Abstract:

The Pitarrilla Ag-Zn-Pb deposit, Mexico, is hosted by Cretaceous, Eocene, and Oligocene strata that record a complex volcano, sedimentary, structural, and hydrothermal history. Deformed Cretaceous rocks form the basement to unconformably overlying Eocene and Oligocene volcanic strata. The Eocene volcaniclastic strata were sourced directly during explosive arc volcanism and from the erosion of subaerial arc volcanoes, with the clastic material transported by sedimentary gravity flows and deposited into a below storm wave base basin that developed within a back-arc extensional setting. Uplift of the arc during the Eocene, was accompanied by extension, and voluminous silicic pyroclastic volcanism, which is manifested by ignimbrite and pyroclastic surge deposits dated at 49.8 ±1.0 Ma. Erosion during the Eocene and early Oligocene was accompanied or followed by northeast- and north-northwest-trending faulting, the emplacement of rhyolitic and andesitic sills and dikes, and a 31.59 ± 0.52 Ma rhyolitic dome.<br/>Trace, rare earth element, and Pb and Nd isotopic data indicate that the Eocene and Oligocene intermediate volcanic strata are typical of calc-alkaline arc dacites and andesites, and were derived from a mantle derived mafic magma that was contaminated by older arc crust during subduction. In contrast, the Eocene and Oligocene felsic volcanic strata are atypical of arc rhyolites, and were derived from magmas generated by partial melting and assimilation of arc crust during periods of crustal thinning that developed in response to episodes of arc extension and rifting.<br/>The Pitarrilla Ag-Zn-Pb deposit is characterized by iron oxide- and sulfide-associated mineralization, which defines a mushroom-shaped and vertically zoned ore system centered on rhyolitic dikes and sills that constitute the feeder system for an early Oligocene bimodal volcanic center manifested by a rhyolitic dome, and rhyolitic and andesitic sills and dikes. The main Ag-Zn-Pb mineralization event is interpreted to have occurred during or after emplacement of the early Oligocene rhyolitic dome. The sulfide-associated mineralization is rooted in the Cretaceous sedimentary basement and is represented by an areally restricted, but vertically extensive zone of disseminated and vein-hosted Ag-Zn-Pb (-Cu-As-Sb) sulfide mineralization and strata-bound replacement mineralization that was localized within conglomerates that occur at the Cretaceous-Eocene unconformity. The disseminated and vein sulfide mineralization extends upwards into the overlying Eocene and Oligocene volcanic strata and rhyolitic sills, where it grades into a laterally more extensive, crudely semiconformable zone of disseminated silver and base metal (Ag-Zn-Pb) mineralization that was later altered by supergene processes to form the iron oxide-associated mineralization. Two gold prospects containing gold bearing quartz (± calcite, fluorite) veins that crosscut deformed Cretaceous sedimentary strata occur immediately at the north-northwest and northeast of Pitarrilla. The timing and relationship of the gold mineralization to the silver and base metal mineralization is uncertain.<br/>The restricted lateral extent and vertical stacking of alteration types at Pitarrilla, from iron carbonate and chlorite (± sericite) at depth, upward to tourmaline and argillic alteration defines a crudely discordant alteration pipe that contains the bulk of the sulfide- and iron oxide-associated mineralization. This alteration pipe is surrounded by semiconformable zones of propylitic and weak argillic alteration. The tourmaline alteration, which is atypical of epithermal deposits, may be a product of an earlier and deeper porphyry system that preceded a later magmatic hydrothermal system responsible for the silver and base metal mineralization. Alteration in proximity to the mineralization was so intense that the HFSE, REE, and Nd and Pb isotopes that<br/>are widely used in petrogenetic studies were mobile for some host rocks during hydrothermal alteration at Pitarrilla.