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The Evolution of Supergene Enrichment in the Morenci

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THE EVOLUTION OF SUPERGENE ENRICHMENT IN THE MORENCI PORPHYRY COPPER DEPOSIT, GREENLEE COUNTY, ARIZONA by Merritt Stephen Enders _________________________ A Dissertation Submitted to the Faculty of the DEPARTMENT OF GEOSCIENCES In Partial Fulfillment of the Requirements For the Degree of DOCTOR OF PHILOSOPHY In the Graduate College THE UNIVERSITY OF ARIZONA 2 0 0 0 2 Merritt Stephen Enders The Evolution of Supergene Enrichment in the Morenci Porphyry Copper Deposit, Greenlee County, Arizona Doctor of Philosophy 3 STATEMENT BY AUTHOR This dissertation has been submitted in partial fulfillment of requirements for an advanced degree at the University of Arizona and is deposited in the University Library to be made available to borrowers under rules of the Library. Brief quotations from this dissertation are allowable without special permission, provided that accurate acknowledgement 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 or her judgement the proposed use of the material is in the interests of scholarship. In all other instances, however, permission must be obtained from the author. SIGNED: _________________________________ 4 ACKNOWLEDGEMENTS This work was initiated because I started a conversation with Dr. P.J Ryan, executive vice president of Phelps Dodge Mining Company, over a beer at the company’s guesthouse in Clifton in May 1994. I invited him to spend a day in the field, unencumbered by meetings, just to review the geology of the district. So many questions came up during our trip, that Pat suggested we find a student to support for some basic geologic research. I spent the five following months looking for such a student but came up empty handed. At the time, I was the Chief Geologist at Morenci, which was mostly an administrative job, and frustrated by not having the time to study the geology of this world class copper deposit. In January 1995, I met with T.R. Snider, president of Phelps Dodge Morenci Inc, and volunteered to go back to school and conduct some research myself. When Tim said he thought that was a good idea, I almost fell over. Today, six years after my initial conversation, I am profoundly grateful to P.J. Ryan, T.R. Snider, J.S. Whisler, A.L. Lawrence, H.M. Conger, W.H. Wilkinson, M.W. Bartlett, and R.J. Stegen for the opportunity and support to study the Morenci district. I feel great about the legacy I leave behind and the experience that I will always carry with me. Thank you. There were expectations associated with this opportunity, however. Not only was I expected to conduct original scientific research from an academic perspective, I was also expected to create value for Phelps Dodge. So, I decided to study supergene mineralization, because of its significant economic impact to the company and myriad of unanswered research questions. To do this, I designed a broad, multi-disciplinary research program to look for evidence at scales from the regional to the microscopic using multiple analytical methods. To get the job done, I leveraged my time and resources by collaborating with a number of other researchers. Charles Ferguson of the Arizona Geological Survey and David Parker of Phelps Dodge contributed their time and expertise on the revised geologic map of the Clifton-Morenci area. Spence Titley provided encouragement, advice, insight, and energy for the detailed study of the supergene mineralization; and when there were no obvious answers, he consoled me by sitting on the muck pile and saying, “God just made it that way.” Gordon Southam and Chris Knickerbocker of the Department of Biosciences at Northern Arizona University opened up the world of bacteria/mineral interactions and provided important laboratory support. Gentlemen, without your help, this study would not have been possible, thank you. There were so many others who contributed to this work and I have tried to acknowledge your help in the text. If I have missed someone, please accept my apologies and know that I am grateful. I would particularly like to thank Roberto Sotelo and Linda Dufek for their help in preparing the figures, Mark Hertel for help with the Medsystem modeling, Tim Orr for the digital cartography, Wes Bilodeau for use of the XRD lab, Ralph Stegen, Bob North and David Parker for the edits, and Mark Barton, Joaquin Ruiz and Bill Chavez for the advice. Finally, I would like to thank Sydney and Maegan for their understanding, support and encouragement to fulfill my dreams. 5 DEDICATION I stand on the shoulders of giants. The hard work and dedication of the geologists who have worked at Morenci and studied the geology of this great district over the last 100 years was of fundamental importance in establishing our understanding today. The careful and thorough documentation of their observations and interpretations has been extremely helpful. So, I would like to dedicate this dissertation to the geologists who have come before us, to those of you working at Morenci today, and to the next generation of geoscientists to study the district. I have tried to respect and acknowledge the work that has already been done, to give credit and thanks to those of you who have helped me during my residence, and to leave behind a record of my work and ideas for your use in the future. Best wishes and good prospecting. Yabadabado! 6 TABLE OF CONTENTS LIST OF FIGURES..................................................................................................... 13 LIST OF ILLUSTRATIONS....................................................................................... 17 LIST OF TABLES...................................................................................................... 18 ABSTRACT ............................................................................................................... 20 INTRODUCTION....................................................................................................... 21 Study Site ...................................................................................................... 22 Location and Physiography................................................................. 22 Hydrogeology ..................................................................................... 23 Climate and Weather .......................................................................... 24 Previous Work .............................................................................................. 24 Morenci District................................................................................. 24 Supergene Enrichment ........................................................................ 26 Classic Concepts of Supergene Enrichment................................................ 27 Previous Notions of Supergene Enrichment at Morenci............................ 30 General Approach......................................................................................... 36 REGIONAL GEOLOGY OF THE CLIFTON-MORENCI AREA – IMPLICATIONS FOR LANDSCAPE EVOLUTION AND SUPERGENE ENRICHMENT IN THE MORENCI MINING DISTRICT 48 Introduction................................................................................................... 48 Pre-existing Geologic Mapping.......................................................... 48 Landscape Evolution and Supergene Enrichment................................ 50 Methods.............................................................................................. 52 Regional Setting............................................................................................ 54 Morenci Block.................................................................................... 56 Stratigraphy ............................................................................ 56 Structure ................................................................................. 57 Mineralization ........................................................................ 58 Duncan Basin...................................................................................... 59 Mid-Tertiary volcanic rocks................................................... 60 Mid-Tertiary to Recent sedimentary rocks ............................. 61 Volcanic Geochronology............................................................................... 62 Materials and Methods ....................................................................... 63 Basalts .................................................................................... 63 Rhyolites................................................................................. 63 7 TABLE OF CONTENTS - Continued Results ................................................................................................ 64 Basalts .................................................................................... 64 Rhyolites................................................................................. 64 Interpretation ...................................................................................... 64 Sequence 1 Older andesitic lava flows ................................. 65 Sequence 2 Younger andesitic lava flows ............................. 66 Sequence 3 Gila Mountains silicic volcanic center............... 69 Sequence 4 Enebro Mountain Formation............................... 70 Sequence 5 Conglomerate of Bonita Creek............................ 71 Sequence 6 Younger
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