Mid-Tertiary geology and geochronology of the Clifton-Morenci Area, Greenlee and Graham Counties, Arizona and adjacent New Mexico

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). Since then, the role of pre-volcanic enrichment has been questioned (Cook, 1994; Enders, 2000). Understanding the evolution of supergene enrichment at Morenci is of critical importance, not only because of this deposit’s economic significance, but also in terms of our understanding of the evolution of porphyry copper systems in general. As a result, Phelps Dodge Morenci Inc. initiated a research program in 1997 to address unanswered questions concerning the Tertiary geologic history of the Clifton-Morenci area. The principal result of this effort was a Ph.D. program undertaken by M. Stephen Enders at the University of Arizona that included a re-examination of the regional structure and Tertiary stratigraphy of the Clifton-Morenci area. The detailed geologic map, structural cross-sections, and the stratigraphic, geochronologic, and structural observations contained in this report are the principal products of this aspect of the research program. (69 pages)Documents in the AZGS Document Repository collection are made available by the Arizona Geological Survey (AZGS) and the University Libraries at the University of Arizona. For more information about items in this collection, please contact azgs-info@email.arizona.edu