EXPLORATION OF GENETIC LINKS BETWEEN BRECCIA PIPES AND PORPHYRY COPPER DEPOSITS IN A LARAMIDE HYDROTHERMAL SYSTEM, SOMBRERO BUTTE, PINAL COUNTY, ARIZONA By Amy B. Eichenlaub ________________ A Manuscript Submitted to the Faculty of the DEPARTMENT OF GEOSCIENCES In Partial Fulfillment of the Requirements For the Degree of MASTER OF SCIENCE In the Graduate College THE UNIVERSITY OF ARIZONA 2007 2 STATEMENT BY THE AUTHOR This thesis has been submitted in partial fulfillment of requirements for the Master of Science degree at The University of Arizona and is deposited in the Antevs Reading Room to be made available to borrowers, as are copies of regular theses and dissertations. Brief quotations from this manuscript are allowable without special permission, provided that accurate acknowledgment of the 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 Department of Geosciences when the proposed use of the material is in the interests of scholarship. In all other instances, however, permission must be obtained from the author. _______________________________________________ ___________________ (author’s signature) (date) APPROVAL BY RESEARCH COMMITTEE As members of the Research Committee, we recommend that this thesis be accepted as fulfilling the research requirement for the degree of Master of Science. Eric Seedorff___________________________________ __________________ Major Advisor (type name) (signature) (date) Mark D. Barton_________________________________ __________________ (type name) (signature) (date) Spencer R. Titley________________________________ __________________ (type name) (signature) (date) 3 ACKNOWLEDGMENTS I would like to thank Eric Seedorff for his assistance in all aspects of this project; particularly for his time in carefully editing of this manuscript. I would also like to thank Mark D. Barton and Spencer R. Titley for comments and advise on previous versions of this manuscript. I am grateful to Bell Resources Corporation for their financial support for my graduate work. I would also like to especially thank Timothy Marsh for his ongoing support and input throughout this study. I would like to thank many others that assisted me during my graduate work. Thanks to Doug Kriener for his help in formatting and general support during writing. Thanks to David Keeler for helping to keep my sanity and making sure that I took lunch breaks. Thank you to Ken Dominick for his assistance in the use of the electron microprobe and Gary Salais for his careful sample preparation. 4 DEDICATION To Dr. & Mrs. Thomas H. Lake, my grandfather and grandmother who funded my undergraduate education and have supported me throughout my life, To my father, mother, and brother for their love and support, To Kevin for his unconditional love and understanding throughout the past few years, To T.S. Laudon for his inspiration. ‘There’s a land where the mountains are nameless, And the rivers all run God knows where; There are lives that are erring and aimless, And deaths that just hang by a hair; There are hardships that nobody reckons; There are valleys unpeopled and still; There’s a land—oh, it beckons and beckons, And I want to go back and I will.’ Robert Service 5 TABLE OF CONTENTS LIST OF ILLUSTRATIONS ........................................................................................... 8 LIST OF TABLES .......................................................................................................... 10 ABSTRACT ..................................................................................................................... 11 INTRODUCTION .......................................................................................................... 13 MINING AND EXPLORATION HISTORY ............................................................... 16 GEOLOGIC OVERVIEW ............................................................................................. 18 STRUCTURAL SETTING ............................................................................................ 20 LOCAL GEOLOGY- THE AUDACIOUS AREA ...................................................... 22 Laramide volcanic rocks ............................................................................................... 22 Laramide intrusive rocks............................................................................................... 23 Tertiary volcanic rocks ................................................................................................. 24 METHODS AND APPROACH ..................................................................................... 25 Field mapping and core logging ................................................................................... 25 Petrographic analysis .................................................................................................... 26 Electron microprobe analysis ........................................................................................ 27 Geochemical analysis of drill core ................................................................................ 27 BRECCIA PIPES ............................................................................................................ 29 Abundance, geometry, and dimensions of breccia pipes .............................................. 29 Weathering profile ........................................................................................................ 30 Fabric of breccias .......................................................................................................... 31 Clasts ......................................................................................................................... 31 6 TABLE OF CONTENTS-Continued Matrix-supported breccia .......................................................................................... 32 Clast-supported breccia ............................................................................................. 33 Petrographic and microprobe analysis .......................................................................... 33 Hypogene alteration of clasts and wall rocks ............................................................... 36 Grade distribution ......................................................................................................... 39 Geochemical comparison between breccias ................................................................. 41 PORPHYRY ENVIRONMENT .................................................................................... 43 Vein types ..................................................................................................................... 44 Alteration ...................................................................................................................... 44 Breccia pipes and porphyry copper deposits ................................................................ 45 Fluid sources for mineralization ................................................................................... 46 INTERPRETATION OF SOMBRERO BUTTE ......................................................... 48 Geochemistry ................................................................................................................ 48 Mineralizing source ...................................................................................................... 49 DISCUSSION .................................................................................................................. 51 Mechanism of breccia pipe formation .......................................................................... 51 Separation and collection of an aqueous phase ............................................................. 53 Conditions leading to preferential formation of breccia pipes ...................................... 54 Significance of breccia pipes to exploration for porphyry systems .............................. 55 CONCLUSIONS ............................................................................................................. 57 ACKNOWLEDGMENTS .............................................................................................. 59 7 TABLE OF CONTENTS-Continued REFERENCES ................................................................................................................ 60 FIGURE CAPTIONS ..................................................................................................... 67 APPENDIX I Actlabs geochemistry procedures……...………………………………89 APPENDIX II Electron microprobe data...………………………………………….100 APPENDIX III Sombrero Butte geologic summary logs……………..………..…...109 APPENDIX IV Electron microprobe X-ray distribution maps………….………...112 8 List of Illustrations FIGURE 1, General location map of the Sombrero Butte Project……………………….71 FIGURE 2, General geologic map of Sombrero Butte……………………………..........72 FIGURE 3, Outcrop photographs at Sombrero Butte……………………………………73 FIGURE 4, Generalized geologic cross section of Sombrero Butte, looking northwest…………………………………………………………………………………74 FIGURE 5, Generalized geologic cross section of Sombrero Butte, looking northeast………………………………………………………………………………….75 FIGURE 6, Fabrics of breccia………………………………….......................................76 FIGURE 7, Drill hole photographs from SB-03, Campstool breccia pipe……………....77 FIGURE 8, Drill hole photographs from SB-02, Magna breccia pipe…………………..78 FIGURE 9, Photomicrographs showing alteration at Sombrero