0361-0128/05/3560/1605-12 $6.00 1605 Introduction THE SOUTHWESTERN North American province (Arizona, New Mexico and northern Mexico) has one of the highest concen- trations of porphyry copper deposits in the world. The area has been the subject of extensive geological and geochemical studies, but most of these have focused on porphyry copper deposits in Arizona and New Mexico (e.g., Titley, 1982, 2001). In contrast, geological information on the Mexican porphyry copper deposits is sparse. Sillitoe (1976) presented a recon- naissance analysis of 29 Mexican porphyry Cu-Mo deposits, providing the first general overview of these deposits. One limitation of that work was the limited geochronology avail- able at the time. Damon et al. (1983a) published a series of K- Ar dates and interpreted the data in relationship to regional tectonics. Barton et al. (1995) provided a comprehensive tab- ulation and synthesis of porphyry copper deposits in Mexico. This paper examines the age and duration of the hydrother- mal systems and the associated mineralization. The Re-Os isotope system has been recognized as a geo- chemical tool for directly dating sulfide minerals (i.e., Luck and Allegre, 1980; McCandless et al., 1993; Suzuki et al., 2000). Re and Os are chalcophile and siderophile elements (Luck and Allegre, 1980) and hence are concentrated in sul- fide minerals rather than in associated silicate phases. Molyb- denite is particularly useful for this method because it has high concentrations of Re (in the parts per million to wt percent range) and essentially no initial or common osmium. As a re- sult, the Os present in molybdenite is almost entirely the product of the decay of 187 Re. Furthermore, the low errors as- sociated with the age determination (usually between 0.4 and 0.6%) allow us to identify different molybdenite mineraliza- tion pulses that occurred within a very short period of time. Re-Os molybdenite data have been used extensively to deter- mine the age of single deposits (e.g., Chesley and Ruiz, 1997; Stein et al., 1998), but only recently has this isotopic system been used to assess the duration of hydrothermal systems by dating different molybdenite events in single deposits (e.g., Selby and Creaser, 2001; Barra et al., 2003; Maksaev et al., 2004). Few studies have attempted to constrain mineralization Laramide Porphyry Cu-Mo Mineralization in Northern Mexico: Age Constraints from Re-Os Geochronology in Molybdenite FERNANDO BARRA, † Department of Geosciences, University of Arizona, Tucson, Arizona 85721, and Instituto Geología Económica Aplicada, Universidad de Concepción, Concepción, Chile JOAQUIN RUIZ, VICTOR A. VALENCIA, Department of Geosciences, University of Arizona, Tucson, Arizona 85721 LUCAS OCHOA-LANDÍN, Departmento de Geología, Universidad de Sonora, Hermosillo, Mexico JOHN T. CHESLEY, AND LUKAS ZURCHER Department of Geosciences, University of Arizona, Tucson, Arizona 85721 Abstract Twenty-five new Re-Os molybdenite ages for nine porphyry copper-molybdenum deposits from northern Mexico constrain the timing of mineralization and the longevity of porphyry systems in this region. The ages of all the deposits are between 50 and 61 Ma (i.e., Paleocene to early Eocene) and are associated with the Laramide orogeny. The oldest deposits are those from the Cananea district and include the El Alacrán prospect (~61 Ma), the Maria deposit (60 Ma), the current Cananea mine, and La Colorada breccia (~59 Ma). The age of mineralization at Cumobabi also is 59 Ma, and it was followed by the emplacement of deposits at Suaqui Verde and Cuatro Hermanos at about 57 to 56 Ma. The Malpica prospect, the southernmost porphyry copper prospect in western Mexico, was emplaced at 54 Ma. The La Caridad deposit in Nacozari and the El Crestón prospect both have Re-Os molybdenite ages of 54 Ma. The Tameapa prospect has a protracted hydrothermal mineralization history with multiple molybdenite mineralization events at ~57, 52 to 53, and 50 Ma. Total Re and 187 Os concentrations for molybdenites range from 10,424 to 26 ppm and from 6,641 to 14 ppb, respectively. The deposit with the highest Re and Os concentrations is El Alacrán, and the lowest concentra- tions are found in El Crestón. The ages determined support the formation of porphyry copper deposits in northern Mexico during the Laramide orogeny. In general, the ages of porphyry copper deposits in the North American province range from ~76 Ma at Bagdad, Arizona, to ~50 Ma at Tameapa, Sinaloa. The ages also show that porphyry copper mineralization occurred continuously between these two dates, with two main episodes at ~60 Ma (Cananea district, México, and Sierrita and Copper Creek, United States) and at ~54 Ma (La Cari- dad, El Crestón, Malpica in Mexico and Morenci, United States). † Corresponding author: e-mail: fbarra@geo.arizona.edu ©2005 Society of Economic Geologists, Inc. Economic Geology, v. 100, pp. 1605–1616