Sulfide-rich Proterozoic Mafic Rocks and Arizona Porphyry Copper Deposits - A Connection?
by Arend Meijer
Abstract: The Paleoproterozoic Pinal terrane comprises the basement (i.e., oldest) rock of southern Arizona. It has the characteristics of a fore-arc/subduction complex and most likely formed a large fraction of the original crust of southern Arizona. In addition to tectonic mélanges and thick sequences of turbidites, it contains Paleoproterozoic mafic rocks that include oceanic crust (based on trace element characteristics) overlain by ferruginous chert. In analogy with modern-day oceanic crust, some fraction of these oceanic rocks likely contain stockworks of sulfide veins and even massive sulfides originally produced at Paleoproterozoic oceanic spreading centers. There are also mafic metavolcanic rocks with arc affinities within the Pinal terrane and these could also have associated sulfide bodies. Most of the known porphyry copper deposits in Arizona are situated within the Pinal terrane. Further, isotopic and trace element data suggest most porphyry copper plutons in southern Arizona contain large fractions of recycled Paleoproterozoic crust with some consisting almost entirely of recycled crust. This suggests the metals in the porphyry copper deposits were likely also derived dominantly from the continental crust. Paleoproterozoic oceanic crust within the Pinal terrane and the sulfides that may be associated with it are here argued to have been sources for a large portion of the metals and the sulfur found in the late Mesozoic-early Tertiary porphyry copper deposits of southern Arizona. Paleoproterozoic arc-related fore-arc metavolcanic rocks and Neoproterozoic mafic rocks may have been additional sources. In analogy with Phanerozoic subduction complexes, the distribution of Paleoproterozoic oceanic crust with depth (up to 35 km) in the Pinal subduction complex is likely irregular. Further, the distribution of sulfide bodies within Paleoproterozoic mafic crust was likely also irregular. Such irregular distributions of source rocks in southern Arizona suggest a model in which magmas with mineralization potential (i.e., “wet” magmas of intermediate composition) would only be extensively mineralized with copper, other metals, and sulfur if their sources included significant proportions of sulfide-rich Proterozoic mafic crust. Thus, a mineralized pluton could have formed in fairly close proximity to a barren pluton of approximately the same age if the former contained a large component of sulfide-rich Proterozoic mafic crust while the latter did not.
Bio: Dr. Meijer received a doctorate in geology in 1974 from the University of California at Santa Barbara. The title of his doctoral dissertation was “A Study of the Geochemistry of the Mariana Island Arc System and its Bearing on the Genesis and Evolution of Volcanic Arc Magmas”. His dissertation advisors included Drs. George R. Tilton, Daniel E. Karig, and Clifford A. Hopson. Following the completion of his dissertation work, Dr. Meijer conducted post-doctoral studies with Dr. Krishna A. Sinha at Virginia Polytechnic Institute and Dr. Lee T. Silver at California Institute of Technology before joining the Geosciences faculty at the University of Arizona in 1976. In addition to teaching undergraduate and graduate courses, he conducted field and laboratory studies focused on volcanic rocks in the Mariana island arc system. In 1984, he accepted a position at Los Alamos National Laboratory in New Mexico. In addition to work on classified projects, he carried out studies of the U-Th-Pb isotope systematics of ultramafic nodules from the mantle. In 1994, Dr. Meijer started his own consulting company to provide geochemical expertise to chemical companies, mining companies, and oil companies. He retired from the company in 2008. Dr. Meijer has published over 50 papers in peer-reviewed journals in addition to several books. Although originally from southern California, he prefers the geological variety and freedom of movement available in southern Arizona.