HYPOGENE ALTERATION AND MINERALIZATION IN THE DOS POBRES PORPHYRY CU(-AU-MO) DEPOSIT, SAFFORD DISTRICT, ARIZONA: A GOLD- AND MAGNETITE-RICH VARIANT OF ARIZONA PORPHYRY COPPER SYSTEMS By Daniel Russin _________________ A Thesis Submitted to the Faculty of the DEPARTMENT OF GEOSCIENCES In Partial Fulfillment of the Requirements For the Degree of MASTER OF SCIENCE In the Graduate College THE UNIVERSITY OF ARIZONA 2 0 0 8 2 STATEMENT BY THE AUTHOR This thesis has been submitted in partial fulfillment of requirements for the Master of Science degree at The University of Arizona and is deposited in the Antevs Reading Room to be made available to borrowers, as are copies of regular theses and dissertations. Brief quotations from this manuscript are allowable without special permission, provided that accurate acknowledgment of the source is made. Requests for permission for extended quotation from or reproduction of this manuscript in whole or in part may be granted by the Department of Geosciences when the proposed use of the material is in the interests of scholarship. In all other instances, however, permission must be obtained from the author. Daniel Russin______________________________ ___________________ (author’s signature) (date) APPROVAL BY RESEARCH COMMITTEE As members of the Research Committee, we recommend that this thesis be accepted as fulfilling the research requirement for the degree of Master of Science. Mark D. Barton_____________________________ __________________ Major Advisor (type name) (signature) (date) Eric Seedorff_______________________________ __________________ (type name) (signature) (date) Jon Patchett________________________________ __________________ (type name) (signature) (date) 3 Hypogene alteration and mineralization in the Dos Pobres porphyry Cu(-Au-Mo) deposit, Safford District, Arizona: A gold- and magnetite- rich variant of Arizona porphyry copper systems. Daniel Russin* Mark D. Barton Eric Seedorff Center for Mineral Resources, Department of Geosciences, University of Arizona, Tucson, Arizona 85721-0077 * E-mail: [email protected] Abstract The Dos Pobres Cu(-Au-Mo) deposit (211 million metric tonnes sulfide resource 0.73% Cu with up to 1 ppm Au) is located in the Safford district of southeastern Arizona and is one of several Au-bearing porphyry systems in Arizona. The deposit is centered on Paleocene (57 Ma) quartz monzodioritic porphyry dikes that intrude and alter Late Cretaceous (67-73 Ma) basaltic andesites. The porphyry dikes locally contain igneous anhydrite. The volcanic rocks dip gently (10-15°) to the northeast; they and the ENE- trending porphyry dikes are cut by a NW-striking down-to-the-west normal fault that down-drops the southwestern portion of the Dos Pobres system by roughly 1 km. This study focuses on the characterization and distribution of veins, hydrothermal alteration, and ore minerals below the base of weathering, utilizing core logging and petrography 4 coupled with whole-rock geochemical and electron microprobe analyses, as well as U-Pb and Re-Os geochronology. Hypogene veins at Dos Pobres are divided into groups based on their mineralogy, textures, and alteration envelopes. Five early vein types have envelopes that are dominated by biotite and/or K-feldspar. These are: hairline biotite (biotite ± magnetite ± bornite); sugary quartz (quartz + K-feldspar ± sulfide (bornite chalcopyrite) ± biotite ± anhydrite); comb quartz (inward-growing quartz + K-feldspar ± sulfide (bornite < chalcopyrite) ± biotite); complex biotite (biotite + K-feldspar + quartz + sulfide (pyrite < bornite < chalcopyrite) ± anhydrite); and green mica veins (biotite + sericite + K-feldspar ± sulfide (bornite chalcopyrite) ± anhydrite ± andalusite). Two types of veins with chlorite ± sericite envelopes are sulfide-sericite (sulfide (chalcopyrite > pyrite) ± quartz ± sericite ± chlorite ± anhydrite) and clotty sulfide-chlorite (quartz ± sulfides (chalcopyrite > pyrite) ± chlorite ± sericite ± anhydrite). These veins cut those with biotite and/or K-feldspar envelopes. Veins consisting of chlorite + epidote + calcite + quartz ± sulfide (pyrite > chalcopyrite) with chlorite + epidote + calcite envelopes are also common and cut those with chlorite ± sericite envelopes. Zeolite veins without alteration envelopes cut all other vein types. Potassic alteration assemblages with pervasive biotite ± K-feldspar ± magnetite comprise the best developed and earliest alteration suite. It is most intense in proximal locations where quartz + K-feldspar veins locally compose upwards of 30 vol percent of the rock. The intensity of this alteration diminishes outward and upward. Scattered biotite + actinolite-hornblende + magnetite alteration is interpreted to represent 5 the fringes of pervasive potassic alteration. Hydrolytic alteration, characterized by sericite- and chlorite-rich replacement of feldspars and biotite is later and/or peripheral, partially overprinting potassic alteration. Propylitic alteration (chlorite ± epidote ± calcite) forms a weakly defined zone that overprints the earlier assemblages and shows a gradational boundary with unaltered host rock. Whole-rock geochemical analyses indicate that the Dos Pobres rocks have unextraordinary igneous compositions but that many of them have experienced significant metasomatic addition of K2O, whereas hydrolytic alteration is quantitatively minor. Hypogene sulfides are abundant and systematically distributed beneath the base of oxidation (depth ~350 m). Early bornite (± chalcocite ± digenite) dominates the deep core of the deposit coincident with the most intense K-silicate alteration and quartz veins. There is a transition from these through bornite-chalcopyrite assemblages into chalcopyrite-dominated veins. The chalcopyrite-bearing veins typically cut bornite-rich veins and surround the bornite-dominated core but are also associated with K-silicate alteration. Pyrite (± chalcopyrite)-bearing veins have hydrolytic envelopes and are most abundant in a ring-shaped zone surrounding the core. Molybdenite is rare but is most commonly associated with chalcopyrite in deep, flanking veins with associated hydrolytic alteration. Gold occurs as tiny (mostly <10 μm) grains of electrum (~10-15 wt % Ag) and sylvanite in early bornite. Silver is also present in early bornite, occurring as electrum, hessite, sylvanite, and a silver sulfide mineral (argentite?). Silver also occurs as hessite and sparse argentiferous galena (up to ~2.5 wt % Ag) that commonly rim and crosscut all 6 Cu-Fe sulfides. Supergene bornite, chalcocite, and rare covellite locally replace hypogene sulfides beneath the oxide zone. Dos Pobres is unusual among Arizona porphyry deposits in that it is relatively gold- and bornite-rich, and magnetite-bearing and is intimately associated with relatively mafic (quartz monzodiorite - low silica granodiorite) porphyry dikes. Conversely, it has considerably less pyrite and acid alteration than most other Arizona porphyry deposits. These features are like those in many other Au-rich porphyry systems, and they have also stimulated comparisons with andesite-hosted iron-oxide(-Cu-Au) (IOCG) systems such as Candelaria. At Dos Pobres, as in most Cu-Au-Mo porphyry deposits, Cu-Fe sulfides are deposited with voluminous early quartz veins, magnetite is widespread but minor in abundance, bornite is the dominant early ore mineral, deposition of pyrite postdates most deposition of Cu, and there is a close spatial and temporal association of mineralization with relatively mafic porphyry intrusions – all features consistent with introduction and cooling of magmatic fluids. These characteristics differ profoundly from andesite-hosted IOCG deposits, which typically have abundant Fe-oxides (tens of percent), REE enrichment, voluminous Na-Ca and/or K-Ca alteration, comparatively sparse quartz veins, and generally late copper mineralization. Introduction The Dos Pobres porphyry Cu(-Au-Mo) deposit is located in the North American Laramide porphyry belt, which extends from western Mexico into Arizona and New 7 Mexico. This region is richly endowed with porphyry Cu-(Mo) deposits, yet Dos Pobres, one of at least four porphyry deposits in the Safford district (Langton and Williams, 1982), represents one of the few porphyry Cu(-Au-Mo) deposits, a distinctive family of porphyry deposits that are commonly associated with relatively mafic intrusive rocks (Seedorff et al., 2005), that has been documented to date. The spatial and temporal association of Dos Pobres with comparatively Au-poor porphyry Cu-Mo deposits in the region and perhaps in the Safford district poses questions about comparisons between these systems and the processes that form them. Many hypotheses have been presented, including differences in composition and/or thickness of underlying crust (Hollister, 1975), differences in emplacement depth or wall-rock properties (Kesler, 1973), vertical zoning of Cu and Au coupled with different erosion levels (Titley, 1982), composition of mineralizing intrusions (Kesler, 1973; Sillitoe, 1979), and other complex magmatic geochemical factors (Sillitoe, 2000). As more of these deposits have been discovered and described, it is apparent that these systems show differences among themselves and that none of these hypotheses is sufficient – thus there is the need for continued systematic study of the deposits themselves and for their comparison with others. This study, which was sponsored by Phelps Dodge Exploration (now Freeport- McMoRan Copper and Gold), was undertaken with the twin