Description:
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)