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Geology of the Fee Sna L Peak Area Baboquivari Geology of the Fresnal Peak area, Baboquivari Mountains, Arizona Item Type text; Thesis-Reproduction (electronic) Authors Donald, Peter Gordon, 1934- Publisher The University of Arizona. Rights Copyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author. Download date 02/10/2021 03:15:09 Link to Item http://hdl.handle.net/10150/551311 GEOLOGY OF THE FEE SNA L PEAK AREA BABOQUIVARI MOUNTAINS, ARIZONA by Peter G. Donald A Thesis Submitted to the Faculty of the DEPARTMENT OF GEOLOGY In Partial Fulfillment of the Requirements For the Degree of MASTER OF SCIENCE In the Graduate College UNIVERSITY OF ARIZONA 1959 -*• ■-» t vvT...-: i V . 1 : r - - t V '• ■r V *. T ’" T - •* -X — .1 •' i * . --. - - . : ’*j 1 ‘ vl I STATEMENT BY AUTHOR This thesis has been submitted in partial fulfillment of require­ ments for an advanced degree at the University of Arizona and is de­ posited 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 their judgment 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 W. C. £ACY Date Professor of Geology GEOLOGY OF THE FRESNAL PEAK AREA BABOQUIVARI MOUNTAINS, ARIZONA by Peter G. Donald ABSTRACT The Fresnal Peak area is located on the eastern slope of the Baboquivari Mountains approximately 50 miles southwest of Tucson. Granites of undetermined age crop out in the eastern and southwestern portions of the area. The eastern granite exposure con­ tains numerous pegmatite-aplite segregations and fracture injections. Continental type sediments, deposited during late Cretaceous-early Tertiary times, lie marginal to igneous bodies and dip at low angle to the west. Granite-sedimentary rock contacts are marked by shear or fault zones striking N 10-40 W and dipping to the southwest. Structural features are dominated by a tension joint pattern striking N 50-80 W and N 05-25 E. Fractures dip at a high angle to the southwest and east. These are commonly followed by acid and basic dikes of Tertiary age. Narrow zones of mineralization are observed at several points within the Fresnal Peak area. These are characterized by high temperature replacement, extreme selectivity, and lack of continuity. A late magmatic genesis is suggested. t TABLE OF CONTENTS Page INTRODUCTION....................... 1 Location ........................... Method of Investigation . Topography and Drainage Acknowledgments ............. MAJOR ROCK UNITS ............. Igneous Rocks ................... ' • • • Otero G ranite .......... Cataclastic Alaskite Fresnal Granite .... tO CO Ol Ol CJl H* Sedimentary Rocks ..... 11 Conglomerate I .... 11 Conglomerate n ... 12 Arkose ..................... 12 Conglomerate in .. 13 . APLITE-PEGMATITE ......... 14 Pegm atite ............ 14 Aplite ................................ 16 DIKES........................................ 18 D iscussion....................... 18 Felsite ................... 18 Rhyolite Porphyry 19 Hornblende Granite 20 Intermediate.......... 20 Basics ............ 22 S tru c tu re ....................... 25 iv Page STRUCTURE............................................................................................ 27 J o in ts ................................................................................................ 27 D iscussion................................................................................ 27 F a u lts ................................................................................................. 29 D iscussion............................................. 29 GEOLOGIC HISTORY .... 32 Sequence of Events .............................................. .................. .. 32 MINERAL DEPOSITS.............................................................................. 34 Introduction ................... ..................................... .. ............. 34 Diablo P ro s p e c t.............................................................................. 34 Host Rock ............................................................................ 35 Rhyolite ............................ ................. .................... .. 36 A ndesite............................................................................ .. 36 . Primary Assemblage..................... 37 P yrite ....................................... 37 C halco p y rite...................................................... 37 Specularite ................................. 38 Molybdenite.............^................ ............................ 38 O thers .............................................................................. 38 Secondary Assemblage 39 Gangue and Alteration 39 S tru c tu re ................... .. 39 Genesis ....................... 40 Minor Occurrence Sulphide Mineralization 41 Sediments ................... ........................................................... 41 Pegmatites ................... ................ .. 42 Genetic Relationship ................... ................................................. 42 SELECTED REFERENCES ..................................... ........................... 45 - V LIST OF PLATES Plate Page 1. Geologic map of the Fresnal Peak area ..............................in pocket 2. Geologic map of the Diablo Copper Prospect ......... in pocket 3. Surface geology of the. Gold Bullion Mine .................... in pocket 4. Index map of the Fresnal Peak area ...................................... 2 5; View of the Fresnal Peak area west from the Tucson- Sasabe highway ....................................................................... 3 6. Relationship of sedimentary and granitic rocks .................... 6 7. Conglomerate I .................................................. 10 8. "Bull" quartz pegmatite cut by andesite dike ....................... 15 9. Rhyolite porphyry dike- ........................................... 21 - 10. Andesite dike showing sharp intrusive contact with granite country rock ......................................................... 23 11. Northeast-northwest joint patterns ................................... 28 12. Sulphide-bearing pegmatite .................... .......;........... 43 vi INTRODUCTION Location The Fresnal Peak area is located on the eastern slope of the Baboquivari Mountains approximately 50 miles southwest of Tucson and 3 miles northeast of Baboquivari Peak (PL 4). The map area ex­ tends from Contreras Canyon on the north to Brown Canyon on the south, a distance of some 3. 5 miles, and from the Papago Indian Reser­ vation eastward for approximately 4.25 miles (PL 5). Method of Investigation The thesis was completed during the summer and fall of 1958. Seventy days were spent in the field. Geology was mapped on a standard U= S. Geological Survey 15-minute quadrangle sheet enlarged to a scale of 4 inches to 1 mile. Aerial photographs and resection methods were used in determining locations. Structural projections were carried by stereoscopic exam­ ination. A brief reconnaissance study of the Gold Bullion Mine was plotted on a quadrangle sheet enlarged to a scale of 1 inch equals 500 feet. Brunton and tape mapping methods, based on a scale of 1 inch 2 TUCSON PLATE 4 Index Map of Arizona Showing Location of (2 66) the Fresn a I Peak area PLATE 5 View of the Fresnal Peak area west from the Tucson-Sasabe highway. 4 equals 40 feet, were employed for detailing in specific problems. Topography and Drainage The Baboquivaris extend for approximately 26 miles north- northwest from a point near the town of Sasabe at the international border to San Juan Pass just south of the Coyote-Quinlan Mountains and highway 86. The eastern slope of the range is steep and deeply dissected in the vicinity of Fresnal Peak. The map area is characterized by ex­ treme relief. Fresnal Peak at an elevation of approximately 7, 000 feet rises more than 3,000 above inter mountain valleys. The high ridges parallel northeast and northwest structural features. Lower slopes are deeply incised by a drainage pattern of similar trend and are covered by a thin veneer of alluvium which reflects the character of underlying bedrock. Acknowledgments The writer is grateful to Dr. W. C. Lacy, for his many con­ structive suggestions and direction of this thesis. Thanks go also to the Millers of Elkhorn Ranch who provided a base of operations within the Fresnal Peak area. MAJOR ROCK UNITS Both sedimentary and igneous rocks are observed within the Fresnal Peak area (PI. 6). The eastern one-half of the map is dom­ inated by a granitic exposure classified for the purpose of this paper as the Otero granite. A second granite forms the high ridge of Fresnal Peak in its southwest corner. A thick sedimentary section lies to the west of the Otero gran­ ite and is separated from both igneous exposures by well-defined fault or shear zones. Sedimentary rocks are tentatively divided into four
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