03 .09 tr .0 .0 .4 .30 No. 12 M. de Vacas, ]. Aldama .0068 67.2 10,.3 6.6 0.8 tr 0.14 0.8 4.66 3.16 .04 0.18 .0 .03 .6 .05 No. 13 Durjzrio, J. Aldama .0041 64.3 12,.3 4.4 .5 tr .15 1.3 3.00 3.40 .05 .54 .0 .0 6.8 1.71 No. 14 C. di: Bartolo, ]. Aldama .0138 73.3 12,.6 3.9 1.3 tr .15 1.3 3.2 2.56 .065 .0054 .0 .0 0.0 0.09 No. 15 C. de Bartolo, ]. Aldama .0049 73.8 13,.6 2.3 1.4 tr .12 O.S 4.56 1.95 .055 .0047 .0 .0 .35 .13 No. 16 C. de Elena, ]. Aldama .0058 72.5 14 .9 2.1 1.28 tr .15 .6 3.02 1.62 .060 .0013 .0 .0 .4 .07 No. 17 £1 Naranjo, Zac. .0047 70.6 13 .2 4.2 0.64 tr .28 .6 1.45 3.03 .6 .54 .0 .001 1.08 .70 Table 3. Chemical Analyses of Rhyolitic Rocks of the Black Range Tin Area in New Mexico (Data courtesy of Mr. John L. Lufkin. Analyses by New Mexico Bureau of Mines.)

Chemical Composition (%) impie 3. Si02 AI2O3 Fe203 FeO MnO MgO CaO NA20 K2O H2O P2O5

1 75.7 12.7 1.06 0.12 0.053 0.62 4.86 3.65 3.8 0.29 0.009 2 74.4 12.0 1.91 0.05 0.077 1.85 1.67 2.46 3.9 0.14 0.009 3 76.0 12.4 2.06 0.07 0.059 0.52 0.46 3.81 4.1 0.26 0.006 4 72.8 12.8 1.71 0.08 0.070 0.33 0.95 3.93 4.2 0.20 0.049 5 75.5 12.5 1.81 0.07 0.066 0.45 0.56 3.84 4.3 0.12 0.056 6 73.3 12.1 5.02 — 0.163 0.25 0.50 3.48 4.6 0.10 0.051 7 75.2 12.8 2.12 0.06 0.049 0.33 0.39 4.06 4.1 0.18 0.105 8 78.4 11.21 1.59 0.05 0.044 1.07 1.05 5.05 4.1 0.61 0.023 9 75.7 12.7 2.10 0.07 0.077 0.17 0.33 5.20 4.5 0.11 0.014 10 77.1 11.8 1.50 0.06 0.049 0.33 1.31 4.43 4.1 0.21 0.002 11 75.0 11.9 1.47 0.04 0.053 0.55 0.40 3.55 6.4 0.13 0.008 12 71.2 10.7 0.92 0.08 0.057 0.67 2.16 2.03 2.4 2.81 0.002

Rock descriptions: 1. Reddish vitrophyre with sanidine, quartz, plagioclase in spherulitic glass matrix. 2. Tuff breccia 3. Flow-banded rhyolite 4. White, argillized flow-banded rhyolite, sanidine, quartz, devitrified groundmass 5. Vesicular rhyolite 6. Vesicular rhyolite, hematite, pseudobrookite, and monazite 7. Flow-banded rhyolite 8. Flow-banded rhyolite 9. Argillized white rhyolite 10. Gray porphyritic rhyolite 11. Argillized facies of gray rhyolite (sample 10) 12. Vitric tuff. Table 4. Chemical Analyses of Tin-barren Mexican Rhyolitic Rocks

(Analyses by Consejo de Recursos Naturales No Renovables)

% % % % 9//o % % % % % LxC @ % % PPm ppm No. SiC>2 AI2O3 Fe2C>3 FeO Ti02 CaO MgO CO Na20 K2O 900°C H2O P2O5 MnO Sn

1 70.4 13.8 3.0 0.13 0.09 0.65 0.33 2.05 4.55 3.25 1.30 0.24 129 0

2 69.2 16.5 1.70 0.91 0.19 1.50 0.10 — 2.07 4.42 2.30 0.30 0.12 348 0

3 72.2 14.5 4.3 0.62 0.14 0.80 0.01 — 2.02 3.98 0.60 0.40 0.00 375 0

4 73.0 14.6 1.54 0.93 0.16 0.40 0.12 — 1.43 4.48 2.20 0.25 0.12 193 0

5 75.2 14.4 1.11 1.44 0.14 0.30 0.02 — 0.97 3.47 1.55 0.25 0.00 193 0

6 72.8 15.4 1.66 1.34 0.19 0.60 0.02 — 1.78 3.28 0.85 0.10 0.06 193 0

7 73.2 14.5 2.83 1.20 0.17 0.30 0.03 — 1.43 5.30 0.80 0.01 0.01 193 0

Samples 1, 2, and 3 were taken at kilometer 35, 114, and 122, respectively, of the highway between Queretaro and San Luis Potosi.

Samples 4, 5, 6, and 7 were taken from around the San Jose Dam near the city of San Luis Potosi. Table 5. Description of Rock Samples Included in Table 2

Sample Macroscopic Rock Mineral Composition No. Location Rock Name Description (microscopic observation)

1 S . de Pinos Rhyolitic Pink with purple shades; Devitrified rhyolitic glass, quartz, limo- tuff compact, crystalline, nite, hematite, rock fragments, zircon, porphyritic, pyroclastic; cristobalite, sanidine, oligoclase, ande- no visible alteration sine, magnetite.

2 S . de Pinos Rhyolitic Light yellow; compact, Devitrified rhyolitic glass, oligoclase, tuff clay alteration, Fe andesine, quartz, magnetite, hematite, oxidation limonite, rock fragments, zircon, cris­ tobalite, sanidine, sericite, clay; pyroclastic texture.

3 S. de Pinos Ignimbrite Dull yellow; compact, Devitrified rhyolitic glass, quartz, mag- crystalline, porphyritic, netite, limonite, hematite, cristobalite, slight Fe oxidation sanidine, oligoclase, andesine; pyro­ clastic texture.

4 S. de Pinos Ignimbrite Dull reddish; compact, Devitrified rhyolitic glass, quartz, mag- crystalline, porphyritic netite, hematite, limonite, cristobalite, oxidation zircon, sanidine, rock fragments, oligo­ clase, andesine; pyroclastic texture

5 S . de Pinos Ignimbrite Dull reddish; compact, Quartz, devitrified rhyolitic glass, sanidine, crystalline, porphyritic oligoclase, andesine, zircon, magnetite, hematite, limonite, sericite, biotite; pyro­ clastic texture Table 5. Description of Rock Samples Included in Table 2—continued

Sample Macroscopic Rock Mineral Composition No. Location Rock Name Description (microscopic observation)

6 S. de Pinos Ignimbrite Dull reddish; compact, Quartz, devitrified rhyolitic glass, sani­ crystalline, porphyritic dine, oligoclase, andesine, zircon, mag­ netite, hematite, limonite, sericite, biotite; pyroclastic texture

7 S. de Pinos Rhyolitic Dull gray and reddish; Devitrified rhyolitic glass, quartz, mag­ tuff compact, crystalline, netite, hematite, limonite, oligoclase, porphyritic andesine, sericite, sanidine; pyroclastic texture

8 El Naranjo, Flow- Dark pink; compact, No microscopic description. Quartz, Zac., banded crystalline, porphyritic, feldspars, hematite, specularite, kaolin; surface rhyolite kaolinization dense

9 El Naranjo, Flow- Reddish; compact, No microscopic description. Quartz, Zac., bot­ banded crystalline, porphyritic, feldspars, hematite, specularite, kaolin; tom of mine rhyolite kaolinization dense

10 El Infiernito, Rhyolitic Pink with white spots; Devitrified rhpolitic glass, quartz, sanidine, Sapioris tuff compact, crystalline, magnetite, trydimite, spherulitic growths porphyiitic, kaolin- ization Table 5. Description of Rock Samples Included in Table 2—Continued

Sample Macroscopic Rock Mineral Composition No. Location Rock Name Description (microscopic observation)

11 C. de Elena, Rhyolitic Dull gray; compact, crys­ Quartz, sanidine, devitrified rhyolitic J.Aldama tuff talline, porphyritic glass, biotite, clay, hematite, limonite, pyroclastic texture

12 Mesa de Ignimbrite Light pink; compact, Quartz, devitrified rhyolitic glass, sani­ las Vacas, crystalline, porphyritic, dine, biotite, rock fragments, hematite, J. Aldama kaolinization limonite, kaolin, clay; pyroclastic texture

13 Durazno, Rhyolitic Light pink; agglomeratic, Quartz, devitrified rhyolitic glass, sani­ J. Aldama tuff poorly cemented, kaolin­ dine, biotite, volcanic rock fragments, ization clay, hematite, limonite, trydimite

14 C. de Biotite Dark pink; compact, Quartz, sanidine, devitrified rhyolitic Bartolo, Rhyolite crystalline, porphyritic glass, biotite, clay, hematite, limonite, J. Aldama hypocrystalline hypidiomorphic texture

15 C. de Rhyolitic Dark pink; crystalline, Quartz, sanidine, rock fragments, Bartolo, tuff compact, porphyritic devitrified rhyolitic glass, hematite, mag­ J. Aldama netite, limonite, clay, chlorite; pyroclas­ tic texture Table 5. Description of Rock Samples Included in Table 2—Continued

Sample Macroscopic Rock Mineral Composition Mo. Location Rock Name Description (microscopic observation)

16 C. de Elena Flow- Pink; compact, crystal­ Quartz, sanidine, devitrified glass, hema­ J. Aldama banded line, porphyritic tite, limonite, kaolin; hypocrystalline rhyolite texture

17 El Naranjo, Rh yolitic Pink; crystalline, Quartz, sanidine, devitrified rhyolitic Zac. tuff kaolinized, semi- glass, rock fragments, hematite, specu- rnid-level compact larite, kaolin; pyroclastic texture

Glossary:

Rhyolitic Tuff. "Indurated pyroclastic rock of acid composition and grain generally finer than 4 mm, that is, the indurated equivalent of volcanic ash or dust" (Wentworth and Williams, 1932, p. 50).

Ignlmbrite. "Tuffaceous rock of acid composition thought to have been formed from showers of intensely heated minute fragments of volcanic magma, that because of their high temperature, were viscous and adhered together after they reached the ground" (Marshall, 1935, p. 4-10). Table 6. Descriptions of Rock Samples Included in Table 4

Sample Mineral Composition No. Rock Name Megascopic Description (microscopic observation)

1 Ignimbrite Light pink with white spots; compact, Devitrified glass, rock fragments, crystalline, porphyritic quartz, sanidine, magnetite, hematite, limonite, sericite, clay; pyroclastic texture

2 Biotite Dark pink-brown with white spots; Quartz, sanidine, devitrified glass, rhyolite flow-banded, crystalline, compact, biotite, magnetite, hematite, limonite, porphyritic clay; hypocrystalline, spherulitic

3 Ignirnbrite Brownish with white spots; compact, Quartz, rock fragments, devitrified crystalline, porphyritic glass, andesine, oligoclase, sanidine, hematite, limonite, clay; pyroclastic texture

4 Rhyolitic Reddish brown with white spots; Quartz, sanidine, andesine, oligoclase, tuff compact, crystalline, porphyritic rock fragments, biotite, hematite, limo­ nite, clay; pyroclastic texture

5 Ignimbrite Reddish brown; compact, crystal­ Quartz, sanidine, devitrified glass, line, porphyritic andesine, oligoclase, hematite, limo­ nite, clay; pyroclastic texture Table 6. Descriptions of Rock Samples Included in Table 4--Continued

Sample Mineral Composition Mo. Rock Name Megascopic Description (microscopic observation)

6 Ignimbrite Reddish brown; compact, crystalline, Devitrified glass, rock fragments, porphyritic quartz, sanidine, andesine, oligoclase, hematite, limonite, clay; pyroclastic

7 Ignimbrite Reddish brown; compact, crystalline, Quartz, rock fragments, devitrified porphyritic glass, spherulites, hematite, limonite, clay; pyroclastic 22 adapted by the author for the system of the National University of

Mexico (Burroughs 6500) .

All the samples used in this study were selected, based on macroscopic appearance, from a large number to represent as closely as possible the sample group from which they were derived. They were collected within tin districts from zones where no obvious tin mineral­ ization was present. The perfect fit linear regression equation obtained for the samples of rocks of the Juan Aldama district is as follows:

Sn = Ci + 0.218 Si02 + 0.371A1203 - 0.402 Fe203

+ 0.363 FeO + 0.117 MgO + 0.732 CaO - 0.107 K20

- 0.463 Na20 - 0.376 Ti02 - 0.452 P205 - 0.084 S

- 0.360 C02 - 0.162 H20.

In this, as well as in all similar equations presented in this paper, "C" is a constant.

Except for Si02, A1203, FeO, MgO, and CaO, there seems to be a negative correlation between tin content and the other major chem­ ical components. CaO is the anion which best correlates directly with

Sn.

A regression analysis using log Sn as the dependent variable was run, and the correlation seemed to improve a little, as shown in the following perfect fit equation.

log Sn = C2 + 0.244 Si02 + 0.452 A1203 - 0.450 Fe203

+ 0.350 FeO + 0.088 MgO + 0.669 CaO + 0.030 K20

- 0.458 Na20 - 0.436 Ti02 - 0.525 P2Os - 0.029 S

- 0.412 C02 - 0.254 H20. Sierra de Pinos

A multiple regression analysis was also done with the chemical compositions of rhyolitic rocks of Sierra de Pinos, using the tin content as the dependent variable. The perfect fit regression equation obtained was as follows:

Sn = C3 + 0.781 Si02 - 0.243 AI2O3 - 0.584 Fe203

- 0.269 FeO - 0.137 MnO + 0.971 MgO + 0.965 CaO

+ 0.290 K20 + 0.983 Na20 - 0.181 P205 + 0.972 C02

+ 0.974 H20.

The results obtained from this area are different than those from

Juan Aldama. In Pinos there is an almost perfect direct correlation be­ tween tin and MgO, CaO, Na20, C02, and H20 contents, while in Juan

Aldama only CaO seemed to be directly related to tin. The correlation of tin to Si02 also improved greatly in Sierra de Pinos.

The regression equation using log Sn as the dependent variable shows correlation coefficients very similar to those of the normal equa­ tion, as shown below:

log Sn = C4 + 0.758 Si02 + 0.265 AI2O3 - 0.606 Fe203

- 0.230 FeO - 0.180 MnO + 0.950 MgO + 0.948 CaO

+ 0.282 K20 + 0.968 Na20 - 0.162 P2Os + 0.958 C02

+ 0.964 H20.

The regression equations forthejuan Aldama and the Sierra de

Pinos districts are quite different, and it is not possible at this time to draw any conclusions about the possible relation of tin to any major anions (other than to CaO) in the rhyolitic rocks of the tin-bearing 24 districts. It is possible that each tin district may have its own distinc­ tive regression equation.

As shown in Table 7, the SiC>2 and AI2O3 contents of the rocks of Juan Aldama and Sierra de Pinos are strikingly similar. The barren

rhyolitic rocks have a slightly higher content of both Si02 and AI2O3 than their productive Mexican counterparts; however, they have lower

Si02 than the Black Range rocks.

Tin Deposits and Districts Examined

As mentioned previously, the number of tin deposits in Mexico is very large and the available information about them is rather limited.

For this reason, only those areas examined by the author are included in this section. Information about other districts can be obtained elsewhere as indicated in the selected bibliography. No discussion of small placer deposits in some of the areas visited will be included.

America-Sapioris District

This district is located in the northern part of the State of

Durango (Fig. 1, in pocket) in the Sierra de San Francisco and covers an area of approximately 15 x 5 km.

The geologic sequence in this area is typical of Mexican tin districts. Mineralized rhyolites, breccias, and tuffs overlie a green andesite which crops out in the valley to the east of the sierra. The thickness of the rhyolitic sequence is not precisely known in the area, but it is possible that it reaches up to 1,000 meters around the America area. There is also a red conglomerate that overlies the rhyolitic sequence. 25

Table 7. Average Chemical Composition of Some Tin-bearing and Barren Mexican Rhyolitic Rocks

Content %

Component 1 2 3 4 5

sio2 71.334 71.585 75.025 72.386 72.494 > o I—' 11.645 11.143 12.134 14.814 11.4875 to CO

Fe2°3 4.356 5.300 1.939 2.306 3.4025

FeO 0.958 2.243 0.0682 0.9385 —

MnO — 0.240 0.0681 — —

MgO 0.160 0.201 0.595 0.909 —

CaO 0.845 1.071 1.220 0.650 —

K2O 3.271 2.401 4.208 4.211 3.141

Na20 2.529 4.489 3.791 1.6786 3.6695

TiOz 0.209 — — 0.1543 —

P2°5 0.129 0.299 0.0278 0.0814 —

BaO — — — — —

S 0.003 — — — —

co2 1.021 1.371 — — —

H2O 0.405 0.697 0.513 0.3728 —

1. Average of 10 samples from the Juan Aldama tin district. 2. Average of 7 samples from the Sierra de Pinos tin district. 3. Average of 12 samples from the Black. Range tin district (data courtesy of Mr. John L. Lufkin) . 4. Average of 7 barren samples from the States of Queretaro and San Luis Potosi. 5. Average of 2- barren samples from the States of Hidalgo, Mexico, Nayarit, Guanajuato, Queretaro, Chihuahua, Durango, Jalisco, and Guerrero (Ordonez, 1900, p. 71). 26

Many veins in the area strike northwesterly, but a great number of structures follow a northeasterly direction. Most of them dip 50° to

80° S.

The America-Sapioris district includes the two areas indicated in the name and also the Potrillos district. The geologic characteristics are the same for the three zones, and their deposits are said to belong to one area or another depending only on proximity to one or the other village.

To general knowledge the mines of this district have not been worked since around 1900; however, there is systematic exploration being carried on in the area by large mining companies.

La Grant Mine. This is probably the largest mine in the district.

It is located about 2 km northeast of La America village.

Mineralization is in a rhyolitic tuff and occurs in small vein- lets and in major well-defined veins with strikes from N. 50° E. to

N. 75° E. and dips of 50° to 70° S. Widespread alteration and high porosity of the tuff suggest that there may be some low-grade tin dis­ semination. A chip sample taken from the mine wall where no obvious veins were present analyzed 0.2% tin.

Kaolinization is observed in the tuff near the vein walls.

From the present workings, it appears that there were large accumulations of ore at the intersections of two or more veins. There are stopes up to 6 meters wide from which, it is reported, ore with 5% to 12% tin was mined. However, most main veins have been mined to widths of no more than 1.5 meters. Undiscovered veins and mineralized pockets may be present within the La Grant mine, but, due to their 27 limited potential, they could only be of interest for medium- to small- scale operations. Hence, a broad exploration program to look for veins would not be justified. On the other hand, it is possible that a low- grade, large-tonnage disseminated deposit exists in the rhyolitic rocks of the mine.

La Candelaria Mine. This is probably the second largest mine in the district. It is located about 1,000 meters northeast of the village of La America. This deposit, like La Grant, consists of small vein and pocket fillings in an altered rhyolitic tuff. Most main veins have a

N. 60°-70° W. strike and 60° to 80° S. dip. There is a wide shaft

(about 3.0 x 3.0 meters) to a depth of at least 60 meters in the mine, but it is not safely accessible. Mineralized extensions and future ex­ ploration work within the area of this mine would have to be conditioned to the same limitation stated for the La Grant mine.

Taltomates Mine. Located about 3 km east of the village of

La America, it consists of one single main vein with a general strike of

N. 30° W. and dip of N. 45° E. with multiple branching veinlets ana fissures. There are no large stopes in this mine, and all the ore may have been produced from the single main vein.

La Polvillenta Mine. This is another deposit near the village of

La America, consisting also of a system of veins in an altered rhyolitic tuff. Alteration, mineralization, and geologic conditions are practically the same as for the mines mentioned above. A local gambusino, who was employed as a guide, stated that a general sample of all around the mine taken by him analyzed 2.5% Sn. However, a chip sample taken by the author avoiding obvious veinlets showed only 0.09% Sn. 28 The rhyolite at the surface of this mine shows a great amount of lithophysae with silica (possible chalcedony) filling.

El Infiernito Mine. This is a prospect rather than an old tin deposit like the others mentioned above. It is a shaft that was sunk in

1970 by Canmex, S .A., an exploration company of Canadian and Mexican capital, in the vicinity of the village of Sapioris.

The shaft is in a sequence of dense rhyolites and rhyolitic tuffs highly silicified at the surface and kaolinized at depth. There are many small veinlets with mineralization of cassiterite, specularite, limonite, and chalcedony. It appears that the rhyolites and tuffs also carry a small amount of disseminated cassiterite. In some places the rock shows almost complete replacement by silica with a minor amount of cassiterite. There are several faults up to 2 meters wide with a typical fault breccia that has been silicified and mineralized by tin-carrying fluids. The tin is found as cassiterite around the breccia fragments.

The assay results of samples taken from this shaft are not known, but they were apparently rather low.

El Pinto Area. This is an area with many small prospects. The main prospect is an adit in rhyolitic tuff driven by Canmex in "1970. From a general inspection of the area, it seems that most veins were very nar­ row but of long extension along strike. The number of prospects in this area is very large, suggesting wide spreading of the mineralization. Al­ though exploration for vein-type deposits in this area would be of ques­ tionable value, it couid be worthwhile to investigate the possibility of low-grade disseminated tin mineralization within the area. 29

Even though most of the mineralized areas known at present in the America-Sapioris district consist of vein fillings of rather small ton­ nage, there is the possibility of a low-grade, high-tonnage deposit that may prove profitable for a medium- to large-scale operation.

Avino Area

The Avino area is located about 75 km northeast of the City of

Durango near the town of San Jose de Avino. This tin area, with only a few workings , is now being worked under the name of Mina La Esperanza.

Here the tin mineralization seems to be confined to one breccia horizon which is interbedded with a dense rhyolite. This mineralization is re­ stricted to the spaces between the breccia fragments. Additional similar deposits may exist within this area. Location of mineralized breccias could be done relatively easily by a combination of stream sediment and soil geochemical surveys and a good photogeological study.

La Barrosa Mine

This mine is located about 100 km northwest of the City of

Durango and consists of a rhyolitic breccia with fragments from 3 to 80 cm. The cementing material is composed of a clay of rhyolitic deriva­ tion with botryoidal cassiterite, specularite, limonite, quartz, and chal­ cedony. The emplacement of the mineralization seems to have been controlled exclusively by the distribution of the circulation channelways.

The breccia is overlain by a dense rhyolite horizon which pos­ sibly acted as a deflector for the mineralizing solutions, forcing them to spread through the more permeable breccia structure. 30 Los Remedios Mine

Cerro de Los Remedios is a low hill located on the west side within the city limits of Durango. All deposits in this hill are in veins of rhyolite. On occasion, the faults widen and produce breccia zones where rich bonanzas were formed. The veins are oriented N. 35° E. and have variable dips between 60° and 90° NW. The average width of the fault is about 1.0 meter, but there are sections where it narrows to 10 cm and others where it widens up to 2.0 meters.

Cassiterite seems to be limited to some vein fillings within the breccia and to crusts formed on the breccia fragments; no disseminated tin seems to exist either in the breccia fragments or in the wall rocks.

Other mineralized structures may exist within the area; however, be­ cause of the expected limited tonnage, it would not be advisable to carry on any major exploration program.

El Narnajo Mine

This mine is located about 30 km southeast of the town of

Sombrerete and is the main mine of a tin district that includes the deposits of Buena Suerte, Volaaero, and others of less importance.

The general geology of the area is typical of the Mexican tin districts. It includes extensive flows of rhyolite with pyroclastic lenses of variable thickness and extension, consisting of rhyolitic tuffs and breccias. The central area of the district is in a rhyolitic breccia 15 to 20 meters thick, with fragments from 20 to 80 cm lying between two flows of rhyolite. Near the mineralized zones the breccia and the rhyo­ lite are intensely kaolinized, and, in some places,.they are also silicified. 31

Dikes of rhyolitic porphyry are present, which commonly cut through all the rhyolitic sequence, but may, as at the Naranjo mine, only reach to the base of the upper rhyolitic flow, which acted as a seal for both dikes and mineralization. These relationships suggest that the dikes were emplaced during the last stages of igneous activity and very likely were almost simultaneous with the mineralization. Samples from these dikes carried 0.09% and 1.3% Sn. It is thought by the author that dikes within tin areas normally have a higher tin content than those of barren areas and hence geochemical studies of these dikes could lead to the discovery of blind tin deposits. This relationship has been proved in the Cornish tin province (Lee Moreno, 1968) . A similar relationship has been observed by the author in the porphyry dikes of the silver- copper-zinc mineralized area of La Paz, San Luis Potosi.

The mineralization is restricted mostly to the breccia, but a few stringers extend to the upper and lower rhyolite. Tin is present as cas- siterite in crusts on fracture walls within the breccia and surrounding the breccia fragments. The cementing material of the breccia carries fine cassiterite powder, which is mined with the rest of the cementing mater­ ials, kaolin, limonite, clays, quartz, opal, chalcedony, tridymite, specularite, and cristobalite. A small amount of fluorite was also observed.

There is preferential concentration of tin mineralization along the contact zones of the breccias with the enclosing rhyolites, near a porphyry dike, and along the extensions of the veins that come from the lower rhyolite. These veins were obviously channelways for the emplace­ ment of the mineralization in the breccia. 32

No systematic sampling of the deposit was done by the author, but a general chip sample gave 0.8% Sn. It is possible that the average tin grade of the mine would prove extraction by bulk methods to be economic.

As stated for the Esperanza mine inAvino, detection of similar enriched breccia structures could be accomplished with a minimum expen­ diture by well-planned geochemical stream sediment and soil studies.

Study of the porphyry dikes of the El Naranjo district would probably prove to be rewarding.

Juan Aldama District

The mining district of Juan Aldama is located on the Sierra de

Flores, which begins at about 3 km southeast of the town of Juan Aldama and extends along a belt of approximately 15 x 8 km.

The geology of the area is again typical of that in other tin areas of Mexico. At the base, there are calcareous rocks of possible

Cretaceous age. These are overlain by a series of rhyolitic rocks, which include small lenses of breccias and tuffs enclosed between two flows of dense rhyolite.

The district includes nearly 30 main prospects,with average development of 10 meters along narrow veins or lenses, and possibly around a hundred minor workings with only 2 to 3 meters of development work. Only a few of the main prospects will be mentioned, although most of the workings in the area were visited by the author.

Cerro de San Bartolo. This is a hill which has about 15 small small mine workings. All were dug following narrow veinlets in rhyolite. Occasionally, the fractured zones widen and form small brecciated zones where mineralization was emplaced. There is also a talus breccia in the area which contains small amounts of cassiterite near those places where the parent rhyolite contained a mineralized veinlet. The mines of

El Cristal, Panchillo, and La Puntilla are among the largest in this hill.

All three are veins in rhyolite. The thickness of the veins varies from 1 to 5 cm,and the veins normally follow a strike between northeast and northwest with dips of 70° to 90° SW. to SE. Commonly, the veins con­ tain a crust of tridymite and cristobalite of around 3 to 6 mm in thickness over which alternate layers of cassiterite and specularite were deposited.

The wall rock near mineralized zones is usually kaolinized.

Mesa del Venado. A number of small prospects also exist in this hill; their geologic environment is very similar to that of the Cerro de San Bartolo. The mines of Santa Leonor, La Mula, and El Zanjon are the largest of the area. A very generalized location map of this area is shown in Figure 3 . Santa Leonor mine is on the northwest side of Mesa del Venado. It has a shaft about 20 meters deep and some short cross­ cuts at the bottom which were not accessible at the time of the visit.

In this mine there are two main veins in rhyolite about 12 meters apart, with a strike S. 45° E. and dip of 60° SW. and variable thickness, which in no place seemed to exceed 0 .1 meters, although the width of the work­ ings reach 1.2 meters. Between the two main veins is intense fracturing that gives the impression of a breccia structure. Most of the fractures in this area carry some tin mineralization, although the rhyolite frag­ ments have a very low tin content (0.05% Sn). Specularite, fluorite, and forms of silica are normally present in the veins. 34

103° 241 I03°22' 103° 20'

j 7~7 °X" \ 2 4° 18' Sta. 'T\ N Hue i a \ \ '-H * \J? JUAN /[ ^ALDAMA \

\ V 24°!6' Mesq-^j l .\ Masa~^Sdrtolo ,lp's"Vccusi\\ Jfir

/

Reticnaci 24° 14'

I Km

Figure 3 . Location Map of Mesa del Venado in the Juan Aldama Tin District

Scale 1:125,000 35

La Mula mine was developed along another vein in rhyolite.

Where the vein widens, there are zones of breccia in which the mineral­ ization was preferentially concentrated. The present workings include a shaft about 20 meters deep with some short drifts at the bottom that fol­ low richer veinlets within the fault breccia. Tin mineralization occurs as cassiterite in crusts on the wall rock and around the rhyolite breccia fragments. There is also specularite, quartz, and a minor amount of fluorite within the vein. The rhyolite of the wall rock, as well as that of the breccia fragments, shows intense alteration (mainly kaolinization) and also carries some mineralization to a distance of about 75 cm from the vein wall. Bracho (1961) reported a sample of a breccia fragment from this mine that assayed 10.24% Sn, but this sample may have had a crust of cassiterite attached ana was not, as he stated, a true "magmatic tin segregation" within the rhyolite.

El Zanjon mine is a big trench, about 15 meters deep and about

25 meters long, which was dug along a vein in rhyolite with a strike

N. 40° W. and dip 85° NE. The vein has an average thickness of 30 cm, including the gangue and some fault fragments which appear on occasion.

The mineralization is of the type described for La Mula mine.

Mesa de Las Vacas, Redonda, and Cerro de Mota are the other most important hills on the northwestern portion of the Sierra de Flores.

They all have small mine workings, whose mineralization is like that of

Cerro San Bartolo ana Mesa de Las Vacas. All these prospects have been worked by local gambusinos following the veinlets with higher tin content.

El Durazno Mine. This is a mine located on the southeastern portion of the Sierra de Flores on the western side of the Cerro del 36 Durazno. It has a development of around 100 meters in vertical, in­ clined, and horizontal workings along a vertical vein in rhyolite that has a strike of N. 70° W. Its average thickness is about 15 cm. Tin is present as cassiterite with specularite and chalcedony attached to the wall rock. This is possibly the deepest mine in the region.

La Vieja Mine. This is one of the oldest mines known in the area. It is located in one of the highest points of the Espinazo de la

Vieja in the southeastern portion of the Sierra de Flores . It has a ver­ tical shaft of about 10 meters deep and various workings with a total extension of about 50 meters.

Tin mineralization is within a vein 15 cm thick combined with specularite and chalcedony. An alteration zone in the wall rock extends about 75 cm from the vein and contains a small amount of mineralization.

Conclusions on Mining Economics of the Tuan Aldama District.

Tin mineralization in the area is of two types: (1) vein filling and (2) free cassiterite in recent unconsolidated sedimentary deposits.

There is no place in the Juan Aldama district where tin dissem­ ination in the rhyolite could be observed. All mine workings examined are veins in rhyolite, which in some places have fault breccias within the main structures.

The lower rhyolite does not crop out in the area but has been detected at the bottom of La Mula mine. This formation does not contain tin minerals as basic components, but doubtlessly it contains a few vein- lets that are extensions of the mineralized zones located in the upper horizons. 37

Breccia and tuff lenses found in the area do not have a homo­

geneous content of economic tin mineralization. Locations of mineral­

ization in these rocks is controlled by the presence of permeable zones

at the time of emplacement of the tin mineralization.

The upper rhyolite is seen capping practically all hills of the

area; it generally has a conspicuous columnar fracturing caused by

cooling. This formation acted to deflect the mineralizing fluids that

were responsible for depositing tin in the lower horizons. In a few

places tin veinlets were formed in the upper rhyolite.

The mine workings of the area are very numerous, but most of

them have developments under 5 meters in length and many were dug

only for exploration. The largest prospects of the area are La Mula,

El Zanjon, Santa Leonor, La Vieja, and El Durazno. These mines ob­

viously rendered some economic benefit to their proprietors, but hardly

any of them (or all together) would justify any investment for a large-

scale operation.

A few placers have been worked by the local gambusinos, but

their mineralization is very erratic and their bulk grade very low. All of the more important placers and workings of the district were sampled

by the author, and in none was the average tin grade more than 0.2% Sn, most of them being less than 0.1% Sn.

The stream sediments and valley fill of the area contain large amounts of dark heavy minerals which give good panning concentrates.

This led some people to think that cassiterite was abundant in the allu­ vium of the area. Testing was done at the laboratories of the Consejo de Recursos Naturales No Renovables to investigate the nature of the 38 heavy concentrates. Samples were taken from areas which gave the higher weights of concentrate. A concentration was made by means of a laboratory-size gravimetric shaking table. The magnetic and nonmagnetic fractions of the concentrate were separated, and the materials of each group were carefully studied under the microscope and by X-ray spec­ trograph^. The results obtained were as follows:

Original Concentrate 0.27% Sn (100% weight) magnetite, hematite, quartz, feldspars, cassiterite, zircon, topaz, ilmenite

Magnetic Fraction 0.008% Sn (46.25% weight) quartz, feldspars, hematite, cassiterite, zircon, topaz, ilmenite

Nonmagnetic Fraction 0.45% Sn (52 .45% weight) zircon, topaz, cassiterite, hematite, ilmenite

The concentrate was about one-hundredth of the original sample by weight, thus making the general tin grades of the pseudo- placers in the order of 0.0027%.

Ten other samples were checked for tin content in a laboratory in the United States; the results correspond to those found in the Mexican laboratories.

It can then be concluded that alluvial accumulations of the Juan

Aldama district are not economic for large-scale operation; the same can be said for the hard-rock prospects of the area.

Selective exploitation by local gambusinos may be feasible in some places, just as has been done up to the present, but no major sys­ tematic exploration program there should be encouraged. Sierra de Pinos

The Sierra de Pinos district is located in the eastern portion of the State of Zacatecas and southwestern part of San Luis Potosi. It has an approximate length of 40 km in a northwest-southeast direction and an average width of 15 km.

The older rocks of the area are a sequence of Cretaceous lime­ stones that, according to Petroleos Mexicanos (PE-MEX) (Hoja Tolosa,

S.L.P.), includes the Tamaulipas Inferior, La Pena, Cuesta del Cura,

Indidura, and Caracol Formations. These rocks were intruded, possibly in late Mesozoic and early Tertiary times, by a granitic body that crops out near the town of Pinos, Zacatecas, and in the well-known Penon

Blanco north of that town. Finally, a rhyolitic emission during mid-

Tertiary covered the old rocks . This last formation was deposited in various layers of rhyolite and lenses of interbedded tuffs and breccias.

A geologic map of the area is presented in Figure 2 (in pocket) .

Tin has been mined in the area in numerous small prospects of little importance. These are shown in Figure 2. With the exception of the Huizache and Veta del Llano mines, none are being systematically mined at present, although there are always around a hundred local gambusinos that work several of the old mines for a minimum recovery.

The total production is not known but possibly is around 20 kg of tin per week. This constitutes a complement to the poor economy of the region.

All of the 174 workings of the area were examined and chip sampled. The results of this work are in a lengthy unpublished report of the Consejo de Recursos Naturales No Renovables (Lee Moreno, 1971) .

Only a few of the most important prospects will be discussed here. 40 Veta del Llano. This mine is being presently worked by about

12 local gambusinos. It is a vein in a light, dense rhyolite that strikes

N. 60° E. and is practically vefffcal. The vein has an average width of

10 cm. The entire mine consists of a big trench excavated about 10 meters along the strike of the vein to an approximate depth of 65 meters.

In February 1971, the miners found an ore shoot that carried about 300 kg of practically pure botryoidal cassiterite; this occurred in a widening of the vein and is an example of what the gambusinos hope to find while following the narrow veinlets. The cassiterite is associated with specularite and SiC^. The wall rock is kaolinized and carries no economic mineralization. A grab sample of the wail rock taken by the author adjacent to the new ore shoot showed only 0.006% Sn. A channel sample from the entire width of the working (1.5 meters) analyzed 0.04%

Sn, and a grab sample from the dump avoiding obvious mineralized frag­ ments contained 0.24% Sn.

Vetas Las Lisas . This is a group of about 50 small workings within about 100 square meters in a gray rhyolitic ignimbrite which shows intense kaolinization and minor oxidation of iron minerals . A few of the workings have about 7 or 8 meters of development, but most of them have less than 3 meters. The dumps cover most of the surface, and it is not clear whether the structure is a stockwork-type multifractured zone or an actual cemented breccia. In any case, due to the large extension of the mineralized area, it is possible that it might be worked as a large- tonnage, low-grade operation, if the general average grade proved to be economic. 41

A grab sample taken from a dump avoiding obviously mineralized fragments gave 0.006% Sn. Two channel samples taken across two other workings analyzed 0.004% Sn and 1.67% Sn, respectively.

Vetas Guadalupe. This is another large group of small work­ ings within a small area. Individually, they follow narrow mineralized veins in rhyolite, but all together they are located in a highly fractured body of kaolinized rhyolite which, like Las Lisas group, could constitute a breccia at depth. The dominant strike of the veins is N. 20° E. and the dip almost vertical. Cassiterite is again associated with specularite and silica. A channel sample from one of the workings analyzed 0.00 7%

Sn, and a grab sample from the dump gave 0.08% Sn.

Vetas de Las Adjuntas. This is possibly the largest working in the area with a development of around 90 meters. It is a vein deposit with many secondary fractures that give the impression of a smail stock- work. A channel sample from various places inside the mine showed

0.18% Sn.

Mina San Tose. The San Jose mine is also known as Vetas del

Saucillo. It has many diggings from 3 to 15 meters in length which follow small veins in a brecciated rhyolitic tuff. The strike of the main struc­ tures is between N. 30° W. and N. 65° W. with a dip of 60°-70° NE.

There is kaolinization and slight oxidation of iron minerals in all the mineralized area. Although the mine has been selectively worked by gambusinos, there is a possibility that it might be worked by bulk methods if the general grade proved to be adequate. During the study, four channel chip samples were taken around the main workings of the group. The results obtained are as follows: 42 Type of Working Approximate Development % Sn

Irregular holes 15 meters 0.21

Shaft and adit 30 meters 0.24

Shaft 20 meters 0.18

Irregular holes 30 meters 0.48

Average grade of above 0.288%

It was not possible to get to the lowest parts of the workings because of their state of abandonment, but it is thought that they do not extend too far beyond the last sampled point, since, due to lack of facil­ ities, gambusino workings very seldom extend deeper than 20 or 30 meters.

Mina Nueva. The area of Mina Nueva includes a group of five small prospects within an area of approximately one square kilometer.

All prospects are veins in a rhyolitic ignimbrite, but the rock at Mina

Nueva is also brecciated. The breccia fragments here are about 0.5 to

1.0 meters in diameter and are cemented by clay derived from the same rock and by hematite, limonite, and cassiterite. The last occurs in small crusts 1 to 2 mm wide attached to the breccia fragments. The rock is kaolinized in all the area and shows slight oxidation of iron minerals.

One selected sample of mineralized rhyolite from the interior gave 30%

Sn. A grab sample from a small dump found inside the Mina Nueva ana­ lyzed 3.83% Sn, and a chip sample from various places inside the work­ ing showed only 0.006% Sn.

This area is within a geochemical anomaly that will be discussed later in detail. 43 A limited drilling program was carried on in this zone. Most of the holes were drilled down to only 50 meters through a rhyolitic tuff with various degrees of kaolinization. The average tin content was

750 gms/ton. Tin was observed only under the microscope as cassiterite in very small grains within the tuff, usually near or along very narrow fractures and mixed with specularite and silicate filling litophysae. The higher tin values were always found in the rock near fractures, which in no place were wider than 4 mm.

It is thought that, even though there was high-grade mineral­ ization in this area, the volume of tin retained in the minute fractures and other voids of the tuff is not enough to make it economic at present on a large-scale mining basis. Exploitation of the richer veins within the area could only be economic in some places for the local gambusinos.

Mina El Huizache. Exploitation of this mine is temporarily halted due to the lack of an available concentration plant near the area.

Mine El Huizache is a typical tin deposit in a breccia. The rock is a rhyolitic tuff with fragments from a few centimeters to 2 or 3 meters.

The breccia seems to have been produced between two normal faults that have a general strike of N. 45° E. and join about 200 meters east of the main mine entrance. They form a small graben. Because of the brecciation, it is hard to identify any important individual fractures inside the mine; however, it is possible to notice a dominant strike of

N. 50° W. for the longest fractures observed. The tin mineralization is normally present around the breccia fragments and in some fractures be­ tween blocks. Alteration is present as kaolinization, jasperization, hematitization, and silicification and usually diminishes from the edge 44 to the center of the breccia fragments. It was observed that the areas with higher content of jasper were also richer in tin values.

A continuous channel sample was taken from all of the workings of this mine, and an average tin content of 0.39% was obtained, with values ranging from 0.1% to 0.6% Sn. Drilling results indicate a highly erratic distribution of mineralization and a strong decrease of tin values at depth. Although a good structural pattern exists in this mine, there may not have been enough affluence of tin mineralization to make it economic for a large-scale operation.

Conclusions on the Sierra de Pinos Area. Most of the workings of the area have been dug in veins, in tuffs, and rhyolites and lack im­ portance for medium- or large-scale operations. The prospects with the most development are Mina San Jose (or El Saucillo) , Vetas de Las Ad- juntas, Vetas Guadalupe, Vetas Las Lisas, and Veta del Llano. All are vein deposits in highly fractured rock of a stockwork type, which at some time permitted profitable small-scale operations. Some of these areas might yield considerable tonnages of low-grade tin ores that might be mined by open pit methods. The combined tonnages of various mines might justify the reopening of the concentration plant at Pinos,

Zacatecas. If the general tin grades of these areas do not make a medium- or large-scale operation feasible, an organized mining program utilizing the local gambusinos might yield some production. However, any extensive exploration program to find more vein-type deposits in the area cannot be justified. 44 to the center of the breccia fragments. It was observed that the areas with higher content of jasper were also richer in tin values.

A continuous channel sample was taken from all of the workings of this mine, and an average tin content of 0.39% was obtained, with values ranging from 0.1% to 0.6% Sn. Drilling results indicate a highly erratic distribution of mineralization and a strong decrease of tin values at depth. Although a good structural pattern exists in this mine, there may not have been enough affluence of tin mineralization to make it economic for a large-scale operation.

Conclusions on the Sierra de Pinos Area. Most of the workings of the area have been dug in veins, in tuffs, and rhyolites and lack im­ portance for medium- or large-scale operations. The prospects with the most development are Mina San Jose (or El Saucillo), Vetas de Las Ad- juntas, Vetas Guadalupe, Vetas Las Lisas, and Veta del Llano. All are vein deposits in highly fractured rock of a stockwork type, which at some time permitted profitable small-scale operations. Some of these areas might yield considerable tonnages of low-grade tin ores that might be mined by open pit methods. The combined tonnages of various mines might justify the reopening of the concentration plant at Pinos,

Zacatecas. If the general tin grades of these areas do not make a medium- or large-scale operation feasible, an organized mining program utilizing the local gambusinos might yield some production. However, any extensive exploration program to find more vein-type deposits in the area cannot be justified. 45

Mineralogy and Origin

In previous section there has been mention of particular mineral species that occur in the various areas visited. In this section some specific minerals that are thought to be particularly relevant to the study will be discussed with a little more detail but still from a general point of view.

Mineral Species

Cassiterite occurs in rhyolitic rocks only as open-space filling, usually accompanied by iron oxides. Where the openings were rather narrow or small, deposition of cassiterite took place in such a way that gives an aspect of bein a disseminated syngenetic rock component; how­ ever, when seen under the microscope, especially in thin section, one realizes that the tin mineral is epigenetic and was deposited along minute channelways after the rock had been solidified. No difference was ob­ served between the cassiterite of the large fractures and that of the narrow fissures within the rock. None of the samples observed by the author really showed an authentic magmatic dissemination of cassiterite.

Common vein minerals are cassiterite, specular hematite, cristobalite, tridymite, chalcedony, topaz, magnetite, zircon, and il- menite. Less often, fluorite, tourmaline, and opal were also observed.

A few mineralized rock samples from the Juan Aldama district were sent to the Bureau de Recherches Geologiques et Minieres of France for metal- lographic studies, and they reported the presence of austinite,

CaZn(As04) (OH),and a nonidentified mineral which they supposed was holdenite, 8MnO *4ZnO-AS205 • 5H2O . No sulfides were observed in any 46 of the samples studied, and only a few grains were seen in some of the

Juan Aldama stream concentrates.

Cassiterite is present in well-formed red crystals and in bot- ryoidal aggregates (wood tin), which sometimes show intergrown bands of specularite, thus suggesting a simultaneous deposition of both min­ erals. The wood tin is commonly brown or black, but it has been seen in practically all shades of white, yellow, orange, and dull red.

Wood tin seems to have, on occasion, grown around minute specularite crystals, but mostly it is observed as a fibrous and banded form apparently grown around other cassiterite grains or around minute rock fragments. X-ray studies of two samples of wood tin from Juan

Aldama and Sierra de Pinos showed that this material is perfectly crys­ talline and not truly colloidal, as it has been mentioned elsewhere in the literature. The banded aspect is produced by concentric or semi- concentric layers of minute cassiterite crystals deposited from a boiling or gaseous fluid.

A sample of wood tin from Juan Aldama was sent for metallo- graphic study to the Bureau de Recherches Geoiogiques et Minieres of

France, whose report (1970) reads as follows: "... rhythmic deposits of cassiterite with a colloidal appearance but well crystallized, and with gradual steps from dark to light and again to dark deposits. The whole material is completely crystallized." No differences in chemical com­ position were reported from band to band of wood tin, based on micro­ scopic studies; however, it is possible, in the opinion of the author, that the differences in color of the bands are in fact due to different chemical trace elements present in the cassiterite, as shown in Table 8. 47

Table 8. Trace Element Analyses of Various Cassiterite Samples from the Sapioris District

(Analyses by Bureau de Recherches Geologiques et Minieres)

Element 1 2 3 4 5

Ni 5 5 5 5 5 As 1 1 1 1 1 Mn 160 365 95 25 315 Ge 6 6 6 6 6 Sr 5 5 5 5 5 Ba 10 11 5 5 5 Cr 5 5 5 5 5 Yb 2 2 2 2 2 Sc 2 2 2 2 2 Y 10 10 10 10 10 Co 5 5 5 5 5 Cu 114 36 75 78 70 V 10 10 10 10 10 Mo 7 7 200 1100 20 Bi 11 6 16 45 7 Ga 2 2 51 650 2 Pb 6000 6 20 320 1150 B 40 20 195 280 35 Be 6 30 120 5 16 Zn 5000 70 330 3000 1200

Sample 5 is a sample of cassiterite in a siliceous (possibly quartz) matrix. 48

Smith (1947) synthetized cassiterite of various colors from different chemical combinations.

All vein materials are thought to have been deposited almost simultaneously. Usually, specularite and cassiterite are found sur­ rounded by tridymite and cristobalite, but in many samples from the

Juan Aldama district cassiterite is seen intergrown with these silica forms or encrusting cristobalite. The identity of the cassiterite and cristobalite in this particular sample was established by X-ray spec­ trograph^ at the laboratories of Consejo de Recursos Naturales No

Renovables.

Cassiterite in association with cristobalite has also been re­ ported by Fries (1940) in the rhyolitic series of the Black Range, New

Mexico, and by Knopf (1917) in the rhyolites of Lander County, Nevada .

The metallographic report from the Bureau de Recherches Geo- logiques et Minieres (1970) indicates that a sample of cassiterite, also from the Juan Aldama district, shows "... abundant inclusions of hema­ tite /specular hematite/ which presents with cassiterite all the charac­ teristics of syncrystallization, besides, the hematite presents very fine exsolutions of cassiterite. . . ."

Botryoidal cassiterite or wood tin is known to occur mainly as a hypogene mineral in vein deposits particularly associated with rhyolitic lavas and related rocks (tuffs, breccias, etc.). However, it may also be formed by supergene processes in a zone of oxidation in tin deposits.

For example, supergene cassiterite was formed in Bolivia replacing stannite or some other tin sulfide, never after cassiterite. There is no evidence in any of the tin areas visited of the existence of sulfides 49

(other than the few grains of pyrite found in some Juan Aldama stream

sediments), and no leached capping was found. These facts suggest that the cassiterite of all tin deposits visited is of hypogene origin.

Tourmaline was not always observed, but where it is found

(Sapioris) it suggests fumarolic action or mineralizing gas from a mag- matic fluid.

Topaz was found in most of the samples studied. According to

Dana and Ford (1932, p. 614), "... this mineral is formed in granites and rhyolites as a result of pneumatolytic action after crystallization of the original magma. It is often accompanied by fluorite, cassiterite and tourmaline."

Cristobalite has two polymorphs: high cristobalite and low cristobalite. The variety identified as an intergrowth with cassiterite was the latter. Frondel (1962, p. 2 73) notes that "... low cristobalite is a tetragonal metastable form of high cristobalite that exists at lower temperatures. High cristobalite is stable from 14 70° to the melting point at 1728° at atmospheric pressure, but exists metastably down to a dis- placive inversion at about 268° where it converts to low cristobalite."

So, in order to have low cristobalite, we must first have high cristobalite.

Tridymite may also be a product of inversion of high cristobalite and forms at a temperature of 1470°.

All the evidence suggests that tin mineralization in the areas studied was originally carried in a pneumatolytic or boiling phase and that it is of hypogene origin. Solution Chemistry 9 4 Tin can only occur in magmas as Sn* or Sn . According to

Ringwood's (1955) and Goldschmidt's (1958) principles of magmatic behavior, these ions will proxy for elements with similar ionic radii and electronegativity, that is, Ca^ , Cd^, and Fe^ for Sn^, and Fe^,

Fe^, Mg^, Sc^, and Ti^ for Sn*. However, the behavior of the two ions in the magma must be different. Ringwood (1955) suggests that com­ plexes in which the central ion possesses a charge of four or more will become enriched in the residual magma, since the larger the charge in the central cation the less readily the complex will be accepted into a silicate structure. On the other hand, ions with smaller charges can easily be incorporated into the silicate lattice as long as the require­ ments of ionic radius and electronegativity are satisfied. According to the above, Sn^ should be accumulated in the residual magma, while Sn^ should enter the silicate melt. Ringwood (1955, p. 198) also contends that inasmuch as the radius of Sn^ (0.92 A) is very similar to that of

Ca^ (0.99 A), "it might be expected that Sn^ would be camouflaged in

Ca-bearing minerals entering into early fractions in excess of Ca, and thus becoming impoverished in the magma." However, Sn^ does not be­ have as expected and also stays in the residual magma. Ringwood ex­ plains this phenomenon in terms of the higher electronegativity of Sn^

(1.65) as compared to that of Ca (1.0). Under these conditions, Sn-O bonds are more covalent and hence weaker than Ca-O bonds, thus allow­ ing Sn2 to stay in residual magmas.

Many experiments have been made in order to investigate the behavior of tin during transport and deposition in processes connected with magmatism. Perhaps the best known hypothesis is that proposed by

Daubree (1849) , who states that tin is carried out of a crystallizing magma as the volatile tetrafluoride. This reacts with water in fissures of the enclosing rock and hydrolizes to form cassiterite as indicated in the following reaction:

SnF4 + 2H20 Sn02 + 4HF.

According to Daubree, the hydrofluoric acid thus liberated would attack the feldspars to form topaz and quartz. Fluorine in this way is also freed to form fluorine micas and fluorite.

Smith (194 7) synthesized cassiterite from a sodium stannate solution at temperatures around 450°C at pressures he assumed to be around 2000 atmospheres. He observed that a decrease in the alkalinity of the mixed stannate solution would precipitate cassiterite due to the consumption of sodium by quartz and the formation of sodium silicate.

Smith's basic equations are as follows:

Na2Sn03 + H2O — 2NaOH + Sn02

Na2SnC>3 + Si02 —- Na2Si03 + Sn02

Na2Sn03 + CO2 7* Na2CC>3 + Sn02.

Barsukov (1956) opposed Daubree's thesis, contending that since tin is definitely amphoteric it should occur as a cation only in acidic media, while it is well known that phenomena normally observed in tin deposits, such as albitization of feldspars and muscovitization of biotite in granitic rocks, must have taken place in an alkaline medium.

Under such conditions SnF^ could not possibly exist.

He then proposed the idea of formation of cassiterite from a sodium stannate complex as follows: 52

Na2Sn(OH,F)6 + 2H20 — Sn(OH)4 + 2NaF + 2HF

Sn (OH) 4 SnO(OH)2 + H20

SnO (OH) 2 Sn02 + H20

Na2Sn(OH,F)6 Sn02 + 2NaF2 + 2HF.

In this way, hydrofluoric acid was also produced which gave rise to the formation of topaz, fluorite, and quartz after reacting with feldspars.

Later, Barsukov, Kurilchikova, and Vernadsky (1966) ran some new experiments using tin hydroxide (which they derived as shown above) as the basic tin component and combined it with chlorine, carbon dioxide, boron, silicic acid, and fluorine. They concluded that "tin cannot be transported by hydrothermal solutions except in the form of hydroxy- fluo-stannate complexes which formed according to the following reac­ tion:

n_m Sn4 + nF + mOH — SnFn(OH)&~ ."

Perhaps Barskukov's ideas may account for the transportation of tin in fluorine-bearing deposits, while Smith's results may perhaps be applicable to the formation of non-fluorine deposits. It is clear that the chemistry of the carrying solutions was not necessarily the same for all tin deposits, and formation of cassiterite may have occurred by more than one specific process.

The Emplacement of Rhyolitic Rocks

Generally, the emplacement of Mexican rhyolites can be sum­ marized, as follows. During Miocene time there was intense volcanic activity, mainly of rhyolitic magma, which resulted in the emplacement of layers of rhyolite and lenses of related pyroclastics. During the last stages of volcanic activity there were intrusions of acidic dikes and the escape of the gases held in the magmatic residuum. These gases min­ eralized and altered the walls of the openings in the pyroclastics and rhyolites.

The mode of emplacement of volcanic rocks and related pyro­ clastics has for a long time been a subject of debate; some factors, such as ultimate source of igneous materials and the mechanism affect­ ing their movements, are still controversial. However, it is generally well accepted that the volatile content in the system had a large in­ fluence in the eruption mechanism.

Pyroclastic materials that have been transported through the air are likely to lose most of their volatile constituents . Most of the pyroclastic rocks found in the studied area have considerable vesicula- tion which reflects an initial high content of volatiles. This fact sug­ gests that these pyroclastics were deposited by ash and lava flows rather than from aerial clouds.

The temperature of deposition of these deposits is uncertain, although welding and devitrification are often present in the rhyolitic tuffs studied. Various experiments mentioned by Ross and Smith (1961) indicate melting temperatures from 640°C for a granite under 5000 atmos­ pheres pressure of water to 1050°C for dry granite. However, they con­ sider that since rhyolitic magma is erupted under a sudden release of pressure its temperature must have been lower than that of a dry fusion of granite, indicating that a temperature of about 900°C seemed probable.

They also mention experimental work done by Smith, Irving, and Friedman

(no other reference is given) which has shown that welding of glassy rhyolitic ash ana pumice could take place at temperatures as low as 580°C under a water-vapor pressure of 20 atmospheres and a mechanical load of about 35 atmospheres. They believed that these conditions could exist in flows with a thickness of 800 feet and a density of 1.5. This condition is not common for Mexican pyroclastics but is not unknown—

La America area has a possible thickness of rhyolites of around 1000 meters. Boyd and Kennedy (1951) reported welding temperatures of a rhyolitic pumice from 775°C to 900°C. Ross and Smith (1961) conclude that rhyolitic tuffs are derived from a magma with an initial temperature no greater than 1000°C and probably below 900°C .

Porosity of the Battle ship Rock Tuff was also studied by Ross and Smith (1961) . They found that porosity was higher (between 50 and

70 percent) in the lower and upper 50 feet of a total thickness of 250 feet. Although no porosity measurements have been made in the Mexican tuffs, it appears that a relation similar to that of the Battleship tuff exists here. This may be one of the reasons why mineralization is more concentrated at the bottom and top of the tuffaceous lenses in the areas of higher porosity.

Apparent Geologic Controls

Observed tin mineralization occurs as open-space filling along narrow fractures in rhyolites, within voids in porous or fractured portions of the pyroclastic lenses, and between breccia fragments. Stratigraphic field relations and observed rock textures suggest that the tin-bearing rocks were emplaced at the surface. It is also believed that these rocks have not been highly eroded and hence their present characteristics are very much like those at the time of formation. All cassiterite is believed to be hypogene and to have been deposited almost simultaneously with the vein materials during the last stages of igneous activity. Tin-bearing areas seem to be located in indistinct domelike structures where rhyolite flows display irregular strikes and dips appar­ ently due to the effects of flowing. This peculiar attitude and the com­ mon larger thicknesses of the volcanic section near mineralized areas suggests that tin-bearing mineralization was emplaced near the eruptive centers. Only a few deposits were observed in thin rhyolitic flows. No regional structures seem to be present in the areas studied, and veins appear to have been emplaced along fractures produced by effects of cooling or by minor stresses derived from the same volcanic activity. Tin mineralization in the tuffs seems to be preferentially located at the bottom and top of the tuffaceous horizons, possibly due to a high­ er degree of porosity in these areas. Areas adjacent to porphyry dikes that cut through the pyroclastic lenses and the contact of the tuffs with the upper layer of rhyolite also seem to have constituted good traps for the emplacement of mineralization. Normally, accumulations of cassiter- ite and associated minerals in the tuffaceous horizons are located in zones with some degree of fracturing or shattering, and no tin deposits were observed in dense and intact tuffs. Of the mineralized breccias studied only those formed in fault intersections or in horsts or grabens were of economic significance. No breccia-forming structures observed had an apparent extension of more than 500 meters and most had less than 100 meters. The rock fragments were always rotated, but the small extent of tilting suggests that the breccia-forming forces were not very large. Alteration of wall rock was observed in every place where there is tin mineralization. Jasperization, kaoiinization, silicification, and 56 oxidation of iron minerals were the common types of alteration observed. Feldspar phenocrysts often remain unaltered, but in many places they are conspicuously kaolinized. Alteration seldom extends beyond 50 cm from the vein walls but occasionally was observed to go as far as 2 meters. Apparently, the mineralizing fluids and the rhyolitic magma had a common origin, and the degree of abundance of tin minerals within a certain area is primarily dependent on the amount of tin present in the original source. In general, tin mineralization observed was of small extent, thus suggesting a low tin content in the mineralizing fluids, or a lack of proper conditions for its deposition. Kelly and Turneaure (1970) and Turneaure (1971) review the geo­ logic characteristics of Bolivian tin deposits; they assume depths of deposition of 2000 to 4000 meters for tin-tungsten deposits and 350 to

1000 meters for the "shallow-seated" Tertiary deposits, with tempera­ tures of formation that range from 235°C minimum for the bismuthinite- pyrrhotite assemblage to 530°C for the precipitation of early quartz, apatite, and cassiterite. Present knowledge of the Mexican tin deposits in rhyolites does not permit a comparison with the Bolivian type of deposit, since all fac­ tors, from deposition conditions to general mineral assemblages, are quite different. However, if the concept of hydrothermal regeneration of Turneaure (1971, p. 220) could be applied to the Mexican deposits, there would be a possibility for the existence of major accumulations of tin values at depth within deep-seated equivalents of the rhyolitic rocks. Due to the large thickness of the rhyolitic section studied, it was not possible to investigate the nature of the rocks at depth near the emanative centers, but the possibility of a mineralized subvoicanic or intrusive phase of the Bolivian type at depth must be kept in mind for future studies. PRODUCTION TECHNOLOGY

Tin production in Mexico comes mainly from small deposits mined by gambusinos. Only three tin companies utilize wage-earning mining labor for the exploitation of their deposits; they are Campania

Estanera de Avino, Mina El Naranjo, and Compania Estanera Mexicana.

Typically, gambusinos only mine vein-type, alluvial, and elluvial deposits. When a prospect is found to contain economic min­ eral values, an informal association is made between the finder and other gambusinos to mine it. The finder provides powder and tools, and his associates are paid with all the ore that they can mine during a cer­ tain time interval, depending on the richness of the deposit. Concen­ tration of the ore is done in planillas mexicanas , which are small trenches about 2x1x0.5 meters, each having an inclined bottom.

Ground ore is placed on the higher part of the planilla and water is then constantly added to this pile by means of a small pan; thus, the gangue material is washed down, leaving the heavy minerals remaining near the higher part of the trench. This procedure obviously results in poor re­ covery, which very seldom exceeds 50 percent.

Compania Estanera Avino works the tin mine of La Esperanza.

This company has installed a gravity concentration plant of 30 tons per day. When in operation, mill head grade is reported to be 0.5% Sn; how­ ever, this plant does not work regularly, mainly due to lack of ore and to metallurgical recovery problems. Present operations at the mine involve following higher grade shoots, which may be several tens of meters wide.

57 If an average of 0.3% or 0.4% Sn can be proved in a deposit, exploita­ tion is possible by open pit methods.

Mina El Naranjo has a two-table gravity concentration plant at the mine. The installation treats 20 tons per day, and at present is operated exclusively with material from old mine dumps. Mill head grade is not known precisely, but it is assumed to be between 0.5% and 1% Sn.

Compania Estanera Mexicana owns the mine of El Huizache, which is not being worked at present due to lack of known reserves and to unavailability of a nearby concentration plant. This company owns a gravimetric concentration table installation, which is presently shut down. A metallurgical test run by this company on ore from El Huizache gave the following results: with mill head grades of 1.92% Sn (selected material), a concentrate of 20% Sn was obtained in one pass over the table with a recovery of 53.2 percent. When a second pass was amde, eliminating only the fine tailings, a concentrate with 20.08% Sn was ob­ tained with a recovery of 24.99 percent for a total recovery of 63 .63 per­ cent, producing a concentrate with an average grade of 20.08% Sn.

Comision de Fomento Minero owns a 30 ton per day gravity and flotation concentration plant in the town of Pinos, Zacatecas. This plant is not being operated at present due to lack of a consistent guaranteed feed. The combined production from various mines might justify its re­ opening .

All tin concentrates produced in Mexico are sold to the four tin smelters that exist in Mexico; three of them are located in the city of 59 San Luis Potosi and one in Mexico City. Total smelting capacity of these plants is around 60 tons of concentrates per month.

Most data on milling and smelting economics in Mexico still remain unpublished, and it is not often possible to gain access to this type of information.

Recovery of very fine grained cassiterite has been a tremendous problem in most areas where medium-taonnage deposits have been found.

It is possible that unless a metallurgical method is developed for im­ proved treatment of this type of ore, no large-tonnage, low-grade deposit will be put into production in Mexico for a long time. GEOCHEMICAL STUDY

Orientation Study

Tin associated with rhyolitic rocks is known to occur mainly in

Mexico and, to a smaller extent in Bolivia and the United States. In most places, these deposits are worked by small mining outfits or by individuals, usually with rather limited benefits. These factors have contributed greatly to the scarcity of published technical and geologic papers on this topic.

To the writer's knowledge there is no single publication that deals with the application of geochemical exploration techniques to the search for tin in this type of environment. The current work has thus been a combination of orientation and exploration surveys.

A study was made of the distribution of tin in stream sediments and in soils of tin-bearing areas to investigate the magnitude and type of response to be expected from other unknown mineralized areas.

Similarly, a mechanical analysis was made and a study was performed on the distribution of tin in various size fractions of stream sediments, soils, and bank materials. Finally, the dispersion of tin in soils and stream sediments derived from the vicinity of a known tin deposit was also investigated.

Tin in Natural Surface Waters

For a general point of view, one can perhaps mention Schuiling's

(1967) observations on the solubility of tin. He calculated the stability fields of the system Sn-^O and deduced that the utilization of 60 61 geochemical prospecting by analyses of surface waters is useless in the exploration for tin deposits. " . . .as even waters which would be in equilibrium with cassiterite would have such low tin concentrations as to be below the detection limit." He calculated the solubility of cas­ siterite in a typical surface water to be 10" at 25°C and 1 atm. This value is many orders of magnitude below the detection limit by any of the analytical methods known to date.

Schuiling also recognized that the addition of other ions, such as chlorine, or a change in the pressure and temperature conditions would cause the total solubility of tin in water to be higher. In spite of this factor, he concluded that analyses of surface waters for the explor­ ation of tin deposits are not feasible.

Schuiling's observations are confirmed by experiments carried on by students of the Camborne School of Metalliferous Mining of Eng­ land under the direction of Dr. K. F. G. Hosking (Hosking, 1967) . For this study, water samples emerging from South Crofty tin mine were ana­ lyzed and in all instances their tin content was less than 2.5 ppm.

Varlamoff (1969) notes that in sulfide deposits where stannite,

CuFeSnS4, is present in the weathering zone, the secondary mineral varlamoffite is likely to be formed replacing stannite partially or com­ pletely, with the production of pseuaomorphs. He further adds that

" . . . varlamoffite is soluble in weak acids and may be reduced easily to less than micron size particles. These particles once suspended in water, form a kind of emulsion that can be transported easily over long distances and eventually dispersed in the soil." 62

Varlamoffite apparently has been also produced by Bonnici et al. (1964) by hydrothermal treatment of cassiterite at 300°C at a pres­ sure of 80 atmospheres, thus suggesting that varlamoffite can also be formed in non-sulfide deposits.

It is finally worth mentioning that even though cassiterite is not soluble under normal surface conditions, it can be reduced by mechanical transportation to micron grain sizes which are carried in suspension by stream waters over great distances.

It thus seems that analyses of waters for tin in geochemicai exploration are useless in most situations and that, even though there might be places where micron-size particles of tin minerals may be present as suspended colloids in natural waters in anomalous concen­ trations, hydrogeochemical methods of prospecting for tin should be avoided unless there are strong special reasons to use them.

Tin in Vegetation

Geobotanical and biogeochemical methods of prospecting have not had the development of other geochemicai methods because they re­ quire a certain specialization in botany and because the distribution of plants that can be used for this purpose is generally erratic.

Many authors have studied the reactions of several plants to varying concentrations of ore minerals in soils, but it appears that only a few have included tin in their investigations. Millman (195 7) made a biogeochemical study in a copper-tin area in southwest England. He analyzed leaves and twigs of various trees for their Pb, Ag, Zn, Sn, and

Gu content and concluded the following: 63 1. ...lead is concentrated preferentially in the twigs of several species as compared with their leaves, and copper, tin, zinc, and silver are concentrated in the leaves.

2. ...no common species was found to show preferential concentration of tin.

3. ...comparison of the analyses of plants with those of the underlying soils, shows that although the biogeocnem- ical method of prospecting is suitable for detecting lead, zinc, and copper mineralization, the method is unsatis­ factory for tin in view of the low-ratio uptake of that metal by plants, even with substantial quantities of cas- siterite present in the soil.

4. ...for tin, analyses of soils would be much more satis­ factory, since the uptake of this metal by plants is so small.

Millman's conclusions are obviously limited to the plant species and conditions of the area he studied, but his conclusions can be gen­ eralized without great danger of mistake. Of course, there are other plants, such as Equisetum sp., which are capable of extracting tin from the soil, but again their ratio of uptake is very low as compared to the amount of tin present in the soil.

Since most Mexican tin deposits are practically monometallic, the only possible metal to be used as a pathfinder is tin itself. Also, vegetation in the Mexican tin provinces is rather scarce because of the semidesert climate. For these reasons, it is evident that geochemical prospecting for tin by plant analyses is unsuitable for Mexico.

Tin in Stream Sediments

Geochemical prospecting for tin by stream sediment samples has been a successful exploration tool in various places, for example,

Thailand, Malaysia, Cornwall, etc.; it was thus thought proper to test the possibility of using this method in Mexico. 64 During the initial stage, stream sediment, bank, and soil samples were taken from several tin districts of Durango and Zacatecas.

They were separated into several grain sizes, and the different fractions were analyzed for tin. This study was done to investigate the distribu­ tion of tin in the various size fractions and thus select the proper size for analyses. The results obtained are shown in Table 9.

Table 9. Distribution of Tin in Different Size Fractions of Soils, Stream Sediments, and Banks from Durango and Zacatecas

Soil Stream Sed. Bank

Mesh % Sn % Sn % Sn (B .S .S .) weight PPm weight PPm weight PPm

+20 19.96 42 52.97 100 19.78 42

Coarse -20+40 17.23 63 24.85 36 24.31 13 Sand -40+60 14.91 41 9.93 105 18.64 18

-60+80 12.85 45 4.15 45 10.80 33

-80+100 6.34 76 1.41 73 5.14 35

Fine -100+120 6.38 33 1.34 22 4.41 9 Sand -120+150 11.96 62 2.38 21 7.46 32

-150 10.29 30 2.89 17 9.36 8

Soil values: average of 11 samples.

Stream Sed values: average of 11 samples.

Bank values: average of 6 samples.

Table 9 shows that, while the smaller 10 percent by weight of the soil and bank samples is within the -150 mesh fraction, the smaller 10 percent of the stream sediments is in the -60 mesh fraction. The smaller 60 percent is very similar for soils and banks and covers all the

-40 mesh fraction, while for the stream sediments it nearly includes the

-10 mesh fractions.

The uniformity coefficient (maximum size of smallest 60 percent divided by maximum size of smallest 10 percent) is higher for the soils and banks than for the stream sediments. These values are not surpris­ ing if one considers that stream sediments are by nature more hetero­ geneous than soils or banks.

Tin seems to be preferentially accumulated in the -40+60 frac­ tion of stream sediments but shows fair detectable amounts from 20 to

100 mesh. The -80 mesh particles appear to travel longer distances from the source in stream sediments; for this reason and to avoid further grinding, this fraction was selected for general stream sediment, surveys.

This same approach was also extended to soil surveys.

A study of the distribution of tin was made downstream from the mine of El Huizache in the Sierra de Pinos to investigate the magnitude of the tin values in the stream sediments and to determine the distances they traveled in this particular environment. It was found that within a distance of 3.0 km the values stayed within an interval of 7.5 to 10 ppm; thus, samples with 10 ppm are considered anomalous (Fig. 2, in pocket) .

This low value is possible due to the fact that the tin mineralization at

El Huizache is intimately associated with silica, and, as previously described, the mineralized breccia is highly silicified. This makes the tin minerals difficult to liberate into the stream sediments. In the Juan

Aldama district, where silicification is not as strong, stream sediment values range from 6 to 400 ppm Sn, the higher value possibly correspond­ ing to a sample where minute cassiterite grains were included. In La

Barrosa area, where silicification is even less, downstream values ob­ tained were as high as 500 ppm, as shown in Table 10.

Table 10. Distribution of Tin Values Downstream from La Barrosa Mine

Tin content (ppm) in various mesh sizes (B.S.S.) Distance from Mine -20 -20+40 -40+60 -60+80 -80

100 m 175 50 50 400 450

600 m 120 50 50 500 500

1500 m 375 50 50 75 420

It appears that even though general geologic conditions are very similar in all the rhyolitic tin provinces the values of this metal to be considered as anomalous in stream sediments derived from different mines are quite different from one area to another. It also seems that the differences in background are dependent more on the mineralogical association of the ore rather than on the amount of ore present. Deter­ minations of background values must then be made specifically for each individual area.

It is perhaps worthwhile to note that many stream sediment samples from the Sierra de Pinos area were analyzed for cold extractable metals to determine whether or not this procedure would help to locate anomalous concentrations of tin. The distribution of heavy metals was found to be so erratic that their use as pathfinders was considered useless in this area. Perhaps the method could be used in other areas where the base metal content is higher than in Sierra de Pinos.

Tin in Soils

Geochemical soil surveys can provide cheap and reliable infor­ mation in many areas where suboutcropping ore may be concealed under a residual cover, either in known mineralized districts or in new fields where other evidences indicate the possibility of occurrence of subout­ cropping ore.

To obtain satisfactory results from a geochemical soil survey, it is essential to establish by experiments the geochemical procedure best suited to meet local conditions. If possible, it is best to under­ take these experiments in the vicinity of known ore deposits of the type one expects to find. In this study, a preliminary orientation survey was carried on in the Juan Aldama tin district and on the mine of El Huizache of the Sierra de Pinos district.

Mineralogical Composition of Soils. An experiment was car­ ried out on a soil sample from near Cerro de Elena in the Juan Aldama tin district to investigate the overall mineralogical composition and the nature of the heavy minerals present in the area. For this purpose, a small trench 1.0 x 1.0 x 1.0 meters was dug in alluvial soil and an 8 kg representative sample of this material was taken. The heavy mineral fraction of the sample was separated by gravimetric concentration in a laboratory-size concentration table. A sample of the concentrate was then subjected to magnetic separation. The nonmagnetic fraction was concentrated in bromoform, and all the different phase of the sample were studied at the central laboratories of the Consejo de Recursos Naturales No Renovables by X-ray diffraction to identify the mineral components. They were also analyzed for their tin content by atomic absorption.

The results obtained from this study were as follows:

Original sample: 2759 ppm Sn magnetite, hematite, quartz, feldspars, cassiterite, zircon, topaz, ilmenite

Magnetic fraction: 83 ppm Sn magnetite, hematite, ilmenite

Nonmagnetic fraction: 450 ppm Sn quartz, feldspars, hematite, cassiterite, zircon, topaz, ilmenite

Heavy nonmagnetic 1720 ppm Sn fraction: zircon, topaz, cassiterite, hematite, ilmenite

Medium nonmagnetic 5250 ppm Sn fraction: quartz, cassiterite, feldspars

Light nonmagnetic 2660 ppm Sn fraction: quartz, feldspars, cassiterite.

These results suggest that, while cassiterite is strongly attached to quartz and feldspars, it can be easily separated from the heavy non- metallic (that is, zircon, topaz, etc.) and magnetic minerals.

A soil sample from Mesa del Venado was also studied by the same method. The results obtained were as follows:

Magnetic fraction: 100 ppm Sn magnetite, hematite, quartz, plagioclase

Nonmagnetic fraction: 100 ppm Sn hematite, plagioclase, quartz, ilmenite 69

Heavy nonmagnetic 200 ppm Sn concentrate: hematite, plagioclase

The fact that no cassiterite is reported among the minerals of this sample even though the analyses indicate some tin, is possibly due to the small concentration that escaped X-ray detection.

Finally, a complete mineralogical study of 10 representative alluvial soil samples from various tin districts of Durango and Zacatecas was ordered from Bureau de Recherches Geologiques et Minieres. The result of this work is shown in Table 11.

Besides the minerals already mentioned, Table 11 shows the existence of others which are obviously rock-forming minerals (that is, pyroxenes, amphiboles) and have little relation to the mineralizing stage.

Tin in Soils of El Huizache Mine. To investigate the super­ ficial geochemical expression of a deposit of this type, soils were col­ lected, on 50-meter intervals, along six lines with 100-meter separations, directly over the Huizache mine. Tin values obtained in these samples range from 1.25 to 30 ppm and they have a very erratic distribution, which is possibly an expression of the irregular location of fractures which served as channelways for mineralization. The low values are possibly due, as in the case of stream sediments from this area, to the difficulty of the cassiterite being liberated from its highly siliceous matrix.

The lines extended about 1 km on each side of the known ore body. No clear break between anomalous and normal values was ob­ served. If an area iike that of the Huizache mine were detected by a 70

Table 11. Mineralogical Composition of Alluvial Soil Samples from Various Tin Districts

(Analyses by Bureau de Recherches Geologiques et Minieres)

Sample No. Mineral 1 2 3 4 5 6 7 8 9 10

Cassiterite 0 X X X X X 0 0 0 0 Amphibole 0 0 0 0 Epidote tr tr tr Garnet tr Hematite X X X X X 0 X X X X Ilmenite 0 0 0 0 X X X X X Magnetite 0 0 0 0 0 0 0 0 0 0 Pyrite tr Pyroxene tr tr tr Rutile 0 0 0 0 0 0 0 0 Sheelite tr tr Tourmaline tr Zircon 0 0 0 0 0 0 X X X 0 Calcite 0 0 0 0 0 0 0 0 Durangite 0 0 tr tr Fluorite tr tr 0 tr Sphene 0 tr tr Topaz 0 0 0 0 0 0 0 0 0

Sample Location: 1 to 4—America-Potrillos district 5—Sierra de San Francisco (Las Barro- sitas) 6—Sapioris 7—La Barrosa mine 8 to 10—Juan Aldama district.

Tr = trace; X = more than 10% in concentrate; 0 = 1% to 10% in concentrate. stream sediment survey, by geology, or by any other method, it is pos­ sible that a geochemical soil survey would not permit the outlining of the extension of the ore body, although it might suggest its approximate location.

Trace Elements in Mineralized Rocks

A complete trace element study was ordered from the Bureau de Recherches Geologiques et Minieres on several mineralized rock samples from the Mesa del Venado area taken from the walls of old gambusino-type mines. These samples are obviously only representative for this area, but they also allow some general deductions on possible trace element associations in other areas and hopefully the use of some of them in geochemical prospecting. The results obtained from this study are shown in Table 12.

An empirical interpretation of these results permits the follow­ ing observations. Fluorine content is generally high, although it does not seem to be directly related to tin content. Mercury, silver, ger­ manium, cobalt, molybdenum, bismuth, boron, and cadmium concentra­ tions do not show any variation among the samples analyzed; this is possibly due to an analytical error derived from limitations in the methods used. Gold, nickel, manganese, strontium, barium, gallium, niobium, vanadium, ytterbium, tantalum, and beryllium contents are erratic with respect to tin. Finally, yttrium, lead, zinc, and copper contents seem to have some direct relation to tin content. Table 12. Trace Element Content of Mineralized Rocks of Mesa del Venado, Juan Aldama, Zac.

(Analyses by Bureau de Recherches Geologiques et Minieres)

Sample No. and Analyzed by Atomic Location Absorption Analyzed by Direct Reading Spectrometry

Sn Hg Au F Ni Ag Sn Mn Ge Sr Ba Cr Yb Sc Y Co Cu V Mo Bi Gd Pb B Be Cd Zn

No. 1 Mala 3300 1 60 5,500 7 1 3,950 190 6 107 62 5 16 2 115 5 36 10 7 3 31 146 20 9 6 2000

Nc. 2 Las Amanitas 400 1 55 11,000 25 1 307 83 6 490 350 14 11 6 90 11 21 14 7 3 22 50 20 3 6 100

No. 3 Santa Leonor 1020 1 75 240 5 1 1,000 250 6 25 64 5 29 2 215 5 13 10 7 3 28 46 20 11 6 315

No. 4 Benjamin 710 1 75 2,200 5 1 660 300 6 7 40 5 19 2 140 5 17 10 7 3 30 77 20 12 6 195

No. S Balde (Tunel) 1600 1 75 1,800 5 1 2,000 265 6 7 15 5 17 2 117 S 20 10 7 3 30 32 20 11 6 150

No. 6 Balde (Tiro) 220 1 75 10,500 5 1 128 210 6 13 17 5 15 2 110 5 12 10 7 3 28 33 20 12 6 125

No. 7 Palmas 350 1 75 3,000 6 1 232 150 6 200 80 5 12 2 80 5 16 10 7 3 18 50 20 3 6 220

No. 6 £1 Agua 680 1 67 1,500 5 1 420 145 6 27 107 5 17 2 160 5 16 10 7 3 26 36 20 8 6 75

No. 9 Chorrera 640 1 85 4,200 7 1 610 340 6 80 80 5 16 2 120 5 50 13 7 3 27 112 20 9 6 355

No. 10 La Picuya 460 1 75 2,900 5 1 355 220 6 22 38 5 14 2 92 5 30 18 7 3 28 200 20 11 6 675

No. 11 El Sanjon 320 1 75 2,500 5 1 187 260 6 32 110 5 12 2 66 5 16 10 7 3 26 56 20 9 6 95

Nc. 12 Cruz 2400 1 70 5,500 5 1 2,540 155 6 93 60 5 17 2 85 5 15 10 7 3 28 106 20 9 6 65"

All results are given In ppm except Au in ppb. 73

Multiple Regression Analysis. To have a more definite idea about the correlation of tin with the various trace elements of Table 12, a multiple regression analysis was made on these results using the same computer program, previously discussed. The perfect fit linear regres­ sion equation thus obtained was as follows:

Sn = C5 - 0.349 Au - 0.058 F - 0.155Ni - 0.071 Mn

- 0.104 Sr - 0.244 Ba + 0.227 Yb + 0.058 Y + 0.215 Cu

- 0.303 V + 0.534 Ca + 0.318 Pb + 0.159 Be + 0.551 Zn, where C represents a constant.

Zinc seems to be the element with the best correlation to tin in these samples, and the values obtained are large enough to make it an interesting element to be investigated as a possible pathfinder for tin in this environment.

Lead and copper have smaller correlation coefficients, but they still show some degree of direct relation to tin content. It is possible that analyses of these elements may offer some assistance in some specific cases of tin exploration.

Gallium showed the second best direct correlation to tin after zinc. The geologic meaning of this relation is not clear at this point; perhaps this element together with ytterbrium and yttrium should be given a little more attention on a more extensive sampling program and a de­ tailed laboratory investigation.

Strangely, although fluorine values are high, this element shows a negative numerical correlation to tin. This suggests that, although tin and fluorine must be genetically related, the content of one is not necessarily dependent on the concentration of the other. It seems 74 that even with this handicap, the possibilities of using fluorine as a pathfinder for tin should be investigated in more detail.

The logarithmic regression equation obtained for the same values of Table 10 was as follows:

log Sn = C6 - 0.239 Au - 0.26 7 F - 0.184 Ni + 0.04 Mn

- 0.163 Sr - 0.251 Ba + 0.404 Yb + 0.244 Y

+ 0.219 Cu - 0.270 V + 0.562 Ca + 0.277 Pb

+ 0.209 Be + 0.506 Zn.

This equation is very similar to the normal equation. Perhaps the most striking change is that the correlation of F, Yb, ana Y to log Sn improved greatly from the values obtained for the relations Yb-Sn and Y-Sn. The correlation of tin to the other elements used remains practically the same.

It is perhaps worthwhile to mention that, although not shown in the above data, the correlation coefficient obtained for Yb to Y was

0.901. Other nearly perfect direct correlation coefficients were as fol­ lows:

Ni-Sr 0.936

Ni-Ba 0.940

Sr-Ba 0.901.

Trend Surface Analysis at Juan Aldama

The area of Mesa del Venado in the Juan Aldama district (Fig. 3) was chosen for a trend surface analysis of tin values in soils, because the distribution of mine workings (Fig. 4) and the general geologic con­ stitution had already been investigated. This provides a relatively good basis for interpretation of results. 75

Tigea sc. 1:20,000 Tiqea

Tbr

Qal Qtal

Tbr

Tbr Qtal Tigea

EXPLANATION

LITHOLOGIC UNITS

| Qtal | TALUS BRECCIA AND CLAY (UNDERLAIN BY TUFF)

[ Qal j ALLUVIUM

| Tbr | AUTOCLAST1C BRECCIA (UNDERLAIN BY TUFF)

| Tigea j EXTRUSIVE ACID IGNEOUS ROCK

| Tr | RHYOLITE

SYMBOLS

• MINE WORKINGS

Figure 4. Geologic Map and Location of Tin Mining Works of Mesa del Venado in Juan Aldama, Zacatecas.—After Payan, 1971 76

The object of this study was mainly to investigate the efficiency of trend surface analysis in representing selective areas with higher tin concentrations which might be considered good exploration targets for further work.

Soil samples were taken on a grid of 100 x 100 meters at vari­ able depths between 25 and 50 cm. To facilitate the handling of data, a grid of only 10 x 10 samples (or 1 sq km) was considered for the study.

However, this area covered most of the Mesa del Venado prospects. All samples were homogenized and sieved to -80 mesh and analyzed for their tin content by atomic absorption.

Basis of the Method. All trend functions are described by mathematical equations. Fitting of any of these functions to a set of data can be accomplished by various methods of which the least squares method is by far the best known and easiest to use. In fitting trend sur­ faces by the least squares method, we seek to minimize the sum of squares of the vertical distances between the data points and the given surface. The problem is then to calculate the equation coefficients so that the sum of the squared deviations is the least possible.

The method of estimating these coefficients is the same for all trend surfaces regardless of their order. The general procedure is illus­ trated below for the case of a first order trend surface. The equation for a first order trend surface is:

ztrend = A + Bx + Cy.

We then have to find the values of A, B, and C so that the sum of the squared deviations is minimized. The deviation may be 77 represented by "Di" and is equivalent to the difference between the ob­ served and the trend values.

= = Di ^0bs ~ ^trend ^obs ~ A - Bx - Cy. The sum of squared deviations may now be expressed as a function of A,

B, and C as follows:

F«A,B,C) = Di2 = (zobs - A - Bx - Cy)2.

The minimum value of this function occurs when the partial derivatives equal zero.

-|f- = 2 (Z0bs - A - Bx - Cy) (- 1) =0

= 2 (Z0bs ~ A - Bx - Cy)(-x) = 0

= 2 (Zobs - A - Bx - Cy)T-y) = 0.

For n number of data points, the above equations can be reduced as follows:

= ZZQbs An + Blx + CZy

x = 2 XZQbs AXx + BXx + CZxy Jzobsy = A*-y + Bixy + ciy2. Thus, we have a system of three equations and three unknowns which can be solved by matrix inversion or by the Gauss elimination method.

For a first degree trend surface only the three equations shown above are produced, but this number is increased to 10 for a third degree trend surface. The solution for a system of this nature would be formid­ able without a computer. There are 110 terms (not counting duplications) to determine before one could even attempt to solve the 10 equations, and some of these terms go up to sixth order. 78

A computer program to fit trend surfaces up to third order was developed from group class work at The University of Arizona. This pro­ gram was later modified by the author to calculate trend residuals and to plot both trend surfaces and trend surface residuals.

Results Obtained. All analytical values were treated as one whole population because no significant difference was noted in tin values from one geologic material to another. This is not surprising because in fact all units in the area are of rhyolitic composition.

The trend surface equations obtained for tin values in soils of

Mesa del Venado were as follows:

FIRST ORDER EQUATION

Z = 48.153 - 0.017x - 0.902y

SECOND ORDER EQUATION

Z = 39.121 - 1.087X + 4.286y + 0.250x2 - 0.286xy

- 0.376y2

THIRD ORDER EQUATION

Z = 56.659 - 13.327X + 2.109y + 1.777x2 + 2.197xy

- 1.114y2 - 0.046x3 - 0.169x2y - 0.079xy2

+ 0.076y3.

Initially, a hand-contoured map of observed tin values was made to be used as a comparison with computer contouring; this is shown in Figure 5. Broadly, it shows two main anomalous highs, one on the east and another on the west of the area. No great influence was noticed here of contamination from the old mine dumps. In fact, the isopleths with the lowest values are around the center of the area where there is an accumulation of small prospects. 79

45 20 45 40 40

100+ 42 38

70 40 39 50 '45^ 100; 40

45

42/ 40 100 +

Figure 5. Hand-contoured Map of Observed Tin Values in Soils of Mesa delrVenado

Contour values in ppm. 80 The contour maps of the first, second, and third order trend surfaces and residuals are shown in Figures 6, 7, and 8, respectively.

The first order trend surface obviously only shows the general trend of the tin values in a place. The second and third order trend surfaces clearly shown an increase of values toward the center on a north-south trend and a decrease on a northeast-southwest direction. This configura­ tion broadly reflects the location of the barren upper rhyolite as opposed to the breccias which are directly underlain by mineralized tuffs which normally carry higher tin values (Fig. 4) . This relation is best observed in the third order surface .

Residual surfaces are rather similar for the three orders; this is not surprising if one considers that the goodnesses of fit were 17,

23, and 31, respectively for the first, second, and third order equations.

It appears that the anomalous spots of the hand-contoured map show as negative computed residuals. This may be the result of the smoothing effect of the trend surface in areas where there are much lower concentrations of tin than expected. It may be concluded in this case that there seems to be an inverse relationship between tin values in soils and high residuals and that second and third order trend surfaces broadly reflect the differences in tin concentration between the upper rhyolite and the tuffs.

Geochemical Experiment at Sierra de Pinos

To test the efficiency of geochemical prospecting in the search for tin deposits associated with rhyolitic rocks, an experimental geo­ chemical survey was carried on in the Sierra de Pinos, Zacatecas. This 47

46

45

44

43 -50

42 -20

-2 40 -2 39 -.4

FIRST ORDER TREND SURFACE FIRST ORDER TREND SURFACE RESIDUALS

Figure 6. First Order Contour Maps of Mesa del Venado

Contour values in ppm.

oo 50

44

46 48 50 52 54

48 46 -30 44

42 -30 40

38 -4 36 34 32

0 200 m SECOND ORDER TREND SURFACE SECOND ORDER TREND SURFACE RESIDUALS

Figure 7. Second Order Contour Maps of Mesa del Venado

Contour values in ppm

CO to •36' 38 -4 40 48 60 30 42 56 43 54 44 52

48 43 -30 46

42 44

-50

42 -15 -30

0 200 m THIRD ORDER TREND SURFACE THIRD ORDER TREND SURFACE RESIDUALS

Figure 8. Third Order Contour Maps of Mesa del Venado

Contour values in ppm.

00 CO 84 area was chosen following the geological and geochemical conclusions previously mentioned, keeping in mind that, according to the type of deposits that occur in this environment, only the search for large- tonnage, low-grade deposits would justify a systematic exploration program.

Stream Sediment Survey

Samples were taken in all streams which have a length above the average in the area at intervals from 300 meters to 1 km (Fig. 2) .

Utilization of a helicopter greatly facilitated this part of the study. A total of 700 samples were collected in three weeks by two field crews, covering an area of approximately 600 sq km. Reconnaissance geologic mapping was also done during this period, based on a previously made photointerpretation. All of these samples were analyzed for tin by the colorimetric method of gallein and by cold extractable heavy metals.

The values obtained by this last method were very erratic and did not yield good results in this exploration program.

Using the statistical criteria mentioned earlier, a value of 17 ppm was considered as the threshold of significance. A smaller target of 65 sq km was thus selected in the southern portion of the initial area for further study which was called Picacho-Adjuntas (Fig. 9, in pocket).

Soil Survey "B" Horizon

Soil Samples were collected from the "B" horizon of the Picacho-

Adjuntas area at depths from 15 to 30 cm and at 100-meter intervals along lines spaced 250 meters with a strike N. 45° W. (perpendicular to the main fracturing direction). It was thought that if a large-tonnage, 85 low-grade tin deposit existed in the area it should have a geochemical surface expression in soils, either as a leakage anomaly or as a result of direct weathering of the deposit itself.

Statistical studies were made on soils coming from different lithologic units, excluding obviously high values, and it was found that all samples could be considered to belong to the same population within the 85% confidence limits. As at Mesa del Venado, this result should be expected, considering that all materials in the area are rhyolitic.

A mean value of 9 ppm Sn was found for soils of the area, with a standard deviation of 5 ppm. Based on the results obtained over El

Huizache mine, a value of 20 ppm, equivalent to the mean plus twice the standard deviation, was selected as threshold.

A second order anomalous interval from 12 to 20 ppm was also allowed to assist in the interpretation.

Distribution of tin in these soil samples was rather erratic, possibly due to the occasional presence of minute "kidneys" of wood tin derived from minor veinlets scattered heterogeneously throughout the area. Anyhow, it was possible to outline several anomalous areas which were divided into first and second order anomalies, using the criteria given above (Fig. 9).

Two other important targets resulted from this interpretation, one located south of rancho El Saucillo around Mina San Jose and the other on the eastern side of Picacho around Mina Nueva.

Soil Survey "C" Horizon

To test the response in different materials obtained from an anomalous area, soil samples were collected from the "C" horizon in the 86

San Jose and Mina Nueva area. The results obtained were plotted to­ gether with those from stream sediment and "B" soil horizon samples in

Figures 10 and 11.

"C" horizon samples were taken at the bottom of small trenches

0.75 to 1.5 meters deep. Sample sites were located at 50-meter inter­ vals along a contour line which more or less surrounded the anomalous area previously outlined by stream sediment and "B" horizon samples. A crossed line was also sampled in the higher parts to enclose completely the anomalous area.

Tin values of these samples ranged from 8 to 33 ppm for the

Mina San Jose area, with a mean value of 13 ppm and a standard devia­ tion of 6 ppm. With a threshold value of 19 ppm, equal to the mean plus once the standard deviation, the mineralized area of Mina San Jose could be very well outlined.

In the Mina Nueva area, values for the mean and standard devi­ ation were equal to 10 ppm and 5 ppm, respectively. Again a threshold value of 15 ppm, the mean plus once the standard deviation, outlined the area where the mineralized workings are located.

Samples from the "B" horizon would also have detected these anomalous areas, although with less definition. In view of the great length of time involved in the collection of "C" horizon samples, it is considered advisable that only "B" horizon samples be taken in any future surveys of this sort.

Trend Surface Analysis

All soil values from the "B" horizon were treated numerically, using the same trend surface computer program described for the Juan 87

. N* -J.

\o

• o 0« ;0

/

EXPLANATION

• STREAM SEDIMENT SAMPLE B ANOMALOUS \ 18.00 P. P.M. Sn • NORMAL ^ 18.00 P. P.M. Sn o SOIL SAMPLE ( "C" HORIZON ) • ANOMALOUS 15.00 P. P.M. Sn o NORMAL *=—= 15.00 P.P.M. Sn O SOIL SAMPLE ( "B" HORIZON ) PICACHO • ANOMALOUS 14.00 P.P.M. Sn

0 NORMAL 14.00 P. P.M. Sn LOCATION MAP MINE WORKING APROX. SCALE 1140,000

Figure 10. Geochemical Orientation Survey on the Area of Mina San Jose 88

C •

• O *° o o

sc. 1:20,000

EXPLANATION

• STREAM SEDIMENT SAMPLE G ANOMALOUS 18.00 P.P.M. Sn • NORMAL ^ 18.00 P.P.M. Sn O SOIL SAMPLE ("c" HORIZON ) « ANOMALOUS 19.00 P.P.M. Sn >ICACHO O NORMAL ^ 19.00 P.P.M. Sn O SOIL SAMPLE ("B" HORIZOH )

LOCATION MAP 9 ANOMALOUS 14.00 P.P.M. Sn APROX. SCALE I'.40,000 o NORMAL ^ 14.00 P.P.M. Sn ® MINE WORKING A2 DIAMOND DRILL HOLE

Figure 11. Geochemical Orientation Survey in the Area of Mina Nueva Aldama area. Tin values were smoothed prior to the numerical treatment to reduce the number of data to keep within program and computer memory limitations. This was done by averaging groups of nine data points and plotting the obtained value at the central location. These were hand contoured and plotted together with the map of location of mine work­ ings in Figures 12 and 13. The trend surface analysis gave the three following equations, with 19%, 25%, and 31% goodness of fit, respec­ tively:

FIRST ORDER EQUATION

Z = 9.082 - 0.262x + 0.260y

SECOND ORDER EQUATION

Z = 6.461 + 0.099x + 0.260y - 0.003x2 - 0.045xy

+ 0.022y2

THIRD ORDER EQUATION

Z = 6.005 + 1.166x + 0.084y - 0.251x2 + 0.018xy

+ 0.030y2 + 0.013x3 - 0.005x2y + O.OOOlxy2

- 0.0004y3.

It was again observed that while the actual trend surfaces broadly reflect the general trend concentrations according to lithology the surface residuals tend to express the high-value concentrations by negative anomalies, such as those located on the northeast and central portions of Figures 14, 15, and 16.

The anomaly of Mina Nueva was the only one to coincide with a residual high, which is clearly observed in the second and third order residual maps (Figs. 15 and 16) . In this instance, the third order sur­ face was only significant to its sixth term, so it is possible that sc. 1:100,000 EXPLANATION

LITHOLOGIC UNITS

• ALLUVIUM

•RESIDUAL SOIL

j ('c.b^ it ^ ( , J jjk, V*s/ vV A., • TALUS •RHYOLITIC FLOWS WITH SEMI' 0 f^-^vcav COLUMNAR STRUCTURE AND OCCASIONAL FLOW BANDED V/WO1 / / (,' TEXTURE ( , PiCACHO/r •RHYOLITIC TUFFS AND IGNIM • V BRITES, GENERALLY PSEUDO • STRATIFIED LOCATION MAP SYMBOLS Scole I ! 375,000 • MINE WORKINGS

NOTE ! Tin valuos wert smoothed prior to contouring

Figure 12. Geologic Map and Location of Tin Mining Works of Sierra de Pinos 91

I Km

Figure 13 . Hand-contoured Map of Observed Tin Values in Soils of Sierra de Pinos

Contour values in ppm . 92

FIRST ORDER TREND SURFACE

FIRST ORDER TREND SURFACE RESIDUALS

Figure 14. First Order Contour Maps of Sierra de Pinos

Contour values in ppm. SECOND ORDER TREND SURFACE

SECOND ORDER TREND SURFACE RESIDUALS

Figure 15. Second Order Contour Maps of Sierra de" Pihos

Contour values in ppm. 94

THIRD ORDER TREND SURFACE

r "^trrrr-J. /* /^5 N

THIRD ORDER TREND SURFACE RESIDUALS

Figure 16. Third Order Contour Maps of Sierra de Pinos

Contour values in ppm . 95 second order surface calculations would have been enough to arrive at the same conclusions.

After comparing geochemical results with the geologic observa­ tions of the area, it was thought advisable to test the Mina Nueva anomaly by diamond drilling .

Diamond Drilling at the Mina Nueva Anomaly

A series of only three shallow drill holes was planned to in­ vestigate at depth the nature of the material that produced the geochem­ ical surface anomalies found. The holes were drilled at points spaced from 100 to 200 meters within the anomaly. DDH No. 1 and DDH No. 2 were vertical and DDH No.3 was inclined 40° with a direction N. 50° E., which was more or less perpendicular to one of the main fracture direc­ tions .

Since tin in this environment is likely to occur in very fine cassiterite grains that may be washed off small veins during drilling, sludge samples were taken at 2-meter intervals and analyzed together with the cores for tin.

Generally, the holes were drilled through an upper thin layer of rhyolite with a thickness of 4 to 10 meters and continued in rhyolitic tuff to the bottom which ranged from 54 to 5 7 meters. Alteration intensity varied from none to about 50 percent total rock alteration and consisted mostly of kaolinization with silicification and minor oxidation of iron minerals. 96

Tin grades in the drill holes were as follows:

Drill Hole No. 1

Core 0.073% Sn

Sludge 0.088% Sn

Drill Hole No. 2

Core 0.081% Sn

Sludge 0.063% Sn

Drill Hole No. 3

Core 0.104% Sn

Sludge 0.106% Sn

Due to the low content of tin, it was difficult to observe the minerals by the unaided eye. Careful analysis suggested that tin was present in the cores as cassiterite in very minute brown to black grains contained in the lithophysae and in small vein fillings. Petrographic studies of some samples verified the nature of the rock and the mode of occurrence of tin.

Although the tin values detected are obviously anomalous and suggest the existence of an area where there was tin mineralizing action, they do not reach the minimum grade of 0.4% Sn required for an economic operation in the area.

Therefore, it may be concluded that the geochemical study of

Sierra de Pinos has successfully led to the selection of a significant target whose grade unfortunately remained under the minimum economic grade required. Other geochemical anomalies selected during the same study, which also showed good geologic conditions, are now being drilled in Sierra de Pinos. It is possible that all anomalous areas 97 detected there will be found to contain sub-economic tin grades only workable by gambusino-type methods, but there is also a chance that one of them could reach the required values to guarantee a well-planned mining operation. CONCLUSIONS

Most of the tin now being mined in Mexico comes from small mines associated with rhyolitic rocks. These are worked in a primitive way by local miners, called gambusinos. Typically, the ore from these mines is produced by the exploitation of small veins which contain cassiterite as the only identified tin mineral.

Geologic observation in many tin districts of Durango and

Zacatecas shows that cassiterite is also present within the lithophysae of many rhyolites and rhyolitic tuffs and in the spaces between rhyolitic breccia fragments.

Common vein minerals are cassiterite, specular hematite, cristobalite, tridymite, chalcedony, topaz, magnetite, zircon, and ilmenite. Less often fluorite, tourmaline, and opal are observed. In­ timate intergrowth of these various minerals suggests that most vein materials must have been deposited almost simultaneously. The presence of obvious high-temperature species suggests that mineralization was carried originally in a pneumatolytic or a boiling phase.

Cassiterite is nearly insoluble under normal conditions, and there is no evidence of the presence of other soluble tin minerals, such as stannite, in the area. This fact and the fact of the mineralogical association suggest that the deposits are of hypogene origin.

Porphyry dikes in the areas visited are of rhyolitic composition and are always spatially related to the main rhyolitic body. This sug­ gests that porphyry dikes were derived from the same igneous system as

98 the rhyolitic magma. Concentration of tin mineralization along the con­ tacts of the dikes with the intruded rocks suggests that mineralization came almost simultaneously, or perhaps a little after, the porphyry dikes. Both dikes and mineralization were emplaced during the last stages of igneous activity.

All deposits examined during this study are of small extent, and even the rich ore shoots encountered in some of them had big voids left by insufficiency of the filling materials. This suggests that min­ eralizing solutions were not abundant or that proper conditions for mineral deposition did not exist. In spite of this, the general geologic conditions observed make it reasonable to believe that there might exist tin deposits with large tonnage and low grade which could be mined by open pit methods. A first economic estimate requires a minimum of 10 million tons with average grade of 0.4% Sn. Reconnaissance tin explora­ tion must be undertaken in areas where there are possibilities of locat­ ing this type of deposit. The deposits may occur as epigenetic disseminations in rhyolitic tuffs or as vein and void fillings in large bodies of breccia.

Tin mineralization in tuffs is preferentially located at the bot­ tom and top of the tuffaceous lenses, in the vicinity of porphyry dikes and near the contact with the upper layer of rhyolite. Large bodies of breccia of possible economic significance are normally located at the intersection of faults or in horst and graben structures. No brecciated bodies observed extended more than 500 meters and most of them were smaller than 100 meters. 100 Alteration is always related to mineralization although in most places alteration is not intense. Jasperization, kaolinization, silicifi- cation, and oxidation if iron minerals are the common types observed.

Vein walls are seldom altered to more than 50 cm from the vein edges, but occasionally alteration extended as far as 2 meters.

Linear regression equations of the chemical composition of tin- productive rhyolites seem to be quite different from one district to an­ other. SiC>2 and CaO contents, however, appear to be related directly to tin content of the rock. Of the few barren and productive rhyolites com­ pared, the former average slightly higher in silica and alumina content than the latter; other than that no important chemical difference was ob­ served. At this point, it is not possible to present any conclusive evi­ dence about significant differences between barren and tin-productive rhyolites. Perhaps a study based on a more extensive sample population and an investigation of the content of tin or other elements in individual rhyolite-forming minerals would contribute to the solution of this problem.

Trace elements in mineralized rock samples show a good direct correlation of zinc to tin content. Lead and copper also have a direct correlation to tin through a rather small coefficient, which may not be considered significant.

Gallium content is also directly related to tin content. The geologic meaning of this relation is not yet clear. This element, as well as ytterbium and yttrium, should be investigated in more detail for their association with tin mineralization. Fluorine has the highest concentra­ tion values of the 28 elements investigated in the mineralized samples. 101

Strangely enough, its correlation coefficient to tin is low and negative, thus suggesting that, although these two elements must be genetically related, the content of one is not necessarily dependent on the concen­ tration of the other.

Tin seems to be preferentially concentrated in the -40+60 mesh fraction of stream sediment but shows fair detectable concentration from

20 to 135 mesh. On the other hand, possibly due to the smaller size,

-80 particles seem to travel longer distances. For this reason, the -80 mesh fraction is suggested for general stream sediment reconnaissance.

Orientation survey would help greatly to make the right selection.

Tin values in stream sediments from the stream that drains El

Huizache mine are very low. To consider this stream anomalous, a value of 10 ppm had to be taken for the threshold. Values much higher than this were found elsewhere in apparently unmineralized areas. It is con­ cluded that large tin values in stream sediments are important and should be followed up; however, low values do not necessarily rule out tin occurrences. Liberation of tin minerals into stream sediments will de­ pend largely on the nature of the matrix or the form in which tin is present and on the intensity of the erosion. What is anomalous in one area may not be so in another.

Cassiterite is commonly associated in stream sediments and in all kinds of soils of tin-bearing areas with magnetite, specular hema­ tite, topaz, cristobalite, and other silica polymorphs; care must be exercised when trying to identify pan concentrates by visual observa­ tions. It is sometimes mistakenly assumed that all the heavy minerals 102 concentrated from a stream sediment or soil of a tin-producing area are cas site rite.

Soil samples do not necessarily reflect the real extension of a tin deposit of the kind studied. Vein deposits may be inadvertently overlooked in a soil survey due to their limited expression. Brecciated ore bodies, like that at the El Huizache mine, show erratic distribution of tin values in soils, possibly due to the heterogeneous array of frac­ tures and other mineralizing channels. Tin values in tuffaceous rock masses are usually initially low, and, since soil dispersion is neces­ sarily irregular, concentration of tin minerals in soil will often not fol­ low a definite pattern. Soil anomalies over this type of deposit, like that of the Mina Nueva, may show some grouping of anomalous values neighboring a group of background values. In a general soil survey, the

-80 mesh fraction may be used for analysis, but for a sharper outline of a given bedrock source, the samples should be ground prior to analysis.

Since all lithologic materials present in the areas studied are of a rhyolitic nature and presumably are genetically related, the tin con­ tent in soil samples taken over the various materials tended to belong to one statistical population. This fact permitted treating these values equally for trend surface analyses.

Trend surface fitting generally reflected the normal slight changes of concentration from the upper rhyolitic layers to the underlying tuffs. Anomalous spots obtained by regular hand contouring generally were shown as negative computed trend surface residuals. On a regional reconnaissance survey, trend surface studies could perhaps help in selecting the areas of higher tin concentrations. However, in small 103

areas, like the ones used for this study, the output is comparable to

that obtained by empirical hand contouring. The only advantage of the

computer method is the rapidity of obtaining the configuration which can

be done with or without calculating the trend surfaces.

It is generally thought that geochemical prospecting may be a

useful tool where used wisely in exploration for tin associated with

rhyolitic rocks. Like any prospecting technique, it may be disastrous

if applied loosely and without scientific context.

Since most of the cassiterite associated with possible large- tonnage, low-grade deposits in Mexico is present in a very fine frac­ tion, it is probable that no deposit of this type will be put into operation in Mexico for a long time unless a metallurgical method is discovered to

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Millman, A. P., 1957, Biogeochemical investigations in areas of copper-tin mineralization in south-west England: Geochim. et Cosmochim. Acta, v. 12, p. 85-93.

Onishi, H.( and Sandell, E. B., 1957, Meteoric and terrestrial abun­ dance of tin: Geochim. et Cosmochim. Acta, v. 12, no. 3, p. 262-270.

Ordonez, E., 1900, Las rhyolitas de Mexico: Mexico, D. F., Inst. Geol. de Mexico.

Payan, R., 19 71, Estudio geologico de la region de Juan Aldama, Zaca- tecas: Unpub. Engineering thesis, Escuela Superior de Inge- nieria y Arquitectura, Mexico, D.F.

Petrova, Z. I., and Legeydo, V. A., 1965, Geochemistry of tin in the magmatic process: Geoch. Intl., v. 2, no. 2, p. 301-307.

Rankama, K., andSahama, Th. G., 19 50, Geochemistry: Chicago, University of Chicago Press .

Rattigan, J. H., 1963, Geochemical ore guides and techniques in ex­ ploration for tin: Australasian Inst. Min. Met. Bull. 207, p. 137-151.

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Ross, S.C., and Smith, L. R., 1961, Ash-flow tuffs, their origin, geologic relations and identification: U.S. Geol. Surv. Prof. Paper 366.

Sainsbury, C. L., 1969, Tin resources of the world: U.S. Geol. Surv. Bull. 1301.

Sainsbury, C. L., and Hamilton, S. C., 1967, Geology of lode tin deposits: London, Intl. Tin Council Conference, p. 313-355.

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Secretaria del Patrimonio Nacional, Direccion general de minas y petro- leo: Mexico, D.F., Public files.

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Wood, G. A., 1956, A rapid method for the determination of small amounts of tin in soils, in. Geochemical Exploration Symposium, Vol. 2: Mexico City, 20th Int. Geol. Congress, p. 461-474. 107° 106° 105° 104° PARRAL

VILLA OCAMPO

LA ZARCA Cuchillas ' de Zarca y Cienega de GOMEZ Escobar PALMITO PAL AC

v Sierra de , * la Candela

Potrillos X Los Lobos •-/ Sapioris America SANTIAGO ^PAPASQUIARO X Mesa Altai La Soiedad X Cerro de ) X la\Mesa /Los ArticulosX _ v . . ^Bomex X x Los Pielagos I Qy La^arr^sa Penon BiancoX^ lc' El Tomii 'Jerre- '^del Gal lo I \ \ v . V < ® \ X x Piedras ...-WJ. Ca^iatlan |_a Esperanza El ^ El Lebrillo x Azules X \ CAMftTLAN Progre Huachumeta X Las Cuevillas X Las Palmas ( X GPE. VICTORIA de Abaio El Maguey •X Rio Ver Cienega de^Batrjes El Piloncillo Sierra de Sla.Mari X * / X El i jehuento DI^RANG Ochoa / Ci * f X' Sierra \Sta. Lu( Bayacora berlad VICENTE Los MolinUlosi' GUERRERO Quebrada ^ * Cerro Bkjnc'o "X" de Ma Gtfdc Cerro de / San Miguel / Guanajuatillo

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f \j | JALISCO |

EXPLANATION

PRODUCTIVE TIN-BEARING AREA 0 EXAMINED

V PRODUCTIVE TIN-BEARING AREA

X NON-PRODUCTIVE TIN-BEARING AREA

RAILROAD

c/- IMPROVED ROAD

UNIMPROVED ROAD

— K. RIVER • | TL*S 4.UTIU y- Cienega de GOME -v Escobar V0 PALMITO PAL/!

"V v Sierra de , * la Candela /

Potnllos X Los Lobos Sapioris America SANTIAGO (PAPASQLIIARQ X Mesa Attaii La Soledad X C'erro de } X la\Mesa

/Los Arficulos X _ v . •/ /§ome* * X llos Pielagos I Qy La^Bari^sa •Penon Blanco X\ Jc El ToauL X C'err^-'del Gallo ' ^ ' ~T ^ v . V ' ® X \ Cofiatlari E ranza \ x v Piedras La spe El Lebrillo « Azules X \ CAWftTLAN Huachumeta X Las Cujeviilas de Abajo X Las Palmas ( X GPE. VICTORIA \ y Ei Maguey iX Rio Ver Cienega de Batrjes El Piloncillo Sierra de Sla.M X DlpRANG X El Huehuento Ochoa * f X- Sierrc Bayacora \ Sta. L Libertad I VICENTE^ Los Molln>l Iosr \ GUERRERO Quebmda ) * Cerro Blanco V de^Stfacamaya ^ y Cerro de / { San Miguel \ / Guanajuatillc ' x de^Temoyo/* Cerro Gordo rJ fx San Andres i v /' Las Ventanas lP^° ^ Ange^ -k >. J , *X ri,y ' 1 • iY/,- f U, ; V Los Gavi/anei -r j m^eiu i ! v V f-C /y ; ] j J s _ >4- o { % \ \ j (/•?{ ) /' /

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•Penon Blanco \ ® \ TECOLOTES , \ Cdriatlarl |_ Esperanzo CONCEPCION 'vLr»., Q DEL ORO CAMftTLAN is- ,^•-—^.^.4. i ./ GPE. VICTORIA •V w Juan Aldoma • l 'CAMACHO /A . ! AI . ^ JUAN ALDAMA res Sierra de Sla.Mariax X .sy Miguel Ochoa NIEVES N ieves * f ,/"\ x S " X- Sierra de \ R/O Grande X, Bayacora \ Sta. Lucia <;

5°fc,''° / VICENTEN^NJ _ I V GUERRERO y^aralc^rhuites , ^ * Cerro Blanco X El Naranjo

) x Cerro de ® X NSain AltcL San Miguel ^ / Guanajuatillo ,V / d^.Temoya/'' /! X Chapultepec fj fa San Andres de Teul FRESNILLO ' "X jO]o de Angel/ Las Ventanas X

U. , Los Gavi/ane# ZA$AT£CAS \ x! : Valparaiso i ( ^ HuejuquiMa y /LO buemad SALINAS V V X ) N <•- < Sierra de Jere, ) f /> ) i / Suslican X_/ , .J- I !.' ) — .JjSPetongo; llanueva Tepezol PINOS Rincon Asienlos / ( •.Romos Huizache v

\ X .Sierra del Paiaro

•• / X 'Calvillo Paso de ^X^j X En^arnacion de Diaz /

JUCHIPILA . Jx N 1^" r / TeocaWiche I / / V. / EXPLANATION

PRODUCTIVE TIN-BEARING AREA (g) EXAMINED

V PRODUCTIVE TIN-BEARING AREA JLOTES ^ -cf \ CONCEPCION ^ .. ' DEL IORO ^ X NON-PRODUCTIVE TIN-BEARING AREA

/ MJVCHO I / RAILROAD / IMPROVED ROAD

«„•' UNIMPROVED ROAD

K RIVER

23°

SALINAS

PINOS / con de (5 Romos Huizache

2? FIGURE- I \ y .Sierra del Paiaro INDEX MAP OF STATES OF ZACATECAS \ \y AND DURANGO SHOWING LOCATION nation Diaz y OF TIN BEARING AREAS \ 7 DEPT. OF MINING AND GEOL. ENGR,

JOSE LUIS LEE M. DISSERTATION 1972

GEOL. ENGR. |: X ' V

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LITHOLOGIC UNITS

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STATE OF ZACATEVAS ^ / y/ j ^ SCALE y (/ , _ \J J>y 0_l0 / / J o Kmr / /^T" |>Vy?

LOCATION MAP

EXPLANATION

LITHOLOGIC UNITS

| Qo> 1 ALLUVIUM RESIDUAL SOIL pMBfel TALUS

|Tr | RHYOLITIC FLOWS WITH SEMICOLUMNAR STRUCTURE Akin rtrr.ACinMai pi ntt/ BAuncn tcvtiioc PINOS

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r v LOS TANQUES 0 0

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GEOLOGIC S'l

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RHYOLITIC FLOWS WITH SEMICOLUMNAR STRUCTURE

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RHYOLITIC TUFFS AND IGNIMBRITES , GENERALLY

PSEUDOSTRATIFIED

ii ZONE OF ALTERATION (MAINLY KAOLINIZATION)

GEOLOGIC SYMBOLS

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TOPOGRAPHIC SYMBOLS

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SLOPE

GEOCHEMICAL SYMBOLS

O STREAM SEDIMENT SAMPLE

TIN CONTENT IN P. P.M.

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APROXIMATE SCALE I % 25,000

0 1000

f

FIGURE-2 GEOLOGIC MAP AND STREAM SEDIMENT

GEOCHEMICAL SURVEY OF SIERRA DE

PINOS, ZAC. MEXICO

GEOLOGICAL ENGINEERING

JOSE LUIS LEE M. I DISS. 1972 W v

X x,.v,, x

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STATE OF ZACATF.CAS • Qtal '(.//

Kms.

location map

EXPLANATION

lithologic units

ALLUVIUM Qr RESIDUAL SOIL

VETAS LAS LISAS Ofal TALUS Tr RHYOLITIC FLOWS WITH SEMICOLUMNAR STRUC - TURE AND OCCASIONAL FLOW BANDED TEXTURE

Tt RHYOLITIC TUFFS AND IGNIMBRITES, GENERALLY ^ _\VETAS GUADALUPf^ PSEUDOSTRATIFIED

Za ZONE OF ALTERATION (MAINLY KAOLINIZATION)

geologic symbols

FRACTURE OR FAULT

NORMAL FAULT >- STRIKE AND DIP LITHOLOGIC CONTACT

topographic symbols Tt

aa BARRANCA y

Qal

VETA DEL

Qtal

Qtal 'T COLONIA DE GARCIA SALINAS

\ SAN JUAN DE APROXIMATE SCALE 25,000 ' LOS HERRERA 1000

Mts Qal CERRO DEL : HUERFANO

EL SA

VETAS LF

%

picacho

, 'h'/H/f Kms. / // - Z: ^ i location map

EXPLANATION

lithologic units

Qal ALLUVIUM Qr RESIDUAL SOIL

VETAS LAS LISAS Qtal TALUS

Tr RHYOLITIC FLOWS WITH SEMICOLUMNAR STRUC - TURE AND OCCASIONAL FLOW BANDED TEXTURE

RHYOLITIC TUFFS AND IGNIMBRITES, GENERALLY VETAS GUADALUPf^& % PSEUDOSTRATIFIED

Za ZONE OF ALTERATION (MAINLY KAOLINIZATION)

geologic symbols

FRACTURE OR FAULT

NORMAL FAULT

STRIKE AND DIP

LITHOLOGIC CONTACT

topographic symbols

^ STREAM - DIRT ROAD

OD HOUSES •• VILLAGE

-{

DAM X MINE WORKING

SCARP SLOPE

geochemical symbols

c3 FIRST ORDER SOIL ANOMALY 20 P. P.M. Sn CP SECOND ORDER SOIL ANOMALY 12 - 20 P.P. M. Sn

FIGURE —9 GEOCHEMICAL SOIL SURVEY "B" HORIZON AREA PICACHO-AD JUNTAS

geological engineering

jose luis lee m.| diss. | 197;