Four pairs of active volcanoes along the northern Central American volcanic front have erupted basalt‐andesite magmas that show consistent intrapair behavioral and compositional differences. These differences are found in records of volcanic activity and complete major and minor element data on over 200 samples. From northwest to southeast along the volcanic front the four volcano pairs are Cerro Quemado‐Santa María, Tolimán‐Atitlán, Acatenango‐Fuego, and Santa Ana‐Izalco. The volcano pair relations help explain compositional differences, apart from those reflecting variation in crustal thickness of about 15 km along the volcanic front, providing insight into across‐arc variations and closely spaced subvolcanic plumbing systems. Intravolcano pair spacing is less than 5 km compared with an average intervolcano spacing of 25 km along the entire volcanic front. Within each volcano pair, the seaward volcano has had more frequent historic activity, erupting magmas that are generally more mafic, lower in large ion lithophile elements and higher in Na2O/K2O than magmas erupted from its landward counterpart. Each paired volcano site lies in close proximity to a rhyolitic caldera, situated north or northeast of the volcano pair. However, rare earth element data at the Tolimán‐Atitlán volcano pair imply that mixing between caldera rhyolite and the mafic magma of the paired volcanoes does not occur. Petrographie, isotopic, and other geochemical data from the Tolimán‐Atitlán volcano pair suggest that separate but contemporaneous magma bodies beneath each volcano evolve and pass through the crust at different rates. Atitlán magmas are processed through the crust more efficiently and with greater frequency than Tollman magmas, which undergo longer periods of stagnation interrupted by mafic injection and rapid eruption. This relation appears to hold at the other paired volcano sites and is further evidence that closely spaced volcanoes, with similar subcrustal magma sources, evolve over separate magmatic plumbing systems that traverse the crust. The pairing pattern probably reflects the regional southward migration of the volcanic front.
The DeLamar silver mine, located in southwest Idaho, produced over 17 million ounces of silver and 230 thousand ounces of gold by bulk mining methods between 1977 and late 1987. The mine occurs within the Silver City rhyolite--an extensive and complex sequence of middle Miocene silicic volcanic flows and domes that unconformably overlie a thick pile of Miocene alkali olivine basalt. The basalt overlies Late Cretaceous granite which is correlative with the Idaho batholith. All three major rock types contain epithermal silver-gold veins, although rhyolites such as those at DeLamar, are the only hosts for low-grade, large tonnage mineralization.The DeLamar mine occurs within a sequence of six pervasively altered silicic volcanic
units. From oldest to youngest these units have the following informal designations: (1) porphyritic latite, (2) quartz latite, (3) tuff breccia, (4) porphyritic rhyolite, (5) banded rhyolite, and (6) millsite rhyolite. The porphyritic rhyolite unit is spatially, temporally, and genetically related to silver-gold mineralization and is the principal host rock at DeLamar.This unit was emplaced as coalescing and overlapping lava domes and was preceded by cogenetic eruption of tuff breccia in the form of tuff rings. The emplacement of tuff rings and lava domes was controlled by structures related to a shallow level magma body and regional basin-and-range faulting. Onlap of banded rhyolite preceded hydrothermal activity and served as an important control on localizing ore deposition. The mineralogic character of ore and alteration at DeLamar is similar to adularia-sericite-type volcanic-hosted epithermal deposits, but the geologic setting is more like that of acid-sulfate deposits.
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