Volcanic and Structural Controls of Mineralization in the Dos Cabezas Mountains of Southeastern Arizona U.S. GEOLOGICAL SURVEY BULLETIN 1676 Volcanic and Structural Controls of Mineralization in the Dos Cabezas Mountains of Southeastern Arizona By HARALD DREWES, DEAN P. KLEIN, and SCOTT D. BIRMINGHAM U.S. GEOLOGICAL SURVEY BULLETIN 1676 DEPARTMENT OF THE INTERIOR DONALD PAUL MODEL, Secretary U.S. GEOLOGICAL SURVEY Dallas L. Peck, Director UNITED STATES GOVERNMENT PRINTING OFFICE: 1988 For sale by the Books and Open-File Reports Section U.S. Geological Survey Federal Center, Box 25425 Denver, CO 80225 Any use of trade names is for descriptive purposes only and does not imply endorsement by the U.S. Geological Survey. Library of Congress Cataloging-in-Publication Data Drewes, Harald, 1927- Volcanic and structural controls of mineralization in the Dos Cabezas Mountains of southeastern Arizona. (U.S. Geological Survey bulletin ; 1676) Bibliography: p. Supt. of Docs, no.: I 19.3:1676 1. Ore-deposits Arizona Dos Cabezas Mountains. 2. Volcanic ash, tuff, etc. Arizona Dos Cabezas Mountains. 3. Geology Arizona Dos Cabezas Moun­ tains. I. Klein, Dean P. II. Birmingham, Scott D. III. Title. IV. Series. QE75.B9 no. 1676 557.3s 86-600158 [QE390] [553'.09791'53] CONTENTS Abstract 1 Introduction 2 Geographic setting 2 Geologic setting 3 Mineralization setting 4 Geology 5 Stratigraphy and petrography 5 Precambrian rocks 6 Paleozoic rocks 9 Mesozoic rocks 10 Latest Cretaceous and Paleocene rocks 11 Mid-Tertiary intrusive rocks 13 Young surficial deposits 14 Structural geology 14 Southwestern terrane 14 Northeastern terrane 16 Apache Pass fault zone 18 Mineral deposits 19 Geochemistry 20 Precious metals 25 Common base metals 28 Other metals 28 Geophysics 29 Regional aspects 31 Central Dos Cabezas area 34 Southwestern terrane 37 Northeastern terrane 38 Apache Pass fault zone 40 Summary 42 References 44 PLATE [Plate is in pocket] 1. Generalized geology and aeromagnetic features of the Dos Cabezas Moun­ tains, Arizona, and surrounding area FIGURES 1. Location of Dos Cabezas Mountains and the mining districts and camps of adjacent ranges 3 2. Generalized geologic map of the Dos Cabezas Mountains 6 3. Diagrammatic section through the southwestern structural block near Dos Cabezas village 15 4. Diagrammatic section of the northeastern structural block near Elma mine 16 5. Diagrammatic section of the Apache Pass fault zone 18 6. Map of the mining districts, mines and prospects, and geochemical sample localities of the Dos Cabezas Mountains 21 Contents III FIGURES 7. Map showing distribution and degree of concentration of silver, lead, molybdenum, and copper in the Dos Cabezas Mountains 26 8. Map showing distribution and degree of concentration of gold, beryllium, manganese, and arsenic in the Dos Cabezas Mountains 27 9. Map showing distribution and degree of concentration of barium, bismuth, tungsten, and zinc in the Dos Cabezas Mountains 30 10. Upward continued aeromagnetic map of the Dos Cabezas Mountains area 32 11. Bouguer gravity map of the Dos Cabezas Mountains area 33 12. Gravity profiles and models of bedrock density distributions across the Dos Cabezas Mountains 35 13. Generalized aeromagnetic map of the central Dos Cabezas Mountains 36 14. Aeromagnetic map of the central Dos Cabezas Mountains from original contour maps 39 15. Original aeromagnetic maps that cover parts of the Dos Cabezas and Sim- mons Peak 71/2-minute quadrangles 41 16. Distribution of main geological and geophysical features viewed as sup­ porting a favorable assessment of the mineral potential of the Dos Cabezas Mountains, Arizona 43 TABLES 1. Stratigraphic summary of Dos Cabezas Mountains 8 2. Semiquantitative spectrographic and atomic absorption analyses of rock samples from the Dos Cabezas Mountains 22 3. Magnetic susceptibility of rocks in the central Dos Cabezas Mountains based on measurements at outcrops or on hand specimens 37 IV Contents Volcanic and Structural Controls of Mineralization in the Dos Cabezas Mountains of Southeastern Arizona By Harald Drewes, Dean P. Klein, and Scott D. Birmingham Abstract Still other faults are inferred to underlie alluvium or some of the volcanic rocks, and one of these faults appears to mark a The Dos Cabezas Mountains, about 50 km southeast of subsurface change from a thin to a thick sequence of the Willcox, Ariz., have been the site of a moderate amount of base- volcanic pile. The known and inferred collapse of part of the and precious-metals mining and have a strong potential for the volcano-plutonic field between the Ivanhoe and Elma mines 'discovery of additional mineral deposits. This encouraging and Howard Peak, combined with the evidence of many brec­ evaluation has been obtained from the combined results of cia pipes and other known intrusive rocks in the area, suggests geologic mapping, geochemical sampling, and geophysical that this area was the major volcanic center. Faults and other surveys. These results have also led us to identify some general structures, then, are seen as the major avenues of guiding the objectives and some specific targets for exploration in the cen­ movement of fluids at deep to moderate depths and to sites tral part of the mountains. of rocks that may have accumulated ore deposits. Geologic mapping indicates that the central part of the Rocks favorable to mineralization include Paleozoic and mountains is underlain by a volcano-plutonic field of dacitic Mesozoic sedimentary rocks and some of the dacitic breccia. to rhyolitic (granodioritic to granitic) composition of Late These are particularly good host rocks where they lie near the Cretaceous-Paleocene age. Much of the remaining volcanic borders of stocks and where they are faulted. From regional field consists of dacitic-flow breccia and breccia pipes. Several relationships, we find stocks of Late Cretaceous-Paleocene age granitoid stocks, some porphyry masses, and assorted aphanitic to be most commonly associated with mineralization. Fur­ to porphyritic dikes intrude the volcanic pile. Very locally the thermore, the occurrence of these favorable host rocks at a volcanic pile is underlain by Mesozoic sedimentary rocks; in transitional structural level between plutonic and volcanic en­ most places Precambrian crystalline rocks make up the base­ vironments is propitious for mineralization. ment. Breccia pipes containing clasts of Paleozoic or Mesozoic Geochemical sampling, mainly of mine dumps and pros­ sedimentary rocks suggest that such rocks also underlie con­ pects, shows that base and precious metals are widely dis­ cealed parts of the volcanic pile. An area of such an igneous persed. In some places, silver is seen to occur with lead, and system, especially one having this age, composition, and altera­ elsewhere it occurs with copper. High silver values occur main­ tion intensity, offers a prime general target for mineral ly in the north-central part of the area and along the southwest exploration. strand of the Apache Pass fault zone. Silver occurs as commonly Faults have had a major control in the distribution of the with copper alone as it does with lead, with which it volcano-plutonic rocks. The Apache Pass fault zone is a major presumably appears as argentiferous galena. Gold occurs in northwest-trending, steeply dipping, diversely and recurrent­ pyritiferous quartz veins or argentiferous galena in widely ly reactivated basement structure, forming the southwest side dispersed sites. Copper appears nearly throughout the volcanic of the volcano-plutonic field. Magmatic and hydrothermal fluids field. Zinc is also widespread, but is more erratic in its presence probably migrated upward along deeper levels of this fault than is copper. Lead is more common in the northwestern and zone. A subordinate fault splaying off the Apache Pass fault northeastern parts of the volcanic field than elsewhere. Quartz zone lies near the northeast side of the field. A ring-dike system veinlets and propylitic alteration are widespread, and locally lies along part of this splay fault and follows the margin of the intense clay mineral alteration is also present. In summary, there field where it diverges from the subordinate fault. Other subor­ are many signs that hydrothermal fluids migrated through the dinate faults trend northeast, offsetting the Apache Pass fault sedimentary and volcanic rocks of the central Dos Cabezas. zone and probably guiding fluid movement in shallower levels. Geophysical surveys, including aeromagnetic and gravity Abstract studies, indicate the presence of subsurface structural features and geophysical investigation aimed at searching for cer­ and help to show the configuration of structural features ex­ tain kinds of mineral deposits. posed at the surface. Blind stocks or dikes are suggested by the surveys at several sites along the Apache Pass fault zone or within the area of the volcanic field, where they are of poten­ Geographic Setting tial economic interest. Blind stocks or dikes are also indicated along the southwestern foothills, where they are of lesser significance chiefly because the associated exposed dikes are The Dos Cabezas Mountains lie immediately south­ of mid-Tertiary rather than Late Cretaceous-Paleocene age. The east of Willcox in the southeast corner of Arizona (fig. 1). geophysical signature of the volcanic field indicates that the The mountains are usually given less attention than field may be thickest near the breccia pipes and that the neighboring ranges because they are lower and less ex­ volcanic rocks may thin abruptly, perhaps through subsurface tensive. Both Interstate Highway 10 and the Southern faulting, between the Ivanhoe and Elma mines. Finally, the Pacific Railroad skirt the northeast side and northwest surveys show that the sedimentary rocks of the Apache Pass end of the mountains. Access to the mountains is mainly fault zone decrease in dip southwestward, where they ap­ from the paved State Highway 186 and the village of Dos parently diverge from the steeply dipping fault planes of the Cabezas to the southwest and from an unpaved county extensive Apache Pass zone. road and Bowie to the northeast.