U-PB ZIRCON AGE AND PB ISOTOPIC CONSTRAINTS ON THE AGE AND ORIGIN OF VOLCANOGENIC MASSIVE SULFIDE DEPOSITS IN THE GUERRERO TERRANE OF CENTRAL MEXICO

The Guerrero Terrane (GT) of central and western Mexico mainly comprises volcanic and sedimentary strata of Jurassic to Cretaceous age, with geochemical signatures consistent with formation in back-arc and juvenile arc settings. Volcanogenic massive sulfide (VMS) deposits are widely distributed within the GT, and define several spatially distinct VMS districts. The exact age(s) of the VMS host rocks in the various districts has until this point only been loosely constrained by available fossil ages, very limited U-Pb zircon dating, and regional stratigraphic correlation. We have obtained U-Pb zircon ages for volcanic host rocks from several VMS districts in the GT: SE Nahuatl subterrane - Campo Morado (137.4 to 145.9 Ma; 4 samples) and Mamatla (138.7 to 139.7 Ma; 2 samples); NW Nahuatl subterrane - Cuale (155.9 to 162.4 Ma; 2 samples); and SE Tepehuano subterrane - San Nicolas-El Salvador (147.9 to 150.6 Ma; 5 samples) and Leon-Guanajuato (146.1 Ma; 1 sample). Our data indicate that most of the VMS deposits dated thus far formed within a short period between latest Jurassic and earliest Cretaceous. However VMS deposits in the Cuale District in western Jalisco are somewhat older (late Middle Jurassic). Pb isotopic analyses of sulfides from throughout the GT show a clear distinction between syngenetic (mainly VMS) and epigenetic deposits. Most Pb isotopic compositions for syngenetic occurrences from various districts define overlapping fields. The La Blanca deposit in Colima and the Madrono occurrence in the Leon-Guanajuato District are hosted within a similar limestone unit and yield similar sulfide Pb compositions and considerably older Pb model ages than the other syngenetic deposits in the GT. The significance of the older model ages for these two deposits is still uncertain.