Abstract

The Guerrero terrane of central and western Mexico comprises volcanic and sedimentary strata of mainly Mesozoic age, with geochemical signatures indicating formation in juvenile back-arc to slightly more evolved arc environments. At least 60 volcanogenic massive sulfide (VMS) deposits have been discovered thus far within the Guerrero terrane. They comprise two main belts: the Coastal belt mainly comprising deposits in the Zihuataneo subterrane (including the Cuale-Bramador district), and the Central belt, which includes deposits in the Teloloapan subterrane (Campo Morado, Tlanilpa-Azulaquez, Tizapa, Rey de Plata, etc.), the Guanajuato subterrane (El Gordo, Las Gavalinas), and the Zacatecas subterrane (San Nicolas and El Salvador deposits). The age of host rocks for the VMS mineralization in the various subterranes was poorly constrained prior to this study. Eleven new U-Pb zircon ages for volcanic host rocks from the Campo Morado, Tlanilpa-Azulaquez, Leon-Guanajuato, and San Nicolas-El Salvador VMS districts, together with recently determined U-Pb ages from the Cuale area, demonstrate that VMS deposits in the Guerrero terrane range from latest Middle Jurassic to early Early Cretaceous (Callovian to Valanginian) in age. The oldest ages were obtained from the Cuale district (total age range of 157.4 ± 4.1 Ma for three units) and the youngest are from the Tlanilpa-Azulaquez district (total age range of 139.7 ± 2.5 Ma for two samples). Volcanogenic massive sulfide deposits in the Central belt resemble those of the bimodal-siliciclastic deposit type, whereas those in the Coastal belt are more similar to the bimodal-felsic type. Published geochemical and radiogenic isotope data from the various subterranes suggest that VMS deposits in the Coastal belt and most of those in the Central belt are hosted within juvenile to slightly evolved arc settings. The San Nicolas and El Salvador deposits of the Zacatecas subterrane are the only deposits that unequivocally formed in a back-arc setting. Our new U-Pb dating results, together with existing stratigraphic, geochemical, and radiogenic isotope data from throughout the Guerrero terrane, are consistent with the Guerrero terrane having developed as a west-facing continental margin arc that was built on mainly oceanic crust (and overlying Early Mesozoic siliciclastic fans shed from the east) along the western margin of nuclear Mexico. Slab rollback in the Middle Jurassic led to the development of a “Rocas Verdes-type” continental back-arc inboard of the arc, which is now preserved as the Arperos basin in the central and southern Guerrero terrane and the host rocks for the San Nicolas and El Salvador deposits farther to the north in southeastern Zacatecas State.

We have determined Pb isotope compositions of sulfides from VMS deposits and several other styles of mineralization from throughout the Guerrero terrane. A compilation of these and previously published data shows that isotopic compositions from the various VMS deposits fall in a relatively confined field that overlaps the average upper crustal growth curve. This suggests that metals were mainly sourced from continentally derived sedimentary strata that are interlayered with or immediately underlie the host volcanic section and/or from the host volcanic rocks themselves (or subvolcanic intrusive equivalents) that had assimilated significant quantities of upper crustal material either at the point of origin or during subsequent ascent and eruption. Lead isotope compositions from the Francisco I Madera deposit in Zacatecas State indicate that most of the sulfides in this deposit are epigenetic rather than syngenetic in origin and are likely Tertiary in age.

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