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March 3 2003

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David Maher geodave@mindspring.com
Ph.D. candidate
Hometown: New Berlin, Wisconsin

  • BS, Geology, University of Wisconsin - Oshkosh (1991)

  • MS, Economic Geology, Petrology, Oregon State University (1995)
    Major Advisor: John Dilles

A new look at middle Tertiary dismemberment and rotation of Laramide porphyry systems in southern Arizona (Abstract submitted to SEG for the April 2002 meeting in Denver, CO.)

In southern Arizona, crustal extension has dismembered, displaced, and rotated numerous porphyry copper and associated igneous systems. From Globe-Miami to San Manuel, reevaluation of well-mapped quadrangles and new observations permits an improved assessment of middle Tertiary crustal extension and its profound impact on the distribution of igneous-related Laramide copper mineralization. This provides an improved framework for exploration for hidden mineral deposits (e.g., Wilkins and Heidrick, 1995) and development of accurate deposit models. Analogous dismemberment and rotation during Tertiary extension has been documented in well-studied systems such as Yerington, in western Nevada, and Robinson, in east-central Nevada (Dilles et al., 2000, and Gans et al., 2001).

The Globe-Miami and San Manuel to Kelvin areas in Arizona exhibit similar relationships to Yerington, where domains of =100% extension with multiple generations of faults collectively cut and rotate paleo-up from <10 to >90. The area around Ray Arizona is more akin to Robinson where regional tilt domains overlap. Here, multiple generations of faults with opposing displacement directions result in a number of smaller fault blocks that underwent a more complex tilt history. The present orientation of individual blocks is the sum of the tilts of the overlapping fault events.

Models of hydrothermal alteration in porphyry systems were strongly influenced by exploration-driven work in this region in the 1950s and 1960s when dismemberment of porphyry systems by normal faults was first recognized (e.g., Lowell and Guilbert, 1970). Evolving understanding of regional-scale, post-mineral structure (e.g., Dickinson, 1991; Wilkins and Heidrick, 1995; Howard and Foster, 1996; Richard and Spencer, 1998) remains critical to reconstruction of both hydrothermal and related igneous systems. Key issues include determination of paleo-up, identification of all deposit fragments (including eroded portions), and clarifying what constitutes variation in mineralogical zoning patterns versus structural juxtaposition of different portions of hydrothermal systems. Tertiary extension was also important to the supergene history through rapid exhumation of deposits and modification of groundwater flow patterns.

Many features provide pierce points and help establish that extension exceeded 100% across a large fraction of the region. Key features include contacts between Mesoproterozoic granitoids and metamorphic rocks; Neoproterozoic diabase dikes and sills; Paleozoic platform sequences and Laramide stocks, dikes and volcanic rocks. Mid-Tertiary volcanic and sedimentary rocks show complex fanning relationships and development of unconformities contemperanous with extension. These complex relationships are seen in map view as approximate paleo-cross sections and establish the structural pierce points in the faulted blocks that have been displaced more or less laterally relative to the present day surface (e.g., as well-documented at Yerington, Proffett and Dilles, 1984). The structural pierce points observed in the study in southern Arizona allow the reconstruction of numerous fault blocks that contain mineral deposits, and highlight areas that may contain faulted segments of known deposits.

Given that many of the mineral deposits in southern Arizona are fault-bounded, exploration for deposits in this terrain has long utilized structural interpretations. A revised interpretation of regional extension opens windows for finding hidden deposits or displaced fragments of known deposits. In a number of these cases, such as at San Manuel (Lowell, 1968) the offset portions of the deposit have been identified. Others, however, such as at Kelvin or Superior East, have not. Reconstructing the structural history of the region provides an opportunity to identify and discover missing portions of the individual porphyry systems and understand how individual deposits within districts are related to each other. Application of regional structural understanding to the deposit scale also has substantial economic implications from feasability through production. For example, understanding faults within deposits is key to generating accurate reserve calculations, mine planning, and anticipating dilution effects and predicting ore-waste boundaries for ore control.

Selected publications:

Dickinson, W. R., 1991, Tectonic setting of faulted Tertiary strata associated with the Catalina core complex in southern Arizona: Boulder, CO, United States, Geological Society of America (GSA), 106 p.

Dilles, J. H., Proffett, J., and Einaudi, M. T., 2000, Field trip day two: Magmatic and hydrothermal features of the Yerington Batholith with emphasis on the porphyry Cu-(Mo) deposit in the Ann-Mason area, in Thompson, T. B., ed., Society of Economic Geologists Guidebook, 32, p. 67-89.

Gans, P. B., Seedorff, E., Fahey, P. L., Hasler, R. W., Maher, D. J., Jeanne, R. A., and Shaver, S. A., 2001, Rapid Eocene extension in the Robinson district, White Pine County, Nevada; constraints from (super 40) Ar/ (super 39) Ar dating: Geology (Boulder), v. 29, p. 475-478.

Howard, K. A., and Foster, D. A., 1996, Thermal and unroofing history of a thick, tilted Basin-and-Range crustal section in the Tortilla Mountains, Arizona: Journal of Geophysical Research, B, Solid Earth and Planets, v. 101, p. 511-522.




dmaher1_v.jpg | dmaher2_v.jpg

caption: Dave records paleo-up in Hackberry Wash, central Tortilla Mountains, AZ. [click on pictures for full size images]
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University of Arizona
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