Mid-Tertiary geology and geochronology of the Clifton-Morenci area, Greenlee and Graham Counties, Arizona, and adjacent New Mexico by Charles A. Ferguson1, M. Stephen Enders2, Lisa Peters3, and William C. McIntosh3 Arizona Geological Survey Open-file Report 00-07 August, 2000 Arizona Geological Survey 416 W. Congress, Suite 100, Tucson, AZ 85701 Also available in Adobe® PDF format with DI-18 and DGM-01 1 Arizona Geological Survey, 416 W. Congress, #100, Tucson, AZ 85701 email:
[email protected] 2 Phelps Dodge Morenci, Inc., 4521 Highway 191, Morenci, AZ 85540 3 New Mexico Institute of Mining and Technology, 801 Leroy Place, Socorro, NM 87801 Mid-Tertiary geology and geochronology of the Clifton- Morenci area, Greenlee and Graham Counties, Arizona, and adjacent New Mexico INTRODUCTION The geology of the Clifton-Morenci area has been investigated for nearly 100 years beginning with the pioneering work of W. Lindgren at the start of the 20th century. For most of the century, geological investigation focused on mineralization and the complex suite of Eocene porphyritic intrusive stocks and dikes in the area. Very little attention was directed at the regional structural and stratigraphic relationships in the overlying, and largely unmineralized Tertiary volcanic rocks. From the beginning, geologists have understood that supergene enrichment played a critical role in making the Morenci district a world class porphyry copper deposit (Lindgren, 1905a). For much of the past 90 years, enrichment was thought to have occurred during two phases of exposure and erosion: pre-volcanic and post-volcanic (Lindgren, 1905a; Moolick and Durek, 1966; Langton, 1973; North and Preece, 1993).