This study was undertaken to obtain reconnaissance isotopic ages on tin, mercury, fluorine, and antimony mineralization in northern Mexico and to determine their temporal relation to the metallogenic evolution of the continental volcanic arc in this region. Deposits were dated using conventional K-Ar and 40 Ar/ 39 Ar analyses of whole rocks and phenocryst separates for altered and unaltered pre- and postore samples. Ore-related rhyolites in three tin deposits (Sombrerete, La Ochoa, and Cerro de los Remedios) formed at 30 to 31 m.y. ago and mineralization probably took place at the same time. The Guadalcazar stock, which hosts disseminated cassiterite, formed at about 28 m.y. ago. Mineralization in the Canoas mercury district formed at 26 to 27 m.y., although mineralization in the other two districts studied (Sain Alto and El Cuarenta) could be limited only to post-37 m.y. and post-40 m.y., respectively. Fluorite deposits hosted by limestone in the Zaragoza and Rio Verde districts formed at 30 to 31 m.y., and smaller, volcanic-hosted deposits in the Inde district could have formed as early as 38 m.y. The only radiometric age related to an antimony district obtained in this study was 36 m.y. for the granodiorite stock at Santa Maria de la Paz, north of the Wadley district. These ages demonstrate that the Sn-Hg-F-Sb mineralization was essentially contemporaneous with the latest phases of base and precious metal mineralization in northern Mexico and suggest that formation of deposits containing these metals is a special stage in the metallogenic evolution of volcanic arcs, which requires a relatively thick crust that is conducive to extensive magmatic differentiation and silicic volcanism.

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