Some geologic and exploration characteristics of porphyry copper deposits in a volcanic environment, Sonora, Mexico

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Authors Solano Rico, Baltazar, 1946-

Publisher The University of Arizona.

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Link to Item http://hdl.handle.net/10150/566651 SOME GEOLOGIC AND EXPLORATION CHARACTERISTICS

OF PORPHYRY COPPER DEPOSITS IN A VOLCANIC

ENVIRONMENT, SONORA, MEXICO

by

Baltazar Solano Rico

A Thesis Submitted to the Faculty of the

DEPARTMENT OF MINING AND GEOLOGICAL ENGINEERING

In Partial Fulfillment of the Requirements For the Degree of

MASTER OF SCIENCE WITH A MAJOR IN GEOLOGICAL ENGINEERING

In the Graduate College

THE UNIVERSITY OF ARIZONA

19 7 5 STATEMENT BY AUTHOR

This thesis has been submitted in partial fulfillment of re­ quirements for an advanced degree at The University of Arizona and is deposited in the University Library to be made available to borrowers under rules of the Library.

Brief quotations from this thesis are allowable without special permission, provided that accurate acknowledgment of source is made. Requests for permission for extended quotation from or reproduction 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 judg­ ment the proposed use of the material is in the interests of scholar­ ship. In all other instances however, permission must be obtained from the author.

SIGNED:

APPROVAL BY THESIS DIRECTOR

This thesis has been approved on the date shown below:

/CW 7 , / f WILLIAM C . PETERS Date Professor of Mining and Geological Engineering To Amalia

iii ACKNOWLEDGMENTS

The author is indebted to Minera Tri6n, S.A. de C.V., Hermo- sillo, Sonora, and its staff for help and encouragement as well as finan­ cial support. Especially mentioned are Ings. Alberto J. Terrones and

Rigoberto Reynoso. The Government of Mexico through the Consejo

Nacional de Ciencia y Tecnologia provided payment of fees and a monthly stipend.

The author acknowledges Dr. William C. Peters, professor and kind person, whose patience and guidance are high appreciated, and

Drs. Spencer R. Titley and DeVerle P. Harris, members of his thesis com m ittee.

The following persons contributed with their knowledge, com­ ments, and significant data: Gustavo Calderdn, former graduate student at The University of Arizona; Russell Corn, Regional Geologist, Kerr-

McGee Corporation, Tucson, Arizona; Darrel Dean, graduate student.

The University of Arizona; Adelaide Gutierrez, Consulting Geologist,

Hermosillo, Sonora; Amador Osoria, Manager of Exploration, Industrial

Minera Mexico, S.A ., Mexico, D.F.; Luis Palafox, Senior Geologist,

Compania Minera Constelacidn, Hermosillo, Sonora; Claude Rangin,

Research Geologist, Institute de Geologia, and professor at the Univer- sidad de Sonora, Hermosillo, Sonora; and Ariel Echavarri P ., professor at the Universidad de Sonora.

iv TABLE OF CONTENTS

Page

LIST OF ILLUSTRATIONS...... v ii

LIST OF TABLES...... v iii

ABSTRACT...... ix

1 . INTR O D U C TIO N ...... 1

2 . REGIONAL GEOLOGY OF S O N O R A ...... 3

Pre Cam brian...... 3 P a l e o z o i c ...... 4 M e s o z o i c ...... 5 T ria ssic-L o w er J u r a s s ic ...... 5 C r e t a c e o u s ...... 6 C e n o z o ic ...... 7 T ertiary...... 7 Structural G e o lo g y ...... 8 P a leo g eo g ra p h y ...... 11

3 . RELATIONSHIP BETWEEN VOLCANIC ROCKS AND PORPHYRY COPPER D E P O SIT S...... 17

Distribution of Volcanic Rocks ...... 17 Porphyry Copper Type Occurrences ...... 29

4 . SOME CHARACTERISTICS OF PORPHYRY TYPE OCCURRENCES IN VOLCANIC HOST RO C K S ...... 31

Porphyry Occurrences in Sonora ...... 34 Petrography and Alteration ...... 34 M ineralization ...... 42 S tru ctu re...... 43 G e o c h e m istr y ...... ' ...... 47 C a p p in g ...... 50

5 . SPECIFIC FACTORS AFFECTING EXPLORATION...... 52

T e c t o n i c s ...... 52 Middle Tertiary Volcanic C over ...... 53 Structural G e o lo g y ...... 55 Outcrop Characteristics and Photogeology ...... 55

v v i

TABLE OF CONTENTS— Continued

Page

W eathering and C lim a t e ...... 56 G eoch em ical E xp loration ...... 57 M in e r a liz a tio n ...... 59 Petrography and Alteration ...... 59 G e o p h y s ic s ...... 60

6. METALLOGENIC CONSIDERATIONS...... 62

7 . CO NCLUSIO NS...... 73

SELECTED BIBLIOGRAPHY 78 LIST OF ILLUSTRATIONS

Figure Page

1. Geologic map of Sonora, M exico ...... in p ock et

2. Structural m ap ...... 9

3. Precambrian paleogeography ...... 12

4 . P a le o z o ic p aleogeograp h y and P a leo z o ic iso p a ch m a p ...... 14

5. Late Triassic-Early Jurassic paleogeography ...... 15

6. Early Cretaceous paleogeography, showing reported s e c t io n s ...... 16

7. Distribution of Laramide and Cretaceous volcanism ...... 18

8. Laramide and Cretaceous volcanic localities ...... 19

9. Cretaceous and lower Tertiary volcanic rocks in S on ora, M e x ic o ...... 20

1 0 . L ocation o f reported p r o s p e c t s ...... 30

11. Distribution of Tertiary postmineral volcanic cover ...... 54

12. Interpreted distribution of magmatic activity along M ex ic o 's north w est c o a s t ...... 67

13. Sillitoe's idealized model of a porphyry copper deposit .... 69

14. Diagrammatic sketch of a vertical cross section at El Alacrctn, Sonora ...... 71

... LIST OF TABLES

Table Page

1. Some characteristics of porphyry occurrences in S o n o r a ...... 35

2 . Rock g eo ch em ica l v a l u e s ...... 48

v iii ABSTRACT

A review of the geology, including a brief description of the

structural geology and paleogeography of Sonora, Mexico is presented.

Basement rocks cropping out are mainly older PreCambrian metamorphic rocks and younger Pre Cambrian sedimentary rocks. A north-south trough formed during Paleozoic time is now characterized by quartzites and

limestones and Triassic-Jurassic marine and terrigenous sedimentary rocks. Intense volcanic activity occurred during Cretaceous and Tertiary

tim e .

The Lower Cretaceous and Laramide volcanic rocks are de­

scribed, and their possible relationship with some porphyry copper type

occurrences in Sonora is discussed. The petrographic, alteration, struc­

tural and mineralization characteristics of these occurrences are also

d e scr ib e d .

Among the several factors considered important in the search

for porphyry copper deposits in Sonora are tectonics, mid-Tertiary post­

mineral volcanic cover, and the effects of weathering and climate on

outcrop characteristics. The use of photogeology, geochemistry, and

other exploration techniques is examined.

A review of copper metallogenesis concepts indicates a direct

relationship between Laramide volcanism and the occurrence of porphyry

copper type deposits. This concept would be useful in delineating zones

of better chances for discovery.

ix CHAPTER 1

INTRODUCTION

Even though the general geology of southern Arizona is compara­ ble and almost identical to that of Sonora, one of the most striking dif­ ferences noticed by the exploration geologist is the greater abundance of volcanic rocks in Sonora, both in distribution and thickness and in varia­ tio n .

Volcanic rocks of Triassic-Jurassic age have not been reported in Sonora as they have been in southeastern Arizona. Nevertheless, the opening of the Cretaceous period was marked by sedimentation in a shal­

low sea and volcanism continued, with several minor interruptions, during the Late Cretaceous and Tertiary periods. Rocks of these ages

are found widespread throughout all of Sonora, and they serve the geol­

ogist both as a guide to favorable host rocks and as a nightmare.

Since very little radiometric dating has been done, the geolo­

gist must commonly base his work on regional structure and stratigraphic

and petrographic relationships.

It is one of the main purposes of this thesis to make a prelimi­

nary distinction between possibly mineralized pre-Laramide and Laramide

volcanic rocks and middle and upper Tertiary extrusive units (Fig. 1, in

p o c k e t).

Data are mostly derived from bibliographic research and six

years of experience in mine, project, and reconnaissance exploration in

1 2

Sonora. Most basic geologic information was derived from investigations

done before 1946 by American and Mexican pioneers, among whom were

Aguilera (1896), Durable (1900, 1901), Flores (1929), Taliaferro (1933),

R. E. King (1934, 1939), Valentine (1936), Imlay (1939), Cooper and

Arellano (1946), and Wilson and Rocha (1946).

More recently, geological reconnaissance has been done in

western and northwestern Sonora by Anderson and Silver (1973), Anderson

(1974), Merriam (1972a), and several others. Radiometric dating has

been done by Damon et al. (1962), Livingston (1973b), and Institute de

Geologfa (1962).

A program of systematic geologic mapping is underway through

the Institute de Geologfa of the Universidad Nacional Autondma de

Mexico and the Universidad de Sonora, and several theses and disser­

tations have been completed recently. Hopefully, a better knowledge of

.the geology of this region will be reached in the near future .

The geologic mapping done by R. E. King (1934), the work that

settled basic stratigraphy and tectonics in central Sonora, has been one

of the most fruitful sources of information. CHAPTER 2

REGIONAL GEOLOGY OF SONORA

Precambrian

Precambrian basement, a continuation of the southwestern

American craton, is exposed in northwestern Sonora as far south as 50 km north of Hermosillo, the state capital. It is composed mainly of two different lithological types: (1) Altar Schist and (2) Aibo Granite. The first type presents a thick sequence of metasediments, schists, quartz­ ites, and some limestones, and metavolcanic rocks, metarhyolites, meta rhyodacites, and meta-andesites. The Aibo Granite, found 20 km south of Caborca, is a highly altered, reddish, micrographic granite that has been dated at 150 and 710 m .y.; however, field relationships indicate an older age for this granite and a possible age resetting. Both types are intruded by dikes and sills of diabase and lamprophyre and are un- conformably covered by a fairly distinctive basal conglomerate. regional metamorphism is at greenschist facies with local contact effects of hornblende hornfels facies.

Fries (1962b) estimates the age of the metamorphic event at not less than 1600 to 1700 m .y., and Damon et al. (1962) give a radio- metric age of 1680 m .y. for the Altar Schist. More recently, Anderson and Silver (1973) reported a series of metarhyolites northwest of Bamori as being the oldest rock yet found with an age of 1790 m .y.

3 4

These rocks are equivalent to the Vishnu, Yavapai, and Pinal metamorphic complexes as well as that of Death Valley, California, formed during the Mazatzal Revolution and named "older Precambrian" in Arizona (Fries, 1962b).

Another important area of Precambrian rocks is the metamorphic terrain around Magdalena (Salas, 1970). Smaller zones occur at La Me­ rit a (Taliaferro, 1933), Sierra de los Ajos, southeast Esqueda (Adelaide

Gutiirrez, personal commun., 1974), and southeast Tuape.

Younger Precambrian rocks, unconform ably overlying the meta­ morphic and igneous basement, consist of more than 2,000 m of quartz­ ites, phyllites, limestones, and dolomites with a very low degree of metamorphism or even fresh—Goyotillo Group at Magdalena (Salas,

1970)—and have abrupt changes eastwards. The uppermost part of the

carbonate succession can hardly be distinguished from the unconform-

ably(?) overlying Cambrian sedimentary rocks. The younger Precambrian rocks can be correlated with the Grand Canyon Series and the Apache

Group in Arizona and with the Pahrump Series in California (Fries,

1962b ).

P a le o z o ic

The Paleozoic era was characterized by the development of a

shallow, elongate, north-south trough (Sonoran basin) that filled up more

rapidly in its northwestern corner. Cambrian sedimentary rocks are

found near Caborca and are of predominantly calcareous composition

with minor proportions of quartzites and shale (Cooper and Arellano,

1946). A quartzitic member of the group, the Arroyos Formation, has 5 similar counterparts in north-central Sonora, at Cananea (Capote quartz­ ite) , and northeast Sonora (Bolsa Formation).

Ordovician and Silurian rocks are generally missing except for a thin carbonate section at La Casita and Sierra Cobachi in south-central

Sonora (R. E. King, 1939) and possibly a carbonate facies near Caborca.

Similarly, Devonian through Permian sedimentary rocks are predominantly carbonates, as shown by the Martin and Escabrosa formations and the

Naco Group in northeast Sonora, by the Murcielagos, Venada, and

Monos Limestones in northwest Sonora, El Tigre Formation in Bavispe, east Sonora, and the Willard, Colorada, and Cobachi Mountains in cen­ tral Sonora.

M e so z o ic

Triassic-Lower Jurassic

Predominantly clastic rocks of continental origin are attributed to the Triassic-Early Jurassic period. Dumble (1900) named the Barranca

Formation and described it at its type location at La Barranca as con­ sisting of two members of massive sandstone (generally quartzites) divided by a thin-bedded sandstone and shale member containing coal and graphite. The total thickness of the formation is about 500 m. The

Barranca Formation crops out mainly in central Sonora, although R. E.

King (1934) suggested a possible extension southeastward to near

Moris and Lluvia de Oro in western Chihuahua.

The Barranca Formation appears to have a gradual change to a marine facies northwestward; in the neighborhood of La Colorada, fos-

siliferous limestones appear interbedded, and near Caborca, over 3,000 m of sandstones, quartzites, shales, and limestones of "Juratrias" age were reported by Baker (R. E. King, 1934). Apparently the area of depo­ sition of the Barranca Formation was restricted to the northwestern half and southern portions of Sonora, since no outcrops have been reported in northeastern Sonora.

C retaceou s

During early and middle Cretaceous time, Sonora was possibly subdivided into three elongate tectonic provinces. The westernmost province was characterized by continental or island-arc type volcanism represented by andesitic flows and tuffs of the Lista Blanca Formation and by granite-granodiorite intrusions. At the same time, thick succes­ sions of argillaceous and limy materials were being deposited to the east with intercalated andesites, breccia flows, and tuffs at Sahuaripa

(Potrero and Palmar Formations) (R. E. King, 1939), Lampazos(?) (Solano

R ., 1970), Nacozari(?) (McAnulty, 1970), and Santa Ana (Salas, 1970).

Correlation of these areas is difficult, considering that "Cretaceous sedimentary rocks in the Southwestern Province accumulated in isolated basins and are characterized by extremely rapid facies changes over short distances" (Lowell, 1973, p. 4); this in addition to the lack of information.

The sedimentary sequence of Cretaceous age in the northeastern corner of the state is generally lacking in volcanic material and repre­ sents shelf-type sediments or miogeosynclincal environments. These assemblages have been reported at the Cabullona Basin (Taliaferro, 1933;

Viveros M ., 1965). 7

Upper Cretaceous sedimentary rocks are reported only at the

Cabullona Basin and probably did not extend further south than Nacozari.

These rocks are also detrital and include continental sandstones and some volcanic material (rhyolitic tuffs) (Taliaferro, 1933). It is probable that most of the state was being subjected to either erosion or volcanic activity during Cretaceous tim e.

C en o zo ic

Tertiary

The Cretaceous-Tertiary boundary was marked by folding and faulting of previously deposited marine rocks and widespread intrusion of plutonic bodies, immediately followed by the intrusion of hypabyssal stocks, plugs, sills, and dikes, as well as by the coeval extrusion of volcanic ejecta. However, there is a suggestion of a previous orogenic folding of Cenomanian age (Claude Rangin, personal commun., 1975).

It was during this period,the Laramide orogeny—that main mineraliza­ tion processes were active, as pointed out by Damon and Mauger (1966) and many others.

This period of magmatic intrusion and mineralization started approximately during Late Cretaceous time and died out before the middle

Eocene, that is, from 80 to 50 m .y. B.P. The character of Laramide vol- canism, although not differentiated yet, seems to be predominantly an­ desitic compared to the generally more acidic volcanism in the mid-

Tertiary tim e. A second pulse of magmatism occurred between 10 and 40 m .y. B.P.; however, it was essentially volcanic with minor intrusive activity. Major activity took place during the time marking the boundary 8 between Miocene and Oligocene. Mid-Tertiary volcanism was better developed toward the eastern portion of the state within the Barrancas geomorphologic province (Alvarez, 1968). R. E. King (1939) gives a good stratigraphic and petrographic description of this province.

The Basin and tiange orogeny may be considered contempora­ neous with this period of volcanism or a little later. It caused elongate parallel zones of normal faulting trending north-northwest between which thick, poorly sorted clastic sediments were laid down in local, narrow basins. The Baucarit Formation of late Tertiary age is present almost all over the state, although it is better developed in the parallel Basin and

Range province in eastern Sonora.

A late stage of Pliocene-Pleistocene volcanism of generally basaltic composition developed during the formation of the basins and ranges and is found interlayered with the Baucarit Formation and uncon­ form ably overlie it. Large extensions of basaltic mesas are located west of Sonoyta, south of Guaymas, south of Moctezuma, and southeast of

Agua Prieta.

Structural Geology

The structural map (Fig. 2) is based on structural alignments derived from physiographic landforms, some ERTS-1-derived data, and a few reports dealing with regional geology of Sonora. Although not appar­ ent in the map, there are some east-west and east-northwest structures that were probably formed during PreCambrian or early Paleozoic time; these reflect major folds, some faults, and the generally east-west trending paleo-shorline during these eras. K < Y \ ' X

x\v \ V

\ / -----

Figure 2. Structural map 1

10

It can be easily observed on the map that four major systems of faults and fractures are predominant: northeast, northwest, north- northwest, and north-northeast. The first two are evidently related, with few exceptions, to pre-Laramide and Laramide events. The last two fault and fracture systems affect middle and upper Tertiary volcanics and are related to M iocene-early Pliocene tectonism.

Laramide and pre-Laramide structures are in general parallel to

Titley's (1972a) "Cretaceous discontinuities or faults" in Arizona; their trend ranges from N . 60° W. to N . 40° W. One of particular interest is the Cananea Nacozari trend (Echavez's "Central Belt," 1968), which is probably a deep-rooted structure that has suffered several periods of ac­ tivation, since it shows both Laramide and middle Tertiary mineralization.

Northeast alignments range from N. 35° E. to N . 60° E.; however, a N.

80° E.-trending system was probably formed during PreCambrian or Lara­ mide time, or both.

Laramide and pre-Laramide structures, partially obliterated by

Tertiary faulting, are generally normal, although some thrusts are present, such as that in the La Morita-Cabullona area in northeast Sonora. It should be pointed out that the absence of northeast- and northwest- trending structures on the map (Fig. 2) in the southwestern two-thirds of the state is due to lack of information.

As mentioned earlier, middle and late Tertiary tectonism was characterized by formation of elongate horsts and grabens with alluvium and volcanic filling (Pliocene Baucarit Formation), giving rise to the typical landforms of the "Sonoran Ranges and Parallel Basins" physio­ graphic subprovince. 11

Although most faults associated with this system are normal,

R. E. King (1939) and W isser (1966) report long, westward-overlapping thrust faults in the eastern limits of the subprovince near Sahuaripa.

Tectonism is still active; in 1887 a fault 10 km long was formed in the

neighborhood of Arizpe, Sonora.

The importance of this tectonic framework becomes evident when its influence on the location of Laramide and mid-Tertiary mineral­

ization is analyzed in Chapter 4.

Pale oqe ography

The most important feature of Sonora's geologic development was the formation of a shallow trough from as early as PreCambrian time until the Jurassic. The Sonora basin was affected by two main erogenic

events and several periods of epeirogenesis.

The concentration of Precambrian outcrops in the northwestern

corner of the state, the presence of small windows of Pinal Schist in

northeastern Sonora, and the recent dating of a granite in the Cananea

Mountains at 1440 + 15 m .y. by Anderson and Silver (1973), as well as

the presence of other Precambrian outcrops in the neighboring states of

Chihuahua and New Mexico (U.S.A .), suggest the extension of this

craton toward the east and northeast (Fig. 3).

Early Paleozoic seas covered a similar extent of land as those

of younger Precambrian age and apparently advanced to the south as a

narrow north-northwest-trending basin during Ordovician and Silurian

tim e. Carboniferous seas returned to cover the original early Paleozoic Figure 3, Precambrian paleogeography 13 area, and it was not until the Permian that a general southward trans­ gression covered more than two-thirds of the state (Fig. 4). Paleozoic seas deposited miogeosynclinal-type sediments; the existence of a eu- geosynclinal facies near the southern border of the state has been sug­ gested by De Cserna (1960).

After diastrophic movements of the Appalachian orogeny, the trough was reduced in east-w est extension and the seas receded to the north, the basin was filled with quartzites and terrigenous sediments in its southern half and with shaly marine sediments in the northern part

(F ig. 5 ).

The Cretaceous period was characterized by thick sections of sedimentary and volcanic rocks, both representing the north we sternmost portion of the Mexican M esozoic geosyncline. Its western limit is not well defined, possibly due to erosion; however, the Cretaceous volcanic terrain was probably an active arc or peninsula, since marine eugeosyn- clincal assemblages have been reported in the Pacific coast of northern

Baja California (Fig. 6). This idea has also been suggested by Bonneau

(1972) and by Claude Rangin (oral communic., 1975). 14

Possible mbximun extent of Pcleoz seas

Figure 4. Paleozoic paleogeography and Paleozoic isopach map MARINE FACIES

Figure 5. Late Triassic-Early Jurassic paleogeography 16

^ ELI A

| / | Volcanic enviroment

I 2 I Vc I conic "sedimentary environment

m Merino shel -type environmen

References: 1. Whetstone M ts., Az. (King, 8. Sierra de San Javier (King, 1934) 1934) 2. Bisbee district, Az (King, 1934) 9. Near Arivechi (King, 1934) 3. Cabullon Basin, Son. 10. NW of Ghinipas (King, 1934) (Taliaferro, 1933) 11. Sierra Gopercuin (McAnulty, 4. Santa Ana, Son. (King, 1934; 1970) Salas, 1970) 12. Sierra de Lampazos (Solano R ., 5. El Antimonio (King, 1934) 1970) 6. Sierra Sta. Rosa (King, 1934) 13. Sierra de El Tigre (Imlay, 7. Sierra Moradillas (King, 1934) 1939)

Figure 6. Early Cretaceous paleogeography, showing reported s e c tio n s CHAPTER 3

RELATIONSHIP BETWEEN VOLCANIC ROCKS AND PORPHYRY COPPER DEPOSITS

Distribution of Volcanic Rocks

Figure 7 attempts to show the distribution of volcanic rocks of

Cretaceous and early Tertiary age. Not included are those clearly of an eugeosynclinal assemblage with abundant sedimentary material.

The best detailed descriptions of volcanic rocks in Sonora have been made by Valentine (1936) at Cananea, McAnulty (1970) in the north­ ern Nacozari district, ChiapaC. and Thoms (1971) at the Pilares de Naco- zari area, Wilson and Rocha (1946), and R. E. King (1939) in east-central

Sonora. The following is a brief description of Cretaceous-early Tertiary volcanics, stratigraphic sections studies, and locations reported (Figs.

8 and 9 ).

Identification of lower Cretaceous volcanic rocks has apparently been done only in south-central Sonora and is mainly based on Dumble

(1900) and R. E. King (1939). The basis on which this age was assigned is that fossils of Cretaceous age were identified in limestones inter­ calated between the volcanic rocks and on the similarity between these rocks and those found by King in eastern Sahuaripa, Sonora (Albian

Potrero Formation). The thickness of lower Cretaceous volcanic rocks in this area is well over 300 m in a central zone comprising the Tonichi,

Onavas, Tecoripa, Santa Clara, La Dura, and Tezopaco areas; it prob­ ably thinned out westward. This central area is characterized by the

17 18

Figure 7. Distribution of Laramide and Cretaceous volcanism 13 0

I* La Margarita TV No cozorl 1 3 Son Marcia! 2 r a n a n g a c 8." P11 o r e s H.* Arroyo Torohumoro 3r E| Ala cron 9r Op u I o 6.- On o v o s 4* Cobullono IO.* Mocttzumo ISr ©Seycoro

S~Esquedo II - Sohuorlpo, Yoqol River IT.- La Dura 6 r Gr o n o dt fa s 12.• Lo Color a d o

Figure 8 Laramide and Cretaceous volcanic localities Thickness Nome Description Age UrLA MARGARITA {NE Coborco) Not Quartz latite pyroclastic flows pyroclastic rocks Reported "Pre-Mineral" "Siiiclfied volcanic*"

(Amador 0 so rioters comn) (1973) 2)r CANANEA . Son, -VTiT-v^-v— Tuff and agglomerate beds w m San Pedro andesite flows Pink and white tuffs Over MESA 1500 F m. Coarse agglomerate m Fine-grained purple andesitic tuffs Bed of coarse agglomerate containing boulders of Elisa Rhyolite up to5m 5vS:-fc^ Green and gray tuffs and agglomerates with* .'thin Interbedded flows p m Mariquita r Holocrystalline andesite , amygdaloldal at the base ;texture Late UNCONFORMITY becomes finer towards the top. Cretaceous- EE?3EE Granular lithic fuff and flow agglomerate in which the fragments and the Earl y matrix are composed of trachylic and andesitic material. Tertiary ±2100 HENRIETTA m • Elisa Rhyolite 7 Thick porphyritic rhyolite flows, IIght colored with phenocrystals of quartz, orthoclose, oligoclase and occa­ Svbapc sional biotite . « Fine-grained tuffs, dacite and latite flows and coarse agglomerates.

UNCONFORMITY Rhyolite r white to light bluff in color ; quartz phenocrystalos In a quartz orthoclase matrix, pseudoflowage. Agglomerate at •

''*r~ ELENITA Pink and purple trachyte, flow structure and large orthoclase ±2100 F m. phenocrystals. m J Increasing agglomerate flows towards the bottom One andesite, 1 n - — # soom layer , several sand layers with delta cross-bedding . Ocossionol 1 tuff 1 aye r s . l (Valentine .193 6 )

Figure 9. Cretaceous and lower Tertiary volcanic rocks in Sonora, Mexico Thickness Nome Description Age 3)r EL ALACRAN . ( SE Cananea, Son. ) P + * A Sequence of andesitic and docitic tephra. Grain size Not ^ r g , . g - K _ . • decreases from a volcanic center . Possible correlation to the base Laramldet?) Reported of the Henrietta Fm at Cananea.

(Darrel Dean.pers.comn.) ( 1974)

4)r CABULUON A , Son.

Over 200 m More than 800 Ft of tuffs and agglomerate ' Lot e 1 J r a Crete cows ♦ —Comas ss. Camas tuffaceous sandstone l i i Upper Cretaceous limestone (Taliaferro, 1933)

5 ).— EAST OF ESOUEDA , SON. Late Several Thousand feet of shale and tuff; No fossils. ' ? < 4 3 < = Cretaceous -r-i-r-r— (Imlay , 1939) .

6)— GR ANAOITAS .Son. Over Mid 20 m Tertiary Andesitic Breccia und tuff. g S £ c£ c UNCONFORMITY t 50 m Moctezumc(?) Rhyolitic flows and tuff . Moctezuma Fm (? ).Uranium mineralization Early 4- ^ 4= Docitic flows and tuffs Tertiary 100 m UNCONFORMITY Pre- Cenozoic limestones

(Antunez et ol, I968(?))

Figure 9. Continued Thickness Nome Description Age 7)— NORTHERN NACOZARI DISTRICT Over 25 m La tile flow breccia and lobar VALLE Porphyritic biotite and hornblende andesite flows, and layers of 300 — —— m Andesite andesitic tuff and sandstone. Basalt interbedded with andesite at top of sequence; occurs at Southern Nocozori valley also. Early Over Tertiary — 150m Florida member —Reddish brown masive. welded oloaioclose crystal-tuff and Cretaceous intercalated water-lain;non welded tuff and sandstone abundant in upper part. ~ Over e j i a o * 140m . Castillo member — Ten to white water-lain tuff. sandstone and agglomerate Rhyolite and thin ash flow tuff and minor basalt. Over Reoreso member Buff to reddish-brown massive silicified and welded 300m tuff and minor intercalated water-loin tuff and andesitic sandstone

X (McAnulty, 1970) 8)7” P1 LARES , (SE Nacozari , Son.) Over Pll are s 200 m La 11 te La title breccia flows , fine-grained matrix

1 60 Porv enir Massive andesitic flows , andesitic breccias and labor deposits. m SMS • F m . t v » * V ; » Early 140 Rosario Tertiary — m Andesitic sandstones with some andesitic flows F m • Cretaceous ? Paulina Fm. Lit hie ignimbrite , andesitic and la title fragments. Esperanza Fm. ^Orr.-I n '" Rhyolitic ignimbrite and tuffs. Andesitic sandstone ? Andesi tic sandstones and conglomeratic sandstones UNCONFORMITY "West PHares Andesitic flows , da c 1 tic flows and breccias Andesites”

(Chlapa 8 Thoms,1971)

Figure 9. Continued Thickness Nome Descrip 11 on 9)rLA V1B0RA , (NE of Villa Hidalgo , S on.) ‘ Cretaceous? Over More than 200 m • of gray andesitic agglomerates Early T ? 200 m breccias and flows. s a f e (Solano , I 972)

I0)r MOCTEZUMA FM., (Near Dlvisoderos, Son.)

Early Moctezumo Alternate brown to Pink rhyolitic and andesitic tuffs and 90T m Tertiary Fm flows with occasional Ignlmbrltlc members. 50m (Antunez et al* , I 9 6 8(?)) 11)7- 20 Km WNW of SAHUAR1PA (Yoqui River Area) La tile f lotus Andesite flows and breccias N.R. Pre-Mineral Andesitic breccias Andesitic porphyry 8 ______(Amador Osorio, pers.comn.) ( 1973) elrLA COLORADA, Son. Rhyolitic flows Agglomerate Early N.R. Hornblende andesite flows In Sierra de Moradilla, Cretaceous

( King , 19 39)

Figure 9. Continued Th ickness _N om e D e s c r i p M o n I3)~L1STA BLANCA Fm . (SW Son Marcta! Son.)

Andesitic flows with a black glossy lava' at the base Tuff with a brown vesicular lava at the base Over Agglomerate and slaty andesite. Early 300m L i s t a Cret aceous BIonca Tuff, with a basal layer of tuffaceovs agglomerate. Fm Andesitic agglomerate Volcanic conglomerate with tuff lenses Increasing upwards,.The - ....— conglomerate contains angular to rounded boulders of andesite agglo­ a merate , porphyritic andesite, h ornblende andesite, gray vesicular lava and slate ( From 20 to 65m .) ( King, I 939 after Dumble)

14)-TARAHUMARA Fm .

Over Over 200 m of andesitic and latltlc flows, agglomerate and Early 200m m breccia , occas Iona I Interca 1 atlons of sedimentary beds Cretaceous

(Wilson and Rocha, 19 4 6)

I5)r ARROYO OBISPO (9 km W of Onavos)

Andesite flows Sand SE of So. de Son Javier , up to Early Limes tones 1000 mts of volcanic breccias overlain Cretaceous Agglomerate by tuff and andesite.

( King, 1 939 )

Figure 9. Continued Thickness Name Description Age

16)- GUEYCORA. ( 15 Km. WSW of Tocupefo )

Early N.R. Andesite flows and tuffs containing few layers of limestones. Basal conglomerate. Cretaceous i i i § ( King, 1939)

17)— LA DURA - EL REALITO

-f* -f- -f- Over Lista More than 1000m of dense dork-gray lava flows of ande­ Early sitic composition Occasional limestone beds interlay ered. 000m Bl'anc a Cretaceous Fm ?

Indurated agglomerate. # La Barranca Fm. (King , 1939)

Figure 9. Continued 26

Lista Blanca Formation, with basal conglomerates and volcanic agglom­ erates . Andesitic flows were apparently thicker and more continuous in this area and change to more tuffaceous and agglomeratic materials w est­ ward and to eugeosynclinal assemblages eastward.

Minor latite flows are reported west of Sahuaripa and at Santa

Clara; however, early Cretaceous volcanism was predominantly ande­ sitic and probably deposited under subaerial conditions in local basins that probably connected to the existing eastern sea. Considering the present-day exposures of a eugeosynclinal type of rocks in northern

Baja California, by moving the peninsula back to its original position, we find that the zone of volcanism was probably an elongate north- northwest peninsula or possibly an island arc limited to the east and west by seas. Extension of this volcanic belt to the north cannot be made with existing data; however, similar conditions for western United

States are thought to have been possible during middle and Late Creta­ ceous time (Guilbert and Sumner, 1968) and eugeosynclinal assemblages are reported as far north as Nacozari (McAnulty, 1970) and Santa Ana

(Salas, 1970).

For some reason, volcanism appears to have extended toward the northeast during the first stages of the Laramide Revolution, cover­ ing those areas of previous Early and middle Cretaceous volcanism (Fig.

5) and depositing eugeosynclinal sediments only on the Cabullona Basin

(T aliaferro, 1 9 3 3 ).

The nature of Late Cretaceous and Laramide volcanism was high­

ly variable as reflected in variations in rock composition and texture.

Although correlations are difficult to make due to the scarce information 27 available and to the above variations, several general observations can be m ad e.

The area of thickest deposition of volcanic material was along a northwest zone passing through Cananea and Nacozari, possibly thin­ ning westward. The continuity of these rocks to the east is unknown, owing to the middle Tertiary volcanic cover. Thicknesses over 5,400 m are reported at Cananea (Valentine, 1936), 450 m at El Alacr&n (Darrel

Dean, personal commun., 1974), and well over 1,200 m at Nacozari

(McAnulty, 1970). The effect of erosion is generally unknown.

At Cananea, three distinct periods of eruption have been recog­ nized—Elenita, Henrietta, and Mesa formations—the oldest generally being flows of acidic nature, followed by mixed acidic and intermediate rocks with predominance of flows over pyroclastic material, and the youngest composed of mostly andesitic types. A rough trend of decreas­ ing content in silica is indicated by this section (Loera F ., 1973).

Approximately 100 km southwest near Santa Ana, Salas (1970) reported an igneous complex composed of rhyolitic flows with minor bodies of andesite, temporally related to andesites of clear eugeosyn- clinal origin. At Sinoquipe, two thick (+ 200 m), acidic flows (dacite?), separated by a tuff horizon, rest upon an andesitic conglomerate. To the west of this area, within the Altar Desert geomorphic province,

Cretaceous-Tertiary volcanic rocks crop out less frequently. They are possibly of pyroclastic origin and andesitic composition as at Atil or of latitic composition as at La Margarita (Amador Osoria, personal com­ mun. , 1973). East of Cananea, in the Sierra Manzanal, over 450 m of andesitic and latitic pyroclastics are possibly correlated with the 28 lowermost unit of the Elisa Rhyolite at Cananea (Darrel Dean, personal commun., 1974). At Nacozari, three main "old volcanic groups" are recognized, one of which is probably of Late Cretaceous age. Chiapa G. and Thoms

(1971) described the West Pilares Andesites as a series of andesitic and dacitic flows and volcanic breccias. These older volcanic rocks are al­ so mentioned by de la Campa (1968) at La Garidad and by Bolich (1969) southeast of La Garidad. It is possible that these rocks are correlated to those west of Nuevo Hidalgo (Oputo) and at La Vibora where at least

200 m of andesites and andesitic breccias crop out.

Unconformably overlying the older volcanic rocks, a series of acidic tuffs, welded tuffs, sandstones, conglomerates, and minor flows are reported as the Ejido Rhyolite in northern Nacozari and are tentative­ ly correlated with the Paulina and Esperanza formations of Ghiapa G. and Thoms (McAnulty, 1970). These rocks are overlain by a thick se­ quence of andesitic flows and minor sandstones and breccias (Valle Andesite, Porvenir and Rosario Formations ?) and capped by a latitic breccia flow (Pilares latite ?).

The Esperanza rhyolitic ignimbrite was dated by Ghiapa G. and

Thoms (1971) by the K-Ar method at 52.2 + 1.9 m .y ., and a sample of a latite flow south of the La Garidad (antigua) mine was dated at 51.3 +,

1.0 m .y ., placing the extrusive event during the Laramide orogeny.

The northeastern part of the district is separated from the Lara­ mide volcanics by the Garidad fault and is composed of conglomerates with interlayered rhyolitic tuffs. One of these units was dated at 24.4

± 0 .4 m .y. by the K-Ar method. This date marks "the end of conglom­ erate deposition and the beginning of middle Tertiary volcanism in this locality" (Livingston, 1973b, p. 14). 29

Porphyry Copper Type Occurrences

Figure 10 shows the location of some areas in which typical characteristics of alteration, intrusion, and mineralization can be re­ lated to those shown in porphyry copper deposits. If the distribution of these areas is overlain on a geologic map, a close relationship can be seen. From a group of 24 localities, approximately 80 percent are en­ closed in either a volcanic or subvolcanic environment. Even though the relationship of these areas may be coincidental, some petrogenetic ties are apparent in that both mineralized subvolcanic rocks and enclosing volcanics can hardly be distinguished; they are petrographically and geochemically similar.

In addition, some of the areas present structural features typi­ cal of volcanic origin, and there is a widespread abundance of tourma­ line breccias with volcanics and intrusive fragments. The widespread hydrothermal alteration in some volcanic units suggests that on a region­ al scale the Sonoran Laramide copper subprovince may be closely related to the tectonic processes that produced the generally Late Cretaceous Laramide volcanism. Furthermore, if Laramide volcanism was a single event throughout the state, as it appears to have been, volcanic units

should be correlatable on a more local basis and possibly the emplace­ ment of subvolcanic porphyries would have occurred at similar strati­ graphic levels. Consequently, the detailed regional study of a volcanic

stratigraphy aided by geochronologic dating can be used as an effective exploration tool.

The following chapter will be devoted to the description of hydrothermally altered areas in volcanic fields, and a more detailed explanation of these relationships will be given. 1 El Alamo 9 La Caridad 17 Opodepe 2 El Ala cron 10 Cerro Colorado 18 Oputo 3 Los Alisos 11 Cumpas area 19 Cerro de Oro 4 San Antonio 12 El Cusi - Abra 20 Cerro Pic echo 5 San Antxio de la Huerta 13 Cerro San Felipe 2 1 Pilares de Teras 6 Batamote - Jucaros 14 Florida -Barrigon 2 2 La Verde 7 Cabullona I 5 Huachinera 2 3 La Vibora 8 Can an ea I 6 San lanacio 24 Piedras Verdes

Figure 10 Location of reported prospects CHAPTER 4

SOME CHARACTERISTICS OF PORPHYRY OCCURRENCES IN VOLCANIC HOST ROCKS

Eyde (1972) pointed out that even though porphyry copper type mineralization is normally related to intrusive rocks, it may occur in both intrusive and intruded host rocks; he also mentioned that approxi­ mately 30 percent of the total copper produced in 1972 was obtained from mineralized intruded rocks.

Bearing this fact in mind, Titley (1966) classified porphyry cop­ per deposits into three categories:

1. Intrusion—in which mineralization is essentially contained in

the intrusive igneous rock. j

2. Composite—in which mineralization is found in both intrusive

and intruded rock, regardless of the nature of the latter.

3. Wall rock—for those deposits developed almost entirely in the

intruded host rock.

The occurrence of wall-rock porphyry copper type deposits has been observed to be of more economic importance in sedimentary rocks, especially limestone contact deposits, volcanic rocks, and less common­

ly in metamorphic and related rocks. The relationship between favorable

sedimentary rocks and porphyry coppers has been fairly well studied,

and the use of their stratigraphy is a common tool employed in explora­ tion. Nevertheless, the study of structural features, lateral petrographic

31 32 variations, and especially of stratigraphic characteristics of volcanic rocks related in age to main periods of mineralization does not seem to be widely used in exploration.

Outstanding examples of wall-rock type of deposits in volcanic rocks occur worldwide: Safford, Esperanza, Ajo, and Cananea in south­ western North America; Bethlehem in British Columbia; El Teniente, El

Salvador, Toquepala, and many others in South America; Panguna, Ples- yumi, and Freida R. in the South Pacific area; and Majdanpeck within the

Alpine copper-molybdenum belt. While the geologic setting and charac­ teristics of each deposit will vary, there are more known coincident

features than differences, especially among those within the same sub­ province, giving rise to a concept of their close genetic relationship.

From a regional standpoint, their location may be controlled by

major tectonic features, such as a subducting plate in the Southwest

Pacific region or the intersection of trans current and strike-slip faults

related to plate tectonics (Howell and M olloy, 1960), or minor local

features, such as faulting as at Cananea, Esperanza, and Safford. Local­

ly, the control of emplacement of porphyry stocks may also be related to

zones of brecciation (El Teniente, Cananea) or fracturing.

Mineralizing intrusions may vary from quartz diorite (Ajo) to

dacitic porphyries (El Teniente) and be emplaced in volcanic rocks wide­

ly varying in composition and mechanics of extrusion. Andesitic flows

and breccias are present as host rocks at Safford (Robinson and Cook,

1966), Cananea (Velasco, 1966), El Teniente (Hollister, 1973), and at

Panguna (Titley, 1973). Dacites are also found at Panguna (Titley, 1973) 33 and at Esperanza, and rhyolitic flows are common at Esperanza (Lynch,

1966) and at Ajo (Concentrator Volcanics) (Dixon, 1966).

Brecciation seems to be a phenomenon particularly common to volcanic "wall-rock" porphyry type deposits. Mineralized tourmaline breccia pipes are found at Cuajone, Toquepala, and Cananea; breccia­ tion is also present at El Teniente (Hollister, 1973), Cananea, and

Safford (Robinson and Cook, 1966), although barren at Safford.

Mineralization in these deposits is that common for typical porphyry coppers; nevertheless, some differences may exist between volcanic wall-rock orebodies according to the structure of emplacement.

The most common is in fractures and disseminated; it may be a true stockwork, such as that of El Salvador, Chile or combined, such as Ajo in which veinlets and disseminations are present. The other type of de­ posit is commonly present in the Andean metallogenic belt in the form of breccia pipes. These may or may not be filled with tourmaline.

Ore mineralogy is commonly composed of the pyrite-chalcopyrite molybdenite assemblage, with minor changes and occasional presence of bornite and enargite. In composite models, vertical zoning is apparent, with bornite in the deepest levels, changing upward to chalcopyrite and probably to enargite and copper sulfosalts in the proposed uppermost part of these system s. However, tourmaline breccia pipes have a more varied mineralogy, and zoning may exist in only a few hundred feet.

They commonly have pyrite, chalcopyrite, bornite, and less frequently, molybdenite.

The alteration products found in volcanic rocks range from potassic to propylitic. Alteration seems to be partially controlled by 34 composition of the rock type, texture of the rock, and location relative to the center of focus o alteration. In such a manner, it can be seen that Safford presents strong biotitization in andesitic host rocks as does

El Teniente; at New Cornelia (Ajo), ore mineralization occurs within a quartz-orthoclase assemblage in the volcanic rocks. However, the most common alteration found in volcanic "host-rock porphyries" is quartz sericitic with variable degrees of argillization (Cananea, Esperanza,

Red Mountain, for example). Propylitic alteration is generally found in most occurrences as a surrounding halo.

Alteration and mineralization assemblages are spatially extended, with some areal extents as much as 3,600 x 2,700 m (pyritic halo at

Cuajone, Peru) or 1,800 x 1, 200 m (Panguna, New Guinea).

Porphyry Occurrences in Sonora ! i Petrography and Alteration

Table 1 records some characteristics of the 24 occurrences whose locations are shown in Figure 10. The information given in the table is based on reported data, personal observation, and oral commun­ ications from different sources. If the data includes microscopic study of host rocks, wall rock, and alteration, the name of the occurrence is marked with an asterisk. The rest have only crude field descriptions.

Associated Intrusive Rocks. In column 1, the petrographic composition of the intrusive rocks related to the mineralization process is shown; where several intrusive rocks are found, the possible se­ quence of intrusion is presented from left to right. Most of the intrusive rocks tend to have a quartz-free, intermediate or acidic composition, Table 1. Some characteristics of porphyry occurrences in Sonora

Alteration and Regional Mineralized Intrusive Rock W all Rock Zoning, Mineralization Structure Structure

1. El Alamo * tonallte porphyry; granodiorite; potassic-phyllic- p y rite, NW, NE . E-W granodiorite porphyry; Laramlde andesite argilllc-propylitic chalcopyrite, quartz monzonlte por­ (plus tourmaline) molybdenite phyry

2. El Alacran* granodiorite porphyry; Laramide andesite; argillic-propylitic p y rite, NNW, NW quartz monzonlte por­ Laramlde rhyolite (plus tourmaline chalcopyrite; phyry and tuffs and alunite) chalcocite

3. Los Alisos* andesite porphyry; Laramide andesite phyllic-argillic- p y rite, b ■■B .E-W monzonlte porphyry propylitic (plus chalcopyrite tourmaline and alunite) 4. San Antonio quartz porphyry Laramide andesite; argillic-propylitic p y rite, NNW, . NW, NNE Laramide rhyolite chalcopyrite NW

5. San Antonio dlorlte porphyry Triassic-Jurasslc argillic p y rite, E-W , NW E-W(?) de la Huerta metasediments chalcopyrite, chalcocite

6. Batamote- diorite porphyry Laramlde andesite; siliclfication, p y rite, NE, NW NE Jucaros Laramide rhyolite; sericite chalcopyrite, Laramide tuffs (argillic) chalcocite, molybdenite

7. Cabullona dlorlte porphyry; diorite; Laramide argillic, garnet pyrite, NE ENE quartz porphyry limestone and chalcopyrite, shale molybdenite (sphalerite, galena)

8. Cananea quartz porphyry Paleozoic meta­ phyllic-argillic- p y rite, NW, NE, sediments, propylitlc; skam chalcopyrite, NNW Laramide andesite. (tourmaline and bornite, alunite) chalcocite Table 1. Some characteristics of porphyry occurrences in Sonora—Continued

‘ Alteration and Regional Mineralized Intrusive Rock W all Rock Zoning Mineralization Structure Structure

9. La Caridad * quartz monzonlte granite; argillic; phyllic; p y rite, NW, NE porphyry; quartz granodiorite tourmaline chalcopyrite, porphyry bornite, ch alco cite, molybdenite

10. Cerro quartz monzonlte granite;. . phylllc-argilllc- p y rite, NNW, NNW Colorado granodiorite; propylitic chalcopyrite ENA(?) . Laramide andesite

11. Cumpas area acidic, variable Laramlde volcanics argillic; propylitic pyrite, NNW(?) chalcopyrite, molybdenite (tungsten, lead, gold, silver)

12. El Cusl-Abra* quartz monzonlte granite; grano­ phyllic-argillic- pyrite ? E-W , NW ENE porphyry; andesite diorite; Laramide propylitic (plus porphyry andesite tourmaline, breccia)

13. Cerro San quartz monzonlte granite; grano­ phyllic-argillic- p y rite, E-W, Felipe porphyry diorite; Laramide propylitic molybdenite, ENE • andesite chalcocite?, chalcocite?, chalcopyrite? chalcopyrite?

14. Florida- andesite porphyry metasediments silicification; p y rite, NNE, NE, NNW, NW, Barrigon skarn chalcopyrite, NW NNE molybdenite

15. Huachinera quartz porphyry diorite argillic; propy­ pyrite ? NNE, NW NNE litic

16. San Ignacio * quartz monzonlte Laramide andesite phyllic?-arglllic- pyrite ? NE,ENE porphyry propylItic (plus . tourmaline, breccia) Table 1. Some characteristics of porphyry occurrences in Sonora—Continued

. Alteration and Regional Mineralized Intrusive Rock Wall Rock Zoning Mineralization Structure Structure

17. Opodepe quartz porphyry granite; grano- phyllic; argillic- p y rite, NW several facies diorite; Triasslc- propylitic chalcocite ?; Jurassic molybdenite metasediments

18. Oputo quartz porphyry? Laramide volcanics argil lie; p y rite, ? propylitic chalcocite

19.Cerro de Oro tonalite porphyry; granite; grano- potass ic-phyllic- NW, ENE quartz monzonite diorite; Laramide argilllc porphyry andesite 20. Cerro diorlte porphyry; Laramide andesite argillic- p y rite, N -S , NW, E-W , N-S Picacho * andesite porphyry; propylitic (plus tetrah ed rite, ENE tonalite porphyry; tourmaline breccia, chalcocite Laramide andesite alunite, and specularite)

21. Pilares de quartz diorlte . Laramide volcanics; argllllc- pyrite NE?NNW Teras porphyry; quartz Paleozoic sediments propylitic porphyry

22. La Verde quartz monzonite granite, grano- phylllc?-argillic- p y rite, ENE, NE, porphyry; quartz diorite; Laramide propylitic chalcopyrite, NW porphyry andesite chalcocite

23. La Vibora quartz porphyry . Laramide andesite silicification; p y rite, NE argillic chalcopyrite, chalcocite

24. Piedras quartz porphyry metasediments argil lic-serlcite pyrite, WNW ENE Verdes (quartz monzonite (shales) chalcocite porphyry?) (chalcopyrite), (molybdenite)

♦Data includes microscopic study of host rocks, wall rock, and alteration 38 being commonly quartz monzonitic. Reported "quartz porphyries" have arbitrarily been assigned to this category. Less frequently, quartz latite porphyries and tonalite or dacite porphyries are found.

At the other end of the range, trachyte and latite porphyries are rarely seen associated with mineralized areas. Both andesitic and dio- ritic intrusive rocks are generally of late magmatic origin, and some are definitely postmineral. They are commonly found as narrow dikes and

small irregular m asses.

A field description of the typical "quartz porphyry" would be as follows: light-gray to light-brown porphyritic rock with 15 to 40 percent

altered phenocrysts, ranging in size from 0.5 to 5 mm. The matrix is

generally aphanitic and silicified. Free silica in the form of ovoidal

quartz eyes occurs in minor quantities, generally less than 5 percent by

volume. Detialed petrographic analysis would define this rock in the

range from quartz monzonite to dacite.

Associated Volcanics. Most of these prospects are found in

volcanic rocks of pre -Laramide or Laramide age. The wall rocks are

mainly andesitic or less frequently more acidic, with interbedded tuffs.

The next most common wall rock is granite or granodioritic intrusive

rocks and occasionally sedimentary rocks of Cretaceous age of Paleo­

zoic metasediments.

As mentioned earlier, excellent examples of porphyry-related

mineralized areas in volcanic rocks are encountered in the Nacozari

district as well as at Cananea and Sierra El Manzanal. The Pilares brec­

cia is intruded in a series of latitic breccia flows and andesitic flows. 39 breccias, and pyroclastics. A similar condition is shown at El Alacran in the eastern part of the Sierra El Manzanal, but volcanism is mainly andesitic-dacitic. At Gananea, part of the mineralized area is found within the Mesa andesitic volcanics, as it is in the Ghela-Millipas area north of La Mariquita.

Three other regions where volcanic rocks are commonly intruded by Laramide stocks are the La Verde-Washington district west of Cumpas,

El Novillo area along the Yaqui River near the Novillo Dam, and the south­ ern Yaqui River region between Soyopa, Tecoripa, La Dura, and Yecora.

Even though the character of volcanism may vary widely from place to place, the most general type of rocks are agglomerates and flows of andesitic type.

Sedimentary Associations. M esozoic sedimentary rocks, includ­ ing the Morita, Mural, and Gintura Formations of Early Cretaceous age, are intruded by a fine-grained diorite and by mineralized "quartz porphyry" dikes and stocks at Santa Rosa east of Cabullona, Sonora. Similarly, metasediments of undetermined age, which are mineralized and strongly altered, occur at Pie dr as Verdes north of Alamos. Less frequently, the

Barranca Formation of Triassic-Jurassic age occurs with strong evidences of mineralization as in San Antonio de la Huerta and west of Opodepe.

Alteration. The degree to which alteration is present at these localities is controlled by many variables, such as reactivity of wall rock, closeness and level of intrusion, intensity of fracturing, and in­ tensity and duration of alteration processes .

In column 3, Table 1, the different reported alteration assem­ blages as well as their possible zoning are shown. It can be seen that 40 the presence of phyllic, argillic, and propylitic alterations are quite common; however, the second is almost ubiquitously found in all pros­ p e c ts .

Potassic alteration has been reported for a .few. At La Caridad,

Echavarri P. (1971) found this type of alteration very localized, not well developed, and only at the bottom levels of the deposit, it is be­ lieved to have been formed under deuteric crystallization processes of biotite rather than under true activity of hydrothermal solutions. At A1

Alamo near Suaqui Grande, Sonora, a quartz dioritic porphyry with hypo- gene biotite shows development of quartz veining, secondary biotite, and occasional K-feldspar flooding of rock and in veinlets. This area is bounded on the west by a quartz monzonite porphyry with a phyllic alteration assemblage, quartz-pyrite-sericite-montmorillonite (super­ gene?), which, in turn, is surrounded by a series of andesites and andesitic agglomerates with structurally controlled propylitic alteration, pyrite-chlorite-epidote-calcite.

The ratio of primary sericite to biotite plus K-feldspar within the biotitic zone, as well as the low sulfur to metal ratio (especially iron), suggests that this zone is located at the fringe of a potassic core or within a transitional potassic-phyllic contact (Solano R. and Domin­ quez L., 1972).

The Aurora prospect is reported as having epigenetic K-feldspar in veinlets and phenocrysts associated with a stage of alteration that preceded a main tourmalinization stage (Berlanga G ., 1971). At El Ala- crein, biotitization occurs associated with minor amounts of topaz and 41 fluorite within a pseudo-elliptical intrusion breccia shell (Darrel Dean, personal commun., 1974).

Phyllic alteration is particularly well developed at La Caridad,

Cananea, El Alacrin, San Felipe, Opodepe, and Piedras Verdes. It is, of interest to note that all these areas are located within physically large, complex sulfide alteration systems.

As mentioned earlier, the presence of argillic alteration seems to be very common in all Sonoran prospects; however, most of the re­ ported areas have not been studied in detail and only shallow drilling has been done. It is suggested that the widespread and abundant pres­ ence of argillaceous minerals may be partially due to supergene effects, as mentioned by Rose and Baltosser (1966) for Santa Rita and by Tit ley

(1972b ).

The propylitic alteration to which mention is made in column 3,

Table 1, includes quartz-chlorite-epidote-carbonates (mainly calcite), and minor amounts of albite and zeolites. It occurs in 18 out of the 24 listed prospects.

Two other types of alteration are considered in this chapter: tourmalinization and alunitization.

Varela (1971) pointed out the occurrence of tourmaline-bearing copper mining districts in the Western Hemisphere south of the Patagonia

Mountains through M exico, Central America, and the western coast of

South America. Similarly, EchSvarri P. (1967) writes: "It looks like if the presence of Boron in the magmas is terminated by the structural zone so called 'Texas Belt' showing a major enrichment of alkalies northwards, within the United States, where potassic alteration is more common" 42

(Berlanga G ., 1971, p. 43). Tourmaline is a very common product of hydrothermal alteration in the Sonoran province and especially in those areas where volcanic rocks are intruded and altered. It is commonly found in small (2-5 mm), black and occasionally green, translucent rosettes disseminated throughout the mass of a pervasively sericitized rock. It also occurs as a matrix in breccia-pipe type structures as black, m assive, schorl-type tourmaline and gray quartz-tourmaline meta-

somatically replacing the host rock and in fracture fillings and veinlets.

The distribution of tourmaline is highly variable. It has been found in the uppermost part of mineralized systems decreasing with depth, as at

La Garidad (EchAvarri P ., 1971) and in the periphery of hydrothermally altered systems as at Aurora; however, it seems to be more susceptible to structural control and granite-volcanic contact as clearly shown by the hundreds of small quartz-tourmaline breccias that occur in this sub­ province .

Alunite is another alteration mineral commonly found. Even though its origin may be subject to discussion as to whether it is of hydrothermal or supergene derivation, the fact is that in those prospects

in which alunite has been reported Laramide(?) vblcanism is common.

Mineralization

Physical occurrence of metallic mineralization is extremely

variable in character and distribution; however, in volcanic rocks af­

fected by porphyries it tends to be very fine grained and disseminated.

Under certain local structural conditions, metallization may appear in 43 veinlets and veins in stockwork structures and occasionally as m assive, particularly in wider veins and in breccia pipes.

As shown in column 4, Table 1, mineralization is mostly re­ stricted to pyrite in all areas, with less chalcopyrite, bornite, and molybdenite and supergene chalcocite and cove Hite. Sphalerite, galena, scheelite, wolframite, and members of the tetrahedrite-freibergite group are occasionally found.

Structure

Structural control of alteration and mineralization is another important aspect in the development of porphyry copper-type deposits in volcanic rocks, varying from broad regional zones of weakness or fault zones, such as the Cananea-Nacozari belt, to the small local structures common to all prospects. The preferential strike patterns characteristic l of pre-Laramide and Laramide faulting have been briefly mentioned in

Chapter 2 and will be discussed further in the next chapter. Of particu­ lar interest in this discussion are some structures that seem to be typ- pical and are frequently found in most prospects.

Fracturing and Doming Associated with Intrusive and Intrusion

Breccias. Intrusion of hypabyssal stocks or breccias produces doming of the overlying volcanic rocks. After release of the pressure, fractur­ ing will be created, commonly parallel to the doming. Fracturing is generally low angle (15 to 30 degrees) and concentric, be it circular or elliptical, to the central feature. The patterns commonly show "brec- ciated horizons" of altered volcanic rocks and are limited by steeply 44 dipping faults. This type of brecciation has been particularly observed in the lower Yaqui River region.

Intrusive Breccias. Intrusive breccias may be divided into two subclasses according to cementing material, notwithstanding that their processes of formation appear to be the same.

1. In tourmaline breccias, light-colored porphyry or volcanic rock

fragments show generally strong silicification and variable re­

placement by sericite, clay minerals, or quartz-tourmaline.

Fragment size ranges from a few millimetres up to half a metre,

with an average of about 5 cm. The fragments are generally ir­

regular and angular and show both displacement and rotation

relative to each other.

Voids are commonly present, representing up to 20 percent

of the rock volume. The matrix is a dark-gray, fine-grained

mixture of quartz and tourmaline; the latter may be present in

the schorl variety and less frequently in the green (Na, Li) vari­

ety. The rest of the matrix may be hematite and jarosite with

veinlet-controlled, disseminated or massive sulfides (pyrite),

alunite, traces of muscovite, jasperoid, and clay minerals.

Occasionally, intrusive breccias are found with vertical

elongate fragments near the walls of the feature. These frag­

ments become increasingly horizontal toward the top of the

structure, suggesting fast injection and collapse (Kents, 1964).

A good example is the Mina de Fierro breccia in the Eleyn pros­

pect at Mesa de Galindo, Sonora. 45

The shape is generally irregular. However, at a regional

scale they may be found as parts of larger, elongate, semicir­

cular, "cauldron-type" structures.

Some of these breccias show two different stages of tour-

malinization, the first represented by disseminated rosettes of

schorl-tourmaline and the second by quartz-tourmaline filling.

2. Sericite breccias are commonly vertically elongate bodies with

a quasi-elliptical horizontal section bounded by a series of

concentric, steeply dipping faults. Fracturing and faulting are

locally intense, and separation and rotation of angular frag­

ments are common.

Rock fragments are light colored, strongly silicified, and

pervasively sericitized to a degree that recognition of the orig­

inal rock is commonly difficult. Their size generally ranges

between 1 and 20 cm. The matrix consists mainly of quartz,

silica, sericite, limonites, and occasionally alunite. Miner­

alization is normally present as disseminated cubes of pyrite

and corresponding oxidation products. This type is exempli­

fied by the Cerro Pehascoso breccia at Mesa de Galindo,

S on ora.

Tourmaline and sericite breccias can also be classified as subsidence type and heavy type, according to Kents' (1964) description.

' Contact Breccias. Of particular interest in this discussion are many alteration zones associated with plutonic-volcanic contact phenonv ena. Some of these" areas present definite structural orientation and are 46 characterized by intense silicifaction and pervasive sericite or chlorite alteration with disseminated pyrite or magnetite. Brecciation and altera­ tion are characteristically tabular and parallel to the contact, and they frequently contain tourmaline-silica filling as well as pyrite and chal- copyrite. Tungsten mineralization (wolframite) can be also present (Luis

Palafox, personal commun., 1975). Metasomatic alteration effects ob­ served in these areas occur notably bounded by fault zones, and they present strong fracturing and brecciation right along the contact.

A word of warning is given here to the careful geologic exami­ nation of contact phenomena possibilities, due to the fact that many breccia zones have been drilled with a "breccia pipe" concept in mind and after crossing the fractured zones drill holes enter into a relatively fresh to fresh granitic rock with no evidences of mineralization. This

condition differs from the "breccia pipe" pattern in that vertical extent

of brecciation and mineralization is greater in the latter case, among

other aspects. Nevertheless, the possibility of economic mineralization

along contact zones within intruded volcanic rocks is good, especially

if significant alteration, capping, or geochemical anomalies are present.

It will have to be borne in mind in this case that the target of

exploration is reduced in size and probably tabular shaped and that the

chance of finding significant ore mineralization at depth is minor, since

the intruding pluton is commonly barren.

Stockworks. Stockworks are randomly distributed and commonly

have very limited dimensions. Sometimes they appear completely isolated

within hydrothermally altered volcanic rocks and appear not to bear any

relation with nearby brecciation areas (for example, Guadalupe area. 47 northeast Suaqui Grande, Sonora). Fractures are commonly filled with goethitic and hematitic limonites with a common average of 3 to 5 per­ cent by volume of the original corresponding sulfides. Local concentra­ tion represents as much as 15 percent of the original sulfides.

Geochemistry

In Table 2, some reported surface rock geochemical values are shown to illustrate the range of values considered as normal and anom­ alous. Values reported are from rock-chip samples of altered volcanics, mainly taken within argillic and sericitic zones, except at Aurora, where sampling included an altered quartz monzonite porphyry as well as sever­ al tourmaline breccias. Values for unaltered andesite are the result of a survey within an almost unaltered andesite (slightly propylitized) sur­ rounding a hydrothermally altered zone. All results are whole-rock atomic absorption analyses. As a means of comparison, Hawkes and

Webb's (1962) values for average rocks and felsic rocks are also given.

Copper seems to behave constantly through different prospects, and anomalous values are considered to be over 90 ppm. Anomalous values for other elements are 15 ppm for molybdenum, 0.3 ppm for gold,

7 ppm for silver, 125 ppm for zinc (within a propylitic zone), 90 ppm for antimony, and 120 ppb for mercury.

The reader is reminded that these values are "in the range of," since more valid estimates cannot be made due to lack of information as well as to the numerous variations of sample collection, type and inten­ sity of alteration and mineralization, geographic location, mobility of the element, etc. 48

Table 2. Rock geochemical values

Minimum Maximum M ean Threshold Anomalous Prospect Value Value Background Value Value

Copper , ppm

Aurora 10 1 ,0 0 0 45 100 > 100 Eleyn 0 340 40 90 > 90 M ilp illa s 70 > 70 Alaercin 0 1 ,5 0 0 75 > 75 A liso s 500 50 > 50 Beatriz 16 1 ,0 0 0 72 113 >113

Average rock 90 Average felsic rock 30 U naltered a n d esite 20 230 40 70 >70

Molybdenum, ppm

Aurora 2 150 5 20 >20 Beatriz 2 60 26 37 >37 Alacr&n >10 M ilp illa s 5

Average rock 1.7 Average felsic rock 1 .9

G old , ppm CO Aurora 0 .0 1 00 > 0 .3

Average mafic rock 0 .1 Average felsic rock 0 .0 1

S ilv er, ppm

E leyn 0 60 4 7 ? 7 U naltered a n d esite 0 10 4 6 >6 Average rock 0 .2 Felsic rock 0 .1 5 49

Table 2. Rock geochemical values—Continued

Minimum Maximum M ean T hreshold Anom alous Prospect Value Value Background Value Value

Z in c, ppm

AlacrAn 0 425 125 ^ 125 Beatriz 0 600 72 146 >146

Average rock 80 Felsic rock 60

Antimony, ppm

E leyn 0 320 50 90 £ 90

U naltered andesite 0 60 30 50 250 Average rock 0.3 Felsic rock 0 .4

Mercury, ppb

M ilp illa s 0 430 80 120. 2120

.Average rock 60 Average felsic rock 90 50

C a p p i n g

Two aspects of capping development appear to be characteristic of most volcanic-altered areas associated with porphyry copper deposits in Sonora; (1) there is a high degree of leaching with minor bleaching of the rock and (2) there is a predominance of fine-grained disseminated mineralization compared to that in mineralized fractures and veinlets.

The rock generally shows strong silicification with surface exposures having empty casts from sulfides and soft elongate plagioclases. Very commonly the original content of pyrite is over 3 percent by volume of fine (1 mm) close or interconnected grains, giving rise to highly acidic solutions that penetrate and form a strongly leached, deep oxidation environment.

Two types of capping are commonly associated with hydrother- mally altered sulfide systems in Sonora; jarositic and hematitic. Jaro- sitic cappings show generally a mixture of light colors (green, gray, yellow) with bright red produced by coatings of exotic iron oxides. They normally show abundance of pyrite still under oxidation and a strong de­ velopment of clays. Hematitic cappings have a predominance of dissem­ inated indigenous hematite that generally shows a characteristic reddish-live maroon streak color. Both types of cappings are very con­ spicuous due to their bright red brick color, which contrasts with the generally dull-colored surroundings.

Grain size of disseminated original sulfides is larger in hema­ titic cappings than in jarositic cappings. The presence of fracture- controlled mineralization, especially quartz-hematite, is common in 51 hematitic cappings, but mineralization is more regularly distributed in jarositic cappings than in hematitic ones.

The growth of vegetation, as well as the variety of plants, appears to be slowed in this type of terrain. A low type of brush, locally known as Tarachique, is particularly well suited to grow in these hostile acidic environments. CHAPTER 5

SPECIFIC FACTORS AFFECTING EXPLORATION

T ecto n ics

Notwithstanding the lack of basic information and geologic map­ ping in Sonora, any exploration effort must be conducted with a basic knowledge of the regional tectonic conditions. Of special interest will be the discrimination of those structures that are more likely to be min­ e r a liz e d .

It is apparent to this writer that some east-w est alignments, especially in the northwestern part of the state, that appear within the older rocks in form of faults, folds, and schistosisty of the rock are i likely to be of PreCambrian age. More important, however, appear to be those structures, ranging from N. 40° W. to N . 60° W ., that are com­ monly found within pre-Tertiary rocks and commonly associated with

N . 450-55° E. and east-northeast or N. 70o-80° E. systems of fractures.

It is believed that the intersection of these systems of structures was favorable to the location of hydrothermally altered mineralized subvol- canic complexes. This assumption is partly suggested by the position of the Cananea, Alacr&n, and Caridad orebodies at the intersection of the

Caracahui-Cananea N. 45° E. trend, a N. 70° E. trend, and the Cumpas-

Nacozari belt with the major northwest Cananea-Nacozari mineralized belt. Lineaments of capping exposures, altered zones, mineral districts, and major faults and structures closely follow this orthogonal pattern.

52 53

The exploration geologist must also be aware of structures that were superimposed after the Laramide mineralizing process. The w ell-

documented Basin and Range orogeny shows a remarkable parallel pat­ tern of north, north-northwest, and north-northeast faults (Fig. 2). It

is of interest to take into account the effect of this faulting. It may

serve as a clue to downthrown mineralized bodies now covered by upper

Tertiary Baucarit Formation or later alluvium filling. One example is the

La Vibora area, northeast of Nuevo Hidalgo, Sonora, where the system

appears to be faulted in its western limit. Deeper, primary mineralized

zones that may exist closer to the surface may not be recognized due to

Basin and Range faulting.

Middle Tertiary Volcanic Cover

Bearing also upon the possibility of finding blind ore bodies, the

extension and distribution of mid-Tertiary volcanic cover as well as upper

Tertiary clastic and volcanic cover must be accounted for (Fig. 11). Un­

der favorable structural conditions, the same chance of discovery exists

in those areas where the cover is thin as in those in which erosion has

reached deeper levels.

Considering only these two factors, the most likely area to be

considered, and the most heavily explored to date, is the physiographic

subprovince of parallel valleys and ranges. The desert and coastal areas

are generally deeply eroded and covered by vast plains of alluvium and

Tertiary sediments; It is my opinion that maximum igneous activity of

Laramide age was centered toward the eastern portion of the state, as

will be discussed later. 54

/1 L / ^ ^ / r^O ¥ Vi 0 "rt &

7 i )\— i * *» r1 ■ A m ------\ V ^ \ V %L -

v^V / " " /

v l

( J

Figure 11. Distribution of Tertiary postmineral volcanic cover 55

In the Barrancas section of the Sierra Madre Occidental, the mid-Tertiary cover has been considered to be over 1,500 m thick (R. E.

King, 1939), which, in addition to the poor communications and rugged­ ness of the terrain, makes more difficult the possibility of having eco­ nomic success in geologically attractive projects. Nevertheless, the importance of this area and the possibility of it being mineralized sug­ gest that efforts should be allocated in some part to its exploration.

Structural Geology

The importance of regional trends of mineralization upon the location of mineral deposits has been mentioned. Now, within specific areas a thorough knowledge of the local structural conditions is impor­ tant. It is seen that those areas with particularly well-developed struc­ tures, be these heavy fracturing, brecciation, dome structures, or caul- - I dron subsidence, are more suited for mineral deposition, particularly in true volcanic flows and pyroclastic material.

Detailed mapping of these structures at a scale of 1:5000, at

least, or preferably 1:1000, has proved to be of particular usefulness in

picking up structures that are not readily apparent in smaller scale map­

ping. Mapping of fracture density may also be used under these condi­

tions to outline areas with probable good ground preparation before the

mineralizing processes.

Outcrop Characteristics and Photoqeoloqy

Exploration in Sonora has leaned heavily on certain character­

istics of the capping, such as redness of the soil and reflectance of the

rock, especially during reconnaissance flights and photogeology. 56

Another tool that could be used, especially for preliminary mapping and reconnaissance work, is ERTS-1 imagery. However, the geologist must bear in mind the light reflection characteristics of the objective when choosing the most appropriate band or combination of bands. Lyon

(1972, p. 23) mentions that "a soil is red in color because it preferen­ tially absorbs the blues and greens, and reflects the red tone. Thus in the blue channel, it appears darker and the red channel shows to be lighter in tone."

Infrared imagery, particularly band 7, gives high resolution and detail in lineament and structural features under the climatic conditions and poor tree coverage in Sonora. False color photographs give resolu­ tions under different conditions and are suitable for multiple purposes, depending on the bands and filters used. For geologic mapping and cap­ ping locations, the combination of bands 4, 5, and 7 (green, red, and infrared) with red, green, and blue filters, respectively, should give good results (Gustavo Calder6n, personal commun., 1974).

Weathering and Climate

The climate in Sonora is extreme. It is hot and dry during eight months of the year approximately. Temperatures reach 48°C during sum­ mer and drop to below zero during the winter. Humidity is generally low with the exception of the short periods of rain. For exploration purposes, the climate in Sonora may be considered as suitable all year round, with the exception of the extremely hot summer days when field work is nor­ mally performed early in the morning and late afternoon and in the rainy

season when sudden storms frequently destroy dirt roads and make trans­ portation difficult and hazardous. 57

Present weather conditions are not suitable for the development of supergene enrichment blankets, since exposures are now undergoing rapid erosion. According to Livingston (1973c), enrichment of south­ western North American porphyry copper deposits occurred before mid-

Tertiary volcanism, giving rise to the possibility of locating supergene blankets beneath barren Oligocene-Miocene volcanics.

Geochemical Exploration

As a result of climatic conditions, development of soils is sel­ dom complete. Soils are typically desert type with good development of a calcareous C horizon. Organic and leaching zones are generally lacking.

Mountainous areas show a better development of soils where vegetation is abundant. There is very little soil where topography is rugged; here, only the C horizon and fragmental talus are found.

! This phenomenon precludes the use of soils in geochemical ex­ ploration, even though in some prospects geochemical exploration has been properly applied and successful. Stream sediment sampling is mostly restricted to the first stages of reconnaissance, and the samples are commonly of silts taken on the banks and within the beds of the a rro y o s.

Huff (1971) made a comparative study of different types of sampling of drainages in the Lone Star mining district, Arizona, and he concluded that minus 80 mesh silt and fine sand samples, as well as pebbles, have good resolving power in separating washes draining min­ eralized areas from those draining barren areas. Huff's additional com­ ments given in the following paragraphs include; 58

1. High content of copper in fine sediments seems to be caused

mainly by coating of the grains, as well as by some absorption

in the smallest grain sizes.

2. Spectrographic determination of ultrasonic concentrates and

magnetic heavy minerals have similar results, but in a wider

spectrum of copper values.

3. Analysis of 30 to 80 mesh fractions do not effectively separate

mineralized from nonmineralized areas. The spread of values

and content of copper in magnetic heavy minerals were found to

be the same in mineralized and unmineralized areas.

4. Best results in stream surveys are obtained for relatively mobile

and highly mobile elem ents, such as copper and zinc. Anoma­

lous e zones can be detected as far as 10 km from the source of

dispersion.

5. In the Lone Star district, detectable amounts of molybdenum,

silver, and lead are present within mineralized basins. How­

ever, high concentrations of chromium, cobalt, nickel, and

tin are characteristic of unmineralized basins and probably de­

rived from the post-mineralization volcanic cover.

Huff's conclusions are applicable to actual environment conditions in the southwestern United States and Sonora.

Probably the most widely used geochemical method is rock sampling along traverses collecting chips of rocks. Copper and molyb­ denum values are normally well retained by indigenous limonites unless leaching is high. An advantage to rock sampling is that interpretation of anomalies is easier and the elements are less subject to transport than 59 with soils. Nevertheless, care must be taken when interpreting results; a strongly leached capping may give rise to the location of background values in an area overlying important mineralization. A similar effect is found where the py-cpy ratio is very high (10:1, 12:1) because the pro­ duction of HgS flushes away the rock's copper content.

Mineralization

Capping interpretation is an exploration tool that has been widely used in Sonoran prospects and that under careful determination can be very useful. It has been mentioned that most of the Sonoran cap­ ping is hematitic, and most have been drilled with the purpose of locat­ ing supergene blankets in mind. However, it must be remembered that under present economic conditions, primary sulfide ore bo dies with lower grade can be successfully mined. Therefore, a careful reinterpretation of studied capping, looking for signs of primary mineralization at depth, may be worthwhile. The location of low-sulfur, goethitic capping with

significant geochemical anomalies may be a good target for further ex­

ploration .

These studies call for careful visual estimation of hematite-

jarosite-goethite content (by volume) and interpretation along with geo­

chemical, structural, and alteration maps.

Petrography and Alteration

The exploration of sulfide anomalies based on a theoretical

model have to account for the type of host rock, as previously men­

tioned. Zoning of alteration and mineralization may not be expected to

be as nearly perfect as it is in some locations where intruding and

v 60 intruded rocks have similar compositions and where fluid equilibrium is maintained from one environment to the other.

In a volcanic environment, such as that of Sonora, the enhance­ ment or absence of certain alteration zones can be predicted and care­ fully studied. Due to a relatively high content of ferromagnesian minerals, andesites and related rocks will tend to show a wide and strong zone of propylitic alteration. The potassic zone would probably be better characterized by a high content of biotite than of K-feldspar.

Conversely, acidic rocks, such as rhyolite to rhyodacites, due to their high content of silica, might show strongly developed silicification zones, wider sericitic-argillic zones, and probably less conspicuous propylitic zones.

The geometric distribution of alteration characteristics, along with the geologic distribution of chemical changes, may provide a better understanding in the search for primary sulfide concentrations.

G eo p h y sics

Geophysical methods are widely used and will continue to be increasingly, used in the exploration of blind orebodies, faulted segments of known ones, etc. Most common techniques applied to porphyry copper exploration are magnetic and electric in nature.

Magnetic surveys will be of help in delineating basement rock irregularities in those areas where an alluvium-covered orebody is pos­

sibly located. Copper mineralization associated with either basic or in­ termediate rocks can be explored with this technique where good contrast exists (for example, Promontorio mine, Sonora?). 61

The combined use of resistivity and induced polarization has been of particular use in porphyry copper deposit exploration. Best re­ sults are obtained when interfering factors are accounted for:

1. Mapping of related rock units with significantly different

electric parameters.

2. Estimation of thickness of overburden.

3. Mapping distribution of sulfides and any possibly conductive

material showing (a) extension, (b) physical occurrence (dis­

seminations, grain size, stockwork, veinlets, etc.), (c) semi-

quantitative visual evaluation in percent per volume.

4. Location and attitude of possibly mineralized units.

5. Mapping of important structures, especially shear zones.

6. Mapping of hydrothermal or supergene alteration.

7. Location of all pertinent data that may cause cultural noise.;

Two specific disturbing factors have been found in the Sonoran environment: (1) highly conductive oberburden and (2) highly resistive silicic cap rock, possibly due to supergene precipitation of silica in hydrothermally altered areas.

Among new techniques, complex resistivity measurements ap­ pear to have a number of advantages over induced polarization in provid­ ing information to interpret subsurface geologic structure, rock type, and in some areas alteration. Complex resistivity can provide sulfide dis­ crimination (pyrite, chalcopyrite, chalcocite, cove Hite, molybdenite), mineralization trends, and according to Zonge and Wynn (1974), abso­ lute mineralization estim ates. CHAPTER 6

METALLOGENIC CONSIDERATIONS

A brief discussion of the leading concepts in regional metal- logeny in regard to the location of porphyry copper deposits in the South­ west will be discussed.

Schmitt (1966) proposed that the sources of copper and heat energy for porphyry copper deposits originate in the mantle or lower crust. Porphyry copper deposits were located, according to this theory, in the intersection of orogens and fault zones, specifically the intersec­ tion of the north-south-trending Wasatch-Jerome orogen's central rift with the northwest-trending Texas lineament zone. However, he con­

sidered the Texas zone to be unimportant as a copper localizer without the intersection of major fault zones. It is also a conclusion of his paper that zones of intersection, especially triple or more complex in­ tersections, are of prime interest.

Along with the same line of thought, Mayo (1958, p. 1174)

stated that "heat and fluids including the ore-depositing fluids, are most

likely to rise at or near intersections of major structures where the crust

is fractured, or weakened, to great depth." Mayo described a series of

northeast, northwest, and east-west-trending lineaments whose inter­

sections are classified from first to fourth class. Intersections with

major lineaments, such as the Texas zone, are of prime interest, espe­

cially if more than two are present and igneous activity is evident.

62 63

Similar conclusions, but with more emphasis on the importance of the Texas zone, are reached by Wertz (1970). He recognized three

sets of lineaments: (1) a north-northwest-trending anticlinorium that merges to the north with the Wasatch-Jerome belt; (2) northeast-trending fractures, such as the Questa-Grants-Morenci and the Lordsburg-Tyrone-

Santa Rita lineaments; and (3) the west-northwest-trending Texas belt.

"The orebodies are generally found at intersections, near the most cen­ trally located intrusive with a distance varying from contiguity to one or

two miles (Wertz, 1970, p. 80).

Guilbert and Sumner (1968) recognized the importance of sea­

floor spreading and the consumption of oceanic crust under island arc

structures with associated geosynclinal activity. They also related ore

deposits to oceanic ridges, island arcs, and transform faults and recog­

nized the significance of deep fractures reaching the earth's mantle as a

conduit of mineralizing fluids. Again the Texas zone, as the "single most

dominant control" in location of porphyry copper deposits was pointed

out (Guilbert and Sumner, 1968, p. 111).

Later, Sillitoe (1973) stated that the close relationship between

calc-alkaline rocks and porphyry copper deposits is due to the partial

melting of descending lithospheric plates along the Benioff zone and to

the presence of copper concentrations in the oceanic crust produced by

submarine volcanism along the oceanic rises. It was his postulate that

subduction of the East Pacific ocean floor beneath the American plate

occurred during the M esozoic and early Tertiary at a very flat dipping

angle, causing production of calc-alkalic magma and associated metals

in a very broad, irregular area (southwestern United States). 64

A new and more recent interpretation by Livingston (1973a,

1973c) explained the existence of porphyry copper deposits by means of a general north-westerly drift of the North American plate over an im­ mobile , hot spot in the mantle. The plume, from a deep source anoma­ lously high in copper, started 72 m .y. ago and reached a maximum size of 450 km^ 65 m .y. ago near Tucson, Arizona, and then dissipated as the Laramide volcanic activity waned. This hypothesis explains the northwesterly alignment of porphyry copper deposits and their southeast­ erly decrease in age. A similar hypothesis was supported by Lowell

(1973), although he preferred a spreading center in the upper mantle or lower crust as a source of the copper.

Based mainly on the distribution of porphyry copper deposits, alignments, and paleographic features, Titley (1972a) described the formation of a southeasterly elongated trough at whose foreland the loca­ tion of porphyry copper deposits took place.

Characteristics of the distribution of preo-ore rocks suggest that the intrusions on the foreland were, in great part, vol­ canic related and penetrated to very shallow levels in the crust, possibly along major faults which were deeply penetrat­ ing and which developed major lines of weakness (Titley, 1972a, p. 258).

According to Gilmour (1972), it is generally agreed that the occurrence of porphyry copper deposits are associated with orogenic belts. Three types of igneous activities develop in an orogen:

1. Sodium-rich lavas and pyroclastic deposits (spillites) associ­

ated to basic and ultrabasic intrusions (ophiolites), active

during the geosynclinal phase. 65

2. Intrusion of granitic and granodioritic batholiths associated

with the zone of most intense deformation (orogen).

3. Calc-alkaline basalts, andesites, and rhyolites erupt toward

the end of the orogenic phase and during the succeeding stage

of vertical adjustment.

Gilmour believes that porphyry copper deposits then are associ­ ated with the intrusive stage of calc-alkaline post-orogenic activity. He also points out that most intrusive rocks are porphyritic or granitoid sug­ gesting shallow depths of deposition; observation supported by breccia pipes and pebbles dikes (for example, Urad and Henderson). Volcanic rock alteration-mineralization relationships in many deposits suggest that mineralization and alteration were either contemporaneous with the eruption of the volcanic rocks or followed it. Depth is assumed to be between 1,500 and 10,000 feet (457.5 and 3,050 m), with a mean of

4,000 to 5,000 feet (1,220 to 1,525 m).

"The most compelling evidence in support of the thesis that the parent intrusions of porphyry copper mineralization are related to late orogenic calc-alkaline volcanism is derived from radioactive dating as shown by Damon's work" (Gilmour, 1972, p. 4).

It may be concluded from the foregoing that (1) the importance of linear features is recognized by almost all investigators; (2) the relation­ ship between porphyry copper deposits and calc-alkaline igneous activity, especially volcanism, is fairly obvious; and (3) the possible origin of porphyry coppers related to a subducted paleo slab is being considered.

All of these theories, of course, try to explain the location of 66 southwestern United States porphyry copper deposits; however, none of them has satisfactorily been extrapolated to Sonora and southern M exico.

Figure 12 shows the location of some important copper deposits as they were some 10 m.y. ago. It seems to be possible that a continuous Mex­ ican Pacific copper province does exist as a southern extension of the

Wasatch-Jerome structural zone and as a link with the Central America-

South America belt. This idea is suggested by Lowell (1973) and ex­ pressed by Martinez M. (1974).

Now, if we take into consideration the regional geology of

Sonora, Baja California, western Chihuahua, and northern Sinaloa, there is a noticeable trend of younger igneous activity toward the east; the oldest plutons are generally near the west coast, the middle Tertiary volcanics are near the Chihuahua-Sonora border.

Radiometric studies report ages of the order of 117 to 111 m .y.

(K-Ar) for the Baja California Norte batholith (Barthelmy, 1974). De

Csema (1970) reported a period of emplacement between 96 and 115 m .y.

Apparently the age within this pluton decreases toward the eastern coast

(Ariel EchSvarri P ., oral commun., 1974). Anderson (1974, p. 484) re­

ported similar conclusions, stating, "apparent ages systematically de­

crease from 80-97 million years on the west coast to 60-70 million

years near the Chihuahua border. They suggest a north-south-trending

front of magmatic emplacement which migrated eastward at a rate of one

to 1-1/2 cm ./year." Therefore, it is likely that the Mexican Pacific

copper province is in some way linked to the circum-Pacific magmatic

activity in the M esozoic and Cenozoic. 67

Radiometric date, million years

Zone of predominant Cretaceous - Loramide activity. ^2) Zone of predominant Loramide-Mid Tertiary activity (^ ) Zone of predominant Oligocene-Pliocene activity.

Figure 12. Interpreted distribution of magmatic activity along Mexico's northwest coast 68

We have now established a possible regional setting for the

Sonoran copper subprovince, as well as definite direct relationship be­ tween porphyry copper deposits and calc-alkaline volcanism. Below, a

closer view of the possible genetic link between porphyry copper de­ posits and coeval volcanic rocks will be taken.

In 1973, Sillitoe published an integrated hypothetical model of porphyry copper deposits to which the typical Sonoran project may be related. This model (Fig. 13) infers that a porphyritic mineralized stock

w ill show an outward alteration zoning from a potassic core through a

sericitic argillic zone which fades away into a propylitic assemblage.

Copper-molybdenum mineralization is normally associated with the

potassic and sericitic zones. Downward the stock may grade into an

unaltered pluton of considerably larger dimensions .

This stock may not be, but commonly is , covered by a comag-

matic volcanic pile, which may take the form of a stratovolcano. This

pile may now have been eroded. Mineralized stocks may present cupola­

like protuberances of major plutons or may have intruded them as a oo­

genetic final stage of igneous activity.

The upper part of a porphyry copper volcanic edifice, as en­

visioned by Sillitoe, will have decreasing sericitic alteration which will

be replaced by advanced argillic alteration. Widespread silicification

surrounding propylitic alteration and the presence of epithermal lead-

zinc veins are characteristic, as is the abundance of hydrothermal brec­

cias. The tops are possibly characterized by stratovolcanos with

showings of native sulfur, pyrite, marcasite, and high-temperature

fumaroles, if still active. 69

4 Km*

ALTERATION ROCK TYPES [ SIUCIFICATION a ADVANCED ARGILUC PORPHYRY STOCK •

PROPYUTIC PHANERITIC GRANODIORITE

SERICITIC HYDROTHERMAL INTRUSION BRECCIA

POTASSIUM SIUCATE t From: richard h. s il u t o e , 1973)

Figure 13. Sillitoe's idealized model of a porphyry copper d e p o sit 70

In regard to the porphyry-volcanic relationships, Sillitoe con­

sidered the emplacement of the former into the latter common, rather than being overlaid by it. The depth of emplacement of a porphyry copper is

apparently very shallow, and the vertical extent of different alteration

types probably depends on the depth attained by meteoric waters.

A preliminary conclusion that can be derived from this discus­

sion is that characteristics of Sonoran prospects are located at a medium

to high level within Sillitoe's model. A good example illustrating this

conclusion is El Alacran, southeast of Cananea. The following comments

as well as Figure 14 are drawn from an unfinished M .S. thesis by Darrel

Dean at the Department of G eosciences, The University of Arizona.

El Alacr&n is a 2.5 x 6 km sulfide-associated anomaly elongate

northeasterly and located along the Cananea-Nacozari trend. A semi­

circular quartz latite porphyry is intruded within a pyroclastic andesitic

and dacitic pile dipping away from the central quartz latitic core which

shows intense argillization. The intrusion breccia at the contact shows

biotitic alteration and traces of mineralization. The volcanic pile shows

a concentrically zoned phyllic alteration belt surrounded by a pyrite-

epidote-chlorite zone with base-metal mineralization. Sericite-

tourmaline alunite breccias, as well as pebble dikes and a remarkable

association of volatile mineralization (fluorite-topaz-tourmaline-alunite),

suggest a depth of emplacement not greater than 1 km. The system was

probably degassing to surface. It is Dean's conclusion that El Alaerctn

is centered in a volcanic-subvolcanic edifice in much the same way as

Sillitoe's model. (T)r- Quartz latite porphyry with clay alteration , barren .

NOT TO SCALE ( 2 ) ” Contact intrusion breccia shell with biotitic alteration

( Darrel Dean, Pers. Cortmun. J974-)

Figure 14. Diagrammatic sketch of a vertical cross section at El Alacrhn, Sonora 72 Some similarities in alteration, petrology, and the existence of

hydrothermal breccias exist between El Alacran and Red Mountain, south

of Patagonia, Arizona. In this area, a thick sequence of Laramide pyro­

clastic rocks is intruded by a silicified monzonite porphyry and abundant

intrusion breccias. Volcanic rocks are in general pervasively altered to

quartz—sericite and clay mineral. Moreover, two important characteris­

tics make this particular situation very interesting.

1. Primary mineralization is pyrite and enargite in upper levels;

the latter grades downward to chalcopyrite.

2. The principal concentration of primary mineralization is encoun­

tered where potassic-altered zones appear at a depth of 3,000

feet (915 m) (Russell Corn, oral commun., 1974).

In many respects, both mineralized bodies are similar to the typical

Sonoran prospect developed in a volcanic host rock. CHAPTER 7

CONCLUSIONS

A series of hydrothermally altered areas within volcanic rocks is found scattered throughout the state of Sonora. This type of alteration is commonly produced by the intrusion of fine- to medium-grained grano- dioritic to quartz dioritic stocks or by the intrusion of irregular hyp abys­

sal bodies. The latter are frequently of intermediate to acidic composi­ tion and of porphyritic texture. The most common intruded rock is a

volcanic sequence of Early Cretaceous to Laramide age, andesitic in

composition.

Epigenetic hydrothermal alteration is commonly well zoned, in­

cluding those areas produced by pluton-volcanics contact. The outer­

most alteration is propylitic and includes quartz-chlorite-epidote-

chalcocite, with minor sulfide mineralization. The nature of rocks

affected by this alteration is well preserved, and recognition is fairly

easy. It generally changes to a silica-sericite-argillic minerals assem­

blage toward the center of alteration. The degree to which pervasive

phyllic alteration is found is variable. The change may be transitional

or very sharp, due to various degrees of structural control in case of

volcanic host rocks or forced intrusion in case of hypabyssal brecciated

b o d ie s .

Characteristic mineralization of these areas is disseminated

pyrite, very commonly fine grained, 1 to 3 percent by volume. Occasional

73 74 chalcopyrite, molybdenite, specularite, scheelite(?), and wolframite (?) also occur.

Most areas belong to this sericite-silica-clay alteration group.

However, the importance of extensive argillic alteration is substantially

augmented by supergene processes.

Disseminated black or green, translucent tourmaline is common

and characteristic of strongly sericitized quartz-tourmaline breccias. A

general decrease of this mineral with depth has been found. Similarly,

white to pale or yellow alunite occurs in veinlets within the supergene-

oxidized environment, even though primary alunite has been reported at

La Mariquita (Adelaide Gutierrez A ., oral commun., 1974).

Potassic alteration is only reported at a few prospects and is

characterized by biotitization and K-feldspar flooding. Its importance is

mentioned in connection with high chalcopyrite-pyrite ratios (El AlacrAn

and El Alamo) and presence of molybdenite.

Control of the emplacement of hypabyssal rocks and the associ­

ation of alteration zones are probably marked by N. 40o-60° W ., N .

35o-50° E., and N. 70o-80° E. major lineaments. At a local level,

structural control is also of utmost importance and presents several dif­

ferent characteristics, depending on the type of emplacement: (1) frac­

turing and doming produced by intrusive and intrusion breccias; (2)

tourmaline and sericite intrusive breccias; (3) contact breccias; and

(4) sto ck w o rk s.

Particularly interesting is the case of plutonic-volcanic contact

breccias because they may have structural characteristics similar to

breccia pipes or intrusive breccias. However, brecciation disappears 75 with depth and fresh platonic rock is found with almost no sign of min­ eralization.

In regard to capping expression in these areas, two observa­ tions are pointed out: a generally high degree of leaching and a predominant distribution of limonites finely disseminated. Jarositic and hematitic cappings are common and very conspicuous due to their bright brick red color.

The location of Sonoran hydrothermal sulfide systems suggests a very close relationship to a third stage of igneous activity toward the end of the orogenic phase, as pointed out by Gilmour (1972). Alteration and mineralization followed or were concomitant with a very active mag- matism of Laramide age. It is therefore believed that the Sonoran copper

subprovince is closely related to the areas of strongest volcanic activity of Laramide age, which in turn might have been controlled by the follow­

ing factors:

1. A zone of thin Paleozoic sedimentary rocks (less than 2,000 m?)

as shown in Figure 4.

2. A series of N . 40o-60° W. major breaks and discontinuities.

3. A possible northwest-southeast limit of Cretaceous mio geo-

synclincal environment to the north and volcanism with minor

volcano-sedimentary assemblages to the south.

It has been shown that magmatism decreases in age eastward,

giving rise to a possible spatial east-west range of Laramide volcanism

and consequently to porphyry copper deposits. It has also been proved

that coeval magmatism does exist along Mexico's western Pacific coast. 76

It is then pointed out that there is a possibility of a continuous copper metallogenic province, as partially proved by copper prospects found in northern and southern Sinaloa, as well as in Michoacan and Guerrero.

Once a favorable ground for exploration has been defined, it is believed that Sillitoe's (1972) porphyry copper model in the boundary be­

tween plutonic and volcanic environments can be applied to some extent

in Sonoran prospect areas. The presence of epithermal lead-zinc veins,

pervasive silicification, argillic and propylitic alteration, abundance of

tourmaline, high level of erosion, and absence of deep-seated alteration

assemblages partially support the conclusion that these lie at a medium

to high level within the model.

Based on these conclusions, several suggestions for exploration

can be derived. Reevaulation of many alteration sulfide systems in

Sonora, with a deep target in mind, might prove worthwhile. This ex­

ploration could be greatly helped by shallow exploratory drilling of the

most promising areas, with detailed petrographic and geochemical back

up to define alteration, mineralogical, and geochemical trends. Explora­

tion should be conducted of deep primary sulfide systems outcropping

through a series of tectonic uplifts. Blind orebodies, faulted and alluvi­

um or volcanic covered, are also good targets for exploration.

M assive exploration projects could be conducted in the search

of compos it or wall-rock porphyries (as at Cananea) as well as for in­

trusion porphyries (as at La Caridad). However, the exploration and

development of these types of deposits require large financial resources,

generally difficult to obtain. 77

Therefore, targets of minor size but with low investment re­ quirements and short payback periods may be of more interest. Among these are copper-molybdenum breccia pipes of the La Colorada and

Pilares de Nacozari type, copper-molybdenum-tungsten volcanic brec­ cias of the Washington and Cobre Rico type, copper-molybdenum peg- matitic breccias of the San Judas Tadeo type, and copper-molybdenum- tungsten pegmatitic contact breccias of the Galaviz type.

Exploration techniques envisioned as of future use in Sonora

are: (1) lineament maps produced from ERTS-1 imagery, (2) magnetometry,

(3) induced polarization, and (4) deep drilling with detailed analysis of

hydrothermal alteration minerals and geochemical trends.

It is also suggested that a comprehensive rock geochronology

program for volcanic and plutonic rocks in the state of Sonora, carried

out by Mexico's governmental exploration agencies, should yield vital

information in the better geologic understanding of this entity and, most

important of all, assist in the discovery of major disseminated copper-

molybdenum orebodies. SELECTED BIBLIOGRAPHY

Aguilera, J. G ., 1896, Itineraries geologicos eri Bosquejo geologico de Mexico: Inst. Geol. de Mexico Bull. 4,5,6, p. 118-119.

Alarcon, UbaldoL., and Aguilera, Eduardo, 1973, Informe sobre los estudios geologicos, geoquimicos y geofisicos en el prospecto "El Alacran," Sonora, hasta Marzo de 1970: Consejo de Rec. Nat. no Renovables Bull. 79.

Alvarez, Manuel, Jr., 1968, Geologia, paleografia y tectonica de Mexico: unpublished class notes, Facultadde Ingenierfa, Universidad Nacional Autonoma de M exico, M exico, D.F.

Anderson, Thomas H ., 1974, Late Cretaceous volcanism in Sonora, M exico, and its relationship to circum-Pacific magmatism (abs.): Rocky Mountain Section, G .S.A ., Flagstaff, Arizona, March 1974, G.S.A. Abstracts with Programs, v. 6, no. 5, p . 4 8 4 .

______and Silver, Leon T ., 1973, The Cananea granite—implications of its PreCambrian age (abs.): G.S.A. Annual Meeting, Dallas, Texas, 1973, G.S.A. Abstracts with Programs, v. 5, no. 7, p . 5 3 4 .

Antunez, F., Ruiz, Jesus, and Calderon, Alejandro, n .d ., Resultados de la exploracion geoldgico-minera por la Comisidn de Energia Nuclear: Comisidn Nacional de Energia Nuclear report (1968?).

Aponte Barrera, Mario, 1974, Estratrigraffa del Paleozoico (Cambrico- Pensilvdnico) del centra de Sonora: unpublished thesis, E .S.I.A ., Institute Polite'cnico Nacional, Mexico, D.F.

Balmer, R. W ., 1973, The Bougainville copper project: Mining Congr. Jour., v. 59, no. 4, p. 33-40.

Bancroft, G. J., 1903, Yaqui River country, Sonora: Eng. Mining Jour., v. 76, p. 160-162.

Barthelmy, David A ., 1974, Geologic and K-Ar relationships of copper mineralization and M esozoic plutonic rocks in the El Arco area, Baja California (abs.): Cordilleran Section, G .S.A ., Las Vegas, Nevada, 1974, G.S.A. Abstracts with Programs, v. 6, no. 3, p . 1 4 2 .

78 79

Berlanga Galindo, Edmundo, 1971, Geologfa del Proyecto Auroa, region del bajo Rio Yaqui, Municipio de Onavas, Edo. de Sonora: un­ published thesis, E .S.I.A ., Institute Politecnico Nacional, Mexico, D.F.

______1973, Metallogenic trends of the parallel valleys and ranges sub-province, Sonora, Mexico: unpublished report, Dept, of Mining and Geological Engineering, University of Arizona.

Bolich, Leonard C ., 1969, Petrography of the western portion of the Juriquipa stock, Nacozari, Sonora, Mexico: unpublished M .S. thesis. The University of Texas at El Paso, Texas.

Bolt, L. R ., 1969, Petrology of the San Martin de Porres area, Nacozari, Sonora, Mexico: unpublished M .S. thesis. The University of Tennessee, Knoxville, Tennessee.

Bonneau, Michel, 1972, Donnees nouvelles sur les series cretacees de la Cdte Pacifique du Mexique: La Societe geologique de France Bull., 7th series, v. 14, no. 1-5.

Bowie, A. J ., 1882, Gold mines of Mulatos: unpublished report, source unknown, copy in possession of author.

Brown, R. G ., 1897, A mineralized dyke (Sahuaripa, Sonora, Mexico): Columbia School of Mines Quart., v. 19, p. 90-93. ! Burnham, C. W ., 1959, Metallogenic provinces of the southwestern United States and northern Mexico: New Mexico Bureau of Mines, Mineral Res. Bull. 65, p. 1-76.

Chiapa G., Armando, and Thoms, John A ., 1971, Bosquejo geologico del Distrito Minero de Nacozari: Excursion Guidebook, DC Natl. Convention, Asoc. de Ing. de Minas, Met. y Geologos de M exico, Hermosillo, Sonora, reprint.

Cooper, G. Arthur, and Arellano, Alberto R. V ., 1946, Stratigraphy near Caborca, northwest Sonora, Mexico: Am. Assoc. Petroleum Geologists Bull., v. 30, no. 4, p. 606-619.

Damon, Paul E., Livingston, Donald E., Mauger, Richard L., Giletti, Bruno J ., and Pantoja A lor, Jerjes, 1962, Edad del Precambrico "Anterior" y de otras rocas del Zdcalo de la region de Caborca- Altar de la parte noroccidental del Estado de Sonora: Institute de Geologia Bull. 64, Part 2, p. 11-44.

Damon, P. E., and Mauger, R. L ., 1966, Epeirogeny-orogeny viewed from the Basin and Range province: SME Trans., v. 235, p. 9 9 -1 1 2 . . 80 de la Campa J ., Genaro, 1968, Exploracion geologico minera del area Nuevo Contlnente, Mpio. de Nacozari, Sonora: unpublished thesis, E .S.I.A ., Institute Polite'cnico Nacional.

De Cserna, Zoltan, 1960, Orogenesis in time and space in Mexico: Geol. Rundschau, no. 50, p. 595.

______1961, Tectonic map of Mexico: Geol. Soc. America, scale 1:2 , 000 , 000 .

______1970, M esozoic sedimentation, magmatic activity and deforma­ tion in northern Mexico, in Seewald, Ken, and Sundeen, Dan, eds., The geologic framework of the Chihuahua tectonic belt: Symposium in honor of Professor Ronald K. De Ford, November 1970, Midland, Texas. Sponsored by the West Texas Geol. Society and The University of Texas at Austin. Midland, Texas, West Texas Geol. Society, p. 99-117.

Dixon, D. W ., 1966, Geology of the New Cornelia mine, Ajo, Arizona, in Titley, S. R ., and Hicks, C. L ., ed s., Geology of the por­ phyry copper deposits, southwestern North America: Tucson, Arizona, University of Arizona Press, p. 123-132.

Dumble , E. T ., 1900, Notes on the geology of Sonora, Mexico: AIME Trans., v. 29, p. 122-152.

______1901, Occurrence of oyster shells in volcanic deposits in Sonora, Mexico: Texas Academy of Science, Trans., v. 4, p . 8 2 .

Eardley, A. J., 1951, Structural geology of North America: New York, Harper Bros. Publishing Co.

Echavarri Perez, Ariel, 1967, La alteracidn hidrotermal del yacimiento de Nuevo Continente, Nacozari, Sonora: unpublished report, Consejo de Rec. Nat. no Renovables, Hermosillo, Sonora.

______1971, Petrografla y alteracidn en el depdsito de La Caridad, Nacozari, Sonora, Mexico: paper presented at the DC National Convention, Asoc. de Ings. de Minas, Met, y Gedlogos de M exico, Hermosillo, Sonora.

Echavez Valverde, Joaqufn, 1968, The Parral district. Chihuahua, M exico, as related to the silver metallogenetic province of northern Mexico: unpublished M .S. thesis, University of A rizon a.

Eyde, Theodore H ., 1972, Stratigraphy is a sometimes overlooked guide to porphyry coppers: Mining Eng., v. 24, no. 4, p. 40-62. 81

Field, Cyrus W ., Jones, Michael B ., and Bruce, Wayne R ., 1973, Porphyry copper-molybdenum deposits of the Pacific Northwest: Paper presented at the SME-AIME Annual M eeting, Feb. 25- March 1, 1973, Chicago, 111., Preprint No. 73-S-69.

Flores, Teodoro, 1929, Reconocimientos geoldgicos en la region central del Estado de Sonora: Inst. Geol. Mexico, Bull. 49.

Fries, Carl, Jr., 1962a, Lista de fechas geoquxmicas reportadas para minerales y rocas mexicanas, con un comentario sobre su sig- nificado geoldgico y geotectdnico: Institute de Geologia, UNAM Bull. 64, part 6, p. 85-109.

______1962b, Resena de la geologia del Estado de Sonora, con enfasis en el Paleozoico: Asoc. Mex. de Geol. Petr. Bull., v. V-14, nos. 11—12, p. 257-273.

Gilmour, Paul G., 1972, The relationship between porphyry copper de­ posits and post-orogenic volcanism: paper presented at the SME-AIME Annual Meeting, San Francisco, Feb. 20-24, 1972, Preprint No. 72-1-49.

Guilbert, John M ., and Lowell, J. David, 1974, Variations in zoning pattern in porphyry ore deposits: C.I.M . Bull., Feb., p. 99- 1 0 9 .

Guilbert, John M ., and Sumner, John S ., 1968, Distribution of porphyry copper deposits in the light of recent tectonic advances, in Titley, S. R ., e d ., Southern Arizona Guidebook III: Tucson, Arizona Geol. Society, p. 97-112.

Gutierrez Arce, Adelaide, 1973, Caracteristicas generales de intrusivos hipabisales an el centre de Sonora y su relacion con mineraliza­ tion de tipo pdrfido cuprffero: paper presented at the X National Convention, Asoc. de Ings. de Minas, Met. y Geol. de Mexico, Chihuahua, M exico.

Hafer, C., 1912, The mines of the Sonora Valley, Sonora, Mexico: Mining World, v. 36, p. 903.

Hawkes, H. E ., and Webb, J. S., 1962, Geochemistry in mineral ex­ ploration: New York, Harper and Row Publishers Inc.

Hollister, V. F., 1973, Regional characteristics of porphyry copper de­ posits of South America: paper presented at the SME-AIME Annual Meeting, Feb. 25-March 1, 1973, Chicago, 111., Preprint No. 73-1-2.

Howell, Fred H ., and Mo Hoy, John S ., 1960, Geology of the Braden orebody, Chile, South America: Econ. G eol., v. 55, no. 5, p. 863—905. 82

Huff, L. C ., 1971, A comparison of alluvial exploration techniques for porphyry copper deposits: C.I.M . special vol. 11, p. 190-194.

Hynes, D. C., 1912, Notes on the geology of the Mina Mexico vein: Econ. Geol., v. 7, p. 280.

Imlay, R. W ., 1939, Paleographic studies in northeastern Sonora: Geol. Soc. America Bull., v. 50, p. 1723-1744.

Institute de Geologia, 1962, E studios geocronologicos de rocas M exi- canas: Inst, de Geologia,UNAM, Bull 64.

Kents, Paul, 1964, Special breccias associated with hydrothermal de­ velopments in the Andes: Econ. G eol., v. 59, p. 1551-1563.

King, P. B ., 1969, The tectonics of North America: U .S. Geological Survey Prof. Paper 628. Map scale 1:5,000,000.

King, R. E., 1934, Geological reconnaissance in central Sonora: Amer. Jour. Sci., 5th ser., v. 28, p. 81-101.

______1939, Geological reconnaissance in northern Sierra Madre Occidental of Mexico: Geol. Soc. America Bull., v. 50, p. 1625-1722.

Livingston, D. E., 1973a, Laramide hot-spot, copper deposits and plate motion in Arizona, Sonora and New Mexico: Dept, of Geosciences, University of Arizona, Contr. 79. i

______1973b, Geology, K-Ar ages and Sr isotopy at La Garidad, Nacozari district, Sonora, Mexico: Dept, of Geosciences, University of Arizona, Contr. 80.

______1973c, A plate tectonic hypothesis for the genesis of porphyry copper deposits of the southern Basin and Range province: Earth and Planetary Sci. Letters 20, p. 171-179.

Loera Fragoso, Francisco J ., 1973, Geologia del area de prospeccion Chela M ilpillas, Mpio. de Santa Cruz, Sonora: unpublished thesis, E.S.I.A., Institute Politecnico, Mexico, D.F.

Lopez, Ramos, E. (ed.), 1968, Geologic map of Sonora: Institute de Geologia, UNAM, scale 1:500,000.

______1969, Marine Paleozoic rocks of Mexico: Amer. Assoc, of Petroleum Geologists Bull., v. 53, no. 12, p. 2399-2417.

Lowell, J. David, 1973, Regional characteristics of southwestern North America porphyry copper deposits: paper presented at SME- AIME Annual Meeting, Feb. 25-March 1, 1973, Chicago, 111., Preprint No. 73-S-12. 83

Lowell, J. David, and Guilbert, John M ., 1970, Lateral and vertical alteration-mineralization zoning in porphyry ore deposits: Econ. G eol., v. 65, no. 4, p. 373-408.

Lynch, Dean W ., 1966, The economic geology of the Esperanza mine and vicinity, in Titley, S. R., and Hicks, C. L ., eds.. Geology of the porphyry copper deposits, southwestern North America: Tucson, University of Arizona Press, p. 267-279.

Lyon, R. J. P ., 1972, Exploration application of remote sensing tech­ nology (image forming systems): Mining Cong. Jour., v. 58, no. 6, p. 18-23.

Martinez Muller, Remigio, 1974, Geology of the Malpica area, Sinaloa, Mexico: unpublished M .S. thesis, Colorado School of M ines, Boulder, Colorado.

Mayo, Evans B ., 1958, Lineament tectonics and some ore districts of the Southwest: AIME Trans., v. 211, p. 1169-1175.

McAnulty, William N ., Jr., 1970, Geology of the northern Nacozari district, Sonora, Mexico: unpublished Ph.D. dissertation, University of New Mexico, 169 p.

Merriam, Richard, 1972a, Reconnaissance geologic map of the Sonoyta quadrangle, Sonora, Mexico: Geol. Soc. America Bull., v. 83, no. 11, p. 3533—3536.

______1972b, Evidence for 200 miles of right-lateral displacement in northwestern Sonora (abs.): Cordilleran Section, G .S.A ., Las Vegas, Nevada, G.S.A. Abstracts with Programs, v. 4, no. 3, p . 1 9 8 .

Merrill, J. H ., 1907, Copper in the Sonora quartz veins: Engr. Mining Jour., v. 97, p. 296.

______1908, Mineral resources of Sonora: Mining and Scientific Press, v. 97, p. 296.

Minera Tridn, S.A. de C .V ., 1968-1974, unpublished reports and maps: in company files, Hermosillo, Sonora.

Mishler, R. T ., 1920, Geology of the El Tigre district, Mexico: Mining and Scientific Press, v. 121, p. 585-591.

Mitchell, C. J., 1928, The geology of Sonora: Science, v. 67, p. 373.

Pearce, S. L ., 1910, Piedras Verdes disseminated copper zone. Alamos, Sonora: Engr. Mining Jour., v. 89, p. 920. 84

Ramirez Ruvalcaba, Jose, 1965, Geology of the San Antonio Mountains , State of Sonora, Mexico: unpublished M .S. thesis, University of Arizona.

______1973, Informe actualizado de las investigaciones efectuadas en el Prospecto "Los Alisos": Consejo de Rec. Nat. no Reno- vables Bull. 79.

Richard, F., 1904, Copper deposits in Sinaloa and southern Sonora: Engr. Mining Jour., v. 79, p. 97.

Robinson, R. F ., and Cook, Annan, 1966, The Safford copper deposit, Lone Star mining district, Graham County, Arizona, in Titley, S. R ., and Hicks, C. L ., eds.. Geology of the porphyry cop­ per deposits, southwestern North America: Tucson, University of Arizona Press, p. 205-220.

Rose, A. W ., and Baltosser, W. W ., 1966, The porphyry copper deposit at Santa Rita, New Mexico, in Titley, S. R., and Hicks, C. L ., e d s., Geology of the porphyry copper deposits, southwestern North America: Tucson, University of Arizona Press, p. 205- 2 2 0 .

Russell, B. E., 1908, Nacozari mining district, Sonora: Engr. Mining Jour., v. 86, p. 657.

Salas, Guillermo A ., 1970, Areal geology and petrology of the igneous rocks of the Santa Ana region, northwest Sonora: Soc. Geol. Mexico Bull., v. 31, no. 1, p. 11 -63.

Schmitt, Harrison A ., 1966, The porphyry copper deposits in their re­ gional setting, in Titley, S. R., and Hicks, C. L ., eds.. Geology of the porphyry copper deposits, southwestern North America: Tucson, University of Arizona Press, p. 17-33.

Seewald, Ken, and Sundeen, Dan (eds.), 1970, The geological framework of the Chihuahua tectonic belt: Symposium in honor of Professor Ronald K. De Ford, November 1970, Midland, Texas. Spon­ sored by the West Texas Geol. Society and The University of Texas at Austin. Midland, Texas, West Texas Geol. Society.

Sillitoe, Richard H ., 1972, A plate tectonic model for the origin of por­ phyry copper deposits: Econ. G eol., v. 67, p. 184-197.

______1973, The tops and the bottoms of porphyry copper deposits: Econ. G eol., v. 68, no. 6, p. 799-815.

Solano Rico, Baltazar, 1970, Geologfa y yacimientos minerales del Distrito de.Lampazos, Sonora: unpublished thesis, Fac. de Ingenierfa, Universidad Nacional Autdnoma de M exico, M exico, D.F. 85

Solano Rico, Baltazar, 1972, Informe final del Proyecto Eleyn: unpub­ lished report, Minera Tridn, S.A, de G .V ., Hermosillo, Sonora.

______and Dominguez Lara, Evaristo, 1972, Informe final del Proyecto Lucia: unpublished report, Minera Tridn, S.A. de G .V ., Hermo­ sillo, Sonora.

Spurr, J. E ., 1927, A feldspar-chalcopyrite rock ore from Sonora: Engr. Mining Jour., v. 123, no. 9, p. 604.

Taliaferro, N . L ., 1933, An occurrence of Upper Cretaceous sediments in northern Sonora, Mexico: Jour. Geology, v. 41, p. 12-37.

Tays, E. A. H ., 1910, The Lluvia de Oro mine: Mining and Scientific Press, v. 100, p. 59.

Tellez Herrera, Roberto, 1969, Estudio geologic© miner© de una area en el Distrito Miner© de La Verde, Edo. de Sonora: unpublished thesis, Fac. de Ingenieria, Universidad Nacional Autdnoma de Mexico, Mexico, D.F.

Titley, Spencer R ., 1966, Preface to Geology of the porphyry copper deposits, southwestern North America, Titley, Spencer R ., and Hicks, C. L ., eds.: Tucson, University of Arizona Press.

Titley, Spencer R ., 1972a, Pre-ore environment of southwestern North America porphyry copper deposits: Section 4, Mineral deposits, 24th International Geological Congress, p. 252-260.

______1972b, Some geological criteria applicable to the search for southwestern North American porphyry copper deposits: paper presented at Joint Meeting MMIJ-AIME, Tokyo, Japan, Preprint G -1 2 .

______1973, Geological environment and characteristics of some por­ phyry copper deposits in the southwestern Pacific: paper pre­ sented at SME-AIME Annual Meeting, Feb. 25-March 1, 1973, Chicago, 111., Preprint No. 73-S-65.

Treadwell, J. C., 1905, The Sahuayacan district, Mexico: Engr. Mining Jour., v. 80, p. 1213.

United Nations—Consejo de Recursos Naturales No Renovables, 1969, Survey of metallic mineral deposits in Mexico: New York, United Nations Publ. DF/SF/UN19, Mex. Plan. Zone 9.

Valentine, Wilbur G ., 1936, Geology of the Cananea Mountains, Sonora, Mexico: Geol. Soc. America Bull., v. 47, p. 53-86. 86

Varela, F. E., Jr., 1971, La Turmalina como mineral indice en varias partes del Hemisferio Oeste: paper presented at DC National Meeting, Asoc. de Ings. de Minas, Met. y Geol. de Mexico, Hermosillo, Sonora.

Velasco, J. Ruben, 1966, Geology of the Canane a district, in Tit ley, S. R., and Hicks, C. L ., ed s., Geology of the porphyry cop­ per deposits, southwestern North America: Tucson, University of Arizona Press, p. 245-249.

Viveros Martfnez, Alejandro, 1965, Estudio geoldgico de la Sierra de Cabullona, Mpio. de Agua Prieta, Edo. de Sonora: unpublished thesis, Fac. de Ingeniena, Universidad Nacional Autonoma de Mexico, Mdxico, D .F., 24 p.

Waring, W. G ., 1897, Gold fields of Altar, Mexico: Engr. Mining Jour., v. 63, p. 257.

Weed, W. H ., 1922, Notes on certain mines in the States of Chihuahua, Sinaloa and Sonora, Mexico: AIME Trans., v. 32, p. 396.

Wertz, J. B ., 1970, Arizona's copper provinces and the Texas lineament: Mining Eng., v. 22, no. 5, p. 80-81.

W iese, J. H ., 1946, Tungsten deposits of south Sonora, Mexico: U.S. Geological Survey Bull. 946-D, p. 103-130.

W ilson, Ivan F ., and Rocha, V. S., 1946, Los yacimientos de Carbon de la region de Santa Clara, Municipio de San Javier, Edo. de Sonora: Com. Direct. Invest, de Rec. Minerales Bull. 9, 108 p .

W isser, Edward, 1966, The epithermal precious metals province of northwestern Mexico: Nevada Bureau of M ines, Report 13, Part G .

Zendejas Mendivil, Manuel, 1972, Exploracion de cobre diseminado en el Proyecto Catalinas del Distrito Minero de Canane a, Sonora: unpublished thesis, E.S.I.A., Institute Politecnico Nacional, M e x ic o , D.F.

Zonge, Kenneth L ., and Wynn, Jeffrey C., 1974, Recent advances and applications in complex resistivity measurements: paper pre­ sented at the 44th Annual International SEG Meeting, Dallas, Texas, Nov. 12, 1974. 8 6

# B. Solano Rico, (VI.S. T hesis, Dept, of Mining and Geological Engineering, 1975 JUS Or